16.2 Ceramic Sensors and Actuators

The human brain would be useless without sensors to provide inputs, such as eyes, ears and fingers, and actuators to provide output, namely muscles operating arms, legs, mouth etc. The world we live in depends largely on the control we exercise over our environment. This means we have to measure various physical and chemical parameters, including temperature, pressure and position and use these measurements to produce the required output. It turns out that it is much easier to apply sophisticated computer processing to electrical signals than it is to obtain the signals in the first place, and many systems are limited by the lack of adequate means of measurement or of causing an output with an actuator. This is why the large range of ceramic sensors and actuators are so important to our way of life. Advanced ceramics are critically important because of their stability, low cost and often unique measuring and actuating capability.

16.3 Manufacture

The new advanced scientific ceramics are made from inorganic compounds, much the same as other ceramics, but demand a much more exacting control of raw materials and processing. Instead of digging up clay that is made up of a rather indeterminate mixture of oxides and other chemical compounds, the raw materials for advanced ceramics need to be pure, finely ground oxides mixed in exactly the right proportions to achieve the required characteristics. The absence of “clay” as a plasticiser led to organic ones being used. The densification (removal of pores) no longer depends on added fluxes but depends on “calcining” and “sintering” sometimes under pressure. Calcining is a low temperature pre-firing that converts constituents such as nitrates and carbonates into oxides and causes constituents to start to interdiffuse, thus reducing the extent diffusion needs to occur during the subsequent “sintering” process. Calcining partly fires the constituents so they have to be milled again to around 1-10 microns. Firing is often referred to as “sintering” in the context of advanced ceramic manufacture, but in sintering no molten liquid is formed. Ideally sintering produces fully dense material and eliminates porosity.

The properties of advanced ceramics depend critically on numerous parameters such as the optimal size of the particles, the state of aggregation of the components, their chemical purity and homogeneity. Densification is important to achieve good performance, so the aim is to repeatedly produce a dense ceramic with zero porosity, which is not easy. The powdered raw materials are often mixed with 5% of a binder such as polyvinyl alcohol and pressed into a mould the same shape as that finally required, but larger to allow for shrinkage (the “green” body). The binder is removed on firing.

More predictable is the performance of single crystals that some electroceramic devices use, rather than a polycrystalline form, but the growth of these single crystals is complex and very time consuming.

The raw materials for polycrystalline ceramics can be obtained from highly refined minerals or specific chemical synthesis. Milling oxides, hydroxides and carbonates to fine powders is a cheap way of preparing advanced ceramics, but it is difficult to get the required size and homogeneity so that when fired they are consistent throughout with no voids, also milling balls can contaminate the mix. Higher temperatures of around 1500 ºC are also needed for their sintering. However milling is used for some more tolerant applications.

There are several alternative methods used to prepare pure powders with the sizes needed. Some of these methods are based on the components being in a solution. One of these is precipitation. As an example it can be used for manufacturing ferrites by making solutions of the chlorides or sulphates of iron and other metals, and adding agents to cause the material required to precipitate out. After drying and calcining at 180 to 300 ºC, high purity particles with a narrow size range of 0.05 to 0.5 microns can be obtained. Because of this particle quality, the sintering temperature can be significantly lower, for example 550 instead of the 720 ºC required for strontium hexaferrite if the components are only milled.