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81.
82.
Joseína Moutinho Tavares de Souza Maria do Carmo Rangel 《Reaction Kinetics and Catalysis Letters》2002,77(1):29-34
The replacement of chromium by aluminium in high temperature shift catalysts was investigated. It was found that aluminium-doped hematite is a convenient candidate as an environmental friendly catalyst. 相似文献
83.
84.
85.
86.
Maria Inês Bruno Tavares 《Polymer Testing》2000,19(8):899-904
Analysis of the dynamic behaviour of two highly crystalline polymers, polyethylene (HDPE) and polypropylene (iPP) was carried out by solution and solid state nuclear magnetic resonance (NMR) to obtain the response of the behaviour of both polymers with respect to the molecular chain dynamics, the chain ordination and the molecular packing. The proton spin-lattice relaxation time in the rotating frame (T1Hρ) showed that HDPE is more rigid than iPP, because the chain orientation and the molecular packing are different. T1Hρ parameter also depends on the molecular chain dynamics, as a result of the different sequence distribution in the domains 相似文献
87.
R. J. M. da Fonseca L. P. Sosman A. Dias Tavares Jr. H. N. Bordallo 《Journal of fluorescence》2000,10(4):375-381
Interest in 3d transition metal impurities in ionic crystals has increased due to their important role in the laser activity of these materials. Moreover, recent advances in tunable solid-state lasers and high-power semiconductor laser diode arrays have generated a strong interest in investigating new compounds that emit in the visible and near-infrared spectral regions. In particular, many optical studies have been devoted to Cr3+-doped fluoride crystals as a consequence of the high quality of some Cr3+-based laser materials. In the present investigation, the low-temperature emission spectra of Cr3+ ions in the hexagonal elpasolites Cs2NaAlF6 and Cs2NaGaF6 have been measured. Each compound has two crystallographically inequivalent octahedral sites for the Al3+ and Ga3+ ions that can be occupied by Cr3+ ions. For both materials, the luminescence spectrum presents two zero-phonon lines accompanied by a well-defined vibrational structure. The different peaks of the emission broad band are described in terms of phonons of the lattice and normal modes of the octahedral complex [CrF6]3–. A detailed analysis of the vibrational structure observed leads to the conclusion that the 2
E and 4
T
2 excited states of the [CrF6]3– ion are displaced along the e
g and a
1g
and probably the t
2g
coordinates. 相似文献
88.
Microporous Framework (Nb,Fe)-Silicate with Much Potential to Remove Rare-Earth Elements from Waters
Dr. Zhi Lin Dr. Daniela Tavares Prof. Eduarda Pereira Prof. João Rocha 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(72):e202202435
Nanoparticles of a new small-pore metal silicate formulated as Na2.9(Nb1.55Fe0.45)Si2O10 ⋅ xH2O and exhibiting the structure of previously reported Rb2(Nb2O4)(Si2O6) ⋅ H2O have been synthesized under mild hydrothermal conditions. Replacement of the bulky Rb+ by smaller Na+ ions was accomplished by stabilizing the framework structure via partial occupancy of the Nb5+ sites by Fe3+ ions. Exploratory ion-exchange assays evidence the considerable potential of this new silicate to remove rare-earth elements from aqueous solutions. 相似文献
89.
María S. Aurora-Prado Claudinei A. Silva Marina F. M. Tavares Kevin D. Altria 《Chromatographia》2010,72(7-8):687-694
Microemulsion electrokinetic capillary chromatography has been successfully applied to the separation and determination of water-soluble vitamins (thiamine hydrochloride, riboflavin, niacin, pyridoxine hydrochloride, folic acid, cobalamin, ascorbic acid) and a fat-soluble vitamin (α-tocopherol acetate). The optimal microemulsion buffer contained sodium dodecylsulfate (SDS) as surfactant, butan-1-ol as the co-surfactant, ethyl acetate as the oil and pH 9.2 tetraborate buffer, modified with 15% (v/v) 2-propanol. UV detection at 214 nm gave adequate sensitivity without interference from sample excipients. Under the optimized conditions, the vitamins were baseline separated in less than 7 min. Analytical curves of peak area versus concentration presented coefficients of determination (R 2 ) > 0.99, acceptable limits of quantification between 8.40 and 16.23 μg mL?1 were obtained. Vitamin levels in liquid formulation were quantified with intra-day precision better than 0.99% RSD for migration time and 1.19% RSD for peak area ratio. Recoveries ranged between 98.7 and 101.7%. The method was considered appropriate for rapid and routine analysis. 相似文献
90.
S.S.M. Tavares J.M. Pardal A.M. Gomes 《Journal of magnetism and magnetic materials》2010,322(17):L29-451
Duplex stainless steels are high strength and corrosion resistant steels extensively used in the chemical and petrochemical industry. The best mechanical properties and corrosion resistance are obtained with a microstructure composed by equal parts of ferrite and austenite and free from tertiary phases. Sigma phase is one of these deleterious tertiary phases. In the present work different amounts of sigma phase were precipitated by heat treatments in a UNS S31803 stainless steel. Some specimens were cold rolled before sigma phase precipitation in order to evaluate the effect of deformation on the magnetic measurements. The amount of sigma phase was precisely determined by microscopy and image analysis for each heat treatment condition. The effects of sigma phase on the steel properties were investigated, confirming the detrimental effects of very small percentages on corrosion resistance and toughness. Two magnetic methods were used to detect sigma phase: magnetization saturation measurements in a Vibrating Sample Magnetometer and ferritoscope testing. Both methods were found to be sensitive to small percentages of sigma phase in the microstructure. 相似文献