Study On Two Kinds Of Soft Magnetic Ferrite Materials For High Frequency Power Conversion

The rapid development of information technology requires the development of magnetic components used for high frequency power conversion (HFPC), which have high permeability and saturation magnetic induction, low core loss, and excellent temperature and frequency characteristics for both permeability and core loss. Currently, there is much research focusing on using MnZn and NiZn ferrites for HFPC because they possess these qualities. China is a technologically developing country, and the research and development of MnZn and NiZn ferrites in China still lags behind the research in other countries. Finally, research on soft magnetic ferrites for HFPC will lead to smaller, more lightweight and more reliable devices. To address the above-mentioned trends and market demands, this dissertation will investigate the preparation technology and mechanisms for both MnZn and NiZn ferrites.First, this dissertation summarizes the current state of development, research progress, and basic theory of MnZn and NiZn ferrites for HFPC, and points out the importance and goals of this research both in China an abroad. After recognizing and describing the present situation, it will briefly present the preparation process and characterization methods of soft magnetic ferrites for HFPC.MnZn ferrites were studied to determine their usefulness for HFPC. The influence of processing parameters on the crystal structure, microstructure, permeability, and temperature and frequency characteristics of both permeability and core loss were investigated in detail. These processing parameters include raw materials, composition, powder processing, and additives and sintering techniques. The results are as follows.Adjusting the ratio of Fe₂O₃ to ZnO could reduce core loss and improve permeability in MnZn ferrites. For the MnZn ferrites operating in the frequency range from 1 to 3 MHz, the proper composition Fe₂O₃ : ZnO : MnCO₃ = 53.0: 8.5: 38.5 mol%. Ball milling techniques could influence the powder size and consequently could affect powder activity. Utilizing super-hard Zirconia (ZrO₂) balls, instead of steel balls, could effectively reduce core loss at high frequency while moderately extended milling time improved permeability and decreased core loss. The calcination process had a large effect on the magnetic properties of MnZn ferrites. Proper calcination temperature could enhance the permeability and reduce core loss. For MnZn ferrites used in HFPC the effect of adding multiple additives is not the sum of the effects of adding all of the additives individually, rather, the effect of multiple additives should be considered as a composite effect. We prepared MnZn ferrites for HFPC with three different additive combinations. The combinations were: (1) CaO(0.30 wt%), SnO₂ (0.10 wt%), TiO₂ (0.40 wt%), and (2) CaO(0.30 wt%), SnO₂ (0.10 wt%), TiO₂ (0.20 wt%), and (3) CaO(0.30 wt%), SnO₂ (0.10 wt%), TiO₂ (0.20 wt%), NbO5 (0.03 wt%), and K₂CO₃ (0.03 wt%). The approximate sintering condition is to sinter at about 1230 oC for 3 hours with about 4% partial oxygen pressure.We developed a MnZn ferrite to operate at high frequencies (1³ MHz) over a wide range of temperatures (25¹20 oC). We determined from experimental data the correct value of the Steinmentz exponent (x) in high frequencies. We also determined the frequency exponent (y). The exponent (x) of magnetic induction was 2.58 at 1 MHz and 2.01 at 3 MHz, and the power loss followed the equation PL=kBmxfy. The frequency exponent (y) was 2.22 at 40mT and 2.73 at 10mT. We also analyzed residual loss at high frequencies with theory model, and the model matched very well with experimental results.NiZn ferrites were also studied to determine their usefulness for HFPC. In particular, the effects of additives, and optimal fabrication parameters were investigated. These optimal parameters include composition, ball-milling media and time, and calcination temperature. This dissertation will address the influence SnO₂, Bi₂O₃, and Nb₂O₅ additives on crystal structure, grain growth, densification and magnetic properties. For industrial applications, it is desirable to use the relatively inexpensive CuO to partially replace the more expensive NiO. The effects of this substitution, along with ith the effect of iron-deficient, equimolar and iron-excessive compositions on the crystallization, grain growth, and magnetic properties were investigated. The results are as follows.It was found that the Bi₂O₃ additive segregated at the grain-boundaries as a liquid-phase layer. This phenomenon could be confirmed by backscattering electron image (BSEI) and energy-dispersive X-ray spectrum (EDS). The grain growth mechanism and the microstructure evolution can be understood by using liquid-phase sintering theory. The NiZn ferrite samples containing Bi₂O₃ in concentrations of 0.20, 0.08, and 0.04 sintered at 1180, 1220 and 1250 oC respectively exhibited high permeability (μi) and low core losses (PL) at 50kHz and 150mT. The X-ray diffraction intensity of the NiZn ferrites doped with SnO₂ was stronger than that of the sample without the additive. Due to its low melting point (¹127 oC) moderate SnO₂ enhanced mass-transfer and sintering by forming liquid phase, which accelerated grain growth. However, excess SnO₂ produced too much liquid phase, which retarded mass-transfer and sintering, leading to a decreased grain size. If the concentration of SnO₂ was 0.1¹.5 wt%, NiZn ferrites showed a higher permeability and lower core loss. A single phase spinel structure was observed when the concentration of Nb₂O₅ was≤0.85 wt%. If 1.00wt% of Nb₂O₅ was added, an orthorhombic phase of FeNb₂O₆ was detected in addition to the spinel phase. This phase resulted in the (440) diffraction peak growing to become the main peak. Using XRD, SEM and EDS analyses, we investigated the grain-growth mechanism of NiZn ferrites doped with different concentrations of Nb₂O₅. When Nb₂O₅ was 0.4wt%, NiZn ferrite showed the highest permeability (1783) and lowest core loss (257 kW/m3).Finally, due to considerations relating to costs of fabrication and energy consumption, moderate amounts of CuO were used to replace NiO. We studied the effects of this substitution, as well as iron deficient, equimolar, and iron-excessive compositions on the crystallization, crystal structure, microstructure and magnetic properties. The results showed that iron-deficient composition had a lower crystallization temperature (887.2 oC), while iron-excessive composition exhibited a higher crystallization temperature (974.2 oC). As the Fe₂O₃ ratio increased from iron-deficient to iron-excessive, the X-ray diffraction peaks initially shifted towards lower angles and then moved to higher angles. Correspondingly, an initial increase in lattice parameter followed by a subsequent decrease was observed. The lattice parameter showed a maximum when the Fe₂O₃ ratio was 49.0 mol%. When the system was iron-deficient, ZnO with hexagonal crystal phase was detected in addition to the spinel phase. However, equimolar and iron-excessive compositions exhibited a single spinel phase. As the content of Fe₂O₃ increased, the grain size, density, saturation induction, and initial permeability first increased and then decreased. Core loss at 50kHz and 150mT, however change in the opposite way. Finally, NiCuZn ferrite with an equimolar composition presented the highest initial permeability(1467) and lowest core loss (234 kW/m³).