PORE FORMATION DURING THE PYROLYSIS OF C-NB2O5 AND C-TA2O5 XEROGELS

Binary organic-inorganic gels have bean prepared by mixing a carbonace ous hydrosol and a Nb2O5 or Ta2O5 sol derived by hydrolysis of the alk oxides. The gels are pyrolyzed under an inert atmosphere into precurso rs in which carbon and the metal oxides are mixed very intimately. Hig h temperature treatment converts the precursors into the cubic face ce ntered carbides. The precursors as well as the carbides have been show n to be micro- and mesoporous materials. Measurements of nitrogen adso rption reveal a characteristic change of the shapes of the isotherms ( Type I --> Type IV) and of the hysteresis loops (H4--> H2--> H1) durin g the thermal processes. Pore widening has been observed with rising t emperature. The phenomena of crystallization, carbothermal reduction a nd sintering were found to control the pore shape and size. The result s of the adsorption measurement correlate well with those of the therm oanalytical and X-ray diffraction studies.