Preparation Of Nano- Composite Oxides By Microwave Method

Nano-metal compound oxide has attracted the most attention because of their excellent performance. Its application depends on the research work about the preparation, characterization and properties of nano-metal compound oxides.MgCr2O4, MgAl2O4 and ZnGa2O4 belong to nano-metal compound oxides with a typical spinel-type structure. There are many conventional methods for preparation of spinel-type compounds, such as chemical precipitation method, sol-gel method, hydrothermal method and micro-emulsion method. In this work, a new preparation method—microwave method with the temperature controlled technology would be used to prepare several nano-metal compound oxides mentioned above. In the experiment process, the gel precursor obtained from microwave synthesis equipment was calcined in the muffle furnace to get nano-metal oxide powders. The crystal phases, morphology, the optical and magnetic properties of the obtained powders were identified by powder X-ray diffractometry (XRD), transmission electron microscope (TEM), fluorescence spectrophotometer and physical property measurement system (PPMS-9). The major contents are as follows:(1) MgCr2O4 nano-powder was prepared from magnesium nitrate, chromium nitrate, acrylamide and polyvinyl alcohol. The influence of the microwave radiation power, reaction temperature, calcination temperature and calcination time on the product were studied. The formation mechanism and the adjusting mechanism of the nanoparticles size distribution in the presence of polyvinyl alcohol were also discussed. The optimal reaction parameters are: the molar ratio of metal ions to PVA monomer is 1:3, the molar ratio of metal ions M2+ to acrylamide is 1:1, the microwave power is 600W, the reaction temperature is 98°C, the calcined time is 2h. The spinel phase began to form at about 550°C and a perfect spinel phase would be formed at 700°C. The average grain size of the resulting powder is 33nm and its lattice constant is 8.328?, which is consistent with the TEM results.(2) The ZnGa2O4 and MgAl2O4 powder with good spinel structure were successfully synthesized under the smilar conditions mentioned above. The feasibility of spinel-type oxides synthesis with polyvinyl alcohol and acrylamide as chelating agent by microwave method was analyzed.(3) In MgCr2O4:Co2+, the cobalt nitrate was used as raw material for the doped of Co2+. The introduction of Co2+ did not greatly change the crystal structure and the lattice constant of MgCr2O4. Moreover, all the Co2+ ions completely enter into the crystal lattice of MgCr2O4. The optical features of the sample were characteristic of tetrahedral Co2+ ions replacing tetrahedral Mg2+ ions in the MgCr2O4 crystal structure. When the mole fraction of dopped Co2+ is 3%, the luminous intensity of the product is the strongest.(4) The obtained MgFexCr2-xO4 nano-powder by microwave method is not an admixture of MgCr2O4 and MgFe2O4, but a new kind of solid solution material. The lattice constant of MgFexCr2-xO4 has a perfect linear relationship with doped amount, which accorded to the Vegard Rule. With the increase of dopped Fe content (x), the grain size decreases, while the lattice constant increases. The magnetic characterization results show that MgFexCr2-xO4 nanoparticles exhibit antiferromagnetic behavior. There is a transition from the antiferromagnetic to paramagnetic for the MgFe0.6Cr1.4O4 nanoparticle and the neel temperature is about 20K, while the neel temperature of the MgCr2O4 nanoparticle is about 13K.