全文获取类型
收费全文 | 370538篇 |
免费 | 4377篇 |
国内免费 | 1207篇 |
专业分类
化学 | 171385篇 |
晶体学 | 4473篇 |
力学 | 13093篇 |
综合类 | 7篇 |
数学 | 36480篇 |
物理学 | 95070篇 |
无线电 | 55614篇 |
出版年
2020年 | 2169篇 |
2019年 | 2118篇 |
2018年 | 2186篇 |
2017年 | 2066篇 |
2016年 | 4234篇 |
2015年 | 3469篇 |
2014年 | 4863篇 |
2013年 | 14854篇 |
2012年 | 11346篇 |
2011年 | 14138篇 |
2010年 | 9106篇 |
2009年 | 9711篇 |
2008年 | 14358篇 |
2007年 | 15061篇 |
2006年 | 14601篇 |
2005年 | 13668篇 |
2004年 | 12377篇 |
2003年 | 11058篇 |
2002年 | 10874篇 |
2001年 | 12330篇 |
2000年 | 9835篇 |
1999年 | 8028篇 |
1998年 | 6825篇 |
1997年 | 6599篇 |
1996年 | 6588篇 |
1995年 | 6168篇 |
1994年 | 5873篇 |
1993年 | 5688篇 |
1992年 | 6121篇 |
1991年 | 6004篇 |
1990年 | 5558篇 |
1989年 | 5196篇 |
1988年 | 5403篇 |
1987年 | 4433篇 |
1986年 | 4300篇 |
1985年 | 5970篇 |
1984年 | 6029篇 |
1983年 | 4963篇 |
1982年 | 5326篇 |
1981年 | 5289篇 |
1980年 | 5045篇 |
1979年 | 5092篇 |
1978年 | 5088篇 |
1977年 | 5059篇 |
1976年 | 4976篇 |
1975年 | 4864篇 |
1974年 | 4724篇 |
1973年 | 4860篇 |
1972年 | 2833篇 |
1971年 | 2131篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Conlon P.J. Tong Y.P. French P.M.W. Taylor J.R. Shestakov A.V. 《Electronics letters》1994,30(9):709-710
Sub-100 fs pulse generation from a Kerr lens modelocked Cr4+ :YAG laser has been demonstrated for the first time, yielding femtosecond pulses tunable from 1.49 to 1.56 μm with pulses as short as 90 fs obtained at 1.53 μm 相似文献
992.
Langanay J. Brindel P. Brun E. Provost J.G. Lesterlin D. Hamaide J.P. 《Electronics letters》1994,30(19):1599-1601
The variability of optical frequency modulation with electric field in an electro-optical phase section is analysed. When configured for maximum sensitivity, the electro-optical phase section was used to compensate for the chirp of the gain section: 48 ps and 8.5 GHz wide transform-limited pulses were obtained at 5 GHz 相似文献
993.
A general finite element approach to computing the modes of open-boundary waveguides is presented. This efficient and robust scheme applies equally to situations with real and complex propagation constants. By using a localised boundary impedance condition, the approach preserves the sparsity of the global matrices. A comparison with the well established infinite element technique and the analysis of a 2-D leaky optical structure are given as examples 相似文献
994.
Since the early demonstrations of our concept of pulsed microwave catalysis in chemistry, we have witnessed the coming of age of the technique as an enabling technology in the industrial world, despite the inadequate understanding of the basic mechanisms of how microwaves interact with metallic surface sites and the subsequent energy transfer. We shall present here some very recent developments of the microwave-induced acoustic technique and its applications to a better understanding of the basic surface heating and energy transfer phenomena in pulsed microwave catalytic systems. 相似文献
995.
J. Goschnick J. Schuricht H. J. Ache 《Fresenius' Journal of Analytical Chemistry》1994,349(1-3):203-205
A gravimetric method is presented for the easy determination of sputter erosion rates for powders, applicable also to non-conducting materials. Measurements of the erosion rates of 22 powders gave an average value of 0.4 nm/s ±50% for bombardment with 400 eV Ar+ at 1 mA/cm2. The validity of the data has been demonstrated with technical pigments and outdoor aerosol particles. 相似文献
996.
B. Hornetz H.-J. Michel J. Halbritter 《Fresenius' Journal of Analytical Chemistry》1994,349(1-3):233-235
ARXPS (angle resolved X-ray photoelectron spectroscopy) measurements are used to obtain informations about surfaces and grain boundaries. Data acquired from nanocrystalline carbidic hard coatings have been employed to establish structural models. Magnetron-sputtered coatings of TiC, SiC and TiC/SiC were examined. In such coatings, partly defective TiC nanocrystallites are surrounded by interfacial carbide. This excess carbon shows a binding state similar to that of doped graphite or fullerenes. X-ray amorphous SiC is found in the residue. On top of sputtered SiC coatings, less oxide and graphite is found as compared to TiC/SiC or TiC coatings. 相似文献
997.
Mingyang Zhao Warren T. Ford Stefan H. J. Idziak Nicholas C. Maliszewskyj Paul A. Heiney 《Liquid crystals》1994,16(4):583-599
The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N6 was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N6 has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH2)11 spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N6 mesogen. The polysiloxanes also display a liquid crystalline phase. 相似文献
998.
Christopher J. Booth John W. Goodby Judith P. Hardy Olwen C. Lettington Kenneth J. Toyne 《Liquid crystals》1994,16(6):925-940
The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense. 相似文献
999.
In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δεi, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique. 相似文献
1000.
A series of nitrosyl complexes of empirical formula Kn[M(CN)5NO], where M = V, Cr, Mn and Co and n = 3, or M = Mo and n = 4, have been prepared which are notional analogues of the widely used vasodilator sodium nitroprusside. Their reactivity towards common nucleophiles (OH?, NH2R, NHR2, HS? and RS?), acid and photolysis has been investigated to elucidate the desired properties required of new metal nitrosyls which may have some potential as new non-cyanide-based vasodilators. 相似文献