KAg_(3)Te_(2)with a layered crystal structure has been predicted to be a possible topological insulator.Through electrical transport measurements,we revealed its semiconducting behavior with a narrow band gap of~0.4 e...KAg_(3)Te_(2)with a layered crystal structure has been predicted to be a possible topological insulator.Through electrical transport measurements,we revealed its semiconducting behavior with a narrow band gap of~0.4 eV and p-type character.The infrared transmission spectra of single crystals yielded an optical band gap of~0.3 eV.Angle-resolved photoemission spectroscopy reveals a bulk energy gap at the Brillouin zone center,with no observable surface state,suggesting that KAg_(3)Te_(2)is a topological trivial narrow-gap semiconductor.The experimentally determined effective mass of the holes in KAg_(3)Te_(2)is very small(~0.12 me).The valence band maximum is quasi-two-dimensional,while the conduction band minimum is fully three-dimensional.Such intriguing dimensional anisotropy can be attributed to the distinct orbital contributions from K,Ag,and Te atoms to the respective bands.展开更多
Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are re- quir...Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are re- quired to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteris- tics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the tran- sition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.展开更多
为了提高窄间隙焊接的智能化水平,文中基于被动视觉传感方法,提出一种同步性好、鲁棒性强的窄间隙焊接图像坡口边缘感兴趣区域(region of interest,ROI)窗口的自适应定位方法,解决焊接过程中因坡口宽度变化带来的坡口边缘位置定位难、...为了提高窄间隙焊接的智能化水平,文中基于被动视觉传感方法,提出一种同步性好、鲁棒性强的窄间隙焊接图像坡口边缘感兴趣区域(region of interest,ROI)窗口的自适应定位方法,解决焊接过程中因坡口宽度变化带来的坡口边缘位置定位难、检测精度差的问题.通过调节坡口边缘ROI窗口水平平移量,揭示ROI窗口位置对坡口边缘图像背景与目标占比和边缘特征提取精度的影响规律,建立与电弧位置同步的坡口边缘ROI窗口生成机制,实现焊接过程中坡口边缘ROI图像的精准生成和坡口边缘的精确提取,从而提高窄间隙焊接工艺的适用性.通过变坡口焊接试验,进一步验证了所提方法在坡口边缘定位和检测中的适应性和有效性.结果表明,当坡口边缘ROI窗口顶点横坐标的水平平移量为ROI窗口宽度的50%时,检测的坡口边缘位置与平均位置差值的绝对值小于3.53μm,检测平均时间小于30 ms/帧,满足窄间隙焊接过程参数检测的精度和实时性要求.展开更多
气孔缺陷及焊缝组织不均匀是导致厚壁铝合金摇动电弧窄间隙熔化极气体保护焊(Gas metal arc welding,GMAW)接头力学性能降低的主要起因。为降低焊缝气孔缺陷及改善组织均匀性,本研究将超声频电源通过电容及电阻与摇动电弧窄间隙GMAW系...气孔缺陷及焊缝组织不均匀是导致厚壁铝合金摇动电弧窄间隙熔化极气体保护焊(Gas metal arc welding,GMAW)接头力学性能降低的主要起因。为降低焊缝气孔缺陷及改善组织均匀性,本研究将超声频电源通过电容及电阻与摇动电弧窄间隙GMAW系统进行并联,将超声能量引入5083铝合金窄间隙GMAW熔池内,系统研究电弧超声对5083铝合金窄间隙GMAW焊缝气孔及组织均匀性的影响规律。研究结果表明:利用电弧超声辅助5083铝合金摇动电弧窄间隙GMAW,有利于改善气孔分布的均匀性,减小气孔数量和气孔尺寸,进而降低焊缝的气孔率。单层焊气孔率最大下降幅度为60.8%,多层焊气孔率最大下降幅度为33.9%。施加电弧超声后,焊缝各部位晶粒尺寸均有所减小。无电弧超声和施加电弧超声下的晶粒直径的方差分别为2.17×10^(3)和1.48×10^(3),5083铝合金摇动电弧窄间隙多层焊组织性均匀性得到显著改善。展开更多
为研究窄间隙P-GMAW(pulsed gas metal arc welding,脉冲熔化极气体保护焊)过程中电弧行为对侧壁热输入的影响,分析了焊枪在距离侧壁不同位置的电弧行为变化规律。在双椭球模型基础上,采用坐标变换的方式建立了考虑电弧运动和偏转的热...为研究窄间隙P-GMAW(pulsed gas metal arc welding,脉冲熔化极气体保护焊)过程中电弧行为对侧壁热输入的影响,分析了焊枪在距离侧壁不同位置的电弧行为变化规律。在双椭球模型基础上,采用坐标变换的方式建立了考虑电弧运动和偏转的热源模型,获得了焊接温度场的变化规律。发现增大焊枪与侧壁的距离,电弧的燃烧位置由侧壁向坡口底部逐渐过渡。电弧在侧壁上按脉冲频率变化:电流基值阶段电弧作用在侧壁位置,峰值阶段电弧作用在坡口底部。温度场结果表明,焊枪距离侧壁1.5 mm左右时,焊缝成形良好,改进的热源模型能准确的描述焊枪在距侧壁不同位置时电弧对侧壁的热输入。采用考虑电弧行为的热源模型能够获得更加准确的温度场分布。展开更多
基金Project supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China.
