Microwave Processing of Functional Materials with Mesoscopic Structure

微波加热正在迅速成为无机合成的既定程序。本研究利用微波辐射成功地合成了各种纳米或介观结构的无机材料，并从“微波选择加热”的角度系统分析了“非平衡反应场”的性质。在900 S的微波辐射下，成功地合成了金红石型固溶体TiO2-SnO2。在1100℃处理的样品的衍射图中，观察到(hkl：1≠0)衍射峰由于调幅分解而分裂。透射电子显微镜观察也证实了调幅分解的发生。观察到调制周期为10-30 nm的层状结构。在TiO2-SnO2二元系中，从平衡相图可知，固溶体范围的下限为1350℃(钛/锡=1/1)。然而，在本研究中，固溶体的形成在1100℃时完成，低于相图的预期。实验结果表明，介观结构的形成机理如下。当微波辐射到二氧化钛和二氧化锡的粉末混合物上时，二氧化锡颗粒被选择性地加热到较高的温度，并与二氧化钛颗粒反应生成钛-钛；1-x-gt；锡-xO2固溶体。在一定时间内保持非等温状态，微观尺度内的温度波动会引起调幅分解，从而形成介观片层结构。在Fe_3O_4-FeAl_2O_4二元体系中也合成了类似的介观结构。结果表明，微波选择性加热在纳米、介观调制复合材料和薄膜器件等非平衡结构的制备中起着关键作用。

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.