Conducting Charge or Not

Abstract

Ceramics show the widest range of electrical properties of any class of material. At one extreme we have high-temperature superconductors, which have no resistance to an electrical current. At the other extreme we have electrical insulators. Ceramic superconductors have not yet ful-filled many of the expectations and predications for useful applications, whereas insulating ceramics are used for a number of critical applications such as packages for integrated circuits. Without the use of insulating ceramics the development of powerful personal computers would not have been so rapid. Between the two extremes are ceramics that behave very much like metals, and there are the semiconductors, which are all ceramics. Ceramics with metal-like conductivity are used as electrodes and in thick-film resistors. Semiconductors such as SiC are important for high-temperature electronics. In this chapter we will explain why ceramics show such a diverse range of electrical properties. The important concepts are related to our earlier discussion of bonding and energy bands.

In some ceramics the only species that can move in an applied electric field are the ions in the structure. Generally, the movement of ions is slow, but in a class of ceramics called fast ion conductors, they can move very rapidly. In cubic zirconia the diffusion of oxygen ions at high temperature is particularly fast, and this ceramic is used as the electrolyte in solid oxide fuel cells. Fuel cells are becoming a key part of a diverse energy plan for the twenty-first century.

We will begin by describing the conduction mechanisms in ceramics and looking at some specific applications. We will finish by describing one of the most fascinating developments in ceramics—high-temperature superconductors.