UDK 621.318.1 ISSN 1580-2949 Izvirni znanstveni članek MTAEC9, 37(1-2)87(2003) SOFT MAGNETIC FERRITE MATERIALS MEHKOMAGNETNI FERITNI MATERIALI Andrej Žnidaršič1, Miha Drofenik2 1Iskra feriti, Stegne 29, 1000 Ljubljana, Slovenia 2Institut Jožef Stefan, Jamova 39, 1000 Ljubljana, Slovenia andrej.znidarsicŽiskra-feriti.si Prejem rokopisa - received: 2002-10-21; sprejem za objavo - accepted for publication: 2002-11-25 MnZn ferrites are ceramics with special magnetic properties which are widely used as core materials for inductive components in electronics. For special applications, MnZn ferrites with the following properties are required: low power losses in the frequency range of 500 kHz and a high magnetic saturation flux density. This paper describes the differences in the electromagnetic characteristics of a standard ISKRA material, A, and a new material, B, together with the results of a structural analysis. Key words: magnetic ceramic, MnZn ferrites, magnetic properties, grain boundaries, eddy current, ferrite grains, electrical properties MnZn-feriti so keramični materiali, ki so zaradi svojih specifičnih magnetnih lastnosti pomembni gradniki v elektroniki. Nizke izgube v frekvenčnem območju do 500 kHz in visoka nasičenjska gostota sta pomembni lastnosti pri posebnih aplikacijah. V članku je predstavljena razlika elektromagnetnih lastnosti med obstoječim materialom A in novim materialom B ter analizni pregled rezultatov. Ključne besede: magnetna keramika, MnZn-feriti, magnetne lastnosti, meje med zrni, vrtinčne izgube, feritna zrna, električne lastnosti 1 INTRODUCTION The development and continued success of switch-mode power supplies is constantly challenging the ferrite industry to produce new, high-quality ferrite cores capable of operating at increasingly higher frequencies 1,2. For this reason it is important to reduce the power losses of MnZn ferrites that are used as transformer cores. The electromagnetic characteristics of MnZn ferrite are not only dependent on the composition of the main elements but also on the material’s microstructure3. As a consequence of this, efforts have been made to improve their loss characteristics by controlling the size of the grains and the distribution of the small amounts of additives in the grain-boundary region 4. In MnZn ferrites the grain boundaries exhibit different chemical and physical properties than the ferrite grains. The segregation of impurities and the partial reoxidation of the Fe2+ on the grain boundaries during cooling make the MnZn-ferrite grain boundaries highly insulating in comparison to the grain interior. These insulating layers are, in practice, very thin and therefore exhibit a relatively high electrical capacity. Core loss can be divided into three components: hysteresis loss (Ph), eddy-current loss (Pe) and residual loss (Pr). The proportions of these components in the total loss can vary widely, depending on the measurement conditions such as frequency and magnetic flux density. At low frequencies Ph losses are dominant, and in order to reduce these losses it is important to form a uniform microstructure that is free from lattice defects and pores. At high frequencies the proportion of Pe losses increases, but this can be reduced by increasing the resistance of the cores. Eddy-current loss can be decreased by having grain boundaries with a high electrical resistance and by having a ceramic microstructure with small grains. The resistance of the grain boundaries is determined by additives, which are enriched at the grain boundaries during the sintering process, forming an insulating phase. Small grains can be achieved by applying sintering conditions that suppress the grain growth and by choosing additives that act as grain-growth inhibitors. The sintering parameters must lead to a suppression of the grain growth. So the choice of raw materials, as well as the technological parameters, influences the power losses. In general, two extreme cases regarding the eddy current in the magnetic core of MnZn ferrites can be identified by applying the brick-wall model5. In the first case, when the magnetic grains are isolated and the eddy current in this hypothetical case is localised inside the grains, Figure la, the core behaves as an assembly of individual magnetic grains in which each grain contributes to the eddy-current loss. A different dependence between the total loss and the microstructure holds when the grain boundaries are permeable to the eddy current, Figure lb. According to this model the bulk material can be approximated by a group of small cubes of size D, MATERIALI IN TEHNOLOGIJE 37 (2003) 1-2 87 A. ŽNIDARŠIČ, M. DROFENIK: SOFT MAGNETIC FERRITE MATERIALS separated by high-resistance layers of thickness ?g.b. and resistance Rg.b.. Using this model 5 we obtain, at low D frequencies (?cR < 1), the relation Pe = cBm2 f2 R (mic) g. b. while for