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High temperature absorbing coatings with excellent performance combined Al2O3 and TiC material
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Original Article
High temperature absorbing coatings with excellent performance combined Al2O3 and TiC material Tengqiang Shaoa, Hua Maa,*, Jun Wanga, Mingde Fenga, Mingbao Yana, Jiafu Wanga, Zhaoning Yangb, Qian Zhouc, Heng Luod, Shaobo Qua,* a
Department of Basic Sciences, Air Force Engineering University, Xi’an, 710051, China School of Science, Xi’an University of Posts and Telecommunications, Xi’an, 710121, China c Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnic University, Xi’an, 710072, China d School of Physics and Electronics, Central South University, Changsha, 410083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: EM absorbing material TiC material x band High temperature
A series of challenges are impeding the development of high temperature electromagnetic (EM) wave absorbing materials in x band. In this study, to deal with this problem, a coating, (1-x).wtAl2O3∼x.wtTiC, is designed and prepared using plasma spraying technology. Its permittivity increases with temperature and TiC content, which endows it with a good EM impedance at high temperature. The coating possesses excellent EM absorbing performance at 800 ℃. When the x value equals 0.2 and thickness 1.6 mm, the coating exhibits an EAB of 3.45 GHz at 800℃, and a reflection loss lower than -8 dB in whole x band. The XRD result shows that only two phases exist in the coating. The SEM images illustrate that TiC is unevenly dispersed in Al2O3, causing loss of conductivity and interface polarization. The finding not only broadens the application of TiC-based materials but also indicates the promising future of this material system.
1. Introduction Nowadays, with the advent of high-tech reconnaissance and detection means, higher demands for the radar stealth performance [1–6] also become a top priority for researchers. Radar stealth is a sophisticated technology where radar section cross (RCS) is reduced so that the detection probability by the enemy radar is decreased. At present, two major methods are normally employed to achieve radar stealth. First, changing the shape of the target object to minimize the RCS at a specific radar incident angle. Second, applying radar absorbing materials (RAMs) to the surface of the objects to absorb more EM energy. Meanwhile, in weapons such as fighters, super-aircraft, and missiles, the temperature of certain working parts may reach 700 ℃ or even higher. Therefore, to ensure the battlefield survivability, development of high-temperature-resistant RAMs becomes an urgent task. Relevant studies have been capturing the attention of numerous researchers. However, so far away, most of existing RAMs can only work in a temperature below 300 ℃ [7–14]. Numerous attempts are being made to fabricate RAMs which can achieve radar stealth in x band (8.2∼12.4 GHz). A wide variety of materials are taken into consideration, including ceramics, metal oxides, and carbon-based materials, such as Si3N4, BaTiO3, SiBCN, SiCf/SiC, ZnO, MnO2, carbon fibers, ⁎
CNTs and grapheme [15–31]. Nonetheless, some major challenges still remain there, such as narrow EM absorbing bandwidth and oversized thickness, which substantially restrict their further and large-scale applications. Besides, complexity of fabrication also leads to a relatively lower applicability and economic effectiveness. Hence, if the manufacture of RAMs can be simplified and the cost can be reduced, the practical application will be feasible and convenient. The radar absorbing coatings (RACs) have various advantages such as simple preparation, excellent bonding force and high temperature resistance. Usually, they are composed of conductors and non-conductors, such as Ti3SiC2 and Al2O3 based ceramics coatings, Fe/ FeAl2O4, and CB/mullite coatings [32–43]. By adjusting the proportion of the substances, researchers can obtain desirable EM parameters. Then the RACs can properly match the EM impedance and absorb more EM energy. As we mentioned, some problems still remain unsolved. Therefore, a new RAC with excellent EM absorption performance and good thermal resistance is urgently needed. Among all the options, Titanium carbide (TiC) [44,45], which is already widely used in cermet coatings, has very low density (4.93 g.cm−3), extreme hardness (28–35 GPa), high melting point (3340 K), and good electrical and thermal conductivities. Besides, Alumina (Al2O3) is a common base material for RACs by virtue of its excellent mechanical and thermal
Corresponding authors. E-mail addresses: [email protected] (H. Ma), [email protected] (S. Qu).
https://doi.org/10.1016/j.jeurceramsoc.2020.01.036 Received 22 August 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 0955-2219/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Tengqiang Shao, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2020.01.036
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properties. All above mentioned qualities naturally make the two substances the appropriate choices for new coatings. Thus, in our design, TiC and Al2O3 are utilized to form the new RAC which not only possesses a smaller thickness and a wider absorption bandwidth in x band, but also can work effectively at a high temperature up to 800 ℃. 2. Experimental 2.1. Samples manufacturing and plasma spraying Plasma spraying is employed in the fabrication of the (1x).wtAl2O3∼x.wtTiC coatings (x = 0.2,0.25,0.3). High-purity powders of Alumina and Titanium Carbide, both 99.9 %, purchased from Shanghai Xiangtan Nano Materials Co., Ltd., China, are selected as the raw materials. The preparation comprises two specific steps. First, preparing the spherical powder with good fluidity for plasma spraying. In this step, the powders are mixed together and then placed into a jar mill containing distilled water and zirconia balls where they are ground for 24 h. Where after, as a binder, polyvinyl alcohol (PVA) is added to ensure that the particles of the powder become spherical. Then the powder is processed through spray-drying to obtain good fluidity. There are several key factors affecting the powder quality, the first of which is the slurry composition, as shown in support information (SI). Apart from that, main parameters of the spray-drying also play a role in determining the powder quality, such as air temperature, nozzle speed, slurry flow rate, and atomization airflow rate, as listed in SI. In the second step, the ground powder is processed into a RAC on superalloy base through plasma spraying. Certain process parameters normally influence the powder quality, such as spraying distance, spraying power, main gas flow rate, and powder feeding speed. Relevant parameters are also demonstrated in SI.
