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## Meet the editor

Dr. Uday M. Basheer Al-Naib is a senior lecturer at Universiti Teknologi Malaysia. He completed a postdoctorate in Materials Engineering at Universiti Sains Malaysia (USM) between 2013 and 2014. He received his Ph.D. in Materials Engineering from USM in 2013 and obtained his master's and bachelor's degrees in chemical engineering from the University of Baghdad and the University of Technology, Iraq, respectively. He has more than

30 years of academic and practical experience in the field of ceramic and metal composites, metal alloys, advanced ceramics, and ceramic–metal joining. He is the editor of a book series from IntechOpen. In addition, he is a recognized reviewer for Elsevier. He is acknowledged at the practical level for solving specific problems related to materials industries. This was evident when he published his research work with different publishers and in international material engineering journals with high impact factors. In addition to his own research, Dr. Uday has been acting as a supervisor of several academic theses in different fields of materials engineering. He received his CEng from the Engineering Council, UK, and became a professional member of the Welding Institute in May 2019.

### Contents


#### **Chapter 6 117** Recent Modifications of Zirconia in Dentistry *by Ghassan Albarghouti and Haneen Sadi*

#### **Chapter 7 139**

Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) *by Alexander Chee Hon Cheong and SivaKumar Sivanesan*

Preface

In recent decades, the field of materials science has witnessed a surge in the exploration of advanced ceramics, leading to the development of novel materials with remarkable properties and diverse applications. Among these, zirconia has emerged as a prominent player, captivating the attention of researchers, engineers, and professionals across various industries. The allure of zirconia lies not only in its inherent structural and mechanical properties but also in its exceptional versatility that extends its application horizon from health care to electronics. This book, *Zirconia – New Advances, Structure, Fabrication and Applications*, is a comprehensive compilation of cutting-edge knowledge about zirconia, its advancements, structural intricacies, fabrication techniques, and its wide-ranging applications. Each chapter is thoughtfully crafted to provide a deeper understanding of zirconia, making this book an indispensable resource for students, researchers, practitioners, and anyone intrigued by the world of advanced

The journey begins with an introductory chapter (Chapter 1) that lays the foundation for comprehending the significance of zirconia in modern materials science. This chapter offers a panoramic view of zirconia's historical evolution, its unique properties, and an overview of its myriad applications across different sectors. In Chapter 2, the focus shifts to a detailed exploration of the classification and generations of zirconia. The evolution of zirconia-based materials is traced through different phases and compositions, highlighting the progress made in enhancing its mechanical and optical properties. Chapter 3 delves into the fundamental processes of synthesizing zirconia, elucidating various methods such as solution combustion synthesis, sol-gel synthesis, co-precipitation, and hydrothermal and solid-phase sintering techniques. The chapter also extensively covers the characterization tools and techniques employed to analyze the structural, morphological, and functional properties of zirconia materials.

Chapter 4 embarks on a journey through the diverse processing techniques employed in shaping zirconia materials. From powder compaction to sintering and beyond, this chapter offers insights into the intricate procedures that transform raw zirconia into functional components with tailored properties. Zirconia's emergence as a dental biomaterial has revolutionized the field of restorative dentistry. Chapter 5 delves into the unique attributes of zirconia that make it an ideal candidate for dental applications, including its biocompatibility, mechanical properties, and aesthetic appeal. Building upon the foundation laid in the previous chapter, Chapter 6 takes a closer look at the specific dental applications where zirconia has found substantial traction. From dental crowns to bridges and implants, the versatility of zirconia in enhancing oral health care is thoroughly explored. The book culminates with an in-depth analysis of yttria-stabilized zirconia (YSZ), a prominent subcategory of zirconia. Chapter 7 elucidates the remarkable properties of YSZ that have led to its adoption in various high-tech applications, including solid oxide fuel cells, thermal

ceramics.

barrier coatings, and more.

## Preface

In recent decades, the field of materials science has witnessed a surge in the exploration of advanced ceramics, leading to the development of novel materials with remarkable properties and diverse applications. Among these, zirconia has emerged as a prominent player, captivating the attention of researchers, engineers, and professionals across various industries. The allure of zirconia lies not only in its inherent structural and mechanical properties but also in its exceptional versatility that extends its application horizon from health care to electronics. This book, *Zirconia – New Advances, Structure, Fabrication and Applications*, is a comprehensive compilation of cutting-edge knowledge about zirconia, its advancements, structural intricacies, fabrication techniques, and its wide-ranging applications. Each chapter is thoughtfully crafted to provide a deeper understanding of zirconia, making this book an indispensable resource for students, researchers, practitioners, and anyone intrigued by the world of advanced ceramics.

The journey begins with an introductory chapter (Chapter 1) that lays the foundation for comprehending the significance of zirconia in modern materials science. This chapter offers a panoramic view of zirconia's historical evolution, its unique properties, and an overview of its myriad applications across different sectors. In Chapter 2, the focus shifts to a detailed exploration of the classification and generations of zirconia. The evolution of zirconia-based materials is traced through different phases and compositions, highlighting the progress made in enhancing its mechanical and optical properties. Chapter 3 delves into the fundamental processes of synthesizing zirconia, elucidating various methods such as solution combustion synthesis, sol-gel synthesis, co-precipitation, and hydrothermal and solid-phase sintering techniques. The chapter also extensively covers the characterization tools and techniques employed to analyze the structural, morphological, and functional properties of zirconia materials.

Chapter 4 embarks on a journey through the diverse processing techniques employed in shaping zirconia materials. From powder compaction to sintering and beyond, this chapter offers insights into the intricate procedures that transform raw zirconia into functional components with tailored properties. Zirconia's emergence as a dental biomaterial has revolutionized the field of restorative dentistry. Chapter 5 delves into the unique attributes of zirconia that make it an ideal candidate for dental applications, including its biocompatibility, mechanical properties, and aesthetic appeal. Building upon the foundation laid in the previous chapter, Chapter 6 takes a closer look at the specific dental applications where zirconia has found substantial traction. From dental crowns to bridges and implants, the versatility of zirconia in enhancing oral health care is thoroughly explored. The book culminates with an in-depth analysis of yttria-stabilized zirconia (YSZ), a prominent subcategory of zirconia. Chapter 7 elucidates the remarkable properties of YSZ that have led to its adoption in various high-tech applications, including solid oxide fuel cells, thermal barrier coatings, and more.

We extend our sincere gratitude to IntechOpen for their enthusiastic and professional support in bringing this book to fruition. Their dedication to disseminating knowledge has played a pivotal role in making this compilation a reality. As you embark on this journey through the multifaceted world of zirconia, we hope that the insights and revelations offered within these pages inspire further innovation and exploration in the realm of advanced ceramics.

#### **Dr. Uday M. Basheer Al-Naib**

**1**

Section 1

Introduction to Zirconia

Centre for Advanced Composite Materials (CACM), Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Section 1

## Introduction to Zirconia

#### **Chapter 1**

## Introductory Chapter: Introduction to Zirconia Ceramic – A Versatile and Durable Material with a Wide Range of Applications

*Uday M. Basheer Al-Naib*

#### **1. Introduction**

Zirconium oxide, also known as zirconia, is a ceramic material that has many important properties, including high strength, good toughness, and excellent corrosion resistance [1, 2]. It is a white crystalline material made from the mineral zircon found in various parts of the world. Zirconia exhibits numerous attributes that hold significance within industrial applications including use in the manufacture of ceramics, abrasives, and refractories, and as a structural material in the aerospace and automotive industries [3]. In addition to its practical uses, zirconia is also known for its beautiful, diamond-like appearance, which has made it a popular use in jewelry and other decorative items [4].

Zirconia possesses a multitude of distinct properties that render it a compelling material for a diverse range of applications [5]. With an exceptionally elevated melting point, zirconia demonstrates remarkable resistance to elevated temperatures and thermal shocks. Furthermore, its exceptional performance as an electrical insulator and its minimal coefficient of thermal expansion contribute to its resilience against thermal stresses. Beyond these qualities, zirconia's notable strength and hardness make it a prime candidate for structural applications. Moreover, its pronounced corrosion resistance positions it as a superb option for deployment in demanding environmental conditions [3].

When it comes to its manufacturing process, zirconia is created by subjecting zircon sand to exceptionally elevated temperatures within a specialized furnace. This procedure yields zirconium oxide as the final product [6]. The technique employed is known as sintering, which yields a compact and durable substance amenable to various configurations. Once crafted, zirconium oxide finds diverse applications based on particular traits and needs. Notably, it finds utility in crafting cutting implements and components resistant to wear, and it serves pivotal roles within the aerospace, automotive, and medical sectors. Its biocompatibility renders it valuable for producing medical equipment, including dental implants and other healthcare devices [7].

#### **2. History of zirconia**

Zirconia possesses a captivating and extensive historical background. Its origins trace back to the eighteenth century, marked by the pioneering work of German

scientist Martin Heinrich Klaproth. He succeeded in extracting zirconia from the mineral zircon, an enduringly precious gemstone with a centuries-old legacy [6, 8]. In the nineteenth century, zirconia was first used as an abrasive, and it was later found to possess several unique properties that made it useful for a wide variety of applications. One of the earliest uses of zirconia was in the manufacture of abrasives and it was used to make grinding wheels and other abrasive tools. In the twentieth century, zirconia began to be used in ceramic manufacture, and proved to be an excellent material for use in structural applications due to its high strength and toughness. Zirconia has also been shown to have excellent corrosion resistance and a low thermal expansion coefficient, making it an ideal material for use in harsh environments. Zirconia has continued to be a popular and widely used material in the twenty-first century [9]. In the medical field, zirconia has been used to manufacture dental implants, as well as other medical devices such as prosthetics and surgical tools. In the aerospace industry, zirconia has been used as a structural material due to its high strength and corrosion resistance. It has also been used in the production of cutting tools, wear-resistant parts, and other industrial components [10, 11].

#### **3. Structure of zirconia**

Zirconia displays polymorphism across varying temperatures, manifesting itself in three distinct shapes (**Figure 1**): monoclinic or baddeleyite (from room temperature up to 1170°C), tetragonal (1170–2370°C), and cubic (2370–2700°C, aligning with its melting point) [12]. At room temperature, zirconia exists in a monoclinic crystalline structure, but it can also exist in other crystalline structures depending on the temperature and pressure conditions. One of the unique properties of zirconia is that it can undergo a phase transition from monoclinic structure to a tetragonal structure under severe stress [13]. This alteration in phase brings about heightened toughness and ductility in the material, making it an ideal material for use in structural applications where it may be subjected to high stresses. Beyond its crystalline arrangement, zirconia's notable attributes encompass elevated density and hardness, qualities

**Figure 1.** *Structure of zirconia.*

#### *Introductory Chapter: Introduction to Zirconia Ceramic – A Versatile and Durable Material… DOI: http://dx.doi.org/10.5772/intechopen.113023*

attributed to robust chemical bonds. These characteristics collectively bestow zirconia with remarkable potency and resilience, positioning it as an optimal choice across an extensive array of applications [14].

Zirconia boasts an intricate crystalline arrangement, composed of zirconium oxide molecules arranged in a repetitive pattern [15]. This material exhibits refractory qualities, indicating a remarkable resistance to chemical reactions and an elevated melting point. At ambient conditions, zirconia takes on a monoclinic crystalline structure, characterized by the intersection of two axes at an angle. This configuration remains stable at room temperature and is marked by low density and strength. Yet, under substantial loads, zirconia can undergo a reversible phase transition, shifting from monoclinic to tetragonal. This phase transition is reversible and occurs when the material is stressed beyond a certain threshold [16]. The tetragonal structure exhibits superior density and strength compared to the monoclinic form, and it also demonstrates greater ductility, rendering it more resilient against cracks and fractures when burdened. Beyond its crystalline arrangement, zirconia is distinguished by high density and hardness, attributes attributed to robust chemical bonds within the material. These intrinsic characteristics bestow upon zirconia exceptional strength and endurance, rendering it an optimal choice for a wide array of applications. Moreover, zirconia functions as an outstanding electrical insulator and exhibits a low coefficient of thermal expansion, which safeguards it against thermal strains and positions it as a prime candidate for high-temperature environments [17]. Collectively, zirconia's intricate structure bequeaths it with a distinctive amalgamation of traits, establishing its significance across diverse industries.

Zirconia's distinct attributes are also influenced by a range of structural properties [18]. These encompass its microstructure, grain size, and defects. A material's microstructure consists of the arrangement of its atoms and their spatial relationships. In the case of zirconia, its microstructure can exhibit variability contingent upon the specific production conditions employed. For instance, when zirconia is manufactured via sintering, its microstructure often assumes a more porous nature, yielding a heightened surface area. Conversely, the application of hot isostatic pressing yields zirconia with a denser microstructure and a diminished surface area [19]. Notably, zirconia's microstructure holds considerable sway over its attributes, including strength and toughness [20].

#### **4. Types of zirconia**

There are different types of zirconia used in different applications. The most common types of zirconia include: (1) Monoclinic zirconia: This is the most stable form of zirconia at room temperature and is widely used in grinding and cutting tools, as well as in ceramic and refractory applications, (2) Tetrahedral zirconium: This form of zirconium undergoes a phase transition from monoclinic to tetragonal at a certain temperature known as the "transition temperature" [21]. Quadrilateral zirconia has higher strength and toughness than monoclinic zirconia, making it suitable for applications where these properties are important, (3) Cubic zirconia: This type of zirconia has a high refractive index and is often used as a substitute for diamond in jewelry. It is also used in high temperature thermal protection tubes and other grinding and high temperature applications, and (4) Stable zirconia: This type of zirconia is made by adding small amounts of other elements such as yttrium or magnesia to improve the stability and strength of the material. Stabilized zirconia is widely used in abrasive and high temperature applications, as well as in fuel cell electrodes and other advanced technologies. The type of zirconia used in a particular application depends on the specific properties and performance requirements for that application [22].

#### **5. Production of zirconia**

Zirconia, also referred to as zirconium dioxide, is synthesized through a procedure known as calcination. This process entails subjecting natural zircon, a mineral containing zirconium, to elevated temperatures in the presence of oxygen [23]. This chemical reaction transforms zircon into zirconia (**Figure 2**). The initial stage in zirconia production involves the extraction of zircon from natural deposits, usually undertaken within mining operations that involve excavating the earth to access zircon reservoirs [25]. Once extracted, zircon is purified to eliminate impurities. The refined zircon is then finely ground and combined with other substances like alumina or magnesia to generate a uniform mixture. This blend is introduced into a calcination furnace, where it is subjected to high temperatures (typically around 2000°C) in an oxygen-rich environment. During this calcination process, zircon reacts chemically with oxygen, resulting in its transformation into zirconia. Subsequently, the zirconia powder formed is cooled and collected for subsequent procedures [24]. Following

**Figure 2.** *The production processes of zirconia [24].*

#### *Introductory Chapter: Introduction to Zirconia Ceramic – A Versatile and Durable Material… DOI: http://dx.doi.org/10.5772/intechopen.113023*

calcination, additional processing steps, such as pressing and sintering, may be applied to the zirconia powder. These steps help shape the material into its desired form while achieving specific properties. Eventual zirconia products find application across a broad spectrum of uses, encompassing abrasives, ceramics, refractories, and cutting-edge technologies.

Multiple techniques exist for producing zirconia, with their suitability contingent upon the distinctive attributes and performance prerequisites of the product. Further insights into zirconia production encompass:

Following calcination, the zirconia powder can undergo additional refinement via "hot pressing" or "hot isostatic pressing." This technique entails pressing the zirconia powder under elevated pressure and temperature, resulting in a compact, robust structure with heightened strength and toughness. An alternate method, termed "sintering," involves subjecting the zirconia powder to elevated temperatures (typically around 1600 to 1800°C) in a dedicated sintering furnace. As the powder heats, its particles coalesce, yielding a consolidated piece characterized by improved density and strength. Beyond hot pressing and sintering, another avenue for zirconia processing is "reaction bonding." In this approach, zirconia powder is blended with a metal oxide like alumina or magnesia, and the mixture is then subjected to high temperatures. The ensuing chemical reaction establishes a bond between the zirconia and the metal oxide, yielding a sturdy, fortified piece with heightened strength and toughness [26]. The selection of a specific zirconia production method hinges upon the distinct attributes and performance requisites of the final product [27]. Considerations encompass parameters like density, strength, toughness, and various other properties that influence the optimal manufacturing route.

#### **6. Future developments in zirconia technology**

There are several areas of research and development that are focused on advancing the technology of zirconia. Some potential future developments in zirconia technology include [28]:


### **7. Conclusion**

Zirconia ceramic emerges as a resilient and versatile substance, renowned for its multifaceted properties such as elevated melting point, robust strength, notable toughness, minimal thermal expansion, chemical resilience, and remarkable wear resistance. This amalgamation of attributes equips it for a diverse array of applications, spanning industrial and biomedical domains. Its utility encompasses cutting tools, wear-resistant coatings, dental prosthetics like crowns and bridges, refractory components, catalysts, and abrasives. Furthermore, the burgeoning interest in leveraging zirconia within fuel cell technology is fuelled by its aptitude for oxygen ion conduction at elevated temperatures. Overall, zirconia stands as a highly advantageous and invaluable material, exerting its influence across an extensive spectrum of fields.

