Study of Electromagnetic Shielding Properties of Composites Based on Glass Fiber Metallized with Metal Films

Abstract

The article presents the results of an investigation of composites based on manufactured samples of fiberglass metalized with a submicron film made of brass, neusilber and non-magnetic SS304 stainless steel. It was found that, due to their characteristics, the samples of metallized fiberglass are an effective electromagnetic wave-absorbing filler for various building and construction materials; The developed metallized fiberglass samples are also useful for the creation of EMI-shielding building materials for protection from microwave radiation. With an increase in the proportion of metalized glass fiber, the electromagnetic shielding of the studied composites increases systematically. It is determined that at a concentration of 5 wt.% brass-metallized glass fiber, a test composite material with a thickness of 250 mm is able to shield up to 13.7 and 21.2 dB in the 4G and 5G ranges of cellular communication electromagnetic waves, respectively.

1. Introduction

At present, the issue of the negative impact of high-frequency electromagnetic radiation from the use of wireless information transfer technologies on a person is quite relevant [1,2,3], especially in densely populated urban areas. It is known that the level of high-frequency electromagnetic emission background in cities exceeds the natural level of electromagnetic radiation by 50–60 dB [4] and that, when talking on a cell phone, the level of electromagnetic background is about 23 dBm for the 4G LTE (4G Long-Term Evolution) standard [5]. The transition to 5G technologies for wireless information transmission causes a number of experts to be concerned about the genotoxicity and effects on the human nervous and blood systems of such radiation in the range of 3.6 to 28 GHz [6,7,8,9].

It is possible to solve the problem of the harmful effect on a person of high-frequency electromagnetic fields in the microwave range from transmitting information systems by developing samples of building and structural materials of a new generation with special electromagnetic properties. Such materials make it possible to reduce, due to the effects of microwave electromagnetic energy absorption and electromagnetic interference (EMI) shielding, the value of the intensity of harmful high-frequency electromagnetic waves to acceptable levels.

Previously, it was shown that from an economic and environmental point of view, it is promising to use the products resulting from processing various industrial wastes as raw materials for low-cost microwave-absorbing/EMI-shielding fillers for concrete building and structural materials [10,11,12,13,14,15,16,17]. However, as the results of studies [10,11,12,13,14,15,16,17] show, the use of a large mass fraction of such fillers (from 15 to 50%) is required to achieve effective electromagnetic shielding in the microwave range with electromagnetic radiation frequencies above 3 GHz. Additionally, it is necessary to take into account the practical fact that microsized powder radio-absorbing/radio-shielding fillers mostly lead to a significant decrease in the strength of concrete that is based on them. It is worth noting that undoped concrete materials have been reported to have a shielding effectiveness in the range of 8 to 12 dB (~90% or less) for samples with a 180 ± 25 mm thickness [18,19].

The aim of the work was to study the microstructural and microwave electromagnetic energy shielding properties of composite samples made on the basis of metallized glass fiber. Such a material can be used as a reinforcing fibrous dielectric filler to create environmentally friendly building microwave-absorbing materials, and the question of studying its properties is therefore relevant.

2. Materials and Methods

Samples of twill fiberglass (manufactured by the Russian Federation, STEKLONiT JSC, Ufa, Russia) were purchased commercially. Glass fiber was cut into pieces 4 ± 0.2 mm long using special scissors for cutting glass fiber. Furthermore, to remove technological traces of silicone oil and paraffin emulsion from the surface, used in the production of glass fiber and interfering with the deposition of metal coatings on its surface, the glass fiber was intensively washed in white spirit, then in isopropyl alcohol, and was then dried at 110 °C in an oven.

