Preparation of soft magnetic composite from Fe-6.9wt%Si by
different heat treatment strategies.
Bence Kocsis1,2, Lajos Károly Varga1, Ibolya Zsoldos2
1
Wigner Research Center for Physics Institution for Solid State Physics and Optics,
29-33 Konkoly-Thege út, Budapest 1121, Hungary
2
Széchenyi István University Department of Materials Science and Technology, 1.
Egyetem tér, Győr 9026, Hungary
E-mail: kocsis.bence@sze.hu
Abstract. Present study investigated the effect of isothermal heat treatment strategies between
800 °C and 1150 °C on the magnetic properties of toroidal samples made from Fe-6.9wt%Si
powder. The samples were prepared by classical powder metallurgy method since the classical
sheet forming methods no longer work with the high silicon content. Our results presented here
are part of a series of comparative experiments where we study the effectiveness of the insulating
layers created during and before the compacting of soft magnetic composites (SMCs). Our goal
was to create a soft magnetic composite made of ferromagnetic and inorganic insulating material
with a frequency limit already in the megahertz range and a Snoek limit of few gigahertz. In the
case of samples made from Fe-6.9wt%Si powder, the computed tomography results showed that
significant porosity is to be expected after pressing. Its positive effect occurred during the heat
treatment in the atmospheric agent, where silicon is precipitated and deposited on the surface of
the particle. This coating is an electrically insulating layer at the grain boundaries. Depending
on the heat treatment strategy, 1 or 2 ferromagnetic phases were observed. The frequency limit
approached the target values, but due to the low value of static permeability, the Snoek limit did
not reach the gigahertz range. However, there is a significant improvement in magnetic
properties compared to the heat-treated samples in a protective gas.
1. Introduction
According to today’s demands of high efficient, energy-saving power electronics (motors,
transformers), the conversion and transfer of electromagnetic energy are possible only with the use of
advanced soft-magnetic materials because they have high saturation magnetization and low magnetic
loss [1]. They also have unique magnetic properties, including stable magnetic permeability at higher
frequencies and magnetic anisotropy. Fe-Si is a typical type of soft magnetic alloys that has excellent
soft magnetic properties but still exhibits a drastic increase in eddy current loss when excitation
frequency is above 400 Hz [2]. Soft magnetic composites (SMCs) consist of a core-shell structure where
the core is made up of a ferromagnetic particle and an electrically insulating layer that completely covers
it. They are typically manufactured using classical powder metallurgy techniques, but recently, the 3D
printed parts are beginning to appear. The main steps in classical powder metallurgy production are
powder production by atomization or milling, powder pressing and final heat treatment. In order to
reduce the stress anisotropy, which is caused by the high forces awaking during pressing, various heat
treatment methods are used. During 3D printing, the amount of heat transmitted by the laser generates
significant residual stress in the structure, which can also be eliminated by heat treatment.
In this study, the most widely used magnetic material in the industrial application, Fe-Si alloy was
used, within it, Fe-6.9wt%Si with outstanding properties [3]. Differently, to the usual procedure where
the coating with insulating layer precedes the compaction, our aim is to create an oxide insulating layer
around the ferromagnetic particles by heat treatment after the powder compression. The advantage of
this inorganic coating is that it remains stable at higher temperature ranges as opposed to organic
insulators. This is needed both for special applications and for miniaturization of power electronic
equipment. This can be done by increasing the frequency limit of the iron core without loss of
performance. In addition, if we can minimize dissipation losses, we can improve the energy balance of
the unit by either eliminating cooling or reducing performance. Expressed in terms of the energy stored
in the coil (L – inductance, Imax – maximum current flowing in the coil) and the magnetic field (where
μ0 is vacuum permeability), we obtain the ratio of effective permeability (μeff ) to volume (V), which
shows that maximal permeability and volume reduction give maximum saturation induction (Bmax ).
