Reduction in Core Loss of Soft Magnetic Composites with TiO₂ Coated Fe Powder

Abstract

This study demonstrates the improvement of core loss through the reduction of eddy current loss in soft magnetic composites (SMCs) composed of TiO₂-coated Fe powder and epoxy resin. A thin and uniform TiO₂ insulating layer was successfully deposited on the surface of Fe powder via a sol-gel process, employing titanium (IV) butoxide (TBOT) as the precursor. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses confirmed the formation of a core/shell Fe/TiO₂ structure, with a coating thickness of several tens of nanometers. Increasing the TBOT concentration and coating duration time led to an improved quality factor (Q factor) and a shift of the maximum Q factor values to higher frequency regions. Notably, the permeability was decreased slightly from 14.2 to 13.4, but the core loss, measured at various AC frequencies under 20 mT and then separated into hysteresis loss and eddy current loss at 1 MHz, was significantly reduced from 573 to 435 kW/m³ when the Fe powder was coated with TiO₂ using a 2.5 wt.% TBOT solution for 8 h. This reduction in core loss is attributed to the effective suppression of inter-particle eddy currents by the TiO₂ insulation layer.

1. Introduction

Fe-based materials are well-known soft magnetic materials characterized by high magnetic flux density, high permeability, and low coercivity. Due to these properties, they are extensively used as magnetic core materials in various industrial applications that operate under alternating current (AC) fields [1,2,3,4]. Despite their favorable magnetic properties, soft magnetic materials typically experience significant core loss when subjected to AC fields. This core loss is composed of three main components: hysteresis loss, eddy current loss, and anomalous loss. Among these, the eddy current loss increases rapidly with rising frequency, making it a critical factor to mitigate in high–frequency applications, such as inductors and microwave filters [5,6]. To address this issue, insulation coatings on the surface of Fe powders have gained considerable attention. These coatings effectively reduce eddy current loss by obstructing inter-particle eddy current paths, thereby improving the material’s performance in high-frequency environments.

Magnetic core materials come in various forms, including sintered Fe-based alloys, ferrites, and magnetic powders such as Fe. Sintered Fe-based alloys are known for their high magnetic flux density and suitable permeability at low frequencies. Due to their low electrical resistivity, however, they suffer from high core loss at high frequencies, limiting their use in high-frequency applications [7]. They, however, possess low magnetic flux density compared with Fe-based alloys, which limits their effectiveness in high-power applications [8,9,10,11].

Soft magnetic composites (SMCs), typically produced by compacting powders of magnetic materials such as Fe-based powders, offer high magnetic flux density and high permeability at high frequencies. The powder metallurgy process used in the fabrication presents significant industrial benefits, including net-shape production and cost-effective manufacturing, particularly for mass production. Additionally, magnetic powder cores exhibit three-dimensional isotropic ferromagnetic properties and relatively high electrical resistivity. The application of insulation coating on the surface of powder further enhances electrical resistivity, thereby reducing core loss at high frequencies [12,13,14,15]. In this respect, a uniform and thin insulation coating is crucial. Non-uniform coatings may fail to effectively block eddy current paths due to metal-to-metal contact, while excessively thick coatings can significantly reduce the volume fraction of magnetic metal powder in SMCs, leading to a substantial deterioration in magnetic properties such as permeability. Phosphate coating has been widely used as insulation on Fe powder due to its high electrical resistivity and ease of fabrication [12,13]. Magnesium oxide has also been explored as an insulation coating material [14,15]. These inorganic insulation coatings on Fe powder surfaces have proven effective in reducing core loss and enhancing performance in high-frequency applications.

Since TiO₂ is a well-known insulation material with a high electrical resistivity exceeding 10⁸ Ω⋅cm [16], it represents a promising candidate for reducing eddy current loss in magnetic composites. Previous studies have explored the use of TiO₂ coatings to improve core loss, but the coatings, which were fabricated using surfactants, often suffered from issues such as non-uniformity and excessive thickness [17,18,19]. By contrast, Kim et al. fabricated a uniform and thin TiO₂ coating on Fe powder via a sol-gel method without the use of surfactants to enhance the adhesion between the Ag overlayer and the Fe powder. However, the effect of insulation coating on the core loss of the Ag/TiO₂-coated Fe powder was not reported [20].

