Microwave Absorption Performance of Carbon Black/Polylactic Acid Composite for Fused Filament Fabrication

Abstract

To prepare lightweight, wideband, strong absorption and low-cost microwave absorbing materials, carbon black/polylactic acid (CB/PLA) composite filaments were prepared using a high-energy mixer and twin-screw extruder. Coaxial ring test pieces were printed by fused filament fabrication (FFF) technology with polylactic acid as the matrix and carbon black as the absorbent. The crystal texture and micromorphology of the composites were observed by XRD and SEM. The electromagnetic parameters were measured at 2–18 GHz using a vector network analyzer. The influence of CB content on the absorbing performance of the composites was studied, and the loss mechanism was explained. The results show that when the CB content (refers to PLA) is 25%, the composite has suitable impedance matching, conduction loss, and polarization loss, resulting in the best microwave absorption performance. The minimum reflection loss reached −45.47 dB at 13.68 GHz, and the corresponding effective absorption bandwidth was 5.36 GHz (10.72–16.08 GHz) with a matching thickness of 2.1 mm. At the same time, the density of the composite was only 1.19 g/cm³. This work sheds light on the development of lightweight and high-efficiency microwave absorption materials (MAMs) in a simple and low-cost manner.

1. Introduction

The rapid development of electronic information technology has allowed electromagnetic technology to be widely used in various industries such as mobile phones, network security, radar systems, medical facilities, electronic household appliances, and even aerospace [1,2]. While increasing convenience to human life, the pollution caused by electromagnetic radiation has also aroused widespread concern. This harmful pollution affects the regular operation of electronic equipment, interferes with people’s daily lives, and directly threatens human health [3,4]. Microwave-absorbing materials (MAMs) effectively prevent electromagnetic pollution and protect human health; therefore, they have attracted much attention. The development of lightweight, ultra-thin, high-efficiency, and broadband MAMs is needed [5,6].

Carbon-based materials (such as carbon black, carbon nanotubes, graphene, etc.) have become the research focus of MAMs due to their advantages of low density, high specific surface area, good electrical conductivity, and high thermal stability [7,8]. Carbon black (CB) is widely used because of its low cost, low density, and perfect dielectric and polarization characteristics [9].

Liu et al. [10] used silicon rubber as the matrix and CB and T-ZnO as the absorbents to obtain composite absorbing patches by traditional preparation processes. When the mass fraction of CB and T-ZnO was 8% and 10%, respectively, its minimum reflection loss (RL) was only −14.68 dB, and the effective absorption bandwidth (EAB) was only 3.04 GHz. The absorbing properties of the composite materials need to be further improved. Tang et al. [11] grew graphene oxide (RGO) in situ on CB by liquid phase methods to obtain a double composite dielectric material CB/RGO and studied its dielectric properties and microwave absorption energy. It was found that, when the thickness was 2.2 mm and the addition was 7%, the maximum absorption intensity reached −47.5 dB, and the EAB reached 5.92 GHz. However, this preparation process has many complicated procedures and long reaction times, which is not conducive to large-scale production. He et al. [12] used NBR as the matrix and barium ferrite and CB as the absorbing agents to prepare single-layer, double-layer, and four-layer composites by mold pressing using a flat vulcanizing machine. The results showed that the absorption effect of double-layer and four-layer composite materials was better than that of single-layer, and the maximum absorption strength of four-layer composite materials only reached −11.38 dB. Wang et al. [13] prepared a gradient multilayer microwave absorption patch (RAP) by molding with linear low-density polyethylene (LLDPE) as the matrix and CB as the absorber. When using CB with contents of 5%, 15%, 25%, and 35% as the fourth, third, second, and first layer of RAP (thickness of each layer, 2 mm; total thickness, 8 mm), in the frequency range of 4–18 GHz, the bandwidth of the RL value below −10 dB is 4.65 GHz. By structurally designing absorbing materials, the performance can be increased, but there is still room for improvement. At the same time, the traditional molding cost is high, and the mold depends on the tolerance, which is not conducive to wide applications, especially in manufacturing special structures; 3D printing technology can effectively solve this problem.

