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Electro-chemo-mechanical coupling of nanoporous gold at the microscale
Citation Link: https://doi.org/10.15480/882.4256
Publikationstyp
Doctoral Thesis
Date Issued
2022
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2022-02-25
TORE-DOI
Citation
Technische Universität Hamburg (2022)
Its tunable strengthening, stiffening and actuation behavior, induced by a change of surface state controlled via electrochemical potential, make nanoporous gold (NPG) a promising candidate for many applications. Due to the complex network structure, a fundamental understanding of the functional behavior of NPG requires accurate measurements of the electro-chemo-mechanical coupling. While many experiments at the macro-scale were published, only few experiments have been carried out at the micro-scale. In this work, the micromechanical testing with and without electrochemical control has been carried out, using a modified nanoindentation system with which the effects of deformation length-scale and microstructural length-scale are investigated.
Based on the results of nanoindentation and microcompression without electrochemical environment, the elastic modulus, strength and hardness show a decrease with increasing ligament size. By varying ligament size (L) and sample diameter (D) of NPG micropillars, a critical ratio (α=D/L=20) was found, above which the test structure can be considered a representative volume element (RVE) resulting in reproducible stress-strain behavior and uniform deformation. Below this critical ratio, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a fixed ligament size of L=350 nm. Stochastic behavior along with non-uniform deformation for α<10, indicate that the size of the load-bearing unit is close to 10 times the corresponding ligament size.
In the case of in situ microcompression, a novel loading profile was developed to decouple the contribution of displacement due to the actuation from the compression-induced deformation. The flow stress of pillars under potential jumps exhibited the same trend as the corresponding macroscopic results; strength is enhanced significantly due to the surface adsorption, and this response is reversible. The amount of previous deformation of pillars with a clean surface has no impact on the subsequent flow stress coupled with adsorption. The elastic modulus was found not to depend on the potential, in contrast to what was found with dynamic mechanical analysis at the macro-scale. The stress-strain curves of pillars with varying α indicate that the relative change in strength induced by adsorption (∆σ/σ_off) decreases with increasing ligament size, i.e. the absolute change in strength and the strength itself scale differently with the ligament size. A change in pillar size does not impact the ∆σ/σ_off for a fixed ligament size, even though α is below the ratio of the RVE for the mechanical behavior.
Based on the results of nanoindentation and microcompression without electrochemical environment, the elastic modulus, strength and hardness show a decrease with increasing ligament size. By varying ligament size (L) and sample diameter (D) of NPG micropillars, a critical ratio (α=D/L=20) was found, above which the test structure can be considered a representative volume element (RVE) resulting in reproducible stress-strain behavior and uniform deformation. Below this critical ratio, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a fixed ligament size of L=350 nm. Stochastic behavior along with non-uniform deformation for α<10, indicate that the size of the load-bearing unit is close to 10 times the corresponding ligament size.
In the case of in situ microcompression, a novel loading profile was developed to decouple the contribution of displacement due to the actuation from the compression-induced deformation. The flow stress of pillars under potential jumps exhibited the same trend as the corresponding macroscopic results; strength is enhanced significantly due to the surface adsorption, and this response is reversible. The amount of previous deformation of pillars with a clean surface has no impact on the subsequent flow stress coupled with adsorption. The elastic modulus was found not to depend on the potential, in contrast to what was found with dynamic mechanical analysis at the macro-scale. The stress-strain curves of pillars with varying α indicate that the relative change in strength induced by adsorption (∆σ/σ_off) decreases with increasing ligament size, i.e. the absolute change in strength and the strength itself scale differently with the ligament size. A change in pillar size does not impact the ∆σ/σ_off for a fixed ligament size, even though α is below the ratio of the RVE for the mechanical behavior.
DDC Class
600: Technik
620: Ingenieurwissenschaften
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