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   385367篇
  免费   4759篇
  国内免费   1341篇
电工技术   7640篇
综合类   510篇
化学工业   57657篇
金属工艺   14840篇
机械仪表   12428篇
建筑科学   9867篇
矿业工程   1417篇
能源动力   9671篇
轻工业   36692篇
水利工程   3513篇
石油天然气   4432篇
武器工业   72篇
无线电   50628篇
一般工业技术   73366篇
冶金工业   67388篇
原子能技术   7203篇
自动化技术   34143篇
  2021年   2688篇
  2019年   2745篇
  2018年   4351篇
  2017年   4481篇
  2016年   4788篇
  2015年   3224篇
  2014年   5550篇
  2013年   16898篇
  2012年   9168篇
  2011年   12679篇
  2010年   9943篇
  2009年   11289篇
  2008年   12452篇
  2007年   12579篇
  2006年   11347篇
  2005年   10418篇
  2004年   10099篇
  2003年   9815篇
  2002年   9595篇
  2001年   9561篇
  2000年   9116篇
  1999年   9217篇
  1998年   19828篇
  1997年   14604篇
  1996年   11804篇
  1995年   9386篇
  1994年   8492篇
  1993年   8190篇
  1992年   6759篇
  1991年   6354篇
  1990年   6126篇
  1989年   5878篇
  1988年   5598篇
  1987年   4932篇
  1986年   4823篇
  1985年   5706篇
  1984年   5316篇
  1983年   4866篇
  1982年   4568篇
  1981年   4601篇
  1980年   4301篇
  1979年   4181篇
  1978年   3967篇
  1977年   4690篇
  1976年   5805篇
  1975年   3655篇
  1974年   3490篇
  1973年   3523篇
  1972年   2848篇
  1971年   2671篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
There has been a significant increase in the number of clinical drug trials (particularly phase III) being conducted in developing countries for infectious diseases such as HIV, malaria, and tuberculosis. Laboratory results provided by medical testing laboratories in the region are critical to ensuring the safety of patients and the generation of good quality data. A number of well accepted Good Clinical Practice (GCP) and Good Laboratory Practice (GLP) guidelines govern the conduct of clinical trials internationally. Good Clinical Practice guidelines remain too vague with respect to sample analysis to ensure practical implementation in these laboratories. In their strictest sense, Good Laboratory Practice guidelines refer to the analysis of samples from non-clinical studies. A specific set of minimum standards or requirements for practical implementation of clinical trial requirements in medical testing laboratories in the developing world is urgently required.  相似文献   
992.
Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni5Al3 phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni5Al3 transformation, but rather the M → Ni5Al3 transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni5Al3 determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni5Al3, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.  相似文献   
993.
There is a growing trend of research in neuropsychological assessment in Asia: The mean number of published studies increased from 1.8 in the 1980s to 16.0 in 2 recent years. About 40% of the studies reviewed concerned test development or adaptation, with the latter greatly outnumbering the former. To facilitate the test selection for clinical use, 36 clinical measures were evaluated based on some statistical criteria. The results showed that only 8 neuropsychological tests (5 Chinese. 2 Korean, and 1 Japanese) met the criteria. Given that Asian and Western populations differ on certain cognitive processes, future development of neuropsychological assessment measures in Asia should move from the adaptation of Western tests toward the construction of original tests with better ecological validity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
994.
Prediction of Undrained Sinkhole Collapse   总被引:2,自引:0,他引:2  
Sinkholes are surface depressions or shafts resulting from the collapse of a submerged cavity in soil. The cavities that lead to sinkholes form as a result of underlying geology in limestone areas, or as a result of human activity such as mining or leakage from a sewer. The formation of sinkholes is often sudden and can lead to extensive damage and loss of life, especially in urban areas. Much of the literature on the subject of sinkhole formation is empirical in nature, often being associated with specific locations. This paper presents the results of a study, using numerical modeling, of the undrained stability of the submerged cavities that lead to sinkhole formation. Finite-element limit analysis techniques (using programs developed at the University of Newcastle) are used to obtain upper and lower bound values of a suitable load parameter, which bracket the exact solution. The results are compared to analytical solutions, both from literature and derived independently.  相似文献   
995.
