Exothermic reactions in cold rolled Ni/Al reactive multilayer foils

Self-propagating exothermic formation reactions have been intensively studied in a variety of reactive multilayer foils, such as Ni/Al, Ni/Si, and Nb/Si foils. These foils contain numerous alternating nanoscale layers of elements with large negative heats of mixing. With a small thermal pulse, atoms diffuse normal to the layering and result in a rapid exothermic reaction. The heat produced is conducted down the foil and facilitates more atomic mixing and compound formation, thus establishing a self-propagating reaction. Multilayer foils are generally fabricated by physical vapor deposition (PVD) methods, such as magnetron sputtering or electron-beam evaporation. These methods involve creating a vapor of a material, known as the source material, and then depositing the vapor onto a substrate. The rate at which the vapor is deposited is controllable, allowing the growth of films with thickness ranging in nanometers. Alternatively, multilayer foils can also be made by cold rolling. In this study, Ni/Al multilayer foils were fabricated by a cold rolling method. Self-propagating reactions in these foils were investigated. The microstructure, phase composition and reactive formation process for the cold rolled Ni/Al foils were characterized. Thin sheets of elemental Ni and Al with a Ni/Al thickness ratio of 2/3 were stacked alternatively together, in order to obtain a Ni/Al atomic ratio of 1/1. The stacked sheets were placed in between a folded stainless steel sheet, which was previously hardened by repeated rolling. This assembly was cold rolled a few times using a laboratory rolling mill to reduce the thickness to half of the original thickness. Afterwards the Ni/Al sheets were taken out of the stainless steel sheet, cut into halves, stacked the halves together to recover their original thickness, and then cold rolled without changing the distance between the rollers (this is defined as a rolling cycle). After several rolling cycles, a uniform Ni/Al multilayer foil was achieved. The total thickness of the foil was around 200 m. The cold rolled Ni/Al multilayer foils were ignited by a flame. After heating for several seconds, a self-propagating reaction started in the foil and two distinct reaction steps were observed. First, the reaction front spread along the foil at a relatively slow rate and the foil surface became darker. Then, a second reaction followed, where the reaction front spread along the entire foil very fast, showing visible red light. To characterize the reaction products, reacted Al/Ni foils were ground into powders for symmetric X-ray diffraction (XRD) examination using Cu K radiation. As cold rolled Al/Ni foils were also examined by XRD for comparison. XRD traces for as cold rolled and reacted Ni/Al multilayer foils are plotted in Figure 1. For the as cold rolled foils, all the peaks in the x-ray diffraction scan correspond to Al and Ni, as shown in the upper scan in Figure 1. After reaction, all major peaks correspond to the ordered B2 AlNi compound, which is the equilibrium compound for this composition, as shown in the lower scan in Figure 1. Figure 1 XRD patterns of Ni/Al multilayer foils before and after reaction. Figure 2 shows scanning electron microscopy (SEM) images of an as cold rolled Ni/Al multilayer foil and a reacted foil. No sign of reaction can be observed in the cold rolled Ni/Al multilayer foil. Necked Ni particles are embedded in the Al matrix and aligned along the rolling direction. Most of the Ni particles possess a wavy surface. The average bilayer thickness is in the range of several micrometers. Two phases can be observed in the reacted foil: AlNi as the reaction product and some remaining Ni. There are also some pores in the reacted foil, which result from the contraction during rapid cooling and the density increase during the reaction.