全文获取类型
收费全文 | 66463篇 |
免费 | 11664篇 |
国内免费 | 2902篇 |
专业分类
化学 | 64886篇 |
晶体学 | 992篇 |
力学 | 1566篇 |
综合类 | 31篇 |
数学 | 8234篇 |
物理学 | 5320篇 |
出版年
2024年 | 16篇 |
2023年 | 110篇 |
2022年 | 156篇 |
2021年 | 354篇 |
2020年 | 764篇 |
2019年 | 2581篇 |
2018年 | 2406篇 |
2017年 | 2832篇 |
2016年 | 3128篇 |
2015年 | 5434篇 |
2014年 | 5188篇 |
2013年 | 7273篇 |
2012年 | 5806篇 |
2011年 | 5411篇 |
2010年 | 4498篇 |
2009年 | 4346篇 |
2008年 | 4747篇 |
2007年 | 3999篇 |
2006年 | 3647篇 |
2005年 | 3499篇 |
2004年 | 2943篇 |
2003年 | 2660篇 |
2002年 | 3323篇 |
2001年 | 1727篇 |
2000年 | 1584篇 |
1999年 | 782篇 |
1998年 | 246篇 |
1997年 | 200篇 |
1996年 | 159篇 |
1995年 | 130篇 |
1994年 | 133篇 |
1993年 | 100篇 |
1992年 | 93篇 |
1991年 | 73篇 |
1990年 | 76篇 |
1989年 | 85篇 |
1988年 | 60篇 |
1987年 | 44篇 |
1986年 | 43篇 |
1985年 | 52篇 |
1984年 | 46篇 |
1983年 | 33篇 |
1982年 | 34篇 |
1981年 | 33篇 |
1980年 | 35篇 |
1979年 | 30篇 |
1978年 | 20篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1971年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
G. Montero L. Gonzlez E. Flrez M. D. García A. Surez 《Numerical Linear Algebra with Applications》2002,9(3):239-247
Parallel preconditioners are presented for the solution of general linear systems of equations. The computation of these preconditioners is achieved by orthogonal projections related to the Frobenius inner product. So, minM∈??∥AM?I∥ F and matrix M0∈?? corresponding to this minimum (?? being any vectorial subspace of ??n(?)) are explicitly computed using accumulative formulae in order to reduce computational cost when subspace ?? is extended to another one containing it. Every step, the computation is carried out taking advantage of the previous one, what considerably reduces the amount of work. These general results are illustrated with the subspace of matrices M such that AM is symmetric. The main application is developed for the subspace of matrices with a given sparsity pattern which may be constructed iteratively by augmenting the set of non‐zero entries in each column. Finally, the effectiveness of the sparse preconditioners is illustrated with some numerical experiments. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
72.
73.
74.
75.
76.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献
77.
A Volterra type integral equation in a Hilbert space with an additional linear operator L and a spectral parameter depending
on time is considered. If the parameter does not belong to the spectrum of L unconditional solvability of the considered problem
is proved. In the case where the initial value of the parameter coincides with some isolated point of the spectrum of the
operator L sufficient conditions for solvability are established. The obtained results are applied to the partial integral
equations associated with a contact problem of the theory of elasticity. 相似文献
78.
M.F. Al-Kuhaili 《Journal of luminescence》2006,117(2):209-216
Thin films of sodium salicylate were deposited by spin coating from a solution prepared by dissolving sodium salicylate in methanol. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, UV-visible spectroscopy and spectrofluorometry. The films were crystalline with a strong blue fluorescence peak at an emission wavelength of 419.3 nm. The influence of solution concentration, spin speed and annealing temperature on the fluorescence intensity was studied. Optimum results were obtained with a solution whose concentration was 2.5 M. It was found that lower spin speed resulted in higher fluorescence intensity. Moreover, the fluorescence intensity decreased as the annealing temperature was increased. The durability of the films over a period of 30 days was also investigated, and films annealed at higher temperatures were found to be less degraded with time. 相似文献
79.
Let G=(V(G),E(G)) be a graph. A (n,G, λ)‐GD is a partition of the edges of λKn into subgraphs (G‐blocks), each of which is isomorphic to G. The (n,G,λ)‐GD is named as graph design for G or G‐decomposition. The large set of (n,G,λ)‐GD is denoted by (n,G,λ)‐LGD. In this work, we obtain the existence spectrum of (n,P3,λ)‐LGD. © 2002 Wiley Periodicals, Inc. J Combin Designs 10: 151–159, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jcd.10008 相似文献
80.
Ludmila C. Fidale Sarah Köhler Martin H.G. Prechtl Thomas Heinze Omar A. El Seoud 《Cellulose (London, England)》2006,13(5):581-592
The concentrations of water, W, and electrolytes present in solutions of LiCl in N,N-dimethylacetamide, LiCl/DMAc, and of tetrabutylammonium fluoride. x-hydrate in DMSO, TBAF.xW/DMSO can be accurately and expediently determined by three independent methods, UV–vis, FTIR and EMF measurement. The first relies on the use of solvatochromic probes whose spectra are sensitive to solution composition. It is applicable to W/LiCl/DMAc solutions but not to TBAF.xW/DMSO, because the charge-transfer complex bands of the probes are suppressed by strong interactions with the latter electrolyte. Integration of νOH band of water may be employed in order to determine [W], hence [electrolyte] by weight difference. EMF measurement uses ion-selective electrodes in order to determine [electrolyte], hence [W] by weight difference. Results of the latter method were in excellent agreement with those of FTIR. The reason for the failure of Karl Fischer titration is addressed, and the relevance of the results obtained to functionalization of cellulose under homogenous solution conditions is briefly commented on. 相似文献