Propagation of harmonic waves through micro gap with consideration of frictional contact

Transmission of elastic waves through a micro gap between two solids with consideration of frictional contact is investigated. By using the Fourier analysis technique and the corrective solution method, the nonlinear boundary problem is reduced to a set of algebraic equations. Numerical results exhibit the locations and extents of separation, slip, and stick zones, the interface tractions, and the energy partition. The effects of gap width, frictional coefficients, and the incident angle on the wave transmission are discussed in detail. The results show that higher harmonics are generated due to the local contact/slip at the interface.

Influence of arc pressure on the forming of molten pool in tungsten inert gas arc butt welding with micro gap for tantalum sheet

Arc pressure is the key influencing factor to forming of molten pool. Countering the characteristic of tungsten inert gas arc welding with micro gap for tantalum sheet, according to the fundament of arc physics, a distribution model of arc pressure and forming mechanism of molten pool with micro butt gap are proposed, and the influences of arc pressure on forming of molten pool are discussed. Experimental researches for the dynamic formation process of weld molten pool by using high-speed vidicon camera show that when butt gap is appropriate, that is from 0. 1 to 0. 15 mm, molten metals formed on two workpiece uplift and grow up first, then are fused and form uniform molten pool finally.

Treatment of Invasive Marine Species on Board by Using Micro-gap Discharge Plasma

A pilot-scale experiment of 20t/h for the treatment of ship＇s ballast water is reported in this paper. When the concentration of the dissolved OH. was 0.68 mg/L during the experiment, the rate of destroying bacteria, mono-algae, protozoan reached 100% within 2.67 s. The effect of hydroxyl radicals on biochemical processes was also studied. The attenuation rate of photosynthesis pigment was 100%. And the main reason for the cells＇ death was the strong destruction of the monose, amylose, protein, DNA and RNA in the cells. As an advanced oxidation method, the procedure can destroy invasive marine species when a ship is in the process of discharging its ballast water.

Discharge characteristics of a needle-to-plate electrode at a micro-scale gap

To understand the discharge characteristics under a gap of micrometers,the breakdown voltage and current–voltage curve are measured experimentally in a needle-to-plate electrode at a microscale gap of 3–50 μm in air.The effect of the needle radius and the gas pressure on the discharge characteristics are tested.The results show that when the gap is larger than 10 μm,the relation between the breakdown voltage and the gap looks like the Paschen curve;while below 10 μm,the breakdown voltage is nearly constant in the range of the tested gap.However,at the same gap distance,the breakdown voltage is still affected by the pressure and shows a trend similar to Paschen＇s law.The current–voltage characteristic in all the gaps is similar and follows the trend of a typical Townsend-to-glow discharge.A simple model is used to explain the non-normality of breakdown in the micro-gaps.The Townsend mechanism is suggested to control the breakdown process in this configuration before the gap reduces much smaller in air.

Effects of Hydroxyl Radicals on Introduced Organisms of Ship's Ballast Water Based Micro-Gap Discharge

With the physical method of micro-gap gas discharge, OH. radicals were produced by the ionization of O2 in air and H2O in the gaseous state, in order to explore more effective method totreat the ship＇s ballast water. The surface morphology of Al2O3 dielectric layer was analysed using Atomic Force Microscopy （AFM）, where the size of Al2O3 particles was in the range of 2 μm to 5 μm. At the same time, the biochemical effect of hydroxyl radicals on the introduced organisms and the quality of ship＇s ballast water were studied. The results indicate that the main reasons of cell death are lipid peroxide and damage of the antioxidant enzyme system in Catalase （CAT）, Peroxidase （POD） and Superoxide dismutase （SOD）. In addition, the quality of the ballast water was greatly improved.

Gap junctions enhance the antiproliferative effect of microrna-34a in glioma cells

OBJECTIVE To investigate the effect of gap junctions on the anti-tumor function induced by mi R-34a in glioma U87 cells.METHODS 1.Transfection(miR-34a mimics were transfected into glioma cells to upregulate their expression);2.Co-culture assay(U87cells were transfected with mi R-34a co-cultured with U87 cells that was transfected PCMV-eG FP plasmid);3.Flow cytometry analysis(select the e GFP labed U87 cells);4.RNA isolation and real-time PCR;5.CCK-8 assay;6.Western blotting.RESULTS Mi R-34a mimics transfered between the U87 cells.Parachute assay showed that GJ inhibition(CBX and 18-α-GA)can decrease mi R-34a expression than co-culture group.RA and galanglin enhanced mi R-34a expression than co-culture group.Mi R-34a relative expression reduced after co-culture,while gap junctions composed of Cx43 were down-regulated by sh RNA.Transfected with mi R-34a mimics reduced the survival of U87 cells in a dose-dependent manner.To more specifically establish the role of GJIC in mi R-34a induced growth inhibition of U87 cells,si RNA was used to knockdown the expression of Cx43,the dominant connexin expressed in U87 cells.CCK-8 assay showed that siR NAs have no effect on cell growth,but they could aggravate the growth inhibition of miR-34a to U87 cels.CONCLUSION Gap junctions enhance the antiproliferative effect of miR NA-34a in glioma cells.

