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 × 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.