Formation mechanisms of NaZn13-type phase in giant magnetocaloric La–Fe–Si compounds during rapid solidification and annealing

Abstract

a b s t r a c tThe promise of the LaeFeeSi alloys for energy efficient magnetic refrigeration devices stems from a strong magneto-structural transition in the La(Fe,Si) 13 (1:13) phase near room temperature. The formation of the 1:13 phase during rapid solidification was compared in detailed microstructural studies of the wheel-side and free-side surfaces of melt-spun ribbons. On the free-side, clusters of similarlyoriented crystallites are observed; chemical segregation of La, Fe, and Si leads to a nanoscale texturing of a-Fe and LaFeSi. In contrast, equiaxed 1:13 grains~100e400 nm form the microstructure of the wheelside surface, with a minor a-Fe phase precipitated in the matrix. Upon annealing, the 1:13 phase grows via dissolution of the a-Fe phase on the wheel side and a peritectoid reaction from the free side. For longer annealing times, the completion of the peritectoid reaction improves the magnetic entropy change under a magnetic field change of 1.5 T from 12 J/kg K (2 min) to 17 J/kg K (2 h), and increases the Curie temperature of the ribbons from 189 K to 201 K.

Formation of tree-like and vortex magnetic domains of nanocrystalline α-(Fe,Si) in La–Fe–Si ribbons during rapid solidification and subsequent annealing

“…La-Fe-Si ribbons with a nominal composition of LaFe 11.5 Si 1.5 were fabricated by a melt-spinning technique. Details of the preparation have been reported elsewhere [20,21]. The resulting (as-spun) ribbons were sealed in evacuated quartz ampoules and subjected to isothermal heat treatment at 1273 K for 3 min or 2 h before rapid quenching to room temperature.…”

Viable Materials with a Giant Magnetocaloric Effect

“…Magnetic shape memory alloys [148,204] include Ni-Mn-In [167,175,192,261,283] doped with Co [136,172,241] and Si [122]; Ni-Mn-Sn [115,226]; NiMn-based B2 (Ni-Co)(Mn-Ti) [30], Ni-Co-Mn-Al films [189,190] [293], and metallic glasses [274]. Significant interest was devoted to the potentially viable LaFe 13−x Si x [137,200,284,288,290,292], doped with Co [249] and other additives [35], or partially hydrated [52][53][54][55]. For completeness, we mention Mn 3 CuN 1−x C x [147] and molecular magnets [128].…”

Peritectic Solidification Path of the La(Fe,Si)13 Phase in Dual-Phase Directionally Solidified La-Fe-Si Magnetocaloric Alloys

“…Previous research has reported that supercooling plays an important role in the nucleation competition between a-Fe and La(Fe,Si) 13 . [26] However, the transition area will be dismissed if the large volume fraction of the La(Fe,Si) 13 phase in directionally solidified LaFe 11.6 SI 1.4 alloys is brought about by the nucleation competition, which could imply a different valid mechanism.…”