【作者】 许灿辉；

【导师】 易丹青；

【作者基本信息】 中南大学 ， 材料学， 2013， 博士

【摘要】 作为最重要的含镉触头替代材料,Ag-SnO2触头材料的复合界面结构对其电接触稳定性具有重要影响。但是直到目前,Ag-SnO2材料的界面结构特征及其相关性质仍没有得到深入系统的研究。因此,认识并建立这种材料的界面结构特征与化学成分、制备工艺和性能之间的联系对材料的结构设计、工艺改进及性能预测都具有重要意义。本文采用金相(OM)、X射线衍射(XRD)、电子探针(EPMA)、常规透射电子显微镜(TEM)等分析手段,系统研究了Ag-3Sn-(In, Bi, La)合金内氧化过程中的组织演变规律、相结构特征,探讨了合金元素对Ag-Sn合金内氧化行为的影响机理；以内氧化Ag-3Sn合金为例,对Ag/SnO2界面的结构特征、界面取向关系等进行了详细地表征和分析；采用第一性原理热力学计算方法研究了金属(Ag)和氧化物(SnO25RuO2)的表面性质以及Ag/SnO2界面的界面性质；综合界面微观结构表征、界面第一性原理热力学计算及内氧化扩散动力学分析几方面研究结果,建立了Ag-Sn合金内氧化界面的热力学与动力学模型,并对Ag-Sn合金内氧化界面结构及性质进行了预测分析,得到了以下主要研究结论：(1)Ag-3Sn合金内氧化过程中,原位析出的SnO：颗粒多为长八面体状和长片状,并且与Ag基体间存在明显的界面择优生长关系：(111)Ag//(101)SnO2；内氧化Ag-3Sn-In合金中的相组成为Ag、SnO2和In203,其中In203颗粒容易与Sn02颗粒形成附着生长结构；内氧化Ag-3Sn-Bi合金中的相组成为Ag、SnO2和Bi2Sn207, SnO2和Bi2Sn2O7颗粒容易形成类似鸡蛋的包裹结构；内氧化Ag-3Sn-La合金中的相组成为Ag、SnO2和La2Sn207, La2Sn颗粒尺寸较大,主要在晶界处形成,并伴随有尺寸较大的SnO：颗粒,并在晶粒内部形成了非常细小弥散的Sn02颗粒,尺寸约为10nm。(2)结合Ag-3Sn-(Bi, La)合金内氧化微观结构表征结果,探讨了合金内氧化机理。Ag-3Sn合金内氧化过程中,氧化带的形成受O、Sn原子浓度变化的动态平衡过程控制。提高合金中氧扩散能力或降低Sn原子的反向扩散能力是避免氧化带形成的有效途径。合金元素In抑制了Sn原子的反向扩散行为,同时In203优先形核促使Sn原子扩散至Ag/In2O3界面处发生氧化形核；合金元素Bi具有很好的细化晶粒作用,大量的晶界能有效促进氧原子扩散,加速内氧化过程,SnO2优先形核导致Bi原子容易扩散至Ag/SnO2界面处反应形成Bi2Sn2O7；合金元素La主要以弥散分布的共晶相La5Sn3存在,氧原子容易沿相界扩散,并与La5Sn3优先反应形成La2Sn2O7相,然后才向晶粒内部扩散形成细小弥散的SnO2颗粒。(3)结合TEM、HRTEM.SAED等分析手段对内氧化Ag-Sn合金中的Ag//SnO2界面的晶体学位向关系、晶格畸变进行了详细表征分析。进一步证实了SnO2颗粒与Ag基体间的界面择优生长关系：(111)Ag//(101)SnO2,<110>Ag//<010>SnO2;采用矩阵计算和极图方法确定了SnO2颗粒与Ag基体间的三种不同晶体学位向关系：Type Ⅰ:[110]Ag//[010]SnO2,(111)Ag//(101)SnO2Type Ⅱ:[112]Ag//[010]SnO2,(111)Ag//(101)SnO2TypeⅢ:[110]Ag//[001]SnO2,(111)Ag//(110)SnO2(4)采用第一性原理热力学计算方法研究了面心立方金属Ag和金红石结构氧化物(Sn02,Ru02)的低指数表面稳定性。结果表明：Ag低指数表面表面能大小顺序为：(111)<(100)<(110)；SnO2低指数表面表面能大小顺序为：(110)<(100)<(101)<(001)；RuO2低指数表面表面能大小顺序为：(110)<(101)<(100)<(001)。(5)结合第一性原理计算和表面反应热力学,建立了氧化物(Sn02. RuO2)表面能随温度、氧分压变化的函数关系,绘制了两种氧化物的表面结构稳定性热力学相图,并结合Gibbs-Wulff晶体平衡形态理论,预测了SnO2和RuO2两种氧化物在不同生长条件下的晶体平衡形态及其环境选择性生长行为,探讨了氧化物晶体的环境选择性生长机理。(6)基于Ag//SnO2界面结构表征结果,采用第一性原理热力学计算方法研究了Ag(111)/SnO2(101)界面的原子弛豫、电子结构特征,并采用界面分离功(Wsep)和界面粘附功(Wad)评价了该界面的润湿性和结合强度。结合第一性原理计算和界面反应热力学,建立了Ag(111)/SnO2(101)界面的界面能随温度、氧分压变化的函数关系,并在此基础上首次绘制了Ag(111)/SnO2(101)界面的界面结构稳定性热力学相图。(7)通过对Ag-Sn合金内氧化动力学过程和氧在银合金中溶解热力学过程进行分析,建立了合金内氧化区内“原位氧分压”与环境氧分压、合金成分、内氧化温度、氧化时间、内氧化层距离之间的函数关系,实现不同内氧化工艺条件下合金内氧化区内“原位氧分压”的预测。(8)综合界面结构表征、界面稳定性和界面性质的第一性原理热力学计算,以及内氧化扩散动力学分析等方面研究结果,提出了Ag-Sn合金内氧化界面热力学与动力学模型,定量描述了Ag-Sn合金内氧化过程中的合金成分(Sn含量)—内氧化工艺(内氧化温度、氧分压、时间)—界面结构—界面性能(界面润湿性,结合强度)四者间的相互关系,并对Ag-3Sn合金在空气气氛,1023K和823K条件下的内氧化界面结构特征进行了预测。更多还原

【Abstract】 As the most important substitute for the replacing of conventional toxic Ag-CdO contact material, the electric contact stability of Ag-SnO2was highly affected by its interface structure. However, the interface structure and its properties have not been deeply and comprehensively investigated till now. It is important for us to insight the interrelationship between the interface structure, chemical composition, processing parameters and its properties, which was helpful to the improvement of this material, including the structure optimization, processing improvment and properties prediction.Based on the investigation of microstructure characterization by a combination of metallographical microscope (OM), X-ray diffraction (XRD), Electron probe