Microwave Processing of Functional Materials with Mesoscopic Structure

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Microwave heating is rapidly becoming an established procedure for inorganic synthesis. In this study, various inorganic materials with nano- or meso-scopic structure were successfully synthesized by microwave irradiation and the nature of the "non-equilibrium reaction field" was systematically analyzed in a view point of "microwave selective heating". Rutile-type solid solution, TiO_2-SnO_2, was successfully synthesized within 900 s of microwave irradiation. In the diffraction pattern of the specimen treated at 1100℃, splitting due to spinodal decomposition was observed for (hkl: 1≠0) diffraction peaks. The occurrence of spinodal decomposition was also confirmed by TEM observation. Lamellar structure with the modulation cycle of 10-30nm was observed. In the binary TiO_2-SnO_2 system, lower limit of the solid solution range is at 1350℃ (Ti/Sn=1/1) from the equilibrium phase diagram. In this study, however, formation of solid solution was completed at 1100℃, which was lower than that expected by the phase diagram. The experimental results suggest the following mechanism of the formation of the mesoscopic structure. When microwaves are irradiated on the powder mixture of TiO_2 and SnO_2, SnO_2 particles are selectively heated to higher temperature and react with TiO_2 particles to form Ti_<1-x>Sn_xO_2 solid solutions. Anisothermal condition is maintained in a certain period and the temperature fluctuation within a microscopic scale induces spinodal decomposition, consequently, the formation of mesoscopic lamellar structure. Similar type of meso-scopic structure was also synthesized in binary Fe_3O_4-FeAl_2O_4 system. It is concluded that microwave selective heating plays a key role on fabricating non-equilibrium structures like nano-, meso-scopic modulated composites and thin film devices.