A theoretical study of the stability of Zr-Al-C and Ti-Al-C MAX phases

MAX phases have garnered considerable research attention due to their unusual combination of metallic and ceramic properties that make them desirable materials especially in applications requiring extreme operating conditions. Zr-Al-C MAX phases specifically, are of particular interest in the nuclear industry where their low neutron absorption make them compelling candidates for fuel cladding materials. The synthesis of Zr-Al-C MAX phases, however, has been challenging, with the presence of impurities suggested as necessary to stabilise them [1, 2, 3] and secondary phases considered unavoidable in the reported successful synthesis of Zr$_2$AlC [4] and Zr$_3$AlC$_2$ [5]. This has led to questions as to whether the composition of MAX phases in this system is likely to change when in service. Addressing these uncertainties has been the main objective of this thesis, making the theoretical study of the thermodynamic stability of Zr$_{n+1}$AlC$_n$ of central importance. The stability of the Zr$_{n+1}$AlC$_n$ and closely related Ti$_{n+1}$AlC$_n$ MAX phases in the context of the M-A-X ternary phase diagrams and competing binary and ternary compounds, as a function of temperature, was calculated by applying density functional theory (DFT) within the quasiharmonic approximation. We found that the Zr-based MAX phases are thermodynamically unstable at room temperature, although Zr$_3$AlC$_2$ becomes stable above 500 K. Ti-based MAX phases on the other hand, show higher thermodynamic stability, with Ti$_2$AlC in particular, having the lowest formation energy of the MAX phases on the Ti-Al-C convex hull and appearing stable at all temperatures, in agreement with its reported success in synthesis. In the course of this work we also attempted to identify trends and similarities in predicted structural, elastic, thermophysical and electronic properties as well as the chemical bonding within the MAX phases in the two systems. Chemical bonding differences between the two systems, though, were not found to explain their differences in stability. Based on phonon calculations, Raman-active mode frequencies of Zr-based MAX phases and their most competing phases were also predicted, to assist in identifying phases present in a Zr$_3$AlC$_2$ synthesised sample [6]. Our predicted Zr$_3$AlC$_2$ frequencies of Raman-active modes were within 2% of peaks in the experimental Raman spectra recently measured by Lyons [6].Open Acces

Carbometalates: Intermediate phases in the ternary systems RE-T-C (RE = Y, La, Gd-Er; T = Cr, Fe, Ru)

The main motivation of this work was the preparation and characterization of novel compounds in the ternary systems RE–T–C with T = Cr, Fe and Ru with a special focus on compounds containing C2n- and C3m- or mixed C and C2n- as structural units. This would allow to investigate the applicability of the concept of complex anions to this class of materials

ATOMIC SCALE SIMULATION OF ACCIDENT TOLERANT FUEL MATERIALS FOR FUTURE NUCLEAR REACTORS

The 2011 accident at the Fukushima-Daiichi power station following the earthquake and tsunami in Japan put renewed emphasis on increasing the accident tolerance of nuclear fuels. Although the main concern in this incident was the loss of coolant and the Zr cladding reacting with water to form hydrogen, the fuel element is an integral part of any accident tolerant fuel (ATF) concept. Therefore, to license a new commercial nuclear fuel, the prediction of fuel behavior during operation becomes a necessity. This requires knowledge of its properties as a function of temperature, pressure, initial fuel microstructure and irradiation history, or more precisely the changes in microstructure due to irradiation and/or oxidation. Amongst other nuclear fuels, uranium diboride (UB2) and uranium silicide (U3Si2) are considered as potential fuels for the next generation of nuclear reactors due to their high uranium density and high thermal conductivity compared to uranium dioxide (UO2). However, the thermophysical properties and behavior of these fuels under extreme conditions are not well known, neither are they readily available in the literature. Therefore, in this thesis, density functional theory (DFT) and classical molecular dynamic (MD) simulations were used to investigate the thermophysical properties, radiation tolerance and oxidation behavior of UB2 and U3Si2 as potential fuels or burnable absorbers for the next generation of nuclear reactors. UB2 was studied in order to understand its thermophysical properties as a function of temperature. The phonon-assisted thermal conductivity (kph) exhibits large directional anisotropy with larger thermal conductivity parallel to the crystal direction. This has implications for the even dissipation of heat. The increase in thermal conductivity with temperature is justified by the electronic contribution to the thermal transport, especially at high temperatures. This shows that UB2 is a potential ATF candidate. In terms of radiation tolerance, Zr is more soluble in UB2 than Xe, while uranium vacancy is the most stable solution site. Furthermore, as the concentration of Zr fission product (FP) increases, there is a contraction in the volume of UB2, while an increase in Xe results in swelling of the fuel matrix. In terms of diffusion, the presence of an FP in the neighboring U site increases the migration of U in UB2, making U migrate more readily than B as observed in the ideal system. The thermophysical properties of U3Si2 as a possible ATF were studied and discussed considering the neutronic penalty of using a SiC cladding in a reactor. The calculated molar heat capacity and experimental data are in reasonable agreement. Due to the anisotropy in lattice expansion, a directional dependence in the linear thermal expansion coefficient was noticed, which has also been experimentally observed. The thermal conductivity of U3Si2 increases with temperature due to the electronic contribution while the phonon contribution decreases with increasing temperature. A comparison of the thermal conductivity in two different crystallographic directions sheds light on the spatial anisotropy in U3Si2 fuel material. The inherent anisotropic thermophysical properties can be used to parametrize phase field models by incorporating anisotropic thermal conductivity and thermal expansion. This allows for a more accurate description of microstructural changes under variable temperature and irradiation conditions. Due to the metallic nature of U3Si2, the oxidation mechanism is of special interest and has to be investigated. Oxidation in O2 and H2O was investigated using experimental and theoretical methods. The presence of oxide signatures was established from X-ray diffraction (XRD) and Raman spectroscopy after oxidation of the solid U3Si2 sample in oxygen. Surface oxidation of U3Si2 can be linked to the significant charge transfer from surface uranium ions to water and/or oxygen molecules. Detailed charge transfer and bond length analysis revealed the preferential formation of mixed oxides of U-O and Si-O on the U3Si2 (001) surface as well as UO2 alone on the U3Si2 (110) and (111) surfaces. Formation of elongated O−O bonds (peroxo) confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Experimental analysis by Raman spectroscopy and XRD of the oxidized U3Si2 samples has revealed the formation of higher uranium oxides such as UO3 and U3O8. Overall, this work serves as a step towards understanding the complex anisotropic behavior of the thermophysical properties of metallic UB2 and U3Si2 considered as potential accident tolerant nuclear fuel. The calculated anisotropy of thermophysical properties can be used to parametrize phase field model and to incorporate in it anisotropic thermal conductivity and thermal expansion