**6. Classification of porous ceramic**

porous ceramics such as the replica method, the sacrificial phase technique, direct foaming methods, paste extrusion and most recently developed rapid prototyping technique [13].

The fields of application and specific forms of porous ceramics are wide and varied according to their manufacturing processes [14]. Some of its useful applications are in the manufacture of filter. As these porous structures are used to filter high-pressure gas at high temperature and are used as an aid to remove the contaminants. In the field of petroleum treatment, porous ceramics are used as a substrate for catalysts in the process of filtration. They are also used in recovering hydrogen from the crude oil. Other applications are thermal insulators in filter membrane to separate metal impurities from molten metals such as steel, iron and aluminum. Today, porous ceramic structures prepared from different materials based on their application are used widely in biomedical field. For example, porous calcium phosphate materials can be used to replicate bone architecture and allow the growth of osseous tissue on an artificial substrate, thereby forming an artificial living bone structure. There porous hydroxyapa-

Porous ceramics possess a number of suitable properties, which combine the features of ceramics, and porous materials such as low density, lightweight, low thermal conductivity, low dielectric constant, thermal stability, high specific surface area, high specific strength, high permeability, high resistance to chemical attack and high wear resistance [13]. Either porous ceramics are reticulate (interconnected voids surrounded by a connection of ceramic) or foam (closed voids within a continuous ceramic matrix). Reticulated porous ceramics are usually used for molten metal, industrial hot-gas filters, catalyst supports, and diesel engine exhaust filters.

Pore size and porosity percentage are controlled by the particle size distribution of starting ceramic powders, fabrication techniques, types of binder used, concentration of binder and sintering conditions respectively [1]. Generally, the particle size of raw ceramic powder should be geometrically in the range between two to five times larger than that of pores in order to provide the desired pore size. The Porosity percentage reductions with increased making conditions such as pressure, sintering temperature and time. Furthermore, the fabrication influences such as the amount and type of additives, green densities, and sintering conditions (temperature, pressure atmosphere, etc.) significantly affect for the porous ceramics microstructures.

The general properties for porous ceramics can be designed for specific environmental application by controlling their composition and microstructure [16]. Changes in open and closed porosity, distribution of pore size and shapes of pore can have a main effect on the properties of porous ceramics. All of these microstructural features are in turn greatly affected by the processing way used to produce of the porous ceramic. For mechanical properties of porous ceramics, they are determined by their structural parameters, such as percentage of porosity, pore size, and shape. Furthermore, the solid microstructure phase of grain growth and solid phase continuity greatly affect the mechanical properties. Several important issues relating to

tite can be used to replace bone and also as a drug delivery system [15].

**4. Porous ceramic structure and properties**

4 Recent Advances in Porous Ceramics

**5. Mechanical behavior of porous ceramic**

Classification of pores is one of the basic requirements of inclusive characterization of porous ceramics (**Figure 3**). There are different classes of pores described porous ceramic in the literature,

**Figure 3.** Schematic of classification of porous ceramic [1].

but they are difficult to give a consistent general classification of porous ceramics including catalyst carriers in various chemical processes, electrolyte carriers in fuel elements, adsorbents. As well as filtration of liquids, hot gases, melted metals and alloys, membranes for separation and purification of gas and liquids etc. The purpose of these classifications is to organize pores in classes by grouping them based on their common characteristics like structure, size, shape, accessibility etc. Therefore, porous ceramics can be classified according to the different characteristic attributes such as chemical composition of initial ceramic materials, porosity percentage, physical state of these products, refractoriness correlated to service temperatures, destination and application area [1].

**9. Porous hydroxyapatite ceramic and its biomedical applications**

Hydroxyapatite (HA) porous ceramics are substitute materials for bone and teeth in repairing and regeneration applications due to their chemical and biological similarity to human hard tissue [20]. In the design of these porous ceramic for bone repairing or regeneration, it is important to control their pore structures. Pore ceramic structure can be designed using the size and morphology of the Hydroxyapatite particles that are utilized to build these porous ceramics. Porous hydroxyapatite ceramic exhibits strong joining to the bone, the pores provide a strong mechanical interlock leading to a firmer fixation of the structure. Porous hydroxyapatite is more resorbable and osteoconductive than HA dense counterpart [21]. The surface area of porous Hydroxyapatite form is greatly increased which allows more bone cells to be carried in comparison with dense hydroxyapatite. The most common techniques used to make porosity in a biomaterial are gas foaming, salt leaching, freeze-drying, phase separation and sintering depending on the material used to make the scaffold. The minimum pore size

Introductory Chapter: A Brief Introduction to Porous Ceramic

http://dx.doi.org/10.5772/intechopen.74747

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required to regenerate mineralized bone is generally considered to be around 100 μm.

Porous ceramics have a good activity and high absorption materials. The reaction rate and conversion increase significantly for the reactive fluid that flows through the porous ceramic networks [22]. The ceramic catalyst carrier plays a major role in promoting the chemical reaction. Due to the chemical corrosion resistance and thermal shock of porous ceramics, they can be used in highly required service conditions, like the reactor in chemical engineering and the vehicles gas exhaust treatment. As well as the fine metal particles are usually supported on the heterogeneous catalyst carriers, which are generally ceramic. Catalysis becomes also progressively more important in environmental pollution control. The catalyst effectively reduced pollution from automotive and industries applications. The ceramic used must have connected porosity and the pore size can differ between 6 nm and 500 μm. Alumina, titania, zirconia silica and silicon carbide are the most popular choices for catalyst supports. These ceramic powders are formed into a variety of shapes such as cylinder bars or hollow beads or clover-leaf shaped sections. They are then sintered to their final density. Porous ceramics also can be used as carriers in the recycling of steam, oxidation of ammonia, recombination of methane, destruction of volatile organic compounds (VOCs) by incineration and decomposition of organics by photocatalysis.

Porous ceramic membranes can be used to separate water, oil, liquids, solids, dust in gas, yeast or thallus and blood cells and to clarify alcohol in the food, chemical and medical industries. In addition, these membranes act as biological reactors in the recovery of fermented fluid. During the last few decades, the ceramic membrane applications have increased because of their excellent chemical, mechanical and thermal stability, and high separation efficiency [23]. High-permeability ceramic membranes can only be obtained in an asymmetric multilayer

**10. Porous ceramics and catalyst carriers**

**11. Porous ceramics and membranes**

## **7. Ceramic foams**

Ceramic foams are porous brittle materials with closed, open-celled structures or partially interconnected porosity [17]. Ceramic foams are a special class of porous materials included of large voids with linear dimensions in the range between 10 and 5 mm. Foams are also called cellular ceramic materials because their structure can be represented by a lattice of a repeatable unit called "cell". They are fabricated from a broad kind of ceramic materials; specifically both oxide and non-oxide, which includes pure oxides, aluminosilicates and carbides that are being considered for the whole range of possible applications. These include filtration, catalysis, impact-absorbing structures, thermal insulation, performs for metal-ceramic composites, biomechanical implants, high specific strength materials and high efficiency combustion burners. The ceramic foams have been produced in a variety of materials with different shape sizes, densities and degree of interconnectivity. Foams or cellular are usually made with the density between 10 and 40% of theoretical and the pore sizes less than 1 mm. Ceramic foams can be made with a variety of microstructures with controlled properties through several versatile and simple methods, such as direct foaming, replica, sacrificial template techniques [5].
