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Item type:Publication, Access status: with files , On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au(2023-08); ; In this study, the consequence of intrinsic and extrinsic size effects on mechanical responses of nanoporous gold is investigated via microcompression testing. By varying the micropillar diameter (D) between 1 µm and 20 µm and the ligament size (L), 50 nm and 350 nm, a critical ratio (α = D/L = 20) was found, above which the test structure can be considered a representative volume element, resulting in identical mechanical response and uniform deformation. Below that value, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a measurement series with a fixed ligament size of 350 nm where the flow stress decreased by more than 50% (from approximately 5 to 2.5 MPa) and the elastic modulus was reduced from approximately 0.5 GPa to almost zero. Stochastic behavior along with non-uniform deformation and failure is observed for α < 10, suggesting that the size of the load-bearing units in this material is about 10 times the corresponding ligament size.Publicationtype: Journal ArticleScopus© Citations 4 22 72 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electro-chemo-mechanical coupling of nanoporous gold at the microscale(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.Publicationtype: Doctoral Thesis285 941 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Image segmentation and analysis for densification mapping of nanoporous gold after nanoindentation(Springer, 2021-07-14); ; ; ; Segmentation of scanning electron microscopy (SEM) images of focused ion beam (FIB) cross-sections through indented regions in nanoporous gold (np-Au) is carried out. A key challenge for image analysis of open porous materials is the appropriate binarization of the pore and gold ligament regions while excluding material lying below the cross-sectional plane. Here, a manual approach to thresholding is compared to global and local approaches. The global thresholding resulted in excessive deviations from the nominal solid fraction, due to a strong gray-scale gradient caused by the tilt angle during imaging and material shadowing. In contrast, the local thresholding approach delivered local solid fractions that were free of global gradients, and delivered a quality comparable to the manual segmentation. The extracted densification profiles vertically below the indenter as well as parallel to the surface showed an exponential-type decay from the indenter tip towards the nominal value of 1 far from the indenter. Graphic abstract: [Figure not available: see fulltext.]Publicationtype: Journal ArticleScopus© Citations 3 148 233 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electro-chemo-mechanical coupling of nanoporous gold at the microscaleThe observation of reversible strengthening and stiffening of nanoporous gold (NPG) under electrochemical potential has opened opportunities to exploit this material for multifunctional applications. Yet the complex structural geometry and length-scales involved make a definitive understanding of structural correlations to the behaviors difficult at best. Achievement of coupled electro-chemo-mechanical testing at the micrometer scale is a key step toward this goal. Here, we introduce an experimental approach to investigate the elastic and plastic behaviors of NPG under electrochemical potential at the microscale using a modified nanoindentation setup and multiple load function. The in situ experiments in electrolyte show a significant increase by 32% in strength of pillars in a positive potential regime where oxygen adsorption occurred. This response was found to be reversible, which agrees with macroscopic results, while the elastic modulus was shown to be insensitive to the applied potential - an observation inconsistent with recent bulk dynamic mechanical analysis results.Publicationtype: Journal ArticleScopus© Citations 8 377 314