A lightweight concrete composite material with improved EMI shielding by using a chiral particle array

Electromagnetic interference (EMI) shielding is a technique employed to protect electronic devices from electromagnetic power. Various methods exist for manufacturing a shielded room, including covering walls with aluminum foil or using a steel chamber. Currently, globally shielded buildings are being developed for diverse purposes.^1–9 On the other hand, modern buildings use particle-loaded concrete instead of traditional ones. This type of cement-based composite can significantly reduce transmitted power and shield against electromagnetic waves while improving the structure’s mechanical properties. A variety of particles, both macroscopic and microscopic, can be incorporated into the composite to achieve this objective.^10–22 

Various additives are employed in the design of composites for EMI shielding applications.^10–29 For instance, carbon filament or carbon fiber additives were utilized.²³ Another study incorporated nickel filament into the design of shielding composites.^24–26 Carbon-based additives were incorporated into a cement matrix to construct a robust slab for EMI shielding.^10–21 Steel fibers are also implemented in a cement matrix.²² Recently, carbon nanofibers have been used for a similar purpose.^27–29 

Most of these studies are inappropriate for being employed in a wide frequency range. It is a limitation that requires further investigation. On the other hand, as previously mentioned, the mechanical properties of concrete are crucial from various perspectives. Macroscopic additives are more effective than microscopic additives in protecting the concrete composite against macroscopic cracks. In most of the studies that have utilized the macroscopic conductive additive, such as steel fibers, the value of the additive is notably high to gain a high level of shielding effectiveness. This deficiency will lead to a significant increase in the mass density of the composite. Lighter composites are often preferred over heavier ones in many applications. Consequently, it is crucial to devise a novel approach to reduce the number of additives and design lightweight concrete composites while maintaining a substantial degree of shielding effectiveness in a wide frequency range. This study proposes an approach to enhance the electromagnetic shielding capability of a concrete composite while minimizing the use of metal additives.

Macroscopic conductive additives can be added to concrete in various geometries and sizes. Helical-shaped particles represent a well-known type of chiral inclusion that can be employed for electromagnetic interference (EMI) shielding applications, particularly due to their resonant properties.^30–32 

The additives can be arranged in an array or distributed randomly, each with advantages and disadvantages. A traditional method for constructing concrete composites for EMI shielding applications involves the addition of steel rods into the concrete paste in the mixer. This method randomly distributes the steel additives into the concrete as the host medium. However, new techniques have been recently developed for organizing the conductive particles in concrete paste.^33–35 

This paper proposes a novel structure for concrete composite materials for electromagnetic interference (EMI) shielding applications. The structure encompasses an array of helical-shaped conductive particles as chiral additives that are embedded in a concrete material as the host medium. In specific circumstances, the helical particles have the ability to replicate the behavior of a below-cutoff cylindrical waveguide. As a consequence, there is a significant decrease in the transmitted power from the array of helical particles within its resonance frequency range. This phenomenon helps achieve considerable shielding effectiveness with a small number of conductive additives, resulting in a lightweight concrete composite compared to the traditional ones, which utilize randomly distributed rods.

Many recent valuable studies on concrete composites in the field of EMI shielding focus on microscopic additives and nanoparticles, particularly carbon-based additives such as carbon fiber, carbon nanotube, and graphene oxide.^11–21 Besides microscopic additives, implementing macroscopic additives in concrete composites is beneficial.³⁶ Macroscopic additives are more industrially practical and have proven effective through empirical testing because the homogeneous dispersion of nanoparticles in the composite remains challenging and an active area of research. The proposed structure can enhance the mechanical characteristics of the concrete composite, significantly improving its ductility. Moreover, it can be combined with microscopic additives. Simultaneously implementing the proposed method plus nanoparticles is advantageous and can be utilized in designing ultra-high-performance concrete (UHPC).^36,37

This paper is organized into four sections. Section II describes the structure of two distinct approaches for designing concrete composite materials and the geometry of their conductive additives for computer full-wave simulation. Two categories of the samples are provided: the first is the proposed structure with the samples including an array of helical particles, and the second is a traditional structure with the samples including randomly distributed sinusoidal rods. At the end of the section, the results of the full-wave simulations are presented. Section III represents the authenticity of the full-wave simulations. An experimental sample is constructed to compare its measured shielding effectiveness with the computer simulation results. Likewise, the experimental measurement results and the corresponding comparison are presented at the end of the section. Finally, Sec. IV provides a conclusion.

The proposed structure comprises an array of helical particles embedded within the concrete slab, collectively forming a concrete composite. A multi-layer array is implemented to achieve significant shielding effectiveness. However, designing a wideband structure is one of the key constraints. To achieve this objective, the size of the helical particles in each layer differs from that of the other layers, thus broadening the applicable bandwidth.

