Synthesis of Nanocrystalline Oxide Ceramic Materials under Carbon Nanoreactor Conditions
Carbon-coated nanocrystalline oxides TiO2@C, Al2O3@C and calcium aluminate C12A7@C were synthesized, and the effect of carbon coating on their thermal stability and reactivity was studied. It was found that the carbon coating prevents sintering of the titania nanoparticles and contributes to stabilization of their size. Thus, the particle size of anatase can be stabilized at level of 10–12 nm even after heat treatment at 750 °C. Deposition of the carbon coating on -Al2O3 prevents growth of the alumina nanoparticles and substantially improves their thermal stability. Complete Al2O3 conversion to corundum occurs after heat treatment at 1250 °C, whereas practically no corundum is formed under similar conditions for Al2O3@C. Dispersed calcium aluminate C12A7 particles resistant to sintering at high-temperature were synthesized inside the carbon shell. Meanwhile, intense sintering at high temperatures was observed for samples without the carbon coating. The carbon shell prevents sintering making it possible to synthesize samples in a disperse state even at 1450 °C. According to the TEM data, characteristic sizes of C12A7 particles inside the carbon coating range from 100 nm to several microns. So, the core-shell approach can be used to stabilize the size of the oxide core nanoparticles up to temperatures when their reaction with the coating material occurs. In the case of the carbon shell, this is the temperature when the carbothermal reduction of the oxide core takes place.