Metodos baseados na fotoacustica para caracterização de materiais magnetocaloricos / Methods based on the photoacoustic for characterization of magnetocaloric materials

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

In this work the photoacoustic technique was used to develop experimental setups to investigate magnetocaloric materials, which have been of great interest due to their applications in refrigeration systems. The value (4,5 ± 0,2).10-6 m2/s was determined, at room temperature, as the thermal di usivity of the Gd, based on the two-beam photoacoustic method, by means of both amplitude and phase of the normalized signal. The thermal diffusivity at temperatures around the magnetic transition (TC) was obtained from the photoacoustic signal phase, in temperature scans, in a reproductible and accurate way, with errors below 5%. A method for measuring the magnetocaloric e ect (MCE), based on the acoustic detection, was developed. The temperature oscillations on such materials, due to the application of an alternating magnetic field, produce acoustic waves which can be detected by a microphone. The details concerning the calibration procedure and the proposed methodology to determine the MCE were discussed, and measurements on Gd and Gd 5(Ge1-xSix)4 compouds were performed in powder and pellet samples. For a 20 kOe applied field, the maximum MCE values, related to magnetic second order transitions, are between 3 K, for the Gd, and 4,5 K, for Si - rich compounds. For the samples with x = 0,5, the maximum MCE values are around 1,7 K, related to magneto-structural first order transitions. It is also possible, for some samples, to observe both first and second order transitions. The obtained results point out this acoustic type detection as an effcient technique to measure the magnetocaloric effect, as well as to detect spurious phases present in some of these compounds. The errors in the MCE measurements are about 15-20%, compared to conventional methods.

ASSUNTO(S)

magnetoacustica magnetoacoustic thermal diffusivity fotoacustica magnetocaloric effect difusividade termica photoacoustic efeito magnetocalorico

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