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   1983篇
  免费   31篇
  国内免费   4篇
电工技术   21篇
综合类   1篇
化学工业   237篇
金属工艺   56篇
机械仪表   46篇
建筑科学   77篇
矿业工程   7篇
能源动力   26篇
轻工业   115篇
水利工程   21篇
石油天然气   9篇
无线电   120篇
一般工业技术   201篇
冶金工业   842篇
原子能技术   28篇
自动化技术   211篇
  2021年   10篇
  2020年   12篇
  2019年   11篇
  2018年   20篇
  2017年   16篇
  2016年   29篇
  2015年   19篇
  2014年   18篇
  2013年   66篇
  2012年   37篇
  2011年   37篇
  2010年   35篇
  2009年   47篇
  2008年   53篇
  2007年   44篇
  2006年   40篇
  2005年   41篇
  2004年   29篇
  2003年   33篇
  2002年   23篇
  2001年   38篇
  2000年   39篇
  1999年   52篇
  1998年   256篇
  1997年   156篇
  1996年   118篇
  1995年   79篇
  1994年   65篇
  1993年   72篇
  1992年   11篇
  1991年   27篇
  1990年   26篇
  1989年   28篇
  1988年   21篇
  1987年   26篇
  1986年   30篇
  1985年   30篇
  1984年   23篇
  1983年   14篇
  1982年   26篇
  1981年   18篇
  1980年   26篇
  1979年   14篇
  1978年   11篇
  1977年   26篇
  1976年   62篇
  1975年   15篇
  1974年   10篇
  1973年   10篇
  1971年   9篇
排序方式: 共有2018条查询结果,搜索用时 15 毫秒
61.
Consider a mappingF from a Hilbert spaceH to the subsets ofH, which is upper semicontinuous/Lipschitz, has nonconvex, noncompact values, and satisfies a linear growth condition. We give the first necessary and sufficient conditions in this general setting for a subsetS ofH to be approximately weakly/strongly invariant with respect to approximate solutions of the differential inclusion . The conditions are given in terms of the lower/upper Hamiltonians corresponding toF and involve nonsmooth analysis elements and techniques. The concept of approximate invariance generalizes the well-known concept of invariance and in turn relies on the notion of an ∈-trajectory corresponding to a differential inclusion. Dedicated to Professor R. V. Gamkrelidze The first and the third authors are supported in part by the Natural Sciences and Engineering Research Council of Canada and Le Fonds FCAR du Québec. The second author is visiting Rutgers University. Supported in part by the Russian Foundation for Fundamental Research, Grant 96-01-00219, by the Natural Sciences and Engineering Research Council of Canada, Le Fonds FCAR du Québec, and by the Rutgers Center for Systems and Control (SYCON).  相似文献   
62.
Benthic invertebrate communities were examined in Compensation Creek, a man‐made stream in south‐central Newfoundland, Canada. Samples taken in September 2006 and September 2007 from large woody debris (LWD) were compared with samples from benthic environments to determine whether LWD supported a more diverse and abundant invertebrate community. Benthic habitats in a nearby natural stream were also sampled. Taxa composition was similar between the man‐made and the natural stream, highlighting successful colonization for the majority of taxa. Within Compensation Creek, taxa richness was higher in benthic habitats than on LWD, likely influenced by the successional age of the stream and surrounding habitat. The more complex benthic substrate provided refugia and allowed for the accumulation of fine detritus as a food source. Scrapers were almost completely absent from LWD and collector‐gatherer abundance was greater in the benthos. Collector‐filterer abundance was more than six times greater near the pond outflow than farther downstream when discharge was high, but abundances were almost equal when discharge was reduced. Riparian vegetation has not fully established around the man‐made stream, whereas it is overhanging and extensive at the natural stream, leading to more leaf‐litter input for shredders. As the morphology of Compensation Creek changes, the invertebrate community will continue to develop and likely increase utilization of accumulated detritus at LWD. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
63.
Dimethyl sulphide formation in malt occurs primarily during the kilning process, and is significantly dependent upon the maximum kilning temperature. During mashing a considerable fraction of the dimethyl sulphide of malt is transferred to the wort. Since this is not always completely removed by the wort boiling process, the remaining dimethyl sulphide, although decreased during fermentation, may be sufficient to impart a noticeable dimethyl sulphide aroma to the finished beer.  相似文献   
64.
