Preface

*Pottery and ceramics* have been an important part of human culture for thousands of years. From prehistoric storage jars to tiles on the space shuttles, pottery and ceramics have played a key role in innumerable human endeavors. The art of ceramics is one of the oldest known, dating to prehistoric times.

Once heated (fired) to between 350⁰ and 800⁰C, the clay is converted to ceramic and will never dissolve again. Ceramic products are used as structural including bricks, pipes, floor and roof tiles, Refractories, such as kiln linings, gas fire radiants, steel and glass making crucibles, Whitewares, including tableware, wall tiles, pottery products and sanitary ware or technical, such items include tiles used in the Space Shuttle program, gas burner nozzles, biomedical implants etc. Frequently, the raw materials do not include clay. *Calcination* refers to thermal treatment which removes volatile fraction. Calcination treatment prior to sintering improves the densification of ceramics. For instance, in the case of hydroxyapatite, a bioceramic, calcining at 900 °C prior to sintering at 1250 °C results in a higher bending strength (about 55MPa) with finer grain size.

The chapters covered by this book on Sintering of Ceramics include emerging new techniques on sintering which include Spark plasma sintering, Magnetic Pulsed compaction Low Temperature Co-fired Ceramic technology for the preparation of 3 dimesinal circuits, Microwave sintering of thermistor ceramics, Synthesis of Biocompatible ceramics, Dielectrics and opto-electronic materials, Sintering of Rare Earth Doped Bismuth Titanate Ceramics prepared by Soft Combustion, nanostructured ceramics, alternative solid-state reaction routes yielding densified bulk ceramics and nanopowders, Sintering of intermetallic superconductors such as MgB2, impurity doping in luminescence phosphors using soft techniques etc. Other advanced sintering techniques such as radiation thermal sintering for the manufacture of thin film solid oxide fuel cells, are also covered.

*Spark plasma sintering* (SPS) is a form of sintering where both external pressure and an electric field are applied simultaneously to enhance the densification of the amorphous or metallic/ceramic powder compacts. This form of densification uses lower temperatures and shorter amount of time than typical sintering. The theory behind it is that there is a high-temperature or high-energy plasma that is generated between

#### XIV Preface

the gaps of the powder materials; materials can be metals, inter-metallic, ceramics, composites and polymers. It is a novel technique used to sinter ceramics, metals and composites within a few minutes and to obtain densities greater than 99.9%.

Selective laser sintering (SLS) is an additive manufacturing technique that uses a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass that has a desired 3-dimensional shape.

A *microwave* furnace can sinter dental ceramics within 1/12th the time of a conventional oven. Microwave has the added advantage of producing better mechanical strength and resistance to low temperature degradation. In order to be effective, the ceramic powders to be sintered are to be surrounded by microwave susceptors such as ferric oxide. Microwave is the best choice for sintering dental ceramics such as Zirconia. Dental laboratories, overloaded with requests, usually take seven or more days to process the permanent dental restoration with a conventional oven.

*In magnetic pulsed compaction (MPC),* it is possible to apply a high pressure (up to 5 Gpa) to a ceramic powder for a short period of time (~ 500 µs) which results in improved density.

*Vacuum sintering.* ZrO2 based ceramic materials have unique properties (high abrasion resistance, strength and breakdown viscosity), and are being used in different technologies. A basic characteristic of the fabrication of such materials is that during traditional sintering at 16000C in air to obtain high densities causes a Tetragonal Monoclinic transformation on cooling, which destroys the sample, due to grain enlargement. However sintering at vary low partial oxygen pressure (10-6 Pa) stabilizes the high temperature tetragonal phases of ZrO2 similar to oxide additives (CaO, MgO, Y2O3, etc).

Recycling green building materials using *cold-bonding technique* incorporates the principles of cement chemistry which results in the reduction of energy consumption and CO2 emission.

> **Arunachalam Lakshmanan**  Saveetha Engineering College, Thandalam, Chennai, India

**Part 1** 

**New Sintering Techniques** 

**1** 

*Brazil* 

**Microwave Fast Sintering of Ceramic Materials** 

Microwaves are electromagnetic waves with wavelengths ranging from 1m to 1 mm, which correspond to frequencies between 0.3 and 300 GHz. This frequency range lies just above radio waves and just below visible light on the electromagnetic spectrum (Katz, 1992). The possibility of processing ceramics by microwave heating was discussed over 50 years ago by Von Hippel (1954a), and experimental studies on microwave processing of ceramics were started in the mid 1960s by Tinga and Voss (Tinga & Voss, 1968). Since then, the results of many investigations into microwave sintering and joining of ceramics have been reported (Bykov et al., 2001). Activity in this field began to accelerate in the mid-1970s because of a shortage of natural gas, prompting the investigation of microwave heating and sintering of

While most of today's industrial microwave applications involve the relatively lowtemperature processing of food, wood, rubber, polymers, etc., interest in high-temperature microwave processing of materials has been growing. In recent years, microwave heating has been widely employed in the sintering and joining of ceramics (Bykov et al., 2001;

Most of the reposts published in the literature assert that microwave-driven processes are faster than conventional heating processes. This faster speed is manifested as a reduction in the densification time of ceramic powder compacts, often allied to lower sintering temperatures (Bykov et al., 2001). In general, the kinetics of synthesis and sintering reactions are reportedly augmented by two or three orders of magnitude or even more when conventional heating is substituted for microwave radiation (Oghbaei & Mirzaee,

Microwave heating is a process whereby microwaves couple to materials, which absorb the electromagnetic energy volumetrically and transform it into heat. This differs from conventional methods in which heat is transferred between objects through the mechanisms of conduction, radiation and convection. Because the material itself generates the heat, heating is more volumetric and can be very rapid and selective (Sutton, 1989). Thus, microwave sintering techniques allows for the application of high heating rates, markedly

**1. Introduction**

Huang et al., 2009).

shortening the processing time.

2010).

several ceramic materials in the late 1970s and 1980s.

Romualdo R. Menezes1, Pollyane M. Souto2

*1Department of Materials Engineering, Federal University of Paraiba 2Department of Materials Engineering, Federal University of São Carlos* 

and Ruth H.G.A. Kiminami2
