Properties of soft magnetic composites based on Fe fibres coated with SiO2 by hydrothermal method

Journal Pre-proof Properties of soft magnetic composites based on Fe fibres coated with SiO2 by hydrothermal method B.V. Neamţu, A. Belea, F. Popa, E. Ware, T.F. Marinca, I. Vintiloiu, C. Badea, M. Pszola, M. Nasui PII:

S0925-8388(20)30585-5

DOI:

https://doi.org/10.1016/j.jallcom.2020.154222

Reference:

JALCOM 154222

To appear in:

Journal of Alloys and Compounds

Received Date: 13 November 2019 Revised Date:

4 February 2020

Accepted Date: 5 February 2020

Please cite this article as: B.V. Neamţu, A. Belea, F. Popa, E. Ware, T.F. Marinca, I. Vintiloiu, C. Badea, M. Pszola, M. Nasui, Properties of soft magnetic composites based on Fe fibres coated with SiO2 by hydrothermal method, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.154222. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Credit author statement

B.V. Neamtu: conceived the presented idea; contributed data analysis; performed the magnetic analysis; wrote the paper. A. Belea: performed the experimental parts in collaboration with M.Nasui; F. Popa: performed the SEM-EDX analysis; E. Ware: performed the TEM-EDX analysis; T.F. Marinca: contributed to the interpretation of the results especially FTIR and M(H); I. Vintiloiu: was involved in data interpretation, C. Badea: prepared the toroidal samples and was involved in data correlation; M. Pszola: helped carry out the magnetic measurements; M. Nasui: carried out the experiment, contributed data or analysis tools; performed the analysis; wrote some parts from the paper. All authors discussed the results and contributed to the final manuscript.

1

Properties of soft magnetic composites based on Fe fibres coated with SiO2 by

2

hydrothermal method

3

B.V. Neamţu1, A. Belea1, F. Popa1, E. Ware2,3, T.F. Marinca1, I. Vintiloiu4, C. Badea5,

4

M. Pszola6, M. Nasui2*

5 6

1

7

105, Muncii Avenue, 400641 Cluj-Napoca, Romania,

8

2

9

Cluj-Napoca, Memorandumului Street, no 28, 400114 Cluj-Napoca, Romania,

Technical University of Cluj-Napoca, Materials Science and Engineering Department,103-

Center for Superconductivity, Spintronics and Surface Science, Technical University of

10

3

11

Kingdom

12

4

Advanced Development Group Brose Fahrzeugteile GmbH & Co. KG., Wurzburg, Germany

13

5

Technical University of Cluj-Napoca, Department of Physics and Chemistry, Cluj-Napoca,

14

Romania

15

6

Imperial College London, Exhibition Road, South Kensington, London SW 7 2AZ, United

Institute of Electrical Machines, RWTH Aachen University, Aachen, D-52062, Germany

16 17

*Corresponding author: [email protected]; Phone number: +40264 599855

18 19

Abstract

20

The paper reports on the preparation and characterisation of a new type of soft

21

magnetic composites in which the ferromagnetic particles are replaced by Fe fibres. The

22

fibres were coated via hydrothermal method with a SiO2 layer having a thickness of 150 –

23

200 nm. Also, a hybrid coating was developed by adding a supplementary layer of polymer

24

on top of the SiO2 layer. The coated fibres were compacted via cold pressing at the pressure

25

of 700 MPa. Our results indicate that using only SiO2 as the insulating layer leads to

1

1

compacts characterised by lower hysteresis losses. However, it seems that during compaction,

2

the SiO2 layer is damaged leading to the development of large eddy currents at high

3

frequencies. Conversely, the additional polymer layer manages to prevent the development of

4

excessive eddy currents in the compact, but with a significant increase of the hysteresis losses

5

of the compacts being observed. A significant improvement of the magnetic characteristics of

6

the fibre-based soft magnetic composites can be achieved by an annealing treatment with the

7

condition of maintaining the integrity of the insulating layer, or minimising the diffusion

8

between the fibres and the SiO2 coating during the annealing.

9 10

Keywords: soft magnetic composites, fibres based composites, SiO2 coating, hybrid coating,

11

hydrothermal coating.

12 13 14

1. Introduction

15

A composite material must possess unique properties that are usually obtained by

16

combining two or more materials that, typically, have very different properties. These two

17

materials work together to give to the composite a unique set of properties. A typical soft

18

magnetic composite (SMC) consists of two phases: (i) a ferromagnetic phase (usually

19

ferromagnetic particles) characterised by high saturation magnetisation, high magnetic

20

permeability, low coercivity, etc. Concurrently, the ferromagnetic phase has low electrical

21

resistivity which leads to excessive core losses via eddy currents when is subjected to high-

22

frequency magnetising fields. (ii) a dielectric phase that should have electrical resistivity as

23

high as possible. Also, the dielectric phase must have high thermal stability (allowing

24

application of heat treatments to the SMCs) and high thermal conductivity (allowing heat

25

dissipation) [1-3].

2

1

The ferromagnetic particles are initially coated with the dielectric phase and then

2

compacted via pressing to form 3D magnetic cores [1-3]. The ideal SMC should combine the

3

high magnetic properties of the ferromagnetic phase with the high electrical resistivity of the

4

dielectric phase. Hence, the resulting material should be able to work at medium to high

5

frequency and to possess simultaneously several interesting properties such as: isotropy of

6

magnetic properties, high saturation induction, very low eddy current losses due to their high

7

resistivity, high permeability, low coercivity, high Curie temperature, reduction in size and

8

weight of electromagnetic devices, the possibility to create complicated 3D magnetic cores

9

via powder metallurgy, etc [2]. Both classes of materials that dominate the global market of

10

soft magnetic materials, i.e. electrical steels and soft magnetic ferrites, do not simultaneously

11

possess such characteristics [3].

12

As ferromagnetic phase, pure Fe powders are the most used but other Fe-based, Ni-

13

based or Fe-Co powder alloys are also intensively investigated [4-8]. Amorphous and

14

nanocrystalline materials (especially Fe-based amorphous alloys) are interesting materials

15

that start to be used as ferromagnetic phase, gradually replacing the polycrystalline powders

16

[9, 10]. Different shapes and sizes of the ferromagnetic phase were investigated such as

17

irregular particles, spherical particles, fakes and more recently fibres [7 - 9, 11 - 13]. For

18

example, it was proved that fibres based soft magnetic composites (FSMCs) present superior

19

initial relative permeability as compared to classical SMCs based on powders due to their low

20

demagnetising factor [13].

21

A major aspect of publications related to the preparation and characterisation of

22

SMCs, is a focus on finding methods to effectively insulate the ferromagnetic particles. For

23

insulating materials, organic, inorganic or hybrid coatings are generally used. As organic

24

coating, thermosetting polymers such as epoxy resins, polyesters, acrylic resins are most

25

commonly used [1, 4, 5, 7, 8]. The polymeric layer insulates the particles from each other

3

1

and, at the same time, binds the particles between them giving a certain mechanical strength

2

to the compact. However, due to the fact that organic coatings cannot withstand high

3

temperatures without degradation, annealing at high temperatures of this type of SMCs is not

4

feasible. This generally leads to compacts characterised by high coercivity, high hysteresis

5

losses, and low magnetic permeability. On the other hand, inorganic coatings can withstand

6

high temperature annealing. A large variety of inorganic coatings were tested up to now such

7

as Al2O3, SiO2, ZrO2, MgO, FePO4, different soft magnetic ferrites, etc [14 - 20]. Thin

8

inorganic coatings provide satisfactory insulation of the ferromagnetic particles but they offer

9

no mechanical strength to the compact since they are not acting as a binder, as in the case

10

with polymeric layers. Recently, hybrid coatings consisting of an inorganic layer and an

11

organic layer were used in attempts to take advantage of the strengths of the two types of

12

insulating layers [8, 21].

