全文获取类型
收费全文 | 232873篇 |
免费 | 2248篇 |
国内免费 | 801篇 |
专业分类
化学 | 116896篇 |
晶体学 | 3999篇 |
力学 | 10764篇 |
综合类 | 1篇 |
数学 | 25511篇 |
物理学 | 78751篇 |
出版年
2020年 | 1790篇 |
2019年 | 2047篇 |
2018年 | 2720篇 |
2017年 | 2651篇 |
2016年 | 3919篇 |
2015年 | 2389篇 |
2014年 | 3969篇 |
2013年 | 10079篇 |
2012年 | 7539篇 |
2011年 | 9396篇 |
2010年 | 6687篇 |
2009年 | 6629篇 |
2008年 | 8702篇 |
2007年 | 8740篇 |
2006年 | 8280篇 |
2005年 | 7501篇 |
2004年 | 6923篇 |
2003年 | 6114篇 |
2002年 | 6061篇 |
2001年 | 7093篇 |
2000年 | 5374篇 |
1999年 | 4236篇 |
1998年 | 3563篇 |
1997年 | 3472篇 |
1996年 | 3296篇 |
1995年 | 3009篇 |
1994年 | 2952篇 |
1993年 | 3048篇 |
1992年 | 3152篇 |
1991年 | 3333篇 |
1990年 | 3087篇 |
1989年 | 3027篇 |
1988年 | 3068篇 |
1987年 | 2955篇 |
1986年 | 2800篇 |
1985年 | 3809篇 |
1984年 | 4006篇 |
1983年 | 3198篇 |
1982年 | 3410篇 |
1981年 | 3339篇 |
1980年 | 3234篇 |
1979年 | 3335篇 |
1978年 | 3424篇 |
1977年 | 3351篇 |
1976年 | 3448篇 |
1975年 | 3141篇 |
1974年 | 3150篇 |
1973年 | 3195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 15 毫秒
951.
Simultaneous DSC-TG and DTA-TG were used to investigate the calatytic effect of the metal on the thermal decomposition of
a cellulose matrix containing small copper particles. The techniques were also used to demonstrate the effect of the metal
particles on the subsequent activation of the carbon matrix, a process which develops the pore structure necessary to expose
the metal particles to the gas phase. Temperature programmed desorption was used to study the initial mass loss found on activation.
To quantify the catalytic effect of the copper particles on the activation process an estimate was made of the activation
energy of the catalysed and uncatalysed reactions. The work gives valuable information on the processes involved in the preparation
of a new range of metal-carbon catalysts.
In celebration of the 60th birthday of Dr. Andrew K. Galwey 相似文献
952.
P. N. Timonin 《JETP Letters》2002,76(1):37-39
In terms of the isotropic elastic crystal model, it is shown that the formation of planar layers consisting of edge dislocation pileups is advantageous in energy if the shear modulus of the crystal is far lower than the bulk modulus. As pressure rises, the dislocation radius decreases, which can destroy the crystal structure. 相似文献
953.
954.
Christopher J. Booth John W. Goodby Judith P. Hardy Olwen C. Lettington Kenneth J. Toyne 《Liquid crystals》1994,16(6):925-940
The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense. 相似文献
955.
In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δεi, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique. 相似文献
956.
Oxyethylene macrocyclic compounds (crown-ethers) act as ligands of intracrystalline cations of certain layered silicates as montmorillonites. Stable intercalation materials are formed which are used to prepare organic-inorganic membranes by encapsulating these intercalation compounds with a poly-butadiene thin coating. Electrochemical Impedance Spectroscopy (EIS) is used to study the resulting composite membranes in contact with aqueous electrolytes. From the impedance plots, the ionic resistance of the membranes is obtained. The thickness of the polybutadiene coating is an important factor determining the ability of ions to pass across the membrane. Marked differences in the ionic resistance are observed as a function of the nature of the interlayer macrocyclic compound. For non-intercalated montmorillonite membranes, the ionic resistance is strongly reduced, whereas for some crown-ether intercalated materials such as 18-crown-6 and dibenzo 24-crown-8, iono-selective membranes are obtained. Concerning the nature of the electrolyte, cations exhibiting greater hydration energies show higher difficulties to pass through the membrane and, consequently, the ionic resistance increases. 相似文献
957.
958.
959.
P. G. Hjorth S. L. Kokkendorff S. Markvorsen 《Proceedings of the American Mathematical Society》2002,130(1):175-181
We study finite metric spaces with elements picked from, and distances consistent with, ambient Riemannian manifolds. The concepts of negative type and strictly negative type are reviewed, and the conjecture that hyperbolic spaces are of strictly negative type is settled, in the affirmative. The technique of the proof is subsequently applied to show that every compact manifold of negative type must have trivial fundamental group, and to obtain a necessary criterion for product manifolds to be of negative type.
960.
David P. Williamson 《Mathematical Programming》2002,91(3):447-478
In this survey, we give an overview of a technique used to design and analyze algorithms that provide approximate solutions
to NP-hard problems in combinatorial optimization. Because of parallels with the primal-dual method commonly used in combinatorial
optimization, we call it the primal-dual method for approximation algorithms. We show how this technique can be used to derive
approximation algorithms for a number of different problems, including network design problems, feedback vertex set problems,
and facility location problems.
Received: June 19, 2000 / Accepted: February 7, 2001?Published online October 2, 2001 相似文献