16.19 Electrostriction

All dielectric materials undergo a small change in dimension when subjected to an electric field. This phenomenon is known as electrostriction and expands the material regardless of field direction. It is not reversible so pressure does not result in polarisation. Electrostrictive devices are used in fuel injectors, printer heads, precision machining and deformable mirrors. The electrostrictive effect in some ferroelectric ceramics near the Curie transition region can be very high, and have advantages over piezoelectric ceramics, but suffer from temperature sensitivity.

16.20 Magnetostriction

Magnetostrictive tag

Ferromagnetic materials exhibit the same sort of change in dimensions when subjected to a magnetic field. The effect has found uses in sensors as well as in coded tags in retail shops to prevent shoplifting. The tags are read by detectors in posts by the exit doors.

16.21 Piezo-Polymer Composites

Although originally used for submarine detection, PZT is not an ideal ceramic for sonar as it is a poor acoustic match with water. To increase the ultrasonic power that can be beamed into the water, a polymer can be used as a quarter wave matching section between the ceramic and the water. The polymer would be typically 6 mm thick at 100 kHz.

Another composite structure is made up of ceramic pillars surrounded by polymer. This is used in equipment for non-invasive ultrasonic investigation of human tissue, such as viewing a foetus. The device is a 20 mm diameter disc 0.6 mm thick that is made up of an array of square section pillars 150 microns apart that operates at 3MHz. The space between the pillars is filled with epoxy making the whole a good acoustic match, and around 20 times more effective than PZT alone. With associated electronics, the acoustic beam from such an array of transducers can be steered as in phased-array radars.

Another method of manufacturing certain ceramics is known as “Polyderived Ceramics”. The ceramic powder is mixed with a polymer and pyrolysed, typically at 1,100 ºC, to produce fibres for filters, brake parts and thermal insulation. It also could have application for repairing space vehicles, as it could be designed to become a ceramic during high temperature re-entry.

16.22 Gas Detection

Quite early on it was noticed that the resistivity of certain ceramics depended on the local atmosphere, being affected for example by the moisture in the air, which led to the use of ceramics as gas detectors. There are two basic forms of ceramic gas sensor, one based on a change in its resistivity and the other based on the principle of the “fuel cell”.

When ceramics are sintered at a low temperature they can be made very porous so certain specific gases can be readily absorbed. Many different ceramics can be used, allowing the detection of individual gases such as carbon monoxide and dioxide, methane, oxides of nitrogen, hydrogen sulphide and ammonia. Typically grain size is around 1 micron, with pores 0.05 to 0.3 microns.

Toyota Lambda probe

In the early 1960’s the first zirconium-based oxygen detector called a Lambda probe was introduced and used widely in the automotive industry to detect oxygen in vehicle exhausts. The zirconia is doped with yttria or calcia so that it is conductive to oxygen ions, the same phenomenon that is used in fuel cells. The ceramic is formed as a tube with a porous platinum plating inside and outside as electrodes. It is used to improve engine efficiency by measuring the oxygen, enabling the control of CO and NOx.

They are also used to measure the oxygen content in diver’s breathing gas, as well as used widely in the aerospace, transportation and power industries. They can even be used to dip directly into molten steel to measure its oxygen content. In 1998 a planar version was introduced significantly reducing the mass and cost of the sensing element.