high frequencies (?cR > 1), the equivalent Thus, the eddy-current relation is P ??? D g. b. g. b. power loss Pe, which prevails in the frequency range above 500 kHz, can be effectively suppressed by decreasing the average grain size (D), by increasing the grain-boundary resistance R(m ic) and by increasing the grain-boundary width <5g.b.. The saturation Bs of a material mainly depends on the composition of the ferrite. The maximum saturation for MnZn ferrites is in the region 55 mol % Fe2O3, 35 mol % MnO and 10 mol % ZnO. However, this saturation can be enhanced by increasing the density of the sintered material. Concerning the initial permeability, it must be stressed that a high µi and low core losses at high frequencies are in contradiction when it comes to the microstructure because small grains lead to an internal shearing and so to a lower initial permeability. This means that the composition has to be chosen as a compromise between the saturation Bs, the initial permeability µi, and the SMP (minimum of power loss). There are many contradictory requirements for low-core-loss ferrites. To achieve the lowest possible core losses, both compromise and novel processing techniques are required. The development process must ensure that the required characteristics of the core are D (grain size) D (grain size) JJ&yAV, grain boundary grains D thickness m "*1 T Č +)\ i»)»)»)fr) L 0000D 0000D „P000D a) b) Figure 1: Brick-wall-microstructure model: (a) schematic picture of a real and an ideal microstructure of a material with isolated magnetic grains exposed to micro-eddy currents. (b) a sketch of an actual and an idealised ferrite microstructure with grain boundaries permeable to the eddy current. Slika 1: Model opečne strukture: (a) Shema dejanske in idealne strukture materiala z izoliranimi magnetnimi zrni, (b) Shema dejanske in idealne strukture materiala, prevodnega za vrtinčne tokove grains \ gr thickness v / o— č- Čs \ ) J / y Č — Č achieved. In high-frequency power ferrites the minimisation of the eddy-current losses is the most important factor in determining the optimum core characteristics. When developing a process for the production of ferrites, the problem should not be approached by trying to optimise each magnetic property. Because of the interdependency of the magnetic properties, improving one property may lead to the degradation of several others. The way to approach the problem is to find the compromise that best fits the design requirements. When designing the process, reproducibility must also be taken into account, as must the cost. The two main factors governing the reproducibility of a process are control of the processing variables and the chemical homogeneity. The number of uncontrollable processing variables is reduced by switching to more sophisticated equipment (DTA, TGA, X-ray). Chemical homogeneity is achieved mainly through good-quality starting powders and intensive mixing procedures. Major improvements in material properties through processing usually occur as a result of the following: a careful choice of composition; the selection of raw materials and processes that introduce the minimum amount of impurities and produce a homogeneous product; proper and improved control; and an understanding of the firing process. 2 EXPERIMENTAL The weighed raw materials, Fe2O3, Mn3O4 and ZnO, were mixed and then pelletized with a small amount of water. The red pellets were calcinated in a rotary kiln. After that the black pellets, together with water and the additives, were fine milled in an attritor. The average grain size of the ferrite powder after milling was 0.8 µm. The slurry was spray dried with a PVA and PEG binder combination. Toroids were pressed and sintered in order to measure the magnetic properties. For the determination of inductance (L/L0) vs. maximum flux density, air-gapped toroids were made. ¦----------------*----------------¦ 200 175 150 25 60 80 100 120 temperature Š°C] Figure 2: Core loss as a function of temperature at 400 kHz and 50 mT (measured for toroids of dimensions: d0 = 22 mm, di = 1 4 mm, h = 7 mm) for the standard material A and the new design B Slika 2: Izgube jedra v odvisnosti od temperature pri 400 kHz in 50 mT (merjeno na toroidnem jedru d0 = 22 mm, di = 1 4 mm, h = 7 mm) za standardni material A in novi material B 225 125 100 88 MATERIALI IN TEHNOLOGIJE 37 (2003) 1-2 A. ŽNIDARŠIČ, M. DROFENIK: SOFT MAGNETIC FERRITE MATERIALS Tabela 1: Comparison of the sintered density, initial permeability µi (25 °C), power losses and the BS saturation for A and B Tabela 1: Primerjava sintrane gostote, začetne permeabilnosti µi (25 °C), močnostnih izgub in nasičenjske gostote za A in B Initial permeability /ü AB 2000 1800 Flux density Bmax (m T) 10kHz; 250A/m 100°C 120°C 340 280 380 360 10kHz; 1200A/m 100°C 120°C 370 300 420 390 Curie temperature TC (°C) 200 230 Relative loss