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Electron microscopy characterisation of polycrystalline silicon carbide

  • Authors: Ndzane, Nolufefe Muriel
  • Date: 2014
  • Subjects: Electron microscopy , Silicon carbide
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10553 , http://hdl.handle.net/10948/d1020634
  • Description: This dissertation focuses on an electron microscopy investigation of the microstructure of SiC layers in TRISO coated particles deposited by chemical vapour deposition under different experimental conditions, which include temperature, concentration of gases and deposition time. The polycrystalline β-SiC was deposited from the decomposition of methyl trichlorosilane MTS in the presence of hydrogen (H2) as carrier gas. Scanning electron microscopy (SEM), using the backscattered electron (BSE) mode, was used to image the microstructure of and defects in the SiC layers of TRISO particles. Electron backscatter diffraction (EBSD) in the SEM was used to determine the SiC grain sizes and distribution thereof in TRISO particles deposited under different conditions. For samples with a poor EBSD indexing rate, transmission Kikuchi diffraction and transmission electron microscopy (TEM) investigations were also carried out. From the results, the effects of growth temperature on the SiC microstructure, specifically on the grain size and shape and the porosity were determined. The effects of cooling or non-cooling of the gas inlet nozzle on the SiC microstructure were also investigated. TEM and scanning TEM (STEM) analyses of the SiC layers in TRISO particles were performed to image the defects and reveal the crystallinity of SiC layers. The microstructure and composition of SiC tubes fabricated by reaction bonding (RB) was also investigated by using electron microscopy and Raman spectroscopy. SEM-BSE imaging of RBSiC samples allowed the identification of impurities and free silicon in the RBSiC. Finally, the penetration of the metallic fission product, palladium, in reaction bonded SiC at a temperature of a 1000ºC is determined. A brief comment on the suitability of RBSiC as candidate for fuel cladding in a PWR is made. A short discussion of the suitability of the characterisation techniques used is included at the end.
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  • Date Issued: 2014

Ruthenium and palladium assisted silver transport in silicon carbide

Characterization of hydrogen and helium implanted silicon carbide

  • Authors: O'Connell, Jacques Herman
  • Date: 2009
  • Subjects: Silicon carbide , Hydrogen , Helium
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10539 , http://hdl.handle.net/10948/979 , Silicon carbide , Hydrogen , Helium
  • Description: This dissertation focuses on the characterization of hydrogen implanted 3C-SiC and helium implanted 6H-SiC and the interaction of silver and palladium with the SiC. The main technique used for the analysis of the implanted SiC was transmission electron microscopy (TEM), although scanning electron microscopy (SEM) and nanoindentation hardness testing were also used. Both H and He implantations were done at an ion energy of 100 keV and the total dose for both species was 1017 ions/cm2. Specimens were annealed at 1000 °C for 20 min in an inert atmosphere. The observed depth of ion damage agreed closely with that predicted by TRIM 2008. The damaged region in the He implanted specimens had a high density of small bubbles but no cracks were observed. Severe cracking was observed along the damaged region in the H implanted specimens. A second phase hexagonal (4H-SiC) was detected in two grains in the 3C-SiC and some grown in voids were also observed. The wettability of SiC by a 2:1 by volume mixture of Ag and Pd is extremely low but is significantly increased through the addition of Si to the mixture. The Si containing metal mixture was found to migrate along the grain boundaries of polycrystalline 3CSiC while sealing the point of entry. Pd attacks SiC and severe etching was observed on the surface of 6H-SiC. An epitaxially orientated Pd2Si inclusion was observed in the 6H-SiC. No effect of implantation damage on the interaction of Pd and Ag with SiC was observed. Pd etched the surface of the He implanted 6H-SiC down through the damaged region with no evidence of implantation damage left after 67 hours of annealing and thus no conclusions could be drawn. The metal mixture interacted with the non implanted surface of the H implanted 3C-SiC after dissolving the Si substrate and therefore the effect of the ion damage on the interaction could not be investigated. Nanoindentation hardness measurements showed a marked increase in hardness of He implanted 6H-SiC annealed for 20 min at 1000 °C over that of as implanted and virgin material. There was also a large decrease in hardness corresponding to the depth of the ion damage.
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  • Date Issued: 2009

Analysis of the extended defects in 3C-SiC

  • Authors: Olivier, Ezra Jacobus
  • Date: 2008
  • Subjects: Crystals -- Defects , Crystallography , Silicon carbide
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:10525 , http://hdl.handle.net/10948/730 , Crystals -- Defects , Crystallography , Silicon carbide
  • Description: The dissertation focuses on the analysis of the extended defects present in as-grown and proton bombarded β-SiC (annealed and unannealed) grown by chemical vapour deposition (CVD) on (001) Si. The proton irradiation was done to a dose of 2.8 × 1016 protons/cm2 and the annealing took place at 1300°C and 1600°C for 1hr. The main techniques used for the analysis were transmission electron microscopy (TEM) and high resolution TEM (HRTEM). From the diffraction study of the material the phase of the SiC was confirmed to be the cubic beta phase with the zinc-blende structure. The main defects found in the β- SiC were stacking faults (SFs) with their associated partial dislocations and microtwins. The SFs were uniformly distributed throughout the foil. The SFs were identified as having a fault vector of the type 1/3 <111> with bonding partial dislocations of the type 1/6 <121> by using image simulation. The SFs were also found to be predominantly extrinsic in nature by using HRTEM analysis of SFs viewed edge-on. Also both bright and dar-field images of SFs on inclined planes exhibited symmetrical and complementary fringe contrast images. This is a result of the anomalous absorption ratio of SiC lying between that of Si and diamond. The analysis of the annealed and unannealed irradiated β-SiC yielded no evidence of radiation damage or change in the crystal structure of the β-SiC. This confirmed that β-SiC is a radiation resistant material. The critical proton dose for the creation of small dislocation loops seems to be higher than for other compound semiconductors with the zinc-blende structure.
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  • Date Issued: 2008
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