Fused filament fabrication (FFF) process is the most widely used 3D printing process, and polylactic acid (PLA) is the most common polymer for preparing filament. PLA comes from a wide range of sources. It is a kind of green and renewable resource with good biodegradability, mechanical properties, and formability. In this paper, CB/PLA composite filaments were first prepared by a high-energy mixer and a twin-screw extruder, and then, the coaxial ring samples were printed via FFF technology to test the electromagnetic parameters. The influence of CB content on the dielectric properties of composites is analyzed in detail, the loss mechanism is explained, and the composite’s absorbing performance are discussed. The filament prepared in this paper can be used for 3D printing of complex structure absorbing metamaterial, breaking through the limitations of traditional absorbing coatings. This provides a new choice to meet the performance requirements of wideband, strong absorption, and light weight.

2. Materials and Methods

2.1. Materials

The materials are listed in

Table 1. Experimental materials.

Materials	Performance	Producer
PLA powder		Huachuang Plastic Technology Co., Ltd., Dongguan, China
CB	specific surface area 55–70 m2/g, resistivity 2.5 Ω∙m	Zhengyuan Technology Co., Ltd., Tianjin, China
Aluminate coupling agent (AL)		Feiteng Chemical Co., Ltd., Nanjing, China

2.2. CB/PLA Composites Preparation

PLA and CB were placed in a vacuum drying oven (VDM50, IRM Technology GmbH, Frankfurt, Germany) and dried at 80 °C for 6 h to eliminate excess moisture. Aluminate coupling agent with a mass fraction of 2% was added to PLA. A certain proportion of CB (wt%, relative to the PLA) was added, and samples were put in a dual-motion high-energy mixer (JHT-5, Gold Co-Powder Technology Co., Ltd., Zhengzhou, China). The rotating speed of the barrel and blade was 28 r/min and 17 r/min, respectively; the mixing time was 40 min to ensure uniform mixing [14].

The uniformly mixed composite powder was added to a twin-screw extruder (SJZS-10B, Ruiming Experimental Instrument Manufacturing Co., Ltd., Wuhan, China) to prepare CB/PLA composite filament (as illustrated in Figure 1a). The extruder parameters were: c zone temperature 145 °C, molten zone temperature 165 °C, melt-conveying zone 3 temperature 170 °C, head temperature 170 °C; the screw speed was 18 r/min, and the filament diameter was controlled at 1.75 ± 0.05 mm.

Using a desktop 3D printer (CR-5, Chuangxiang 3D Technology Co., Ltd., Shenzhen, China), the self-made CB/PLA composite filament was printed as coaxial ring pieces to measure the electromagnetic parameters (as illustrated in Figure 1b). The outer diameter of the coaxial ring was 7 mm, the inner diameter was 3.04 mm, and the thickness was 2 mm. A thin cylindrical piece with a diameter of 25 mm, thickness 3 mm was printed to test the DC conductivity (as illustrated in Figure 1c). The printing parameters were: printing speed 40 mm/s, layer height 0.1 mm, nozzle temperature 210 °C, bed temperature 40 °C, and filling density 100%.

2.3. Experimental Design

If the CB content is too high (more than 25%), the surface quality of the filament will be poor or even cannot be extruded. For this reason, CB content (relative to PLA) was controlled in the range of 0% to 25%. Aluminate coupling agent (AL) can be used to improve the dispersion of CB in PLA matrix and the extrusion performance of the polymer. Its addition amount (relative to PLA) was selected as 2 wt.%. The composition is shown in

Table 2. Ratio of CB and AL contents refers to PLA.

Samples	CB/%	CB/% Refers to Total	AL/%
0% CB/PLA	0	0	2
5% CB/PLA	5	4.76	2
10% CB/PLA	10	9.09	2
15% CB/PLA	15	13.04	2
20% CB/PLA	20	16.67	2
25% CB/PLA	25	20	2

.

