Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Preparation and characterisation of Fe/Fe 3 O 4 fibres based soft magnetic composites B.V. Neamţu a,∗ , M. Pszola b , H. Vermeşan c , G. Stoian d , M. Grigoraş d , A. Opriş a , L. Cotojman a , T.F. Marinca a , N. Lupu d , I. Chicinaş a a Technical University of Cluj-Napoca, Materials Science and Engineering Department, 103-105, Muncii Avenue, 400641, Cluj-Napoca, Romania b Institute of Electrical Machines, RWTH Aachen University, Aachen, D-52062, Germany c Technical University of Cluj-Napoca, Department of Environmental Engineering and Sustainable Development Entrepreneurship, 103-105, Muncii Avenue, 400641, Cluj- Napoca, Romania d National Institute of Research & Development for Technical Physics, 700050, Iasi, Romania ARTICLE INFO Keywords: Fibres based soft magnetic composites Fe 3 O 4 insulating layer DC and AC magnetic Characterisation Core loss separation ABSTRACT Soft magnetic composites (FSMCs) have been prepared by using Fe fibres coated with a layer of Fe 3 O 4 , 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 Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 leads to a slight deterioration of the compact's magnetic properties. However, as the thickness of the Fe 3 O 4 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. 1. Introduction In the past decades, the study of soft magnetic composites (SMCs) has become an increasingly important research direction bringing to- gether researchers from various fields like material science, physics and chemistry. Several review papers dealing with state of the art and the perspectives of SMCs already exist [1–3]. A soft magnetic composite is classically prepared via powder technology routes and must contain at least two phases: a ferromagnetic phase and a dielectric phase. The proper combination of those two phases should lead to a material with a unique set of properties such as low eddy current losses at medium to high frequencies, high saturation induction, relatively high magnetic permeability, relatively low coercivity and isotropy of the physical properties [1–3]. Mainly, the preparation of SMC consists of the coating of the ferromagnetic phase with the dielectric phase and the con- solidation of these composite particles via pressing. The published research in the field of SMCs aimed to elucidate the influence of the characteristics of the ferromagnetic phase as well as those of the di- electric phase on the AC and DC magnetic characteristics of the ob- tained magnetic cores [1]. Concerning the ferromagnetic phase, various types of powders were used, such as pure Fe, Fe-based, Ni-based or Fe–Co powder alloys [4–8]. Also, amorphous and nanocrystalline materials, especially Fe-based amorphous alloys, are starting to be used as ferromagnetic phase [9,10]. The shape, size and particle size distribution for the ferromag- netic phase were also intensively investigated [7–9,11]. Much attention was paid to the dielectric phase since this should be present in the material in the lowest amount possible, but at the same time, it must electrically insulate the ferromagnetic particles between them. Organic, inorganic or hybrid coatings are generally used as a dielectric phase. The most commonly used organic coatings are ther- mosetting polymers such as epoxy resins, acrylic resins, polyesters, etc. https://doi.org/10.1016/j.ceramint.2020.08.165 Received 22 June 2020; Received in revised form 9 August 2020; Accepted 18 August 2020 ∗ Corresponding author. E-mail address: bogdan.neamtu@stm.utcluj.ro (B.V. Neamţu). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: , Ceramics International, https://doi.org/10.1016/j.ceramint.2020.08.165