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Fe/ZrSiO4 composite compacts with high thermal stability and excellent soft magnetic performance were prepared successfully by mechanical milling and spark plasma sintering. The effect of ZrSiO4 content on microstructure and magnetic properties of the Fe/ZrSiO4 composite compacts was investigated in detail. ZrSiO4 could be coated uniformly on the surface of Fe particles. With the increasing ZrSiO4 content, the core losses are effectively reduced, but the effective permeability and the saturation magnetic magnetization gradually decrease, while the coercivity has shown the opposite trend accordingly. The composite compact with 7.5 wt% ZrSiO4 has shown the superior structure and soft magnetic properties such as high saturation magnetic magnetization (202.20 emu/g), stable effective magnetic permeability (29.39), and lower core loss (4.53 W $\cdot $ kg−1 at 5 kHz). The results indicated that the Fe/ZrSiO4 composite compacts had potential application prospects in electromagnetic conversion equipment at medium and high frequencies. [ABSTRACT FROM AUTHOR]

SEM images with EDS mapping of the uncoated (a) and coated FeSiAl compacts (b–g). • FeSiAl/Fe 3 O 4 particles was obtained by ball milling. • The Fe 3 Si/Al 2 O 3 composite compacts formed by SPS. • The saturation magnetization increased and the coercivity decreased. • Ultra-low inter-particle eddy current losses (0.841 W/kg at 50 kHz). FeSiAl/Fe 3 O 4 core-shell composite particles and transformed Fe 3 Si/Al 2 O 3 composite compacts have been prepared via ball milling coating combined with subsequent spark plasma sintering (SPS) process. The formation mechanism of the Al 2 O 3 insulating layer and the magnetic properties of the Fe 3 Si/Al 2 O 3 composite compacts have been investigated systematically. The FeSiAl particles are coated with uniform Fe 3 O 4 nanoparticles by ball milling method. And during SPS process, the new phase Fe 3 Si/Al 2 O 3 composite compacts are formed due to the high temperature solid phase reaction between FeSiAl alloy core and Fe 3 O 4 coatings. The Fe 3 Si phase possesses excellent soft magnetic properties and the uniform Al 2 O 3 layer shows good insulativity. Therefore, the Fe 3 Si/Al 2 O 3 composite compacts exhibit superior soft magnetic performance such as enhanced saturation magnetization, reduced coercivity, good permeability stability, high resistivity and thus low core loss at high frequency compared with the original FeSiAl compacts. The investigated results in this work indicate that FeSiAl alloy particles coated with iron oxides can be transformed into soft magnetic composite materials with new insulating layer via high temperature solid phase reaction, which provides a promising approach to develop new-style soft magnetic composites with excellent electro-magnetic properties. [ABSTRACT FROM AUTHOR]

• FeSiAl@MnO 2 composite particles were obtained by ball milling. • Novel core–shell structured FeSiAl@MnO/Al 2 O 3 were formed by SPS. • High resistivity (32.46 mΩ·cm) is conducive to the frequency stability of permeability. • Total core loss of FeSiAl@MnO/Al 2 O 3 SMCs is only 6.71 W/kg, which decreases by 94.1%. In this paper, a novel core–shell structured FeSiAl@MnO/Al 2 O 3 soft magnetic composites (SMCs) has been fabricated by the combination of ball milling with MnO 2 and spark plasma sintering (SPS) process. The structure and phase evolution of MnO/Al 2 O 3 insulation layer also were investigated. Meanwhile, the influences of various MnO 2 content on magnetic properties of FeSiAl SMCs have been revealed. The results indicate that MnO/Al 2 O 3 insulation layer with high resistivity (32.46 mΩ·cm) is conducive to the frequency stability of permeability. In particular, the total core loss of FeSiAl@MnO/Al 2 O 3 SMCs is only 6.71 W/kg, which is 94.1% lower than that of original FeSiAl (113.6 W/kg) at 50 kHz. It implies that the novel core–shell structure is a powerful method to increase resistivity and reduce total core loss of SMCs, which can significantly improve energy utilization. [ABSTRACT FROM AUTHOR]

