服役条件下方钴矿基热电元件的界面应力分析（英文）

热电器件中,界面可靠性是影响整体稳定和功率输出的关键因素。对于方钴矿(SKD)器件,热电臂和电极通过扩散阻挡层(DBL)连接。在高温下,DBL与SKD、电极之间会发生反应并生成复杂的界面结构,导致界面附近的热、电、力学性能发生变化。本研究根据实际界面结构建立了包含微观结构的有限元模型,并将其用于分析方钴矿基元件的界面应力状态。采用单层模型对DBL材料参数进行了筛选,发现热膨胀系数(CTE)和弹性模量(E)对第一主应力有显著影响。采用包含界面微结构的多层模型定量模拟了不同老化温度、时间下元件内部的应力分布,结果表明在SKD/Zr和SKD/Nb中,CoSb₂反应层最为薄弱,随着老化时间的延长,反应层的厚度增加,界面应力变大。同时,元件的拉伸试验结果与计算结果吻合较好,验证了模型的准确性与可行性。本研究为提升SKD/DBL元件的结构稳定性提供了指导,同时也为精确模拟多层结构中的应力状态提供了研究思路。     

Fe/Bi_(0.5)Sb_(1.5)Te_3热电元件高温稳定性研究（英文）

热电元件的界面高温稳定性是决定热电器件服役性能和应用前景的重要因素,而阻挡层和热电材料之间的界面扩散和界面电阻则是评价热电元件高温稳定性的主要标准。为了进一步提升P型碲化铋热电器件的界面稳定性,本研究采用高通量筛选的方法选定适用于P型碲化铋的Fe阻挡层材料。通过一步烧结的方法制备了Fe/P-BT的热电元件,并系统研究了高温加速老化实验下的Fe/P-BT的界面微观结构的演变和界面电阻率的稳定性。在老化过程中, Fe/P-BT的界面连接良好且Fe-Sb-Te的三元扩散层的成分基本不变。扩散层厚度与时间的平方根成线性关系,生长激活能为199.6 kJ/mol。Fe/P-BT的界面电阻率较小且随着老化时间延长缓慢增大,在350℃老化16 d后仍然低于10μ?·cm~2。基于界面扩散动力学的寿命预测表明Fe可以用作Bi_(0.5)Sb_(1.5)Te_3热电元件的阻挡层材料。     

Pt/YSZ电极结构老化特性研究

借助于I-V极化曲线、交流阻抗测试及电子扫描电镜等技术和设备,对以Y₂O₃稳定ZrO₂多晶固体氧离子导体(YSZ)为基体的Pt/YSZ电极结构电学性能和电极形貌随时间的老化变化特性进行了研究;为定量表述电极形貌的变化特性,作者提出了一种有关Pt/YSZ电极结构电极界面的形貌模型,采用此模型对不同老化时间下的电极Pt/空气/YSz三相界面的长度进行了定量计算.实验测试、理论模型计算等结果都表明:Pt/YSZ电极结构的电学性能和电极形貌均随老化时间出现了明显的变化.     

方钴矿热电材料/Ti88Al12界面稳定性研究

热电器件的界面稳定性是决定其服役可靠性和寿命的关键因素。对于方钴矿热电器件, 为了抑制高温电极与方钴矿材料之间的相互扩散, 需要在两者之间加入阻挡层。本工作选用Ti₈₈Al₁₂作为阻挡层, 利用一步法热压烧结制备n型Yb_0.3Co₄Sb₁₂/Ti₈₈Al₁₂/Yb_0.3Co₄Sb₁₂和p型CeFe_3.85Mn_0.15Sb₁₂/Ti₈₈Al₁₂/CeFe_3.85Mn_0.15Sb₁₂样品, 研究Ti₈₈Al₁₂阻挡层与热电材料间的界面接触电阻率及微结构在加速老化实验中的演化规律。结果表明: 在相同的老化条件下, n型样品的界面接触电阻率增加速度比p型样品慢, 其激活能分别为84.1 kJ/mol和68.8 kJ/mol。对于n型样品, 由元素扩散反应生成的金属间化合物中间层的增长及最终AlCo/TiCoSb层的开裂是导致界面接触电阻率增加的主要原因; 而p型热电材料与Ti₈₈Al₁₂的热膨胀系数的差异加速了p型样品中界面裂纹的产生。     

