Microstructure evolution and soft magnetic property optimization of core-shell FeSiBCCr@SiO2&ZrO2 amorphous magnetic powder cores

In this study, core-shell structured FeSiBCCr@SiO2&ZrO2 amorphous magnetic powder cores (AMPCs) were successfully prepared by the sol-gel method using tetraethyl orthosilicate (TEOS) and zirconium oxychloride hydrate (ZrOCl2·8H2O) as raw materials. The growth mechanism and potential chemical reactions of the SiO2&ZrO2 mixed insulation layer were analyzed by XRD, XPS and SEM?EDS. In addition, the effect of the insulation layer content on the soft magnetic properties of FeSiBCCr@SiO2&ZrO2 AMPCs was systematically investigated. The SiO2&ZrO2 insulation layer can significantly increase the resistivity, effectively improve the DC bias performance and reduce the core loss of AMPCs. S2* (1.2 mL TEOS and 1.4 g ZrOCl2·8H2O) had a minimum core loss of 2290 mW/cm3 at high frequency (50 mT, 1 MHz). It had a high effective permeability of 48.2 and a superior direct current (DC) bias performance of 77.9% at 8 kA/m. This study provides a valid method to practically produce SMPCs with high resistivity, low core loss at high frequency and excellent DC bias performance through a novel insulation coating process.     

Ultra-low core loss FeSiAl-based soft magnetic composites with ultra-thin MoO3 composite insulating layer

In this work, core/shell structured FeSiAl/MoO₃ (spherical FeSiAl covered by ultra-thin MoO₃ composite insulating layer) soft magnetic composites (SMCs) have been fabricated by a two-step heat treatment process. The influences of ammonium molybdate (AHM) content, first-step annealing temperature and second-step annealing temperature on magnetic, mechanical properties and electrical resistivity (ρ) have been comprehensively investigated. It is shown that the coating integrity and the thickness of MoO₃ nanoparticles layer can be regulated by the content of AHM, leading to the improvement of ρ. Moreover, composite insulating layer with the thickness of 47 ± 8 nm is formed and completely coated on FeSiAl particles with 15 wt% AHM, resulting in the fact that the highest radial crushing strength (K = 62.3 MPa) and lowest core loss (P_cv = 128.8 mW/cm³) is obtained. In addition, P_cv is separated into two components: hysteresis loss component and eddy current loss component. Further studies display that eddy current loss is only half of hysteresis loss. As a result, the FeSiAl SMCs with 47 ± 8 nm MoO₃ composite insulating layer possess the lowest core loss of 128.8 mW/cm³ at 50 mT/100 kHz. The low core loss of the FeSiAl/MoO₃ SMCs with ultra-thin composite insulating layer has a great potential in the fields of conversion and power transmission.     

Preparation and characterisation of Fe/Fe3O4 fibres based soft magnetic composites

Soft magnetic composites (FSMCs) have been prepared by using Fe fibres coated with a layer of Fe₃O₄, this layer playing the role of insulating material. The coating was made via blackening method by simply immersing the fibres in the blackening bath for 5, 10 and 15 min. The formation of the Fe₃O₄ coating on the surface of the fibres was confirmed by X-ray diffraction. The SEM investigation, used to evaluate the thickness of the coatings, has proved that increasing the coating duration leads to the increase of the coating thickness and complete coverage of the surface of the fibres. Differential scanning calorimetry and thermomagnetic measurements were used to investigate the thermal stability of the composite fibres. The fibres coated with Fe₃O₄ were compacted at a compaction pressure of 700 MPa to obtain toroidal magnetic cores. The obtained cores were characterised in DC and AC magnetisation regime. The analysis of the quasi-static hysteresis loops evidenced that increasing the thickness of the Fe₃O₄ leads to a slight deterioration of the compact's magnetic properties. However, as the thickness of the Fe₃O₄ layer increases, the development of eddy currents at a larger scale is hindered as proved by the AC magnetic investigations. A model for analytic separation of the core losses is proposed. By applying this model to the prepared samples, we are now able to discriminate between the occurring losses and adjust the preparation process of new samples to the targeted characteristics.     

