正电子湮没研究Al、Nb共掺CaCu3Ti4O12陶瓷高介电常数机理

关于CaCu₃Ti₄O₁₂陶瓷的高介电常数机理，目前广泛接受的是非本征的内阻挡层电容模型。该模型认为在多晶中元素变价、缺陷和非化学计量比等导致的半导化晶粒被绝缘晶界层，即内阻挡层所包围。其中内阻挡层的厚度对材料的介电性能影响较大，而扫描电子显微镜（SEM）测试表明样品晶界呈稀烂的果酱状，SEM难以测量晶界区域绝缘内阻挡层厚度。本文采用正电子湮没技术表征其厚度，通过对CaCu₃Ti₄O₁₂陶瓷共掺不同浓度的Al、Nb（CaCu₃Ti_4-xAl_0.5xNb_0.5xO₁₂，x=0.2%、0.5%、5.0%）改变其晶粒和晶界的微观结构，研究CaCu₃Ti₄O₁₂陶瓷高介电常数机理。正电子湮没结果显示，掺杂样品符合多普勒展宽谱S参数的变化趋势与平均寿命的变化趋势一致。x=0.5%掺杂样品的介电常数最高，其平均寿命、S参数和湮没长寿命成分均最小，阻挡层最薄。实验结果验证了描述CaCu₃Ti₄O₁₂陶瓷高介电常数机理的内阻挡层电容模型的预测。

Study on Colossal Dielectric Constant Mechanism of Al/Nb Co-dopedCaCu3Ti4O12 Ceramic by Positron Annihilation Technique

For the colossal dielectric constant mechanism of calcium copper titanate, the most widely accepted model is an inner barrier layer capacitance model. It is said that in polycrystalline calcium copper titanate ceramic, its semiconducting interior grains are surrounded by thin insulating grain boundaries, i.e. internal barrier layer, which has great influence on its dielectric properties. Results of scanning electron microscope (SEM) show that the sample grain boundary is jam like. It is hard for SEM to measure the thickness of the insulating internal barrier layer in grain boundary region. So positron annihilation technique (PAT) was used to characterize the thickness of the internal barrier layer in this work. The research was carried out by co-doping various concentrations of Al/Nb on its Ti⁴⁺ sites to change the microstructure of calcium copper titanate ceramic grain and grain boundary （CaCu₃Ti_4-xAl_0.5xNb_0.5xO₁₂, x=0.2%, 0.5%, and 5.0%）. PAT results show that the change trends of sample mean lifetime and Doppler parameter S are similar. Besides, the mean lifetime, Doppler parameter S and the longer lifetime component are minimum at x=0.5%. This implies the thinnest thickness of its barrier layer. The obtained experimental results support the prediction of internal barrier layer capacitance model that describes the origin of the colossal dielectric constant for the calcium copper titanate ceramics.