The solution processing and stoichiometry of ZrC

Abstract

In the aftermath of the incident at Fukushima Daiichi, evaluation of alternative Accident-Tolerant Fuel (ATF) was initiated. The purpose of ATF systems is to achieve greater safety margins by delaying the initiation of severe core damage that can lead to more managing time to implement mitigation strategies during an accident situation. One of the candidate materials identified for new cladding materials for ATF is zirconium carbide (ZrC). This thesis reviewed the crystal chemistry and phase chemistry of ZrC in an effort to derive the fundamental parameters that influence the synthesis of ZrC. Standard fabrication methods were also reviewed in order to find and apply a method to synthesize and characterize stoichiometric and non-stoichiometric ZrC powders. Solution-based processing was reviewed in order to find a method to synthesize ZrC powders. After a brief overview of some of the properties of zirconium carbide, it was found that the stoichiometry of ZrC greatly influences the parameters as well as the mechanical and thermal properties. The review found that a thorough understanding of the thermodynamic, mechanical, and heat transport properties of ZrC is limited and that a careful and systematic detailed characterization of ZrC, as a function of stoichiometry, with emphasis on resulting microstructures, is required. Various synthesis routes of the formation of quality ZrC that can be used in the nuclear industry were reviewed. The review showed that solution-based synthesis routes hold the greatest advantage regarding control of the stoichiometry. The Pechini method was chosen to synthesize ZrC powders as a solution-based synthesis technique for ZrC. The Pechini method is also a simple and benign method for precursor preparation via the formation of an in situ polymerizable complex, in situ charring and in situ reaction at 1300 °C. The Pechini method was used to prepare stoichiometric and non-stoichiometric ZrC powders. The variation of the stoichiometry of ZrC powders was investigated by varying the carbon content during the formation of ZrC powders. Carbothermal reduction reactions were carried out at various temperatures (1000 °C to 1500 °C) and the resulting powders were characterised by X-ray powder diffraction (XRD) analysis to identify the phases present. According to XRD analysis, the formation of ZrC starts at temperatures around 1200 °C and is substantially completed by 1500 °C. X-ray photoelectron spectroscopy (XPS) analysis showed that the synthesized ZrC powders are in close agreement to ZrC powders available commercially. In order to achieve different stoichiometries of ZrC, the molar ratio of the starting materials, citric acid and zirconium oxychloride was varied, by varying the amount of citric acid added to the zirconium oxychloride during synthesis. The stoichiometries of the synthesized ZrC powders, using the Pechini method and carbothermal reduction, were determined by considering the XRD and Inert Gas Fusion (IGF) analyses results. Stoichiometries varying between ZrC₀.₄₉O₀.₅₁ and ZrC₀.₉₆O₀.₀₄ were derived for the synthesized ZrC powders. The amounts of carbon and oxygen (determined by IGF) in each sample revealed that even samples with an excess amount of initial carbon formed ZrC powders with a stoichiometry of ZrC₀.₉₆O₀.₀₄ Comparing the lattice parameter of the synthesized ZrC powders with respect to the determined stoichiometric carbon content, the best results for ZrC synthesis via the Pechini method were achieved after carbothermal reduction at 1300 °C for 2 hours. The Pechini method was chosen as synthesis method due to no oxygen being reported in the ZrC powders synthesized from this method, but like all of the other solution-based synthesis methods, the Pechini method also forms oxygen containing ZrC powders. Based on these results, a process to remove the oxygen from the ZrC powders was investigated. By adding magnesium powder to the dried Pechini gels before carbothermal reduction, magnesiothermic reduction of the ZrO₂ in the dried gels was performed. XRD analysis of the reaction products of the magnesiothermic reduction of the dried gels of the Pechini method, pyrolysed at 900 °C, results in a powdered mixture of 56.0 wt. % ZrC and 44.0 wt. % MgO. These weight percentages when compared to the theoretical weight percentages calculated for ZrC and MgO (56.15% ZrC and 43.85% MgO) shows a near complete conversion into ZrC and MgO. The observation of MgO in the powdered samples shows that magnesium acts as an “oxygen-getter” during the synthesis of ZrC via magnesiothermic reduction. After comparison of the d-spacing values of the ZrC data file JCPDS 00-035-0784, with the d-spacing values of S1.00-1500 it is concluded that the stoichiometry of the ZrC used to generate the ZrC data file JCPDS 00 035 0784, is closer to the stoichiometry of S1.00-1500, which is ZrC₀.₉₆O₀.₀₄.