• FeSiAl@Fe 3 O 4 core-shell structure was formed by NaOH oxidation. • Fe 1.82 Si 0.18 @Al 2 O 3 SMCs were obtained by SPS. • Effects of oxidation time on the microstructure and formation mechanism were investigated. • Enhanced magnetic permeability stability, high resistivity and low magnetic loss. Fe 1.82 Si 0.18 @Al 2 O 3 soft magnetic composites (SMCs) have been fabricated by NaOH oxidation combined with subsequent spark plasma sintering (SPS). The microstructure and generation mechanism of Fe 3 O 4 coatings on the surface of FeSiAl powders have been investigated systematically. Effects of oxidation time on the magnetic properties also have been revealed. In addition, the displacement reaction between Fe 3 O 4 and Al during SPS process results in the formation of Al 2 O 3 insulation layer, leading to a remarkable variation of the magnetic performances of SMCs. The increased NaOH oxidation time had positive influences on the electrical resistivity, frequency stability of permeability and core loss. The saturation magnetization first decreases then increases with the increase of oxidation time. These above results indicate that the fabrication method in this paper creates a way to form Fe 3 O 4 nanoparticles and Al 2 O 3 insulation layer, which enhance the magnetic performance of FeSiAl SMCs and will facilitate the design of efficient magnets in the future. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Fe-6.5wt.%Si particles could be well insulated by controllable SiO 2 insulating layer. • The thickness of SiO 2 layer was controllable by proper ammonia concentration. • Excessive addition of ammonia led to inhomogeneous SiO 2 insulating layer. • The composites with ammonia of 0.02 ml/g had continuous and thicker SiO 2 layer. • The composites with ammonia of 0.02 ml/g exhibited lower total core loss. Abstract In this study, the intergranular insulating Fe-6.5wt.%Si/SiO 2 soft magnetic composites (SMCs) were prepared successfully using in-situ chemical deposition followed by the spark plasma sintering (SPS) process. The effects of ammonia concentration on the microstructure and magnetic properties of the composites have been studied systematically. The Fe-6.5wt.%Si alloy particles could be well insulated by the uniform SiO 2 insulating layer, and its thickness increases with increasing the ammonia concentration from 0 to 0.02 ml/g. However, further increasing the ammonia concentration to 0.03 and 0.04 ml/g would result in the discontinuous and uneven SiO 2 insulating layer. Correspondingly, the saturation magnetization and effective permeability of the composite compacts first decrease and then increase with increasing the ammonia concentration from 0.00 to 0.04 ml/g, whereas the coercivity and resistivity vary in the opposite tendency. Note that the overall performances such as the frequency stability of effective permeability, higher resistivity and lower total core loss, reach the optimal value for the sample with the ammonia concentration of 0.02 ml/g. [ABSTRACT FROM AUTHOR]

Abstract The intergranular insulating Fe-Si soft magnetic composites (SMCs) with uniform three-shell SiO 2 -Fe 2 SiO 4 -SiO 2 insulating layer were prepared successfully via simple water oxidation combined with the spark plasma sintering process (SPS). The formation mechanism of the insulating layer and the magnetic properties of the Fe-Si SMCs have been studied systematically. Note that the simple water oxidation process could well ensure the uniformity of the oxide layer consisting of Si and Fe 3 O 4. And then, the high temperature solid-state reactions during the SPS process would result in the conversion of the oxide layer (Si-Fe 3 O 4) to the insulating layer with special three-shell structure (SiO 2 -Fe 2 SiO 4 -SiO 2). As a consequent, the Fe-Si SMCs exhibit excellent performance suah as high saturation magnetization (181.9 emu/g), stable effective permeability (60.5) and very low core loss (7.8 W/kg at 50 kHz). The results indicate that the preparation method in this study is simple and effective to produce high performance Fe-Si SMCs used in the field of electromagnetic conversion equipment. Graphical abstract SEM images of the Fe-Si particles before (a) and after water oxidation (b,c,d); SEM images of the uncoated (e) and coated Fe-Si compacts (f) and the corresponding line scanning (g). fx1 Highlights • Fe 3 O 4 coating on the surface of Fe-Si particles was obtained by water oxidation. • The SiO 2 -Fe 2 SiO 4 -SiO 2 layer formed between Fe-Si particles during SPS process. • The saturation magnetization of Fe-Si SMCs can reach 181.9 emu/g. • The coated Fe-Si SMCs exhibited very low core loss (7.8 W/kg at 50 kHz). [ABSTRACT FROM AUTHOR]

