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Research Papers: Nuclear Power

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

[+] Author and Article Information
Johan P. R. de Villiers, James Roberts, Noko Ngoepe, Alison S. Tuling

 University of Pretoria, Pretoria 0002, South Africa

J. Eng. Gas Turbines Power 131(6), 062904 (Jul 17, 2009) (7 pages) doi:10.1115/1.3098426 History: Received November 27, 2008; Revised December 01, 2008; Published July 17, 2009

The SiC layers in experimental tristructural-isotropic (TRISO) coated particles with zirconia kernels were evaluated for their phase composition, impurity levels, crystal perfection, and twinning of the crystallites in the layers. This evaluation was necessary to compare the different SiC layers and relate these properties to various quality tests and ultimately to manufacturing parameters in the chemical vapor deposition (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. The SiC in some samples is disordered. This is characterized by planar defects, of different widths and periodicities, giving rise to streaking in the diffraction pattern along the [111] direction of the 3C polytype. Polarized light microscopy in transmission easily distinguishes between the cubic (beta) and noncubic (alpha) SiC in the layers and 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 (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 the use of scanning electron microscope (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 (82–94%) 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 less important 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/outer pyrolytic carbon (OPyC) 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 ppm to 18 ppm for some isotopes.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Thin section of a coated particle in plane polarized light

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Figure 2

(a) Enlarged section of a SiC layer in plane polarized light; (b) SiC layer in crossed polarized light. The cubic polytype remains dark when the microscope table is rotated.

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Figure 3

(a) Typical SADP of ordered 3C SiC along the [110] zone axis; (b) the calculated SADP. G1=[110], G2=[001]. The stacking direction is [111].

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Figure 4

(a) Experimental and (b) calculated SADP of the 6H SiC polytype. G1=[001], G2=[110]. The streaking along [111] is due to disorder.

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Figure 5

SADP of (a) a disordered crystallite and (b) an ordered crystallite of the 3C polytype. Streaking occurs along the [111] direction. The position of the objective aperture for the dark field image in Fig. 6 is shown in (a).

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Figure 6

Dark field image from the (002) diffraction spot, marked in Fig. 5. The disorder with variable periodicity is present, indicated by arrows A.

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Figure 7

Transmission electron micrograph showing elongated disordered crystallites

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Figure 18

Distribution of C13 and Ni and Cu in three particles from the same sample. The erratic distribution of the Ni and Cu is an indication of a possible contribution by contaminant particles.

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Figure 8

SADP of a twinned SiC–3C crystal. Some disorder is visible, and both twinning and multiple diffraction spots are present.

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Figure 9

Interpretation of the twinning shown in Fig. 8 with the different domains contributing to the SADP

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Figure 10

Image showing both twinning and stacking disorder; the diffraction pattern also shows the presence in two different directions

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Figure 11

Successive Raman spectra of sample PO-6 taken from the inside of the SiC layer to the outside. Si is the predominant phase on the inside and gradually diminishes outwards.

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Figure 12

Successive Raman spectra of sample PO-5 taken from the inside of the SiC layer to the outside. The main phase is SiC–3C. No Si is present in the SiC layer.

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Figure 13

The Si fraction in the SiC layers of coated particles of samples PO-1–PO-10. Samples PO-6 and PO-8 have excessive amounts of Si in the layer.

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Figure 14

Electron backscatter image of the SiC layer in sample PO-2 (CPT-T-B8). The presence of a discontinuous Si layer with a higher backscatter intensity can be seen. The position of the line scan across the SiC layer, shown in Fig. 1, is also shown.

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Figure 15

Variation of Si, C, and Al counts across a SiC layer shown in Fig. 1; analysis 14 corresponds to the position of the Si layer.

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Figure 16

Coated particles after examination by LA-ICP-MS. The holes drilled by the laser ablation are shown, (particle diameter ∼700 μM).

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Figure 17

Distribution of impurities and major isotopes in the coated particles. The C13 and Si29 distributions show the position of the SiC and outer pyrolytic carbon layers. A contamination layer exists at the particle edge and at the SiC and OPyC interfaces.

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