### **Author details**

Uday M. Basheer Al-Naib Centre for Advanced Composite Materials (CACM), Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Malaysia

\*Address all correspondence to: ummb2008@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Introductory Chapter: Introduction to Zirconia Ceramic – A Versatile and Durable Material… DOI: http://dx.doi.org/10.5772/intechopen.113023*

#### **References**

[1] Kalavathi V, Bhuyan RK. A detailed study on zirconium and its applications in manufacturing process with combinations of other metals, oxides and alloys—A review. Materials Today: Proceedings. 2019;**19**:781-786

[2] Qi B et al. ZrO2 matrix toughened ceramic material-strength and toughness. Advanced Engineering Materials. 2022;**24**(6):2101278

[3] Vasile BS et al. Ceramic composite materials obtained by electronbeam physical vapor deposition used as thermal barriers in the aerospace industry. Nanomaterials. 2020;**10**(2):370

[4] Hurrell K, Johnson ML. Gemstones: A Complete Color Reference for Precious and Semiprecious Stones of the World. New York, USA: Chartwell Books; 2016

[5] Correia DM et al. Ionic liquid– polymer composites: A new platform for multifunctional applications. Advanced Functional Materials. 2020;**30**(24):1909736

[6] Arnold B. Zircon, Zirconium, Zirconia-Similar Names, Different Materials. Berlin, Germany: Springer; 2022

[7] Fathi R et al. Past and present of functionally graded coatings: Advancements and future challenges. Applied Materials Today. 2022;**26**:101373

[8] Halka M, Nordstrom B. Transition Metals. New York, USA: Infobase Holdings, Inc; 2019

[9] Mogensen MB. Materials for reversible solid oxide cells. Current Opinion in Electrochemistry. 2020;**21**:265-273

[10] Okada A. Automotive and industrial applications of structural ceramics in Japan. Journal of the European Ceramic Society. 2008;**28**(5):1097-1104

[11] Hoornaert T, Hua Z, Zhang J. Hard wear-resistant coatings: A review. In: Advanced Tribology: Proceedings of CIST2008 & ITS-IFToMM2008. China; 2010. pp. 774-779

[12] Bocanegra-Bernal M, De La Torre SD. Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics. Journal of Materials Science. 2002;**37**:4947-4971

[13] Zhang N, Zaeem MA. Competing mechanisms between dislocation and phase transformation in plastic deformation of single crystalline yttriastabilized tetragonal zirconia nanopillars. Acta Materialia. 2016;**120**:337-347

[14] Geetha M et al. Ti based biomaterials, the ultimate choice for orthopaedic implants–A review. Progress in Materials Science. 2009;**54**(3):397-425

[15] Rushton MJ et al. Stoichiometry deviation in amorphous zirconium dioxide. RSC Advances. 2019;**9**(29):16320-16327

[16] Swain M. Shape memory behaviour in partially stabilized zirconia ceramics. Nature. 1986;**322**(6076):234-236

[17] Xu L et al. A new class of highentropy fluorite oxides with tunable expansion coefficients, low thermal conductivity and exceptional sintering resistance. Journal of the European Ceramic Society. 2021;**41**(13):6670-6676

[18] El-Wazery M, El-Desouky A. A review on functionally graded ceramicmetal materials. Journal of Materials and Environmental Science. 2015;**6**(5):1369-1376

[19] Mayo MJ. Processing of nanocrystalline ceramics from ultrafine particles. International Materials Reviews. 1996;**41**(3):85-115

[20] Rathee G et al. Emerging multimodel zirconia Nanosystems for highperformance biomedical applications. Advanced NanoBiomed Research. 2021;**1**(9):2100039

[21] Gorelov V. High-temperature phase transitions in ZrO 2. Physics of the Solid State. 2019;**61**:1288-1293

[22] Zhao W et al. High-temperature oxidation behavior of Zr-4 and Zr-Sn-Nb alloy in different oxidation ambient. Journal of Alloys and Compounds. 2021;**887**:161396

[23] Ende M et al. Dry annealing of radiation-damaged zircon: Single-crystal X-ray and Raman spectroscopy study. Lithos. 2021;**406**:106523

[24] Trevisi R et al. Radiological protection in industries involving NORM: A (graded) methodological approach to characterize the exposure situations. Atmosphere. 2023;**14**(4):635

[25] Liu H. Investigation on joint mining of rare earth elements and zircons from the Yangtze River, China. In: Journal of Physics: Conference Series. Hangzhou, China: IOP Publishing; 2021

[26] Bapat RA et al. Review on synthesis, properties and multifarious therapeutic applications of nanostructured zirconia in dentistry. RSC Advances. 2022;**12**(20):12773-12793

[27] Katoh Y et al. Properties of zirconium carbide for nuclear fuel applications. Journal of Nuclear Materials. 2013;**441**(1-3):718-742

[28] Olhero S et al. Conventional versus additive manufacturing in the structural performance of dense aluminazirconia ceramics: 20 years of research, challenges and future perspectives. Journal of Manufacturing Processes. 2022;**77**:838-879

Section 2

## Classification of Zirconia

#### **Chapter 2**

## Classification and Generations of Dental Zirconia

### *Ali Dahee Malallah and Nadia Hameed Hasan*

#### **Abstract**

Zirconium oxide (ZrO2) is polymorphic (temperature dependent) structure; zirconia can take three crystallographic forms at ambient pressure. Under normal conditions, pure zirconia is monoclinic (m). At (1170°C), the substance converts to a tetragonal crystal structure (t), then to a cubic crystal structure (c) at (2370°C), and finally to a fluorite structure above (2370°C), melting at (2716°C). During the heating and cooling cycles, the Zirconium oxide ceramic undergoes a hysteretic, martensitic t- m transformation, which is reversible at 950°C upon cooling. For dental applications, various types of zirconium-dioxide (zirconia) materials are available. These materials have a variety of chemical compositions, crystal configurations, manufacturing processes, and important variations in their mechanical and optical properties. Numerous generations of zirconia materials have been developed, ranging from the use of zirconia crystals as reinforcement elements in zirconia toughened alumina (ZTA) to partially stabilized zirconia (PSZ) and the conventional (3Y-TZP) to the appearance of new translucent zirconia materials such as cubic stabilized zirconia (CSZ).

**Keywords:** zirconia, monolithic, tetragonal polycrystal zirconia, partially stabilized zirconia, zirconia in dentistry

#### **1. Introduction**

Zirconium (Zr) is a chemical element whose name Zr is taken from the name of mineral zircon and the name zircon comes from the Persian word "Zar-Gun," which means "golden color".Zr is a transition metal with an atomic mass of (91.224 g/mol) and an atomic number of (40). The melting point of Zr is (1855°C), while the boiling point is (4371°C). Zr was discovered in 1789 by Martin Heinrich Klaproth, a German scientist and isolated in 1824 by Swedish chemist Jöns Jacob Berzelius (Gautam *et al.,* 2016; Nistico, 2021). Helmer and Driskell reported the first biomedical application of Zr in 1969; however Christel (1988) was the first to use Zr to make a ball head for a complete hip replacement [1, 2].

Zr is never found in nature as a naturally occurring metal. It occurs naturally in igneous rocks in combination with other elements such as iron, titanium, and silicon oxide. The most abundant source of Zr is zircon (ZrSiO4), which is found mostly in Australia, South Africa, Brazil, India, Russia, and the United States. Many other mineral species contain Zr, including baddeleyite. Despite the fact that zirconia was

first used in orthopedics in 1969 for hip head replacement, it was not used in dentistry until the 1990s. Zirconia-based ceramics such as tetragonal zirconia which is partially stabilized by yttria have been successfully incorporated into daily dental work to construct fixed dental prostheses (FDPs) and dental (CAD/CAM)system based on their excellent biological, mechanical, and physical properties. Many endeavors, on the other hand, have undergone numerous improvements in composition and microstructure in order to improve their optical properties without minimizing their mechanical properties [3–5].

Zirconia is a non-etchable polycrystalline material as it has no glassy phase within its structure that can be bonded to tooth structures using both traditional and resin cements; however, resin cements are favored because they have a great marginal seal, better retention, and improved ceramic fracture resistance. Zirconia –based ceramics are conventionally used as substructure materials that require a veneer facing for clinically acceptable appearance due to their high opacity and whitish optical appearance with excellent biocompatibility and lower the risk of pulp irritation that may occur due to the lower thermal conductivity. Ceramic facing, on the other hand, has some drawbacks, such as poor tensile strength and crack toughness. Porcelain is vulnerable to cracking when stressed due to intrinsic flaws in the crystal structure and the presence of voids. As a result, veneer chipping has been described as a major cause of failure and the most common complication in all-ceramic crowns [6, 7].

A new generation of zirconia, known as "monolithic zirconia," has recently been introduced. Monolithic zirconia restorations, according to the manufacturers, has a higher translucency than conventional zirconia and therefore does not need a veneer layer. Monolithic zirconia restorations can have a number of clinical advantages. Without the use of a veneer, the amount of tooth loss and the risk of chipping are reduced, as well as the restorations' actual strength will be increased [8, 9].

#### **2. Classification of zirconia**

Over the past 20 years, there have been more varieties of dental zirconia, making it occasionally challenging to select the right kind for each restoration. Utilizing CAD/ CAM technologies, it is now possible to create dental restoratives with extremely precise fitting owing to advancements in digital technology. Metal-free restorations are also appealing for biological and cosmetic reasons. For dental applications, various types of zirconium-dioxide (zirconia) materials are available. These materials have a variety of chemical compositions, crystal configurations, manufacturing processes, and important variations in their mechanical and optical properties. Numerous generations of zirconia materials have been developed, ranging from the use of zirconia crystals as reinforcement elements in zirconia toughened alumina (ZTA) to partially stabilized zirconia (PSZ) and the conventional (3Y-TZP) to the appearance of new translucent zirconia materials such as cubic stabilized zirconia [10, 11]. **Figure 1** is showing recent classification of yttria stabilized zirconia.

Zirconia can be classified according to the following criteria into:

#### **2.1 Zirconia microstructure**

The monoclinic phase of pure zirconia (ZrO2) is stable at ambient temperature, while the tetragonal and cubic crystal phase systems change with temperature. When *Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

#### **Figure 1.**

*Structural schematic diagram and classification of yttria-stabilized dental zirconia [10].*

zirconia is solidly dissolved in yttrium(Y), calcium (Ca), magnesium (Mg), cerium (Ce), or other ions with an ionic radius larger than that of zirconium (Zr), and accordingly divided into [12]:

#### *2.1.1 Partially stabilized zirconia (PSZ)*

The tetragonal and cubic phase systems become stable at room temperature when zirconia is solidly dissolved in yttrium (Y), calcium (Ca), magnesium (Mg), cerium (Ce), or other ions with a greater ionic radius than that of zirconium (Zr) [1, 13]. Cubic-stabilized zirconia (CSZ) is the cubic phase zirconia that is stable at room temperature when yttria (Y2O3) is introduced in amounts greater than 8 mol%. Tetragonal and cubic phases are mixed at room temperature when yttria is 3 to 8 mol%, and this material is known as partially stabilized zirconia (PSZ). Tetragonal zirconia polycrystal (TZP), also known as toughened zirconia, is zirconia that is close to 100% tetragonal at room temperature when yttria is approximately 3 mol%. This 3 mol% yttria tetragonal zirconia polycrystal (3Y-TZP) was one of the earliest zirconia used as "white metal" in dentistry [10].

#### *2.1.2 Fully stabilized zirconia (FSZ)*

It is a cubic zirconia that includes no less than 8% yttrium oxide(Y2O3), hence the term "fully" stabilized zirconia (FSZ). presenting tetragonal and cubic crystals on its microstructure, being totally inert to the aging in autoclave because cubic zirconia does not exhibit the t/m transformation toughening phenomenon. The volume of cubic crystals is greater. They are less firmly bonded and, as a result, have increased light scattering at the grain boundaries due to reduced residual porosity. Additionally, incident light is emitted more uniformly in all spatial directions because the cubic crystal structures are more isotropic than the tetragonal structures. To increase the translucency of dental zirconia, several producers have recently started producing it in its full cubic stabilized form (due to increased cubic phase) [14–17].

#### **2.2 Zirconia used with porcelain facing or not**

It can be divided into:

#### *2.2.1 Monolithic (full contour restoration)*

The terms "monolithic" is Greek word that means: mono is "single" and "lithos" is "stone," This indicates that the materials have a consistent appearance. As microstructures, monolithic materials consist of two or more phases such as in zirconia it has three phases monoclinic, tetragonal and cubic phases. With the continuous development of new and more translucent Y-TZP and the advancement of CAD-CAM technology that facilitate the fabrication of monolithic Y-TZP crowns and FDPs. These systems aim to eliminate the problem of veneering porcelain chipping and provide acceptable esthetics with characterization, which may be an esthetic option in the molar area without a reduction in strength. Full-contour zirconia restorations, however, do not have adequate translucency because the matrix and zirconia particles have different refractive indices. To produce restorations with great translucency and to achieve zirconia's mechanical qualities, a number of brands of monolithic zirconia have been introduced. To increase zirconia's translucency, a full cubic stabilized monolithic zirconia (FSZ) has been produced. Comparing newly developed Y-TZP monolithic materials to traditional zirconia, the translucency has enhanced and low temperature degradation has been limited due to the materials' lower alumina content, relatively fine grain size, and presence of optically isotropic cubic zirconia particles. A monolithic multilayer zirconia has been developed. It is a polychromatic, translucent zirconia with combined shade and translucency gradient [18–20].

#### *2.2.2 Core build up (veneered with porcelain facing)*

A high strength ceramic substructure made of zirconia or alumina is the foundation of bi-layered crowns, which are then veneered with ceramic or dental porcelain such as feldspathic porcelain. Despite having good esthetic qualities, the resulting restorations are prone to failure, such as chipping of the veneering ceramic. An alternative approach is to eliminate the veneer and produce a full contour monolithic zirconia crown but the possibility of wearing the opposing natural teeth is still a concern. Furthermore, the potential loss of strength brought on by low temperature aging or degradation (LTD), which may be induced in an aqueous environment. By completely covering the zirconia restoration with a ceramic veneer, it can prevent it from coming into direct contact with the oral cavity and potentially prevent this occurrence [20, 21].

#### **2.3 Colored and non-colored zirconia**

Dental zirconia has changed over the past 20 years from its original white, opaque appearance to translucent and chromatic as well as polychromatic (multi-layered) forms. Zirconia has become as the most versatile restorative material, offering a wide spectrum of translucency and colors.

#### *2.3.1 Non dyed (monochromatic white)*

Non dyed blanks have a hard-white monochrome color, which can be an esthetic disadvantage in many indications. The restorations milled in the white body condition

#### *Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

can be manually and individually colored with coloring oxides after the milling process and sintered afterwards to overcome this drawback. The form-milled openpore framework is immersed in the suitable colored liquid for a brief period of time to dye it. Alternatively, brushes can be used to create color gradients that are comparable to various colored liquids of varying intensities. The sintering procedure is performed after removing the excess remaining color while it is still wet and drying the framework. Utilizing liner or stain is another way to make white zirconia more esthetically beautiful [17].

#### *2.3.2 Dyed polychromatic (multilayer)*

Construction of polychromatic zirconia in the form of a gradational multilayered zirconia disc with two to seven or even more layers of color, mimicking the appearance of layered porcelain in full-contour monolithic restorations, and the creation of zirconia blanks with multiple, different shaded layers make full-contour zirconia restorations more esthetically pleasing than those made with conventional monochromatic zirconia. After sintering, full-contour zirconia restorations with esthetics that outperform monochromatic restorations can be obtained. The Katana Multi-Layered zirconia by Kuraray Noritake Dental Japan was the first polychromatic zirconia to be introduced to the dental market in February 2015. They are intended for the production of full contour zirconia crowns with a greater level of esthetics. Since then, virtually every significant dental company has introduced their own version of multi-layer zirconia [20, 22].

#### **2.4 Fully sintered or partially sintered zirconia blanks**

#### *2.4.1 Fully sintered or hot isostatic procedure (HIP)*

Performed by a hot isostatic press at 1000 bar and 50°C below the sintering temperature at 1400–1500°C under high pressure and an inert gas atmosphere to reduce the material porosity and ensure high values of toughness and translucency of zirconium ceramics by such procedure. Carrying out HIP on Y-TZP results in a grayblack material that usually requires subsequent heat treatment to oxidize and restore whiteness. Then, a specially designed milling system used to machine the blanks, but with a low machinability and high hardness of a fully sintered Y-TZP, such milling system has to be particularly robust [23, 24].

#### *2.4.2 Partially sintered or cold isostatic pressing*

With cold isostatic pressing, the powders are shaped into ceramic blanks. The powder of the partially sintered blanks typically contains a binder that enhance its pressing, but during the step of a pre-sintering it will be eliminated. The most widely used procedural method for Y-TZP shaping is cold isostatic pressing, which yields stable, chalk-like non-sintered green-stage objects with a very high primary density. Sintering without pressure in the oxidizing atmosphere of a specific furnace allows the green objects to be further stabilized and condensed up to approximately 95% of their theoretical density., the pre-sintering temperature of the heat treatment effects on the machined blank roughness, a higher pre-sintering temperature gives a rougher surface, so the choice of a proper pre-sintering

temperature will be thus critical. These materials remain softer than the HIP zirconia and are easier to mill [24].

