全文获取类型
收费全文 | 431770篇 |
免费 | 4092篇 |
国内免费 | 1402篇 |
专业分类
化学 | 225515篇 |
晶体学 | 6899篇 |
力学 | 19259篇 |
综合类 | 3篇 |
数学 | 49780篇 |
物理学 | 135808篇 |
出版年
2020年 | 3257篇 |
2019年 | 3821篇 |
2018年 | 4796篇 |
2017年 | 4631篇 |
2016年 | 7113篇 |
2015年 | 4586篇 |
2014年 | 7239篇 |
2013年 | 19487篇 |
2012年 | 14140篇 |
2011年 | 17474篇 |
2010年 | 12039篇 |
2009年 | 11862篇 |
2008年 | 15961篇 |
2007年 | 16063篇 |
2006年 | 15058篇 |
2005年 | 13716篇 |
2004年 | 12519篇 |
2003年 | 11137篇 |
2002年 | 10988篇 |
2001年 | 12579篇 |
2000年 | 9603篇 |
1999年 | 7557篇 |
1998年 | 6431篇 |
1997年 | 6394篇 |
1996年 | 6006篇 |
1995年 | 5541篇 |
1994年 | 5328篇 |
1993年 | 5439篇 |
1992年 | 5817篇 |
1991年 | 5994篇 |
1990年 | 5622篇 |
1989年 | 5555篇 |
1988年 | 5633篇 |
1987年 | 5418篇 |
1986年 | 5160篇 |
1985年 | 7029篇 |
1984年 | 7361篇 |
1983年 | 6000篇 |
1982年 | 6478篇 |
1981年 | 6279篇 |
1980年 | 6131篇 |
1979年 | 6274篇 |
1978年 | 6575篇 |
1977年 | 6314篇 |
1976年 | 6566篇 |
1975年 | 5957篇 |
1974年 | 6075篇 |
1973年 | 6389篇 |
1972年 | 4223篇 |
1971年 | 3422篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
942.
943.
The rheological behavior of a mixture of two liquid-crystal polymers, hydroxypropyl cellulose and ethyl cellulose, in acetone solution is studied. The total polymer concentration in the solvent is held constant (40%) as the ratio of the two polymers is varied. The mixtures are anisotropic, isotropic, or biphasic (isotropic/anisotropic), depending on the concentration. Curves of viscosity vs shear rate for all the mixtures studied show three regions of viscosity, as described by Onogi and Asada for liquid-crystal polymers. The viscosity as a function of the weight ratio of the two polymers at constant shear rate exhibits deviations from additivity of viscosities of the two components at all concentrations. In mixtures of two polymers in the melt, deviations are also observed; the negative ones are attributed to phase separation and the positive ones to homogenous mixing (comparison with the phase diagram). All the mixtures studied (anisotropic, isotropic, or biphasic), show ranges of shear rates where the first normal-stress difference is negative, as is generally observed for anisotropic liquid-crystal polymers. It is concluded that the isotropic solutions become anisotropic under shear, as they are not far from the critical concentration. © 1994 John Wiley & Sons, Inc. 相似文献
944.
Michael S. Kent Matthew Tirrell Timothy P. Lodge 《Journal of Polymer Science.Polymer Physics》1994,32(11):1927-1941
Solution properties for random and diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been measured by dynamic and total intensity light scattering in solvents of differing quality. The results are compared with the corresponding properties for PS and PMMA homopolymers of similar molecular weight, in order to determine if interactions between unlike monomers are significant. The hydrodynamic radius (Rh) and diffusion second virial coefficient (kd) for the random copolymer are found to be larger than the corresponding values for the homopolymers in a solvent which is near-theta for the two homopolymers, whereas no such effect is observed for the block copolymer. This suggests that most intrachain interactions occur a relatively short distance along the chain backbone. In a mutual good solvent Rh and kd of the random copolymer are comparable to the average of the values for the homopolymers, indicating that in a good solvent monomer/solvent interactions dominate over monomer/monomer interactions. For an isolated diblock copolymer in a mutual good solvent, there is no evidence that interactions between unlike monomers lead to additional expansion of the entire molecule, as measured by Rh, nor expansion of the individual blocks as probed by light scattering with one block optically masked. However, at low but finite concentration there is evidence (the coefficients of the binary interaction terms in the viscosity and the mutual diffusion coefficient, and the second and third virial coefficients) that a weak ordering effect may exist in block copolymer solutions, far from the conditions where microphase separation occurs. Finally, measurements of ternary polymer-polymer-solvent solutions show no dependence on monomer composition or monomer distribution for the tracer diffusion of probe PS-PMMA copolymers in a PMMA/toluene matrix. This indicate that the frictional interaction is largely unaffected by interactions between unlike monomers. However, there is evidence that the thermodynamic interaction is more unfavorable between a random copolymer and the homopolymer matrix than between a diblock and the matrix. © 1994 John Wiley & Sons, Inc. 相似文献
945.
