Using magnesium to maximize heat generated by reactive Al/Zr nanolaminates

In this study, we explore the effect of magnesium content on the ability of reactive nanocomposite foils to generate heat, by comparing three chemistries: Al:Zr, Al–8Mg:Zr, and Al–38Mg:Zr. These correspond to foils with alternating aluminum and zirconium layers where the Al is either pure, an 8 at.%Mg alloy, or a 38 at.%Mg alloy, respectively. Measurements performed in a specially designed bomb calorimeter show that Al–8Mg:Zr foils perform the best, generating the greatest gravimetric heat in air, oxygen, and nitrogen environments. Both Mg-containing foils release a visible plume of particles and vapor upon reacting, which was recorded with a high speed camera. This ejected mass includes Mg vapor and particles of all three metals. Both the vapor and particles oxidize rapidly in air, resulting in single metal-oxide particles. The reacted foils, particularly the Al–8Mg:Zr samples, contain voids and higher levels of oxygen and nitrogen throughout their thicknesses than reacted Al:Zr foils. To explain the higher heats of reaction for the Al–8Mg:Zr foils, we suggest that the out-diffusion and evaporation of Mg generates a high concentration of vacancies that enhance oxygen and nitrogen diffusion throughout the foil, thereby increasing the degree of oxidation and nitridation.