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   63325篇
  免费   2914篇
  国内免费   3899篇
林业   8143篇
农学   5155篇
基础科学   1812篇
  6372篇
综合类   24923篇
农作物   4917篇
水产渔业   2700篇
畜牧兽医   7629篇
园艺   3746篇
植物保护   4741篇
  2024年   218篇
  2023年   711篇
  2022年   1195篇
  2021年   1425篇
  2020年   1455篇
  2019年   1639篇
  2018年   1336篇
  2017年   1738篇
  2016年   2460篇
  2015年   2425篇
  2014年   3206篇
  2013年   3555篇
  2012年   4440篇
  2011年   4251篇
  2010年   3498篇
  2009年   3534篇
  2008年   3367篇
  2007年   3795篇
  2006年   3274篇
  2005年   2847篇
  2004年   2182篇
  2003年   1833篇
  2002年   1620篇
  2001年   1679篇
  2000年   1403篇
  1999年   1308篇
  1998年   1079篇
  1997年   950篇
  1996年   940篇
  1995年   1065篇
  1994年   994篇
  1993年   700篇
  1992年   679篇
  1991年   576篇
  1990年   600篇
  1989年   531篇
  1988年   507篇
  1987年   323篇
  1986年   157篇
  1985年   133篇
  1984年   112篇
  1983年   73篇
  1982年   52篇
  1981年   52篇
  1980年   48篇
  1979年   32篇
  1978年   37篇
  1977年   48篇
  1976年   14篇
  1975年   13篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
891.
Additional time delay of arrival resulted from non-line of sight(NLOS)propagation and muti-path effect in(cellular) environment is the dominant factor resulting in TDOA location error,study on the statistical property of TDOA error is useful for improving the accuracy of TDOA location.Based on the exponential distribution property of NLOS propagation delay,and considering the system error resulted from the signal detection,the authors establish a statistical model of arrival time and a model of TDOA error distribution by statistics when non-light of sight and multi-path propagation exist.The models reflect the statistical property of arrival time and TDOA error in cell network,the simulation shows that the proposed models have a good accuracy and a satisfactory behavior.  相似文献   
892.
A theoretical model is developed for the dropwise condensation heat transfer on the horizontal circular surface with radial gradient surface energy based on the heat transfer model of individual condensate drop and the size distribution model of condensate drop on homogeneous condensation surface.The effect of variation of contact angle on the gradient surface on condensation heat transfer and condensate drop size distribution is taken account of in this model.The theoretical calculation method was obtained to predict the dropwise condensation heat transfer coefficient on the horizontal circular surface with radial gradient surface energy under various wall subcooled temperature,contact angle profile on wall surface,and working fluid.The effects of surface energy gradient,wall subcooled temperature,and thermophysical properties of condensate on the condensation heat transfer are discussed respectively.The calculation results show that the condensation heat transfer coefficient increases slightly with the increase of wall subcooled temperature.As latent heat and surface tension increasing,the condensation heat transfer coefficient increases.A larger surface energy gradient induces a larger condensation heat transfer coefficient.  相似文献   
893.
A video Monitor &Control system of tunnel based on DSP/BIOS is proposed,which is different from traditional video Monitor & Control system.It uses DSP to process image,and uses enhanced video input processor to qcquire image.This system satisfies all requirements of video supervision,and is a low cost system.It has a good application foreground.The system has been confirmed as a good veracity and practicability system through testing in the Zhongliang Mountain tunnel of Chongqing.  相似文献   
894.
The authors determine the index system and the index weight of capability of real estate by applying Delphi method and AHP method, and found the fuzzy comprehensive evaluation model of capability of real estate. They creates the model of capability Vs. price based on value engineering theory. The authors analysis detailedly deciding method of capability Vs. price of real estate combined with a practice example and application. The method adapts to a house as well as the type of the house. The method resolves the problem which capability Vs. price of real estate was only described by qualitatively for ages, which supplies theory basis to developer and consumer.  相似文献   
895.
The predictive intelligent control scheme is proposed, which is based on approximation theory and numerical method of Chcbyshev orthogonal polynomial. It has the practical value to the cold storge using widely electromagnetic valve and can be also applied similar systems in orther area.  相似文献   
896.
In this paper the reimprovement for G. I. Taylor theory of plastic deformation in impact of cylindical projectiles is done. The impulse in the reimproved theory is G. I. Taylor' s 1/2, and 3/4 of G. I. Taylor' s improved theory. The analytical solution of the reimproved theory shows that it gives better agreement with exprimental results than those of the original Taylor' s theory and, the improved theory.  相似文献   
897.
The green is as a symbol of no pollution,no public harm and environmental protection. The green automobile is the inevitable results that the automobile technique continuously developes. A new style vehicle--the green automobile is the urgent solicit of environmental protection is developed. The automobile industry can develop continuously. This article mainly discusses the importance of the green auto_mobile's research and development,the development status in quo and prospect of the green auto_mobile technique in the world,and states the relationship between the green automobile technique and environmental protection from green technical system theoretically clarify the study of green automobile,including its design and the system,the manufacture and its technology,the energy,the rebuild engineering and a series of subjects. How to evaluate the cycle and the evaluable mark of the green automobile,the prospect of green automobile are pointed out.  相似文献   
898.
In TT&C system,unified spread spectrum framework and adaptive antenna digital beamforming technologies are used to track, telemetry and command objects. Constant modulus array (CMA) and adaptive signal canceller estimate the direction of arrival (DOA) of every signal and use the optimum weighted vector to form the beams. The phase weighted vectors estimated from DOA improve the processing gain as well as being utilized in the transmitting of tele command signal to form the beam of specified object. Its processing performance is researched and simulated in this paper.  相似文献   
899.
With the rapid development of the manufacture, the technology of information and net, the traditional management of enterprise cannot meet the demands of the market, to get a better development and benefit, enterprises have to keep a competitive and cooperative relation with each other. Thus is the beginning of the dynamic union. Combining the theory of dynamic union with BPR, this thesis proposes the BPR facing the dynamic union and expounds the principle of it, the re-engineering of the full life cycle and the evaluation method of re-engineering on the base of analyzing the characteristic of dynamic and demands of process.  相似文献   
900.
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