In this work, we aimed to fabricate the core/shell structure of Fe/TiO₂ using a sol-gel method to further explore its potential in soft magnetic composites. We investigated the microstructures, magnetic properties, and core loss characteristics of SMCs composed of both uncoated and TiO₂-coated Fe powder, focusing on the effectiveness of the TiO₂ coating in reducing core loss.

2. Materials and Methods

In this study, spherical Fe powder with an average particle size of approximately 1 μm was used. Titanium (IV) butoxide (TBOT, 97%, Sigma-Aldrich Co., Burlington, MA, USA) was used as the precursor for TiO₂ coating on Fe powder. The TiO₂ insulation coating was synthesized via a sol-gel process. To prepare the coating solution, distilled water and ethanol were mixed in a 1:10 volume ratio. For the hydrolysis reaction to fabricate a TiO₂ coating layer on Fe powder, the pH value was adjusted to 5 by adding 3 M HCl solution. Fe powder was dispersed in the solution by ultrasonication for 30 min. TBOT was then added to the mixed solution in various concentrations (1 and 2.5 wt.% of Fe powder) and for different reaction times (1 and 8 h). After the coating reaction, the mixed solution was washed with ethanol and then dried in an oven at 50 °C for 12 h. Fe and TiO₂-coated Fe powders were mixed with 4 wt.% epoxy resin to prepare SMC samples. The mixtures were then pressed into a toroidal mold and cured at 160 °C for 1 h.

The surface morphology of the TiO₂-coated Fe powder was observed by scanning electron microscopy (SEM, MERIN Compact, ZEISS, Oberkochen, Germany). The coating layer was characterized for thickness and element composition using transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan) and energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, Bristol, UK). The magnetic properties of SMCs, which were fabricated into toroidal cores by mixing magnetic powder with epoxy resin, were measured under AC frequency. The permeability and quality factor (Q factor) were measured using an impedance analyzer (4294A, Agilent, Santa Clara, CA, USA), and core loss was measured using a B-H analyzer (SY-8218, IWATSU, Tokyo, Japan).

3. Results

3.1. Microstructure of Fe and Ti-Coated Fe Powders

The morphologies of both the uncoated and TiO₂-coated Fe powders were examined using SEM. As shown in Figure 1, the uncoated Fe powder exhibits a spherical shape with a diameter of approximately 1 μm. By contrast, the TiO₂-coated powder shows a relatively rough surface; however, the TiO₂ coating is not distinctly visible in SEM images. The morphology and thickness of the TiO₂ coating layer were characterized using scanning transmission electron microscopy (STEM). Figure 2 shows the STEM micrographs of the Fe powders with TiO₂ insulation layers coated with different concentrations of TBOT and the coating reaction times: (a) 1 wt.% and 1 h, (b) 2.5 wt.% and 1, and (c) 2.5 wt.% and 8 h. It was observed that insufficient TBOT concentration and reaction time resulted in the non-uniform fabrication of the coating layer on the Fe particles, whereas the 2.5 wt.%-8 h sample exhibited a uniform and continuous coating layer of consistent thickness. Additionally, the STEM micrographs and elemental mapping of the 2.5 wt.%-8 h sample showed a uniform TiO₂ coating layer with a thickness of ~20 nm, as shown in Figure 3. The presence of Ti in the coating layer was confirmed through EDS analysis (Figure 3c). These STEM and EDS results demonstrate that a thin and uniform TiO₂ insulation layer can be effectively fabricated on the surface of Fe powders by the sol-gel method.

In this study, TiO₂-coated Fe powder was successfully fabricated using the sol-gel process, resulting in a uniform coating layer with a thickness of several tens of nanometers. Although there has been extensive research on insulation coating for Fe or metal alloy powders, previous studies have often lacked clarity regarding the morphology and composition of the insulation layers, which frequently had thin and non-uniform thicknesses [12,13,14,15,17,18,19]. Moreover, it was often unclear whether the insulation coating was applied uniformly to each individual particle, as most studies focused on cross-sectional observations of SMCs that included resins between the magnetic particles rather than examining single particles [13,14,15,18,19]. Consequently, these studies did not adequately demonstrate the precise thickness and morphology of the insulation coatings on individual powder particles, often resulting in coatings that were rough and inconsistent in thickness [17,19].

