The “sol-gel” process that was developed in 1981 is particularly useful as it can be used to produce different shaped particles, such as fibres and for thin films. The “sol” is a solution or dispersal of solid particles in an organic solvent, which is destabilised by the addition of an agent, often water, when the viscosity increases to form a “gel”. The liquid is removed by various complex processes. Subsequent calcining and sintering results in a product in the form of a powder, fibre or film. As an example, for the yttrium barium cuprate superconductor, the temperatures are 1000 ºC for calcining and 1223 ºC for sintering. This process was first used to form microspherical particles (100 micron diameter) for use as fuel in high temperature nuclear reactors. The market for sol-gel products is expected to be $2Bn by 2017.

There is still a measure of trial and error needed in the manufacture of advanced ceramics, because the theory of the performance of them is incomplete and the processes producing the effects are extremely complex. However, starting from pure raw materials means that results can be precisely reproduced from one sample to another if processed in the same way, so electronic ceramics can be specially formulated for specific characteristics.

There are several different ceramic structures used for advanced ceramic devices – polycrystalline, glass ceramic and single crystal:

16.4 Polycrystalline

The manufacturing process for some polycrystalline ceramics, although more precise, can follow the usual processes used to make ceramics. The raw materials, which can be oxides, nitrides or carbides, are prepared to produce the fine particle size ranges required. The components are mixed together in the correct proportions, heated and calcined at 500 to 800 ºC. The aggregate is then ground/milled finely (less than one micron in some cases) and shaped. Shaping can be achieved in a number of ways. Pressure is used to increase the density and get high surface area contact between the particles. “Die pressing” with a die and punch is the most common method for small components with fairly simple shapes, however uneven forces cause inhomogeneity. “Isostatic rubber moulding” has a more uniform density where the powder is put into a flexible mould and immersed in fluid under pressure so that the pressure is even overall. This method allows uniform density to be achieved with complex shapes and is good for tube shaping. “Extrusion” and “slip casting” need the powders to be plastic using water or a polymer, which gives the problem of subsequently heating to remove the medium without affecting the shape – perhaps taking 36 to 48 hours in an oven. The formed piece is still sometimes referred to as “greenware”. Once dried the components are sintered at several hundred degrees above the calcining temperature or around 70% of the melting temperature to complete the reaction of the constituents and maximise the density of the ceramic, which typically has 5-20 micron grain size.

16.5 Glass Ceramic

Glass ceramic for armoured car windows
- source Schott

As mentioned earlier, when a composition containing sufficient inorganic material such as SiO2 is heated to its melting point and held there for sufficient time it melts and becomes a homogenous liquid. If it is then cooled sufficiently slowly it becomes increasingly viscous and a solid glass will be formed. It has become a “supercooled liquid” that is the state of all glasses. If these conditions are not met a polycrystalline structure will result, normally with uncontrolled growth of crystals. “Glass Ceramics” are produced when the crystal growth is controlled to form uniform and small crystals. They were developed from work on photosensitive glasses. Glass ceramics are formed by heat-treating a glass containing a nucleating agent such as gold, silver or titanium dioxide at around 500 to 700 ºC, which produces a large number of nuclei to start off crystal growth. The temperature is then raised to 200 to 500 ºC above the softening temperature of the original glass, when crystals grow until up to 100% crystallisation is achieved. The resulting structure is like fired vitrified pottery but has a greater amount of very small crystals so it is fine-grained and non-porous, and therefore very strong. Its properties are controllable so its expansion coefficient can be from zero to as high as that of metals.