Elucidation of grain-alignment mechanism of ceramics polycrystals by reactive diffusion technique

通过对La_2SiO_5/La_2Si_2O_7/La_2SiO_5扩散偶在1873 K等温加热50 h，制备了具有高度c轴取向的磷灰石型La_（9.50）Si_（6026.25）多晶材料。使用光学显微镜，X-射线衍射和阻抗谱，其特征在于所得到的多晶。对退火后的电偶进行机械加工，得到由织构多晶组成的薄板电解质。在723 ~ 1073 K温度范围内，氧离子电导率沿着c轴由2.0 × 10 ~（-2）S/cm增加到7.9 × 10 ~（-2）S/cm。该材料的电导率在723 - 973 K时约为1.25。2.5比c轴取向的磷灰石多晶La9.33Si6O26的高10倍。这两种材料具有相同的导电激活能（0.35 eV），因此导电机制必须相同。La9.50Si6O26.25和La9.33Si6O26在室温下的晶体结构（空间群P63/m）都显示出与Si原子键合的O原子（12 i位）的明显位置无序，以及La原子（4f和6 h位）的非谐位移。前一种结构的特征还在于沟道氧化物离子（2a和4 e位点）的位置无序以及间隙氧化物离子（6 h位点）的存在，这将有助于沿着c轴的更高电导率。

We have prepared the highly c-axis-oriented polycrystalline material of apatite-type La9.50Si6026.25 by isothermal heating of the sandwich-type La2SiO5/La2Si2O7/La2SiO5 diffusion couple at 1873 K for 50 h. The resulting polycrystal was characterized using optical microscopy, X-ray diffractometry and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the c-axis steadily increased from 2.0 X 10-2 S/cm to 7.9 X 10-2 S/cm with increasing temperature from 723 to 1073 K. The conductivity of this material was, at 723 - 973 K, ca. 2.5 times higher than that of the c-axis-oriented apatite polycrystal of La9.33Si6O26. These two materials have the identical activation energy of conduction (0.35 eV), and hence the conduction mechanism must be the same. Both crystal structures of La9.50Si6O26.25 and La9.33Si6O26 at ambient temperature (space group P63/m) showed the appreciable positional disordering of 0 atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The former structure is further characterized by the positional disordering of channel oxide ions (2a and 4e sites) as well as the presence of interstitial oxide ions (6h site), which would contribute to the higher conductivity along the c-axis.