P M Narins 《The Journal of the Acoustical Society of America》1992,91(6):3551-3557
Tympanic membrane (TM) velocities of vocalizing male frogs, Eleutherodactylus coqui, were measured in the field using laser vibrometry. The animal's call produced levels of 114-120 dB SPL measured at distances of 12-20 mm from the eardrum and mean TM displacements in response to the two notes of the male's own advertisement call were 1.84 and 2.08 nm, respectively. These values are close to the TM displacements obtained in response to playback of the two-note call at much lower sound levels. Thus TM vibrations are attenuated during phonation in the frog, and several candidate mechanisms for auditory sensitivity reduction are discussed. 相似文献
946.
R. Szymczak K. Zaveta H. Szymczak H. -Q. Guo H. Kronmü ller 《Journal of magnetism and magnetic materials》1994,130(1-3):363-366
The temperature dependence of magnetization of a nanocrystalline Fe60Co30Zr10 alloy annealed at various temperatures to varying crystal sizes was investigated down to 5 K in the field range up to 5 T. The fitting procedure gave T3/2 as the leading term irrespective of the crystal size. We suggest that the increase of the coefficient by this term for smaller crystals may be connected to an increased relative volume of their surface. 相似文献
947.
Sulfide cluster-derived ensembles are promising models of the active sites in commercial hydrotreatment catalysts. A series of sulfide clusters were adsorbed intact onto high-surface-area γ-alumina, magnesium oxide and activated carbon supports, then pretreated to produce highly dispersed catalytic ensembles with sizes similar to those of their precursor clusters. The activities of the bimetallic cluster-derived catalysts were significantly higher than those of the monometallic catalysts. We took this as evidence that direct interactions between molybdenum and the promoter element cause the promotional effect observed in commercial hydrotreatment catalysts. The hydrodesulfurization and hydrodenitrogenation activities correlated with the extent of molybdenum reduction. Our results suggested that the active sites in promoted hydrotreatment catalysts are centered on molecular-scale ensembles containing molybdenum, sulfur and the promoter element. 相似文献
948.
M. D. Zipper G. P. Simon P. Cherry A. J. Hill 《Journal of Polymer Science.Polymer Physics》1994,32(7):1237-1247
Homopolymers and blends of polycarbonate/ThermX have been investigated by differential scanning calorimetry, dynamic mechanical analysis, density measurements, and positron annihilation lifetime spectroscopy. The study focuses on the chain mobility and free volume in the amorphous miscible regions of the blends and how this mobility and free volume are affected by increasing crystallinity. It is proposed that judicious annealing, which results in increasing crystallinity (while avoiding ageing or transesterification), induces a constraint on the amorphous region leading to an increase in Tg and coalescence of free volume sites. © 1994 John Wiley & Sons, Inc. 相似文献
949.
The nature of the propagation center in the cationic polymerization of N-benzoyl-8-octanelactam initiated by octanoylium hexachlcroantimonate, SbCl5, and Ph3CAsF6 in perdeuterated tetrachloroethane or its mixture with o-dichlorobenzene was studied using 1H, 13C, 19F, 31P, 75As, and 121Sb nuclear magnetic resonance (NMR) of model oligomers and the products of their end-capping with triphenylphosphine. In all cases, the nature of the propagation center has been found to be of an acylium ion pair with an SbCl6? or AsF6? counterion coordinated with the nearest benzoylamide group and cosolvated by the solvent. © 1994 John Wiley & Sons, Inc. 相似文献
950.
V. K. Rybin L. P. Revina L. A. Baratova N. V. Makarov 《Chemistry of Natural Compounds》1992,27(4):518-519