CVD Diamond Coated Cylinders for Advanced Thermionic Fuel Element

Abstract

The recent success of several efforts directed toward the development of a diamond film sheath insulator for a thermionic fuel element (TFE) has bolstered interest in the use of diamond films as high temperature electrical insulators. We have been pursuing, under SDIO's Innovative Science and Technology sponsorship, development of a method to deposit diamond film on the refractory metal outer surface of a cylindrical TFE. This diamond coating will provide electrical insulation between each of the several TFEs which exist within a typical thermionic space nuclear power reactor. The diamond film's high thermal conductivity will also enhance the liquid metal cooling of the TFE's outer surface, which must be maintained at a temperature of 1000 K. We have significantly advanced the program by the development of a three dimensional diamond CVD process. This deposition process has been successful in uniformly and completely coating molybdenum “TFE size” cylinders, 5/8 inch in diameter and 3 inches long. The quality of these diamond coated cylinders has been analyzed by scanning electron microscope, Raman spectroscopy, and measurement of bulk electrical resistivity properties. The positive results of the analyses has led us to initiate liquid alkali metal exposure/compatibility and nuclear irradiation tests on the diamond coated cylinders to verify the durability of the diamond films in the thermionic reactor environment. This paper will present the results of the analysis done on the diamond coated molybdenum cylinders and discuss the current efforts in evaluating the beneficial impact of diamond film as a thermionic sheath insulator.