全文获取类型
收费全文 | 18643篇 |
免费 | 855篇 |
国内免费 | 55篇 |
专业分类
化学 | 13513篇 |
晶体学 | 114篇 |
力学 | 380篇 |
数学 | 2841篇 |
物理学 | 2705篇 |
出版年
2023年 | 158篇 |
2022年 | 143篇 |
2021年 | 242篇 |
2020年 | 318篇 |
2019年 | 313篇 |
2018年 | 295篇 |
2017年 | 292篇 |
2016年 | 655篇 |
2015年 | 605篇 |
2014年 | 673篇 |
2013年 | 1237篇 |
2012年 | 1422篇 |
2011年 | 1568篇 |
2010年 | 888篇 |
2009年 | 684篇 |
2008年 | 1327篇 |
2007年 | 1289篇 |
2006年 | 1159篇 |
2005年 | 1086篇 |
2004年 | 907篇 |
2003年 | 732篇 |
2002年 | 657篇 |
2001年 | 261篇 |
2000年 | 220篇 |
1999年 | 160篇 |
1998年 | 116篇 |
1997年 | 128篇 |
1996年 | 170篇 |
1995年 | 110篇 |
1994年 | 91篇 |
1993年 | 109篇 |
1992年 | 95篇 |
1991年 | 63篇 |
1990年 | 64篇 |
1989年 | 51篇 |
1988年 | 52篇 |
1987年 | 57篇 |
1986年 | 74篇 |
1985年 | 109篇 |
1984年 | 95篇 |
1983年 | 57篇 |
1982年 | 84篇 |
1981年 | 79篇 |
1980年 | 74篇 |
1979年 | 64篇 |
1978年 | 64篇 |
1977年 | 49篇 |
1976年 | 55篇 |
1974年 | 38篇 |
1973年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Ana Luísa Daniel‐da‐Silva João Carlos Moura Bordado José Miguel Martín‐Martínez 《Journal of Polymer Science.Polymer Physics》2007,45(22):3034-3045
The degree of phase separation in several moisture‐cured poly(urethane urea)s (PUUs) was studied by FTIR spectroscopy, wide angle X‐ray diffraction (WAXD), and temperature‐modulated differential scanning calorimetry (TMDSC). This latter technique was shown to be particularly useful in analysing the degree of phase separation in PUU polymers. Both phase mixing and phase segregation coexisted in the PUUs and the degree of phase separation increased as the urea hard segment (HS) content in the PUU increased. The maximum solubility of urea HSs into the polyol soft segments (SSs) was achieved for 50 wt % urea HS content in diol‐based PUUs, whereas for triol‐based PUUs the highest solubility between HS and SS was reached for lower urea HS amount. Finally, the higher the urea HS content the higher the extent of phase separation in the PUU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3034–3045, 2007 相似文献
22.
André Markoff 《Mathematische Annalen》1886,27(2):177-182
Sans résumé 相似文献
23.
Monika Goikoetxea María J. Barandiaran José M. Asua 《Journal of polymer science. Part A, Polymer chemistry》2007,45(24):5838-5846
The mechanisms involved in the formation of n‐butanol during the synthesis of butyl acrylate containing latices were investigated. The experimental results showed that neither the hydrolysis of butyl acrylate nor of the ester bond in the butyl acrylate segments of the polymer played a major role in the formation of n‐butanol, which was mainly generated from the polymer backbone, by transfer reactions to polymer chain followed by cyclization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5838–5846, 2007 相似文献
24.
Ricaurte Rodríguez Manuel Nogueras Justo Cobo John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(7):o376-o378
Molecules of the title compound, C12H13ClN4, are linked by two independent N—H...N hydrogen bonds into a chain of edge‐fused R22(8) rings. The significance of this study lies in its attempt to rationalize the patterns of supramolecular aggregation in the title compound and in a range of analogous 4,6‐disubstituted 2‐aminopyrimidines. 相似文献
25.