μeff
Bmax 2
=
V
μ0 ∙ Imax 2 ∙ L
We also aimed to achieve some gigahertz values of Snoek limit, which is a number for the
characterization of soft magnetic composites. In this case, the complex permeability spectrum was
obtained by multiplying the static permeability and the frequency limit. This value includes the physical
characteristics that are important to our development.
2. Material and Methods
2.1. Material
Iron-silicon alloys are one of the most prominent known soft magnetic materials in the fields of power
electronics, telecommunications, military industry, and vehicle manufacturing. When the alloy reached
6.5-6.9wt%Si content (high silicon electrical steel) it has excellent soft magnetic properties such as high
permeability, low magnetic loss and near-zero magnetostriction. Above 3wt% Si, classical rolling
technologies no longer work because the plastic deformation of the material is close to zero at room
temperature [4]. In this case, the Fe-6.9wt%Si bulk material was produced by a home built cold-crucible
induction melting equipment. The ingot of about 20 grams was powdered by grinding with a SPEX 8000
M ball mill machine. The resulting wide-size distribution of powder was separated on a sieve shaker.
We were used to the smaller powder particles than 74 μm to the final sample preparation. After the
production of powder raw material, the morphology and chemical composition of the particles were
investigated. Scanning electron microscopy (SEM) images of Fe-6,9wt%Si powder are illustrated in
Figure 1.
Si – 6.65wt%
Fe – 93.35wt%
(a) 15.0 kV 10.9 mm x300 SE
Si – 6.99wt%
Fe – 93.01wt%
Si – 6.41wt%
Fe – 93.59wt%
80 µm
(b) 15.0 kV 11.0 mm x500 SE
50 µm
Si – 6.81wt%
Fe – 93.19wt%
50 µm
50 µm
(c) 15.0 kV 11.0 mm x500 SE
(d) 15.0 kV 10.9 mm x500 SE
Figure 1. Fe-6,9wt%Si powder composition analysis by scanning electron microscope
2.2. Methods
Toroidal samples (D = ~ 12 mm, d = ~ 7mm; h = ~ 2.5 mm) were prepared in two steps. In the first step,
the powder was compressed using a conventional hydraulic press at 1.34 GPa. In the second step, after
the pressure was released, the samples were subjected to oxidation heat treatment in a chamber furnace.
This procedure was always started in a heated furnace, maintaining the isotherm conditions. Heat
treatment temperatures were 800 °C; 900 °C; 1000 °C; 1100 °C; 1150 °C. The annealing was repeated
in the air atmosphere and in the argon shielding gas. We also examined the effect of the duration of heat
treatment on the magnetic properties of the SMC. Table 1 summarizes the different heat treatment
strategies.
heat treatment time
atmosphere
Table 1. Heat treatment strategies
800 °C 900 °C 1000 °C 1100 °C
3h
3h
3h
3h
air
air
air
air
1h
air
1150 °C
3h
3h
air
Ar
5h
air
Heat treatment at 800 °C and 900 °C for 3 hours were performed in an air atmosphere, however, the
samples did not sinter properly due to the relatively low temperature and were broken during handling.
Among the magnetic properties, we analyzed the complex permeability spectrum in the frequency
range of 0.1 – 30 000 kHz, from which the product of the static permeability and the frequency limit
gives the so-called Snoek limit. The Snoek limit can be considered as a figure of merit for comparing
the SMCs from a magnetic point of view. Saturation magnetization was not part of our current
measurement series.
3. Results and discussion
Scanning electron microscopy was used to verify the effect of heat treatment strategies. The particle
morphology, coating, composition and the changes of composition were also examined.
Point 1
El AN C norm.
2
O 8 30.52
Si 14 0.40
1
Fe 26 69.07
Point 2
El AN C norm.
