Soft Magnetic Properties of MnZn Ferrites Prepared by Powder Injection Moulding

In this study, properties of soft-magnetic manganese zinc ferrite manufactured by powder injection moulding – PIM technology were presented. A powder consisting of Mn1xZnxFe2O4 and a small amount of hematite α-Fe2O3 was mixed with an organic binder (wax and thermoplastic) to form ferrite feedstock. Then, injection moulded toroidal samples were solvent, thermally debinded prior to sintering in air atmosphere. Magnetic properties were investigated by thermomagnetic measurements (Faraday balance μ–T curve) and a hysteresis graph (B-H curve at different frequencies up to 1 kHz). The Curie temperature was TC ≈ 390 K for the green sample but after heating up to 740 K, TC increased up to about 570 K. The high increase in normalized magnetic permeability of about 700 % was observed due to the melting and burning of the binder. The hysteresis loop of sintered MnZn ferrite toroidal cores had an R-shape with saturation of 0.44 T and the remanence ratio of 0.49. The low value of coercivity (only 0.047 kA/m) was related to the presence of α-Fe2O3 crystalline phase as well as optimum density attained (ρ ≈ 4800 kg/m3) i.e. the low porosity observed. The relative magnetic permeability attained μr ≈ 2⋅10 and core power losses Ps ≈ 21 W/kg for the sintered sample (at 1000 Hz; 0.39 T) are in agreement with the MnZn ferrite commercial samples prepared by standard ceramic route.


Introduction
Soft magnetic ferrites are used as cores in modern electronic components such as recording heads, filters, switching power supply transformers, amplifiers, etc. MnZn ferrites attracted attention due to a wide range of relative magnetic permeability values (from 10 3 to 10 4 and therefore low magnetic losses) as well as due to increased thermal stability (high saturation magnetic flux density at high temperatures (B s > 0.4 T at 370 K) and a relatively high Curie temperature) [1,2].Operating frequencies are usually in the 1 kHz to 1 MHz range but the frequencies in GHz range were used in some applications.Furthermore, excellent corrosion resistance and chemical stability enable their application in extreme exploitation conditions.
Recently, a variety of preparation routes have been examined for MnZn ferrite production: mechanochemical processing [3,4], chemical co-precipitation method [5], sol-gel [6] or microemulsion [7].This paper deals with MnZn ferrites produced by Powder Injection Moulding (PIM) technology.This technology can produce a number of ferrites in very shorter time compared to the classic method, i.e. it offers large scale manufacturing of small and geometrically complex parts.PIM technology is appropriate for the production of components for both soft and hard ferrites [8][9][10][11].Soft magnetic properties of the MnZn ferrite sample prepared by PIM method and afterwards sintered in air were presented.The primary motivation behind the research on soft magnetic properties of MnZn ferrites prepared by PIM route is to improve their magnetic permeability, as it is well known that high permeability is accompanied by low power losses in magnetic cores.

Experimental
MnZn toroidal cores (about 15 mm high, external diameter 18 mm, internal diameter 10 mm) prepared by PIM preparation route were kindly supplied by FOTEC, Wiener Neustadt, Austria.
A powder consisting of Mn 1-x Zn x Fe 2 O 4 with small addition of α-Fe 2 O 3 was mixed with an organic binder (wax, thermoplastic and additives) to form ferrite feedstock.The starting MnZn powder was originally prepared for the pressing technology i.e. in mass ferrite production (90 % of all particles are finer than 2.8 μm).All details about PIM production parameters were already published [12].Injection moulded toroidal samples were solvent, thermally debinded and sintered in air atmosphere (sintering temperature was 1610 K with a holding time of 3.5 h).
Thermomagnetic measurements were performed by a Faraday balance (based on the effect of an inhomogeneous magnetic field of about 8 kA/m on the ferromagnetic sample) [13].Magnetic force measurements were performed with a sensitivity of 10 -6 N in a protecting argon atmosphere up to 800 K. Permeability was compared to the starting value obtained for the green sample at room temperature μ S (293 K), which is normalized magnetic permeability μ(T)/μ S (293 K).
Soft magnetic properties were measured on toroidal core samples at room temperature.The coercivity H c , saturation magnetic induction B s , remanence magnetic induction B r , relative magnetic permeability μ r and core power losses P s were determined from B-H hysteresis loops measured by Brockhaus Tester MPG 100 D [14].Maximum excitation was H m = 6 kA/m and the set of used frequencies ranged from 50 Hz to 1 kHz.Microstructure was observed by Polivar Met Reichart optical microscope (software LEICA Q 500 MC).

