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   2693篇
  免费   118篇
  国内免费   131篇
系统科学   171篇
丛书文集   62篇
教育与普及   40篇
理论与方法论   12篇
现状及发展   13篇
研究方法   9篇
综合类   2635篇
  2024年   18篇
  2023年   29篇
  2022年   42篇
  2021年   69篇
  2020年   36篇
  2019年   17篇
  2018年   32篇
  2017年   51篇
  2016年   42篇
  2015年   83篇
  2014年   121篇
  2013年   81篇
  2012年   102篇
  2011年   141篇
  2010年   105篇
  2009年   96篇
  2008年   132篇
  2007年   131篇
  2006年   111篇
  2005年   124篇
  2004年   96篇
  2003年   95篇
  2002年   78篇
  2001年   90篇
  2000年   85篇
  1999年   138篇
  1998年   99篇
  1997年   93篇
  1996年   109篇
  1995年   89篇
  1994年   76篇
  1993年   60篇
  1992年   69篇
  1991年   61篇
  1990年   45篇
  1989年   34篇
  1988年   37篇
  1987年   13篇
  1986年   8篇
  1985年   1篇
  1981年   3篇
排序方式: 共有2942条查询结果,搜索用时 31 毫秒
21.
Hu Z  Xia Y  Guo X  Dai J  Li H  Hu H  Jiang Y  Lu F  Wu Y  Yang X  Li H  Yao B  Lu C  Xiong C  Li Z  Gui Y  Liu J  Zhou Z  Shen H  Wang X  Sha J 《Nature genetics》2012,44(2):183-186
Non-obstructive azoospermia (NOA) is one of the most severe forms of male infertility. Its pathophysiology is largely unknown, and few genetic influences have been defined. To identify common variants contributing to NOA in Han Chinese men, we performed a three-stage genome-wide association study of 2,927 individuals with NOA and 5,734 controls. The combined analyses identified significant (P < 5.0 × 10(-8)) associations between NOA risk and common variants near PRMT6 (rs12097821 at 1p13.3: odds ratio (OR) = 1.25, P = 5.7 × 10(-10)), PEX10 (rs2477686 at 1p36.32: OR = 1.39, P = 5.7 × 10(-12)) and SOX5 (rs10842262 at 12p12.1: OR = 1.23, P = 2.3 × 10(-9)). These findings implicate genetic variants at 1p13.3, 1p36.32 and 12p12.1 in the etiology of NOA in Han Chinese men.  相似文献   
22.
23.
在物理课堂中利用计算机仿真演示实验进行辅助教学一直受到许多关注与研究.已有各种技术被应用到进行物理仿真演示的多媒体教学软件的开发中.本文论述了如何利用3dsMax中的Reactor动力学系统这一新技术进行物理力学现象的仿真演示,这种技术具有不需编程、创作快捷、形象直观且容易学习、实现交互性好等特点.  相似文献   
24.
变压器的安装调试运行必须遵循施工顺序和操作方法,达到现行的国家标准和验收规范的规定,方可运行生产.本文主要探讨变压器安装和调试应注意的问题.  相似文献   
25.
Performance of wireless mesh networks can be greatly improved by spatial reuse (SR), therefore, lots of algorithms have been proposed to maximize the number of concurrent transmissions. However, most methods only considered the interference brought by the sender nodes. In this paper, we study the interference model in WiMAX mesh networks and propose a minimal SR-pair interference model to perform concurrent transmissions in both uplink and downlink directions. Simulation results show that our model is more efficient and stable, i.e., it increases the number of concurrent transmissions and its performance is constant and independent of transmission directions. Biography: XIONG Qing(1978–), male, Ph. D., research direction: wireless mesh networks.  相似文献   
26.
Objective: The age-related change is important part of degenerative disc disease. However, no appropriate animal model or objective evaluation index is available. This study aimed to investigate the features of intervertebral disc degeneration in aging process of rats. Methods: 22-month-old Sprague-Dawley (SD) rats were used as spontaneously occurring intervertebral disc degeneration models and 6-month-old rats as young controls. Expression of collagen types II and X was measured by immunohistochemistry. Degenerations of intervertebral discs were scored according to Miyamoto’s method. Numbers and areas of afferent vascular buds were measured. The thicknesses of non-calcified and calcified layers were measured and statistically analyzed. Results: There were less collagen type II expression and more collagen type X expression in the calcified layer of the cartilage endplates and nucleus pulposus in the rats of the aged group than in the young control. There were fewer and smaller afferent vascular buds in the rats of the aged group than in the young control group. The ratio of the non-calcified to the calcified layers in the rats of the aged group significantly decreased, compared with that of the young control group (P<0.01). Conclusion: Rats can spontaneously establish intervertebral disc age-related degeneration. The expression of collagen types II and X, numbers and areas of afferent vascular buds, the ratio of the non-calcified to the calcified layers, and water and glycosaminoglycan contents in the nucleus pulposus are sensitive indexes of intervertebral disc degeneration.  相似文献   
27.
Sikkema-Kantorovitch算子在B_α空间的逼近   总被引:1,自引:0,他引:1  
研究了Sikkema-kantorovitch算子在Bα空间的逼近估计.  相似文献   
28.
The rapid prototyping and manufacturing technology (RPM), is an integration of many different disciplines. It is based on an advanced dispersed-accumulated forming principle and originated from 1980s. It generates an entity by first forming a series of layers according to the dispersed section information of the digital model, and then piling the formed layers sequentially together. It is capable of forming parts with complicated structures and non-homogeneous materials. Traditional RPM techniques are mainly used as prototypes in product invention process, such as stereolithography, three-dimensional printing, laminated object manufacturing, and fused deposition modeling. Later, with the progress of material and enabling technology, many new RPM techniques emerged out and have been already applied in the fields such as rapid tooling/moulding, direct formed usable part, nano-/micro-RPM, and biomanufacturing. This high flexible digital manufacturing method has a likely ability to become an almighty forming technology.  相似文献   
29.
对文题内容研究表明,Na_2CO_3对生物质的湿式裂解是良好的催化剂。Ni-Mo催化剂有利于甲烷生成,Ni-Mo和Na_2CO_3混合催化剂效果最好。可使气体转化率达55%以上。生物质的湿式裂解气化可得气体、液体和焦三种产品。其中气体产品主要含CO_2、CO、H_2、CH_4和少量的C_mH_n。脱(?)CO_2后可得热值为12500~16700kJ/m~3左右的中热值煤气。对液体产品的组分用荧光光谱法进行了分析,发现有菲、(?)醌等几十种物质。湿式裂解气化的最佳反应时间为150min、反应温度380℃以上。若以液体产品为主,则反应时间为80min较佳,反应温度相对降低。讨论了不同催化剂及Na_2CO_3用量对甘蔗渣湿式裂解气化的影响。  相似文献   
30.
本文导出了 TiO_2吸附 Cu(Ⅱ)、Cd(Ⅱ)的等温式代入实验数据由计算机求取参数 X 和 Y。结果表明,该等温式描绘的等温线与实验点吻合很好。X、Y 反映了 PH 和吸附质性质对等温线的影响程度。  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