Structure and microwave absorbing properties of carbon-filled ultra-high molecular weight polyethylene

1 Introduction

Widespread use of electric and electronic devices leads to the necessity of developing effective means of protection against the electromagnetic radiation (EMR) [1]. A common way of protection against EMR is shielding. This method is based on two mechanisms: the primary mechanism of protection from the EMR is the reflection of electromagnetic waves from the screen surface and the secondary one is the attenuation (absorption) of the refracted wave in the body of the screen. Sheet metal, metal mesh, conducting coatings and composite materials are widely used as screens. High-frequency EMR penetrates only into a thin surface layer of conducting material (skin effect) due to high concentration and mobility of charge carriers, which interact with the external electromagnetic field (EMF). The attenuation of the EMF in the conductor proceeds exponentially: a distance, at which the amplitude decreases by e times, is the depth of skin layer δ and is determined by the following equation:

where f is the oscillation frequency, Hz; σ is the conductivity of the screen, S/m; μ_оis the permeability of vacuum (4π·10⁻⁷H/m); μ_r is the relative permeability. For instance, for copper (μ_r=1 and σ=5.8·10⁷ S/m), δ_Cu is 1.05 μm at a frequency of 4 GHz, and the length of extending plane wave λ_Cu=2πδ_Cu is 6.6 μm.

The efficiency E of the screen, which comprises a metal coating on the radioparent substrate with a thickness of λ_Cu, was considered in [2]. It was shown that E depends on the thickness and properties of the screen as given by the equation:

where Z_{c air} is the characteristic resistance of air, which is equal to 377 Ω, and Z_{с met} is a module of the characteristic resistance of metal, which can be calculated as Z_{с met}=(2πfσμ_оμ_r/σ)^1/2; Z_{с met} is a hundred or thousand times smaller than Z_{c air}. The Z_{c air}/4Z_{с met} ratio in Equation (2) is an approximated value of the product of refractive indexes at the air-metal and metal-air interfaces. Calculations [3] showed that the above screen reflects about 99.99999% of the incident power flow of high-frequency radiation.

However, the screening does not solve the problem of human protection against the microwave radiation, because, in this case, the electromagnetic wave changes the direction of propagation only. Protecting an object in this way, one can get through resonance and reflected waves’ local fields around the object, and the EMF strength may be significantly higher than the EMF of the incident wave. It means that effective protective material should work as an absorber of EMR rather than as a reflector. Recently, a number of nanostructures materials, including metal oxides and hybrid materials based on them, were developed and tested as effective absorbers of EMR [4], [5], [6], [7], [8], [9]. Such powders are suitable for thin protective coating formation; however, for some applications, bulk articles, such as plate, screen or shroud, are required. That is why the polymer EMR protective composites, in which the active filler is distributed over the solid matrix, attract significant interest [10].

Carbon materials, which exhibit a high specific surface area, high strength and conductivity and the possibility of covalent interaction of their surface with various functional groups, are promising for application as fillers for EMR absorption composites. Carbon materials are widely used in polymer composites that combine low density, high physical-mechanical and tribological properties with good EMR absorption characteristics [11], [12]. An additional advantage of polymers filled with carbon materials is a combination of heat-insulating properties with low flammability [13], [14], [15]. Various modifications of carbon materials, such as carbon black [16], [17], short carbon fibers [18], expanded graphite [19], [20], onion-like carbon [21], [22], carbon nanotubes [23], [24], [25], [26], [27], [28], [29], [30], graphene [30], [31], carbon nanofibers [28], [29], [32], and fullerenes [29], were considered as fillers for EMR absorption polymer composites. A comparison of various carbon fillers shows that the best microwave absorption properties were shown by composites containing carbon nanotubes and nanofibers [11], [28], [29]. According to [11], in order to achieve high microwave absorption properties, carbon particles should be appropriately dispersed in the polymer matrix to establish a 3D network with close contact between the particles. This conclusion is in good agreement with the results of our previous work [3]. In [3], it was shown that, in order to increase the EMR absorption properties of polymer composites, carbon nanoparticles should form well-organized conductive net structure, in which carbon particles or their agglomerates form cells 1–500 μm in size. Such a structure in the composite can be formed using activated carbon with a high specific surface area of at least 160–180 m²/g and an average particle size of 10–30 nm. Simultaneous existence of conducting grid and insulated carbon particles in the dielectric matrix is one of the main conditions of the dipole composite structure formation. Such structure provides high absorption properties of polymer composite.