This design method employs the resonant property of an infinite periodic planar array of helical particles to achieve a structure that can effectively attenuate the transmitted electromagnetic power and is appropriate for EMI shielding applications. Theoretically, the substantial attenuation of the transmitted power from the infinite periodic arrays of helical particles [such as the arrays whose unit cells are displayed in Figs. 1(d) and 2(b)] results from the helical particles’ ability to imitate the TE₁₁ mode of a cylindrical waveguide and its corresponding surface current on its helical wire. This phenomenon occurs in the resonance frequency range below the cutoff frequency of the TE₁₁ mode of a cylindrical waveguide with a radius similar to that of the helical particle. Figure 1 shows the comparison of the electric field pattern of an array of conductive cylinders (cylindrical waveguides) and an array of seven-turn helices. The comparison is provided at 2 GHz in the resonance frequency range below the TE₁₁ mode cutoff frequency of 3.1 GHz, and at 3.5 GHz, above the cutoff frequency. It shows a considerable similarity in different cross-sections and frequencies. The scattering of an individual chiral particle or canonical helix is maximized when the wavelength is approximately equal to the circumference of the particle.³⁰ The resonance frequency of a helix-loaded composite occurs at a wavelength equal to 0.75 times the total length of the helix wire, which is almost equal to the circumference of the helix in particles with a small number of turns and pitch angle.^31,32 Similarly, for an infinite periodic planar array of helical particles with a slight pitch angle and a small number of turns, as illustrated in Figs. 2(a) and 2(b), the central frequency of the resonant frequency band is the frequency whose wavelength is approximately equal to the circumference of the helical particle, which is presented in the unit cell of the array.

Based on our design objectives, a six-layer structure has been proposed, comprising six parallel infinite periodic planar arrays that are dense enough, as shown in Figs. 2(d)–2(g). All of the planar arrays are located in concrete as a host medium.

The elements in each planar array have identical orientation and geometry, but the arrays can differ in element and unit cell size. The parallel planar arrays with different unit cell sizes were selected to achieve a wideband structure. In this example, each planar array’s particle and unit cell size is a scale of its previous array. We intend to design an array of helical particles for shielding applications in the frequency range between 2 and 4 GHz. Thus, the dimensions of the largest particle (layer 6) and the second smallest particle (layer 2) are designed so that the center of their resonance band, respectively, occurs at the frequencies of 1.7 and 4.2 GHz. Furthermore, an extra layer (layer 1) with a smaller particle size and higher resonance frequency (5.2 GHz) is added to the structure to improve the results in the desired frequency band.

Generally, the distances between the arrays are assumed to be arbitrary and dependent on the utility and design. In this study, two distinct six-layer arrays of helical particles are designed and simulated by a full-wave simulation software, CST Studio Suite. The primary distinction between these six-layer arrays is the thickness of their corresponding concrete slabs. One structure is designed with a thickness of 2.5 cm, as shown in Figs. 2(d) and 2(e), while the other is designed with a thickness of 11 cm, as shown in Figs. 2(f) and 2(g). Table I presents the characteristics of the materials utilized in all of the simulations in this paper, as recorded in the software database.

A helical particle is described by three independent parameters: the radius of the helix (R), pitch angle, and number of turns. As mentioned before, in each planar array of helices with a small pitch angle, the radius R is determined according to the intended resonance frequency of the array (c = 2πR ≃ λ_resonance).^30–32 Consequently, the helix radius (R₆ = 12 mm) for the planar array in layer 6, containing the largest particle, has been established to resonate at 1.73 GHz. The helix radii for the smaller layers (R₅, …, R₁) are scaled multiples of R₆, with each multiplier being less than 1. The number of turns depends on the desired application. In this study, the number of turns should be an odd multiple of a quarter-turn (1.75 turns) to achieve a bi-anisotropic slab that can effectively interact with two orthogonal polarizations of the incident wave’s electric field. The specifications of the helical particles’ geometry for each array are presented in Tables II and III.

A concrete composite incorporating randomly distributed additives based on traditional methods for full-wave simulation was designed to compare with the proposed structure, which encompasses arranged helical additives. It is illustrated in Figs. 2(h) and 2(i). Sinusoidal steel rods, shown in Fig. 2(c), were selected for these composites due to their prevalence of use and impact on concrete’s mechanical properties. The length of these fibers is 2.5 cm, and their wire thickness is 400 µm. The thickness of the samples was 2.5 cm, equivalent to the thickness of one of the concrete slabs, including an array of helices.

In the full-wave simulations, the behavior of the arrays was studied for normally incident, linearly polarized electromagnetic waves in the frequency range from 1 to 4 GHz. The incident wave propagates in the -z direction.

