Stabilizing effect of the carbon shell on phase transformation of the nanocrystalline alumina particles

Intensive phase transformations of alumina are known to occur at temperatures above 1000 °C. In the present work, high temperature behaviour of pure Al₂O₃ and the carbon coated Al₂O₃@C sample with core-shell structure was comparatively studied using low-temperature nitrogen adsorption, transmission electron microscopy, powder X-ray diffraction (XRD) analysis and solid-state nuclear magnetic resonance (NMR). The solid-state NMR ²⁷Al method has allowed us to identify and estimate the concentration of all phases appeared during the transformation of pseudoboehmite γ-Al₂O₃ into corundum α-Al₂O₃. The data obtained correlate well with the results of XRD analysis and low-temperature nitrogen adsorption. It is shown that the deposition of carbon coating with formation of core-shell Al₂O₃@C system stabilizes the size of oxide core and prevents the formation of corundum phase until the temperatures of 1350–1400 °C, which are close to the temperature of carbothermal reduction of alumina. The stabilization of the size of the oxide core nanoparticles was considered as a main factor preventing the formation of corundum phase at high temperatures. At the same time, the carbon coating does not affect the phase transformation of γ-Al₂O₃ to δ-Al₂O₃.