Introduction

The patent covers a method of preparing the magnetic material AlFe2B2 by means of a combustion synthesis reaction triggered by microwave exposure, with subsequent purification procedure of the compound based on the magnetic separation technique to remove the reaction by-products obtained from combustion. The method is characterized by extremely effective, fast and selective heating of raw materials by using specific waveguides that allow microwaves to be used in a predominantly electric or magnetic field. The AlFe2B2 compound is an inexpensive, lightweight, rare-earth-free magnetocaloric material and can be used in the development of sustainable refrigeration and waste heat recovery technologies.

 

Technical features

The invention consists of using microwaves, predominantly in electric field (E mode) or magnetic field (H mode), to trigger a combustion reaction for making the compound AlFe2B2 from elemental powders (Fe, Al, B) or pre-alloyed powders (Fe-Al, Fe-B,..). The use of a special reactor/waveguide allows operation with (mostly) high-frequency electric or magnetic (microwave) fields. The frequencies that can be used are all those provided by the ISM band (6.765 MHz to 246 GHz). The use of microwaves, in this particular configuration, allows for selective (only the precursors increase in temperature and not the surrounding air or material) and volumetric (very small internal temperature gradient within the sample) heating of the raw materials to be processed. This leads to a very fast temperature rise in the material, resulting in an initiation of the combustion reaction within seconds of microwave application. Volumetric heating makes it possible to speed up the reaction and improve the yield, the powders being preheated by the microwaves and thus not taking energy away from the synthesis by combustion. The use of a predominantly electric (E mode) or magnetic (H mode) field results in different heating efficiency, and thus different reaction efficiency.

 

Possible Applications

• Realization of active elements for magnetic chillers with applications in the refrigeration industry (refrigerators, freezers, refrigeration in general, air conditioning, heat pumps);
• Realization of active elements for the development of new solutions for thermomagnetic energy harvesting by low-enthalpy heat recovery;
• Development of large-scale microwave applicators for industrial-scale synthesis of AlFe2Be compound.

Advantages

The method of heating raw materials generates significant advantages in terms of speed, efficiency and simplicity with which the desired phase can be obtained compared to the compound preparation methods present in the state of the art (arc or induction melts followed by lengthy high-temperature heat treatments and cleaning with concentrated acids with high environmental impact).