Evaluation of the Phase Composition, Crystallinity and Trace Isotope Variation of SiC in Experimental TRISO Coated Particles

The silicon carbide layers in experimental TRISO coated particles with zirconia kernels were evaluated for their phase composition, their impurity levels and the crystal perfection and twinning of the crystallites in the layers. This evaluation was necessary to compare the different SiC layers and to relate these properties to various quality tests and ultimately to manufacturing parameters in the CVD coater. Identification of the various polytypes was done using electron diffraction methods. This is the only method for the unequivocal identification of the different polytypes. The 3C, and 6H polytypes were positively identified. A feature of the SiC in some samples is the disordered nature of the phase. The disorder is characterised by planar defects, of different width and periodicity, giving rise to streaking in the diffraction pattern along the [111] direction of the 3C polytype. Polarised light microscopy in transmission is a useful tool to easily distinguish between the cubic (beta) and non-cubic (alpha) SiC in the layers. It also provides valuable information about the distribution of these phases in the layers. Raman spectroscopy was used to examine the distribution of Si in the SiC layers of the different samples. Two samples contain elevated levels of Si, of the order of 50%, with the highest levels on the inside of the layers. The elevated Si levels also occur in most of the other samples, albeit at lower Si levels. This was also confirmed by use of SEM electron backscatter analysis. Rietveld analysis using X-ray diffraction is presently the only reliable method to quantify the polytypes in the SiC layer. It was found that the SiC layer consists predominantly (82% to 94%) of the 3C polytype, with minor amounts of the 6H and 8H polytypes. Impurities in the SiC and PyC could be measured with sufficient sensitivity using laser ablation inductively coupled mass spectrometry (LA-ICP-MS). The SiC and PyC layers are easily located from the intensity of the C¹³ and Si²⁹ signals. In most cases the absolute values are of less concern than the variation of impurities in the samples. Elevated levels of the transition elements Cu, Ni, Co, Cr and Zn are present erratically in some samples. These elements, together with Ag¹⁰⁷ and Ag¹⁰⁹, correlate positively, indicating impurities, even metallic particles. Elevated levels of these transition elements are also present at the SiC/OPyC (Outer Pyrolytic Carbon) interface. The reasons for this are unknown at this stage. NIST standards were used to calibrate the impurity levels in the coated particles. These average from 1 to 18 ppm for some isotopes.