全文获取类型
收费全文 | 397篇 |
免费 | 6篇 |
国内免费 | 4篇 |
专业分类
化学 | 233篇 |
晶体学 | 7篇 |
力学 | 1篇 |
数学 | 36篇 |
物理学 | 130篇 |
出版年
2022年 | 3篇 |
2021年 | 17篇 |
2020年 | 10篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 11篇 |
2015年 | 8篇 |
2014年 | 8篇 |
2013年 | 20篇 |
2012年 | 21篇 |
2011年 | 37篇 |
2010年 | 22篇 |
2009年 | 23篇 |
2008年 | 31篇 |
2007年 | 24篇 |
2006年 | 14篇 |
2005年 | 19篇 |
2004年 | 15篇 |
2003年 | 14篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1966年 | 1篇 |
排序方式: 共有407条查询结果,搜索用时 15 毫秒
71.
M. Fleisher D. Jansone G. Andreeva L. Leite E. Lukevics 《Chemistry of Heterocyclic Compounds》2001,37(6):688-693
The condensation of pyridinecarbaldehydes with 3-cyano-4,5,5-trimethyl-2(5H)-furanone and 3-cyano-4,6,6-trimethyl-5,6-dihydro-2(2H)-pyranone leads to the formation of the products of crotonic condensation and compounds of the Michael adduct type. A model investigation of the mechanism by the semiempirical AM1 quantum-chemical method for the case of the reaction of 3-pyridinecarbaldehyde with 3-cyano-4,5,5-trimethyl-2(5H)-furanone showed that the products can be formed in parallel from one and the same intermediate compound. 相似文献
72.
José MaurÍcio A. Caiut Shirley Nakagaki Omar J. De Lima Cesar Mello Carlos A.P. Leite Eduardo J. Nassar Katia J. Ciuffi Hérica C. Sacco 《Journal of Sol-Gel Science and Technology》2003,28(1):57-64
The hybrid organic-inorganic catalyst constituted by {5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraaza[14]annulenato} nickel (II), NiTMTAA, encapsulated in an alumina matrix has been prepared. NiTMTAA was synthesized by the reaction of nickel acetate with o-phenylenediamine in the presence of 2,4-pentanedione under argon atmosphere. The alumina hybrid material was obtained by a non hydrolytic sol-gel route, through the condensation of aluminum chloride with diisopropylether in the presence of NiTMTAA. The material has been prepared through precipitation from a gel. Characterization of the alumina hybrid material has been performed by ultra violet-visible spectroscopy, electron spectroscopic imaging, surface area, atomic absorption, infrared spectroscopy, and thermogravimetric analysis. The ultra violet-visible absorption spectrum of the hybrid material has bands characteristic of the NiTMTAA compound showing that the structure of NiTMTAA has been preserved in the hybrid material. The new material has a surface area of 300 m2/g. The electron image was that of a non-crystalline microstructure. Comparison between the leaching of NiTMTAA from NiTMTAA adsorbed on commercial neutral alumina confirm that in the non-hydrolytic materials the NiTMTAA is entrapped and not only adsorbed on the alumina surface. The use of conventional hydrolytic sol-gel process leads to the complete leaching of NiTMTAA from matrix, underlining the importance of the non-hydrolytic alumina gel process in the matrix preparation. The new catalysts prepared were tested for their ability to catalyze the epoxidation of (Z)-cyclooctene using iodosylbenzene as oxygen donor, giving moderate yields in the epoxidation (40%), while the homogeneous NiTMTAA is inactive due to NiTMTAA bleaching. These results emphasize the effect of the non-hydrolytic alumina matrix to prevent chemical degradation of NiTMTAA. 相似文献
73.
74.
Received: 8 April 1998/Revised version: 3 June 1998 相似文献
75.
