The Effect of Si on the Formation of the ${\rm La}({\rm Fe},{\rm Si})_{13}$ Phase Synthesized by the Reduction-Diffusion (R/D) Process

Abstract

In this contribution, the influence of Si content on the formation of the of LaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(13-x)</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(x)</sub> (1:13 phase) synthesized by the reduction-diffusion (R/D) process is addressed. The raw materials used were Fe and Si, in powder form, La oxide (La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), and Ca granules as reduction agent. The amount of Si per unit formula, represented by x, was varied from 1.3 to 1.7, for three different compositions. After a leaching step, subsequent to the actual R/D process and necessary to extract the reaction byproducts, such as CaO and nonreacted Ca, an alloy in powder form is obtained, with an average particle size of about 10 μm. In order to know the relative phase-amount evolution with the increasing Si content for these three different alloys, X-ray diffraction (XRD), supported by Rietveld refinement, was performed. Magnetic measurements carried out on a vibrating sample magnetometer (VSM) were used as an alternative means to infer the amount of formed 1:13 phase. The agreement found for the two methods (XRD and VSM) was good and showed that, for predetermined conditions, the amount of 1:13 phase formed, is directly related to the Si content added in powder form, as raw material. Moreover the transition-temperature (TC) of the target 1:13 phase, at which the magnetocaloric effect is highest, was accessed by differential scanning calorimetry (DSC) in order to check homogeneity and stoichiometry deviations from the predicted composition. The different values measured for distinct preparation conditions, point out to different Si contents incorporated to the targeted 1:13 phase, what directly affects phase formation and stability. The well-defined DSC-peaks, however, point out to good phase homogeneity.