This paper presents the results of elaboration and research of polymer composites filled with carbon components and designed for EMR protection.

2 Experimental

Technical carbon of the UM-76 (JSC Omsktekhuglerod, Omsk, Russia) nanopowder with particles 20 nm in size and a specific surface area of 160–190 m²/g and TIVAR® UHMW-PE-1000 (Quadrant AG, Lotte, Germany) ultra-high molecular weight polyethylene (UHMWPE) were used as the start materials. Carbon powder was dispersed for 1 h using the turbulent mixer-disperser with a rotation rate of 20,000 rpm in the hot waterglass (sodium silicate) with an addition of surfactant (JSC Spetskhimprodukt Plant, Moscow, Russia). Granulated commercial UHMWPE was milled in Pulverisette 14 rotor mill (Frich GmbH, Idar-Oberstein, Germany) to obtain a fine powder.

To measure EMR absorption properties of dispersed carbon powder, 50 µm thick carbon layer was deposited on the radiotransparent surface (glass) by aerosol spraying method. To obtain composite samples, ball-milling technique was used. Earlier [33], [34], [35], it was shown that high-energy ball milling is a suitable method for UHMWPE based composite production. UHMWPE and carbon powders, pretreated as it was described above, were milled together in Pulverisette 5 planetary ball mill (Frich GmbH, Idar-Oberstein, Germany); stainless steel balls 5 mm in diameter were used as the milling bodies, the milling time was 2 h. Hot isostatic pressing used APVM-902 press (NITI TESAR, Saratov, Russia) was used to obtain bulk composite samples.

Microwave absorption properties were studied by measuring the coefficients of reflection and transmission of electromagnetic radiation in the frequency range from 2.6 GHz to 37.5 GHz using the standing wave ratio meter (JSC EMCC VEGA, Moscow, Russia). The structure of composites was studied using JSM-6700F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The mechanical properties were investigated by tensile tests using IR5040-5-11 machine (JSR ZIP, Ivanovo, Russia).

3 Results and discussion

Figure 1 gives the dependence of EMF attenuation on the EMR frequency for the carbon coating on the glass. Within the investigated range of frequencies, the absorption of EMR is significantly higher than the reflection. The absorption magnitude for the coating varies from −5 to −13 dB, which indicates high EMR protection properties of the used carbon powder. Such high magnitude of absorption is close to the values obtained for the nanosystems, which realize resonance absorption mechanism of EMR [36]; thus, we can propose that a similar absorption mechanism may be realized in our case.

Figure 1:

Dependence of EMF attenuation on the EMR frequency for 50-µm thick carbon layer deposited on the glass. 1, reflection losses; 2, absorption losses.

Figure 2 shows SEM micrographs of bulk composite samples. A change in the filler content results in changes in the composite structure, which can affect its absorption property. For the composites with a low content of fillers, nearly non-oriented distribution of carbon particles was observed (Figure 2A). The particles exhibit a nearly equiaxial shape with an average size of about 10 μm. Obviously, such particles represent the agglomerates of initial carbon nanopowder. An increase in the filler content has an effect on the agglomerates’ size, and small particles of carbon fillers coexist with coarse agglomerates in this case (Figure 2B). Moreover, at high filler content, a noticeable orientation of carbon fillers inside the matrix was observed. An increase in the filler content up to 20 wt% (Figure 2C) also leads to a change in the polymer matrix structure. The micrograph of this sample shows a visible cracking of polymer matrix, which can lead to the deterioration of composite mechanical properties.