3.2. Magnetic Properties of Soft Magnetic Composites of Fe and Ti-Coated Fe Powders

3.2.1. Permeability and Q-Factor

To evaluate the effect of TiO₂ insulation coating on the magnetic properties of SMCs, the effective permeability and Q factor were measured under AC fields. Figure 4 shows the plots of (a) effective permeability (μ_e) and (b) Q factor values as a function of applied frequency. The permeability, Q factor values at 1 MHz, Q_max value, and frequency at Q_max (MHz) of the SMCs are presented in

Table 1. Magnetic properties of SMCs of Fe and TiO₂-coated Fe powders.

	Uncoated	1 wt.%-1 h	2.5 wt.%-1 h	2.5 wt.%-8 h
Permeability at 1 MHz	14.2	13.6	13.4	13.4
Q factor at 1 MHz	30.1	28.8	28.7	28.2
Qmax value	61.1	107.6	110.9	113.2
Frequency at Qmax (MHz)	9.07	21.7	21.7	24.3

.

As shown in Figure 4a, SMCs composed of TiO₂-coated Fe powder exhibited low permeability values compared with those made from uncoated Fe powder. It is well established that permeability is proportional to the volume fraction of magnetic material in the core of SMCs. As the concentration of TBOT and coating reaction time were increased, permeability was gradually decreased. This indicates that the reduction in permeability was due to the increased volume fraction of non-magnetic material in the core, and thicker coating layers can lead to lower permeability in the TiO₂-coated powder samples.

Furthermore, as shown in Figure 4b, the TiO₂-coated powder samples exhibited significantly higher Q factor values at high-frequency regions compared with the uncoated powder sample. The maximum Q factor (Q_max) for the TiO₂-coated powder samples exceeded 100, with the frequencies corresponding to these Q_max values shifting to around 20 MHz. By contrast, the uncoated powder sample had a Q_max value of 61.1 at 9.07 MHz. The Q factor, which represents the ratio of inductive reactance to resistance in an AC field, is a critical measure of performance in high-frequency applications. The observed shift in the Q factor to higher frequency regions indicates that the TiO₂ insulation coating effectively reduces magnetic energy loss at high frequencies, thereby enhancing the Q factor. In conclusion, the application of TiO₂ insulation coating on Fe powder improves the high-frequency performance of SMCs under AC fields, despite a slight reduction in magnetic permeability.

3.2.2. Core Loss Improvement by TiO₂ Insulation Coating

Core loss measurement was performed at various AC frequencies under a magnetic flux density of 20 mT and then separated into hysteresis loss and eddy current loss at a frequency of 1 MHz. The core loss can be written as the following equations [6]:

P_tot = P_hys + P_ed + P_an,

(1)

P_tot/f = W_hys + k_edf,

(2)

where P_tot, P_hys, P_ed, and P_an are total core loss, hysteresis loss, eddy current loss, and anomalous loss, respectively. W_hys and k_ed are the coefficients of hysteresis loss and eddy current loss, respectively, and f is applied frequency. Equation (1) shows that P_tot consists of P_hys, P_ed, and P_an. Since P_an is considered an artifact of the classical eddy-current model, which ignores domain structures and domain wall motion, leading to artificially low values of core losses, Equation (2), which consists of only the terms of hysteresis loss and eddy current loss, was used for core loss separation [6]. Figure 5a shows the variation of P_tot/f versus f for SMCs of Fe and TiO₂-coated Fe powders. The hysteresis loss and eddy current loss were calculated from the fitting using Equation (2) and are shown in Figure 5b.

Table 2. Total core loss, hysteresis loss, and eddy current loss of SMCs of Fe and TiO₂-coated Fe powders at 1 MHz.

	Uncoated	1 wt.%-1 h	2.5 wt.%-1 h	2.5 wt.%-8 h
Total core loss (kW/m3)	573	521	459	435
Hysteresis loss (kW/m3)	458	419	418	424
Eddy current loss (kW/m3)	115	102	41	11

shows total core loss, hysteresis loss, and eddy current loss at 1 MHz, and the core loss values of the TiO₂-coated Fe powder samples were lower than those of the uncoated Fe powder sample. Additionally, an increase in TBOT concentration and coating reaction time resulted in a further decrease in core loss values. This reduction in core loss for the TiO₂-coated Fe powder samples can be primarily attributed to a decrease in eddy current loss, as confirmed by the analysis of both hysteresis and eddy current losses. The TiO₂-coated samples exhibited lower hysteresis loss values compared with the uncoated sample, and a significant reduction in eddy current loss was observed. Specifically, the eddy current loss was decreased from 115 kW/m³ for the uncoated powder sample to 11 kW/m³ for the sample coated with TiO₂ in 2.5 wt.% TBOT solution for 8 h.