Ultra-low core loss FeSiAl soft magnetic composites with in-situ double oxidation layers of outer Fe3O4 layer and inner super-thin Al2O3/SiO2 hybrid layer

How to synchronously reduce eddy current loss and hysteresis loss still remains a challenge for achieving low core loss of soft magnetic composite (SMC). In-situ surface oxide effectively combines the formation of insulating layer and the release of internal stress during the molding process. In this study, FeSiAl SMC has been fabricated by powder metallurgy method with in-situ oxidated FeSiAl powder, in which FeSiAl powder are covered by outer Fe₃O₄ insulating layer and inter super-thin Al₂O₃/SiO₂ hybrid layer. Fe₃O₄ layer alleviates dilution magnetic effect, ensuring high saturated magnetization and effective permeability. The super-thin Al₂O₃/SiO₂ hybrid layer enhances electrical resistance, reducing eddy loss. Effects of the in-situ oxidation time on insulating layer and soft magnetic performances of SMC are investigated in detail. Synchronous reduction of eddy current loss and hysteresis loss is achieved through high resistance accompanied with proper insulating layer thickness and low coercive force provided by the special microstructure. For powder with 90 min oxidation at 500 °C, core loss of SMC is low up to 64 mW/cm³ at 50 mT and 100 kHz and 363 mW/cm³ at 100 mT and 100 kHz, while the permeability is kept at 50 until 1100 kHz and is stable until 140 °C. Meanwhile, DC bias performance reaches 51.2% at 100 Oe applied field and Q value is 104.8 at 400 kHz.     

Influence of fibres diameter on the AC and DC magnetic characteristics of Fe/Fe3O4 fibres based soft magnetic composites

Fibres-based soft magnetic composites (FSMCs) have been prepared by using Fe fibres of different diameters (65, 125, 250 and 500 μm). The Fe fibres were coated with a 3 μm thick layer of Fe₃O₄ via the blackening process and subsequently compacted at 700 MPa. The X-ray diffraction analysis (XRD) was used to prove the formation of the Fe₃O₄ coating on the surface of the fibres. The thickness and the uniformity of the coating were analysed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The DC measurements performed on the composite cores revealed that the saturation induction increase from 1.36 to 1.68 T, the maximum relative permeability increase from 550 to 940, and the coercive field decrease from 796 to 454 A/m as the fibre's diameter increase from 65 to 500 μm. By using thinner fibres (65 and 125 μm), composites with low losses and stable initial relative permeability, in the frequency range 50 Hz–10 kHz, can be obtained. To distinguish between different types of losses dissipated by our compacts, and the influence of the fibre's diameter on the different components of the total losses, a numerical model for loss separation is proposed. The comparative evolution of the AC magnetic characteristics of the FSMCs and powder-based SMCs is presented. According to the presented results, this new type of composites can be successfully used to prepare magnetic cores designated to work in the medium to high-frequency range.     

Highly stable carbon-coated Fe/SiO2 composites: Synthesis, structure and magnetic properties

A new method to coat Fe nanoparticles with carbon by pyrolysis of acetylene is reported. The Fe nanoparticles were beforehand coated with silica layers by sol-gel combined hydrogen method. The antioxidation capability of Fe/SiO₂ composites has enhanced greatly after coated with amorphous carbon shells. By the introduction of only 7.5 wt.% nonmagnetic silica and carbon, these composites have relatively high specific magnetization of 200.27 emu/g. The insulating amorphous silica and carbon shells prove effective to reduce the eddy current at high frequency.     

