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   7966篇
  免费   689篇
  国内免费   130篇
系统科学   1196篇
丛书文集   99篇
教育与普及   77篇
理论与方法论   282篇
现状及发展   880篇
研究方法   11篇
综合类   6240篇
  2024年   25篇
  2023年   31篇
  2022年   66篇
  2021年   87篇
  2020年   52篇
  2019年   28篇
  2018年   755篇
  2017年   768篇
  2016年   477篇
  2015年   112篇
  2014年   120篇
  2013年   131篇
  2012年   379篇
  2011年   1091篇
  2010年   946篇
  2009年   586篇
  2008年   667篇
  2007年   891篇
  2006年   119篇
  2005年   143篇
  2004年   215篇
  2003年   229篇
  2002年   134篇
  2001年   66篇
  2000年   56篇
  1999年   81篇
  1998年   61篇
  1997年   70篇
  1996年   72篇
  1995年   54篇
  1994年   64篇
  1993年   42篇
  1992年   51篇
  1991年   34篇
  1990年   27篇
  1989年   22篇
  1988年   16篇
  1987年   9篇
  1986年   6篇
  1984年   1篇
  1967年   1篇
排序方式: 共有8785条查询结果,搜索用时 15 毫秒
141.
High-speed all-optical logic circuits have attracted much attention because of their important roles in signal processing in next-generation optical networks.The digital encoder is widely used in binary calculation,multiplexing,demultiplexing,address recognition and data encryption.A priority encoder allows the existence of multiple valid inputs simultaneously,identifies the priority of the request signals and encodes the priority.We propose and experimentally demonstrate an all-optical 4-bit priority encoder for return-to-zero signals at 40 Gbit/s based on cross-gain modulation in semiconductor optical amplifiers.Detuning fil-ters after semiconductor optical amplifiers are employed to improve the output performance.Correct logic bit sequences and clear open eye patterns with extinction ratios exceeding 10 dB are achieved.  相似文献   
142.
It is difficult to date pyroclastic rocks, for almost all the dating methods, due to the multiple sources during their formation. ^40Ar/^39gAr incremental heating results on groundmass selected from the samples show that the age spectra are meaningless geologically. However, singe crystal total fusions of CO2 lasing on the sanidine separates could yield rational 4^40Ar/^39Ar results and distinguish their sources in this study. Timing on three formations of the Moshishan Group, after avoiding the exotic and altered grains by lasing on the single sanidine separate, was reported in this paper. The lowermost portion of the Chawan Formation gives an age of 113.7±0.3 Ma; the lower part of the Xishantou Formation was formed 116.4±0.4 Ma ago and the bottom of the Gaowu Formation took its shape at 118.4±0.4 Ma. These new ages are much younger than the previous ones, suggesting that these thick volcanic formations had been formed in very short durations.  相似文献   
143.
In order to develop highly secure database systems to meet the requirements for class B2, the BLP (Bell-LaPudula) model is extended according to the features of database systems. A method for verifying security model for database systems is pro- posed. According to this method, an analysis by using Coq proof assistant to ensure the correctness and security of the extended model is introduced. Our formal security model has been verified secure. This work demonstrates that our verification method is effective and sufficient.  相似文献   
144.
针对Hopfield网络求解TSP问题经常出现局部最优解,将粒子群算法(PSO)与Hopfield神经网络结合,提出一种基于粒子群的Hopfield神经网络方法. 实验证实这种方法能够以更大概率收敛到全局最优.  相似文献   
145.
Copper/liquid microcapsule composite coatings with polyvinyl alcohol (PVA), gelatin or methyl cellulose (MC) as shell materials were prepared by electrodeposition. The influence of shell materials on the corrosion resistance of the composite coatings in 0.1 M H2SO4 was investigated by means of electrochemical techniques, scanning electron microscopy (SEM), and energy dispersion spectrometry (EDS). The results show that the participation of microcapsules can enhance the corrosion resistance of the composite coatings compared with the traditional copper layer. Based on the analysis of electrochemical test results, the release ways of microcapsules were deduced. Gelatin and MC as the shell materials of microcapsules are easy to release quickly in the composite coating. On the contrary, the releasing speed of PVA microcapsules is relatively slow due to their characteristics.  相似文献   
146.
一种基于Cloud-P2P计算架构的大规模病毒报告分析机制   总被引:1,自引:0,他引:1  
通过引入新型的云计算与对等计算融合模型Cloud-P2P,提出基于Cloud-P2P计算架构的大规模病毒报告分析机制. 该机制充分利用海量终端的计算能力,将针对大规模病毒报告的分析任务进行有效分割,并通过移动Agent有序部署到服务器端和终端节点上执行,从而有效降低服务器端的负载,提高系统的处理效率. 详细阐述了Cloud-P2P融合计算模型、基于移动Agent的任务部署与执行点选择机制以及基于Cloud-P2P计算架构的大规模病毒报告分析机制和原型系统,并从负载平衡、安全性、可靠性等方面进行了性能分析. 分析结果表明,机制具有良好的理论与实用参考价值.   相似文献   
147.
