全文获取类型
收费全文 | 624篇 |
免费 | 13篇 |
国内免费 | 9篇 |
专业分类
化学 | 266篇 |
晶体学 | 5篇 |
力学 | 11篇 |
数学 | 271篇 |
物理学 | 93篇 |
出版年
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 16篇 |
2016年 | 19篇 |
2015年 | 19篇 |
2014年 | 15篇 |
2013年 | 48篇 |
2012年 | 39篇 |
2011年 | 37篇 |
2010年 | 32篇 |
2009年 | 43篇 |
2008年 | 37篇 |
2007年 | 50篇 |
2006年 | 33篇 |
2005年 | 34篇 |
2004年 | 19篇 |
2003年 | 25篇 |
2002年 | 19篇 |
2001年 | 3篇 |
2000年 | 8篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1961年 | 1篇 |
1929年 | 1篇 |
排序方式: 共有646条查询结果,搜索用时 15 毫秒
571.
[reaction: see text] A simplified synthesis of N-heterocyclic carbene (NHC)Pd-carboxylate complexes and their activity in Suzuki-Miyaura cross-coupling reactions are described. Coupling of sterically hindered aryl and activated alkyl chlorides bearing beta-hydrogens has been successfully achieved. 相似文献
572.
Dan Donescu Sever Serban Cristian Petcu Cristina Lavinia Nistor Marius Ghiurea Mihai Cosmin Corobea 《Colloid and polymer science》2007,285(13):1455-1462
The styrene (St), butyl acrylate (BuA), and methyl methacrylate (MMA) polymerization in microemulsion in the presence of sodium
dodecylsulfate is studied. This process is conducted in the presence of some comonomers having groups that can participate
in sol–gel processes: 3(trimethyloxysilyl) propyl methacrylate (MPTS), triethoxy vinylsilane (VTES), and a comonomer with
a sulfate group, styrene sodium sulfonate (StSO3Na). It has been observed that stabile latexes are obtained by radical polymerization at pH = 7, followed by a sol–gel process
in the presence of ammonia. Latex particles sizes and zeta potential grow with MTPS concentration and in StSO3Na presence. VTES effect depends on its reactivity in St, MMA, and BuA copolymerization. Glass transition temperature and
thermal decomposing temperature are influences by functional comonomer concentration and chemical structure. The Fourier transform
infrared spectrum and inorganic residue growth after organic part thermal decomposition shows the presence of silica in obtained
latexes. 相似文献
573.
Mironel Enescu Roxana Ionescu Gheorghe Dumbraveanu Mihai L. Pascu 《Photochemistry and photobiology》1993,57(2):367-370
Binding of sodium dodecyl sulfate (SDS) to bovine serum albumin (BSA) and human serum albumin (HSA) in aqueous solutions at room temperature induces significant changes in the phosphorescence lifetime of tryptophan (Trp) residues. A steep rise of the phosphorescence lifetime from 1.9 ms to 10.0 ms for BSA and from 1.9 ms to 5.5 ms for HSA is observed when the total SDS concentration increased from 0.0 mM to 0.22 mM at 1 mg/mL protein concentration. As the total SDS concentrationis further inccreased to 2.2 mM, a slower increase in the phosphorescence lifetime is observed, from 10.0 ms to 19.5 ms for BSA and from 5.5 ms to 7.2 ms for HSA. It appears that the phosphorescence lifetime modifications are mainly due to an increase of protein matrix rigidity around Trp residues. The observed differences (between HSA and BSA) allow us to distinguish the contribution of the two Trp residues to the BSA phosphorescence. 相似文献
574.
Alexandru T. Balaban Cornelia Uncuţ/a Mihai Elian Filip Chiraleu Antonie Dinculescu 《Tetrahedron》1984,40(13):2547-2553
Pairwise chemical shift nonequivalence of the 2,6-methyl and 3,5-protons in 1H NMR spectra, as well as of the 2,6-methyl, 2,6-ring and 3,5-ring carbons in 13C NMR spectra, was observed for N-alkyl-2,4,6-trimethylpyridinium salts 2. Dynamic NMR spectroscopy demonstrates appreciably higher activation free energies ΔG# for rotation around the N(sp2)-C(sp3 bond than ΔG# for the analogously substituted mesityl derivatives, in agreement with the shorter N-C bond distance than for the C-C bond. 相似文献
575.
