9.1.1 Selection of Raw Materials

One of the important factors in the fabrication of bulk ceramic is the choice of raw materials i.e. starting ingredients. For ceramic materials, raw materials are typically oxides or carbonates. It is important to choose powders with the highest possible purity, as it is evident from defect chemistry that impurities, even in small quantities, lead to significant changes in the electrical characteristics. Another aspect of choosing raw material is to see their moisture content. It may vary from 0.1% to ~1% for non-hygroscopic materials while it can be much larger for hygroscopic materials. One can conduct the thermogravimetric analysis (TGA) to verify the weight change upon heating due to moisture loss to accurately determine the moisture content. One more important parameter in the selection of powders is the particle size. While smaller particle size promotes their reactivity, tendency of agglomeration also increases as the particle size decreases. One needs to optimize this after careful phase and microstructure analysis using standard X-ray and microscopy methods.

9.1.2 Powder Preparation

After carefully weighing the powers in desired proportions depending on the desired stoichiometry, the powders are mixed together. Mixing is usually carried out using either pestle-mortar or ball-mills. Care should be taken to avoid any contamination at this stage while breaking the agglomerates as effectively as possible. In case of multi-component systems, it is important to see whether there is any segregation of the constituents. The mixed powders can also be prepared using chemical route if the starting precursors are chemical precursors such as alkoxides, nitrates or citrates by dissolving them in appropriate solvents such as methoxyethanol, glacial acetic acid or citric acid, followed by mixing under controlled conditions. The solution can be then dried to yield the starting powers for calcinations by evaporating the moisture and volatile substances such as carbon and nitrogen. Again, thermal analysis of the mixture gives an idea of the temperatures needed for drying and calcination.

9.1.3 Calcination

The next step in the powder preparation is calcinations which is typically a step for achieving the phase formation. Calcination is a high temperature phenomenon in which powders are heated to high temperature in a chosen ambience and then held for a long time to achieve the phase formation. High temperatures are required to facilitate the diffusion of constituents to enable their reaction with each other. Calcination also reduces the diffusion requirements and shrinkage that occur during sintering and densification stages.

9.1.4 Shaping

Shaping is typically done using a die and compact and is usually carried out at room temperature using high pressures. Sometimes binders are also required to allow the particle to remain bound to each other. This process is achieved either by uniaxial dry pressing or using iso-static pressing if uniformity in density is a stringent requirement. Wet processes for shaping include slip casting and band casting where a slurry is cast into molds followed by drying.

9.1.5 Densification and Finishing

Densification is basically a sintering operation in which compact from shaping stage are put into high temperature furnaces to achieve the densification and make the compact stronger. Typically this stage is characterized by the use of high temperatures and accompanied by shrinkage during the process. Care must be taken to prevent cracking in the compacts due to shrinkage stresses. This requires good control of heating and cooling rates during sintering process. Finally the dense sintered compacts are provided finishing operations either by machining, glazing or metalizing.