An electrochemically prepared precursor for the formation of tantalum carbide

Abstract

Metallic tantalum was electrochemically dissolved in an organic electrolyte containing n-propylamine. This resulted in the formation of a viscous fluid. Heating of the solution resulted in the formation of a glassy amorphous solid. Calcination of this precursor in an atmosphere of argon or ammonia resulted in the formation of tantalum carbide, with nitrogen contents in the range 0.6 to 2.5 wt %. Calcination at comparably low temperatures led to products with extremely broadened X-ray diffraction (XRD) patterns. Tantalum carbide has some remarkable properties such as an excellent resistance against oxidation and hydrolysis as well as an extremely high melting point of 3985 °C [1, 2]. Its main current use is as a hard coating on hard metals, such as tungsten carbide, to increase the corrosion and wear resistance [1, 2]. Coating tantalum nitride on various materials, such as carbon fibres, should drastically increase their oxidation resistance. By contrast to titanium, for example, different tantalum-nitride phases with the stoichiometries Ta2N, TaN and Ta3Ns, and different titanium-carbide phases with the stoichiometries Ta2C and TaC are known [3]. TaC and TaN both possess a cubic crystal structure; the solubility of TaN in the TaC lattice is around 45% [4]. Tantalumcarbide and nitride coatings have been solely obtainable, up to now, by physical means [1, 2] or by chemical vapour deposition [5]. This letter describes the formation of an oxygen-free tantalum-carbide precursor, by an electrochemical route, and its calcination behaviour. Electrochemically prepared polymeric precursors have already been used for the formation of aluminium nitride [6-13], titanium nitride and titanium carbide [12-15], and chromium nitride and chromium carbide [16]. This route also enables the formation of coatings [11-14] on various substrates and fibres and it is possibly applicable to all metal nitrides and many carbides relevant in materials science. Metallic tantalum was anodically dissolved in a purely organic electrolyte. This consisted of an organic amine (400 mL n-propylamine), acetonitrile (150 mL) to increase the polarity, and tetrabutylammonium bromide (11 g) as a supporting electrolyte to achieve sufficient ionic conductivity. A doublewalled glass vessel contained the electrodes and the electrolyte. Both the cathodes and the anodes were formed by sheets of metallic tantalum (3 mm thick). The distance between two electrodes was about 2 mm. A condenser, fixed at the top of the vessel,