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   5310篇
  免费   141篇
  国内免费   24篇
化学   3725篇
晶体学   49篇
力学   137篇
数学   671篇
物理学   893篇
  2023年   23篇
  2021年   45篇
  2020年   54篇
  2019年   68篇
  2018年   43篇
  2017年   47篇
  2016年   118篇
  2015年   77篇
  2014年   109篇
  2013年   215篇
  2012年   289篇
  2011年   344篇
  2010年   192篇
  2009年   153篇
  2008年   283篇
  2007年   314篇
  2006年   339篇
  2005年   304篇
  2004年   241篇
  2003年   233篇
  2002年   215篇
  2001年   83篇
  2000年   71篇
  1999年   40篇
  1998年   51篇
  1997年   55篇
  1996年   78篇
  1995年   43篇
  1994年   48篇
  1993年   44篇
  1992年   32篇
  1991年   45篇
  1990年   49篇
  1989年   40篇
  1988年   50篇
  1987年   32篇
  1986年   40篇
  1985年   74篇
  1984年   74篇
  1983年   35篇
  1982年   75篇
  1981年   70篇
  1980年   61篇
  1979年   51篇
  1978年   55篇
  1977年   56篇
  1976年   43篇
  1975年   51篇
  1974年   43篇
  1973年   46篇
排序方式: 共有5475条查询结果,搜索用时 390 毫秒
101.
We have observed a large nonresonant third-order nonlinear susceptibility, X (3) (-; , 0, 0) in the isotropic phase of a nematic liquid crystal 4-n-hexyl-4-cyanobiphenyl (K18). The highest value of X (3) obtained at 632.8 nm is 1.16274×10-18 m2V-2 corresponding to a temperature 29.3°C. The observed second-order pretransitional temperature T * from our measurements is 1.2°C below the first-order nematic to isotropic transition temperature. The dependence of the Kerr constant on (T-T *)-1 is found to be in good agreement with the predictions of the Landau-de Gennes model.  相似文献   
102.
The molecular structure of the cytotoxic sesquiterpene lactone eupatocunin has been determined by X-ray crystallographic analysis of theo-bromobenzoate derivative. Two structural isomers, arising from hindered rotation, are present within the single crystal. The molecules are disordered (ratio 0.750.25) with the bromine atom occupying alternate ortho positions on the aromatic ring. Molecular mechanics has been used to examine the energies of molecular conformation by torsion angle variation.  相似文献   
103.
Analysis of low concentrations of metabolites is required for new fields of biological research, such as metabolomics. In this review, recent work in our laboratory aimed at developing improved strategies for on-line sample preconcentration of metabolites by capillary electrophoresis (CE) is presented. Dynamic pH junction, sweeping and dynamic pH junction-sweeping represent three complementary methods for electrokinetic focusing of large volumes of sample directly on-capillary. Focusing selectivity and focusing efficiency are two factors that can be used to assess the suitability of each method for different classes of metabolites. Buffer properties can be selected to enhance the focusing of specific types of metabolites based on knowledge of the analyte physicochemical properties. The application of on-line preconcentration CE for trace analysis of metabolites in real samples of interest, such as biological fluids and cellular extracts, is also demonstrated. Under optimum conditions, up to three orders of magnitude increase in concentration sensitivity can be realized for several classes of metabolites, including catecholamines, purines, nucleosides, nucleotides, amino acids, steroids and coenzymes. Recent work on hyphenating on-line preconcentration with multiplexed CE is highlighted as a promising platform for sensitive and high-throughput analyses of metabolites.  相似文献   
104.
Spiro[oxirane-2, 4′-piperidines] have been prepared by the action of dimethyloxosulfonium methylide on 4-piperidones. The spiro[oxirane-2,4′-piperidines] act as alkylating agents to introduce (4-hydroxy-4-piperidyl) methyl moieties onto heteroaromatic compounds such as 4(3H)-quinazolone.  相似文献   
105.
A theoretical study is made of the states of nonrigid molecules, extending earlier work to apply Longuet–Higgins' symmetry group of feasible permutation/inversions in two previously untreated cases. The first involves nonrigid molecule processes between different stereoisomers, that is between molecular equilibrium configurations of different geometrical shapes. The second takes into account nonrigid molecule processes in linear molecules. The theory is applied to the specific case of nonrigid molecule effects in the butyl ion C4H (case i), and the form of the nonrigid molecule energy levels and spectra are determined. The theory is also applied to determine the nonrigid molecule energy levels in linear H2F2 (case ii).  相似文献   
106.
Polycrystalline samples of bilayered brownmillerite-like Ca2.5Sr0.5GaCo0.15Mn1.85O8 and Ca2.5Sr0.5Ga1.2Mn1.8O8 have been prepared and characterised by magnetometry and neutron diffraction over a wide temperature range. The structural chemistry and magnetic properties are compared to those of Ca2.5Sr0.5GaMn2O8. Ga enrichment has a significant effect on the former but not on the latter, whereas changes in both occur when paramagnetic Co3+ cations enter the parent phase on the 4-coordinate sites. The coupling between the environment around the 4-coordinate cations and the transition to an antiferromagnetic ordered state that was observed in Ca2.5Sr0.5GaMn2O8 is not apparent in the cation-substituted compositions, although both show long-range antiferromagnetic order at low temperatures.  相似文献   
107.
Kinetic isotope measurements using [4,4-2H2]NADH and [4-1H, 4-2H]NADH have been used to investigate the mechanism of the electrochemical oxidation of NADH at poly(aniline)-poly(vinyl sulfonate)-modified electrodes. The experiments show a primary kinetic isotope effect for the reaction of 4.2. This is consistent with literature values for the corresponding isotope effect for the oxidation of NADH by two-electron oxidants in homogeneous solution. The result demonstrates that transfer of H from NADH to the modified electrode occurs in the rate-limiting step within the reaction complex.  相似文献   
108.
We present a model of the weak interactions in which a custodial symmetry that is not an invariance of the starting Lagrangian emerges in the effective low-energy sector of the theory. This symmetry maintains the relationM w=M z cos w to all orders in the Higgs self-couplings to any required degree of accuracy, while leaving the quark mass spectrum completely unconstrained. The model is a local left-right symmetric chiral flavor gauge theory of the electroweak interactions in which the symmetry is spontaneously broken by fundamental Higgs fields which transform the same way under the chiral group as fermion Dirac and Majorana masses.  相似文献   
109.
110.
The equilibrium and fluctuation methods for determining the surface tension, sigma, and bending modulus, kappa, of a bilayer membrane with a fixed projected area are discussed. In the fluctuation method the elastic coefficients sigma and kappa are measured from the amplitude of thermal fluctuations of the planar membrane, while in the equilibrium method the free energy required to deform the membrane is considered. The latter approach is used to derive new expressions for sigma and kappa (as well as for the saddle-splay modulus), which relate them to the pair-interactions between the amphiphiles forming the membrane. We use linear response theory to argue that the two routes lead to similar values for sigma and kappa. This argument is confirmed by Monte Carlo simulations of a model membrane whose elastic coefficients are calculated using both methods.  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