全文获取类型
收费全文 | 424篇 |
免费 | 16篇 |
国内免费 | 1篇 |
专业分类
化学 | 310篇 |
晶体学 | 2篇 |
力学 | 4篇 |
数学 | 28篇 |
物理学 | 97篇 |
出版年
2023年 | 5篇 |
2022年 | 17篇 |
2021年 | 12篇 |
2020年 | 14篇 |
2019年 | 6篇 |
2018年 | 15篇 |
2017年 | 9篇 |
2016年 | 17篇 |
2015年 | 10篇 |
2014年 | 17篇 |
2013年 | 20篇 |
2012年 | 19篇 |
2011年 | 29篇 |
2010年 | 17篇 |
2009年 | 15篇 |
2008年 | 19篇 |
2007年 | 28篇 |
2006年 | 11篇 |
2005年 | 13篇 |
2004年 | 19篇 |
2003年 | 10篇 |
2002年 | 18篇 |
2001年 | 13篇 |
2000年 | 10篇 |
1999年 | 13篇 |
1998年 | 8篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
排序方式: 共有441条查询结果,搜索用时 15 毫秒
91.
Behaviour of Ca2Fe2O5 with Nb substitution and sintering temperatures seen by Mössbauer spectroscopy
S. Duhalde C. Saragovi I. J. Moraes M. G. Terrile R. H. P. Francisco 《Hyperfine Interactions》1991,66(1-4):261-266
Mössbauer spectroscopy of samples of Ca2Fe2?xNbxO5+x with x values ranging from 0 to 0.8 and sintering temperatures of 1200°C and 1300°C shows the presence of two magnetic fields and a paramagnetic signal. The behaviour of the parameters as a function of x and of the sintering temperatures are discussed and compared with XRD results. 相似文献
92.
Cleide M. F. Soares Onelia A. dos Santos Heizir F. de Castro Flavio F. de Moraes Gisella M. Zanin 《Applied biochemistry and biotechnology》2004,113(1-3):307-319
The hydrolysis of tetraethoxysilane using the sol-gel process was used to produce silica matrices, and these were tested for the immobilization of lipase from Candida rugosa by three methods: physical adsorption, covalent binding, and gel entrapment in the presence and absence of polyethylene glycol (PEG-1450). The silica matrices and their derivatives were characterized regarding particle size distribution, specific surface area, pore size distribution (Brunauer, Emmett, and Teller [B.E.T.] method), yield of grafting (thermogravimetric analyzer [TGA]), and chemical composition (Fourier transform infrared). Immobilization yields based on recovered lipase activity varied from 3.0 to 32.0%, and the highest efficiency was attained when lipase was encapsulated in the presence of PEG. 相似文献
93.
J. C. S. Moraes E. M. Telles F. C. Cruz A. Scalabrin D. Pereira G. Carelli N. Ioli A. Moretti F. Strumia 《International Journal of Infrared and Millimeter Waves》1992,13(11):1801-1823
A systematic investigation of13CD3OH infrared transitions around CO2 laser line emissions is performed by means of optoacoustic detection. Using a waveguide CO2 laser of 290 MHz tunability and, in some cases, an acoustooptic modulator to provide an extension of 180 MHz, 108 transitions are observed. Measurements of pump off-set and relative intensity have been performed. The data may be compared with those previously reported of high resolution Fourier Transform spectroscopy.Work supported by: FAPESP, CNPq, FAEP — Brasil CNR — ItaliaPermanent address: Depto de Ciências, FEIS-UNESP (15378) I.Solteira-SP, Brasil 相似文献
94.
Luiz Frederico Rodrigues Julio Cezar Paz de Mattos Valderi Luiz Dressler Dirce Pozebon Érico Marlon de Moraes Flores 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007
Trace impurities of Cd, Cu and Pb were determined in alumina based catalysts using direct solid sampling graphite furnace atomic absorption spectrometry (DSS-GF AAS). The analyzed catalysts are widely used in petrochemical processes. The following analytical parameters were evaluated: pyrolysis and atomization temperatures, feasibility of calibration with aqueous solutions, the necessity for palladium as chemical modifier and the sample mass introduced into the atomizer. Test samples between 0.05 and 8.5 mg were used. Palladium was investigated as chemical modifier but no improvement in analytical performance was obtained and its use was considered unnecessary for all elements. The results obtained by DSS-GF AAS were compared with those of inductively coupled plasma optical emission spectrometry (ICP OES) and also with conventional solution analysis by GF AAS (Sol-GF AAS). Characteristic masses were 1.4, 9 and 20 pg, for Cd, Cu and Pb, respectively. Using DSS-GF AAS the relative standard deviation was always less than 10% and the results agreed with those obtained by Sol-GF AAS and ICP OES. Calibration using aqueous solutions showed good linearity within the working range (R2 better than 0.99). Limits of detection (3σ, n = 10) for Cd, Cu and Pb using the proposed procedure were 0.2, 22, and 1.2 ng g− 1, respectively. 相似文献
95.