After installing glass fiber samples in a flat glass cup and a metal target in the working chamber of the Quorum Q150 ES magnetron sputtering unit (Quorum Technologies, Lewes, UK), it was pumped out to a pressure of 2 × 10⁻³ Pa and filled with argon to a pressure of 0.3 Pa. Before the deposition of the metal coating, the glass fiber samples were subjected to ion cleaning by bombarding them with argon ions for 3 min at a bias voltage of 500 V. After the ion bombardment of the glass fiber samples, a discharge current of 80 mA was applied to the target. The composition of metal targets for magnetron sputtering according to the EDXA energy dispersive microanalysis data corresponded to the usual compositions of these alloys (

Table 1. Parameters of deposition of metal films on glass fiber.

Metal	Composition, wt.%	Deposition Rate	Current Strength
brass	Cu–60.78%, Zn–39.22%.	40 nm/min	80 mA
neusilber	Cu–57.31%, Zn–29.90%, Ni–12.79%	34 nm/min	80 mA
AISI SS304	Fe–70.71%, Cr–19.71%, Ni–8.04%, Mn–1.05%, Si–0.49%	20 nm/min	80 mA

).

The deposition of metal films of metals on the surface of the prepared glass fiber was carried out using a Quorum Q150 ES magnetron sputtering installation (Quorum Technologies, Lewes, UK) from the corresponding metal targets with a diameter of 57 mm and a thickness of 0.3 mm. The deposition parameters are given in Table 1. The deposition of metal films was carried out in 4 stages, with stirring of the cut glass fiber after each stage of deposition of a metallized layer 250 nm thick.

The resulting samples of metallized glass fiber were mixed in molten paraffin in various mass ratios and pressed into the shape of a toroid of the required size to measure the electromagnetic characteristics.

The microstructure of glass fiber and metallized glass fiber samples was studied using JEOL JSM-7500F (JEOL, Tokyo Japan) and EVO HD15 (ZEISS, Oberkochen, Germany) scanning electron microscopes in secondary electron (SEI) and reflected electron (composition image (COMPO)/back scattered detector (BSD)) modes. The shooting mode in reflected electrons was chosen because, in this case, the image reflects the real phase composition of the sample and has a good phase contrast. Qualitative elemental X-ray spectral microanalysis (EDA) and the construction of distribution maps of chemical elements were carried out using the INCA X-Sight and INCA X-Max energy dispersive microanalysis systems (Oxford Instruments, Abington, UK) on a scanning electron microscope. During the microscopic measurements, glass fiber samples were placed on carbon tape on special brass and duralumin cylindrical holders.

To determine the electromagnetic properties of the fabricated composites based on metallized fiber with paraffin at a glass fiber mass fraction of 2.5 and 5%, the transmission loss characteristics were measured in a 10-cm HP-11566A coaxial cell (Agilent HP, Santa Clara, CA, USA) with a toroid size of 7.0 mm × 3.05 mm and a height 10 mm. We used a KC901V Deepace vector network analyzer (VNA) (Deepace technology, Dongguan, China) with an operating frequency range of 15 MHz to 7.0 GHz. The magnitude of the electromagnetic shielding of the investigated composites SE was determined experimentally by measuring the complex transfer coefficient S₂₁ according to the following Formula (1) [20]:

SE_Tot = 20·log10|S₂₁|

(1)

3. Results

3.1. Electron Microscopy and EDX Microanalysis

The microstructure of the glass fiber surface and the EDA spectrum, which reflects the composition of the used glass fiber as well as the prepared metallized glass fiber, are shown in Figure 1, Figure 2, Figure 3 and Figure 4. The thickness of the pure glass fiber according to the microscopic measurement was 15.5 ± 0.5 µm, and its chemical composition was characteristic of alkali-resistant glass fiber.

To study the microheterogeneous structure of the cut of the prepared metallized glass fiber, the COMPO mode of reflected electrons was chosen, due to the fact that, in this case, the image reflected the real phase composition of the sample and, in contrast to the SEI mode of secondary electrons, makes it possible to obtain an image with a sufficiently high phase contrast [21]. As is known, the emission of reflected electrons strongly depends on the atomic number and, accordingly, the atomic mass of the chemical elements, and the areas of the sample with lower average atomic masses look much darker in the photograph of the microstructure.

