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   4197篇
  免费   285篇
  国内免费   383篇
林业   636篇
农学   323篇
基础科学   278篇
  1331篇
综合类   1528篇
农作物   161篇
水产渔业   79篇
畜牧兽医   241篇
园艺   85篇
植物保护   203篇
  2024年   19篇
  2023年   71篇
  2022年   120篇
  2021年   118篇
  2020年   97篇
  2019年   135篇
  2018年   80篇
  2017年   169篇
  2016年   206篇
  2015年   153篇
  2014年   204篇
  2013年   196篇
  2012年   286篇
  2011年   286篇
  2010年   238篇
  2009年   238篇
  2008年   235篇
  2007年   270篇
  2006年   232篇
  2005年   221篇
  2004年   180篇
  2003年   163篇
  2002年   134篇
  2001年   131篇
  2000年   117篇
  1999年   74篇
  1998年   72篇
  1997年   60篇
  1996年   69篇
  1995年   46篇
  1994年   56篇
  1993年   48篇
  1992年   36篇
  1991年   30篇
  1990年   23篇
  1989年   15篇
  1988年   24篇
  1987年   5篇
  1986年   5篇
  1985年   2篇
  1981年   1篇
排序方式: 共有4865条查询结果,搜索用时 15 毫秒
31.
A new type of inorganic anchoring material is developed as a substitution for anchoring adhesive to overcome some imperfections in the process of planting reinforcing steel bar.The mix proportion of the anchoring material is determined by experiments.The compressive strength,adhesive force to concrete,volume stability and anchoring force with steel bar are studied.The early strength of the anchoring inorganic material is quite high: for 1d up to 38.5MPa and 3d up to 60MPa.It is a good adhesive to concrete and the adhesive strength of 28d is about 2MPa.The anchoring material will have tiny volume expansion with the age,which is beneficial to enhancing the anchoring force.The essential component of the inorganic anchoring material is cement-based material,so some defects of the organic macromolecular material can be avoided,such as poor stability and requirement for vigorous construction conditions.Its characteristics in early strength,adhesive force to concrete and volume expansion can satisfy the requirement of the construction to the planting reinforcing steel bar and the loading force,thus it can be used as a substituting material for anchoring adhesive.  相似文献   
32.
Geogrid reinforced soil retaining wall(GRSRW) is composed of panel,geogrid and soil,its coupled hydromechanical structural characteristics are increasingly emphasized in practice.In this paper,based on the characteristics of the reinforcement-soil interaction,the grogrid and soil around it with a definite thickness were regarded as a kind of composite material with strain-compatible condition.A saturated porous media model was introduced to simulate the soil and reinforcement-soil composite material hereby and the mathematic model for numerical analysis on structural characteristics of GRSRW was built up.The FEM equations were deduced from the standard Galerkin procedure,resolving these equations,we can gain the relevant dynamic response of saturated GRSRW.  相似文献   
33.
Pearlitic steel is composed of numerous pearlitic colonies with randomly distributed orientations,and each colony is composed of many alternatively arranged parallel lamellas of ferrite and cementite.The pearlitic steel with fine interlamellar spacing possesses excellent mechanical properties,such as high strength,good resistance against wear,and high fatigue life,etc. Based on the inconsistency of the deformation between the two phases,a strain_equivalent and non_local constitutive model is proposed. Using this model,the effect of the interlamellar spacing of peailitic steel,a fundamental parmeter of the microstructure of pearlitic material, on its macroscopic mechanical behavior can be described without changing the local material parameters for each phase of the material.The asymmetrical cyclic plasticity of both the hot_rolled and the off_line full_length quenched pearlitic steel PD3 is analyzed.The comparison between the computational and the experimental results shows satisfactory agreement.  相似文献   
34.
Bill of material of product is data about product structure, and plays an important role in product information management. The paper analyzed characteristics of product BOM, and put forward a product BOM management method based on single layer BOM, witch can well meet requirements of product BOM.  相似文献   
35.
To embed or heap industry solid wastes somewhere will occupy the earth and pollute the environment seriously. So utilizing the industry solid wastes as regeneration resource is important for sustainable development of city. Based on application and research for years in utilizing industry wastes and widely investigation, the produce status of industry solid wastes in Chongqing such as fly ash, desulfuration gypsum, blastfurnace slag and chromium slag are introduced. At the same time,this paper introduces application and research on these wastes in field of building material in Chongqing. Analyze problems and countermeasures during utilizing industry solid wastes as regeneration resource in Chongqing. As a further the prospect of utilizing industry solid wastes as regeneration resource of building material is discussed.  相似文献   
36.
黎榆A繁殖母本有效穗和结实率与产量的关系   总被引:5,自引:0,他引:5  
繁殖田母本有效穗和结实率是影响产量的主要因素。通过对10个不同父母本栽插行比的分析结果表明:黎榆A繁殖产量以父母本栽插行比为2:18最高,达292.09kg/667m^2。增加有效穗对于增产的效应要比增加结实率大;有效穗与产量,结实率与产量的偏相关系数均呈正相关,且有效穗与产量的相关达显著水平,多元回归方程为:y-155.6283 4.8660x 0.2342x2.  相似文献   
37.
为检测X射线对日本沼虾精子的灭活效果,利用X射线对雄性日本沼虾进行不同时间的照射处理,将处理后的雄虾与刚完成生殖脱壳的雌虾交配。待雌虾产卵后,计数雌虾抱卵天数,并在显微镜下观察卵的畸形情况。X射线管照射强度为100 k V,5 m A,源距10 cm,照射时间为0、30、60、90、120 min。结果发现:照射30 min组的抱卵天数及卵的畸形率较0 min组无变化,卵的畸形率为0,卵能正常发育至孵化出膜。随着照射时间的增加,在30~90 min之间,雌虾抱卵天数逐渐减少,且所抱卵的畸形率呈上升趋势。照射120 min组的抱卵天数较90 min组变化不大,卵的畸形率达100%。在本试验照射时间范围(0~120 min)内,灭活日本沼虾精子遗传物质的最佳照射时间为90~120 min。  相似文献   
38.
基于稀土超磁致伸缩材料的微致动器设计   总被引:7,自引:0,他引:7  
选择Terfenol-D作为微致动器主要材料,采用弹簧对GMM棒施加一定的预压力以获得较大的磁致伸缩应变,采用偏置磁场消除GMM棒的倍频现象并使之工作在线性段,对磁路进行了分析和优化设计。针对电磁线圈工作发热及涡流损耗引起的GMM棒温升,采取了强制水冷方法并专门设计了一个恒温冷却系统用以消除材料的热变形对致动精度的影响。  相似文献   
39.
利用两相复合材料位移基本解,将受复合载荷作用下复合材料三维裂纹问题,归为求解一组超奇异积分方程,并分析了对裂纹尖端应力奇性和方程组求解理论方法;进而以反映裂纹前沿应力奇性的密度函数基本解表示位移间断,根据Taylor积分及其特征展开特性,给出方程组数值求解方法;最后,以三维矩形裂纹为例,讨论了应力强度因子(SIFs)收敛性及其变化规律。  相似文献   
40.
利用复合材料模型模拟加筋地基,按照土塑性力学分析加筋地基的变形和破坏,并按传统滑移线解法推导出浅基础加筋地基的承载力设计公式,利用实例比较探讨了本文方法与传统极限分析法的优缺点。  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