全文获取类型
收费全文 | 1000篇 |
免费 | 18篇 |
国内免费 | 4篇 |
专业分类
化学 | 595篇 |
晶体学 | 7篇 |
力学 | 18篇 |
数学 | 153篇 |
物理学 | 249篇 |
出版年
2021年 | 6篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 15篇 |
2016年 | 23篇 |
2015年 | 17篇 |
2014年 | 11篇 |
2013年 | 96篇 |
2012年 | 40篇 |
2011年 | 59篇 |
2010年 | 30篇 |
2009年 | 21篇 |
2008年 | 40篇 |
2007年 | 59篇 |
2006年 | 46篇 |
2005年 | 52篇 |
2004年 | 43篇 |
2003年 | 36篇 |
2002年 | 34篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 20篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 21篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 5篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1972年 | 6篇 |
1932年 | 3篇 |
1930年 | 6篇 |
1911年 | 3篇 |
排序方式: 共有1022条查询结果,搜索用时 296 毫秒
991.
Lan-Ying Qin Andrew G. ColeAxel Metzger Kurt W. SaionzIan Henderson 《Tetrahedron letters》2011,52(8):849-852
A synthesis of novel substituted 3,8,11-triazaspiro[5,6]dodecan-7-ones using a combination of solution-phase and solid-phase chemistries is described. A solution-phase approach was used to produce a key piperidine intermediate that was then incorporated into a solid-phase synthesis. The combined synthetic strategy was applied to provide a series of substituted 3,8,11-triazaspiro[5,6]dodecan-7-ones in good yield and high purity. 相似文献
992.
The reaction of a fluorinated iridium complex with cerium(IV) and organic reducing agents generates an intense emission with a significant hypsochromic shift compared to contemporary chemically-initiated luminescence from metal complexes. 相似文献
993.
Xie Z Henderson EJ Dag Ö Wang W Lofgreen JE Kübel C Scherer T Brodersen PM Gu ZZ Ozin GA 《Journal of the American Chemical Society》2011,133(13):5094-5102
There has always been a fascination with "impossible" compounds, ones that do not break any rules of chemical bonding or valence but whose structures are unstable and do not exist. This instability can usually be rationalized in terms of chemical or physical restrictions associated with valence electron shells, multiple bonding, oxidation states, catenation, and the inert pair effect. In the pursuit of these "impossible" materials, appropriate conditions have sometimes been found to overcome these instabilities and synthesize missing compounds, yet for others these tricks have yet to be uncovered and the materials remain elusive. In the scientifically and technologically important field of periodic mesoporous silicas (PMS), one such "impossible" material is periodic mesoporous hydridosilica (meso-HSiO(1.5)). It is the archetype of a completely interrupted silica open framework material: its pore walls are comprised of a three-connected three-dimensional network that should be so thermodynamically unstable that any mesopores present would immediately collapse upon removal of the mesopore template. In this study we show that meso-HSiO(1.5) can be synthesized by template-directed self-assembly of HSi(OEt)(3) under aqueous acid-catalyzed conditions and after template extraction remains stable to 300 °C. Above this temperature, bond redistribution reactions initiate a metamorphic transformation which eventually yields periodic mesoporous nanocrystalline silicon-silica, meso-ncSi/SiO(2), a nanocomposite material in which brightly photoluminescent silicon nanocrystallites are embedded within a silica matrix throughout the mesostructure. The integration of the properties of silicon nanocrystallinity with silica mesoporosity provides a wealth of new opportunities for emerging nanotechnologies. 相似文献
994.
Alexander G. Nikolaev Sheldon H. Jacobson Shane N. Hall Darrall Henderson 《Computational Optimization and Applications》2011,49(3):407-433
This paper presents a framework for analyzing and comparing sub-optimal performance of local search algorithms for hard discrete
optimization problems. The β-acceptable solution probability is introduced that captures how effectively an algorithm has performed to date and how effectively
an algorithm can be expected to perform in the future. Using this probability, the necessary conditions for a local search
algorithm to converge in probability to β-acceptable solutions are derived. To evaluate and compare the effectiveness of local search algorithms, two estimators for
the expected number of iterations to visit a β-acceptable solution are obtained. Computational experiments are reported with simulated annealing and tabu search applied
to four small traveling salesman problem instances, and the Lin-Kernighan-Helsgaun algorithm applied to eight medium to large
traveling salesman problem instances (all with known optimal solutions), to illustrate the application of these estimators. 相似文献
995.
