Influence of electronic and metallographic structures on hydrogen redistribution in La(Fe,Si)13-based magnetocaloric compounds

Abstract

Hydrogen redistribution between the two magnetic states that coexist at transition temperature, which is called the splitting phenomenon, was investigated in La(Fe,Si)13-based magnetocaloric compounds. Asymmetric splitting into two portions with different volumes and H concentrations was observed on analysis of thermomagnetization curves, and this asymmetric growth was determined, using first-principle calculations, to be correlated with the electronic band structure. Excellent entropy change was observed in (Ce,La)(Fe,Mn,Si)13H specimens, on which full hydrogenation was performed to inhibit the splitting phenomenon. The splitting phenomenon was found to be incompletely suppressed in these specimens after a short duration of homogenization annealing, and it gradually disappeared with increasing annealing duration. Magneto-optical observation with a magnetic transfer film shows that first heterogeneous nucleation appears at the spot-like sites; this is followed by the instant growth of a cloud-shaped droplet. The pinning of the droplet boundary by the spot-like nucleation site was also confirmed. Comparison of the metallographic structure revealed that the spot-like heterogeneous nucleation sites correspond to the grain-boundary triple points formed by imperfect grains, which would be swallowed by other stable grains during a ripening action. The droplet boundary, which is central to the hydrogen diffusion process, runs inside of crystalline grains, and in consequence, the main diffusion path of hydrogen was considered to be the body diffusion.