Radhika Barua1 Brian Lejeune2 Brandt Jensen3 Ryan Ott3 Matthew Kramer3 Laura Lewis2 4

1, Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Richmond, Virginia, United States
2, Department of Chemical Engineering, Northeastern University, Boston, Massachusetts, United States
3, Division of Materials Science & Engineering, Ames Laboratory, Ames, Iowa, United States
4, Department of Mechanical Engineering, Northeastern University, Boston, Massachusetts, United States

Material processing schemes play a critical role in guiding the development of emerging magnetocaloric materials for energy-related applications such as magnetic refrigeration and thermomagetic energy conversion.1 To this end, the intermetallic boride AlFe2B2 has attracted considerable attention due to its low cost, promising thermal properties that promote effective heat transfer (specific heat capacity Cp=120 Jmole-1K-1; thermal conductivity κ=5.6 Wm-1K-1), and moderate magnetocaloric response near room temperature (adiabatic temperature change ΔTad =1 K and magnetic entropy change ΔS=2.6 Jkg-1K-1 at μ0Happ = 2 T).2,3 In this work, a number of synthesis methods to form single-phase AlFe2B2 alloys were investigated. Further, the magnetocaloric potential of AlFe2B2 samples synthesized via conventional metallurgical routes (casting) and additive manufacturing technology (laser engineered net shaping (LENS)) was evaluated.

Suction-casting allows fabrication of samples of various composition, including Al1.2MxFe2B2 (M=Ga and/or Ge, x≤0.1). Experimental data obtained using structural and magnetic probes indicate that the unit cell volume, saturation magnetization (Ms), Curie temperature (Tc) and specific heat capacity (Cp) of the samples increase with increased Ga and Ge content (x). Relative to the unmodified parent AlFe2B2 sample, a larger than two-fold improvement in magnetocaloric effect (MCE) was observed in the Al1.1Ge0.05Ga0.05Fe2B2 specimen (ΔS= 6.5 Jkg-1K-1, ΔTad = 2.2 K at μ0Happ=2 T). Intriguingly, the solid solubility of Ga and Ge in AlFe2B2 was determined to be negligible and it is deduced from calorimetric data that additions of these substituent elements alter the solidification route for formation of the AlFe2B2 phase. The enhanced MCE of the Al1.2-x(Ga/Ge)xFe2B2 samples is ascribed to a combination of chemical bonding and electronic effects arising from a hypothesized enrichment of Fe atoms on the Al sites within the (ac) plane of the AlFe2B2 lattice.

These results provide fundamental insights regarding the phase stability of the Al-Fe-B ternary system, and guide development of AlFe2B2 samples of complex geometries using laser engineered net shaping (LENS). The MCE of cylindrical LENS samples (5 mm dia; 30 mm length) was found to be comparable to that of corresponding undoped suction-cast samples (ΔSLENS=2.8 Jkg-1K-1, ΔTad,LENS=1.1 K at μ0Happ=2 T). Further, the feasibility of 3D-printing honeycomb shaped samples was explored. It is surmised that AlFe2B2 is amenable for construction of magnetocaloric heat exchangers where the working material may be shaped as channel structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid. Overall, this study provides strategies for maximizing the magnetofunctional potential of AlFe2B2.

References:1J. Lyubina, J Phys. D: Appl. Phys. 50(5), 053002, 2017; 2R. Barua et. al., J. Alloys and Comp. 745, 505, 2018; 3B.T. Lejeune et. al., Materialia, 2018.