factor P/V (mW/cm3) /= 300kHz; B = 100mT 25°C 100°C 120°C 700 530 550 700 390 400 /= 400kHz; ß = 50mT 25°C 100°C 120°C 200 180 180 200 120 125 3 RESULTS Figure 2 shows the power loss of the standard high-frequency power-transformer material, A, compared with the new material, B. By introducing the design principles described above, the power loss could be decreased from 180 mW/cm3 to 120 mW/cm3 at 400 kHz, 50 mT, 100 °C (Figure 3). Table 1 summarises the sintered density, the initial permeability and the saturation of A and the new B. For the upgraded version of B, the density was increased by 0.1g/cm3 and the saturation from 300 mT to 390 mT (at 1200 A/m) at 120 °C. The average grain size during sintering can be decreased when a suitable sintering profile is applied 6, Figure 4. This program is a combination of an initially high oxygen concentration and then, above 900 °C, a low oxygen concentration. For an oxygen concentration of above 20 vol % the microstructural development in MnZn ferrites is dominated by exaggerated pore growth, while at lower oxygen concentrations, below about 20 vol %, the grain-boundary mobility in MnZn ferrites during sintering is promoted because of the increase in the concentration of oxygen vacancies, which are the slowest moving species and hence promote volume diffusion. On the other hand, grain growth in ferrites is largely determined by the attachment or separation of pores from the grain boundaries, which depends on the ratio of pore size to grain size. Thus, a combination of an initially high oxygen partial pressure, which promotes exaggerated pore growth, and a subsequent low oxygen partial pressure, which enhances volume diffusion, can inhibit grain growth and consequently decrease the average grain size in MnZn ferrites, and so increase the final sintered density. The increase of the grain-boundary width and the resistance can be achieved by the segregation of Ca2+ during sintering and subsequent cooling of the MnZn-ferrite cores 7. The distinction between usable flux density and saturation flux density is critical for magnetic materials such as MnZn ferrites that exhibit hard saturation. Hard saturation is a sharp decrease of the magnetic permeability as the excitation results in flux-density values located in the non-linear region of the hysteresis loop 8,9. -25 -15 -5 15 25 35 45 55 65 75 85 95 105 115 122 141 158 177 185 198 202 219 226 228 230 232 233 234 235 temperature Š°] Figure 3: Initial permeability vs. temperature for the standard material A and the new design B Slika 3: Začetna permeabilnost v odvisnosti od temperature za standardni material A in novi material B Figure 4: Typical firing scheme for the sintering of high-performance power ferrites Slika 4: Optimalne razmere pri sintranju visokokvalitetnih močnost-nih feritov 7000 -, 6000 - 5000 - 4000 - 3000- 2000- 1000- MATERIALI IN TEHNOLOGIJE 37 (2003) 1-2 89 A. ŽNIDARŠIČ, M. DROFENIK: SOFT MAGNETIC FERRITE MATERIALS 1,0 =B=č*ČČČ_ jč"B \Č \ ¦ ..........\' A \ B ¦ \ \ 0,10 0,15 0,25 BŠT] Figure 5: Inductance of toroid cores with an air-gap, made from the standard material, A, and the new design, B, as a function of BPK_MAX at 120 °C Slika 5: Primerjava induktivnosti toroidnih feritnih jeder z zračno režo materialov A in B v odvisnosti od točkovne nasičenjske gostote pri T = 120 °C Useable flux density is defined as the calculated flux density at the point of minimum cross-sectional area (Amin) in the magnetic core that corresponds to the dc bias condition. A dc current (IDC) equivalent to the peak current is typically used to measure the inductance for a specific peak-current condition and the maximum peak-flux density in the core (BPK_MAX) is established as (Figure 5): L is the inductance at 0 ADC, I is the specified test current, Amin is the minimum cross-sectional area, and Np is the number of turns. 4 CONCLUSIONS The new material, B, is high-frequency region up to very appropriate for the 500 kHz. Compared with the standard material, A, the power losses can be decreased by 40 %. This was achieved mainly by creating a microstructure with smaller grains and a higher electrical resistance of the grain boundaries. At the same time the saturation was increased by choosing an appropriate composition and by enhancing the density. With these properties, B, the upgraded version of A, is very suitable for DC-DC applications. 5 REFERENCES 1 Goldman, Modern Ferrite Technology, Van Nostrand Reihold 1990, New York 2 C. R. Hendrics, W. R. 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Plutzer, Recent Progress in the Development of MnZn Ferrites for Applications at high Frequencies in Electronics, 7th Conference of the European Ceramic Society, 2000, 1247-1250 9 G. E Schaller, Excerpt on usable flux density from IEEE paper on review of power mag. internal paper, March 13, 2000 90 MATERIALI IN TEHNOLOGIJE 37 (2003) 1-2