Characterization of TiVZrNb based high entropy alloys for hydrogen storage applications

Abstract

The aim of this work is to synthesise different high entropy alloys (HEAs) and to characterise them with respect to their hydrogen storage capacity and structural properties. The alloys are all based on TiVZrNb in different ratios and additionally with a fifth element added. The focus here was on a series of experiments involving aluminium. Five TiVZrNb HEAs were synthesised with 0, 5, 10, 15 and 20 at % Al. In addition, experiments were carried out with 5 at % chromium,manganese and germanium each. The technique of arc melting was used to synthesise the alloys. X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF) were used to characterise the purity and structure. To characterise the hydrogen storage properties, the samples were hydrogenated using high pressure hydrogen gas and subsequently analysed via thermogravimetric analysis (TGA). XRD was performed directly after synthesis, after hydrogenation and after de-hydrogenation. All XRD data were analysed using the Rietveld refinements method where possible. Furthermore, it is planned that the pressure composition isotherms will be measured through existing collaboration which will form the basis of futurework.All samples that were hydrogenated show good hydrogen absorption characteristics with H/Mratios up to 2,1 and wt % H values between 2,1 and 2,9, whereby the trend in the set with aluminium is towards better values with less Al. Desorption temperatures lie between 248,1and 304,8 °C. With regard to the purity of the samples, a clear trend can be observed: the higher the amount of Al, the lower the purity and the more phases occur.The second set of experiments shows similar results. The H/M ratios lie between 1,8 and 1,9with wt % H values of 2,5 to 2,7. The on set temperatures here are between 229,9 and 275,5 °C.For good purity, the samples containing Cr and Mn can be favoured here, the Ge containing sample shows formation of further phases and lower purity.The fundamentally good potential of these samples for use as hydrogen storage materials must be investigated by further analysis in order to be able to make precise statements about their usability.It should also be mentioned that a third set of experiments was carried out. This involved two samples based on the base alloy TiVZrNb with different element ratios with the aim of producing amorphous samples.