13

This paper reports on the preparation and characterisation of a new type of SMCs

14

which is fibres based soft magnetic composites (FSMCs). In this new type of magnetic

15

composites, the ferromagnetic particles are replaced with Fe fibres having the length of

16

several tens of centimetres. Two different types of FSMCs were prepared, one consisting of

17

Fe fibres coated with SiO2 and the other consisting of Fe fibres coated with a layer of SiO2 on

18

top of which is a layer of polymer. The SiO2 coating was realised via hydrothermal method

19

which, currently, is used increasingly, by researchers working in the field of SMCs, to create

20

the insulating layer between ferromagnetic particles [16, 22].

21

This new type of material is designed to combine the strength point of Fe-Si laminates

22

and SMCs. It can be used at higher frequencies as compared to electrical steels (generally

23

limited to several hundred Hz) but is not able to operate (at least in this moment) at

24

frequencies of several tens or hundreds of kHz that are characteristic to SMCs. So, this new

25

type of material will fill the gap between electrical steel and SMCs.

4

1 2

2. Experimental

3

Fibres based soft magnetic composites (FSMCs) were prepared using Fe fibres coated

4

with a thin layer of SiO2 or a hybrid coating consisting of a layer of SiO2 and a second layer

5

of polymer (Araldite AT1). The Fe fibres used in this study have lengths in the range of

6

several tens of centimetres (20-40 cm) and thickness within the range of 80 – 120 µm. These

7

fibres are commercially available (manufactured by STAX, Germany) and were produced

8

using a steel wool cutting machine. The chemical composition of Fe fibres, in wt.%, is:

9

99.82% Fe, 0.078 % Mn, 0.004 % P, 0.0031 % S, 0.010 % Cu, 0.005 % N, 0.027 % Ni, 0.021

10

% Si, 0.005 % Al, 0.021 % Cr, 0.003 % Mo. Before coating, the Fe fibres were subjected to a

11

recrystallization heat treatment at a temperature of 650 °C for 2 hours in high purity Ar

12

atmosphere. This heat treatment was done in order to remove the stresses induced by the

13

production process and to increase the fibres compressibility.

14

Before the hydrothermal process, the fibres were treated as follows: 10 minutes of

15

ultrasonic cleaning, with an alkaline solution of pH= 9, and then 1– 2 minutes pickling with a

16

mixture of hydrochloric acid and nitric acid to eliminate any organic material from the

17

surface of the fibres. The Fe fibres were subjected to a hydrothermal process using analytical

18

reagents and an ethanol solvent bath. The reagents were: tetraethyl orthosilicate TEOS

19

(99.9%) with ammonium hydroxide (32%) as activator. The molar ratio used was

20

TEOS:EtOH = 1:4. After homogenization of the mixture between TEOS and ethanol, the

21

ammonium hydroxide solution (NH4OH) was added as activator, followed by magnetic

22

stirring for 2 hours. The solution obtained was introduced into a Teflon beaker, the metallic

23

fibres were inserted, and the autoclave was sealed. The high-pressure autoclave was

24

maintained at 200 °C for 8 hours and then cooled to room temperature. After removal from

25

solution, the fibres were rinsed with acetone and then dried (at 100 °C). After hydrothermal

5

1

treatment, the Fe fibres were treated at 600 °C for 1 hour in a high purity Ar atmosphere with

2

the aim of removing any residual organic parts and to form the SiO2 coating. One part of the

3

Fe fibres coated with SiO2 was used to create FSMCs compacts and the other part was coated

4

with a supplementary layer of polymer and used to obtain the second type of FSMCs

5

compacts. The polymer layer was added as follow: first, a liquid solution was obtained by

6

dissolving the Araldite AT 1 polymer in acetone. The Fe fibres coated with SiO2 were

7

immersed in this solution and continuously mixed until complete evaporation of the acetone.

8

After complete evaporation of the acetone, each fibre is covered with a thin layer of polymer

9

situated on top of the SiO2 layer. The amount of polymer that forms the polymer layer

10

represents 1wt.% of the mass of the fibres.

11

The uncoated fibres, as well as the fibres coated with SiO2, were investigated using a

12

Jeol-JSM 5600 LV scanning electron microscope equipped with an EDX spectrometer

13

(Oxford Instruments, Inca 200 software). The thickness of the SiO2 coating was evaluated via

14

transmission electron microscopy (TEM). HELIOS NanoLab600 Focussed Ion Beam (FIB)

15

was used to cut a thick lamella from the prepared sample. A JEOL JEM-2100F at 200 kV

16

field emission transmission electron microscope (TEM), which combines high spatial

17

resolution and analytical performance, was used to acquire images of areas of interest as well

18

as for EDX mapping. The mapping was performed in Scanning Transmission Electron

19

Microscopy (STEM) mode, which has a lattice resolution of 0.2 nm.

20

Fourier Transform Infrared (FTIR) Spectroscopy was performed for the coated samples

21

using a Bruker Tensor 27 FTIR Spectrometer in Attenuated Total Reflection mode (ATR).

22

All spectra were normalised for the highest band in the 400–4000 cm−1 range with a

23

resolution of 2 cm-1. For each spectrum, a ratio of 32 sample scans to 1 background scans was

24

used.

6

1

The fibres coated with polymer were wrapped on the core rod of the die and then cold

2

pressed at a pressures of 700 MPa. Using this method of mould filling, we assume that the

3

fibres are mainly oriented along the magnetic field created by the excitation coil. However, it

4

is difficult to assume that all the fibres are perfectly aligned parallel to the applied field since

5

the fibres are not perfectly straight. The typical dimensions of the toroidal samples were: the

6

inner diameter of 12 mm, the outer diameter of 18 mm and the height of about 3.5 mm. The

7

toroidal compacts prepared from fibres coated with SiO2 and polymer were polymerized

8

through a heat treatment at 180 °C for 1 hour in the air. After the characterisation of the

9

toroidal compact based on fibres coated with SiO2, the compact was subjected to an

10

annealing treatment to remove stresses induced by the compaction process. The parameters of

11

the annealing were: annealing temperature of 500 °C, dwell time of 60 min, heating/cooling

12

speed of 10 °C/min, atmosphere – high purity Ar.

13

The AC and DC magnetic properties of the toroidal compacts were determined using a

14

computer-controlled Remagraph - Remacomp C - 705 hysteresisgraph produced by Magnet

15

Physik Dr. Steingroever GmbH. The AC magnetic measurements were performed in the

16

frequency range of 50 Hz – 10 kHz with maximum flux density (Bmax) of 0.1 T. The main

17

magnetic characteristics determined by AC measurements were total core losses (P/f) and

18

initial relative permeability (µri). DC magnetic measurements were performed to determine

19

the maximum relative permeability (µrmax), the coercivity (Hc), and the saturation induction

20

(Bs) of the compacts. The parameters of the measurements were: frequency of 0.5 Hz and

21

excitation magnetic field of 8 kA/m. The electrical resistivity of the compacts was determined

22

by 4 points probe measurements setup.

