Innovative ferrite nanofibres reinforced soft magnetic composite with enhanced electrical resistivity

Highlights

• Ferrite nanofibres in SMC improve the structural and electromagnetic properties.

• SMC samples show very high electrical resistivity.

• Magnetization study reveals soft magnetic properties of all samples with ferrite nanofibres.

Abstract

Soft magnetic materials with an excellent performance are desired for functional applications. We present an innovative method for manufacturing the soft magnetic composites that may pave the way for magnetic cores with improved electromagnetic properties. In this paper, soft magnetic composites based on FeSi powder coated with the hybrid organic-inorganic coating composed of boron phenol-formaldehyde resin and Ni_0.3Zn_0.7Fe₂O₄ ferrite fibres were fabricated to investigate the effects of ferrite nanofibres on the structural and electromagnetic properties. A uniformity of hybrid organic-inorganic coating is reflected in a high value of the electrical resistivity. A low porosity and extraordinary high values of mechanical hardness and flexural strength were found in prepared soft magnetic composites.

Introduction

Magnetic materials made of soft magnetic composites (SMCs) are extensively developed as a viable alternative to the laminated steel materials in a range of new applications, such as transformers, inductors, sensors, fast switching solenoids and electrical motors. In comparison with laminated soft magnetic materials, SMCs based on the ferromagnetic powder materials and dielectric coatings allow for revolutionized design of electromagnetic devices with improved efficiency and reduced weight and costs, without sacrificing magnetic performance [1]. Basically, the SMCs are designed as a ferromagnetic powder surrounded by a thin electrically insulating layer [2], which exhibit unique magnetic properties such as an isotropic magnetic behaviour, low eddy-current loss, as well as, a relatively lower total core loss at medium and high frequencies. Till now, the most used ferromagnetic core materials for designing of SMCs are Fe [[3], [4], [5]] or Fe-Si alloys [6,7], which are characterized by high electrical resistivity, Ni-Fe alloys [8,9] with high permeability and Fe-Co alloys [10] with high magnetic saturation magnetization.

Generally, the dielectric coatings are divided to organic or inorganic materials. The advantages of organic coatings lie in simple coating process, what leads to a creation of uniform insulation of particles ensuring production of the materials with high density and high electric resistivity of final green compacts [11]. The main disadvantages of SMCs with organic component are the thermal treatment, which is strictly limited by the thermal resistance of the organic insulating material. On the other side, for application of core materials where the higher temperature is needed in the preparation process, it is more desirable to use the pure inorganic coatings (electric motors, batteries, house appliances, machine tools etc.). Moreover, inorganic coatings including phosphates [12], oxides [13], silica [14], sodium silicates [15] or ferrites [16] and other ceramic compounds ensure that a final thermal treatment is adequate to remove residual stress after compaction and a preparation of green compacts. The common disadvantage of all inorganic coatings is creation of shrinkage during heat treatment leading to formation of cracks and possibility of metal-on-metal contacts yielding to final exfoliation of inorganic coatings. Another feature of inorganic coatings is formation of bi-layered coating because of nature of ferromagnetic core, which can interact with coating at high temperature during synthesis procedure [17]. Several efforts have been conducted to form hybrid organic-inorganic coatings in SMCs, e.g. by the sol-gel-prepared nano-sized SiO₂ in the phenol-formaldehyde resin [18,19], by direct addition of SiO₂ nanoparticles in the epoxy-modified silicone resin [19] or by the incorporation of the mixture of SiO₂ and Fe₃O₄ particles into epoxy-modified silicone resin [20]. Such the modifications tend to the enhanced mechanical strength, electrical resistivity and magnetic properties. Chemical routes of in-situ preparation of SiO₂ nanoparticles in the organic coatings allows to induce the chemical bonding between inorganic and organic part of the coating and finally cause the homogeneous formation of coating [18] and improved thermal stability [19]. The challenging task remains the preparation of hybrid SMC with ferrite coating for industrial applications. Ferrites are well-known ferrimagnetic materials, which possess unique electromagnetic properties, high electrical resistivity, controllable saturation magnetization, moderate thermal expansion coefficients, energy-transfer efficiency [21]. There is a variety of physical or chemical methods employed to prepare the spinel type ferrite: sol-gel autocombustion method [22], chemical precipitation [17] or mechanical mixing procedure [23]. However it is known that basic magnetic characteristics of spinel-type ferrites can be controlled by changing the type of divalent cations in a crystal structure [24] or by the size and shape of ferrite nanoparticles [[25], [26], [27]]. One-dimensional nanostructures in the form of fibres have received considerable attention due to their tunable mechanical properties such as high mechanical strength, toughness and Young's modulus. By the preparation of hybrid organic-inorganic coatings, it is possible to combine the benefits of both types of coatings and prepare the new family of SMC with desirable properties.

In this work, it is reported the preparation of soft magnetic composite based on FeSi powder coated with the hybrid organic-inorganic coating composed of boron phenol-formaldehyde resin (PFRB) and Ni_0.3Zn_0.7Fe₂O₄ ferrite fibres. PFRB was synthesized by polycondensation reaction of phenol and formaldehyde in the presence of boric acid and the synthesis, characterization and advantage of thermal resistance was reported previously in detail [28]. The needle-less electrospinning was used for preparation of Ni_0.3Zn_0.7Fe₂O₄ soft magnetic fibres in large scale. The hybrid coating was deposited on a surface of spherical FeSi powder and processed by PM technology for a bulk sample for mechanical, electrical and magnetic tests. The morphology of prepared ferrite fibres as well as final SMCs samples were characterized by SEM, TEM, XRD and EDX analysis. Electro-magnetic properties were studied and the complex permeability and core losses dependence of hybrid organic-inorganic coatings composition are discussed.