Comparative microscale investigations of the carbothermal synthesis of (Ti, Zr, Si) carbides with oxide intermediates of different volatilities

Abstract

The carbothermal reduction process of TiO 2, SiO 2, ZrO 2, SiO 2/ZrO 2, and ZrSiO 4 was investigated using three different types of carbon: carbon black, active carbon, and graphite. The progress of each reaction was investigated by a new thermoanalytical method consisting of dynamic and isothermal heating procedures accompanied by quantitative in situ gas detection (CO). The solid products of 11 starting mixtures of oxide/carbon variations were characterised ex situ by chemical, phase, and morphological analyses. The results for the reduction of TiO 2 and SiO 2 are in agreement with known reaction mechanisms. Contrary to earlier assumptions, the carbothermal reduction of ZrO 2 represents neither of the two boundary mechanisms. A comparative discussion of the crystallographic structures of the oxides and the carbides supports these experimental findings. ZrSiO 4 decomposes to ZrO 2 and SiO 2 and reacts as a mixture of them. The type of carbon used influences the reaction rate (in the case where no volatile oxide intermediates exist, i.e., for TiO 2) or the morphology of the reaction products (in the case where volatile oxide intermediates exist, i.e., for SiO 2).