Microwave ignition of the combustion synthesis of aluminides and field-related effects

• Microwaves ignited combustion synthesis of aluminides in separate E and H fields.
• H field ignition allows higher reproducibility and lower energy requirement.
• Curie point of the ferromagnetic element plays a crucial role in synthesis kinetic.
• H field ignition increases reactivity of less exothermic reactions.
• Microwaves can be used to decrease the cooling rates of the synthesized products.

Combustion synthesis of aluminide intermetallics starting from reactive powders mixtures comprising a ferromagnetic component (Fe, Co and Ni) has been performed by igniting the exothermic reactions in a microwave single mode applicator, in order to investigate possible effects related to the positioning of the reactive sample into the region of predominant electric or magnetic field.In addition to an increase of the reproducibility of the experiments and the possibility to avoid electric arcs generation, the ignition of the combustion reactions in predominant magnetic field allowed to significantly reduce the ignition times, with a global reduction of the power required to synthesize the intermetallics. This implies that the amount of the reactive volume under ignition conditions is reduced proportionally, thus moving from thermal explosion towards Self-propagating High-temperature Synthesis mode of combustion.Moreover, in the case of Fe–Al system, the disposition of the load into the region of predominant magnetic field increased the yield of the desired intermetallic compound, to the detriment of α-Fe(Al) solid solution co-product. Furthermore, the possibility to continue to furnish energy to the reactive Fe + Al sample after combustion synthesis ignition has been proven for both configurations. Results show that in case of exposure to the predominant magnetic field, the sample cooling rate resulted lower, hence the permanence at high temperatures after synthesis could be proficiently exploited to increase adhesion to an underlying substrate, in case combustion synthesis technique is used to obtain intermetallic-based coatings.