2.4. Test and Characterization

The PLA, CB particles, and composite samples were characterized by scanning electron microscope (JSM-IT500, JEOL Ltd., Tokyo, Japan) to observe their microstructure and mixing uniformity. PLA and CB/PLA composites were analyzed by Cu-K-α radiation X-ray diffraction (XRD-Smart Las mar, Rigaku, Tokyo, Japan) with a scanning speed of 10°/min and a scanning range of 5–45°. This was to investigate whether the extrusion process had any effect on the crystal structure. CB/PLA composites were analyzed by thermogravimetric analysis (METTLER TOLEDO, TGA/DSC3+HF1600) with a heating rate of 10 °C/min and a temperature range of room temperature −500 °C. This was to determine the accuracy of the real CB content. The DC conductivity of the composites was tested with a double-electrical four-probe tester (RTS-9, Four-Probe Technology Co., Ltd., Guangzhou, China). The electromagnetic parameters were tested in the range of 2–18 GHz with a vector network analyzer (Keysight Agilent E5071C, Santa Rosa, CA, USA) by the coaxial transmission line method. They were all used to reveal the absorbing mechanism of the composite materials. The coaxial transmission line method is shown in Figure 2. The two ports of the coaxial transmission line were connected to port 1 and port 2 of the vector network analyzer. The scattering coefficient of the material was measured, which was used to calculate the electromagnetic parameters of the material (complex dielectric constant and complex permeability).

3. Results and Discussion

3.1. Structure and Micromorphology of Raw Materials and Composites

Figure 3a shows the microscopic morphology of the PLA particles under SEM (50×). PLA particles had irregular shapes and large size. Figure 3b shows the microscopic morphology of CB particles under SEM (5000×). CB particles had a high specific surface area and surface energy with nanometer diameter, so agglomeration and adsorption occurred. It contributes to the formation of conductive networks, thus increasing the conductivity and dielectric constant of the material. Figure 3c is the microscopic topography of CB/PLA composites. In general, the CB particles were uniformly distributed in the matrix with good compatibility, which is beneficial to improving the microwave absorption performance.

The XRD patterns of the CB/PLA composites with different CB contents are shown in Figure 4. With the increase of CB, the crystal diffraction peak of the CB/PLA composite was still the crystal diffraction peak of PLA. Although the crystal diffraction peak intensity of the composite material was slightly weakened, the position of the diffraction peak remained basically unchanged. It shows that the increased CB content had little effect on the crystal structure.

The TG curves of the CB/PLA composites are shown in Figure 5. From room temperature to 270 °C, the curve was almost horizontal, and no mass loss occurred. It indicated that the composites had good thermal stability at this stage. The residual rate of the pure PLA sample was 0.68%, indicating that it was almost completely decomposed. The CB residual rate of 5–25% CB–PLA samples were 4.83%, 8.74%, 13.23%, 16.37%, and 20.19%. These are coincident with the ratio of CB to total composite that is listed in the last column of Table 2 (4.76, 9.09, 13.04, 16.67, 20). It proved that the control of various contents of loading CB was precise.

3.2. Electromagnetic Parameters and Loss Mechanisms of Composites

Since all materials were nonmagnetic, we only focused on analyzing dielectric properties (complex permittivity and dielectric loss tangent). In detail, the real (${\epsilon }^{\prime }$) and imaginary (${\epsilon }^{″}$) parts of complex permittivity, respectively, represent the storage and dissipation capacity for electromagnetic energy, while the dielectric loss tangent ($\tan {\delta }_{\epsilon }={\epsilon }^{″}/{\epsilon }^{\prime }$) stands for the dielectric loss ability [15,16]. Figure 6a,b shows the relationship between the real and imaginary parts of complex permittivity and frequency (2–18 GHz) for different CB/PLA composites. Dielectric loss mainly includes conduction loss and polarization relaxation loss (electron polarization, ion polarization, dipole polarization, and interfacial polarization). Among them, electron polarization and ion polarization usually occur in the THz and PHz range, so it is not considered in the GHz range [17,18].