O 8 41.26
Si 14 19.39 SE Si Fe O
50
µm
50 µm
Fe 26 39.35 20.0 kV 12.8 mm x500
20.0 kV 12.9 mm x500
Figure 2. Scanning electron microscopy images of compacted and heat treated Fe-6,9wt%Si sample (t
= 1 h, T = 1150 °C on air). Point 1 indicates the core and point 2 indicates the location of the point
analysis of the shell structure. The figure on the right shows the elemental mapping results
Based on the SEM images it can be stated that a core-shell structure was obtained which core is Sipoor Fe. However, silicon and oxygen-rich coating appears on the grain boundaries and acts as an
electrically insulating material. This silicon diffuses from the pre-alloyed powder particles to the surface
of the grains (Fig. 2., 2. point). In addition to the silica layer on the porous sites and on the surface of
the toroidal sample, a significant volume fraction of iron oxide coating is also observed. A sample (1100
°C, 3h) which was prepared by the same conditions, except the heat treatment atmosphere, it was argon,
did not show any precipitation or composition change.
Figure 3. The heat treatment temperature (left) and time-dependence (right) of complex permeability
spectra
The magnetic permeability spectra of toroidal samples were measured using the Agilent RLC meter
type 4294 A measuring the series equivalent inductance (Ls) and resistance (Rs) data. The different
exciting field amplitudes current in a wide frequency range between 100 Hz and 30 MHz [5]. The sample
treated in argon shielding gas had a relatively high static permeability (μs= 954), but it’s limit frequency
was only flim= 1 kHz. The Snoek limit obtained by multiplying these values is only 954 kHz since SMC
had only one component, which had not electrically insulating layer that separates the particles.
On the other hand, SEM images of air-annealed samples showed a composite of at least 2
components. In this case, it is clear from the permeability spectra in Figure 3, that the temperature values
of the heat treatment strategies have no effect on the frequency limit. The reason for this is that in all
cases a sufficient size of the insulation layer is formed. However, the static permeability values
decreased from the previous 954 value to 20-55 as a result of heat treatment in air. This is presumably
due to the broadening of the B-H curve. Due to the appearance of insulating layers, which resulted in a
drastic reduction in iron loss, at the same time increased the internal demagnetization (N) factor (μeff =
1
), causing a significant decrease in effective permeability.
1
μ
+N
Figure 4. Snoek limit evolution as a function of heat treatment temperature and period of time in air
atmosphere
Figure 4 shows the evolution of the Snoek limit in each case. In contrast to the 954 kHz sample
treated with argon, the best sample treated with air was found to be 1.1 GHz. It should be mentioned
that at 1150 °C, two magnetic “phases” can be observed in our sample after 1h, 3h, and 5h heat treatment.
This is well illustrated by the local maxima of the imaginary part of the permeability spectrum. The 1.1
GHz Snoek limit came from the second higher limit frequency. If these values are not taken into account,
the sample heat-treated at 1000 °C for up to 3h proved to be the best (Snoek limit = 0.74 GHz). It is also
clear from this index that the heat treatment in the air has a positive effect on the magnetic properties of
the samples.
4. Conclusions
After these experiments, it turned out that self-sustaining toroidal samples can be sintered by heat
treating in the air at 1150 °C. The silicon content of the particles is reduced and a silicon oxide insulating
coating is formed at the interface of the particles. It can be converted in situ to soft magnetic composites.
Developing a uniform and thin coating and controlling the Si content of the particles is a rather complex
task. SEM images revealed that the internal oxidation at the grain boundaries was not complete and
consequently, there were regions in the sample where the powder particles were not electrically insulated
from each other. This heterogeneity causes the presence of two magnetic “phases” with different
insulations and different demagnetizing factors and different frequency limits. The samples treated at
1150 °C proved to be the best soft magnetic iron cores based on their Snoek limit value. With respect to
the frequency limit, the temperature of the heat treatment has no significant effect, in each case
approximately a value of 25 MHz was obtained. By increasing the duration of the heat treatment (see
Fig. 3 right), the ratio of the first magnetic “phase” with a lower limit frequency is significantly reduced.
Even with a further heat-treated sample, this magnetic phase can be completely eliminated.
