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   278707篇
  免费   5331篇
  国内免费   2131篇
电工技术   5845篇
技术理论   5篇
综合类   1706篇
化学工业   45461篇
金属工艺   11483篇
机械仪表   8389篇
建筑科学   7375篇
矿业工程   1839篇
能源动力   6096篇
轻工业   27511篇
水利工程   3112篇
石油天然气   6164篇
武器工业   270篇
无线电   29851篇
一般工业技术   51555篇
冶金工业   50195篇
原子能技术   5750篇
自动化技术   23562篇
  2021年   2915篇
  2020年   1917篇
  2019年   2431篇
  2018年   3739篇
  2017年   3679篇
  2016年   4051篇
  2015年   3129篇
  2014年   5070篇
  2013年   12261篇
  2012年   8389篇
  2011年   11044篇
  2010年   8800篇
  2009年   9400篇
  2008年   10170篇
  2007年   10309篇
  2006年   9042篇
  2005年   7905篇
  2004年   7268篇
  2003年   7027篇
  2002年   7324篇
  2001年   7259篇
  2000年   6429篇
  1999年   6325篇
  1998年   13909篇
  1997年   10199篇
  1996年   7785篇
  1995年   5880篇
  1994年   5348篇
  1993年   5238篇
  1992年   4104篇
  1991年   3870篇
  1990年   3942篇
  1989年   3886篇
  1988年   3640篇
  1987年   3117篇
  1986年   3144篇
  1985年   3540篇
  1984年   3392篇
  1983年   3151篇
  1982年   2769篇
  1981年   2987篇
  1980年   2729篇
  1979年   2893篇
  1978年   2776篇
  1977年   2924篇
  1976年   3856篇
  1975年   2502篇
  1974年   2318篇
  1973年   2366篇
  1972年   2010篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
871.
First available appraisals for 67,644 Jersey cows were used to estimate relative magnitudes of additive and selected sources of nonadditive genetic variance and covariance for 13 type traits scored linearly from 50 to 99 points. Covariances among four types of relatives were used to estimate components of genetic variance and covariance for each of the 13 traits. Direct additive, direct dominance, and additive maternal genetic variances and the direct-maternal additive genetic covariance were estimated using covariances among paternal half-sibs, full-sibs, daughters and dams, and maternal half-sibs. Estimates of additive genetic variance were 11 to 36% of variance within herd-appraiser subclasses with largest values for stature, strength, rump angle, rump width, and udder depth. Dominance components were two to four times the magnitude of additive components for dairy character, rump width, fore udder, rear udder height, rear udder width, and suspensory ligament but much smaller for other traits. Maternal effects and the direct-maternal additive genetic covariance appeared small for all linear type traits.  相似文献   
872.
873.
874.
The StoryGrid project undertook studying the role new interface technologies might play in education, particularly at the high school level. Unfortunately, technology often seizes center stage in high school classrooms; i.e., it becomes the topic of instruction. We believe that learning about technology would be most successful when technology is not the topic, but simply a tool used during instruction. StoryGrid, therefore, was designed to support and to enhance existing narrative activity in classrooms by adhering to the following goals: trigger reflection and interpretation, accommodate individual expression and encourage student discourse.  相似文献   
875.
876.
The Uniform Covering by Probabilistic Rejection (UCPR) algorithm was used, in conjunction with the Soil and Water Assessment Tool (SWAT) model, to identify P loads from point source and nonpoint source polluters in the upper Etowah River basin (UERB) in Georgia. The key findings of the research are as follows. The mean absolute error was preferred over the root mean square error as a search criterion for the UCPR algorithm when water quality observations are scarce. The undocumented P load from point sources in the UERB was consistently estimated as about 43 kg/d by the proposed method; but the method was not able to identify the broiler litter application rate to the poultry/beef operation pastures. Point sources (both documented and undocumented), poultry/beef operation pastures, and forests are the three major contributors of P. During 1992-1996, on average they accounted for 36.4, 31.7, and 17.2% of P load from the UERB, respectively.  相似文献   
877.
Many spectral‐recovery methods using RGB digital cameras assume the underlying smoothness of illuminant and reflectance spectra, and apply low‐dimensional linear models. The aim of the present work was to test whether a direct‐mapping method could be used instead of a linear‐models approach to recover spectral radiances and reflectances from natural scenes with an RGB digital camera and colored filters. In computer simulations, a conventional RGB digital camera with up to three colored filters was used to image scenes drawn from a hyperspectral image database. Three measures were used to evaluate recovery with the direct‐mapping method: goodness‐of‐fit, root‐mean‐square error, and a color‐difference metric. It was found that with two and three filters both spectral radiances and reflectances could be recovered sufficiently accurately for many practical applications. With little increase in computational complexity, an RGB camera and a few colored filters can provide significantly better recovery of natural scenes than an RGB camera alone. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 352–360, 2007  相似文献   
878.
The mechanism of formation and properties of emulsion-synthesized globular pseudoheterogeneous catalysts are considered. The results of application of these catalysts to coal hydrogenation and other industrial-chemistry processes are reported.  相似文献   
879.
The design of vehicles transporting hazardous materials has important public safety and economic implications. Conventional wisdom among industry and government has held that a thicker tank on railroad tank cars and trucks reduces risk. However, a thicker tank increases vehicle weight and thus leads to an increase in the number of shipments required to transport the same amount of product and consequently greater exposure to accidents. In this research we develop a model that analyzes the tradeoff between increased damage resistance and greater exposure to accidents in which the objective function is minimization of the probability of release. The model accounts for the reduction in tank car release probability as a function of tank thickness, and the increased exposure to accidents that occurs due to the increased number of shipments needed for the heavier car. Three variables affecting this optimal thickness are considered in this paper: the volumetric capacity of the tank, the probability of release from other, non-tank sources, and the weight capacity of the car. Sensitivity analyses using the model indicate that for any particular configuration of tank car there is an optimal thickness. This optimal thickness is affected by several factors and there is no single optimum for all tank cars.  相似文献   
880.
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