Fig. 1. The XRD patterns of the 80 %.wtAl2O3∼20 %.wtTiC coating at normal temperature and 800℃.
molten. Meanwhile, there also exist certain small pores generated during spraying. All these phenomena are typical characteristics of sprayed ceramics. In addition, the different colors in the images represent the uneven distribution of TiC and Al2O3, which indicates that TiC is actually unevenly dispersed in Al2O3 matrix. This is also confirmed by Fig. 2(b). The unevenly distributed contents of the two substances correspondingly induce uneven conductivity in these micro areas. Meanwhile, the EDS spectra is adopted to analyze the mass fraction variation of the elements in the coatings, as shown in SI. The measured results show that the contents of Ti, C, Al and O are close to the theoretical values, indicating almost no element loss in the coatings. Fig. 3 demonstrates the complex permittivity (ε = ε′ − jε′′) and the dielectric loss (tanδ) of 80 %wt.Al2O3–20 %wtTiC coating in x band between 25℃–800℃. It can be found that, as temperature rises, the real and imaginary parts of permittivity (ε ′ and ε′′) and tanδ also increase. More specifically, when temperature increases from 25℃ to 800℃, the values of ε ′, ε′′ and tanδ correspondingly increase from 15.27 to 23.06, from 2.78 to 7.39 and from 0.182 to 0.316, respectively. As for the other two coatings, their ε and tanδ values are illustrated in SI. Normally, complex permittivity reflects the polarizing characteristics of coatings and conductivity is a major parameter for electrical loss materials. Therefore, the complex permittivity is mainly influenced by relaxation polarization and electrical conductivity. According to Debye’s theory, the correlation among relaxation polarization time, ε ′ and ε′′ can be described as following:
2.2. Characterization of powders and coatings These coatings are carefully examined. First, their phase structure is inspected by using an X-ray diffractometer (Philips X-Pert ProDiffractometer, Almelo, The Netherlands). Second, the microstructure is observed using a scanning electron microscope (SEM, model JSM-6360, JEOL, Tokyo, Japan). Third, 22.86 × 10.16 × 1 mm RACs are measured using an Agilent Technologies E8362B PNAnetwork analyzer with the waveguide in X band. The aim here is to obtain electromagnetic scatter parameters (S parameters) which could be converted into complex permittivity through calculation. Fourth, reflection loss (RL) of a 180 × 180 mm coating is measured under high temperature through an arch reflection test system. For the sake of accuracy, in this step, the temperature is elevated gradually and kept at each temperature for 10 min.
τ (T ) = τ0 exp(Ea/ KT ) 3. Results and discussions
ε′ = ε∞ + Fig.1(a) demonstrates the X-ray diffraction patterns of the 80 %.wtAl2O3∼20 %.wtTiC coating at 25℃ and 800℃. Only TiC and Al2O3 phases can be observed in the XRD. This suggests that TiC normally does not react with Al2O3 and that TiC is hardly oxidized when the coating is formed or at high temperature. All these properties are crucial to EM absorbing capability. In this design, TiC with high ε ′ and ε′′ is used as absorbent which is dispersed in Al2O3, the matrix material with low ε ′ and ε′′. The two materials are coupled with each other and then desirable ε ′ and ε ′′ are achieved, which then creates a good EM impedance matching. Therefore, no chemical reaction between the two is actually a favorable condition for the design. As for the other two coatings, and pure Al2O3 and TiC, their XRD pattern results are illustrated in SI. Fig. 2 are the SEM image of the 80 %.wtAl2O3∼20 %.wtTiC coating after being heated for 30 min at 800 ℃. It can be found that its surface is not completely even and flat. It is, however, irregular and almost
ε′′ =
εs − ε∞ 1 + ω2τ 2
εs − ε∞ σ (T ) •ωτ (T ) + 1 + ω2τ 2 (T ) ε0 ω
(1) (2)
(3)
Where τ0 , Ea and K represent specific relaxation time, activation energy and Boltzmann constant respectively; εs , ε ́ and ε∞ stand for static permittivity, free space permittivity and high frequency permittivity, respectively. According to Eqs. (1) and (2), τ (T ) increases with temperature, which then induces the increase of ε ́ . Meanwhile, under high frequency, ωτ ≫ 1, Eq. (1) can be simplified as follows:
ε′′ =
εs − ε∞ σ (T ) + ωτ (T ) ε0 ω
(4)
For the first term, its value increases with temperature, and for the second one, its value is determined by σ (T ) . The coating’s conductivity is predominantly determined by the added TiC and mainly comes from 2
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Fig. 2. The SEM images of the 80 %.wtAl2O3∼20 %.wtTiC coating after heated at 800℃.
