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Connecting topology, mechanical behavior, and hydrogen interaction dynamics in nanoporous Nb and Pd
Citation Link: https://doi.org/10.15480/882.15930
Publikationstyp
Doctoral Thesis
Date Issued
2025
Sprache
English
Author(s)
Sohn, Seoyun
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2025-08-22
Institute
TORE-DOI
Citation
Technische Universität Hamburg (2025)
Studying the mechanical performance of nano- and microscale porous materials enhances our understanding of small-scale solids and improves materials design for functional applications. This study investigates the microstructure and mechanical behavior of millimeter-sized samples with a nanoscale random network structure, integrating metal hydrides to expand their functionality. Notably, metals such as Nb and Pd can form solid solutions with hydrogen across a broad concentration range at relatively low temperatures. Therefore, nanoporous (np) Nb and np Pd, representing body-centered cubic (BCC) and face-centered cubic (FCC) crystal structures respectively, are fabricated and studied, with np Au serving as a benchmark.
In the first part of the thesis, the mechanical behavior of np material is investigated with a focus on the role of its microstructure size and topology. Results from macro-compression tests and X-ray nanotomography of np Nb confirm that coarsening degrades yield strength and that its Young’s modulus deviates from scaling laws developed for np Au, a widely-investigated model system produced via aqueous dealloying. The scaled genus, a measure of the network's connectivity, of np Nb is lower than that reported for np Au, and this reduced connectivity provides an obvious explanation for the low modulus. From these observations, a novel scaling law that explicitly involves the scaled genus is established. Furthermore, the comparison between np Au and np Nb implies that structural dispersion should be acknowledged as an additional structural descriptor to draw an analogy between liquid-metal dealloyed and electrochemically dealloyed materials.
The second part of the thesis focuses on the interaction dynamics between np Pd and hydrogen. Compared to np Nb, np Pd demonstrates significantly higher hydrogen absorption efficiency. The study aims to understand the role of geometry on the hydrogen charging kinetics by tuning the ligament size of np Pd and to understand the limiting subprocess. Hydrogen ad/absorption and desorption kinetics are analyzed using electrochemical impedance spectroscopy and potential jump tests. The results suggest that the interfacial injection of hydrogen is the controlling factor of the sorption rate. This injection process is analyzed from a thermodynamic perspective, with the Pd-H miscibility gap taken into account. The Butler-Volmer equation is adapted to model the injection rate consistent with the equation of state for the composition-dependent chemical potential at equilibrium in an interacting solid solution. The model successfully predicts the characteristic charging time observed in the np Pd-H system.
Overall, this thesis establishes a coupled relationship between mechanics, microstructure, and hydrogen absorption kinetics, providing insights into the optimization of structural design in np materials, thereby paving the way for their applications as integrated material systems.
In the first part of the thesis, the mechanical behavior of np material is investigated with a focus on the role of its microstructure size and topology. Results from macro-compression tests and X-ray nanotomography of np Nb confirm that coarsening degrades yield strength and that its Young’s modulus deviates from scaling laws developed for np Au, a widely-investigated model system produced via aqueous dealloying. The scaled genus, a measure of the network's connectivity, of np Nb is lower than that reported for np Au, and this reduced connectivity provides an obvious explanation for the low modulus. From these observations, a novel scaling law that explicitly involves the scaled genus is established. Furthermore, the comparison between np Au and np Nb implies that structural dispersion should be acknowledged as an additional structural descriptor to draw an analogy between liquid-metal dealloyed and electrochemically dealloyed materials.
The second part of the thesis focuses on the interaction dynamics between np Pd and hydrogen. Compared to np Nb, np Pd demonstrates significantly higher hydrogen absorption efficiency. The study aims to understand the role of geometry on the hydrogen charging kinetics by tuning the ligament size of np Pd and to understand the limiting subprocess. Hydrogen ad/absorption and desorption kinetics are analyzed using electrochemical impedance spectroscopy and potential jump tests. The results suggest that the interfacial injection of hydrogen is the controlling factor of the sorption rate. This injection process is analyzed from a thermodynamic perspective, with the Pd-H miscibility gap taken into account. The Butler-Volmer equation is adapted to model the injection rate consistent with the equation of state for the composition-dependent chemical potential at equilibrium in an interacting solid solution. The model successfully predicts the characteristic charging time observed in the np Pd-H system.
Overall, this thesis establishes a coupled relationship between mechanics, microstructure, and hydrogen absorption kinetics, providing insights into the optimization of structural design in np materials, thereby paving the way for their applications as integrated material systems.
Subjects
Nanoporous
Palladium
Niobium
Hydrogen
Mechanical behavior
DDC Class
620.11: Engineering Materials
530: Physics
Funding Organisations
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Name
Dissertation_Sohn_2025_Final_24092025.pdf
Size
41.51 MB
Format
Adobe PDF