纳米多晶体的热力学函数及其在相变热力学中的应用

以往关于纳米材料热力学的研究，绝大多数以界面的热力学函数表征整体纳米材料的热力学性质，这种近似处理，对于尺寸超过几十纳米的较粗纳米材料，在相变热力学中对特征转变温度和临界尺寸等重要参量的预测，将导致很大误差. 应用“界面膨胀模型”和普适状态方程，研究了纳米晶界的热力学特性，进一步发展了纳米晶整体材料热力学函数的计算模型，给出了单相纳米多晶体的焓、熵和吉布斯自由能随界面过剩体积、温度，以及晶粒尺寸发生变化的明确表达式. 以Co纳米晶为例，分析了界面与整体纳米多晶体热力学函数的差异，确定了相变温度与晶粒尺寸的依赖关系，以及一定温度下可能发生相变的临界尺寸.关键词：纳米多晶体热力学函数相变热力学

Thermodynamic functions of nanocrystals and its application to the study of phase transformations

In the literature, a majority of studies on thermal properties of nanocrystals have taken the thermal features of interfaces as a representative of those of the whole nanocrystal. This treatment may cause big errors in predicting some important parameters such as the transition temperatures and critical sizes of phase for transformation in nanocrystalline materials, when the grain size is larger than several tens of nanometers. In the present paper, a model to calculate the thermodynamic functions of the whole nanocrystal has been developed with the consideration of the effects of both the interfaces and the crystal in the grain interior. The expressions of the enthalpy, entropy and Gibbs free energy, as a functions of the excess free volume at interfaces, temperature and grain size, are derived for single_phase nanocrystals. Using the Co nanocrystal as an example, thermal features of interfaces and the bulk nanocrystal have been compared with the conventional. Furthermore, the characteristics of β_Co→α_Co phase transformation in nanocrystalline materials are studied, the transformation temperatures at different levels of grain size, as well as the critical grain sizes at different temperatures, are predicted and compared with experimental results.