Abstract: Based on the requirements of high initial permeability (μi), saturation flux density (Bs), and low power loss (Rc) of PC44 and PC50 power ferrite materials for high-frequency switching power supply transformers, the influence of key technologies such as formulation, additives, and sintering processes on the preparation of these materials is discussed. Introduction With the development of power electronics technology, the demand for multifunctional and high-density electronic devices has increased. Switching power supplies, as indispensable components of electronic devices, urgently require miniaturization and lightweight design. To achieve miniaturization of switching power supplies, the miniaturization of the switching power supply transformer is paramount. PC44 and PC50 power ferrite materials and cores with higher operating frequencies have been developed to meet this demand. The performance of ferrites is not solely determined by their chemical composition and crystal structure; it is also necessary to study and control their density, grain size, porosity, and their distribution within and between grains. Therefore, for the preparation of high-performance power ferrite materials, formulation is fundamental, and sintering is crucial. The formulation and density determine the saturation magnetic flux density Bs (power ferrite cores typically operate under a DC bias field; a high Bs is necessary to ensure the core has high DC superposition characteristics) and Curie temperature (fc). The addition of effective additives and matching with appropriate sintering processes are crucial to the ferrite's performance, influencing the degree of solid-state reaction and the final phase composition, density, and grain size. This allows for more effective control of the microstructure of soft magnetic ferrites, ensuring a harmonious balance of the material's main characteristic parameters. 1. Selection of Main Formulation for High-Performance Power Ferrites To improve power conversion efficiency and avoid saturation, power ferrite materials used in high-frequency switching power supply transformers are required to have high Bs, high initial permeability (μi), and high amplitude permeability (μa). Simultaneously, to prevent transformer overheating and breakdown at high frequencies, the material's power loss (Rc) should be as low as possible, ideally exhibiting a negative temperature coefficient. In essence, the three key magnetic performance parameters for evaluating the quality of power ferrite materials are μi, Bs, and Rc, along with their frequency, temperature, and time stability. These parameters form a contradictory unity, with some even being highly conflicting. The overall approach to organically unifying them involves controlling the magnetocrystalline anisotropy constant K1-t curve and the ferrite's microstructure. This involves optimizing the formulation, additives, and sintering process to ensure K1 has good temperature characteristics, adjusting its minimum value to a suitable position, and bringing it close to zero. The magnitude of μi contributes most directly to the high inductance factor (AL) of the magnetic core; therefore, ensuring a high μi value in the ferrite is essential. However, μi and the material's cutoff frequency fr are mutually restrictive. Increasing the material's operating frequency and increasing μi are contradictory; in practical applications, both must be balanced. The Bs and Curie temperature tc of power ferrites are determined by the formulation and density. For the main formulation of power ferrites, domestic and foreign soft magnetic researchers have conducted in-depth and systematic research, and have made it into a phase diagram (without additives) as shown in Figure 1 to make it more intuitive. After years of research, TDK Corporation of Japan has further defined the value range in the Mn-Zn ferrite composition phase diagram. The formulation at its center is approximately: FezO3:MnO:ZnO=53.5:36.5:10 (mole fraction), which is basically consistent with the main formulation of PC44 of many domestic enterprises: FezO3:MnO:ZnO=53.3:36.5:10.2 (mole fraction). For PC44 and PC50, since their Bs are relatively high, an over-Fe formulation must be used. This is because when the Fe2O3 content is in the range of (51~55) mO1%, the Bs increases with the increase of Fe2O3 content (conversely, excessive ZnO content will cause the material to reach high temperature, or the Bs and tc decrease). The optimal formulation combination can be determined through orthogonal process experiments, combined with the selection of doping and sintering processes. 