全文获取类型
收费全文 | 28584篇 |
免费 | 701篇 |
国内免费 | 232篇 |
专业分类
化学 | 19366篇 |
晶体学 | 128篇 |
力学 | 649篇 |
综合类 | 1篇 |
数学 | 5433篇 |
物理学 | 3940篇 |
出版年
2022年 | 183篇 |
2021年 | 278篇 |
2020年 | 350篇 |
2019年 | 313篇 |
2018年 | 277篇 |
2017年 | 293篇 |
2016年 | 601篇 |
2015年 | 627篇 |
2014年 | 660篇 |
2013年 | 1412篇 |
2012年 | 1409篇 |
2011年 | 1710篇 |
2010年 | 958篇 |
2009年 | 819篇 |
2008年 | 1467篇 |
2007年 | 1470篇 |
2006年 | 1556篇 |
2005年 | 1395篇 |
2004年 | 1330篇 |
2003年 | 1101篇 |
2002年 | 1049篇 |
2001年 | 454篇 |
2000年 | 409篇 |
1999年 | 396篇 |
1998年 | 388篇 |
1997年 | 435篇 |
1996年 | 401篇 |
1995年 | 363篇 |
1994年 | 393篇 |
1993年 | 345篇 |
1992年 | 321篇 |
1991年 | 321篇 |
1990年 | 280篇 |
1989年 | 281篇 |
1988年 | 266篇 |
1987年 | 220篇 |
1986年 | 247篇 |
1985年 | 365篇 |
1984年 | 372篇 |
1983年 | 317篇 |
1982年 | 352篇 |
1981年 | 323篇 |
1980年 | 338篇 |
1979年 | 272篇 |
1978年 | 303篇 |
1977年 | 281篇 |
1976年 | 224篇 |
1975年 | 195篇 |
1974年 | 199篇 |
1973年 | 171篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Gamil A. Guirgis Mohammad A. Qtaitat Y. A. Pentin C. M. Whang J. F. Sullivan J. R. Durig Peter Klaeboe 《Fresenius' Journal of Analytical Chemistry》1995,352(5):499-507
The Raman (50 to 3200 cm–1) and infrared (50 to 3500 cm–1) spectra of chlorodimethylmethoxysilane, Cl(CH3)2SiOCH3, in the vapor and solid phases have been recorded. Raman spectra of the liquid including depolarization ratios have also been recorded. Optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing the RHF/3–21G* and RHF/6–31G* basis sets. The calculations from both of these basis sets indicated the gauche conformer to be significantly more stable than the trans conformer. Since the gauche has twice the multiplicity of the trans form it is unlikely that the trans conformer will be detected in the fluid phases at room temperature. This is supported by the fact that no infrared or Raman bands were found to vanish in the spectra of the crystalline solid. The vibrational frequencies have been calculated using appropriate scaling factors, and the vibrational spectra are interpreted in detail. The results have been compared with those obtained for some related molecules.Dedicated to Professor Dr. H. Kriegsmann on the occasion of his 70th birthdayFor part XX, see J Raman Spectrosc 26:in press (1995)Analytical R/D Department, Organic Products Division, Miles Inc., Bushy Park Plant. Charleston, SC 9411, USAChemistry Department, Mu'tah University, P.O.Box 7, Mu'tah-Karak, JordanDepartment of Chemistry, Moscow State University, Moscow, B-234, RussiaDepartment of Ceramic Engineering, Inha University, Nam-Ku, Incheon 160, KoreaDepartment of Chemistry, University of Oslo, P.O.Box 1033, 0315 Oslo, Norway 相似文献
992.
Neil M. Atherton Peter H. Blustin Ceris A. Humphreys Ahmed S. Shalabi 《Magnetic resonance in chemistry : MRC》1984,22(7):456-458
The rates of the exchange of sodium ions between sodium tetraphenylborate and sodium–nitrobenzene or sodium–p-nitrobenzonitrile were measured from analysis of the line broadening effect on the ESR spectra off the paramagnetic ion pairs. The rate is significantly lower for the substituted nitro compound, and it is suggested that this reflects a difference in mechanism for the two systems. 相似文献
993.
