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Article

Experimental Study on Electromagnetic Shielding Characteristics of a Fe-Based Amorphous Soft Magnetic Composite

by
Jaehoon Yoo
1 and
Sangmin Lee
2,*
1
Department of Protection and Safety Engineering, Seoul National University of Science & Technology, Seoul 01811, Korea
2
Center for Security and Strategy, Korea Institute for Defense Analyses, Seoul 02455, Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(12), 6158; https://doi.org/10.3390/app12126158
Submission received: 19 May 2022 / Revised: 12 June 2022 / Accepted: 15 June 2022 / Published: 17 June 2022
(This article belongs to the Special Issue Innovative Protection Facility and CBRNE Effects)
Browse Figures
Versions Notes

Abstract

:
An Fe-based amorphous soft magnetic composite with flexibility and elasticity was fabricated to shield harmful electromagnetic waves in industrial and military defense applications. Through the combination and structural arrangement of the amorphous soft magnetic sheet and the conductive sheet, the inlet (POE) form of electromagnetic waves was artificially diversified, and shielding performance was measured according to the criteria of MIL-STD-188-125-1 in the range from 1 kHz to 10 GHz, in consideration of the electromagnetic pulse (EMP) protection. As a result, the shielding effectiveness of 80 dB was achieved in a triple “sandwiched” structure by alternately stacking an iron-based amorphous soft magnetic material on top of a flexible conductive sheet at a 90-degree angle, rather than in parallel.

1. Introduction

Fe-based amorphous soft magnetic composites were developed to prevent the malfunction of electronic devices due to electromagnetic interference in automobiles, ships and airplanes, as well as to shield a facility from electromagnetic pulse (EMP) activity. As the development of automatic-driving automobiles accelerates, and electric vehicles are being equipped with high-power motors, the density of electronics in automobiles has been increasing rapidly [1,2,3]. Electromagnetic waves emitted from the electrical signal cables in the car may cause interference, such as causing engines or electrical motors to accelerate without cause, and lead to an accident [4,5,6,7]. In 1962, the U.S. Navy’s John Forrestal aircraft carrier was on the brink of sinking from a huge fire as a result of malfunction in a missile due to electromagnetic interference [8].
Electronic and information communication devices, including computers and mobile phones, are small and typically transportable. Accordingly, electronic components have become more highly integrated, and the signal processing speed has accelerated as well. One of the most important factors that affects the reliability of these electronics and components is electromagnetic interference (EMI) [9,10,11,12]. Interference caused by electromagnetic noise can cause malfunctions of the electronic equipment, as well as potential health problems (e.g., disease) by increasing the temperature of cells in a human body [13,14,15,16]. Therefore, many countries have identified electromagnetic waves as carcinogens and are strengthening their regulations [17,18,19,20,21].
Meanwhile, electromagnetic compatibility (EMC) protects the inside of electronic devices from electromagnetic noise generated in external environments [22]. With the rapid development of electronic devices, the demand for these EMI/EMC-related materials and components is growing, and electromagnetic wave shielding technologies have been developed as well [23,24,25,26,27,28,29].
Materials that prevent or reduce electromagnetic noise have been widely used in electronic devices and have recently been expanded to many new electronics, such as televisions, personal computers, smart mobile devices and phones, etc. [30]. Conventionally, the main module of the device was shielded with a thin film, such as copper, a conductive material, for protection against electromagnetic waves [31]. However, high shielding efficiency using only conductive materials for electromagnetic waves has limited application for the waves generated by magnetic fields. Therefore, conventional shielding materials were replaced with soft magnetic materials capable of shielding both the magnetic and electric fields’ excellent permeability [32,33,34]. Table 1 shows the shielding effects of a representative soft magnetic material as compared to the conductive materials.
Magnetic field shielding is more important than electric field shielding due to the frequency band of electromagnetic waves emitted from electronic devices being relatively low in the kHz range. However, electromagnetic wave shielding could be effective with only a conductive material, as the frequency increases beyond the MHz region.
It is essential to use an electrical conductor with soft magnetic characteristics for the purpose of reducing the electromagnetic wave noise generated inside electronic devices and protecting them from external EMP attacks [35,36,37,38]. Therefore, an amorphous soft magnetic material with high permeability and excellent electrical conductivity in the kHz frequency range was adopted as an electromagnetic wave-shielding material in this study. Table 2 shows soft magnetic properties of the magnetic specimen with high permeability and low coercivity.
The objective of this study was to measure electromagnetic wave-shielding efficiency under various conditions using an amorphous soft magnetic sheet, which was a precursor (base material) for the amorphous soft magnetic fibers, and to identify the optimal composite material production. We improved the flexibility and elasticity of the material by braiding the amorphous soft magnetic fibers containing more than Fe of 70 at.%, considering the magnetic properties and mechanical characteristics. In this paper, the potential of thin, light EMP protection materials applicable in the frequency range of 1 kHz–10 GHz was demonstrated.