Grafting of cyclodextrins onto filter paper 总被引:1,自引:0,他引:1
Rita C. de Bergamasco Gisella M. Zanin Flavio F. de Moraes 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):75-78
Grafting of cyclodextrins and cyclodextrins derivatives on cellulosic surface, such as paper or filter paper, provides hosting
cavities that can include a large variety of chemicals for specific cellulose finishing. In this study grafting of monocholorotriazinyl-β-cyclodextrin
(MCT-β-CD) and β-cyclodextrin (β-CD) to filter paper has been performed. β-cyclodextrin has been bonded to filter paper using
1,4-butanediol diglycidyl ether as the crosslinking agent. The untreated and treated filter papers were characterized by differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA), demonstrating the covalent binding of cyclodextrins to filter
paper. The quantification of β-CD and MCT-β-CD grafted to filter paper was determined by the dye extinction method with the
inclusion of phenolphthalein. The final β-CD content amounted to 15.9 μmol per gram of support (1.8% by weight), and 72.8 μmol
per gram of support (11.3% by weight) for MCT-β-CD. 相似文献
96.
97.
A systematic investigation of a novel epoxide and thioepoxide ring expansion reaction promoted by gaseous acylium and thioacylium ions is reported. As ab initio calculations predict, and 18O-labeling and MS3 pentaquadrupole experiments demonstrate, the reaction proceeds by initial O(S)-acylation of the (thio)epoxides followed by rapid intramolecular nucleophilic attack that results in three-to-five-membered ring expansion, and forms cyclic 1,3-dioxolanylium, 1,3-oxathiolanylium, or 1,3-dithiolanylium ions. This gas-phase reaction is analogous to a condensed-phase reaction long since described by H. Meerwein (Chem. Ber. 1955, 67, 374), and is termed as "the gas-phase Meerwein reaction"; it occurs often to great extents or even exclusively, but in some cases, particularly for the most basic (thio)epoxides and the most acidic (thio)acylium ions, proton transfer (eventually hydride abstraction) competes efficiently, or even dominates. When (thio)epoxides react with (thio)-acylium ions, the reaction promotes O(S)-scrambling; when epoxides react with thioacylium ions and the adducts are dissociated, it promotes S/O replacement. An analogous four-to-six-membered ring expansion also occurs predominantly in reactions of trimethylene oxide with acylium and thioacylium ions. 相似文献
98.
99.
Solid particles can be transported as a mobile granular bed, known as bed-load, by pressure-driven flows. A common case in industry is the presence of bed-load in stratified gas–liquid flows in horizontal ducts. In this case, an initially flat granular bed may be unstable, generating ripples and dunes. This three-phase flow, although complex, can be modeled under some simplifying assumptions. This paper presents a model for the estimation of some bed-load characteristics. Based on parameters easily measurable in industry, the model can predict the local bed-load flow rates and the celerity and the wavelength of instabilities appearing on the granular bed. 相似文献
100.
Bharathi Avula Yan‐Hong Wang Rita M. Moraes Ikhlas A. Khan 《Biomedical chromatography : BMC》2011,25(11):1230-1236
A new rapid UPLC‐UV‐MS method has been developed that permits the analysis of four lignans (4′‐O‐demethylpodophyllotoxin, podophyllotoxin, α‐peltatin and β‐peltatin) in P. peltatum L. Podophyllotoxin is a natural lignan that is being used as a precursor for the semi‐synthetic anti‐cancer drugs etoposide, teniposide and etopophos. The chromatographic separation was achieved using a reversed‐phase C18 column with a mobile phase of water and acetonitrile, both containing 0.05% formic acid. Analyses of P. peltatum leaves collected from different colonies within a single site indicated a significant variation in 4′‐O‐demethylpodophyllotoxin, α‐peltatin, podophyllotoxin and β‐peltatin content. Within 3.0 min four main lignans could be separated with detection limits of 0.1, 0.3, 0.3 and 0.2 μg/mL, respectively. 4′‐O‐demethylpodophyllotoxin and α‐peltatin appeared most prominently among the lignans obtained. The podophyllotoxin content was found in the range of 0.004–0.77% from 16 samples collected from 6 colonies within the same site. The content of podophyllotoxin is directly proportional to the content of 4′‐O‐demethylpodophyllotoxin and inversely proportional to α‐peltatin and β‐peltatin content. LC‐mass spectrometry coupled with electrospray ionization (ESI) interface method is described for the identification of four lignans in various populations of plant samples. By applying principal component analysis and hierarchical cluster analysis, Podophyllum samples collected from various colonies within a location were distinguished. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献