In the future new ways of internal oxidation will be experimented under flowing O2+N2 and O2+Ar
and O2+CO2 gas mixtures preventing the “burning” of the samples during the long duration heat
treatment necessary for homogeneous internal oxidation of each powder particles. In further
experiments, we will sinter a pre-oxidized Fe-Si powder, the oxide layer of which is chemically formed
under controlled conditions by the Stöber method [6]. New types of alloys and insulating materials will
be introduced in the series of experiments, which will be compared with 3D printed toroidal samples.
Acknowledgements
Present study was supported by EFOP-3.6.2-16-2017-00016 “Dynamics and control of autonomous
vehicles meeting the synergy demands of automated transport systems”.
References
[1]
Luo Z, Fan X, Hu W, Luo F, Wang J, Wu Z, Liu X, Li G and Li Y 2019 Properties of
Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe6.5wt%Si/Fe3O4 composite particles J. Magn. Magn. Mater. 166278
[2]
Wu Z Y, Jiang Z, Fan X A, Zhou L J, Wang W L and Xu K 2018 Facile synthesis of Fe6.5wt%Si/SiO 2 soft magnetic composites as an efficient soft magnetic composite material at
medium and high frequencies J. Alloys Compd. 742 90–8
[3]
Garibaldi M, Ashcroft I, Hillier N, Harmon S A C and Hague R 2018 Relationship between laser
energy input, microstructures and magnetic properties of selective laser melted Fe-6.9%wt Si
soft magnets Mater. Charact. 143 144–51
[4]
Mo Y, Zhang Z, Pan H and Xie J 2016 Improved Plasticity and Cold-rolling Workability of Fe6.5wt%Si Alloy by Warm-rolling with Gradually Decreasing Temperature J. Mater. Sci.
Technol.
[5]
Varga L K and Kovac J 2018 Decomposing the permeability spectra of nanocrystalline finemet
core AIP Adv. 8 047205
[6]
Stöber W, Fink A and Bohn E 1968 Controlled growth of monodisperse silica spheres in the
micron size range J. Colloid Interface Sci. 26 62–9
Preparation of soft magnetic composite from Fe-6.9wt%Si by
different heat treatment strategies.
Bence Kocsis1,2, Lajos Károly Varga1, Ibolya Zsoldos2
1
Wigner Research Center for Physics Institution for Solid State Physics and Optics,
29-33 Konkoly-Thege út, Budapest 1121, Hungary
2
Széchenyi István University Department of Materials Science and Technology, 1.
Egyetem tér, Győr 9026, Hungary
E-mail: kocsis.bence@sze.hu
Abstract. Present study investigated the effect of isothermal heat treatment strategies between
800 °C and 1150 °C on the magnetic properties of toroidal samples made from Fe-6.9wt%Si
powder. The samples were prepared by classical powder metallurgy method since the classical
sheet forming methods no longer work with the high silicon content. Our results presented here
are part of a series of comparative experiments where we study the effectiveness of the insulating
layers created during and before the compacting of soft magnetic composites (SMCs). Our goal
was to create a soft magnetic composite made of ferromagnetic and inorganic insulating material
with a frequency limit already in the megahertz range and a Snoek limit of few gigahertz. In the
case of samples made from Fe-6.9wt%Si powder, the computed tomography results showed that
significant porosity is to be expected after pressing. Its positive effect occurred during the heat
treatment in the atmospheric agent, where silicon is precipitated and deposited on the surface of
the particle. This coating is an electrically insulating layer at the grain boundaries. Depending
on the heat treatment strategy, 1 or 2 ferromagnetic phases were observed. The frequency limit
approached the target values, but due to the low value of static permeability, the Snoek limit did
not reach the gigahertz range. However, there is a significant improvement in magnetic
properties compared to the heat-treated samples in a protective gas.