23 24

3. Results and discussions

25

7

1

3.1.Characterisation of Fe fibres

2

The cross-section and the longitudinal aspect of the Fe fibres used in this study are

3

presented in figure 1. It can be noticed that the cross-section of the fibres has a semi-circular

4

shape. The surface of the fibres contains two different surfaces with very different

5

morphologies. The plane surface is characterised by a relatively low roughness while the

6

semi-circular surface is characterised by higher roughness. These two types of surfaces are

7

created during the fibres production process which is a cutting process similar to planing.

8

(a)

(b)

(c)

9

Figure 1. SEM images of Fe fibres used for the FSMCs preparation. (a) Fibre’s cross-section,

10

(b) morphology of the semi-circular surface and (c) morphology of the plane surface of the

11

fibres.

12 13

3.2.Characterisation of the SiO2 coating

14 15

As it is well known, the hydrolysis of TEOS gives Si-OH groups that start to react with

16

other Si-OH groups through a polycondensation reaction which gives a three-dimensional

17

silica network. By heating this network in the temperature range of 600-800 °C, SiO2 is

18

obtained [23]. Figure 2 presents the normalised FTIR spectra of the hydrolysis steps of TEOS

19

and iron fibres before/after the deposition process. The assignments of the TEOS vibration

20

bands have been accomplished according to literature sources [24, 25]. The vibration bands 8

1

are assigned as follow: 480 cm-1 is assigned to the bending vibration - δ of C-C-O group, 795

2

cm-1 corresponds to the asymmetrical valence vibration (stretching) – νasym of SiO4 group,

3

945 cm-1 is attributed to the rocking vibration - ρ of C-H bonds in CH3 from TEOS, 1070 cm-

4

1

5

asymmetric stretching and 1165 cm-1 is the rocking vibration of C-H bonds from CH3 groups.

6

The addition of ethanol and ammonia on the TEOS leads to a decrease in intensity of TEOS

7

bands (1101, 945 and 795 cm-1) and the increase of bands situated at 480 cm-1. The bands

8

from 1070 and 1165 cm-1 appear to be unaffected. These changes in intensities can be

9

assigned to the total or partial overlapping of the TEOS and ethanol vibrations (ethanol C-O

10

symmetrical stretching vibration is at 1048 cm-1 and asymmetrical stretching vibration is at

11

1086 cm-1), and to the hydrolysis process respectively. Indeed, the bands that are more

12

intense or keep their intensity are overlapping with the vibrations of Si-O-Si bonds: at about

13

460, 1168 and 1200 cm-1 [23]. These data give information related to the hydrolysis process

14

of TEOS; the ruptures of the TEOS chemical bonds.

460

Fe fibres coated with SiO2

480

1165

TEOS+ Ethanol+NH4OH

795

945

1070

Fe-fibres

1101

Absorbance (u.a.)

1014

500

is the symmetrical stretching - νsym of Si-O-Si group, 1101 cm-1 correspond to C-O

TEOS 2000

1800

1600

1400

1200

1000

800

600

400

-1

Wavenumber (cm )

15 16

Figure 2. FTIR analysis of the hydrolysis steps of TEOS and iron fibres before and after

17

deposition process.

9

1 2

The IR spectrum of the metallic fibres after the cleaning process does not present any

3

clear vibration band; in the fingerprint region of Metal-Oxygen bonds (700-400 cm-1), no

4

obvious vibration can be identified. After hydrothermal synthesis/thermal treatment, the

5

bands corresponding to TEOS disappear. In the spectrum two new bands are identified, both

6

relatively large: one centred at 1014 cm-1and another approximately 500 cm-1. These bands

7

are assigned to the asymmetric stretching vibration of the Si-O and to the bending vibration

8

modes in O-Si-O from SiO2. No obvious vibration band of the precursors can be identified

9

into the spectrum after heat treatment. This clearly indicates the removal of residual organic

10

parts and the formation of SiO2 on the surface of the fibre.

11

With the aim of investigating the success of the hydrothermal method to create a

12

continuous SiO2 coating, SEM-EDX analysis was performed. Since the plane and semi-

13

circular surfaces are characterised by different morphologies, EDX analysis was performed

14

on both surfaces. In figure 3 b-d we present the distribution maps of Fe, Si and O on the

15

plane surface of the coated fibres while in figure 3 f-h the distribution maps of Fe, Si and O

16

on the semi-circular surface are presented. Regardless of the surface analysed (with high or

17

low roughness), it can be observed that the Si and O are uniformly distributed on the surface

18

of the analysed fibre indicating that the fibre is coated with a uniform layer of SiO2.

(a)

(b)

(c)

10

(d)

(e)

(f)

(g)

(h)

1

Figure 3. EDX analysis of the fibres surfaces coated with SiO2. (a) and (e) are SEM images

2

of the plane and semi-circular surface of the fibres (b, c, d) and (f, g, h) are distribution maps

3

of the Fe, Si and O on the plane and semi-circular surface of the fibres.

4 5

The Fe fibres coated with SiO2 were analysed also by X-ray diffraction (XRD) but no

6

signal from the coating layer was detected. The absence of Bragg reflexions corresponding to

7

SiO2 can be explained either by the fact that the layer is too thin (detection limit for XRD is

8

3-5%) or that the layer has an amorphous structure. However, EDX analysis indicated that the

9

Si:O ratio is 1:2 indicating the formation of SiO2 on the fibre’s surface. This is in good

10

agreement with FTIR analysis that highlighted only the Si-O vibration bands on the coated

11

fibres surface.

12

In order to quantify the thickness of the SiO2 layer, supplementary TEM and EDX

13

analyses was performed. The results are presented in figure 4. In figure 4a we present the

14

TEM image of a section through a Fe fibre coated with SiO2 layer. From figure 4a it can be

15

observed that the SiO2 layer is continuous on the surface of the Fe fibre, confirming the EDX

16

analysis presented in figure 3 a, b. Also, it can be noticed that the top surface of the SiO2

17

coating is not uniform, it has high roughness (figure 4a, b). The thickness of the SiO2 layer,

18

as determined from figure 4b, is in the range of 150 - 200 nm. However, due to the roughness

19

of the deposited layer, its thickness can reach as much as 300 - 400 nm in some areas. These

20

observations are confirmed by the EDX analysis presented in figure 4 c, d.

11

1 2 3

(a)

(b)

(c)

(d)

4 5

Figure 4. TEM images (a, b) and EDX analysis (c, d) of the different sections through a fibre

6

coated with a layer of SiO2.

7

Additional, EDX analysis revealed the existence of two different types of interfaces

8

between the Fe fibres and the SiO2 layer. One type of zone, where almost no diffusion

9

between the fibre and the SiO2 coating is noticed (figure 4c) and another type of zone where

10

the diffusion took place, the diffusion depth being approximated to several tens of nm (figure

11

4d). Most probably, the diffusion took place during the heat treatment applied to the fibres

12

immediately after the hydrothermal coating. This treatment was performed at 600 °C for one 12

1

hour in Ar atmosphere. However, why the diffusion took place in some areas yet not in others

2

remains an open question. It is reasonable to assume that the existence of zones, on the

3

surface of the fibres, where the density of the structural defects (such as dislocations or

4

vacancies) is higher as compared to other zones, favours the diffusion process. These zones

5

are created during the production of the fibres, which is essentially a cutting process.

