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

Journal Article · 06 January 2020 · Journal of Alloys and Compounds

DOI: https://doi.org/10.1016/j.jallcom.2020.153693 · OSTI ID:1799068

Zhang, X.  ^[1]; Lejeune, B. T.  ^[2]; Barua, R. ^[2]; McCallum, R. W. ^[3]; Lewis, L. H. ^[2]

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1. Northeastern Univ., Boston, MA (United States); OSTI
2. Northeastern Univ., Boston, MA (United States)
3. McCallum Consulting LLC, Santa Fe, NM (United States)

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 Al_1.2-xGa_xFe₂B₂ (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 ΔS_mag. The water-immersed pulverized samples exhibit exponential-decay-like degradations in magnetization and in ΔS_mag. The Ga-containing compound exhibits significantly smaller reductions in magnetization and in ΔS_mag 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 findings 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.

• Research Organization: Northeastern University, Boston, MA (United States)
• Sponsoring Organization: USDOE; USDOE Advanced Research Projects Agency - Energy (ARPA-E)
• Grant/Contract Number: AR0000754
• OSTI ID: 1799068
• Journal Information: Journal of Alloys and Compounds, Journal Name: Journal of Alloys and Compounds Journal Issue: C Vol. 823; ISSN 0925-8388
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English