空域PCR芯片/微装置中温度控制技术

基于微电子机械系统(MEMS)技术而发展起来的空域聚合酶链式反应(空域PCR)芯片/微装置由于其具有反应速度快、所需DNA样品体积小以及自动化程度高等优点,已经日益引起人们的关注。空域PCR芯片/微装置系统中精确的温度控制是DNA成功扩增的关键因素之一。主要介绍国内外空域PCR芯片/微装置中温度控制技术进展,包括空域PCR系统温度循环特征;薄膜电阻加热器/传感器的设计原则;空域PCR反应系统温度均一性等。     

CuZnAl（Mn）合金双程记忆性能的衰减特性

本文探讨了CuZnAl,CuZnAlMn合金的弹簧试样在相变热循环过程中双程记忆性能的变化规律,表明引起铜基形状记忆合金双程记忆性能衰减的主要因素是由于位错增殖;在有外加应力条件下热循环时,还要考虑应力诱发马氏体对记忆性能的贡献。少量Mn的加入使CuZnAl合金在热循环过程中位错增殖加速,同时使应力诱发马氏体容易产生,结果导致在有外加应力条件下热循环时Mn能缓和记忆性能的衰减。     

Reliability of lead-free interconnections under consecutive thermal and mechanical loadings

To simulate more realistically the effects of strains and stresses on the reliability of portable electronic products, lead-free test assemblies were thermally cycled (−45°C/+125°C, 15-min. dwell time, 750 cycles) or isothermally annealed (125°C, 500 h) before the standard drop test. The average number of drops to failure increased when the thermal cycling was performed before the drop test (1,500 G deceleration, 0.5 ms half-sine pulse). However, the difference was not statistically significant due to the large dispersion in the number of drops to failure of the assemblies drop tested after the thermal cycling. On the other hand, the average number of drops to failure decreased significantly when the isothermal annealing was carried out before the drop test. The failure analysis revealed four different failure modes: (1) cracking of the reaction layers on either side of the interconnections, (2) cracking of the bulk solder, (3) mixed mode of component-side intermetallic and bulk solder cracking, and (4) voidassisted cracking of the component-side Cu₃Sn layer. The assemblies that were not thermally cycled or annealed exhibited only type (1) failure mode. The interconnections that were thermally cycled before the drop test failed by mode (2) or mode (3). The drop test reliability of the thermally cycled interconnections was found to depend on the extent of recrystallization generated during the thermal cycling. This also explains the observed wide dispersion in the number of drops to failure. On the other hand, the test boards that were isothermally annealed before the drop testing failed by mode (4).     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

提高铅酸蓄电池循环寿命的研究

通过对不同极板厚度、不同电解液密度的铅酸蓄电池的初期容量、国标循环寿命和不同限压值的恒流限压充电对循环寿命的研究,以及对寿命终止电池的解剖分析,得出结论:适当增加正极板厚度,降低电解液密度,选择最佳的恒流限压充电的限压值能够提高电池的循环寿命。     

Predicting the onset of high cycle fatigue damage: an engineering application for long crack fatigue threshold data

Fatigue crack propagation rates have been measured for two titanium-based aeroengine disc alloys using compact tension test-pieces. The loading block employed simulates two features of the engine flight pattern. A major stress cycle represents the start-stop operation which leads to low cycle fatigue. In-flight vibrations, which may give rise to high cycle fatigue, are represented by superimposed minor cycles of high frequency. The threshold values associated with the minor cycles have been used to predict the onset of minor cycle activity. These predictions are successful for Ti-6A1-4V, whilst for Ti-5331S they are found to be either accurate or safe. T-5331S exhibits the greater fatigue lives by virtue of a marginally greater resistance to the onset of minor cycle crack growth combined with slower major cycle crack growth rates prior to this event.     

CO Oxidation Kinetics over a Nanostructured Cu0.1Ce0.9O2?y Catalyst: A CO/O2 Concentration Cycling Study

Feed composition cycling as a transient kinetic technique provides detailed information about elementary steps in CO oxidation over a nanostructured Cu_0.1Ce_0.9O_2–y catalyst. This catalyst has a great potential as a future PROX reactor catalyst since it has great oxygen storage capacity as well as high reoxidation rate under oxygen rich conditions.     

.NET解决VB6中"循环引用"问题的分析与比较

计算机中的内存资源是非常宝贵的,在多任务环境中,有效、充分利用内存是系统稳定运行的关键,面向对象的程序设计中,对象占用内存资源使用与回收是程序设计中较难解决的问题,通过对VB6环境中“循环引用”问题的分析,比较了NET及VB6环境下的两种“垃圾处理”机制以及存在问题、优缺点。阐述了NET对VB6中“循环引用”问题的解决方法。     

Fatigue Fracture in VNS-25 Stainless Steel

The paper covers the experimental study of the influence of cycling frequency ranging from 20 Hz to 10 kHz and the cycle asymmetry on the crack growth resistance of VNS-25 stainless steel. It is demonstrated that as the cycling frequency is raised, the crack growth rate decreases monotonically, while threshold values of the stress intensity factor K_th grow. The latter is due to the fact that at high cycling frequencies the plastic deformation processes remain incomplete and the contribution of the fracture mechanisms that facilitate greater energy intensity of fatigue crack growth processes becomes more significant. A comparison of K_th and fatigue life values as a function of cycling frequency for smooth and notched specimens demonstrates that if represented in relative coordinates these functions are described by a common relationship. This allows a quantitative assessment of the effect of the strain cycling rate on the above-mentioned properties of a material from the results of tests of one type only. It has been found that the values K_th in asymmetrical loading cycles are related to those in symmetrical cycles, as well as to a coefficient of material's sensitivity to the static component, and linearly depend on the load ratio.     

Microstructural Evolution and Cracking of Pb-free Ball Grid Array Assemblies under Thermal Cycling

Two kinds of CBGA （ceramic ball grid array） assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy （SEM） and finite element method （FEM）. Before thermal cycling, intermetallic compounds （IMCs） Cu6Sn5 and Ag3Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu3Sn was observed near the Cu pad in both two assemblies and the layers of Cu6Sn5 and Ag3Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.