The function of organic solar cells is based upon charge photogeneration at donor/acceptor heterojunctions. In this paper, the origin of the improvement in short circuit current of poly(3‐hexylthiophene)/6,6‐phenyl C61‐butyric acid methyl ester (P3HT/PCBM) solar cells with thermal annealing is examined. Transient absorption spectroscopy is employed to demonstrate that thermal annealing results in an approximate two‐fold increase in the yield of dissociated charges. The enhanced charge generation is correlated with a decrease in P3HT's ionization potential upon thermal annealing. These observations are in excellent quantitative agreement with a model in which efficient dissociation of the bound radical pair into free charges is dependent upon the bound radical state being thermally hot when initially generated, enabling it to overcome its coulombic binding energy. These observations provide strong evidence that the lowest unoccupied molecular orbital (LUMO) level offset of annealed P3HT/PCBM blends may be only just sufficient to drive efficient charge generation in polythiophene‐based solar cells. This has important implications for current strategies to optimize organic photovoltaic device performance based upon the development of smaller optical bandgap polymers.  相似文献   
65.
In this report, we conducted a secondary analysis of the Treatment of SSRI-Resistant Depression in Adolescents (TORDIA) study to explore the impact of specific cognitive–behavioral therapy (CBT) treatment components on outcome. In TORDIA, 334 youths (ages 12 to 18 years) with major depressive disorder who had failed to respond to an adequate course of selective serotonin reuptake inhibitor (SSRI) medication were randomized to a medication switch (either to an alternative SSRI or venlafaxine) with or without 12 weeks of adjunctive CBT. Participants who had more than 9 CBT sessions were 2.5 times more likely to have adequate treatment response than those who had 9 or fewer sessions. CBT participants who received problem-solving and social skills treatment components, controlling for number of sessions and other confounding variables, were 2.3 and 2.6 times, respectively, more likely to have a positive response. These preliminary findings underscore the importance of receiving an adequate number of sessions to attain an adequate clinical response. Finally, social skills and problem solving may be active elements in CBT for adolescent depression and should be considered in treatment by those working with seriously depressed youths. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
66.
Within 12 months of the discovery of X rays in 1895, papers appeared in the literature reporting adverse effects from high exposure. In 1925, the first International Congress of Radiology, held in London, considered the need for a protection committee, which it established at its second congress in Stockholm in 1928. This paper celebrates the 80th anniversary of ICRP by tracing the history of the development of its policies, and identifying a few of the personalities involved from its inception up to the modern era. The paper follows the progress from the early controls on worker doses to avoid deterministic effects, through the identification of stochastic effects, to the concerns about public exposure and increasing stochastic risk estimates. The key features of the recommendations made by ICRP from 1928 up to the most recent in 2007 are identified.  相似文献   
67.
Pharmacological effects of acute treatment with venlafaxine (VEN), a clinically active antidepressant [a noradrenaline (NA) and 5-hydroxytryptamine (5-HT) reuptake inhibitor without any affinity for neurotransmitter receptors] were studied in mice and rats. VEN inhibited the reserpine- or apomorphine-induced hypothermia and enhanced the L-5-HTP-induced head twitches in mice. It reduced the immobility time in Porsolt's test in mice and rats, but either did not change the locomotor activity (mice) or decreased it (rats). VEN reduced the locomotor hyperactivity induced by amphetamine (AMP), apomorphine (APO) and quinpirole (QUI), as well as the APO-induced stereotypy; the stereotypy induced by AMP in rats was prolonged. VEN neither changed the clonidine-induced aggressiveness in mice nor the behavioral syndrome induced by oxotremorine in rats. The obtained results indicate that VEN, given acutely, shows a pharmacological profile similar to that of tricyclic NA and 5-HT reuptake inhibitors. In contrast to the antidepressants mentioned above, VEN does not exhibit an alpha 1-adrenolytic or a cholinolytic activity (in vivo tests).  相似文献   
68.
A new preprocessing technique which removes unwanted notches and random noise generated by electronic scanners is presented. Using a 3 element by 3 element mask, the central picture element is altered from black to white, or vice versa, if the surrounding elements correspond to a predetermined pattern. A significant improvement in both subjective image quality and coding efficiency is observed.  相似文献   
69.
A simple regulator for stabilizing the temperature of a liquid (4)He bath near 4.2 K is described. The device controls the pressure of the (4)He gas, and achieves a stability of +/-50 muK for the hold-time of the cryostat.  相似文献   
70.
The author describes the particular technical features of coffee standards used commercially and/or legislatively. Existing national standards on green coffee and their harmonisation of terminology, sampling and test method are described and discussed. Comparisons are made of the various legislative standards of different countries with respect to instant coffee and their harmonisation within the EEC member countries. Roasted coffee is considered to a lesser extent as it is not an important element in the world trade for coffee. The role of various international organisations in standardisation is described. It is emphasised that the flavour of coffee is not amenable to international standardisation.  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