全文获取类型
收费全文 | 232771篇 |
免费 | 2248篇 |
国内免费 | 800篇 |
专业分类
化学 | 116823篇 |
晶体学 | 3996篇 |
力学 | 10757篇 |
综合类 | 1篇 |
数学 | 25521篇 |
物理学 | 78721篇 |
出版年
2020年 | 1787篇 |
2019年 | 2043篇 |
2018年 | 2719篇 |
2017年 | 2651篇 |
2016年 | 3913篇 |
2015年 | 2389篇 |
2014年 | 3967篇 |
2013年 | 10072篇 |
2012年 | 7529篇 |
2011年 | 9392篇 |
2010年 | 6684篇 |
2009年 | 6628篇 |
2008年 | 8703篇 |
2007年 | 8733篇 |
2006年 | 8272篇 |
2005年 | 7496篇 |
2004年 | 6917篇 |
2003年 | 6111篇 |
2002年 | 6060篇 |
2001年 | 7090篇 |
2000年 | 5373篇 |
1999年 | 4235篇 |
1998年 | 3563篇 |
1997年 | 3472篇 |
1996年 | 3293篇 |
1995年 | 3009篇 |
1994年 | 2951篇 |
1993年 | 3043篇 |
1992年 | 3151篇 |
1991年 | 3332篇 |
1990年 | 3084篇 |
1989年 | 3027篇 |
1988年 | 3066篇 |
1987年 | 2955篇 |
1986年 | 2800篇 |
1985年 | 3808篇 |
1984年 | 4005篇 |
1983年 | 3198篇 |
1982年 | 3410篇 |
1981年 | 3336篇 |
1980年 | 3230篇 |
1979年 | 3335篇 |
1978年 | 3422篇 |
1977年 | 3350篇 |
1976年 | 3448篇 |
1975年 | 3141篇 |
1974年 | 3150篇 |
1973年 | 3194篇 |
1972年 | 2235篇 |
1971年 | 1849篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
152.
The excitation of eigen surface waves by tubular electron beams in cylindrical discharge devices is studied. The influence of the wave‐field azimuthal structure on the excitation efficiency and nonlinear stage of the plasmabeam instability is investigated both numerically and analytically. Analytical expressions for the saturation amplitude and excitation efficiency of the wave under study are derived. They are found to agree well with results obtained by numerical modelling of the plasma‐beam interaction presented in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
153.
P.A. Ramachandran 《Numerical Methods for Partial Differential Equations》2006,22(4):831-846
Time‐dependent differential equations can be solved using the concept of method of lines (MOL) together with the boundary element (BE) representation for the spatial linear part of the equation. The BE method alleviates the need for spatial discretization and casts the problem in an integral format. Hence errors associated with the numerical approximation of the spatial derivatives are totally eliminated. An element level local cubic approximation is used for the variable at each time step to facilitate the time marching and the nonlinear terms are represented in a semi‐implicit manner by a local linearization at each time step. The accuracy of the method has been illustrated on a number of test problems of engineering significance. © 2005 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2006 相似文献
154.
P Vincent A BrioudeC Journet S RabasteS.T Purcell J Le BrusqJ.C Plenet 《Journal of Non》2002,311(2):130-137
We report here the successful inclusion of carbon nanotubes (CNs) into a TiO2 matrix prepared by a sol-gel method. The presence of CNs in the sol-gel matrix and the structure of the film were analyzed principally by transmission electron microscopy. Complementary information about the behavior of embedded carbon nanotubes versus heat treatment and ion irradiation were obtained by X-ray photoelectron spectroscopy. The elaboration of an inorganic matrix containing embedded carbon nanotubes leads to a new nanocomposite. The possible applications of this nanocomposite are discussed. 相似文献
155.
E. P. Gordov 《Russian Physics Journal》1981,24(11):1039-1041
An evolution operatior is described, which acts on the operators of a problem to take it into the solutions of the corresponding problem of semiclassical electrodynamics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 66–69, November, 1981. 相似文献
156.
Dominant nonlinear electric dipole polarizations at the interaction of fundamental and second harmonic waves in centrosymmetric media without and with memory are treated in this paper. The process of forming a permanent spatially periodic quadratic susceptibility grating in the medium with memory is described from the phenomenological point of view.Dedicated to the Czechoslovak Student Movement '89. 相似文献
157.
The variational problem of contact equilibrium of a punch and an elastic body is considered. An equivalent formulation of the problem is given in variational inequality form. Existence and uniqueness of the solution is investigated in a particular case. A penalty method is proposed for approximate solution of the problem.Translated from Vychislitel'naya i Prikladnaya Matematika, No. 56, pp. 97–103, 1985. 相似文献
158.
A firm's logistics cost, including shipping and inventory-carrying, is a substantial percentage of its sales. Nevertheless, typical inventory-control methods ignore or insufficiently represent the shipping cost. This paper describes a recursive algorithm that determines the reorder cycle-time that minimizes total logistics cost. It allows for a realistic accounting of shipping cost, which is modelled here as a function of shipping distance and weight. The algorithm uses a relaxation procedure to identify a suitable initial approximation to the optimal order cycle-time and then, through a series of recursive steps, moves to the optimal result. We demonstrate the algorithm with a single item, with a group of items that share a common order cycle, and with multi-items when item demands are random variables. Experience with this algorithm indicates that it converges to the optimal result in a very few steps. 相似文献
159.
J. P. Levine 《Inventiones Mathematicae》1989,96(3):571-592
160.
P. David J. Hartfiel H. Janszen R. von Mutius J. Arvieux L. Farvacque B. Berthier B. Bonin J. C. Lugol 《Zeitschrift für Physik A Hadrons and Nuclei》1987,326(4):367-372
The excitation function of the fission probability P E E x) for238U has been measured in the reaction238U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB nf , the threshold for second chance fission, and aboveB nf up toE x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP f at excitation energies well aboveB nE . The role of the quasi-elastic knock-out process in this reaction is discussed. 相似文献