This article compiles results from a century of social psychological research, more than 25,000 studies of 8 million people. A large number of social psychological conclusions are listed alongside meta-analytic information about the magnitude and variability of the corresponding effects. References to 322 meta-analyses of social psychological phenomena are presented, as well as statistical effect-size summaries. Analyses reveal that social psychological effects typically yield a value of r equal to .21 and that, in the typical research literature, effects vary from study to study in ways that produce a standard deviation in r of .15. Uses, limitations, and implications of this large-scale compilation are noted. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
996.
This paper tests the assertion that experienced contractors are more competitive than inexperienced contractors by measuring the effect of experience on bidding competitiveness for building contracts procured by a regular client. Contractors are grouped into experienced and inexperienced contractors, and two levels of contractors’ experience are identified: (1) bidding experience only and (2) bidding plus construction experience. Results of two-sample t tests show that experienced contractors are more competitive than inexperienced contractors. Contractors’ competitiveness in respect of school contracts and other contracts are further compared in evaluating the relationship between the project type and experience effect. Experienced contractors are, on average, more competitive in competing for school contracts. It emerges that the factor of project type familiarity amplifies the experience effect. A comparison of experienced contractors’ competitiveness in bidding for (1) new building works and (2) alteration and extension works shows that the effect of experience would be more significant when contract works packages are highly standardized.  相似文献   
997.
The effect of prior oxidation at 1473 K on the creep behavior of an Fe-Ni-Cr-Al alloy, hardened by ordered NiAl precipitates, has been investigated at 873 K over a stress range of 275 to 450 MPa. The alloy in the as-electroslag remelted (ESR) as well as the ESR-plus-hot-worked conditions was considered. Prior oxidation causes creep strengthening in the Fe-Ni-Cr-Al alloy, resulting in a decrease in minimum creep rate and increase in time to rupture, in contrast to the observations reported on nickel-based superalloys. Creep strengthening is, however, accompanied by a significant reduction in creep ductility. Oxidation-induced creep strengthening in the current alloy can be attributed to the improved adherence of surface oxide caused by the presence of yttrium. An effective stress that incorporates the contributions of load transfer as well as substructural strengthening is used to account for the observed oxidation-induced creep strengthening. While creep strengthening is more pronounced in the ESR cast alloy, the loss in creep ductility is more intense in the ESR wrought alloy. Increasing the oxidation time beyond 1 hour has a minimal effect on creep strengthening of both the alloys, though it lowers significantly the creep ductility of the wrought alloy. The observed differences in creep behavior of the alloy in the two different conditions could be attributed to the differences in grain size as well as morphology and related oxidation-induced damage.  相似文献   
998.
One important area of modern condensed matter research is the investigation of the nature of the superconducting cuprates. Much progress in this field has been obtained with the technique of neutron scattering. We here present a review of neutron scattering studies of the high-temperature superconductor La2?xSrxCuO4, performed at Risø National Laboratory. We review the work on the mapping of the incommensurate spin fluctuations, the investigation of the gap in the fluctuation spectrum, the magnetic properties of the vortices appearing in an applied field, and the quantum critical behaviour of the system. We discuss our findings in the light of results of neutron scattering from other groups, on other cuprate systems, and results from other experimental methods, e.g. NMR, μSR, STM, X-ray diffraction, and ARPES. We end with a discussion on the implications of the experimental results for the progress in the general understanding of high-temperature superconductivity. PACS numbers: 61.12.Ex, 74.20.Mn, 74.72.Dn, 75.25.+z, 78.70.Nx.  相似文献   
999.
高频微型脉冲管制冷机的实用化研究   总被引:2,自引:0,他引:2  
脉冲管制冷机经过多年的发展,性能不断改善,目前已开始进入实用化阶段。高频微型脉冲管制冷机结构紧凑,应用方便,并且主要工作在需求量较大的液氮温区,因此将首先得到一定规模的应用。介绍了作者针对脉冲管制冷机在红外和超导方面的应用所作的实用化工作,讨论了尚待解决的问题。  相似文献   
1000.
Today, with the increasing popularity of multicore processors, one approach to optimizing the processor's performance is to reduce the execution times of individual applications running on each core by designing and implementing more powerful cores. Another approach, which is the polar opposite of the first, optimizes the processor's performance by running a larger number of applications on a correspondingly larger number of cores, albeit simpler ones. The difference between these two approaches is that the former focuses on reducing the latency of individual applications or threads (it optimizes the processor's single-threaded performance), whereas the latter focuses on reducing the latency of the applications' threads taken as a group (it optimizes the processor's multithreaded performance). The panel, from the 2007 Workshop on Computer Architecture Research Directions, discusses the relevant issues.  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