全文获取类型
收费全文 | 385197篇 |
免费 | 3576篇 |
国内免费 | 1168篇 |
专业分类
化学 | 203647篇 |
晶体学 | 6040篇 |
力学 | 16605篇 |
综合类 | 9篇 |
数学 | 44046篇 |
物理学 | 119594篇 |
出版年
2021年 | 3088篇 |
2020年 | 3412篇 |
2019年 | 3852篇 |
2018年 | 5279篇 |
2017年 | 5109篇 |
2016年 | 7244篇 |
2015年 | 4332篇 |
2014年 | 6826篇 |
2013年 | 17008篇 |
2012年 | 12887篇 |
2011年 | 15596篇 |
2010年 | 11075篇 |
2009年 | 10886篇 |
2008年 | 14657篇 |
2007年 | 14826篇 |
2006年 | 14021篇 |
2005年 | 12595篇 |
2004年 | 11484篇 |
2003年 | 10315篇 |
2002年 | 10074篇 |
2001年 | 11199篇 |
2000年 | 8722篇 |
1999年 | 6651篇 |
1998年 | 5574篇 |
1997年 | 5604篇 |
1996年 | 5157篇 |
1995年 | 4592篇 |
1994年 | 4520篇 |
1993年 | 4428篇 |
1992年 | 4849篇 |
1991年 | 4951篇 |
1990年 | 4758篇 |
1989年 | 4582篇 |
1988年 | 4556篇 |
1987年 | 4496篇 |
1986年 | 4257篇 |
1985年 | 5596篇 |
1984年 | 5961篇 |
1983年 | 4819篇 |
1982年 | 5194篇 |
1981年 | 5065篇 |
1980年 | 4802篇 |
1979年 | 5050篇 |
1978年 | 5281篇 |
1977年 | 5268篇 |
1976年 | 5289篇 |
1975年 | 4959篇 |
1974年 | 4932篇 |
1973年 | 5202篇 |
1972年 | 3388篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
B. A. Usievich D. Kh. Nurligareev V. A. Sychugov K. M. Golant 《Optics and Spectroscopy》2006,101(6):930-934
A system of tunnel-coupled rectilinear waveguides is studied. The dependence of the number of modes in this system on the number of waveguides N, the distance between the waveguides, and the number of modes in a separate waveguide is considered. It is shown that the modes of the m = N and N + 1 orders in such a system are Bragg modes; i.e., the angle between the direction of their propagation and the system axis is close to the Bragg angle. The effective refractive indices n * of these modes change stepwise. The step size Δn * is found to be dependent on the distance s between the waveguides and on the number of modes in a separate waveguide. A system of single-mode waveguides with the number of guided modes M = 34 < N = 50 is studied experimentally. It is shown that the Bragg modes of the system lie among the leakage modes of the system and have rather low losses. It is demonstrated that the localization of the Bragg modes among the leakage modes may be favorable for their selection upon light generation. 相似文献
953.
The variation of the structure of the control parameter space for a nonautonomous nonlinear oscillator is demonstrated with a harmonically excited nonautonomous oscillatory circuit with a piecewise linear capacitance. 相似文献
954.
M. G. Voronkov E. A. Grebneva O. M. Trofimova A. I. Albanov N. F. Chernov N. N. Chipanina 《Russian Journal of General Chemistry》2006,76(12):1851-1853
Reaction of tetrafluorosilane with tris(2-hydroxyethyl)-and tris(2-trimethylsiloxyethyl)amine results in formation of 1-fluorosilatrane and fluorosilatrane in 75 and 53% yield, respectively. Reaction of tetrafluorosilane with bis(2-trimethylsiloxyethyl)amine and its N-methyl derivative leads to the hitherto unknown 1,1-difluoroquasisilatranes (N → Si) F2Si(OCH2CH2)2NR (R = H, Me) containing donor-acceptor bond N → Si and pentacoordinate silicon atom. The structure of the synthesized compounds was proved by 1H, 13C, 15N, 19F, 29Si NMR and IR spectroscopy. 相似文献
955.
A. A. Lamberov E. V. Dement’eva Kh. Kh. Gil’manov S. R. Egorova N. V. Romanova 《Russian Journal of Applied Chemistry》2006,79(9):1453-1457
The effect of the paste molding pressure on the physicomechanical and texture characteristics of catalysts was studied. An indirect criterion is suggested to evaluate the molding pressure in industrial extruders, whose optimal value ensures good physicomechanical properties of the catalysts and kinetic control over the catalyzed reaction. The results obtained were verified in paste molding on various industrial extruders, and this enabled the optimal choice of the molding equipment. 相似文献
956.
Summary We study the integration theory for general integral metrics when restricted to upper integrals q, finding improvements in the relation between the classes of the q-integrable and the ql-integrable functions. We give new results and notions which lead to the desirable characterizations of q-integrable functions as ql-integrable f with q(|f|) < ∞, and of ql-integrable functions via the integrability of their upper truncations, under natural conditions which are fulfilled in most
finitely additive integration theories. 相似文献
957.
958.
This paper proposes a new method that extends the efficient global optimization to address stochastic black-box systems. The
method is based on a kriging meta-model that provides a global prediction of the objective values and a measure of prediction
uncertainty at every point. The criterion for the infill sample selection is an augmented expected improvement function with
desirable properties for stochastic responses. The method is empirically compared with the revised simplex search, the simultaneous
perturbation stochastic approximation, and the DIRECT methods using six test problems from the literature. An application
case study on an inventory system is also documented. The results suggest that the proposed method has excellent consistency
and efficiency in finding global optimal solutions, and is particularly useful for expensive systems. 相似文献
959.
Off-the-shell anomalous factors of the two-body Coulomb transition matrices appear in the integral form of the Faddeev second-order
nuclear-electronic amplitude, for proton-hydrogen charge transfer scattering in a typical nlm → n′l′m′ transition. A symmetric-impulse approximation (SIA) is applied to eliminate these factors and an induction method is proposed
to analytically calculate the remaining integrals. The nuclear-electronic amplitude is derived for the general case, and for
totally symmetric collisions, in terms of generalized hypergeometric functions of two variables, F
4, and of one variable, 3
F
2, respectively. The angular distribution of the second-order nuclear-electronic charge transfer amplitude shows the Thomas
mechanism as a peak or a hump for symmetric and asymmetric collisions. There also exists a peak in the forward angular distribution
of the second-order nuclear-electronic amplitude, which partly cancels the kinematic peak in the angular distribution of the
charge transfer differential cross sections. 相似文献
960.
N. Ponpandian A. Narayanasamy D. Prabhu K. Ganesan M. Manivel Raja K. Chattopadhyay 《Journal of magnetism and magnetic materials》2006
The Fe73.5Cu1Nb3Si13.5B9 FINEMET alloy has been prepared by the rapid solidification technique. The critical behaviour of this alloy in the amorphous as well as in its nanocrystalline states has been studied near their respective Curie temperatures. From the values of the critical exponents one can conclude that the alloy behaves like a 3D Heisenberg ferromagnet in the amorphous and nanocrystalline states. But there exists a slight increase in the value of β for the alloy annealed at 823 K (the nanocrystalline state) as observed in most of the amorphous alloys. 相似文献