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   23038篇
  免费   615篇
  国内免费   1166篇
系统科学   932篇
丛书文集   764篇
教育与普及   828篇
理论与方法论   109篇
现状及发展   115篇
研究方法   78篇
综合类   21990篇
自然研究   3篇
  2024年   157篇
  2023年   278篇
  2022年   481篇
  2021年   542篇
  2020年   308篇
  2019年   164篇
  2018年   249篇
  2017年   315篇
  2016年   347篇
  2015年   700篇
  2014年   867篇
  2013年   835篇
  2012年   940篇
  2011年   1028篇
  2010年   998篇
  2009年   1101篇
  2008年   1237篇
  2007年   1122篇
  2006年   899篇
  2005年   892篇
  2004年   710篇
  2003年   665篇
  2002年   826篇
  2001年   745篇
  2000年   736篇
  1999年   1048篇
  1998年   836篇
  1997年   922篇
  1996年   759篇
  1995年   709篇
  1994年   693篇
  1993年   524篇
  1992年   504篇
  1991年   426篇
  1990年   380篇
  1989年   341篇
  1988年   265篇
  1987年   139篇
  1986年   89篇
  1985年   33篇
  1984年   4篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1980年   2篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Human β-definsin-2(hBD-2)is mainly induced by bacterial factors and pro-inflammation mediators in epithelial cells.As the major cause of community-acquired pneumonia,whether Streptococcus pneumoniae(S.pneumoniae)stimulation induces hBD-2 expression in airway epithelial cells is elusive.In this study,we found that S.pneumoniae stimulation induced hBD-2 expression in a time-and concentration-dependent manner in primary human airway epithelial cells.To further reveal the mechanism of S.pneumoniae inducing hBD-2,we found that S.pneumoniae stimulation activated NF-jB signaling pathway.Specific NF-jB inhibitor,PDTC,could reverse the induction of hBD-2 by S.pneumoniae.We also found that cellular inner Ca2+ signaling is involved in the S.pneumoniae-induced hBD-2.Taken together,our findings indicated that S.pneumoniae can stimulate the expression of hBD-2 in airway epithelial cells and NF-jB and inositol triphosphate-dependent intracellular calcium release is involved in this induction.  相似文献   
992.
This paper presents a new implementation of a high-definition image-processing engine,which mainly targets the 3-dimensional(3D)visualization and stereo video stream display of binocular display equipment.The engine is compatible with the mainstream analog and digital stereo videos in component format and is able to receive stereo composite video broadcast signals using an integrated analog stereo video decoder.The four modules include a spatiotemporal scaling transform engine,a 2D–3D converter,an image animating engine,and a 2D scalar operating in pipeline architecture to implement the video format conversion and the stereo effect enhancement.Furthermore,the data access,hardware structure,and system-level configurations are optimized.Finally,the proposed architecture is realized by 0.18 lm CMOS technology.The application-specific integrated circuit verification results show that the engine can generate a strong feeling of 3D immersion and highdefinition image quality with minimal flicker.The chip has wide compatibility and an uppermost 1080P-processing capacity,which has approximately 3.5 million gates with about 43 mm2 die size.  相似文献   
993.
MicroRNAs (miRNAs or miRs) are endogenous non-coding RNAs that negatively regulate gene expres- sion by binding to the 3' non-coding regions of target mRNAs, resulting in their cleavage or blocking their translation. miRNAs may have an impact on cell differentiation, proliferation, and survival, and their deregulation can be inclined to diseases and cancers, including thyroid tumors. The purpose of this review is to summarize the existing findings of deregulated miRNAs in different types of thyroid tumors and to exhibit their potential target genes, especially to demonstrate those involved in tumor invasion and metastasis. In addition, new findings of circulating miRNA expres- sion profiles, single nucleotide polymorphism (SNP) in thyroid tumors, and the correlation of somatic mutations with deregulated miRNA expression in thyroid tumors were all included in this review.  相似文献   
994.
主要讨论了基本上成立的一些概念,并给出与这些概念相关的一些性质,同时用基本上成立的概念简述一些定理和性质的叙述、证明及它们之间的关系.  相似文献   
995.
Biological network alignment is an important research topic in the field of bioinformatics. Nowadays almost every existing alignment method is designed to solve the deterministic biological network alignment problem.However, it is worth noting that interactions in biological networks, like many other processes in the biological realm,are probabilistic events. Therefore, more accurate and better results can be obtained if biological networks are characterized by probabilistic graphs. This probabilistic information, however, increases difficulties in analyzing networks and only few methods can handle the probabilistic information. Therefore, in this paper, an improved Probabilistic Biological Network Alignment(PBNA) is proposed. Based on Iso Rank, PBNA is able to use the probabilistic information. Furthermore, PBNA takes advantages of Contributor and Probability Generating Function(PGF) to improve the accuracy of node similarity value and reduce the computational complexity of random variables in similarity matrix. Experimental results on dataset of the Protein-Protein Interaction(PPI) networks provided by Todor demonstrate that PBNA can produce some alignment results that ignored by the deterministic methods, and produce more biologically meaningful alignment results than Iso Rank does in most of the cases based on the Gene Ontology Consistency(GOC) measure. Compared with Prob method, which is designed exactly to solve the probabilistic alignment problem, PBNA can obtain more biologically meaningful mappings in less time.  相似文献   
996.
以TI公司CC2530模块为例,根据Zigbee路由协议,在总结现有路由算法的基础上,给出了一种新的获取Zigbee网络拓扑信息的方法 .仿真结果表明,系统自动获取的网络拓扑信息与Zigbee无线网络中的理论地址分配机制是一致的,为Zigbee网络的有效、安全管理提供了一种可靠的手段.  相似文献   
997.
We theoretically investigate terahertz(THz)emission and detection from h110i-oriented electro-optic(EO) crystals adapted for Yb-doped femtosecond pulse laser.According to the principles of phase-matching condition, the dispersion relation between optical velocity and THz pulse,THz absorption spectra, and coherence lengths of CdTe, GaP,and GaAs crystals below the phonon resonant frequency are calculated correspondingly. The optical rectification and EO sampling process of above crystals with the same thickness of0.1 mm are simulated. As a consequence, we found that the optimal emission frequency of CdTe is at 2.65 THz, however,it reaches 6.56 THz of GaAs and 4.77 THz of GaP. With the help of frequency response function, the calculated cut-off frequency of CdTe is only 3.45 THz, while GaAs and GaP achieve 7.15 and 6.37 THz correspondingly. Finally, the EO sampling sensitivity of GaAs is higher than CdTe and GaP when the crystal's thickness exceeds 1.58 mm. The strong THz absorption of CdTe saturates distinctly the EO sampling sensitivity with its thickness increasing.  相似文献   
998.
Photoelectrochemical (PEC) water splitting for solar energy conversion into chemical fuels has attracted intense research attention.The semiconductor hematite (...  相似文献   
999.
利用扫描电镜分析和机械性能测试对15-2-1型高铬铸铁的二次硬化效应进行了研究,并结合合金耐磨性分析探讨了二次硬化效应在高铬铸铁锚喷衬板中的作用.  相似文献   
1000.
用中性溶液法在硼硅酸盐玻璃片上制备了低反射涂层,并在Na2HPO4和AlCl3组成的浸涂液中于95℃条件下恒温浸泡6 h,结果表明硼硅酸盐玻璃片在400~900 nm波长内的平均透过率由原来的91.97%提高至98.97%,平均反射率降低至1.41%.中心波长处反射率仅为0.85%.为提高涂膜的耐候性,以低质量分数的十...  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