For instance, as shown in Fig. 4(b), as the thickness increases, the fm gradually decreases to a lower level. Additionally, with a thickness of 1.6 mm, the coating can exhibit excellent EM absorbing capability at 800℃. When the RL are below -5 dB and-10 dB, the corresponding bandwidths can reach 4.2 GHz (whole x band) and 3.1 GHz, respectively. Here, we define the frequency bandwidth corresponding to RL below -10 dB as effective absorption bandwidth (EAB). Accordingly, the thickness for the maximum EAB value is deemed the optimum thickness (dm). To study the coating’s properties, the data of its maximum EM absorbing performance at 800℃ is acquired for detailed comparison and analysis, as shown in Fig. 5(a). As x is 0.2, the values of EAB and dm turn out to be 3.1 GHz and 1.6 mm, respectively. The mechanism will be analyzed from the following two aspects. First, according to Eq. (1), the quarter wavelength (λ1/4) of the incident waves under the frequency of fm can be obtained. When x is 0.2, corresponding λ1/4 and fm are respectively 1.53 mm and 10 GHz, as shown in Fig. 5(b). The λ1/4 here is nearly equal to the the coating’s dm. Hence, when the coating thickness approaches λ1/4, interference occurs between the waves reflected from the surface and those from the metal substrate, which creates excellent absorbing capability. Second, the normalized complex EM impedance (Z) is also calculated and its values are shown in Fig. 6(a) and only the coating (x = 0.2) is discussed in the paper, Z values of the other two coatings are shown in SI. From the acquired data, it can be observed that, the imaginary part of the impedance (Z˝) is extremely small and approaches 0, meanwhile, its real part (Z ′) is close to 1. This indicates that the coating has a better absorbing capability in the whole x band, particularly when placed under 9.8 GHz where its Z˝ and Z ́ are respectively 0 and 1. This moment also marks the minimum RL of the coating. Fig. 6(b) shows the correlation between the temperature and the RL of the 20 %.wtAl2O3– 80 %.wtTiC coating with a thickness of dm. It can
electronic rather than ionic conductivity [46], namely, valences electrons Ti-4 s will migrate into the C-2p and Ti-3d orbits in the compound. Hence, according to the electronic conduction theory, TiC’s conductivity can be formulated as follows:
σ (T ) = Ae−U / kT
(5)
Where A represents invariant, U stands for potential barrier and k is Boltzmann invariant. Obviously, the conductivity increases with temperature. Hence, as per previous analysis, ε′′ will also increase as temperature rises. Fig. 4 comprises a RL graph based on calculated values. Following equations are employed to calculate relevant data of the three coatings.
RL = 20 log
Zin =
fm =
Zin − 1 Zin + 1
μ tanh (j 2πf με dc −1) ε
c 4d |μ||ε|
(6)
(7)
(8)
Where Zin is the normalized input impedance, f represents the EM frequency, c and d stand for the speed of light in vacuum and the thickness of the coating, respectively. fm is the frequency for the minimum RL. From the above equations, it can be seen that thickness is a major parameter tightly correlated with RL and fm. The corresponding mechanism can be explained as follows. As shown in Fig. 4(a), the coating’s wave impedance (Zm) does not match the free-space impedance (Z0 = 377Ω). Meanwhile, the EM absorption performance is correlated with the Zin which is determined by the Zm and the thickness. Hence, a conclusion can be drawn that both the RL and the fm vary with thickness.
Fig. 3. Permittivity of 80 %wt.Al2O3 – 20 %wt.TiC coating at various temperature in x band. 3
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Fig. 4. (a) The diagram of input impedance for thickness change; (b) The RL of 80 %wt.Al2O3 – 20 %wt.TiC coating at various thickness in x band.
impedance matching. More EM waves thus can enter its interior. Second, changing the EM loss of the coating. At the micro level, TiC is unevenly dispersed in Al2O3, resulting in uneven distribution of element contents and conductivity. Therefore, interface polarization will occur between different microscopic regions [66]. In addition, due to the presence of TiC, conductivity loss will also happen in each region. Meanwhile, at high temperature, the thermal motion in the coating is intensified. In different microscopic regions, TiC and A2O3 particles that are close to each other are aligned along the dipole moments direction under the EM field, which can cause certain distribution of positive and negative charges. This can serve as a pair of electric dipoles, thus causing dipole relaxation polarization. The polarization will inevitably consume part of the incident EM waves. Therefore, such microstructure not only endows the coating with good impedance matching, but also with conductivity loss, interfacial polarization loss, and relaxation polarization loss. Hence, the coating can absorb substantial incident electromagnetic waves. To examine the theoretical RL values at high temperature, the reflectivity of the 80 %.wtAl2O3∼20 %.wtTiC coating is measured. Fig.7(a) shows the comparison between the measured and the calculated RL values at 800℃ with a coating thickness of 1.6 mm. It can be founded that, the experimental value (3.45 GHz) of EAB is very close to its theoretical counterpart (3.1 GHz). Likewise, the difference between the calculated and the measured values of RLmin (-20.62 dB and -21.77 dB) is very subtle, too. The two values of fm are also very close (10.02 GHz and 10.18 GHz). All abovementioned errors can be ascribed to the fact that the coating thickness is not one hundred percent even. This causes the deviations between theoretical and measured values.
be found that, as temperature increases, fm decreases but EAB rises. To be more specific, as the temperature increases from 25℃ to 800℃, fm decreases from a value over 12.4 to 10.0 GHz but EAB increases from 0 to 3.1 GHz. Based on all above information, it can be seen that, ε ́ and ε˝ increase gradually with temperature and this also makes fm decline to a lower value. Besides, the normalized complex impedance is also measured at various temperatures to further explain the mechanism of EAB and relevant data is shown in Fig. 6(c) and (d). It can be found that, when Z ́ and Z˝ are close to 1 and 0 respectively, their corresponding bandwidths increase with temperature, indicating that, when temperature rises, Zin and Z0 can better match each other within a wider frequency range. Besides, as discussed above, the tanδ increases with temperature. Hence, more EM waves can enter the coating and can be consumed with the increase of temperature. This indicates that the EAB will become broader as temperature rises. The corresponding data of the other two coatings are shown in SI. To sum up, the EM absorbing property of the coating is essentially determined by its microstructure. The XRD results and the microstructure analysis indicate that there mainly exist two ways to affect the EM absorbing capability. First, changing the coating’s macroscopic EM parameters. As discussed above, TiC has high ε ́ and ε˝, therefore its mass fraction can influence the macroscopic EM parameter. From micro perspective, TiC is dispersed in Al2O3 and the ε is not high, which implies that a conductive network cannot be formed [40,64]. According to the percolation theory [65], the electrical conductivity of the coating is jointly determined by Al2O3 and TiC when the content of the latter does not exceed the percolation value. Therefore, while the two substances are mixed in a proper proportion, the coating can possess good
Fig. 5. (a)The best EM absorbing performance of (1-x)wt.Al2O3– x.wtTiC coatings at 800℃; (b) The calculated quarter wavelength of each coating at their best EM absorbing performance. 4
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Fig. 6. (a) The Z of coating with x = 0.2 at 1.6 mm in 800℃; (b) The correlation between temperature and RL of the coating at 1.6 mm; (c) (d) Z as a function of the temperature for this coating at 1.6 mm under 25-800℃.
relatively narrower and corresponding working temperatures are also comparatively lower.
