全文获取类型
收费全文 | 318533篇 |
免费 | 4392篇 |
国内免费 | 1103篇 |
专业分类
化学 | 173948篇 |
晶体学 | 4475篇 |
力学 | 13087篇 |
综合类 | 7篇 |
数学 | 37185篇 |
物理学 | 95326篇 |
出版年
2020年 | 2017篇 |
2019年 | 1991篇 |
2018年 | 1992篇 |
2017年 | 1901篇 |
2016年 | 3928篇 |
2015年 | 3310篇 |
2014年 | 4499篇 |
2013年 | 14116篇 |
2012年 | 10846篇 |
2011年 | 13533篇 |
2010年 | 8354篇 |
2009年 | 8334篇 |
2008年 | 12497篇 |
2007年 | 12718篇 |
2006年 | 12368篇 |
2005年 | 11337篇 |
2004年 | 10188篇 |
2003年 | 8978篇 |
2002年 | 8824篇 |
2001年 | 10178篇 |
2000年 | 7780篇 |
1999年 | 6181篇 |
1998年 | 4938篇 |
1997年 | 4798篇 |
1996年 | 4837篇 |
1995年 | 4430篇 |
1994年 | 4165篇 |
1993年 | 3987篇 |
1992年 | 4495篇 |
1991年 | 4337篇 |
1990年 | 4073篇 |
1989年 | 3867篇 |
1988年 | 4181篇 |
1987年 | 3844篇 |
1986年 | 3716篇 |
1985年 | 5411篇 |
1984年 | 5447篇 |
1983年 | 4435篇 |
1982年 | 4814篇 |
1981年 | 4852篇 |
1980年 | 4594篇 |
1979年 | 4724篇 |
1978年 | 4729篇 |
1977年 | 4694篇 |
1976年 | 4667篇 |
1975年 | 4577篇 |
1974年 | 4405篇 |
1973年 | 4570篇 |
1972年 | 2608篇 |
1971年 | 1912篇 |
排序方式: 共有10000条查询结果,搜索用时 27 毫秒
991.
992.
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
993.
994.
A number of biphenyl, terphenyl analogues and ethynes which contain a pyrazine ring have been made and their liquid crystal transition temperatures, together with examples of birefringence measurements, are reported. All the 2,5-disubstituted pyrazine systems are liquid crystalline showing high birefringence values for the biphenyl and terphenyl analogues, whereas the 1,5-disubstituted systems are not liquid crystalline. The pyrazine ethyne systems exhibit very high birefringence values. X-ray diffraction has been used to identify the liquid crystal phases of 2-n-nonyloxy-5-(4'-propylbiphenyl-4-yl)pyrazine. 相似文献
995.
We study general relativity in the framework of non-commutative differential geometry. As a prerequisite we develop the basic notions of non-commutative Riemannian geometry, including analogues of Riemannian metric, curvature and scalar curvature. This enables us to introduce a generalized Einstein-Hilbert action for non-commutative Riemannian spaces. As an example we study a space-time which is the product of a four dimensional manifold by a two-point space, using the tools of non-commutative Riemannian geometry, and derive its generalized Einstein-Hilbert action. In the simplest situation, where the Riemannian metric is taken to be the same on the two copies of the manifold, one obtains a model of a scalar field coupled to Einstein gravity. This field is geometrically interpreted as describing the distance between the two points in the internal space.Dedicated to H. ArakiSupported in part by the Swiss National Foundation (SNF) 相似文献
996.
The chemical mechanism of the precipitation of lead titanium peroxohydroxide particles in a solution of nitrilotriacetate (NTA) complexes of lead and titanium peroxo-hydroxide is envisioned as the interaction between cations of lead hydroxide and anions of polymeric titanium peroxo-hydroxide. 相似文献
997.
V. D. Fedotov N. P. Obuchov R. A. Zadikhanov J. Spěváček J. Straka 《Applied magnetic resonance》1993,4(4):491-511
The combined analysis of1H and13C NMR relaxation data in solid lysozyme and some typical homopolypeptides was carried out by using “model-free” approach. Three types of relaxation transitions (γ’, γ and β) were revealed in the temperature range investigated. The microdynamical parameters of these motions were determined. From the comparison of these parameters with those of selected synthetic polymers it follows that the molecular motions in proteins and synthetic polymers are of the same nature. All these motions show pronounced anisotropic character. In the investigated temperature range no molecular motions corresponding to α-relaxation (liquid-like) transition were revealed. The hydration effects on parameters of the motions in proteins were considered. The most pronounced effect takes place for β-transition. The effect of Brownian rotation of protein molecule in solution on measured correlation function of local motions was also discussed. 相似文献
998.
The stability behaviour of a thin-film superconductor under a localized release of thermal disturbance is investigated. Two-dimensional conjugate film/substrate conduction equation with anisotropic thermal conductivity of the film, and Joule heat are employed to investigate effects of substrate and thermal properties on the intrinsic stability and quenching recovery. To consider the thermal boundary resistance between film and substrate, an interfacial-layer model (ILM) with very low diffusivity and an acoustic mismatch model (AMM) are employed. Results show that the thermal boundary resistance influences strongly the intrinsic stability. Thermal boundary resistance increases intrinsic stability if the thermal conductivity of the substrate or the disturbance energy is large. Higher Biot numbers and thermal conductivity ratios of film to substrate in longitudinal direction influence stability favorably. We demonstrate also that operation of a film/substrate system, such as YBCO/MgO, is either intrinsically stable or irrecoverably unstable.The authors wish to express their sincere appreciation to Dr. R. C. Chen for his invaluable advice and suggestions during the course of this paper. This research was supported by the National Science Council of the R. O. C. through grant NSC 83-0401-E-009-006. The computations were performed on the IBM ES/9000 at the National Center For High-Performance Computing. 相似文献
999.
用漫反射红外光谱和光声红外光谱研究了金属羰基化合物(CpFe(CO)2)2Cp=η^5-C5H5与酸性,中性和碱性Al2O3及TiO2的相互作用,结果表明,在Al2O3表面生成的洗生物种类及浓度与Al2O3的酸碱度明显相关,在酸性Al2O3表面,主要存在衍生物(CpFe(CO)2Fe-H-Fe(CO2Cp)^+及少量的CpFe(CO)2(-O-);在中性Al2O3表面存在的CpFe(CO)2(-O 相似文献
1000.
M. J. Fernndez J. M. Casares E. Glvez J. Bellanato 《Journal of heterocyclic chemistry》1993,30(3):815-817
The methyl 2,6-diphenyl-1-methyl-4-oxopiperidine-3,5-dicarboxylates 1 were synthesized by the Mannich procedure from methyl 3-oxoglutarate, benzaldehyde and methylamine. Keto-enol tautomerism as well as configurational isomerism at C-2 were observed. The stereochemistry of the Ib and Ic enolic forms were determined by 1H and 13C nmr data. 相似文献