1. Introduction
According to today’s demandsDevelopment of high efficient, energy- saving power electronics such as(
motors, or transformers),, the conversion and transfer of electromagnetic energy areis possible only with
the use of advanced soft-magnetic materials because, as they have the benefits of high saturation
magnetization and low magnetic loss [1]. They also have unique magnetic properties, including stable
magnetic permeability at higher frequencies and magnetic anisotropy. Fe-Si is a typical type of soft
magnetic alloys thatwhich has excellent soft magnetic properties, but still exhibits a drastic increase in
eddy current loss when excitation frequency is above 400 Hz [2]. Soft magnetic composites (SMCs)
consist of a core-shell structure where the core is made up of a ferromagnetic particle and an electrically
insulating layer that completely covers it. They are typically manufactured using classical powder
metallurgy techniques, but recently, the 3D printed parts are beginning to appear. The main steps in
classical powder metallurgy production are powder production by atomization or milling, powder
pressing and finally heat treatment. In order to reduce the stress anisotropy, which is caused by the high
forces awaking during pressing, various heat treatment methods are used. During 3D printing, the
Formatted: Space After: 0 pt
amount of heat transmitted by the laser generates significant residual stress in the structure, which can
also be eliminated by heat treatment.
In this study, the most widely used magnetic material in the industrial application, Fe-Si alloy was
used, within it, Fe-6.9wt%Si with outstanding properties [3]. Differently, to the usual procedure where
the coating with insulating layer precedes the compaction, our aim is to create an oxide insulating layer
around the ferromagnetic particles by heat treatment after the powder compression. The advantage of
this inorganic coating is that it remains stable at higher temperature ranges as opposed to organic
insulators. This is needed both for special applications and for miniaturization of power electronic
equipments. This can be done by increasing the frequency limit of the iron core without loss of
performance. In addition, if we can minimize dissipation losses, we can improve the energy balance of
the unit by either eliminating cooling or reducing performance. Expressed in terms of the energy stored
in the coil (L – inductance, Imax – maximum current flowing in the coil) and the magnetic field (where
μ0 is vacuum permeability), we obtain the ratio of effective permeability (μeff ) to volume (V), which
shows that maximal permeability and volume reduction give maximum saturation induction (Bmax ).
μeff
Bmax 2
=
V
μ0 ∙ Imax 2 ∙ L
We also aimed to achieve some gigahertz values of Snoek limit, which is a number for the
characterization of soft magnetic composites. In this case, the complex permeability spectrum was
obtained by multiplying the static permeability and the frequency limit. This value includes the physical
characteristics that are important to our development.
2. Material and Methods
2.1. Material
Iron-silicon alloys are one of the most prominent known soft magnetic materials in the fields of power
electronics, telecommunications, military industry, and vehicle manufacturing. When the alloy reached
6.5-6.9wt%Si content (high silicon electrical steel) it has excellent soft magnetic properties such as high
permeability, low magnetic loss and near- zero magnetostriction. Above 3wt% Si, classical rolling
technologies no longer work because the plastic deformation of the material is close to zero at room
temperature [4]. In this case, the Fe-6.9wt%Si bulk material was produced by a home built cold-crucible
induction melting equipment. The ingot of about 20 grams wasere powderized by grinding with a SPEX
8000 M ball mill machine. The resulting wide-size distribution of powder was separated on a sieve
shaker. and Wewe were used to the smaller powder particles than 74 μm to the final sample preparation.
After the production of powder raw material, the morphology and chemical composition of the particles
were investigated. Scanning electron microscopy (SEM) images of Fe-6,9wt%Si powder are illustrated
in Figure 1.
Formatted: Subscript
Si – 6.65wt%
Fe – 93.35wt%
(a) 15.0 kV 10.9 mm x300 SE
Si – 6.41wt%
Fe – 93.59wt%
80 µm
Si – 6.99wt%
Fe – 93.01wt%
(c) 15.0 kV 11.0 mm x500 SE
(b) 15.0 kV 11.0 mm x500 SE
50 µm
Si – 6.81wt%
Fe – 93.19wt%
50 µm
(d) 15.0 kV 10.9 mm x500 SE
50 µm
Figure 1. Fe-6,9wt%Si powder composition analysis by scanning electron microscope
Formatted: Space After: 6 pt
2.2. Methods
Toroidal samples (D = ~ 12 mm, d = ~ 7mm; h = ~ 2.5 mm) were prepared in two steps. In the first step,
the powder was compressed using a conventional hydraulic press at 1.34 GPa. In the second step, after
the pressure was released, the samples were subjected to oxidation heat treatment in a chamber furnace.
