全文获取类型
收费全文 | 324133篇 |
免费 | 2471篇 |
国内免费 | 798篇 |
专业分类
化学 | 169971篇 |
晶体学 | 5293篇 |
力学 | 15708篇 |
综合类 | 13篇 |
数学 | 34298篇 |
物理学 | 102119篇 |
出版年
2021年 | 2791篇 |
2020年 | 2945篇 |
2019年 | 3522篇 |
2018年 | 4865篇 |
2017年 | 4888篇 |
2016年 | 6754篇 |
2015年 | 3611篇 |
2014年 | 6183篇 |
2013年 | 13766篇 |
2012年 | 10855篇 |
2011年 | 12955篇 |
2010年 | 9845篇 |
2009年 | 10000篇 |
2008年 | 12344篇 |
2007年 | 12039篇 |
2006年 | 11134篇 |
2005年 | 9981篇 |
2004年 | 9343篇 |
2003年 | 8568篇 |
2002年 | 8420篇 |
2001年 | 10147篇 |
2000年 | 7506篇 |
1999年 | 5702篇 |
1998年 | 4493篇 |
1997年 | 4605篇 |
1996年 | 4282篇 |
1995年 | 3847篇 |
1994年 | 3956篇 |
1993年 | 3682篇 |
1992年 | 4385篇 |
1991年 | 4583篇 |
1990年 | 4322篇 |
1989年 | 4375篇 |
1988年 | 4227篇 |
1987年 | 4246篇 |
1986年 | 3973篇 |
1985年 | 5013篇 |
1984年 | 5113篇 |
1983年 | 4202篇 |
1982年 | 4163篇 |
1981年 | 3987篇 |
1980年 | 3812篇 |
1979年 | 4383篇 |
1978年 | 4449篇 |
1977年 | 4573篇 |
1976年 | 4483篇 |
1975年 | 4206篇 |
1974年 | 4151篇 |
1973年 | 4252篇 |
1972年 | 3100篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
L. T. Ashchepkov 《Computational Mathematics and Mathematical Physics》2006,46(7):1168-1175
A general nonlinear programming problem with interval functions is considered. Two reductions of this problem to the deterministic nonlinear programming problem are proposed, and illustrative examples are discussed. 相似文献
112.
A Modified Quasi-Newton Method for Structured Optimization with Partial Information on the Hessian 总被引:2,自引:0,他引:2
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the
Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both
function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last
two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method
is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization
Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December
9–11, 2004 相似文献
113.
On mechanical characteristics of nanocrystals 总被引:2,自引:0,他引:2
The dependence of the elastic moduli of a nanocrystal on its size is investigated theoretically with reference to a two-dimensional single-crystal strip. It is shown that the uncertainty (of a fundamental nature) in the size of a nanocrystal causes the determination of many of its mechanical characteristics to be ambiguous. It is found that the Cauchy-Green relations are modified and the elastic-constant tensor ceases to be symmetric; the size and shape of a nanocrystal render its mechanical properties more anisotropic. For a single-crystal strip, the Poisson ratio decreases and the Young modulus increases with decreasing thickness of the strip; in the case of a very thin crystal film (two atomic layers thick), these elastic moduli can differ from their macroscopic values by a factor of two. The size effects which make the continuum elasticity theory inapplicable to nanocrystals are estimated. The size effects that occur when the molecular dynamics method is applied for modeling macroscopic objects are also discussed. 相似文献
114.
Nonlinear Theory of Spectra of Thermally Stimulated Currents in Complex Crystals with Hydrogen Bonds
Tonkonogov M. P. Ismailov Zh. T. Timokhin V. M. Fazylov K. K. Kalitka V. A. Baimukhanov Z. K. 《Russian Physics Journal》2002,45(10):1008-1018
The nonlinear theory of thermally stimulated depolarization currents is developed. The theory explains the processes of hetero- and homocharge relaxation in complex crystals with hydrogen bonds and allows the relaxation oscillator parameters to be calculated using the quadratic approximation for the external electric field. 相似文献
115.
The equilibrium geometry and hyperfine interaction constants of the nearest and next-to-nearest neighbor atoms are calculated for a negatively charged silicon vacancy in the high-spin state in cubic SiC. The calculations are performed within the cluster approach in terms of the density-functional theory (DFT). It is shown that the results of calculations with the use of a 70-atom cluster are in good agreement with experimental data. A detailed consideration is given to spin polarization in the electron subsystem and the applicability of a simple LCAO model that is commonly used in the interpretation of the electron paramagnetic resonance data for semiconductors. The spin density distribution for the defect under investigation is analyzed in terms of localized orbitals. 相似文献
116.
117.
A.R. DegheidyM.T. Attia M. Sallah 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,74(3):285-297
The time-independent radiative transfer problem in a scattering and absorbing planar random medium with general boundary conditions and internal energy source is considered. The medium is assumed to consist of two randomly mixed immiscible fluids, with the mixing statistics described as a two-state homogeneous Markov process. The problem is solved in terms of the solution of the corresponding free-source problem with simple boundary conditions which is solved using Pomraning-Eddington approximation in the deterministic case. A formalism, developed to treat radiative transfer in statistical mixtures, is used to obtain the ensemble-averaged solution. The average partial heat fluxes are calculated in terms of the albedoes of the source-free problem. Results are obtained for isotropic and anisotropic scattering for specular and diffused reflecting boundaries. 相似文献
118.
A method for stabilizing frequency based on using the soliton state of the Bose-Einstein condensate of alkali metal atoms as an atomic source was suggested. The critical total number of lithium condensate particles at which the existence of a quasi-one-dimensional soliton in the condensate was possible and the lifetime of such a soliton were estimated. The attainable accuracy of measuring reference transition frequencies in the suggested standard was shown to be substantially higher than with the known quantum frequency standards. 相似文献
119.
U. T. Yakhshiev 《Physics of Atomic Nuclei》2002,65(3):562-566
The properties of Skyrmions in finite nuclei are considered. The deformation effect is taken into account through the external-field-induced distortion of the profile function of a chiral field. The masses of classical Skyrmions and the distribution of their baryon number versus the Skyrmion position within a nucleus are discussed. 相似文献
120.
Summary In virtually all neutron activation analysis laboratories, researchers are given film badges known as thermoluminescence dosimeters
(TLD's) that are monitored on a monthly basis. Sometimes additional pocket dosimeters are worn to get a daily reading of exposure,
while in other instances ring badges are worn to assess beta doses. However, more than often little consideration is given
to beta-exposure rates that arise from the plethora of radionuclides produced in the many types of samples irradiated. A common
mistake is to assess gamma-exposures of activated samples that rely on the use of a beta-shield on the survey meter. Our experimental
results have shown that there are many high energy beta-particles from neutron activated samples that can easily penetrate
the beta-shield and thus give an underestimation of the total beta-exposure. 相似文献