Corrosion of amorphous and nanocrystalline Fe-based alloys and its influence on their magnetic behavior

Nanocrystalline soft magnetic materials with low coercivity, high saturation magnetization and high permeability are commonly used as cores in transformers and generators in stress and field sensors. The influence of factors connected with corrosion is almost impossible to eliminate. In the present work, a comparative study of the electrochemical behavior of Fe₇₈Si₁₃B₉ and Fe_73.5Si_13.5B₉Nb₃Cu₁ amorphous and nanocrystallized alloys, tested in 0.5 M NaCl solution, has been performed by linear polarization and electrochemical impedance spectroscopy methods. Changes of magnetic properties including coercivity, induction and magnetic retentivity were analyzed. These properties were investigated as a function of the structure of primary amorphous ribbons and as a function of corrosion environment type, in which longitudinally and transversely cut ribbon specimens were exposed for 15 days. The best magnetic properties were found for the Fe₇₈Si₉B₁₃ ribbon after a structural relaxation at a temperature of 350 °C for an hour and for the Fe_73.5Si_13.5B₉Nb₃Cu₁ ribbon after a primary crystallization at a temperature of 550 °C for an hour. Corrosion did not cause the direct degradation of the magnetic properties of the Fe₇₈Si₉B₁₃ and Fe_73.5Si_13.5B₉Nb₃Cu₁ alloys. The corrosion processes occurring on the surface of the Fe_73.5Si_13.5B₉Nb₃Cu₁ alloy ribbon with the amorphous structures improve induction B_s. Most probably it is connected with the decrease of undesirable stresses blocking a motion of magnetic domain walls on the ribbon surface. Changes of corrosion mechanism depending on structure and applied solution were analyzed. The electrochemical impedance experiment were performed at open circuit potential for amorphous and nanocrystalline specimens. Two electrochemical corrosion mechanisms of Fe_73.5Si_13.5B₉Nb₃Cu₁ alloy in 0.5 M NaCl solution were found. Charge transfer control mechanism is typical for amorphous (as received) alloys. Mixed mechanism-mass transport and charge transfer controlled was observed for nanocrystalline Fe_73.5Si_13.5B₉Nb₃Cu₁ alloy.     

Filling effect on the structural,electrical, and magnetic properties of PVAc/Co composites

Polyvinyl‐acetate/‐cobalt (PVAc/Co) composite films were prepared using a casting technique. The structural and physical properties were studied using X‐ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, dielectric measurements, direct current magnetic susceptibility (χ_dc), and electron spin resonance (ESR). The XRD patterns revealed that the incorporation of Co particles increases the amorphization of PVAc and Co oxide formations. The DSC results suggest that the thermal properties obviously improved. Frequency and filler concentration dependence of the dielectric constant (ε′) and AC conductivity (σ_AC) were measured at room temperature in the frequency range 20 Hz to 3 MHz of pure PVAc and PVAc/Co composite films. The dielectric constant shows usual dielectric dispersion behavior. The dielectric constant and AC conductivity increased with the increase in Co content. The variation of σ_AC is attributed to hopping of polarons and bipolarons in the composites. The filling level dependence of the effective magnetic moment (μ_eff) has been evaluated. The ESR spectra exhibit a peak of an increasing depth as Co content increases. The control of thermal stability, dielectric and magnetic moment of the composites films is interesting for applications such as electric and magnetic sensors. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The effects of Barium Strontium Titanate (BST) on the soft magnetic properties and loss performance of MnZn ferrites

With the increasing power density of the switched mode power supply (SMPS) developed nowadays, higher efficiency is required from the magnetic core, where the MnZn ferrites are often adopted. However, the relatively high operating temperature of the SMPS often results in reduced resistivity of the MnZn, which increases the eddy current loss. To enhance the resistivity of MnZn ferrite at high temperature range (>100 °C), donor-doped barium strontium titanate (BST) with a positive temperature coefficient of resistivity (PTCR) is prepared and dopped in the MnZn ferrite. The influence of BST addition from 0.000 wt% to 0.020 wt% on the MnZn ferrite is investigated over a wide temperature range from 25 °C to 140 °C. The XRD result suggests ionic exchange between the spinel phase and perovskite phase. The SEM result shows a refined and more uniform microstructure of MnZn ferrite brought about by the BST addition. At the maximum of 0.020 wt%, the BST addition shows almost no influence on density and the saturation magnetic induction. However, the initial permeability is slightly reduced by the BST addition, due to the microstructural change. Moreover, the BST concentrating at the grain boundaries improves the DC-resistivity across the temperature range from 25 °C to 140 °C. Due to the addition of BST, the reduction in eddy current loss at 300kHz/100 mT is around 35% at 25 °C, and ∼20% reduction at 140 °C.     

