全文获取类型
收费全文 | 329218篇 |
免费 | 4340篇 |
国内免费 | 1155篇 |
专业分类
化学 | 170874篇 |
晶体学 | 4458篇 |
力学 | 13035篇 |
综合类 | 7篇 |
数学 | 36350篇 |
物理学 | 94861篇 |
综合类 | 15128篇 |
出版年
2020年 | 1987篇 |
2018年 | 1991篇 |
2016年 | 3894篇 |
2015年 | 3260篇 |
2014年 | 4457篇 |
2013年 | 14015篇 |
2012年 | 11021篇 |
2011年 | 13758篇 |
2010年 | 8369篇 |
2009年 | 8289篇 |
2008年 | 12670篇 |
2007年 | 12961篇 |
2006年 | 12638篇 |
2005年 | 11595篇 |
2004年 | 10397篇 |
2003年 | 9246篇 |
2002年 | 9112篇 |
2001年 | 10857篇 |
2000年 | 8344篇 |
1999年 | 6483篇 |
1998年 | 4912篇 |
1997年 | 4749篇 |
1996年 | 4798篇 |
1995年 | 4379篇 |
1994年 | 4128篇 |
1993年 | 3954篇 |
1992年 | 4774篇 |
1991年 | 4610篇 |
1990年 | 4371篇 |
1989年 | 4147篇 |
1988年 | 4430篇 |
1987年 | 4105篇 |
1986年 | 3950篇 |
1985年 | 5615篇 |
1984年 | 5615篇 |
1983年 | 4583篇 |
1982年 | 4881篇 |
1981年 | 4900篇 |
1980年 | 4680篇 |
1979年 | 4997篇 |
1978年 | 4977篇 |
1977年 | 4915篇 |
1976年 | 4822篇 |
1975年 | 4766篇 |
1974年 | 4744篇 |
1973年 | 4715篇 |
1972年 | 2715篇 |
1971年 | 2243篇 |
1970年 | 2193篇 |
1968年 | 2144篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
L. J. Yamí n S. E. Blanco J. M. Luco F. H. Ferretti 《Journal of Molecular Structure》1997,390(1-3):209-215
The isomerization mechanism of 2′(OH)chalcone (1) in flavanone (2) was studied. The calculations were performed with the semiempirical method AM1, using totally optimized molecular geometries. A 6-step mechanism including several equilibrium states was proposed. It was concluded that: (a) At the conformational equilibrium of 1 there could be 43.9% of s-cis conformer; (b) The acid dissociation of 1 trans-s-trans is considerable; (c) The EE, ΔHf and net charges show that the rotation of ring A of 1 and the formation of ring C of 2 occurs without greater impairments; (d) Although the keto structure is the most stable one, the enolate of 2 is present in the reaction medium; (e) The conversion of enol of 2 in the keto form would be the limiting step of the analyzed isomerization rate. 相似文献
993.
994.
A. Sachdev R. I. Masel J. B. Adams 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):310-312
The embedded atom method (EAM) was used to determine the equilibrium shapes of small platinum and palladium clusters (N=5–60 atoms). The stability of various polyhedral symmetries was performed at 0K. A search for the presence of any other lower energy structures was also carried out. Pt clusters reconstructed to lower energy clusters even at the magic numbers. Pd also reconstructed to lower energy structures except at the magic number sizes where the icosahedron was found to be the most stable. 相似文献
995.
996.
After carefull analysis in a turbulent zero-pressure gradient flow, various simple algebraic turbulence models were applied to the almost separated flow on the upperside of an airfoil at incidence. The Johnson-King and Horton non-equilibrium (or rate equation) models give clearly improved results. 相似文献
997.
998.
DNA fingerprinting transforms the art of cell authentication. 总被引:4,自引:0,他引:4
The increasing diversity of new cell cultures is seriously stretching the capabilities of traditional methods of identification. DNA fingerprinting is set to play an important role in increasing confidence in the authenticity of cultures in research and industry. 相似文献
999.
Chandhok M. Grizzle J.W. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1998,26(2):181-189
A semianalytical model for capacitively coupled radio frequency (RF) sheaths of asymmetric (unequal electrode area) systems has been developed. It can be applied in the high-frequency (ω > ω pi) regime at different pressures. An analytical approximation to the pressure-dependent ion density profile is used. The time-varying electric field and potential within the sheath are obtained by solving Poisson's equation. The current balance and zero net DC current conditions are applied to solve for the RF sheath parameters and DC bias voltage. The DC voltage ratio between the powered and grounded electrode sheaths increases as the pressure decreases, which results in a larger DC bias voltage at lower pressures 相似文献
1000.