| Tantalum metal has a wide range of applications in electronics, metallurgical, mechanical, chemical and aerospace industries due to its remarkable properties. Especially in electronics, tantalum capacitors have attracted a considerable attention because of its large specific capacitance, small size and high reliability. Currently, the main commercial tantalum powder is being produced by the sodiothermic reduction of K2TaF7and carbothermal reduction of Ta2O5, which are regarded to be energy intensive, low efficiency, complicated, and environment unfriendly. Electrochemical reduction of solid Ta2O5to tantalum powder in molten salts promises a novel, fast, low carbon and non-fluorine industrial development for the production of tantalum because of a number of metris:low energy consumption, convenient operation and environment benefit. However, there also remain some challenges, such as further understanding of the mechanism and establishment of the overall mass transfer dynamics for oxygen removal from the cathode, and also in decreasing the particle size and improving the purity of the product. In this thesis, the mechanism of direct electro-reduction of solid Ta2O5in molten salt has been investigated using by metallic cavity electrode from the microscopic point of view, and established a theoretical model of the overall mass transfer dynamics for oxygen removal from the cathode. Ultrafine (nanoscale) capacitor tantalum powders were also prepared in different molten salts.The main topics and results of the research are summarized as follows:1. Cyclic voltammetry and chronoamperometry of Ta2O5powder in molten CaCl2at850℃were investigated by using the metallic cavity electrode (MCE). The (CaO)xTa2O5complexes were identified by XRD, transmission electron microscopy (TEM) and electron diffraction pattern. The influence of electrolysis time and potential on the sintering of Ta powder and the mechanism of direct electro-reduction of solid Ta2O5in molten salt has been investigated from the microscopic point of view. Experimental results indicated that the redcution process includes the following steps:Ta2O5â†'Calcium tantalum oxygen compositeâ†'Ta. The morphology of Ta powders of direct electro-reduction of solid Ta2O5in molten salt is consistent with the raw materials in macroscopic view, but from the microscopic view, the Ta powders were porous and similar nodular shapes, which were made up of Ta powder with about a few tens of nanometers of the particles size. The more negative potential, the longer electrolysis time, and the lower oxygen content in Ta particles, Ta powder would be easier to sinter, resulting in the particle size increases.2. The rate-determining mechanism of the solid cathode process has been fundamentally investigated. A theoretical model correlating the precursor porosity, P (in volume percentage), the metal-to-oxide molar volume ratio, R, and the cathode volume shrinkage S (in volume fraction, experimentally determined)(the PRS model) with the deoxidation speed of the solidmetal oxide cathode inmolten chlorides has been developed, allowing accurate prediction of the optimal cathode porosity by a simple equation, Popt=3R+S-1/3R×100%. Tests of this equation in practices suggest Popt to be an intrinsic parameter to the oxide. For example, the predicted values of Popt for the electrolysis of TiO2and Ta2O5cathodes are about67%and25%respectively, and a cathode porosity far away from Popt would suppress the deoxidation seriously. The established model has been well verified by the electrolysis of the oxides of Si, Ti and Ta.3. In this study, an ultrafine Ta powder (primary particle size,～70nm) with low oxygen contents (～7000ppm) was prepared by electro-reduction in eutectic CaCl2-NaCl molten salt at700℃. It has a large BET surface area (2.9499m2·g-1). Electro-reduction of Ta2O5precursors with different particle sizes in a eutectic CaCl2-NaCl molten salt at700℃has been studied by potentiostatic electrolysis, constant voltage electrolysis together with CV, XRD, SEM, EDX and elemental analyses. The results reveal that the initial step in the electro-reduction process undergoes through several stable calcium-containing compounds which were likely CaTa4O11, Ca2Ta2O7and Ca4Ta2O9. These were then reduced to metallic Ta. At voltage higher than2.2V, ultrafine tantalum powders can be prepared by direct electrolysis of thin and porous pellets of Ta2O5powder. The same particle size of the tantalum powder could obtain by electro-reduction of two different particle size of the oxide precursor shows that no direct relation with particle size between the oxide precursor and the product.4. Ta powder with uniform particle sizes and similar nodular shapes was successfully prepared in the MgCl2-KCl-NaCl eutectic salt at700℃by mixing with a conductive additive and changing the porosity of the oxide pellet. Electrochemical reduction of solid Ta2O5to Ta metal in molten LiCl at700℃has been studied. Experimental results indicated that the redcution process includes the following steps:Ta2O5â†'LiTaO3â†'Ta. |
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