全文获取类型
收费全文 | 314109篇 |
免费 | 4433篇 |
国内免费 | 901篇 |
专业分类
化学 | 167220篇 |
晶体学 | 4379篇 |
力学 | 12972篇 |
综合类 | 2篇 |
数学 | 40630篇 |
物理学 | 94240篇 |
出版年
2021年 | 2544篇 |
2020年 | 2967篇 |
2019年 | 3234篇 |
2018年 | 4347篇 |
2017年 | 4071篇 |
2016年 | 6629篇 |
2015年 | 4730篇 |
2014年 | 6297篇 |
2013年 | 15135篇 |
2012年 | 12801篇 |
2011年 | 14705篇 |
2010年 | 9991篇 |
2009年 | 9430篇 |
2008年 | 13258篇 |
2007年 | 13139篇 |
2006年 | 12230篇 |
2005年 | 11284篇 |
2004年 | 10174篇 |
2003年 | 8753篇 |
2002年 | 8426篇 |
2001年 | 8435篇 |
2000年 | 6635篇 |
1999年 | 5138篇 |
1998年 | 4309篇 |
1997年 | 4132篇 |
1996年 | 4191篇 |
1995年 | 3634篇 |
1994年 | 3686篇 |
1993年 | 3731篇 |
1992年 | 3879篇 |
1991年 | 3928篇 |
1990年 | 3725篇 |
1989年 | 3594篇 |
1988年 | 3589篇 |
1987年 | 3441篇 |
1986年 | 3238篇 |
1985年 | 4474篇 |
1984年 | 4628篇 |
1983年 | 3678篇 |
1982年 | 3966篇 |
1981年 | 3850篇 |
1980年 | 3689篇 |
1979年 | 3780篇 |
1978年 | 3888篇 |
1977年 | 3756篇 |
1976年 | 3890篇 |
1975年 | 3511篇 |
1974年 | 3559篇 |
1973年 | 3602篇 |
1972年 | 2455篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
D. P. Kouzov 《Journal of Mathematical Sciences》2002,108(5):721-735
A condition for regularization of boundary-contact integrals in two-dimensional problems of acoustics is considered. With an example of the simplest boundary-contact problem of acoustics, it is shown that a known sufficient condition for regularization is also a necessary one. The study of analytic properties of the transition admittance of an infinite plate that is in contact with a medium is conducted in order to obtain this statement. The considerations are carried out in general terms. The results obtained may be used in arbitrary models of plate theory, and they also may be adapted to boundary-contact problems of another physical nature. Bibliography: 9 titles. 相似文献
102.
103.
We discuss a mass change that has its origin in the action of forces on an object. This phenomenon, well-known in the context of quantum field theory (mass renormalization), can be discussed systematically in both classical and quantum mechanics in a framework given by Stückelberg. We employ this framework to resolve an interesting conflict of opinions between Schopenhauer and Hegel in the mid-19th century. We show that Hegel, Kant, and Schopenhauer demonstrated remarkable prescience in their views as seen from a modern perspective. 相似文献
104.
The results of experimental investigations on thermal nonequilibrium ionization in CO2: N2: He mixtures are presented. Measurements of electron density, ne, in vibrationally excited nitrogen were made in a supersonic flow with different CO2 contents as well as in a CO2: N2: He = 1 : 5 : 4 mixture laser gas. The mixtures were heated in a shock tube and expanded through a supersonic nozzle. Furthermore, supersonic mixing of N2 and CO2 + He was used in some experiments. The measured values of ne in the plenum chamber and in the supersonic nozzle are reported, and the processes responsible for nonequilibrium ionization in a laser-active medium are discussed. 相似文献
105.
106.
P. R. Latthe P. S. Shinge Bharati V. Badami P. B. Patil S. N. Holihosur 《Journal of Chemical Sciences》2006,118(3):249-256
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been
found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e). 相似文献
107.
Dikandé A. M. 《The European Physical Journal B - Condensed Matter and Complex Systems》2004,42(2):247-253
Small and large-amplitude elastic deformations of the armchair structure of single-walled carbon nanotubes are investigated with emphasis on the cylindrical geometry. As starting model, we consider a discrete one-dimensional lattice of atoms interacting via a Lennard-Jones type two-body potential. In an expansion scheme using cylindrical coordinates where radial displacements are assumed negligible compared to the angular motions, a sine-lattice Hamiltonian is derived. In the limit of small-amplitude angular displacements, the dispersion spectrum of acoustic phonons is derived and the associate characteristic frequency is given as a function of parameters of the model. In the large-amplitude regime, lattice vibrations give rise to kink-type deformations which move undergoing lattice dispersion and lattice discreteness effects. The dispersion law of the kink motion is obtained and shown to lower the effect of lattice discreteness, giving rise to a vanishing Peierls stress for kink sizes of the order of a few lattice spacings. Implications of the coupling of two armchair structures on the stability of vibrational modes of an individual armchair nanotube are also discussed. A gap of forbidden modes is predicted in the phonon spectrum while the energy needed to create a kink deformation in individual nanotubes is shifted in the presence of a wall-to-wall interaction.Received: 2 August 2004, Published online: 14 December 2004PACS:
81.07.De Nanotubes - 62.30. + d Mechanical and elastic waves-vibrations - 63.22. + m Phonons in low-dimensional nanoscale materials - 63.20.Ry Anharmonic lattices modes 相似文献
108.
G.P. Romanelli D. Bennardi D.M. Ruiz G. Baronetti H.J. Thomas 《Tetrahedron letters》2004,45(48):8935-8939
Substituted coumarins are synthesized from phenols and β-ketoesters by the Pechmann reaction, using a Wells-Dawson heteropolyacid (H6P2W18O62·24H2O) as catalyst by a solvent-free procedure. This one requires low reaction times, 130 °C temperature and as little as 1 mol % of Wells-Dawson acid, obtaining good to excellent yields of coumarins. The catalyst showed to be reusable with no differences in the yields. The results are compared with those of the reactions performed in toluene solution. The presented synthetic procedure is a convenient, clean and fast alternative for synthesizing 4-substituted coumarins (17 examples). 相似文献
109.
Sauer, Shelah, Vapnik and Chervonenkis proved that if a set system on n vertices contains many sets, then the set system has full trace on a large set. Although the restriction on the size of the
groundset cannot be lifted, Frankl and Pach found a trace structure that is guaranteed to occur in uniform set systems even
if we do not bound the size of the groundset. In this note we shall give three sequences of structures such that every set
system consisting of sufficiently many sets contains at least one of these structures with many sets. 相似文献
110.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures. 相似文献