However, it is worth noting that the experimental value of EAB reaches 3.45 GHz. Meanwhile, when RL is less than -8 dB, the experimental values of bandwidth can cover the whole x band, which implies that the coating possesses excellent absorbing capability under high temperature. Fig. 7(b) shows an intuitive comparison among the EABs of various absorbing materials at elevated temperature in the x band. It can be found that, for some materials, the EAB range can cover the whole x band. After being processed into finished coatings, however, their thicknesses mostly exceed 2.5 mm and some are even bigger than 4 mm. Meanwhile, their working temperatures are normally below 600℃, some are even lower than 400℃. These disadvantages greatly restrict their large-scale application. In contrast, some materials can be processed into comparatively smaller thicknesses, but their EABs are just
4. Conclusion In this work, to better solve high temperature EM absorbing issue in x band, a new RAC material, whose major ingredients are (1x).wtAl2O3-xwt.TiC, is proposed. The coating is processed using plasma spraying. To study the properties of the new coating, EM parameters are employed to calculate the theoretical RL values. The results imply that it possesses a comparatively better EM absorbing capability at 800℃. Meanwhile, the experimental values of RL are also obtained to examine and confirm the new coating’s qualities. The numbers demonstrate that, with a thickness of 1.6 mm, the 80 wt.%Al2O3 -20 wt.%TiC material has
Fig. 7. (a) The comparison between the measured and calculated value of the coating with x = 0.2 in the thickness of 1.6 mm under 800 ℃; (b) A comparison of absorbing properties among various absorbing materials [12–16,27,28,47–63]. 5
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excellent absorbing capability at 800℃ and the EAB can reach 3.45 GHz. Besides, microscopic analysis shows that, when TiC is unevenly dispersed in Al2O3 matrix in a proper proportion, conductivity loss and interface polarization can occur. Macroscopically, the material possesses good EM impedance matching that allows in more EM waves and eventually weaken them. Based on all the facts, we are confident that this new RAC is a promising and competitive material in high temperature EM absorbing for the foreseeable future.
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Declaration of Competing 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. Acknowledgements The authors are grateful to the support from the National Natural Science Foundation of China (Grants 61671467 and 61331005) and the National key R&D program of China (Grant 2017YFA0700201) Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2020.01. 036. References [1] M.S. Pinho, M.L. Gregori, R.C. Nunes, B.G. Soares, Performance of radar absorbing materials by waveguide measurements for X-and Ku-band frequencies, J. Eur. Polym. 38 (2002) 2321–2327. [2] F.B. Meng, H.G. Wang, F. Huang, Y.F. Guo, Z.Y. Wang, H. David, Z.W. Zhou, Graphene-based microwave absorbing composites: a review and prospective, Compos. Part B 137 (2018) 260–277. [3] L. Kong, Z. Li, L. Liu, R. Huang, M. Abshinova, Z. Yang, C. Tang, P. Tan, C. Deng, S. Matitsine, Recent progress in some composite materials and structures for specific electromagnetic applications, Int. Mater. Rev. 58 (2013) 203–259. [4] X.W. Yin, L.F. Cheng, L.T. Zhang, N. Travitzky, P. Greil, Fibre-reinforced multifunctional SiC matrix composite materials, Int. Mater. Rev. 62 (2017) 117–172. [5] K. Zikidis, A. Skondras, C. Tokas, Low observable principles, stealth aircraft and anti-stealth technologies, J. Comput. Modell. 4 (2014) 129–165. [6] W.Q. Wang, Moving-target tracking by cognitive RF stealth radar using frequency diverse array antenna, IEEE T Geosci. Remote 54 (2016) 3764–3773. [7] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes, Adv. Mater. 16 (2004) 401–405. [8] J.B. Kim, S.K. Lee, C.G. Kim, Comparison study on the effect of carbon nano materials for single-layer microwave absorbers in X-band, Compos. Sci. Technol. 68 (2008) 2909–2916. [9] Y. Zhang, Y. Huang, T. Zhang, H. Chang, P. Xiao, H. Chen, Z. Huang, Y. Chen, Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (2015) 2049–2053. [10] Q.L. Wen, W.C. Zhou, J.B. Su, Y.C. Qing, F. Luo, D.M. Zhu, Dielectric and microwave absorption properties of plasma sprayed short carbon fibers/glass composite coatings, J. Mater. Sci.: Mater. Electron. 27 (2015) 1783–1790. [11] X. Tang, K. Hu, Preparation and electromagnetic wave absorption properties of Fedoped zinc oxide coated barium ferrite composites, Mater. Sci. Eng. B 139 (2007) 119–123. [12] H.Y. Wang, D.M. Zhu, W.C. Zhou, F. Luo, High temperature electromagnetic and microwave absorbing properties of polyimide/multi-walled carbon nanotubes nanocomposites, Chem. Phys. Lett. 633 (2015) 223–228. [13] H.Y. Wang, D.M. Zhu, W.C. Zhou, F. Luo, Electromagnetic and microwave absorbing properties of polyimide nanocomposites at elevated temperature, J. Alloys Compd. 648 (2015) 313–319. [14] M.M. Lu, X.X. Wang, W.Q. Cao, J. Yuan, M.S. Cao, Carbon nanotube-CdS core–shell nanowires with tunable and high-efficiency microwave absorption at elevated temperature, Nanotechnology 27 (2016) 065702. [15] Q. Zhou, X.W. Yin, F. Ye, Z.M. Tang, R. Mo, L.F. Chen, High temperature electromagnetic wave absorption properties of SiCf/Si3N4 composite induced by different SiC fibers, Ceram. Int. 45 (2019) 6514–6522. [16] M. Li, X.W. Yin, G.P. Zheng, M. Chen, M.J. Tao, L.F. Cheng, L.T. Zhang, Hightemperature dielectric and microwave absorption properties of Si3N4–SiC/SiO2 composite ceramics, J. Mater. Sci. 50 (2015) 1478–1487. [17] F. Ye, L.T. Zhang, X.W. Yin, Y.J. Zhang, L. Kong, Q. Li, Y.S. Liu, L.F. Cheng, Dielectric and EMW absorbing properties of PDCs-SiBCN annealed at different temperatures, J. Eur. Ceram. Soc. 33 (2013) 1469–1477.
