Technical Paper: PIM International, Vol.3 No. 1 March 2009, pages 50-55, 2819 words

Author: B. S. Zlatkov [1], M. V. Nikolic [2], H. Danninger [3] and O. Aleksic [2]

[1] FOTEC Forschungs- und Technologietransfer GmbH, Viktor Kaplan-Strasse 2, 2700 Wiener
[2] Institute for Multidisciplinary Research, Kneza Viseslava 1, 11000 Beograd, Serbia
[3] Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, 1060 Wien, Austria

Abstract

Ba - hexaferrite specimens with an oriented structure were produced by the powder injection moulding technology (PIM). Ferrite powder (BaFe12O19) with an average particle size of 0.4 µm was mixed with a low viscosity binder (wax, thermo-plastics, solvents and additives) to form a feedstock to be used in the PIM process.

The feedstock was injected into a cylindrical mould by an injection moulding machine. The influence of the injection parameters such as feedstock temperature and pressure on final magnetic properties was analysed. A solenoid with a high DC current was positioned around the cylindrical mould to orient single domain particles and clusters in the cylinder main axis direction. The optimal magnetic field intensity was determined that is needed for efficient magnetic orientation of single domain particles and clusters in the heated feedstock during the PIM process. Both isotropic (non-oriented) and anisotropic (oriented) green samples were produced. The green samples were then solvent debinded in acetone and dried slightly above room temperature. The remaining binder was then combusted in air during the sintering process.

The sintering procedure was optimised in order to attain a maximum energy product value - (BxH)max in two steps: optimization of the sintering temperature and optimization of the sintering time. Magnetic properties of the green and sintered samples were measured parallel to the specimen axis using a hysteresis-graph. It showed that sintering for 2 h at 1200C resulted in optimum magnetic properties, which are well comparable to those of ferrites produced by conventional routes.

Introduction

Hard ferrites are used today mainly for the production of different permanent magnetic cores. Sintered ferrite cores are not well suited for additional mechanical treatment, such as cutting and grinding, to fit tolerances, and because of this a gnear net shapeh technology such as PIM (powder injection moulding) is an attractive alternative [1]. PIM technology enables high flexibility in shaping complex hard ferrite components in a very reproducible way with high reliability and repeatability [2]. Hard ferrites, 'hexaferrites', are based on Fe2O3 and metal oxides MO (M = Pb, Ba, Sr) and have a hexagonal structure. BaFe12O19 and SrFe12O19 are used most often [3,4]. The hexagonal lattice has maximum magnetic anisotropy in the direction normal to the hexagonal basis e.g. parallel to the z-axis. Magnetic, optical, electrical and other properties are different in the z-axis compared to the a and b directions. In Table 1 the main lattice parameters and crystal density are given for the most common hexaferrites. An arrow parallel to the z-axis marks the maximum magnetic anisotropy (e.g. easy magnetisation axis).......

Further sections of this article include:

- Experimental procedure
- Results and discussion
- Conclusion

Figures and Tables:

Fig. 1 SEM image of BaFe12O19 starting powder

Fig. 2 Cross section of the mould used for the injection of barium hexaferrite feedstock in a magnetic field
1-Green part, 2-Magnetizing coil, 3-Injection cavity (nonmagnetic stainless steel 316L), 4-Nozzle of injection cylinder, 5-Nozzle side of the mould, 6-Mould parting surface, 7-Ejection side, 8-Ejector (non magnetic steel), 9-Ejector head, 10-Magnetic field direction

Fig. 3 Green barium ferrite cores to be shaped by PIM technology with a central hole to ½ the axis length

Fig. 4 Demagnetisation curves of barium hexaferrite samples shaped by PIM and sintered at different temperatures: GA1-anisotropic green sample, I1-Isotropic sample sintered at 1200°C/1h, A1-anisotropic sample sintered at 1180°C/1h; A2-anisotropic sample sintered at 1200°C/1h, A3-anisotropic sample sintered at 1220°C/1h; A4-anisotropic sample sintered at 1240°C/1h, A5-anisotropic sample sintered at 1260°C/1h

Fig. 5 Remanent induction Br (a) and coercive force Hcb (b) vs. sintering temperature for isotropic (red) and anisotropic (blue) barium hexaferrite samples obtained by PIM technology measured parallel to the applied magnetic field

Fig. 6 Microstructure (optical microscopy) of isotropic (a) and anisotropic (b) barium hexaferrite PIM samples sintered at 1260°C/1 h in air; magnification of 200 x. The easy magnetisation axis is normal to the hexagonal planes e.g. normal to the picture surface

Fig. 7 Microstructure (optical microscopy) of anisotropic barium hexaferrite PIM sample sintered at 1200°C/1 h in air; magnification of 200 x

Fig. 8 Demagnetisation curves of barium hexaferrite samples made by PIM sintered with different sintering time: GA1-anisotropic green sample, I1-Isotropic sample sintered at 2h/1200°C, A1-anisotropic sample sintered 0.5h/1200°C, A2-anisotropic sample sintered at 1h/1200°C , A3-anisotropic sample sintered at 2h/1200°C, A4-anisotropic sample sintered at 4h/1200°C

Fig. 9 Remanent induction Br (a) and coercive force Hcb (b) vs. sintering time for isotropic and anisotropic barium hexaferrite samples obtained by PIM technology measured parallel to the applied magnetic field

Table 1 Simplified sketch of the hexaferrite lattice (a), showing magnetic alignment B, and the main lattice parameters (b)

Table 2 Specification of BaFe12O19 starting powder

Table 3 Variation of PIM-parameters for anisotropic (aligned) samples

Table 4 Density and dimensional change values of barium hexaferrite samples made by PIM obtained for different sintering temperatures

Table 5 Maximum energy product (BxH)max of anisotropic barium hexaferrite samples vs. sintering temperature

Table 6 Optimisation of sintering time for barium hexaferrite samples made by PIM

Table 7 Maximum energy product (BxH)max of anisotropic barium hexaferrite samples vs. sintering time