Microwave Synthesis And Characterization Of Several Inorganic Materials

Microwave heating is a process in which the materials couple with microwaves, absorb the electromagnetic energy and transform into heat. Microwave heating is caused by molecular dipole rotation within the dielectric. The friction of molecules causes energy loss, so that the material can be heated to reaction temperature. This technology has features of bulk heating, selective heating and non-thermo effect. Since some materials cannot absorb microwave effectively, solid state hybrid microwave heating has been developed. The material having strong capability to absorb microwave, such as Si C, Cu O or Mo Si2, is used to cover the reactants, working as an auxiliary heating.In this paper, Co Sb3, Ni50Mn37Sb13 and Ni S2 were synthesized by solid state hybrid microwave heating and their properties are characterized. The main results are as follows:(1) Thermoelectric properties of bulk Co Sb3 synthesized by one-step solid state hybrid microwave heatingThermoelectric material is a new functional material that can convert heat directly into electric. Co Sb3-based skutterudite compounds have attracted great attention as a group of promising thermoelectric materials because of their high conductivity and high Seebeck coefficient. Bulk Co Sb3 thermoelectric material was synthesized by one-step solid state hybrid microwave heating. Conventional preparation methods for Co Sb3 are time and energy consuming, while microwave heating costs for a short time, low energy and is easy to manipulate. X-ray powder diffraction and scanning electron microscopy analyses reveal that the obtained products are of single phase cubic structure, homogeneous and dense. Fast microwave heating creates the microstructure in nanoscale resulting in low thermal conductivity, which leads to a comparable dimensionless figure of merit 0.08 at 750 K to those reported previously for the samples after long time solid state reaction and fast sintering.(2) Transport and magnetic properties of Heusler alloy Ni-Mn-Sb prepared by solid state hybrid microwave heatingMagneto shape memory alloy exhibits large field induced shape memory effect, exchange bias(EB), ferromagnetic(FM) and antiferromagetic(AFM) exchange interaction. Ni-Mn-X magneto shape memory alloy undergoes cubic austenite to tetragonal martensite phase transition and transforms from ferromagnetism to antiferromagnetism. Heusler alloy Ni50Mn37Sb13 has been prepared by solid state hybrid microwave heating within 40 min. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses reveal that the obtained samples are of single phase cubic structure and homogeneous. Temperature dependence of electrical resistivity and magnetization measurements confirm the austenite to martensitic transition of the first order nature with the absence of EB. Compared with the data of samples from conventional arc-melting method, the sample synthesized by fast microwave heating has lower martensitic transition temperature, smooth magnetization change in the martensitic region and disappeared EB, implying strongly suppressed AFM interaction.(3) Synthesis of Ni S2 by Solid State Hybrid Microwave MethodTransition metal chalcogenide compounds have attracted great attention because of their key roles in the study of condensed state mechanism and the preparation of photosensitive, magnetic devices. Conventional solid-state reaction is a time and energy consuming method for Ni S2. In order to solve this problem, single phase Ni S2 crystals have been synthesized by solid-state hybrid microwave heating from the mixture of Ni and S powders. The reaction time has been shortened significantly from several days to 30 minutes with the suitable microwave heating temperature determined basically. The structure, composition and morphology of the as-prepared products were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectrum.