全文获取类型
收费全文 | 1214篇 |
免费 | 40篇 |
专业分类
化学 | 737篇 |
晶体学 | 7篇 |
力学 | 26篇 |
数学 | 216篇 |
物理学 | 268篇 |
出版年
2023年 | 10篇 |
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 32篇 |
2018年 | 34篇 |
2017年 | 20篇 |
2016年 | 43篇 |
2015年 | 48篇 |
2014年 | 39篇 |
2013年 | 75篇 |
2012年 | 88篇 |
2011年 | 79篇 |
2010年 | 52篇 |
2009年 | 59篇 |
2008年 | 70篇 |
2007年 | 74篇 |
2006年 | 59篇 |
2005年 | 58篇 |
2004年 | 32篇 |
2003年 | 21篇 |
2002年 | 31篇 |
2001年 | 26篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 16篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1990年 | 6篇 |
1988年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1967年 | 4篇 |
1966年 | 7篇 |
1965年 | 5篇 |
1964年 | 9篇 |
1963年 | 6篇 |
排序方式: 共有1254条查询结果,搜索用时 15 毫秒
901.
Perfect graphs constitute a well-studied graph class with a rich structure, which is reflected by many characterizations with
respect to different concepts. Perfect graphs are, for instance, precisely those graphs G where the stable set polytope STAB(G) equals the fractional stable set polytope QSTAB(G). The dilation ratio of the two polytopes yields the imperfection ratio of G. It is NP-hard to compute and, for most graph classes, it is even unknown whether it is bounded. For graphs G such that all facets of STAB(G) are rank constraints associated with antiwebs, we characterize the imperfection ratio and bound it by 3/2. Outgoing from
this result, we characterize and bound the imperfection ratio for several graph classes, including near-bipartite graphs and
their complements, namely quasi-line graphs, by means of induced antiwebs and webs, respectively.
相似文献
902.
Hydride and dihydrogen dicarbonylrhenium(I) complexes with phosphites, phosphonites and phosphinites
Sandra Bolaño Flor Fernández-García Soledad García-Fontán María del Carmen Marín 《Journal of organometallic chemistry》2009,694(18):2994-2999
Photoirradiation of a toluene solution of [ReH(CO)3(L)] [S. Bolaño, J. Bravo, R. Carballo, S. García-Fontán, U. Abram, E.M. Vázquez-López, Polyhedron 18 (1999) 1431-1436] [L = 1,2-bis(diphenylphosphinoxy)ethane] in the presence of PPhn(OR)3−n (n = 0, 1; R = Me, Et) leads to the replacement of a CO ligand by the corresponding monodentate phosphite or phosphonite ligand to give new hydride compounds of formula [ReH(CO)2(L)(L′)] [L′ = P(OMe)3 (1); P(OEt)3 (2); PPh(OMe)2 (3); PPh(OEt)2 (4)]. Protonation of compounds 1-4 in CD2Cl2, with HBF4.OMe2 or with HOOCCF3 at 193 K in a NMR tube, gave the corresponding dihydrogen complexes. When the temperature was increased from 193 to 293 K, the η2-H2 ligand was replaced by OMe2 or −OOCCF3 groups (depending on the acid employed) to give new stable complexes and the loss of H2 gas. 相似文献
903.
Luciana P. da Silva Leandro Luetkmeyer Cristina R.G. Furtado Maria Inês Bruno Tavares Elen B.A.V. Pacheco 《Polymer Testing》2009,28(1):53-56
Proton spin-lattice relaxation time (T1H) in the solid state was used to determine the molecular mobility of PP/regenerated tire-rubber blends (PP/RgR), employing low-field NMR. The blends were prepared with different quantities of regenerated rubber (5, 10, 15 and 20 wt%). The addition of 5 wt% maleic anhydride functionalized polypropylene (PP-g-MAH) was carried out to evaluate the processing behavior of the reclaimed material. Important differences were observed concerning the molecular mobility according to the content of regenerated rubber. A decrease in the relaxation time of the blends with an increase in PP-g-MAH content indicates that enhanced mobility of PP/regenerated tire-rubber blends was obtained and, thus, PP-g-MAH acts as a plasticizer and/or impact modified. 相似文献
904.