#### **2.5 Zirconia generations**

#### *2.5.1 First generation*

The first generation of 3 mol% yttria (3Y-TZPs) had flexure strengths greater than 1 GPa and contained 0.25 wt% alumina (Al2O3) as a sintering aid. However, that zirconia displayed considerable opacity, due to the inherent birefringence of noncubic zirconia phases as light is scattered at the grain boundaries, pores, and additional inclusions. They were primarily used as framework materials in posterior and anterior fixed dental prostheses (FDPs) and porcelain-veneered crowns [25, 26].

#### *2.5.2 The second generation*

The alumina content of the second iteration of 3 mol% yttria (3Y-TZP), used in dentistry was reduced from 0.25 wt% to 0.05 wt%. The 3Y-TZP with 0.05 wt% alumina is more translucent than the 3Y-TZP with 0.25 wt% alumina, but because there is less alumina to stabilize the tetragonal phase, it is more prone to low-temperature degradation. Some confusion in nomenclature has occurred as both 0.05 wt% alumina- containing 3Y-TZP and 5 mol% yttria-stabilized zirconia polycrystal (5Y-ZP) have been called "translucent zirconia"; however, these zirconia materials have different mechanical and optical properties [26, 27].

#### *2.5.3 The third generation*

The third iteration of dental zirconia is doped with 5 mol% yttria, which creates a partially stabilized zirconia with around 50% cubic phase zirconia is produced. Zirconia's cubic phase is isotropic in many crystallographic directions, which reduces the amount of light scattering at grain boundaries. The cubic zirconia thus seems more transparent. Since stabilized cubic zirconia does not change at room temperature, it will not degrade at low temperatures or undergo transformation toughening. It has diminished mechanical qualities but will not change with time [26].

#### **2.6 Zirconia according to mol% concentration of yttria**

#### *2.6.1 3 mol% yttria-partially stabilized zirconia (3Y-PSZ)*

Mechanical and optical properties of zirconia depend on the yttria mol% content. Of all the several forms of zirconia, 3Y-PSZ has the greatest values for opacity, flexural strength, and fracture toughness. The material's high color value and opacity restrict its use to posterior restorations [26].

#### *2.6.2 4 mol% yttria-partially stabilized zirconia (4Y-PSZ)*

4Y-PSZ zirconia has translucency and mechanical properties in between 3Y-PSZ zirconia and 5Y- PSZ zirconia making it an attractive in between material for esthetic zone [26].

#### *2.6.3 5 mol% yttria-partially stabilized zirconia (5Y-PSZ)*

5Y-PSZ zirconia has enhanced translucency, but reduced mechanical properties [26]. **Table 1** is showing zirconia by the yttrium content and its effect on physical and mechanical properties.

#### **3. Zirconia ceramics for dental applications**

The following are the three most common forms of zirconia used in dentistry today [1]:

a.Tetragonal zirconia polycrystals doped with yttrium cation (3Y TZP)

Traditional 3Y-TZP zirconia is the most commonly used zirconia material in bilayered dental restorations as a substitute for the metal substructure. The 3Y-TZP is the toughest zirconia material currently available in dentistry. As a stabilizer, 3Y-TZP contains 3 mol% Yttria (Y2O3). The stabilizer Y3+ cations and Zr4+ ions are distributed randomly over the cationic sites, but electrical neutrality is attained by creating oxygen vacancies [29].

The mechanical properties of 3Y-TZP are highly influenced by the particle size. 3Y-TZP becomes less vulnerable to spontaneous tetragonal to monoclinic (t-m) transformation when the particle size is less than (1 μ). Furthermore, the transformation is not possible below a certain particle size (0.2 μ), resulting in decreased fracture toughness. A particle size greater than (1 μ), on the other hand, makes 3Y-TZP less stable and more vulnerable to spontaneous t-m transformation, resulting in huge volume increase and a decrease in fracture toughness. As a result, it appears that the critical particle size for 3Y-TZP is 1(μ) [30].

According to this, the sintering conditions that result in an increased particle size have a significant effect on the final ceramic product's stability and strength. Larger particle sizes result from higher sintering temperatures and longer sintering times. Depending on the producer, the final sintering temperatures of 3Y-TZP range from (1350 to 1550C). Furthermore, the particle size and phase stability of 3Y-TZP used in dental applications can be affected by variations in sintering temperatures during firing [30–32].

b.Partially stabilized zirconia doped with magnesium cation (Mg-PSZ)

The quantity of magnesium oxide in commercial materials ranges from 8 to 10 mol%. To control the fracture toughness of Mg-PSZ, high sintering temperatures (1680 and 1800 C) and tightly controlled cooling cycles are required. Because of the less stable molecular composition of this material, early t-m transition may occur, leaving insufficient tetragonal zirconia for the material to transform and toughen upon further fracture formation. Lower mechanical properties are the outcome of the higher prevalence of the monoclinic form. Due to the existence of surface porosity associated with a large particle size (30–60 μ), magnesium stabilized zirconia (Mg-PSZ) has not been widely used in biomedical applications due to the risk of excessive wear [1].


#### **Table 1.**

*Defining zirconia used in dentistry by the yttrium content and its effect on physical and mechanical properties [28].*

#### c.Zirconia toughened alumina (ZTA)

Another way to take advantage of stress-induced zirconia transformation is to use zirconia toughened alumina (ZTA), which is composed of an alumina matrix and zirconia. The commercially accessible dental product In-Ceram Zirconia (VidentTM, Brea, CA) was developed by adding 33 vol.% of 12 mol % ceria-stabilized zirconia (12Ce-TZP) to In-Ceram Alumina. Slip casting or soft machining may be used to create In-Ceram Zirconia restorations. Increased porosity is likely related to In-Ceram zirconia's lower mechanical properties as compared to 3Y-TZP dental ceramics [33].

#### **4. Zirconia microstructure**

Zirconium oxide (ZrO2) is polymorphic (temperature dependent) structure; zirconia can take three crystallographic forms at ambient pressure. Under normal conditions, pure zirconia is monoclinic (m). At (1170°C), the substance converts to a tetragonal crystal structure (t), then to a cubic crystal structure (c) at (2370°C), and finally to a fluorite structure above (2370°C), melting at (2716°C). During the heating and cooling cycles, the Zirconium oxide ceramic undergoes a hysteretic, martensitic t- m transformation, which is reversible at 950°C upon cooling [2, 34]. **Figure 2** is showing Crystallographic phases of zirconia and temperature hysteresis.

Monoclinic phase (m) is a deformed prism with parallelepiped sides that has lower mechanical properties and may lead to a reduction in the cohesion of ceramic particles [35].

Tetragonal phase (t): is a straight prism with rectangular sides that possesses increased mechanical properties. Precipitation of a finely dispersed tetragonal phase within a cubic matrix, capable of being changed into the monoclinic phase when the matrix's pressure was alleviated by a crack propagated within the matrix [35].

Cubic phase (c): in the shape of a square-sided straight prism has a large grain size, which reduces light scattering and birefringence at grain boundaries. The cubic phase of zirconia is isotropic in different crystallographic directions, which reduces light scattering at grain boundaries. The cubic zirconia becomes more transparent as a result. At room temperature, stabilized cubic zirconia does not transform, so it will not undergo transformation toughening or low-temperature

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

**Figure 2.** *Crystallographic phases of zirconia, temperature and hysteresis [2].*

degradation. In other words, it has weakened mechanical properties but will not change over time [28, 35].

#### **5. Generations of zirconia in dentistry**

There are four main generation of zirconia in dentistry and new multilayer novel generation of zirconia:


In comparison to the previous two generations of zirconia materials, those cubic zirconia materials exhibit optical qualities that are comparable to those of glass ceramics. However, on a material level, the high translucency obtained by increasing the yttria level reduces the material's intrinsic mechanical properties to a significant degree. As a result, cubic zirconia materials have a flexural strength of around half that of standard 3Y-TZP materials (approximately 600 MPa) [18, 28].

iv. **(Fourth generation 4Y-TZP 0.05** Al2O3**)**: as mentioned before 4Y-PSZ zirconia has a translucency and mechanical properties that fall between 3Y-PSZ zirconia and 5Y-PSZ zirconia, making it an appealing material for the esthetic region [26].

#### v.**Novel Multilayer zirconia**

Construction of zirconia blanks with several, differently-shaded layers and creation of polychromatic zirconia for more esthetic full-contour zirconia restorations than conventional monochromatic zirconia in the form of a gradational multilayered zirconia disc with two to seven or even more layers of color, simulating the appearance of layered porcelain in a full-contour monolithic restorations. Full-contour zirconia restorations with superior esthetics to the monochromatic restorations can be obtained immediately after sintering [15].

This multi-layered technology uses only pigmentation to simulate the shadegradient of natural teeth while maintaining the same yttria percentage in the zirconia blank, resulting in a color gradient with no difference in flexural intensity between the enamel and dentin layers [23, 36].

What could be called the age of the next generation of modern multi-layered transparent zirconia materials has begun in the last years. These materials (for example, IPS e.max ZirCAD Prime and Katana Multi-Layer) comprise two to four layers of varying translucency inside the same product. This is accomplished by a novel manufacturing procedure that blends two zirconia materials (3Y-TZP and 5Y-TZP) to create a single unique material. By lowering the cubic content of the previous generation to a balanced level with 3Y-TZP, these new materials achieve a balance of strength and esthetics [36, 37].

A multi-layered technology was introduced by combining two generations of zirconia (combination of two different percentages of yttia) in one blank with the aim of combining the benefits of both generations of zirconia. This is mainly a combination of a high-flexural-strength 3Y-TZP in the dentin/body region to enhance flexibility and a high-translucency 5Y-TZP in the incisal or occlusal region to improve esthetics. Even the 5Y-TZP, which has the lowest flexural strength of the zirconia generations, possessed superior mechanical qualities and a translucency comparable to lithium disilicate ceramics [7, 38].

#### **6. Clinical applications of zirconia in dentistry**

The construction of veneers, full and partial coverage crowns, implants, fixed partial dentures (FPDs), posts and/or cores, implants, and implant abutments are among the range of zirconia's modern clinical applications. Extra-coronal attachments, surgical drills, cutting burs, and orthodontic brackets are a few additional zirconia-based auxiliary components that are offered as commercial dental products [39].

#### **6.1 Zirconia crown**

Space, para-functional habits, malocclusion, short clinical crowns, tooth mobility, tooth inclination considerations, and basic clinical sequence are all the same as they are for other all-ceramic crowns. The tooth preparation clinical guidelines for zirconia crowns are also the same as those for metal-ceramic restorations.

An appropriately constructed diamond set is typically used to accomplish the tooth preparation for a zirconia crown, which should give a desirable distribution of the functional stresses. In general, 1.5 mm to 2.0 mm of incisal or occlusal reduction and 1.2 mm to 1.5 mm of axial reduction are needed to prepare a tooth for a zirconia restoration. All dihedral angles should be tapered, and the axial convergence angle of the crown preparation should be around 6 degrees. The preparation should end with a uniform 0.8 mm to 1.2 mm subgingival (about 0.5 mm) deep chamfer or marginal shoulder with rounded internal angles [39]. **Figure 3** is showing a full coverage of 12 maxillary single zirconia crowns.

#### **6.2 Zirconia fixed partial denture**

In comparison to other traditional all-ceramic systems like lithium-disilicate glass ceramics and zirconia-reinforced glass-infiltrated alumina, Y-TZP FPDs were found to have a significantly higher load bearing capacity. It has also been reported that veneering further increased fracture resistance [39, 40]. The most recent framework material for the production of all-ceramic FPDs in either anterior or posterior sites is Y-TZP, which is based on the superior mechanical qualities of zirconia (such as high flexural strength and fracture resistance) [41]. **Figures 4** and **5** are showing zirconia framework in anterior and posterior regions respectively.

#### **6.3 Zirconia as post and core**

Meyenberg et al. reported using zirconia in post-and-core systems for the first time in 1995. They found no failure after an average observation period of 11 months [42]. Since then, zirconia posts (ZPs) have emerged as a potential treatment option for teeth with reduced structural integrity and filled roots teeth, particularly for patients with high esthetic needs. Because there are different observation periods, post surface treatments, cement systems, tooth preparation designs, and post designs, the success rate of ZPs in the literature ranges greatly, from 81.3%2 to 100%18. In order to prevent rotation of the posts and cores, for instance, the length of the posts should be prepared to be longer than that of the clinical crowns [43]. Additionally, the structural integrity of the root-filled teeth must have a ferrule with a minimum height of 1.5 mm to counterbalance the lateral forces experienced during post insertion, the wedging effect of posts, and the functional lever forces [44–46]. these restorations also have some problems. For instance, the high elastic modulus may lead to a less uniform stress distribution throughout the tooth, and complications such as tooth fracture and post debonding still exist [47]. **Figure 6** is showing all-zirconia post and core of a maxillary left endodontically treated lateral incisor (tooth 12)) where Laboratory work performed by Mr. F Ferraresso (Saluzzo, Italy) and Dr. SO Koutayas (Corfu, Greece).

#### **6.4 Zirconia implants titanium**

Zirconia is emerging as a promising alternative to conventional Titanium based implant system for oral rehabilitation with superior biological, esthetic, mechanical

#### **Figure 3.**

*A total of 12 maxillary single zirconia crowns (teeth 16 to 26). Top: full coverage preparation of the abutment teeth. Middle: Zirconia. Bottom: final clinical situation after crown adhesive cementation [39].*

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

#### **Figure 4.**

*Anterior six-unit zirconia fixed partial denture restoration (teeth 13 to 23). Top: Zirconia framework in situ. Middle: Zirconia framework after laboratory completion. Bottom: Final clinical situation after cementation [39].*

#### **Figure 5.**

*Posterior four-unit zirconia fixed partial denture restoration (teeth 47 to 44). Top: Zirconia framework. Middle: Zirconia framework after laboratory completion. Bottom: final clinical situation after cementation [39].*

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

#### **Figure 6.**

*Single crown restoration of a maxillary left endodontically treated lateral incisor (tooth 12) with the use of an all-zirconia post and core: a) initial situation after endodontic treatment b) prefabricated zirconia post with core analogue model, c) two- piece all-zirconia post and core after milling of a Y-TZP core d) bonding of the post and core restoration using an adhesive resin (Panavia 21, Kuraray), e) completion of the tooth preparation, f) final clinical situation after crown placement [39].*

and optical properties [48]. To date, there are five commercially available zirconia implant systems on the market. One- and two-piece designs are offered by only one system (Sigma, Incermed, Lausanne, Switzerland), whereas one-piece designs are only offered by CeraRoot, CeraRoot Dental Implants, Barcelona, Spain; Z- Look3, Z-Systems, Constance, Germany; whiteSKY, Bredent Medical, Senden, Germany; and zit-z, Ziterion, Uffenheim, Germany. A customized zirconia root-analogue implant with a micro- and macro-retentive implant surface was also described in a recent clinical trial, but neither the zirconia material nor the milling machine were fully explained [49]. Zirconia implants do not have any clinical long-term data despite some encouraging preliminary clinical results. 93% of survival rate after one year, according to reports (189 one-piece implants, Z-Systems) [50] 98% (66 one-piece implants, Z-Systems), [51] and 100% (one-piece implants, CeraRoot) [52]. **Figure 7** is showing: Zirconia implant supported zirconia crown (tooth 12) where laboratory work performed by Mr. W Woerner (Freiburg, Germany).

#### **6.5 Zirconia implant abutment**

In the most recent systematic review, published in2013, Bidra and Rungruanganunt compared the survival, mechanical, biological, and esthetic outcomes of implant

#### **Figure 7.**

*Zirconia implant supported zirconia crown (tooth 12. Top: Zirconia implant placement after tooth extraction. Middle: 4 months later; placement of retraction cord prior to impression. Bottom: after final cementation of zirconia crown [39].*

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

abutments (Ti and Zir). Due to greater color integration, they came to the conclusion that Zir abutments were preferred from an esthetic standpoint, especially for patients with thin mucosal tissues. They had improved gingival color, according to a recent analysis of their esthetic outcomes, and Zirconia had comparable soft-tissue recession, probing depths, bleeding on probing, marginal bone level, and patient reported outcomes as Ti [53, 54]. Zirconia abutments, however, had greater mechanical issues than Ti abutments. Therefore, the main obstacle to the widespread application of Zir abutments is their lack of mechanical strength [55]. **Figure 8** is showing zirconia prefabricated implant abutment of an upper right lateral incisor (tooth 12) where Clinical and laboratory work performed by Dr. SO Koutayas (Corfu, Greece) and Dr. D Charisis (Athens, Greece), respectively.

#### **6.6 Zirconia dental auxiliary components**

different zirconia-based auxiliary components such as cutting burs and surgical drills, extra-coronal attachments, and orthodontic brackets are also available as commercial dental product [39, 56].

#### *6.6.1 Zirconia orthodontic brackets*

Orthodontic brackets made of Y-TZP are stronger, more resistant to wear and deformation, less likely to adhere plaque, and more esthetically pleasing. Additionally, they share the same frictional qualities as polycrystalline alumina brackets and show good sliding properties with both stainless steel and nickel-titanium arch wires [56, 57].

