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   69944篇
  免费   4561篇
  国内免费   6054篇
林业   5993篇
农学   5205篇
基础科学   3598篇
  7396篇
综合类   32269篇
农作物   4716篇
水产渔业   2904篇
畜牧兽医   10820篇
园艺   4524篇
植物保护   3134篇
  2024年   233篇
  2023年   1223篇
  2022年   2858篇
  2021年   3030篇
  2020年   2864篇
  2019年   2653篇
  2018年   1987篇
  2017年   3097篇
  2016年   2311篇
  2015年   3411篇
  2014年   3705篇
  2013年   4168篇
  2012年   6014篇
  2011年   5972篇
  2010年   5737篇
  2009年   5238篇
  2008年   5103篇
  2007年   4557篇
  2006年   3911篇
  2005年   3027篇
  2004年   1850篇
  2003年   1302篇
  2002年   1398篇
  2001年   1288篇
  2000年   1110篇
  1999年   636篇
  1998年   306篇
  1997年   272篇
  1996年   212篇
  1995年   187篇
  1994年   158篇
  1993年   149篇
  1992年   130篇
  1991年   125篇
  1990年   69篇
  1989年   64篇
  1988年   53篇
  1987年   37篇
  1986年   29篇
  1985年   11篇
  1984年   11篇
  1983年   4篇
  1982年   6篇
  1981年   10篇
  1979年   1篇
  1964年   1篇
  1963年   1篇
  1962年   12篇
  1956年   25篇
  1955年   3篇
排序方式: 共有10000条查询结果,搜索用时 703 毫秒
991.
992.
993.
Jointing of titanium alloy and stainless steel is very often in the fields like aviation, chemical plants etc. The combination of materials has the advantages of both titanium alloy and stainless steel and will reduce the consumption of titanium resource. The phase transformation diffusion bonding of TA17 titanium alloy to 0CriSNigTi stainless steel are carried out on Gleeble - 1500D. The effcet of maximum thermal cycle temperature, the tensile strength, the fracture surface and microstruture of the joints are investigated. The results show that TiFe and TiFe2 intermetallics and some solid solutions are formed along the interface, when maximum thermal cycle temperature increase, the area proportion of intermetallics in the fracture surface increase, which are detrimental to the mechanical properties of the transition joints.  相似文献   
994.
为对菲牛蛭抗凝血活性物质的提取进行优化,通过采取不同方法对菲牛蛭活体(6种)、干体(6种)及唾液腺分泌物(3种)进行提取得出最适提取方法。结果表明对于菲牛蛭活体和干体均是采取80%预冷丙酮提取得到的抗凝血物质含量高,最高分别达至(61.20±1.13)AT-U/g和(89.19±1.13)AT-U/g;对于菲牛蛭唾液腺分泌物则是采取10%的三氯乙酸提取得到的抗凝血物质含量高,最高达(24.41±0.98)AT-U/g。研究结果得出了适合菲牛蛭活体、干体和唾液腺分泌物中提取抗凝血活性物质的最适方法,有利于推动水蛭素的大规模生产和广泛的临床应用。  相似文献   
995.
996.
Aiming at ensuring that product family improvement is based on customer needs while taking full advantage of enterprise history database, firstly, customer needs in the product family improvement are expressed. Then a method of realizing the mapping of customer needs to functional requirements is presented, which is based on quality function development and association rule mining. Finally, an example illustrates the good application of the method.  相似文献   
997.
The existence of ettringite in sulphoaluminate cement hardened paste is mainly in the form of pole-like crystal,which causes unfavorable effect on the cement.In this paper the strength and microstructure of the sulphoaluminate cement containing ultra-fine CaCO_3 weze discussed.Experimet results show that the strength of sulphoaluminate cement was obviously improved by adding 3% of ultra-fine CaCO_3.After curing for 28d,the compressive strength of cement paste and mortar is 100.6MPa and 94.1MPa respectively,and the bending strength of the mortar reached 12.5MPa.SEM study on hardened paste of sulphoaluminate cement containing ultra-fine CaCO_3 showed that its structure was dense and homogeneous,there was hardly any massive crystall detected.  相似文献   
998.
According to the character of PD in GIS, the authors design four kinds of GIS defection models. The GIS gray intensity images are constructed based on mass specimens gathered by the ultra - high frequency and high speeds systems, Aiming at the PD characteristics and its defections, A PCA-FDA method is put forward based on PD images. The principal component analysis is employed to condense the dimension of PD images, then the optimal sets of statistically uncorrelated discriminant vectors are extracted, and the minimum distance classifier is constructed as classifier. The identified results show that this method can effectively elevated the discrimination of the four kinds of defects in GIS PD.  相似文献   
999.
In this paper,four models are selected as the representatives and applied to nonlinear dynamic analysis program for space frames which is based on the beam-column elements using the finite element flexibility method and the fiber model.Park's test on combined axial and bending columns under cyclic loading are taken as the calibrations,comparisons to material and structural member level are drawn and the analytical results of various models are discussed.  相似文献   
1000.
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