Microwave Processing of Functional Materials with Mesoscopic Structure

微波加热正迅速成为无机合成的一个既定程序。本研究成功地利用微波辐射合成了各种具有纳米或介观结构的无机材料，并从“微波选择性加热”的角度系统地分析了“非平衡反应场”的本质。在900 s内成功合成了金红石型固溶体TiO_2-SnO_2。在1100℃处理的样品的衍射图中，观察到（hkl：1 × 0）衍射峰由于亚稳分解而分裂。透射电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的片层结构。在TiO_2-SnO_2二元系中，从平衡相图上看，固溶区的下限为1350℃（Ti/Sn=1/1）。然而，在本研究中，固溶体的形成在1100℃完成，这低于相图所预期的温度。实验结果表明，介观结构的形成机制如下。当微波辐照TiO_2和SnO_2的混合粉末时，SnO_2颗粒被选择性地加热到较高的温度并与TiO_2颗粒反应形成Ti_<1-x>Sn_xO_2固溶体。在一定时间内保持非等温状态，在微观尺度上的温度波动引起失稳分解，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。微波选择性加热在制备纳米、介观调制复合材料和薄膜器件等非平衡结构中起着关键作用。

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.