全文获取类型
收费全文 | 456927篇 |
免费 | 12599篇 |
国内免费 | 6970篇 |
专业分类
化学 | 224382篇 |
晶体学 | 6788篇 |
力学 | 25209篇 |
综合类 | 336篇 |
数学 | 75039篇 |
物理学 | 144742篇 |
出版年
2022年 | 4448篇 |
2021年 | 5337篇 |
2020年 | 5625篇 |
2019年 | 5947篇 |
2018年 | 16589篇 |
2017年 | 16545篇 |
2016年 | 15563篇 |
2015年 | 7604篇 |
2014年 | 10479篇 |
2013年 | 20257篇 |
2012年 | 20774篇 |
2011年 | 29341篇 |
2010年 | 19656篇 |
2009年 | 19794篇 |
2008年 | 22795篇 |
2007年 | 24332篇 |
2006年 | 15166篇 |
2005年 | 14077篇 |
2004年 | 13085篇 |
2003年 | 11997篇 |
2002年 | 10924篇 |
2001年 | 11788篇 |
2000年 | 8963篇 |
1999年 | 7235篇 |
1998年 | 6064篇 |
1997年 | 5812篇 |
1996年 | 5543篇 |
1995年 | 4758篇 |
1994年 | 4569篇 |
1993年 | 4349篇 |
1992年 | 4618篇 |
1991年 | 4805篇 |
1990年 | 4517篇 |
1989年 | 4342篇 |
1988年 | 4040篇 |
1987年 | 4151篇 |
1986年 | 3918篇 |
1985年 | 4834篇 |
1984年 | 4832篇 |
1983年 | 4023篇 |
1982年 | 4087篇 |
1981年 | 3797篇 |
1980年 | 3735篇 |
1979年 | 3991篇 |
1978年 | 3965篇 |
1977年 | 3955篇 |
1976年 | 3924篇 |
1975年 | 3718篇 |
1974年 | 3647篇 |
1973年 | 3696篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
This note introduces a refinement to a previously proposed tabu search algorithm for vehicle routing problems with time windows. This refinement yields new best known solutions on a set of benchmark instances of the multi-depot, the periodic and the site-dependent vehicle routing problems with time windows. 相似文献
942.
943.
Monodisperse polyaniline nanoparticles (PAPSSA) were synthesized from an oxidative dispersion polymerization using poly(sodium 4-styrenesulfonate) (PSSA) as both a polymeric stabilizer and a dopant agent due to its acidity. The nanoparticles were being stabilized with two different molecular weight of PSSA. Size effect of PAPSSA particles were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The d.c. electrical conductivity of composite films on the glass substrate was measured by a four-probe method. It was found that the electrical properties of the composite films are affected by the content of nano-sized polyaniline and different molecular weights of stabilizer in the poly(vinyl alcohol) (PVA) matrix. 相似文献
944.
A method is developed for a stress analysis in an infinite elastic body containing two neighboring periodically cophasaly curved fibers located along two parallel lines. The stress distribution is studied when the body is loaded at infinity by uniformly distributed normal forces in the fiber direction. The investigation is carried out within the framework of a piecewise homogeneous body model with the use of exact three-dimensional equations of elasticity theory. Numerical results related to the stress distribution considered and the influence of interaction between the fibers on this distribution are given. 相似文献
945.
