Magnetocaloric Effect in Alloys with Distributed Exchange Interactions

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Non Technical Abstract The magnetocaloric effect is the temperature change a magnetic material undergoes on application or removal of a magnetic field. Materials exhibiting a large magnetocaloric effect are of interest for refrigeration applications because magnetocaloric refrigeration does not use ozone depleting gases necessary in conventional gas compression refrigerators. Magnetic refrigeration is up to 20% more efficient than conventional refrigeration and has potential to lessen environmental impact and decrease energy demands of cooling, that is a significant portion of US yearly energy usage, Magnetocaloric cooling refrigerators have been announced for consumer products as early as 2020. Magnetocaloric cooling could also be used for thermal management, resulting in more efficient motors both for commercial and military defense vehicles. The materials to be studied are new alloys that fit applications requiring durable materials, as they have previously been studied for use in extreme temperature and pressure environments making them viable for active cooling and thermal management of engines. The project will engage outreach programs at Florida Polytechnic University and mentor high school science students. Technical Abstract The magnetocaloric effect (MCE) refers to the temperature change of a magnetic material on application of a magnetic field, H. Materials with a large magnetocaloric response near room temperature can be used for magnetic cooling. Critical rare earths metals (REs)compounds were first studied because of large magnetocaloric effects near room temperature. However, the increasing cost and scarcity of REs limits their use for commercial refrigeration, so transition metal-based replacementss have been investigated. Magnetocaloric materials are classified by undergoing: (1) a 1st order magneto-structural phase transition, or (2) a 2nd order magnetic transition. 1st order transitions have a large, narrow peak magnetic entropy changes in a magneto-structural phase transition, but this is accompanied by thermal hysteresis making them less desirable for multi-cycle cooling. 2nd order phase transitions have a small but broad magnetic entropy change. The broad entropy change gives these 2nd order phase transition materials a larger working temperature range increasing their refrigeration capacity, RC. Optimal magnetocaloric materials require a relatively large peak entropy change, a large RC and working temperature range, low thermal hysteresis, and resistance to thermomechanical fatigue. The studies will focus on the role of crystallographic disorder and pressure on exchange interactions in multi-component high entropy alloys to assess the maximum magnetocaloric effect in these materials. These multi-component alloys have tunable Curie temperatures, Tc, magnetocaloric response, and refrigeration capacity. We will model how positional disorder affects the magnetic phase transition and magnetocaloric response of the system. Quaternary and quinternary Fe-Co-Ni-based alloys are of interest because distributed J(R) allows Tc tuning and control of RCs, through the breadth of the 2nd order transition, while the Fe-Co-Ni basis allows for a larger average magnetic moment of the system, which increases the magnetocaloric response. We will (1) observe structure under pressure, P, at Argonne National Lab with pressure cells that can reach P > 6 GPa.; (2) extend random exchange models with band theory to understand Tc tuning in multicomponent systems and P-dependence of the Bethe-Slater curve; (3) investigate pairwise exchange interactions using band theory that includes spin-orbit interactions in fully relativistic formalisms to confirm J(R)'s predicted by the Bethe-Slater curve; (4) study pressure dependent atomic spacing in fcc-based HEA?s, measured at synchrotron facilities, to be used with calculated J(R)'s to predict T-dependent magnetization and MCE; (5) confirm M(T) predictions by P-dependent susceptibility measurements; and (6) measure magnetic hyperfine field distributions using Mossbauer spectroscopy below Tc in technologically relevant alloys. The project will engage outreach programs at Florida Polytechnic University and mentor high school science students.