Browsing by Author "Valente, M. A."
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- Investigation of Cr substitution effect on the evolution of La0.67Ca0.2Ba0.13Fe1−xCrxO3 (x = 0 and 0.03) electrical properties under frequency and temperature variationPublication . Dhahri, Ah.; Bougoffa, Amira; Zaouali, A.; Benali, A.; Graça, M. P. F.; Valente, M. A.; Costa, B. F. O.; Abdelmoula, N.; Mariano, José F. M. L.In this study, La0.67Ca0.2Ba0.13Fe1-xCrxO3 (x = 0 and 0.03) powders were prepared by auto-combustion method. The examination of the purity and crystalline structure of the samples by X-ray diffraction showed a cubic structure formed with the Pm (3) over barm space group. Scanning electron microscopy was used to study the morphology and grains size, indicating an agglomeration of nanometric particles. Impedance spectroscopy in the temperature range from 200 to 360 K was used in order to understand the evolution of the electrical behavior of the substituted perovskites. The real and imaginary parts of the impedance were adjusted with two different equivalent circuits. The activation energies extracted from the imaginary part of the impedance, and the electrical conductivity was very closed indicating a transformation of the electrical behavior. This was confirmed by a decrease in the resistance after substitution of Fe cations by Cr ones. AC conductivity studies showed a Jonscher's behavior and a change of conduction process from a succession of NSPT and CBH model to the existence of only CBH one after the substitution.
- Resonant gravity-wave drag enhancement in linear stratified flow over mountainsPublication . Teixeira, M. A. C.; Miranda, P. M. A.; Argaín, José Luís Almaguer; Valente, M. A.High-drag states produced in stratified flow over a 2D ridge and an axisymmetric mountain are investigated using a linear, hydrostatic, analytical model. A wind profile is assumed where the background velocity is constant up to a height z(1) and then decreases linearly, and the internal gravity-wave solutions are calculated exactly. In flow over a 2D ridge, the normalized surface drag is given by a closed-form analytical expression, while in flow over an axisymmetric mountain it is given by an expression involving a simple 1D integral. The drag is found to depend on two dimensionless parameters: a dimensionless height formed with z(1), and the Richardson number, Ri, in the shear layer. The drag oscillates as z(1) increases, with a period of half the hydrostatic vertical wavelength of the gravity waves. The amplitude of this modulation increases as Ri decreases. This behaviour is due to wave reflection at z(1). Drag maxima correspond to constructive interference of the upward- and downward-propagating waves in the region z < z(1), while drag minima correspond to destructive interference. The reflection coefficient at the interface z = z(1) increases as Ri decreases. The critical level, z(c), plays no role in the drag amplification. A preliminary numerical treatment of nonlinear effects is presented, where z(c) appears to become more relevant, and flow over a 2D ridge qualitatively changes its character. But these effects, and their connection with linear theory, still need to be better understood.