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   413626篇
  免费   4579篇
  国内免费   960篇
电工技术   7269篇
综合类   392篇
化学工业   65562篇
金属工艺   17700篇
机械仪表   13145篇
建筑科学   8939篇
矿业工程   3195篇
能源动力   9275篇
轻工业   35175篇
水利工程   4986篇
石油天然气   11150篇
武器工业   52篇
无线电   42923篇
一般工业技术   82579篇
冶金工业   75088篇
原子能技术   11106篇
自动化技术   30629篇
  2021年   3723篇
  2019年   3601篇
  2018年   6249篇
  2017年   6255篇
  2016年   6766篇
  2015年   4066篇
  2014年   6937篇
  2013年   17571篇
  2012年   10713篇
  2011年   14244篇
  2010年   11352篇
  2009年   12554篇
  2008年   13064篇
  2007年   13077篇
  2006年   11491篇
  2005年   10295篇
  2004年   9690篇
  2003年   9403篇
  2002年   9303篇
  2001年   9281篇
  2000年   8877篇
  1999年   8895篇
  1998年   21531篇
  1997年   15152篇
  1996年   11676篇
  1995年   8940篇
  1994年   7908篇
  1993年   8001篇
  1992年   6227篇
  1991年   5943篇
  1990年   5995篇
  1989年   5870篇
  1988年   5728篇
  1987年   4989篇
  1986年   5080篇
  1985年   5784篇
  1984年   5495篇
  1983年   5078篇
  1982年   4710篇
  1981年   4754篇
  1980年   4703篇
  1979年   4632篇
  1978年   4691篇
  1977年   5056篇
  1976年   6266篇
  1975年   4164篇
  1974年   3951篇
  1973年   4160篇
  1972年   3596篇
  1971年   3292篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
'Software defined radio' (SDR) is a technology that will appear in future generations of mobile phones, i.e. following the third-generation mobile phone technology that is currently being defined and developed. Early versions of 'pragmatic' SDR will allow the terminal to be reconfigured at any level of its protocol stack. Ultimately, the 'pure' SDR technology will allow a mobile phone or terminal to have its air interface software configured or reconfigured by other software (or software parameters) that have been downloaded to the terminal, e.g. over the air, or from a remote server via the Internet and one's personal computer (PC). A number of security issues arise with downloaded code that implements the air interface functions, and these may not be obvious simply from looking at the way PC software is updated on-line today. This paper starts with an outline of the code that allows a mobile phone to operate over a particular air interface. This sets the baseline for a discussion of the security issues surrounding the change of this code from one that is fixed and downloaded once only, to code that is reconfigurable during the life of a product.  相似文献   
92.
93.
S N Maitra 《Sadhana》1985,8(4):373-385
The burn time and burnout velocity of a multistage rocket flown vertically in vacuum with constant thrust tangential to the flight path and a prescribed initial/final thrust-to-weight ratio in an arbitrary stage have been determined. The present paper also deals with optimal staging under given conditions of flight.  相似文献   
94.
95.
96.
Existing duality principles in structural optimisation are briefly reviewed and then they are extended to structures with segment-wise constant cross-sections. All theories are discussed in the particular context of optimal plastic beam design with symmetric convex specific cost functions and are confirmed by independent calculations on illustrative examples. It is shown that the optimal solution is always associated with a displacement field in which the mean absolute curvature value for each segment equals the subgradient of the specific cost function, with respect to the maximum absolute moment value for that segment. Moreover, the dual problem consists of the maximisation of the difference of two terms: the first one is the integral of the product of load and deflection (external work), and the second is the sum of products of segment lengths and the mean complementary cost values (taken with respect to the mean absolute curvature for that segment). Finally, some tentative proposals for a class of non-convex optimisation problems are presented. For special cases, the proposed general statements reduce to theorems by Heyman, Foulkes and Hemp.  相似文献   
97.
98.
Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 2, pp. 14–15, February, 1989.  相似文献   
99.
The usefulness of selected PCR-protocols for the detection of Salmonella in 117 samples of animal origin (17 raw minced meat, 27 raw chicken meat, 8 raw sausages, and 25 egg samples, as well as 18 poultry faecal, and caecal swabs samples) and DNA-fingerprinting typing is shown. To establish an accurate PCR-procedure for Salmonella detection the following parameters were evaluated: two pre-PCR concentration procedures, centrifugation and immunomagnetic separation (IMS) using Dynabeads anti- Salmonella; the specificity and sensitivity of 10 sets of primers; and different conditions of the amplification reaction. In light of the results obtained from the use of PCR-based procedures alone or in combination with conventional methods, the following findings can be underlined: First, IMS is more efficient than centrifugation in the recovery of Salmonella. Second, the selected IMS/PCR-detection protocol is less time-consuming (45 h) than the IMS/culture procedure (90 h), and a good concordance between them was found when the Kappa coefficient was calculated (0·87). Third, PCR-ribotyping technique showed a very low discrimination power, being able to differentiate only three profiles. Fourth, RAPD technique using specific primers supports previous works in which it was proposed as a simple and useful tool for discriminating isolates between and within serotypes. Fifth, The efficiency, rapidity, and flexibility of the PCR-protocols applied were high, and they can be performed using two PCR-programs and the same basic equipment.  相似文献   
100.
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