全文获取类型
收费全文 | 331890篇 |
免费 | 3526篇 |
国内免费 | 935篇 |
专业分类
化学 | 166076篇 |
晶体学 | 5245篇 |
力学 | 15628篇 |
综合类 | 7篇 |
数学 | 38871篇 |
物理学 | 110524篇 |
出版年
2021年 | 3135篇 |
2020年 | 3382篇 |
2019年 | 3799篇 |
2018年 | 5222篇 |
2017年 | 5461篇 |
2016年 | 7432篇 |
2015年 | 4199篇 |
2014年 | 7094篇 |
2013年 | 16018篇 |
2012年 | 12488篇 |
2011年 | 14986篇 |
2010年 | 11099篇 |
2009年 | 11008篇 |
2008年 | 13354篇 |
2007年 | 13303篇 |
2006年 | 12253篇 |
2005年 | 10613篇 |
2004年 | 9926篇 |
2003年 | 8771篇 |
2002年 | 8748篇 |
2001年 | 10449篇 |
2000年 | 7734篇 |
1999年 | 6015篇 |
1998年 | 5010篇 |
1997年 | 4847篇 |
1996年 | 4532篇 |
1995年 | 3907篇 |
1994年 | 3861篇 |
1993年 | 3755篇 |
1992年 | 4142篇 |
1991年 | 4347篇 |
1990年 | 4137篇 |
1989年 | 4059篇 |
1988年 | 3831篇 |
1987年 | 3964篇 |
1986年 | 3724篇 |
1985年 | 4667篇 |
1984年 | 4704篇 |
1983年 | 3927篇 |
1982年 | 4006篇 |
1981年 | 3749篇 |
1980年 | 3640篇 |
1979年 | 3924篇 |
1978年 | 3915篇 |
1977年 | 3914篇 |
1976年 | 3903篇 |
1975年 | 3662篇 |
1974年 | 3620篇 |
1973年 | 3657篇 |
1972年 | 2601篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. Ferreira da Silva I. Pepe S.A. Tomás W.M. de Azevedo R. Ahuja 《Applied Surface Science》2006,252(15):5365-5367
In this paper we present spectroscopic properties of doped and undoped titanium dioxide (TiO2) as nanofilms prepared by the sol-gel process with rhodamine 6G doping and studied by photoacoustic absorption, excitation and emission spectroscopy. The absorption spectra of TiO2 thin films doped with rhodamine 6G at very low concentration during their preparation show two absorption bands, one at 2.3 eV attributed to molecular dimmer formation, which is responsible for the fluorescence quenching of the sample and the other at 3.0 eV attributed to TiO2 absorption, which subsequently yields a strong emission band at 600 nm. The electronic band structure and optical properties of the rutile phase of TiO2 are calculated employing a fully relativistic, full-potential, linearized, augmented plane-wave (FPLAPW) method within the local density approximation (LDA). Comparison of this calculation with experimental data for TiO2 films prepared for undoped sol-gels and by sputtering is performed. 相似文献
992.
High temperature annealing effect on structural and magnetic properties of Ti/Ni multilayer (ML) up to 600 °C have been studied and reported in this paper. Ti/Ni multilayer samples having constant layer thicknesses of 50 Å each are deposited on float glass and Si(1 1 1) substrates using electron-beam evaporation technique under ultra-high vacuum (UHV) conditions at room temperatures. The micro-structural parameters and their evolution with temperature for as-deposited as well as annealed multilayer samples up to 600 °C in a step of 100 °C for 1 h are determined by using X-ray diffraction (XRD) and grazing incidence X-ray reflectivity techniques. The X-ray diffraction pattern recorded at 300 °C annealed multilayer sample shows interesting structural transformation (from crystalline to amorphous) because of the solid-state reaction (SSR) and subsequent re-crystallization at higher temperatures of annealing, particularly at ≥400 °C due to the formation of TiNi3 and Ti2Ni alloy phases. Sample quality and surface morphology are examined by using atomic force microscopy (AFM) technique for both as-deposited as well as annealed multilayer samples. In addition to this, a temperature dependent dc resistivity measurement is also used to study the structural transformation and subsequent alloy phase formation due to annealing treatment. The corresponding magnetization behavior of multilayer samples after each stage of annealing has been investigated by using Magneto-Optical Kerr Effect (MOKE) technique and results are interpreted in terms of observed micro-structural changes. 相似文献
993.
Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) transmission, and Hall effect measurements were performed to investigate the structural, optical, and electrical properties of as-grown and in situ-annealed Hg0.7Cd0.3Te epilayers grown on CdTe buffer layers by using molecular beam epitaxy. After the Hg0.7Cd0.3Te epilayers had been annealed in a Hg-cell flux atmosphere, the SEM images showed that the surface morphologies of the Hg0.7Cd0.3Te thin films were mirror-like with no indication of pinholes or defects, and the FTIR spectra showed that the transmission intensities had increased in comparison to that of the as-grown Hg0.7Cd0.3Te epilayer. Hall-effect measurements showed that n-Hg0.7Cd0.3Te epilayers were converted to p-Hg0.7Cd0.3Te epilayers. These results indicate that the surface, optical, and electrical properties of the Hg1 − xCdxTe epilayers are improved by annealing and that as-grown n-Hg1 − xCdxTe epilayers can be converted to p-Hg1 − xCdxTe epilayers by in situ annealing. 相似文献
994.
995.
N.A. Buznikov A.S. Antonov A.B. Granovsky C.G. Kim C.O. Kim X.P. Li S.S. Yoon 《Journal of magnetism and magnetic materials》2006
The giant magnetoimpedance effect in composite wires consisting of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau–Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the magnetic material. 相似文献
996.
In this paper, we study an evasion problem in systems described by equations of parabolic type. We consider four versions of constraints on the control parameters. 相似文献
997.
S.A. Grigorian 《Topology and its Applications》2006,153(18):3598-3614
Finite-sheeted covering mappings onto compact connected groups are studied. We show that for a covering mapping from a connected Hausdorff topological space onto a compact (in general, non-abelian) group there exists a topological group structure on the covering space such that the mapping becomes a homomorphism of groups. To prove this fact we construct an inverse system of covering mappings onto Lie groups which approximates the given covering mapping. As an application, it is shown that a covering mapping onto a compact connected abelian group G must be a homeomorphism provided that the character group of G admits division by degree of the mapping. We also get a criterion for triviality of coverings in terms of means and prove that each finite covering of G is equivalent to a polynomial covering. 相似文献
998.
999.
Danny E. Akin W. Herbert Morrison III Luanne L. Rigsby Franklin E. Barton II David S. Himmelsbach Kevin B. Hicks 《Applied biochemistry and biotechnology》2006,129(1-3):104-116
Information is presented on structure, composition, and response to enzymes of corn stover related to barriers for bioconversion
to ethanol. Aromatic compounds occurred in most tissue cell walls. Ferulic acid esterase treatment before cellulase treatment
significantly improved dry weight loss and release of phenolic acids and sugars in most fractions over cellulase alone. Leaf
fractions were considerably higher in dry weight loss and released sugars with esterase treatment, but stem pith cells gave
up the most phenolic acids. Results help identify plant fractions more appropriate for coproducts and bioconversion and those
more suitable as residues for soil erosion control. 相似文献
1000.
Experimental data is presented for the interaction between a propagating flame and a simple vortex flow field structure generated
in the wake of solid obstacles. The interaction between gas movement and obstacles creates vortex shedding forming a simple
flow field recirculation. The presence of the simple turbulent structure within the gas mixture curls the flame front increasing
curvature and enhancing burning rate. A novel twin camera Particle Image Velocimetry, PIV, was employed to characterise the
flow field recirculation and the interaction with the flame front. The technique allowed the quantification of the flame/vortex
interaction. The twin camera technique provides data to define the spatial variation of both the velocity of the flow field
and flame front. Experimentally obtained values of local flame displacement speed and flame stretch rate are presented for
simple flame/vortex interactions. 相似文献