Nb–based alloys and their application for hydrogen permeation membranes
thesisposted on 27.02.2017 by Wong Tek Kong, Timothy
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The ultimate goal of this work has been to produce some novel and versatile membranes for hydrogen permeation and purification. Nb-alloy membranes were chosen due to their high hydrogen permeability, Φ, and their lower cost when compared to conventional Pd-alloy membranes. The formation range of amorphous in Nb-Ni-Zr and Nb-Ti-Co alloys produced by melt-spinning was investigated. It was found that the (Nb90Zr7Ni3)1-X(Ni50Zr50)X alloys with Nb contents below 45 at. % (X > 0.5) yielded amorphous ductile ribbons, while those above this Nb content resulted in crystalline brittle ribbons. In the case of the Nb-Ti-Co alloy, the eutectic composition resulted in ductile amorphous ribbons when thin ribbons were spun, however, attempts to prepare wide ribbons failed. The (Nb90Zr7Ni3)1-X(Ni50Zr50)X alloys with a niobium content, Nb ~ 45 – 20 at. % (0.5 < X < 0.8), resulted in NiZr + Nb phases. A nano-scale grain refinement of the NiZr and bcc-Nb microstructure was observed after heating to 923 K. Coarsening of the Nb and NiZr duplex structure was observed in samples heated to 1173 K. The effect of annealing on the hydrogen permeation properties and microstructure of Nb-Ni-Zr alloys was investigated. These alloys consist of the primary bcc Nb phase surrounded by a Nb+NiZr phase mixture. The hydrogen permeability of Nb40Ni30Zr30 was found to increase with increasing annealing temperature and was the highest when annealed at 1123 K for 1 h, resulting in a Φ 3.9 times larger than that of Pd and 2.3 times larger than that in the as-cast state. Two modes of crack propagation were observed, namely transgranular and intergranular cracking. The modes of fracture were found to be composition dependent, with samples with high Nb content suffering from transgranular cracking and those with low Nb content from intergranular cracking. The mode of fracture was found to be unaffected by annealing. For the first time, the hydrogen permeability, solubility and diffusivity of a chemically identical alloy membrane were investigated in an amorphous, nanocrystalline and crystalline state. It was found that the hydrogen permeability of the Nb20Ni40Zr40 sample with a nanostructure was about twice that of the amorphous sample and one order of magnitude higher than the crystalline sample. Even though the n-sample and the a-sample have higher hydrogen permeabilities, the hydrogen diffusivity and solubility states of each were found to be different. It was shown that the improvement in hydrogen permeation in the samples with a nanostructure was not solely based on improved hydrogen diffusivity, when compared to the amorphous sample, but also due to the improved hydrogen solubility. The prospect of using Cu-Nb alloys as hydrogen permeation membranes was explored for the first time. The preliminary hydrogen permeation tests on as-cast Cu-15 vol.% Nb and rolled Cu-20 vol.% Nb samples resulted in a Φ of 1.7x10-9 and 1.93x10-9 molH2m-1s-1Pa-0.5 at 673 K respectively. Both membranes were found to be mechanically strong, hydrogen embrittlement resistant and ductile. An in-situ analysis was conducted by XRD. It was found that Nb was responsible for the permeation of hydrogen and Cu was unaffected by H. The catalytic Pd coating was lost to a Cu rich Cu-Pd phase in areas in direct contact with the Cu matrix at temperatures above 573 K. To circumvent this issue and also to potentially improve the hydrogen permeability coefficient of the membrane, a modelling exercise was carried out to investigate the effect of altering the morphology of the Nb and Cu phases in Cu-Nb alloys. Based on the simulation, it was found that Nb and Cu phases arranged in parallel to the direction of the flow of hydrogen compared to one arranged in series, would yield gains varying from five to eight orders of magnitude at 723 K and 523 K respectively. The simulation also showed that the Φ of a Cu-15 vol.% Nb alloy membrane following the parallel model would be between 1 to 2 orders of magnitude higher than that of pure Pd membranes at 723 K and 523 K respectively. It was found that arranging the Nb and Cu phases in parallel would yield the most efficient increase in Φ with increasing Nb content. The models of the Cu-Nb alloy with phases arranged in parallel indicate that it follows the unique hydrogen permeation characteristics of Group V elements, which is to have an increasing hydrogen permeability with decreasing temperature. This carries positive implications in improving the efficiency of these membranes. In an attempt to confirm the model, a Cu/Nb/Cu multifilamentary alloy was tested. SEM analysis revealed that the Pd catalyst coating remained intact in areas directly above the Nb filaments at 673 K, thus significantly increasing the operational temperature of the Cu-Nb alloy membrane system. In summary, the optimum compositions to produce ductile amorphous ribbons in Nb-Ni-Zr and Nb-Ti-Co alloys were identified. The heat treatments to obtain chemically identical alloy membranes in amorphous, nanocrystalline and crystalline state were successfully investigated. The hydrogen permeability, solubility and diffusivity of these membranes were studied. For the first time Cu-Nb superconducting wires were studied for their application as hydrogen permeation membranes and were shown to have a high resistance to hydrogen embrittlement, good durability at high temperature and reasonable hydrogen permeation properties. Further insight into the possible advantages of using Cu-Nb alloys used in the ‘parallel mode’ was given. Therefore, this study further supports Nb-based alloy membranes as potential alternatives to Pd-based alloy membranes.