CARBOTHERMIC REDUCTION OF TITANIUM-DIOXIDE BY HIGH-TEMPERATURE X-RAY-DIFFRACTION WITH CONTROLLED PRESSURE

Polycrystalline compressed mixtures of titanium dioxide (rutile structure) and graphite progressively undergo reaction with carbon monoxide under controlled pressure (10(5) greater-than-or-equal-to P(CO) (Pa) greater-than-or-equal-to 10(-1)) in a graphite resistance high temperature X-ray diffractometer at around 1500 K. Reduction of TiO2 by CO2 via a carbon transfer through the gas phase (CO/CO2 couple), is shown to be predominant compared with direct reduction by graphite. A complex mechanism has been found that accounts especially for the formation of the monoclinic beta-Ti3O5 oxide and of the cubic TiC1-xOx monocarboxide obviously out of equilibrium, which is in contrast to the existence of a monovariant field (beta-Ti3O5, TiC1-xOx, C, CO) asserted by others. The lattice parameters of these metastable phases beta-Ti3O5 and TiC1-xOx have been determined between 293 K and 1883 K. An estimation of the composition of the oxycarbide has been made as a function of the temperature, taking into account the observed variations in the lattice parameter at 293 K and the available literature data. It has been emphasized that the quenched samples display a very significant heterogeneity, principally because of the alpha- and gamma-Ti3O5 phases.