Enhancement of Electromagnetic Wave Shielding Effectiveness by the Incorporation of Carbon Nanofibers–Carbon Microcoils Hybrid into Commercial Carbon Paste for Heating Films

Carbon microcoils (CMCs) were formed on stainless steel substrates using C2H2 + SF6 gas flows in a thermal chemical vapor deposition (CVD) system. The manipulation of the SF6 gas flow rate and the SF6 gas flow injection time was carried out to obtain controllable CMC geometries. The change in CMC geometry, especially CMC diameter as a function of SF6 gas flow injection time, was remarkable. In addition, the incorporation of H2 gas into the C2H2 + SF6 gas flow system with cyclic SF6 gas flow caused the formation of the hybrid of carbon nanofibers–carbon microcoils (CNFs–CMCs). The hybrid of CNFs–CMCs was composed of numerous small-sized CNFs, which formed on the CMCs surfaces. The electromagnetic wave shielding effectiveness (SE) of the heating film, made by the hybrids of CNFs–CMCs incorporated carbon paste film, was investigated across operating frequencies in the 1.5–40 GHz range. It was compared to heating films made from commercial carbon paste or the controllable CMCs incorporated carbon paste. Although the electrical conductivity of the native commercial carbon paste was lowered by both the incorporation of the CMCs and the hybrids of CNFs–CMCs, the total SE values of the manufactured heating film increased following the incorporation of these materials. Considering the thickness of the heating film, the presently measured values rank highly among the previously reported total SE values. This dramatic improvement in the total SE values was mainly ascribed to the intrinsic characteristics of CMC and/or the hybrid of CNFs–CMCs contributing to the absorption shielding route of electromagnetic waves.


Introduction
The trend toward miniaturization and multifunctionality of electronic devices has exacerbated the problem of electromagnetic interference (EMI). As the applicable frequency range of electronic devices enters the higher frequency region, the shielding of electromagnetic wave radiation emitted from electronic devices is required to prevent not only the malfunction of other electronics, but also a significant threat to human health. This is because increased exposure can cause the device to malfunction and affect human health. The only solution to prevent damage from harmful radiation and protect electronics is to provide a shield that filters out the interference.
The shielding of electromagnetic waves typically proceeds via three main routes: reflection loss, absorption loss, and internal reflection loss [1]. For an absorption loss route greater than 10 dB, reflection and absorption loss routes are usually regarded as the main shielding routes [1][2][3][4]. In the relatively low-frequency range (typically less than 2.0 GHz), the reflection loss route is thought to be the crucial mechanism for preventing EMI [4,5]. However, at high operating frequencies (above 2.0 GHz), the absorption loss route is considered the main mechanism for preventing EMI [4,5]. High electrical conductivity is the key parameter for the shielding mechanism via the reflection loss route, while both high from the CNFs-CMCs hybrid material incorporated with carbon paste was investigated across the operating frequencies in the 1.5-40 GHz range. The results were compared with those for heating films made using a native commercial carbon paste or controllable CMCsincorporated carbon paste. The morphologies and electrical conductivities of different types of heating films were investigated. The main shielding mechanism of the heating film made from the hybrids of CNFs-CMCs incorporated carbon paste is suggested and discussed. Figure 1 shows the magnified FESEM images of the surface morphologies for samples A-I. Under a flow rate of 20 sccm SF 6 , the diameters of the CMCs increased with the SF 6 flow injection time (see Figure 1a-c). Under the 50 sccm and 100 sccm SF 6 flow rates, the CMCs diameters also tended to increase with increasing SF 6 flow injection time (see Figure 1d-f and Figure 1g-i, respectively). This reveals that the lowest SF 6 flow injection time in this study (5 min) can give rise to the CMCs with the smallest diameters, irrespective of the SF 6 flow rate. In addition, Figure 1 shows that overall the diameters of the CMCs are independent of the SF 6 flow rate. These results strongly suggest that the SF 6 flow injection time, instead of the SF 6 flow rate, can directly influence the formation of CMCs with a specific diameter.

