全文获取类型
收费全文 | 5323篇 |
免费 | 78篇 |
国内免费 | 33篇 |
专业分类
化学 | 3488篇 |
晶体学 | 69篇 |
力学 | 150篇 |
数学 | 892篇 |
物理学 | 835篇 |
出版年
2020年 | 41篇 |
2016年 | 62篇 |
2015年 | 49篇 |
2014年 | 67篇 |
2013年 | 215篇 |
2012年 | 184篇 |
2011年 | 210篇 |
2010年 | 127篇 |
2009年 | 108篇 |
2008年 | 218篇 |
2007年 | 209篇 |
2006年 | 240篇 |
2005年 | 222篇 |
2004年 | 171篇 |
2003年 | 195篇 |
2002年 | 166篇 |
2001年 | 97篇 |
2000年 | 91篇 |
1999年 | 71篇 |
1998年 | 55篇 |
1997年 | 86篇 |
1996年 | 83篇 |
1995年 | 90篇 |
1994年 | 91篇 |
1993年 | 93篇 |
1992年 | 72篇 |
1991年 | 81篇 |
1990年 | 74篇 |
1989年 | 66篇 |
1988年 | 68篇 |
1987年 | 54篇 |
1986年 | 64篇 |
1985年 | 99篇 |
1984年 | 95篇 |
1983年 | 70篇 |
1982年 | 107篇 |
1981年 | 91篇 |
1980年 | 96篇 |
1979年 | 94篇 |
1978年 | 89篇 |
1977年 | 90篇 |
1976年 | 75篇 |
1975年 | 73篇 |
1974年 | 88篇 |
1973年 | 95篇 |
1972年 | 44篇 |
1971年 | 48篇 |
1970年 | 42篇 |
1969年 | 37篇 |
1968年 | 37篇 |
排序方式: 共有5434条查询结果,搜索用时 15 毫秒
21.
The evolution of the viscoelastic behavior of an epoxy resin at various stages of curing has been followed with the changes in the retardation spectrum. The creep J(t) and recoverable creep compliance Jr(t) curves of the neat epoxy resin Epon l00lF (Shell) were determined at temperatures between 30 and 77°C. The viscosity decreased over 8 orders of magnitude as the temperature was increased. Specimens with eight stages of network development were prepared by reacting all of the epoxy resin's oxirane rings with amine hydrogens from varying ratios of a monofunctional amine (methyl aniline) and a tetrafunctional amine 4,4′-diamino diphenyl sulfone (DDS). Preparations in which 25, 35, and 40% DDS were used did not result in a molecular network, so they were viscoelastic liquids. With 45% DDS, the product had a nascent network and was judged to be just beyond the point of incipient gelation. The remaining preparations from 0.50, 0.60, 0.70, and 1.0 DDS yielded tighter less compliant molecular networks. The creep and recoverable compliance curves were measured over a range of temperatures above the glass transition temperature, Tg. They were reduced to Tg, and retardation spectra L(ln τ) were calculated. 相似文献
22.
Donald Craig 《Tetrahedron: Asymmetry》1991,2(12):1177-1180
The synthesis and thermally-induced intramolecular Diels-Alder (IMDA) reaction of (S)s-(+)-S-[(1E,7E)-1,7,9-decatrienyl]-S-phenyl-N-(p-tolylsulphonyl)sulphoximine 4 is described. A model is proposed for the cis-selectivity of the cycloaddition based on previous results from the reactions of sulphonyl-substituted trienes. The stereochemistry of the major cis-fused cycloadduct is determined by single-crystal X-ray analysis of that obtained from reaction of racemic 4. 相似文献
23.
24.
Kenneth A. Caldwell Michael L. Gross 《Journal of the American Society for Mass Spectrometry》1994,5(2):72-91
Accurate mass measurements were used to assign elemental compositions and tandem mass spectrometry was used to characterize the peak-at-every-mass background ions produced by kiloelectron-volt-particle bombardment of neat fast-atom bombardment matrices. The majority of the background ions observed in the mass spectrum of neat glycerol was identified. On the basis of the experiments with glycerol, a theory for the formation of background ions is presented. Results are discussed according to the chemical and physical changes that ygoe;on-volt-particle bombardment produces in the matrix. 相似文献
25.
26.
Isabel Echeverria Ping-Chung Su Sindee L. Simon Donald J. Plazek 《Journal of Polymer Science.Polymer Physics》1995,33(17):2457-2468
Creep and differential scanning calorimetry (DSC) measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures ranged from 160°C to Tg with aging times ranging from 10 min to 8 days. The only measurable effect of physical aging on the short-time creep curves is a shift of the creep compliance to longer times. Andrade plots of the compliance versus the cube root of time are linear at short times with the slope β decreasing with increasing aging time to a constant value once equilibrium is reached. Log β3 is related directly to the degree to which the creep curves shift to longer times with physical aging, and is used in this work as a measure of physical aging. A reduced curve of log β3 versus log aging time is obtained for the aging temperatures investigated by appropriate vertical and horizontal shifts. The enthalpy change during aging increases linearly with the logarithm of the aging time, ta, leveling off at equilibrium at values which increase with decreasing aging temperature. Hence, both nonequilibrium and equilibrium temperature shift factors can be calculated from the DSC data. Good agreement is observed between the equilibrium temperature shift factors obtained from the creep and DSC data. The temperature dependence of the nonequilibrium temperature shift factors is found to be an order of magnitude smaller than that of the equilibrium shift factors. The time scales to reach equilibrium for enthalpy and for mechanical measurements are found to be the same within experimental error. © 1995 John Wiley & Sons, Inc. 相似文献
27.
28.
The purpose of this paper is to review the mechanisms and available theoretical methods for modeling the strength and failure of thin film/substrate systems 相似文献
29.
The high structural similarity of decarboxylase antibody 21D8 and esterase antibody 48G7 suggests that 21D8 might also possess hydrolytic activity. Kinetic investigations show that 21D8 does promote the selective hydrolysis of methyl 4‐nitrophenyl carbonate and sodium 4‐(acetoxy)benzenesulfonate with catalytic proficiencies (kcat/Km)/kun of ca. 105 M ?1. The ability of 21D8 to accelerate a reaction for which it was not developed suggests that certain antibody scaffolds are intrinsically predisposed toward catalysis, a property that can be enhanced and refined during affinity maturation in response to a transition‐state analog. At the same time, however, the restricted structural diversity of the immune system may ultimately limit the catalytic efficiency that can be achieved. 相似文献
30.
In multivariate statistical analysis, several authors have studied the total positivity properties of the generalized (0 F 1) hypergeometric function of two real symmetric matrix arguments. In this paper, we make use of zonal polynomial expansions to obtain a new proof of a result that these 0 F 1functions fail to satisfy certain pairwise total positivity properties; this proof extends both to arbitrary generalized ( r F s ) functions of two matrix arguments and to the generalized hypergeometric functions of Hermitian matrix arguments. In the case of the generalized hypergeometric functions of two Hermitian matrix arguments, we prove that these functions satisfy certain modified pairwise TP2properties; the proofs of these results are based on Sylvester's formula for compound determinants and the condensation formula of C. L. Dodgson [Lewis Carroll] (1866). 相似文献