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   197637篇
  免费   2356篇
  国内免费   630篇
电工技术   4059篇
综合类   123篇
化学工业   26636篇
金属工艺   7509篇
机械仪表   5461篇
建筑科学   4043篇
矿业工程   746篇
能源动力   4660篇
轻工业   14396篇
水利工程   1694篇
石油天然气   2842篇
武器工业   7篇
无线电   27301篇
一般工业技术   37429篇
冶金工业   45425篇
原子能技术   3563篇
自动化技术   14729篇
  2021年   1246篇
  2019年   1281篇
  2018年   2126篇
  2017年   2109篇
  2016年   2172篇
  2015年   1489篇
  2014年   2617篇
  2013年   7951篇
  2012年   4466篇
  2011年   6113篇
  2010年   4935篇
  2009年   5777篇
  2008年   6153篇
  2007年   6203篇
  2006年   5637篇
  2005年   5309篇
  2004年   5257篇
  2003年   5094篇
  2002年   4903篇
  2001年   5309篇
  2000年   4920篇
  1999年   5437篇
  1998年   15637篇
  1997年   10328篇
  1996年   7934篇
  1995年   5792篇
  1994年   5025篇
  1993年   5028篇
  1992年   3386篇
  1991年   3265篇
  1990年   3219篇
  1989年   3045篇
  1988年   2785篇
  1987年   2239篇
  1986年   2302篇
  1985年   2611篇
  1984年   2317篇
  1983年   2064篇
  1982年   1902篇
  1981年   2044篇
  1980年   1791篇
  1979年   1660篇
  1978年   1656篇
  1977年   2022篇
  1976年   2702篇
  1975年   1422篇
  1974年   1369篇
  1973年   1316篇
  1972年   1125篇
  1971年   956篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
221.
222.
The paper covers a simple idea. If we sample an aperture, we can obtain valid patterns over a limited angular region about the normal to the aperture. The same expression can be used with near-field measurements. I reduced the expression to a nomograph. A nomograph allows one to rapidly test various choices. In the second half of the paper, I answer questions caused by the February column which discussed polarization (Milligan, IEEE Antennas. Propag. Mag., vol.38, no.1, p.56-8, 1996)  相似文献   
223.
The authors have fabricated 1.3-μm InAsP-InP separate-confinement-heterostructure (SCH) strained-layer double-quantum-well (SL-DQW) laser diodes (LDs) by metalorganic chemical vapor deposition (MOCVD). A low threshold current density of 410 A/cm 2 was obtained. The CW threshold current was as low as 1.8 mA at 20°C, and maximum CW operating temperature of 120°C was obtained. These characteristics are almost the same as those of well-designed GaInAsP-InP SL-QW LDs. Further improvement of the characteristics of InAsP-InP LDs is expected by optimizing the device structure  相似文献   
224.
225.
A low threshold current density of ~100 A/cm2 has been obtained at 1.55 μm using a graded-index separate-confinement-heterostructure strained InGaAsP single-quantum-well laser. The design of the laser structure is based on results calculated from the viewpoint of effective carrier injection into the well  相似文献   
226.
227.
The soluble 14 kDa beta-galactoside-binding lectin from bovine heart, a member of the S-type lectin family, has been crystallized in a form suitable for X-ray diffraction analysis. The crystals, in the absence of a saccharide ligand, diffract beyond 2.5 A resolution. They are obtained from polyethylene glycol 6000 at pH 6.0. Crystals grow as monoclinic plates, space group P2(1), with cell dimensions: a = 35.47 A, b = 64.33 A, c = 58.78 A and beta = 91.7 degrees. The asymmetric unit contains two molecules related by a 2-fold non-crystallographic axis. Two lectin monomers in the asymmetric unit give a Vm of 2.4 A3/Da, i.e. a solvent content of approximately 50%. The complex of lectin with the saccharide ligand, N-acetyllactosamine, crystallizes in the space group P2(1)2(1)2 with cell dimensions: a = 63.55 A, b = 82.13 A and c = 62.39 A. Crystals of this complex diffract beyond 2.0 A resolution. Two complexes in the asymmetric unit lead to a Vm value of 2.8 A3/Da (57% solvent).  相似文献   
228.
The biotransformation of oxazepam by Bifidobacterium bifidum was studied. The major metabolite was purified by chromatographic methods and found to be desmethyldiazepam using NMR, IR and other physicochemical data.  相似文献   
229.
According to some ultrastructural studies, the pericapillary axon terminals in the central nervous system (CNS) are functionally connected with the capillary vessel wall. Thus, it may be expected that the population of pericapillary axon terminals will be morphologically distinct from the terminals at a further distance from the capillary walls. To test this hypothesis, morphometrical analysis of 3,048 axon terminals was performed, comparing terminals situated in the close vicinity of the capillary vessel with those at a distance from the vessels in the lateral, basal, medial, central and cortical nuclei of the amygdaloid body of eight cats. The cross-sectional area and circumference of each identified axon terminal profile were measured, and the shape of synaptic vesicles and the presence of synaptic contacts and granular vesicles were recorded. The statistical evaluation of results was performed by means of the Newman-Keuls' test, Wilcoxon's test, Fisher's contingency-table test and the test for two coefficients of structure. The morphometric examination revealed two ultrastructurally distinct groups of axon terminals, pericapillary and distant terminals, in all the nuclei of the amygdaloid body. The differentiating features were the shape of the synaptic vesicles, the number of synaptic contacts, and the size of the axon terminals. These results further support the hypothesis of a functional connection between axon terminals and the capillary vessel wall in the CNS.  相似文献   
230.
In the last 2 years (1994-95), two symposium volumes and three reviews have been published that were fully devoted to peptide antibiotics (antibacterial peptides or antimicrobial peptides). Since the field has been growing rapidly, this review is largely a follow-up of new results published in the last 2 years. Sequencing of the 16S RNA of the small ribosomal subunit indicate that the microbial world is much larger than generally appreciated. The importance of the natural flora is stressed and its effect on the evolution of peptide antibiotics and immunity in general is discussed.  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