全文获取类型
收费全文 | 295790篇 |
免费 | 3014篇 |
国内免费 | 1695篇 |
专业分类
化学 | 145559篇 |
晶体学 | 3971篇 |
力学 | 15440篇 |
综合类 | 61篇 |
数学 | 55381篇 |
物理学 | 80087篇 |
出版年
2020年 | 1754篇 |
2019年 | 1896篇 |
2018年 | 12631篇 |
2017年 | 12532篇 |
2016年 | 9534篇 |
2015年 | 3229篇 |
2014年 | 3805篇 |
2013年 | 9144篇 |
2012年 | 11266篇 |
2011年 | 19478篇 |
2010年 | 12044篇 |
2009年 | 12265篇 |
2008年 | 15087篇 |
2007年 | 17276篇 |
2006年 | 8553篇 |
2005年 | 8923篇 |
2004年 | 8315篇 |
2003年 | 7946篇 |
2002年 | 6937篇 |
2001年 | 6921篇 |
2000年 | 5330篇 |
1999年 | 3799篇 |
1998年 | 3212篇 |
1997年 | 3135篇 |
1996年 | 3203篇 |
1995年 | 2732篇 |
1994年 | 2791篇 |
1993年 | 2673篇 |
1992年 | 2879篇 |
1991年 | 2955篇 |
1990年 | 2752篇 |
1989年 | 2672篇 |
1988年 | 2615篇 |
1987年 | 2552篇 |
1986年 | 2575篇 |
1985年 | 3344篇 |
1984年 | 3401篇 |
1983年 | 2838篇 |
1982年 | 3086篇 |
1981年 | 2847篇 |
1980年 | 2651篇 |
1979年 | 2833篇 |
1978年 | 3030篇 |
1977年 | 3050篇 |
1976年 | 3074篇 |
1975年 | 2809篇 |
1974年 | 2917篇 |
1973年 | 2979篇 |
1972年 | 2334篇 |
1971年 | 1853篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
831.
C. Uranga C. Connell G.M. Borstad L.R. Zink M. Jackson 《Applied physics. B, Lasers and optics》2007,88(4):503-505
A three-laser heterodyne system was used to measure the frequencies of twelve previously observed far-infrared laser emissions
from the partially deuterated methanol isotopologues 13CD3OH and CHD2OH. Two laser emissions, a 53.773 μm line from 13CD3OH and a 74.939 μm line from CHD2OH, have also been discovered and frequency measured. The CO2 pump laser offset frequency was measured with respect to its center frequency for twenty-four FIR laser emissions from CH3OH, 13CD3OH and CHD2OH.
PACS 07.57.Hm; 42.55.Lt; 42.62.Eh 相似文献
832.
Oil migration in filled chocolates limits shelf life due to texture changes and loss of visual appeal. Spatial and temporal
oil concentration changes of a two-layer chocolate and peanut butter filling system, which models a filled chocolate, were
monitored by magnetic resonance imaging. Formulations of chocolate varied with respect to particle size, milk fat content
and emulsifier level. Sorption-time experimental data was used to establish the best fit diffusion coefficient for a Fickian
diffusion model. On the basis of the comparison of spatial experimental data to the model, Fickian diffusion alone does not
account for oil migration.
Authors' address: Michael J. McCarthy, Department of Food Science and Technology, University of California, One Shields Avenue,
Davis, CA 95616, USA 相似文献
833.
We study a small quantum system (e.g., a simplified model for an atom or molecule) interacting with two bosonic or fermionic
reservoirs (say, photon or phonon fields). We show that the combined system has a family of stationary states parametrized
by two numbers, T
1 and T
2 (‘reservoir temperatures’). If T
1 ≠ T
2, then these states are non-equilibrium stationary states (NESS). In the latter case we show that they have nonvanishing heat
fluxes and positive entropy production and are dynamically asymptotically stable. The latter means that the evolution with
an initial condition, normal with respect to any state where the reservoirs are in equilibria at temperatures T
1 and T
2, converges to the corresponding NESS. Our results are valid for the temperatures satisfying the bound min (T
1,T
2) > g
2 + α, where g is the coupling constant and 0 < α < 1 is a power related to the infra-red behaviour of the coupling functions.
