X-ray Imaging of Nanoporous Gold: Evolution of Morphology and Internal Strain during Dealloying and Coarsening

Nanoporous gold (np-Au) has numerous potential applications, which include sensors, actuators, catalysists, super-capacitors and support structures for the electrodes of Li-ion batteries. The research presented in this dissertation focuses on (i) nanopore formation process during dealloying (selective dissolution of Ag from Ag-Au alloys), (ii) dealloying-induced stresses and strains, and (iii) mophological evolution of np-Au during thermal coarsening.;The dealloying process was imaged using high-resolution (sub-50nm) full-eld x-ray microscopy, so-called 'transmission x-ray microscopy'. A sharp dealloying front between the np-Au and Ag-Au was directly observed. The propagation velocity of this dealloying front for various alloy compositions (Ag-xAu, x=20--40 at. %) and acid concentration (7.3--14.6 M) was quantied. The propagation velocity (vd) remains constant with time to a depth of at least 3 micron, which indicates an 'interface-controlled' dealloying process. vd is found to be exponentially proportional to the Ag/Au atomic ratio and to the acid concentration, which can be explained by the exponential dependency between the dealloying rate and the over-potential and bond energy.;To characterize in three-dimensions (3D) the morphology of the sponge-like nanoporous structure of np-Au, Fresnel coherent diraction imaging (CDI) was used. The coarsening occurring concurrently with the dealloying process was found to cause a slight densification in the np-Au structure.;The dealloying-induced stresses and strain were found to cause a macroscopic volume reduction in Ag-20Au and Ag-25Au alloys. Crack formation and a linear shrinkage of 38% of shrinkage were found in the Ag-20Au alloy. For Ag-25Au, the shrinkage is only < 5 % and thus does not lead to cracking: finite-element modeling predicts that only the np-Au near the dealloying front (distance < 1.4 micron) may exhibits a plastic deformation.;This dealloying-induced strain was further investigated in an early stage of dealloying of a Ag-30Au nano-crystal, using CDI in the Bragg geometry, which is sensitive to the lattice distortion. The average tensile and compressive strain increased by 0.01--0.02% after a 10 s period of dealloying. A 60--80 nm dealloyed region exhibited compressive strain. By contrast, the interior of the nano-crystal developed tensile strain.;X-ray nano-tomography was used to study the structural evolution of np-Au during thermal coarsening when both the gold ligaments and pores increase in size (from sub-50 nm to a few microns). The morphological anisotropy of this material increases with the coarsening time, which can be explained by a preference for the crystallographic orientation with the lowest surface energy. This leads to faceting during coarsening, which was revealed by micro-beam Laue diraction. This anisotropic coarsening behavior results in a change of the shape of the interface between gold ligaments and pores.