Results and Discussion
CMCs-related materials could enhance SE values.
In the present work, controllable CMCs were obtained by manipulating the additive gas, SF6, and flow injection time. In addition, the hybrid formation of numerous small carbon nanofibers (CNFs) on the surfaces of CMCs (CNFs-CMCs) can be achieved by the injection of H2 flow with the cyclic process of SF6 flow. The SE of the heating film made from the CNFs-CMCs hybrid material incorporated with carbon paste was investigated across the operating frequencies in the 1.5-40 GHz range. The results were compared with those for heating films made using a native commercial carbon paste or controllable CMCs-incorporated carbon paste. The morphologies and electrical conductivities of different types of heating films were investigated. The main shielding mechanism of the heating film made from the hybrids of CNFs-CMCs incorporated carbon paste is suggested and discussed. Figure 1 shows the magnified FESEM images of the surface morphologies for samples A-I. Under a flow rate of 20 sccm SF6, the diameters of the CMCs increased with the SF6 flow injection time (see Figure 1a-c). Under the 50 sccm and 100 sccm SF6 flow rates, the CMCs diameters also tended to increase with increasing SF6 flow injection time (see Figure 1d-f and g-i, respectively). This reveals that the lowest SF6 flow injection time in this study (5 min) can give rise to the CMCs with the smallest diameters, irrespective of the SF6 flow rate. In addition, Figure 1 shows that overall the diameters of the CMCs are independent of the SF6 flow rate. These results strongly suggest that the SF6 flow injection time, instead of the SF6 flow rate, can directly influence the formation of CMCs with a specific diameter.    Figure 2 shows FESEM images of sample J, which were obtained by the injection of H 2 flow and the cyclic process of SF 6 flow. As shown in Figure 2a, a woolen yarn-type surface is present in sample J. The magnified FESEM image of sample J also indicated the existence of many small-sized CNFs on the CMCs surfaces (see Figure 2b,c). As previously reported, the small-sized CNFs around the CMCs seem to have a hybridized aspect between the CNFs and CMCs by the cyclic process of SF 6 flow [32][33][34][35][36][37][38][39][40]. This result suggests that the cyclic SF 6 flow with the injection of H 2 gas could produce CNFs-CMCs hybrids [40].

Results and Discussion
surface is present in sample J. The magnified FESEM image of sample J also indicated the existence of many small-sized CNFs on the CMCs surfaces (see Figure 2b,c). As previously reported, the small-sized CNFs around the CMCs seem to have a hybridized aspect between the CNFs and CMCs by the cyclic process of SF6 flow [32][33][34][35][36][37][38][39][40]. This result suggests that the cyclic SF6 flow with the injection of H2 gas could produce CNFs-CMCs hybrids [40]. The presence of different nanocarbon formation attributes in sample J compared to samples A-I can be explained as follows. It is known that producing a hybrid nanocarbon from typical nanocarbon materials, such as CNTs with CNFs and CMCs with carbon nanocoils, is very difficult because the transition metals used as catalysts to promote hybrid nanocarbon growth tend to easily diffuse into the interior of the carbon substrate during the reaction [41,42]. For the surfaces of CMCs during the reaction, the tiny Ni catalysts used for hybrid nanocarbon growth would not be sufficient because tiny Ni catalysts could easily diffuse into the interior of CMCs in an amorphous solid state [43][44][45].
Consequently, numerous small-sized CNFs could not be formed on the surfaces of the CMCs of samples A-I because of the lack of tiny Ni catalysts on the surfaces of the CMCs during the reaction.