2 Selection of Additives for High-Performance Power Ferrites The chemical composition of power ferrites is not the only factor determining the properties of ferrites. The distribution of cations and crystal point defects in the crystal sites plays a crucial role. Improving the microstructure of ferrites by adding additives and adjusting the process helps to achieve a harmonious unity of the main characteristic parameters of the material. According to the basic magnetic theory, the cutoff frequency fr of power ferrite materials is related to the grain size d of the ferrite by the following equation (1). Where: Ms is the saturation magnetization of the material; β is the damping coefficient. As can be seen from equation (1), , is inversely proportional to d (μ1-1). Therefore, by adding additives and adjusting the sintering process to refine the grains and reduce the grain size, the cutoff frequency of the material can be increased (which also increases its operating frequency). However, the infinite reduction of the grain size will inevitably increase the power loss. On the other hand, the level of μ1 (which is closely related to the sintering temperature) also affects the size of fr. For PC44 and PC50 materials that typically operate at high frequencies of several hundred kHz, power loss mainly consists of two parts: hysteresis loss Rh and eddy current loss Pe. Since hocBm3 (Bm is the working magnetic flux density), it can be seen that in order to reduce Ph, the material's Bs should be high, the compositional uniformity should be good (using high-purity raw materials), and the consistency of grain size should be improved and the material density should be increased to minimize internal stress. Eddy current loss is expressed by equation (2). Pe = (π2/4)·r2·lf2·Bm2/p (2) Where: r is the average grain size; p is the resistivity. It can be seen that there are two main ways to reduce the power loss of materials at high frequencies: increase the resistivity; control the ferrite grains within the optimal range (if the grains are too small, Pe will be smaller, but Ph will be larger). The most effective way to control grain size and resistivity is to reasonably add additives and improve the sintering process. As is well known, the addition of beneficial additives such as SnO2, TiO2, and Co2O3 can further control the K1 value of the material, making it very small over a wide temperature range; the composite addition of CaO and SiO2 can increase the resistivity of the material and reduce its power loss. In fact, many additives have practical value in improving the performance of Mn-Zn ferrites. Their main functions can be divided into three categories: the first category of additives segregates at grain boundaries, affecting grain boundary resistivity; the second category affects the microstructure changes during ferrite sintering, and by controlling the sintering temperature and oxygen content, the microstructure can be improved, power loss reduced, and the temperature and time stability of the material's magnetic permeability increased, as well as frequency expansion; the third category is dissolved in the spinel structure, affecting the material's magnetic properties. Additives of elements such as Ca and Si belong to the first and second categories; elements such as Bi, Mo, V, and P belong to the second category; and the main functions of elements such as Ti, Cr, Co, Al, Mg, Ni, Cu, and Sn belong to the third category. Figure 2 shows the effect of six additives, including MoO and CuO, on the permeability of Mn-Zn ferrite, where μ1 and μ2 represent the permeability of ferrite without additives and ferrite with a small amount of additives, respectively. Figure 3 shows the effect of SiO2 doping on the permeability of Mn-Zn ferrite. Figure 4 shows the effect of TiO2 addition on the μi-t curve of Mn-Zn ferrite. Figures 5(a) and 5(b) show the effect of composite addition of SiO2 and CaO on the resistivity and specific loss coefficient (tanδ6/μi) of Mn-Zn ferrite at 100 kHz, respectively. Researchers at Tohoku Metal Co., Ltd. in Japan, while developing SB-1M (equivalent to PC50) materials, discovered that a portion of the commonly used composite additives SiO2 and CaO dissolves within the grains, thereby increasing hysteresis loss. Under conditions of 500 kHz to 1 MHz, its effect on reducing power loss is not good. To this end, they carried out fruitful research work, hoping to find additives that can more effectively improve resistivity without increasing hysteresis loss. Table 1 lists their research results. Among these 8 additives, Al2O3, SnO2, and TiO2 are dissolved in the grains and have almost no effect on improving resistivity. Other additives are mainly free in the grain boundaries. Among these additives, HfO2 has the most significant effect on improving resistivity [2] and has the best effect on reducing eddy current loss. When developing high-performance power ferrite materials, we should make full use of the achievements of our predecessors and not waste too much effort on the exploration of formulations and additives. The general principle of formulation and doping is to make the magnetocrystalline anisotropy constant K1 and magnetostriction constant λs as close to zero as possible. The following principles should be noted when selecting additives: 1) The total amount of additives (wt%) should be controlled within 0. 