Manuel Cardona 《Solid State Communications》2005,133(1):3-18
Considerable progress has been made in recent years in the field of ab initio calculations of electronic band structures of semiconductors and insulators. The one-electron states (and the concomitant two-particle excitations) have been obtained without adjustable parameters, with a high degree of reliability. Also, more recently, the electron-hole excitation frequencies responsible for optical spectra have been calculated. These calculations, however, are performed with the constituent atoms fixed in their crystallographic positions and thus neglect the effects of the lattice vibrations (i.e. electron-phonon interaction) which can be rather large, even larger than the error bars assumed for ab initio calculations.Effects of electron-phonon interactions on the band structure can be experimentally investigated in detail by measuring the temperature dependence of energy gaps or critical points (van Hove singularities) of the optical excitation spectra. These studies have been complemented in recent years by observing the dependence of such spectra on isotopic mass whenever different stable isotopes of a given atom are available at affordable prices. In crystals composed of different atoms, the effect of the vibration of each separate atom can thus be investigated by isotopic substitution. Because of the zero-point vibrations, such effects are present even at zero temperature (T=0).In this paper, we discuss state-of-the-art calculations of the dielectric function spectra and compare them with experimental results, with emphasis on the differences introduced by the electron-phonon interaction. The temperature dependence of various optical parameters will be described by means of one or two (in a few cases three) Einstein oscillators, except at the lowest temperatures where the T4 law (contrary to the Varshni T2 result) will be shown to apply. Increasing an isotopic mass increases the energy gaps, except in the case of monovalent Cu (e.g. CuCl) and possibly Ag (e.g. AgGaS2). It will be shown that the gaps of tetrahedral materials containing an element of the first row of the periodic table (C,N,O) are strongly affected by the electron-phonon interaction. It will be conjectured that this effect is related to the superconductivity recently observed in heavily boron-doped carbon. 相似文献
26.
Thermodynamic fluctuations in systems that are in nonequilibrium steady states are always spatially long ranged, in contrast to fluctuations in thermodynamic equilibrium. In the present paper we consider a fluid subjected to a stationary temperature gradient. Two different physical mechanisms have been identified by which the temperature gradient causes long-ranged fluctuations. One cause is the presence of couplings between fluctuating fields. Secondly, spatial variation of the strength of random forces, resulting from the local version of the fluctuation-dissipation theorem, has also been shown to generate long-ranged fluctuations. We evaluate the contributions to the long-ranged temperature fluctuations due to both mechanisms. While the inhomogeneously correlated Langevin noise does lead to long-ranged fluctuations, in practice, they turn out to be negligible as compared to nonequilibrium temperature fluctuations resulting from the coupling between temperature and velocity fluctuations. 相似文献
27.
Anatoliy Samoilenko Manuel Pinto Sergei Trofimchuk 《Proceedings of the American Mathematical Society》2005,133(1):145-154
We apply the Krylov and Bogolyubov asymptotic integration procedure to asymptotically autonomous systems. First, we consider linear systems with quasi-periodic coefficient matrix multiplied by a scalar factor vanishing at infinity. Next, we study the asymptotically autonomous Van-der-Pol oscillator.
28.
José Francisco Rodrigues 《Monatshefte für Mathematik》2004,29(2):157-177
This paper presents further developments in the study of strong solutions and their coincidence sets to the obstacle problem for linear transport operators. Under natural mild assumptions, the strong convergence of the solutions and the characteristic functions of their coincidence sets is obtained in the passage of second order to first order problems. An application to a steady-state chemotaxis problem is given. The extension to the two obstacles problem of first order is also presented, having in view an application to a porous media model in presence of saturation arising in petroleum engineering. 相似文献
29.
Máté Matolcsi 《Acta Mathematica Hungarica》2005,108(1-2):129-136
Summary He present work deals with estimations of the n-th linear polarization constant c(H)n of an n-dimensional real Hilbert space H. We provide some new lower bounds on the value of sup║y║=1 │1,y> ... n,y>│, where x1, ... ,xn are unit vectors in H. In particular, the results improve an earlier estimate of Marcus. However, the intriguing conjecture c(H) n= nn/2 remains open. 相似文献
30.
A method employing a wide pore polymeric reversed phase column has been developed for the separation of most of the chlorophylls and related compounds previously described as occurring in marine microalgae. The high selectivity toward molecular shape of this kind of stationary phase has enabled compounds of very similar structure, such as chlorophylls c1, c2 and Mg-divinylpheoporphyrin a5 monomethyl ester, and chlorophyll a and the phytol-substituted chlorophyll c-like pigments, which commonly coelute on monomeric bonded phases, to be resolved in a single run. Some of these pigments, formerly thought to be a single compound, have, in fact, been demonstrated to be groups of two or more. The method has been successfully applied to both algal cultures and natural sea water samples. When visible light absorbance detection was used, the method proved suitable for separation of various carotenoids. 相似文献