On Carburization of Tantalum Oxide

Abstract

Carburization into TaC of mixtures of Ta₂O₅ and graphite was carried out and the influence of the carburizing condition on property of the products was investigated in some detail. A variety of Ta₂O₅ powders, differing with each other in crystallinity and the amout of major chemical impurities (such as Nb₂O₅ and a lower oxide of Ta), were used as starting materials to be carburized.
Carburization was carried out in either vacuum or open high-frequency-furnace. In the latter case, prevention of oxidation of the products was necessary, since towards the end of carburizing reaction generation of CO gas decreases. Consequently, any of the following precautions were taken when the generation of CO gas tends to decrease ; (a) to shut exhaust pipe of the graphite reaction vessel, and (b) to introduce H₂ gas into the vessel.
The following results were obtained:
(1) In open high-frequency-furnace, carburization reaction into TaC of mixtures of β-Ta₂O₅ and graphite began at about 1000°C and proceeded slowly with temperature. On the other hand, in the case of mixtures of amorphous Ta₂O₅ and graphite, the reaction suddenly started at about 1300°C. In both cases, however, combined carbon content of the products became almost constant at 1400°C and with elevation in temperature it approached to the stoichio-metric value (TaC_1.00).
(2) When graphite of nearly stoichiometric amount required for formation of TaC was blended with Ta₂O₅ and heated at 1500°C, the resulting carbide was invariably non-stoichiometric in chemical composition (TaC_1-x, x>0). It was found that formation of stoichiometric TaC required elevation in carburization temperature above 1800°C. Furthermore, in the temperature range involved in the carburizing experiments, no evidence was obtained for the presence of Ta ₂C phase in the products.
(3) When 60% Graphite of stoichiometric amount required for formation of TaC was blended with Ta₂O₅ and heated at 1800°C in vacuum furnace, α-Ta₂O₅ and Ta ₂C were obtained. As 90% Graphite was blended, and heated at the same condition, TaC, Ta ₂C and an unknown product (which was neither TaC, Ta ₂C, α-Ta ₂O5, β-Ta₂O₅, Ta, nor Graphite) were obtained. In open furnace, however, when 90% Graphite was blended, and heated at 1800°C, Ta ₂C was never present and mixture of two kinds of TaC with different lattice constant were obtained.
(4) Crystallinity of starting Ta ₂C ₅ was found to govern particle size of the resulting TaC ; amorphous Ta ₂O25 led to finer particles in the products in comparison with the case of crystalline β-Ta₂O₅.
(5) Lowering of the amount of graphite in the initial mixtures below stoichiometric .amount required for the formation of TaC invariably led to coarsening of particles. of the products.
(6) Particle size of TaC products obtained by open furnace, carburization was found to be considerably greater than that of the products obtained by vacuum furnace carburization.
(7) Morphological feature of the products was not influenced by crystallinity of starting Ta₂O₅. It was found, however, that β-Ta₂O₅ containing a lower oxide of Ta led to aggregates of very fine and irregular-shaped particles in the products and coarsening of particles did not occur even at high temperatures.