全文获取类型
收费全文 | 1299篇 |
免费 | 87篇 |
国内免费 | 11篇 |
专业分类
化学 | 964篇 |
晶体学 | 5篇 |
力学 | 30篇 |
综合类 | 1篇 |
数学 | 194篇 |
物理学 | 203篇 |
出版年
2023年 | 15篇 |
2022年 | 17篇 |
2021年 | 42篇 |
2020年 | 59篇 |
2019年 | 49篇 |
2018年 | 30篇 |
2017年 | 33篇 |
2016年 | 64篇 |
2015年 | 61篇 |
2014年 | 48篇 |
2013年 | 89篇 |
2012年 | 69篇 |
2011年 | 99篇 |
2010年 | 38篇 |
2009年 | 38篇 |
2008年 | 64篇 |
2007年 | 54篇 |
2006年 | 56篇 |
2005年 | 47篇 |
2004年 | 56篇 |
2003年 | 33篇 |
2002年 | 29篇 |
2001年 | 11篇 |
2000年 | 8篇 |
1998年 | 16篇 |
1997年 | 14篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1971年 | 6篇 |
排序方式: 共有1397条查询结果,搜索用时 281 毫秒
181.
Manfred J. R. Cantow Roger S. Porter Julian F. Johnson 《Journal of polymer science. Part A, Polymer chemistry》1967,5(6):1391-1394
Column systems for gel permeation chromatography are usually calibrated by eluting a series of sharp polymer fractions of known molecular weights and by correlating their elution volumes with molecular size or weight. A different method for calibration is proposed in which only one polymer sample, with a broad, well-characterized molecular weight distribution, is used. The cumulative distribution and the integrated, normalized GPC chromatogram are successively superimposed, and molecular weights and corresponding elution volumes are correlated. It is found that calibration curves obtained in this manner show a definite curvature. A possible explanation and correction for this behavior is given, based on the concentration dependence of elution volumes. 相似文献
182.
183.
The adaptation of the lattice-gas model to embody features possessed by water is further explored. On the basis of Martin's functional derivative formulation of Ising problems, a perturbation scheme is developed which allows calculation of the free energy to any desired order in the interaction potential at fixed density. The free energy correct to second order in the interaction strength is utilized here for calculation of other thermodynamic properties of the model. With reasonable choices of values of the interaction parameters these thermodynamic properties of the model can be brought into agreement with those of real water. 相似文献
184.
Julian I Palmore 《Journal of Differential Equations》1981,40(2):279-290
We prove the existence of many homographic solutions of the n-body problem in E4 by topological methods. Homographic solutions are associated with relative equilibria. Homothetic solutions always give rise to central configurations. In Euclidean space E4 central configurations are a proper subset of the relative equilibria for any n ? 3 and for any (mi)?R+n. We compare the existence and classification of homographic solutions of the n-body problem in E3 with the Newtonian potential and that of homographic solutions of the n-body problem in E4. Classifying relative equilibria leads to classifying homographic solutions. 相似文献
185.
The room-temperature photolysis of N2O (10–100 torr) at 2139 Å to produce O(1D) has been studied in the presence of CH4 (10–891 torr). The reactions of O(1D) with CH4 were found to be The method of chemical difference was used to measure the rate constant ratio k4/(k2 + k3), where reactions (2) and (3) are The CH3 radicals produced in reaction (4) react with the O2 and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C2H6} gives an accurate measure of k4/(k2 + k3). For the translationally energetic O(1D) atoms produced in the photolysis, k4/(k2 + k3) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k4/(k2 + k3) drops to 1.35 ± 0.3. 相似文献
186.
Julian Schwinger Kimball A. Milton Wu-Yang Tsai Lester L. DeRaad Donald C. Clark 《Annals of Physics》1976,101(2):451-495
The nonrelativistic problem of the scattering of two dyons (including the case of electron scattering by magnetic monopoles) is systematically studied, both classically and quantum mechanically, with a view toward the discrimination between various combinations of electric and magnetic charges. We analyze the classical cross section with particular attention to the interesting phenomena which occur for large angle scattering, the “rainbows” and “glory,” where the cross section becomes infinite. Quantum mechanically, we find that these infinities do not occur and that, when the partial wave scattering amplitude is summed, a very elaborate structure emerges for the cross section, which depends sensitively upon the electric and magnetic charges of the particles, as well as on their relative speed. We further discuss a large modification, leading to spin flip and nonflip amplitudes, due to the dipole moments of the particles. Numerical results are presented for a variety of values of these parameters. In principle, these results could be used to distinguish the δ-ray distributions produced by the various species of electrically and magnetically charged particles. Quite apart from the experimental implications of our numerical results, we have made a number of theoretical improvements and extensions. Numbered among these are the consideration of dyons and particles having dipole moments, and the explicit demonstration, based on the methods of angular momentum, that the differential cross section is independent of the choice of singularity line. 相似文献
187.
A total synthesis of 13-deoxytedanolide is described. The synthesis features a highly stereoselective fragment assembly aldol reaction of methyl ketone 4 and aldehyde 5 to establish the complete carbon skeleton of the natural product in the form of aldol 15. The facile formation of the remarkably unreactive hemiketal 16 thwarted attempts to elaborate 15 to tedanolide. However, deoxygenation of the C(13)-hydroxyl of 16 provided the 13-deoxy hemiketal 17 that was smoothly elaborated to 13-deoxytedanolide. 相似文献
188.
189.
The reaction of O(1D) with CH4 was studied to determine the efficiency of H2 production in a direct process, and it was found to be 0.11 ± 0.02. Thus the two channels which account for all of the reaction between O(1D) and CH4 in the gas phase are 相似文献
190.
Use of a thermomechanical analyzer, TMA, affords a rapid, precise method for determining linear expansion coefficients and linear expansivities of cross-linked polymeric insulation material. Completed cable samples were dissassembled and insulation specimens were cut longitudinally and examined with repeated temperature cycling: ethylene—propylene copolymer rubbers and filled and unfilled crosslinked polyethylenes. The TMA results indicated that all specimes were oriented, likely incurred from extrusion processing. The facility of TMA for investigating potential anisotropic behavior from prior orientation is illustrated by examples of TMA measurements on radial specimens, similarly free of mechanical constraints. 相似文献