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   253861篇
  免费   3845篇
  国内免费   1050篇
电工技术   4352篇
综合类   205篇
化学工业   37592篇
金属工艺   8258篇
机械仪表   7306篇
建筑科学   6842篇
矿业工程   549篇
能源动力   7289篇
轻工业   28820篇
水利工程   2130篇
石油天然气   1854篇
武器工业   5篇
无线电   33509篇
一般工业技术   46505篇
冶金工业   46620篇
原子能技术   3380篇
自动化技术   23540篇
  2021年   1840篇
  2020年   1381篇
  2019年   1707篇
  2018年   2638篇
  2017年   2655篇
  2016年   2941篇
  2015年   2222篇
  2014年   3733篇
  2013年   12087篇
  2012年   6860篇
  2011年   9813篇
  2010年   7537篇
  2009年   8507篇
  2008年   9016篇
  2007年   9004篇
  2006年   8330篇
  2005年   7545篇
  2004年   7249篇
  2003年   7137篇
  2002年   6633篇
  2001年   7039篇
  2000年   6350篇
  1999年   6842篇
  1998年   16414篇
  1997年   11391篇
  1996年   8629篇
  1995年   6554篇
  1994年   5723篇
  1993年   5588篇
  1992年   4010篇
  1991年   3780篇
  1990年   3558篇
  1989年   3376篇
  1988年   3281篇
  1987年   2599篇
  1986年   2493篇
  1985年   3165篇
  1984年   2806篇
  1983年   2566篇
  1982年   2357篇
  1981年   2398篇
  1980年   2203篇
  1979年   2054篇
  1978年   1884篇
  1977年   2189篇
  1976年   2740篇
  1975年   1590篇
  1974年   1490篇
  1973年   1565篇
  1972年   1146篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
The structure of the recently introduced Brazilian government programme for monitoring pesticide residues in six major agricultural products is described. Residues found from the analysis of samples of tomatoes and strawberries are discussed in detail. The much greater frequency of detection of pesticide residues, many of which are not approved for use in Brazil, in produce being sold on the domestic market, as opposed to that produced for export, is of major concern. It is perceived that this new monitoring programme will become permanent and be used to underpin enforcement actions that will need to be taken to reduce the numbers of incidents of illegal residues occurring, particularly in fresh fruits and vegetables.  相似文献   
102.
Neutron-irradiated quartz crystal is one of the interesting materials for the study of the glassy dynamics. The temperature and frequency dependence of the sound velocity in amorphous materials at temperatures above the low-temperature maximum is not well understood. In this paper we present the results of our measurements of the variation of the ultrasound velocity (270 – 650 MHz) in neutron-irradiated quartz crystals with different irradiation doses in temperature range 5 - 35 K. We have found that the temperature dependence of the ultrasound velocity is linear in the sample with a dose < 10 19 n/cm2 and is sublinear in samples with a dose > 10 19 n/cm2 . We discuss our results in the framework of the different theoretical approaches. PACS numbers: 61.43. j, 62.65.+k, 63.  相似文献   
103.
104.
105.
106.
107.
A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In 0.12Ga0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In0.12Ga0.88As was estimated to be about 380 meV  相似文献   
108.
A combination of concept (the product portfolio and the product life cycle) and empirical research (the PIMS study) can aid in clarifying the key issues involved in the selection of a firm's core strategies. Strategy control is multistage: periodic re-evaluation of strategy alternatives by use of the marketing audit; an analytical framework for control of chosen strategies by breaking down components and reporting their performance variances; and 'marketing segment' analysis for control of the detailed marketing plan.?  相似文献   
109.
Polystyrene packaging material taint was sensorily evaluated in cocoa powder for drinks and chocolate flakes using short-cut signal detection measures on differences between control and test samples and on recognition of styrene. No differences were observed in cocoa powder for drinks and plain chocolate flakes treated with 0.5 dm2 polystyrene of 1 mm thickness. However, differences were detected in milk chocolate flakes and plain chocolate flakes, which were in contact with a larger area or thicker polystyrene packaging material. The latter results were confirmed by the styrene recognition test, so polystyrene is a potential source of off-flavour for chocolate products. The amount of residual styrene in the polystyrene used was about 320 ppm, while the amounts of styrene ranged from 7 to 132 ppb in cocoa drinks and from 414 to 1447 ppb in chocolate flakes.  相似文献   
110.
The application of the Trefftz method for calculating wave forces on offshore structures is presented. Indirect and direct formulations using complete and non-singular systems of Trefftz functions for the Helmholtz equation are posed in this paper. An effective technique using different interpolation functions for the velocity potential and wave force are suggested to improve the computational accuracy of the wave force. The numerical examples show that the present method is highly efficient and accurate.  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