Summary

Migration of defects plays a very crucial role in determining conduction in ceramic materials. Migration of ionic defects such as vacancies or interstitials is basically a diffusive process and is governed by the law of diffusion. The diffusivity of species is a strongly temperature dependent parameter with an exponential dependence on the temperature. The diffusivity related to electrical conductivity is shown by Nernst-Einstein relationship. As a result, conductivity in the ionic systems also follows a temperature dependent Arhenious type behavior. In ionic systems, to utilize the ionic conduction of materials, it is essential that ionic conductivity is a dominant contribution towards the total conductivity which typically happens at either higher temperatures or at reasonably high doping levels unless the electron concentration is extremely low. The examples of ceramic materials with high ionic conductivity are materials like fast ion conductors and a few ceramic glasses. The presence of composition gradients of charged defects also leads to electrical potential gradients and this when combined with the chemical potential gradients, gives rise to an expression for electro chemical-potential, called Nernst equation. This equation is often employed in designing the sensors for use in automotives where differential pO₂ gives rise to a voltage which can be used as a feedback to regulate the fuel/air supply in the engines. Ceramic conductors are used in a variety of applications, for example, ZnO as varistors, doped ZrO₂ as solid electrolytes in fuel cells and as oxygen sensors in the automobile exhaust.