La-Na-Co계 M형 페라이트 소결자석의 합성과 자기적 특성

Abstract

Although, the sintered ferrite magnets has lower (BH)max value, which is figure of merit of permanent magnets, with respect to rare-earth (RE) - transition metal (TM) - based magnet. Cost-performance and chemical stability of the ferrite magnet is superior to RE-TM magnet because the ferrite magnet is consists of iron oxide (hematite). For the applications of industrial motors, electric vehicle driving motors, and generators the RE-TM-based magnets have been widely used. The ferrite magnets are increasingly replacing the RE-TM magnets for those applications, for which enhancing performance of the ferrite is demanded. In this study, firstly, we investigated the dependence of fraction of M-type phase in the synthesized sample on the mole ratio in the raw materials. Effect of Co-substitution for Fe in the M-type La-Na system ferrite was also examined. Secondly, we investigated the M phase fraction and magnetic properties of M-type La-Na system ferrite magnets with varying Co-substitution. La-Na-Co ferrite sample was prepared by solid state reaction method using raw materials of α-Fe2O3, La2O3, Co3O4 in water and then dried. The dried powder were mixed with Na2CO3 in dry method to avoid the detrimental reaction between Na2CO3 and water. The mixed powder was further molded into pellets and calcinated in air at 1250 or 1300 ℃ for 3 h. The calcinated powder was crushed to sizes of 3 – 10 ㎛ using disk mill. The crushed powder was milled to size of 0.8 ㎛ using ball mill without addiitives in water. The milled slurry sample was wet-pressed into cylinder type samples under a magnetic field of approximately 1 T. Firstly, sample with/without Co-substitution composed of multiphases of M-type ferrite and other phases. Fraction of M-type ferrite in the samples was depending on mole ratio in the raw material. Curie tempertures of Co-free and Co-substitution M-type ferrite were approximately 432 ℃ and 428 ℃, respectively. Saturation magnetisation of the Co-free La0.5Na0.5 system M-type ferrite approximately 72.5 emu/g. Secondly, La-base M-type single phase (LaNa)Fe8-xCoxO19 sintered ferrite magnet was successfully fabricated by dopping and substituting with Na = 0.2 and Co = 0.1, respectively. For this sintered magnet, M-phase fraction decreased with increasing Co-substitution. (La0.8Na0.2)Fe7.9Co0.1O19 sintered magnet has good magnetic properties ; Ms = 4.7 kG, Br = 4.59 kG, which is considered to be superior to commercial ferrite magnet.