2. Experiments

2.1. Test Specimens

The amorphous soft magnetic sheets were manufactured by the melt-spinning method, and they were 20–25 μm thick and 210 mm wide. It was necessary to measure the electromagnetic wave shielding effectiveness (SE) of the base material sheet first, since this was cut into thin pieces to obtain soft magnetic fibers. The point-of-entry (POE) shape of electromagnetic waves was artificially diversified through various combinations and structural arrangements of amorphous soft magnetic sheets and conductive sheets. The shielding effectiveness (SE) was measured in a range from 1 kHz to 10 GHz according to the criteria of MIL-STD-188-125-1 and MIL-DTL-83528D. Protection frequency range against nuclear EMP threat had been required for up to 1 GHz, but it was recently extended to 10 GHz against non-nuclear EMP threat. In this study, the shielding region was evaluated for up to the maximum 10 GHz to protect against both nuclear and non-nuclear EMP threat.
Electromagnetic waves can be passed through the space (holes) between the fabrics depending on the frequency if the fabric is made of amorphous soft magnetic fibers. It was assumed that the shielding efficiency would be improved if the holes were blocked by a flexible conductive material. In other words, we expected to satisfy the shielding performance with a composite material composed of an amorphous soft magnetic material and a conductive material.
The experiment was conducted under the following conditions:
First, a long slit-shaped defect was intentionally formed between the amorphous soft magnetic sheets without the addition of the conductive material layer. Then, a vertical wave and a horizontal wave were irradiated to the slits according to the criteria of MIL-STD-188-125-1 and MIL-DTL-83528D. As a result of the SE test, no significant values were obtained without a conductive layer.
Second, several amorphous soft magnetic sheets with various slit conditions were prepared to analyze the influence of the conductive layer on SE. Adhesive conductive tape was added to the soft magnetic sheet to produce composite materials, as shown in Figure 1. The effect of the conductive tape on the results was negligible. To increase the accuracy of SE measurement, the size of the specimen was manufactured to be 1000 mm in width and length.
Third, the best composite material satisfying the SE of 80 dB was selected for fibers in the next steps. In this paper, only the preceding research necessary for the optimal fiber composite material candidate was conducted.
A specimen of a multilayer structure was manufactured by adding conductive tapes of various widths or varying the overlapping method of the slits of a soft magnetic sheet, as shown in Table 3.
As shown in Figure 2, conductive tape lengths with widths of 0, 20, 50 and 100 mm were attached to the slits of the amorphous soft magnetic sheet, in specimens #1 to #4, respectively. In specimens #5 and #6, conductive tape with widths of 50 and 100 mm was attached to the slit of the amorphous soft magnetic sheet, and an additional conductive tape with a width of 20 mm was overlapped. In specimen #7, conductive tape was superimposed between amorphous soft magnetic sheets in a sandwich structure. In specimen #8, conductive tape was stacked between amorphous soft magnetic sheets in a sandwiched structure at an angle of 90 degrees.