Moreover, a control sample was simulated to compare its shielding effectiveness with the reinforced samples and to remove the effect of the shielding property of the concrete material. The control sample is a specimen made of the host medium, with the same size and without any conductive additives, as illustrated in Fig. 2(c) for comparison only with the reinforced slabs with 2.5 cm thickness.

The other well-known figures of merit are the additive volume fraction (AVF) and the additive mass fraction (AMF). The additive volume (or mass) fraction is defined as the ratio of the volume (or mass) of all additives to the total volume (or mass) of the composite. Equations (2) and (3) represent these parameters.

This technique minimizes the effect of possible differences between experimental and simulation methods, thereby making the comparison more valid and logical.

Figure 3 shows the comparison of the EMI shielding property between two distinct types of concrete composites, arranged and randomly distributed additives, illustrated in Figs. 2(d)–2(i).

Figure 3(a) shows the comparison of the normalized shielding effectiveness of different composites within the 1–4 GHz frequency range. In these simulations, the shielding effectiveness is calculated for a normal incident wave. Figure 3(a) demonstrates that the composite, including the array of helices, exhibits significantly greater attenuation of transmitted power from the slab than the composite, including the randomly distributed sinusoidal rods in the frequency range of 1.7–4 GHz. This is the desired frequency band for which the array has been specifically designed. On the other hand, although the helical arrays were not specifically designed for the 1–1.7 GHz frequency range, it can be observed that the values of shielding effectiveness for these composites, demonstrated in the line graphs (II) and (III), are superior to those of randomly distributed sample (I) within this range of frequency. However, it should be noted that the additive volume fraction, mass fraction, and mass density of these specimens, including the helical array, are significantly lower than that of the specimen, including randomly distributed sinusoidal rods. Table IV presents these figures of merit values for all specimens.

Furthermore, the efficiency parameter was calculated using Eq. (4) for the simulated samples in the 1–4 GHz frequency range. The efficiency of conductive additives for each specimen is presented in Fig. 3(b). It shows that the concrete composites comprising the array of helices have a significantly higher efficiency of conductive additives.

To validate the results of the full-wave simulations, an experimental measurement was provided. Due to the complexities in constructing the proposed structure, including helical-shaped particles, as well as high costs and the need for advanced civil engineering equipment, the second concrete composite comprising randomly distributed sinusoidal rods was constructed. This test setup is developed to validate the accuracy of the simulation setup. The experimental samples, analogous to the simulated samples with the same volume fraction, mass fraction, and geometry of sinusoidal rods, were, thus, realized. The sinusoidal rods are made of steel and coated with copper. A sinusoidal rod can be observed in Fig. 4(a). Table V presents the mass ratio of various components to the cement powder in the concrete paste. Upon the addition of the rods to the concrete paste during the mixing process, they become randomly dispersed within the concrete after drying, as illustrated in Fig. 4(b).

In addition, similar to simulated samples, a control sample of the same size and without any conductive additives was made. Figure 4(c) shows both the reinforced and control samples.

The samples’ cross-section size was 165 × 82 mm², almost equal to the dimensions of the WR650 waveguide. The waveguides are shown in Fig. 4(d). After seven days of the dehydration process, the electromagnetic shielding of the samples was measured by the WR650 waveguide and Agilent E5071C ENA Vector Network Analyzer in the 1.15–1.72 GHz frequency band. Figures 4(e) and 4(f) show the test setup and the network analyzer, respectively.

Moreover, in the simulations in the frequency range of 1.15–1.72 GHz, an oblique incident wave was implemented in full-wave simulations to realize the TE₁₀ excitation of the rectangular waveguide in the experimental setup.

Figure 5 provides a comparison within the frequency range of 1.15–1.72 GHz. The line graphs (I) and (II) show the acceptable deviation in the experimental measurement and the full-wave simulation results for the compositions. It also demonstrates that the simulated model results are reliable and can be utilized to assess the performance of diverse composites and analyze the values of their shielding effectiveness for EMI shielding applications.

The results indicate that embedding a chiral steel particle array in a concrete host medium enhances the slab’s shielding effectiveness while reducing the additive quantity and composite mass density compared to traditional randomly distributed rods. This improvement is achieved by utilizing evanescent wave propagation in a below-cutoff circular waveguide, mimicked by helical particles, which facilitates high shielding effectiveness. Furthermore, implementing a multi-layer structure enables significant shielding effectiveness across a wide frequency range, aligning with the design objectives for the concrete composite. Ultimately, besides improving the electromagnetic shielding properties of the composite, the proposed method can enhance the concrete’s mechanical characteristics, making it suitable for designing ultra-high-performance concretes.

The authors have no conflicts to disclose.

Ayoub Hamidi: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (lead); Resources (equal); Software (lead); Validation (equal); Writing – original draft (lead); Writing – review & editing (equal). Ahmad Cheldavi: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (lead); Validation (equal); Writing – review & editing (equal). Asghar Habibnejad Korayem: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.