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   22053篇
  免费   2483篇
  国内免费   3646篇
化学   14893篇
晶体学   257篇
力学   1793篇
综合类   72篇
数学   3745篇
物理学   7422篇
  2024年   314篇
  2023年   794篇
  2022年   797篇
  2021年   897篇
  2020年   1043篇
  2019年   685篇
  2018年   828篇
  2017年   786篇
  2016年   956篇
  2015年   1032篇
  2014年   1211篇
  2013年   1461篇
  2012年   1554篇
  2011年   1405篇
  2010年   1272篇
  2009年   1178篇
  2008年   1067篇
  2007年   1202篇
  2006年   1306篇
  2005年   886篇
  2004年   646篇
  2003年   531篇
  2002年   524篇
  2001年   594篇
  2000年   379篇
  1999年   450篇
  1998年   398篇
  1997年   298篇
  1996年   221篇
  1995年   212篇
  1994年   168篇
  1993年   175篇
  1992年   204篇
  1991年   143篇
  1990年   116篇
  1989年   116篇
  1988年   259篇
  1987年   356篇
  1986年   330篇
  1985年   100篇
  1984年   60篇
  1972年   52篇
  1971年   90篇
  1970年   54篇
  1968年   76篇
  1967年   128篇
  1966年   84篇
  1965年   76篇
  1964年   79篇
  1963年   57篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The microscopic theory of the blue phases of chiral liquid crystal is proposed. Beginning with the potential between two molecules, by using the cell model of liquid, applying statistical physical method, the distribution function and the free energy of the system are obtained. By using variational approach and zero-order approximation, the differential equation that the order parameter tensor of the blue phase can satisfy is obtained. Then we change the differential equation to the eigenequation problem in quantum mechanics. Considering the symmetry of the blue phases,the order parameter tensors of blue phases Ⅰ and Ⅱ can be made up of the eigenvectors. Our results are the same as the results of Ginzberg-Landau‘s phenomenological theory. The parameters in the order parameter tensors that we calculate in the located system are close to the predecessors‘ results.  相似文献   
2.
Han  Ningning  Lu  Jian  Li  Shidong 《Numerical Algorithms》2022,90(3):1197-1223
Numerical Algorithms - Iterative algorithms based on thresholding, feedback, and null space tuning (NST+HT+FB) for sparse signal recovery are exceedingly effective and efficient, particularly for...  相似文献   
3.
Qi Ji 《Tetrahedron》2006,62(11):2529-2536
Regio- and stereocontrolled synthesis and conformational analysis of a series of benzimidazole nucleosides were achieved. A simple method by 1H NMR 1D NOE experiment was developed for estimation of syn or anti conformation of benzimidazole nucleosides. Substituents at C2 of benzimidazole demonstrated to play a key role both in the unexpected regioselectivity of the glycosidic reaction and in the conformation distributions of the final products.  相似文献   
4.
A water-soluble nanometer-scale metallocapped polyrotaxane has been prepared by the inclusion complexation of azo-calixarenes with metallo-bridged bis(beta-CD)s, displaying highly selective binding for Ca(2+).  相似文献   
5.
《Fluid Phase Equilibria》2005,238(2):193-203
The present paper reports liquid–liquid equilibrium data for the system refined cottonseed oil + commercial linoleic acid + ethanol + water at 298.2 K. The experimental data were used for adjusting parameters of the NRTL and UNIQUAC models. The global deviations between calculated and experimental concentrations were 0.80% for the NRTL model and 1.44% for the UNIQUAC equation. The influence of the solvent on the distribution coefficient of tocopherols was also studied. UNIQUAC and NRTL interaction parameters between tocopherols and the other pseudocomponents were determined assuming that the nutraceutical compound is present at infinite dilution in the liquid–liquid equilibrium system. The obtained parameter set enables the simulation of liquid–liquid extractors.  相似文献   
6.
The coordination compound [Co(C10H7COO)2(H2O)3]2n · 4nH2O was prepared by the reaction of 1-naphthoic acid and cobalt(II) acetate tetrahydrate in basic solution, and was fully characterized by X-ray diffraction, element analysis, FTIR, TG-DTA and luminescent spectra. In the crystal the six-coordinated Co(II) centers are linked into one-dimensional zigzag chains by water molecules, which are further assembled into a two-dimensional network through weak inter-chain C–H···π interactions. The solid complex exhibits favorable fluorescent properties similar to those of free ligand at room temperature, which can be assigned to the intraligand electronic transfer.  相似文献   
7.
8.
Thirteen novel sulfonylureas containing thiadiazole moiety were synthesized in a two-step reaction. Their structures were determined using IR, ^1H NMR, HRFTMS, and elemental analysis. Herbicidal activities of these compounds were determined in the green house bio-assay. The results show that four compounds among them exhibit some activity toward four tested herbs.  相似文献   
9.
《Tetrahedron》1987,43(6):1117-1122
  相似文献   
10.
Many authors have suggested new forms to describe the surface anchoring energy of the liquid crystal-wall interface, replacing the Rapini-Papoular (RP) formula g s = (1/2) A sin2 theta. If the RP function is considered as the primary approximation, and a lowest order modification is included, then the surface anchoring energy can be represented by g s = (1/2) A sin2 theta(1 + zeta sin2 theta). zeta characterizes the modification to the RP formula and varies for the different energy forms. It is well known that the RP formula predicts a second order Freedericksz transition. This paper points out that the transition can be first order if the modification is taken into account, in which case at the threshold point the tilt angle of the director at the middle layer of the cell, thetam, is finite. The conditions for the existence of the first order transition are obtained; zeta < 0 is required for a first order transition. The approximate expression of the threshold field is also given.  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