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   1480263篇
  免费   24369篇
  国内免费   7000篇
电工技术   34313篇
综合类   6510篇
化学工业   269490篇
金属工艺   63781篇
机械仪表   41977篇
建筑科学   45529篇
矿业工程   11420篇
能源动力   50170篇
轻工业   109922篇
水利工程   14798篇
石油天然气   37513篇
武器工业   186篇
无线电   195815篇
一般工业技术   280172篇
冶金工业   147981篇
原子能技术   33886篇
自动化技术   168169篇
  2021年   15505篇
  2020年   11803篇
  2019年   14557篇
  2018年   15345篇
  2017年   14682篇
  2016年   21028篇
  2015年   17268篇
  2014年   28607篇
  2013年   87586篇
  2012年   34675篇
  2011年   46745篇
  2010年   41883篇
  2009年   50363篇
  2008年   43794篇
  2007年   40795篇
  2006年   44162篇
  2005年   38714篇
  2004年   40967篇
  2003年   40957篇
  2002年   40070篇
  2001年   36829篇
  2000年   35342篇
  1999年   34110篇
  1998年   40009篇
  1997年   36237篇
  1996年   33422篇
  1995年   29673篇
  1994年   27970篇
  1993年   27816篇
  1992年   26065篇
  1991年   23096篇
  1990年   23403篇
  1989年   22414篇
  1988年   20954篇
  1987年   19302篇
  1986年   18684篇
  1985年   21989篇
  1984年   22282篇
  1983年   20247篇
  1982年   19199篇
  1981年   19270篇
  1980年   17849篇
  1979年   18479篇
  1978年   17699篇
  1977年   17187篇
  1976年   17735篇
  1975年   15965篇
  1974年   15538篇
  1973年   15589篇
  1972年   13039篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Metal fatigue in welded joints in offshore structures is considered. Due to the considerable variability of conditions, a probabilistic approach is used. Theoretical studies of various aspects of the fatigue reliability problem in welded joints are presented. These include a study of the Palmgren-Miner rule, a modified linear model on S/N data, the use of the rainflow method of counting, and a closed form expression for the probability of fatigue failure. A probability model is derived as a suggested basis for an approach to fatigue design.  相似文献   
992.
The application of recently developed algorithms to antenna systems design is demonstrated by the worst-case tolerance optimization of linear broadside arrays, using both spacings and excitation coefficients as design parameters. The resulting arrays are optimally immunized against deviations of the design parameters from their nominal values.  相似文献   
993.
994.
We describe a simple way to achieve CW single-frequency laser operation with a grating as the sole tuning element. It is Shown, both experimentally and theoretically, that by proper choice of cavity parameters, the competing hole burning modes can be completely suppressed. Experiments to demonstrate the theoretical calculations were carried out in a CW color center laser using Tl0(1) centers. Linewidths of 0.01 cm-1were obtained and this figure can probably be much improved by proper cavity stabilization. The method can be readily extended to any compact gain medium.  相似文献   
995.
A study of the reasons for delay in software development is described. The aim of the study was to gain an insight into the reasons for differences between plans and reality in development activities in order to be able to take actions for improvement. A classification was used to determine the reasons. 160 activities, comprising over 15000 hours of work, have been analyzed. The results and interpretations of the results are presented. Insight into the predominant reasons for delay enabled actions for improvements to be taken in the department concerned. Because the distribution of reasons for delay varied widely from one department to another, it is recommended that every department should gain an insight into its reasons for delay in order to be able to take adequate actions for improvement  相似文献   
996.
Elasticity is discussed as an aspect of viscoelasticity, which is described by the tube model. The effects of both crosslinks and entanglements contribute to this model and a discussion of how these effects can be quantified is given. At high enough concentration, entanglements ensure the existence of elastic effects even without crosslinks, and a theory is presented on how this dynamical phase change comes about.  相似文献   
997.
C. Viney  A.M. Donald  A.H. Windle 《Polymer》1985,26(6):870-878
Banded textures produced in a thermotropic liquid crystal polymer by shearing between glass slides are examined by using both transmission electron and polarized light microscopy. The periodic variation in director orientation about the shear axis, as measured by light microscopy, is shown to be distinctly different from that indicated by electron diffraction. Measurements of birefringence and observation of Zernicke phase contrast indicate periodic variations in optical properties of the polymer, in step with the bands. Such effects are accounted for in terms of a synchronous rotation of the planar aromatic groups about the molecular chain axes. Evidence for an out-of-plane component of molecular orientation is also presented.  相似文献   
998.
An investigation into the effects of pressure (helium gas) on the isothermal fluid behavior includes: (1) the effect of pressure on the rate of melting and coking as evidenced by the rate constants k(melt) and k(coke); (2) the effect of pressure on the energies of activation of melting and coking; (3) the effects of pressure on the characteristic times; (4) the effects of pressure on the maximum isothermal fluidity. Results from the effects of pressure on k(melt) revealed that it was generally the high total sulfur, low nitrogen, low reactives/mineral matter ratio, medium rank coals which show the greatest increase in k(melt), whereas the highest rank coals show the least decrease in k(coke). The energies of activation of melting and coking were not significantly affected by pressure. The investigation also reveals increases or decreases in the respective times of softening, maximum fluidity, resolidification and total time of fluid behavior under isothermal pressurized conditions. There appears the possibility that these shifts may be rank dependent. Additionally, the lower rank coals show the largest relative increase in their fluidities when subjected to pressure. Empirical relationships were derived in order to quantitatively predict the maximum isothermal fluidity for most (fluid) coals at a given pressure.  相似文献   
999.
The synthesis of three new azaphenothiazine ring systems and an evaluation of their use as intermediates for new dyes and pigments is described. 2-Amino-5-bromopyrazine-3[4H]-thione (11) was prepared and converted to the novel 1,4,6,9-tetraazaphenothiazine ring system. The reaction of 4,5-diaminopyrimidine-6[1H]-thione (15) with 2,3-dichloropyrazines gave the isomeric 1,4,6,8-tetraazaphenothiazine ring system, another new heterocycle in this series. With 2,3-dichloroquinoxaline a previously unknown tetracyclic tetraazaphenothiazine ring system was isolated in satisfactory yields. The properties and reactions of these new heterocyclic systems are presented. Mixed nitric and sulphuric acids convert them to their 5-sulphoxides. Structural assignments were based on chemical evidence and their UV, IR, NMR and mass spectra. Mechanistic pathways to these compounds are also proposed.  相似文献   
1000.
A simple apparatus for elongational test of molten polymers is presented. Its realiability is demonstrated by means of stress growth in constant stretching rate experiments and relaxation test on a low density polyethylene sample.  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