全文获取类型
收费全文 | 28767篇 |
免费 | 813篇 |
国内免费 | 775篇 |
专业分类
化学 | 4641篇 |
晶体学 | 881篇 |
力学 | 523篇 |
综合类 | 71篇 |
数学 | 21112篇 |
物理学 | 3127篇 |
出版年
2023年 | 97篇 |
2022年 | 238篇 |
2021年 | 194篇 |
2020年 | 204篇 |
2019年 | 637篇 |
2018年 | 692篇 |
2017年 | 438篇 |
2016年 | 421篇 |
2015年 | 397篇 |
2014年 | 846篇 |
2013年 | 1660篇 |
2012年 | 837篇 |
2011年 | 1861篇 |
2010年 | 1797篇 |
2009年 | 2004篇 |
2008年 | 2200篇 |
2007年 | 2194篇 |
2006年 | 1641篇 |
2005年 | 1228篇 |
2004年 | 1146篇 |
2003年 | 889篇 |
2002年 | 758篇 |
2001年 | 587篇 |
2000年 | 552篇 |
1999年 | 623篇 |
1998年 | 553篇 |
1997年 | 450篇 |
1996年 | 583篇 |
1995年 | 541篇 |
1994年 | 549篇 |
1993年 | 470篇 |
1992年 | 415篇 |
1991年 | 276篇 |
1990年 | 230篇 |
1989年 | 237篇 |
1988年 | 154篇 |
1987年 | 177篇 |
1986年 | 165篇 |
1985年 | 238篇 |
1984年 | 202篇 |
1983年 | 115篇 |
1982年 | 179篇 |
1981年 | 164篇 |
1980年 | 111篇 |
1979年 | 74篇 |
1978年 | 105篇 |
1977年 | 86篇 |
1976年 | 68篇 |
1975年 | 20篇 |
1974年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Francis Jordan 《Topology and its Applications》2006,153(14):2386-2412
An abstract scheme using particular types of relations on filters leads to general unifying results on stability under supremum and product of local topological properties. We present applications for Fréchetness, strong Fréchetness, countable tightness and countable fan-tightness, some of which recover or refine classical results, some of which are new. The reader may find other applications as well. 相似文献
32.
A Volterra type integral equation in a Hilbert space with an additional linear operator L and a spectral parameter depending
on time is considered. If the parameter does not belong to the spectrum of L unconditional solvability of the considered problem
is proved. In the case where the initial value of the parameter coincides with some isolated point of the spectrum of the
operator L sufficient conditions for solvability are established. The obtained results are applied to the partial integral
equations associated with a contact problem of the theory of elasticity. 相似文献
33.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献
34.
Thin films of, N-N′diphenyl 1-4phenylene-diamineane are prepared using vacuum sublimation technique. The electrical conductivity from room temperature down to 127 K is studied. It is found that the conduction of charge carriers obeys T−1/2 dependence on temperature. The average hopping distance, hopping energy, density of states and their variation due to post-deposition heat treatment are studied. Schottky diodes are fabricated with gold as ohmic contact and aluminium as Schottky contact. From the observed current voltage characteristics the saturation current density, diode ideality factor and the barrier height are determined. Their variation with air annealing is also investigated. 相似文献
35.
A Garcia-Quiroz David L AzevedoE.C da Silva 《Journal of Physics and Chemistry of Solids》2002,63(10):1863-1866
Properties of oxygenated carbon nitride films have attracted the attention of physics researchers due to their magnetic and physical properties, as well as for their usefulness in the industry. The free radicals were investigated using electron paramagnetic resonance applied in the study of spin concentration due to the different mechanism of preparation of carbon nitride films by RF-discharge with different kinds of plasma. Unpaired spin concentrations, in the order of 1020 per cm3, were measured and their time recombination dependency was important in those films. The films were grown by plasma enhanced chemical vapor deposition using mixtures of hydrocarbons, N2 and O2 in different proportions. 相似文献
36.
A. P. Vink E. van der Kolk P. Dorenbos C. W. E. van Eijk 《Optics Communications》2002,210(3-6):277-284
The optical properties of Ce3+ in CaSO4, SrSO4 and BaSO4 are reported. The Ce3+ ion shows 4f05d1→2F5/2,2F7/2 luminescence in all three sulphates. Co-doping with Na+ does not change the local surrounding of the Ce3+ ion, but enhances the amount of Ce3+ ions built in. Under optical excitation, besides the typical Ce3+ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce3+ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce3+ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton. 相似文献
37.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
38.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献
39.
基于校园网的实验管理系统的设计 总被引:2,自引:1,他引:1
主要讨论了应用实验室管理系统的必要性,并从用户需求分析、系统总体设计、功能模块的设计等方面对B/S实验室管理系统的开发进行了详细的阐述。 相似文献
40.
Using Brown's construction (J. Algebra 15 (1970) 103) of an exact 6-term sequence for a fibration of groupoids we show how an exact 9-term sequence can be associated to a fibration of bigroupoids. Applications to topology and algebra are given. 相似文献