cBN Insulator for TFE Trilayer

An improved insulator material is proposed for use in space nuclear reactors. This material is for the insulator of the trilayer of a thermionic fuel element (TFE). This insulator must withstand rigorous conditions of long life at high temperature under exposure to nuclear radiation. Previous work has studied alumina, YAG, and diamond as possible RFE insulator materials. cBN is similar to diamond in many ways: high thermal conductivity, refractory, and large bandgap for high electrical resistivity. This material can potentially enable longer life in space nuclear reactors. Like diamond, cBN has been expensive to produce in bulk. However, recent experiments have shown that cBN can be readily deposited in thin film coatings. This is accomplished through a laser ablation deposition process. Laser deposition using long wavelength lasers (Nd: YAG operating in fundamental or second harmonics) have only produced BN films of hexagonal crystal structure. But laser deposition using excimer lasers operating in the UV show good promise. Work in the excimer laser deposition of boron nitride films on SI indicated that the resulting BN films of cubic zincblende structure. Regions of films also exhibit technique in cBN film preparation. BN films will be prepared by excimer laser deposition. The films will be characterized by scanning electron microscopy, raman spectroscopy, and x-ray diffraction. The effect of laser wavelength on the CN film crystal formation will be studied.