Thermophysical properties and thermodynamic stability of zirconium carbide as a function of non-stoichiometry
Researcher: Dr Heather Jackson
Supervisor: Professor Bill Lee
Funding: Overseas Research Scholarship Award
Winner of the Tony Evans Prize for best PhD in Ceramics at Imperial 2011
Effects of non-stoichiometric composition in zirconium carbide (ZrCx) ceramics on crystallographic, microstructural, and thermophysical properties relevant to nuclear fuel applications were investigated through complementary experimental and computational studies. ZrCx compositions (32.1-50.3 at% C) were synthesised by carbothermal reduction of ZrO2 and rapidly densified by spark plasma sintering (SPS). A novel thermal analysis technique employed pulsed laser heating and optical pyrometry to generate time-temperature thermogram data suitable for the identification of latent heat exchanges indicative of extremely high temperature (> 3000 K) liquidus, solidus, and eutectic transitions in the Zr-C system. Liquidus temperatures were measured for hypoeutectic ZrC+C compositions, an area of the phase diagram lacking in accurate measurements. Discontinuous changes in temperature indicated that emissivity of ZrC decreases upon the transition from solid to liquid; based on an estimated solid emissivity of 0.6, liquid emissivity decreased to 0.46-0.58. Dendritic microstructures confirmed that ZrC existed in the liquid phase, and diffraction identified the recrystallised material as ZrC. Microstructures nearest the melted surface (100-700 μm) were homogeneous and consistent with the equilibrium phase diagram. Material at greater depth was heat-affected but contained metastable phases due to solute segregation. The shape and temperatures of phase transitions were largely insensitive to laser pulse timescale and repeated melting, supporting the attainment of local thermodynamic equilibrium.
The ground state carbon-deficient ZrCx structure was simulated from first principles using the CASTEP code. Carbon vacancies induced local atomic displacements and global lattice contraction. Bulk modulus decreased and formation enthalpy became less negative with increasing vacancy concentration. Ab initio calculations were combined with a statistical mechanical representation of non-stoichiometry in ZrCx to characterise the energetic favourability of various spatial distributions of vacancies. A preference was demonstrated for non-random distribution of carbon atoms and vacancies on the carbon sublattice, characterised by vacancies taking mutual third nearest neighbour positions along <211> directions, avoiding first and second nearest neighbour positions, but tolerating first nearest neighbours at sufficiently high vacancy concentration.