The wideline proton NMR spectra of polyethylene powder samples were analyzed in terms of contributions from three components: (1) a rigid part with immobile chains, (2) a soft region with liquid-like character which produces a Lorentzian contribution to the spectrum, and (3) an intermediate region in which the rotation of methylene groups about C-C bonds is partially hindered. The relative mass fractions as well as chain mobilities varied greatly among samples produced by different polymerization techniques. The NMR crystallinity agreed well with that estimated by WAXD and was much higher than DSC crystallinity, indicating an inclusion of the contribution from a crystalline-amorphous interphase. The crystalline defects in the rigid part could be significantly affected by processing parameters when employing the same type of polymerization technique. The intermediate region in the NMR spectra was analyzed according to the comparison between bimodal high density polyethylene and corresponding linear unimodal one. It was found that the mass fraction of the NMR interphase could be an indication of the percentage of tie molecules between crystalline lamellae and thus may significantly affect the mechanical properties of polymeric material. 相似文献
576.
We prove by elementary means that, if the Bergman orthogonal polynomials of a bounded simply-connected planar domain, with
sufficiently regular boundary, satisfy a finite-term relation, then the domain is algebraic and characterized by the fact
that Dirichlet’s problem with boundary polynomial data has a polynomial solution. This, and an additional compactness assumption,
is known to imply that the domain is an ellipse. In particular, we show that if the Bergman orthogonal polynomials satisfy
a three-term relation then the domain is an ellipse. This completes an inquiry started forty years ago by Peter Duren.
To Peter Duren on the occasion of his seventieth birthday
The first author was partially supported by the National Science Foundation Grant DMS- 0350911.
Received: October 15, 2006. Revised: January 22, 2007. 相似文献
577.
Tudor Mihai Stoenescu Mingyan Liu Demosthenis Teneketzis 《Mathematical Methods of Operations Research》2007,65(2):199-228
In this two-part paper we present a general framework for addressing the optimal rare control problem in multirate multicast
where the objective is the maximization of a social welfare function expressed by the sum of the users’ utility functions.
Specifically, we propose a market-based mechanism that satisfies the informational constraints imposed by the decentralization
of information in multirate multicast service provisioning, and achieves an optimal solution to the corresponding centralized
optimization problem. In Part I we discover properties of an optimal solution to the centralized problem. Based on these properties,
we develop a distributed algorithm that determines how link prices are split among users whose connections along a multicast
tree share the same link. 相似文献
578.
A comparison between ultrasonic array beamforming and super resolution imaging algorithms for non-destructive evaluation 总被引:1,自引:0,他引:1
In this paper the total focusing method, the so called gold standard in classical beamforming, is compared with the widely used time-reversal MUSIC super resolution technique in terms of its ability to resolve closely spaced scatterers in a solid. The algorithms are tested with simulated and experimental array data, each containing different noise levels. The performance of the algorithms is evaluated in terms of lateral resolution and sensitivity to noise. It is shown that for the weak noise situation (SNR > 20 dB), time-reversal MUSIC provides significantly enhanced lateral resolution when compared to the total focusing method, breaking the diffraction limit. However, for higher noise levels, the total focusing method is shown to be robust, whilst the performance of time-reversal MUSIC is degraded. The influence of multiple scattering on the imaging algorithms is also investigated and shown to be small. 相似文献
579.
580.
In this paper we study a transient birth and death Markov process penalized by its sojourn time in 0. Under the new probability
measure the original process behaves as a recurrent birth and death Markov process. We also show, in a particular case, that
an initially recurrent birth and death process behaves as a transient birth and death process after penalization with the
event that it can reach zero in infinite time. We illustrate some of our results with the Bessel random walk example. 相似文献