C.S. Alves

The magnetic and magnetocaloric properties of Gd5Ge2Si2 compound under hydrostatic pressure

The Gd5Ge2Si2 compound presents a giant magnetocaloric effect with transition temperature at around 276 K and is a very good candidate for application as an active regenerator material in room temperature magnetic refrigerators. Recently it has been shown that pressure induces a colossal magnetocaloric effect in MnAs, a material that presents a giant magnetocaloric effect and a strong magnetoelastic coupling, as also happens with the Gd5Ge2Si2 compound. This motivated a search of the colossal effect in the Gd5Ge2Si2 compound. This work reports our measurements on the magnetic properties and the magnetocaloric effect of Gd5Ge2Si2 under hydrostatic pressures up to 9.2 kbar and as a function of temperature. Contrary to what happens with MnAs, pressure increases the Curie temperature of the compound, does not affect the saturation magnetization and decreases markedly its magnetocaloric effect.

Giant magnetocaloric effect in Gd5(Si2Ge2) alloy with low purity Gd

Gd5(Ge1-xSix), x < 4 based alloys are potential candidates for magnetic refrigeration in the range ~20 - ~290 K. However, one of the greatest obstacles for the use of that technology in large scale is the utilization of high pure Gd metal (99.99 wt. (%)) to produce the GdGeSi alloys, since the impurity elements decrease the intensity of the magnetocaloric effect (EMC)1. In this work, we prove that annealing of the Gd5Ge2Si2 can promote remarkable values for the EMC in comparison to those obtained for the alloy with high pure Gd. Also, the as cast alloy and the annealed alloy are not monophasic, but have at least two crystalline phases in their microstructure. Results for X-ray analysis, optical and electronic microscopy and magnetization measurements are reported.

Isothermal variation of the entropy (ΔST) for the compound Gd5Ge4 under hydrostatic pressure

In the present work, the isothermal variation of the entropy (ΔST) for the compound Gd5Ge4 was studied at different applied hydrostatic pressures (from 0 up to 0.58 GPa). In all pressure ranges, we observe the giant magnetocaloric effect. The ΔST data for the compound Gd5Ge4 at zero applied pressure present two peaks: the lowest temperature peak is due to irreversible processes and the highest temperature peak is due to magnetostructural transitions. Increasing the pressure, the lowest temperature peak displaces to lower temperatures and disappears. The magnitude of the other peak has a nonlinear behavior with pressure. Different protocols were used to obtain ΔST at zero applied pressure and the results indicate that ΔST strongly depends on the initial and final states of Gd5Ge4 compound. We also present a T-P magnetic phase diagram built from the available magnetic data.

A influência da pressão de compactação sobre as propriedades magnéticas e estruturais da Liga de Gd5Ge2,15Si1,85 obtida por metalurgia do pó

The magnetocaloric effect (MCE) has been studied seeking their application in cooling systems near room temperature (domestic employ). The magnetic refrigerators work with nontoxic solids that substitute the refrigerant gases used in hermetic compressors, without relying on ozone-depleting coolants. Some alloys of Gd5(GexSi4-x) present the giant magnetocaloric effect (GMCE), and the Gd5Ge2Si2 alloy, announced in 1997 by Pecharsky e Gschneidner, have a first-order magneto-structural transition at the Curie Temperature (Tc = 273 K) and MCE around 30 J.(kg.K)-1, being probable their use in those cooling systems. However, when that material is sintered, it loses its initial characteristics during the powder metallurgy process. Therefore, this study will treat of the compacting pressure interferences on the structural and magnetic properties of the Gd5Ge2.15Si1.85 alloy.

Eletrochemical hydrogenation of Nd2Fe17 and Nd2Fe15.5Ga1.5

A study of hydrogenation of the metallic alloys Nd2Fe17 and Nd2Fe1,5Ga1.5 using electrochemical hydrogenation in a solution of 0.1 N NaOH at 313 K and 2 A/m2 current density is presented. The pure and electrochemical hydrogenated samples were characterized by Mossbauer spectroscopy and X-ray diffraction. Analysis after hydrogen desorption reveal no presence of new phases or oxides. Similarities and differences with gas hydrogenation are observed.