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   251699篇
  免费   4105篇
  国内免费   1043篇
电工技术   4327篇
综合类   201篇
化学工业   37110篇
金属工艺   8217篇
机械仪表   7263篇
建筑科学   6785篇
矿业工程   541篇
能源动力   7236篇
轻工业   28638篇
水利工程   2120篇
石油天然气   1852篇
武器工业   5篇
无线电   33392篇
一般工业技术   46175篇
冶金工业   46451篇
原子能技术   3366篇
自动化技术   23168篇
  2021年   1798篇
  2020年   1346篇
  2019年   1667篇
  2018年   2571篇
  2017年   2593篇
  2016年   2875篇
  2015年   2163篇
  2014年   3655篇
  2013年   11954篇
  2012年   6754篇
  2011年   9664篇
  2010年   7444篇
  2009年   8394篇
  2008年   8916篇
  2007年   8930篇
  2006年   8278篇
  2005年   7485篇
  2004年   7208篇
  2003年   7093篇
  2002年   6591篇
  2001年   7020篇
  2000年   6324篇
  1999年   6822篇
  1998年   16376篇
  1997年   11367篇
  1996年   8581篇
  1995年   6543篇
  1994年   5707篇
  1993年   5566篇
  1992年   3996篇
  1991年   3774篇
  1990年   3548篇
  1989年   3369篇
  1988年   3276篇
  1987年   2589篇
  1986年   2487篇
  1985年   3156篇
  1984年   2797篇
  1983年   2559篇
  1982年   2344篇
  1981年   2390篇
  1980年   2201篇
  1979年   2048篇
  1978年   1879篇
  1977年   2176篇
  1976年   2724篇
  1975年   1584篇
  1974年   1489篇
  1973年   1558篇
  1972年   1144篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
An unpackaged microchip is used as the sensing element in a miniaturized gaseous proportional chamber. This letter reports on the fabrication and performance of a complete radiation imaging detector based on this principle. Our fabrication schemes are based on wafer-scale and chip-scale postprocessing. Compared to hybrid-assembled gaseous detectors, our microsystem shows superior alignment precision and energy resolution, and offers the capability to unambiguously reconstruct 3D radiation tracks on the spot.  相似文献   
992.
Modafinil is indicated for the management of excessive daytime sleepiness; however, recent studies have examined a broad range of potential uses. Given that clinical uses of modafinil may be expanding, this study compared modafinil and d-amphetamine effects on subjective and performance measures. Across 11 sessions, 11 healthy adults were tested after oral doses of placebo (5 sessions), modafinil (1.75 mg/kg, 3.50 mg/kg, or 7.00 mg/kg), and d-amphetamine (0.035 mg/kg, 0.070 mg/kg, 0.140 mg/kg) under double-blind, randomized conditions. Assessments of cognitive performance and subjective effects were completed before drug administration, 30 min after drug administration, and at hourly intervals after drug administration for 5 hr. Modafinil increased ratings on the Amphetamine and Morphine Benzedrine Group scales of the Addiction Research Center Inventory (ARCI) and increased ratings on the Vigor and Total Positive scales of the Profile of Mood States. d-Amphetamine increased visual analog ratings of feeling stimulated and liking the drug and increased ratings on the Morphine Benzedrine Group scale of the ARCI. Both medications significantly reduced visual analog scale ratings of feeling sleepy, and modafinil decreased ratings on the ARCI Pentobarbital-Chlorpromazine-Alcohol Group scale. Both medications sustained performance that deteriorated across time on the Sternberg Number Recognition Test. Modafinil also enhanced performance rate on the Digit-Symbol Substitution Task above baseline levels and increased response rate on the Repeated Acquisition of Response Sequences Task. These results suggest that modafinil engenders alerting effects and increases performance in healthy non-sleep-deprived individuals comparable with that of d-amphetamine. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
993.
994.
Cyclamate and its metabolite cyclohexylamine affect male fertility in high dose animal studies, but this affect has not been investigated in epidemiological studies. This paper reports the first epidemiological study designed to investigate the possibility of a relationship between cyclamate and cyclohexylamine and male fertility in humans, in which 405 cases of clinically defined infertility in men and 379 controls were surveyed. Semen evaluation, urine analysis for cyclamate and cyclohexylamine and dietary questionnaires were compared between cases and controls. No evidence was found of a significant association between cyclamate intake and male infertility; neither high cyclamate nor high cyclohexylamine excretion were associated with elevated risk. The lack of association remained after adjusting by age, area of residence, education, total energy intake and other variables. No significant correlations were observed between cyclamate intake, metabolism or excretion, and sperm count and motility. The results demonstrate no effect of cyclamate or cyclohexylamine on male fertility at the present levels of cyclamate consumption.  相似文献   
995.
Hsu  C.-L. Wang  F.-J. 《Software, IET》2007,1(5):188-205
Although many workflow models have been proposed, analyses on artifacts are seldom discussed. A workflow application with well structured and adequate resources may still fail or yield unexpected results in execution due to inaccurate artifact manipulation, for example, inconsistency between data flow and control flow, or contradictions between artifact operations. Thus, artifact analysis is very important since activities cannot be executed properly without accurate information. This paper presents a three-layer workflow model for designing a workflow and characterises the behaviour of an artifact by its state transition diagram. By abstracting common usages of artifacts, six types of inaccurate artifact usage affecting workflow execution are identified and a set of algorithms to detect these inaccurate usages in workflow specifications is presented. An example is demonstrated and then related works are compared.  相似文献   
996.
997.
Biaxial fatigue of friction stir welded stiffened panels Within the framework of the European WelAir project, cruciform specimens made from stiffening FSW overlap joints were fatigued in the DLR biaxial test rig. To resemble the loading situation of pressurized fuselage structures, proportional loading without any phase shift, but with different load ratios λ between the loading components in both directions was applied.Natural crack initiation and subsequent crack growth were governed by the stiffness gradient caused by introducing the stringer. Cracks initiated and propagated at run‐in and run‐out locations in a direction perpendicular to the weld seam. The shortest fatigue life was observed for uniaxial loading in welding direction (λ = 0). An additional stress component perpendicular to the joint line (λ > 0) resulted in a higher number of cycles to failure. Similar to single stringer panels, increasing the load ratio also increased the number of cycles to failure for FSW clip‐stringer structural members, but additionally gives a different location of the fatal crack.  相似文献   
998.
999.
Since the end of the Cold War and due to the needs to produce titanium more cost‐efficient and thus more attractive for its civilian use, the electroslag remelting (ESR) process has become an increasingly important topic of international research programmes, because titanium and titanium aluminides can be chemically refined by ESR in some degree. Using ESR, titanium turnings from machining steps and scrap from foundries can be remelted, refined and provided as secondary titanium for the market at relatively favourable prices. This article investigates the removability of the main impurities out of titanium and titanium‐aluminium alloys by electroslag remelting using the active slag system CaF2‐Ca‐(CaO). Thermochemical and kinetic aspects of the ESR process are considered.  相似文献   
1000.
This paper presents a new concept for symmetric boundary element method (SBEM) applicable to 2-D steady-state and transit potential problems. Two kinds of SBEM formulations are derived. Symmetry is obtained simply through matrix manipulation, and no hypersingularity appears. Therefore, SBEM is much easier than the traditional symmetric Galerkin BEM. Compared with the traditional asymmetric BEM, the present SBEM can reduce the computational cost for time domain problems only. However, when applied to BEM/FEM coupling procedure, SBEM can reduce the computational cost for both steady-state and time domain problems. Three numerical examples are included to illustrate the effectiveness and accuracy of the present formulations.  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