• ZnO coatings formed on the surface of the iron particles by sol-gel synthesis combined with pyrolysis process. • ZnO as inorganic coatings is used to synthesize Fe/ZnO soft magnetic composite. • The Fe/ZnO soft magnetic composite exhibited low core loss (W 30mT/10kHz = 91.11 kW/m3). In this work, Fe/ZnO soft magnetic composite (SMC) is prepared successfully by sol-gel method combined with a pyrolysis process. Formation mechanism and effect of annealing on magnetic properties for the Fe/ZnO SMCs are studied in detail. The results show that Fe/zinc acetate hydrogel mixture will be decomposed into the Fe/ZnO composites by heating above 340 °C. And ZnO is well coated on the surfaces of Fe particles. The Fe/ZnO composite compact has lower total core loss and better frequency stability of magnetic permeability compared with pure Fe compact. In addition, annealing treatment shows an enhanced effect on the magnetic properties for the Fe/ZnO composite compact such as reduced coercivity and increased permeability. The annealed Fe/ZnO SMCs can keep stable magnetic permeability (26) from 50 Hz to 10 kHz, high magnetic induction (1.65 T) and a relatively low core loss of 91.11 kW/m3 measured under a magnetic flux density of 30 mT at 10 kHz. Therefore, the Fe/ZnO soft magnetic composites prepared by the above-mentioned method provide a great potential application in electromagnetic conversion equipment. [ABSTRACT FROM AUTHOR]

• Addition of Co in Fe-6.5Si alloy can improve its M s and ρ. • Doping Co can both decrease hysteresis loss and eddy current loss of the SMCs. • Fe-6.5Si-1.0Co SMCs shows lower P and same value of μ compared with Fe-6.5Si SMCs. Co element was doped in Fe-6.5Si alloy and the corresponding soft magnetic composites were fabricated successfully by conventional powder metallurgy techniques in this work. Saturation magnetization of Co-doped Fe-6.5Si powder increases compared to powders without Co and the coercivity increases because of grain refinement. Co addition is helpful to improve the electrical resistivity of Fe-6.5Si alloy. After doping Co, it shows a significant trend towards a decrease for core loss of Fe-6.5Si SMCs by reducing both eddy current loss and hysteresis loss. The effective permeability of Co-doped Fe-6.5Si SMCs decreases monotony with the Co content changing from 0 to 7.0 wt%. Fe-6.5Si-1.0Co SMCs shows best magnetic performances (100 mT/50 KHz) with core loss of 781.03 mW/cm3 and effective permeability of 103.36. [ABSTRACT FROM AUTHOR]

• Fe 2 SiO 4 /SiO 2 coated Fe-Si SMCs transformed from Fe-6.5wt%Si/Fe 3 O 4 composites were prepared. • The transformation mechanism was revealed in detail. • Effect of Fe 3 O 4 coating content on structure and magnetic properties was studied. • The novel Fe-Si SMCs exhibited high Ms (190.4 emu/g) and very low P cv (9.02 mW/cm3). In this work, Fe 2 SiO 4 /SiO 2 coated Fe-Si soft magnetic composites (SMCs) have been prepared successfully via ball milling Fe-6.5wt%Si/Fe 3 O 4 composite particles combined with subsequent spark plasma sintering (SPS) process. The formation mechanism of Fe 2 SiO 4 /SiO 2 insulating layer and effect of Fe 3 O 4 coating content on the microstructure and properties for the Fe-Si SMCs were investigated in detail. It is interesting that high temperature SPS process would lead to the occurrence of redox reactions between Fe 3 O 4 and Si from Fe-Si alloy cores, and resulted in the transformation from Fe 3 O 4 coating layer to Fe 2 SiO 4 /SiO 2 insulating layer. As a consequence, the novel Fe-Si SMCs exhibited superior performances including high saturation magnetization (190.4 emu/g) and low core loss (9.02 mW/cm3 at 10 kHz) decreased by 89.1% compared to that of the uncoated Fe-Si SMCs (83.08 mW/cm3 at 10 kHz). The results indicated that the method of preparing insulating layer by the redox reactions is a simple and effective way to synthesize new SMCs with high performance for electro-magnetic energy conversion equipments. [ABSTRACT FROM AUTHOR]

