(A) study on the reaction properties of compression-bonded Ni/Al microfoils

Abstract

The mechanical bonded reactive Al/Ni multilayers have numerous applications such as high temperature bonding and local heat sources. Moreover, combustion of the mechanical bonded Al/Ni multilayers generate not only high temperatures with a little providing energy to ignite the binary systems but also restrict to local heating. In this paper, a compression-bonded Ni-sputtered Al foil multilayer process is proposed in order to study the phase formation, thermal kinetic, velocity and reaction heating in mechanically bonded multilayers. The compression-bonded multilayers exhibit advantages over the sputtered multilayers due to the negligible formation of an intermixed region and low process costs. Moreover, the process provides uniform and continuous layers which were not observed in the conventional mechanical bonded multilayers. In the $1^{st}$ chapter, the Al/Ni multilayers with various atomic ratio was investigated as thermal annealing (TA) during slow heating via differential scanning calorimetry (DSC). The reaction paths were different significantly to the varying Al/Ni ratios. When the Al/Ni ratio of the multilayers was set to 3:1 and 3:2, the corresponding compounds ($Al_3Ni$ and $Al_3Ni_2$, respectively) were formed. Meanwhile, the reaction was not completed when sufficient Ni, above 40 at.%, was provided in the reaction. As the Ni atomic ratio increases from 41.2 to 57.6%, the reaction heating decreased from the theoretical values by up to 24.4%. The microstructures after combustion wave exhibited entirely agreed with phase diagram via solidification. In the $2^{nd}$ chapter, the variations of the exothermic reaction behaviors of compression-bonded Ni-sputtered Al foil multilayers are investigated through controlling the interfacial layers of the multilayers. The interfacial Al oxide layers between each Ni/Al bilayer are intentionally preserved for the pursuit of thermal explosion (TE) during slow heating with up to a few tens of K/min. In contrast, the TA occurs in the highly shear-deformed multilayers with many broken Ni layers, because the ductile Al filled the gaps in the broken Ni layers to form reactive Al/Ni interfaces. Increased onset temperatures of 865 to 893 K and activation energies, 469.89 kJ/mol, are measured in the multilayers exhibiting the TE. The onset temperature is increased by 60-80 K when compared with the multilayers exhibiting the TA. The activation energy for the $Al_3Ni$ formation reaction of the multilayers exhibiting the TA is 224.14 kJ/mol. The native oxide layers between each of the Ni/Al bilayers are preserved in order to delay the Al/Ni interdiffusion using uniform compression and reduced shear stress during the compression bonding. The influences of the Al oxide layers on the exothermic reaction behavior of the compression-bonded multilayers are discussed. In the $3^{rd}$ chapter, the Al/Ni multilayers with various atomic ratios are fabricated by various bilayer thicknesses. The microstructures that are formed after the self-propagating combustion using laser ignition result in equilibrium phases in the Ni-Al binary system. Homogeneous intermetallic compounds for $Al_3Ni_2$ and AlNi are obtained through controlling the Ni layer thickness. The onset temperatures of the multilayers are below 700 K for all multilayer samples, except the specimen with Ni of 18.6 at.%. The maximum temperatures correspond to the liquidus temperatures of the intermetallic compounds. The self-propagating direction is divided into a transverse propagating direction and a gross propagating direction. The measured gross propagation velocities vary widely without exhibiting a clear trend. However, the transverse propagation velocity is dependent on the measured maximum temperatures, while the effects of the bilayer thickness are not discernible. The measured transverse propagation velocities are similar to the reported propagation velocities for sputtered multilayers with similar bilayer thicknesses. It is indicated that the transverse propagation is progressed as partially stable as conventional stable self-propagation. In the last chapter, we obtain stable self-propagating reactions in the multilayers through controlling the total number of multilayers. While stable self-propagation is not obtained in multilayers with 40 bilayers, it is constantly observed in the multilayers with 70 to 90 bilayers. The total reaction velocity does not have trends due to the similar bilayer thickness, while the stable self-propagation velocity is more consistent than the total reaction velocity. The minimum total number of bilayers required in order to obtain a stable self-propagation reaction for the 9 μm bilayer thickness is experimentally identified as between 60 and 70 bilayers.