FORMATION OF NBC AND TAC FROM GEL-DERIVED PRECURSORS

Binary hydrogels, in which a niobium or tantalum oxide gel and a pyrol ysable organic compound are combined, were prepared as preceramic mate rials. Carbonaceous gel or saccharose is used as the organic gel const ituent, and alkoxides or peroxo acids are the starting materials for t he transition metal component. Under pyrolysis at 600-700 degrees C, t he gels are transformed into carbide precursors in which a reactive ca rbon and finely dispersed oxide particles are mixed intimately. The su bsequent carbothermal reduction proceeds at lower temperatures es comp ared to physical mixtures of oxides and carbon black. The high-tempera ture treatment was monitored by TG/DTA, X-ray diffraction and nitrogen adsorption. Microporosity is generated in the temperature range 700-8 00 degrees C, and mesoporosity is additionally formed at higher temper atures. The changes in porosity ave controlled by the processes of cry stallization, carbothermal reduction and sintering. The final products at 1400-1500 degrees C are assemblages of fine particles of the face- centred cubic carbides with grain sizes of 1 mu m and less. (C) 1997 E lsevier Science Limited.