Effects of Multi-Components on the Microwave Absorption and Dielectric Properties of Plasma-Sprayed Carbon Nanotube/Y2O3/ZrB2 Ceramics

Carbon nanotube (CNT)-reinforced Y2O3/ZrB2 ceramics were fabricated via planetary ball milling and atmospheric-pressure plasma spraying for the first time. The phase composition, micromorphology, and electromagnetic (EM) wave absorption performance of the Y2O3/ZrB2/CNT hybrid was investigated from 8.2 to 12.4 GHz. Both the real and imaginary parts of the complex permittivity were enhanced as the ZrB2 and CNT content increased. The Y2O3/ZrB2/CNT hybrids corresponded to a ZrB2 content of 15 wt.%, and the CNT content was 2 wt.% and showed an exceptional EM wave absorption capability, with a minimum reflection loss of −25.7 dB at 1.9 mm thickness, and the effective absorption band was in a full X-band. These results indicate that an appropriate CNT or ZrB2 content can tune the complex permittivity and absorption performance of the Y2O3/ZrB2/CNT ceramics.


Introduction
Electromagnetic (EM) pollution has become a serious problem to human health and the environment due to the growth of the electronic industry and radio communication technologies [1][2][3]. To solve this problem, designing and fabricating suitable microwave absorption materials is an urgent task for researchers in the field of materials science [4][5][6][7][8]. Currently, polymer-based absorbing materials used as fillers have limited utility under special environments due to their relatively poor mechanical performance. In contrast, ceramic-based absorbers have received extensive attention due to their excellent mechanical properties [9][10][11]. Y 2 O 3 is a sintering aid that can be added into ceramics such as SiC, SiAlON, and Al 2 O 3 ; this doping promotes the solid-phase sintering of materials and can effectively improve the mechanical properties of ceramics [12,13]. In addition, zirconium diboride (ZrB 2 ) has a high strength and electrical conductivity (∼6 × 10 4 Ω −1 ·cm −1 ), high melting point (3200 • C), good thermal shock resistance, and good oxidation resistance at a high temperature. Thus, it has attracted attention as a microwave-absorption material [14].
Such excellent features can be attributed to the unique crystal structure of ZrB 2 . Typically, the graphite-like layered structure of B atoms and the electronic structure of the Zr outer layer leads to the formation of highly conductive ZrB 2 [15]. The high conductivity implies a large value of ε so that the material has a good dielectric loss capability, thus causing the EM energy to be dissipated in the form of Joule heat by inducing a high current [16]. However, the strong covalent bonds of ZrB 2 imply that hot pressing or spark plasma sintering at temperatures over 2000 • C are usually required to meet the densification of ZrB 2 -based ceramics [17]. To solve the densification problem, coatings deposited by the atmospheric-pressure plasma spraying (APS) process can effectively tune the multilayer Nanomaterials 2021, 11, 2640 2 of 9 structure and porosity and thus obtain a high bonding strength; this has become a key method for preparing ZrB 2 -based ceramic coatings.
APS outperforms traditional methods of ceramic coating, such as thermal spraying [18], laser cladding [19], micro-arc oxidation [20], and combustion synthesis [21]. It is an exceptional technology that can meet commercial requirements due to its reduced cycle times, high deposition efficiencies, low cost, and excellent microwave absorption performance. More importantly, the integration of highly conductive carbon nanotubes (CNTs) can be further optimized to control the complex permittivity and the microwave-absorption capabilities of the Y 2 O 3 /ZrB 2 ceramic [22][23][24].
Here, Y 2 O 3 /ZrB 2 /CNT hybrids with a low CNT and ZrB 2 content were fabricated by APS technology for the first time. The phase composition, micromorphology, and EM properties of the hybrid were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and a vector network analyzer (VNA), respectively. Comparative analysis of the EM properties of ceramics doped with different contents of ZrB 2 and CNT showed that the plasma-sprayed Y 2 O 3 /ZrB 2 /CNT hybrids, corresponding to a ZrB 2 content of 15 wt.% and the CNT content of 2 wt.%, have the best EM wave-absorption capability, with a minimum reflection loss (RL min ) of −25.7 dB at 1.9 mm thickness, as well as an effective absorption band (EAB) in the full X-band. Therefore, Y 2 O 3 /ZrB 2 /CNT ceramics prepared via plasma-spraying could provide a theoretical basis and technical support for mass production in the field of ceramic-based high-temperature materials.