6 7

3.3. DC magnetic characteristics of the FSMCs

8

The hysteresis curves of the compacts prepared from Fe fibres coated with SiO2 and

9

Fe fibres coated with SiO2 and 1 wt.% of polymer are presented in figure 5. Also, table 1

10

presents the main magnetic characteristics of the FSMCs as derived from their hysteresis

11

loops. It worth mentioning that the saturation state of the compacts was not achieved by the

12

applied magnetic field of 8 kA/m. Due to this fact, the saturation induction and coercive field

13

presented in this study are those corresponding to an applied field of 8 kA/m.

1.5

Fe fibres + SiO2 Fe fibres + SiO2+ 1 wt.% Araldite

1.0

B (T)

0.5 0.0 -0.5 -1.0 -1.5 -8

14

-6

-4

-2

0 2 H (kA/m)

4

6

8

15

Figure 5. Comparison of hysteresis loops of FSMCs prepared from Fe fibres coated with

16

SiO2 and Fe fibres coated with SiO2 and 1 wt.% polymer (Araldite).

17 13

1 2 3 4 5 6

Table 1. Magnetic characteristics of the FSMCs prepared from Fe fibres coated with SiO2

7

and Fe fibres coated with SiO2 and 1 wt.% polymer (Araldite). Sample

Fe@SiO2

Density

Electrical

Saturation Coercive Maximum relative

resistivity

induction

field

permeability

(g/cm3)

(Ω·mm)

(T)

(A/m)

-

7.12

0.311

1,39

367

797

6.55

0.348

1,26

353

733

Fe@SiO2+1wt.% polymer 8 9

As expected, the addition of 1wt.% of polymer leads to a decrease in the density and

10

increase in the electrical resistivity of the compact. Similar observations, a decrease of

11

compact’s density by increasing the amount of dielectric, were made on soft magnetic

12

composites based on ferromagnetic powders or fibres [7, 13]. Such behaviour was explained,

13

mainly, on the basis of the difference between the densities of the ferromagnetic and

14

dielectric phases (the dielectric have lower density as compared to the density of the

15

ferromagnetic phase). As can be remarked from the data presented in table 1, an increase of

16

about 10% of the electrical resistivity of the compacts was obtained by the addition of 1 wt.%

17

of polymer. Unsurprisingly, the compact characterised by higher density presents the higher

18

value of the saturation induction and maximum relative permeability (see figure 5 and table

19

1). However, the coercivity of this compact is about 4% higher as compared to the coercivity 14

1

of the compact with lower density. Considering that the preparation process for both

2

compacts was the same (same type of fibres, same SiO2 coating, same compaction pressure,

3

etc.), the presence of the polymer layer should be responsible for the lower coercive field of

4

the compacts with lower density. Indeed, the polymer can act as a lubricant during the

5

compaction process leading thus to a reduction of the amount of stressed induced in the

6

ferromagnetic phase [13]. This will lead to a diminution of the coercive field of the compact.

7

It has been previously proved that the use of a small amount of lubricant leads to SMCs

8

compacts with enhanced magnetic properties (lower magnetic losses via hysteresis and higher

9

initial relative permeability) [26].

10 11

3.4.AC magnetic characteristics of the FSMCs

12

The variation of core losses (P/f) and initial relative permeability (µri) of the compacts

13

prepared from Fe fibres coated with SiO2 and hybrid coated (SiO2 and polymer) is presented

14

in figure 6a and b respectively. Concerning the core losses, it can be observed that the

15

compact based on hybrid coated fibres (SiO2 and polymer) presents the largest losses in the

16

entire frequency range investigated in this study. It is known that the total power losses of a

17

magnetic core, which is subjected to an alternating magnetic field, are mainly given by the

18

sum of the hysteresis losses and eddy current losses. The hysteresis losses are the main part

19

of the total power losses when the frequency of the magnetising field is low. The hysteresis

20

losses are strongly influenced by the material purity, the density of the magnetic core as well

21

as the presence of stressed regions (induced by the compaction process). At higher

22

frequencies, the dominating losses are the eddy current losses caused by the development of

23

large eddy currents in the sample. This type of loss is mainly influenced by the electrical

24

resistivity of the compacts [1, 3]. Analysing the curve representing the core losses of the

25

compact prepared from hybrid coated fibres, it can be noticed that the predominant losses are

15

1

the hysteresis losses (the losses of the compacts at low frequency). In the frequency range 50

2

Hz – 5 kHz an increase of the total core losses by 19 % can be noticed. However, in the

3

frequency range, 5 kHz – 10 kHz the core losses present a steeper increase (increase with 26

4

%) indicating the development of eddy currents leading to a more dissipative character of the

5

compact. In the case of the compact prepared from fibres coated with SiO2, a quasi-

6

continuous increase of the core losses can be observed as the frequency increase in the

7

frequency range 50 Hz – 5 kHz. This indicates the development of eddy currents once the

8

frequency of the magnetizing field increases. The same steeper increase of the core losses in

9

the frequency range 5 kHz – 10 kHz was noticed in the case of the compact prepared from Fe

10

fibres coated with SiO2. The larger hysteresis losses of the compact prepared from hybrid

11

coated Fe fibres can be explained by the presence of the supplementary insulating layer (the

12

polymer layer). It was previously proved that increasing the amount of dielectric leads to an

13

increase of the hysteresis losses while the development of eddy currents is hindered [7, 27].

14

The development of large eddy currents in the compact based on fibres coated with SiO2

15

indicates that the insulating layer was damaged during the compaction process. This leads to

16

the creation of electrical contact between fibres and the eddy currents developing at a larger

17

scale (not in a single fibre but rather in fibre bundles). This is in accordance with the

18

electrical resistivity measured on the compacts that evidenced that the electrical resistivity of

19

the compact based on hybrid insulated fibres is 11% higher as compared to the electrical

20

resistivity of the compact based on fibres coated with SiO2.

21

16

(a)

(b)

1 2

Figure 6. Evolution of the total core losses (a) and initial relative permeability (b) of the

3

compacts based on Fe fibres coated with SiO2 and, SiO2 and polymer. Bmax = 0.1 T, f = 50 Hz

4

– 10 kHz.

5

To summarise, it worth mentioning that the difference between the total core losses of

6

the compacts decreases from 113% at 50 Hz to only 20% at 10 kHz. This indicates that the

7

polymer layer manages to prevent the development of excessive eddy currents in the

8

compact. However, the addition of the supplementary insulating layer (the polymer layer)

9

leads to a significant increase in the hysteresis losses of the compacts. Conversely, using only

10

SiO2 as the insulating layer leads to lower hysteresis losses but it seems that it fails to hinder

11

the development of the eddy currents due to its deterioration during the compaction process.

