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   10777篇
  免费   833篇
  国内免费   385篇
系统科学   1409篇
丛书文集   130篇
教育与普及   82篇
理论与方法论   299篇
现状及发展   907篇
研究方法   10篇
综合类   9158篇
  2024年   16篇
  2023年   68篇
  2022年   114篇
  2021年   142篇
  2020年   99篇
  2019年   72篇
  2018年   793篇
  2017年   812篇
  2016年   530篇
  2015年   217篇
  2014年   265篇
  2013年   265篇
  2012年   550篇
  2011年   1269篇
  2010年   1117篇
  2009年   731篇
  2008年   843篇
  2007年   1044篇
  2006年   226篇
  2005年   283篇
  2004年   299篇
  2003年   378篇
  2002年   417篇
  2001年   307篇
  2000年   214篇
  1999年   189篇
  1998年   103篇
  1997年   108篇
  1996年   81篇
  1995年   58篇
  1994年   72篇
  1993年   65篇
  1992年   53篇
  1991年   42篇
  1990年   47篇
  1989年   36篇
  1988年   38篇
  1987年   19篇
  1986年   6篇
  1985年   2篇
  1984年   1篇
  1981年   2篇
  1980年   1篇
  1967年   1篇
排序方式: 共有10000条查询结果,搜索用时 528 毫秒
991.
通过Pro/E建立六关节弧焊机器人的CAD模型,导入ADAMS软件并通过点驱动的方式,求解各关节角度随时间的变化;通过ADAMS的后处理模块,将测量所得各关节角度随时间的变化曲线转换为样条曲线,在各关节上添加驱动电机,观察求解的准确性;求解所得的各关节角度随时间变化的数据转换为相应的脉冲函数关系,在物理样机上进行驱动。  相似文献   
992.
Bafilomycin A1 (Baf) induces an elevation of cytosolic Ca2+ and acidification in neuronal cells via inhibition of the V-ATPase. Also, Baf uncouples mitochondria in differentiated PC12 (dPC12), dSH-SY5Y cells and cerebellar granule neurons, and markedly elevates their respiration. This respiratory response in dPC12 is accompanied by morphological changes in the mitochondria and decreases the mitochondrial pH, Ca2+ and ΔΨm. The response to Baf is regulated by cytosolic Ca2+ fluxes from the endoplasmic reticulum. Inhibition of permeability transition pore opening increases the depolarizing effect of Baf on the ΔΨm. Baf induces stochastic flickering of the ΔΨm with a period of 20 ± 10 s. Under conditions of suppressed ATP production by glycolysis, oxidative phosphorylation impaired by Baf does not provide cells with sufficient ATP levels. Cells treated with Baf become more susceptible to excitation with KCl. Such mitochondrial uncoupling may play a role in a number of (patho)physiological conditions induced by Baf.  相似文献   
993.
During the first leg of a Chinese research cruise to the Southeast Indian Ridge (SEIR) in 2007, we revisited the K-segment of the SEIR and observed a water column turbidity anomaly at a CTD cast station. This station was located ~400 m from a dredge site on a plume anomaly detected in 1996 by US scientists. The characteristics of the turbidity anomaly detected in our survey are different from those observed 11 years previously, implying that either the detected anomalies have distinct and different plume sources or the robustness of the plume has changed in the intervening time. In addition, turbidity anomalies were observed at two localities on the K-segment in a deep tow-yo profile. If these two anomalies are not connected, the plume incidence calculated would be about 0.28, larger than the predicted value of 0.25 for a 65 mm/a spreading rate. This increase in plume incidence on the K-segment seems to be related to the influence of the St. Paul-Amsterdam hotspot, located about 300 km to the northwest.  相似文献   
994.
Henry反应是一类重要的碳碳键形成反应,广泛用于各类天然产物和医药中间体合成中.使用碱性蛋白酶催化芳香醛和硝基甲烷之间的Henry反应,在室温的条件下,得到对应的产物,产率高达88%.  相似文献   
995.
采用布拉格光纤光栅(FBG)作为编解码器的跳频时延(WHTS)OCDMA系统中,FBG制作不完善会导致FBG编解码器之间的不匹配,这种不匹配会引入一种新的噪声,称之为FBG编解码噪声.就这种噪声对跳频时延OCDMA系统的影响进行了分析,并就一定不匹配程度对跳频时延OCDMA系统的影响情况作出仿真,对其新的误码率函数进行了推导,发现FBG编解码噪声对不同的系统速率,系统性能带来不同程度的影响.  相似文献   
996.
Coordination polymers (or metal-organic frameworks, MOFs), as one of the essences of crystal engineering and coordination chemistry, have attracted considerable atten-tion from chemists,  相似文献   
997.
A number of recently evolved animals possess poison glands for feeding and/or defense.However,examples of such animals are rare in the fossil record.We report a fossil arthropod Isoxys curvirostratus from the Early Cambrian Chengjiang biota of China.This species is regarded as the oldest known venomous arthropod based on the presence of venomous glands in its head region.The adult animal is 2-5 cm long and the body is covered entirely with a carapace.The presence of large stalk eyes and a pair of stout grasping appendages with a terminal spine suggest it was raptorial.Interestingly,the two pear-shaped,three-dimensionally preserved objects that are present in the head region and at the base of the grasping appendages closely resemble the venom glands of some living arthropods in size,shape,and position.These features indicate that the presence of venomous predators could date back 520 million years.Furthermore,our observations suggest that the feeding strategies and organs adapted for this purpose had already reached a high level of diversity and anatomical sophistication in the Early Cambrian ecosystems.  相似文献   
998.
A novel method for characterizing the nonlinearity of all-optical sampling is proposed based on the normalized transfer function.Simulation results demonstrate the effectiveness of our method.Furthermore,an all-optical sampling experiment is performed to verify our method.Both simulation and experiment show consistency between our method and the measurements.The method only requires normalization and polynomial fitting of the transfer curve,and enables direct expression of the nonlinearity with the coefficients of the normalized transfer function.  相似文献   
999.
高分辨气相色谱法快速测定汽油辛烷值   总被引:9,自引:0,他引:9  
用高分辨气相色谱详细分析了汽油组成,按各组分的辛烷值特性分组,并与标准方法所测定的辛烷值进行关系,得出了每组的质量有效辛烷值及准确计算的MON和RON,本方法将国内外普遍采用的31组分组改缩至21组,分析时间只需35min。  相似文献   
1000.
关中地区区域开发对策探讨   总被引:4,自引:0,他引:4  
基于关中地区区域经济发展现状及所处的发展阶段,对影响区域经济发展的主要问题与潜力作了深入的分析研究,提出了相应的产业结构优化、技术开发、点轴开发—网络发展的空间开发对策.  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