IRON BASED SOFT MAGNETIC COMPACTED MATERIALS

Soft magnetic materials play an important role in broad applications, such as transformers and electrical motors. There is an interest in bulk soft magnetic materials because of the demand for miniaturization of cores. We have prepared bulk samples in the form of the small cylinders with good soft magnetic properties. The frequency dependence of magnetic properties is studied, and it is attributed mainly to the structure of the initial powder and domain wall damping. The good combination of various shapes and good soft magnetic properties indicates the possibility of future development as a new soft magnetic compacted material.


INTRODUCTION
Soft magnetic alloys are used in many areas including transformers for electrical energy distribution, power electronics for small and large-scale power management, pulse power devices, telecommunication devices, and sensors.The large values for the maximum induction and relative magnetic permeability, low coercive fields and low core losses are the essential requirements for soft magnetic alloys used in applications.There is a limitation of laminated steels and magnetic ribbons in applications consisting of their shape limitation.The main advantage of the compacted materials is the shape flexibility.The amorphous Vitroperm (Fe 73 Cu 1 Nb 3 Si 16 B 7 ) material is produced by rapid solidification as an originally amorphous ribbon, which is subsequently annealed above its crystallization temperature [1].One of the ways to prepare bulk material is compaction of powder produced by milling of amorphous or nanocrystalline ribbons [2].Soft magnetic composites (SMCs) are typically produced by powder processing methods, allowing a net-shape production for a wide variety of shapes and sizes [3].Magnetic powder parts are produced from powder particles each covered by insulating coatings, which cause a barrier to particle-to-particle eddy current paths under ac magnetization hence minimizing eddy current losses.The amount of insulating materials should be minimized to maintain the permeability and saturation magnetization at a high level.Low resistivity material is used for dc applications but alloys with high resistivity are needed to minimize eddy current loss for high-frequency operation.

EXPERIMENTAL
We have prepared two series of the bulk samples.The initial material of the first series is -amorphous ribbon Fe 73 Cu 1 Nb 3 Si 16 B 7 , supplied by Vacuumschmelze, Germany via melt spinning technique.The ribbon was milled (R) or cryomilled (L) using a RETSCH PM4000 planetary ball mill.The samples were consolidated at 700 MPa for 5 min at 500°C.The second series -consisting of Iron powders Somalloy (S), provided by Höganäs AB, Sweden and flakes Fe 73 Cu 1 Nb 3 Si 16 B 7 (VPM), supplied by Vacuumschmelze, Germany.The samples were consolidated at 800 MPa for 5 min.at room temperature.The compacts were cured for 60 min.in an electric furnace in Argon atmosphere at a temperature of 520°C (L, R ) or at a 530°C (S-VPM), respectively.The electrical resistivity was measured by the Van der Pauw method.The DC hysteresis loops at maximum flux density of 0.1 T and 0.2T were measured by a fluxmeter based hysteresisgraph.The AC hysteresis loops were measured by AC hysteresis graph MATS-2010SA.Complex permeability spectra were measured with an impedance analyser HP 4194A.