环形间隙式层流元件设计及流量特性研究

为了解决传统层流流量计(LFM)线性度不佳、长径比较大、加工使用不便和结构易受流体影响等诸多问题,受双锥流量计的启发,提出了一种环形间隙式层流元件结构,介绍了测量原理和流道内非线性压力损失来源。保持该结构外套筒体和圆柱锥体同轴心,其流道截面为同心圆环,通过CFD仿真确定了锥形导流结构的合理锥形角度,确定了层流元件的尺寸参数。将取压孔设置在流道中充分发展的层流段,理论上消除了传统毛细管式LFM进出口处流动局部损失和层流发展段的动能损失。制作3种不同间隙大小的试件并进行试验,结果显示,当测量流量值小于53 mL/min时,层流元件的测量误差在3%以内;当测量流量值在(130~6189)mL/min时,测量误差在±2%以内;层流元件的压差和流量之间具有优秀的线性关系。说明环形间隙式层流元件结构可有效克服传统LFM的非线性影响,同时测量流量范围可随间隙大小变化而改变。

保压候凝对页岩气井固井质量的影响及机理分析

页岩气井开发过程面临钻压同步的情况,常伴有套变现象发生,影响后期压裂改造。为此,油田采取保压候凝措施预防套变。通过跟踪页岩气井保压候凝后续固井质量情况,发现采取该措施的页岩气井存在质量分界现象,即保压候凝后,在某一井深范围会出现质量反转,固井质量差异明显。通过统计数据与测井曲线图特征分析,探索总结其影响规律,并结合弹性力学厚壁筒理论对其进行了机理分析。对于抗外挤强度高的套管,在满足抗外挤条件下,不建议候凝期间采取管内保压措施。如果必须采取保压措施,建议采用弹性水泥浆、树脂水泥浆等体系进行浆体设计,并合理控制保压值以减小微间隙的产生。另外,还可在重叠段关键位置使用遇烃遇水自膨胀管外封隔器封堵气窜通道,降低后期环空带压机率。

数字普惠金融对城乡收入差距的影响及其机制

以2011—2021年全国284个地级市为样本,使用北京大学数字普惠金融指数及其三个分维度指数衡量数字普惠金融发展水平,构建泰尔指数衡量城乡收入差距,从宏观视角来看,数字普惠金融发展能够缩小城乡收入差距,这一缩小效应在中西部地区明显较强。在此基础上,使用北京大学数字普惠金融指数及其三个分维度指数匹配CFPS2012—2020五期数据集,从农村家庭这一微观社会单元入手,揭示数字普惠金融通过缓解农村家庭正规信贷约束、改善农村家庭生产经营、提高农村家庭收入水平等微观机制赋能农村居民增收从而缩小城乡收入差距。

CO_(2)腐蚀-应力耦合下固井水泥环密封完整性

CO_(2)地质封存过程中,与地层围岩中的水反应后腐蚀着固井水泥环,腐蚀损伤和套管内压(应力)耦合作用极大地影响着水泥环的密封完整性。基于CO_(2)腐蚀实验,获得不同腐蚀程度水泥石材料力学性能参数,采用混凝土损伤塑性(CDP)本构模型和Mohor-Coulomb准则描述腐蚀前后水泥环的应力-应变行为,利用ABAQUS软件建立考虑CO_(2)腐蚀与应力耦合作用的井筒组合体(套管-水泥环-地层围岩)有限元分析模型,分析和探讨了套管内压和腐蚀时间对水泥环完整性的影响。结果表明,较高套管内压下,井筒水泥环发生弹塑性变形,出现结构损伤,套管与水泥环界面易形成微间隙;受腐蚀和套管内压的耦合作用,水泥环更易于出现完整性失效问题,相比较于未腐蚀水泥环,腐蚀水泥环受压后径向应力、等效塑性应变、微间隙以及拉伸和压缩损伤均较大,与之相反,塑性半径是减小的;微间隙与拉伸和压缩损伤受腐蚀时间的影响不明显。

