Microstructural evolution of lead-free Sn-Bi-Ag-Cu SMT joints during aging

It was reported in a previous study that the Sn-6Bi-2Ag-0.5Cu solder alloy had great potential to replace leaded alloys. This alloy was prepared by mechanical alloying, and had the advantage of providing a high percentage of supersaturate solution of bismuth in tin. In the present paper, the microstructural evolution of surface-mount joints during aging was examined. In the as-soldered joints, small bismuth and Ag₃Sn particles of about 1 mum in size were found to be finely dispersed in a nearly pure tin matrix with a small amount of eta-Cu₆Sn₅ phase in the bulk of solder. During aging, microstructural evolution of solder joints occurred. These include Cu-Sn intermetallic compound (IMC) layer growth at the interface between solder and copper pad on the printed circuit board, as well as bismuth phase and Ag₃Sn phase coarsening. The shear strength of the solder joints decreased parabolically with the increase in IMC layer thickness, such that tau_s=22.22-radic22.05(t-1.88), where tau_s is the shear strength in MPa and t (>1.88) is the total IMC layer thickness in micrometers. The microstructure of solder appeared to be stable under aging at elevated temperatures up to about 160degC. Above this temperature, brittle and porous IMC epsiv-Cu₃Sn appeared at the copper/eta-Cu₆Sn ₅ interface. Fracture was found to occur at the Cu-Sn IMC layer-solder interface and in the bulk of solder     

Impact of nonuniform graded dopant profile in polysilicon gate on gate leakage current

In a MOSFET, a nonuniform, graded vertical dopant profile in the polysilicon gate causes a potential drop at the polysilicon/oxide interface. In this paper, the effect of this potential drop on the gate leakage current has been evaluated for the first time. The extent of variations of this affected gate leakage current with gate oxide thickness, gate length, and gate and drain bias conditions have been assessed with device simulation for an nMOS at 0.13 /spl mu/m low-voltage process. The results provide a guideline to the severity of this effect from the point of view of device and circuit operation and standby power consumption.     

高层建筑屋面悬挑曲梁结构施工技术

本文通过工程实例,说明采用型钢和钢管组合支撑体系,解决高层建筑屋面悬挑曲梁结构的支模难题,保证工程质量和施工安全,并探讨施工中型钢的节点连接做法。     

Parathion degradation and toxicity reduction in solar photocatalysis and photolysis.

The solar photocatalytic degradation of methyl parathion was investigated using a circulating TiO2/solar light reactor. Under solar photocatalysis condition, parathion was more effectively degraded than solar photolysis and TiO2-only conditions. With solar photocatalysis, 20 mg/L of parathion was completely degraded within 60 min with a TOC decrease of 63% after 150 min. The main ionic byproducts during photocatalysis recovered from parathion degradation were mainly as NO3-, NO2- and NH4+, 80% of the sulphur as SO4(2-), and 5% of phosphorus as PO4(3-). The organic intermediates 4-nitrophenol and methyl paraoxon were also identified, and these were further degraded in solar photocatalytic condition. Two different bioassays (Vibrio fischeri and Daphnia magna) were used to test the acute toxicity of solutions treated by solar photocatalysis and photolysis. The Microtox test using V. fischeri showed that the toxicity expressed as EC50 (%) value increased from 5.5% to >82% in solar photocatalysis, indicating that the treated solution is non-toxic, but only increased from 4.9 to 20.5% after 150 min in solar photolysis. The acute toxicity test using D. magna showed that EC50 (%) increased from 0.05 to 1.08% under solar photocatalysis, but only increased to 0.12% after 150 min with solar photolysis, indicating the solution is still toxic. The pattern of toxicity reduction parallels the decrease in TOC and the parathion concentrations.     

Induction of immune tolerance with heart-thymus composite allotransplantation in rats

Objective To study on the role of thymus transplantation for heart allograft in rats. Methods Vascularized heart-thymus combined transplantation was performed with microsurgical technique. Graft survival, histopathology, level of IL-2, IL-4 and its mRNA expression in serum and cardiac grafts were investigated. Results Heart-thymus combined transplantation achieved effect in the prolongation of cardiac graft survival with short-term administration of cyclosporine. Conclusions Vascularized thymus transplantation induced immune tolerance in thymectomized rats.     

鄂尔多斯盆地中侏罗世直罗—安定期沉积构造特征

中侏罗世直罗—安定期伴随大华北盆地分化瓦解,直罗—安定组除在鄂尔多斯盆地呈整体分布外,在其周邻地区亦有大量零星分布的同期可对比地层。在现今盆地内,直罗组主要为河流相沉积;安定组以干旱气候条件下的河流、湖泊相沉积为主。直罗—安定组的湖相沉积主要位于盆地东南部的古沉积中心部位,其堆积中心位于盆地西部的乌海—鄂托克前旗—平凉一线,堆积中心与沉积中心偏离。直罗组沉积前古构造格局为南高北低、东高西低。直罗—安定期的古地质面貌呈西北高东南低特点,南、北分异表现甚微。结合周邻同期地层岩性、岩相和现今构造单元形成时限等综合分析,认为直罗—安定期原始盆地的边界大致为:西在贺兰山之西,东达吕梁山之东、太行山以西,北抵大青山,南至秦岭。     

Pervaporation separation of aqueous mixtures using crosslinked poly(vinyl alcohol), I. Characterization of the reaction between PVA and amic acid

For the purposes of new membrane material development for pervaporation separation based on crosslinked polyvinyl alcohol (PVA), IR spectroscopy and DSC were used to characterize the crosslinking reaction and the imidization of the (PVA)-amic acid system. The IR spectra and the thermal analysis were performed on specimens that had been reacted for several different times at 150°C. The crosslinking reaction between the hydroxy groups of PVA and the carboxylic groups of amic acid was faster than the imidization of the amic acid. The best reaction times were about 30 min for the crosslinking reaction and 90 min for the imidization. The effects of the reaction time and the amic acid content on the thermal and mechanical properties have also been investigated.