Estimating the in-operando stabilities of AlFe2B2-Based compounds for magnetic refrigeration

The in-operando stabilities of AlFe2B2-based (aka, 1-2-2) compounds were investigated by mimicking two separate operating conditions (immersion in a heat transfer fluid and magnetic field cycling) of an active magnetic regenerator (AMR) refrigerator. Parent AlFe2B2 and Ga-containing samples of nominal composition Al1.2-xGaxFe2B2 (x = 0, 0.05 and 0.1) were prepared by casting and subsequent annealing. The chemical stability of pulverized (Al,Ga)Fe2B2-based compounds subjected to water immersion at room temperature for up to 960 h was evaluated. In a separate test, the mechanical stability of dry (Al,Ga)Fe2B2 cast-disk-shaped samples was studied utilizing a device constructed to mimic the AMR field cycling process. The materials’ responses as a function of water-immersion time or of magnetic field cycles were assessed by probing the crystal structure, the microstructure, the magnetic characters and the magnetocaloric response as quantified by the magnetic entropy change ΔSmag. The water-immersed pulverized samples exhibit exponential-decay-like degradations in magnetization and in ΔSmag. The Ga-containing compound exhibits significantly smaller reductions in magnetization and in ΔSmag after 960 h of water immersion relative to those of the Ga-free composition. In the separate magnetic-field-cycling test conducted under a dry condition, the structural and the magnetocaloric responses of both the Ga-free and Ga-containing disks remain similar after ∼5 × 105 cycles in and out of a 0.5 T magnetic field. The decrease in the magnetocaloric response during water immersion is attributed to the formation of oxide and/or hydroxide phases that reduced the AlFe2B2 phase fraction. The stable structure and the constant magnetocaloric response observed after magnetic field cycling are attributed to the extremely small unit cell volume change of AlFe2B2 through its magnetic phase transition. Overall, these results demonstrate that the AlFe2B2-based compounds exhibit good chemical stability upon water immersion that is improved by small Ga additions, and robust mechanical integrity upon dry magnetic field cycling, suggesting that AlFe2B2-based compounds are promising as robust AMR magnetic refrigeration working materials.