数字金融与区域创新差距

数字金融作用于区域创新发展路径,对推动区域创新均衡协调发展具有重要意义。结合北京大学数字普惠金融指数、省市级宏观数据和上市公司等多维度数据,基于泰尔指数、基尼系数和阿特金森指数测度了区域创新发展差异,多方位考察数字金融指数及数字覆盖范围、使用程度、支持服务力度与区域创新差距之间的关系,并探索数字金融影响区域创新差距的传导机制和微观基础。研究发现,数字金融的发展有利于缩小省内各地级市间的创新差距,并且数字金融的普惠效应在东部地区和省份内部创新差异较小的地区更强。进一步的传导机制分析表明,数字金融能够促进传统金融与信息基础设施的有效结合,提高信息基础设施对区域创新差距的边际影响。但值得注意的是,数字金融并不能显著推动省份之间创新的均衡化发展。微观分析结果表明,数字金融能够缩小企业间创新差距,但是不利于东部和中西部地区之间创新的均衡化发展。因此要进一步深化数字金融发展,实施差异化的发展策略,为区域创新的平衡发展提供强劲动力。研究结论不仅能够为推动区域技术创新、缩小地区创新差距提供理论基础,还可以为区域协调发展提供一定的启示。

基于激光烧蚀微推进技术的GAP靶带致密性研究

从用于激光烧蚀微推进技术靶带的药剂配方、涂覆厚度、固化温度等方面入手,研究了各因素对靶带固化速率以及致密性的影响。对比研究了甲苯二异氰酸酯(TDI)、六亚甲基二异氰酸酯(HDI)、异佛尔酮二异氰酸酯(IPDI)、异氰酸酯(N100)等固化剂对聚合物端羟基聚叠氮缩水甘油醚(GAP)的固化速率和固化效果。结果显示,IPDI固化效果最佳。通过激光扫描共聚焦显微镜和场发射扫描电子显微镜观测了样品的靶带表面气孔率,并确定了涂覆厚度为200μm、固化温度为40~50℃的最佳工艺条件,使靶带气孔率在符合激光微推进系统要求下达到最低。

烟尘超低排放技术在湿法脱硫塔上的应用研究

在不对锅炉原有除尘系统进行改动且停炉时间最短的条件下,仅在湿法脱硫塔的上部,安装专利产品—全旋流微湿电除尘除雾装置,使烟尘满足超低排放标准。本文结合应用实践对“全旋流微湿电除尘除雾技术”进行分析研究,为面临烟尘排放难题的企业提供一种新的烟尘解决方案。

甲烷/空气混合气体在针板电极下的微间隙放电特性

研究爆炸性环境下微间隙放电过程及特性对本质安全电路的研究具有指导性意义,但目前爆炸性环境下的微间隙放电特性尚不明晰,且大多从宏观角度进行研究,缺少微观层面的理论分析,该文根据程控微间隙放电实验装置的电极结构,基于流体动力学理论模拟研究了甲烷/空气混合气体在低电压微间隙放电动态过程。通过改变极距、甲烷/空气混合气体体积分数的变化来分析对电子密度、各种正离子数密度、电场强度和平均电子能量的影响,从微观层面分析了微间隙放电机理,并通过试验验证了仿真结果的真实性和可靠性。经研究发现:阴极板附近的正离子鞘层区使得阴极板场强发射发生畸变,畸变为原来的3~5倍,电场强度大于场致发射所需临界电场强度,放电过程中存在场致发射;甲烷体积分数在3.5%~13.5%范围变化时,体积分数每增加5%,CH_(4)^(+)数密度约增加2×10^(15)m^(-3);当针板距离小于10μm时,电场畸变较大,满足场致发射条件,汤森放电和场致发射共同存在,平均电子能量上升两次;大于或等于10μm时,电场强度畸变程度变弱,低于场致发射电场强度临界值,放电机制为汤森放电,平均电子能量只上升一次。

掺杂吸收剂对激光烧蚀GAP推进性能的影响

对激光烧蚀微推力器而言,聚合物靶材的激光烧蚀推进性能至关重要。为了研究掺杂剂对靶材推进性能的影响,寻找优化靶材性能的方法,以含能聚合物聚叠氮缩水甘油醚(GAP)作为烧蚀靶材,以纳米碳粉和红外染料作为掺杂剂,采用高精度扭摆、能量计和高倍显微镜测量的方法,以冲量耦合系数、比冲和能量转化效率参数作为量化指标,改变聚合物的掺杂浓度、靶材厚度以及激光入射方式,对掺杂不同吸收剂后的GAP的推进性能进行了研究。结果表明:纳米碳粉以局部热区的形式加速聚合物的气化分解,推进性能受掺杂浓度的影响较小;掺杂纳米碳粉的GAP表现为面吸收特性,随着靶材厚度增加,喷射产物的气化程度降低,聚合物推进性能下降;较薄的掺杂纳米碳粉的GAP的等离子体阈值低,表现出高比冲和高能量转化效率,其中能量转化效率最优值接近300%,适合作为透射式激光烧蚀微推力器的烧蚀靶材。红外染料对1064nm波长的激光具有较好的吸收性能,掺杂红外染料使GAP对激光能量的吸收增强,化学能释放更充分;聚合物的推进性能对红外染料的掺杂浓度十分敏感,在掺杂浓度为5%时性能表现优秀;不同浓度红外染料掺杂下GAP的烧蚀深度不同,当靶材的厚度与烧蚀深度相近时,GAP的推进性能表现最好。

