Sub-micron or nano-sized spherical particle with the uniform particle diameter is applied relatively widespread. The most common is being the calibration standard of equipment. If the particle shows an irregular morphology and the assembled powder has a worse flowability, the powder is not favored for the uses of packing material, for instance, dry pressing material. Then the powder belongs to low-leveled products for engineering applications. In optics, sub-micron or nano-sized spherical particles can be applied in manufacturing photonic crystal having periodic structure. The particles which have a uniform size and spherical morphology can also be provided for sintering to get dense components.
The most common sub-micron or nano-sized spherical particle with the uniform particle size is silicon dioxide (SiO2) reproduced by Stöber method (1968). At present, this technology is the most mature method for the production of high quality SiO2 particles, and has the advantages of easily-controlling particle size and distribution. In addition to SiO2, the sub-micron or nano-sized spherical particle with the uniform particle size is yttrium oxide (Y2O3), cerium dioxide (CeO2), titanium dioxide (TiO2) and zinc oxide (ZnO), etc. However, at present, there is no any spherical indium oxide-based particles available and the producing method thereof.
Indium oxide (In2O3) has a crystal structure of bixbyite, and the crystalline structure is cubic system. The coordination number of indium ion is 6, and indium ions in the structure are situated in the octahedral sites. The crystal lattice constant of In2O3 is 1.0117 nanometer (nm), and the space group thereof is Ia3. A unit cell includes 14 octahedral structures of InO6, wherein the coordination number of oxygen is 4, and the density of In2O3 with high purity is 7.075 g/cm3.
In2O3 is a kind of n-type semiconductor oxide, and the bandgap is 3.6 eV. At present, tin (Sn) is the most common additive of In2O3 because the size of tin ion is extremely close to that of indium ion. However, the valence number of tin ion is more than that of indium ion. A donor-type defect of In2O3 is formed, and the electrical conductivity of indium oxide can be increased enormously. While a trace amount of tin is added in In2O3, indium tin oxide (ITO) is formed. ITO is an excellent electrical conductive oxide, and has been applied in many photoelectric elements, such as transparent electrical conductive film.
The most common producing method of the indium oxide-based particle is the co-precipitation method and the hydrothermal method.
In the co-precipitation method, the precipitant in two or more cation solutions is reacted and oversaturated for obtaining uniform-ingredient precipitate. The precipitate can be further synthesized as the metallic compound in solution having various ingredients. The advantages of the co-precipitation method lies in that the particle size and the particle morphology can be controlled. If the particle is formed in nano-size, the specific surface area of the nano-sized particle is high and becomes very high activity. In addition, the co-precipitation method has the advantages of uniform particle size, ease process at room temperature, and low production cost, etc. However, the drawbacks of the co-precipitation method lies in that the precipitate if in poor control usually forms as gelation or agglomeration. This agglomerated precipitate is needed to disperse or to wash for many times, so as to remove the residues (e.g. the cationic or anionic impurity).
The hydrothermal method assists the growth of the crystalline phase of the In2O3-based particle by utilizing the function of the aqueous solution under controlled temperature and pressure. The advantage of the hydrothermal method lies in that the particles with specified particle size and specified crystal formation are formed in the hydrothermal environment which has the operation temperatures lower than the crystallization temperature of oxide precipitates. This is greatly reducing the drawback of agglomeration formed by high temperature calcination and sintering. However, the batch operation in high-pressured reactor is essential in the hydrothermal method. The cost increases mostly due to the disadvantage of discontinuous production.
It is therefore attempted by the applicant to deal with the above situation encountered in the prior art.