全文获取类型
收费全文 | 941篇 |
免费 | 57篇 |
国内免费 | 7篇 |
专业分类
化学 | 687篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 171篇 |
物理学 | 136篇 |
出版年
2023年 | 6篇 |
2022年 | 6篇 |
2021年 | 18篇 |
2020年 | 34篇 |
2019年 | 35篇 |
2018年 | 19篇 |
2017年 | 17篇 |
2016年 | 39篇 |
2015年 | 38篇 |
2014年 | 41篇 |
2013年 | 64篇 |
2012年 | 55篇 |
2011年 | 62篇 |
2010年 | 52篇 |
2009年 | 23篇 |
2008年 | 83篇 |
2007年 | 52篇 |
2006年 | 54篇 |
2005年 | 43篇 |
2004年 | 26篇 |
2003年 | 24篇 |
2002年 | 24篇 |
2001年 | 12篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 5篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有1005条查询结果,搜索用时 15 毫秒
991.
Denote by pn/qn,n=1,2,3,…, the sequence of continued fraction convergents of the real irrational number x. Define the sequence of approximation coefficients by θn:=qn|qnx−pn|,n=1,2,3,…. A laborious way of determining the mean value of the sequence |θn+1−θn−1|,n=2,3,…, is simplified. The method involved also serves for showing that for almost all x the pattern θn−1<θn<θn+1 occurs with the same asymptotic frequency as the pattern θn+1<θn<θn−1, namely 0.12109?. All the four other patterns have the same asymptotic frequency 0.18945?. The constants are explicitly given. 相似文献
992.
The three-component condensation of phenylhydrazine, α-bromo aryl ketones and aryl isothiocyanates leading to 1,3,4-thiadiazine derivatives is described. 相似文献
993.
Christina Schubert Dr. Mateusz Wielopolski Lars‐Hendrik Mewes Dr. Gustavo de Miguel Rojas Dr. Cornelia van der Pol Dr. Kathryn C. Moss Prof. Martin R. Bryce Prof. Jacques E. Moser Prof. Timothy Clark Prof. Dirk M. Guldi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(23):7575-7586
A new series of donor–bridge–acceptor (D–B–A) compounds consisting of π‐conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron‐donor and a fullerene (C60) electron‐acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono‐ and bi‐fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p‐phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy‐matching between the Fc and the oFL units, which results in strong electronic‐coupling. Hence, intramolecular charge‐transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc–oFL–C60 conjugates results in transient radical‐ion‐pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge‐separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge‐separation rates, resulting in beta values between βCS 0.08 and 0.19 Å?1. In contrast, charge recombination depends strongly on the electron‐donor–acceptor distance, but not at all on the linker. A value of βCR (0.35±0.01 Å?1) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D–B–A systems. 相似文献
994.
Rainer Streubel Hendrik Wilkens Christoph Neumann 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1563-1565
Catalytic reaction of 2 H -azaphosphirene complex 1 with ortho -, meta -, and para -benzodinitrile ( 2a-c ) led in all cases to the 2 H -1,4,2-diazaphosphole complexes 3a-c if ferrocenium hexafluorophosphate was used as catalyst. In the case of the meta - and para -benzodinitriles 2b-c , the bis-2 H -1,4,2-diazaphosphole complexes 4b-c were additionally obtained. Under the same reaction conditions, acetone, diethylketone and cyclohexanone ( 5a-c ) reacted with complex 1 to yield j 3 -1,3,2-oxazaphospholene complexes 6a-c in good yields. 相似文献
995.
