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   12819篇
  免费   688篇
  国内免费   2954篇
化学   14795篇
晶体学   111篇
力学   152篇
综合类   158篇
数学   220篇
物理学   1025篇
  2024年   24篇
  2023年   197篇
  2022年   1100篇
  2021年   873篇
  2020年   776篇
  2019年   620篇
  2018年   559篇
  2017年   573篇
  2016年   694篇
  2015年   596篇
  2014年   615篇
  2013年   1276篇
  2012年   890篇
  2011年   703篇
  2010年   494篇
  2009年   688篇
  2008年   651篇
  2007年   669篇
  2006年   558篇
  2005年   491篇
  2004年   435篇
  2003年   384篇
  2002年   522篇
  2001年   231篇
  2000年   257篇
  1999年   234篇
  1998年   209篇
  1997年   158篇
  1996年   149篇
  1995年   144篇
  1994年   124篇
  1993年   90篇
  1992年   78篇
  1991年   71篇
  1990年   47篇
  1989年   63篇
  1988年   49篇
  1987年   20篇
  1986年   20篇
  1985年   18篇
  1984年   21篇
  1983年   11篇
  1982年   21篇
  1981年   9篇
  1980年   9篇
  1979年   9篇
  1978年   7篇
  1977年   4篇
  1976年   6篇
  1975年   4篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
71.
Four MnIII-porphyrin complexes, chloro(tetraphenylporphinato)MnIII(1,6-diaminohexane), bromo(tetraphenylporphinato)MnIII(1,6-diaminohexane), azido(tetraphenylporphinato)MnIII(1,6-diaminohexane), and thiocyanato(tetraphenylporphinato)MnIII(1,6-diaminohexane), have been synthesized. These complexes have been characterized using UV-Vis, IR, ESI-mass spectra, elemental analyses, magnetic susceptibility measurements, and conductivity measurement. The molar conductance values of these complexes in ethanol indicate non-electrolytes. The utility of these complexes in de-polymerization of coal using humic acid as the coal model has been tested by the optical density method.  相似文献   
72.
A series of Co(II), Ni(II), Cu(II), and Zn(II) complexes of a tridentate hydrazone were prepared and characterized by various spectro‐analytical techniques and magnetic moment studies. The complexes were found to be monomeric and non‐electrolytes. The copper complex is electrochemically active in the applied potential range. The compounds synthesized in the present study have shown promising antiproliferative activity when screened using the in vitro method against two human cancer cell lines: HeLa and HepG2. The Escherichia coli DNA‐binding properties of all the compounds were investigated with UV–visible absorption spectrophotometric titrations, viscosity measurements, DNA melting experiments and gel electrophoreses measurements. The compounds were demonstrated to act as DNA intercalators with appreciable DNA‐binding constant values. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
73.
Recently, thioredoxin reductase as a target for treatment of tumors has attracted the attention of scientists. 1,2‐[Bis(1,2‐benzisoselenazolone‐3(2 H)‐ketone)]ethane (ethaselen, BBSKE, PCT: CN02/00412), designed and synthesized previously, is an effective thioredoxin reductase inhibitor; presently it is in phase II clinical trials, targeting gastric cancer, lung cancer and colon cancer. To seek more novel and effective anticancer drugs, we have developed many selenazole‐based compounds. Evaluation of the thioredoxin reductase inhibitory effect and investigation of the mechanism of anticancer drugs require abundant thioredoxin reductase, but since commercial thioredoxin reductase is expensive its use is often limited. Therefore, the preparation of thioredoxin reductase is necessary. Base on the above investigation, in this work we have prepared thioredoxin reductase and evaluated selenazole‐based compounds, and found that 44 compounds have high inhibitory effect on thioredoxin reductase with IC50 < 10 µ m , of which 16 compounds have IC50 values below 1 µ m . This is helpful in investigating and elucidating the mechanism of this kind of compound. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
74.
柑皮挥发油对衣蛾驱虫作用的初步研究   总被引:4,自引:0,他引:4  
用水浴回流法从柑皮中提取挥发油,分离并签定出16种组分,主要组分为橙皮甙等黄酮类化合物,同时进行了挥发油对衣蛾驱虫作用的实验.实验显示,不同浓度的柑皮挥发油均对衣蛾有较强的驱避及杀伤作用.柑皮挥发油气.味芳香,对人体无毒,价廉,不污染环境,有着很好的应用前景.  相似文献   
75.