#### *6.6.2 Zirconia prefabricated zirconia attachments*

The use of Prefabricated zirconia attachments in clinical applications is based on the material's durability and strength. The literature on clinical performance and effectiveness, however, is nonexistent. There are currently two different types of Y-TZP attachments available on the market: an extracoronal, cylindrical, or ball attachment for removably attached partial dentures, and a ball attachment for overdentures that is a part of a zirconia post (Biosnap, Incermed) and is available in three diameters for three levels of retention (Proxisnap, Incermed) [39].

#### *6.6.3 Zirconia cutting and surgical instruments*

Newly created zirconia cutting instruments (such as drills and burs) can be employed in soft tissue trimming, maxillofacial surgery, implantology, and other fields. These tools have been shown to be resistant to chemical corrosion, and they provide maximum cutting efficiency while operating smoothly and with less vibration. Lastly, alumina-toughened zirconia (ATZ) can be used to create surgical equipment such as scalpels, tweezers, periosteal elevators, and depth gauges using injection molding (Z- Look3 Instruments, Z-Systems) [39].

#### **7. Conclusions**

It is obvious that zirconia is continuous developing and growing ceramic material for wide applications in dentistry from the traditional core or framework material

#### **Figure 8.**

*Single implant all-ceramic crown restoration with the use of a zirconia prefabricated of an upper right lateral incisor (tooth 12). Top: abutment connection. Middle: Zirconia abutment after laboratory modification and Ti screw. Bottom: final clinical situation after crown adhesive cementation [39].*

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

to the newly developed monolithic multilayer zirconia Dental zirconia continues to increase and is classified into many species in the yttria system alone. They are classified with yttria content, monochromatic/polychromatic, uniform/hybrid composition, and monolayer/multilayer. Zirconia with a higher yttria content is more translucent and less strong mechanically. Zirconia applications seem to consolidate a well-established position in clinical dentistry, due to the improvements in CAD/CAM technology and to the material's exceptional physical properties. Existing clinical studies demonstrated a promising survival potential regarding tooth-supported restorations therefore, a suitable zirconia should be selected depending on whether strength or esthetics are desired. Therefore, it is concluded that an adequately selected zirconia is a suitable material because of its mechanically, esthetically, and biologically excellent properties. Zirconia abutments provide a favorable bioesthetic addition to implant dentistry, however, long-term clinical assessment is needed for accurate evaluation of implant-supported zirconia restorations.

#### **Acknowledgements**

Great thanks to the college of dentistry/university of Mosul/Iraq for their continuous support and encouragement.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Abbreviations**


#### **Author details**

Ali Dahee Malallah\* and Nadia Hameed Hasan Department of Conservative Dentistry, College of Dentistry, University of Mosul, Mosul, Iraq

\*Address all correspondence to: ali.dhahi@uomosul.edu.iq

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Classification and Generations of Dental Zirconia DOI: http://dx.doi.org/10.5772/intechopen.109735*

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Section 3

## Production of Zirconia

#### **Chapter 3**

## Zirconia: Synthesis and Characterization

*Bincy Cyriac*

#### **Abstract**

Main resource of zirconia is the mineral zircon which occurs in beach sand and placer deposits. Alkali fusion and thermal plasma dissociation are the frequently adopted procedures to convert zircon to zirconia. Synthesis of different zirconia phases (monoclinic, cubic, and tetragonal) can be accomplished by the precise control of different operating parameters and stoichiometry of the reagents. Mesoporous and nano-zirconia which find wide application in catalysis and electronics are synthesized by different methods like solution combustion synthesis, sol–gel synthesis, hydrothermal synthesis, co-precipitation, and solid-phase sintering. Recently, biosynthesis of zirconia has taken a quantum leap due to environmental concerns. The synthesized zirconia is characterized by various chemical, physical, and instrumental methods to find out composition, crystal structure, size, and morphology.

**Keywords:** zircon, zirconia, plasma dissociated zircon, synthesis of zirconia, characterization of zirconia

#### **1. Introduction**

Zirconia (ZrO2) is one of the important materials which finds wide usage depending on its purity and crystal structure in various fields such as ceramics, refractories, electronics, and others. Diversified applications of zirconia as a hightechnology material for industrial applications are due to its superior mechanical, thermal, electrical, chemical, and optical properties [1]. It is an ideal material for the production of ceramics, electronic materials, and pigment due to the combination of properties like hardness, strength, high melting point, and biocompatibility. Zirconia ceramics have excellent biocompatibility with the human body, a property which helped it to replace alumina for prosthesis devices in hip joints, femoral ball beads, and dental implants. Optically clear cubic zirconia, known as synthetic diamond, is widely used in jewelery.

Zirconium oxide exhibits three well-defined crystal structures, i.e., monoclinic, tetragonal, and cubic (**Figure 1**) [2]. The monoclinic phase is stable up to 1170°C, and above this temperature it is transformed into tetragonal phase. The tetragonal phase is stable up to 2370°C and then transforms to the cubic phase, which is stable up to the melting temperature of 2680°C. On cooling to the transformation temperature, the structure reverts back to the original phase [3]. Out of these, tetragonal to monoclinic

**Figure 1.**

*Crystal structures of ZrO2: (a) cubic, (b) tetragonal, and (c) monoclinic. Red and blue spheres correspond to oxygen and zirconium atoms, respectively.*

transformation is of great importance due to large volume change. This volume reduction is very advantageous for improving strength and toughness of ceramics.

Baddeleyite (ZrO2) is the naturally occurring zirconia, and it occurs in carbonatite rocks [4]. However, the major source of zirconia is its silicate mineral zircon (ZrSiO4), and it mainly occurs as a constituent of beach sand and placer deposits along with rutile, ilmenite, and monazite. Beach sands of Australia, India, Brazil, and USA are rich in zircon. In terms of zirconia, world reserves of zirconium are around 64 million tones. Australia (35%) is the major producer of zircon followed by South Africa (28%), USA, and Mozambique (7% each) [5]. In placer deposits, zircon occurs as a major constituent along with ilmenite, rutile, and quartz, while the minor constituents are sillimanite, garnet, and magnetite. Zircon is separated from beach sands and placer deposits by physical beneficiation method as given in **Figure 2**.

Zircon has the general composition of 67% zirconia and 32.8% silica. Zircon is zirconium hafnium silicate mineral with general formula (Zr, Hf)SiO4. Zircon usually contains some hafnium, typically about 1%. **Table 1** gives the chemical composition of Egyptian zircon.

**Figure 2.** *Physical beneficiation of beach sand [6].*

*Zirconia: Synthesis and Characterization DOI: http://dx.doi.org/10.5772/intechopen.111737*


#### **Table 1.**

*XRF analysis data of chemical composition of Egyptian zircon [8].*

Zircon is chemically very stable. Zircon is considered a refractive material due to its low coefficient of thermal expansion and high melting point. Extraction of zirconium, zirconia, and other products from zircon requires rigorous chemical and thermal treatments to break the bonds between ZrO2 and SiO2 (**Figure 3**) [1]. A variety of techniques have been proposed for the extraction of zirconia from zircon.

**Figure 3.** *Crystal structure of zircon [7].*

#### **2. Decomposition of zircon**

The two major routes used for the decomposition of zircon are chemical and thermal process.

#### **2.1 Chemical process**

The chemical decomposition is divided into different categories based on the reagents used as fluxes. They are fused with:

1.Sodium hydroxide (caustic soda) [6, 8–11].

2. Sodium carbonate [12].

3.Calcium oxide and magnesium oxide [13].

4.Calcium carbonate (lime) [14].

5.Carbochlorination [15].

#### *2.1.1 Fusion with sodium hydroxide*

This is the widely adopted commercial method for the extraction of zirconia from zircon sand. In this method, zircon is fused at 600°C with caustic soda to produce sodium zirconate (Na2ZrO3) and sodium silicate (Na2SiO3) (eq. 1):

$$\text{ZrSiO}\_4 + 4\text{NaOH} \rightarrow \text{Na}\_2\text{ZrO}\_3\downarrow + \text{Na}\_2\text{SiO}\_3 + 2\text{H}\_2\text{O} \tag{1}$$

Water-soluble sodium silicate is removed by washing, and the residual sodium zirconate is treated with hydrochloric acid to produce zirconyl oxychloride (ZrOCl2- ZOC) (eq. 2):

$$\text{Na}\_2\text{ZrO}\_3 + 4\text{HCl} \rightarrow \text{ZrOCl}\_2 + \text{NaCl} + 2\text{H}\_2\text{O}.\tag{2}$$

Zirconium oxychloride obtained is further acidified to precipitate it as zirconium oxychloride crystals:

$$\text{ZrOCl}\_{2(l)} + 8\text{H}\_2\text{O} + \text{HCl} \rightarrow \text{ZrOCl}\_2\text{.8H}\_2\text{O(s)} + \text{HCl.} \tag{3}$$

The residual solids are separated by filtration which contains mainly sodium zirconate, hydrous zirconia, and some silica. This residue is dissolved in 5 M HCl at 90°C to obtain a clear solution of zirconium oxychloride. The zirconium oxychloride obtained is neutralized with ammonia to precipitate zirconium hydroxide which in turn on calcination at 900°C gives pure zirconia, assaying >99.5%, with a monoclinic structure.

Flow sheet of alkali fusion of zircon is given in **Figure 4**.

#### *2.1.2 Fusion with sodium carbonate*

In this method, zircon is fused with sodium carbonate at 1100°C for several hours in an electric furnace to produce sodium zirconium silicate (Na2ZrSiO5):

*Zirconia: Synthesis and Characterization DOI: http://dx.doi.org/10.5772/intechopen.111737*

#### **Figure 4.**

*Flow sheet for alkali fusion of zircon [16].*

$$\text{ZrSiO}\_4 + \text{Na}\_2\text{CO}\_3 \to \text{Na}\_2\text{ZrSiO}\_5 + \text{CO}\_2.\tag{4}$$

Sodium zirconium silicate reacts with excess of sodium carbonate to form sodium zirconate and sodium silicate according to the eqn:

$$\mathrm{Na\_2ZrSiO\_5} + \mathrm{Na\_2CO\_3} \to \mathrm{Na\_2ZrO\_3} + \mathrm{Na\_2SiO\_3} + \mathrm{CO\_2}.\tag{5}$$

Sodium zirconate remaining after the washing is treated with HCl to produce zirconium oxychloride crystal (ZOC) which in turn is converted into zirconia.

#### *2.1.3 Fusion with calcium oxide and magnesium oxide*

This method is mainly adopted for the preparation of cubic zirconia. Mixtures of zircon and CaO/MgO in the same molar ratio are fused at 1200°C. The complete disintegration of zircon produces zirconia (ZrO2) and calcium magnesium silicate according to the equation:

$$\text{ZrSiO}\_4 + \text{CaO} + \text{MgO} \to \text{ZrO}\_2 + \text{CaMgSiO}\_4 \downarrow \tag{6}$$

Leaching of the fused mass with hydrochloric acid removes the silica totally. The zirconium oxychloride obtained upon calcination gives cubic zirconia. The temperature and the composition of CaO and MgO play an important role in the concentration of cubic and monoclinic zirconia obtained. At lower percentage of CaO/MgO mixture, monoclinic zirconia content increases whereas at higher CaO/MgO percentage cubic zirconia increases. The fusion of zircon with CaO and MgO separately results in the

formation of monoclinic zircon, while their combination in the same molar ratio as zircon produces cubic zircon. At a lower temperature of 1200°C monoclinic zirconia content increases, while at 1400–1500°C cubic zirconia content increases.

#### *2.1.4 Fusion with calcium carbonate (lime)*

This method is not an industrial method for the decomposition of zircon. Zircon on fusion with lime and subsequent cooling, produce a very fine powder of calcium silicate and coarse crystals of calcium zirconate enabling the physical separation of the two an easy process:

$$\text{2ZrSiO}\_4 + \text{5CaCO}\_3 \to \text{2CaZrO}\_3 + (\text{CaO})\_3(\text{SiO}\_2)\_2 + \text{CO}\_2.\tag{7}$$

Calcium zirconate is converted into ZOC by treating with HCl.

#### *2.1.5 Carbochlorination*

This reaction takes place in a chlorinator. Zircon is heated with chlorine gas at 1100°C by induction heating of the graphite walls of chlorinator. Products, consisting of zirconium tetrachloride, silicon tetrachloride, and carbon monoxide, which are in gas phase are cooled down to 200°C:

$$\text{ZrSiO}\_4 + 4\text{Cl} + 4\text{C} \to \text{ZrCl}\_4 + \text{SiC}\_4 + \text{CO}.\tag{8}$$

On cooling, zirconium tetrachloride solidifies first followed by silicon tetrachloride facilitating their separation. Zirconium tetrachloride is converted into zirconium oxychloride on treatment with water.

Zircon decomposition by alkali fusion has the capability of large-scale production and high efficiency, in comparison with chloride and lime sintering method. But in the former method the temperature profile and atmospheric control inside the furnace plays a crucial role. Silica carryover to zirconium fraction increases at high temperature. Once the sodium silicate is extracted from the sodium zirconate and dissolved in hydrochloric acid, two distinct routes can be followed to precipitate various zirconium chemicals. The most common route is to precipitate zirconium oxychloride crystals (ZOC), with subsequent purification from all contaminants (crystal route). Less known is the process (liquid route) that involves the direct precipitation of zirconium basic sulfate (ZBS). This route will yield a less pure product, with contaminants such as silica and titanium. An important factor in this route is the prevention of silica gel formation, which could hamper final product filtration.

#### **2.2 Thermal decomposition of zircon**

Even though chemical decomposition by alkali fusion at very high temperature is the time-tested method for the extraction of zirconia from zircon, it has certain shortcomings. High treatment cost, complexity of the process, and environmental issues associated with effluent treatment are some of them. These drawbacks compelled the search for cost-effective and environmentally friendly alternatives for zircon decomposition. The promising thermal plasma technology offers a one-step

alternative for this conversion. Use of thermal plasma dissociation for the decomposition of zircon was carried out first by Wilks and co-workers in 1966 [17]. Zircon is dissociated to ZrO2 and SiO2 when heated above 1700°C [18, 19] as expressed by the following reaction:

$$\text{ZrSiO}\_4 \rightarrow \text{ZrO}\_2 + \text{SiO}\_2.\tag{9}$$

The above reaction is reversible and the oxides recombine to form the silicate on cooling. However, if zircon is heated to temperatures exceeding 1900°K and the reaction products are quenched rapidly, the reversible reaction is prevented. Recombination of products back to zircon is prevented by rapid cooling. The silica can be separated from the product by simple acid or alkali leaching to get pure zirconium oxide [20]. For thermal plasma decomposition, a direct current non-transferred argon arc plasma torch is used to generate plasma [21, 22]. The thermal dissociation is thermodynamically reversible, and both oxides recombine to form the silicate [19]. But if the cooling rate is fast enough (quenched) the recombination is avoided, and the product of the dissociation is known as plasma-dissociated zircon (PDZ). During the heating process, the zirconium silicate crystal structure re-arranges into amorphous silica and zirconium oxide phases. On fast cooling, amorphous glassy silica matrix entraps the fine zirconium oxide particles which can be easily disintegrated by acid attack. At high temperature, the stable zirconia phase is the tetragonal zirconia (t-ZrO2) as inferred from phase diagram of zirconia (**Figure 5**). The resulting tetragonal zirconia during the cooling might transform into monoclinic zirconia which is the stable phase at room temperature [23]. Plasma decomposition of zircon is greatly affected by forward power, plasma and carrier gas flow, and location of power feed port.

The microstructure of PDZ is influenced by the cooling rate after melting which depends strongly on the initial zircon particle size [24]. Thus, relatively slow cooling results in spherulitic crystals of monoclinic ZrO2 in SiO2 glass, whereas the rapid cooling gives extremely fine (< 10 nm) crystals of tetragonal ZrO2 crystals in glassy silica matrix. Major advantages of thermal plasma processing over conventional methods are the ability of plasma reactors to attain high energy density and high temperatures, ability to control the processing atmosphere, increased reaction kinetics, eco-friendly nature of the process, the rapid cooling rate which prevents back reaction, and adaptability to process a variety of materials [25].

Plasma thermal dissociation processes have undergone a variety of process changes to obtain zirconia with high purity and definite crystal structure. Low-power transferred arc plasma (TAP) in which air is used as plasma gas instead of conventional argon gas renders the process cost-effective [24]. Carbothermal plasma dissociation in which coke is added to zircon during plasma dissociation is another adaptation. Energy consumption and overall operating cost of production are drastically reduced by these processes compared to conventional methods which are energy-intensive and expensive [21]. In flight, removal of silica is achieved in carbothermal plasma dissociation [26]. The thermally aided dissociation process becomes a thermo-chemically driven reaction at a much lower temperature. Another major feature is that the oxides of Al, Fe, and Ti and Mg that are present as impurities in zircon mineral are completely removed as vapor during plasma processing.

**Figure 5.** *Phase diagram of zircon [23].*

#### **3. Alternate methods for the synthesis of zirconia**

The above-mentioned procedures are used generally for the synthesis of macro- or micro-crystalline zirconia which is mainly used as ceramics due to its hardness, abrasiveness, high melting point, and low frictional resistance. At nanoscale, it becomes immensely valuable owing to its high thermal stability, luminescence, refractive index, chemical stability, high specific area, biocompatibility, and ability to exhibit significant antibacterial, antioxidant, and antifungal properties. Such outstanding characteristics have motivated the scientific community to explore zirconia-based nanomaterials in a wide range of technological fields as functional materials, *viz.*, catalysts [27], sensors [28], semiconductor devices [29], ceramics [30], and implants [31]*.* Apart from that, it can also be employed as a dielectric, electro-optic, and piezoelectric material due to its favorable optical and electrical properties. Zirconia nanoparticles are synthesized by chemical and physical methods.

