Abstract
Reactive particles consisting of nickel and aluminum represent an adaptable heat source for joining applications, since each individual particle is capable of undergoing a self-sustaining exothermic reaction. Of particular interest are particles with intrinsic lamellar microstructures, as they provide large contact areas between the reactants nickel and aluminum. In this work, the exothermic reaction as well as the microstructure of such lamellar reactive particles produced by high energy planetary ball milling were investigated. Based on statistically designed experiments regarding the milling parameters, the heat of reaction was examined by means of differential scanning calorimetry (DSC). A statistical model was derived from the results to predict the heat of reaction as a function of the milling parameters used. This model can be applied to adjust the heat of reaction of the reactive particles depending on the thermal properties of the joining partners. The fabricated microstructures were evaluated by means of scanning electron microscopy (SEM). Through the development of a dedicated SEM image evaluation algorithm, a computational quantification of the contact area between nickel and aluminum was enabled for the first time. A weak correlation between the contact area and the heat of reaction could be demonstrated. It is assumed that the quantification of the contact areas can be further improved by a higher number of SEM images per sample. The findings obtained provide an essential contribution to enable reactive particles as a tailored heat source for joining applications.
Highlights
Combustion synthesis, which is known as self-propagating high-temperature synthesis (SHS), is a process in which at least two reactants, which are denoted as reactive system, are transformed into a product in a highly exothermic reaction [1,2]
A defined amount of energy can be released depending on various influencing factors, such as the stoichiometric ratio of the reactants contained. Important types of these reactive particles are core–shell structures obtained by wet chemical synthesis [8,9] as well as lamellar microstructures obtained by high energy ball milling (HEBM) of metallic powders [10,11]
It should be taken into account that higher factor levels of the milling parameters result in a higher energy input into the reactive particles, which promotes the formation of lamellar microstructures
Summary
Combustion synthesis, which is known as self-propagating high-temperature synthesis (SHS), is a process in which at least two reactants, which are denoted as reactive system, are transformed into a product in a highly exothermic reaction [1,2]. A defined amount of energy can be released depending on various influencing factors, such as the stoichiometric ratio of the reactants contained Important types of these reactive particles are core–shell structures obtained by wet chemical synthesis [8,9] as well as lamellar microstructures obtained by high energy ball milling (HEBM) of metallic powders [10,11]. With HEBM, the activation temperature of homogeneous Ni + Al powder mixtures can be significantly reduced, while the heat of reaction is increased [10,11] This is caused by a reduction in particle size [10], and by an increase of the contact area between nickel and aluminum due to the formation of lamellar microstructures within the particles [15,16,17,18]. The presented SEM image evaluation algorithm is a key element, as it allows a computational determination of the contact area between the reactants
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