In this work, Fe–Si/Fe 2 SiO 4 soft magnetic composites (SMCs) are prepared successfully via sintering Fe-6.5 wt%Si/α-Fe 2 O 3 core-shell composite powders obtained by ball milling. The transformation mechanism from Fe-6.5 wt%Si/α-Fe 2 O 3 to Fe–Si/Fe 2 SiO 4 is revealed in detail. The effect of α-Fe 2 O 3 coating content on microstructure and magnetic properties for the Fe–Si/Fe 2 SiO 4 SMCs is also investigated. During the high temperature sintering process, α-Fe 2 O 3 coating will in situ react with diffused Si from Fe–Si alloy cores, and result in the generation of Fe 2 SiO 4 insulating layer and Fe atoms. When the α-Fe 2 O 3 coating content increases from 0.0 wt% to 7.5 wt%, the insulating layer becomes thicker. Nevertheless, with further increasing the α-Fe 2 O 3 coating content from 7.5 wt% to 12.5 wt%, the generation of excessive Fe breaks the continuity of the insulation layer. As a result, the saturation magnetization decreases gradually, while the coercivity exhibits an opposite trend. The core loss decreases firstly and then increases. Therefore, the Fe–Si/Fe 2 SiO 4 SMCs coated with 7.5 wt% α-Fe 2 O 3 exhibit superior comprehensive performance such as very low core loss (39.6 mW/cm3 at 50 kHz), very high saturation magnetization (191.4 emu/g) and stable permeability (44) from 50 Hz to 200 kHz. Image 1 • Fe–Si/Fe 2 SiO 4 SMCs transformed from Fe-6.5 wt%Si/α-Fe 2 O 3 composites were prepared. • The transformation mechanism was revealed in detail. • Effect of α-Fe 2 O 3 coating content on magnetic properties was studied. • The Fe–Si SMCs coated with 7.5 wt% α-Fe 2 O 3 exhibited very low P cv (39.6 mW/cm3) and high Ms (191.4 emu/g). [ABSTRACT FROM AUTHOR]

• Fe-Si/IMI SMCs were prepared successfully via sintering Fe-Si/γ-Fe 2 O 3 powders. • The transformation mechanism from γ-Fe 2 O 3 layer to IMI layer was revealed. • Optimized performance was achieved by controlling γ-Fe 2 O 3 coating content. • The 5.0 wt% γ-Fe 2 O 3 coated Fe-Si SMCs exhibited very low core loss (23.34 mW/cm3 at 50 kHz) and high Ms (188.3 emu/g). Fe-Si soft magnetic composites (SMCs) coated with novel insulating-magnetic-insulating (IMI) layer were prepared via ball milling Fe-Si/γ-Fe 2 O 3 powders and subsequent spark plasma sintering (SPS) process. We had investigated the formation mechanism of the IMI layer and optimized the microstructure and magnetic properties of the Fe-Si/IMI SMCs by adjusting the γ-Fe 2 O 3 coating content. The analysis results showed that the ball milled Fe-Si/γ-Fe 2 O 3 powders exhibited superior magnetic properties. And γ-Fe 2 O 3 would react with Si and produced Fe 2 SiO 4 with good insulation at the interface between γ-Fe 2 O 3 coating layer and Fe-Si alloy particles during SPS process, which finally resulted in the formation of a novel three-shell structure of Fe 2 SiO 4 -γ-Fe 2 O 3 -γ-Fe 2 O 3 (IMI) layer. By controlling the γ-Fe 2 O 3 coating content from 0.0 wt% to 10.0 wt%, optimized performances such as very low core loss (23.34 mW/cm3 at 50 kHz), high quality factor (18.4) and saturation magnetization (188.3 emu/g), were obtained when the γ-Fe 2 O 3 coating content is 5.0 wt%. [ABSTRACT FROM AUTHOR]

• Fe-Si SMCs coated with variable Fe 2 SiO 4 /SiO 2 insulating layer were prepared. • The effect of sintering temperature on magnetic properties was investigated. • Optimized magnetic properties were obtained by adjusting sintering process. • The Fe-Si SMCs sintered at 1000 °C exhibited very low core loss (169.7 mW/cm3). Fe-Si SMCs with different insulating layer (Fe 2 SiO 4 /SiO 2) have been prepared successfully via sintering the oxidized Fe-Si powders at different temperature. We have investigated the effect of sintering temperature on microstructure and magnetic properties for the Fe-Si SMCs in detail. The insulating layer is mainly Fe 2 SiO 4 when the sintering temperature is 800 °C and 900 °C. Noting that a three-shell insulating layer composed of Fe 2 SiO 4 and SiO 2 is formed when the sintering temperature reaches 1000 °C. However, excessive sintering temperature including 1100 °C and 1200 °C, will result in the generation of SiO 2 and many Fe atoms. Thus, the core loss and quality factor increase with increasing the sintering temperature from 800 °C to 1200 °C except for 1000 °C due to the unique three-shell insulating layer, while the saturation magnetization increases gradually. And optimized magnetic properties including very low core loss of 169.7 mW/cm3, Ms of 183.6 emu/g, μ e of 59 and Q of 6.9 have been obtained for the Fe-Si SMCs sintered at 1000 °C. [ABSTRACT FROM AUTHOR]