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排序方式: 共有199条查询结果,搜索用时 15 毫秒
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Insung Hwang Dongcheol Kim Munjin Kang Jae-Hyun Kwak Young-Min Kim 《Metals and Materials International》2017,23(2):341-349
Using alternating current (AC)- and direct current (DC)-type welders, the resistance spot weldability of lightweight steel was evaluated under various electrode forces, welding currents, and times. The acceptable welding conditions were specified; however, these had very narrow ranges and there was little difference between the conditions determined for the AC- and DC-type welding. In both types of welding with electrode forces of of 300 kgf and 400 kgf, the acceptable weld currents were 5.0 kA and 5.5 kA, respectively. Also, the nugget size increased with the welding current. Under the acceptable welding conditions, there were no significant changes in the maximum tensile shear strength and nugget size, as 6.4-6.6 kN and 4.1-4.3 mm, respectively. The microstructure of weld metals was consisted of martensite, austenite and ferrite. And the small fraction of martensite was founded in the heat affected zone (HAZ), therefore the weld metal had the greatest hardness, and HAZ softening did not occur in this study. Considering the fracture surface, cleavage and ductile fracture were investigated because of the existence of martensite and ferrite in the welds. 相似文献
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Nanogenerators: Highly‐Efficient,Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates (Adv. Mater. 16/2014) 下载免费PDF全文
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Junjie Guo James R. Morris Yungok Ihm Cristian I. Contescu Nidia C. Gallego Gerd Duscher Stephen J. Pennycook Matthew F. Chisholm 《Small (Weinheim an der Bergstrasse, Germany)》2012,8(21):3283-3288
A scanning transmission electron microscopy investigation of two nanoporous carbon materials, wood‐based ultramicroporous carbon and poly(furfuryl alcohol)‐derived carbon, is reported. Atomic‐resolution images demonstrate they comprise isotropic, three‐dimensional networks of wrinkled one‐atom‐thick graphene sheets. In each graphene plane, nonhexagonal defects are frequently observed as connected five‐ and seven‐atom rings. Atomic‐level modeling shows that these topological defects induce localized rippling of graphene sheets, which interferes with their graphitic stacking and induces nanopores that lead to enhanced adsorption of H2 molecules. The poly(furfuryl alcohol)‐derived carbon contains larger regions of stacked layers, and shows significantly smaller surface area and pore volume than the ultramicroporous carbon. 相似文献
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Liu X Biju KP Park J Park S Shin J Kim I Md Sadaf S Hwang H 《Journal of nanoscience and nanotechnology》2012,12(4):3252-3255
Yttria-stabilized zirconia (YSZ) layers of various thicknesses were designed and introduced before Pr0.7Ca0.3MnO3 (PCMO) film was deposited on W bottom electrodes with a submicron via-hole structure. By changing the thickness of the YSZ barrier layer (3, 5, 9, and 13 nm), a tunable memory window can be realized while low power consumption (P(max) < 4 microW) is maintained. Resistive switching (RS) in a Pt/PCMO/YSZ/W stack with a thin YSZ layer can be ascribed to an oxidation/reduction reaction caused by a ring-type PCMO/W contact, while RS with a thick YSZ layer may be related to oxygen migration across the YSZ layer between the PCMO film and the W bottom electrode and the increase (decrease) of the effective tunnel barrier height of the YSZ layer. Excellent RS behavior characteristics, such as a large R(HRS)/R(LRS) ratio (> 10(3)), die-to-die uniformity, sweeping endurance, and a retention time of more than 10(3) s, can be obtained by optimizing the thickness of YSZ layer. 相似文献
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Monodisperse particles of conducting polypyrrole (PPy) were directly synthesized in large quantities by emulsion polymerization with FeCl3 in organic solvents in the presence of polyvinylpyrrolidone (PVP). The particles were almost spherical form capped with PVP and their sizes ranged from 30 to 60 nm with a narrow size distribution when molecular weight of PVP was 3700k. The particle sizes were decreased with an increase in molecular weight of PVP: as molecular weight is increased from 40k to 3600k, the size is decreased from 90–110 to 60–80 nm, respectively. These PPy particles are easily dispersed in organic solvents such as water, methanol, butanol isopropanol and these solutions can also be blended with organic binder polymers by casting for film formation. The conductivity of pelletized PPy particles was 10–15 S/cm. 相似文献
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Kyung-Ran Hwang Jong-Soo ParkSon-Ki Ihm 《International Journal of Hydrogen Energy》2011,36(16):9685-9693
Si-modified Pt/CeO2 catalysts were prepared for a water-gas shift (WGS) reaction and the effects of this silica addition on the textural and structural characteristics, reducibility and WGS reaction performance of Pt/CeO2 were investigated. The surface areas of the prepared catalysts increased and both interplanar spacing and average crystalline size of ceria gradually decreased with Si content, resulting in less crystalline and smaller particles. Si addition up to 20 wt. % facilitated the bulk reduction of ceria by inducing significant hydrogen consumption. The oxygen defects in the support, associated with lower valence state cerium, increased with the Si addition. These modifications offer a promising potential to increase the density of hydroxyl groups on the surface of the ceria and consequently increase the concentration of surface intermediate species. The addition of Si to ceria improved the catalytic performance for the WGS reaction, in spite of its irreducible nature. Pt catalysts supported on Si-modified ceria, with a Si content of 5-10 wt.%, exhibited a 2.5-fold increase in reaction rate and turnover frequency (TOF) compared to that of Pt/CeO2. 相似文献
10.
Richard Hahnkee KimSeok Ju Kang Insung BaeYeon Sik Choi Youn Jung ParkCheolmin Park 《Organic Electronics》2012,13(3):491-497
In this study, we investigated the molecular and microstructures of thin poly(vinylidene fluoride-chlorotrifluoro ethylene) (PVDF-CTFE) copolymer films with three different CTFE compositions of 10, 15, 20 wt% with respect to PVDF in relation with their ferroelectric properties. All PVDF-CTFE annealed at 130 °C showed consecutive TTTT trans conformation with β type crystals while films molten and re-crystallized from a temperature above their melting points exhibited α type crystals with characteristic TGTG conformation. Microstructures of the films treated with the two different thermal histories also supported the formation of β and α type crystals with hundreds of nanometer scale sphere caps and micron level spherulites, respectively. Interestingly, PVDF-CTFE films with both α and β type crystals gave rise to relatively high remnant polarization of approximately 4 μC/cm2 in metal/ferroelectric/metal capacitors regardless of the composition of CTFE. The ferroelectric polarization of a PVDF-CTFE film independent of thermal processing history allowed a wide processing window and easy fabrication protocol, resulting in a non-volatile ferroelectric field effect transistor memory which exhibited saturated hysteresis loops with the current ON/OFF ratio of approximately 103 at ±60 V sweep and reliable data retention. 相似文献