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981.
Flexible circuit boards are being widely used in the electronic packages. Solders are often used to assemble chip resistors and other components on them. In practice, solder alloys work in high homologous temperatures and experience cyclic temperature loadings. As a result, damage may accumulate in solder materials quickly and this will eventually lead to the failure of the solder joints. In this work, computer modelling technique has been used to predict such damage accumulations in chip resistor solder joints under a range of thermal cycling conditions. It has been documented that the higher and the lower dwell temperatures of a thermal cycle dominate the damages in solder joints. Both the ramp time and the cycle duration have strong influence on the damage accumulation. In general, faster ramp time and longer cycle duration cause more damages. The types of materials used to produce flexible circuit board have also significant impact on the damage accumulation. Polyimide (PI), Polyethylene Naphthalate (PEN) and Liquid Crystal Polymer (LCP) based flexible circuit boards have been compared for their effect on damage in solder joints and the results show that the highest damage could be found in the chip resistor solder joint on the PI-based flexible circuit boards and least damage could be found for the LCP based flexible circuit boards. The results also show that the thicknesses of the constituent layers of different materials in flexible circuit boards are linearly proportional to the damage accumulation in solder joints. 相似文献
982.
Yong-Hun Cho In-Su Park Yoon-Hwan Cho Oh Joong Kwon Yung-Eun Sung 《Electrochimica acta》2010,56(2):717-721
The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance. 相似文献
983.
A two‐step pulsed pressure‐swing adsorption (PPSA) process has been modeled to assess the extent to which an oxygen concentrator might be miniaturized for medical applications. The process consists of a single bed of packed adsorbent particles that is alternately pressurized and depressurized at the feed end. An enriched oxygen product is withdrawn at ambient pressure from the product end when the bed is pressurized at the feed end. The product end remains closed during depressurization. The model development addresses the manner in which axial dispersion enters into the describing equations and the formulation of proper boundary conditions, both of which have not been handled rigorously in some prior modeling studies. The describing equations are solved using COMSOL® Multiphysics software. The effect on the performance of the adsorption time, desorption time, bed length, particle diameter, and imposed pressure drop across the bed have been investigated. An interesting novel result is that for a chosen particle size, bed length, and applied pressure drop, there is an optimum combination of adsorption and desorption times that maximizes the product purity. The results suggest that there are operating windows for both 5A and partially Ag‐exchanged Li‐substituted 13X zeolite adsorbents wherein the product oxygen purity is greater than 90%. At a given product flow rate within this operating window, the extent of miniaturization is limited by the (maximum) cycling frequency that is practically achievable. Sizing of an oxygen concentrator for personal medical applications is also discussed. A principal conclusion is that a compact oxygen concentrator capable of producing a highly oxygen‐enriched product is possible using commercially available adsorbents and implementable operating conditions. © 2009 American Institute of Chemical Engineers AIChE J, 2010 相似文献
984.
985.
Improved Reversibility of Fe3+/Fe4+ Redox Couple in Sodium Super Ion Conductor Type Na3Fe2(PO4)3 for Sodium‐Ion Batteries
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986.
Bas de Geus Grégory Vandenbulcke Luc Int Panis Isabelle Thomas Bart Degraeuwe Elke Cumps Joris Aertsens Rudi Torfs Romain Meeusen 《Accident; analysis and prevention》2012
The purpose of this study is to gain insight into bicycle accidents. Bicycle accident data and weekly exposure data were prospectively collected for one year to calculate the incidence rate (IR) of bicycle accidents. An accident was included if it occurred during utilitarian cycling, resulting in an acute injury with corporal damage. If an accident occurred, a detailed questionnaire was filled out to collect detailed information about its circumstances and consequences. 相似文献
987.
Abstract This paper applies the submodeling technique in analyzing thermal cycling reliability of high performance flip‐chip ball grid array package assemblies. The packages have one‐piece tunnel‐type heat spreaders with different lead widths, connected to chips using different thermal interface materials. The global model contains no solder bumps to simplify the analysis. The calculated displacement field of the global model is then interpolated on the boundary of the submodel that contains the critical solder bump. The submodel is solved using the prescribed displacement boundary conditions together with external thermal loads to evaluate thermomechanical reliability of the critical solder bump. 相似文献
988.
对苏丹膨胀土地区数个工程案例进行了调查分析,以常用的干作法钻孔灌注桩基础为例,分别采用理论计算与现场工程案例相结合的方式进行了论证,结果表明,干湿循环交替的膨胀土环境对单桩最终极限承载力影响非常大,并提出了经过实践检验的膨胀土地区桩基工程灾害防治措施。 相似文献
989.
990.
Alfredo Garcia Fernando Agustin GomezCarlos Llorca Antonio Angel-Domenech 《Accident; analysis and prevention》2015
Cycle track design guidelines are rarely based on scientific studies. In the case of off-road two-way cycle tracks, a minimum width must facilitate both passing and meeting maneuvers, being meeting maneuvers the most frequent. This study developed a methodology to observe meeting maneuvers using an instrumented bicycle, equipped with video cameras, a GPS tracker, laser rangefinders and speed sensors. This bicycle collected data on six two-way cycle tracks ranging 1.3–2.15 m width delimitated by different boundary conditions. The meeting maneuvers between the instrumented bicycle and every oncoming bicycle were characterized by the meeting clearance between the two bicycles, the speed of opposing bicycle and the reaction of the opposing rider: change in trajectory, stop pedaling or braking. The results showed that meeting clearance increased with the cycle track width and decreased if the cycle track had lateral obstacles, especially if they were higher than the bicycle handlebar. The speed of opposing bicycle shown the same tendency, although were more disperse. Opposing cyclists performed more reaction maneuvers on narrower cycle tracks and on cycle tracks with lateral obstacles to the handlebar height. Conclusions suggested avoiding cycle tracks narrower than 1.6 m, as they present lower meeting clearances, lower bicycle speeds and frequent reaction maneuvers. 相似文献