连续分布界面相对SiCp/Al复合材料热导率影响的数值模拟

采用有限元方法对SiCp/Al复合材料的导热性能进行了数值模拟, 建立了含界面相颗粒增强铝基复合材料测试模型, 研究了不同界面相种类、厚度对复合材料热导率的影响。结果表明: 当界面相与SiC/Al结合理想时, 且界面相在颗粒表面呈连续分布时, 复合材料热导率随着界面层热导率的增加而增大, 但增加的幅度由快变慢; 复合材料热导率随界面层厚度的变化取决于界面层厚度t与颗粒粒径a的比值, 当t/a很小或t/a较大时, 热导率随界面层厚度的变化很小, 当t/a较小时, 热导率随界面层厚度的变化则与界面层热导率有关。     

锆钛酸铅压电陶瓷元件高温老化行为及其微观机制

长时高温作用会显著加速压电陶瓷元件电学性能的退化，极大地影响压电元件使用寿命。本工作在85、125和150℃下对PZT压电陶瓷元件进行长时老化处理，研究并分析了高温老化过程中PZT陶瓷元件电学性能退化规律及其微观机制。压电常数与介电常数随老化周期延长均退化明显，初期退化速率较慢，后期退化加快并逐渐趋于稳定，介电常数和压电常数退化均符合Boltzmann变化规律。85℃和125℃下老化49 d时，元件介电常数退化率达到最大，分别为31.01%和36.48%;压电常数退化率达到最大，分别为11.57%和19.02%。PZT压电元件介电性能退化主要由表面Ag电极内结构退化、Ag/PZT界面弱化及PZT陶瓷退极化三方面所贡献，其中界面弱化和陶瓷退极化占主导。长时高温作用促使Ag电极逐渐缓慢氧化生成Ag₂S,晶粒收缩产生大量孔洞，导致Ag电极致密度下降和Ag/PZT界面分层，降低了Ag/PZT的电容率；此外，长时高温作用加速了PZT陶瓷电畴转向无序状态，促进氧空位位移钉扎电畴壁，导致了PZT压电元件介电性能退化。     

Au-CZT与Au/Cr-CZT接触性能比较

本文通过附着力测试,应力模拟和IV特性及多道能谱分析对Au-CZT与Au/Cr-CZT接触进行研究。结果表明采用Au/Cr复合电极可提高电极附着强度和热稳定性。在老化实验中,Au-CZT界面附着力下降了61.98%,而Au/Cr-CZT仅降低28%。Au/Cr电极器件的漏电流较低,241Am射线下能谱响应更佳。分析其原因可能是Au与CZT间的Cr层降低了接触层内的热应力,合金化过程促进了金-半界面的互扩散,使Au和CZT更易形成欧姆接触,综合考虑Au/Cr复合电极能获得比Au电极更理想的接触性能。     

Pt/YSZ电极结构老化特性的复阻抗研究

借助于交流复阻抗测试技术和电子扫描电镜设备,对以YzO3稳定ZrO2固体氧离子导体(YSZ)为基体的Pt／YSZ电极结构电学性能和电极形貌随时间的老化变化特性进行了研究;根据作者提出了一种有关Pt／YSZ电极结构电极界面的形貌模型,对不同老化时间下的电极Pt／空气／YSZ三相界面的长度进行了定量计算。实验测试和模型计算等结果都表明：Pt／YSz电极结构的电学性能和电极形貌均随老化时间出现了明显的变化。     

一种改进的内聚力损伤模型在复合材料层合板低速冲击损伤模拟中的应用

针对传统内聚力损伤模型(CZM)无法考虑层内裂纹对界面分层影响的缺点,提出了一种改进的适用于复合材料层合板低速冲击损伤模拟的CZM。通过对界面单元内聚力本构模型中的损伤起始准则进行修正,考虑了界面层相邻铺层内基体、纤维的损伤状态及应力分布对层间强度和分层扩展的影响。基于ABAQUS用户子程序VUMAT,结合本文模型及层合板失效判据,建立了模拟复合材料层合板在低速冲击作用下的渐进损伤过程的有限元模型,计算了不同铺层角度和材料属性的层合板在低速冲击作用下的损伤状态。通过数值模拟与试验结果的对比,验证了本文方法的精度及合理性。     