12

The evolution of the initial relative permeability (µri) of the compacts in the frequency

13

range 50 Hz – 10 kHz is presented in figure 6b. It can be observed that the µri of both

14

compacts is constant up to the frequency of 1 kHz. However, the µri of the compact based on

15

fibres coated only with SiO2 is 27% higher compared to that corresponding to the compact

16

based on fibres coated with SiO2 and polymer. It is generally accepted that, at low

17

frequencies, a lower amount of dielectric (a thinner layer of insulator) leads to composites

18

with higher initial relative permeability [27]. Nevertheless, thinner insulating layers present

17

1

the risk of being damaged during the compaction process, thus allowing the development of

2

eddy currents at a larger scale (as discussed in the case of core losses). This will

3

automatically lead to an important decrease in the initial relative permeability of the

4

compacts. Indeed, if we analyse the evolution of the initial relative permeability of the

5

compacts in the frequency range 1 kHz – 10 kHz, the following can be noted: (i) the µri of the

6

compacts based on fibres coated with SiO2 decrease by 24 % (practically reaching its cutting

7

frequency); (ii) the µri of the compacts based on fibres coated with SiO2 and polymer

8

decrease by only 2 %. As a concluding remark, although a hybrid coating leads to lower µri,

9

the constant value of µri in the investigated frequency range is an important result.

10 11 12

3.5.Influence of annealing on DC and AC magnetic characteristics of FSMC

13

The influence of annealing at 500 °C for 1 hour on the magnetic properties of the

14

compact based on Fe fibres coated with SiO2 was investigated. The compact based on hybrid

15

insulated fibres was not subjected to annealing since the polymer layer cannot withstand

16

temperatures higher than 250 °C without degradation.

17

The DC hysteresis loops of the compact based on Fe fibres coated with SiO2 before

18

and after annealing are presented in figure 7 along with the main magnetic characteristics of

19

the compacts. As can be observed, the coercive field (Hc) and the maximum relative

20

permeability (µrmax) of the compact are substantially improved. The decrease of the Hc and

21

increase of compact’s µrmax can be explained by the removal of the stresses induced by the

22

compaction process. However, a slight decrease of the compact’s saturation induction (Bs)

23

can be noted. This was attributed to a slight diffusion of the Si and O atoms from the SiO2

24

coating into the surface of the Fe fibres. The diffusion of nonmagnetic atoms into a

25

ferromagnetic material (Fe fibres) will naturally reduce its magnetic properties [28, 29].

18

1

2 3

Figure 7. Comparison of the DC hysteresis loops of the compact based on Fe fibred coated

4

with SiO2 before and after annealing at 500 °C for 1 hour.

5

We recall that the TEM-EDX analysis of the coated fibres revealed that there are

6

zones on the surface of the fibres in which the diffusion of Si and O atoms in the surface of

7

the fibres occurred. The annealing treatment applied to the compact, at 500 °C for 1 hour,

8

favour the diffusion process leading to a wider diffusion layer.

9

The influence of the annealing at 500 °C for 1 hour over the core losses (P/f) and

10

initial relative permeability (µri) of the compact based on fibres coated with SiO2 is presented

11

in figure 8.

19

(a)

(b)

1 2

Figure 8. Influence of annealing at 500 °C for 1 hour over the core losses (a) and initial

3

relative permeability (b) of the compact based on Fe fibres coated with SiO2.

4 5

It can be remarked that this annealing leads to a slight reduction of the hysteresis

6

losses of the compact. Although a more important reduction of hysteresis losses was

7

expected, a decrease of only 17 % was achieved. Our inference is there is a competition

8

between two phenomena having opposite effects on the value of hysteresis losses. On one

9

hand, the removal of stresses induced by compaction leads to a decrease of hysteresis losses.

10

On the other hand, the diffusion of Si and O atoms into the surface of the fibres leads to an

11

increase of the hysteresis losses since they will act as impurities. Removal of stresses has the

12

predominant effect (since the hysteresis losses decrease after annealing) suggesting that

13

indeed only the surface of the fibres is contaminated by the diffusion of Si and O atoms. This

14

is in good accordance with DC magnetic measurements that evidenced a decrease of the

15

coercive field of the compact after annealing (figure 7). When the frequency of the

16

magnetizing field increases, a steep growth of the core losses is observed, suggesting the

17

development of eddy currents. Most likely, the partial diffusion of Si and O atoms from the

18

insulating layer into the surface of the fibres leads to a larger amount of electrical contacts

20

1

between the adjacent fibres, and thus the development of eddy currents at a larger scale is

2

promoted. The diffusion of Si and O atoms into the surface of the fibres will lead to an

3

increased electrical resistivity of the layer in which the diffusion took place. However, the

4

electrical resistivity of the phases that can be formed by diffusion is lower as compared to the

5

electrical resistivity of the SiO2. For example, the electrical resistivity of Fe oxides falls

6

within the range of 10-1 – 104 Ω·mm, the electrical resistivity of Fe-Si alloys is about 10-5

7

Ω·mm, while the electrical resistivity of SiO2 is about 1014 Ω·mm. Indeed, the electrical

8

resistivity of the compact, measured after annealing, decreased to 0.263 from 0.311 Ω·mm.

9

The same phenomena (stresses release and surface contamination by diffusion) affects

10

the initial relative permeability (µri) of the compact. As can be observed, a very important

11

increase of the compact µri is induced by the annealing. However, the value of the µri is no

12

longer constant (not even up to 1 kHz as in the case of the un-annealed sample) but decreases

13

as the frequency increases. An abrupt decrease of the compact’s µri can be remarked at 500

14

Hz; this corresponds to the frequency where the core losses present a stepper increase as a

15

result of the excessive increase of the eddy currents. The same phenomena are responsible for

16

the diminution of the µri of the compact. The fact that the µri of the compact starts to decrease

17

at a lower frequency after the heat treatment, further support our inference that a part of Si

18

and O atoms diffuse into the fibres’ surface, favouring the electrical contact between adjacent

19

fibres. To summarise the above observations, it worth mentioning that a significant

20

improvement of the magnetic characteristics of the FSMCs can be achieved by annealing

21

with the condition of maintaining the integrity of the insulation layer or minimising the

22

diffusion between the fibre and the SiO2 coating during annealing.

23

3.6. Comparison of DC and AC magnetic characteristics of FSMCs and SMCs

24

A comparison of the AC and DC magnetic properties of the FSMCs and different

25

SMCs materials reported in the literature is presented in table 2. Comparing the magnetic 21

1

induction (Bs) reached by the compact prepared from Fe fibres coated with SiO2 (FSMC)

2

with the magnetic inductions reported in the literature for different Fe-based SMCs, it can be

3

observed that the induction of the FSMC compact is among the highest. Obviously, the

4

induction of the compacts depends mainly on their density and the magnitude of the applied

5

magnetic field. It worth to be mentioned that compacts with relatively high density can be

6

obtained by using fibres instead of powders as was shown in this paper and ref. [13].

7

Table 2. Comparison of the AC and DC magnetic properties measured on FSMCs compacts

8

based on Fe fibres coated with SiO2 and different SMCs reported in the literature.

Sample

DC

AC Hc

B

P@10 kHz

[A/m]

[T]

[W/kg]

797

367

0.1

1.55

911

371

40

1.79

189

6.85

5

0.6

NiFeMo powder @ 1wt.% resin

6.05

-

FeSiAl powder @ 7.5 wt.%Fe3O4

6.73

Fe powder @ 7 vol.% Ni-Zn ferrite and resin Fe powder @ 1wt.% resin

Density

Happlied

Bs

µri

Ref.