#### **3.1 Chemical methods for the synthesis of nano-zirconia**

Important chemical methods used for the synthesis are solution combustion synthesis (SCS), sol–gel synthesis, hydrothermal synthesis, and co-precipitation.

#### *3.1.1 Solution combustion synthesis (CSC)*

Solution combustion synthesis (SCS) has been widely used for the synthesis of metal oxides with desired morphology. This involves the complex exothermic reactions in solution between fuel which is an organic compound and zirconium metal solution. SCS is a self-sustained redox reaction initiated by a source of energy (thermal or electric) between a fuel and an oxidant (usually metal nitrates) [32]. The fuel is typically composed by organics containing carbon and hydrogen that facilitate the liberation of heat by the formation of CO2 and H2O during the combustion process. The metal precursors are the source of the metal cations which decides the final metal oxide. One of the methods for the synthesis of zirconia nanopowders is by annealing stochiometric mixtures of zirconium oxy nitrate hydrate in alcohol and urea [33]. Nanocrystalline zirconium oxide powder has been prepared by the glowing combustion method using sucrose as the fuel and zirconyl nitrate as an oxidant in aqueous solution [34]. Solution combustion synthesis is an attractive technique for the preparation of ZrO2 nanopowders and thin films, owing to its simplicity, energy and time savings, cost-effectiveness, versatility, and higher purity compared to conventional methods [35].

#### *3.1.2 Sol–gel synthesis*

Sol–gel synthesis is the stepwise formation of metal oxide nanoparticles in which the sol or colloidal solution of solid particles formed first is transformed into a gel which is an interconnected network of polymerized metal oxide in a solvent. Drying of the gel results in aerogels having nanocrystalline size [36]. For the preparation of zirconia by sol–gel process, zirconium alkoxide in alcohol (precusor) is hydrolyzed with ammonia. Resultant sol is stirred continuously until the gel is formed. The gel formed is dried in conventional oven or by supercritical drying methods. Supercritical drying produces aerogels with smaller size compared to conventional drying methods:

Zr OC ð Þ 3H7 <sup>4</sup> þ C3H7OH þ NH4OH ! Zr OH ð Þ<sup>4</sup> ð Þþ sol C3H7OH þ NH3*:* (10)

$$\text{Zr}\,(\text{OH})\_{4(\text{Sol})}\,\text{(on polycondensation)} \to \left[\text{Zr}(\text{OH})\_{4}\right]\_{\text{n}}\text{xC}\_{3}\text{H}\_{7}\text{OH.xH}\_{2}\text{O.}\tag{11}$$

$$\left[\text{Zr(OH)}\_{4}\right]\_{\text{n}}\text{xC}\_{3}\text{H}\_{7}\text{OH}.\text{xH}\_{2}\text{O}\ (110^{\circ}\text{C}) \rightarrow \text{Zr(OH)}\_{4} + \text{C}\_{3}\text{H}\_{7}\text{OH} + \text{H}\_{2}\text{O}.\tag{12}$$

$$\text{Zr}\,(\text{OH})\_4\,(400^\circ \text{C}) \to \text{ZrO}\_{2(\text{T})}\,(500 \,-700^\circ \text{C}) \to \text{ZrO}\_{2(\text{T})} + \text{ZrO}\_{2(\text{M})}\tag{13}$$

In this method, the reactivity of the precursors can be modified using chelating agents, which influence the gelation and ultimately the modification of the size and shape of the particles. The different chelating reagents that have been employed for the preparation of nano-zirconia are acetylacetone, acetic acid, citric acid, and different sugars (e.g., sucrose, maltose, and glucose). The advantages of sol–gel synthesis over other techniques are the ease in controlling the purity, homogeneity, and physical characteristics at low temperature [37].

#### *3.1.3 Hydrothermal synthesis*

Hydrothermal method for the synthesis of nanocrystalline zirconia is usually carried out by heating a mixture of precursors (zirconium compounds) with appropriate reagent which are reducing/oxidizing/hydrolyzing in a suitable solvent in a closed vessel [31, 38]. This method ensures the production of nanocrystals with definite morphology, crystal structure, stability, and size. In some methods, surfactants are added to the mixture to avoid agglomeration. Agglomeration can be further eliminated by hydrothermal corrosion methods. This method involves the use of corroding mediums such as sulfuric and hydrochloric acid to break down the hard agglomerate to dispersed fine nanoparticles. By carefully adjusting the reaction parameters, zirconia with definite crystalline structure and required size can be obtained. By introducing suitable dopants at the synthesis stage, zirconia with different physical properties can be synthesized. By introducing Eu3+ during the synthesis of cubic zirconia, white light-emitting nano-zirconia is produced which can be used in light-emitting diodes and electronic flashes [39]. Zirconia nanocrystals with different size can be synthesized by careful control of reaction parameters and concentration of reactants. Spindle/rod-like structures are synthesized by this method [40]. Surfactant-assisted hydrothermal synthesis of zirconia produces thermally stable zirconia crystals with different morphology [41]. Surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 are generally used for this type of synthesis. By tuning the reaction conditions during synthesis, definite phase composition and morphology can be achieved (**Figure 6**).

#### *3.1.4 Microwave-assisted hydrothermal synthesis*

This is another method for the synthesis of nano-zirconia. The advantages of this method over the other methods are volumetric heating (the entire volume of solution is evenly heated, instead of relying on heat diffusion processes across the reaction vessels) and short reaction time (reaction time can be as short as a few minutes) [42]. Moreover, the accurate control of morphology and crystal structure is possible by adjusting microwave parameters. Different zirconia phases can be obtained by adjusting pH, temperature, reaction time, pressure, and the precursor used [43]. In this method, microwave digestion of stochiometric composition of zirconia percussor,

$$\begin{array}{c|c} \text{ZrO(NO}\_3\text{)}\_2 + \text{NH}\_4\text{OH} & \begin{array}{c} \text{g}\text{O}^{\text{O}}\text{C}/\text{n2}\text{hres} \\ \hline \text{CTAB} & \text{CTAB} \\ \hline \text{CTAB} & \text{CTAB complex} \end{array} & \begin{array}{c} \text{Zr(OH)}\text{2}, \text{CTAB} + \text{ZrO(OH)(NO}\_3\text{)} \rightarrow \text{NH}\_4\text{OH} \\\\ \text{Calculation} \\\\ \text{Calculation} \\\\ \text{Messorrous} \\\\ \text{ZnO} & \text{ZrO2} \end{array} \end{array}$$

**Figure 6.** *Surfactant-assisted nano-zirconia synthesis [41].*

generally a zirconyl compound (e.g., zirconyl chloride, zirconyl hydroxide, zirconyl nitrate hydrate, and zirconium alkoxides) and an alkali generally sodium hydroxide is carried out at specified temperature and pressure.

#### *3.1.5 Co-precipitation technique*

This is an easy method for the synthesis of zirconia nanoparticles. Co-precipitation method is a promising alternative to other methods due to its inherent simplicity, ecological compatibility, precise stoichiometry, structural control, and large-scale production. However, size, shape, and dispersion of powders prepared by this method depends strongly on precipitants used [44]. Hence, the selection of appropriate precipitating reagent is the most important factor in co-precipitation method. Ammonium hydroxide (NH4OH), ammonium bicarbonate (NH4HCO3), ammonium carbonate [(NH4)2CO3], sodium hydroxide (NaOH), and urea are the typical precipitants used in precipitation method. pH of the precipitating medium has significant influence on the homogeneity and composition of the precipitate [45]. Precipitants are added to zirconium precursor, typically zirconium oxychloride or nitrate at a controlled rate. As the critical solution concentration of the zirconium hydroxide is reached, nucleation starts followed by growth phase. Zirconium hydroxide on calcination gives zirconia nanoparticles. Morphology of the nanoparticle obtained can be controlled by the ratio of the reactants and calcination temperature [46]. Surface area of the zirconia prepared by this method depends on nature of the precursor, rate of addition of base, pH of the solution, and presence of surfactants and organic anions like oxalate and citrate [47]. At an increased calcination temperature, size of the crystal and its agglomeration increased.

#### **3.2 Solid-phase sintering**

In this method, precursors are mixed and milled in a high-intensity ball mill. Mesoporous zirconia with worm-like structure is obtained by milling zirconyl chloride mixed with block polymer surfactant along with sodium hydroxide. The mixture is autoclaved and allowed to crystallize [48]. Sintering method is used for the densification of the material. Preparation parameters have strong influence on the structure of zirconia synthesized. Pore structure changes from microporous to mesoporous with change in parameter. A controlled grain size with good densification can be achieved by proper selection of heating schedule. Densification of powders prepared by chemical vapor methods can undergo one step sintering at 1000°C. Powders prepared by solution technique cannot be sintered at a lower temperature. Sintering process depends on mass transport mechanism [49]. Four underlying mechanisms of sintering are surface diffusion, spread throughout volume boundary, evaporation, and condensation. Temperature and residence time of the material under sintering are decisive in particle size and bond strength. The smaller the particle size, greater will be the bond strength due to the increased contact between the particles (**Figure 7**).

#### **3.3 Biosynthesis of zirconia**

Biosynthesis is a greener approach to zirconia synthesis which helps to enhance its biocompatibility and reduce the environmental concerns. This process involves the synthesis of zirconia nanoparticles by reducing, stabilizing, and capping of the metal precursor using natural and renewable agents like microbes and plant parts [51–55].

**Figure 7.**

*A model showing sintering process [50].*

**Figure 8.** *Biosynthesis of zirconia [56].*

Major steps involved in this process are reduction and stabilization. There are intracellular and extracellular pathways adopted for reduction. In intracellular pathway, metal ion is transported into the cell by interaction with the negative receptors of the cell wall. The enzymes of the cell cause the reduction of metal. In extracellular synthesis, zirconium ions are reduced by an enzyme – nitrate reductase. After the reduction, the zirconia nanoparticles are formed v*ia* nucleation, aggregation, and growth (**Figure 8**). The protein species released by the cell wall act as capping agents which stabilize zirconia nanoparticles [56].

#### **4. Characterization of zirconia**

Characterization is the essential requirement to understand the physical, chemical, structural, and morphological properties of a newly synthesized material. Characterization involves the identification of different components present in the material, their quantification, structural elucidation, and identification of morphological properties of the material. Quantification of different components in zirconia is carried out by chemical procedures and by instrumental techniques like X-ray photoelectron spectroscopy (XPS), wavelength-dispersive X-ray fluorescence spectroscopy

*Zirconia: Synthesis and Characterization DOI: http://dx.doi.org/10.5772/intechopen.111737*

(WD-XRF), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Structural elucidation is carried out by instrumental techniques like X-ray diffraction (XRD) spectroscopy, Fourier-transformed infrared spectroscopy (FTIR), N2 adsorption measurements, and Raman spectrometry. Morphology of synthesized material is usually assessed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

#### **4.1 Chemical characterization of zirconia**

Complete chemical characterization of zirconia is the quantification of elements present in zirconia. Multielement analysis of zircon samples is usually carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectroscopy (ICP-MS) after the quantitative dissolution [57]. Zirconia being a highly refractory material, quantitative decomposition of the material cannot be accomplished by usual acid decomposition procedures. Fusion of zirconia with sodium carbonate, sodium metaborate, and lithium metaborate are reported methods for complete dissolution. Microwave-assisted digestion of zirconia with a mixture of hydrofluoric acid, sulfuric acid, and hydrochloric acid is also used for decomposition of zirconia. Once in solution state, the concentration of elements is determined by either ICP-AES or ICP-MS. Electrothermal vaporization inductively coupled plasma mass spectrometry (ET-ICP-AES) in presence of a modifier is an effective method for the characterization of high-purity zirconia [58]. ICP-MS has also been used for the single-particle analysis of zirconia colloids in water [57]. Laser ablation inductively coupled plasma mass spectrometry is an excellent technique for the chemical characterization of zirconia samples.

#### **4.2 X-ray photoelectron spectroscopy**

Surface chemical characterization of zirconia is carried out using XPS. XPS also known as electron spectroscopy for chemical analysis (ESCA) is a quantitative spectroscopic technique which gives valuable information on the surface elemental composition and chemical state of the components of the material under study [59]. XPS provides surface analysis data and the average depth of analysis for an XPS measurement is approximately 5 nm. XPS is typically accomplished by exciting a samples surface with mono-energetic Al kα or Mg Kα X-rays which prompts the emission of photoelectrons from sample surface. An electron energy analyzer is used to measure the energy of the emitted photoelectrons. From the binding energy and intensity of a photoelectron peak, the elemental identity and chemical state of the element can be determined. Surface defects can be inferred from chemical state of the elements on the surface. Wide-scan spectrum, core-level spectrum, and the upper valence band (UVB) spectrum are the major analysis modes used for characterization in XPS. The wide-scan spectrum gives elemental information of the surface. **Figure 9a** shows the wide-scan spectrum of zirconia. It shows peaks corresponding to Zr and oxygen. Presence of any peak other than that of Zr and O2 shows contamination or doping. **Figure 9b** shows the core-level spectrum of zirconia. XPS core-level peaks are used to obtain information on the chemical state of the surface, and crystalline phase. **Figure 10** gives the Zr-3d5/2, Zr-3d3/2, and O-1 s core-level peaks of thin films of zirconia with different oxygen content. The chemical shift observed in the films with higher oxygen content corresponds to different oxidation state of zirconia. Peak shift in core-level peaks, Zr-3d5/2, Zr-3d3/2, and O-1 s of **Figure 11** corresponds to phase change.

**Figure 9.** *The wide-survey scan spectrum (a) and Zr 3d core-level XPS spectra (b) of zirconia [60].*

#### **Figure 10.**

*(a) Core-level Zr 3d (XPS) spectra of bulky metallic ([O/Zr] = 0.07), partially oxidized films ([O/Zr] = 0.25 and [O/Zr] = 0.9) thin oxide films([O/Zr] = 1.3) with different oxidation states: Zr0 for metallic, Zr1+, Zr2+, Zr3+ for suboxides and Zr4+ for stoichiometric oxide. (b) Core-level O1s spectra (XPS) for the corresponding Zr3d spectra [61].*

Identification of different crystalline phases by shift in core-level peaks is extremely difficult as the variation in crystal structure does not produce large shift in the core levels. However, by analyzing the valence band of XPS, phase transformation data can be obtained. **Figure 12** shows the upper valence band XPS (UVBXPS) spectrum of zirconia surfaces grown by dry thermal oxidation in the temperature range 300–450°K. Oxide films grown below 400°K are predominantly amorphous, and formation of tetragonal phase starts above this temperature. There is a pronounced change in the shape of upper valence band spectra with increase in oxidation temperature. This can be attributed to the gradual change in the formation of tetragonal

**Figure 11.** *XPS spectra of (a and b) tetragonal and (c and d) monoclinic zirconia films annealed at 650o c and 850o c [62].*

**Figure 12.** *XPSUVB spectra of thermally grown ZrO2 films at temperatures 300°K and 450°K [63].*

phase (increase in crystallinity) with temperature above 400°K. Change in the shape of the spectra with crystallinity is due to the increase in Zr-O bond ionicity and changes in the first coordination spheres of both Zr and O.

#### **4.3 Fourier-transformed infrared spectroscopy (FTIR)**

FTIR spectrum is an effective tool for the identification of functional groups present in a molecule [64]. When infrared radiation passes through a molecule, it causes changes in the dipole moment of the molecule which corresponds to a definite vibrational energy. Since every functional group is composed of different atoms and bonds with different bond strengths, frequencies of vibrations are unique to individual groups and classes of functional groups (e.g., O-H and C-H stretching frequency appear around 3200 cm<sup>1</sup> and 2900 cm<sup>1</sup> , respectively). Since the collection of vibrational energy bands for all the functional groups of a molecule is unique to every molecule, these peaks can be used for identification using library searches of comprehensive sample database. FTIR spectrum of ZrO2 nanoparticles and surfactant-modified zirconia are shown in **Figure 13**. The absorption peak at 608 cm<sup>1</sup> corresponds to Zr-O stretching vibrations. A shift in the position of this peak is observed in the surfactant-modified zirconia. Two extra broadbands at 1607 cm<sup>1</sup> 1nd 1400 cm<sup>1</sup> in the spectra of surfactant modified ZrO2 are attributed to the bending vibration of C-H bonds.

#### **4.4 BET surface area analysis**

Brunauer–Emmett–Teller (BET) surface area analysis is used to measure total surface area, total pore volume, and pore size distribution of a material [64]. In this method, multipoint analysis of analytes-specific surface area (m2 /g) is measured through gas absorption analysis. When inert gases like N2 is flown over the solid sample, the gas molecules absorb on to analyte surface and in pores due to the weak van der Waals forces forming a monolayer of gas on the surface. After the adsorption layers are formed, the sample is removed from the nitrogen atmosphere and heated to release the adsorbed nitrogen from the material and quantified. The data obtained is used to

#### **Figure 13.**

*FTIR spectra of zirconia nanoparticles (a) and surfactant-modified zirconia nanoparticles (b) from 4000 to 400 cm<sup>1</sup> [65].*

**Figure 14.** *BET adsorption desorption curve for zirconia nanoparticles [66].*

plot the BET isotherm which is a plot of amount of gas adsorbed as a function of pressure. At normal temperatures, the interaction between solid and gases is minimal, and the sample surface is cooled with liquid nitrogen to obtain appreciable adsorption. Surface area is calculated using BET equation from the adsorption branch of isotherm, and pore size distribution was calculated from desorption branches using the Barrett– Joyner–Halenda (BJH) method [64]. **Figure 14** shows the BET adsorption–desorption curve of ZrO2 nanoparticles. The surface area and size of the nanoparticle obtained from this curve are 44m2 /g and 24 nm, respectively [66].