Affine-Invariant Distances, Envelopes and Symmetry Sets 总被引:3,自引:0,他引:3
Affine invariant symmetry sets of planar curves are introduced and studied in this paper. Two different approaches are investigated. The first one is based on affine invariant distances, and defines the symmetry set as the closure of the locus of points on (at least) two affine normals and affine-equidistant from the corresponding points on the curve. The second approach is based on affine bitangent conics. In this case the symmetry set is defined as the closure of the locus of centers of conics with (at least) 3-point contact with the curve at two or more distinct points on the curve. This is equivalent to conic and curve having, at those points, the same affine tangent, or the same Euclidean tangent and curvature. Although the two analogous definitions for the classical Euclidean symmetry set are equivalent, this is not the case for the affine group. We present a number of properties of both affine symmetry sets, showing their similarities with and differences from the Euclidean case. We conclude the paper with a discussion of possible extensions to higher dimensions and other transformation groups, as well as to invariant Voronoi diagrams. 相似文献
994.
We derive for Hecke-Maass cusp forms on the full modular group a relation between the sum of the form at Heegner points (and
integrals over Heegner cycles) and the product of two Fourier coefficients of a corresponding form of half-integral weight.
Specializing to certain cycles we obtain the nonnegativity of theL-function of such a form at the center of the critical strip. These results generalize similar formulae known for holomorphic
forms.
Partially supported by NSF grant # DMS-9096262.
Partially supported by NSF grant # DMS-9102082. 相似文献
995.
996.
We describe an adaptive mesh refinement finite element method-of-lines procedure for solving one-dimensional parabolic partial differential equations. Solutions are calculated using Galerkin's method with a piecewise hierarchical polynomial basis in space and singly implicit Runge-Kutta (SIRK) methods in time. A modified SIRK formulation eliminates a linear systems solution that is required by the traditional SIRK formulation and leads to a new reduced-order interpolation formula. Stability and temporal error estimation techniques allow acceptance of approximate solutions at intermediate stages, yielding increased efficiency when solving partial differential equations. A priori energy estimates of the local discretization error are obtained for a nonlinear scalar problem. A posteriori estimates of local spatial discretization errors, obtained by order variation, are used with the a priori error estimates to control the adaptive mesh refinement strategy. Computational results suggest convergence of the a posteriori error estimate to the exact discretization error and verify the utility of the adaptive technique.This research was partially supported by the U.S. Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR-90-0194; the U.S. Army Research Office under Contract Number DAAL 03-91-G-0215; by the National Science Foundation under Grant Number CDA-8805910; and by a grant from the Committee on Research, Tulane University. 相似文献
997.
With the help of drug design, 8-, 9-, 10- and 11-methylacronycines 24, 25, 26 and 27 have been selected and synthesised to improve the cytostatic potency of acronycine ( 31 ). The condensation of phloroglucinol with 6-methylanthranilic acid gave 7-hydroxy-1,9-dimethyldibenzo[b,j][1,7]phenanthroline-8,14(5H,13H)-dione ( 32 ) as the main product. 相似文献
998.
We construct a family of infinite Markov systems on [?1, 1] with the property that the rational functions from these systems are not dense inC[?1, 1]. This gives counterexamples to a long-standing conjecture of D. J. Newman. 相似文献
999.
Alkyl 2-acyloxy α-D-erythro-hex-2-enopyranoside diesters, readily available from D-hexoses, are versatile intermediates fro the expeditious preparation of six-carbon chiral synthons containing alternating and/or consecutive C-methyl and hydroxyl groups. 相似文献
1000.
The syntheses of p-terphenylphosphonic acid and p,p″-terphenyl-bis-phosphonic acid have been developed, and the resulting products were used to prepare layered Th(IV) and Zr(IV) phosphonates. The monoacidic terphenyl species formed a bilayered structure while the diacid created a pillared geometry. Mixed component phases were also produced. Synthesis, characterization, X-ray and thermal properties of these materials are discussed. 相似文献