microanalysis (EPMA) and transmission electron microscope (TEM), the internal oxidation mechanism of Ag-3Sn-(In, Bi, La) alloy were disscussed in details; The interfacial structures and crystallographic orientation relationships of the Ag/SnO2interface in-situ formed in internal oxidized Ag-3Sn alloy were characterized; With the first principle thermodynamic calculation, the surface properties of cubic metal (Ag) and rutile-type oxide (SnO2, RuO2), and the interface properties of Ag/SnO2interface were assessed. Combined with interface structure characterization, interfacial properties thermodynamic assessment and the internal oxidation diffusion kinetics, a modeling of the thermodynamic and kinetics of internal oxidized interface in Ag-Sn alloy was proposed, by which the interrelationship between composition, interface microstructure, processing parameters and properties of internal oxidized interface in Ag-Sn alloy was analyzed quantitatively. The main conclusions were presented as follows:(1) The SnO2particles formed in-situ in the internal oxidized Ag-3Sn alloy are mainly constituted by elongated octahedral shaped and plate shaped particles, with highly preferred interface orientation relationship of (111)Ag//(101)SnO2.The phase compositions of Ag-3Sn-In alloy are Ag, SnO2, In2O3. The In2O3and SnO2particles are always precipitate together. The phase compositions of Ag-3Sn-Bi alloy are Ag, SnO2and The SnO2and Bi2Sn2O7particles formed as wrapping structure. The phase compositions of Ag-3Sn-La alloy are Ag, SnO2and La2Sn2O7. The La2Sn207particles with large size formed at grain boundary, with small quantity of large SnO2particles. Large quantity of small SnO2particles with size of about～10nm formed and dispersed uniformly inside the grains.(2) The formation of oxidation band in Ag-3Sn alloy was controlled by a dynamic equilibrium between the concentration changes of oxygen and Sn atoms during the internal oxidation process. Enhence the diffusivity of oxygen or reduce the back-diffusion behaviour of Sn atoms is an alternative method to prevent the formation of oxidation band. With the addition of In, the back-diffusion behavour of Sn atoms was restained effectively by the priority nucleation of In2O3, which promote the precipitation of SnO2at the Ag/In2O3interface. The grains were refined markedly by the addition of Bi element and resulted in a large quantity of grain boundaries. Oxygen diffused quickly along the grain boundaries and fully internal oxidation was processed. The diffusion of Bi atoms to Ag/SnO2interface was promoted by the priority nucleation of SnO2and reacted with SnO2to form Bi2Sn2O7particles. The oxygen in Ag-3Sn-La alloy is easily diffused along the phase boundary of Ag and eutectic La5Sn3, and reacts with La5Sn3to form La2Sn2O7. The dispersed SnO2particles with nano size were formed by the diffusion of oxygen from grain boundaries to the inside of grains.(3) The Ag/SnO2interface structure in internal oxidized Ag-3Sn alloy was characterized by a combination of TEM, HRTEM, and SAED. The in-situ formed SnO2precipitates highly preferred growing with interfacial orientation of (111)Ag//(101)SnO2,<110>//<010>SnO2Three kinds of orientation relatioships (ORs) between Ag matrix and SnO2precipitates were identified in the internal oxidized Ag-3at.