16.1 Introduction

Up to this point the scientific applications of ceramics were mainly due to their insulating, structural and thermal characteristics. After so long thinking ceramics are great insulators, it is somewhat of a surprise to find they can exhibit a wide spectrum of electrical properties. It is the combination of these electrical and magnetic properties and ceramic’s characteristic stability that makes them the ideal choice for many technological applications. Their robustness is their appeal and ceramics even have preference over metals in some relatively higher conductivity applications.

The range of properties that can be achieved is very broad, including: magnetic (ferrites); superconductive; non-linear resistive; capacitive; and piezoelectric, with sub-sets pyroelectric (heat sensitive), ferroelectric (electric field sensitive) and electro-optic (light sensitive).

All ceramic materials that can exist in the form of a crystal have been split into 32 crystal classes, of which only 20 are piezoelectric (generating electricity from mechanical pressure). Ten of these are spontaneously polarised, so they can exhibit electrostatic polarisation (dipoles in the crystal line up) with no applied electric field. These are also temperature sensitive so pyroelectric (polarisation changes with temperature). The important class of ferroelectrics belong to a sub-group of this pyroelectric class. They are called ferroelectric because of their similar “domain” structure to iron-based magnetic material, but having an electric rather than a magnetic field.  In a ferroelectric, “domains” are regions of uniform spontaneous electric field polarisation. Ferroelectrics also have the property that an external electric field can reverse their internal polarisation.

The earliest recorded account of pyroelectricity was by the Greek, Teophrastus (371-287 BC), who wrote that the mineral, lyngourion, probably tourmaline, became charged when heated and attracted small pieces of wood. In the 18^th century there were investigations of pyroelectricity related to work on electrostatics. In 1880 the Curie brothers, Pierre and Jacques, discovered piezoelectricity in tourmaline, zincblende and quartz and Valasek in USA discovered ferroelectricity in 1921.

Modern scientific applications of ceramics took off when these exciting properties of ceramics were seen to be key to the development of the electronic and information age, which was based on the simultaneous development of semiconductors and these electronic ceramics. Ceramics and glasses are important enabling technologies in many electronics markets. The range of properties of electronic ceramics is the largest known of any material class. For example, the resistivity of ceramics varies from zero for superconductors to the most resistive of materials.

The high strength of the electronic bonding within the ceramic gives it the characteristics of generally high melting point, corrosion resistance, brittleness and compressive strength together with low thermal conductivity. It is the variation in this electronic bonding within different crystal structures, and in some cases particularly the characteristics of boundaries between grains within the body of the ceramic material that produces the enormous range of electronic and magnetic properties of ceramics. There is no comprehensive theory to explain the effects exhibited by some of these ceramic materials, and much has been achieved by trial and error. As an example the addition of magnesium oxide to alumina produces the translucent ceramic used to encapsulate high-pressure sodium vapour streetlights, but it is not understood exactly what role the magnesium oxide performs. Some of the electroceramic materials used are highly specialised, prepared from compositions often not found in nature, carefully processed and fabricated into complex shapes to meet specific applications.

Although semiconductor advances have overtaken some ceramic devices, technology moves ever onward. A recent discovery involves a new oxide crystal structure that switches from metallic to semiconducting when illuminated, giving the potential for a memory device 1000 times denser than a DVD!