Construction of a chemical vapour deposition reactor and the deposition of ZrC layers

Abstract

Generation IV high temperature nuclear reactors use TRISO (tristructural isotropic) particles for containment of radioactive fission products. In these particles silicon carbide (SiC) is the main barrier for containing solid fission products. ZrC has been proposed either as an additional layer or replacement of SiC layer of the TRISO fuel particle. This is because ZrC would be a better barrier than SiC against the diffusion of Ag and will be more resistant against palladium attack. This suggestion is based on ZrC‘s excellent physical properties like: low neutron capture cross section, good thermal shock resistance, excellent thermal stability, etc. However these properties depend on a number of factors such as chemical composition, microstructure, morphology, the presence of impurities, etc. Many attempts have been made to grow ZrC layers. However, the growth of high quality polycrystalline ZrC layers with good stoichiometry has remained a challenge. This study focuses on designing a chemical vapour deposition (CVD) reactor for growing ZrC layers and studying the properties of ZrC layers grown under different conditions. CVD was chosen as the most effective process to grow ZrC layers. CVD enables the production of relatively pure uniform layers with good adhesion and reproducibility at fairly good deposition rates. A thermal CVD reactor system operating at atmospheric pressure was designed to grow ZrC layers. Radio frequency (RF) induction heating was used as the energy source, with a vertical-flow design, using thermally stable materials. The ZrC layers were grown while varying the substrate temperature, gas flow ratios, substrate-gas inlet gaps, deposition time and partial pressures. The precursors utilised to grow ZrC were zirconium tetrachloride (ZrCl4) and methane (CH4) in an excess of argon and hydrogen. The deposited ZrC layers were then characterised by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and field emission scanning electron microscopy (FE-SEM). To optimise the process parameters, response surface methodology was applied. A central composite design was used to investigate the effect of temperature (1200 °C–1600 °C) and molar ratio of CH4/ZrCl4 (6.04–24.44) on the growth rate, atomic ratio of C/Zr and crystallite size of ZrC layers. The temperature of 1353.3 °C and the CH4/ZrCl4 molar ratio of 10.41 were determined as the optimal conditions for growing near-stoichiometric ZrC layers. The optimum values for C/Zr atomic percentage ratio, growth rate and average crystallite size were 1.03, 6.05 ?m/h and 29.8 nm respectively. The influence of the gap between the gas inlet and the substrate (70–170 mm) on the growth rate, surface morphology, phase composition and microstructure of ZrC layers deposited at different temperatures was investigated. The growth rate of ZrC layers prepared at 1400 °C was higher than at 1200 °C, and decreased with increase in the gap at both temperatures. The boundary layer thickness increased with an increase in substrate-inlet gap. The diffusion coefficients of the reactants increased with temperature. A model was used to illustrate the diffusion of source materials through the boundary layer to the reacting surface. An increase in the gap from 70 mm to 170 mm at a temperature of 1400 °C, caused the layers to become more uniform. An increase in particle agglomeration was also observed. By contrast, at 1200 °C the surface crystallites had complex facets that decreased in size as the gap increased from 70 mm to 170 mm.