Rubber Isostatic Pressing (RIP) for Ferrite Magnets

J. PHYS. IV FRANCE 7 (1997) Colloque Cl, Supplkment au Journal de Physique I11 de mars 1997 Rubber Isostatic Pressing (RIP) for Ferrite Magnets M. Sagawa, H. Nagata, T. Watanabe and 0. Itatani Intermetallics Co., Ltd., Erie 401, 22-1 Oiagecho, Matsumuro, Nishikyo-ku, Kyoto 615, Japan Abstract. Applying RIP (Rubber Jsostatic Pressing) to the femte sintered magnets, the following have been developed: Ferrite sintered magnets having energy product as high as 5.12 MGOe RIP setup for arc-segment magnets (1) (2) 1. Introduction RIP has been proposed as a new t e c h q u e of pressing powder for the rare earth and fenite sintered magnets[l][2]. The combination of high packing density of the powder in the mbber-mold caviq and magnetic alignment by a high, pulsed magnetic field was shown to be essential for obtaining good compactsfor sintered magnets having very high energy products[l]. T h ~technique is being introduced lately in the industrial production of the sintered NdFeB magnets. s If RIP is used in the production of the femte magnets, the following effects are expected: (1) The degree of orientation in the green compacts is improved remarkably, leading to substantial improvement in rernanenceof the sintered magnets. (2) Because a very fine powder can be used in RIP, ferrite magnets with very high coercivity can be produced. Apart from the improvement in the magnetic properties, an important subject in applying RIP to the fenite magnets is to develop a setup of RIP for producing arc-segment magnets, because the arc-segment is the most important shape in the femte sintered magnets. The purposes of this paper are: (1) to develop femte sintered magnets with high remanence by improving the degree of orientation, (2) to develop fenite sintered magnets with coercivity as high as possible by using very line powder, and (3) to develop RIP setup for the arc-segment fenite magnets 2. Results The starting material was Sr0.GFe203 with small addition of Si02 and CaC03. For the high remanence magnef 1 wt% of calcium stearate was mixed in the powder during milling. The average grain size was 0.7pm as measured by the Fisher subsieve sizer. For the high coercivity magnets, powder with an average grain size of 0.3 p was prepared by the method of m Taguch et al[3]. 2 % of calcium stearate was mixed during milling. Both the powders were then dried by flowing air. A RIP set desc~ibed before [l] was used. The packing density was 2.0 g/cm3,pulsed magnetic field for alignmentwas 3T, and the pressing pressure was 0.3 to 0.8 ton/cm2. Both the two powders could be compacted easily in good shape by RIP. The density of the compacts was 2.8 g/cm3when the pressure was 0.5 ton/cm2. Compacts were then sintered in air at 1240°C for the high remanence magnets and 1200°C for the high coercivity magnets both for 2h. Density reached 5.0 g/cm3. In Fig. 1 are shown demagnetization curves of the high remanence magnet and high coercivity magnet measured on cylindrical specimens, 20 mm in diameter and 13 mm long. The high remanence magnet has surprisinglyhigh remanence. The degree of orientation, which is estimated from the ratio of the remanence and the highest value of magnetization in the first quadrant of the hysteresis curve, reaches 99 %. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19971120 Cl-308 JOURNAL DE PHYSIQUE IV Upper punc Fig. 1 Demagnetization curves for (a)the high remanence magnet p r = 4.57 kG, Hci = 2.93 kOe, @H)max = 5.12 MGOe] and (b)the high coercivity magnet [ Br = 4.19 kG,Hci = 4.00 kOe, @H)max= 4.26 MGOe] Fig. 2 RIP setup for arc-segment fenite magnets The degree of orientation for the high coercivity magnet was 96 %. It is likely that agglomeration of the powder was not completely eliminated during dtylng. Even so, the level of the magnetic properties of the high coercivity magnet is extremely high considering that this magnet is prepared by dry pressing. In addition, it should be noted that a dry powder as fine as 0.3 was compacted easily in perfect shape by RP. By o r d i n q die pressing, compaction of such fine dry powder of ferrite would be impossible without mixing a large quantity of organic binder. If wet pressing is used, compaction of such powder may be possible, but filtration problem would be serious. the magnetic field for In Fig. 2 is shown an RIP setup for arc segments that was developed for ferrite magnets. In this -p, the alignment is applied perpendicular to the direction of the pressing axis. The core, that contacts the internal m a t u r e of the arc segments, has to consist of hard material like metal: if this part is made of rubber, the compacts would always have vertical cracks. If the die is made of non-magnetic metal and the core of magnetic metal, the arc segments have radial orientation. It has proved that arc-segment femte magnets having excellent magnetic properties can be produced in good shape by the setup shown in Fig. 2 in the laboratory, and development of automated RIP apparatus installing this setup is in progress. Acknowledgment We thank H. Taguchi of TDK and H. Nobuhara of MATE Co.,Ltd for providing femte powders used in our experiments in the present paper. Reference [l] M. Sagawa, H. Nagata, Z E T r a n s M G ,(1993) 2747 EE ~ [2] M. Sagawa, H. Nagata, 0. Itatani, T. Watanabe, Proc. of 13'1ntemational Workshop on Rare Earth Magnets and their Applications, (1994) supplement 13 [3] H. Taguchi, F. Hirata, T. Takeishi, T, Mori, Digests of 6& ~nternationalCon$ On Ferrite (1992) 306