Nb/Mg3SbBi界面层热稳定性研究

Zintl相Mg₃(Sb,Bi)₂基热电材料因在中低温区(27～500℃)表现出优异的热电性能而受到广泛关注。然而,由于Mg、Sb元素比较活泼,在长期高温服役下易与电极发生剧烈界面扩散反应,导致热电器件的性能和服役寿命衰减。因此,选择能有效阻挡元素剧烈互扩散并且具有低界面接触电阻率阻挡层材料至关重要。本研究首先利用热压工艺制备出300℃最高ZT～1.4的n型Mg₃SbBi(Mg_3.2SbBi_0.996Se_0.004)样品,然后采用Nb粉作为扩散阻挡层一步烧结制备Mg₃SbBi/Nb/Mg₃SbBi“三明治”结构样品,系统研究界面层的组成、微结构以及电阻随老化时间演变过程。加速老化实验(525℃/70 h; 525℃/170 h; 525℃/360 h)研究发现, Nb阻挡层中的Mg-Sb/Bi组分发生偏析,表面产生裂纹,抛光处理后界面连接完好,无裂纹和孔洞,界面扩散层厚度随老化时间延长缓慢增加至1.6μm。Nb/...     

CoSb3/MoCu热电接头的一步SPS法制备及性能评价

Mo-Cu合金电极被成功应用于CoSb₃基热电发电元件. 通过调节合金中铜的含量,Mo₅₀Cu₅₀合金取得了与CoSb₃良好的热匹配. 借助于放电等离子烧结(SPS), Mo-Cu合金通过钛层成功实现了与CoSb3热电材料的连接. 在CoSb₃/Ti/Mo-Cu界面区域,没有发现微裂纹,扫描电镜（SEM）显示在CoSb₃/Ti界面处生成了TiSb相. 500℃的热时效实验表明,CoSb₃/Ti界面区域的TiSb相厚度略有增加,显示了良好的热稳定性. 随着热时效时间的延长,接头的剪切强度降低. 四点探针测试表明,界面区域界面电阻很小,界面接触电阻率在20～30μΩ·cm²,显示了热电接头良好的电接触.     

Fe/Bi0.5Sb1.5Te3热电元件高温稳定性研究

热电元件的界面高温稳定性是决定热电器件服役性能和应用前景的重要因素,而阻挡层和热电材料之间的界面扩散和界面电阻则是评价热电元件高温稳定性的主要标准.为了进一步提升P型碲化铋热电器件的界面稳定性,本研究采用高通量筛选的方法选定适用于P型碲化铋的Fe阻挡层材料.通过一步烧结的方法制备了Fe/P-BT的热电元件,并系统研究了...     

Interfacial reaction between Au–Sn solder and Au/Ni-metallized Kovar

Gold-tin (Au–Sn) solder and Kovar alloy have been widely used in many fields such as mechanical engineering, atomic energy industry, aerospace facility, and electronic devices. Solder bonds strongly to the metallized substrate by forming intermetallic compounds (IMCs) at the interface. The IMC layer may adversely affect the reliability of the joints due to excessive growth and thermal fatigue during storage and service. Therefore, knowledge of the interfacial reactions between the Au–Sn solder and Au/Ni-metallized Kovar in microelectronic and optoelectronic packaging is essential. In this study, the microstructural evolution and interfacial reactions between the Au–Sn solder and Au/Ni-plated Kovar substrate were studied during aging at 180 and 250 °C for up to 1,000 h. The microstructure of the Au–Sn/Ni/Kovar joint was stable during aging at 180 °C. The solid-state interfacial reaction was much faster at 250 °C than at 180 °C. The joints aged at 250 °C fractured along the interface, thereby demonstrating brittle failure possibly because of the brittle IMC layer at the interface. The complete consumption of the thin Ni layer significantly weakened the joint interface during aging at 250 °C and clearly demonstrated the need for a thicker Ni layer in order to ensure the high temperature reliability of the Au–Sn/Ni/Kovar joint above 250 °C.     

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from ?40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.     

