Influence of the grain size on the reactivity of TiO2/C mixtures

Mixtures of TiO2 and C of different grain sizes do not show the same reacti vity during the synthesis of titanium carbide TiC. The finest powders (empt y set approximate to 0.2 mu m) lead to the serial reaction TiO2 -->Ti3O5 -- > Ti2OC --> TiC, while the coarsest powder leads to the intermediate format ion of Ti2O3. For temperatures between 1503 and 1873 K, the diffusional oxy gen loss leading to the suboxides is very rapid. It is followed by a comple x carburization forming Ti2OC via a rate Limiting process governed by the c onsumption of carbon via C + CO2 --> 2CO. CO2 comes from an equilibrium ass ociated with the suboxide grains, such as 2Ti(3)O(5) + 13CO = 3Ti(2)OC + 10 CO(2), this explains the coupling observed between temperature and partial pressure of carbon monoxide. The last stage of the reaction, associated wit h the diffusion of carbon inside the grains of the oxycarbide, is slowed do wn probably because of the inhomogeneity of the mixtures at the end of the reaction. (C) 2000 Elsevier Science S.A. All rights reserved.