Hollow Hemisphere Shell Formation by Pure Kirkendall Porosity

Abstract

Nanoshell formation has been studied experimentally in Ag/Au and Ag/Pd systems in a hemispherical geometry at different temperatures. The void formation in these systems is the result ofpureKirkendall-porosity formation, because it is caused mainly by the inequality of the intrinsic atomic fluxes and other effects (e.g. stresses), inevitably present during nanoshell formations in solid state reactions (oxides, sulphides), can be less important or can be neglected. The kinetics of the process was followed by Transmission Electron Microscopy. Both the growth and shrinkage regimes of the process were observed at the same temperature and even the temperature dependence of the characteristic time (tcr) describing the crossover of the two different regimes was observed. We succeeded to show that tcrshifts to smaller values with increasing temperature. This confirms the theoretical results:the growth and the shrinkage regimes are controlled by the faster as well as the slower diffusion coefficients (DAas well as DB), respectively. It is also illustrated that, confirming recent theoretical predictions, the pore radius linearly depends on the initial particle radius and the slope of this straight line increases with the average composition of the faster component.