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   258286篇
  免费   4764篇
  国内免费   626篇
电工技术   3924篇
综合类   212篇
化学工业   41787篇
金属工艺   11188篇
机械仪表   8005篇
建筑科学   5839篇
矿业工程   2274篇
能源动力   5830篇
轻工业   18967篇
水利工程   3358篇
石油天然气   7904篇
武器工业   26篇
无线电   24676篇
一般工业技术   54492篇
冶金工业   45977篇
原子能技术   6898篇
自动化技术   22319篇
  2021年   3415篇
  2020年   2588篇
  2019年   2979篇
  2018年   5541篇
  2017年   5516篇
  2016年   5943篇
  2015年   3676篇
  2014年   5618篇
  2013年   12276篇
  2012年   8764篇
  2011年   10353篇
  2010年   8204篇
  2009年   8616篇
  2008年   9023篇
  2007年   8670篇
  2006年   7201篇
  2005年   6502篇
  2004年   5821篇
  2003年   5649篇
  2002年   5353篇
  2001年   5091篇
  2000年   4775篇
  1999年   4930篇
  1998年   13404篇
  1997年   9160篇
  1996年   6868篇
  1995年   4990篇
  1994年   4350篇
  1993年   4549篇
  1992年   3288篇
  1991年   3269篇
  1990年   3213篇
  1989年   3162篇
  1988年   3112篇
  1987年   2751篇
  1986年   2837篇
  1985年   3146篇
  1984年   2945篇
  1983年   2754篇
  1982年   2533篇
  1981年   2522篇
  1980年   2555篇
  1979年   2541篇
  1978年   2545篇
  1977年   2743篇
  1976年   3323篇
  1975年   2309篇
  1974年   2165篇
  1973年   2272篇
  1972年   2009篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
An algorithm is described for processing the results of two- coordinate measurements of a profile representing a partial circle. Information is presented on the instrument structure and on the functions executed by the microprocessor.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 22–24, September, 1993.  相似文献   
993.
994.
To determine possible relationships between DNA hypomethylation and chromosome instability, human lymphoblastoid cell lines from different genetic constitutions were studied with regard to 1) uncoiling and rearrangements, which preferentially affect the heterochromatic segments of chromosomes 1 and 16; 2) the methylation status of the tandemly repetitive sequences (classical satellite and alphoid DNAs) from chromosomes 1 and 16, and of the L1Hs interspersed repetitive sequences. The methylation status largely varied from cell line to cell line, but for a given cell line, the degree of methylation was similar for all the repetitive DNAs studied. Two cell lines, one obtained from a Fanconi anemia patient and the other from an ataxia telangiectasia patient were found to be heavily hypomethylated. The heterochromatic segments of their chromosomes 1 and 16 were more frequently elongated and rearranged than those from other cell lines, which were found to be less hypomethylated. Thus, in these lymphoblastoid cell lines, alterations characterized by uncoiling and rearrangements of heterochromatic segments from chromosomes 1 and 16 seem to correlate with the hypomethylation of their repetitive DNAs. Two-color in situ hybridizations demonstrated that these elongations and rearrangements involved only classical satellite-DNA-containing heterochromatin. This specificity may be related to the excess of breakages affecting the chromosomes carrying these structures in a variety of pathological conditions.  相似文献   
995.
APP is a transmembrane precursor of beta-amyloid. In dominantly inherited familial Alzheimer's disease (FAD), point mutations V6421, V642F and V642G have been discovered in APP695. Here we show that expression of these mutants (FAD-APPs) causes a clone of COS cells to undergo apoptosis associated with DNA fragmentation. Apoptosis by the three FAD-APPs was the highest among all possible V642 mutants; normal APP695 had no effect on apoptosis, suggesting that apoptosis by APP mutants in this system is phenotypically linked to the FAD trait. FAD-APP-induced apoptosis was sensitive to bcl-2 and most probably mediated by heteromeric G proteins. This study presents a model system allowing analysis of the mechanism for FAD-APP-induced cytotoxicity.  相似文献   
996.
997.
This paper analyses the determination of visibility of journals in which researchers of INRA (National Institute for Agricultural Research) publish. The corpus is comprised of 671 articles published over a period of four years in 258 journals. The advantage of the method applied for determining the visibility of journals is that it combines two approaches: a) bibliometric indicators (coverage by the ISI-publications and by two specific French databases) and b) experts' opinions (10 economists and sociologists). The main results are: a) There is a convergence between the opinion of the experts and «visibility» in the databases, b) The impact factor confirms the main opinions of the experts. The first journals ranked by the experts and JCR Social Sciences are the same but represent only 40 per cent of the total journals analysed. The other journals were revealed by the experts and French databases.«Whoever seeks to pass a balanced but lucid judgement on the general trends of the activity carried out by the profession to which he belongs, is liable to say either banalities or to hurt his colleagues»The notion which we retain of core is that which is successively and conjointly validated by the experts and by the major databases representative of the field.This study was financed by the Department of Rural Economy and Sociology of INRA.  相似文献   
998.
The photoconductive properties of a carotenoid polyene,β-Apo-8′ carotenal in polycrystalline form has been studied. The growth of the photocurrent shows an overshoot in the growth-time curve before steady state value is attained. This behaviour of photocurrent is proposed to be due to higher value of recombination coefficient than trapping coefficient. From the temperature dependence study it is observed that the steady state photocurrent, at first increases with increase of temperature, attains a maximum at a particular temperatureT max and then decreases with temperature. TheT max value agrees with the temperature above and below which steady state photocurrent is attained differently. Monomolecular and bimolecular recombination processes at two temperature regimes are proposed to account for the observed behaviour. The dependence of photocurrent with excitation light intensity and wavelength study provide information on the carrier generation processes. The fast decay of photocurrent have been observed at different temperatures and from this study the decay constant is calculated and it is found to be temperature independent.  相似文献   
999.
Hydrogen is the lightest element in nature, and so, its detection and quantitative analysis is difficult by the conventional methods utilized for other elements. In the recent years the technique of elastic recoil detection analysis (ERD) using 1–2 MeV He+ beam has been developed to quantitatively and simultaneously analyze hydrogen and its isotopes in solids. Such a facility has been set up using the 2 MeV Van-de-Graaff accelerator at IIT Kanpur. It facilitates H and D analysis in a material up to a depth of ∼ 1μm with a detection sensitivity of 0·1 at.% and depth resolution of about 300 ?. The application potential of this setup is illustrated by presenting the results of measurements performed on Al:H:D systems prepared by plasma source ion implantation and highT c YBCO pellets exposed to humid atmosphere.  相似文献   
1000.
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