Powdering and shaping

10.2.1 Powdering and shaping

A wide range of methods are available to produce ceramic powders, depending on the quality of the product desired. Cheap, coarse ceramic products such as porcelain and refractories are produced from powders made by crushing of natural minerals. Fine or engineering ceramics are sintered from high purity and extremely fine powders, which can be produced from a wide range of methods, as summarized in Table 10.2. In the solid route, mechanical crushing by means of ball milling is a widely used method for particle size reduction. In ball milling, the particles to be ground are placed inside a rotating container together with a grinding medium, usually another ceramic in the shape of balls or rods. As the mill turns, the grinding medium tumbles inside the container, thus grinding the particles into smaller sizes. Contamination from the mill lining as well as from the grinding medium is a common problem in ball milling. In the liquid route in Table 10.2, the material is turned into a liquid phase by either liquefaction or solution methods. A common method to liquefy ceramics is by plasma jet, and the resultant liquid trickle is then atomized to produce a fine powder upon solidification. Ceramic powders can also be synthesized from solution by dissolving the appropriate soluble salts into a solvent. The solvent can then be physically removed from the solution, by methods such as heating, spray drying, freeze drying, etc., to increase the concentration so as to precipitate out the solute. Alternatively, the solute can be separated from the solution by a suitable precipitation reaction.

Another common method is ‘sol–gel’, in which a stable dispersion of submicron-sized particles (the

Non-metallics I – Ceramics, glass, glass-ceramics 517 ‘sol’) is first formed in a liquid. The sol is then allowed to form a ‘gel’, which is a softly bonded,

polymer-like network throughout the gel, by methods such as evaporating part of the solvent, ageing or addition of a suitable electrolyte. Finally, the remaining solvent is removed or the solute precipitated out. Synthesis can also be done in the gaseous state. In the physical vapor deposition (PVD) method, the raw material is vaporized at a high temperature, followed by condensation to a fine powder form. In the chemical vapor deposition (CVD) method, the ceramic is produced by a chemical reaction of the vapor of metallic compounds.

Ceramic powder particles, after mixing with additives, including binders and sintering catalysts, can be compacted and shaped using methods such as die pressing, rubber mold pressing, extrusion and injection molding, and slip casting. In die pressing, the powder mixture is filled into a die and pressed into a strong solid product by uniaxial pressures between 200 and 2000 kg cm −2 . Die pressing is used for making refractories, tiles, electronic ceramics, nuclear fuel pallets and other products with relatively simple shapes and requirements for large quantities. Die pressing works well with mold shapes with a small aspect ratio, i.e. shapes that are short compared with their height. Long specimens are difficult to be compacted uniformly as the powder slurry has high viscosity, and internal friction between the particles and the die wall dissipates the pressure along the length of a long specimen, resulting in less compaction at the bottom part of the specimen, and thus non-uniform densification (shrinkage) and mechanical strength after sintering.

Long shapes such as rods or pipes can be produced by extrusion and more complicated shapes by injection molding. Slip casting is a technique for making shell-shaped objects with a small thickness. In slip casting, the ceramic slurry is poured into a hollow mold made of porous plaster. The mold absorbs water content from the slurry, causing the layer in contact with the mold surface to harden.

The excessive slurry in the mold interior is then poured out, leaving only the hardened layer attached to the mold surface. After further drying, the casting is moved from the mold. Some applications such as electrical substrates (e.g. for capacitors), plate-fin heat exchangers and exhaust emission control devices require large area-to-volume ratio. Such sheet or tape shapes can be produced by a variant technique of slip casting, in which the slurry is spread on a flat surface. Drying of the slurry by, for example, blowing warm air on it causes the slurry sheet to harden.