Zhong‐Zhen Yu Mingshu Yang Qingxin Zhang Chungui Zhao Yiu‐Wing Mai 《Journal of Polymer Science.Polymer Physics》2003,41(11):1234-1243
Nylon‐66 nanocomposites were prepared by melt‐compounding nylon‐66 with an alkyl ammonium surfactant pretreated montmorillonite (MMT). The thermal stability of the organic MMT powders was measured by thermogravimetric analysis. The decomposition of the surfactant on the MMT occurred from 200 to 500 °C. The low onset decomposition temperature of the organic MMT is one shortcoming when it is used to prepare polymer nanocomposites at high melt‐compounding temperatures. To provide greater property enhancement and better thermal stability of the polymer/MMT nanocomposites, it is necessary to develop MMT modified with more thermally stable surfactants. The dispersion and spatial distribution of the organic MMT layers in the nylon‐66 matrix were characterized by X‐ray diffraction. The organic MMT layers were exfoliated but not randomly dispersed in the nylon‐66 matrix. A model was proposed to describe the spatial distribution of the organic MMT layers in an injection‐molded rectangular bar of nylon‐66/organic MMT nanocomposites. Most organic MMT layers were oriented in the injection‐molding direction. Layers near the four surfaces of the bar were parallel to their corresponding surfaces; whereas those in the bulk differed from the near‐surface layers and rotated themselves about the injection‐molding direction. The influence of the spatial distribution of the organic MMT on crystallization of nylon‐66 was also investigated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1234–1243, 2003 相似文献
946.
Combining layered positional order as smectic order and chirality can generate complex architectures since twist parallel
to the layers is not allowed. This paper will review some new experimental results on different phases resulting from the
competition between smectic positional order and twist orientational order. It concerns the TGBA and the NL*, that is the liquid line phase as well as the SmQ phase. Chiral effects in the isotropic phase will also be discussed. 相似文献
947.
Salimgareeva V. N. Polevoi R. M. Kolesov S. V. Ostakhov S. S. Ponomareva V. A. Leplyanin G. V. 《Russian Journal of Applied Chemistry》2003,76(5):804-808
The luminescence and scintillation properties of poly(methyl methacrylate) containing 1,1,3-trimethyl-3-phenylindan were studied. 相似文献
948.
S. S. Andreev A. D. Akhsakhalyan M. A. Bibishkin N. I. Chkhalo S. V. Gaponov S. A. Gusev E. B. Kluenkov K. A. Prokhorov N. N. Salashchenko F. Schafers S. Yu. Zuev 《Central European Journal of Physics》2003,1(1):191-209
We present research investigations in the field of multilayer optics in X-ray and extreme ultra-violet ranges (XUV), aimed
at the development of optical elements for applications in experiments in physics and in scientific instrumentation. We discuss
normal incidence multilayer optics in the spectral region of “water window”, multilayer optics for collimation and focusing
of hard X-ray, multilayer dispersing elements for X-ray spectroscopy of high-temperature plasma, multilayer dispersing elements
for analysis of low Z-elements. Our research pays special attention to optimization of multilayer optics for projection EUV-lithography
(ψ-13nm) and short period multilayer optics. 相似文献
949.
Given a map f: X→Y and a Nielsen root class, there is a number associated to this root class, which is the minimal number of points among all
root classes which are H-related to the given one for all homotopies H of the map f. We show that for maps between closed surfaces it is possible to deform f such that all the Nielsen root classes have cardinality equal to the minimal number if and only if either N R[f]≤1, or N R[f]>1 and f satisfies the Wecken property. Here N R[f] denotes the Nielsen root number. The condition “f satisfies the Wecken property is known to be equivalent to |deg(f)|≤N R[f]/(1−χ(M
2)−χ(M
10/(1−χ(M
2)) for maps between closed orientable surfaces. In the case of nonorientable surfaces the condition is A(f)≤N R[f]/(1−χ(M
2)−χ(M
2)/(1−χ(M
2)). Also we construct, for each integer n≥3, an example of a map f: K
n
→N from an n-dimensionally connected complex of dimension n to an n-dimensional manifold such that we cannot deform f in a way that all the Nielsen root classes reach the minimal number of points at the same time. 相似文献
950.
We give an account of the appearance and first developments of the statistical model of atoms proposed by Thomas and Fermi, focusing on the main results achieved by Fermi and his group in Rome. Particular attention is addressed to the unknown contribution to this subject by Majorana, anticipating some important results reached later by leading physicists. 相似文献