【作者】 阮中尉；

【导师】 刘立胜；

【作者基本信息】 武汉理工大学 ， 固体力学， 2012， 博士

【摘要】 目前,能源短缺问题日益严重,开发新型环保能源替代材料已迫在眉睫。热电材料作为一种能将热能和电能相互转换的功能材料,具有广阔的应用前景。其中,方钴矿Skutterudites化合物是一种具有开放笼状结构的窄带半导体材料,它具有高的Seebeck系数以及较高的电导率,通过各种优化手段有望成为一种非常具有前景的高效中温热电转换材料。在热电器件服役过程中,材料一直受到循环热载荷和循环应力载荷的作用,这些疲劳载荷会降低热电器件的可靠度和减少热电器件的的使用寿命。研究的方钴矿材料疲劳特性,对方钴矿热电器件可靠性评估和寿命预测具有重要的工程应用意义。本文以方钴矿热电材料为研究对象,采用熔融-扩散反应法合成粉末、放电等离子体快速烧结法制备纯方钻矿块体材料和块体Ce0.5Fe1.5CO2.5Sb2,经过切割,磨削和抛光后制成棱柱体块体试样。试样分组进行试验,主要有三大类试验,基本力学性能试验、低周疲劳试验和强度衰减试验,主要实验仪器为Instron5882电子万能材料试验机和MTS810陶瓷试验系统。本文的主要试验和理论研究内容包括以下几个方面。首先,进行了棱柱体纯方钴矿的基本力学性能实验,分别有压缩力学性能试验,弯曲力学性能试验。然后进行了方钴矿的低周疲劳试验,低周疲劳试验在MTS810陶瓷试验系统上进行,采用应力控制,循环应力应力比为R=0.1,加载频率为2Hz。试验环境为室温(25℃)、空气介质。单轴静载压缩试验测定出的压缩强度作为低周疲劳试验的选择依据。在不同的应力水平下进行材料疲劳寿命测试；对试样试验前的微观表面进行SEM表征。通过SEM观察和理论分析阐述试样表面裂纹形成机制和过程。并对试件疲劳断口进行SEM观察。疲劳试验结果表明：方钴矿材料具有良好的压缩疲劳性能,由于其为多晶脆性材料,疲劳破坏以脆断为主,疲劳破坏过程实质上就是裂纹扩展过程。本文中假定在不同的应力水平下,试样的低周疲劳寿命为独立的随机分布变量,且分别满足对数正态分。方钴矿块体材料和块体Ce0.5Fe1.5CO2.5Sb2材料的S-N曲线采用两参数线性方程来描述,通过对试验数据的统计处理,获得了各种可靠度下S-N曲线方程的系数,最后得到了P-S-N的表达式。然后进行了下一项疲劳试验,研究了疲劳载荷应力比、加载频率和加载波形(三角波、方波和正弦波)对方钴矿疲劳特性的影响。最后进行了棱柱体纯方钴矿试样的疲劳剩余压缩强度试验,试验结果表明,先对试样施加一定循环次数的疲劳载荷,再通过单轴静载压缩试验测定出剩余压缩强度的方法,能够较好的揭示方钴矿热电材料疲劳剩余强度的衰减规律,根据试验结果和可靠性分析方法,绘出了P-P-N曲线(P为可靠度,R为剩余强度,N为疲劳载荷的循环次数)。P-R-N曲线能在表示出剩余强度衰减的同时给出了相应的可靠度。更多还原

【Abstract】 Nowadays, development of new environmentally-friendly energy alternative materials has been urgent due to the increasingly serious energy crisis. Thermoelectric materials are semiconducting functional materials with wide application prospect, which can interconvert heat and electricity directly. Among them, skutterudites compounds is one of the narrow bandgap semiconductor materials with open structure, it has high Seebeck coefficient and high electrical conductivity and can became be a promising thermoelectric material used in medium temperature by some special optimization method. Under service conditions, thermoelectric devices often suffer from the cyclic thermal loading and cyclic thermal stress loading. All of these loads not only cause damage to the devices, but also reduce its service life. Better understanding of the fatigue behavior of skutterudits compounds is essential for reliability estimation and service life prediction of thermoelectric devices.In this work, Ce0.5Fe1.5Co2.5Sb2compounds CoSb3are taken as the research object, they were prepared by using melting method combined with the spark plasma sintering technique. After cutting, grinding and polishing, the specimens were obtained. All tests are divided into three types. The first tests are static compression tests and three-poind-bending tests. The second type tests are stress-controlled low-cycle fatigue tests. The last tests are residual strength degradation tests. An MTS810servohydraulic machine and Instron5882machie were used for the test.First the static compression tests and three-point-bending tests were condcted. The purpose of these tests was to obtain the compressive strength, flexural strength and Young’s modules. Compression-compression low cycle fatigue tests were then conducted. A sinusoidal waceform of constant amplitude with a frequency of2Hz and stress ratio R of0.1was employed. Tests were carried out at room temperature and in air. The maximum stress of the cyclic loading was chosed to be60%-90%of the static compressive strength. The surfaces and fracture surfaces of the specimens were observed by scanning electron microscopy. Based on surface crack observation and fatigue fracture surface analyses, fatigue cracks initiated from pre-existing defects in the specimen surface due to stress concentration. The fatigue fracture surface was basically characterized by intergranular fracture, which suggests brittle characteristics of the materials. Besides, thegrain size remains basically unchanged afterfatigue loading.All fatigue life tests date exhibit significant scatter, so in this study, the static strength, fatigue life are asuumed to be presented by two-parameter normal distribution. The S-N curve of CoSb3and Ceo.5Fe1.5Co2.5Sb2were described by two parameters linear equation. Based on statictical data processing, the coefficents of S-N equations were obtained under different probability. Thus P-S-N equation the materials are obtained. Further, fatigue tests were employed to analyze the influence the stress ration, the loading frequency and loading waveform (triangular wave, square wave and sine waveform) on the fatigue life of CoSb3. The static fatigue tests were also carried out. The results showed, under the same conditions, the fatigue lifes of static fatigue is bigger than those of cyclic fatigue.Lastly, residual strength degradation tests of CoSb3were employed. The tests results can describe the degradation behaviour and can analyze the effect of the cyclic loading on the residual strength. Based on the test results and probabilistic analysis method, the P-R-N curves of CoSb3were obtained. The curves can predict the residual strength at a given number of fatigues loading cycles with given probability. The residual fatigue life also can be predicted based on P-R-N curves and the linear accumulative damage rule.更多还原