Results and discussion
Fig. 1 shows the microstructure of the PIM MnZn ferrite sample sintered in air.Samples sintered in air exhibit low porosity and sample density ρ = 4 780 kg/m 3 [12].Zlatkov et al. showed that lower density and, thus, higher porosity were obtained for MnZn samples sintered in argon and nitrogen, indicating the effect of sintering atmosphere on the resulting sample density and porosity [12].It is also concluded that, under oxidizing atmosphere (air), a mixture of two phases Mn 0.6 Zn 0.4 Fe 2 O 4 (68 wt.%) and α-Fe 2 O 3 (32 wt.%) was evolved.During heating, the decrease in magnetic permeability vs. temperature is the result of approaching the Curie temperature (T C is about 470 K for the green sample).After the first heating run of the green sample up to 490 K and upon cooling to room temperature, normalized magnetic permeability increases by 100 % and the Curie temperature increase up to about 570 K.After the second heating run up to 620 K, the normalized magnetic permeability increases by 120 %.The highest increase of about 700 % was observed after the third heating run up to 740 K due to the melting and burning of the binder.Improved thermal stability of magnetic permeability with an almost constant value (increase of about 550 % until 500 K) was attained after the fourth heating run up to 770 K.
Hysteresis loops of the MnZn ferrite green sample, the green sample heated up to 770 K and the sintered sample are shown on Fig. 3. Extreme hysteresis loops are observed.The hysteresis loop of MnZn ferrite samples sintered in air has an R-shape with a remanence ratio of 0.48; whereas both the green sample and the green sample heated up to 770 K have an F-shape of the loop with a very low remanence ratio 0.12.Both the green sample and the green sample heated up to 870 K exhibit high values of coercivity (0.63 kA/m and 0.69 kA/m respectively due to the presence of the binder).
The low coercivity value of the sintered sample (only 0.047 A/m) is correlated to the optimum density already attained (ρ = 4 800 kg/m³ [16]) which is in agreement with the lowest level of optically visible porosity (see Fig. 1) as well as with the crystalline phase observed (α-Fe 2 O 3 ) [12].Therefore, the sample sintered in air exhibits attractive magnetic softness due to a high coercivity decrease of about 90% as well as due to observed increase in remanence.
The upper halves of hysteresis loops obtained by successively increasing excitation (commutation curves) were used for the construction of initial magnetization curve ( The permeability reaches a maximum near the knee of the magnetization curve and further increases up to saturation.A maximum of about μ r ≈ 2.5 •10 3 at the excitation of 0.06 kA/m can be observed followed by a constant decrease up to μ r ≈ 10 3 at the excitation of about 0.25 kA/m.These results are also in good agreement with MnZn ferrite commercial samples (initial permeability μ i = 2.5 •10 3 ± 25% [16]).
In order to estimate the magnetic properties of the MnZn ferrite sintered sample for applications in electrical engineering, B-H hysteresis loop measurements were made at 1000 Hz in different excitation magnetic fields (H m = 2 kA/m; 4 kA/m and 6 kA/m).These hysteresis loops excited up to saturation are shown in Fig. 6.One can see similar loop width (a small increase in coercivity), almost the same remanence as well as a slow increase to saturation.The role of magnetisation reversal is the real parameter of dynamic B-H hysteresis.The core losses that are proportional to the surface area of the hysteresis loop include hysteresis and eddy-current losses.The excitation in magnetic field amount had to 6 kA/m, the frequency 50, 300, 500, 800 and 1000 Hz.The effect of frequency with the sinusoidal flux density is shown in Fig. 7 (loops obtained at 500 Hz and 1000 Hz were presented).Increasing frequency induces a higher rate of magnetic reversal, the hysteresis loops become broader and hence higher magnetic losses.This behaviour is caused by eddy-current and at higher frequencies additionally by spin relaxation processes.Frequency dependence of the total power loss P s referred to unit mass is shown in Fig. 8.For example, the total power losses of the sintered sample are 21 W/kg (at 1000 Hz, 0.39 T).Hysteresis losses are determined mainly by coercivity (loop width) and excitation (loop height).Eddy current losses depend primarily on electrical resistivity: therefore high electrical resistivity is very important for low values of magnetic eddy current losses.Electrical conductivity in MnZn ferrite has been attributed to electron hopping mechanism between the two valence states of iron on octahedral sites [15].Therefore, high electrical resistivity of MnZn ferrite is achieved by maintaining the Fe +3 valences of octahedral Fe ions.Due to the high electrical resistivity (0.15-10 Ωm [16, 17]), eddy current losses are limited and due to the low hysteresis losses one can see low total core power losses.
As the hysteresis losses are proportionally to the frequency (~ f) and eddy-current losses are proportionally to the square of frequency (~ f 2 ) it can be performed separation between these components.The results of the separation of the magnetization reversal losses of the MnZn ferrite sintered sample are shown in Fig. 9.The results obtained for P s = 21 W/kg for the sintered sample (at 1000 Hz; 0.39 T) are comparable to those of P s = 27 W/kg (at 25 000 Hz; 0.2 T) for commercial Unimagnet MnZn WM2K5 ferrite [17].
Investigated PIM MnZn ferrite satisfied the criteria for switching power supply transformers.It is an optimum magnetic material for coil products used in various transformers for cars and inverter transformers for LCDs.

Conclusion
In this study, soft magnetic properties of MnZn ferrite toroids manufactured by PIM technology have investigated.Feedstock was prepared from a fine starting powder used in mass ferrite production (consisting of Mn 1-x Zn x Fe 2 O 4 with small addition of hematite -Fe 2 O 3 ) and an organic binder.Due to the presence of the remaining binder sintering of MnZn ferrite obtained by PIM technology is a sensitive process.After injection moulding and debinding, the sample was sintered in air at 1610 K for 3.5 hours.The magnetic results are encouraging for the sintered sample due to the remarkable improvement in permeability and hence low power losses.The characteristics obtained are comparable with commercial samples prepared by the traditional technology and recommended investigated PIM MnZn ferrite for switching power supply transformers.

Fig. 3 .
Fig. 3. Hysteresis loops of MnZn ferrite green sample, green sample heated up to 770 K and sample sintered in air at 1610 K, 3.5 h (excitation H m = 6 kA/m at a frequency of 1 000 Hz).

Fig. 4 .Fig. 5 .
Fig. 4. Upper halves of hysteresis loops of the MnZn ferrite sintered sample obtained by increasing excitation and construction of initial magnetization curve.

Fig. 7 .
Fig. 7. Hysteresis loops of the MnZn ferrite sintered sample at different frequencies (for the sake of clarity, the loops obtained at 500 Hz and 1000 Hz were presented; H m = 6 kA/m).

Fig. 8 .
Fig. 8.Total power looses P s referred to the core mass.