J. Bodega

Hydriding/dehydriding properties of magnesium–ZrCr2 composites

Abstract Composites of Mg and the intermetallic compound (IMC) ZrCr 2 have been prepared by mechanical alloying. The crystallographic structure, composition and morphology of the composites were analysed by means of X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). XRD patterns show that no alloy formation takes place during milling. EDX data show that milling time is a key parameter to control the morphology of the composites. Short milling times of 10 min give composites with the IMC attached to the Mg particle surface, while it is irregularly distributed in the Mg bulk for longer milling times of 70 min. Hydriding/dehydriding properties of the composites were studied by the solid–gas reaction method. Composites with a distribution of the IMC in the Mg bulk are easily activated and show faster hydriding/dehydriding kinetics than untreated Mg, which indicates that ZrCr 2 acts as a catalytic compound for the Mg–hydrogen reaction.

Ultrasonic irradiation as a tool to modify the H-desorption from hydrides: MgH2 suspended in decane

Effects of ultrasonic irradiation on magnesium hydride (MgH(2)) suspended in decane were investigated with the purpose of improving its hydrogen desorption process. Firstly, we have found that the presence of MgH(2) improves the sonolysis of decane enhancing the amount of hydrogen evolved during the sonication process. The sonicated-MgH(2) maintains its microstructural properties practically unaltered but a drastic reduction of the particle size of MgH(2) (down to approximately 20mum) as well as a high pressure MgH(2) phase are observed. However, no substantial modifications of H-kinetic properties of hydride occur as is determined by thermal desorption measurements. This could be attributed to decomposition of decane during sonication which leads to the formation of carbon compounds that hinder the thermal decomposition of MgH(2).

On the van der Pauw’s method applied to the measurement of low thermal conductivity materials

The electrical van der Pauws method has recently been extended to measure the thermal conductivity of different elements and compounds. This technique provides an easy way to determine the sample in-plane thermal conductivity by avoiding the influence of the thermal contact resistances. However, the reported calculated error values appear to be underestimated when dealing with the materials with low thermal conductivity (<5 W/Km) at room temperature. The causes of this underestimation are investigated in this communication and it has been found that they are due to the drastic influence of conduction heat losses through the thermo-resistance wires as well as the resulting modification of the sample temperature map. Both phenomena lead to experimental values of the sample thermal conductivity, which are systematically higher than the tabulated ones. The magnitude of this systematic error is ∼100% dealing with the samples of macroscopic dimensions, and low thermal conductivity indicated that the obtained accurate measurements can be quite challenging.