PHASE FORMATION AND MAGNETOCALORIC EFFECTS IN LaFe_(11.5)Si_(1.5_B_x

摘要

The phase formation, structure, and magnetocaloric effect (MCE) in as-cast and annealed LaFe_(11.5)Si_(1.5)B_x (x =0, 0.2, 0.5 and 1.0) compounds have been studied. Rietveld analyses of the x-ray powder diffraction patterns indicate that the cubic NaZn_(13)-type La(Fe, Si)_(13) (1:13) phase is the main phase in the as-cast LaFe_(11.5)Si_(1.5)B_(0.5) and LaFe_(11.5)Si_(1.5)B_(1.0) alloys. Unlike in ternary silicides, heat treatment at 1373 K for 2 weeks does not facilitate the formation of the 1:13 phases in the quarternary boron compounds. Wavelength dispersive spectrometery (WDS) analysis shows that the boron atoms do not occupy the interstitial positions as carbon and hydrogen do in the La(Fe, Si)_(13) compounds, but form Fe_2B. The Curie temperatures of LaFe_(11.5)Si_(1.5)B_x alloys increases in a nearly linear fashion from 183 to 196 K from x=0 to x=1.0. The maximum magnetic entropy changes, |△S|, for a magnetic field change of 0-50 kOe decrease from 21.3 to 14.5 J/kg K with increasing B content. The first order field induced itinerant electron metamagnetic transition, which accounts for the large |△S| value, changes to second order when the B content reaches x=1.0.