In-Situ Powder Diffraction Studies of Metal-Hydrogen Microstructures

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Author(s)
Primary Supervisor
Gray, Evan
Other Supervisors
Kennedy, Shane
Year published
2004
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In-situ powder diffraction has been used to elucidate the microstructures of several metal-hydrogen systems. Information derived fiom diffraction is correlated with the defect structures that can be observed with TEM. The in-situ technique, in which diffraction data are collected from a sample loaded to a known hydrogen concentration on the beam line, is invaluable in the determination of metal-hydrogen crystal structures because it allows a rigorous test of space group feasibility through comparison with an accurately known hydrogen content. Research on the LaNi5 system focused on activation and degradation properties. In ...
View more >In-situ powder diffraction has been used to elucidate the microstructures of several metal-hydrogen systems. Information derived fiom diffraction is correlated with the defect structures that can be observed with TEM. The in-situ technique, in which diffraction data are collected from a sample loaded to a known hydrogen concentration on the beam line, is invaluable in the determination of metal-hydrogen crystal structures because it allows a rigorous test of space group feasibility through comparison with an accurately known hydrogen content. Research on the LaNi5 system focused on activation and degradation properties. In particular, the y phase has been studied under both extrinsic and intrinsic degradation conditions up to 300°C and 1.1 kbar hydrogen pressure to address the question of thermodynamic instability with extended cycling. En route to unravelling the complicated inter-phase relationship that develops between the a, y and P phases at elevated temperatures, new structure solutions for the y and P phases were derived. A multi-staged mechanical relaxation occurs that is sensitive to the occupation of the 6m position in the p and y phases. Under these conditions, the y phase can morph fiom a P-like structure to an a-like structure. The question of 'trapped' hydrogen was resolved with all D accounted for in interstitial positions in the nphase. The strain relationships between coexisting a and P phases have been investigated in the LaNi5-,Sn,H, system for x = 0.0, 0.1 and 0.2. The substituted alloys display a peculiar prismatic compression of the a phase that can be directly correlated with the removal of a/3 less than 21 10 more than {0i 10) dislocation cores, revealed as a reduction in the severity of anisotropic line broadening. lt is apparent that characteristics of the a-p interface in the parent LaNi5 system are retained in substituted materials, and that variation in the ratio of the densities of a/3 less than 21 10 more than (01 10) and a13 less than 21 10 more than10001) dislocations causes compression of the a phase between layers of phase. The first ever in-situ neutron diffiaction study of the PdDx system above the critical point was executed. D was found to occupy both octahedral and tetrahedral positions. This result is controversial, however the evidence is compelling. Tetrahedral occupation was retained at room temperature and through the two-phase region.
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View more >In-situ powder diffraction has been used to elucidate the microstructures of several metal-hydrogen systems. Information derived fiom diffraction is correlated with the defect structures that can be observed with TEM. The in-situ technique, in which diffraction data are collected from a sample loaded to a known hydrogen concentration on the beam line, is invaluable in the determination of metal-hydrogen crystal structures because it allows a rigorous test of space group feasibility through comparison with an accurately known hydrogen content. Research on the LaNi5 system focused on activation and degradation properties. In particular, the y phase has been studied under both extrinsic and intrinsic degradation conditions up to 300°C and 1.1 kbar hydrogen pressure to address the question of thermodynamic instability with extended cycling. En route to unravelling the complicated inter-phase relationship that develops between the a, y and P phases at elevated temperatures, new structure solutions for the y and P phases were derived. A multi-staged mechanical relaxation occurs that is sensitive to the occupation of the 6m position in the p and y phases. Under these conditions, the y phase can morph fiom a P-like structure to an a-like structure. The question of 'trapped' hydrogen was resolved with all D accounted for in interstitial positions in the nphase. The strain relationships between coexisting a and P phases have been investigated in the LaNi5-,Sn,H, system for x = 0.0, 0.1 and 0.2. The substituted alloys display a peculiar prismatic compression of the a phase that can be directly correlated with the removal of a/3 less than 21 10 more than {0i 10) dislocation cores, revealed as a reduction in the severity of anisotropic line broadening. lt is apparent that characteristics of the a-p interface in the parent LaNi5 system are retained in substituted materials, and that variation in the ratio of the densities of a/3 less than 21 10 more than (01 10) and a13 less than 21 10 more than10001) dislocations causes compression of the a phase between layers of phase. The first ever in-situ neutron diffiaction study of the PdDx system above the critical point was executed. D was found to occupy both octahedral and tetrahedral positions. This result is controversial, however the evidence is compelling. Tetrahedral occupation was retained at room temperature and through the two-phase region.
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Thesis Type
Thesis (PhD Doctorate)
Degree Program
Doctor of Philosophy (PhD)
School
School of Science
Copyright Statement
The author owns the copyright in this thesis, unless stated otherwise.
Item Access Status
Public
Subject
In-situ powder diffraction
metal hydrogen systems