全文获取类型
收费全文 | 396940篇 |
免费 | 4486篇 |
国内免费 | 1786篇 |
专业分类
化学 | 219403篇 |
晶体学 | 6440篇 |
力学 | 16193篇 |
综合类 | 72篇 |
数学 | 42285篇 |
物理学 | 118819篇 |
出版年
2020年 | 3277篇 |
2019年 | 3594篇 |
2018年 | 4440篇 |
2017年 | 4563篇 |
2016年 | 6732篇 |
2015年 | 4370篇 |
2014年 | 6675篇 |
2013年 | 16985篇 |
2012年 | 13351篇 |
2011年 | 16293篇 |
2010年 | 11556篇 |
2009年 | 11419篇 |
2008年 | 14981篇 |
2007年 | 14956篇 |
2006年 | 14033篇 |
2005年 | 12756篇 |
2004年 | 11504篇 |
2003年 | 10369篇 |
2002年 | 10075篇 |
2001年 | 11826篇 |
2000年 | 9033篇 |
1999年 | 6738篇 |
1998年 | 5428篇 |
1997年 | 5355篇 |
1996年 | 5290篇 |
1995年 | 4763篇 |
1994年 | 4763篇 |
1993年 | 4507篇 |
1992年 | 5317篇 |
1991年 | 5283篇 |
1990年 | 5042篇 |
1989年 | 5012篇 |
1988年 | 4947篇 |
1987年 | 4899篇 |
1986年 | 4761篇 |
1985年 | 6221篇 |
1984年 | 6319篇 |
1983年 | 5125篇 |
1982年 | 5408篇 |
1981年 | 5241篇 |
1980年 | 4832篇 |
1979年 | 5336篇 |
1978年 | 5495篇 |
1977年 | 5635篇 |
1976年 | 5480篇 |
1975年 | 5013篇 |
1974年 | 5066篇 |
1973年 | 5089篇 |
1972年 | 3770篇 |
1968年 | 3222篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
V. K. Malyutenko A. I. Liptuga V. A. Botte V. I. Chernyakhovskii 《Journal of Applied Spectroscopy》1991,55(4):1024-1028
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 4, pp. 624–629, October, 1991. 相似文献
72.
Summary Considered here are model equations for weakly nonlinear and dispersive long waves, which feature general forms of dispersion and pure power nonlinearity. Two variants of such equations are introduced, one of Korteweg-de Vries type and one of regularized long-wave type. It is proven that solutions of the pure initial-value problem for these two types of model equations are the same, to within the order of accuracy attributable to either, on the long time scale during which nonlinear and dispersive effects may accumulate to make an order-one relative difference to the wave profiles.This research was supported in part by the National Science Foundation. A considerable portion of the project was completed while the first author was resident at the Institute for Mathematics and Its Applications, University of Minnesota. 相似文献
73.
74.
75.
When solving large complex optimization problems, the user is faced with three major problems. These are (i) the cost in human time in obtaining accurate expressions for the derivatives involved; (ii) the need to store second derivative information; and (iii), of lessening importance, the time taken to solve the problem on the computer. For many problems, a significant part of the latter can be attributed to solving Newton-like equations. In the algorithm described, the equations are solved using a conjugate direction method that only needs the Hessian at the current point when it is multiplied by a trial vector. In this paper, we present a method that finds this product using automatic differentiation while only requiring vector storage. The method takes advantage of any sparsity in the Hessian matrix and computes exact derivatives. It avoids the complexity of symbolic differentiation, the inaccuracy of numerical differentiation, the labor of finding analytic derivatives, and the need for matrix store. When far from a minimum, an accurate solution to the Newton equations is not justified, so an approximate solution is obtained by using a version of Dembo and Steihaug's truncated Newton algorithm (Ref. 1).This paper was presented at the SIAM National Meeting, Boston, Massachusetts, 1986. 相似文献
76.
Partially supported by the general research fund at the University of Kansas 相似文献
77.
The acoustic field inside a shell excited by a spatially inhomogeneous harmonic pressure field is studied. The shell is assumed
to have a finite length, a set of orthogonal stiffening ribs, two ends bounding the acoustic volume, and a sound-insulating
structure, which includes layers of sound-insulating material, resonant elements, and an interior panel. The shell is considered
to be orthotropic with boundary conditions corresponding to a free support. For the acoustic field in the closed volume, analytical
expressions are derived with allowance for the elastoacoustic interaction of the shell with the sound-insulating layers and
with the medium both inside the shell (with arbitrary impedance values at the ends) and outside it. These expressions are
used to investigate the effect of different types of resonant systems on the sound field inside the shell. 相似文献
78.
The effect of hydrogen on the photoluminescence and planar conductivity of GaAs/InGaAs quantum-well heterostructures with an island Pd layer at the anodically oxidized surface was studied. Unlike continuous deposited Pd layers, island layers do not cause the formation of defects in the GaAs surface region and yet the Pd layer maintains high catalytic activity with respect to hydrogen. It is found that the thermal treatment of such a structure in a hydrogen atmosphere causes atomic-hydrogen passivation of the defects in quantum wells. Studies of the characteristics of planar photoresistors with an island Pd layer acting as hydrogen sensors show that their hydrogen detectivity is approximately two orders of magnitude higher than that of diode structures with continuous Pd layers. 相似文献
79.
Electron mobility has been calculated in a number of binary II–VI compound semiconductors using a displaced Maxwellian distribution function and taking the various scattering mechanisms into consideration at different lattice temperatures and for various amounts of ionized impurity concentrations. It is observed that the low field mobility values can be expressed by a cubic power relationship with lattice temperature and with ionized impurity concentration using a least mean square fit technique with an accuracy better than 5 per cent. Similarly, the field dependence of mobility can also be expressed as a power series of the applied electric field. It is suggested that these equations can be profitably used for a quick estimation of mobility values as a check on experiments and also as sufficiently accurate formulae for simulation and modelling purposes. 相似文献
80.
The spin-1 Ising model, which is equivalent to the three-component lattice gas model, is used to study wetting transitions in three-component surfactant systems consisting of an oil, water, and a nonionic surfactant. Phase equilibria, interfacial profiles, and interfacial tensions for three-phase equilibrium are determined in mean field approximation, for a wide range of temperature and interaction parameters. Surfactant interaction parameters are found to strongly influence interfacial tensions, reducing them in some cases to ultralow values. Interfacial tensions are used to determine whether the middle phase, rich in surfactant, wets or does not wet the interface between the oil-rich and water-rich phases. By varying temperature and interaction parameters, a wetting transition is located and found to be of the first order. Comparison is made with recent experimental results on wetting transitions in ternary surfactant systems.This paper is dedicated to J. K. Percus in honor of his 65th birthday. 相似文献