Experimental
, ZrB 2 (99.5%) and CNT were used as raw materials. The Y 2 O 3 /ZrB 2 / CNT ceramics were fabricated with different Y 2 O 3 contents (76, 78, 81, 83, and 85 wt.%); the corresponding samples were denoted as Y76, Y78, Y81, Y83, and Y85 for simplicity. The mass percentages (wt.%) of the different Y 2 O 3 /ZrB 2 /CNT ceramics are listed in Table 1. The desired amount of Y 2 O 3 , ZrB 2 particles, and CNT were evenly mixed using planetary ball milling. The fluidity of the powder was increased with spray drying technology, leading to spray-dried powders with a size of 30-80 µm. The Y 2 O 3 /ZrB 2 /CNT ceramic coating was prepared via the APS process, in which Ar and N 2 were used as plasma gases, with flow rates of 50 and 10 standard liters per minute (slpm), respectively. Experimental parameters for the spray conditions were optimized for good deposition efficiency. The net arc input power and powder flow rate were 25 kW and 4 g/min, respectively; the spraying distance and coating thickness were 100 mm and 2 mm, respectively. Finally, the ceramic samples were polished to a thickness of 1.2 mm for measuring EM parameters. The phase compositions of the plasma-sprayed samples were characterized through XRD with Cu-Kα radiation. The microstructures of the samples were observed by SEM (JEOL JSM-6360LV). The EM parameters of the Y 2 O 3 /ZrB 2 /CNT specimens with a dimension of 22.86 mm (length) × 10.16 mm (width) × 1.2 mm (thickness) were investigated in a full X-band (8.2-12.4 GHz) by VNA (Agilent technologies E8362B). Figure 1 shows the XRD pattern of the plasma-sprayed Y 2 O 3 /ZrB 2 /CNT samples. The different Y 2 O 3 /ZrB 2 /CNT ceramics were only composed of Y 2 O 3 , ZrB 2 , and CNT crystalline phases. The peak intensity of Y 2 O 3 obviously but gradually decreased with a Nanomaterials 2021, 11, 2640 3 of 9 decreasing Y 2 O 3 content. The peak intensity changes in the ZrB 2 and CNT were the same as Y 2 O 3 when studied in the Y 2 O 3 /ZrB 2 /CNT specimens. There was no oxide peak for ZrB 2 in the XRD spectrum, suggesting that the plasma-spray parameters were suitable, and the deposition efficiency was satisfactory.

Results and Discussion
Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 10 Figure 1 shows the XRD pattern of the plasma-sprayed Y2O3/ZrB2/CNT samples. The different Y2O3/ZrB2/CNT ceramics were only composed of Y2O3, ZrB2, and CNT crystalline phases. The peak intensity of Y2O3 obviously but gradually decreased with a decreasing Y2O3 content. The peak intensity changes in the ZrB2 and CNT were the same as Y2O3 when studied in the Y2O3/ZrB2/CNT specimens. There was no oxide peak for ZrB2 in the XRD spectrum, suggesting that the plasma-spray parameters were suitable, and the deposition efficiency was satisfactory. SEM images of Y2O3/ZrB2/CNT ceramics are shown in Figure 2. ZrB2 and CNT were evenly mixed with the Y2O3 matrix. The presence of a microporous structure during the plasma-spraying process may be related to unmelted agglomerates or resolidified agglomerates. Here, the theoretical density of Y2O3/ZrB2/CNT ceramics could be deduced according to the density of ZrB2 (6.08 g·cm 3 ) and Y2O3 powder (5.01 g·cm 3 ). Correspondingly, the relative densities of the Y2O3/ZrB2/CNT hybrid decreased with the Y2O3 powder content. Furthermore, the decrease in Y2O3 content increased the porosity of the material, but the increase in CNT content better filled the pores of the Y2O3/ZrB2/CNT ceramics ( Figure 2a  In general, EM waves can induce two kinds of currents in the absorber: conduction current and displacement current. Within that, the conduction current comes from moving charge carriers and increased conductivity, which in turn, increases the value of ε . The ε value is then related to the dielectric loss (tan δ ε = ε /ε ) capability of the absorber [25,26]. The displacement current is impacted by the dielectric polarization of local charge carriers, and the enhanced ε value can be attributed to the dielectric polarization and space charge polarization effects [27,28]. Figure 3 presents plots of ε and ε over 8.2 to 12.4 GHz, as the content of Y 2 O 3 changed. In the full X-band, both ε and ε of the five samples slightly decreased as the frequency increased. In the Y85 sample, the ε was about 12.5, and the ε was only 0.8, indicating that the high content of Y 2 O 3 and the relatively low content of ZrB 2 lead to lower complex dielectric properties when there was no CNT. This also shows that the Y85 sample may possess a lower dielectric loss ability and poor absorbing ability.

