The substitution of Mn by Ni in (Mn,Fe)2(P,Si) compounds and its magnetocaloric consequences

Abstract

With green energy well on its way to becoming a necessity rather than a commodity it is vital to not only focus on improving the source of energy, but also the efficiency of the eventual energy usage. Vapor-compression based household refrigerators are one of the most used electrical appliances in the common household and consume electricity throughout the day, so finding a more efficient alternative would be beneficial. One such alternative uses the magnetocaloric effect as cooling mechanism and could theoretically reach a much higher efficiency. However, the search for the right material with the right characteristics for the commercial realization of this technology is still ongoing and this thesis aims to assist in making progress in said search. The focus of this thesis is on the substitution of manganese with nickel in Mn-Fe-P-Si compounds and how the magnetocaloric properties of these compounds hold up against similar compounds that had manganese substituted with cobalt as this lowered the Curie temperature and decreased the thermal hysteresis, which are favourable results. Unfortunately, there are a few factors such as price and criticality that motivated replacing cobalt with a close relative such as nickel in hope of achieving similar results. Measurements done with a superconducting quantum interference device (SQUID) indicated that the synthesized Mn-Fe-Ni-P-Si compounds have very similar magnetic properties to the Mn-Fe-Co-P-Si compounds. Differential scanning calorimetry (DSC) measurements showed that both compounds reduce thermal hysteresis similarly, but the nickel compounds are capable of lowering the curie temperature much more with increased doping compared to the cobalt compounds. X-ray powder diffraction (XRD) analysis proved that nickel is substituted in the proper place and that the decreasing Curie temperature is the result of a change in the ratio of the unit cell parameters a and c. Nickel thus showed to be an excellent replacement of cobalt to be used to substitute manganese with to lower the Curie temperature of the initial Mn-Fe-P-Si compound and reduce thermal hysteresis. More research could be done on further tuning the composition by for example increasing the silicon doping as this has shown to increase the Curie temperature rather than decrease it whilst also reducing the thermal hysteresis. Combined with the results from this thesis it could lead to a composition with more favourable magnetocaloric characteristics.