Age splitting of the La(Fe1−xSix)13Hy first order magnetocaloric transition and its thermal restoration

Abstract

The La(Fe1−xSix)13Hy system has a first order ferromagnetic phase transition with a large magnetocaloric effect for 0.11 < x < 0.13. Such materials produced the highest currently published cooling power of a magnetic refrigerator. Adjusting the H content, y allows selection of any Curie point Tc from ∼200 K to 330 K, but hydrogen-unsaturated material (y < 1.6) with a first order transition exhibits an unusual instability. If La(Fe1−xSix)13Hy is held within a few K of its initial Tc, an initially single magnetic transition, with a sharp differential scanning calorimetry peak, gradually splits into two transitions separated by a large temperature interval. The ultimate splitting interval depends almost linearly on (1.6-y) and Tc. If the material is held more than 10 K above or below Tc, an initially sharp transition is retained, and a split transition is restored to its original sharp single transition. The recovery rate increases with temperature. For temperatures above 320 K, the recovery rate is rapid enough to allow overnight recovery of magnetocaloric material that is in a split state. This method was employed to maintain high performance of La(Fe1−xSix)13Hy in a magnetic refrigerator. In order to verify that the recovery process involves the macroscopic movement of hydrogen within the solid, a portion of 0.2 mm diameter particle material with a split transition was ground into particles of 0.05 mm diameter. The unground 0.2 mm particles and the ground 0.05 mm particles were held at 13 K above Tc. The 0.3 mm particles recovered their initial single transition, but the 0.05 mm particles, when examined as a group, retained their split line, presumably because they had been separated into particles with differing total hydrogen fraction y.