Results and Discussion
When the ZrB 2 content was 15 wt.%, the ε and ε values of the Y83 sample were slightly enhanced; the Y81 sample had significantly improved ε and ε values. We made a control Y85 ceramic without CNT, and we found that the ε and ε of Y85 could increase from 12.8 to 34.6 and 0.8 to 18.0, respectively. When the content of ZrB 2 was 20 wt.%, the ε and ε of Y78 were higher than the Y83 sample. The relationship of Y76 and Y81 was the same as the result of CNT at 2 wt.%. These data indicated that Y76 had the highest values of ε and ε : 41.1 and 23.4 versus the other four samples. The Y85, Y83, and Y81 samples had ε and ε values that gradually increased as the content of Y 2 O 3 decreased. These changes were attributed to the enhancement of the CNT content, which in turn, increased In general, EM waves can induce two kinds of currents in the absorber: conduction current and displacement current. Within that, the conduction current comes from moving charge carriers and increased conductivity, which in turn, increases the value of ε″. The ε″ value is then related to the dielectric loss (tan δε = ε″/ε′) capability of the absorber [25,26]. The displacement current is impacted by the dielectric polarization of local charge carriers, and the enhanced ε′ value can be attributed to the dielectric polarization and space charge polarization effects [27,28]. Figure 3 presents plots of ε′ and ε″ over 8.2 to 12.4 GHz, as the content of Y2O3 changed. In the full X-band, both ε′ and ε″ of the five samples slightly decreased as the frequency increased. In the Y85 sample, the ε′ was about 12.5, and the ε″ was only 0.8, indicating that the high content of Y2O3 and the relatively low content of ZrB2 lead to lower complex dielectric properties when there was no CNT. This also shows that the Y85 sample may possess a lower dielectric loss ability and poor absorbing ability.
When the ZrB2 content was 15 wt.%, the ε′ and ε″ values of the Y83 sample were slightly enhanced; the Y81 sample had significantly improved ε′ and ε″ values. We made a control Y85 ceramic without CNT, and we found that the ε′ and ε″ of Y85 could increase from 12.8 to 34.6 and 0.8 to 18.0, respectively. When the content of ZrB2 was 20 wt.%, the ε′ and ε″ of Y78 were higher than the Y83 sample. The relationship of Y76 and Y81 was the same as the result of CNT at 2 wt.%. These data indicated that Y76 had the highest values of ε′ and ε″: 41.1 and 23.4 versus the other four samples. The Y85, Y83, and Y81 samples had ε′ and ε″ values that gradually increased as the content of Y2O3 decreased. These changes were attributed to the enhancement of the CNT content, which in turn, increased the conductivity of the material and improved the dielectric properties  the Y78 and Y76 samples, the ε′ and ε″ values of the materials exhibited a significant i crease when the content of ZrB2 (20 wt.