二元纳米共晶相图的尺寸效应

The investigation of nanophase diagrams has attracted more and more attentions because of the wide applications of nanomaterials. The nanophase equilibrium is metastable in nature and is difficult to measure in experiment. Although the in situ variable temperature TEM measurement can observe the phase change temperature and solid solubility, the results are semi-quantitative. Fortunately,the theoretical method is an alternative way to study it. At present, thermodynamics and computer simulation have become the main theoretical methods to study nanophase diagrams. The purpose of this project is to understand the inbeing of the binary eutectic phase diagrams of nanoalloys and it will be worked out by thermodynamic, computeral simulant and experimental methods. Firstly, the size effect on thermodynamic functions, liquidus,solidus and solvus of binary eutectic phase relationship will be determined according to macroscopic thermodynamic parameters and atomic interaction parameters, as well as the models of size effect on solid-liquid and solid-solid phase change temperature and interaction parameters. Secondly,the computer simulations will be carried out to determine the size effect on interface energy and solid solubility of selected eutectic systems (Pb-Sn, Pb-Bi, Sn-Bi, Cd-Zn, Al-Ge et al.). Then, the validity of thermodynamic functions can be verified by comparing the results of thermodynamic models and computer simulations.Lastly, electron diffraction studies will be carried out to determine the liquidus, solidus and solid solubility of selected eutectic systems. The validity of the thermodynamic model of size effect on binary eutectic phase diagrams will be checked by comparing with the experimental results. The results of this project would supply an easy way to understand the inbeing of binary eutectic phase diagrams of nanoalloys and provide guidances for the thermodynamic study of nanophase diagrams.

由于纳米材料的广泛应用，纳米相图的研究已成为研究的热点。纳米相平衡处于亚稳状态，实验上测量有一定的难度，目前的测量手段只有在透射电镜上原位变温观察相变温度和结构测量获得固溶度, 而且其结果只是半定量的。因此，热力学和计算机模拟等理论研究成为研究纳米相图的主要手段。本申请依托组元的宏观热力学参数和二元交互作用系数，采用组元的固-液、固-固相变温度、组元间的交互作用系数尺度效应的热力学解析式，拟获得二元合金的各种尺寸依赖的热力学函数，相应的尺寸依赖的液相线、固相线、固溶线和二元纳米共晶相图。同时采用计算机模拟方法验证共晶体系（Pb-Sn，Pb-Bi，Sn-Bi，Cd-Zn，Al-Ge等）各热力学函数的尺寸效应的有效性。并对选定体系的纳米粒子在电镜下通过电子衍射方法直接测量液相线和固相线以及固溶度并与热力学计算结果对比，确认热力学理论的正确性，为纳米相图的热力学研究提供参考。

由于纳米材料的广泛应用，纳米相图的研究已成为研究的热点。纳米相平衡处于亚稳状态，实验上测量有一定的难度，目前的测量手段只有在透射电镜上原位变温观察相变温度和结构测量获得固溶度, 而且其结果只是半定量的。因此，热力学和计算机模拟等理论研究成为研究纳米相图的主要手段。本项目从经典热力学出发，结合实验以及计算机模拟，研究了热力学函数的尺寸效应，液相线、固相线、固溶度随尺寸的变化规律并建立了二元纳米共晶相图。研究发现：（1）由于纳米孔洞具有负曲率，随着孔洞尺寸的减小，熔化温度明显增大，而且，自孔洞表面开始的熔化温度均高于块体材料的熔化温度；（2）凝固温度随着尺寸的减小而降低，并且当尺寸减小到达某一临界值时，凝固温度与熔化温度之间的滞后消失，即熔化温度与凝固温度相等；（3）对于特定的形状，形状因子不是一个定值，而是随着尺寸的减小而减小，而且形状因子与尺寸对热力学参数具有相反的作用趋势，尺寸对纳米材料参数的影响起主要作用，形状因子其次要作用，但是两者的作用都应考虑；（4）随着尺寸的减小，二元纳米合金的熔化温度而降低，原子间交互作用力减小，当尺寸小于2 nm时，交互作用力趋近于零，当尺寸大于2 nm时，其尺寸效应明显，但是随着尺寸的继续增大，例如大于10 nm时，交互作用力尺寸效应不明显趋近于大块值；（5）随着尺寸的减小，共晶温度降低，固相线、液相线随着尺寸的减小而下移，而且固溶度增加。本项目的研究结果将为纳米相图的热力学研究提供参考