Synthesis of coatings on SiC fibers and their effects on microstructure and mechanical properties of SiCf/SiBCN composites

In this study, the amorphous C, ZrB₂, and BN single-layer coatings as well as C/BN, C/ZrB₂, ZrB₂/BN, and C/ZrB₂/BN composite coatings were prepared on SiC fibers (SiC_f) by an in situ synthesis and solution impregnation–pyrolysis method. Subsequently, SiC_f/SiBCN composites were fabricated by hot-pressing sintering at 1900℃/60 MPa/30 min to explore the influence of different coatings on the microstructure and mechanical performance of resulting composites. After the preparation of single-layer-coated SiC_f, the SiC_f(BN) or SiC_f(ZrB₂) tended to be overlapped with each other, whereas the dispersion of amorphous C–coated SiC_f was satisfying. Besides, some uneven areas and attached particles have appeared on fiber surfaces of the SiC_f(BN) or SiC_f(ZrB₂), whereas smooth and dense surfaces of amorphous C–coated SiC_f were observed. Because the uniformity of ZrB₂ coatings can be partially damaged by the subsequent coating process of BN, the composite coatings of ZrB₂/BN and C/ZrB₂/BN were thereby not suitable for strengthening SiBCN matrix. The SiC_f/SiBCN composites with C/ZrB₂ coatings have desirable comprehensive mechanical properties. Nevertheless, the conventional toughening mechanisms such as fiber pull-out and bridging, and crack deflection are not available for these composites because the serious crystallization of SiC_f leading to great strength loss, resulting in catastrophic brittle fracture.     

Effects of NiO nanoparticles on the magnetic properties and diffuse phase transition of BZT/NiO composites

A new composite system, Ba(Zr_0.07Ti_0.93)O₃ (BZT93) ceramic/NiO nanoparticles, was fabricated to investigate the effect of NiO nanoparticles on the properties of these composites. M-H hysteresis loops showed an improvement in the magnetic behavior for higher NiO content samples plus modified ferroelectric properties. However, the 1 vol.% samples showed the optimum ferroelectric and ferromagnetic properties. Examination of the dielectric spectra showed that the NiO additive promoted a diffuse phase transition, and the two phase transition temperatures, as observed for BZT93, merged into a single phase transition temperature for the composite samples.     

Flow behaviour and microstructure evolution in novel SiO2/PP/LCP ternary composites: effects of filler properties and mixing sequence

When silica (SiO₂) fillers were introduced into the polypropylene (PP) and liquid‐crystalline polymer (LCP) blend, it was found that the mixing sequence, the filler size, and the filler surface nature affected the rheology of the composites and the morphology of the LCP phase in the ternary composite. In particular, the compatibility between the filler and the PP matrix was found to exert a strong influence on the droplet‐fibril transition. The incorporation of the hydrophobic silica to the LCP/PP blend facilitated the fibrillation of LCP because the hydrophobic filler demonstrated affinity towards the hydrophobic PP matrix. The preferential residence of the hydrophobic silica in the PP phase would minimise the LCP domain disruption leading to the formation of LCP fibrils with high aspect ratios. The use of fine filler and in situ blending, which promoted the filler–LCP interaction, could prevent coalescence, inhibit deformation and hinder fibril development as well. © 2003 Society of Chemical Industry     

Effects of SiC content on the densification,microstructure, and mechanical properties of HfB2–SiC composites

To investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB₂ and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB₂ grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO₂ with higher fraction of SiC. By SiO₂ enrichment, the glass viscosity increased and higher HfO₂ remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB₂ Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m^0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans-granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross-section which implied a weak crack deflection.     