It can be seen that the ${\epsilon }^{\prime }$ and ${\epsilon }^{″}$ of pure PLA (0%CB/PLA) tend to be constant in the whole frequency range of 2–18 GHz with slight fluctuation, about 2.3 and 0.1, respectively, indicating that pure PLA is an almost lossless material. In general, with the increase in CB content, the real and imaginary parts of the complex permittivity decrease with the increase in frequency, and the change trend of the two is similar on the whole. Meanwhile, as the CB content increases from 5% to 25%, the real and imaginary parts of the complex permittivity increase significantly, which may be attributed to the conduction loss. The conduction loss is related to the conductive network formed by the CB in the matrix material, according to free electron theory [19,20]:

$$\sigma =2\pi {\epsilon }_{0}f{\epsilon }^{″}$$

(1)

where $\sigma$ is the conductivity, ${\epsilon }_0$ is the permittivity of the vacuum, and $f$ is the frequency of the electromagnetic wave. The conduction loss is mainly determined by the imaginary part of the complex permittivity, and they are positively correlated.

Figure 7 shows the measured conductivity of the CB/PLA composites. When CB content was 15%, the conductivity of the composite reached 0.0015 S/cm. At this time, CB initially formed a conductive network in the matrix material. With a further increase in CB content (to 20%), the conductive network was gradually formed, and the conductivity increased significantly. This means that the conduction loss is significantly enhanced.

Figure 6c shows the relationship between the dielectric loss tangent and the frequency for different CB/PLA composites. With the increase in CB content, the dielectric loss tangent value increases continuously; the dielectric loss of pure PLA is the smallest and the curve fluctuates slightly, indicating dipole polarization losses. The changing trend of the dielectric loss tangent of 10% CB/PLA, 15% CB/PLA, 20% CB/PLA, and 25% CB/PLA is nearly the same as the curve of the imaginary part of the complex permittivity. There were multiple resonant peaks at 10–18 GHz, which implied the combined effect of conduction and polarization losses. The 25% CB/PLA had the maximum dielectric loss, and a significant peak was found at 18 GHz, corresponding to interfacial polarization [21,22]. Interfacial polarization usually occurs at the interface of inhomogeneous media and is mainly caused by the accumulation of electron or ion agglomeration at the interface under the action of an applied electric field [23,24]. With the continuous increase in CB content, the interfacial polarization is continuously enhanced, and the peak value rises continuously. In addition, the small peak at low frequency mainly corresponds to dipole polarization, which can be attributed to the surface defects of CB and the existence of polar functional groups [25].

The dielectric loss can be explained by the Debye medium theory [26,27]. The Cole–Cole curve, which is the relationship of ${\epsilon }^{\prime }$ and ${\epsilon }^{″}$, can be obtained as shown in Equation (2) [28,29,30]:

$${\left({\epsilon }^{\prime }-\frac{{\epsilon }_s+{\epsilon }_{\infty }}{2}\right)}^2+{\left({\epsilon }^{″}\right)}^2={\left(\frac{{\epsilon }_s-{\epsilon }_{\infty }}{2}\right)}^2$$

(2)

where ${\epsilon }_s$ and ${\epsilon }_{\infty }$ are the static permittivity and relative dielectric permittivity at infinite frequency limit, respectively. Each semicircle in the Cole–Cole curve represents a polarization relaxation process, and the appearance of a tail at the end of the curve indicates conductance loss [31,32]. As shown in Figure 8, all composites exhibited a few semicircles, suggesting that there may be multiple dielectric relaxation losses: interfacial polarization and dipole polarization, which is in accord with the relaxation peaks (as described in Figure 6c). Compared with 0% CB/PLA, in 5% CB/PLA, the tail was more apparent with a higher slope, indicating that conduction loss increased with CB content. At the same time, the number and diameter of the semicircles decreased, which may be ascribed to strong conductivity and hidden polarization loss [20,33].