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[56] W.Q. Cao, X.X. Wang, J. Yuan, W.Z. Wang, M.S. Cao, Temperature dependent microwave absorption of ultrathin graphene composites, J. Mater. Chem. A 3 (2015) 10017–10022. [57] X.Y. Yuan, L.F. Cheng, Y.J. Zhang, S.W. Guo, L.T. Zhang, Fe-doped SiC/SiO2 composites with ordered inter-filled structure for effective high-temperature microwave attenuation, Mater. Design. 92 (2016) 563–570. [58] H.L. Lv, Y.H. Guo, Z.H. Yang, T.C. Guo, H.J. Wu, G. Liu, L.Y. Wang, R.B. Wu, Doping strategy to boost the electromagnetic wave attenuation ability of hollow carbon spheres at elevated temperatures, ACS Sustainable Chem. Eng. 6 (2018) 1539–1544. [59] W. Zhou, R.M. Yin, L. Long, H. Luo, W.D. Hua, Y.H. Ding, Y. Li, Enhanced hightemperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range, Ceram. Int. 44 (2018) 12301–12307. [60] Y. Wang, F. Luo, W.C. Zhou, D.M. Zhu, Dielectric and microwave absorption properties of TiC-Al2O3 /Silica coatings at high temperature, J. Electron. Mater. 46 (2017) 5225–5231. [61] J.B. Su, W.D. Zhou, Y. Liu, Y.C. Qing, F. Luo, D.M. Zhu, High-temperature dielectric and microwave absorption property of plasma sprayed Ti3SiC2/cordierite coatings, J. Mater. Sci.: Mater. Electron. 27 (2016) 2460–2466. [62] C.H. Peng, S.C. Pang, C.C. Chang, High-temperature microwave bilayer absorber based on lithium aluminum silicate/lithium aluminum silicate-SiC composite, Ceram. Int. 40 (2013) 47–55. [63] Z.N. Yang, F. Luo, Y. Hu, D.M. Zhu, W.C. Zhou, Dielectric and microwave absorption properties of TiAlCo ceramic fabricated by atmospheric plasma spraying, J. Ceram. Int. 45 (2016) 8525–8530. [64] C.W. Nan, Y. Shen, M. Jing, Physical properties of composites near percolation, J. Annu. Rev. Mater. Res. (2010) 131–151. [65] Q. Zhou, X.W. Yin, F. Ye, Z.M. Tang, R. Mo, L.F. Cheng, High temperature electromagnetic wave absorption properties of SiCf/Si3N4 composite induced by different SiC fibers, J. Ceram. Int. 45 (2019) 6514–6522.
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Original Article
High temperature absorbing coatings with excellent performance combined Al2O3 and TiC material Tengqiang Shaoa, Hua Maa,*, Jun Wanga, Mingde Fenga, Mingbao Yana, Jiafu Wanga, Zhaoning Yangb, Qian Zhouc, Heng Luod, Shaobo Qua,* a
Department of Basic Sciences, Air Force Engineering University, Xi’an, 710051, China School of Science, Xi’an University of Posts and Telecommunications, Xi’an, 710121, China c Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnic University, Xi’an, 710072, China d School of Physics and Electronics, Central South University, Changsha, 410083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: EM absorbing material TiC material x band High temperature
A series of challenges are impeding the development of high temperature electromagnetic (EM) wave absorbing materials in x band. In this study, to deal with this problem, a coating, (1-x).wtAl2O3∼x.wtTiC, is designed and prepared using plasma spraying technology. Its permittivity increases with temperature and TiC content, which endows it with a good EM impedance at high temperature. The coating possesses excellent EM absorbing performance at 800 ℃. When the x value equals 0.2 and thickness 1.6 mm, the coating exhibits an EAB of 3.45 GHz at 800℃, and a reflection loss lower than -8 dB in whole x band. The XRD result shows that only two phases exist in the coating. The SEM images illustrate that TiC is unevenly dispersed in Al2O3, causing loss of conductivity and interface polarization. The finding not only broadens the application of TiC-based materials but also indicates the promising future of this material system.
