全文获取类型
收费全文 | 659篇 |
免费 | 20篇 |
国内免费 | 6篇 |
专业分类
化学 | 372篇 |
晶体学 | 9篇 |
力学 | 23篇 |
综合类 | 1篇 |
数学 | 97篇 |
物理学 | 183篇 |
出版年
2023年 | 9篇 |
2022年 | 16篇 |
2021年 | 18篇 |
2020年 | 13篇 |
2019年 | 14篇 |
2018年 | 19篇 |
2017年 | 22篇 |
2016年 | 29篇 |
2015年 | 26篇 |
2014年 | 18篇 |
2013年 | 63篇 |
2012年 | 34篇 |
2011年 | 36篇 |
2010年 | 29篇 |
2009年 | 25篇 |
2008年 | 30篇 |
2007年 | 30篇 |
2006年 | 25篇 |
2005年 | 20篇 |
2004年 | 15篇 |
2003年 | 10篇 |
2002年 | 10篇 |
2000年 | 6篇 |
1999年 | 4篇 |
1998年 | 7篇 |
1997年 | 10篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1960年 | 2篇 |
1959年 | 5篇 |
排序方式: 共有685条查询结果,搜索用时 46 毫秒
91.
Reaction of tetralinol 5 with phenylboronic acid removed the isopropylidene group with concomitant formation of phenylboronate 12. Oxidation of the latter with PCC gave keto-ester 13, a key intermediate in the synthesis of enantiopure anthracyclinones. Attempts at cleavage of the isopropylidene group in 5 by the usual procedures led first to substitution and epimerization at the benzylic position, then removal of the acetonide followed by formation of the cyclic ether 11a as the final product. 相似文献
92.
This paper describes the relative acoustical performances established by scale model testing of a number of relatively novel noise barriers in typical highway situations. The various barriers were thin, wide, T-profiled, cylindrically topped, corrugated, inclined, Y-profiled, arrow-profiled and of the thnadner principle, and some were treated with sound absorptive material. The highway situations involved a single barrier with a protected receiver (i.e., a receiver behind the barrier), a single barrier with a receiver on the opposite side of the highway, and parallel barriers, one on each side of the highway. In the single barrier, protected receiver case, higher noise reduction was found for wide top barriers, especially those of T-profile, and especially T-profile absorptive top barriers with cap widths of 0·6 m (2 ft) or more and of small cap thickness. Absorptive side treatment was effective in reducing a small, but measurable sound increase found when a reflective sided barrier is installed on the opposite side of the highway to receivers, and in reducing the degradation in performance that occurs when there is a barrier on each side of the highway rather than on just one. 相似文献
93.
M. M. Osman 《Helvetica chimica acta》1972,55(1):239-244
The acidity of malondialdehyde as well as the stability of its 1 :1-complexes with NiII and CuII have been determined (Table). In comparison with the enolates of aliphatic β-dike-tones the anion C3H3O2? has not only a strikingly reduced basicity, but is also a grossly inferior ligand for metal cations. 相似文献
94.
O. Ersoy 《Optical and Quantum Electronics》1975,7(4):247-252
A theoretical model is developed to account for the effect of diffusion on gain saturation in gas lasers. It is shown that the effect can be characterized by the functionK(k 2 a, k 2 b), which depends on pressure, temperature, radius of the beama, and the radius of the cylindrical cavityb. It is concluded that diffusion can be very effective whenk 2 a is small andb is several times larger thana. 相似文献
95.
96.
Serife Nur Deveci Tuncer Acar Osman Alagoz 《Mathematical Methods in the Applied Sciences》2020,43(5):2772-2782
In this study, we introduce newly defined Gamma operators which preserve constants and e2μ·, μ>0 functions. In accordance with this purpose, we focus on their approximation properties such as uniform convergence, rate of convergence, asymptotic formula, and saturation results. Superior properties of introduced operators have been tested both theoretically and numerically in certain senses to highlight the performance of the new constructions of Gamma operators. 相似文献
97.
Motivated by the U.S. influenza vaccine market, we study the impact of random yield and production capacity on the number of firms, total supply, consumer surplus and social welfare in a market with identical suppliers. We formulate a two-stage game with endogenous entry, where each entering firm aims to maximize its profit under yield uncertainty subject to a production capacity constraint. We show that if firms produce full capacity in the equilibrium, then there are fewer firms in the equilibrium compared to the social optimum even for small levels of yield uncertainty. Furthermore, we prove that if firms do not produce full capacity in the equilibrium, they will not produce full capacity in the social optimum. 相似文献
98.
We present new valid inequalities for 0-1 programming problems that work in similar ways to well known cover inequalities. Discussion and analysis of these cuts is followed by their revision and use in integer programming as a new generation of cuts that excludes not only portions of polyhedra containing noninteger points, also parts with some integer points that have been explored in search of an optimal solution. Our computational experimentations demonstrate that this new approach has significant potential for solving large scale integer programming problems. 相似文献
99.
Y B Türkoğulları N Aras İ K Altınel C Ersoy 《The Journal of the Operational Research Society》2010,61(6):1000-1012
A wireless sensor network is a network consisting of distributed autonomouselectronic devices called sensors. Sensors have limited energy and capabilityfor sensing, data processing, and communicating, but they can collectivelybehave to provide an effective network that monitors an area and transmitinformation to gateway nodes or sinks, either directly or through other sensornodes. In most applications the network must operate for long periods of time,so the available energy resources of the sensors must be managed efficiently. Inthis paper, we first develop a mixed integer linear programming model tomaximize network lifetime by optimally determining locations of sensors andsinks, activity schedules of deployed sensors, and data flow routes from sensorsto sinks over a finite planning horizon subject to coverage, flow conservation,energy consumption, and budget constraints. Unfortunately, it is difficult tosolve this model exactly even for small instances. Therefore, we propose twoapproximate solution methods: a Lagrangean heuristic and a two-stage heuristicin which sensors are deployed and an activity schedule is found in the firststage, whereas sinks are located and sensor-to-sink data flow routes aredetermined in the second stage. Computational experiments performed on varioustest instances indicate that the Lagrangean heuristic is both efficient andaccurate and also outperforms the two-stage heuristic. 相似文献
100.
M. F. Abdel-Wahab M. S. Abdo Y. M. Megahed O. H. Osman 《Isotopes in environmental and health studies》2013,49(2):56-58
Labelling the venom of scorpion was achieved by means of a technique previously published by the author. The radioisotopes, used were both 125I and 131I in a nascent form. Fractionation was done by the use of Sephadex G-100. Both the radioactivity and the extinction curves denoted the presence of three well defined peaks. The toxicity of the first fraction was found to be the most lethal. While the second peak was in the magnitude of 20 percent of the first, the third peak proved to be nontoxic to mice up to a concentration of 6 mg/kg body weight. 相似文献