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   33269篇
  免费   4050篇
  国内免费   2194篇
林业   2656篇
农学   1765篇
基础科学   1548篇
  3351篇
综合类   17039篇
农作物   2402篇
水产渔业   1598篇
畜牧兽医   5264篇
园艺   2394篇
植物保护   1496篇
  2025年   17篇
  2024年   815篇
  2023年   779篇
  2022年   1573篇
  2021年   1504篇
  2020年   1408篇
  2019年   1404篇
  2018年   968篇
  2017年   1575篇
  2016年   1064篇
  2015年   1593篇
  2014年   1839篇
  2013年   1926篇
  2012年   3011篇
  2011年   2915篇
  2010年   2707篇
  2009年   2527篇
  2008年   2523篇
  2007年   2275篇
  2006年   1916篇
  2005年   1530篇
  2004年   961篇
  2003年   587篇
  2002年   642篇
  2001年   597篇
  2000年   537篇
  1999年   192篇
  1998年   25篇
  1997年   17篇
  1996年   8篇
  1995年   7篇
  1994年   4篇
  1993年   12篇
  1992年   11篇
  1991年   6篇
  1990年   2篇
  1989年   1篇
  1988年   1篇
  1987年   6篇
  1986年   8篇
  1985年   3篇
  1981年   1篇
  1962年   6篇
  1956年   9篇
  1955年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In this paper, the authors adopt a method of structural analysis and stereography in combination with acoustic emission Kaiser Effect test in laboratory to find out the main direction of initial stresses. For overcoming the shortcomings of Kaiser effect by comparing it with traditional methods of measuring in-situ stresses, it is shown that this method is simpler, more economical and convenient for a large number of measurements and for studying the law of changes of regional in-situ stresses.  相似文献   
92.
Fingerprint classification can provide an important indexing mechanism in a fingerprint database. An accurate and consistent classification can greatly reduce fingerprint matching time for large database. In the paper, by combining genetic algorithm and neural network is presented a fingerprint classification algorithm which is able to achieve an accurate classification. By inputting the global feature represented by directional image to three layer neural network trained by genetic algorithm, the fingerprints were classified into six categories: whorl, right loop, left loop, arch, double loop and undiscerning type successfully.  相似文献   
93.
In order to study the influence on water temperature raise of river water source heat pump tail water discharged in different methods, the real sensor network is established by using digital temperature sensors. The simulation experiments of different discharging methods of river water source heat pump are carried out, including submerged-type discharge, surface-type discharge and jet-type discharge with 12 effluent ways. By using weighted average method, the preferred values of such factors as the average temperature raise,the temperature raise variance, the biggest temperature raise value, and the number of temperature measure points above the temperature raise average value are calculated. Moreover, the experiment results are analyzed by the three-dimensional figures of water temperature raise, which shows that jet-type discharge is better than submerged-type discharge and surface-type discharge,and the double-port way is better than single-port way. In addition, the double-port way along with the current in surface-type discharge method is the one that exerts least influence on river water temperature raise.  相似文献   
94.
By using near crack line analysis method and Mohr-Coulomb strength criterion,the elastic-plastic solution for near crack line of an eccentric cracks loaded by the pore water pressure under large scale yielding condition is analyzed.The elastic stress fields,the plastic stress field,the unit normal vector of the elastic-plastic boundary near the crack line,the elastic-plastic stress fields near crack line are identified.The change pattern of the length of the plastic zone and the pore water pressure is presented.Besides,the elastic-plastic analytical solution for an eccentric cracks loaded by the pore water pressure is obtained.  相似文献   
95.
A new composite structure which is composed of steel beam and concrete girder is introduced in fuel electric plant construction. With different height of concrete girders with or without anchor bar, two scale models from an actual project were designed and tested. It is shown that the maximum restraining moment of ends of steel beams is approximately 60kN*m; influence of anchor bar is little; the one with 600mm high concrete beam is damaged by shear, and the others steel beam is damaged by bending failure. Finite element models were built for supplementary analysis, and the result indicates that critical value of the height of concrete beam of different failure modes is 700mm; stiffness of shear key and thickness of floor have great influence on the structure; size of anchor bar has little influence; and width of the end of the steel beam has certain influence.  相似文献   
96.
The aim is to evaluate the seismic properties of ancient timber structure after strengthening and analyze the failure process and corresponding failure state. Based on the hysteretic behavior and energy dissipation principle of the dovetail column-frame strengthened with CFRP and Arches Brackets under the low reversed cyclic loading, the “potential of destruction-resisting” of the two energy-consuming components is obtained. The dissipated energy of each energy-consuming component under the various earthquake conditions is calculated combining with the shaking table test of ancient timber structure. The model of seismic damage evaluation of the two energy-consuming components is established on the basis of the “potential of destruction-resisting” and the dissipated energy. By means of the energy distribution coefficient, the relationship of the failure state between energy-consuming components and overall strengthened structure is discovered, and the model of seismic damage evaluation of the overall structure under the various earthquake conditions is presented. With the derived model of seismic damage evaluation, the failure coefficient of the energy-consuming components and the overall strengthened structure is quantitatively calculated. According to the failure state, the corresponding damage grade of overall strengthened structure is obtained. The results can provide a reliable theoretical basis for predicting the destruction before earthquake and re-reinforcement to the strengthened ancient timber structures after earthquake.  相似文献   
97.
Due to the sharp contradiction between rapidly developing housing construction in small-size town and quite low technological level, in this paper, the significance of setting up integrally technological system is emphasized and the suggestion that the technology should be integrated with the sustainable development, regional context and industrialization is put forward.  相似文献   
98.
水稻早世代稳定株系的农艺性状分析   总被引:4,自引:0,他引:4  
育种实践表明,水稻品种间和亚种间杂交后代农艺性状发生疯狂分离,纯合个体的出现一般需经7~10代,5~8年[1,2]。采用花培[3,4]和远缘杂交染色体消除技术[5,6]虽已应用于缩短作物育种时间,但仍耗时耗力。在作物如小麦、大麦、花生、水稻 [8~13]等育种实践中,偶尔会遇到杂种F2不分离的现象。吴先军等[11]提出了水稻早世代稳定现象的概念,即水稻杂交后代F3以前出现农艺性状整齐一致、以后世代不分离的遗传现象。由于水稻早世代稳定现象在育种中具有巨大的应用潜力,对其遗传机制和育种应用进行深入而系统的研究具有重大的理论探讨价值和现实意义。  相似文献   
99.
A hall for a workshop of metasynthetic engineering (HWME) is constructed based on network services, in which the expert, machinery and knowledge systems are combined with services. This hall is an expansion of the traditional workshop hall. The  相似文献   
100.
Computable general equilibrium (CGE)model's solving is important for its reseachers . Though there are many softwares to solve the CGE model, it is very important for its reseachers to understand CGE model's solving principle. The paper introduces the methods,principles and problems of CGE model's solving.The software -MATLAB is used to solve CGE Model, and reseach the existing problem of CGE model's solving.  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