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Title: Microstructural and phase changes during the electrochemical reduction of tantalum oxide in a CaCl2-CaO molten salt
Author: Barnett, R. P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2009
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An investigation into the electroreduction of tantalum pentoxide in a molten CaCl2-CaO melt was carried out, focusing on the choice of anode, the chemical reaction occurring between the oxide and the melt, and the microstructural and phase changing occurring during both this chemical reaction and the actual electroreduction. Comparisons of graphite and SnO2-based anodes showed that both are capable of being used in the FFC Cambridge process with little change in the product microstructure. Experiments in which no voltage was applied to the system showed Ta2O5 forming a series of calcium tantalum oxides on exposure to the melt, with orientation in the direction of a CaO concentration gradient. CaTa4O11, CaTa2O6 and Ca2Ta2O7 were observed, with the more calcium-rich oxides occurring closer to the surface. A variety of morphologies were observed - 50μm CaTa4O11 hexagonal crystals, 100μm long columns and thinner 20μm needles respectively. Reduction experiments showed Ta2O5 forming a similar series of calcium tantalum oxides, but with Ca3(CaTa2)O9 as the final stage before the formation of tantalum. The reduction follows this series of calcium tantalum oxides of the form (Ta2O5)(CaO)with n = (0, 0.5, 1, 2, 4). The tantalum product contains tiny 0.2 – 0.5μm particles in a spongelike structure, with some dendritic growths and larger 4μm particles near the surface. Final oxygen contents of 6700ppm were achieved, with carbon contaminations in the 1200ppm range.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available