首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   42篇
  免费   0篇
化学   25篇
力学   3篇
数学   6篇
物理学   8篇
  2022年   3篇
  2021年   2篇
  2020年   2篇
  2018年   2篇
  2017年   2篇
  2016年   2篇
  2015年   2篇
  2014年   1篇
  2013年   6篇
  2012年   5篇
  2011年   3篇
  2010年   2篇
  2009年   1篇
  2008年   1篇
  2007年   1篇
  2006年   1篇
  2005年   1篇
  2002年   2篇
  2000年   1篇
  1999年   2篇
排序方式: 共有42条查询结果,搜索用时 31 毫秒
1.
4-(5,5-Dioxodibenzothiophen-2-yl)-4-oxo-2-butenoic acid ( 1 ) was condensed with compounds containing active methylene groups under Michael reaction conditions to form the Michael adducts 2a-c , 3a-c , and 4a-b . The behavior of Michael adduct towards the action of hydrazine hydrate was investigated. The compounds were tested for biological properties.  相似文献   
2.
Abstract

The reaction of nucleophilic phosphacumulene ylides with visnaginone and khellinone afforded the corresponding phosphanylidene and furochromene derivatives. Moreover, pyranochromenes were obtained from the reaction of chromene carbaldehydes with phosphacumulenes. On the other hand, the phosphanylidene-cyclobutylidenes and their dimers were produced from the reaction of furochromene carbaldehydes with the same phosphonium reagents.  相似文献   
3.
4.
Road signs must provide a conspicuous signal to a wide variety of drivers over a broad range of environmental and geometric conditions. Recently, there are an increasing number of applications in which light emitting diodes (LEDs) are used as the light source, including critical transportation signaling. In the presence of fog, the resulting visual signal is disturbed due to light scattering by airborne water droplets. By measuring LED brightness with human spectral sensitivity in various densities and various droplet sizes (10, 30, 50, and 100 μm), it is understood that the particle size distribution (fog droplet size) and density of fog does affect visibility in fog. The colored LEDs that contain a yellow component had high brightness evaluation, blue component had low brightness evaluation in all densities and different droplet sizes. The result in this paper can contribute to air and land traffic safety and the prevention of accidents.  相似文献   
5.
New two-dimensional (2D) bismuth and three-dimensional (3D) lead based coordination polymers containing pyridine-2,5-dicarboxylate ligands (H(2)pydc) have been synthesized hydrothermally and characterized by single crystal X-ray diffraction. Bi(3)(μ(3)-O)(2)(pydc)(2)(Hpydc)(H(2)O)(2) (1), which crystallizes in the space group P1? (a = 8.7256(5) ?, b = 11.1217(7) ?, c = 14.0933(9) ?, α = 85.239(1)°, β = 98.582(1)°, γ = 71.106(1)°), has a 3D structure that contains Bi(6)O(4) clusters that connect into 2D sheets via linking ligands. The sheets form a 3D supramolecular structure via hydrogen bonding along the z-axis. Pb(pydc)(H(2)O) (2), which crystallizes in the space group P2(1)/c (a = 10.8343(14) ?, b = 11.2099(15) ?, c = 6.6573(9) ?, β = 90.697(2)°), contains 1D chains of corner-sharing distorted face capped trigonal prisms that are connected into a 3D framework via the pydc ligand. In addition, the ligands are hydrogen bonded to each other. Both 1 and 2 are single component "white" light emitting phosphors and are shown to exhibit "white" luminescence that covers a much wider spectral range than is observed for the as received H(2)pydc ligand.  相似文献   
6.
Bulk irradiation of crystalline α-quartz was performed with ∼170-fs laser pulses with a wavelength of 800 nm focused below the sample surface. Investigations were carried out using transmission electron microscopy on a cross-sectional specimen prepared using focused ion beam techniques. We observed alternating amorphous–crystalline structures with sharp transitions and associated density changes, surrounded by a highly strained crystalline structure. The alternating sub-surface structures are parallel to the laser’s electric field polarization and exhibit a spacing which is close to the laser wavelength in air. Cracking was also observed in the near proximity of these structures.  相似文献   
7.
The ordered double-perovskites Sr(2)MOsO(6) (M = Cu, Ni) consisting of 3d and 5d transition-metal magnetic ions (M(2+) and Os(6+), respectively) are magnetic insulators; the magnetic susceptibilities of Sr(2)CuOsO(6) and Sr(2)NiOsO(6) obey the Curie-Weiss law with dominant antiferromagnetic and ferromagnetic interactions, respectively, and the zero-field-cooled and field-cooled susceptibility curves of both compounds diverge below ~20 K. In contrast, the available density functional studies predicted both Sr(2)CuOsO(6) and Sr(2)NiOsO(6) to be metals. We resolved this discrepancy on the basis of systematic density functional calculations. The magnetic insulating states of Sr(2)MOsO(6) are found only when a substantially large on-site repulsion is employed for the Os atom, although it is a 5d element. The cause for the divergence between the zero-field-cooled and field-cooled susceptibility curves in both compounds and the reason for the difference in their dominant magnetic interactions were investigated by examining their spin exchange interactions.  相似文献   
8.
设a<b是整数,G=(V(G),E(G))是一个图.G的一个支撑子图F称为G的一个[a,b]-因子,若对任意的υ∈EV(G),有a≤d_F(υ)≤b.本文得到了下列结果:设1≤a≤b是整数,G是一个阶为n的图,最小度δ(G)≥a且>(a+b)(2a+2b-3)如果对于G的任意两个不相邻的顶点u,υ有N_G(u)UN_G(υ)≥an,则G有一个[a,b]-因子.  相似文献   
9.
ABSTRACT

Design of structures using a variety of two-dimensional finite elements is considered in this paper. An efficient technique of computing first-order derivatives of pointwise stress constraints for simple and higher-order two-dimensional (membrane) finite elements is presented. Computational aspects of design sensitivity vector calculation, using a semi-analytical method versus traditional methods are presented. Implementation of a fully-stressed design approach to find a suitable initial estimate leads to increased computational efficiency. These aspects of the design procedure are illustrated through analysis of numerical examples. Experience indicates that a suitable mix of low and higher-order elements yields the most efficient and accurate design model.  相似文献   
10.
Water imbibition during the waterflooding process of oil production only sweeps part of the oil present. After water disrupts the oil continuity, most oil blobs are trapped in porous rock by capillary forces. Developing an efficient waterflooding scheme is a difficult task; therefore, an understanding of the oil trapping mechanism in porous rock is necessary from a microscopic viewpoint. The development of microfocused X-ray CT scanner technology enables the three-dimensional visualization of multiphase phenomena in a pore-scale. We scanned packed glass beads filled with a nonwetting phase (NWP) and injected wetting phase (WP) in upward and downward injections to determine the microscopic mechanism of immiscible displacement in porous media and the effects of buoyancy forces. We observed the imbibition phenomena for small capillary numbers to understand the spontaneous imbibition mechanism in oil recovery. This study is one of the first attempts to use a microfocused X-ray CT scanner for observing the imbibition and trapping mechanisms. The trapping mechanism in spontaneous imbibition is determined by the pore configuration causing imbibition speed differences in each channel; these differences can disrupt the oil continuity. Gravity plays an important role in spontaneous imbibition. In upward injection, the WP flows evenly and oil is trapped in single or small clusters of pores. In downward injection, the fingering phenomena determine the amount of trapped oil, which is usually in a network scale. Water breakthrough causes dramatic decrease in the oil extraction rate, resulting in lower oil production efficiency.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号