A Critical Relative Density and a Break-and-Reconnect Model for Annealing-induced Densification in Nanoporous Gold

Abstract Single-crystal or coarse-grained nanoporous gold (NPG) prepared by dealloying is considered stable against sintering or densification during annealing. Herein, we report that this phenomenon only applies to high-density NPG with relative density ( φ ) exceeding a critical value of 0.31, which densifies very slightly during annealing. In low-density NPG with φ 0.31, annealing induces excessive shrinkage that increases with decreasing φ . Sintering along grain boundaries may account for the small densification of high-density NPG, but it fails to explain the excessive shrinkage of low-density NPG. This densification may be linked to ligament pinch-off, which dominates the coarsening of NPG with φ below ∼ 0.30. However, ligament pinch-off does not directly lead to densification. Instead, we suggest that reattachment of dangling ligaments (generated by pinch-offs) is responsible for the large shrinkage in low-density NPG. This break-and-reconnect scenario is supported by quasi-in situ scanning electron microscopy observations and the evolution of scaled genus density for different type of NPGs during annealing. Our findings suggest that annealing-induced densification can be suppressed by increasing the relative density of NP metals to above 0.31, which is critical to many mechanical and functional explorations.