全文获取类型
收费全文 | 835篇 |
免费 | 24篇 |
国内免费 | 5篇 |
专业分类
化学 | 703篇 |
晶体学 | 5篇 |
力学 | 4篇 |
数学 | 95篇 |
物理学 | 57篇 |
出版年
2023年 | 4篇 |
2022年 | 6篇 |
2021年 | 9篇 |
2020年 | 20篇 |
2019年 | 17篇 |
2018年 | 7篇 |
2017年 | 7篇 |
2016年 | 21篇 |
2015年 | 17篇 |
2014年 | 13篇 |
2013年 | 32篇 |
2012年 | 70篇 |
2011年 | 73篇 |
2010年 | 42篇 |
2009年 | 47篇 |
2008年 | 61篇 |
2007年 | 61篇 |
2006年 | 66篇 |
2005年 | 48篇 |
2004年 | 30篇 |
2003年 | 42篇 |
2002年 | 48篇 |
2001年 | 2篇 |
2000年 | 11篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 13篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1991年 | 5篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1930年 | 2篇 |
排序方式: 共有864条查询结果,搜索用时 15 毫秒
771.
Elisa Marenna Esther Fanelli Pasquale Pernice 《Journal of solid state chemistry》2009,182(5):1229-1234
With the final goal to obtain thin films containing stoichiometric lithium niobate nanocrystals embedded in an amorphous silica matrix, the synthesis strategy used to set a new inexpensive sol-gel route to prepare nanocomposite materials in the Li2O-Nb2O5-SiO2 system is reported. In this route, LiNO3, NbCl5 and Si(OC2H5)4 were used as starting materials. The gels were annealed at different temperatures and nanocrystals of several phases were formed. Futhermore, by controlling the gel compositions and the synthesis parameters, it was possible to obtain LiNbO3 as only crystallizing phase. LiNbO3-SiO2 nanocomposite thin films on Si-SiO2 and Al2O3 substrates were grown. The LiNbO3 average size, increasing with the annealing temperature, was 27 nm for a film of composition 10Li2O-10Nb2O5-80SiO2 heated 2 h at 800 °C. Electrical investigation revealed that the nanocrystals size strongly affects the film conductivity and the occurrence of hysteretic current-voltage curves. 相似文献
772.
773.
Chromatography in authenticity and traceability tests of vegetable oils and dairy products: a review 总被引:1,自引:0,他引:1
The new applications of various chromatographic techniques such as gas-liquid chromatography, high-performance liquid chromatography and electrophoretic methods employed for the analysis in macro- and micro-components in vegetable oils and dairy products are compiled and critically evaluated. The employment of these methods for authenticity tests and traceability is discussed. 相似文献
774.
Fraga MF Uriol E Borja Diego L Berdasco M Esteller M Cañal MJ Rodríguez R 《Electrophoresis》2002,23(11):1677-1681
A new approach to the evaluation of the relative degree of genomic DNA methylation through the quantification of 2'-deoxynucleosides is proposed. Detection and quantification of 5-methyl 2'-deoxycytidine in genomic DNA has been performed using micellar high-performance capillary electrophoresis (HPCE) with UV-Vis detection. This approach has been demonstrated to be more sensitive and specific than other HPCE methods for the quantification of DNA methylation degree and also to be faster than other HPLC-based methods. The detection and quantification of nucleosides through enzymatic hydrolyses notably increases the specificity of the technique and allows its exploitation in the analysis of poorly purified and/or concentrated DNA samples such as those obtained from meristematic plant regions and paraffin-embedded tissues. 相似文献
775.
Cserháti T Forgács E Darwish Y Morais H Mota T Ramos AC 《Journal of chromatography. A》2002,949(1-2):269-273
Multiwavelength spectrometry and reversed-phase high-performance liquid chromatography have been employed for the study of the effect of reduced glutathione and storage time on the pigments of paprika powders. The evaluation of the data by principal component analysis and cluster analysis proved that the storage time exerts the highest effect of the decomposition rate of pigments. The stability of some pigment fractions was modified in the presence of reduced glutathione. However, the effect was of secondary importance. Multiwavelength spectrometry combined with HPLC can be successfully used for the study of the stability of pigments of paprika powder. 相似文献
776.
Bröring M Tejero EC Pfister A Brandt CD Pérez Torrente JJ 《Chemical communications (Cambridge, England)》2002,(24):3058-3059
The metalation reactions of the 2,2'-bidipyrrin 4 with different rhodium(I) precursors yield the complexes 5 and 6 and the unusual corrinoid 7, depending only on the type of the ancillary ligand employed. 相似文献
777.
778.
The proper characterization of a commercial qualitative method for determining aflatoxin B1 in some nuts is described. A qualitative method that provides binary responses of the yes/no type means that the performance parameters have been properly adapted and defined. Performance characteristics such as the cut-off limit, the detection limit, sensitivity, specificity, false-positive and negative rates, and the unreliability or uncertainty region are defined and then estimated by means of the performance characteristic curves. The commercial test kit showed the cut-off limit at 1.6 ng/g, with a sensitivity rate of 95% and a false-negative rate of zero. A modification can be performed to shift the cut-off to 2.0 ng/g, keeping the same values for the sensitivity and false-negative rate. 相似文献
779.
Barrena E Palacios-Lidón E Munuera C Torrelles X Ferrer S Jonas U Salmeron M Ocal C 《Journal of the American Chemical Society》2004,126(1):385-395
The structure and stability of alkanethiols self-assembled on Au(111) have been studied as a function of the molecular chain length by means of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). Below saturation, phases consisting of molecules with different tilt angles and periodicities are formed. Differences in the mechanical stability of these phases are revealed by AFM experiments and discussed in terms of the competition between intermolecular and molecule-substrate interactions as a function of chain length. For long molecules, intermolecular interactions play a dominant role which stabilizes the formation of closed packed 30 degrees tilted ( radical 3x radical 3)R30 degrees structures. For short molecules, the van der Waals interaction with the gold substrate favors the formation of a 50 degrees tilted phase in which the molecules are arranged in a rectangular configuration. 相似文献
780.
Sominsky L Rozental E Gottlieb H Gedanken A Hoz S 《The Journal of organic chemistry》2004,69(5):1492-1496
When a solution of a carbonyl compound in alcohol (primary or secondary) is heated to ca. 300 degrees C, a disproportionation reaction, in which a carbonyl compound is reduced to the corresponding alcohol and the alcohol is oxidized to the corresponding ketone, takes place. This uncatalyzed variation of the Meerwein-Ponndorf-Oppenauer-Verley reaction gives, in certain cases, e.g., reduction of acetophenone or benzaldehyde by i-PrOH, almost quantitative yields. Yields are higher with secondary alcohols such as i-PrOH than with a primary alcohol such as EtOH. The reactions were also performed in a flow system by passing at a slow rate the same solutions through a glass or a metal coil heated to elevated temperatures. Ab initio calculations performed at the B3LYP/6-31G* level show that thermodynamically i-PrOH is a more potent reducing agent than EtOH by ca. 4 kcal/mol. The computations also show that in cases of aromatic carbonyl compounds, part of the deriving force is obtained from the entropy change of the reaction. The major contributor to the high yield, however, is the excess alcohol used, which shifts the equilibrium to the right. Calculated entropy of activation as well as isotopic H/D labeling suggest a cyclic transition state. 相似文献