C掺杂ZrO_2四方纯相粉体的甲醇溶剂热制备及热稳定性研究

在ZrO(NO_3)_2·2H_2O-CO(NH_2)_2-CH_3OH溶剂热过程中,水的缺乏使得甲醇通过其甲氧基与Zr~(4+)发生亲核取代或以分子配位,直接参与锆盐的水解-缩聚反应,形成具有ZrO_z(OH)_p(OCH_3)_q·rCH_3OH]_n结构的无机聚合物;同时,甲醇对聚合物低的溶解能力强烈抑制了Ostwald熟化过程,阻碍了溶剂热产物的晶化与热力学支持的结构重排。尿素通过其水解作用与锆盐竞争体系中的水及锆物种骨架上的羟基,这不仅导致无机聚合物中Zr-O-Zr键合相对Zr-OH键合的比例增加,使得溶剂热产物发生结构重排的几率进一步下降;而且也一定程度上增加了溶剂热产物中甲氧基的含量。含有大量甲氧基团的溶剂热产物经400℃焙烧后,形成C掺杂ZrO_2。C掺杂与溶剂效应共同稳定了ZrO_2的四方相。在500-600℃中等温度、空气气氛焙烧过程中,C掺杂ZrO_2四方相结构显示了良好的热稳定性;提高焙烧温度至700℃,游离于颗粒表面的C被完全氧化去除,固溶于晶格中的C也部分脱溶,导致了部分四方相失稳转变成单斜相。

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 withZrO_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.