2.2. Shielding Effectiveness (SE) Test

The low-frequency magnetic field shielding effect of the material was measured by referring to the MIL-DTL-83528D standard that utilized the opening surface of the shield room wall and the MIL-STD-188-125-1 standard, which presented the low-frequency shielding effect measurement standard, as shown in Figure 3. In order to identify the difference between vertical and horizontal polarization, the shielding performance was measured while rotating the electromagnetic wave generator at 90 degrees.
The aperture shielding effectiveness (SE) was calculated by the ratio of the reference mutual inductance (Mi) and the aperture mutual inductance (Ms), as shown in Equation (1) [41].
S E = 20 log 10 ( M i M s )
An effort was made to minimize the measurement error in the entire range of the SE test by making the opening size of the measuring device 1000 mm in width and length. As the size of the opening decreased, the measurement error increased. Therefore, it was possible to increase the accuracy by making a specimen as large as possible.

3. Test Results

3.1. Analysis of Vertical/Horizontal Wave Measurement Results of Eight Specimens

As shown in Figure 4, a maximum 40 dB difference occurred in the 0.1 MHz–300 MHz band compared to the blue line, indicating that the shielding efficiency was high for the vertical (V) wave in the slit direction. That is, since the long slits showed different shielding efficiencies for horizontal and vertical waves, countermeasures for the slit direction were required to increase total shielding efficiency.
The consistent slope from 0.01 MHz to 10 MHz in the shielding standard was attributed to being adjusted to a reasonable reference value, as it was difficult to shield a magnetic field in this range. In addition, since the energy value of the electromagnetic wave was proportional to the Planck constant and frequency, the energy in the low-frequency region was low, so it was judged to be safe despite having low shielding efficiency.
As shown in Figure 5, as a result of placing the conductive tape between the amorphous soft magnetic sheets overlapped with a width of 20 mm to increase the shielding efficiency at the slit, the overall shielding efficiency increased by more than 10 dB in the horizontal and vertical directions. However, there was still a difference in shielding efficiency in the horizontal and vertical directions in the 0.1 MHz–300 MHz band.
As shown in Figure 6, as a result of increasing the width of the overlapping portion of the conductive tape from 20 mm to 50 mm, the overall shielding efficiency increased by more than 10 dB, and the difference between the horizontal and vertical shielding efficiency in the 1 MHz–300 MHz band was reduced.
As shown in Figure 7, the overall shielding efficiency was increased by more than 10 dB by increasing the width of the overlapping sections to 100 mm, and the deviation in the horizontal and vertical directions was reduced in the 0.1 MHz–300 MHz band.
In Figure 8, the shielding efficiency was dramatically increased by attaching an additional conductive tape to the overlapping area. There was a section in which a reversal phenomenon was noted in the horizontal and vertical shielding efficiencies, which showed a significant difference in the 0.1 MHz–300 MHz band, but the overall efficiency values were consistent.
In Figure 9, the overall shielding efficiency was improved as the overlapping area increased by attaching additional conductive tape in an alternating pattern. The reason that the horizontal shielding efficiency was lower than that of the specimen overlapped by 50 mm in the 0.1 MHz–10 MHz band may have been due to manufacturing errors. However, in the other parts, the shielding efficiency was higher when the overlapping specimen was 100 mm, as compared to 50 mm.
As shown in Figure 10, specimen #7 met the shielding efficiency criteria in all bands. Since this specimen was manufactured with a sandwiched structure, the efficiency increased, as there were twice as many soft magnetic layers as other materials. The reason that gaps still occurred in the horizontal and vertical directions in the 0.1 MHz–300 MHz band was attributed to the amorphous soft magnetic layers being arranged in parallel.
The reason that the shielding efficiency increased consistently, up to 100 MHz, in most specimens and then decreased sharply was attributed to the frequency dependence of magnetic permeability. Permeability is the frequency sensitivity of electron spin; magnetic permeability is dependent on the frequency, and frequency dependence is different depending on soft magnetic materials. Nevertheless, a shielding efficiency of 80 dB or more was likely exhibited, even if the magnetic permeability had decreased due to electric field shielding being more efficient at 10 MHz or higher, as compared to magnetic field shielding.
In Figure 11, specimen #8 had a sandwiched structure, and the upper and lower layers were stacked at an angle of 90 degrees, so that the horizontal and vertical measurements were almost identical. In addition, the optimal condition was found by measuring 10–20 dB higher than the reference value in all areas. When the gap between the soft magnetic layers had a long slit shape, it was difficult to increase the shielding efficiency for the electromagnetic waves parallel to the slit direction. Therefore, the shielding efficiency may have been improved, regardless of the horizontal and vertical directions, due to the long slit POE not being formed by stacking it in a sandwiched structure at an angle of 90 degrees. As a result, the amorphous soft magnetic fiber composite material manufactured in this study could provide the highest shielding efficiency when the soft magnetic fibers were crossed at an angle of 90 degrees.