However, a previous report revealed that the injection of H2 gas into the flow of C2H2, and an abundant C2H2 gas flow relative to the SF6 gas flow in the reaction environment are known to facilitate the formation of numerous small-sized CNFs by the generation of numerous tiny Ni catalysts from large-sized Ni catalysts [46]. In this work, the H2 flow injection for sample J was subjected to the injection of H2 flow into the C2H2 flow. Furthermore, the cyclic process in sample J could produce an abundant C2H2 flow environment compared with SF6 gas flow, especially during the SF6 flow-off period [40]. Compared to samples A-I, sample J produced a much higher number of tiny Ni catalysts, which could readily be placed on the surfaces of the CMCs. Consequently, this would result in the formation of small-sized CNFs on the surfaces of the CMCs owing to the increased number of tiny Ni catalysts, probably enough to overcome the insufficiency of Ni catalysts on the surfaces of the CMCs. This can be attributed to the diffusion of Ni catalysts into the interior of the CMCs during the reaction. Figure 3 shows systematic diagrams for the formation of samples A-I (C2H2 + SF6 gas flow system without cyclic process of SF6 gas flow, Figure 3a) and sample J (C2H2 + SF6 + H2 gas flow system with a cyclic process of SF6 gas flow, Figure 3b). Ni fragments were located merely on the heads of CMCs for samples A-I (Figure 3a). However, for Sample J, Ni fragments were present on the CMC head and the tiny-sized Ni fragments were present on the CMC surface. Moreover, the numerous tiny Ni fragments, produced by the injection of H2 gas into the C2H2 flow during the cyclic addition of SF6, were present on the surfaces of the CMCs as shown in Figure 3b. The presence of different nanocarbon formation attributes in sample J compared to samples A-I can be explained as follows. It is known that producing a hybrid nanocarbon from typical nanocarbon materials, such as CNTs with CNFs and CMCs with carbon nanocoils, is very difficult because the transition metals used as catalysts to promote hybrid nanocarbon growth tend to easily diffuse into the interior of the carbon substrate during the reaction [41,42]. For the surfaces of CMCs during the reaction, the tiny Ni catalysts used for hybrid nanocarbon growth would not be sufficient because tiny Ni catalysts could easily diffuse into the interior of CMCs in an amorphous solid state [43][44][45]. Consequently, numerous small-sized CNFs could not be formed on the surfaces of the CMCs of samples A-I because of the lack of tiny Ni catalysts on the surfaces of the CMCs during the reaction.
However, a previous report revealed that the injection of H 2 gas into the flow of C 2 H 2 , and an abundant C 2 H 2 gas flow relative to the SF 6 gas flow in the reaction environment are known to facilitate the formation of numerous small-sized CNFs by the generation of numerous tiny Ni catalysts from large-sized Ni catalysts [46]. In this work, the H 2 flow injection for sample J was subjected to the injection of H 2 flow into the C 2 H 2 flow. Furthermore, the cyclic process in sample J could produce an abundant C 2 H 2 flow environment compared with SF 6 gas flow, especially during the SF 6 flow-off period [40]. Compared to samples A-I, sample J produced a much higher number of tiny Ni catalysts, which could readily be placed on the surfaces of the CMCs. Consequently, this would result in the formation of small-sized CNFs on the surfaces of the CMCs owing to the increased number of tiny Ni catalysts, probably enough to overcome the insufficiency of Ni catalysts on the surfaces of the CMCs. This can be attributed to the diffusion of Ni catalysts into the interior of the CMCs during the reaction. Figure 3 shows systematic diagrams for the formation of samples A-I (C 2 H 2 + SF 6 gas flow system without cyclic process of SF 6 gas flow, Figure 3a) and sample J (C 2 H 2 + SF 6 + H 2 gas flow system with a cyclic process of SF 6 gas flow, Figure 3b). Ni fragments were located merely on the heads of CMCs for samples A-I ( Figure 3a). However, for Sample J, Ni fragments were present on the CMC head and the tiny-sized Ni fragments were present on the CMC surface. Moreover, the numerous tiny Ni fragments, produced by the injection of H 2 gas into the C 2 H 2 flow during the cyclic addition of SF 6 , were present on the surfaces of the CMCs as shown in Figure 3b.