文摘KAg_(3)Te_(2)with a layered crystal structure has been predicted to be a possible topological insulator.Through electrical transport measurements,we revealed its semiconducting behavior with a narrow band gap of~0.4 eV and p-type character.The infrared transmission spectra of single crystals yielded an optical band gap of~0.3 eV.Angle-resolved photoemission spectroscopy reveals a bulk energy gap at the Brillouin zone center,with no observable surface state,suggesting that KAg_(3)Te_(2)is a topological trivial narrow-gap semiconductor.The experimentally determined effective mass of the holes in KAg_(3)Te_(2)is very small(~0.12 me).The valence band maximum is quasi-two-dimensional,while the conduction band minimum is fully three-dimensional.Such intriguing dimensional anisotropy can be attributed to the distinct orbital contributions from K,Ag,and Te atoms to the respective bands.
基金Supported by the 973 Project of China (G1999022308) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-SW-L05)
文摘Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are re- quired to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteris- tics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the tran- sition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.
文摘为了提高窄间隙焊接的智能化水平,文中基于被动视觉传感方法,提出一种同步性好、鲁棒性强的窄间隙焊接图像坡口边缘感兴趣区域(region of interest,ROI)窗口的自适应定位方法,解决焊接过程中因坡口宽度变化带来的坡口边缘位置定位难、检测精度差的问题.通过调节坡口边缘ROI窗口水平平移量,揭示ROI窗口位置对坡口边缘图像背景与目标占比和边缘特征提取精度的影响规律,建立与电弧位置同步的坡口边缘ROI窗口生成机制,实现焊接过程中坡口边缘ROI图像的精准生成和坡口边缘的精确提取,从而提高窄间隙焊接工艺的适用性.通过变坡口焊接试验,进一步验证了所提方法在坡口边缘定位和检测中的适应性和有效性.结果表明,当坡口边缘ROI窗口顶点横坐标的水平平移量为ROI窗口宽度的50%时,检测的坡口边缘位置与平均位置差值的绝对值小于3.53μm,检测平均时间小于30 ms/帧,满足窄间隙焊接过程参数检测的精度和实时性要求.
文摘气孔缺陷及焊缝组织不均匀是导致厚壁铝合金摇动电弧窄间隙熔化极气体保护焊(Gas metal arc welding,GMAW)接头力学性能降低的主要起因。为降低焊缝气孔缺陷及改善组织均匀性,本研究将超声频电源通过电容及电阻与摇动电弧窄间隙GMAW系统进行并联,将超声能量引入5083铝合金窄间隙GMAW熔池内,系统研究电弧超声对5083铝合金窄间隙GMAW焊缝气孔及组织均匀性的影响规律。研究结果表明:利用电弧超声辅助5083铝合金摇动电弧窄间隙GMAW,有利于改善气孔分布的均匀性,减小气孔数量和气孔尺寸,进而降低焊缝的气孔率。单层焊气孔率最大下降幅度为60.8%,多层焊气孔率最大下降幅度为33.9%。施加电弧超声后,焊缝各部位晶粒尺寸均有所减小。无电弧超声和施加电弧超声下的晶粒直径的方差分别为2.17×10^(3)和1.48×10^(3),5083铝合金摇动电弧窄间隙多层焊组织性均匀性得到显著改善。
文摘为研究窄间隙P-GMAW(pulsed gas metal arc welding,脉冲熔化极气体保护焊)过程中电弧行为对侧壁热输入的影响,分析了焊枪在距离侧壁不同位置的电弧行为变化规律。在双椭球模型基础上,采用坐标变换的方式建立了考虑电弧运动和偏转的热源模型,获得了焊接温度场的变化规律。发现增大焊枪与侧壁的距离,电弧的燃烧位置由侧壁向坡口底部逐渐过渡。电弧在侧壁上按脉冲频率变化:电流基值阶段电弧作用在侧壁位置,峰值阶段电弧作用在坡口底部。温度场结果表明,焊枪距离侧壁1.5 mm左右时,焊缝成形良好,改进的热源模型能准确的描述焊枪在距侧壁不同位置时电弧对侧壁的热输入。采用考虑电弧行为的热源模型能够获得更加准确的温度场分布。