Figure 2:

SEM micrographs of UHMWPE-based composites containing (A) 5, (B) 10 and (C) 20 wt% of carbon fillers.

Figure 3 shows a high-resolution micrograph of polymer composite. It is seen that, in addition to the coarse agglomerates, individual carbon particles are retained in the polymer matrix. Carbon particles present both in globular and lamellar form, which allows to create electroconductive net with nanoscale elementary cells. As it was mentioned above, the formation of 3D network with close contact between the particles inside the polymer matrix can provide high microwave absorption properties of the composite.

Figure 3:

SEM micrograph of electrically conductive frame formed by carbon nanoparticles (shown by arrow) in the matrix of UHMWPE for composite containing 1.5 wt% of filler.

Figure 4 shows tensile test curves of polymer composites. The mechanical properties of the composites decrease with increasing content of carbon filler; however, they kept plasticity even at high (20 wt%) filling degree. Thus, we can conclude that the studied composites possess mechanical properties sufficient for industrial application over the entire range of filler content.

Figure 4:

Tensile test curves for UHMWPE-based composites containing (1) 5, (2) 10 and (3) 20 wt% of carbon fillers.

Figure 5 shows the EMR properties of polymer composites as a function of filler content. An increase in the content of carbon filler leads to an increase in losses for the attenuation (curve 1) and absorption (curve 2) of EMR. Figure 5 also shows the voltage standing wave ratio (VSWR) as a function of filler content in the sample (curve 3). VSWR was calculated as follows:

Figure 5:

Dependence of attenuation losses (1), absorption losses (2) of EMR and of VSWR (3) on the content of carbon fillers in UHMWPE-based composites. EMR frequency at measurements was 4 GHz.

where V₁ is a reflected wave voltage and V₀ is an incident wave voltage. As it is seen from Figure 5, VSWR gradually increases with increasing in carbon filler content. An introduction of 20 wt% of carbon filler into the polymer matrix raises the EMR attenuation to approximately 40 dB, i.e. almost by 9×10³ times in relation to the incoming radiation. However, as it was shown above, an increase in the filler content is accompanied by the degradation of the composite microstructure, which results in a significant decrease in their mechanical properties. Thus, it should be noted that even for a sample with a filler content of 5 wt%, the reflection losses are about 50% and the EMR attenuation is about 13 dB, i.e. almost 20 times in relation to the incoming radiation, which in most application is sufficient for reducing the level of EMF to the health standards.

According to [37], to be highly effective for EMR protection, fillers for polymer composites should have a small unit size (due to the skin effect), a high conductivity (for shielding by reflection and absorption), and a high aspect ratio (for connectivity). It is seen that in our case all the three requirements are fulfilled. One of the serious problems of polymer composites filled with nanosized powders is an agglomeration of filler particles, which results in a significant decrease in mechanical and physical properties of composites [38], [39], [40]. In our work, the treatment of commercial technical carbon in hot waterglass with an addition of surfactant prevents the total agglomeration of carbon nanoparticles inside the polymer matrix. As it was shown in Figure 3, in addition to the coarse agglomerates, individual carbon particles are retained in the polymer matrix; therefore, skin effect can be realized in our composites. Moreover, carbon particles in our composites are present not only in globular form, but also in lamellar form (Figure 3) with high aspect ratio, which results in the formation of conductive net structure, providing a significant improvement in electrical and thermal conductivities of composites [41], [42], [43]. As it was proposed in [3], coexistence of conducting net and insulated carbon particles in the dielectric matrix, which is realized in our case, is one of the main conditions of the dipole composite structure formation, which provides high EMR absorption properties of polymer composite.

Conclusions

Carbon nanopowder was used as filler for the UHMWPE matrix to produce composite materials for EMR protection. It was shown that composite structure, including shape and distribution of filler particles in polymer matrix, significantly depends on the filler content in the composite. The mechanical properties of the composites decrease with increasing carbon filler content, whereas the EMR protection characteristics are higher for the samples with high concentration of carbon filler.