Bayes R Bueno JF Hillairet A Davydov YI Depommier P Faszer W Gagliardi CA Gaponenko A Gill DR Grossheim A Gumplinger P Hasinoff MD Henderson RS Hu J Koetke DD MacDonald RP Marshall GM Mathie EL Mischke RE Olchanski K Olin A Openshaw R Poutissou JM Poutissou R Selivanov V Sheffer G Shin B Stanislaus TD Tacik R Tribble RE;TWIST Collaboration 《Physical review letters》2011,106(4):041804
The TWIST Collaboration has completed a new measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters ρ, δ, and P(μ)(π)ξ tests the standard model in a purely leptonic process and provides improved limits for relevant extensions to the standard model. Specifically, for the generalized left-right symmetric model |(g(R)/g(L))ζ|<0.020 and (g(L)/g(R))m(2)>578 GeV/c(2), both 90% C.L. 相似文献
996.
T. J. Atherton J. R. Sambles J. P. Bramble J. R. Henderson S. D. Evans 《Liquid crystals》2013,40(4):353-358
We have discovered an optically uniform type of domain that occurs in twisted nematic (TN) cells that are constructed from substrates chemically patterned with stripes via microcontact printing of self-assembled monolayers; such domains do not occur in TN cells constructed from uniform substrates. In such a cell, the azimuthal anchoring at the substrates is due to the elastic anisotropy of the liquid crystal rather than the conventional rubbing mechanism. A model is presented that predicts the relative stability of the twisted and anomalous states as a function of the material and design parameters. 相似文献
997.
Four homologous series of non‐symmetric linear liquid crystalline trimers, the 4‐[(4‐{n‐[4‐(4‐methoxy‐phenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐methoxyphenylazo)phenoxy]hexyl esters, 4‐[(4‐{n‐[4‐(4‐cyanophenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐methoxyphenylazo)phenoxy]hexyl esters, 4‐[(4‐{n‐[4‐(4‐methoxyphenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐cyanophenylazo)phenoxy]hexyl esters and 4‐[(4‐{n‐[4‐(4‐cyanophenylazo)phenoxy]butoxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐cyanophenylazo)phenoxy]hexyl esters, have been synthesized and characterized. In each series one spacer was held constant while the length of the second was varied from 3 to 12 methylene units. All these trimers were nematogenic and exhibited pronounced odd–even effects in both their clearing temperatures and associated entropies. Only two of the trimers exhibited a smectic phase, and specifically those containing two differing terminal mesogenic groups and an undecyl spacer. This strongly suggests the driving force for smectic phase formation is a combination of molecular shape and a specific interaction between unlike mesogenic units. The smectic phase has been assigned as a triply‐intercalated alternating smectic C phase, the first example of its kind. 相似文献
998.
A homologous series of linear liquid crystal trimers, the 4,4′-bis[ω-(4-methoxyazobenzene-4′-yloxy)alkoxy]azobenzenes, has been synthesized and characterized. The transitional properties of the trimers are compared with those of the corresponding series of dimers, the α,ω-bis(4-methoxyazobenzene-4′-oxy)alkanes, and monomers, the 4-methoxy-4′-alkoxyazobenzenes. Characteristically pronounced odd-even effects were seen for the transitional properties of both dimers and trimers on varying the spacer lengths. The clearing temperatures of the trimers were higher than those of the corresponding dimers, but as the length of the flexible spacers was increased this difference became rather small. The ratios of T NI, and ΔS NI/R for monomer:dimer and dimer:trimer are discussed. These are very similar to reported values for similar materials, suggesting that there may be a rather general relationship between the transitional properties of liquid crystal oligomers as the number of mesogenic units is increased. 相似文献
999.
The thermal stability of poly(vinyl chloride) prepared by radiation polymerization has been examined by thermogravimetric analysis, hydrogen chloride evolution, and visible spectra measurements in films and solutions. Radiation polymerized PVC, prepared with relatively high radiation doses, is much less stable than a commercial suspension polymerization PVC. On the other hand, the stabilities of radiation polymerized PVC, prepared with relatively high radiation doses, is much less stable than a commercial suspension polymerization PVC. On the other hand, the stabilities of radiation polymerized samples, which had received less than 0.45 Mrad, are comparable with commercial PVC. It is suggested that the allylic chlorine atoms produced in the radlysis of the polymer, concurrent with its formation, are chiefly responsible for the instability of the high doses samples. 相似文献
1000.