[g/cm3]

[kA/m]

[T]

Fe@SiO2

7.12

8

1.39

56.2

212

This work

Fe fibres @ 0.5 wt.% resin

7.21

10

0.1

60.6

330

[13]

Fe powder @Fe3O4

7.00

469.5

0.05

12.5

86.7

[30]

Fe powder @ 1.5wt.% phosphate-polyimide

108

260

0.05

12.5

-

[31]

-

-

-

0.1

70

29

[7]

-

1.27

-

891

0.02

≈ 11

120

[32]

7.00

4

0.9

220

420

0.1

23

-

[33]

6.92

10

1.17

238

559

0.1

46

170

[13]

µrmax

9 10 11

It can be noticed that the maximum relative permeability (µrmax) and the initial relative

12

permeability (µri) of the FSMCs are superior to the values reported in the literature for SMCs.

13

This can be attributed to the lower demagnetising factor of fibres as compared to powders.

22

1

The relation between the applied magnetic field (Hext), the demagnetising factor (N), the

2

magnetic field inside the core (Hint) and the magnetization (M) of the core can be written as:

3

Hint = Hext – N·M

(1)

4

If the demagnetizing factor is high, a higher magnetic field has to be applied (Hext) to

5

reach a certain value of magnetisation [1]. This leads automatically to a lower value of

6

magnetic permeability since the relative magnetic permeability is defined as the ratio of B to

7

µ0H. The demagnetising factor has two components: the geometrical demagnetising factor

8

(influenced by the shape of the ferromagnetic phase) and the inner demagnetising factor

9

(influenced by the particle size and the amount of insulator) [1]. The variation of the

10

geometrical demagnetising factor, for different samples such as cylinders or ellipsoid (prolate

11

or oblate) is generally presented as a function of the ratio between the long axis and short

12

axis. As this ratio increase, the demagnetizing factor decrease. In a simplified approach, the

13

shape of the ferromagnetic particles, after the compaction process, can be assimilated to an

14

oblate ellipsoid. Comparing the demagnetising factor for a cylinder (our fibres) with the

15

demagnetising factor corresponding to an oblate ellipsoid (compacted ferromagnetic

16

particles) it can be remarked that the demagnetising factor of particles is one order of

17

magnitude higher as compared to the one corresponding to the fibres [34]. Moreover, as the

18

ratio between the long and short axis increases, the difference between the demagnetising

19

factors increases. Considering the above mentioned, the demagnetising factor corresponding

20

to long fibres will be much lower as compared to the demagnetizing factor of the particles.

21

The inner demagnetising factor is high in the case of SMCs based on powder as

22

compared to FSMCs due to the larger number of airgaps created inside the SMCs compacts.

23

The airgaps are created by the insulating layer that covers each particle [35, 36]. Obviously,

24

in the case of FSMCs, a drastic decrease in the number of air gaps is expected (along the

25

direction of the applied magnetic field), and consequently, the influence of the demagnetising 23

1

fields over the magnetic permeability of the compact is greatly reduced. Summarising, the

2

demagnetising factor for powder-based SMC is higher leading to lower values of magnetic

3

permeability as compared to the magnetic permeability of the FSMCs.

4

Generally, the coercive field reported in the literature for powder-based SMCs is

5

larger as compared to the one corresponding to the FSMCs. The porosity and the distributed

6

air gaps (the insulating layer that covers the particles) act as pinning centres for domain walls

7

propagation leading thus to an increased value of the coercive field.

8

The values for the main magnetic properties reported in this study are comparable

9

with those reported in literature for the composite compacts based on Fe fibres coated with

10

polymer [13].

11 12

4. Conclusions

13

The paper presents our results concerning the preparation and characterisation of new

14

types of soft magnetic composites in which the ferromagnetic particles are replaced by Fe

15

fibres. Two types of fibres were used in this study: Fe fibres coated with a layer of SiO2 and

16

Fe fibres coated with SiO2 and polymer. The SiO2 layer, in both cases, was deposited via

17

hydrothermal method. SEM and TEM investigations showed that the fibres are coated by a

18

uniform layer of SiO2 having thicknesses in the range of 150 – 200 nm. The addition of 1

19

wt.% of polymer leads to an increase of the electrical resistivity of the compacts of 10%.

20

From DC measurements we concluded that the addition of a supplementary layer of polymer

21

on top of the SiO2 layer leads to a decrease of the saturation induction (from 1.39 T to 1.26

22

T), coercive field (from 367 A/m to 353 A/m) and maximum relative permeability (from 797

23

to 733) of the compacts. AC magnetic measurements showed that the compact based on Fe

24

fibres coated with SiO2 exhibits lower hysteresis losses and the higher initial relative

25

permeability. However, as the frequency of the magnetising field increase above 1 kHz, a 24

1

decrease of the initial relative permeability of the compact was found. This was explained by

2

the development of large eddy currents in the sample due to the deterioration of the SiO2

3

layer during compaction. Conversely, the additional polymer layer leads to an increase of the

4

hysteresis losses, but with the development of excessive eddy currents into the sample being

5

prevented. This leads to almost no variation of the initial relative permeability of the

6

compacts in the investigated frequency range of 50 Hz – 10 kHz, indicating that the compact

7

can be used at even higher frequencies. The compact based on Fe fibres coated only with a

8

SiO2 layer was subjected to an annealing treatment which leads to a significant improvement

9

of its magnetic characteristics.

10

As possible applications of this material, according to its magnetic characteristics, we

11

can mention electrical motors for the automotive industry and also small to medium size

12

electrical transformers. For example, the use of such a material could be of utmost interest in

13

the realisation of the axial-flux synchronous-reluctance machines (SynRM) as evidenced in

14

ref [37].

15 16

5. Acknowledgments

17

This work was supported by a grant of the Romanian Ministery of Research and Innovation,

18

CNCS – UEFISCDI, Project number PN-III-P1-1.1-TE-2016-0649 within PNCDI III.

19

6. References

20 21

[1] E.A. Périgo, B. Weidenfeller, P. Kollár, J. Füzer, Past, present, and future of soft

22

magnetic composites, Appl. Phys. Rev. 5 (2018) 031301-37.

23

[2] K.J. Sunday, M. Taheri, Soft magnetic composites: recent advancements in the

24

technology, Met. Powder. Rep. 72 (2017) 425-429.

25

1

[3] H. Shokrollahi, K. Janghorban, Soft magnetic composite materials (SMCs), J. Mater.

2

Process. Technol. 189 (2007)1-12.

3

[4] M. Strečková, L. Medvecký, J. Füzer, P. Kollár, R.Bureš, M.Fáberová, Design of novel

4

soft magnetic composites based on Fe/resin modified with silica, J. Magn. Magn. Mater. 322

5

(2010) 1548-1551.

6

[5] M. Strečková, J. Füzer, L. Medvecký, R. Bureš, P. Kollár, M. Fáberová, V. Girman,

7

Characterization of composite materials based on Fe powder (core) and phenoleformaldehyde

8

resin (shell) modified with nanometer-sized SiO2, Bull. Mater. Sci. 37 (2014) 167-177.

9

[6] W. Xu, C. Wu, M. Yan, Preparation of Fe–Si–Ni soft magnetic composites with excellent

10

high-frequency properties, J. Magn. Magn. Mater. 381 (2015) 116–119.

