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Title: Processing and characterisation of ZrCxNy ceramics as a function of stoichiometry via carbothermic reduction-nitridation
Author: Harrison, Robert
ISNI:       0000 0004 5349 7751
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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Carbothermal reduction-nitridation of ZrO2 has been studied in the context of application of non-oxide zirconium ceramics as fuel components in advanced nuclear fuels. Varying processing parameters of nitridation of ZrCx (where 0.7 x 1) powders revealed the rate increased with dwell time, dwell temperature and higher carbon content of the starting ZrCx powders. A novel mechanism is reported whereby nucleation of small ( 500 nm) ZrN containing crystals occurs on the surface of the ZrCx powder particles, growing separate to the carbide particle and resulting in mixed phases. Sintering of the ZrCxNy powders by hot pressing resulted in higher densities than commercially-available ZrC powders suggesting nitrogen content improves the sinterability of ZrC containing ceramics. Thermal and electrical conductivity of the ZrCxNy ceramics were all higher than the ceramics produced from commercially-available ZrC and ZrN powders. Room temperature thermal conductivities of the ZrCxNy ceramics were found to be 35 and 43 Wm-1K-1 for the lowest and highest N-containing ZrCxNy ceramics and increased with temperature to 45 and 55Wm-1K-1 respectively at 2073 K. Electrical conductivities were in the range 250-450 x 104 -1m-1 for the ZrCxNy ceramics (at 298 K) and again increased with increasing nitrogen content. The increase in thermal conductivity of ZrCxNy with nitrogen content is due to the increase in electrical conductivity. Oxidation studies of ZrN revealed oxidation begins at around 773 K with an initial destabilisation of ZrN occurring at around 673 K. A decrease in oxidation rate was observed between lower (973-1073 K) and higher temperatures (1173-1273 K). This is attributed to dense protective oxide scales forming at higher temperature (1173-1273 K) compared to porous oxide scales forming at lower temperature ( 1073 K). However, this protective layer fails at higher temperature (1373 K), attributed to increased oxygen diffusion through the oxide layer.
Supervisor: Lee, William; Grimes, Robin Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available