The microstructure of the glass fiber metallized with a neusilber film is shown in Figure 3. One can clearly see the deposited neusilber film on the surface of the glass fiber (Figure 3a).

Using the COMPO mode also allows one to clearly see the deposited film of stainless steel on the surface of the fiberglass (Figure 4a). The inset in the upper right corner of the EDA spectrum in Figure 4b shows a map of the distribution of iron atoms in a magnetron-sputtered stainless steel film on a glass fiber surface obtained with the glass fiber being horizontal.

3.2. VNA Measurement

Figure 5a,b shows the EMI shielding effectiveness (SE) of composites based on fabricated metallized glass fiber and 5 mm thick paraffin due to the absorption and reflection mechanisms SE_A and SE_R, according to VNA measurements; the shielding effectiveness due to absorption and reflection is calculated from the following equations:

$$S{E}_A=10·lo{g}_{10}\left(\raisebox{1ex}{$\left({10}^{\frac{S_{21}}{10}}\right)$}\!\left/ \!\raisebox{-1ex}{$\left(1-{10}^{\frac{S_{11}}{10}}\right)$}\right.\right)$$

(2)

$$S{E}_R=10·lo{g}_{10}\left(1-{10}^{\frac{S_{11}}{10}}\right)$$

(3)

Table 2. Average characteristics of electromagnetic shielding efficiency (SE) and the value of dielectric loss tangent tgɛ for samples of microwave shielding composites based on manufactured metallized glass fiber for 4G and 5G mobile communication bands.

Filler Fraction, (wt.%)	SETot (2.5–2.7 GHz)	SETot (4.7–5.0 GHz)	tgɛ (2.5–2.7 GHz)	tgɛ (4.7–5.0 GHz)
-	brass-metallized glass fibers
2.5	−9.15 ± 0.01	−14.93 ± 0.02	0.264 ± 0.002	0.538 ± 0.003
5	−13.72 ± 0.05	−21.15 ± 0.07	0.352 ± 0.007	0.721 ± 0.012
-	neusilber-metallized glass fibers
2.5	−6.07 ± 0.05	−10.48 ± 0.06	0.292 ± 0.002	0.439 ± 0.003
5	−11.84 ± 0.02	−16.95 ± 0.02	0.468 ± 0.005	0.679 ± 0.006
-	SS304-metallized glass fibers
2.5	−2.94 ± 0.04	−5.03 ± 0.06	0.204 ± 0.003	0.245 ± 0.004
5	−6.98 ± 0.04	−10.78 ± 0.05	0.319 ± 0.003	0.383 ± 0.005

shows the corresponding characteristics of electromagnetic shielding (based on a layer thickness of a composite material of 250 mm) for the studied samples of radio shielding composites based on glass fiber and paraffin metallized with brass, neusilber and SS304 stainless steel films for 4G and 5G mobile communication bands.

The frequency dependence of the calculated electromagnetic shielding for composites based on fabricated metallized glass fiber (MGFCM) with 5 wt.%, based on a material layer thickness of 250 mm, is shown in Figure 6.

4. Discussion

The use of the COMPO mode makes it possible to clearly see the deposited brass film on the fiberglass surface (Figure 2). One can observe the inhomogeneity of the film, which, after two depositions of a metal layer of 250 nm each, has a thickness of about 380 nm in the thick part and of about 280 nm in the thin part. After four sputterings with three stirrings of glass fiber, the average thickness of the brass film on the glass fiber was about 760 ± 110 nm.

It can be seen that the end portions of the fiberglass are well covered with a brass film. It is also possible to observe the inhomogeneity of the neusilber film (Figure 2a), which, after four depositions of a metal layer of 250 nm each, has a thickness of about 860 nm in the thick part and of about 620 nm in the thin part. After four stages of sputtering with three mixings of glass fiber, the average thickness of the dense neusilber film on the glass fiber was about 760 ± 130 nm (Figure 2b). Figure 2c shows the detailed structure of the obtained nanostructured brass film, with an average nanoparticle size of about 53 ± 12 nm on the glass fiber surface. In Figure 2d,e, one can also see the distribution of chemical elements that form both the fiberglass itself and the surface film of brass

Figure 3 also shows the detailed structure of the resulting nanostructured neusilber film with an average nanoparticle size of about 51 ± 14 nm on the glass fiber surface. One can also see a noticeable number of defects in the nickel silver film on the surface of the glass fiber.

