The requirements of the advanced metal oxide materials in terms of purity, homogeneity, strength, chemical resistance and electrical properties have been brought up and defined through the research of the last decade. The clear relationship between the initial metal oxide powder properties and the properties of the final bulk material has led to a great deal of research on the possible improvement of powder properties such as size, size distribution, homogeneity and chemical composition. Spray pyrolysis (SP) is one of the methods to contribute to the advancement of this improvement.
The major premises of SP are that the precursors are well-mixed solution and the composition in droplet is homogeneous. It is also often reported, and generally accepted, that phase separation in droplet does not occur. So the multi-component particle is made by SP, and many researches in controlling the morphology and composition is conducted under these premises.
In this work, metal oxide particles such as ZnO, CuO, NiO, $Co_3O_4$, $Y_2O_3$, $CeO_2$, and MgO were also prepared from metal acetate, nitrate and mixed salts. Different morphologies were obtained by changing the mixing ratio. As the fraction of acetate salt became larger in precursor solution, the product metal oxide particle was nanometer-sized particle because of the disintegration of primary particle. However, as the fraction of nitrate salt was increased, the product metal oxide particle became spherical and hollow particles with a rough surface in the range of micron or submicron size. Metal oxide particles prepared from mixed salts kept internal homogeneity. But the internal phase segregation occurred in only ZnO particle. This result was explained by the considerable difference of melting temperature and solubility. This phenomenon is examined by TGA and DTA. The morphologies of the MO were observed by SEM and TEM.
In conclusion, the assumption that the homogeneous composition throughout the particle is in principle correct except when solubility and melting point are extremely different.
CuO, NiO $Co_3O_4$, MgO, $Y_2O_3$, $CeO_2$ 입자들은 전구체 염의 녹는점 차이가 ZnO만큼 크지 않고 혼합염의 녹는점이 대부분 acetate 와 nitrate 사이에 존재하므로 그 차이가 더 작아지기 때문에 상분리 현상이 일어나지 않으며, 이것은 분무열분해법으로 입자제조 시 일반적인 현상이다.
결국 혼합염으로 입자를 제조할 때, 염들간의 녹는점 차이가 크면 분무열분해법의 ‘액적 내의 모든 조성은 일정하며, 균일화합물이다’라는 대전제를 벗어나 상분리 현상이 일어나게 되며, 특별히 이상분리를 원하지 않는다면, 녹는점이 유사한 염을 전구체로 선택하는 것이 좋다.