Interstitial modification of La(Fe,Si)13 phase by acid solutions.

In this article we study a liquid medium route for hydrogenation of La-Fe-Si alloys utilizing diluted 0.83 M acetic(C2H4O2) and formic (CH2O2) acids, where the powdered alloy is immersed in the solution for periods up to 45 minutes depending on the nature of the acid solution. The alloys were characterized by XRD analysis to observe the modification of the lattice and undesired phase content after the hydrogen modification, as well as DSC analysis to characterize the nature of the Curie temperature (TC) transition. It was observed that there is a strong interplay between hydrogen insertion kinetics and La(Fe,Si)13 phase degradation: As the use of stronger acids may shorten the reaction period necessary for a full saturation of the hydrogen sites, uncontrolled exposure periods increase the formation of undesired phases, such as α-Fe. Furthermore, it was shown that this process can be a great alternative to gas-solid reactions used for these alloys, as results show TC shifts from 207 to 340 K in approximately 4 minutes of processing with formic acid, and from 207 to 338 K in 15 minutes of the powder immersion with acetic acid with minimal phase degradation.