3.2. Discussion

As a result of measuring the shielding efficiency for each frequency band as electromagnetic waves passed through the slit of the amorphous soft magnetic sheet, we determined that the waves could pass more easily through the slit in the horizontal direction than in the vertical direction. Therefore, the electromagnetic wave passed through the space between the fibers in the fabric of the amorphous soft magnetic fiber in which the space between the fabrics had not been properly shielded.
In addition, the shielding effect increased as the area of the multi-layered conductive tape attached to the slit of the amorphous soft magnetic sheet increased and was overlapped. Furthermore, it was important that the shielding efficiency increased when the amorphous soft magnetic sheets were stacked alternately at a 90-degree angle rather than when stacked in parallel.
Finally, when manufacturing an amorphous soft magnetic composite material, the gap (slit) must be filled with a conductive material, such as conductive tape, so that there is no space for electromagnetic waves to pass through.
It was difficult to find similar existing research data, so direct comparisons of the results of this study with existing studies were limited. Moreover, this research pioneered a new direction in the field of EMP protection technology by showing the potential of light, flexible electromagnetic shielding materials.
In future research, we will examine the effect of the Fe content of soft magnetic material on its electromagnetic wave-shielding properties in relation to applications in low-frequency regions where magnetic properties are critical.

4. Conclusions

To use Fe-based amorphous soft magnetic composite materials for electromagnetic wave-shielding in industrial applications as well as others, increasing the flexibility and elasticity by weaving the fibers into a narrow width braid was evaluated. However, when the weave of the amorphous soft magnetic fiber was too narrow, the length of the slits between the fibers was reduced, but the number of slits was inevitably increased.
By measuring the electromagnetic wave-shielding performance under various conditions, we confirmed that electromagnetic waves passed through the space between the fibers in fabrics without additional shielding materials. Even if the amorphous soft magnetic fibers and the conductive sheet were not woven together, the shielding efficiency was increased by adding the conductive sheet to the space between the fabric made of the amorphous soft magnetic fibers, and the shielding efficiency was proportional to the area of the conductive sheet and whether it overlapped.
Conductive sheets, which have been widely used in the past, have low magnetic field shielding efficiency in low-frequency regions. Therefore, it was effective to use a composite material by appropriately combining amorphous soft magnetic fibers and conductive sheets in order to fulfill the electromagnetic wave-shielding efficiency in an extended range from 1 kHz to 10 GHz.
The shielding effectiveness of 80 dB was achieved in a triple sandwiched structure by alternately stacking an Fe-based amorphous soft magnetic material on top of a flexible conductive sheet at a 90-degree angle, rather than in parallel. After measuring the shielding efficiency of various combinations of amorphous soft magnetic sheets and conductive sheets, the triple structure composed of an amorphous soft magnetic sheet, a conductive sheet and an amorphous soft magnetic sheet, again, stacked alternately at an angle of 90 degrees, was optimal for electromagnetic waves in vertical and horizontal directions.
In future research, the role of the conductive sheet could be accurately identified by measuring the electromagnetic wave-shielding efficiency when woven with only amorphous soft magnetic fibers at an angle of 90 degrees.
This study showed that conductive tape compensated for the defects in the long slits of the amorphous soft magnetic material. This novel composite material has potential in cable shielding, in which the amorphous soft magnetic fiber is wound in one direction on the outside of the electric wire to shield it.
Given the growing interest in space exploration worldwide, electromagnetic wave-shielding will continue to be a challenge. This thin, light, flexible material for electromagnetic wave-shielding has potential for use in artificial satellites in the future. Since the soft magnetic material used in this study was composed of iron and boron, its application could also protect against harmful radiation, such as gamma rays and neutrons.