Submitted: March 20, 2006. Revised: March 19, 2007. Accepted: May 11, 2007.
Marco Merkli: Partly supported by an NSERC PDF, the Institute of Theoretical Physics of ETH Zürich, Switzerland, the Departments
of Mathematics of McGill University and the University of Toronto, Canada.
Matthias Mück: Supported by DAAD under grant HSP III.
Israel Michael Sigal: Supported by NSERC under grant NA7901. 相似文献
834.
The effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets has been investigated. The magnetic flux loss of two kinds of magnets before and after irradiation was measured. Results show that the effect of γ-ray irradiation on the magnetic properties of sintered NdFeB is not so obvious as that on Fe–Cr–Co magnet. Irradiation-induced damage from γ-ray for the Fe–Cr–Co magnets was characterized for the first time. The decline of permanent magnetic properties of Fe–Cr–Co magnet induced by γ-ray irradiation is reversible except for the maximum energy product (BH)max. The difference of coercivity mechanism between these two kinds of permanent magnets is responsible for the different dependence of magnetic properties loss induced by γ-ray irradiation. 相似文献
835.
Based on observations made during an extensive study of police patrol operations in New York City, we examine the issues of the validity and utility of queueing models of service systems in which adaptive behavior by the (human) customers or servers is likely. We find that in addition to depending on the technical accuracy of its assumptions, the accuracy of such a model will also depend upon the level of managerial control of the system and adequacy of resources. We recommend that queueing models of human service systems be used in a normative fashion and incorporated in the management feedback loop. 相似文献
836.
Michael J. Puls 《Archiv der Mathematik》2007,88(6):500-506
Let p be a real number greater than one. In this paper we study the vanishing and nonvanishing of the first L
p
-cohomology space of some groups that have one end. We also make a connection between the first L
p
-cohomolgy space and the Floyd boundary of the Cayley graph of a group. We apply the result about Floyd boundaries to show
that there exists a real number p such that the first L
p
-cohomology space of a nonelementary hyperbolic group does not vanish.
Received: 4 August 2006 Revised: 2 November 2006 相似文献
837.
This paper is concerned with the spectral analysis of a one-velocity transport operator with Maxwell boundary condition in
L
1-space. After a detailed spectral analysis it is shown that the associated Cauchy problem is governed by a C
0-semigroup. Next, we discuss the irreducibility of the transport semigroup. In particular, we show that the transport semigroup
is irreducible. Finally, a spectral decomposition of the solutions into an asymptotic term and a transient one which will
be estimated for smooth initial data is given. 相似文献
838.
This paper deals with the non-stationary incompressible Navier--Stokes equations for two-dimensional flows expressed in terms of the velocity and pressure and of the vorticity and streamfunction. The equivalence of the two formulations is demonstrated, both formally and rigorously, by virtue of a condition of compatibility between the boundary and initial values of the normal component of velocity. This condition is shown to be the only compatibility condition necessary to allow for solutions of a minimal regularity, namely H1 for the velocity, as in most current numerical schemes relying on spatial discretizations of local type. 相似文献
839.
The use of multigrid methods in complex fluid flow problems is still under development. In this paper a full multigrid procedure has been incorporated in a finite volume solution for predicting fully developed fluid flow in a streamwise periodic geometry. Steady computations in two-dimensional body fitted co-ordinates have shown considerable savings in computation time by this multigrid method. 相似文献
840.
In this paper we examine clathrate formation in the tetraisoamylammonium propionate-water binary system. We have found formation
of four polyhydrates, two of which are metastable over the whole temperature range studied. All polyhydrate crystals were
isolated and their compositions and densities determined; for (i-C5H11)4NC2H5COO·36.5H2O, unit cell parameters were additionally found. The results are compared with data for tetra-n-butylammonium carboxylate
polyhydrates, and the structure of the title compounds is suggested. It is confirmed that the isoamyl radical stabilizes the
tetradecahedral void of the clathrate hydrate framework better than the n-butyl radical.
Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 3, pp. 67–71, May–June, 1994.
Translated by L. Smolina 相似文献