全文获取类型
收费全文 | 411篇 |
免费 | 9篇 |
国内免费 | 1篇 |
专业分类
化学 | 255篇 |
晶体学 | 10篇 |
力学 | 20篇 |
数学 | 80篇 |
物理学 | 44篇 |
无线电 | 12篇 |
出版年
2022年 | 4篇 |
2021年 | 8篇 |
2019年 | 4篇 |
2017年 | 4篇 |
2016年 | 9篇 |
2015年 | 9篇 |
2014年 | 5篇 |
2013年 | 20篇 |
2012年 | 12篇 |
2011年 | 20篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 19篇 |
2007年 | 27篇 |
2006年 | 13篇 |
2005年 | 13篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 11篇 |
2001年 | 3篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1976年 | 4篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1967年 | 3篇 |
1948年 | 2篇 |
1918年 | 2篇 |
1910年 | 2篇 |
1887年 | 2篇 |
1884年 | 4篇 |
排序方式: 共有421条查询结果,搜索用时 765 毫秒
361.
362.
363.
Vincenzo Campisciano Benedetto Taormina Alberto Spinella Leonarda F. Liotta Francesco Giacalone Michelangelo Gruttadauria 《Molecules (Basel, Switzerland)》2022,27(8)
The hydrolysis of 3-ammoniumpropylbis(catecholato)silicate 1, giving two different silica-based materials containing different amounts of tris(catecholato)silicate, is reported. The latter species can be formed through an attack of catechol to the silicon atom in the pentacoordinate complex, in which the silicon-carbon bond is further activated toward electrophilic proton cleavage. The Knoevenagel reaction was used as a probe in order to test the availability of functional groups on the surface of such materials. 相似文献
364.
A study of the interaction between paraquat (methyl viologen) and humic acids, extracted from a soil amended over 30 years with crop residues, cow slurries and cattle manure, was carried out by two emission spectroscopies based on plasmonic effects: surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). To carry out this study Ag nanoparticles were used. The complex formation was tested by analyzing the effect of the herbicide on humic acids, and by varying experimental parameters such as the pH and the laser excitation wavelength. The study of the vibrational bands led to infer information about the interaction mechanism of paraquat with humic acids and to find a correlation between this interaction and the humic acids structural modification induced by the different amendments added to soil. 相似文献
365.
Summary In the present report, it is examined low frequency oscillations built up in a bi-propellant rocket engine equipped with pressurized feeding system.Some of the main parameters having a predominant effect on the phenomenon are investigated and the obtained criteria is found useful for the engine design purposes.All graphs are presented in a non-dimensional form which leads to a generalized use of the above mentioned method.Numerical results were processed by means of a high speed digital computer of the calculation center of Politecnico di Milano.
相似文献
Sommario Nel presente articolo si studiano le oscillazioni a bassa frequenza nei motori a razzo bi-propellente, con sistema di alimentazione e pressurizzazione.Si sono rilevati i parametri fondamentali che influenzano il fenomeno e si sono ottenuti dei criteri di calcolo utili in sede di progettazione.Tutti i grafici sono riportati in forma adimensionale in modo da rendere generale il metodo di calcolo.I risultati numerici sono stati ottenuti con il calcolatore elettronico IBM7040 in dotazione del Centro di calcolo del Politecnico di Milano.
相似文献
366.
367.
Rita Menicagli Corrado Malanga Luciano Lardicci 《Journal of heterocyclic chemistry》1979,16(4):667-672
The Fischer and Brunner reactions as well as the Grignard alkylation of isatin have been used to prepare (S)-3-sec-butyl- and (S)-3-(2-methylbutyl)indoles, respectively, starting from (S)-1-chloro-2-methylbutane. The stereospecificity of the synthetic sequences followed has been investigated and the stereochemical relationships of the title compounds have been established. 相似文献
368.
Mario Nardelli Corrado Pelizzi Giancarlo Pelizzi 《Journal of organometallic chemistry》1976,112(3):263-272
A nitrato-complex of organotin(IV) containing triphenylphosphine oxide, Sn(C6H5)3(NO3) [(C6H5)3PO], has been synthesized and characterized by infrared spectroscopy and X-ray structural analysis. The compound crystallizes in space group P 1¯, with a = 11.817(6), b = 11.086(6), c = 12.471(6), α = 99.6(1),β = 90.8(1), γ = 97.8(1), Z = 2. The structure has been solved from X-ray diffractometer data by Patterson and Fourier methods and refined by least-squares calculations to R = 6.4% for 4301 independent reflections. The structure consists of discrete monomer units in which tin shows trigonal bipyramidal geometry. 相似文献
369.
Massimo Paci Corrado Barone Pier Luigi Magagnini 《Journal of Polymer Science.Polymer Physics》1987,25(8):1595-1605
The thermal properties of solution-prepared blends of poly(butylene terephthalate) (PBT) and liquid crystalline poly(biphenyl-4,4′-ylene sebacate) (PB8) have been studied by differential scanning calorimetry (DSC). The smectic-to-isotropic (s → i) transition of the mesomorphic component is observed at temperatures slightly increasing with the PB8 content, in the 270–280°C range; on cooling, the mesophase formation (i → s transition) takes place at temperatures that decrease markedly with decreasing PB8 content. The temperatures of the crystal-to-smectic and smectic-to-crystal transitions of PB8 are almost invariant with blend composition. The melting and crystallization temperatures of the PBT phase decrease on increasing the content of the liquid crystalline component. These results, together with those of isothermal calorimetry of both the crystallization of the PBT component and the mesophase formation of the PB8 component of the blends, indicate that the two polymers are not immiscible in the isotropic state. In this latter state, however, the two polyesters undergo transesterification, which can be followed through changes in the DSC scans. The effect of the thermal history on the properties of the blends has been studied with particular attention. Both the dynamic and the isothermal calorimetric measurements provide evidence of an increase of the degree of crystallinity of PBT on addition of the liquid crystalline component up to about 35–50% by weight. 相似文献
370.
The special basis in spaces of finite dimensional representation ofS
N and GL(n) is constructed and its properties are studied.
Both authors are partially supported by the National Science Foundation. 相似文献