Author Contributions

Conceptualization, S.L.; methodology, S.L.; software, S.L.; validation, S.L. and J.Y.; formal analysis, S.L. and J.Y.; investigation, S.L. and J.Y.; resources, S.L.; data curation, S.L.; writing—original draft preparation, S.L. and J.Y.; writing—review and editing, S.L.; visualization, S.L. and J.Y.; supervision, S.L.; project administration, S.L. and J.Y.; funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Acknowledgments

This study was performed with support from SPACE & BEAN and Seoul National University of Science and Technology in South Korea.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 06158 g001 550
Figure 1. Top and side views of the amorphous soft magnetic composite specimen.
Figure 1. Top and side views of the amorphous soft magnetic composite specimen.
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Figure 2. Shape of the specimen.
Figure 2. Shape of the specimen.
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Figure 3. Experimental instruments.
Figure 3. Experimental instruments.
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Figure 4. Shielding effectiveness of specimen #1(W = 0).
Figure 4. Shielding effectiveness of specimen #1(W = 0).
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Figure 5. Shielding effectiveness of specimen #2 (W = 20).
Figure 5. Shielding effectiveness of specimen #2 (W = 20).
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Figure 6. Shielding effectiveness of specimen #3 (W = 50).
Figure 6. Shielding effectiveness of specimen #3 (W = 50).
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Figure 7. Shielding effectiveness of specimen #4 (W = 100).
Figure 7. Shielding effectiveness of specimen #4 (W = 100).
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Figure 8. Shielding effectiveness of specimen #5 (W1 = 50, W2 = 20).
Figure 8. Shielding effectiveness of specimen #5 (W1 = 50, W2 = 20).
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Figure 9. Shielding effectiveness of specimen #6 (W1 = 100, W2 = 20).
Figure 9. Shielding effectiveness of specimen #6 (W1 = 100, W2 = 20).
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Figure 10. Shielding effectiveness of specimen #7 (W = 1000).
Figure 10. Shielding effectiveness of specimen #7 (W = 1000).
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Figure 11. Shielding effectiveness of specimen #8 (W = 1000, orthogonal).
Figure 11. Shielding effectiveness of specimen #8 (W = 1000, orthogonal).
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Table
Table 1. Comparison of magnetic and electric field shielding effect between soft magnetic materials and conductive materials.
Table 1. Comparison of magnetic and electric field shielding effect between soft magnetic materials and conductive materials.
Soft Magnetic MaterialsConductive Materials
Magnetic field shielding effectHighNone
Electric field shielding effectHighHigh
Electromagnetic shielding band1 Hz~GHz10 MHz~GHz
EMP Protective bandFull bandPartial band
Table
Table 2. Soft magnetic properties of the amorphous soft magnetic specimen [39,40].
Table 2. Soft magnetic properties of the amorphous soft magnetic specimen [39,40].
Specification of the Specimen
CompositionFe (78 at.%), B (13 at.%), Si (9 at.%)
Permeability (μmax)5 × 105
Coercivity (Hc)2~5 A/m
Magnetization (Bs)1.56 T
Table
Table 3. Types of specimens.
Table 3. Types of specimens.
Specimen No.
12345678
Width of conductive layer(W)0 mm20 mm50 mm100 mm50 mm + 20 mm100 mm + 20 mm1000 mm1000 mm
Arrangement of magnetic layersParallelOrthogonal
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Yoo, J.; Lee, S. Experimental Study on Electromagnetic Shielding Characteristics of a Fe-Based Amorphous Soft Magnetic Composite. Appl. Sci. 2022, 12, 6158. https://doi.org/10.3390/app12126158

AMA Style

Yoo J, Lee S. Experimental Study on Electromagnetic Shielding Characteristics of a Fe-Based Amorphous Soft Magnetic Composite. Applied Sciences. 2022; 12(12):6158. https://doi.org/10.3390/app12126158

Chicago/Turabian Style

Yoo, Jaehoon, and Sangmin Lee. 2022. "Experimental Study on Electromagnetic Shielding Characteristics of a Fe-Based Amorphous Soft Magnetic Composite" Applied Sciences 12, no. 12: 6158. https://doi.org/10.3390/app12126158

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