RESULTS
The inductance and the resistance of the samples were measured to characterize the magnetic permeabilities.In this case, each toroidal sample was modelled as an ideal inductor, in series with an ideal resistor.The real part (µ') and imaginary (µ'') part of the initial complex permeability were determined from the inductance L s and resistance R s of the coil on the toroidal sample using the following relations: ( ) where L s is the self-inductance of sample core, L 0 is derived from geometrical relations showing the inductance of the winding of the coil without the sample core, R 0 is the resistance of the coil without the sample core, N is the number of turns of the coil, h is the height, r 1, r 2 is the inner and outer radius of the toroidal sample, respectively, ω is the angular frequency.
The magnetic properties of the soft magnetic bulk alloys Fe 73 Cu 1 Nb 3 Si 16 B 7 prepared from powder alloy by compaction are influenced by the morphology of the initial powder.This influences the density, the electrical resistivity and electromagnetic properties of the resulting bulk alloys.
In the first series of samples the milling was at room temperature -sample R. In the second series we investigated the milling at temperature of liquid nitrogensample L: sample R -amorphous ribbon milled for 6 hours, consolidated at 500°C for 5 min, annealed at 540°C for 60 min., sample L -amorphous ribbon cryomilled for 6 hours, consolidated at 500°C for 5 min, annealed at 540°C for 60 min.
From the previous experiments [4] the X-ray diffraction patterns of short-time (6h) ball-milling of  At high content of small particles high real µ' part of the permeability cannot be expected.On the other hand, reducing the eddy current loss of bulk alloy from cryomilled powder also hinders the core loss at higher frequency [4].Fig. 3 shows the frequency spectra of real µ' and imaginary µ'' part of the complex permeability in powder cores prepared from different powders.It can be seen that real µ' part of the sample L keep almost stable at low frequency region with slight drop in permeability.On the other hand, the real part of the sample R starts from 6 times higher value of permeability (at 100 Hz) but there is the steep drop in the permeability and after that the value of permeability is decreasing slowly and smoothly.The DC and AC hysteresis loops are given in Fig. 4. The peak permeability as well the shapes of the hysteresis loops, the rectangularity are different with the initial powder material.The hysteresis of a soft ferromagnetic material depends on the irreversible magnetization process and is principally determined by pinning of domain wall motion.The hysteresis losses are partly due to pinning sites from imperfections in the material and stresses introduced in the material at a compaction.
In the case of the bulk metallic Fe-based samples consisting of Iron powders (S) and flakes of Fe 73 Cu 1 Nb 3 Si 16 B 7 (VPM) with the different content (0, 5, 30, 50 wt% of VPM), the volume of pores increases with increasing VPM content and it affects magnetic properties directly.Somaloy powder has average particle diameter of 120 µm, shown in Fig. 5. Vitroperm particles are flat and mostly uniform thickness, shown in Fig. 6.Mean size of particles is 200-225 µm with normal gauss distribution.Shape descriptors indicated irregular shape of the particles.Density measurements exhibited dependence on the VPM content and with an increase in VPM content resulting in a decrease in density.Porosity acts as areas of demagnetization, reducing the saturation magnetization but increasing the specific resistivity of the samples.Although pores and grain boundaries obstruct the movement of domain wall.The complex permeability of the prepared samples shows dependence on their initial powder morphology and the content of two ferromagnetic phases, Fig. 7.The increase in the resistivity leads to an enhancement the real part of permeability at higher frequencies; at the same time the relaxation frequency moves to a higher value [5].The influence of VPM content on the hysteresis loops is given in Fig. 8.The relief of the porosity, density and electrical resistivity influences also the hysteresis loops.
The Fe 73 Cu 1 Nb 3 Si 16 B 7 powder has positive influence on the AC core losses of the S-VPM compacted samples.Result of optical microscopy observation of bulk Fe 73 Cu 1 Nb 3 Si 16 B 7 ribbons indicate that no influence on its structure during the milling and the powder remained amorphous.Particle size of more than 95% of particles after milling at room temperature is from 50 μm to 300 μm, but cryomilled powders have smaller particle sizes, from 20 μm to 150 μm, Fig 1, Fig 2. The resulting particle size distribution may affect the density of the compacted material.The compacted disk L has higher value of the density than disk R, Tab. 1.

Fig. 1 Fig. 2
Fig. 1 Fracture of the milled Vitroperm sample R observed by SEM

Fig. 3
Fig. 3 Comparison of the real µ' and imaginary µ'' part of the complex permeability µ of L and R samples, respectively

Fig. 7
Fig. 7 Comparison of the real µ' and imaginary µ'' part of the complex permeability µ of S-VPM samples, respectively

Table 1
Parameters of the samples