%Sn alloy by using the matrix method and stereographic projection method: Type Ⅰ:[110]Ag//[010]SnO2,(111)Ag//(101)SnO2TypeⅡ:[112]Ag//[010]SnO2,(111),Ag//SnO2TypeⅢ:[110],//[001]SnO2,(111)Ag//SnO2, (4) The surface stabilities of low-index surfaces of cubic metal (Ag) and rutile-type oxide (SnO2, RuO2) were investigated by first-principles calculations. It was found that the ordering of Ag surface energies is:(111)<(100)<(110); the ordering of SnO2surface energies is:(110)<(100)<(101)<(001); the ordering of RuO2surface energies is:(110)<(101)<(100)<(001).(5) Combined the first principle calculation and surface thermodynamic analysis, the surface stabilities of these two oxides were evaluated as functions of oxygen partial pressure and temperature. And the "surface phase diagram" of these two oxides was constructed, which corresponds well with the experimental observations. The environment-dependent surface stabilities assessment of these oxides was look forward to the prediction of crystal morphology, by utilizing the Gibbs-WulfF theorem of equilibrium crystal shape (ECS). The ECS as well as the preferred growth morphology of these oxides was predicted.(6) Based on the experimentally characterization of Ag/SnO2interface structure, the interfacial properties, including the interface atoms relaxation and interfacial electronic structure, of Ag(111)/SnO2(101) interface were investigated by first principle calculations. The interfacial bonding strength of Ag(111)/SnO2(101) interface were assessed by the work of separation (Wsep) and the work of adhesion (Wad).The Ag(111)/SnO2(101) interface energy was presented as a function of temperature and oxygen partial pressure. With the combination of first principle calculation and thermodynamic analysis, a "interface phase diagram" of the Ag(111)/SnO2(101) interface was constructed firstly.(7) With the analysis of the internal oxidation kinetics of Ag-Sn alloy and the thermodynamics process of oxygen dissolved in silver alloy, the local oxygen partial pressure in internal oxidation region was expressed as a complex function of the environmental oxygen partial pressure, alloy composition, temperature, time and distance from the surface to the internal oxidation zone, by which the local oxygen pressure inside internal oxidized region was predicted.(8) Combined with interface structure characterization, interfacial properties thermodynamic assessment and the internal oxidation diffusion kinetics, a modeling of the thermodynamic and kinetics of internal oxidized interface in Ag-Sn alloy was proposed, by which the interrelationship between composition, interface microstructure, internal oxidation parameters and properties of internal oxidized interface in Ag-Sn alloy was analyzed quantitatively. The prediction of internal oxidized Ag/SnO2interface structure and the interfacial properties at any given conditions can be achieved. This modeling was implemented on the internal oxidation of Ag-3Sn alloy under air atmosphere, with oxidation temperature of1023K and823K, respectively.更多还原