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   14256篇
  免费   645篇
  国内免费   1141篇
系统科学   846篇
丛书文集   353篇
教育与普及   257篇
理论与方法论   65篇
现状及发展   138篇
研究方法   49篇
综合类   14332篇
自然研究   2篇
  2024年   54篇
  2023年   140篇
  2022年   249篇
  2021年   295篇
  2020年   219篇
  2019年   116篇
  2018年   177篇
  2017年   222篇
  2016年   228篇
  2015年   418篇
  2014年   597篇
  2013年   591篇
  2012年   689篇
  2011年   771篇
  2010年   748篇
  2009年   835篇
  2008年   935篇
  2007年   911篇
  2006年   770篇
  2005年   671篇
  2004年   535篇
  2003年   564篇
  2002年   731篇
  2001年   633篇
  2000年   564篇
  1999年   559篇
  1998年   365篇
  1997年   368篇
  1996年   338篇
  1995年   290篇
  1994年   274篇
  1993年   231篇
  1992年   207篇
  1991年   168篇
  1990年   145篇
  1989年   144篇
  1988年   120篇
  1987年   76篇
  1986年   45篇
  1985年   13篇
  1984年   4篇
  1979年   4篇
  1978年   2篇
  1977年   2篇
  1972年   3篇
  1971年   3篇
  1970年   3篇
  1969年   2篇
  1968年   3篇
  1967年   2篇
排序方式: 共有10000条查询结果,搜索用时 812 毫秒
921.
讨论了计算机支持的协同工作 (CSCW )的特点和结构 ,结合实际工作需要 ,设计了一个运用CSCW技术进行远程医疗会诊的方案 .分别给出了客户端和服务器端的实现模块 ,采用该方案的专家会诊系统可为患者提供高效、安全的服务  相似文献   
922.
The results of paleogeomagnetics and geochemistry of the coral reef in well Nanyong 2 of Nansha Islands showed that the bottom of the black sedimentary interlayer corresponds to the conversion boundary line between Brunhes Postive Polarity and Matuyama Reversed Polarity (B/M) and the cold/warm (19/20) climatic conversion bounds on δ18O curve, 0.78 Ma ago; and the red sedimentary interlayer corresponds to the ReunionⅠpolarity excursion (reversion) of the geomagnetic field, 2.01—2.04 Ma ago. Comparing with the normal light-coloured coral reef rock, the magnetic susceptibility (c), residual magnetization intensity (Mr) and the content of MnO and Fe2O3 of the black sedimentary layer appeared obviously positive abnormity. The magnetic susceptibility (c), residual magnetization intensity (Mr) and the content of Fe2O3 of the red sedimentary layer also appeared positive abnormity. Combining with the analyzing results of paleontology, we hold that this pair of special and typical sedimentary interlayers was relative to the sudden change of paleoclimate, i.e. the global climatic change and its incidental polarity reversal of the geomagnetic field directly affected the living environment of the paleontological species as well as the dispersion and enrichment of some chemical elements, especially the elements sensitive to redox such as Fe and Mn. For example, the elements Fe and Mn concentrated in the glacial period would be largely oxidized and diluted when the climate warmed up suddenly, and the originally oxidized high valence Fe would be condensed again when the climate cooled suddenly. This is possibly one of the important reasons of appearing and disappearing of the red and black sedimentary events of coral reef in well Nanyong 2.  相似文献   
923.
Cellular immune response is a major barrier to xenotransplantation. Human tumor necrosis factor-α (hTNF-α) possesses cross-species activity and directly amplifies the immune rejection via the upregulation of adhesion molecules on porcine endothelium. We investigated the role of protein tyrosine phosphorylation in the induction of expression of E-sclectin and vascular cell adhesion molecule-1 (VCAM-1), and the augmentation of adhesion of human peripheral blood monocytes (PBMo) and natural killer cells (PBNK), after rhTNF-α-stimulation of porcine aortic endothelial cells (PAEC) in vitro, rhTNF-α-increased adhesiveness of PAEC for both PBMo and PBNK was dose-dependently reduced by pretreatment of PAEC with the selective protein tyrosine kinase (PTK) inhibitor genistein. The inhibitory effect occurred at the early time of PAEC activation triggered by rhTNF-α, and was completely reversible. PTK activity assay indicated that genistein also suppressed rhTNF-α stimulated activation of protein tyrosine kinases (PTKs) in PAEC in a dose-dependent manner. Flow cytometric analysis showed that genistein inhibited the upregulation of E-selectin and VCAM-1 by rhTNF-α. These results suggest that PTKs may regulate the expression of E-selectin and VCAM-1 on PAEC and the adherence of PBMo and PBNK induced by rhTNF-α. Moreover, dietary genistein, used as an adhesion antagonist, may contribute to managing the cell-mediated rejection in the clinical application.  相似文献   
924.