1. Introduction Nowadays, with the advent of high-tech reconnaissance and detection means, higher demands for the radar stealth performance [1–6] also become a top priority for researchers. Radar stealth is a sophisticated technology where radar section cross (RCS) is reduced so that the detection probability by the enemy radar is decreased. At present, two major methods are normally employed to achieve radar stealth. First, changing the shape of the target object to minimize the RCS at a specific radar incident angle. Second, applying radar absorbing materials (RAMs) to the surface of the objects to absorb more EM energy. Meanwhile, in weapons such as fighters, super-aircraft, and missiles, the temperature of certain working parts may reach 700 ℃ or even higher. Therefore, to ensure the battlefield survivability, development of high-temperature-resistant RAMs becomes an urgent task. Relevant studies have been capturing the attention of numerous researchers. However, so far away, most of existing RAMs can only work in a temperature below 300 ℃ [7–14]. Numerous attempts are being made to fabricate RAMs which can achieve radar stealth in x band (8.2∼12.4 GHz). A wide variety of materials are taken into consideration, including ceramics, metal oxides, and carbon-based materials, such as Si3N4, BaTiO3, SiBCN, SiCf/SiC, ZnO, MnO2, carbon fibers, ⁎
CNTs and grapheme [15–31]. Nonetheless, some major challenges still remain there, such as narrow EM absorbing bandwidth and oversized thickness, which substantially restrict their further and large-scale applications. Besides, complexity of fabrication also leads to a relatively lower applicability and economic effectiveness. Hence, if the manufacture of RAMs can be simplified and the cost can be reduced, the practical application will be feasible and convenient. The radar absorbing coatings (RACs) have various advantages such as simple preparation, excellent bonding force and high temperature resistance. Usually, they are composed of conductors and non-conductors, such as Ti3SiC2 and Al2O3 based ceramics coatings, Fe/ FeAl2O4, and CB/mullite coatings [32–43]. By adjusting the proportion of the substances, researchers can obtain desirable EM parameters. Then the RACs can properly match the EM impedance and absorb more EM energy. As we mentioned, some problems still remain unsolved. Therefore, a new RAC with excellent EM absorption performance and good thermal resistance is urgently needed. Among all the options, Titanium carbide (TiC) [44,45], which is already widely used in cermet coatings, has very low density (4.93 g.cm−3), extreme hardness (28–35 GPa), high melting point (3340 K), and good electrical and thermal conductivities. Besides, Alumina (Al2O3) is a common base material for RACs by virtue of its excellent mechanical and thermal
Corresponding authors. E-mail addresses: [email protected] (H. Ma), [email protected] (S. Qu).
https://doi.org/10.1016/j.jeurceramsoc.2020.01.036 Received 22 August 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 0955-2219/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Tengqiang Shao, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2020.01.036
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properties. All above mentioned qualities naturally make the two substances the appropriate choices for new coatings. Thus, in our design, TiC and Al2O3 are utilized to form the new RAC which not only possesses a smaller thickness and a wider absorption bandwidth in x band, but also can work effectively at a high temperature up to 800 ℃. 2. Experimental 2.1. Samples manufacturing and plasma spraying Plasma spraying is employed in the fabrication of the (1x).wtAl2O3∼x.wtTiC coatings (x = 0.2,0.25,0.3). High-purity powders of Alumina and Titanium Carbide, both 99.9 %, purchased from Shanghai Xiangtan Nano Materials Co., Ltd., China, are selected as the raw materials. The preparation comprises two specific steps. First, preparing the spherical powder with good fluidity for plasma spraying. In this step, the powders are mixed together and then placed into a jar mill containing distilled water and zirconia balls where they are ground for 24 h. Where after, as a binder, polyvinyl alcohol (PVA) is added to ensure that the particles of the powder become spherical. Then the powder is processed through spray-drying to obtain good fluidity. There are several key factors affecting the powder quality, the first of which is the slurry composition, as shown in support information (SI). Apart from that, main parameters of the spray-drying also play a role in determining the powder quality, such as air temperature, nozzle speed, slurry flow rate, and atomization airflow rate, as listed in SI. In the second step, the ground powder is processed into a RAC on superalloy base through plasma spraying. Certain process parameters normally influence the powder quality, such as spraying distance, spraying power, main gas flow rate, and powder feeding speed. Relevant parameters are also demonstrated in SI.
Fig. 1. The XRD patterns of the 80 %.wtAl2O3∼20 %.wtTiC coating at normal temperature and 800℃.
molten. Meanwhile, there also exist certain small pores generated during spraying. All these phenomena are typical characteristics of sprayed ceramics. In addition, the different colors in the images represent the uneven distribution of TiC and Al2O3, which indicates that TiC is actually unevenly dispersed in Al2O3 matrix. This is also confirmed by Fig. 2(b). The unevenly distributed contents of the two substances correspondingly induce uneven conductivity in these micro areas. Meanwhile, the EDS spectra is adopted to analyze the mass fraction variation of the elements in the coatings, as shown in SI. The measured results show that the contents of Ti, C, Al and O are close to the theoretical values, indicating almost no element loss in the coatings. Fig. 3 demonstrates the complex permittivity (ε = ε′ − jε′′) and the dielectric loss (tanδ) of 80 %wt.Al2O3–20 %wtTiC coating in x band between 25℃–800℃. It can be found that, as temperature rises, the real and imaginary parts of permittivity (ε ′ and ε′′) and tanδ also increase. More specifically, when temperature increases from 25℃ to 800℃, the values of ε ′, ε′′ and tanδ correspondingly increase from 15.27 to 23.06, from 2.78 to 7.39 and from 0.182 to 0.316, respectively. As for the other two coatings, their ε and tanδ values are illustrated in SI. Normally, complex permittivity reflects the polarizing characteristics of coatings and conductivity is a major parameter for electrical loss materials. Therefore, the complex permittivity is mainly influenced by relaxation polarization and electrical conductivity. According to Debye’s theory, the correlation among relaxation polarization time, ε ′ and ε′′ can be described as following:
2.2. Characterization of powders and coatings These coatings are carefully examined. First, their phase structure is inspected by using an X-ray diffractometer (Philips X-Pert ProDiffractometer, Almelo, The Netherlands). Second, the microstructure is observed using a scanning electron microscope (SEM, model JSM-6360, JEOL, Tokyo, Japan). Third, 22.86 × 10.16 × 1 mm RACs are measured using an Agilent Technologies E8362B PNAnetwork analyzer with the waveguide in X band. The aim here is to obtain electromagnetic scatter parameters (S parameters) which could be converted into complex permittivity through calculation. Fourth, reflection loss (RL) of a 180 × 180 mm coating is measured under high temperature through an arch reflection test system. For the sake of accuracy, in this step, the temperature is elevated gradually and kept at each temperature for 10 min.