%) remained the same, due to the increase in t content of highly conductive CNTs. These results show that increasing the content ZrB2 and CNT improves the ε′ and ε″ of the Y2O3/ZrB2/CNT ceramic. The improve conductivity of the mobile electrons in the absorber was affected by defects such as da gling bonds and vacancies, especially those caused by the interface between the Zr and Y2O3 phases [29][30][31]. Under the effect of the EM field, free carriers in the absorb could accumulate at these interfaces, thus leading to space charge polarization and a increasing ε′. Therefore, the enhanced ε′ value was related to an increased ZrB2 conte and the number of ZrB2/Y2O3 interfaces. Furthermore, a higher ZrB2 and CNT conte would also lead to more permeable electrical pathways, thus improving the ε″ valu Moreover, when the content of CNT was equal, the enhanced ZrB2 content increased t value of ε′ and ε″. The increase in CNT content enhanced the ε′ and ε″ values of t Y2O3/ZrB2/CNT ceramic when the content of ZrB2 was equal. Therefore, we conclude that the presence of ZrB2 or CNT enacted a significant effect on the dielectric properti of Y2O3/ZrB2/CNT ceramics.  increase when the content of ZrB 2 (20 wt.%) remained the same, due to the increase in the content of highly conductive CNTs. These results show that increasing the content of ZrB 2 and CNT improves the ε and ε of the Y 2 O 3 /ZrB 2 /CNT ceramic. The improved conductivity of the mobile electrons in the absorber was affected by defects such as dangling bonds and vacancies, especially those caused by the interface between the ZrB 2 and Y 2 O 3 phases [29][30][31]. Under the effect of the EM field, free carriers in the absorber could accumulate at these interfaces, thus leading to space charge polarization and an increasing ε . Therefore, the enhanced ε value was related to an increased ZrB 2 content and the number of ZrB 2 /Y 2 O 3 interfaces. Furthermore, a higher ZrB 2 and CNT content would also lead to more permeable electrical pathways, thus improving the ε value. Moreover, when the content of CNT was equal, the enhanced ZrB 2 content increased the value of ε and ε . The increase in CNT content enhanced the ε and ε values of the Y 2 O 3 /ZrB 2 /CNT ceramic when the content of ZrB 2 was equal. Therefore, we concluded that the presence of ZrB 2 or CNT enacted a significant effect on the dielectric properties of Y 2 O 3 /ZrB 2 /CNT ceramics.
The EM wave absorption performance of the plasma-sprayed Y 2 O 3 /ZrB 2 /CNT ceramic coatings from 8.2 to 12.4 GHz are shown in Figure 4a-e. In Figure 4a, the minimum reflection loss (RL min ) of the Y85 sample was only −3.8 dB at 9.4 GHz, with a 2.2 mm thickness. The Y83 sample (Figure 4b) had a RL min value of −25.7 dB at 12.3 GHz, with a 1.9 mm thickness. The improved microwave absorption ability may have been caused by the enlarged ε and ε values from CNTs, forming a dense conductive percolation network. This network increased the conductivity loss. Furthermore, the effective absorption band (the RL min was lower than −10 dB) of Y83 ceramics covered the full X-band when the absorber's thickness was 2.0 mm. However, the Y81 ceramic showed an RL min value of −13.0 dB at 1.3 mm thickness from 8.2 to 12.4 GHz (Figure 4c). The decreasing RL min value may be attributed to the highest conductivity based on excessive CNT content. Excessive CNTs cause more EM waves to be reflected from the emitting surface; thus, the waves cannot be absorbed and consumed, i.e., impedance mismatching.
