全文获取类型
收费全文 | 85190篇 |
免费 | 18839篇 |
国内免费 | 321篇 |
专业分类
化学 | 82094篇 |
晶体学 | 346篇 |
力学 | 2091篇 |
综合类 | 1篇 |
数学 | 9344篇 |
物理学 | 10474篇 |
出版年
2024年 | 440篇 |
2023年 | 4361篇 |
2022年 | 1681篇 |
2021年 | 2784篇 |
2020年 | 5167篇 |
2019年 | 2792篇 |
2018年 | 2819篇 |
2017年 | 1101篇 |
2016年 | 6494篇 |
2015年 | 6327篇 |
2014年 | 5916篇 |
2013年 | 7082篇 |
2012年 | 5642篇 |
2011年 | 4161篇 |
2010年 | 4751篇 |
2009年 | 4488篇 |
2008年 | 3858篇 |
2007年 | 3383篇 |
2006年 | 2872篇 |
2005年 | 2576篇 |
2004年 | 2172篇 |
2003年 | 1853篇 |
2002年 | 1753篇 |
2001年 | 909篇 |
2000年 | 809篇 |
1999年 | 666篇 |
1998年 | 615篇 |
1997年 | 673篇 |
1996年 | 713篇 |
1995年 | 623篇 |
1994年 | 629篇 |
1993年 | 732篇 |
1992年 | 593篇 |
1991年 | 510篇 |
1990年 | 428篇 |
1989年 | 394篇 |
1988年 | 413篇 |
1987年 | 430篇 |
1985年 | 595篇 |
1984年 | 599篇 |
1983年 | 534篇 |
1982年 | 604篇 |
1981年 | 562篇 |
1980年 | 570篇 |
1979年 | 503篇 |
1978年 | 508篇 |
1977年 | 551篇 |
1976年 | 542篇 |
1975年 | 508篇 |
1974年 | 493篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
251.
252.
Michael Heller 《International Journal of Theoretical Physics》1992,31(2):277-288
A purely algebraic structure called an Einstein algebra is defined in such a way that every spacetime satisfying Einstein's equations is an Einstein algebra but not vice versa. The Gelfand representation of Einstein algebras is defined, and two of its subrepresentations are discussed. One of them is equivalent to the global formulation of the standard theory of general relativity; the other one leads to a more general theory of gravitation which, in particular, includes so-called regular singularities. In order to include other types of singularities one must change to sheaves of Einstein algebras. They are defined and briefly discussed. As a test of the proposed method, the sheaf of Einstein algebras corresponding to the spacetime of a straight cosmic string with quasiregular singularity is constructed. 相似文献
253.
254.
Michael S. Kent Matthew Tirrell Timothy P. Lodge 《Journal of Polymer Science.Polymer Physics》1994,32(11):1927-1941
Solution properties for random and diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been measured by dynamic and total intensity light scattering in solvents of differing quality. The results are compared with the corresponding properties for PS and PMMA homopolymers of similar molecular weight, in order to determine if interactions between unlike monomers are significant. The hydrodynamic radius (Rh) and diffusion second virial coefficient (kd) for the random copolymer are found to be larger than the corresponding values for the homopolymers in a solvent which is near-theta for the two homopolymers, whereas no such effect is observed for the block copolymer. This suggests that most intrachain interactions occur a relatively short distance along the chain backbone. In a mutual good solvent Rh and kd of the random copolymer are comparable to the average of the values for the homopolymers, indicating that in a good solvent monomer/solvent interactions dominate over monomer/monomer interactions. For an isolated diblock copolymer in a mutual good solvent, there is no evidence that interactions between unlike monomers lead to additional expansion of the entire molecule, as measured by Rh, nor expansion of the individual blocks as probed by light scattering with one block optically masked. However, at low but finite concentration there is evidence (the coefficients of the binary interaction terms in the viscosity and the mutual diffusion coefficient, and the second and third virial coefficients) that a weak ordering effect may exist in block copolymer solutions, far from the conditions where microphase separation occurs. Finally, measurements of ternary polymer-polymer-solvent solutions show no dependence on monomer composition or monomer distribution for the tracer diffusion of probe PS-PMMA copolymers in a PMMA/toluene matrix. This indicate that the frictional interaction is largely unaffected by interactions between unlike monomers. However, there is evidence that the thermodynamic interaction is more unfavorable between a random copolymer and the homopolymer matrix than between a diblock and the matrix. © 1994 John Wiley & Sons, Inc. 相似文献
255.
Guantao Chen Ronald J. Gould Michael S. Jacobson Richard H. Schelp 《Journal of Graph Theory》1994,18(7):663-672
For several years, the study of neighborhood unions of graphs has given rise to important structural consequences of graphs. In particular, neighborhood conditions that give rise to hamiltonian cycles have been considered in depth. In this paper we generalize these approaches to give a bound on the smallest number of cycles in G containing all the vertices of G. We show that if for all x, y ? V(G), |N(x) ∩ N(y)| ≧ 2n/5 + 1, then V(G) is coverable by at most two cycles. Several related results and extensions to t cycles are also given. 相似文献
256.
Erkki Mäkinen 《BIT Numerical Mathematics》1987,27(3):330-339
The splay tree is a self-adjusting binary search tree which has a good amortized performance. This paper studies some properties of top-down splay trees. Different ways to charge for the primitive operations of top-down splaying are discussed. We also give some empirical results concerning the behaviour of different top-down restructuring algorithms.This work was supported by the Academy of Finland. 相似文献
257.
Katalytische Epoxidierung und 1,2‐Dihydroxylierung von Olefinen mit Bispidin‐Eisen(II)/H2O2‐Systemen
258.
259.
P. Schwab A. Kochemasov R. Kullmer D. Bäuerle 《Applied Physics A: Materials Science & Processing》1992,54(2):166-169
Pulsed-laser deposition of YBa2Cu3O7 in N2O permits lowering of the substrate temperature with respect to deposition in O2 atmosphere. Additional photodissociation of N2O near the substrate surface deteriorates the superconducting properties of deposited films.On leave from: Institute of Applied Physics of the Academy of Sciences of the USSR, SU-603600 Nishni Nowgorod, USSR 相似文献
260.
We have studied by ESR and ENDOR spectroscopy the free radicals produced in γ-irradiated inclusion compound formed between the ketone 10-nonadecanone and urea. Only one type of long lived radical is formed by the removal of an α-proton from the ketone. The hyperfine (hf) coupling constants of the α- and β-protons of the radicals have been measured by ESR at different temperatures in the range 110–292 K and at different orientations of the crystals. The hf coupling of the γ-protons of the radical and of the urea protons have been studied by ENDOR. The temperature and angular dependences of the coupling constants have been analyzed in terms of the internal and overall motions of the radical inside the hexagonal channels formed by the urea molecules. It has been found that the radical cannot perform complete reorientations around the long molecular axis, but it undergoes restricted rotational diffusion. This process is explained by assuming a coupling between the rotational and translational degrees of freedom of the radical inside the urea channels. 相似文献