11

[7] B.V. Neamtu, O. Geoffroy, I. Chicinaş, O. Isnard, AC magnetic properties of the soft

12

magnetic composites based on Supermalloy nanocrystalline powder prepared by mechanical

13

alloying, Mater. Sci. Eng. B 177 (2012) 661–665.

14

[8] B. V. Neamţu, M. Nasui, T.F. Marinca, F. Popa, I. Chicinaş, Soft magnetic composites

15

based on hybrid coated Fe-Si nanocrystalline powders, Surf. Coat. Tech. 330 (2017) 219-227.

16

[9] Y.Y. Zheng, Y.G. Wang, G.T. Xia, Amorphous soft magnetic composite-cores with

17

various orientations of the powder-flakes, J. Magn. Magn. Mater. 396 (2015) 97–101.

18

[10] L. Xiaolong, D. Yaqiang, L. Min, C. Chuntao, W. Xin-Min, New Fe-based amorphous

19

soft magnetic composites with significant enhancement of magnetic properties by

20

compositing with nano-(NiZn)Fe2O4, J. Alloy. Compd. 696 (2017) 1323 – 1328.

21

[11] L. Li, Q. Chen, Z. Gao, Y. Ge, J. Yi, Fe@SiO2@(MnZn)Fe2O4 soft magnetic composites

22

with enhanced permeability and low core loss for high-frequency applications, J. Alloy.

23

Compd. 805 (2019) 609-616.

26

1

[12] L. Xiaolong, D. Yaqiang, L. Min, C. Chuntao, W. Xin-Min, New Fe-based amorphous

2

soft magnetic composites with significant enhancement of magnetic properties by

3

compositing with nano- (NiZn)Fe2O4, J. Alloy. Compd. 696 (2017) 1323-1328.

4

[13] B.V. Neamţu, A. Opriş, P. Pszola, F. Popa, T.F. Marinca, N. Vlad, I. Chicinas,

5

Preparation and characterisation of soft magnetic composites based on Fe fibres, J. Mater.

6

Sci. 55 (2020) 1414-1424.

7

[14] M. Yaghtin, A.H. Taghvaei, B. Hashemi, K. Janghorban, Effect of heat treatment on

8

magnetic properties of iron-based soft magnetic composites with Al2O3 insulation coating

9

produced by sol–gel method, J. Alloy. Compd., 58 (2013) 293–297

10

[15] S. Wu, A. Sun, Z. Lu, C. Cheng, X. Gao, Magnetic properties of iron-based soft

11

magnetic composites with SiO2 coating obtained by reverse microemulsion method, J. Magn.

12

Magn. Mater. 381 (2015) 451–456.

13

[16] W. Li, Z. Wang, Y. Ying, J. Yu, J. Zheng, L. Qiao, S. Che, In-situ formation of Fe3O4

14

and ZrO2 coated Fe-based soft magnetic composites by hydrothermal method, Ceram. Int. 45

15

(2019) 3864-3870.

16

[17] ] A.H. Taghvaei, A. Ebrahimi, M. Ghaffari, K. Janghorban, Magnetic properties of iron-

17

based soft magnetic composites with MgO coating obtained by sol–gel method, J. Magn.

18

Magn. Mater. 322 (2010) 808–813.

19

[18] A.H. Taghvaei, H. Shokrollahi, K. Janghorban, Properties of iron-based soft magnetic

20

composite with iron phosphate–silane insulation coating, J. Alloy. Compd. 481 (2009) 681–

21

686.

22

[19] J. Li, J. Yu, W. Li, S. Che, J. Zheng, L. Qiao, Y. Ying, The preparation and magnetic

23

performance of the iron-based soft magnetic composites with the Fe@Fe3O4 powder of in situ

24

surface oxidation, J. Magn. Magn. Mater. 454 (2018) 103-109.

27

1

[20] K. J. Sunday, M. L. Taheri, NiZnCu-ferrite coated iron powder for soft magnetic

2

composite applications, J. Magn. Magn. Mater. 463 (2018) 1–6.

3

[21] J. Füzer, M. Strečková, S. Dobák, Ľ. Ďáková, P. Kollar, M. Fáberová, R. Bureš, Y.

4

Osadchuk, P. Kurek, M. Vojtko, Innovative ferrite nanofibres reinforced soft magnetic

5

composite with enhanced electrical resistivity, J. Alloy. Compd. 753 (2018) 219-227

6

[22] J. Zheng, H. Zheng, J. Lei, L. Qiao, Y. Ying, W. Cai, W. Li, J. Yu, Y. Liu, X. Huang, S.

7

Che, Structure and magnetic properties of Fe-based soft magnetic composites with an Li-Al-

8

O insulation layer obtained by hydrothermal synthesis, Article in press, J. Alloy. Compd.

9

https://doi.org/10.1016/j.jallcom.2019.152617

10

[23] F. Rubio, J. Rubio, J.L. Oteo, A FT-IR study of the hydrolysis of tetraethylorthosilicate

11

(TEOS), Spectrosc. Lett. 31(1) (1998) 199-219

12

[24] P-Y. Chu, D.E. Clark, Infrared spectroscopy of silica sols-effects of water concentration,

13

catalyst, and aging, Spectrosc. Lett. 25 (1992) 201-220.

14

[25] A. Bertoluzza, C. Fagnano, M.A. Morelli, V. Gottardi, M.Guglielmi, Raman and

15

infrared spectra on silica gel evolving toward glass, J. Non-Cryst. Solids 48 (1982) 117-128.

16

[26] J. Lei, J. Zheng, H. Zheng, L. Qiao, Y. Ying, W. Cai, W. Li, J. Yu, M. Lin, S. Che,

17

Effects of heat treatment and lubricant on magnetic properties of iron-based soft magnetic

18

composites with Al2O3 insulating layer by one-pot synthesis method, J. Magn. Magn. Mater.

19

472 (2019) 7–13.

20

[27] M. M. Dias, H. J. Mozetic, J. S. Barboza, R. M. Martins, L. Pelegrini, and L. Schaeffer,

21

Influence of resin type and content on electrical and magnetic properties of soft magnetic

22

composites (SMCs), Powder Technol. 237, 213–220 (2013).

23

[28] B.V. Neamţu, T.F. Marinca, I. Chicinaş, O. Isnard, F. Popa, P. Păşcuţă, Preparation and

24

soft magnetic properties of spark plasma sintered compacts based on Fe-Si-B glassy powder,

25

J Alloy. Compd. 600 (2014) 1-7.

28

1

[29] B.V. Neamţu, I. Chicinaş, O. Isnard, I. Ciascai, H. Chiriac, M. Lăstun, Magnetic

2

properties of nanocrystalline Ni3Fe compacts prepared by spark plasma sintering,

3

Intermetallics 35 (2013) 98-103.

