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   418722篇
  免费   6026篇
  国内免费   1394篇
电工技术   7783篇
综合类   355篇
化学工业   63165篇
金属工艺   14411篇
机械仪表   11987篇
建筑科学   11871篇
矿业工程   922篇
能源动力   11240篇
轻工业   44263篇
水利工程   3378篇
石油天然气   1843篇
武器工业   13篇
无线电   55765篇
一般工业技术   77709篇
冶金工业   76431篇
原子能技术   5620篇
自动化技术   39386篇
  2021年   2771篇
  2019年   2551篇
  2018年   3937篇
  2017年   4008篇
  2016年   4267篇
  2015年   3522篇
  2014年   5862篇
  2013年   19613篇
  2012年   10442篇
  2011年   15002篇
  2010年   11545篇
  2009年   13110篇
  2008年   14205篇
  2007年   14559篇
  2006年   13006篇
  2005年   12134篇
  2004年   11735篇
  2003年   11442篇
  2002年   11058篇
  2001年   11249篇
  2000年   10382篇
  1999年   10882篇
  1998年   24285篇
  1997年   17792篇
  1996年   14072篇
  1995年   11091篇
  1994年   9878篇
  1993年   9470篇
  1992年   7204篇
  1991年   6832篇
  1990年   6463篇
  1989年   6186篇
  1988年   5918篇
  1987年   4930篇
  1986年   4877篇
  1985年   6018篇
  1984年   5642篇
  1983年   4876篇
  1982年   4539篇
  1981年   4494篇
  1980年   4250篇
  1979年   4111篇
  1978年   3811篇
  1977年   4584篇
  1976年   6169篇
  1975年   3145篇
  1974年   3035篇
  1973年   2907篇
  1972年   2339篇
  1971年   2013篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
The measured plastic deformation of Mg-PSZ under stress contains contributions from both transformation and microcracking. A method is described to separate the contributions from transformation and microcracking for samples deformed in uniaxial tension. Such a separation enables the ratio of the longitudinal to transverse strains arising from the transformation to be estimated. The results are compared to the predictions of the shear–dilatation model of Chen and Reyes-Morel. Serious discrepancies are found between the experimental results and the predictions of the model and the possible reason for this is discussed.  相似文献   
942.
This paper describes the fabrication of NMOS-transistors with a geometric gate length of down to 50 nm using conventional optical lithography and a modified sidewall-etchback process. Based on measurements the transistors are characterised and their device parameters are compared to simulations. Finally the procedures for further optimisation of the process will be explained.  相似文献   
943.
944.
Environmental impact of tobacco growing in Tabora/Urambo, Tanzania   总被引:1,自引:0,他引:1       下载免费PDF全文
J. Waluye 《Tobacco control》1994,3(3):252-254
  相似文献   
945.
This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone, rather than a single, sharp crack, and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy Cybond 4523GB (American Cyanamid), there was approximately 5 mm of subcritical crack propagation prior to final fracture. Fracture-load predictions based on the initial uncracked geometry made in previous papers were unaffected by this small change in geometry. For joints bonded with the rubber-toughened epoxy Permabond ESP 310, approximately 50 mm of subcritical crack propagation was observed. It was again found that predictions made in previous papers on the basis of the initial geometry gave a good estimate of the final fracture load even though this subcritical crack propagation significantly altered the geometry, and thus the applied energy release rates. The effect of shear deformations of the adherends was also investigated, and it was found that shear deformations could be neglected in engineering calculations for joints subject to remote tensile loading.  相似文献   
946.
Tabletability of five types of maltodextrin, a filler/binder excipient, was studied by testing their loading potentials with acetaminophen. The formulations consisted of excipient and acetaminophen at five different ratios and magnesium stearate at a 0.5% concentration. These mixtures were compacted employing an Integrated Compaction Research System at a constant punch velocity of 100 mm/sec. under varying applied pressures from 50 to 450 MPa. Compaction data were evaluated using the total work of compaction vs applied pressure plots whilst the post-compaction tests included the measurements of crushing force, disintegration time, and friability of the resulting tablets. Both the energy involved during the compaction of a formulation and the crushing force values of the resulting tablets decreased as the amount of the maltodextrin in a formulation was reduced. Maltodextrins exhibited adequate binding potential at acetaminophen drug loading levels of only up to twenty-five percent. The disintegration times of the tablets containing maltodextrins were generally prolonged and this was found to be due to the formation of a “gel” layer around the tablet which formed on immersion into water. The tabletability of maltodextrins were also compared to that of Fast-Flo lactose, and the compactability of these excipients were found to be similar.  相似文献   
947.
Smart cars, smart highways   总被引:2,自引:0,他引:2  
Advanced electronics is being used to unclog the transportation network and allow private vehicles, trucks, buses and trains to move further faster. already a range of technologies and ideas have been explored, deployed, and tested as part of the Intelligent Vehicle Highway System programs in the United States and Japan, and in the Road Transport Informatics program in Europe. The authors describe how these latest electronics for automobiles and roadways herald far fewer traffic jams, greater safety, and perhaps even a decline in pollution  相似文献   
948.
There is often a need to approximate integrals of highly oscillatory functions when studying scattering and diffraction of electromagnetic waves. This paper presents a method of estimating certain types of these integrals by evaluating one interpolating function and performing one or two relatively easy numerical integrations. The method is demonstrated for the case of a Fresnel integral  相似文献   
949.
The measured equation of invariance (MEI) has been previously introduced to efficiently and accurately handle the boundary truncation for finite methods. The present authors give a theoretical analysis that provides several important insights into the capabilities of the MEI. From the numerical study, they can explain why the MEI works better than one would expect. Both the theoretical and the numerical analyses demonstrate that the accuracy of the solution is dependent on the electrical size of the geometry as well as the distance between the mesh boundary and the geometry. From the analysis, the authors propose a new set of metrons that is less sensitive to the excitation than the previously proposed sinusoidal metrons  相似文献   
950.
Scattering by spherically stratified microwave lens antennas   总被引:2,自引:0,他引:2  
A comprehensive treatment of scattering by sources in the region of a spherically stratified object composed of discrete shells is presented and compared with measured results. The work encompasses the developments necessary for a thorough understanding of the focusing mechanism of a spherical lens. The method used is a modal technique that employs separation of variables in order to determine an eigenfunction representation of the incident and scattered fields. This formulation provides the Green's function required for the calculation of the scattering from a multiple layered sphere excited by an infinitesimal current element. The algorithm allows both near- and far-field predictions. The scattering by more complicated source field distributions is modeled by applying superposition with the appropriate coordinate transformations. From a practical standpoint, the present analysis allows the limitations of spherical lenses to be determined while providing the information required to optimize the lens design. A system was built based on the new design data, and its measured radiation characteristics are compared to the theoretical predictions. Excellent agreement is observed in most cases  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