One can observe the inhomogeneity of the SS304 film (Figure 3), which, after two depositions of a metal layer of 250 nm each, has a thickness of about 410 nm in the thick part and of about 270 nm in the thin part. After four sputterings with three stirrings of glass fiber, the average thickness of the stainless steel film on the glass fiber was about 770 ± 120 nm.

Figure 4c also shows the detailed structure of the obtained nanostructured stainless steel film with an average size of aggregated nanoparticles of about 16 ± 4 nm on the glass fiber surface. One can see that the surface of the stainless steel film consists of agglomerates of nanoparticles with a total size of 40–60 nm, occupying approximately 85% of the total area of the nanostructured film. Nanoparticles in the form of agglomerates in a stainless steel film are connected by bridges. Thus, images of the surface of metallized glass fiber obtained in secondary electrons provide information on the presence of structural inhomogeneities and nanosized pores in a stainless steel film deposited by magnetron sputtering. For stainless steel films deposited by DC magnetron sputtering, a surface structure and porosity with a nanocolumnar structure is a common phenomenon [22,23,24].

Theoretically, the EMI shielding efficiency (SE_T~SE_R + SE_A) mainly depends on the amount of metal deposition on the fibrous surface and the surface activation process [20,25,26]. In our case, the sputtered stainless steel film on the fiberglass had the lowest density and uniformity of the surface metal layer.

As previously measured by us [16,17], the efficiency of the electromagnetic shielding of cement stone samples without any filler in terms of a block thickness of 250 mm is SE (2.5–2.7 GHz) = 7.4 dB for the 4G communication range and SE (4.7–5.0 GHz) = 13.5 dB for 5G communication. It is obvious that these values are clearly insufficient to eliminate the level of high-frequency electromagnetic background harmful to human health in concrete buildings in cities.

The composites we fabricated, based on metallized glass fiber and paraffin, showed, according to VNA measurements, the absence of significant ferrimagnetic properties in the case of all three used metal alloys. Therefore, microwave absorption in such metallized glass fiber materials should be associated with dielectric losses and eddy currents due to inhomogeneities of the microstructure of the metal film, as is clearly visible in Figure 2, Figure 3 and Figure 4.

The mechanism of EMI shielding is basically dependent on three contributions: the reflections of electromagnetic waves from the material, absorption of electromagnetic energy in the material and multiple internal reflections of the electromagnetic waves [20].

The EM energy E_T falling on any absorbing/shielding material is converted according to the following contributions: E_T = E_A + E_R + E_M [21,26]. Since the contributions of energy absorption and reflection E_A + E_R are important for EMI shielding, the primary surface reflection E_R and multiple reflections E_M are singled out in the reflection contribution, whose contribution can be neglected. Then, the formula for the EMI shielding efficiency is calculated from the following Equation (4):

$$S{E}_T=10·\mathit{log}\left(\frac{P_1}{P_T}\right)=20·\mathit{log}\left(\frac{E_1}{E_T}\right)=20·\mathit{log}\left(\frac{H_1}{H_T}\right)=S{E}_R+S{E}_A+S{E}_M$$

(4)

It is known that, due to the relative impedance mismatching between the surface of the shielding material and the EM waves, the contribution to shielding by the mechanism of primary reflection is described by the following expression (5):

$$S{E}_R=20·\mathit{log}\left(\frac{Z_0}{4Z_{in}}\right)=39.5+10·\mathit{log}\left(\frac{\sigma }{2f\pi \mu }\right) \propto \raisebox{1ex}{$\sigma $}\!\left/ \!\raisebox{-1ex}{$\mu $}\right.$$

(5)

At equal concentrations of metallized glass fiber in the composite, a sample with a brass film shows the highest electrical conductivity, and a sample with a stainless steel film shows the lowest electrical conductivity. This is consistent with our calculated contributions SE_R (Figure 5).