For the mass production of CMCs-related nanocarbon samples, we chose the conditions of samples E and J among the various types of CMCs-related nanocarbon formation reaction conditions. Almost 19 g of CMCs per approximately 0.6 g Ni catalyst could be obtained in a one-batch reaction, as shown in Figure 4a.  For the mass production of CMCs-related nanocarbon samples, we chose the conditions of samples E and J among the various types of CMCs-related nanocarbon formation reaction conditions. Almost 19 g of CMCs per approximately 0.6 g Ni catalyst could be obtained in a one-batch reaction, as shown in Figure 4a.    For the mass production of CMCs-related nanocarbon samples, we chose the condi tions of samples E and J among the various types of CMCs-related nanocarbon formation reaction conditions. Almost 19 g of CMCs per approximately 0.6 g Ni catalyst could b obtained in a one-batch reaction, as shown in Figure 4a.   After achieving mass production of CMCs-related materials, we made a blend of 30 wt% sample J with 70 wt% commercial carbon paste. A blend of 30 wt% sample E with 70 wt% commercial carbon paste was also prepared to compare the SE values of samples J and E. These blends were coated on a PET film by a commonly used printing method using a spatula blade. Figure 6 shows the SE values of these coated PET films across the operating frequencies in the 1.5-40 GHz range.
Compared with the total SE values of the PET film coated with 30 wt% sample Eincorporated carbon paste (see Table 1), the value of the PET film coated with 30 wt% sample J-incorporated carbon paste was more than two-fold higher in dB scale across the entire operating frequency range, as shown in Figure 6. After achieving mass production of CMCs-related materials, we made a blend of 30 wt% sample J with 70 wt% commercial carbon paste. A blend of 30 wt% sample E with 70 wt% commercial carbon paste was also prepared to compare the SE values of samples J and E. These blends were coated on a PET film by a commonly used printing method using a spatula blade. Figure 6 shows the SE values of these coated PET films across the operating frequencies in the 1.5-40 GHz range. Compared with the total SE values of the PET film coated with 30 wt% sample Eincorporated carbon paste (see Table 1), the value of the PET film coated with 30 wt% sample J-incorporated carbon paste was more than two-fold higher in dB scale across the entire operating frequency range, as shown in Figure 6.
This dramatic increase in the total SE values of the coated PET film by 30 wt% sample J-incorporated carbon paste seems to be partly ascribed to the enhanced electrical conductivity (from (1.84 ± 0.46) × 10 3 S/m to (2.17 ± 0.23) × 10 3 S/m, see Table 2) by the hybrid formation between the numerous small-sized CNFs and the CMCs in sample J, as in a previous report [30]. Furthermore, the numerous small-sized CNFs on the surfaces of the CMCs in sample J intersected with one another. When an incoming electromagnetic wave reaches these intersected-CNFs, electric current flows into the intersections and dissipates     This dramatic increase in the total SE values of the coated PET film by 30 wt% sample J-incorporated carbon paste seems to be partly ascribed to the enhanced electrical conductivity (from (1.84 ± 0.46) × 10 3 S/m to (2.17 ± 0.23) × 10 3 S/m, see Table 2) by the hybrid formation between the numerous small-sized CNFs and the CMCs in sample J, as in a previous report [30]. Furthermore, the numerous small-sized CNFs on the surfaces of the CMCs in sample J intersected with one another. When an incoming electromagnetic wave reaches these intersected-CNFs, electric current flows into the intersections and dissipates in various directions, thereby inducing an electromotive force and generating a variable magnetic field [25]. The geometry of these CNFs holds and rotates incoming electromagnetic waves within the generated variable magnetic field. Thus, the incoming electromagnetic wave energy is absorbed into these CNFs and is finally converted into thermal energy [26]. Therefore, these intersections can contribute to the absorption mechanism for shielding against electromagnetic waves. Consequently, the total SE values of the PET film coated with the 30 wt% sample J-incorporated carbon paste were higher than those of the 30 wt% sample E-incorporated carbon paste. This matches the measured SE values shown in Figure 6. The skin depth (δ) of a shielding material is defined as δ = (πσf µ) −1/2 [2], indicating that δ 2 is inversely proportional to the electrical conductivity (σ), frequency (f ), and magnetic permeability (µ). Therefore, higher magnetic permeability can efficiently reduce the skin depth of the shielding material, thereby enhancing the SE values. The intrinsic characteristics of CMCs, and the aspect of numerous small-sized CNFs intersecting with one another in sample J, can enhance the magnetic field and then absorb incoming EM waves. Consequently, they can enhance magnetic permeability (µ), resulting in an improvement in the absorption loss of electromagnetic waves. Therefore, the SE values of the PET film coated with the 30 wt% sample J-incorporated carbon paste were much higher than those of the 30 wt% sample E-incorporated carbon paste across the entire operating frequency range.