#### **4.5 X-ray diffraction spectroscopy (XRD)**

Powder X-ray diffraction is a method used for the identification of crystalline structure and phases of a material [67]. When X-rays interact with crystalline samples, they are diffracted at a particular angle which satisfies Bragg's equation (nλ =2dsinθ) which gives the relation between wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. The diffracted Xrays are detected, processed, and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice are obtained due to the random orientation of the powdered material. Conversion of these diffraction peaks to d spacing by comparing it with standard reference patterns allows the identification of crystalline structure. Quantitative characterization of lattice defects and relative stress endured by the crystals can also be elucidated by XRD. As zirconia exists in three different crystalline structures, ascertaining the crystalline phase of the newly synthesized material is the most important part of the characterization of zirconia. Crystalline structure of synthesized zirconia depends on so many factors like temperature of calcination, pH of synthesis, and the reagents used. **Figure 15** shows ZrO2 nanoparticles synthesized at different temperatures (400, 500, and 600°C). The welldefined peaks at 2θ = 30.2, 35.0, 50.4, 60.0, and 62.7°correspond to the diffractions of the (101), (110), (200), (211), and (202) crystalline planes of cubic zirconia. The shape of the peaks at 35.0, 50.4, and 60.0° are slightly asymmetric, suggesting the

**Figure 15.** *XRD patterns of ZrO2 NPs synthesized at different temperatures [68].*

formation of a tetragonal phase. The shoulder at 2θ = 34.5° is due to the diffraction of the (002) crystalline plane of tetragonal phase zirconia. The appearance of peaks corresponding to tetragonal and cubic phase in the XRD spectrum on increasing the temperature from 400 to 600°C gives a clear indication of the conversion of amorphous to crystalline form with increase in temperature.

#### **4.6 Raman spectroscopy**

Raman spectroscopy is a powerful, nondestructive, user-friendly technique to determine the surface characteristics and subsurface damages. This technique is based on Raman effect which in-elastic scattering process which occurs when an incident photon interacts with phonons in the material [69]. When the photons of a particular wavelength interact with the vibrating phonons of the materials, the light is scattered at a frequency and the difference in frequency between the incident and the scattered light provides the information of the lattice vibrations. Raman vibrational spectra provides a structural finger print for the molecular identification. Raman spectra of the molecules are widely influenced by the microstructural changes and impurities in the molecule. Hence, the information from Raman spectra like band position, shift, and intensity can be used to characterize the defects and subsurface damages induced by crystal growth. Raman spectra obtained from the instrument gives information on molecular vibrations and crystal structure of materials [70]. Raman spectra of the crystalline molecules satisfy energy conservation rule, wave vector conservation, and polarization selection rule. But if there are some defects and damages in the crystal, this will induce a reduction in symmetry and breakdown of the selection rules will take place. This may give rise to fresh bands in the Raman spectra or broadening of spectra. A change in the Raman spectra is observed when a material is strained or crystal structure of the material is changed [71].

Raman peaks are widely used to identify different crystalline phases of zirconia.

**Figure 16.** *Raman spectra of different phases of zirconia [72].*

**Figure 16** shows the Raman spectra of monoclinic (A), tetragonal (B), and partly tetragonal partly monoclinic (C) zirconia phases. The two sharp peaks at 142 cm�<sup>1</sup> and 256 cm-1 corresponds to tetragonal phase. Another two bands at around 316 cm�<sup>1</sup> and 460 cm�<sup>1</sup> also correspond to tetragonal phase. The monoclinic phase shows two sharp peaks at 178 cm�<sup>1</sup> and 190 cm�<sup>1</sup> . Broadband at 384 cm-1 also corresponds to monoclinic structure. Partially transformed zirconia of an overlap of bands corresponding to tetragonal and monoclinic phases are observed. Monoclinic phase content (*Vm*) of zirconia samples can be evaluated by the following formula which was first proposed by Clarke and Adar [73]:

$$\text{Vm} = \text{Im178} + \frac{\text{Im189}}{0.97(\text{It145} + \text{It260})} + \text{Im178} + \text{Im189},$$

where *Im* and *It* are the intensity of the peaks corresponding to monoclinic and tetragonal phases at different wavenumbers, respectively.

#### **4.7 Scanning electron microscopy (SEM)**

Scanning electron microscope is a powerful tool for the two-dimensional topographical imaging. The high-energy electrons emitted from the surface of a material after being exposed to a highly focused beam of electrons from an electron gun are used to produce high-definition images with a resolution of 20 to 0.4 nm [74]. There are two modes electron analysis each one giving different information. Backscattered electrons give contrast based on the different chemical composition across an image. Secondary

electrons which are emitted close to the surface of the sample give information on surface topography. Most of the SEM instruments are equipped with energy-dispersive X-ray spectroscopy which is based on the characteristic of X-rays emitted (which is unique to each element) by the element when exposed to an electron beam. A semiquantitative data of chemical composition of the sample is obtained by this method. Surface morphology of zirconia synthesized by different methodologies has different characteristics which are ascertained by SEM data. **Figure 17** shows the SEM data of zirconia with EDS spectra. Peaks in EDS spectra give qualitative data of the material synthesized. Energy-dispersive X-ray spectroscopy (EDS) spectra of pure zirconia show the peaks corresponding to Zr and oxygen. Presence of other peaks can be attributed to dopants, impurities, and incompletely removed reagents. **Figure 18** shows the agglomerated zirconium oxide powder. **Figure 19** shows SEM data of zirconia before and after ball milling at two different magnifications. Surface characteristics of the two are drastically different at 50-μm magnification. However, magnification at 300 nm shows

**Figure 17.** *SEM image and EDX spectra of freshly synthesized zirconia [75].*

**Figure 18.** *SEM picture of agglomerated zirconium oxide powder [76].*

**Figure 19.** *SEM picture of zirconia before (a) and after (b) ball milling [77].*

essentially same surface characteristics. It is inferred from this data that morphology of zirconia remains unaffected even after ball milling.

#### **4.8 Transmission electron microscopy (TEM)**

Transmission electron microscopy data provides topographical and morphological information about specimens using energetic beam of electrons [78]. When a highenergy electron beam is allowed to pass through a thin slice of a material, the electrons interact with atoms of the material. TEM offers a powerful magnification of the order of a million times. A resolution up to 0.05 nm can be achieved by TEM. The highly detailed images provide valuable insight into elemental and compound structure, leading to provide information on surface features, shape, size, and structure. TEM offers valuable information on the inner structure of the sample. **Figure 20** shows the TEM data of nanocrystalline zirconia synthesized by sol–gel method. **Figure 21** is the in situ TEM data of zirconia under heavy ion irradiation. Crystalline materials can be

#### **Figure 20.**

*TEM image of zirconia nanoparticle synthesized by sol–gel method [79].*

**Figure 21.**

*In situ TEM images of ZrO2 before and after ion irradiation [80].*

**Figure 22.** *Particle size and distribution zirconia nanoparticles by AFM [81].*

**Figure 23.**

*AFM images of zirconia ceramics after different surface treatments. (a) APA, (b) 10F5, (c) 10F30, (d) 20F5, (e) 20F30, (f) 30F5, and (g) 30F30.*

made amorphous by ion irradiation. Appearance of voids and phase transformation is observed in the TEM data after ion irradiation.

#### **4.9 Atomic force microscopy (AFM)**

Atomic force microscopy is a powerful tool to characterize the surface of a material down to atomic scale. AFM can be used to obtain nanoscale chemical, mechanical, electrical, and magnetic properties. AFM offers the three-dimensional visualization of individual particles and group of particles. This is a very good tool to elucidate the surface roughness and grain size and shape of zirconia nanoparticles and ceramics. **Figure 22** is an AFM image that shows shows the size of three-dimensional arrangement of zirconia nanoparticles [81].

**Figure 24.**

*SEM images of zirconia ceramics after different surface treatments. (a) APA, (b) 10F5, (c) 10F30, (d) 20F5, (e) 20F30, (f) 30F5, and (g) 30F30.*

#### *Zirconia: Synthesis and Characterization DOI: http://dx.doi.org/10.5772/intechopen.111737*

Tetragonal zirconia used in dental implants are coated with resin. Adhesiveness of the resin to zirconia increases with roughness of ceramic surface. Surface roughness of zirconia ceramics can be increased by either physical abrasion or chemical etching with hydrogen fluoride (HF). AFM pictures of zirconium ceramic material after different surface treatments are given in **Figure 23** [82]. AFM pictures shows changes in roughness with different treatments like (a) abrasion with alumina, (b) etching with 10%HF for 5 minutes, (c) etching with 10%HF for 30 minutes, (d) etching with 20%HF for 5 minutes, (e) etching with 20%HF for 30 minutes, (f) etching with 30% HF for 5 minutes, and (g) etching with 30%HF for 30 minutes. Etching with 30%HF for 30 minutes gave the roughness required for proper adhesiveness of resin to ceramic. SEM images complement AFM data (**Figure 24**). Data obtained by AFM images are supported by the SEM data. SEM images shows the formation of micro and nanopores on the surface by different treatments.

#### **5. Conclusion**

The main source of zirconia, the widely used material in the field of ceramics, electronics, and refractories is zircon, an accessory mineral of placer deposits. Alkali fusion and plasma thermal dissociation are the important decomposition methods adopted widely to convert zircon to zirconia. Various methodologies are adopted for the synthesis of zirconia from its zirconium precursors depending on the particle size and crystal structure. Solution combustion synthesis, sol–gel synthesis, hydrothermal synthesis, and solid-phase sintering are some of the methods adopted for synthesis of different types of zirconia material. Depending on the utility, the synthesized zirconia further undergoes surface modification techniques like surface cleaning and roughening. Doping of zirconia with other materials for increasing the mechanical strength, electrical conductivity, and biocompatibility have been carried out during or after the synthesis. Characterization of synthesized material is essential for ascertaining properties of the synthesized material. Elemental characterization of zirconia can be best accomplished by the destructive solution techniques such as ICP-AES and ICP-MS or by nondestructive solution techniques such as LA-ICP-MS. XPS also can be used for elemental characterization. However, the data obtained pertain only to the surface, but XPS core data can also be used to check the surface chemical state of the zirconium and oxygen. XPS UVB data gives some idea about the crystallinity of the material. Different crystal structure of the synthesized zirconia can be ascertained using XRD data which can be complimented with Raman studies. FTIR data are used to analyze the sample for different functional groups and bonding in the sample. SEM, TEM, and AFM data are used to ascertain the morphology and topography of the prepared material. No single technique is panacea for material characterization, and it is not easy to characterize a material using all the techniques. Hence, judicial choice of combination of techniques for characterization and sufficient knowledge to interpret the data obtained are essential for efficient characterization of a freshly synthesized material.

#### **Author details**

Bincy Cyriac Chemistry Department, Christ Deemed to be University, Bangalore, Karnataka, India

\*Address all correspondence to: bincy.cyriac@christuniversity.in

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 4**

### Processing of Zirconia

*Ghassan Albarghouti and Sojood Mahmoud Farhan Darwish*

#### **Abstract**

This chapter starts with an introduction, including zircon, zircon structure, zirconia ceramics and their structures as they exhibit a distinct crystal structure at various temperatures while maintaining the same chemical composition. Then the properties of zirconia including mechanical, tribological, and electrical properties and thermal conductivity, were discussed. Zirconia's properties offer excellent resistance to corrosion and chemicals. When compared to other high-tech ceramic materials, zirconia is very robust at room temperature. The processing of zirconia was explained, starting with the purification of zirconium compounds followed by powder processing of zirconia, colloidal processing of zirconia, additive manufacturing, and zirconia treatments. Zirconia ceramics are processed from their raw ingredients in order to modify and enhance their physical and chemical characteristics and make them more suitable for use in future processes and in the production of finished goods. Various analysis methods of zirconia, including powder processing and sintering, microstructural analysis, phase characterization, mechanical testing, and tribological characterization were discussed. The last section in the chapter discusses the nano zirconia (ZrO2 nanoparticles). In addition to photocatalytic and piezoelectric uses, dental and optical coatings, nano zirconia has additionally been employed as a catalyst in a variety of organic interactions. It appears in the cubic, tetragonal, and monoclinic structural phases.

**Keywords:** zirconia, structure, properties, processing, analysis, 3D printing

#### **1. Introduction**

#### **1.1 Zircon**

Zircon, which is frequently found in highly silicic stones and contains significant quantities of certain trace elements, is regarded as a resistant auxiliary mineral. It is also a helpful instrument for tracking melt compositional alterations throughout magma development [1]. The quantity of Zr in the principal minerals is often negligible in comparison to that housed by zircon, and there is disagreement on the presence of zircon at boundaries of grains vs. the amount hidden as inclusions within different minerals [2]. Zirconium silicate is another name for the mineral zircon (ZrSiO4). It results from the extraction and processing of historical heavy mineral sand and is most typically found in coastal placers or dune deposits across the world [3]. Zirconium silicates are abundant in essence, along with presence under hydrothermal circumstances (between 300 and 550°C) which seems to have received a lot of interest [4].

The global output of zircon is around 1.1 million t/a, with the majority of it coming from Australia and South Africa. The most important use of zircon is represented by ceramics sector [3]. Zircon may be treated to make zirconia by melting the sand at extremely high temperatures to yield molten zirconium oxide (ZrO2) [5].

It is debatable how zircons behave in suprasolidus metamorphic environments. Experimental findings suggest zircon growth occurs throughout prograde metamorphism in melt-bearing systems, but computational modeling only anticipates zircon development upon cooling and melt crystallization. Zircon in high-pressure zones and elevated temperatures metamorphic rocks is a probable by-product of inconsistent melting events. However, knowledge of how zirconium, a crucial structural component of zircon, is distributed among the numerous products and reactants that occur during incongruent partial melting is still limited. A fresh source of Zr may become accessible to zircon formation after heating under suprasolidus under certain circumstances, or Zr may redistribute from already-existing zircon [2].

#### **1.2 Zircon structure**

Zircon is a mineral that is an orthosilicate accessory. Its chemical formula is ZrSiO4 (Z = 4) with the space group *I*41/*amd*. The zircon structure is made up of chains of consecutive, edge-shared SiO4 and ZrO8 polyhedra that pass along the c axis. The structure is rather wide; it incorporates "channels" of evidently vacant sixfold sites that run in corresponding direction to the {001} orientation and can accommodate big amount of non-formula components [6].

The edges of the ZrO8 triangular dodecahedra are coupled in a zigzag manner across the {100} dimensions. The structure shown in **Figure 1** accounts for the most prevalent three-dimensional behavior of zircon as well as the uncertain cleavage along {110}. Natural zircon incorporates non-formula elements at all times [7]. On the other hand, large cations (U, Th, Y, lanthanides, Hf) often replace Zr in the eightfold dodecahedral position, however, smaller cations may be placed in the sixfold location [8].

#### **1.3 Zirconia ceramics**

For several reasons, zirconia-based ceramics are the most researched and demanding materials. Zirconia (zirconium dioxide, ZrO2), popularly known as "ceramic steel", offers outstanding hardness, strength, and fatigue resistance, as well as good wear characteristics and biocompatibility [9].

Zirconium oxide, silicon nitride, and aluminum oxide are a few frequently employed ceramics that have exceptional chemical and physical qualities including high hardness, high strength, high toughness, and corrosion resistance [1]. Zirconium oxide, on the other hand, outperforms silicon nitride and aluminum oxide in terms of abrasion resistance, toughness, and service temperature [10].

Zirconia is a significantly highly versatile transition metal oxide frequently applied in ceramics, electrolytes for solid-oxide fuel cells and semiconductor technology [11]. The finest qualities come from zirconia stabilized with Y2O3. A crystallographic change on a ZrO2 surface limits fracture formation once is subjected to stress [12].

Baddeleyite, a naturally existing zirconia, is found in igneous rocks such as carbonatite. South Africa was the major producer of baddeleyite until 2002, but these plants have since closed, leaving the Kola Peninsula area of the Russian Federation as the only commercial source of this mineral. The majority of zirconia presently

accessible is made using zircon (34,000 t/a), with a relatively minor natural source by the Russian Federation (6000 t/a) [3].

Zirconia ceramics are traditionally processed using turning, milling, and grinding techniques. These techniques have significant drawbacks, including inadequate productivity and inevitable manufacturing damage, which make it difficult to achieve sufficient quality of the surface. But sandblasting and acid etching have a tendency to pollute the component's surfaces or alter their physical and chemical characteristics. In order to get an immaculate finish and undo surface and subsurface damage from earlier preparation, polishing can significantly enhance the surface quality [10].

