全文获取类型
收费全文 | 4806篇 |
免费 | 226篇 |
国内免费 | 87篇 |
专业分类
化学 | 3283篇 |
晶体学 | 27篇 |
力学 | 153篇 |
综合类 | 10篇 |
数学 | 564篇 |
物理学 | 1082篇 |
出版年
2023年 | 21篇 |
2022年 | 60篇 |
2021年 | 63篇 |
2020年 | 86篇 |
2019年 | 78篇 |
2018年 | 71篇 |
2017年 | 71篇 |
2016年 | 132篇 |
2015年 | 123篇 |
2014年 | 167篇 |
2013年 | 279篇 |
2012年 | 317篇 |
2011年 | 371篇 |
2010年 | 218篇 |
2009年 | 165篇 |
2008年 | 287篇 |
2007年 | 279篇 |
2006年 | 271篇 |
2005年 | 275篇 |
2004年 | 214篇 |
2003年 | 182篇 |
2002年 | 163篇 |
2001年 | 104篇 |
2000年 | 119篇 |
1999年 | 61篇 |
1998年 | 47篇 |
1997年 | 45篇 |
1996年 | 44篇 |
1995年 | 65篇 |
1994年 | 56篇 |
1993年 | 57篇 |
1992年 | 47篇 |
1991年 | 32篇 |
1990年 | 30篇 |
1989年 | 27篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 22篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 35篇 |
1976年 | 28篇 |
1975年 | 24篇 |
1974年 | 24篇 |
1973年 | 14篇 |
排序方式: 共有5119条查询结果,搜索用时 15 毫秒
941.
Polymer solutions are complex fluids that show elasticity and deformation in response to shear flows. A fluorescence resonance
energy transfer (FRET) technique has been applied to measure the end-to-end distances of individual polymer molecules in Couette
flow, using end-tagged reversible-addition fragmentation chain transfer (RAFT) polymerised poly(methyl methacrylate) (PMMA).
Real-time rheofluorescence measurements on these polymers in solution above the critical overlap concentration are reported
at several shear rates. The PMMA in Couette flow shows a systematic decrease in fluorescence, corresponding to a reduction
in end-to-end distance of the polymer molecules with shear exposure. Full reversibility of the fluorescence signal is observed
after the cessation of shear. These results show that polymer solution elasticity arises from compressive deformation of the
polymer molecules in Couette flow. At polymer concentrations above the critical overlap, the polymer molecules are restricted
by their neighbours and the net hydrodynamic forces are compressive rather than extensive. 相似文献
942.
H. F. Chau H. Y. Chan F. K. Chow 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,72(4):641-655
Recently, Yin et al. [Eur. Phys. J. B 49, 205 (2006)] introduced an
efficient small-world network traffic model using preferential next-nearest
neighbor routing strategy with the so-called path iteration avoidance (PIA)
rule to study the jamming transition of internet.
Here we study their model without PIA rule by a mean-field analysis which
carefully divides the message packets into two types. Then, we argue that
our mean-field analysis is also applicable in the presence of PIA rule
in the limit of a large number of nodes in the network.
Our analysis gives an explicit expression of the critical packet injection
rate Rc as a function of a bias parameter of the routing strategy
α in their model with or without PIA rule. In particular, we predict
a sudden change in Rc at a certain value of α. These predictions
agree quite well with our extensive computer simulations. 相似文献
943.
In pursuit of photo‐curable adhesive for optical communication, dual‐curable acrylic oligomers (AOs) having alkoxy silane group, fluorine atoms and vinyl group as a pendent group were synthesized by two‐stage reactions. The isocyanate group containing oligomers were firstly synthesized via radical polymerization of acrylic monomers, and followed by urethane reaction with 2‐hydroxy ethyl methacrylate. The dual curing behaviors, e.g. thermal and photo‐cure, were studied by using photo‐differential scanning calorimetry (DSC) and real‐time IR. An optimum adhesive formulation, based on AO (15 g), epoxy acrylate (80 g), isobonyl methacrylate (17 g) and photo‐initiator (3 g), was obtained. As the content of AO was increased in the optical adhesive formulation, refractive index decreased but transmittance increased due to the increase in fluorine content. The optical transmittance at the range of 1.3 to 1.55 μm was higher than 90%. The addition of colloidal silica with the earlier mentioned formulation was helpful in decreasing crosslinking volume shrinkage and the increasing of glass fiber adhesion. The required properties for the optical adhesive, including chemical resistance and thermal resistance, dimension stability, etc. were also investigated. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
944.
