全文获取类型
收费全文 | 218193篇 |
免费 | 3704篇 |
国内免费 | 525篇 |
专业分类
化学 | 119006篇 |
晶体学 | 2656篇 |
力学 | 8748篇 |
综合类 | 3篇 |
数学 | 27214篇 |
物理学 | 64795篇 |
出版年
2021年 | 1695篇 |
2020年 | 2069篇 |
2019年 | 2177篇 |
2018年 | 2976篇 |
2017年 | 2876篇 |
2016年 | 4652篇 |
2015年 | 3270篇 |
2014年 | 4531篇 |
2013年 | 10580篇 |
2012年 | 9071篇 |
2011年 | 10284篇 |
2010年 | 7070篇 |
2009年 | 6665篇 |
2008年 | 9472篇 |
2007年 | 9378篇 |
2006年 | 8673篇 |
2005年 | 8003篇 |
2004年 | 7265篇 |
2003年 | 6335篇 |
2002年 | 5967篇 |
2001年 | 6074篇 |
2000年 | 4628篇 |
1999年 | 3478篇 |
1998年 | 2917篇 |
1997年 | 2767篇 |
1996年 | 2720篇 |
1995年 | 2409篇 |
1994年 | 2452篇 |
1993年 | 2353篇 |
1992年 | 2594篇 |
1991年 | 2479篇 |
1990年 | 2430篇 |
1989年 | 2350篇 |
1988年 | 2330篇 |
1987年 | 2306篇 |
1986年 | 2181篇 |
1985年 | 2960篇 |
1984年 | 2988篇 |
1983年 | 2551篇 |
1982年 | 2759篇 |
1981年 | 2573篇 |
1980年 | 2524篇 |
1979年 | 2672篇 |
1978年 | 2692篇 |
1977年 | 2595篇 |
1976年 | 2697篇 |
1975年 | 2578篇 |
1974年 | 2541篇 |
1973年 | 2706篇 |
1972年 | 1706篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
981.
In single-objective optimization it is possible to find a global optimum, while in the multi-objective case no optimal solution is clearly defined, but several that simultaneously optimize all the objectives. However, the majority of this kind of problems cannot be solved exactly as they have very large and highly complex search spaces. Recently, meta-heuristic approaches have become important tools for solving multi-objective problems encountered in industry as well as in the theoretical field. Most of these meta-heuristics use a population of solutions, and hence the runtime increases when the population size grows. An interesting way to overcome this problem is to apply parallel processing. This paper analyzes the performance of several parallel paradigms in the context of population-based multi-objective meta-heuristics. In particular, we evaluate four alternative parallelizations of the Pareto simulated annealing algorithm, in terms of quality of the solutions, and speedup. 相似文献
982.
We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlOx and Li2O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O2 exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation. 相似文献
983.
984.
985.
This work develops the dynamics of a perfectly elastic solid model for application to the outer crust of a magnetised neutron
star. Particular attention is given to the Noether identities responsible for energy-momentum conservation, using a formulation
that is fully covariant, not only (as is usual) in a fully relativistic treatment but also (sacrificing accuracy and elegance
for economy of degrees of gravitational freedom) in the technically more complicated case of the Newtonian limit. The results
are used to obtain explicit (relativistic and Newtonian) formulae for the propagation speeds of generalised (Alfven type)
magneto-elastic perturbation modes. 相似文献
986.
The 57Fe Mössbauer technique has been used to investigate the effect of zinc oxide substitution in (25???x)MnO–xZnO–15Fe2O3–60B2O3 glass system (x?=?0, 5, 10, 15 and 20 mol% of ZnO ). Mössbauer absorption spectra for all the samples recorded at room temperature suggest the existence of the two paramagnetic quadrupole doublets. The observed variations in hyperfine parameters have been explained on the basis of cations distribution and exchange interaction at the lattice sites and it is concluded that B–B interaction increases while the metal–metal interaction decreases due to replacement of manganese oxide by zinc oxide. These results suggest that the present glass system exhibits a paramagnetic behaviour that changes towards the weak paramagnetic when manganese oxide was replaced with zinc oxide. 相似文献
987.
988.
Blundell Tom L. Bolanos-Garcia Victor Chirgadze Dimitri Y. Harmer Nicholas J. Lo Thomas Pellegrini Luca Sibanda B. Lynn 《Structural chemistry》2002,13(3-4):405-412
Signaling in living systems needs to achieve high specificity, to be reversible, and to achieve high signal to noise. Signaling mediated by multiprotein systems has evolved that avoids the requirement for high-affinity binary complexes that would be difficult to reverse and which, in the overcrowded cell, would lead to excessive noise in the system. Symmetrical structures are only occasionally formed. When they are, it is principally to colocate components, for example, the tyrosyl kinases of growth factors, where dimers form. Symmetry is, however, often broken, presumably to create more sensitivity and specificity in the signaling system by assembling other components, into higher-order multiprotein systems. The binding of a single heparin to two 1:1 FGF:FGFR complexes is an example, as is the binding of a single ligase to the Xrcc4 dimer, perhaps so creating a further DNA-binding site. 相似文献
989.
The lanthanide sulfotellurides (LS)2Te1+x (0 ≤ x < 0.50) show order-disorder phenomena. Above 450°C, they crystallize in a partially disordered structure, orthorhombic with cell parameters a, b, and c close to 4.1, 5.2, and 13.6 Å, respectively. This structure is determined from diffractometer data on a (HoS)2Te1.34 crystal (R = 0.061). It is built up along the c axis from (L4S) tetrahedra layers, separated by Te planes. Disorder and nonstoichiometry are related to the two vacant Te sites. 相似文献
990.
Sandip B. Bharate 《Tetrahedron letters》2006,47(39):7021-7024
The antimalarial robustadials A and B have been synthesized in two steps starting from commercially available phloroglucinol comprising a key biomimetic three-component reaction that involves in situ generation of an o-quinone methide via Knoevenagel condensation and subsequent Diels-Alder cycloaddition with (−)-β-pinene. 相似文献