The above results show that dielectric loss is the primary loss mechanism of CB/PLA composites. When the CB content is low (0%, 5%), polarization relaxation is the main loss mechanism; when the CB content is high (10%, 15%, 20%, 25%), there is a synergistic effect of polarization relaxation loss and conduction loss.

3.3. Microwave Absorption Performance of Composites

Generally, the microwave absorption performance of composites can be evaluated by the reflection loss (RL) and effective absorption bandwidth (EAB, the frequency range corresponding to RL < −10 dB) [34,35,36]. The smaller the RL, the better the performance. RL below −10 dB indicates that > 90% of the electromagnetic wave can be absorbed [9].

According to the transmit-line theory, RL is usually obtained by Equations (3) and (4) [37,38,39,40]:

$$RL=20\lg \left|\frac{Z_{in}-Z_0}{Z_{in}+Z_0}\right|$$

(3)

$$Z_{in}=Z_0\sqrt{\frac{{\mu }_r}{{\epsilon }_r}}\tanh \left[j\left(\frac{2\pi fd}{c}\right)\sqrt{{\mu }_r{\epsilon }_r}\right]$$

(4)

where $Z_0=\sqrt{\frac{{\mu }_r}{{\epsilon }_r}}\approx 377\Omega$ is the impedance of free space, $Z_{in}$ is the input impedance of the absorber, ${\epsilon }_r$ and ${\mu }_r$ are the relative complex permittivity and permeability, respectively, $j$ is the imaginary unit, $f$ is the frequency of the incident microwave, $d$ is the thickness of the absorbing material, and $c$ is the velocity of light in free space.

Figure 9 is a three-dimensional plot of RL for CB/PLA composites with different thicknesses in the frequency range of 2–18 GHz. Figure 9a shows that pure PLA has no microwave absorption performance and is a wave-transmitting matrix. On the contrary, with the addition of CB, the absorbing properties of the composites gradually increased, but neither 5% nor 10% CB/PLA could achieve an effective absorbing effect. However, with the further increase in CB content, the absorbing properties of the composites were significantly improved and showed strong absorbing properties at smaller thicknesses, especially 25% CB/PLA. When the thickness was 2.1 mm, the minimum RL was −45.47 dB (13.68 GHz), and the EAB was 5.36 GHz (10.72–16.08 GHz; Figure 9f). In order to meet the thin and light requirements of the MAMs, the density of the composite was further tested, and the density of the 25% CB/PLA composite was only 1.19 g/cm³. The density was lower than that of pure PLA (1.25 g/cm³), which meets the requirements of lightweight material with broad application prospects.

The impedance matching Z and attenuation constant $\alpha$ are the two critical factors that affect the material’s excellent absorbing properties, which can be calculated by using Equations (5) and (6) [41,42].

$$Z=\left|\frac{Z_{{}_{in}}}{Z_0}\right|\propto 1$$

(5)

$$\alpha =\frac{\sqrt{2}\pi f}{c}\sqrt{\left({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }\right)+\sqrt{{\left({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }\right)}^2+{\left({\mu }^{″}{\epsilon }^{\prime }+{\mu }^{\prime }{\epsilon }^{″}\right)}^2}}$$

(6)

Figure 10 shows the impedance matching and attenuation constants of the CB/PLA composites. Generally, the impedance matching of the absorbing material should exceed 0.3, and the closer to 1, the better the impedance matching of the material [43,44,45]. Figure 10a,b shows opposite trends: with the increase in CB content, the impedance matching gradually decreased, while the attenuation constant increased continuously. As shown in Figure 10b, the special peak’s position in the figure is the same as in Figure 6c. Ideal MAMs require balance between suitable impedance-matching characteristics and strong attenuation capabilities [46]. A comprehensive comparison shows that, with the highest attenuation constant, the impedance matching of 25% CB/PLA was higher than 0.3 in the frequency range of 2–18 GHz, so the microwave absorption performance of 25% CB/PLA was best.