This procedure was always started in a heated furnace, maintaining the isotherm conditions. Heat
treatment temperatures were 800 °C; 900 °C; 1000 °C; 1100 °C; 1150 °C. The annealing werewas
repeated in the air atmosphere and in the argon shielding gas. We also examined the effect of the duration
of heat treatment on the magnetic properties of the SMC. Table 1 summarizes the different heat
treatment strategies.
heat treatment time
atmosphere
Table 1. Heat treatment strategies
800 °C 900 °C 1000 °C 1100 °C
3h
3h
3h
3h
air
air
air
air
1h
air
1150 °C
3h
3h
air
Ar
5h
air
Heat treatment at 800 °C and 900 °C for 3 hours werewas performed in an air atmosphere, however,
the samples did not sinter properly due to the relatively low temperature and were broken during
handling.
Among the magnetic properties, we analyzed the complex permeability spectrum in the frequency
range of 0.1 – 30 000 kHz, from which the product of the static permeability and the frequency limit
gives the so-called Snoek limit. The Snoek limit can be considered as a figure of merit for comparing
the SMCs from a magnetic point of view. Saturation magnetization was not part of our current
measurement series.
3. Results and discussion
Scanning electron microscopy was used to verify the effect of the heat treatment strategies. The particle
morphology, coating, composition and the changes of composition were also examined.
2
1
20.0 kV 12.9 mm x500
Point 1
El AN C norm.
O 8 30.52
Si 14 0.40
Fe 26 69.07
Point 2
El AN C norm.
O 8 41.26
Si 14 19.39 SE Si Fe O
50 µm Fe 26 39.35 20.0 kV 12.8 mm x500
540
µm
Figure 2. Scanning electron microscopy images of compacted and heat treated Fe-6,9wt%Si sample (t
= 1 h, T = 1150 °C on air). Point 1 indicates the core and point 2 indicates the location of the point
analysis of the shell structure. The figure on the right shows the elemental mapping results
Based on the SEM images it can be stated that a core-shell structure was obtained which core is Sipoor Fe. However, a silicon and oxygen-rich coating appears on the grain boundaries and acts as an
electrically insulating material. This silicon diffuses from the pre-alloyed powder particles to the surface
of the grains (Fig. 2., 2. point). In addition to the silica layer on the porous sites and on the surface of
the toroidal sample, a significant volume fraction of iron oxide coating is also observed. A sample (1100
°C, 3h) which was prepared by the same conditions, except the heat treatment atmosphere, it was argon,
did not show any precipitation or composition change.
Formatted: Centered
Formatted: Space After: 6 pt
Figure 3. The Hheat treatment temperature (left) and time- dependence (right) of complex
permeability spectra.
The magnetic permeability spectra of toroidal samples were measured using the Aglilent RLC meter
type 4294 A measuring the series equivalent inductance (L s) and resistance (Rs) data. The for different
exciting field amplitudes current in a wide frequency range between 100 Hz and 30 MHz [5]. The sample
treated in argon shielding gas had a relatively high static permeability (μ s= 954), but it’s limit frequency
was only flim= 1 kHz. The Snoek limit obtained by multiplying these values is only 954 kHz, since SMC
had only one component, which had not electrically insulating layer that separates the particles.
On the other hand, SEM images of air-annealed samples showed a composite of at least 2
components. In this case, it is clear from the permeability spectra in Figure 3, that the temperature values
of the heat treatment strategies have no effect on the frequency limit. The reason for this is that in all
cases a sufficient size of the insulation layer is formed. However, the static permeability values
decreased from the previous 954 value to 20-55 as a result of heat treatment in air. This is presumably
due to the broadening of the B-H curve. Due to the appearance of insulating layers, which resulted in a
drastic reduction in iron loss, at the same time increased the internal demagnetization (N) factor (μeff =
1
), causing a significant decrease in effective permeability.