1) Less than 2%; 2) CaO (or CaCO3) and SiO2 are usually indispensable additives; 3) High-valence ions such as V2O5, Nb2O5, TiO2, Ta2O5, HfO2, and CO2O3 are added in combination, but the number of components should not be too many, preferably no more than 4. The weight of each additive should generally be controlled below 1000 ppm; 4) Among the above additives, except for Co3+, the K1 value of other ions is negative. For example, the 3F3 material developed by Philips (a material between PC40 and PC50) has the basic technical point of simultaneously adding Ti4+ and CO3+ to control the temperature characteristics of the material and reduce hysteresis loss, as shown in Figure 6. 3 Sintering Process of High-Performance Power Ferrites Sintering is a key process in the preparation of high-performance power ferrite materials. During the sintering process, the heating and cooling rates, the maximum sintering temperature, and the furnace atmosphere are three key factors that must be strictly controlled, as they significantly influence the microstructure, chemical composition, and electromagnetic properties of ferrite materials. A suitable sintering process should be determined comprehensively based on the raw material formulation and additives, pre-firing temperature, furnace structure and length, cooling method, power consumption, and performance considerations for the ferrite. The process should be verified and judged based on the final performance of the material. The heating rate directly affects the density, grain size, and uniformity of the ferrite product. An excessively rapid heating rate will result in uneven grain size and numerous internal pores; a too-slow heating rate will result in low density ferrite with significantly enlarged pores. To obtain ferrite with small and uniform grains (approximately 10–14 μm for PC40 and 3–6 μm for PC50), low porosity, high density, and no cracking defects, the heating rate should not be too rapid below 600℃. Between 600 and 900℃, the heating can be faster. From 900 to 1100℃, during the initial grain growth stage, a steady heating rate is recommended, along with densification treatment. Above 1100℃, the heating rate can be slightly faster, but the maximum sintering temperature should not exceed 1350℃ (to limit grain size). Holding time of 3–4 hours is sufficient. Then, cooling should be performed under nitrogen (N2) protection with an appropriate oxygen partial pressure. Densification measures around 900–1100℃ are essential to reduce the porosity of the ferrite. TDK Corporation of Japan pays particular attention to the heating rate and ambient atmosphere control between 900 and 1100°C. They believe this stage is crucial for achieving a good microstructure in ferrites, especially important for the preparation of high-performance power ferrites such as PC44 and PC50. A common densification method involves steadily heating from 900°C to 1100°C, holding at that temperature for 1 hour, and simultaneously introducing an appropriate amount of N2 to control the oxygen partial pressure. This allows the apparent density of the ferrite to rapidly reach 99% of its true density, with most pores remaining at the grain boundaries. Of course, ensuring sufficient oxygen content in the furnace and unobstructed exhaust pipes is also very important in the heating range below 1000°C. During the cooling stage, oxidation or reduction of the ferrite can occur. Adding an appropriate amount of N2 to control the oxygen partial pressure in the furnace prevents changes in the valence of Mn, Fe, Co, and Cu ions, the formation of dissolved substances, and lattice changes during cooling. Excessive oxidation and reduction lead to the precipitation of other phases such as α-FezO3, FeO, Fe3O4, and Mn2O3, resulting in a sharp deterioration of magnetic properties. Figure 7 shows the equilibrium atmosphere phase diagram of power ferrite with a formulation of Fe2O3:MnO:ZnO = 51.9:26.8:18.3 (mol%). Figure 7 illustrates the importance of the atmosphere for the oxidation state within the spinel phase and Fe2O3 phase boundary. Special attention should be paid to cooling along the isocomposition line first, followed by rapid cooling through the phase boundary at the lowest possible temperature. This minimizes growth kinetics, resulting in minimal desolvation of α-Fe2O3 and the lightest degree of oxidation and formation of other phases. Figure 8 lists typical sintering process curves for power ferrites. 4 Conclusion 1) For the preparation of high-performance power ferrite materials such as PC44 and PC50, the formulation is fundamental, and sintering is crucial. 2) The general formulation and doping principle is to make the magnetocrystalline anisotropy constant K1 and the magnetostriction constant λs approach zero as much as possible. 3) Adding appropriate amounts of additives such as CaO, SiO2, V2OS, TiO2, Co2O3, etc., and matching them with suitable sintering processes can modify the microstructure of high-performance power ferrite materials, making their effect on improving the overall performance of the materials more prominent.