A simulation study on the effects of shear flow on the microstructure and electrical properties of carbon nanotube/polymer composites

We employ a fiber-level simulation technique to simulate carbon nanotube (CNT)/polymer composites in simple shear flow. This model incorporates CNT flexibility, irregular CNT equilibrium shapes and CNT interactions. Electrical conductivity of the composites is determined using a resistor network algorithm. Tunneling resistance of the insulating matrix film between nanotubes is also considered. We show that the rate of imposed shear flow influences the composite conductivity by facilitating the formation or destruction of the conductive aggregates. In addition, the conductivity evolution during shearing for different concentrations is investigated. At low concentration, percolating clusters form and break simultaneously which causes large conductivity fluctuations during the simulations. When sufficiently large concentrations are reached, percolating clusters persist during shearing and the conductivity fluctuations decrease. In agreement with previous research we determine that increasing the shear rate causes alignment of the nanotubes in the flow direction. We show that upon shearing at constant shear rate, the system attains a state with substantially constant electrical conductivity, nanotube orientation and agglomerate size that is a function of the applied shear rate. The state reached for a given shear rate is independent of the initial state of orientation and aggregation.     

Impact of Al2O3-40 wt.% TiO2 feedstock powder characteristics on the sprayability,microstructure and mechanical properties of plasma sprayed coatings

Atmospheric plasma sprayed (APS) Al₂O₃-TiO₂ coatings have found a wide range of industrial application due to their favorable properties, combined with low costs and a high availability. However, the detailed effect of the phase composition and the element distribution of the feedstock powders on the coating properties and the spraying process have only crudely been investigated so far. Here the impact of aluminum titanate (Al₂TiO₅) on the microstructural features and mechanical properties of Al₂O₃-40 wt.% TiO₂ APS coatings is demonstrated by investigating the detailed phase composition and the distribution of aluminum and titanium in three fused and crushed feedstock powders and the respective coatings. Thereby, a direct influence of Al₂TiO₅ content on the deposition efficiency, the porosity, the elastic modulus, and the hardness of the coatings is revealed. The results emphasize the need for a more detailed specification of commercial Al₂O₃-TiO₂ feedstock powders to ensure a high reliability of the coating properties.     

Ti3SiC2-Cf composites by spark plasma sintering: Processing,microstructure and thermo-mechanical properties

MAX phases, and particularly Ti₃SiC₂, are interesting for high temperature applications. The addition of carbon fibers can be used to reduce the density and to modify the properties of the matrix. This work presents the densification and characterization of Ti₃SiC₂ based composites with short carbon fibers using a fast and simple fabrication approach: dry mixing and densification by Spark Plasma Sintering. Good densification level was obtained below 1400 °C even with a high amount of fibers. The reaction of the fibers with the matrix is limited thanks to the fast processing time and depends on the amount of fibers in the composite. Bending strength at room temperature, between 437 and 120 MPa, is in the range of conventional CMCs with short fibers and according to the resistance of the matrix and the presence of residual porosity. Thermo-mechanical properties of the composites up to 1500 °C are also presented.     

Effects of sintering in a high magnetic field on properties of vitrified bond and vitrified CBN composites

Vitrified bond CBN grinding wheels are being widely used due to their superior performance. Also, advantages of vitrified grinding wheels are high elastic modulus, stable chemical property, and low thermal expansion coefficient. Brittleness and low strength are key factors restricting the development of vitrified bond CBN grinding wheels. In this paper, the sintering in a high magnetic field was innovatively introduced into the manufacturing of vitrified bond CBN grinding wheels, and the effects of sintering in a high magnetic field on properties on vitrified bond and vitrified CBN composites were systematically investigated. Vitrified bond was characterized using three-point bending, scanning electron microscopy, X-ray diffraction. It was observed that microstructure of vitrified bond could be changed, grain orientation could be controlled and average grain size could be decreased in a high magnetic field, while vitrified bond strength could be simultaneously improved. High quality vitrified bond could be obtained by appropriately adjusting the strength and direction of high magnetic field. Results demonstrated that vitrified bond properties were improved when the magnetic field strength was 6?T. In order to highlight the high magnetic field effect on the vitrified CBN composites, the ordinary CBN abrasives and nickel plated CBN abrasives were used respectively. Microstructures, bending strengths of vitrified CBN composites were compared in different high magnetic fields. When the magnetic field strength was appropriate (less than 6?T), the binding characteristic of vitrified bond CBN composites with nickel plated CBN abrasives was greatly improved. The highest bending strength value of vitrified CBN composites was 79.5?MPa in 6?T high magnetic field.     

