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   4484篇
  免费   95篇
  国内免费   195篇
系统科学   148篇
丛书文集   155篇
教育与普及   146篇
理论与方法论   17篇
现状及发展   20篇
研究方法   24篇
综合类   4263篇
自然研究   1篇
  2024年   35篇
  2023年   69篇
  2022年   77篇
  2021年   102篇
  2020年   61篇
  2019年   23篇
  2018年   41篇
  2017年   33篇
  2016年   51篇
  2015年   98篇
  2014年   141篇
  2013年   132篇
  2012年   141篇
  2011年   160篇
  2010年   158篇
  2009年   178篇
  2008年   200篇
  2007年   214篇
  2006年   148篇
  2005年   153篇
  2004年   120篇
  2003年   98篇
  2002年   83篇
  2001年   121篇
  2000年   107篇
  1999年   211篇
  1998年   227篇
  1997年   244篇
  1996年   234篇
  1995年   186篇
  1994年   189篇
  1993年   119篇
  1992年   133篇
  1991年   108篇
  1990年   81篇
  1989年   95篇
  1988年   98篇
  1987年   51篇
  1986年   37篇
  1985年   13篇
  1984年   2篇
  1983年   1篇
  1981年   1篇
排序方式: 共有4774条查询结果,搜索用时 10 毫秒
291.
本文描述了产自中国云南省盘菌目Pezizales新属——假地耳属Otideopsis Liu et Cao gen.nov。和新种——云南假地耳O.Yunnanensis Liu et Cao sp.nov.,并对其在盘菌目中的系统地位作了论述。  相似文献   
292.
本文用不同粘度的液体对单螺杆泵的外特性和汽蚀特性进行了实验研究,分析了外特性曲线的变化特点,讨论了粘度变化对特性和能量分配的影响,并对单螺杆泵的汽蚀特性曲线和汽蚀条件下的能量平衡关系进行了初步的分析和探讨。  相似文献   
293.
The Jialing River is one of the main tributaries of the Yangtze River. The average annual runoff accounts for 16 % and the sediment load 26% of the total at Yichang therefore it's one of the main contributors of sediment to the Three Gorges Reservoir. Ever since 1989, our country has implemented "Yangtze River soil and water conservation" project, Till the end of 1996, altogether 25.8 % of erosion area in Jialing River has been improved after large scale conservation has taken effect. The analysis and comparison between records taken before and after the implementation of soil and water conservation on runoff and sediment yield indicated that the sediment load soil erosion in Jialing river basin has been taken under control to some extent. Amount of annual runoff in main conservations have generally dropped by exponential function. Obvious result has been achieved in Jialing River, efficiency of sand reduction is about 10%-25%. Therefore, sediment to Three Gorges Reservoir is decreased accordingly, and it will be beneficial to take advantage of reser- voir's synthetic benefit.  相似文献   
294.
A trusted third party introduced in watermarking protocols would decrease the security and affect the implementation of the protocols. In this paper, a new watermarking protocol with an un-trusted third party (UTTP) was proposed. Based on the idea of all-or-nothing disclosure of secret (ANDOS), all of the buyer, the seller and the third party didn't know the exact watermark, which was embedded in a digital content for tracing piracy. The proposed protocol provided mechanisms to trace piracy and protect customer's right. In addition, the problem that a seller colluded with UTTP to frame the buyer, namely, the conspiracy problem, could be avoided.  相似文献   
295.
0 IntroductionWith the rapid advances of computer networks , moreand more users are connecting together to exchangelarge amounts of information and share useful resources .However ,the security issue is still a major gating factor fortheir full adoption; mainly due to many malicious adversariesresidingin networked environments will eavesdrop,interceptand modify the transmittedinformation.The client-server model , over past years , has enjoyedsignificant and continually increasing popularity in…  相似文献   
296.
企业知识结构及其优化机制   总被引:5,自引:0,他引:5  
主要研究了企业知识结构的内涵和优化方法,提出了知识结构的优化机制,认为从人才结构方面来优化企业知识结构,主要任务是明晰人才结构和知识结构的对应关系,促使企业人才结构不断去适应企业知识结构;从知识使用成本方面优化知识结构,就是要积极吸收和创造能够带来利润的知识,同时把那些不能给企业带来利润的知识外化;从企业知识转化方面优化企业知识结构,就是要加速企业知识的正向转化,阻止和延缓企业知识的逆向转化,促进企业核心知识内部的转化。  相似文献   
297.
曹政才  Fu  Yili  Wu  Qidi  Wang  Shuguo  Wang  Guangguo 《高技术通讯(英文版)》2007,13(2):126-130
Placement and wiring of vast amount of sensor elements on the 3-dimensionally configured robot surface to form soft sensor skin is very difficult with the traditional technology. In this paper we propose a new method to realize such a skin. By implanting infrared sensors array in an elastic body, we obtain an elastic and tough sensor skin that can be shaped freely. The developed sensor skin is a large-area, flexible array of infrared sensors with data processing capabilities. Depending on the skin electronics, it endows its carrier with an ability to sense its surroundings. The structure, the method of infrared sensor signal processing, and basic experiments of sensor skin are presented. The validity of the infrared sensor skin is investigated by preliminary obstacle avoidance trial.  相似文献   
298.
Key agreement protocols are essential for secure communications. In this paper, to solve the inherent key escrow problem of identity-based cryptography, an escrow-free certificate-based authenticated key agreement (CB-AK) protocol with perfect forward secrecy is proposed. Our protocol makes use of pairings on elliptic curves. The protocol is described and its properties are discussed though comparison with Smart's protocol.  相似文献   
299.
300.
Comprehensive field investigations and laboratory analyses show that palaeolakes, including fresh- mesohaline water Megalake Tengger and other semi-con- nected, isolated water bodies, during late Pleistocene covered an area of more than 20000 km2, which is more than half of the Tengger Desert in NW China. Stratigraphic correlation and chronological evidence indicate that before ca. 42000 aBP the area was more arid. The palaeolakes started to develop around 40000 uncal. 14C aBP but until 37000 14C aBP their scope was limited. High water levels established from 35000 14C aBP lasted until 22000 14CaBP. Lake levels regressed between 22000 and 20000 14C aBP but transgressed from 20000 to 18600 14C aBP. Subsequently, water level declined further and the Megalake Tengger finally desiccated at around 18000 14C aBP. Megalake Tengger possessed a fresh-mesohaline water property, implying that the regional precipitation increased significantly. During the period of Megalake Tengger, the climate was warmer-humid than present. The annual rainfall was 250 to 350 mm more than that of today and the temperature was 1.5 to 3.0℃ higher.  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