研究了基于NZ30K合金开发的新型Mg-3Nd-0.2Zn-0.4Zr-0.2Mn镁合金耐腐蚀性、体外降解行为特性及浸提液细胞生物毒性. 采用金相显微镜得到新型镁合金金相显微图, 采用扫描电镜(Scanning Electron Microscope, SEM)获取SEM图; 采用武汉科思特电化学工作站进行电化学测试, 并绘制动电位极化曲线, 以磷酸盐缓冲液(Phosphate Buffer Solution, PBS)模拟体液环境, 记录氢气析出体积并计算腐蚀速率; 利用细胞完全培养基测定pH值、重量变化曲线; 获取大鼠骨髓间充质干细胞(Bone Mesenchymal Stem Cells, BMSCs), 并利用完全细胞培养基制作新型镁合金浸提液, 检测细胞生物活性, 以ZA75镁合金为基础添加0.3%Mn元素制成合金作为对照组, 比较腐蚀电位、体外降解情况以及细胞活性. 结果表明: 新型Mg-3Nd-0.2Zn-0.4Zr-0.2Mn镁合金横截面等轴晶体组织细小均匀性较好, 纵截面呈长条状组织均匀性稍差; 自腐蚀电位较高, 为-1.3912V; 自腐蚀电流密度较低, 为7.37×10-7 A?cm-2; 体外析氢量低, 失重量、pH值变化幅度相对较小; 降解速率下降后呈现小范围上升后趋于平缓; 具有良好的细胞相容性, 可以促进BMSCs细胞增殖分化.  相似文献   
76.
针对注水开发的多层砂岩油藏分层动态分析难度大等问题,在常规井层开发指标计算基础上,结合动、静态劈分方法,综合考虑渗透率、孔隙度、地层系数、含水饱和度、位置系数、措施系数及注水量系数,提出了一种既可将油、水井作为统一整体,又可对小层、方向流动分量开发指标进行定量计算的体现渗流力学本质的方法。用大庆油田N2-O1井组的产液剖面资料进行验证。结果表明,所提方法的计算结果与测量结果吻合度较高,精度平均值达75.11%。用该方法计算开发指标,适用性强,能较真实地反映各小层、各方向的产液情况,对现场应用具有指导意义。  相似文献   
77.
78.
Addition of azurocidin, a protein in granulocytes similar to serine proteases but has no protease activity because of replacement of the active serine residue by glycine, to the incubation mixture containing medullasin induced elastinolytic activity of medullasin. Both medullasin and human leukocyte elastase were already shown to have negligible elastinolytic activity (Aoki, Y. et al. J. Biochem. 114, 122, 1993). Elastinolytic activity of medullasin was induced dose-dependently by the addition of azurocidin. Medullasin activity determined by using apo-ornithine transaminase or casein as substrates or that by N-methoxy-succinyl-(Ala)2-Pro-Val-p-nitroanilide as substrate remained unchanged when azurocidin was added to the tube containing medullasin. Therefore, azurocidin is considered to cause an appearance of elastinolytic activity of medullasin without affecting the protease activity of it.  相似文献   
79.
It is shown that if a volume element V, of space is assumed to have intrinsic energy E, then basic principles of mechanics, thermodynamics and special relativity lead to the equation of state: E=pV, where p is the pressure. When this equation of state is incorporated in the Einstein equations it leads to the prediction that the orbital speed of matter circling a visible galaxy embedded in a spherical galactic halo should be relativistic, in disagreement with observations. However, it also leads directly to the interesting notion that the inertial mass of such a medium can be understood as a resistance to being compressed via Lorentz contraction. It is then shown that the mathematical structure of thermodynamics suggests another plausible definition of pressure if we allow for the possibility that the intrinsic energy of spacetime may not be described by the same work-energy relationship as ordinary matter. This additional possibility leads to the equation of state: E=−pV. While both of these equations of state describe forms of energy that are quite unlike ordinary energy, neither alone is able to account for observed rotational velocity curves of matter orbiting visible galaxies. Therefore, the possibility that space has two distinct components of energy is investigated. This results in a plausible, two-component equation of state in which the former equation of state is tentatively identified as the “dark matter” (DM) component, the latter as the “dark energy” (DE) component. The effective equation of state of space, accounting for the presence of both components, may then be written in the form: p=w ε, where ε is the total energy density, p the total pressure, and w represents the fractional excess of DM over DE (and therefore satisfies: −1≤w≤+1). Given the wide range of possible spacetime properties implied by this equation it appears to be a viable candidate for explaining observations presently attributed to the presence of both DM and DE. Specifically, the static, spherically symmetric solution of Einstein’s field equations, neglecting effects of ordinary matter, predicts the inverse r 2 distribution of intrinsic space energy required to explain observed constant rotational velocity curves for matter in circular orbits around visible galaxies embedded within spherically symmetric galactic halos. The proposed equation of state is also capable of describing regions of space undergoing accelerated expansion as regions where DE is dominant (i.e., w<−1/3).  相似文献   
80.
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