| dc.description.abstract | Growing concern about climate change and the need to reduce greenhouse gas emissions have led to a growing interest in renewable energy. Among them, hydrogen has emerged as a promising substitute for traditional fossil fuels. Nevertheless, safe and efficient hydrogen storage remains a major issue for realizing a hydrogen economy.
There are many ways to store hydrogen, and the metal hydride formula is currently considered as the most practical method, in terms of structural stability, durability, capability and safety. A relatively new way to store hydrogen is the use of high entropy alloys (HEAs). In this work, a high entropy alloy Ti42Zr35Ta3Si5Co12.5Sn2.5 has been studied for its application in hydrogen storage. Particularly, the effects of fabrication process of HEA, and the resulting microstructure on the hydrogen storage capacity have been evaluated.
Three different processes to produce Ti42Zr35Ta3Si5Co12.5Sn2.5 have been employed here, namely, atomization process after laser melting (LM); atomization process after vacuum arc melting; and casting by vacuum arc melting. For the powder produced by the first two methods, sieving was used to separate these powders into different mesh sizes. For the cast HEA, the ingot was ground to produce finer particles. Scanning Electron Microscopy (SEM) was used to measure the particle shapes and sizes, and to characterise the morphology of the HEA so produced. It has been shown by Electron Probe Microanalyzer (EPMA) that the atomization process helped to eliminate any segregation effect in the atomized powders. X-ray Diffraction (XRD) showed that the atomized HEA samples with different particle sizes would have different crystallinity.
Among the sieved Ti42Zr35Ta3Si5Co12.5Sn2.5 powder produced by the first method (Samples A to E), Sample A had the finest sample size (smaller than 53µm), and it also had the lowest crystallinity. The Sample C, with a particle size between 90-106µm, was found to have the higher crystallinity among these atomized samples. Here, only the hydrogen storage properties Samples A (smaller than 53µm) and C (90-106µm) were studied using the Sievert type Pressure Composition Isotherm (PCI) system at 400℃. PCI result of the Sample A shows that there was a plateau at very low pressure (0.0001MPa), with the maximum hydrogen content of 0.5wt%. The hydrogen content increased very slowly to 1.6wt% of hydrogen, when the hydrogen charging pressure went up to 2.7 MPa. For Sample C, it could absorb 0.6wt% of hydrogen at very low pressure at 0.0001 MPa. A second plateau pressure could absorb 2.14wt% at 4.4 MPa. In terms of hydrogen absorption kinetic, it appears that Sample A had a faster absorption at very low pressure. However, both Samples A and C had the similar hydrogen kinetic. The powder produced by the second method absorbed less hydrogen.
The cast material produced by vacuum arc melting has the highest crystallinity than those produced by the atomization process. It also had the highest hydrogen content in the very low-pressure regime. As the pressure went up to 2.7 MPa, it had a hydrogen content similar to those of Samples A and C.
Above findings suggest the Ti42Zr35Ta3Si5Co12.5Sn2.5 HEA can absorb hydrogen at very low pressure. This HEA could absorb 1.6wt% hydrogen under a pressure of 2.7 MPa. All these show very unique hydrogen absorption properties of the HEA studied. The relatively high hydrogen content of 2.14wt% at 4.5 MPa also suggests that this HEA can be a suitable candidate for hydrogen storage. However, to make full use of this HEA, the hydrogen absorption temperature has to be lowered as much as possible. | en_US |