Preparation Of Super-fine Magnesium Gallate By Combustion Method

At present, there are many methods for preparation of super-fine powders, such as high temperature solid state synthesis, chemical precipitation method, hydrothermal method, sol-gel method and microwave synthesis. As a new synthesis method, low-temperature combustion method has advantages of low temperature, fast production, good chemical homogeneity, and no need for calcinations. Though low-temperature combustion method shows a bright future to synthesize super-fine materials, there are few studies on preparation of noble metal compound oxide, especially the synthesis mechanism. In this thesis, super-fine powders MgGa₂O₄, MgGa₂O₄:Co²⁺ and Zn_xMg_1-xGa₂O₄:Co²⁺ were synthesized by low-temperature combustion method. The crystal phases, morphology, and the optical properties of the obtained phosphor powders were identified by powder X-ray diffractometry (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), and fluorescence spectrophotometer. The major conclusions are:(1) The optimal procedure parameters were selected by adjusting type of fuel, ratios of reactants and ignition temperatures. Results show that MgGa₂O₄ were prepared at 500°C by a low-temperature combustion method using urea as a fuel with the ratios of reactants n[Mg(NO3)2]:n[Ga(NO3)3]:n[CO(NH2)2]:n[NH4NO3]=1:2:8:4, without any further calcination. The products were white, fluffy and voluminous. The prepared MgGa₂O₄ crystal has a perfect spinel structure, the lattice constant is ideal, a=8.284 A.(2) In MgGa₂O₄:Co²⁺, all the Co²⁺ ions completely enter into the crystal lattice of MgGa₂O₄, the introduction of Co²⁺ did not greatly change the crystal structure and lattice constant of MgGa₂O₄.The optical features of the sample are characteristic of tetrahedral Co²⁺ ions replacing tetrahedral Mg²⁺ ions in the MgGa₂O₄ crystal structure. The excitation peak at 650 nm is assigned to the ⁴A₂(⁴F)→⁴T₁(⁴P) transition, the intense red emission peak at 680 nm is assigned to the ⁴T₁(⁴P)→⁴A₂(⁴F) transition, and the weaker emission band in the NIR range to the ⁴T₁(⁴P)→⁴T₂(⁴F) transition.(3) The obtained Zn_xMg_1-xGa₂O₄ by low–temperature combustion method is nanocrystals in single phase of mixed spinel structure without any impurity phase. It is not an admixture of MgGa₂O₄ and ZnGa₂O₄, but a new kind of solid solution material. The doped Co²⁺ completely incorporated into the Zn_xMg_1-xGa₂O₄host. The lattice constant of Zn_xMg_1-xGa₂O₄ has a perfect linear relationship with doped amount, which accorded to the Vegard Rule. For all samples, the shapes of the emission spectrum are similar, but the emission intensity decreases with Zn²⁺ ions increase. This phenomenon might be due to the predominant quantum confinement effect.