Theoretical calculation and experimental investigation of interganular van-der Waals forces in ceramics

晶间范德华力（vdW）决定了陶瓷界面的远距离相互作用。在Si_3N_4陶瓷晶间玻璃膜的10阶以上有效。这种薄膜在Si_3N_4陶瓷中普遍存在，而在SiC中不存在。随着高空间分辨率分析电镜技术的发展，人们可以直接从晶间膜中获得电子能量损失谱。由于EELS （ELNES）近边结构的主要特征可以归因于电子偶极子从核心轨道到空占据带的跃迁，因此为了解释实验光谱，需要对空占据带进行精确的电子计算。在本研究中，我们利用Si、N和o组成的模型簇，用离散变分（DV） Xalpha方法进行了第一性原理的分子轨道计算。1)根据局部化学键解释薄膜中ELNES的光谱特征。2)沿膜的晶间vdw力的理论计算。与这些理论工作平行的是，主要通过高空间分辨率电子显微镜进行了表征晶间玻璃膜的实验工作。

Intergranular van-der Waals (vdW) force determines long range interaction of ceramics interfaces. It is effective over 10 hich is the order of the intergranular glassy film of Si_3N_4 ceramics. Such film is ubiquitous in Si_3N_4 ceramics but not in SiC.With recent improvement of high spatial resolution analytical electron microscopy, one can obtain electron energy-loss spectra (EELS) directly from the intergranular film. Since major features of the near edge structures of EELS (ELNES) can be ascribed to electronic dipole transitions from core orbitals to unoccupied band, accurate electronic calculations of the unoccupied band are necessary in order to interpret the experimental spectra.In the present study, we made first principle molecular orbital calculations by discrete variational (DV) Xalpha method using model clusters composed of Si, N and O.There were tow objectives. 1) Interpretation of spectral features of ELNES obtained from the film in terms of the local chemical bonding. 2) Theoretical calculation of intergranular vdWforces across the film. Parallel to these theoretical works, experimental efforts to characterize the intergranular glassy film have been made mainly by means of high spatial resolution electron microscopy.