The 25% CB/PLA exhibited excellent absorbing properties. Figure 11a shows the 25% CB/PLA RL curves under different thicknesses in the frequency range of 2–18 GHz. The peak value of RL gradually moved from high frequency to low frequency as the thickness increased, and the peak value first increased and then decreased. It can be observed from the projection diagram of Figure 9f that the whole displacement of the minimum RL was an approximate fan shape, which can be explained by the quarter-wavelength cancelation theory [47,48]. Figure 11b shows the EAB corresponding to a 1.5–4.2 mm thickness. When the thickness was 2.1 mm, the EAB reached a maximum of 5.36 GHz, and the RL value reached −45.47 dB. Generally, the RL value is lower than −20 dB, indicating that > 99% of electromagnetic waves can be absorbed [49]. In addition, microwave absorption performance can be adjusted by changing the thickness. As shown in

Table 3. Performance comparison with other CB-based composites.

Absorber and Content	EAB/GHz	Minimum RL/dB	Thickness at Minimum RL/mm	Reference
(6%) CB/CF/EP	2.4	−31.8	2	[50]
(7%) CB/T-ZnO/EP	4.25	−17.36	3	[51]
(30%) CB/nmCIP/PU	3.6	−25.8	2	[52]
(5.5%) CB/SiO2f/PI	3.95	−46.18	1.6	[53]
(8%) CB/PAN/EP	6.7	−17.0	6.3	[54]
(30%) CB/CNTs	4.1	−53.6	2.7	[55]
(17%) CB/EP	0.6	−32.1	3	[56]
(25%) CB/PLA	5.36	−45.47	2.1	This work

, compared with other composites using CB absorbers, 25% CB/PLA is advantageous with strong absorption performance, broad bandwidth, small thickness, low cost, and low density, meeting the development requirements of “thin, light, wide and strong” MAMs. Meanwhile, the microwave absorption performance of single-component single-layer composites is similar to that of some multicomponent multilayer materials, avoiding complex processing problems, low efficiency, and high cost.

Based on the above investigation, the microwave absorption mechanism of CB/PLA composites is speculated and illustrated in Figure 12. With increased CB content, the conductive network is gradually completed and continues increasing, so the conduction loss is significantly enhanced. When CB particles are uniformly distributed in PLA, many interfaces, such as the CB/PLA interface, are formed. The number of interfaces increases correspondingly, and the interface loss increases significantly, as the CB content increases. Dipole polarization is promoted due to polar functional groups and surface defects in CB. Additionally, CB and PLA form protrusions and wrinkles at the contact surface, which enhances the reflection and scattering of electromagnetic waves inside the material [25,28]. The uniform distribution of CB particles also induces multiple reflection behaviors, which promote the attenuation of electromagnetic waves; thus, the CB/PLA composite exhibits superior microwave absorption effects.

4. Conclusions

The CB/PLA composite filament is prepared by a high-energy mixer and twin-screw extruder process, and then FFF is used to prepare lightweight, wideband, strong absorption, low-cost MAMs. The method is simple, highly efficient, and low in cost. Meanwhile, 3D printing technology provides a new method for the integrated preparation of material and structural design, which has several application prospects.

With the increase in CB content, the absorption performance of the composites improved significantly. When the CB content was 25%, the composite had the best absorption effect. When the matching thickness was 2.1 mm, the peak value of RL was −45.47 dB, and the EAB was 5.36 GHz (10.72–16.08 GHz). The absorbing mechanisms of CB/PLA composites mainly include the conduction loss, interface polarization loss, dipole polarization loss, and multiple reflections, which are related to the content and distribution of the CB particles. The synergistic effect between suitable impedance matching and multiple loss mechanisms gives the composite superior microwave absorption.