Effect of SiO2 nanoparticles-decorated SCF on mechanical and tribological properties of cenosphere/SCF/PEEK composites

A silicon oxide (SiO₂) nanoparticles-decorated short carbon fiber (SCF) hybrid (SCF-SiO₂) was designed to improve the weak interfacial bonding between fibers and matrix. Nano-SiO₂ was grafted onto carbon fibers by introducing amino group and epoxy group on the surface of carbon fibers and SiO₂, respectively. The chemical composition of SCF-SiO₂ was analyzed by Fourier transform infrared spectrometer and energy-dispersive spectrometry, the microstructure of SCF-SiO₂ were investigated by scanning electron microscope, and then the hybrid filler was introduced into Poly(ether ether ketone) (PEEK). Due to the strong interfacial interaction between filler and matrix, the mechanical and tribological properties of SCF-SiO₂/PEEK composites were significantly better than SCF/PEEK composites. In order to further improve the tribological properties of the composites, micrometer-sized cenosphere (CS) particles were introduced into the aforementioned system to prepare multicomponent composites. The test results of friction and wear indicate that the CS/SCF-SiO₂/PEEK composites have the optimal tribological properties. Compared with pure PEEK, the friction coefficient of CS/SCF-SiO₂/PEEK composites under 200 N load decreases by 56.4% and the specific wear rate decreases by 87.4%. Meanwhile, the thermal decomposition temperature of CS/SCF-SiO₂/PEEK composites is increased by 40 °C compared to pure PEEK. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48749.     

Exchange-coupled of soft and hard magnetic phases on the interfaces of Fe3C/CoFe2O4 nanocomposites

It is extremely important and challenging to develop exchange-coupled nanomagnets with soft/hard magnetic phase to apply for energy-related devices. Herein, we presented a simple strategy to synthesize the Fe₃C/CoFe₂O₄ nanocomposites by a chemical coprecipitation method, in which effective exchange coupling at hard and soft magnetic interfaces was achieved. The as-synthesized Fe₃C/CoFe₂O₄ nanocomposites show exceptional exchange-coupled effect and enhanced magnetic properties. Moreover, this work provides a new soft magnetic phase and an entirely new attempt for nanomagnets based on soft/hard magnetic exchange coupling.     

Fabrication of TiC coated short carbon fiber reinforced Ti3SiC2 composites: Process,microstructure and mechanical properties

The C_f/Ti₃SiC₂ composites were fabricated through spark plasma sintering (SPS) and hot isostatic pressing (HIP), TiC coated C_f and Ti₃SiC₂ powder were used as starting materials. The improved fracture toughness (K_IC) and Vickers hardness (HV₁) of the TiC coated C_f/Ti₃SiC₂ composite fabricated by SPS were 7.59 MPa·m^1/2 and 7.28 GPa. On this foundation, taking the advantage of better sintering process of HIP, the highest K_IC and HV₁ achieved 8.32 MPa·m^1/2 and 9.24 GPa with fiber content of 10 vol%, which increased by 40% and 65% compared with that of monolithic Ti₃SiC₂. The reasonable control of reactive interface is the main factor for the improved mechanical properties of the composites, the TiC coating effectively protected the fiber structure from interfacial reaction compared with that of the non-coated C_f/Ti₃SiC₂. Meanwhile, the artificially designed and weakly bonded TiC coated C_f can fully exert the toughening mechanisms like fiber pull-out and debonding.