Ag-Sn合金的氧化过程与热力学性质

采用密度泛函微扰理论(DFPT)计算了Ag-Sn-O体系中O原子在Ag-Sn合金中的运动与反应, 及该体系中几种化合物的结合能、生成焓以及热容与自由能随温度的变化. 计算发现O原子存在于Ag晶格的四面体间隙, 若存在八面体间隙会使体系能量升高288.23 kJ·mol-1; Sn则在Ag晶格中主要以置换形式存在. 计算得到结合能大小顺序为Ag6O2>SnO2>Ag2SnO3>SnO>Ag2O. SnO2的稳定性最高, 生成焓约-591.1 kJ·mol-1, SnO是一种过渡相, 而Ag2SnO3和Ag6O2为亚稳相, 都能在常温下存在, 这与相关实验结果一致. 原子间相互作用曲线说明Sn—O的成键能力明显高于Ag—O, 氧化时只会形成Sn—O键. 准谐函数近似计算(QHA)表明Ag2SnO3的热容远高于其它化合物, 其德拜温度约500 K, 升温能力仅为SnO2的1/3, 可有效解决AgSnO2电接触材料中温升过快的问题, 而布居分布和Gibbs自由能则进一步说明SnO2是Ag-Sn-O体系中最稳定的相.

Oxidation and Thermodynamic Properties of Ag-Sn Alloy

Diffused and reactive O atom in process of internal oxidation in Ag-Sn-O system was calculated by density functional perturbation approximation theory (DFPT). The results showed binding energy, enthalpy, heat capacity and Gibbs free energy of the phases in Ag-Sn-O system. O atom was on tetrahedron positions in Ag lattice. The system energy of O atom on the octahedron positions was 288.23 kJ·mol-1 higher than that of O atom on the tetrahedron positions. However, Sn atom was on the replacement positions in Ag lattice. The order of cohesive energies (Ec) in Ag-Sn-O system was Ag6O2>SnO2>Ag2SnO3>SnO>Ag2O. The enthalpy of SnO2 was -591.1 kJ·mol-1, which was in agreement with experiment. SnO, Ag6O2, and Ag2SnO3 were the metastable phases in the normal temperature. Because the bonding ability of Sn—O was higher than that of Ag—O, there was no silver-oxide in Ag-Sn-O system. The heat capacity of Ag2SnO3 was much bigger than those of other compounds, and its Debye temperature was about 500 K. SnO2 was the most stable phase in Ag-Sn-O system, which was showed by population analysis and Gibbs free energy.