Hard magnetic composites processed by devitrification of BaO-Fe2O3-B2O3 glasses
Waldemar Kaszuwara*, Artur Witkowski*, Marcin Leonowicz*, Rafał wróblewski*, Piotr Pawlik** *Politechnika Warszawska, Wydział Inżynierii Materiałowej, Centrum Doskonałości NanoCentre, ul. Wołoska 141, 02-507 Warszawa **Politechnika Częstochowska, Wydział Inżynierii Procesowej, Materiałowej i Fizyki Stosowanej, al. Armii Krajowej 19, 42-200 Częstochowa
Quarterly No. 1, 2006 pages 45-51
DOI:
keywords: hard magnetic materials, barium ferrites, glass devitrification
abstract Hard magnetic composites containing barium ferrite precipitates in a ceramic (non magnetic) matrix were investigated. The materials were obtained from basic compounds: BaCO3 and Fe2O3 with addition of B2O3. Mixtures of powders having nominal compositions 4BaOFe2O3B2O3 and 4BaOFe2O32B2O3 were annealed at 1000°C for 24 h in air atmosphere. The annealing products were melted and rapidly solidified by suction casting. A cylindrical die, with 2 mm diameter, was used. The samples were in a form of rods, tubes and tubes with a core. The X-ray analysis revealed that the materials contained Fe2O3 precipitates in an amorphous matrix (Fig. 1). The amount of the B2O3 affected the shape of the precipitates (Fig. 3). The magnetic properties of the as-cast samples were very poor (Figs. 6, 7 and Tab. 2). The hard magnetic composites were obtained by devitrification of the amorphous phase. The devitrification temperature affected the magnetic properties of the materials in a very small extend (Figs. 8, 9). The highest values of the remanence and coercivity were obtained for the 4BaOFe2O3B2O3 samples after annealing at temperatures in the range 900÷950°C and for the 4BaOFe2O32B2O3 specimens annealed at 1100°C, respectively. The both materials contained the BaFe12O19 hard magnetic phase after devitrification (Fig. 1), however in 4BaOFe2O32B2O3 samples the Fe2O3 phase was also found. This fact suggests that large part of the BaO reacted with B2O3, however some free Fe2O3 also remain. On the other hand a higher content of the non magnetic ceramic matrix, from BaO-B2O3 system, forms a magnetic insulating layer between the hard magnetic grains. As a result the coercivity of the 4BaOFe2O32B2O3 was much higher than those for the 4BaOFe2O3B2O3 system (Tab. 2).