τ (T ) = τ0 exp(Ea/ KT ) 3. Results and discussions
ε′ = ε∞ + Fig.1(a) demonstrates the X-ray diffraction patterns of the 80 %.wtAl2O3∼20 %.wtTiC coating at 25℃ and 800℃. Only TiC and Al2O3 phases can be observed in the XRD. This suggests that TiC normally does not react with Al2O3 and that TiC is hardly oxidized when the coating is formed or at high temperature. All these properties are crucial to EM absorbing capability. In this design, TiC with high ε ′ and ε′′ is used as absorbent which is dispersed in Al2O3, the matrix material with low ε ′ and ε′′. The two materials are coupled with each other and then desirable ε ′ and ε ′′ are achieved, which then creates a good EM impedance matching. Therefore, no chemical reaction between the two is actually a favorable condition for the design. As for the other two coatings, and pure Al2O3 and TiC, their XRD pattern results are illustrated in SI. Fig. 2 are the SEM image of the 80 %.wtAl2O3∼20 %.wtTiC coating after being heated for 30 min at 800 ℃. It can be found that its surface is not completely even and flat. It is, however, irregular and almost
ε′′ =
εs − ε∞ 1 + ω2τ 2
εs − ε∞ σ (T ) •ωτ (T ) + 1 + ω2τ 2 (T ) ε0 ω
(1) (2)
(3)
Where τ0 , Ea and K represent specific relaxation time, activation energy and Boltzmann constant respectively; εs , ε ́ and ε∞ stand for static permittivity, free space permittivity and high frequency permittivity, respectively. According to Eqs. (1) and (2), τ (T ) increases with temperature, which then induces the increase of ε ́ . Meanwhile, under high frequency, ωτ ≫ 1, Eq. (1) can be simplified as follows:
ε′′ =
εs − ε∞ σ (T ) + ωτ (T ) ε0 ω
(4)
For the first term, its value increases with temperature, and for the second one, its value is determined by σ (T ) . The coating’s conductivity is predominantly determined by the added TiC and mainly comes from 2
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Fig. 2. The SEM images of the 80 %.wtAl2O3∼20 %.wtTiC coating after heated at 800℃.
For instance, as shown in Fig. 4(b), as the thickness increases, the fm gradually decreases to a lower level. Additionally, with a thickness of 1.6 mm, the coating can exhibit excellent EM absorbing capability at 800℃. When the RL are below -5 dB and-10 dB, the corresponding bandwidths can reach 4.2 GHz (whole x band) and 3.1 GHz, respectively. Here, we define the frequency bandwidth corresponding to RL below -10 dB as effective absorption bandwidth (EAB). Accordingly, the thickness for the maximum EAB value is deemed the optimum thickness (dm). To study the coating’s properties, the data of its maximum EM absorbing performance at 800℃ is acquired for detailed comparison and analysis, as shown in Fig. 5(a). As x is 0.2, the values of EAB and dm turn out to be 3.1 GHz and 1.6 mm, respectively. The mechanism will be analyzed from the following two aspects. First, according to Eq. (1), the quarter wavelength (λ1/4) of the incident waves under the frequency of fm can be obtained. When x is 0.2, corresponding λ1/4 and fm are respectively 1.53 mm and 10 GHz, as shown in Fig. 5(b). The λ1/4 here is nearly equal to the the coating’s dm. Hence, when the coating thickness approaches λ1/4, interference occurs between the waves reflected from the surface and those from the metal substrate, which creates excellent absorbing capability. Second, the normalized complex EM impedance (Z) is also calculated and its values are shown in Fig. 6(a) and only the coating (x = 0.2) is discussed in the paper, Z values of the other two coatings are shown in SI. From the acquired data, it can be observed that, the imaginary part of the impedance (Z˝) is extremely small and approaches 0, meanwhile, its real part (Z ′) is close to 1. This indicates that the coating has a better absorbing capability in the whole x band, particularly when placed under 9.8 GHz where its Z˝ and Z ́ are respectively 0 and 1. This moment also marks the minimum RL of the coating. Fig. 6(b) shows the correlation between the temperature and the RL of the 20 %.wtAl2O3– 80 %.wtTiC coating with a thickness of dm. It can
electronic rather than ionic conductivity [46], namely, valences electrons Ti-4 s will migrate into the C-2p and Ti-3d orbits in the compound. Hence, according to the electronic conduction theory, TiC’s conductivity can be formulated as follows:
σ (T ) = Ae−U / kT
(5)
Where A represents invariant, U stands for potential barrier and k is Boltzmann invariant. Obviously, the conductivity increases with temperature. Hence, as per previous analysis, ε′′ will also increase as temperature rises. Fig. 4 comprises a RL graph based on calculated values. Following equations are employed to calculate relevant data of the three coatings.
RL = 20 log
Zin =
fm =
Zin − 1 Zin + 1
μ tanh (j 2πf με dc −1) ε
c 4d |μ||ε|
(6)
(7)
(8)
Where Zin is the normalized input impedance, f represents the EM frequency, c and d stand for the speed of light in vacuum and the thickness of the coating, respectively. fm is the frequency for the minimum RL. From the above equations, it can be seen that thickness is a major parameter tightly correlated with RL and fm. The corresponding mechanism can be explained as follows. As shown in Fig. 4(a), the coating’s wave impedance (Zm) does not match the free-space impedance (Z0 = 377Ω). Meanwhile, the EM absorption performance is correlated with the Zin which is determined by the Zm and the thickness. Hence, a conclusion can be drawn that both the RL and the fm vary with thickness.