Thermal shock reliability of micro–nano bimodal Cu–Ag sintered joints

In this study, we investigated the thermal shock reliability of die-attach technology using a micro–nano bimodal Cu–Ag paste, which can considerably reduce material costs compared with a nano-Ag paste. A reliability study of Cu-sintered joints can facilitate large-scale applications in the electric vehicle industry as only a few systematic studies have investigated the thermal shock reliability of low-cost Cu-sintered joints. To evaluate the thermomechanical stability and bond strength of the Cu–Ag sintered joints, a thermal shock test between ? 40 and 150 °C for 1000 cycles and die shear tests, respectively, were performed. The thermal shock test results clearly demonstrated that the micro–nano bimodal Cu–Ag sintered joints maintained a high strength (60 MPa) for 1000 cycles. The bimodal Cu–Ag paste die-attach is reliable because of stable microstructures that are free of cracks and interfacial debonding. The results showed that our bimodal Cu–Ag paste die-attach can be used in both Si and SiC power devices operating at high temperatures.

     

Enhancement of optical performance of the light emitting diode packages with advanced thermal design of die-attaching layers

To guarantee long lifetime of light emitting diodes (LEDs), thermal reliability of LED packages should be secured with suitable die-attaching materials. Die-attaching materials are important for the interconnection between optical devices and substrates, as they not only provide electrical transmission but also the emission of thermal budget generated under the operation of the devices. In this study, the joints with different die-attaching materials were closely investigated in conjunction with the optical performance of LED packages. Thermal reliability of the joints was quantitatively analyzed by estimating the thermal resistances of the metallic and intermetallic layers incorporating the joints. The experimental results revealed that insertion of nano-Ag paste between eutectic Au-Sn and Ag finish significantly improved heat emission by effectively suppressing thermal resistance of eutectic Au-Sn layer. However, to ensure long-term reliability, complete removal of numerous nano-voids among Ag nanoparticles should be accomplished to prevent accumulation of thermal budget.     

Crack formation in sapphire/niobium/sapphire joints under compression

Compression tests were carried out on UHV diffusion bonded single crystalline sapphire/Nb/sapphire joints to investigate their mechanical properties, the mechanisms that lead to the failure of the joints and the dislocation-interface interaction. The tests were performed for different orientation relationships (OR) at the interface to study the influence of different bond strength on the mechanical behavior. Additionally, the metal layer thickness was varied for each OR to alter the influence of the interface. The experimental results showed, that with decreasing metal layer thickness the stress needed to form a crack increases drastically, whereas for the Nb/sapphire system the bond strength at the interface seems to have no significant influence on the mechanical behavior of the joint. A theoretical model will be presented that explains the experimentally observed relationship between metal layer thickness and crack stress.     

Effect of post-weld aging treatment on mechanical properties of Tungsten Inert Gas welded low thickness 7075 aluminium alloy joints

This paper reports the influence of post-weld aging treatment on the microstructure, tensile strength, hardness and Charpy impact energy of weld joints low thickness 7075 T6 aluminium alloy welded by Tungsten Inert Gas (TIG). Hot cracking occurs in aluminium welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification. Weld fusion zones typically exhibit microstructure modifications because of the thermal conditions during weld metal solidification. This often results in low weld mechanical properties and low resistance to hot cracking. It has been observed that the mechanical properties are very sensitive to microstructure of weld metal. Simple post-weld aging treatment at 140 °C applied to the joints is found to be beneficial to enhance the mechanical properties of the welded joints. Correlations between microstructures and mechanical properties were discussed.     

Stress analysis for bonded materials with a graded interfacial layer under a rigid punch

It is well known that functionally graded materials can be used to eliminate the stress discontinuity that is often encountered in multilayer composites. In this article, the stress analysis for the coating-functionally graded interfacial layer-substrate structure under a rigid spherical punch is investigated. A linear multi-layer model is used to model the graded interfacial layer with arbitrary varying materials properties along the thickness direction. The spherical indentation problem is formulated in terms of a singular integrate equation with the method of transfer matrix and Hankel integral transform technique. The stress components in the coating-graded interfacial layer-substrate structure are calculated by solving the equation numerically. The results show that stiffens ratio and the gradient index of the graded interfacial layer has a significant effect on the distribution of stress components.