4. Discussion

This study investigated the magnetic properties (permeability, Q factor, and core loss) of soft magnetic composites (SMCs) composed of TiO₂-coated Fe powder and the microstructure of the insulation coating layer. The magnetic properties mentioned above are crucial for the practical application of SMCs under AC fields, as permeability and Q factor are directly related to the device performance, while core loss adversely affects overall magnetic properties. Thus, it is essential to consider these properties collectively when assessing the effects of insulation coatings on the magnetic properties of SMCs. Through comprehensive STEM and EDS analyses, it was confirmed that the SMCs have TiO₂ coating layers with a thickness of ~20 nm on the surface of each Fe core. The findings indicate that the improvement in the Q factor, reduction in core loss (particularly eddy current loss), and a slight decrease in permeability can be achieved by the formation of a TiO₂ insulation coating layer.

More specifically, the reduction in eddy current loss suggests that the TiO₂ insulation layer effectively blocks inter-particle eddy current paths. As the concentration of TBOT and the coating reaction time are increased, leading to a thicker insulation layer, the blocking of inter-particle eddy current paths becomes more effective, thereby further decreasing eddy current loss. This reduction in eddy current loss in SMCs composed of TiO₂-coated Fe powder can be attributed to the insulating effect of the TiO₂ coating, which inhibits the flow of eddy currents between particles. This result is consistent with the observed enhancement in the Q factor, as shown in Figure 4b, indicating that the insulation coating reduced magnetic energy losses at high frequencies, thereby lowering the core loss of the SMCs under AC fields. On the other hand, hysteresis loss is known to be influenced by factors such as materials composition, defects, or impurities, whereas eddy current loss is associated with the electrical resistivity of materials [5,6]. Since impurities in SMCs are known to increase hysteresis loss, the reduction in the hysteresis loss observed in this study can be attributed to the cleaning effect of the sol-gel process, which can remove surface impurities from the Fe particles. Furthermore, the increased volume fraction of non-magnetic material in SMCs of TiO₂-coated Fe powder results in a smaller number of Fe particles per unit volume, which can also contribute to the reduction in hysteresis loss.

Although insulation coating can reduce magnetic permeability and potentially deteriorate performance, the decrease in permeability observed in SMCs composed of TiO₂-coated Fe powder in this study was relatively small (less than 10%) compared with prior results of insulation coatings [4,9,13,17,18,19]. This reduction in permeability is attributed to the thin and uniform TiO₂ coating layer, which is beneficial to maintaining a high proportion of magnetic material within SMCs. By contrast, thicker and non-uniform coatings, such as those formed by aggregated coating materials, can increase the volume fraction of non-magnetic material and porosity in the SMCs. This, in turn, can introduce more pinning sites that impede domain wall motion, resulting in a more pronounced decrease in magnetic permeability. Moreover, the magnetic permeability of SMCs with TiO₂-coated Fe powder showed small changes with varying applied frequencies compared with uncoated powder samples. This indicates that the use of TiO₂ insulation coating is an effective way to maintain magnetic performance across different frequencies. The significant improvement in the Q factor and the reduction in core loss, coupled with only a slight decrease in magnetic permeability, demonstrate that TiO₂ is a highly effective insulation coating material for enhancing the performance of SMCs.

These findings demonstrate that TiO₂ insulation coating is a highly promising candidate for enhancing the performance of SMCs in high-frequency applications. Additionally, this study underscores the importance of comprehensive characterization and analyses of magnetic properties when evaluating the effects of insulation coating on Fe powder.

5. Conclusions

The TiO₂ coating layer was successfully fabricated on the surface of Fe powder using a sol-gel process. STEM and EDS analyses on TiO₂-coated Fe powder confirmed that the uniform TiO₂ coating layer was achieved, and with a TBOT concentration of 2.5 wt.% and the coating reaction time of 8 h, the coating thickness was approximately 20 nm. The TiO₂ insulation coating led to a slight reduction in permeability but significantly improved the Q factor and reduced core loss, primarily by decreasing eddy current loss. These results demonstrate that the TiO₂ insulation coating effectively blocks inter-particle eddy current paths under AC fields. Overall, this study highlights the importance of evaluating magnetic properties when assessing the impact of insulation coatings on Fe powder for SMCs.