全文获取类型
收费全文 | 502026篇 |
免费 | 18501篇 |
国内免费 | 1602篇 |
专业分类
化学 | 286590篇 |
晶体学 | 7205篇 |
力学 | 23384篇 |
综合类 | 13篇 |
数学 | 56562篇 |
物理学 | 148375篇 |
出版年
2023年 | 4551篇 |
2021年 | 6074篇 |
2020年 | 8283篇 |
2019年 | 6680篇 |
2018年 | 8109篇 |
2017年 | 6842篇 |
2016年 | 14063篇 |
2015年 | 10418篇 |
2014年 | 13243篇 |
2013年 | 26352篇 |
2012年 | 19779篇 |
2011年 | 21624篇 |
2010年 | 17670篇 |
2009年 | 17458篇 |
2008年 | 19562篇 |
2007年 | 19358篇 |
2006年 | 17386篇 |
2005年 | 15464篇 |
2004年 | 14260篇 |
2003年 | 12491篇 |
2002年 | 12429篇 |
2001年 | 14568篇 |
2000年 | 10996篇 |
1999年 | 8549篇 |
1998年 | 6918篇 |
1997年 | 6907篇 |
1996年 | 6537篇 |
1995年 | 5775篇 |
1994年 | 5560篇 |
1993年 | 5486篇 |
1992年 | 6010篇 |
1991年 | 6162篇 |
1990年 | 5732篇 |
1989年 | 5656篇 |
1988年 | 5501篇 |
1987年 | 5529篇 |
1986年 | 5217篇 |
1985年 | 6982篇 |
1984年 | 6997篇 |
1983年 | 5780篇 |
1982年 | 6072篇 |
1981年 | 5762篇 |
1980年 | 5662篇 |
1979年 | 5857篇 |
1978年 | 5946篇 |
1977年 | 6046篇 |
1976年 | 5991篇 |
1975年 | 5864篇 |
1974年 | 5731篇 |
1973年 | 5661篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
S. G. Charati A. Y. Houde S. S. Kulkarni M. G. Kulkarni 《Journal of Polymer Science.Polymer Physics》1991,29(8):921-931
Polymer packing density can be conveniently measured by either density-based measurements (direct or estimated free volume) or wide-angle X-ray diffraction (WAXD) spectra. Previously reported diffusivity and permeability values for various polyarylates and polyimides are examined on the basis of the packing density measured by these different techniques. It is shown that in these rigid glassy polymers, the mean intersegmental distance, the d-spacing, obtained from WAXD provides a better measure of the effect of polymer packing on diffusivity than either direct density measurements or estimated free volumes. It has been possible to correlate previously reported diffusivity and sorption data for various polyarylates and polycarbonate on the basis of the WAXD investigation, using the d-spacing and the amorphous peak width, respectively. The effect of polyarylate structure on d-spacing has been studied. A series of polyarylates have been synthesized with different substituents on the bridge carbon. In this series the d-spacing remains constant while the carbonyl group density and the glass transition temperatures are varied. However, small substituents on the aromatic ring cause significant changes in the d-spacing. These results can be used to tailor polyarylates with desired intersegmental distances and chain flexibility. 相似文献
992.
An analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition-dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation-period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor-phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 seconds. 相似文献
993.
Kinetics of oxidation of indigo carmine (IC) by sodium hypohalites, NaOX (X ? Cl or Br), in alkaline buffer of pH 9–11 has been studied spectrophotometrically at λ = 610 nm. The experimental rate law obtained is ?d[IC]/dt = k[OX?][IC][OH?]x where x <1. Variation of ionic strength or dielectric constant of the medium had no effect on the reaction rate while the addition of halide ions slightly retarded the rate. A most plausible mechanism proposed on the basis of experimental results involves the formation of isatin sulphonate which undergoes further oxidation to anthranilate. Activation parameters have been evaluated. 相似文献
994.
A. Ross M. Czisch G.C. King 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,124(2):355-365
A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT2values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τeinternal correlation times being affected somewhat more thanS2order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions. 相似文献
995.
The impurity weakening of crystalline NaCl:Mg and NaCl:Mn associated with purification of the matrix in a melt is demonstrated.
This effect is manifested in a consistent change in the yield strength and mobility of dislocations. Its unusual temperature
dependence is determined: in a number of cases weakening becomes greater as the temperature is lowered within the interval
293–4.2 K. A deviation in the elastic region of the stress-strain curves is observed to increase as the temperature is lowered.
Plasticizing of crystalline NaCl is established: when the deformation temperature is lowered from 293 to 77 and 4.2 K the
rate of rise in the yield strength of Cd2+ and Sr2+ doped samples is lower than that of undoped samples. An explanation for this fact is proposed.
Fiz. Tverd. Tela (St. Petersburg) 39, 313–317 (February 1997)
Deceased. 相似文献
996.
R. P. Sundarraj P. S. Sundararaghavan Dale R. Fox 《The Journal of the Operational Research Society》1994,45(5):549-558
Several production and flexible manufacturing systems can naturally be modelled using queueing networks. In this paper, we consider the problem of acquiring servers for the nodes of an open queueing network, so as to optimize the steady-state mean virtual system parameters subject to a budget constraint. A partial enumeration scheme and a heuristic method have been proposed to solve this problem. Empirical results based on randomly generated test problems are used to identify a class of problems for which the heuristic performs well. 相似文献
997.
We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.This work was supported in part by Swiss NSF grant 20-32644.91. 相似文献
998.
C. A. Beattie M. B. Boisen Jr. L. W. Johnson 《Journal of Optimization Theory and Applications》1991,68(1):203-203
Corrections to the last two equations of Ref. 1 are given. 相似文献
999.
1000.
Spectroscopic determinations of the concentrations associated with the onset of intermolecular interaction in solutions of poly(ethylene terephthalate) (PET) are described. The intersegment forces perturb the electronic spectra and induce ground-state aggregation. These phenomena become observable at different chain densities, reflecting the influence of concentration on the coil dimensions, as well as the dynamical nature of the interactions. An estimate of the equilibrium constant for PET dimer formation in hexafluoroisopropanol is reported. 相似文献