Indeed, the PET film coated with 30 wt% sample J-incorporated carbon paste had total SE values above 20 dB throughout the entire range of operating frequencies. Compared with the previously reported total SE values, the presently measured values seem to be ranked in the top tier (see Table 2). Therefore, we suggest that the hybridized CNFs-CMCs sample can be effectively used in diverse industrial fields.
A blend of 2 wt% sample J with 98 wt% commercial carbon paste was also prepared to determine the optimal amount of sample J to add to the commercial carbon paste. Indeed, the already established manufacturing process for the commercial heating film by SH Korea Co. required the injection of the smallest possible amount of sample J, because a considerable amount of sample J may cause poor adhesion between the paste layer and the PET substrate. The PET film coated with the 2 wt% sample J-incorporated carbon paste seemed to satisfy the adhesion problem, as well as provide good SE values, as shown in Figure 6.
For the manufacturing process of the commercial heating film, we used a coated PET film with 5 wt% sample J-incorporated carbon paste. In this case, we used a typical laminating process using the gravure method of SH Korea Co. The typical thickness of the coated layer using carbon paste was approximately 50 µm. Basically, a single des-HF was composed of two electromagnetic wave shielding layers located at the front and back sides among eight different functional individual thin layers. Therefore, we estimated a total thickness of 100 µm for the coated layers using 5 wt% sample J-incorporated carbon paste. Figure 7 shows the total SE values for the conventional heating film using native commercial carbon paste, des-HF manufactured by 5 wt% sample E-incorporated carbon paste, and des-HF manufactured by 5 wt% sample J-incorporated carbon paste. As discussed in the results depicted in Figure 6, the total SE values of the native conventional heating film using commercial carbon paste were enhanced by the incorporation of 5 wt% sample E, and further enhanced by the incorporation of 5 wt% sample J. The cause for the improvement of the SE values seems to be largely attributable to the fact that numerous small CNFs intersected with one another and/or the intrinsic characteristics of CMCs; however, the electrical conductivity of the coated PET film using the commercial carbon paste was decreased by the incorporation of controllable CMCs, or hybrids of CNFs-CMCs in the commercial carbon paste (see Table 1).