Fig. 3. Permittivity of 80 %wt.Al2O3 – 20 %wt.TiC coating at various temperature in x band. 3
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Fig. 4. (a) The diagram of input impedance for thickness change; (b) The RL of 80 %wt.Al2O3 – 20 %wt.TiC coating at various thickness in x band.
impedance matching. More EM waves thus can enter its interior. Second, changing the EM loss of the coating. At the micro level, TiC is unevenly dispersed in Al2O3, resulting in uneven distribution of element contents and conductivity. Therefore, interface polarization will occur between different microscopic regions [66]. In addition, due to the presence of TiC, conductivity loss will also happen in each region. Meanwhile, at high temperature, the thermal motion in the coating is intensified. In different microscopic regions, TiC and A2O3 particles that are close to each other are aligned along the dipole moments direction under the EM field, which can cause certain distribution of positive and negative charges. This can serve as a pair of electric dipoles, thus causing dipole relaxation polarization. The polarization will inevitably consume part of the incident EM waves. Therefore, such microstructure not only endows the coating with good impedance matching, but also with conductivity loss, interfacial polarization loss, and relaxation polarization loss. Hence, the coating can absorb substantial incident electromagnetic waves. To examine the theoretical RL values at high temperature, the reflectivity of the 80 %.wtAl2O3∼20 %.wtTiC coating is measured. Fig.7(a) shows the comparison between the measured and the calculated RL values at 800℃ with a coating thickness of 1.6 mm. It can be founded that, the experimental value (3.45 GHz) of EAB is very close to its theoretical counterpart (3.1 GHz). Likewise, the difference between the calculated and the measured values of RLmin (-20.62 dB and -21.77 dB) is very subtle, too. The two values of fm are also very close (10.02 GHz and 10.18 GHz). All abovementioned errors can be ascribed to the fact that the coating thickness is not one hundred percent even. This causes the deviations between theoretical and measured values.
be found that, as temperature increases, fm decreases but EAB rises. To be more specific, as the temperature increases from 25℃ to 800℃, fm decreases from a value over 12.4 to 10.0 GHz but EAB increases from 0 to 3.1 GHz. Based on all above information, it can be seen that, ε ́ and ε˝ increase gradually with temperature and this also makes fm decline to a lower value. Besides, the normalized complex impedance is also measured at various temperatures to further explain the mechanism of EAB and relevant data is shown in Fig. 6(c) and (d). It can be found that, when Z ́ and Z˝ are close to 1 and 0 respectively, their corresponding bandwidths increase with temperature, indicating that, when temperature rises, Zin and Z0 can better match each other within a wider frequency range. Besides, as discussed above, the tanδ increases with temperature. Hence, more EM waves can enter the coating and can be consumed with the increase of temperature. This indicates that the EAB will become broader as temperature rises. The corresponding data of the other two coatings are shown in SI. To sum up, the EM absorbing property of the coating is essentially determined by its microstructure. The XRD results and the microstructure analysis indicate that there mainly exist two ways to affect the EM absorbing capability. First, changing the coating’s macroscopic EM parameters. As discussed above, TiC has high ε ́ and ε˝, therefore its mass fraction can influence the macroscopic EM parameter. From micro perspective, TiC is dispersed in Al2O3 and the ε is not high, which implies that a conductive network cannot be formed [40,64]. According to the percolation theory [65], the electrical conductivity of the coating is jointly determined by Al2O3 and TiC when the content of the latter does not exceed the percolation value. Therefore, while the two substances are mixed in a proper proportion, the coating can possess good
Fig. 5. (a)The best EM absorbing performance of (1-x)wt.Al2O3– x.wtTiC coatings at 800℃; (b) The calculated quarter wavelength of each coating at their best EM absorbing performance. 4
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Fig. 6. (a) The Z of coating with x = 0.2 at 1.6 mm in 800℃; (b) The correlation between temperature and RL of the coating at 1.6 mm; (c) (d) Z as a function of the temperature for this coating at 1.6 mm under 25-800℃.
relatively narrower and corresponding working temperatures are also comparatively lower.
However, it is worth noting that the experimental value of EAB reaches 3.45 GHz. Meanwhile, when RL is less than -8 dB, the experimental values of bandwidth can cover the whole x band, which implies that the coating possesses excellent absorbing capability under high temperature. Fig. 7(b) shows an intuitive comparison among the EABs of various absorbing materials at elevated temperature in the x band. It can be found that, for some materials, the EAB range can cover the whole x band. After being processed into finished coatings, however, their thicknesses mostly exceed 2.5 mm and some are even bigger than 4 mm. Meanwhile, their working temperatures are normally below 600℃, some are even lower than 400℃. These disadvantages greatly restrict their large-scale application. In contrast, some materials can be processed into comparatively smaller thicknesses, but their EABs are just
4. Conclusion In this work, to better solve high temperature EM absorbing issue in x band, a new RAC material, whose major ingredients are (1x).wtAl2O3-xwt.TiC, is proposed. The coating is processed using plasma spraying. To study the properties of the new coating, EM parameters are employed to calculate the theoretical RL values. The results imply that it possesses a comparatively better EM absorbing capability at 800℃. Meanwhile, the experimental values of RL are also obtained to examine and confirm the new coating’s qualities. The numbers demonstrate that, with a thickness of 1.6 mm, the 80 wt.%Al2O3 -20 wt.%TiC material has
Fig. 7. (a) The comparison between the measured and calculated value of the coating with x = 0.2 in the thickness of 1.6 mm under 800 ℃; (b) A comparison of absorbing properties among various absorbing materials [12–16,27,28,47–63]. 5
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excellent absorbing capability at 800℃ and the EAB can reach 3.45 GHz. Besides, microscopic analysis shows that, when TiC is unevenly dispersed in Al2O3 matrix in a proper proportion, conductivity loss and interface polarization can occur. Macroscopically, the material possesses good EM impedance matching that allows in more EM waves and eventually weaken them. Based on all the facts, we are confident that this new RAC is a promising and competitive material in high temperature EM absorbing for the foreseeable future.
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