André L. F. Matassoli Igor N. S. Corrêa Márcio F. Portilho Cláudia O. Veloso Marta A. P. Langone 《Applied biochemistry and biotechnology》2009,155(1-3):44-52
The enzymatic alcoholysis of crude palm oil with methanol and ethanol was investigated using commercial immobilized lipases (Lipozyme RM IM, Lipozyme TL IM). The effect of alcohol (methanol or ethanol), molar ratio of alcohol to crude palm oil, and temperature on biodiesel production was determined. The best ethyl ester yield was about 25 wt.% and was obtained with ethanol/oil molar ratio of 3.0, temperature of 50 °C, enzyme concentration of 3.0 wt.%, and stepwise addition of the alcohol after 4 h of reaction. Experiments with 1 and 3 wt.% of KOH and 3 wt.% of MgO were carried out to compare their catalytic behavior with the enzymatic transesterification results. The commercial immobilized lipase, Lipozyme TL IM, showed the best catalytic performance. 相似文献
905.
A new analytical method for determination of coenzyme Q10 (2,3‐dimethoxy‐5‐methyl‐6‐decaprenyl‐1,4‐benzoquinone, CoQ10) in human plasma was developed based on CE using a double tensioactive microemulsion. CoQ10 was quantitatively extracted into 1‐propanol/hexane and quantified by MEEKC. The microemulsion was prepared by mixing 1.4% w/w sodium bis(2‐ethylhexyl) sulfosuccinate, 4% w/w cholic acid, 1% w/w octane, 8.5% w/w butanol, 0.1% w/w PVA and 85% w/w 10 mM Tris buffer at pH 9.0. The optimized electrophoretic conditions included the use of an uncoated silica capillary of 60 cm total length and 75 μm id, an applied voltage of 20 kV, room temperature and 214 nm ultraviolet detection. Selectivity, linearity, LOD, LOQ, precision and accuracy were evaluated as the parameters of validation. Owing to its simplicity and reliability, the proposed method can be an advantageous alternative to the traditional methodology for the quantitation of CoQ10 in human plasma with good accuracy and precision. 相似文献
906.
907.
Zusammenfassung Bei der Kondensation der Weinsäure und Schleimsäure mit o-Phenylendiamin kann man neben den bekannten Bisbenzimidazolylderivaten auch die bisher nicht beschriebenen Monobenzimidazolylderivate in guter Ausbeute bekommen. 相似文献
908.
Kimura T Harada H Ando T Fujita M Levêque JM Luche JL 《Chemical communications (Cambridge, England)》2002,(11):1174-1175
Among the sonolysis products of acetic acid, oxidizing intermediates can play a direct role in reactions run in this solvent, illustrating the fact that organic sonochemistry can originate in organic solvents as aqueous sonochemistry can in water. 相似文献
909.
We prove a simple formula for the transverse Poisson structure to a coadjoint orbit (in the dual of a Lie algebra
) and use it in examples such as
and
. We also give a sufficient condition on the isotropy subalgebra of
so that the transverse Poisson structureto the coadjoint orbit of is linear. 相似文献
910.
Light microscopy analyses of microfilariae of Wuchereria bancrofti treated with DEC revealed a striking loss of the microfilarial sheath. However, no effect was observed on microfilariae of Litomosoides chagasfilhoi treated with DEC. For quantitative analyses microfilariae of W. bancrofti were processed for SEM. Controls, which have not received DEC, had 29.8% of exsheathed microfilariae. Conversely, the number of exsheathed microfilariae increased as increased DEC concentrations: 5 μg/ml (75.9%), 10 μg/ml (80.1%), and 50 μg/ml (87.7%). After DEC treatment some of sheathed microfilariae showed a wrinkled surface, and in some microfilariae, sheaths were observed being liberated almost intact from the larvae surface. But, frequently residues of the lost sheath over the surface were also observed. No damage was observed in the microfilariae cuticle. The present work shows quantitative data on the loss of the microfilarial sheath of W. bancrofti after treatment with DEC. Since no loss of microfilarial sheath was observed in microfilariae of L. chagasfilhoi submitted to the same conditions, DEC may present different mechanisms of action for distinct filarial species. 相似文献