Equilibrium mole fraction solubility of Zocor® (simvastatin) a pharmaceutically important compound, was measured between 279 and 315 K, in fifteen different industrial-relevant organic solvents including: methyl acetate, ethyl acetate, propyl acetate, iso-propyl acetate, butyl acetate, iso-butyl acetate, sec-butyl acetate, tert-butyl acetate, and ethanol, propanol, 1-butanol, 2-butanol, 1-pentanol, 1-hexanol, and 1-octanol. Fusion enthalpy, ΔfusH, melting point temperature, Tm, were measured to be 32,169 J/mol, 412.6 K, respectively; and the difference in the molar heat capacity (at constant pressure) of the liquid, and solid form of simvastatin, ΔCP, was approximated (by extrapolation) to be 230 J/mol K. Dissolution of simvastatin was found to be endothermic, and entropically favorable. The activity coefficient at infinite dilution of simvastatin in each solvent was calculated from the experimental data, then fitted to Gibbs–Helmholtz equation to estimate the limiting partial molar excess enthalpies, , and the limiting partial molar excess entropies, . The data was also fitted to the non-random-two-liquid (NRTL) activity coefficient equation to generate the model interaction parameters for dissolution of simvastatin in the organic solvents studied here. 相似文献
945.
Kelly K. McGrath Kyoungmi Jang Kathleen A. Robins Prof. Dong‐Chan Lee Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(16):4070-4077
Tuning electronic properties and morphologies : We report a unique design platform of n‐type organic semiconductors based on asymmetrically substituted bisphenazines that enable tuning of both electronic properties and morphologies of 1D nanostructures (see figure) by using small substituents with various sizes and electronic demands.
946.
Xiaohang Zhang Christopher A. Paddon Yohan Chan Philip C. Bulman‐Page Paul S. Fordred Steven D. Bull Ho‐Chol Chang Nadeem Rizvi Frank Marken 《Electroanalysis》2009,21(12):1341-1347
Mineral oil microdroplets containing the model antioxidant N,N‐didodecyl‐N′,N′‐diethyl‐phenylene‐diamine (DDPD) are immobilized into a 100×100 pore‐array (ca. 10 μm individual pore diameter, 100 μm pitch) in a boron‐doped diamond electrode surface. The robust diamond surface allows pore filling, cleaning, and reuse without damage to the electrode surface. The electrode is immersed into aqueous electrolyte media, and voltammetric responses for the oxidation of DDPD are obtained. In order to further improve the current responses, 20 wt% of carbon nanofibers are co‐deposited with the oil into the pore array. Voltammetric signals are consistent with the oxidation of DDPD and the associated transfer of perchlorate anions (in aqueous 0.1 M NaClO4) or the transfer of protons (in aqueous 0.1 M HClO4). From the magnitude of the current response, the DDPD content in the mineral oil can be determined down to less than 1 wt% levels. Perhaps surprisingly, the reversible (or midpoint) potential for the DDPD oxidation in mineral oil (when immersed in 0.1 NaClO4) is shown to be concentration‐dependent and to shift to more positive potential values for more dilute DDPD in mineral oil solutions. An extraction mechanism and the formation of a separate organic product phase are proposed to explain this behavior. 相似文献
947.
Tsz Ho Chan 《Archiv der Mathematik》2009,92(4):303-313
In this paper, we continue the study of almost squares; these are integers n representable as n = a · b for some . We show that almost all (in the measure–theoretic sense) short intervals [x, x + (log x)12] contain at least one almost square, and we consider related questions. Moreover, a result of Erdős shows that the exponent
12 cannot be smaller than 0.086 ....
Received: 26 June 2008; Revised: 22 December 2008 相似文献
948.
Heng Huat Chan 《Journal of Number Theory》2009,129(7):1786-1797
With two elementary trigonometric sums and the Jacobi theta function θ1, we provide a new proof of two Ramanujan's identities for the Rogers-Ramanujan continued fraction in his lost notebook. We further derive a new Eisenstein series identity associated with the Rogers-Ramanujan continued fraction. 相似文献
949.
In this paper, we study rings with the annihilator condition (a.c.) and rings whose space of minimal prime ideals, , is compact. We begin by extending the definition of (a.c.) to noncommutative rings. We then show that several extensions over semiprime rings have (a.c.). Moreover, we investigate the annihilator condition under the formation of matrix rings and classical quotient rings. Finally, we prove that if R is a reduced ring then: the classical right quotient ring Q(R) is strongly regular if and only if R has a Property (A) and is compact, if and only if R has (a.c.) and is compact. This extends several results about commutative rings with (a.c.) to the noncommutative setting. 相似文献
950.