RELATIONS MICROSTRUCTURE MAGNETIC PROPERTIES SQUARENESS FACTOR OF PrFeB AND NdFeB SINTERED MAGNETS PREPARED WITH HYDROGEN

Author(s)

Elio Alberto Périgo

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Pesquisas Energéticas e Nucleares (IPEN/BT)
• Defense date: 2009-06-19

Examining board members

Rubens Nunes de Faria Junior; Marcos Flavio de Campos; Fernando Jose Gomes Landgraf; Augusto Camara Neiva; Paulo Antônio Pereira Wendhausen

• Advisor: Rubens Nunes de Faria Junior; Claudio Costa Motta

Abstract

In this work, it has firstly been evaluated the preparation of Pr16Fe76B8 sintered permanent magnets (% at.) by means of high-energy milling using a planetary ball mill. The influence of both milling speed and time has been verified. The best magnetic properties [JR = (1.02 ± 0.02) T, 0JHc = (1.42 ± 0.03) T and (BH)max = (200 ± 4) kJm-3] have been found for a permanent magnet prepared with the magnetic alloy milled during 75 minutes using a rotational milling speed of 200 rpm. In order to improve the remanence, the hydrogen decrepitation process time has been reduced from 60 minutes to 2 minutes. In this case, it has been obtained a sintered magnet with JR = (1.14 ± 0.02) T, 0JHc = (1.44 ± 0.03) T and (BH)max = (250 ± 5) kJm-3 due to the improvement of crystallographic alignment of the hard magnetic phase. During such investigation, a new methodology to quantify the parameter has been developed. Subsequently, for the first time, a quantitative correlation between the microstructure and the squareness factor in anisotropic sintered RE16Fe76B8 (RE = Nd or Pr) magnets has been proposed. The presented expression utilizes the mean size, the mean elongation and the mean roundness of the hard magnetic grains as well as their respective standard deviations. The squareness factor can be improved with a microstructure with rounder grains and with a sharp grain size distribution. The grain size homogeneity is more important to enhance the squareness factor compared to grain shape homogeneity. Furthermore, it has also been verified that the annealing after sintering improves the grain shape homogeneity and the milling enhances the xi grain size homogeneity. Moreover, the effect of the temperature on the squareness factor of anisotropic sintered magnets has also been evaluated. Such parameter is mainly controlled by the samples microstructure, in agreement with the proposed expression. Furthermore, a quantitative correlation between the maximum energy product and the squareness factor has been proposed. Experimental and calculated values have been compared and the discrepancy found has been, in general, inferior to 5%. At last, the minimum squareness factor value of sintered permanent magnets has also been evaluated. Isotropic samples should show the smallest squareness factor due to the reduced degree of crystallographic alignment. In general, for this kind of sample, 0.20 SF 0.30 (AU)