Molecules 2023, 28, x FOR PEER REVIEW 9 of 16 Figure 7. The total SE values for the conventional heating film using native commercial carbon paste, the des-HF manufactured by 5 wt% sample-E incorporated carbon paste, and the des-HF manufactured by 5 wt% sample-J incorporated carbon paste. Figure 8 shows the total SE values, the SE values for the absorption loss, and the SE values for the reflection loss of the des-HF manufactured by 5 wt% sample J-incorporated carbon paste across the operating frequencies in the 1.5-40 GHz range. Above the 4.0 GHz frequency range, the absorption SE values of the des-HF manufactured by 5 wt% sample J-incorporated carbon paste increased and approached the values of the total SE values, Figure 7. The total SE values for the conventional heating film using native commercial carbon paste, the des-HF manufactured by 5 wt% sample-E incorporated carbon paste, and the des-HF manufactured by 5 wt% sample-J incorporated carbon paste. Figure 8 shows the total SE values, the SE values for the absorption loss, and the SE values for the reflection loss of the des-HF manufactured by 5 wt% sample J-incorporated carbon paste across the operating frequencies in the 1.5-40 GHz range. Above the 4.0 GHz frequency range, the absorption SE values of the des-HF manufactured by 5 wt% sample J-incorporated carbon paste increased and approached the values of the total SE values, as shown in Figure 8. This confirms that the higher total SE values of the hybrids of CNFs-CMCs in this work are mainly attributable to the enhanced absorption loss across the operating frequencies in the 4.0-40 GHz range. Therefore, the enhanced total SE values obtained using the hybrids of CNFs-CMCs were mainly ascribed to the enhanced absorption shielding loss, contributed by the intrinsic characteristics of CMCs and the aspect of numerous small-sized CNFs intersecting with one another in sample J. Figure 7. The total SE values for the conventional heating film using native commercial carbon p the des-HF manufactured by 5 wt% sample-E incorporated carbon paste, and the des-HF man tured by 5 wt% sample-J incorporated carbon paste. Figure 8 shows the total SE values, the SE values for the absorption loss, and th values for the reflection loss of the des-HF manufactured by 5 wt% sample J-incorpor carbon paste across the operating frequencies in the 1.5-40 GHz range. Above the 4.0 frequency range, the absorption SE values of the des-HF manufactured by 5 wt% sam J-incorporated carbon paste increased and approached the values of the total SE va as shown in Figure 8. This confirms that the higher total SE values of the hybrids of CN CMCs in this work are mainly attributable to the enhanced absorption loss across the erating frequencies in the 4.0-40 GHz range. Therefore, the enhanced total SE value tained using the hybrids of CNFs-CMCs were mainly ascribed to the enhanced absorp shielding loss, contributed by the intrinsic characteristics of CMCs and the aspect of merous small-sized CNFs intersecting with one another in sample J.

Materials and Methods
As a catalyst for the formation of CMCs, approximately 0.1 g of bunch-type Ni powder (99.7%), with particle diameters ranging from 1 µm to 10 µm, was spread onto a 2 mm-thick, boat-like stainless steel (SUS304) substrate. A thermal chemical vapor deposition (CVD) system was employed for the formation of CMCs, using C 2 H 2 as the source gas and SF 6 as the additive gas. The deposition reaction conditions for the formation of the various CMCs-related samples are listed in Table 3. Ten samples (samples A-J) with different combinations of gas flow rate, gas flow injection time, and gas type were prepared.
Regarding the application of SF 6 gas in sample J, the cyclic process was conducted by simply switching the SF 6 flow on and off continuously. The gas flow sequence mirrored the iterative order of the reaction processes: C 2 H 2 + H 2 + SF 6 flow (C 2 H 2 flow-on, H 2 flow-on, and SF 6 flow-on) followed by C 2 H 2 + H 2 flow (C 2 H 2 flow-on, H 2 flow-on, and SF 6 flow-off), as shown in Figure 9. The cycle period was defined as the sum of the time the source gases were composed of C 2 H 2 + H 2 + SF 6 flow, and the time the source gases consisted solely of C 2 H 2 + H 2 flow. For sample J, the on and off times for the SF 6 flow injection were set at 1.5 min, resulting in a total duration of 3.0 min for one cycle. Because the total cyclic on/off modulation of the SF 6 flow was 15 min, five cycles were performed during the reaction.  The morphologies of the CMCs samples were investigated in detail by field emission scanning electron microscopy (FESEM; S-4200 Hitachi, Tokyo, Japan). The thickness of the sample was measured using a micrometer (406-250-30 Mitutoyo, Nakagawa, Japan) and corrected using cross-sectional FESEM images. Resistivity values were obtained by using a four-point probe (labsysstc-400 Nextron, Busan, Republic of Korea) connected to a source meter (2400 Source Meter Keithley, Cleveland, OH, USA) and by performing calculations using Ohm's law with a correction factor, according to the method proposed by Smits [32]. The four-point probe system consisted of four, straight-lined probes with an equal inter-probe spacing of 3.0 mm. A constant current (I) was supplied through the two outer probes, and the output voltage (V) was measured using the two inner probes [31]. Correction factors (C and F) were obtained from Smits et al. [32]. Surface and volume resistivities were calculated using the following equations [32,33]: where a, d, w, and s denote the length, width, and thickness of the sample and the interprobe spacing, respectively.
