Multi-stage pore development in Ag foams by the reduction of Ag2O and CuO mixtures

Abstract

Pore expansion in solid metals is typically driven by an entrapped gas phase, such that temperature directly determines both the pressure within the pores and the ability of the matrix to plastically deform. This approach fundamentally limits the total porosity, as all pores are active simultaneously, which quickly results in coalescence and percolation. The method introduced here converts dispersed oxide particles to a gaseous reactant using reduction, such that independent stages of pore formation and expansion can be achieved by using more than one oxide chemistry. This is demonstrated using silver and copper oxides distributed within a silver matrix. As the temperature is raised, the silver oxide reduces first and creates porosity. As the temperature is raised further, the copper oxide reduces and creates additional porosity. This allows the pore morphology and grain size to be uniquely controlled while still maintaining the simplicity and scalability of the process. The microstructural development is studied through a combination of isothermal annealing, optical dilatometry, and focused ion beam cross-sectioning, and implications and strategies for other alloy systems are discussed.