Preparation and thermal stability of C-doped zirconia tetragonal particles by the methanol-thermal method

In the solvothermal process of ZrO(NO₃)₂·2H₂O-CO(NH₂)₂-CH₃OH system, methanol can act as both solvent and a reactant. Due to the lack of water, methanol is directly involved in the hydrolysis-condensation reaction of zirconium salt, through the nucleophilic substitution between its methoxy groups and Zr⁴⁺ as well as the coordination as a molecular state, to form inorganic polymers with[ZrO_z(OH)_p(OCH₃)_q·rCH₃OH]_n structure. At the same time, the low solubility of methanol to the polymers strongly inhibits the Ostwald ripening process, thus hindering the crystallization of solvothermal products and also reducing the probability of the thermodynamically supported structural rearrangement. Urea competes with zirconium salt for water in the system and the hydroxyl groups on the skeleton of zirconium species by its hydrolysis reaction, which not only leads to an increase in the amount of Zr-O-Zr bonds in polymers and then a further decrease in the probability of structural rearrangement of the solvothermal products, but also an increase in the content of methoxy group in solvothermal products. When calcined at 400℃, the solvothermal products containing a large amount of methoxy groups transformed into C-doped zirconia. Carbon doping, together with the solvent effect, stabilized the tetragonal phase of zirconia. The tetragonal phase in C-doped zirconia showed comparatively high thermal stability during calcination in air and at the medium temperature range of 500-600℃. Increasing the calcination temperature to 700℃, the free carbon species on the surface of particles was completely removed by oxidation, and the C dissolved in the lattice was also partially removed, resulting in some tetragonal phases lost stability and turned into monoclinic phases.