Zircon U-Pb geochronology of basement metamorphic rocks in the Songliao Basin   总被引:23,自引:0,他引:23  
Zircon LA-ICP MS U-Pb dating of six metamorphic rocks and a metagranite (breccia) from southern basement of the Songliao Basin are reported in order to constrain the formation ages of basement. The basement metamorphic rocks in the Songliao Basin mainly consist of metagabbro (L45-1), amphibolite (SN117), metarhyolitical tuff (G190), sericite (Ser) schist (N103), chlorite (Chl) schist (T5-1), biotite (Bi)-actinolite (Act)-quartz (Q) schist (Y205), and metagranite (L44-1). The cathodoluminesence (CL) images of the zircons from metagabbro (L45-1) and metagranite (L44-1) indicate that they have cores of magmatic origin and rims of metamorphic overgrowths. Their U-Pb isotopic ages are 1808±21 Ma and 1873±13 Ma, respectively. The zircons with oscillatory zoning from amphibolite (SN117) and Chl schist (T5-1), being similar to those of mafic igneous rocks, yield ages of 274 ± 3.4 Ma and 264 ± 3.2 Ma, re-spectively. The zircons from metarhyolitical tuff (G190) and Ser schist (N103) display typical magmatic growth zoning and yield ages of 424 ± 4.5 Ma and 287 ± 5.1Ma, respectively. Most of zircons from Bi-Act-Q schist (Y205) are round in shape and different in absorption degree in the CL images, implying their sedimentary detritals. U-Pb dating yield concordant ages of 427 ± 3.1Ma, 455 ± 12 Ma, 696 ± 13 Ma, 1384±62 Ma, 1649±36 Ma, 1778±18 Ma, 2450±9 Ma, 2579±10 Ma, 2793±4 Ma and 2953±14 Ma. The above-mentioned results indicate that the Precambrian crystalline basement (1808―1873 Ma) exists in the southern Songliao Basin and could be related to tectonic thrust, and that the Early Paleozoic (424―490 Ma) and Late Paleozoic magmatisms (264―292 Ma) also occur in the basin basement, which are consistent with the ages of the detrital zircons from Bi-Act-Q schist in the basement.  相似文献   
148.
Several vanadium compounds have been known for the hypoglycemic and anticancer effects. However, the mechanisms of the pharmacological and toxicological effects were not clear. In this work, we in- vestigated the potential targets of vanadium in mitochondria. Vanadyl ions were found to bind to mi- tochondria from rat liver with a stoichiometry of 244±58 nmol/mg protein and an apparent dissocia- tion constant (Kd) of (2.0±0.8)×10·16 mol/L. Using size exclusion chromatography, a vanadium-binding protein was isolated and identified to be the 60-kDa heat shock protein (HSP60) by mass spectrometry analysis and immunoassays. Additionally, binding of vanadyl ions was found to result in depolymeri- zation of homo-oligomeric HSP60 (GroEL). HSP60 is an indispensable molecular chaperone and in- volved in many kinds of pathogenesis of inflammatory and autoimmune diseases, e.g. type 1 diabetes. Our results suggested that HSP60 could be a novel important target involved in the biological and/or toxicological effects of vanadium compounds.  相似文献   
149.
The reverse construction and analysis of the networks of molecular interactions are essential for understanding their functions within cells. In this paper, a logic network model is constructed to investigate the complicated regulation mechanism of shoot genes of Arabidopsis Thaliana in response to stimuli. The dynamics of the complicated logic network is analyzed, discussed, and simulated. The simulation results show that the logic network of the active genes of shoot eventually evolves into eleven attractors under the stimuli, including five 1-periodic and six 2-periodic attractors. Our work provides valuable reference and guidance for biologists to understand and explain Arabidopsis’ response to external stimuli by experiments.  相似文献   
150.
中国基金管理费激励的有效性   总被引:4,自引:0,他引:4  
管理费激励的有效性取决于基金资金流与基金业绩之间的关系.本文通过建立多变量的回归模型,采用开放式股票型基金的季度数据对中国基金市场存在的"赎回异象"和流业绩关系的特征进行了实证.从实证研究结果,本文发现中国基金市场由于业绩与基金流量之间存在较小的正的系数,以及统计显著性不高等,管理费激励虽然有一定的作用,但效果并不突出.而且在管理费激励制度下,中国基金市场中的流业绩关系没有显著的特征,表明投资者对基金业绩排名不敏感,说明中国基金投资者的理性程度还不够,正处于从非理性到理性的过渡阶段.最后,针对中国基金市场,提出了增强中国基金管理费激励效率的相关政策建议.  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