1
μ
+N
Formatted: Space After: 6 pt
Figure 4. Snoek limit evolution as a function of heat treatment temperature and period of time in air
atmosphere
Formatted: Space After: 6 pt
Figure 4 shows the evolution of the Snoek limit in each case. In contrast to the 954 kHz sample
treated with argon, the best sample treated with air was found to be 1.1 GHz. It should be mentioned
that at 1150 °C, two magnetic “phases” can be observed in our sample after 1h, 3h, and 5 h heat
treatment. This is well illustrated by the local maxima of the imaginary part of the permeability
spectrum. The 1.1 GHz Snoek limit came from the second higher limit frequency. If these values are
not taken into account, the sample heat-treated at 1000 °C for up to 3h proved to be the best (Snoek limit
= 0.74 GHz). It is also clear from this index that the heat treatment in the air has a positive effect on the
magnetic properties of the samples.
4. Conclusions
After these experiments, it turned out that self-sustaining toroidal samples can be sintered by heat
treating in the air at 1150 °C. The silicon content of the particles is reduced and a silicon oxide insulating
coating is formed at the interface of the particles. It can be converted in situ to soft magnetic composites.
Developing a uniform and thin coating and controlling the Si content of the particles is a rather complex
task. SEM images revealed that the internal oxidation at the grain boundaries was not complete and
consequently, there were regions in the sample where the powder particles were not electrically insulated
from each other. This heterogeneity causes the presence of two magnetic “phases” with different
insulations and different demagnetizing factors and different frequency limits. The samples treated at
1150 ° C proved to be the best soft magnetic iron cores based on their Snoek limit value. With respect
to the frequency limit, the temperature of the heat treatment has no significant effect, in each case
approximately a value of 25 MHz was obtained. By increasing the duration of the heat treatment (see
Fig. 3 right), the ratio of the first magnetic “phase” with a lower limit frequency is significantly reduced.
Even with a further heat-treated sample, this magnetic phase can be completely eliminated.
In the future new ways of internal oxidation will be experimented under flowing O 2+N2 and O2+Ar
and O2+CO2 gas mixtures preventing the “burning” of the samples during the long duration heat
treatment necessary for homogeneous internal oxidation of each powder particles. In further
experiments, we will sinter a pre-oxidized Fe-Si powder, the oxide layer of which is chemically formed
under controlled conditions by the Stöber method [6]. New types of alloys and insulating materials will
be introduced in the series of experiments, which will be compared with 3D printed toroidal samples.
Acknowledgements
Present study was supported by EFOP-3.6.2-16-2017-00016 “ Dynamics and control of autonomous
vehicles meeting the synergy demands of automated transport systems”.
References
[1]
Luo Z, Fan X, Hu W, Luo F, Wang J, Wu Z, Liu X, Li G and Li Y 2019 Properties of
Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe6.5wt%Si/Fe3O4 composite particles J. Magn. Magn. Mater. 166278
[2]
Wu Z Y, Jiang Z, Fan X A, Zhou L J, Wang W L and Xu K 2018 Facile synthesis of Fe6.5wt%Si/SiO 2 soft magnetic composites as an efficient soft magnetic composite material at
medium and high frequencies J. Alloys Compd. 742 90–8
[3]
Garibaldi M, Ashcroft I, Hillier N, Harmon S A C and Hague R 2018 Relationship between laser
energy input, microstructures and magnetic properties of selective laser melted Fe-6.9%wt Si
soft magnets Mater. Charact. 143 144–51
[4]
Mo Y, Zhang Z, Pan H and Xie J 2016 Improved Plasticity and Cold-rolling Workability of Fe6.5wt%Si Alloy by Warm-rolling with Gradually Decreasing Temperature J. Mater. Sci.
Technol.
