全文获取类型
收费全文 | 22920篇 |
免费 | 832篇 |
国内免费 | 402篇 |
专业分类
化学 | 14860篇 |
晶体学 | 133篇 |
力学 | 631篇 |
综合类 | 30篇 |
数学 | 3183篇 |
物理学 | 5317篇 |
出版年
2022年 | 219篇 |
2021年 | 442篇 |
2020年 | 358篇 |
2019年 | 358篇 |
2018年 | 350篇 |
2017年 | 273篇 |
2016年 | 661篇 |
2015年 | 581篇 |
2014年 | 678篇 |
2013年 | 1290篇 |
2012年 | 1165篇 |
2011年 | 1365篇 |
2010年 | 938篇 |
2009年 | 826篇 |
2008年 | 1223篇 |
2007年 | 1220篇 |
2006年 | 1085篇 |
2005年 | 1019篇 |
2004年 | 936篇 |
2003年 | 873篇 |
2002年 | 862篇 |
2001年 | 562篇 |
2000年 | 518篇 |
1999年 | 385篇 |
1998年 | 311篇 |
1997年 | 276篇 |
1996年 | 309篇 |
1995年 | 289篇 |
1994年 | 286篇 |
1993年 | 246篇 |
1992年 | 238篇 |
1991年 | 227篇 |
1990年 | 224篇 |
1989年 | 183篇 |
1988年 | 158篇 |
1987年 | 158篇 |
1986年 | 146篇 |
1985年 | 252篇 |
1984年 | 279篇 |
1983年 | 192篇 |
1982年 | 212篇 |
1981年 | 180篇 |
1980年 | 183篇 |
1979年 | 166篇 |
1978年 | 141篇 |
1977年 | 186篇 |
1976年 | 163篇 |
1975年 | 131篇 |
1974年 | 122篇 |
1973年 | 132篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Let Ψ(x,y) (resp. Ψm(x,y)) denote the number of integers not exceeding x that are y-friable, i.e. have no prime factor exceeding y (resp. and are coprime to m). Evaluating the ratio Ψm(x/d,y)/Ψ(x,y) for 1≤slantd≤slantx, m≥slant 1, x≥slant y≥slant 2, turns out to be a crucial step for estimating arithmetic sums over friable integers. Here, it is crucial to obtain formulae with a very wide range of validity. In this paper, several uniform estimates are provided for the aforementioned ratio, which supersede all previously known results. Applications are given to averages of various arithmetic functions over friable integers which in turn improve corresponding results from the literature. The technique employed rests mainly on the saddle-point method, which is an efficient and specific tool for the required design.2000 Mathematics Subject Classification: Primary—11N25; Secondary—11K65, 11N37 相似文献
32.
Journal d'Analyse Mathématique - 相似文献
33.
We investigate heteroclinic orbits between equilibria and rotating waves for scalar semilinear parabolic reaction-advection-diffusion equations with periodic boundary conditions. Using zero number properties of the solutions and the phase shift equivariance of the equation, we establish a necessary and sufficient condition for the existence of a heteroclinic connection between any pair of hyperbolic equilibria or rotating waves. 相似文献
34.
Michael De Castro 《Tetrahedron letters》2004,45(34):6501-6504
2-Deoxy-2-iodo-glycosylamides have been prepared from a variety of protected d-glucals by their reaction with N-iodosuccinimide and amides. Benzyl protected 2-iodoamides, when treated with sodium hydride and 15-crown-5, gave stable C1 N-linked 2-glycooxazolines as the major products. Silyl protected 2-iodoamides afforded the C1 O-linked 2-glycooxazolines; presumably by the rearrangement of unstable N-acylaziridine intermediates. 相似文献
35.
Ray Leslie Withers Carlos Otero-Diaz Adrian Gómez-Herrero Albert Prodan Lasse Norén 《Journal of solid state chemistry》2005,178(10):3159-3168
Ternary derivatives of 1T-TaS2 have been synthesized and the variation in the highly structured diffuse intensity distributions characteristic of such materials carefully monitored to investigate the effect that such substitution has upon the band structures and Fermi surfaces (FSs) of the materials. Removal of d electrons via the replacement of Ta ions with lower valent transition metal ions leads to a systematic increase in the radii of the characteristic structured diffuse intensity distribution. Extended Hückel tight binding calculations of the FSs of the doped samples are carried out and used to predict possible nesting wave-vectors. The results are in reasonably good agreement with the radii of the experimentally observed diffuse intensity distributions. 相似文献
36.
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
37.
W M Niessen M A de Kraa E R Verheij P J Bergers G F La Vos U R Tjaden J van der Greef 《Rapid communications in mass spectrometry : RCM》1989,3(1):1-4
On-line sample pretreatment by means of the phase-system switching approach is an interesting technique for the analysis of aqueous samples, e.g., plasma, by means of supercritical-fluid chromatography. In order to analyse plasma samples the following analytical procedure is used. The plasma sample is injected on to a short precolumn, which is washed with water and subsequently dried with nitrogen. Next, the solutes are desorbed with the supercritical mobile phase, analysed with packed-column supercritical-fluid chromatography and detected with either a UV detector or a mass spectrometer, equipped with a moving-belt interface. The herbicide diuron is selected as a test compound to study the feasibility of this approach. Using a selective detector the procedure is sufficiently sensitive to detect diuron in plasma, but not appropriate to detect the diuron metabolites in a post-mortem plasma sample. These have been identified with liquid chromatography/mass spectrometry. The detection limit of diuron in plasma using the procedure described is about 30 ng/mL. 相似文献
38.
39.
The problem of finding the Euclidean distance between two convex polyhedra can be reduced to the combinatorial optimization problem of finding the minimum distance between their faces. This paper presents a global optimality criterion for this problem. An algorithm (QLDPA) for the fast computation of the distance between convex and bounded polyhedra is proposed as an application of it. Computer experiments show its fast performance, especially when the total number of vertices is large. 相似文献
40.
The basic features of a chemical measurement 总被引:2,自引:0,他引:2