Formation of NbC and TaC from gel-derived precursors

Abstract

Binary hydrogels, in which a niobium or tantalum oxide gel and a pyrolysable organic compound are combined, were prepared as preceramic materials. Carbonaceous gel or saccharose is used as the organic gel constituent, and alkoxides or peroxo acids are the starting materials for the transition metal component. Under pyrolysis at 600–700 °C, the gels are transformed into carbide precursors in which a reactive carbon and finely dispersed oxide particles are mixed intimately. The subsequent carbothermal reduction proceeds at lower temperatures compared to physical mixtures of oxides and carbon black. The high-temperature treatment was monitored by TG/DTA, X-ray diffraction and nitrogen adsorption. Microporosity is generated in the temperature range 700–800 °C, and mesoporosity is additionally formed at higher temperatures. The changes in porosity are controlled by the processes of crystallization, carbothermal reduction and sintering. The final products at 1400–1500 °C are assemblages of fine particles of the face-centred cubic carbides with grain sizes of 1 μm and less.