Neves Pérson P. Maia Lauro J.Q. Bernardi Maria I.B. Zanatta Antônio R. Mastelaro Valmor R. Zanetti Sônia M. Leite Edson R. 《Journal of Sol-Gel Science and Technology》2004,29(2):89-96
A modified polymeric precursor method based on the Pechini process was used to synthesize -BaB2O4 (-BBO) crystalline phase. D-sorbitol (C6H14O6) was used as a polymerizing agent to avoid the loss of boron during the samples' calcination and crystallization. The -BBO stoichiometric crystalline phase was only obtained when sorbitol was added to the solution. The results of Raman spectroscopy show that the amorphous phase is only completely eliminated when the samples are heat-treated at 750°C for 20 h. Thin films of -BBO phase displaying a preferred orientation were obtained when crystallized at 750°C for 2 h and deposited on sapphire substrate. 相似文献
76.
Renato Sonchini Gonçalves Expedito Leite Silva Noboru Hioka Celso Vataru Nakamura Marcos Luciano Bruschi Wilker Caetano 《Natural product research》2018,32(3):366-369
Different from works described in the literature, which use expansive analytical methods to separation of anthraquinones derivatives (AQs), this communication reported a simple and inexpensive methodology to get them. In this way, the expensive commercial AQs: Chrysophanol, physcione and emodine were extracted from plant material (Rhamnus frangula L.) and isolated by classical column chromatography technique under optimised binary mobile phase gradients (CHCl3 : AcOEt(a), a = 1 to 5%) in excellent yields. 相似文献
77.
M. I. B. Bernardi S. C. L. Crispim A. P. Maciel A. G. Souza M. M. Conceiçăo E. R. Leite E. Longo 《Journal of Thermal Analysis and Calorimetry》2004,75(2):475-480
The synthesis and the characterization of Al2O3-based nanocrystalline inorganic pigments are reported. The pigments were synthesized by the polymeric precursor (Pechini
method) using Cr2O3 as chromophore. XRD results only evidenced the corundum phase. The average particle size was about 34 nm. The samples were
also characterized by differential scanning calorimetry (DSC) and thermogravimetry (TG), and CIE-L*a*b* calorimetry. The pigments
obtained in this work presented different colors, ranging from green to rose.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
78.
The mechanism of formation of 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile from 4-hydroxy-4-methylpentan-2-one
and cyanoacetamide in the presence of ammonium acetate has been studied by the AM1 method. It was found that, under the reaction
conditions, the amide is readily converted to an iminol tautomeric form which takes part in subsequent reaction. It was shown
that the reaction is a cascade process forming two intermediates. The final product 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile
is formed from the product of a Knoevenagel condensation via an intramolecular nucleophilic substitution mechanism. On the
basis of the activation energies obtained it can be deduced that the limiting stage is the deprotonation process of the cyanoacetiminol.
Dedicated to Professor L. I. Belen’kii with gratitude and deep respect.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 209–216, February, 2006. 相似文献
79.
Luciana A. I. de Azeredo Leda R. Castilho Selma G. F. Leite Rosalie R. R. Coelho Denise M. G. Freire 《Applied biochemistry and biotechnology》2003,108(1-3):749-755
Streptomyces are important microorganisms because of their capacity to produce numerous bioactive molecules. In the present work protease
production, by Streptomyces sp. 594 isolated from a Brazilian Cerrado soil, was maximized by optimizing a low-cost culture medium composition (casitone
and sugarcane molasses) using statistical experimental design. The final protease activity (56 U/mL) was 2.8-fold and 58-fold
higher than that obtained in the beginning of this study, and in a previous work, using an actinomycete selection medium,
respectively. Protease production, not growth associated, appeared to be modulated by an inducer system, whereby the C/N ratio
seemed to play a significant role. 相似文献
80.
Lisboa-Filho P.N. Zenatti A. Casali G.M. Paskocimas C.A. Ortiz W.A. Leite E.R. Longo E. 《Journal of Sol-Gel Science and Technology》2002,24(3):241-245
Ti-oxide polycrystalline samples have been synthesized by the Pechini method and their low temperature magnetic properties have been characterized. The synthesis procedures allow the preparation of high-quality samples ensuring that the observed low temperature magnetic behavior is intrinsic to the system, not due to the polycrystalline nature of the samples. Magnetic susceptibility measurements showed that the anatase and rutile TiO2 exhibit a paramagnetic behavior. For the CoTiO3 samples, a totally reversible antiferromagnetic transition at 38 K is reported. 相似文献