Vladimir N. Boiko Andrey A. Filatov Yurii L. Yagupolskii Wieland Tyrra Dieter Naumann Ingo Pantenburg Hendrik T.M. Fischer Frank Schulz 《Journal of fluorine chemistry》2011,132(12):1219-1226
Convenient and preparative synthetic procedures of 2,4,6-tris(chlorosulfonyl)- and 2,4,6-tris(fluorosulfonyl)phenol, -chlorobenzene and -aniline have been elaborated. Chlorine exchange for fluorine by KF interaction on 2,4,6-tris(chlorosulfonyl)aniline and especially 2,4,6-tris(chlorosulfonyl)phenol proceeds easily and selectively under anhydrous conditions in dioxane. Unlike, 2,4,6-tris(chlorosulfonyl)chlorobenzene transformation requires the presence of water. On the basis of 2,4,6-tris(fluorosulfonyl)phenol and some of its salts, XRD measurements demonstrated the structural similarity to picric acid and its derivatives in the solid state. 相似文献
996.
Dimitri Zielke Jan Hendrik Petermann Florian Werner Boris Veith Rolf Brendel Jan Schmidt 《固体物理学:研究快报》2011,5(8):298-300
Atomic‐layer‐deposited aluminum oxide (AlOx) layers are implemented between the phosphorous‐diffused n+‐emitter and the Al contact of passivated emitter and rear silicon solar cells. The increase in open‐circuit voltage Voc of 12 mV for solar cells with the Al/AlOx/n+‐Si tunnel contact compared to contacts without AlOx layer indicates contact passivation by the implemented AlOx. For the optimal AlOx layer thickness of 0.24 nm we achieve an independently confirmed energy conversion efficiency of 21.7% and a Voc of 673 mV. For AlOx thicknesses larger than 0.24 nm the tunnel probability decreases, resulting in a larger series resistance. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
997.
The objective of the study was to gather insight into the metabolism of lead-removing microorganisms, coupled with Pb(II) removal, biomass viability and nitrate concentrations for Pb(II) bioremoval using an industrially obtained microbial consortium. The consortium used for study has proven to be highly effective at removing aqueous Pb(II) from solution. Anaerobic batch experiments were conducted with Luria-Bertani broth as rich growth medium over a period of 33 h, comparing a lower concentration of Pb(II) with a higher concentration at two different nutrient concentrations. Metabolite profiling and quantification were conducted with the aid of both liquid chromatography coupled with tandem mass spectroscopy (UPLC-HDMS) in a “non-targeted” fashion and high-performance liquid chromatography (HPLC) in a “targeted” fashion. Four main compounds were identified, and a metabolic study was conducted on each to establish their possible significance for Pb(II) bioremoval. The study investigates the first metabolic profile to date for Pb(II) bioremoval, which in turn can result in a clarified understanding for development on an industrial and microbial level. 相似文献
998.
Hendrik Richter 《Physics letters. A》2008,372(26):4689-4693
The Kaplan-Yorke dimension can be derived using a linear interpolation between an h-dimensional Lyapunov exponent λ(h)>0 and an h+1-dimensional Lyapunov exponent λ(h+1)<0. In this Letter, we use a polynomial interpolation to obtain generalized Lyapunov dimensions and study the relationships among them for higher-dimensional systems. 相似文献
999.
Hendrik Richter 《Physica D: Nonlinear Phenomena》2008,237(2):167-186
Coupled Map Lattices (CML) can be interpreted as spatio-temporal fitness landscapes which may pose a dynamic optimization problem. In this paper, we analyze such dynamic fitness landscapes in terms of the landscape measures modality, ruggedness, information content and epistasis. These measures account for different aspects of problem hardness. We use an evolutionary algorithm to solve the dynamic optimization problem and study the relationship between performance criteria of the algorithm and the landscape measures. In this way we relate problem hardness to expectable performance. 相似文献
1000.
We consider a generalization of the classical Laplace operator, which includes the Laplace–Dunkl operator defined in terms of the differential-difference operators associated with finite reflection groups called Dunkl operators. For this Laplace-like operator, we determine a set of symmetries commuting with it, in the form of generalized angular momentum operators, and we present the algebraic relations for the symmetry algebra. In this context, the generalized Dirac operator is then defined as a square root of our Laplace-like operator. We explicitly determine a family of graded operators which commute or anticommute with our Dirac-like operator depending on their degree. The algebra generated by these symmetry operators is shown to be a generalization of the standard angular momentum algebra and the recently defined higher-rank Bannai–Ito algebra. 相似文献