Thermionic Electron Emission Properties of Nitrogen-Incorporated Polycrystalline Diamond Films

Abstract

Thermionic energy conversion (TEC) is a potentially practical method for the efficient conversion of thermal energy directly into electrical energy. Standard TEC configurations consist of a cathode and an anode separated by some interelectrode gap and electrical connections between the two. When thermal energy is imparted to the cathode, electrons with sufficient energy are thermionically emitted and traverse the interelectrode gap. These electrons are then collected by the cooler anode and cycled back to the cathode through an external load. From this simple description, it is clear that the performance of a TEC device depends on the cathode’s ability to undergo thermionic emission which leads to the purpose of this research. Diamond has several favorable material properties for use in TEC such as low to negative electron affinity, high thermal conductivity, and radiation tolerance. This research investigated diamond’s thermionic emission properties such that an all-diamond TEC device could be realized. A fabrication method was first developed to deposit nitrogen-incorporated diamond thermionic cathodes using microwave plasma-enhanced chemical vapor deposition. Raman spectroscopy indicated that the diamond samples were predominantly diamond with minimal other carbonaceous content. A testing apparatus was then designed capable of accurately studying the thermionic emission operation of the fabricated diamond cathodes in both vacuum and various low pressure gaseous environments. Potential failure mechanisms of diamond cathodes that would inhibit implementation into practical thermionic devices were then identified. To overcome these failure mechanisms, portions of the research were directed toward a better understanding of the emission process from diamond as well as factors that influence it. Finally, new methods were developed that allowed for increased performance, reliability, and operational lifetime. The results obtained in this research were then used to predict the performance of an all-diamond thermionic energy converter.