[5]
Varga L K and Kovac J 2018 Decomposing the permeability spectra of nanocrystalline finemet
Formatted: Reference, Indent: Left: 0 cm, First line: 0
cm, Widow/Orphan control, Adjust space between Latin
and Asian text, Adjust space between Asian text and
numbers
[6]
core AIP Adv. 8 047205
Stöber W, Fink A and Bohn E 1968 Controlled growth of monodisperse silica spheres in the
micron size range J. Colloid Interface Sci. 26 62–9
Dear Editors and Reviewers,
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled
„Preparation of soft magnetic composite from Fe-6.9wt%Si by different heat treatment strategies”.
Those comments are all valuable and very helpful for revising and improving our paper, as well as the
important guiding significance to our researches. We have studied comments carefully and have made
correction which we hope meet with approval. Revised portion are marked in blue. The main
corrections in the paper and the responds to the Editor’s and reviewer’s comments are as flowing:
•
Editorial comments correction:
o Figures 1,2,3,4 – captions are too small, their size should be standardized, scale bars
size should be increased, currently unreadably small.
I increased the size of captions and markings in all 4 figures and I also increased the
size of scale bars and marking labels.
o Affiliations and correspond author’s email inappropriate, reference list format is
incorrect.
I formatted the affiliation paragraph and set the appropriate spacing and indentation.
Correspond author’s email address has been added.
•
Reviewer 1. comments and my corrections:
o Please give the meaning of all quantities present in the expression for ration of the
effective permeability and the volume.
Thank you for your comment. I supplemented the text part of the formula with the
names of the missing quantities.
Expressed in terms of the energy stored in the coil (L – inductance, Imax – maximum current
flowing in the coil) and the magnetic field (where 𝜇0 is vacuum permeability), we obtain the
ratio of effective permeability (𝜇𝑒𝑓𝑓 ) to volume (V), which shows that maximal permeability and
volume reduction give maximum saturation induction (𝐵𝑚𝑎𝑥 ).
𝜇𝑒𝑓𝑓
𝐵𝑚𝑎𝑥 2
=
𝑉
𝜇0 ∙ 𝐼𝑚𝑎𝑥 2 ∙ 𝐿
o
Figure 2: the figure caption should be extended: explain that the right hand side is a
composition map and which colour corresponds to which element, because the
notations in the insert is not well visible. The same applies to the numbers 1 and 2 on
the left hand side figure.
Based on this- and editorial comments I increased the marking labels and captions in
all 4 figures included figure 2. Now the composition map element marking labels are
visible and it is not necessary to explain the color coding of the elements separately in
the caption.
2
1
20.0 kV 12.9 mm x500
Point 1
El AN C norm.
O 8 30.52
Si 14 0.40
Fe 26 69.07
Point 2
El AN C norm.
O 8 41.26
Si 14 19.39 SE Si Fe O
50 µm Fe 26 39.35 20.0 kV 12.8 mm x500
50 µm
Figure 2. Scanning electron microscopy images of compacted and heat treated Fe-6,9wt%Si sample (t
= 1 h, T = 1150 °C on air). Point 1 indicates the core and point 2 indicates the location of the point
analysis of the shell structure. The figure on the right shows the elemental mapping results.
o
I recommend to check the English and correct the misprints (e.g. in the third sentence
below Fig.2) and some awkward formulations (e.g. “and we used the smaller powder
particles than 74 µm to the final sample preparation.
I checked the English and corrected the misprints and some awkward formulations.
The corrections and awkward formulations changes can be found in detail in the
corrected version of the manuscript. Some example of changes:
…We were used to the smaller powder particles than 74 μm to the final sample
preparation.
…power electronic equipments.
According to today’s demands of high efficient, energy-saving power electronics
(motors, transformers), the conversion and transfer of electromagnetic energy are
possible only with the use of advanced soft-magnetic materials because they have
high saturation magnetization and low magnetic loss [1].
The magnetic permeability spectra of toroidal samples were measured using the
Aglilent RLC…
•
Reviewer 2. comments and my corrections:
o English language and style are fine/minor spell check required.
I checked the English and corrected the misprints and some awkward formulations.
The corrections and awkward formulations changes can be found in detail in the
corrected version of the manuscript.