Radiation Induced Conductivity in Ceramics Insulators for Thermionic Systems

Abstract

Insulators in thermionic systems must be able to sustain high temperature, radiation damage and electric fields while maintaining their mechanical stability, high electrical resistivity and thermal conductivity for long periods of time. The most common insulator used in these nuclear systems is alumina, either in the single crystal form (sapphire) or fine grain polycrystals. It has been demonstrated that the simultaneous application of electric field and radiation results in a dramatic increase in conductivity. This paper describes the investigation at Auburn University which examines the radiation‐induced conductivity process in alumina using a variety of controlled radiations. These include high energy protons and alpha particles from a 2 MeV light ion accelerator, gamma radiation from a Co60 source and x‐radiation from an x‐ray tube. The experiments were conducted at temperatures up to 800 K with different radiation intensities. These various types of radiation provide different ratios of electronic energy loss versus nuclear energy loss. The former is expected to yield instantaneous conductivity change whereas the latter is responsible for permanent damage. Both instantaneous radiation‐induced conductivity and permanent effects were determined under controlled environment. It was found that the two process were not completely separable. The structure damage produced by radiation also affects the recoverable radiation effects. Efficiencies, normalized to ionization, for induced conductivity were also measured and compared. Controlling parameters for the electrical degradation process were elucidated based on available data.