Synthesis of nanoscale particles of Ta and Nb 3 Al by homogeneous reduction in liquid ammonia
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The analysis of metallothermic reduction as an electronically mediated reaction predicted that the particle size of solid product could be reduced if the reaction were conducted in a medium that is a mixed conductor (ionic and electronic). This prediction was confirmed by reacting TaCl5 with sodium, each dissolved in liquid ammonia, to produce tantalum powder having an average particle size over an order of magnitude finer than the micron-sized powders produced commercially today. Metallothermic reduction in a mixed conducting medium has been extended to a multicomponent system in order to synthesize nanosized powder of Nb3Al by co-reduction of NbCl5 and AlCl3 both dissolved in liquid ammonia.
I. INTRODUCTION
Reduction in particle size from the current value of around 0.5 m to something nearer 10 nm would significantly improve the performance of tantalum capacitors. Producing nanoparticles is beyond the capabilities of the current technology that produces tantalum powder by metallothermic reduction in a molten salt medium. At approximately 800 °C, potassium heptafluorotantalate (K2TaF7) and sodium are both added to a halide melt (known as a “diluent”) where they react to produce solid tantalum in the form of powder according to K2TaF7(l) + 5 Na(l) = Ta(s) + 5 NaF(l) + 2 KF(l) ,
(1)
where the underline denotes that the species is dissolved in the diluent. Such a process does not present the conditions necessary to sustain the production of unagglomerated, nanoscale powder. Processing at such a high temperature results in an undesirably high rate of particle growth. The fact that sodium is immiscible in the molten salt diluent promotes heterogeneous nucleation of tantalum on metallic reactor components (wall, stirrer) with attendant agglomeration. In contrast to the commonly held view that metallothermic reduction is strictly a chemical reaction and that the process is rate limited by mass transfer, previous work in this laboratory had shown that metallothermic reduction is an electronically mediated reaction, i.e., the kinetic pathway necessarily includes an electron transfer step that under certain circumstances is rate limiting.1 2544
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 13 Mar 2015
The 2 × 2 matrix in Table I presents a taxonomy of chemical reactions involving precipitation. At first glance all the examples might appear to be simple metatheses or exchange reactions. There are important differences, however. The rows of Table I sort reactions by whether or not they involve electron transfer: Reactions in the first row do, while those in the second row do not. Hence, firstrow reactions proceed necessarily by the transport of both ions and electrons. In some cases, electron transport is fast and the overall rate of reaction is controlled by ion transport. Under these circumstances the reaction kinetics are indistinguishable from those of the conjugate second-row reaction. In other cases involving electron transfer, ion transport is fast, and electron transport is
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