A secondary mechanism of EMI shielding is absorption. As we know from the plane wave theory, the amplitude of the EM wave decreases exponentially inside the material as it passes through it. Thus, absorption loss results from ohmic losses and heating of the material due to the currents induced in the medium. For conductive materials, absorption loss (SE_A) in decibels (dB) can be written as:

$$S{E}_A=20·\mathit{log}{e}^{\frac{d}{\sigma }}=8.7d\sqrt{f\pi \sigma \mu } \propto d\sigma \mu \propto \alpha d$$

(6)

where d and α are the thickness and attenuation constants of the material slab, respectively.

It has already been said that at equal concentrations of the metallized glass fiber in the composite, a sample with a brass film shows the highest electrical conductivity, and a sample with a stainless steel film shows the lowest electrical conductivity. Then, for a composite based on a brass film on glass fiber, the contribution of SE_A will be the largest, and for a composite based on a film of SS304 on glass fiber, the contribution of SE_A will be the smallest. This is consistent with our calculated contributions SE_A (Figure 5).

One can see from the data in Figure 5 that with an increase in the frequency of electromagnetic radiation, the efficiency of electromagnetic shielding for all MGFCMs systematically increases. In this case, the maximum losses of electromagnetic energy due to absorption occur in the frequency range of 6 to 7 GHz. In turn, the microwave shielding effectiveness due to reflection, even, in the best case, for a composite based on a brass film, does not exceed a value of 2 dB. Thus, we can conclude that the electromagnetic shielding in the investigated MGFCMs is mainly due to the loss of electromagnetic wave energy due to absorption in the metal film on the glass fiber.

According to Table 2, it can be concluded that the studied samples of composite materials based on metallized nanostructured films on glass fiber with a fraction of 5 wt.% are quite effective (taking into account the small proportion of composite fullness) in terms of microwave shielding for electromagnetic waves, both 4G and 5G mobile bands. It should be noted that paraffin is a radio-transparent material in the studied frequency range of electromagnetic radiation and has an insignificant amount of electromagnetic shielding (less than a 1.1 dB maximal value for a block thickness of 250 mm). Thus, one can consider that the introduction of manufactured metallized fiberglass at a fraction of 5 wt.% into a concrete or gypsum-cement block with a thickness of 250 mm will be able to provide an additional 10.8 to 21.2 dB of shielding in the 5G cellular frequency range.

According to Table 2, one can see that the greater the value of the dielectric loss tangent tgɛ for the studied samples of microwave shielding MGFCMs, the greater the efficiency of their electromagnetic shielding that is observed. It can be concluded that the granular and porous microstructure of the stainless steel film on the surface of the glass fiber leads to the low high-frequency electrical conductivity of metallized fibers and, accordingly, to low values of tgɛ for microwave electromagnetic shielding composites that are based on them. In the case of a brass film, which is characterized by the highest electrical conductivity, a significant high-frequency electrical conductivity is apparently achieved for MGFCMs, which is in good agreement with the significant values of shielding effectiveness due to reflection.

5. Conclusions

According to the data obtained in the course of the performed investigation, glass fibers metallized with submicron films of brass, neusilber, and stainless steel are a good dielectric filler for polymer composite and building materials. At the same time, the studied dielectric micriwave-absorbing metallized glass fiber fillers are more effective at EMI shielding when compared to the previously studied dielectric materials such as rice husk ash [15] or brass micropowder [17]. It is possible to recommend the addition of glass fibers metallized with submicron films of brass, neusilber and stainless steel in the form of a microwave-absorbing and EMI-shielding reinforcing filler to poured concrete. This will make it possible to achieve a level of absorption of high-frequency electromagnetic waves sufficient for practical purposes for 4G and 5G mobile communication bands in the creation of building and structural materials.