The final product of the heating films, namely dual electromagnetic shielding premium heating films (des-HFs), were prepared by a typical laminating process with the gravure method of SH Korea Co. (see Figure 10) using the blends of CMCs-related samples and commercially supplied carbon paste. 2-butoxyethyl acetate was used as the diluting solution during the gravure coating process with these blends. The morphologies of the CMCs samples were investigated in detail by field emission scanning electron microscopy (FESEM; S-4200 Hitachi, Tokyo, Japan). The thickness of the sample was measured using a micrometer (406-250-30 Mitutoyo, Nakagawa, Japan) and corrected using cross-sectional FESEM images. Resistivity values were obtained by using a four-point probe (labsysstc-400 Nextron, Busan, Republic of Korea) connected to a source meter (2400 Source Meter Keithley, Cleveland, OH, USA) and by performing calculations using Ohm's law with a correction factor, according to the method proposed by Smits [32]. The four-point probe system consisted of four, straight-lined probes with an equal inter-probe spacing of 3.0 mm. A constant current (I) was supplied through the two outer probes, and the output voltage (V) was measured using the two inner probes [31]. Correction factors (C and F) were obtained from Smits et al. [32]. Surface and volume resistivities were calculated using the following equations [32,33]: where a, d, w, and s denote the length, width, and thickness of the sample and the interprobe spacing, respectively. The final product of the heating films, namely dual electromagnetic shielding premium heating films (des-HFs), were prepared by a typical laminating process with the gravure method of SH Korea Co. (see Figure 10) using the blends of CMCs-related samples and commercially supplied carbon paste. 2-butoxyethyl acetate was used as the diluting solution during the gravure coating process with these blends.
Surface resistivity: ρs= I C d , s , volume resistivity: ρv=ρs w F where a, d, w, and s denote the length, width, and thickness of the sample and the inter probe spacing, respectively.
The final product of the heating films, namely dual electromagnetic shielding pre mium heating films (des-HFs), were prepared by a typical laminating process with th gravure method of SH Korea Co. (see Figure 10) using the blends of CMCs-related sam ples and commercially supplied carbon paste. 2-butoxyethyl acetate was used as the di luting solution during the gravure coating process with these blends. A single des-HF film was composed of eight different functional individual thi films, as shown in Figure 11. A single des-HF film was composed of eight different functional individual thin films, as shown in Figure 11.
Molecules 2023, 28, x FOR PEER REVIEW Figure 11. Structure of dual electromagnetic shielding premium heating film.
The SE values of the des-HFs were measured using a waveguide method wit tor network analyzer (VNA; 37369C Anritsu, Kanagawa, Japan), as shown in Fi The results were compared with those of a heating film made using commercially ble carbon paste (CKC-300 AF Electrochem, Incheon, Republic of Korea). The s the VNA system consisted of a sample holder with its exterior connected to the sy coaxial sample holder and a coaxial transmission test specimen were set up acco the waveguide method. The scattering parameters (S11 and S21) were measured in quency range of 1.5−40 GHz using the VNA [34][35][36][37][38]. The power coefficients, nam flectivity (R), absorptivity (A), and transmissivity (T), were calculated using the fo equations: R = PR/PI = |S11| 2 and T = PT/PI = |S21| 2 , where PI, PR, PA, and PT are the i reflected, absorbed, and transmitted powers of an electromagnetic wave, respectiv The power coefficient relationships were expressed as R + A + T = 1. The SE of the magnetic waves was calculated from the scattering parameters using the followin tion: The SE values of the des-HFs were measured using a waveguide method with a vector network analyzer (VNA; 37369C Anritsu, Kanagawa, Japan), as shown in Figure 12. The results were compared with those of a heating film made using commercially available carbon paste (CKC-300 AF Electrochem, Incheon, Republic of Korea). The setup for the VNA system consisted of a sample holder with its exterior connected to the system. A coaxial sample holder and a coaxial transmission test specimen were set up according to the waveguide method. The scattering parameters (S 11 and S 21 ) were measured in the frequency range of 1.5−40 GHz using the VNA [34][35][36][37][38]. The power coefficients, namely reflectivity (R), absorptivity (A), and transmissivity (T), were calculated using the following equations: R = P R /P I = |S 11 | 2 and T = P T /P I = |S 21 | 2 , where P I , P R , P A , and P T are the incident, reflected, absorbed, and transmitted powers of an electromagnetic wave, respectively [38]. The power coefficient relationships were expressed as R + A + T = 1. The SE of the electromagnetic waves was calculated from the scattering parameters using the following equation: where, SE Tot , SE R , and SE A denote the total, reflection, and absorption SE values, respectively [37,38].
