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Evolution of microstructure of np-Au up on temperature annealing. (a) SEM micrographs of np-Au obtained after annealing at various temperatures 30, 55, 85, 110, 155, 210, 255, and 500 °C for 6 h. Closed pore structure develops in the sample at higher annealing temperatures. (b) Increase in the ligament diameter L with annealing temperature. (c) Decrease in the ratio Ap/A l of pore to ligament area with annealing temperature. Beyond 300 °C, the nanoporous morphology changes to a disconnected closed pore structure and densification takes place.
Positron lifetime spectra in np-Au compared with those in annealed and cold-worked bulk Au. Measurements were carried out under vacuum of ∼10−6 mbar to avoid the effects of trapped gases in the pores. The spectrums are area normalized.
Variation of positron lifetime parameters. (a) τ1; (b) τ2; (c) I2; and (d) τ3 with annealing temperature in np-Au. The right ordinate in panel (c) shows the increase in the ligament diameter L with temperature, as obtained from SEM studies. The vertical dotted lines mark the distinctive stages, inferred from positron experiments, corresponding to the migration of vacancies at 85 °C, instability at the surface of ligaments at ∼175 °C and the transition to closed pore structures at ∼500 °C.
Positron annihilation lifetime parameters, τ1, τ2, I1, I2 for as-dealloyed np-Au, compared with reference samples of master alloy Ag75Au25 used for the dealloying and cold worked and annealed bulk Au. The bulk Au and Ag75Au25 samples were annealed, respectively, at 850 °C and 700 °C in vacuum better than 10−6 mbar and subjected to air cooling to obtain nearly defect free samples.
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