#### **1.4 Zirconia structure**

There are three different types of zirconia crystal structures: Cubic, tetragonal, and monoclinic. It is possible to achieve high structural stabilization by combining ZrO2 and associating to other metallic oxides like ZrO2—MgO, ZrO2—CaO, or ZrO2—Y2O3 [13]. However, recent research emerged to be more concentrated on zirconia—yttria ceramics, which can be recognized by their fine-grained in the micro range classified as Tetragonal Zirconia Polycrystals (TZP) [14].

At high temperatures (>2370°C), a cubic zirconia also known as (c-ZrO2) fluorite crystal configuration with a unit cell of face-centered cubic (fcc) and a (space group Fm-3m) form. A numerous atom of Zr is linked to eight oxygens to create the fcc cube and the oxygen atoms are arranged along the cube's diagonals [15]. Furthermore, the fluorite structures of the monoclinic and tetragonal are deformed as seen in **Figure 2**.

**Figure 2.**

*The three ZrO2 polymorphs are shown in a schematic in the following order: (a) cubic, (b) tetragonal, and (c) monoclinic [16].*

At intermediate temperatures (1200–2370°C), the structure is transferred to tetragonal (*P*42/*nmc*), while at low temperatures (<950°C), the structure is monoclinic (*P*21/ *c*) [16]. Throughout thermal cycling, undoped zirconia reveals the following phase transitions [17]:

$$\text{cm-ZrO}\_2 \overset{1170^\circ \text{C}}{\underset{950^\circ \text{C}}{\rightleftharpoons}} \text{t-ZrO}\_2 \overset{2370^\circ \text{C}}{\underset{950^\circ \text{C}}{\rightleftharpoons}} \text{c-ZrO}\_2 \overset{2650^\circ \text{C}}{\underset{950^\circ \text{C}}{\rightleftharpoons}} \text{Liiquid} \tag{1}$$

#### **2. Properties of zirconia**

In relation to their usage, this section describes the many varieties and characteristics of zirconia materials. Due to the following qualities, zirconia has recently been the subject of substantial inquiry in the scientific literature:

#### **2.1 Mechanical properties**

Recent studies showed that mechanical parameters including hardness, grain size, and fracture toughness, as seen in **Table 1** have an important bearing on slide wear resistance of bulk ceramics [18]. Zirconia has mechanical properties that are on par with stainless steels. Its traction resistance might be in the 900–1200 MPa range, and it has a 2000 MPa compression tolerance [19]. 3Y-TZP dentistry zirconia incorporates alumina (Al2O3) as a strain-hardening promoter, resulting in great opacity and exceptional mechanical qualities [20].

Although the sintering of ZrO2 stabilized with Y2O3 is substantially more complex, this is the primary type of zirconia recognized for present medical usage. Surface treatments have the potential to alter the physical characteristics of zirconia. Furthermore, prolonged wetness contact may have a negative impact on its assets. The term for this is zirconia aging. Surface grinding can also diminish toughness. The mechanical characteristics of zirconia might deteriorate as it ages. Mechanical forces and moisture exposure are essential for speeding up this process [20].

Due to transition toughening, ZrO2 has the ability to achieve extremely high toughness. It is regarded as a strong contender for numerous cutting tool applications as well as a biomedical material. Nevertheless, the intrinsic fragility of these substances remains the primary bottleneck. This issue has been addressed by developing suitable composites. Novel materials, including carbon nanotubes and fiber-like structures, have been produced in recent years. The fibrous character of the

#### *Processing of Zirconia DOI: http://dx.doi.org/10.5772/intechopen.112121*


**Table 1.**

*Thermal, electrical, and mechanical properties of zirconia [18].*

toughening components may result in increased fracture toughness. Furthermore, compared to the other two non-metallic additions, carbon's electrical conductivity allows for additional electrical qualities to be helpful [21].

Using Eq. (2), it is possible to conduct a three-point bending test and a biaxial flexure test to assess flexural capacity and can be examined using a universal testing device (Autograph AG-X).

$$\sigma = \frac{-\mathbf{0}.2387 \mathbf{P}(\mathbf{X} - \mathbf{Y})}{\mathbf{b}^2} \tag{2}$$

where the center's highest tensile stress is σ (in MPa), the total load producing the rupture is P (in N), and the cross-section of the test piece at the fracture point is b (in mm) [22].

Meanwhile, fracture toughness (KIC) test can be performed using the single-edge pre-cracked beam technique and can be measured by applying Eq. (3). Where **B** represents the material length (in m), **W** represents the sample width (in m), and **a** represents the pre-crack length; **P** represents the breaking force (N), and **S** represents the support span (m) [22].

$$K\_{IC} = \left(\frac{P \times S}{B \times W^{3/2}}\right) \times \left\{\frac{3}{2} \left(\frac{a}{W}\right)^{\frac{1}{2}} \times Y \left(\frac{a}{W}\right)\right\} \tag{3}$$

#### **2.2 Tribological properties**

Zirconia ceramics have been extensively researched as a wear resistant material for engineering fields throughout the last few years. The kind of Hall-Petch law governs the connection between wear resistance and grain size in tetragonal zirconia ceramics. As a result, it is reasonable to predict that nano-structured zirconia coatings will have more wear resistance than standard zirconia. Friction and wear tests may sometimes be performed on an MM-200 wear tester's block-on-ring configuration. As seen in **Figure 3**, the stationary zirconia-coated blocks are

**Figure 3.** *The block on ring test [23].*

pressed up against a moving stainless steel. The tester provides the friction coefficient immediately. Additionally, wear frequencies can be calculated by dividing the wear mass loss by the force implemented, sliding distance, and material density [24].

Throughout wear testing, the surface morphology, particularly big pores, may result in excessive stress concentration and fracture development, resulting in poor resistance. Additionally, it is shown that, as evidenced by Eq. (4), the wear rate of polycrystalline ceramics matched an exponential function. In which **1/K0w** represents the TZP substance wear resistance with no pores (in N.m/mm<sup>3</sup> ) and **P** represents porosity.

$$\frac{1}{k\_w} = \frac{1}{k\_{0w}} \exp\left(-2\Im P\right) \tag{4}$$

Eq. (4) above illustrates that decreasing the porosity of zirconia ceramics resulted in enhanced wear resistance. The existence of gaps reduces the probability of grain boundary sliding, which delays or prevents the occurrence of plastic deflection [24].

#### **2.3 Electrical properties and thermal conductivity**

Zirconia additionally has a high degree of flexibility, a superior conductivity, a poor thermal conductivity, and a strong corrosion resistance as seen in **Table 1** [18]. Because of that, zirconia is utilized in many applications at various high temperatures, such as fuel cells and thermal barrier coatings [18]. Monoclinic zirconia was demonstrated at 1000°C to be an amphoteric semiconductor in a prior work. When oxygen pressures are high (10�<sup>6</sup> to a value of 1 atm), the most common defects are fully ionized zirconium holes, which produced exceptionally well, implying charge transfer via a thermally induced hopping mechanism [25].

On the other hand, at ambient temperature, zirconia in its monoclinic phase has a thermal conductivity of 7.2 Wm�<sup>1</sup> K�<sup>1</sup> . However, at high temperature (1100°C) zirconia begins to have very poor heat conductivity (1.2–2.6 Wm�<sup>1</sup> K�<sup>1</sup> ), making it an ideal candidate for thermal barrier (TBC) coatings [26]. However, the volume variation (5%) caused by phase transformation at elevated heat as well as during cooling the apparatus limits the use of zirconia in aggressive environments. The crumbling of the zirconia-based components is caused by phase transformation and volume change [27].

Yet, when the temperature rises, the phase transition (**Figure 4**) occurs, resulting in structural failure and fractures in the coat. This would be prevented by enriching the zirconia matrix with yttria, which stabilizes the zirconia at high temperatures. Some Zr4+ cations are substituted by Y3+ during this stabilizing procedure [29]. To preserve charge balance, one oxygen gap is formed for every two replacing Y3+ cations. Because of the presence of voids, YSZ is valuable not only for TBCs but also as an electrolyte in oxygen sensors and solid-oxide fuel cells (SOFC) [30].

This kind of stabilization not only assists in preventing phase change but also reduces the thermal conductivity of YSZ, for example, from 1.42 W/m�K (at ambient temperature) to 1.35 Wm�<sup>1</sup> K�<sup>1</sup> at 1200°C since c-phase zirconia exhibits less thermal conductivity than m-zirconia [27].

Zirconia is also a metal oxide which is composed of a weak base and a weak acid and is one of the ceramic semiconductors. Because of its crystalline structure, it can be

**Figure 4.** *Thermal conductivity of ZrO2 [28].*

an insulator utilized as an n-type semiconductor or a high-resistance ceramic. The forms of the nanocrystals determine the potential uses for them. Oxygen sensors, fuel cell electrolytes, and gate dielectrics have all been employed with spherical ZrO2 [31].

#### **3. Processing of zirconia**

This section discusses the purification process of zircon, followed by stating various methods for zirconia processing (thermal dissociation, chlorination, additive manufacturing, and colloidal processing) depending on the characteristics asked for and the purpose or the industry that asks for a zirconia supply.

#### **3.1 Purification of zirconium compounds**

For usage in the electronics sector and the creation of partly stabilized zirconia, highly pure zirconia is commonly used today. A large portion of the world's zirconia supply found naturally as ZrSiO4 [32].

By caustic fritting of zircon sand, the zirconium nitrate solution is obtained. Caustic fritting was accomplished with the aid of magnesium hydroxide carbonate and magnesium oxide. These additives reduce the quantity of soluble silica in nitric acid by less than 1000 ppm. With no additional ingredients, caustic fritting produces some nitric acid-soluble silica, which is eliminated by dehydrating with sulfuric acid [32].

Other procedures currently are utilized to extract zirconia from zircon generally by including heat or chemical disintegration of zircon to provide zirconia and silica. But even so, the resulting zirconia needs to be purified through additional chemical processing. A financially appealing method of producing zirconia is the thermal dissociation of zircon, which results in a combination of zirconia and silica. The dissociated zircon result can be leached either with a strong base, like sodium hydroxide

#### *Processing of Zirconia DOI: http://dx.doi.org/10.5772/intechopen.112121*

(NaOH), to breakdown the silica and end up leaving zirconia or with a strong acid, like sulfuric acid (H2SO4), to solubilize the zirconia and produce a zirconium salt, leaving non-soluble silica. Zirconia may be created using the caustic-leach technique with a purity of up to 99.5%. Still, it must undergo subsequent chemical treatment, usually including dissolving the zirconia in acid to attain greater purity zirconia [32].

In reality, however, acid-leaching precipitates are gel-like, challenging to filter, and enclose numerous contaminants. Because of this, the basic sulfate precipitation using zirconium sulfate solution is favored, even though it is challenging to regulate the circumstances. To precipitate the basic sulfate, sulfuric acid or a sulfate salt is frequently added after the zirconium sulfate is converted to zirconium oxychloride. It has been discovered that creating zirconium compounds with good quality, including zirconia, is feasible by precipitating zirconium from zirconium sulfate solution at a low pH [32].

It is desired to utilize a zirconium sulfate solution with a zirconium level of more than 75 g/L and a sulfate content of more than 180 g/L. The ammonia source is introduced to the aqueous zirconium sulfate solution throughout the procedure of the present invention till the solution's pH value is between 0.1 and 2.5. Whenever the pH is in the interval of 1.0 to 2.0, it is preferable to stop adding the ammonia source. The zirconium compositions created using the procedure of the present invention are white, compressible, unrestricted, finely dispersed substances after washing and drying [32].

#### **3.2 Chlorination**

Chlorinating impure zirconia in the presence of carbon is among the best ways to create highly pure zirconia. To purge contaminants from the ZrCl4, partial solidification and sublimation procedures are performed in the range of temperature between 623 and 673 K. Additionally, highly pure ZrO2 is created by the interaction of pure solid ZrCl4 with water steam. Another crucial stage in the industrial scale of titanium and zirconium is chlorination in the presence of carbon. In the temperature range of 1000 K–1100 K, chlorine and ZrO2 do not really thermodynamically interact, and a reducing agent like carbon is necessary to complete the process. Consequently, the process is known as carbochlorination [33].

By determining the variation in the free energy of the different reactions, it is possible to know why a reducing agent is required. By creating a low oxygen potential environment, the existence of carbon atoms lowers the inclination of the production of oxides and stimulates the creation of chlorides. It has been discovered that between 1075 K and 1275 K, the following reaction (CO2 + C = 2CO) moves more slowly than during the chlorination process of ZrO2 with a <sup>Δ</sup>G of 4.6 kJ mol<sup>1</sup> at 1000 K to a 22.0 kJ mol<sup>1</sup> at 1100 K [34]. On the other hand, thermodynamically speaking, reaction 1 cannot take place at 1100K but other reactions may. It's possible that reaction (ZrO2 + C + 2Cl2 = ZrCl4 + CO2) with a <sup>Δ</sup>G of 248.3 kJ mol<sup>1</sup> and reaction (CO2 + C = 2CO) will produce reaction (ZrO2 + 2C + 2Cl2 = ZrCl4 + 2CO) with a ΔG of 270.4 kJ mol<sup>1</sup> .

#### **3.3 Colloidal processing of zirconia**

During lower sintering temperatures, high-density zirconia structures with decently crystal frames and improved mechanical characteristics may be created using fine nanocrystalline 3Y-TZP powders. However, throughout most of the production of the 3Y-TZP nanocrystalline initial crystals, significant agglomeration takes place [35]. The outer surface of the colloidal matter is decreased by agglomerates, which consist of networks of particles linked by van der Waals forces [36]. Ultimately, accumulation could lead to the development of undesirable variabilities, including defects, fractures, and porous surfaces with big granules, which can impair the mechanical capabilities of zirconia structures. As a result, attempts must be considered to prevent agglomeration due to consolidation. Powders are merged in greenish structures with the necessary shapes using various consolidation techniques, such as solid free-form manufacturing, wet slip casting, dry isostatic and uniaxial pressing. A slip casting is a common colloidal procedure that creates structures with a significant density and few harmful variabilities. By adjusting the intensity of the attracting and repelling interparticle interactions between zirconia particles, steady solutions with high particle dispersion may be created, which are necessary for reducing the production of agglomerates [37]. An electric double surface is produced by generating comparable charges on the interfaces of suspended particles of sufficient size [36].

CAD/CAM, which denotes computer-aided engineering and production, is one way to create zirconia copings and frames. An automatic manufacturing process using a software power tool follows the editing of a 3D model on a screen in the CAD/CAM manufacture of zirconia. There are two methods for creating CAD/CAM zirconia foundations: pre-sintered blanks can be "soft-machined" or completely sintered blanks can be "hard-machined". These foundations have undergone high-quality procedures to produce homogeneous material structures with little to no cavities, defects, or fractures. A binder is blended with partly stabilized zircon powder (silicone oxide 0.02%, yttrium oxide >4%, hafnium oxide >1%, zirconium dioxide 96%, aluminum oxide 1%) to create zirconia [38].

Well-dispersed zirconia solution has just been subjected to direct coagulation casting (DCC) and gel casting are two innovative methods for treating colloidal materials to produce greenish particles [39]. DCC focuses on creating ceramic components closer to their final form from suspensions of ceramic particles in concentration solutions. It combines the DLVO concept with colloidal ceramic formation. The saturated suspension is disrupted by the decomposition generating base, electrolyte, and acid, whether to boost the ionic strength or to let the pH reach the ceramic powder isoelectric point framework [35, 36].

#### **3.4 Powder processing of zirconia**

Because of their acceptable mechanical qualities, minimal neutron absorption cross-section, and superior corrosion tolerance, zirconium that concentrates on alloys has been employed extensively in industrial power stations.

Nevertheless, because zirconium has a great melting point (>1800°C) and a significant attraction for H, O, and other contaminating atoms, it is challenging to produce zirconium workpieces using standard methods. AM will make it easier to produce complex parts that are employed in a variety of industrial applications. The quality of the finished products, however, is heavily dependent on the raw powders. When used in the AM process, powders with desirable morphological properties display improved rheological properties and more optimum particle packing. Research on producing sphere powder from commercial products like Ti64 has advanced significantly. Spherical metal components have been created using various techniques, such as Electrode Induction Melting Gas Atomization, Plasma Atomization, Plasma Rotating Electrode Process, Plasma Spheroidization (most flexible approach), and Vacuum Induction Melting Inert Gas Atomization [40].

#### **3.5 Additive manufacturing (AM)**

3D printing technology commonly referred to as additive manufacturing techniques, opens up new possibilities for material shaping by increasing design flexibility, accelerating production, and minimizing waste and expense. The revolutionary possibilities of using these technologies to produce technical ceramics have been widely acknowledged, despite the ceramic industry's slower adoption of 3D printing technology than the polymer and metal industries. Stereolithography (SLA), one of the several 3D printing technologies currently accessible, offers advantages in the areas of printing resolution and precision, which—along with the exceptional efficiency of exterior coating—make it the ideal suitable AM technology for constructing ceramics [41]. The greatest range for achieving uniform mixing, according to an investigation by Jang et al. on the synthesis of zirconia employing DLP additive manufacturing, was 58 vol% of zirconia by volume. The 3-point bending durability improves as the volume fraction rises. The research did note that the viscosity rose quickly to 56 vol% [42]. SLA technique is built on the layer-by-layer polymerization of a liquid photocurable monomer incorporating ceramic particles; more specifically, it uses a beam of UV laser that shifts from one location to another while tracking the photo-polymerized pattern [43].