When the ZrB 2 content increased to 20 wt.%, the RL min values of Y78 and Y76 were −22.0 dB at 12.2 GHz with a 1.9 mm thickness and −9.8 dB at 12.4 GHz with a 1.2 mm thickness, respectively (Figure 4d,e). Such results further prove that an appropriate CNT content is critical to an optimized absorbing performance in Y 2 O 3 /ZrB 2 /CNT ceramics. Therefore, the excellent RL value of the Y 2 O 3 /ZrB 2 /CNT material corresponded to the ZrB 2 content (15 wt.%) and the CNT content of 2 wt.%. The loss tangent (tan δ) of the prepared composite samples, which is representative of the power loss in the material with respect to the stored reactive power, was estimated as the ratio between the imaginary and real part of the complex effective permittivity [32,33]. The computed values of tan δ are reported in Figure 4f. We notice that tan δ, which increased with the contents of ZrB 2 and CNT in the composite, was always greater than Y85 in the whole frequency range for samples Y76, Y78, Y81, and Y83. As we know, the highest tan δ ε values for the sample represent the impressive storage and loss capabilities for electrical energy. The dielectric loss behavior is mainly related to conductivity loss and polarization loss. Among that, ionic polarization and electronic polarization always work in the frequency of 10 3 -10 6 GHz, and dipole orientation polarization from the frustrated reorientation of dipoles prefers frequency dispersion [34,35]. Thus, the dielectric loss should be determined by the interfacial polarization between Y 2 O 3 , ZrB 2 , and CNT. Furthermore, the highest dielectric loss capability prompts it to consume electromagnetic waves and obtain an excellent reflection loss. dielectric loss behavior is mainly related to conductivity loss and polarization loss. Among that, ionic polarization and electronic polarization always work in the frequency of 10 3 -10 6 GHz, and dipole orientation polarization from the frustrated reorientation of dipoles prefers frequency dispersion [34,35]. Thus, the dielectric loss should be determined by the interfacial polarization between Y2O3, ZrB2, and CNT. Furthermore, the highest dielectric loss capability prompts it to consume electromagnetic waves and obtain an excellent reflection loss.  To better analyze this phenomenon of different microwave absorbers, two essential factors, the EM attenuation constant (α) and impedance matching (|Z in /Z 0 |), need to be considered [36][37][38]. The maximum value of α suggests that more EM waves can enter the material and transform EM energy into thermal energy, as shown in Equation (1) [39]: Figure 5a presents the α value of the five samples in the X-band. The Y85 sample showed an average α value of about 19.0, but the α value of doped-CNT samples had a major effect relative to the lowest α value seen in the undoped-CNT Y85 ceramic. These data further prove that the addition of CNT can improve EM attenuation, leading to good microwave absorption. The average α values of Y76 and Y81 were 319.8 and 267.1, Nanomaterials 2021, 11, 2640 7 of 9 respectively. The average α values of Y78 and Y83 were 188.8 and 176.4, respectively. The Y76 and Y81 values showed a larger α value versus Y78 and Y83, suggesting an exceptional EM wave attenuation ability. However, Y78 and Y83 ceramics had higher reflection loss values than Y76 and Y81: the attenuation constant only indicates that the material has a strong EM attenuation ability. No other factor could be considered, i.e., impedance matching. Figure 5b-f shows the impedance matching of five samples simulated at different absorbing thicknesses. Values of |Z in /Z 0 | of Y85, Y81, and Y76 ceramics were far from 1, but Y83 and Y78 approached 1 as the absorbing thickness changed. These observations suggest that Y83 and Y78 samples absorb more EM waves. Therefore, good microwave absorption is related to both EM attenuation loss and impedance matching.

Conclusions
Y2O3, ZrB2, and CNT powders were uniformly mixed via a planetary ball mill, followed by spray drying. The Y2O3/ZrB2/CNT ceramics were fabricated by APS technology. The ε′ and ε″ values of Y2O3/ZrB2/CNT ceramics increased when the ZrB2 or CNT contents increased, because Y2O3/ZrB2/CNT ceramics have dense conductive percolation pathways. These pathways can respond to the presence of conductance losses and thus improve the microwave absorption ability. The RLmin value of the Y83 sample was −25.7

Conclusions
Y 2 O 3 , ZrB 2 , and CNT powders were uniformly mixed via a planetary ball mill, followed by spray drying. The Y 2 O 3 /ZrB 2 /CNT ceramics were fabricated by APS technology. The ε and ε values of Y 2 O 3 /ZrB 2 /CNT ceramics increased when the ZrB 2 or CNT contents increased, because Y 2 O 3 /ZrB 2 /CNT ceramics have dense conductive percolation pathways. These pathways can respond to the presence of conductance losses and thus improve the microwave absorption ability. The RL min value of the Y83 sample was −25.7 dB at 12.3 GHz with a 1.9 mm thickness. One could control the EM properties of Y 2 O 3 /ZrB 2 /CNT ceramics by tuning the mass percentage of Y 2 O 3 , ZrB 2 , and CNT. This could lead to a high attenuation capability and good impedance matching. This unique Y 2 O 3 /ZrB 2 /CNT hybrid had excellent EM wave absorption properties and is a good candidate for designing ceramic-based high-temperature absorbers.
Author Contributions: All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by R.L. R.L. wrote the manuscript. Y.Q. gave financial support and measurement support for this work. Both authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.