The secondary structures of proteins fall into two classes: regular structure and nonregular structure. Helices and sheets are termed "regular" structures because their residues have repeating main-chain torsion angles, and their backbone N-H and C-O groups are arranged in a periodic pattern of hydrogen bonding. In contrast, the remaining structures with nonrepeating backbone torsion angles are called nonregular secondary structures. In this note, we performed an extensive sequence analysis of nonregular secondary structures and showed that these nonregular parts could be effectively predicted by continuous wavelet transform.  相似文献   
925.
The coding regions of Ag85B MPT-64, and ESAT-6 secreted proteins were cloned initially into the eukaryotic expression vector pJW4303, then transformed to E. coli Top 10 strain for plasmid DNA extraction and further analysis. Plasmids containing the right insertion were sequenced to confirm their identity. COS7 cells were transfected with a mixture containing serially diluted plasmid DNA encoding three secreted proteins and Lipofectin (Gibco). The supernatants and pellets prepared from various cell lines were run on SDS-PAGE gel and the expression of these proteins in COS7 cells were demonstrated by immunoblot using polyclonal or monoclonal antiserum of M.TBH37Rv. 21 days after first vaccination of C57BL-6 mice by all three recombinant eukaryotic expressing vectors, antibody titer for Ag85B reached 1∶3200. 21 days after second vaccination, the antibody titer reached 1∶102400. The highest antibody levels induced by multivalent vaccines after the second injection were equal to or even greater than the highest antibody levels of single DNA vaccine reported in literature after third injections. Antibody titer of MPT-64 was 1∶50 after the first injection and it reached 1∶200 after the second injection. No antigen-specific antibody against ESAT-6 was detected in sera harvested from immunized mice 21 days after both injections. Antigen-specific IFN-g level of Ag85B was 110 pg/mL while no antigen-specific IFN- g level of ESAT-6 and MPT-64 was detected even after third injections. To our knowledge, it is the first time that studies of polyvalent recombinant DNA vaccines against TB were carried out in C57BL-6 mice. Our results indicated that multiple DNA vaccines could be used to enhance protective responses against M.TB.  相似文献   
926.
对与果实成熟相关基因多聚半乳糖醛酸酶 (PG)基因的筛选、鉴定、表达及基因工程的研究进展作一个简要评述  相似文献   
927.
嗅神经鞘细胞移植恢复大鼠损伤脊髓的功能   总被引:1,自引:0,他引:1  
为探讨嗅神经鞘细胞移植恢复大鼠损伤脊髓的功能 ,将成年大鼠分为脊髓半切洞损伤组 (A组 )和脊髓半切洞损伤 嗅神经鞘细胞移植组 (B)组 ,手术后应用联合行为评分 (CBS)感觉诱发电位 (SEP)和运动诱发电位 (MEP)检查 ,测定脊髓功能恢复情况 .结果 2组CBS得分 ,A组 >B组 ,SEP和MEP潜峰时均A组 >B组 ,统计学分析均有显著性差异 (P <0 0 5 ) .表明移植嗅神经鞘细胞能恢复损伤脊髓的功能  相似文献   
928.
调查统计表明,衢州共有野生珍稀濒危植物22种,隶属12科19属,其中国家1级保护植物3种,2级保护植物19种.衢州珍稀濒危植物区系具有种类丰富、过渡性强、成分特有及起源古老等特征.还探讨了它们与邻近地区珍稀植物区系的关系.  相似文献   
929.
Zernike矩的快速算法   总被引:7,自引:0,他引:7  
给出了Zernike矩求解的一种快速算法 .利用Zernike多项式迭代性质 ,找出了Zernike正交矩之间的内在关系 ,这样 ,高阶的Zernike矩可由低价的Zernike矩求出 ,再在Chan等人提出的关于一维几何矩有效算法的基础上 ,得出了一种快速算法 .与已有方法相比 ,该算法大大减少了求解过程中的乘法次数 ,降低了计算复杂度 ,从而提高了运算速度和效率 ;并可以有效用于模式识别、图像分析及重建等领域中  相似文献   
930.
通过添加剂Na2H2Y碱法制备了α-FeOOH,测量了反应速度并用SEM观察了α-FeOOH颗粒的开貌,结果表明该反应对Fe^2 是零组反应,加入添加剂Na2H2Y后反应速度常数k增大。电镜观察发现Na2H2Y有α-FeOOH晶体产生枝晶的作用,当碱比R=3.0,CNa2H2Y=0.8mmol.L^-1时制备的发晶体最佳,用它生产的γ-Fe2O3磁粉性能优良,分析实验结果认为Fe^2 的氧化反应发生在凝聚体[Fe(OH2)]m表面从而使反应对Fe^2 成零级;溶液中螯合物FeY的氧化反应改变了反应速度常数,而且因为它发生在α-FeOOH晶体的弯曲界面区域,从而减少了晶体的缺陷。  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