The main ceramics used as a basis are PZT and BST, in the polycrystalline form for reduced cost, ease of processing and stability. As the pyroelectric effect causes a release of electric charge, it can be measured by an external circuit. A practical structure for a pyroelectric ceramic device would be to have a layer of lead titanate ceramic deposited on a sapphire substrate, with the top surface coated with a thin layer of platinum, transparent to infrared, the final active device typically being 1 mm x 1 mm x 30 microns. General applications include the detection of icing on helicopter blades, liquid level detection in tanks and as part of the heating element in a miniature oven to keep quartz crystals at a constant temperature in constant frequency sources.

Since a small change in infrared radiation falling on a sensitive ferroelectric ceramic such as BST can be measured, they can indicate the temperature of remote objects, such as for the simple activation of security lights. However, they can also be the basis of thermal imaging or heat-detecting cameras.

Thermograph of lion - source Wikipedia

Previous infrared detectors were based on photoconductivity in semiconductors that required expensive cooling with liquid nitrogen to reduce the background noise level in the detector. The use of uncooled, room temperature, pyroelectric ceramic detectors opened up the vast potential of portable night visualisation equipment, particularly for military but also for civil police surveillance, navigation in bad weather and for searching for survivors buried under collapsed buildings.

One thermal imaging principle was based on the vidicon tube, used in television-style cameras until charge-coupled devices replaced them. The alternative is to scan the image with a number of detectors in an array, which can be linear (in a line) or two-dimensional (a rectangle). This also overcomes the relatively slow response speed of the individual detectors. Typically a linear device array would contain 64 elements each measuring 80 x 120 microns, capable of detecting a temperature difference of only 0.15 ºC. To achieve a TV quality image at least a 100 x 100 array is needed. Arrays of detectors using doped barium strontium titanate ceramic are used to produce compact cameras having such quality images.

Later developments were based on a hybrid approach that bonded the ceramic array, pixel by pixel onto a semiconductor readout circuit, much reducing difficulties in signal processing. The ceramic is prepared in the usual way by calcining the raw materials, grinding finely, compacting to shape, firing at 1450 ºC and then dicing into pixels. These are then integrated onto the prepared semiconductor, producing an array of, for example, 245 x 328 pixels. Future developments are aimed to deposit a thin film of ceramic, such as lead calcium titanate, directly onto the semiconductor, reducing cost and size.

Ceramic pyroelectric detectors can be cost-effective and mechanically and chemically robust, but as these materials are also piezoelectric their packaging has to be carefully designed to reduce this effect. They are also used for the analysis of gases, being able to detect their absorption spectra.

16.27 Capacitors

Capacitors are used extensively in electronic circuits to block direct current, letting higher frequency signals through. One of the earliest dielectrics used for capacitors was mica. It is a single-crystal mineral silicate that is naturally formed as thin sheets, so it was easy to interleave these thin sheets of mica with metal foil to form the capacitor. Another early dielectric was steatite that was formed as tubes with 0.1 to 0.5 mm thick walls. The growth of integrated circuit technology has resulted in a huge increase in the consumption of discrete components such as capacitors and resistors. Around 1941, because of the war effort, much work was carried out on dielectrics for capacitors and particularly on anomalously high dielectric constant materials. In the mid to late 1940s, the use of the recently discovered barium titanate brought in the much higher dielectric constants, some 100 times greater than before. The previous range of dielectric constant for ordinary insulators was 5 to 100.