全文获取类型
收费全文 | 18708篇 |
免费 | 2729篇 |
国内免费 | 1949篇 |
专业分类
化学 | 14526篇 |
晶体学 | 178篇 |
力学 | 903篇 |
综合类 | 80篇 |
数学 | 3293篇 |
物理学 | 4406篇 |
出版年
2024年 | 22篇 |
2023年 | 207篇 |
2022年 | 263篇 |
2021年 | 358篇 |
2020年 | 534篇 |
2019年 | 619篇 |
2018年 | 576篇 |
2017年 | 555篇 |
2016年 | 628篇 |
2015年 | 843篇 |
2014年 | 1083篇 |
2013年 | 1709篇 |
2012年 | 1229篇 |
2011年 | 1276篇 |
2010年 | 1067篇 |
2009年 | 1205篇 |
2008年 | 1283篇 |
2007年 | 1151篇 |
2006年 | 1029篇 |
2005年 | 980篇 |
2004年 | 909篇 |
2003年 | 822篇 |
2002年 | 856篇 |
2001年 | 510篇 |
2000年 | 504篇 |
1999年 | 411篇 |
1998年 | 391篇 |
1997年 | 335篇 |
1996年 | 257篇 |
1995年 | 251篇 |
1994年 | 220篇 |
1993年 | 171篇 |
1992年 | 149篇 |
1991年 | 91篇 |
1990年 | 119篇 |
1989年 | 113篇 |
1988年 | 83篇 |
1987年 | 71篇 |
1986年 | 60篇 |
1985年 | 67篇 |
1984年 | 67篇 |
1983年 | 32篇 |
1982年 | 49篇 |
1981年 | 41篇 |
1980年 | 40篇 |
1979年 | 42篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
31.
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
32.
33.
High-temperature series expansions of the susceptibility and second moment to 15th order are calculated for zero external field on the linear chain (LC), plane square (PSQ), simple cubic (SC), and body-centered cubic (BCC) lattices. Checks for specific models against pertinent work in the literature are detailed. 相似文献
34.
《Magnetic resonance in chemistry : MRC》2003,41(6):417-430
A systematic study of the one‐bond and long‐range J(C,C), J(C,H) and J(H,H) in the series of nine bicycloalkanes was performed at the SOPPA level with special emphasis on the coupling transmission mechanisms at bridgeheads. Many unknown couplings were predicted with high reliability. Further refinement of SOPPA computational scheme adjusted for better performance was carried out using bicyclo[1.1.1]pentane as a benchmark to investigate the influence of geometry, basis set and electronic correlation. The calculations performed demonstrated that classical ab initio SOPPA applied with the locally dense Dunning's sets augmented with inner core s‐functions used for coupled carbons and Sauer's sets augmented with tight s‐functions used for coupled hydrogens performs perfectly well in reproducing experimental values of different types of coupling constants (the estimated reliability is ca 1–2 Hz) in relatively large organic molecules of up to 11 carbon atoms. Additive coupling increments were derived for J(C,C), J(C,H) and J(H,H) based on the calculated values of coupling constants within SOPPA in the model bicycloalkanes, in reasonably good agreement with the known values obtained earlier on pure empirical grounds. Most of the bridgehead couplings in all but one bicycloalkane appeared to be essentially additive within ca 2–3 Hz while bicyclo[1.1.1]pentane demonstrated dramatic non‐additivity of ?14.5 Hz for J(C,C), +16.6 Hz for J(H,H) and ?5.5 Hz for J(C,H), in line with previous findings. Non‐additivity effects in the latter compound established at the SOPPA level should be attributed to the through‐space non‐bonded interactions at bridgeheads due to the essential overlapping of the bridgehead rear lobes which provides an additional and effective non‐bonding coupling path for the bridgehead carbons and their protons in the bicyclopentane framework. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
35.
Tsutomu KumagaiMasaaki Tomura Jun-ichi NishidaYoshiro Yamashita 《Tetrahedron letters》2003,44(36):6845-6848
1,3-Dithiol-2-ylidene derivatives containing bis(ethynylpyridine) units were synthesized using a Pd-catalyzed reaction of the corresponding dibromide. X-Ray crystal analysis revealed unique crystal structures depending on the aromatic groups. The absorption spectra and redox properties indicated intramolecular charge-transfer interactions between the 1,3-dithiole unit and the pyridyl parts. 相似文献
36.
A graph G is N2‐locally connected if for every vertex ν in G, the edges not incident with ν but having at least one end adjacent to ν in G induce a connected graph. In 1990, Ryjá?ek conjectured that every 3‐connected N2‐locally connected claw‐free graph is Hamiltonian. This conjecture is proved in this note. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 142–146, 2005 相似文献
37.
In this paper we show that for a given 3-manifold and a given Heegaard splitting there are finitely many preferred decomposing systems of 3g−3 disjoint essential disks. These are characterized by a combinatorial criterion which is a slight strengthening of Casson-Gordon's rectangle condition. This is in contrast to fact that in general there can exist infinitely many such systems of disks which satisfy just the Casson-Gordon rectangle condition. 相似文献
38.
39.
Substructuring preconditioners for saddle-point problems arising from Maxwell's equations in three dimensions 总被引:1,自引:0,他引:1
This paper is concerned with the saddle-point problems arising from edge element discretizations of Maxwell's equations in a general three dimensional nonconvex polyhedral domain. A new augmented technique is first introduced to transform the problems into equivalent augmented saddle-point systems so that they can be solved by some existing preconditioned iterative methods. Then some substructuring preconditioners are proposed, with very simple coarse solvers, for the augmented saddle-point systems. With the preconditioners, the condition numbers of the preconditioned systems are nearly optimal; namely, they grow only as the logarithm of the ratio between the subdomain diameter and the finite element mesh size.
40.
基于相对论平均场理论,研究了各种相互作用参数组(NL1、NL3、NLSH、TM1和GL-97)对中子星物质的性质和中子星整体结构的影响.发现参数组NL1、NL3和NLSH所给出的中子星内部的介子场强度、物质的组成比例、物态方程和中子星的整体特点基本相同,但与TM1和GL-97之间有较大的差别.相对于其他参数组,GL-97给出的介子场强度最弱,中子星的相对数密度最大,物态方程也最软,同时采用GL-97参数组计算的中子星的最大质量也最小. 相似文献