全文获取类型
收费全文 | 279822篇 |
免费 | 3186篇 |
国内免费 | 930篇 |
专业分类
化学 | 149306篇 |
晶体学 | 4805篇 |
力学 | 11554篇 |
综合类 | 7篇 |
数学 | 30896篇 |
物理学 | 87370篇 |
出版年
2018年 | 2534篇 |
2017年 | 2569篇 |
2016年 | 4309篇 |
2015年 | 2780篇 |
2014年 | 4295篇 |
2013年 | 11929篇 |
2012年 | 8669篇 |
2011年 | 10692篇 |
2010年 | 7392篇 |
2009年 | 7170篇 |
2008年 | 9861篇 |
2007年 | 9820篇 |
2006年 | 9488篇 |
2005年 | 8823篇 |
2004年 | 8050篇 |
2003年 | 7252篇 |
2002年 | 7079篇 |
2001年 | 8097篇 |
2000年 | 6109篇 |
1999年 | 4839篇 |
1998年 | 4096篇 |
1997年 | 4018篇 |
1996年 | 3818篇 |
1995年 | 3657篇 |
1994年 | 3447篇 |
1993年 | 3343篇 |
1992年 | 3811篇 |
1991年 | 3729篇 |
1990年 | 3629篇 |
1989年 | 3580篇 |
1988年 | 3581篇 |
1987年 | 3587篇 |
1986年 | 3378篇 |
1985年 | 4490篇 |
1984年 | 4704篇 |
1983年 | 4016篇 |
1982年 | 4414篇 |
1981年 | 4098篇 |
1980年 | 4129篇 |
1979年 | 4258篇 |
1978年 | 4396篇 |
1977年 | 4262篇 |
1976年 | 4314篇 |
1975年 | 4154篇 |
1974年 | 4020篇 |
1973年 | 4373篇 |
1972年 | 2776篇 |
1971年 | 2231篇 |
1968年 | 2200篇 |
1967年 | 2160篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
E. V. Gromachevskaya A. G. Sakhabutdinov V. G. Kul'nevich I. S. Arustamova R. B. Valeev B. A. Bazhenov 《Chemistry of Heterocyclic Compounds》1991,27(5):502-503
It was shown that the reaction of o-hyroxyphenyldiphenylmethanol with benzaldehyde labeled with 17O and 18O isotopes proceeds in such a way that oxygen atoms of the starting phenoloalcohol are retained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 632–633, May, 1991. 相似文献
152.
The spin-1 Ising model, which is equivalent to the three-component lattice gas model, is used to study wetting transitions in three-component surfactant systems consisting of an oil, water, and a nonionic surfactant. Phase equilibria, interfacial profiles, and interfacial tensions for three-phase equilibrium are determined in mean field approximation, for a wide range of temperature and interaction parameters. Surfactant interaction parameters are found to strongly influence interfacial tensions, reducing them in some cases to ultralow values. Interfacial tensions are used to determine whether the middle phase, rich in surfactant, wets or does not wet the interface between the oil-rich and water-rich phases. By varying temperature and interaction parameters, a wetting transition is located and found to be of the first order. Comparison is made with recent experimental results on wetting transitions in ternary surfactant systems.This paper is dedicated to J. K. Percus in honor of his 65th birthday. 相似文献
153.
M. B. Wright 《The Journal of the Operational Research Society》1989,40(2):187-192
This paper addresses a problem common to all railway networks. Given a fixed train timetable and locomotives (or other forms of traction) of various types, each train must be allocated a locomotive. This paper examines the use of stochastic algorithms for such a problem. Two types of algorithm are used—a simple ‘local improvement’ method, performed successively from randomly chosen starting points, and a ‘simulated annealing’ approach. Both are found to give considerably better results than a deterministic method in current use, and the annealing approach is probably the better stochastic method. 相似文献
154.
155.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
156.
Vijaya Gracias Alan F. Gasiecki Joel D. Moore Irini Akritopoulou-Zanze Stevan W. Djuric 《Tetrahedron letters》2006,47(50):8977-8980
A general method for the preparation of diaza-spirocycles is reported. This method used an olefin metathesis in order to construct the desired spirocyclic framework. Beginning with commercially available protected amino ketones, this strategy ultimately produced pharmacologically relevant diaza-scaffolds in an efficient and high-yielding process. 相似文献
157.
In a two-dimensional array of magnetic moments with planar magnetization and relatively weak anisotropy in the basal plane, a stepwise phase transition is induced by an external magnetic field parallel to the easy axis of the system. This transition is similar to the spin flop phase transition in weakly anisotropic Heisenberg antiferromagnets with the significant difference that it is accompanied by the rearrangement of the sublattice structure of the magnet; i.e., it can be interpreted as a topological transition. The transition should manifest itself for arrays of submicron magnetic particles (magnetic dots) on nonmagnetic substrates, which have recently become the object of intensive research. 相似文献
158.
S. K. Godunov V. T. Zhukov O. B. Feodoritova 《Computational Mathematics and Mathematical Physics》2006,46(6):971-982
An algorithm is constructed for calculating invariant subspaces of symmetric hyperbolic systems arising in electromagnetic, acoustic, and elasticity problems. Discrete approximations are calculated for subspaces that correspond to minimal eigenvalues and smooth eigenfunctions. Difficulties related to the presence of an infinite-dimensional kernel in the differential operator are successfully handled. The efficiency of the algorithm is demonstrated using acoustics equations. 相似文献
159.
Aspects of Generic Entanglement 总被引:4,自引:4,他引:0
Patrick Hayden Debbie W. Leung Andreas Winter 《Communications in Mathematical Physics》2006,265(1):95-117
We study entanglement and other correlation properties of random states in high-dimensional bipartite systems. These correlations
are quantified by parameters that are subject to the ``concentration of measure' phenomenon, meaning that on a large-probability
set these parameters are close to their expectation. For the entropy of entanglement, this has the counterintuitive consequence
that there exist large subspaces in which all pure states are close to maximally entangled. This, in turn, implies the existence
of mixed states with entanglement of formation near that of a maximally entangled state, but with negligible quantum mutual
information and, therefore, negligible distillable entanglement, secret key, and common randomness. It also implies a very
strong locking effect for the entanglement of formation: its value can jump from maximal to near zero by tracing over a number
of qubits negligible compared to the size of the total system. Furthermore, such properties are generic. Similar phenomena
are observed for random multiparty states, leading us to speculate on the possibility that the theory of entanglement is much
simplified when restricted to asymptotically generic states. Further consequences of our results include a complete derandomization
of the protocol for universal superdense coding of quantum states. 相似文献
160.
Multi-quantum well heterostructures (MQWHs) of the novel Ga(NAsP)/GaP material system have been grown, pseudomorphically strained to GaP-substrate. The crystalline perfection is verified by transmission electron microscopy (TEM). For As-concentrations in excess of about 70%, a direct band structure and adequate luminescence efficiency for laser device application is observed. Temperature-dependent photoluminescence (PL) investigations show the influence of carrier localisation and non-radiative recombination processes typical for dilute nitride materials. With rising N content in the active material, the emission wavelength shifts towards longer wavelength, leading to Ga(NAs)/GaP MQW structures with photon energies below the indirect band gap of silicon (Si). At the same time the luminescence intensity drops due to an increase in non-radiative carrier traps and/or structural degradation. 相似文献