全文获取类型
收费全文 | 290467篇 |
免费 | 7555篇 |
国内免费 | 4370篇 |
专业分类
化学 | 162186篇 |
晶体学 | 3802篇 |
力学 | 12379篇 |
综合类 | 337篇 |
数学 | 39002篇 |
物理学 | 84686篇 |
出版年
2021年 | 2574篇 |
2020年 | 3003篇 |
2019年 | 3122篇 |
2018年 | 4624篇 |
2017年 | 4825篇 |
2016年 | 5812篇 |
2015年 | 4451篇 |
2014年 | 5747篇 |
2013年 | 12113篇 |
2012年 | 13706篇 |
2011年 | 14884篇 |
2010年 | 10222篇 |
2009年 | 9760篇 |
2008年 | 11684篇 |
2007年 | 11796篇 |
2006年 | 11108篇 |
2005年 | 13808篇 |
2004年 | 12135篇 |
2003年 | 9541篇 |
2002年 | 7708篇 |
2001年 | 7617篇 |
2000年 | 5842篇 |
1999年 | 4582篇 |
1998年 | 3824篇 |
1997年 | 3812篇 |
1996年 | 3934篇 |
1995年 | 3316篇 |
1994年 | 3369篇 |
1993年 | 3127篇 |
1992年 | 3470篇 |
1991年 | 3470篇 |
1990年 | 3186篇 |
1989年 | 3015篇 |
1988年 | 2910篇 |
1987年 | 2817篇 |
1986年 | 2790篇 |
1985年 | 3683篇 |
1984年 | 3681篇 |
1983年 | 3054篇 |
1982年 | 3343篇 |
1981年 | 3037篇 |
1980年 | 2874篇 |
1979年 | 3039篇 |
1978年 | 3249篇 |
1977年 | 3285篇 |
1976年 | 3329篇 |
1975年 | 2987篇 |
1974年 | 3109篇 |
1973年 | 3145篇 |
1972年 | 2426篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
991.
D. Bardzokas M. L. Fil'shtinskii L. A. Fil'shtinskii 《Mechanics of Composite Materials》1997,33(2):189-193
The electrical properties of fiber-metal composites with anisotropic components were averaged within the framework of a regular structure model. The corresponding current problem was reduced to a system of two integral equations with elliptical kernels. The effective electrical conductivity tensor of such materials was defined as several functionals based on integral equation solutions for the boundary problem. The calculation results are given.Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 2, pp. 263–268, March–April, 1997. 相似文献
992.
Protein Conformation of a Lattice Model Using Tabu Search 总被引:1,自引:0,他引:1
We apply tabu search techniques to the problem of determining the optimal configuration of a chain of protein sequences on a cubic lattice. The problem under study is difficult to solve because of the large number of possible conformations and enormous amount of computations required. Tabu search is an iterative heuristic procedure which has been shown to be a remarkably effective method for solving combinatorial optimization problems. In this paper, an algorithm is designed for the cubic lattice model using tabu search. The algorithm has been tested on a chain of 27 monomers. Computational results show that our method outperforms previously reported approaches for the same model. 相似文献
993.
I. L. Nizhnik 《Ukrainian Mathematical Journal》1998,50(11):1789-1791
We investigate a nonlinear chain of two Chua oscillators and its stable solutions.
Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 50, No. 11, pp. 1567–1569, November, 1998. 相似文献
994.
Rhizopus arrhizus lipase (EC 3.1.1.3; triacylglycerol hydrolase) was used in this study to investigate the hydrolysis of palm kernel olein in AOT-isooctane-water reversed micelle system at Wo = 13, pH = 7, and T = 30?C. The hydrolytic reaction obeys Michaelis-Menten kinetics for substrate concentrations in the range (0.175 and 0.877M). The apparentK m and Vmax for the substrate were 0.397M (equivalent to 9.06% w/v) and 5523 U/mg protein, respectively. Product inhibition with a dissociation constant of the enzyme-product complex,K I = 9.74 mM, was confirmed. Experimental results from the change of product concentrations with respect to time correlated sufficiently with those predicted theoretically from the rate equation for a reaction time up to 100 min. However, the discrepancy between the observed results and the predicted ones would increase with reaction time. Possible reasons for this deviation were discussed. 相似文献
995.
介绍F-P干涉滤波器实现中心波长可调的两种机理,给出通过改变腔距来改变中心波工的滤波器的典型结构,并且给出了相应的电压驱动电路,实验表明选择合适的光放大器前置滤波器可以提高系统的灵敏度。 相似文献
996.
Richard Haberman 《Studies in Applied Mathematics》1991,84(1):57-69
Oscillatory dispersive waves propagating in a slowly varying medium are analyzed for Klein-Gordon equations with perturbations. The method of multiple scales is extended to include two fast scales, the usual traveling-wave phase and time, in order to allow initial conditions not usually permitted. An exact wave-action equation is introduced if the traveling wave is stable, involving averages over the periodic wave as well as time. This is equivalent to an extended averaged Lagrangian principle. The equation for the slow modulations of the phase shift of the traveling wave is derived from the higher order terms in the exact action equation and is shown to be the same as in earlier more restrictive studies. 相似文献
997.
V. Ya. Popkova L. E. Vinogradova L. A. Leites V. K. Osmanov 《Russian Chemical Bulletin》1991,40(10):2075-2078
The reaction of ammonia with 2-chloroperfluoro-1-cyclohexene-1-thiocyanate leads to 2-amino-7-iminoperfluoro-4,5,6-trihydrobenzo-1,3-thiazole (II) due to intramolecular heterocyclization in the Thorpe reaction with the concurrent exoiminization. Analysis of the IR spectra of (II) in CCl4 and in the solid phase indicated that molecules of (II) in the crystal form hydrogen-bonded centrosymmetric dimers (D), which are in equilibrium with free molecules of (II) in solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2366–2370, October, 1991.The authors express their gratitude to D. I. Zagorevskii for taking the mass spectrum of (II) and to L. S. German for a useful discussion of this work. 相似文献
998.
999.
Keith Vaughan Ronald J. LaFrance York Tang Donald L. Hooper 《Journal of heterocyclic chemistry》1991,28(7):1709-1713
3-Alkyl-4-arylazomethylene-3,4-dihydro-1,2,3-benzotriazines 5a and 5b undergo facile 1,4-addition of methanol or ethanol to afford stable crystalline hydrazones, 6 and 8 which have been characterised by spectroscopic analysis. In particular, the nmr spectrum of 8 shows novel features due to the diastereotopic nature of the CH2 protons in the two O-ethyl groups. Compound 5b shows a property unique to the compounds in this series; refluxing this arylazomethylenetriazine in ethanol affords a rearranged product, characterised as the 1-aryl-2-cinnolinylhydrazine ( 9 ). The formation of 9 is rationalised as a ring opening-ring closure process analogous to the Dimroth rearrangement. The cinnoline 9 displays some novel chemistry arising from the facility of the arylhydrazino substituent to react with acid to give a fragmentation product, 3-methylindazole ( 7 ). 相似文献
1000.
P. Schuurmans J. Wouters P. De Moor N. Severijns W. Vanderpoorten J. Vanhaverbeke L. Vanneste 《Hyperfine Interactions》1992,75(1-4):423-430
Anisotropic α-emission from nuclei in the decay chain from223Ra to207Tl has been observed using low temperature nuclear orientation in combination with high resolution α-detection at 4.2 K. Information
on partial wave amplitudes as well as hyperfine interactions has been extracted. 相似文献