A study on relation between acoustic emission and characteristic displacement field on the sample with multi en echelon structures——The theoretic and experimental explorations of strain gap

Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographically by real-time holographic interferometry and shadow optical method of caustics, as soon as the loading process started. In the meantime, the AE (acoustic emission) signals were recorded by a micro crack information storage-analysis sys-tem. According to damage theory and location of micro fracture, we have studied the stain gap and gained: a) It is necessary that strain gap appears under the condition of linear elasticity theory, and its situation is relatively stable, corresponding to stress concentration. b) Micro fractures, which appear initially at area of high stress, occur rarely at the strain gap, and their locations are finally in the zone between the stress concentration area and the strain gap, which indicate the clusters or groups. However, the major macro fracture (final rupture) started from the shadow areas, and then grew quickly towards the strain gaps, which resulted in failure of sample.

Analysis of Fatigue and Microhardness in Metallic Powder Mixed EDM

The objective of this work research is to investigate the potential of using metallic powder mixed with electrical discharge machining (EDM) dielectric when machining hard electrically conductive materials. Nowadays, the development of industries requires hard materials for various applications. Machining the hard materials using the traditional processes lead to tool break and poor machined product. Even when the conventional EDM can machine hard material as long as it is electrically conductive materials, the machined parts still present drawbacks. Metallic powder mixed with EDM dielectric (PMEDM) was hypothesized to improve the machined part. The presence of metallic powder ensures uniform distribution of spark and the electrical density of the spark decreases which reduces craters, cracks and voids on machined surface. The transfer and deposit of alloying elements during powder mixed electrical discharge machining improve the machined surface properties particularly micro-hardness and fatigue. Discharge current (IP), gap voltage (GapV), ON-time (ON) and aluminum powder are selected as machined variable parameters and the output responses are fatigue performance, micro-hardness and surface topography. The workpiece material selected is molybdenum high speed steel. Micro-hardness was determined using micro-hardness tester device. The fatigue performance was determined using empirical equation. Analysis of material transfer was done using energy dispersive spectroscopy (EDS) attached to FESEM. EDS analysis involves the generation of an X-ray spectrum from the entire scan area of the SEM. The use of PMEDM improved the fatigue, the micro-harness and the machined surface morphology as the above-mentioned parameters increased.

二维活塞/缸体副微间隙流场的剪切空化实验研究

二维液压元件能充分利用二维活塞的面密封以及周向旋转配流等特点以获得更好的工作性能,但面临二维活塞与缸体配合微间隙中油液强剪切可能诱发空化的问题。为探究该处微间隙内油液的一维运动工况,利用自制的外筒旋转同心圆筒装置开展纯剪切流动实验研究,分别就不同转速、流场压力、油液上方空气占比、内筒外壁面配流槽结构以及微间隙厚度进行可视化研究。实验结果表明:流场压力与发生剪切空化的临界剪切应力成线性增大关系,压力越大油液空化所需剪切应力越大;若油液上方空间内留有空气,则上方空气含量越高对剪切空化的抑制作用越强;而二维活塞的配流槽结构一方面会减小油液的剪切扭矩,另一方面会改变纯剪切流场,从而抑制剪切空化的发生;微间隙厚度的减小将使发生剪切空化的临界剪切应力减小,从而降低剪切空化发生的难度。

微间距气压对场致发射影响的分析与计算

为研究微米间隙中气体的击穿特性,在3~100μm间隙距离中进行1~100 kPa可变气压的放电试验。试验发现10μm以下间距击穿电压曲线偏离巴申曲线,固定间距只改变压强时,击穿电压随气压的变化与较长间隙不同,3~5μm间距的击穿电压会随气压升高而降低。通过求解阴极附近离子数密度公式,发现微间距条件下阴极附近有较高的离子数密度,离子数密度会随气压的升高而增加,随间距增大而减小。离子数密度越高,在阴极表面附近形成离子附加电场就越强,与外电场耦合后促进场致发射,击穿会更易发生。计算了离子增强场效应系数γ’,并将其与Townsend二次发射系数γ对比,发现离子增强场效应对5μm以下的间隙放电影响最强烈,随间距的增加其影响逐渐减弱,10μm后击穿过程可用Townsend理论来解释。