全文获取类型
收费全文 | 10175篇 |
免费 | 2576篇 |
国内免费 | 1118篇 |
专业分类
化学 | 5828篇 |
晶体学 | 185篇 |
力学 | 352篇 |
综合类 | 66篇 |
数学 | 240篇 |
物理学 | 7198篇 |
出版年
2024年 | 15篇 |
2023年 | 105篇 |
2022年 | 297篇 |
2021年 | 333篇 |
2020年 | 475篇 |
2019年 | 431篇 |
2018年 | 418篇 |
2017年 | 454篇 |
2016年 | 538篇 |
2015年 | 447篇 |
2014年 | 594篇 |
2013年 | 872篇 |
2012年 | 798篇 |
2011年 | 817篇 |
2010年 | 679篇 |
2009年 | 665篇 |
2008年 | 649篇 |
2007年 | 651篇 |
2006年 | 615篇 |
2005年 | 517篇 |
2004年 | 501篇 |
2003年 | 421篇 |
2002年 | 477篇 |
2001年 | 351篇 |
2000年 | 301篇 |
1999年 | 218篇 |
1998年 | 191篇 |
1997年 | 143篇 |
1996年 | 133篇 |
1995年 | 118篇 |
1994年 | 102篇 |
1993年 | 70篇 |
1992年 | 79篇 |
1991年 | 51篇 |
1990年 | 50篇 |
1989年 | 41篇 |
1988年 | 39篇 |
1987年 | 24篇 |
1986年 | 29篇 |
1985年 | 32篇 |
1984年 | 27篇 |
1983年 | 14篇 |
1982年 | 21篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1978年 | 13篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
11.
Natalia L. Calvo Sebastian O. SimonettiRubén M. Maggio Teodoro S. Kaufman 《Analytica chimica acta》2015
Exposure of cimetidine (CIM) to dry heat (160–180 °C) afforded, upon cooling, a glassy solid containing new and hitherto unknown products. The kinetics of this process was studied by a second order chemometrics-assisted multi-spectroscopic approach. Proton and carbon-13 nuclear magnetic resonance (NMR), as well as ultraviolet and infrared spectroscopic data were jointly used, whereas multivariate curve resolution with alternating least squares (MCR-ALS) was employed as the chemometrics method to extract process information. It was established that drug degradation follows a first order kinetics. 相似文献
12.
We give a sheaf theoretic interpretation of Potts models with external magnetic field, in terms of constructible sheaves and their Euler characteristics. We show that the polynomial countability question for the hypersurfaces defined by the vanishing of the partition function is affected by changes in the magnetic field: elementary examples suffice to see non-polynomially countable cases that become polynomially countable after a perturbation of the magnetic field. The same recursive formula for the Grothendieck classes, under edge-doubling operations, holds as in the case without magnetic field, but the closed formulae for specific examples like banana graphs differ in the presence of magnetic field. We give examples of computation of the Euler characteristic with compact support, for the set of real zeros, and find a similar exponential growth with the size of the graph. This can be viewed as a measure of topological and algorithmic complexity. We also consider the computational complexity question for evaluations of the polynomial, and show both tractable and NP-hard examples, using dynamic programming. 相似文献
13.
14.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
15.
A new dinuclear iron(Ⅲ) complex has been synthesized and structurally characterized by X-ray crystallography: [FeⅢ2(L)(C6H5COO)(SO4)(CH3OH)2]·CH3CN·CH3OH(1, H3 L = N,N'-bis(salicylidene)-1,3-diamino-2-propanol). Complex 1 belongs to orthorhombic space group Pna21 with a = 11.4400(8), b = 22.9705(2), c = 12.5712(9) , V = 3303.5(4) 3, Z = 4, F(000) = 1576, Dc = 1.531 g·cm–3, Mr = 761.36, μ = 1.007 mm–1, S = 1.014, the final R = 0.0505 and wR = 0.1018. The crystal packing is stabilized by intermolecular O–H···O hydrogen bonds, forming an extended one-dimensional chain structure. The temperature dependence of magnetic susceptibility measurement shows that antiferromagnetic interaction is propagated between the metal centers. Fit as dinuclear arrangement gave parameters of J = 19.7 cm-1, g = 1.89 and R2 = 0.9999. 相似文献
16.
Regis Guyonnet 《Journal of statistical physics》1991,65(1-2):269-289
The NMR properties of nuclei linked to long linear polymer molecules are sensitive to the influence of hard walls. In this context, the residual energy of tensorial spin-spin interactions is calculated using a path integral approach. Several thermodynamic quantities of the polymer system (free energy, equation of state,...) are also expressed, taking chain stiffness effects and the presence of two repulsive walls into consideration. 相似文献
17.
18.
We have obtained expressions of the accelerating effect in Kerr-Newman Kasuya field. These expressionsinclude four parameters: mass m, angular momentum a, electric charge q, and magnetic charge φ. Furthermore we studyits special case (vi = 0). We get the following conclusion. In the gravitation field of souse mass with electric charge qand magnetic charge b, the acceleration of test particle has not only radial component but also transverse component.When θ = 0, the acceleration is minimum, and when θ = π/2, the acceleration is maximum. Furthermore, we discussthe effects of electric charge q and magnetic charge φ respectively. 相似文献
19.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
20.
空间遥感相机长条反射镜较圆反射镜的整体刚度相差很大,整个组件在随火箭发射过程中因力学环境引起的自身位置改变导致面形的微小畸变从而对成像质量造成严重影响。因此,反射镜组件的设计就成为整个相机系统研制的重点与难点之一。对其进行科学、严谨的力学环境仿真是设计中的有效辅助手段。对某相机的长条反射镜组件动力学响应进行了较为深入的研究,借助MSC-patran,用有限元法进行了求解。获得了组件的动力学特性。 相似文献