共查询到20条相似文献,搜索用时 46 毫秒
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J.S. Amaral P.B. Tavares M.S. Reis J.P. Araújo T.M. Mendonça V.S. Amaral J.M. Vieira 《Journal of Non》2008,354(47-51):5301-5303
Ferromagnetic manganites of general formula (where A is a trivalent rare-earth ion and B is a divalent dopant) are candidates for magnetic cooling applications, since they can present either second- or first-order magnetic phase transitions, and the perovskite structure allows for substantial chemical substitution and tuning of the magnetocaloric properties. The consequent chemical distribution from substitution affects the magnetic and magnetocaloric properties of the compound. The change of relative cooling power with chemical disorder is discussed by the use of the molecular mean-field model. We present experimental results of the ferromagnetic, second-order phase transition system. 相似文献
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E. Márquez J.M. González-Leal A.M. Bernal-Oliva R. Jiménez-Garay T. Wagner 《Journal of Non》2008,354(2-9):503-508
We have analysed in detail the effect of silver-content on the optical properties of Ag-photodoped amorphous (As0.33S0.67)100?xTex (with x = 0, 1, 5 and 10 at.%) chalcogenide thin films; the chalcogenide host layers were prepared by vacuum thermal evaporation. Films of composition Agy[(As0.33S0.67)100?xTex]100?y, with y ? 18 at.%, were successfully obtained by successively photodissolving about 20- or 40-nm-thick layers of silver. The optical constants (n, k) have been accurately determined by an improved envelope method [J.M. González-Leal, R. Prieto-Alcón, J.A. Angel, D.A. Minkov, E. Márquez, Appl. Opt. 41 (2002) 7300], based on the two envelope curves of the optical-transmission spectrum, obtained at normal incidence. The dispersion of the refractive index of the Ag-photodoped chalcogenide films is analysed in terms of the Wemple–DiDomenico single-effective-oscillator model: , where Eo is the single-oscillator energy, and Ed the dispersion energy. We found that the refractive index of the Ag-doped samples strongly increases with the Ag-content, whereas the optical band gap, , decreases also notably. For instance, in the particular case of x = 10 at.%, the largest Te-content, decreases from 2.17 down to 1.67 eV. It should also be mentioned that, in the case of the undoped samples, when the Te-concentration increases from zero up to 10 at.%, the value of decreases from 2.49 down to 2.17 eV. 相似文献
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《Journal of Non》2005,351(6-7):472-476
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Apparent activation energies (Eg) of glass transition, glass transition temperature (Tg) and crystallization temperature (Tx) of amorphous alloys of composition Fe91?xBxZr5Nb4 (FBZN, 5 ? x ? 30 at.%) and Fe61?xCoxZr5B30Nb4 (FCZBN, 0 ? x ? 15 at.%) were obtained by using differential scanning calorimeter (DSC) measurement and Kissinger equation, and correlations between Eg and Tg, Tx and glass-forming ability (GFA) were studied in the present work. It was found that the Tg and Tx are not independent each other for each glass composition in the two alloy systems, but related by a formula, Tx = αTg+β, where α and β are constants, and were measured by nonisothermally scanning in the DSC together with the Lasocka’s equation. The Eg was found to be directly proportional to α and β, respectively, and had a correlation with Tx and Tg, , indicating that Eg determines linear relationship between Tx and Tg. Supercooled liquid region ΔTx is used as characterization of GFA of the Fe based metallic glasses and related to Eg and Tg by a formula: , indicating that Eg and Tg can characterize GFA of the Fe based metallic glass well. 相似文献
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《Journal of Non》2005,351(40-42):3365-3369
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《Journal of Non》2006,352(42-49):4785-4789
In this work we carried out positron annihilation lifetime spectroscopy (PALS) on pure poly(propylene glycol) (PPG 4000) and an electron spin resonance (ESR) study on PPG 4000 doped with the spin probe TEMPO. The spectral and dynamic features of both the microscopic probes such as the ortho-positronium (o-Ps) lifetime, τ3, the spectral parameter, T50 G, and the correlation time of spin probe, τc, as a function of the temperature were compared. New empirical relationships between the characteristic PALS temperatures ( and ) and the characteristic ESR temperatures ( and ) have been revealed. In addition, these characteristic PALS and ESR temperatures can be connected with the Schönhals and Stickel (, ) temperatures of the primary α relaxation process of PPG 4000 as obtained by dielectric spectroscopy (DS) studies in the literature. Next, the mean free volume hole sizes of the matrix were related to the spin probe size and to the Arrhenius or the non-Arrhenius motional regimes of TEMPO. Finally, the temperature dependences of the reorientational correlation time have been analysed using the Vogel–Fulcher–Tamman–Hesse (VFTH) equation and its generalized form, i.e., the Bendler–Shlesinger–Fontanella (BSF) one, the latter derived from the defect diffusion model of the dynamics and transport properties. The fitting parameters are then compared with those of the primary α relaxation process. 相似文献
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