The power coefficient relationships were expressed as R + A + T = 1. The SE of the electromagnetic waves was calculated from the scattering parameters using the following equation: where, SETot, SER, and SEA denote the total, reflection, and absorption SE values, respectively [37,38].

Conclusions
Controllable CMCs, especially the controlled-diameter size of CMCs, could be achieved by manipulating the injection gas parameters under C 2 H 2 + SF 6 gas flow in a thermal chemical vapor deposition system. The diameters of the CMCs were independent of the SF 6 flow rate, but strongly dependent on the SF 6 flow injection time. The cyclic process of SF 6 flow with the injection of H 2 flow could produce the formation of CNFs-CMCs hybrid. The formation of the hybrids of CNFs-CMCs by the cyclic process of SF 6 flow with the injection of H 2 flow was explained by the generation of a large number of tiny Ni catalysts to overcome the deficiency of tiny Ni catalysts on the surfaces of the CMCs. This was attributed to the facile diffusion of Ni catalysts into the interior of the CMCs during the reaction. Systematic diagrams were deployed to assist the different formation of the controllable CMCs and hybrids of CNFs-CMCs, according to the different gas injection aspects.
For one batch reaction, approximately 20 g of the hybrids of CNFs-CMCs with numerous small-sized CNFs around the CMCs was obtained by about 0.6 g Ni catalyst onto a 2mmthick, boat-like stainless steel (SUS304) substrate.
Compared with the total SE values of the PET film coated with 30 wt% controllable CMCs-incorporated carbon paste, the values of the PET film coated with 30 wt% hybrids of CNFs-CMCs incorporated carbon paste were more than two-fold higher in dB scale across the entire operating frequency range. This dramatic increase in the total SE values was partly ascribed to the enhanced electrical conductivity, due to the hybrid formation between the numerous small CNFs and the CMCs. Furthermore, the intrinsic characteristics of CMCs and the behavior of numerous small-sized CNFs intersecting with one another in the CNFs-CMCs hybrids can enhance the magnetic field and absorb incoming EM waves. Consequently, they can improve the absorption loss of electromagnetic waves. Therefore, the SE values of the PET film coated with the hybrids of CNF-CMCs incorporated carbon paste were much higher than those with 30 wt% controllable CMCs incorporated carbon paste across the entire operating frequency range.
Although the electrical conductivity of the coated PET film using the commercial carbon paste was decreased by the incorporation of the hybrids of CNFs-CMCs in the commercial carbon paste, the total SE values of the conventional heating film using the commercial carbon paste were significantly enhanced by the incorporation of 5 wt% hybrids of CNFs-CMCs. The cause for this dramatic enhancement of the SE could be attributable to the fact that numerous small-sized CNFs intersect with one another and the intrinsic characteristics of CMCs. Data Availability Statement: All data used and/or analyzed during the current study are shown within the study. If further data are required, theycan be made available by the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.