A near fabrication path to the manufacture of progressed ceramics with extremely complicated geometries is provided by additive manufacturing (AM). It does not involve the original building of a mold, making it a quick-response solution for engineers and designers to create new items [44].

A possible method for creating intricately shaped ceramic components with high accuracy is digital light processing (DLP) [43]. A different type of SLA utilizes a UV projector as the light source for treating an entire resin layer by layer, improving the speed of the process. Due to the variety of utilizations for which this material can be applied, including restorative dentistry and frameworks, fuel cells for monolithic support, tooling and blades, and other precision parts for various applications, including mechanical and thermal ones, 3D-printed zirconia is becoming more and more popular among technical ceramics [44].

DLP, which has excellent printing resolution, a high interfacial polish, and a rapid building process, is becoming increasingly popular for manufacturing ceramic components. However, compared to traditional ceramic processing, DLP is still in its developing phase [44].

Making completely dense, defect-free ceramics with characteristics equal to those of materials treated using traditional technologies is considered a difficult task. This requires the supervision of numerous manufacturing steps, including printing optimization techniques, the slurry preparation, and the sintering heating cycles. Nevertheless, even though DLP and SLA are established for zirconia 3D printing technologies, most articles briefly discuss the mechanical characteristics of the resulting materials [43] within the beginning, zirconia green entities are created for the specimen preparation using a heavily zirconia-loaded slurry on a DLP 3D printer utilizing a UV light with a wavelength of 405 nm. In **Figure 5**, a diagram of the DLP printing principles is displayed. For all builds, the depth can be adjusted at 25 μm. For further characterization and assessment, a variety of zirconia green bars, disks, and parts are acquired.

**Figure 5.** *Diagram illustrating the DLP printing principles [39].*

The extra slurry is then washed off the artificial green parts in an ultrasonic bath. The debinding and sintering then performed in a tubular furnace with an air flow [44].

#### **3.6 Thermal dissociation/calcination**

Calcination is the practice of heating a substance at 400, 600 and 800°C without allowing it to fuse to affect changes in its physical or chemical composition [45]. Electrospinning, a newly improved combination of the ceramic precursor zirconium acetylacetonate and the bond polymer polyacrylonitrile, may be utilized to create zirconia incorporating nanofibers. It is discovered that the steps that follow the ceramic precursor conversion occur simultaneously with the transformation of electrospun zirconium acetylacetonate/polyacrylonitrile fibers into zirconia nanofibers:

$$\text{Zr}(\text{C}\_{5}\text{H}\_{7}\text{O}\_{2})\_{4} \rightarrow \text{ZrO}(\text{OH})(\text{CH}\_{3}\text{COO})\_{3} \rightarrow \text{ZrO}(\text{CH}\_{3}\text{COO})\_{2} \rightarrow \text{t}-\text{ZrO}\_{2} \rightarrow \text{t} \tag{5}$$

$$-\text{ZrO}\_{2} + \text{m}-\text{ZrO}\_{2} \rightarrow \text{m}-\text{ZrO}\_{2}$$

Fibers are heated in two stages; first, they are heated to a temperature of 500°C with a heat rate of 1°C/min, and subsequently to a specified temperature with a heat rate of 5°C/min, calcining them at various temperatures between 500 and 1300°C for 1 hour. A low-temperature profile was adopted for the first annealing stage to ensure the sensitive removal of the ceramic precursor's breakdown products and binding polymer to minimize fiber breaking [46].

Additionally, furnaces for manufacturing glassware have often been built using and repaired with zircon refractories. Its breakdown tendency is a crucial zircon property that might affect furnace and glass performance. A decrease in the proportion of aluminum, titanium, iron, and alkali is necessary to lessen the rate of zircon deterioration [47].

Generally, the below process may be used to describe zircon dissociation:

*Processing of Zirconia DOI: http://dx.doi.org/10.5772/intechopen.112121*

$$\text{ZrSiO}\_4 \rightarrow \text{3ZrO}\_2 + \text{SiO}\_2 \tag{6}$$

where zircon is transformed into zirconia and silica, often by thermal methods at a temperature of around 1676°C. Zirconia acquires a tetragonal shape at 1173°C, whereas underneath this temperature, it exists as a monoclinic phase [47].

Zirconia precipitates are calcined at 800°C for 2 hours (h) in the air, which changes the monoclinic crystal structure and improves the crystallinity as well as the dimensions of the precipitate. Additionally, it was noted that zirconia with a cubic and tetragonal structure undergoes a partial transformation from its monoclinic state. In additional research, acidic and ammoniacal zirconium chloride solutions in different concentrations were used to create nanoscale zirconia precipitates at temps between 110 and 150°C for times ranging from 1 to 4 hours [48].

#### **3.7 Zirconia treatments**

Surface treatment to form a strong chemical connection with such composite resin is one method for successfully mending chemically inert zirconia foundations. Consequently, the effectiveness of dental restorations is determined by the adhesiveness of zirconia and composite resin.

It is extremely hard to chemically or physically change the zirconia ceramic surfaces because of its superior stiffness and chemical inertness to avoid corrosion [49]. In addition, unlike other types of ceramics, such as glass, zirconia is not etchable, making it difficult to carry out the adhesion processes [50]. According to earlier research, sandblasting significantly enhanced the ceramic surface's wettability, surface energy, and roughness. Because of this, the resin cement is retained by microinterlocking due to the bonding area's enhancement [49].

However, implant technology has been working hard to change zirconia in terms of morphological and bioactive features essential for suitable cell interaction and differentiation throughout most of the neighboring bone healing. Numerous physicochemical techniques have been utilized to improve zirconia properties, such as PVD, grit blasting, laser treatment, PVD, micro-machining, and CVD [51].

Although it has been asserted that zirconia pretreatment techniques like tribochemical silica coating, sol-gel processes, silicon nitride hydrolysis, and vapor-phase deposition technique increase resin adherence to zirconia, the long-term bond performance of tribochemical silica coating is in doubt. Condensed silanol layers produced by the sol-gel technique and hydrolyzed from tetraethyl orthosilicate are prone to cracking when heated, and coatings produced by silicon nitride hydrolysis or vapor-phase deposition require complex equipment and take a long time, making them unsuitable for use in clinical settings. With 5% HF inscription for 90–120 seconds at a pressure of 0.3 Mpa, a new glass ceramic spray deposition (GCSD) approach has recently been described to increase the zirconia bond strength. It is possible to create an extremely thin layer that does not alter the zirconia's physical characteristics by spraying glass-ceramic powders over zirconia surfaces which are subsequently sintered [52].

#### **3.8 Powder sintering**

During sintering, powder components are condensed and go under consolidation at a high temperature but still below their melting temperature. A particle substance is transformed into a rigid, compact design. Somewhere at macro scales, changes in geometry, dimension changes (shrinkage), and density variation may be seen, as well as changes in mechanical qualities like mechanical strength (as a result of shrinkage) [53].

To achieve a greater compactness and performance, more machined and sintering operations are required [44]. However, these existing techniques methods have a variety of drawbacks, including the inability to produce highly complex geometry because of the common usage of molds, their increased price, and their prolonged processing times. Furthermore, machining ceramic materials is challenging due to their extraordinary hardness and brittleness. Additionally, flaws and unwanted shrinkages may also be produced in the ceramic parts [54].

DLP-produced sintered ceramic components may have shrinkage problems. An effective way to address them is to scale the CAD/CAM model appropriately for each axis before printing to account for shrinks [44]. Through DLP, a single layer of material is first made, and by repeating this process, a body with three dimensions is developed. The three-dimensional cross-linked polymer that makes up the green body has ceramic fragments caught inside it. The components are then cleaned, debonded, and sintered to create the final thick ceramic part. This technique makes it possible to create intricate three-dimensional ceramic structures with excellent precision and creative freedom [55].

The microstructure changes throughout sintering, and this development is characterized by a rise in grain compaction and rearrangement and cohesive bonds among granules develop in the early stages. Because of the mass transmission, the bonds between particles expand as the sintering process continues. Surface and grain boundary diffusion are often the primary mass transport modes in sintering. Surface tension and bonds'stresses cause particle attraction, which causes the system to contract. The decrease and occasionally the actual eradication of material porosity is a result of neck development and shrinking [53].

Owing to the possibility of the ceramic material's cracking or breaking, thin restorations for minimally invasive dentistry might be challenging to create employing the subtractive method. In addition, subtractive manufacturing usually results in significant cutting tool usage and significant volumes of generated waste (zirconia powder) after milling. Additionally, zirconia's resilience and lifetime are improved by surface treatment and adhesive bonding with primers [55].

#### **4. Analysis methods**

#### **4.1 Microstructural analysis**

For microstructural characterization, Raman microanalysis can be demonstrated. On the other hand, for studying the chemical composition, high vacuum X-ray energy dispersive microanalysis, X-ray photoelectron spectroscopy (XPS) are usually used. XPS is accelerated under a voltage of 14 kV, emission of current of o.2 A, pressure of 10<sup>9</sup> mbar, energy resolution of 0.30 eV, maximum power of 2.8 kW, sampling area of 6 0.5 mm sampling area, and around 90° take-off angle. Finally, to charge compensation, an electron flood gun works at 4 eV. All spectra can be shown and recorded on the binding energy with an excellent resolution of 150 eV pass energy [56].

#### *Processing of Zirconia DOI: http://dx.doi.org/10.5772/intechopen.112121*

In order to recognize and define the pattern of the tetragonal (which can be demonstrated by 145 and 262 cm<sup>1</sup> Raman shift) and monoclinic (by 180 and 190 cm<sup>1</sup> Raman shift) ZrO2 phases just at the surface area, then the cervical collar that is prepared for XRD investigation is examined. The following requirements are used to conduct a Raman microscope: 100 m slit, Ar laser (532 nm), 10 mW at the sample, a grating of 1800 grit/mm, a confocal hole of 1000 μm, and a sampling duration of 10s.

The microscope's optical system independently finds and examines three locations at every implant surface site (collar/root). 20 35 m regions are chosen and examined at 5 m increments with a 5 s scan time in order to map the monoclinic ZrO2 phase [56].

#### **4.2 Phase characterization**

For morphological analysis, LV-SEM is used. And for the roughness, optical profilometry is used. In X-ray diffraction (XRD), a sample from each implant is embedded in epoxy resin and sliced into longitudinal sections using a microtome while being continually cooled by water to test for the presence of tetragonal and monoclinic ZrO2 phases across the whole implantation. The materials are smoothed using SiC paper and polished using 3 M diamond paste, and then for 3 minutes, ultrasonic cleaning in distilled water is applied [56]. The zirconia X-ray diffraction spectra is displayed in **Figure 6**. Their X-ray diffraction patterns may identify zirconia's monoclinic and metastable tetragonal phases [57].

The threaded portions of the implants' 3D surface roughness characteristics are evaluated using optical profilometry. At 10–100 magnification, an optical profiler may be utilized to look at the gap between two subsequent implant threads. The roughness metrics of a 3D surface, including arithmetic mean deviation (Sa), root average square deviation variance (Sq), and max peak to valley altitude (Rt) are then measured using Veeco Vision software [56].

**Figure 6.**

*Diagrammatic representation of the parameters (L, W, and H) change along the zirconia DLP pathway [44].*

#### **4.3 Mechanical testing**

Using the techniques specified in **Table 2** at ambient temperature, the mechanical characteristics, including fracture toughness, Vickers hardness, and flexural strength, are thoroughly studied. A Weibull assessment of the tensile and flexural data is thus carried out to determine the possibility that zirconia would rupture. By employing the single-edge V-notched beam (SEVNB) technique and indentation, correspondingly, the fracture toughness value (KIC) is assessed (**Table 3**) [44].

It is crucial to pick a reference based on the actual indentation fracture morphology since the selection of the equations also depends upon the type of crack. A femtosecond laser is used to construct an incredibly sharp V-notch (0.5 m, indicated by a red dashed line as seen in **Figure 7**) to measure fracture toughness accurately [49, 58, 59].

**Figure 7** shows how U-grooves are created at particular thicknesses using a reduced-speed blade and diamond rims with a 200 m thickness under irrigation water. A 2.9 W femtosecond laser is now being used to create a crisp V-notch just at the bottom of the U-grooves. With a repeating rate of 100 kHz, the femtosecond lasers produced 290 fs linearly polarized pulses at 515 nm [50, 58].

#### **5. Future perspectives of zirconia**

#### **5.1 Nano zirconia**

Nano solids, which are tiny structures of ZrO2, have been extensively proposed. ZrO2 on a nanoscale, which exhibits better mechanical properties and superior biocompatibility, is frequently included into many technologies utilized in tissue engineering and dental applications [51]. Even though sophisticated powder synthesis processes can generate nano-sized particles [60–62]. Nano zirconia also showed a promising future for fuel cell applications. Cubic ZrO2 nanoparticle production is challenging because of the range of phases that the reactions create. In a previous study, a simple precipitation method was established to adjust the form and crystallinity of cubic (Arkelite) and monoclinic (baddeleyite) zirconia nanoparticles. The


**Table 2.**

*Zirconia's carbochlorination reactions and the change in Gibbs free energy [34].*


#### *Processing of Zirconia DOI: http://dx.doi.org/10.5772/intechopen.112121*

**Table 3.**

*Investigation techniques for mechanical properties [44].*

#### **Figure 7.** *XRD spectra of zirconia [57].*

manufactured cubic and monoclinic zirconia can be put into the Nafion® membrane to increase the fuel cell's efficiency. The polytetrafluoroethylene backbone and perfluorinated ether sidechain are characteristics of the proton conductor electrolyte Nafion®. Additionally, due to the transpiration of water that leads the membrane to buckle, Nafion® loses conductivity at a higher temperature of 100 C [31].

Toward the latter situation, the rise in clinical defects in the dental sector has been noticed owing to veneer chipping due to pressure accumulation throughout fabrication. Additionally, the use of ceramics' nanocrystalline structure has gained the interest of engineers since they strengthen their resistance to low-temperature deterioration (LTD) [63].

Current efforts involve synthesizing nanoparticle agglomerates within 30 mm aggregates. The high-velocity oxy-fuel (HVOF) method can be subjected to spray nano-zirconia powders made in radio frequency (RF) plasma. This method employs relatively low flame temperatures (<3000°C) to ensure that the particles are only a little heated. The nano-zirconia particle-reinforced coatings are examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of the post-spray treatment on the coatings are investigated [62].

Wet-chemical synthesis techniques, such as co-precipitation, hydrothermal synthesis, and sol–gel preparation, are primarily used to manufacture nano-ZrO2 powders. The physical approach cannot achieve nanoscale results, and the gas-chemical process is too expensive to be repeated in a real-world setting [64].

The nanoparticles'surfaces are treated with various surfactants that have different attaching head groups and carbon chain lengths. Because of their distinct nature and properties, quantum dots (QDs) fall within the special category of nanomaterials. A QD is defined as an individual semiconducting nanocrystal that ranges in dimension from 2 to 10 nm. Several techniques, including soot vapor deposition, microwave synthesis, laser ablation or chemical oxidation of graphite, and graphite oxidation heat-induced oxidation of a molecular precursor, can be used to create QDs. In recent research, scientists devised a simple approach for producing a highly effective photocatalytic composite by adsorbing carbon quantum dots (CQDs) onto the surface of ZrO2 NPs as demonstrated in **Figure 8**. The developed

#### **Figure 8.**

*(A) Diagrammatic representation of SEVNB samples. (B) Equipment for creating a razor-sharp V-notch using a blade [49, 58].*

**Figure 9.** *TEM pictures of ZrO2 nanoparticles with CQD embellishments [65].*

photocatalyst could be useful for the rapid fading of textile colors. The composite's production method, which entailed calcination of an ammonium citrate solution, is used to make QDs. Then, using a conventional solvent-based chemical method, produced QDs are adsorbed onto the surface of ZrO2 NPs with ultrasound assistance **Figure 9** [65].

Other studies show another development regarding a nano-structured glasszirconia in order to increase the resilience of the interface between dental zirconia substrate and veneered porcelain. A new SiO2–Li–Al–O3 material to create a glasszirconia nanostructure, prepped (SLA) glass was infiltrated through the outermost layer of fully sintered dental zirconia [66].

#### **6. Conclusions**

Zirconia has been indicated and used in various industries including, dentistry, glassware, and electronics. This comes from different reasons especially, its mechanical, electrical, thermal conductivity, and tribological properties. Due to the high demand, zirconia supply had to be produced after purifying zirconium compounds by chemical processing using acids or bases. Different processes might be performed to produce pure zirconia, such as chlorination under a high-temperature range between 623 K and 673 K, colloidal processing of zirconia, and additive manufacturing which uses a 3D printing technology with high precision and resolution. Zirconia's microstructure is analyzed by XPS and X-ray energy, phase characterization by XRD, and Mechanical properties are tested analyze the fracture toughness and flexural strength. The uses of zirconia have been constantly increasing over the last years, and the most discussed structures are nano zirconia integrated with other systems such as CQDs to enhance its characteristics.

#### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Ghassan Albarghouti\* and Sojood Mahmoud Farhan Darwish Birzeit University, Ramallah, Palestine

\*Address all correspondence to: gdaghra@birzeit.edu

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 4