4

[30] L. Qian, J. Peng, Z. Xiang, Y. Pan, W. Lu, Effect of annealing on magnetic properties of

5

Fe/Fe3O4 soft magnetic composites prepared by in-situ oxidation and hydrogen reduction

6

methods, J Alloy. Compd. 778 (2019) 712-720

7

[31] Y. Chen, L. Zhang, H. Sun, F. Chen, P. Zhang, X. Qu, E. Fan, Enhanced magnetic

8

properties of iron-based soft magnetic composites with phosphate-polyimide insulating layer,

9

J Alloy. Compd. 813 (2020) 152205-10

10

[32] F. Luo, X. Fana, Z. Luo, W. Hu, G. Li, Y. Li, X. Liu, J. Wang, Ultra-low inter-particle

11

eddy current loss of Fe3Si/Al2O3 soft magnetic composites evolved from FeSiAl/Fe3O4 core-

12

shell particles, J. Magn. Magn. Mater. 484 (2019) 218–224

13

[33] Z. Birčáková, J. Füzer, P. Kollár, M. Streckova, J. Szabó, R. Bureš, M. Fáberová,

14

Magnetic properties of Fe-based soft magnetic composite with insulation coating by resin

15

bonded Ni-Zn ferrite nanofibers, J. Magn. Magn. Mater. 485 (2019) 1–7.

16

[34] R. M. Bozorth, D. M. Chapin, Demagnetizing factors of rods, J. App. Phys. 13 (1942)

17

320-326.

18

[35] M. Anhalt, Systematic investigation of particle size dependence of magnetic properties

19

in soft magnetic composites, Journal of Magnetism and Magnetic Materials, 320 (2008)

20

e366-e369.

21

[36] P. Kollár, Z. Birčáková, V. Vojtek, J. Füzer, R. Bureš, M. Fáberová, Dependence of

22

demagnetizing fields in Fe-based composite materials on magnetic particle size and the resin

23

content, Journal of Magnetism and Magnetic Materials, 388 (2015) 76–81.

29

1

[37] A.C. Pop, F.P. Piglesan, R. Martis, I. Vintiloiu, C. Martis, First Insights on the

2

Electromagnetic Design of Axial-Flux Synchronous-Reluctance Maschine, Proceedings of

3

the IECON 2018, (2018) 5702-5709.

4 5 6 7 8 9 10 11 12 13

List of figures caption

14

Figure 1. SEM images of Fe fibres used for the FSMCs preparation. (a) Fibre’s cross-section,

15

(b) morphology of the semi-circular surface and (c) morphology of the plane surface of the

16

fibres.

17

Figure 2. FTIR analysis of the hydrolysis steps of TEOS and iron fibres before and after

18

deposition process.

19

Figure 3. EDX analysis of the fibres surfaces coated with SiO2. (a) and (e) are SEM images

20

of the plane and semi-circular surface of the fibres (b, c, d) and (f, g, h) are distribution maps

21

of the Fe, Si and O on the plane and semi-circular surface of the fibres.

22

Figure 4. TEM images (a, b) and EDX analysis (c, d) of the different sections through a fibre

23

coated with a layer of SiO2.

24

Figure 5. Comparison of hysteresis loops of FSMCs prepared from Fe fibres coated with

25

SiO2 and Fe fibres coated with SiO2 and 1 wt.% polymer (Araldite). 30

1

Figure 6. Evolution of the total core losses (a) and initial relative permeability (b) of the

2

compacts based on Fe fibres coated with SiO2 and, SiO2 and polymer. Bmax = 0.1 T, f = 50 Hz

3

– 10 kHz.

4

Figure 7. Comparison of the DC hysteresis loops of the compact based on Fe fibred coated

5

with SiO2 before and after annealing at 500 °C for 1 hour.

6

Figure 8. Influence of annealing at 500 °C for 1 hour over the core losses (a) and initial

7

relative permeability (b) of the compact based on Fe fibres coated with SiO2.

8

9

31

Highlights • • • • •

Fibres based soft magnetic composites (FSMC) were prepared and characterised; The Fe fibres were coated with a SiO2 layer via hydrothermal method; The thickness of the SiO2 layer is 150-200 nm. A hybrid coating was developed by adding a polymeric layer on top of SiO2 layer; AC and DC characterisation showed the potential of FSMCs for further applications.

1 Author declaration [Instructions: Please check all applicable boxes and provide additional information as requested.] 1. Conflict of Interest Potential conflict of interest exists: We wish to draw the attention of the Editor to the following facts, which may be considered as potential conflicts of interest, and to significant financial contributions to this work: The nature of potential conflict of interest is described below: No conflict of interest exists. We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

2. Funding Funding was received for this work. All of the sources of funding for the work described in this publication are acknowledged below: This work was supported by a Grant of the Romanian National Authority for Scientific Research CNCS – UEFISCDI, Project number PN-III-P1-1.1-TE-2016-0649.

3. Intellectual Property We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

4. Research Ethics We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant

2 bodies and that such approvals are acknowledged within the manuscript. IRB approval was obtained (required for studies and series of 3 or more cases) Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).

5. Authorship The International Committee of Medical Journal Editors (ICMJE) recommends that authorship be based on the following four criteria: 1. Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND 2. Drafting the work or revising it critically for important intellectual content; AND 3. Final approval of the version to be published; AND 4. Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All those designated as authors should meet all four criteria for authorship, and all who meet the four criteria should be identified as authors. For more information on authorship, please see http://www.icmje.org/recommendations/browse/roles-andresponsibilities/defining-the-role-of-authors-and-contributors.html#two. All listed authors meet the ICMJE criteria. We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE. B.V. Neamtu: conceived the presented idea; contributed data analysis; performed the magnetic analysis; wrote the paper. A. Belea: performed the experimental parts in collaboration with M.Nasui; F. Popa: performed the SEM-EDX analysis; E. Ware: performed the TEM-EDX analysis; T.F. Marinca: contributed to the interpretation of the results especially FTIR and M(H); I. Vintiloiu: was involved in data interpretation, C. Badea: prepared the toroidal samples and was involved in data correlation;

3 M. Pszola: helped carry out the magnetic measurements; M. Nasui: carried out the experiment, contributed data or analysis tools; performed the analysis; wrote some parts from the paper. All authors discussed the results and contributed to the final manuscript.

One or more listed authors do(es) not meet the ICMJE criteria. We believe these individuals should be listed as authors because: [Please elaborate below]

We confirm that the manuscript has been read and approved by all named authors. We confirm that the order of authors listed in the manuscript has been approved by all named authors.

6. Contact with the Editorial Office The Corresponding Author declared on the title page of the manuscript is: Corresponding Author: M.Nasui

This author submitted this manuscript using his/her account in EVISE. We understand that this Corresponding Author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that the email address shown below is accessible by the Corresponding Author, is the address to which Corresponding Author’s EVISE account is linked, and has been configured to accept email from the editorial office of American Journal of Ophthalmology Case Reports: [[email protected] ]

4 Someone other than the Corresponding Author declared above submitted this manuscript from his/her account in EVISE: No other than the Corresponding Author declared above submitted this manuscript

We understand that this author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors, including the Corresponding Author, about progress, submissions of revisions and final approval of proofs.

5

We the undersigned agree with all of the above.

Author’s name (Fist, Last)

Date

1.

Bogdan Neamtu Viorel

13.11.2019

2.

A. Belea

13.11.2019

3.

F. Popa

13.11.2019

4.

E. Ware

13.11.2019

5.

T.F. Marinca

13.11.2019

6.

I. Vintiloiu

13.11.2019

7.

C. Badea

13.11.2019

8.

M. Pszola

13.11.2019

Signature

6 9.

M. Nasui

13.11.2019