Microwave absorption properties of Ni/C@SiC composites prepared by precursor impregnation and pyrolysis processes

Xinli Ye , Junxiong Zhng , Zhoeng Chen , Juneng Xing , Yun Jing ,Fqin Xie , Xiomin M

a School of Civil Aviation, Northwestern Polytechnical University, Xi'an, 710072, PR China

b Suzhou Qinyun Fiber Assemblies Technology Co. LTD, Suzhou, 215400, PR China

c School of Textile and Clothing, Nantong University, Nantong, 226019, PR China

d College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, PR China

e College of Textile Science and Engineering, Jiangnan University, Wuxi, 214122, PR China

f National Equipment New Material & Technology (Jiangsu)Co. LTD, Suzhou, 215400, PR China

Keywords:Silicon carbide composites Precursor impregnation and pyrolysis Microwave absorbing performance Electromagnetic parameter Absorbing mechanism

ABSTRACT In the present study,the unique three-dimensional graphene coated nickel(Ni/C)foam reinforced silicon carbide (Ni/C@SiC) composites were first obtained via the precursor impregnation and pyrolysis (PIP)processes. The microstructure images indicated that the SiC fillers were successfully prepared in the skeleton pores of the Ni/C foam. The influence of the PIP cycles on the microwave absorption performances was researched, and the results indicated that after the primary PIP process, Ni/C@SiC-I possessed the optimal microwave absorbing performance with a minimum reflection loss(RL)of-25.87 dB at 5.28 GHz and 5.00 mm.Besides,the RL values could be below-10.00 dB from 5.88 GHz to 7.74 GHz when the corresponding matching thickness was 3.85 mm. However, the microwave absorption properties of Ni/C@SiC-II and Ni/C@SiC-III were tremendously degraded as the PIP times increased. At last, the electromagnetic parameter, dielectric loss, attenuation constant as well as impedance matching coefficient were further investigated to analyze the absorbing mechanism, which opened a new path for the certain scientific evaluation of the absorbing materials and had extremely important to the defence technology.

1. Introduction

In recent years,with the explosive growth of the radar detective and wireless communication such as microwave heating devices,communication antenna, and high-power radar, the microwave absorbing materials have received tremendous concern both at home and abroad.Given the rapid developments in protecting the sensitive circuits and environment from the microwave radiation,researchers have already been going on to investigate the microwave absorption properties of various materials featuring thermal stability, high strength, and cost efficiency to tackle the electromagnetic microwave pollution[1-7].More importantly,the weapon systems are in urgent need of the electromagnetic microwave absorbers to improve the safety performance and viability especially at low frequency band of 2.00-8.00 GHz, which is remarkably significant to the defence technology [8].

Among the reported electromagnetic microwave absorbers like metal or metal oxide absorbers [9,10], graphene or reduced graphene oxide foams[11,12],and so on,the silicon carbide(SiC)with outstanding temperature stability, excellent oxidation resistance,and good mechanical strength,was considered to be an important and ideal candidate [13,14]. Usually, the SiC ceramics were commonly obtained via the pre-ceramic precursor derived ceramic(PDC) method or chemical vapor infiltration/deposition (CVI/CVD)method [15]. In our previous work, we always chose the threedimensional carbon foam as the base material, and adopted the CVD technology to deposit the SiC coating or the CVI method to grow the SiC nanowires, finally fabricating the carbon/silicon carbide(C/SiC)composite.As except,the intensity of the main specific diffractive peaks of SiC was strong, and the super microwave absorbing property had significance of promoting the realization of the engineering application. However, the minimum reflection loss(RLmin) was often obtained at high frequency [16,17]. Also, Yin et al.modified the surface of the SiC fibers by the boron nitride(BN)coating and compared the electromagnetic reflection coefficient with and without the BN coating. The results showed that the dielectric loss and electromagnetic absorption ability increased after the CVI progress [18]. Xia et al. built a core-shell structure including SiC and silicon dioxide (SiO2) via the CVD process along with the introduction of the water vapor,which possessed an RLminvalue of -32.72 dB at relatively high frequency [19]. The SiC produced by the CVI/CVD process exhibited high dielectric and microwave absorbing features,while the PDC-derived SiC composites possessed a comparatively low performance owing to the special nanostructure [15]. Besides, many researchers mainly focused on the enhancement of the anti-ablative performances as well as the mechanical properties of the PDC-derived SiC composites [20].Although the PDC-derived SiC composites had drawn more and more interest and attention for their operational simplicity and low cost, only limited attention had been given to the microwave absorbing performance, particularly at low frequency [21,22].

In the present work, the nickel (Ni) foam was acted as the attractive material,which had been widely used in supercapacitors,electrode materials,and electrocatalysts owing to the low-density,high porosity, magnetic characteristic, and chemical stability[23-26]. Besides, a graphene layer was deposited to improve its poor impedance matching performance and then the SiC particles were inducted into the skeleton pores to gain desirable interfacial polarization [27]. Herein, we introduced a three-dimensional graphene coated nickel (Ni/C) foam reinforced SiC (Ni/C@SiC) composite obtained by the precursor impregnation and pyrolysis (PIP)processes, mainly focusing on the impact of the PIP cycles on the microwave absorption abilities at low frequency. It indicated that the infiltration-pyrolysis cycle showed a great influence on the dielectric constant and the final performances.

2. Experimental procedure

2.1. Preparation of three-dimensional Ni/C foam

The Ni foam skeleton was covered by a deposited graphene layer via the CVD method to obtain the three-dimensional graphene coated Ni (Ni/C) foam. The Ni foam with the size of 50×25×2 mm3used as the template was firstly compressed into 50 × 25 × 1 mm3and then immersed with 2 M HCl for 3 h to completely remove the surface oxides and impurities.After that,it was put in a horizontal tube furnace and heated to 1000°C. Then,methane (CH4), hydrogen (H2), and argon (Ar) were pumped and the corresponding flow ratio was set as 50:100:800 sccm for 2 h[28].

2.2. Preparation of three-dimensional Ni/C foam

The three-dimensional Ni/C@SiC composite was fabricated by the PIP process. Firstly, the polycarbosilane (PCS) precursor was mixed evenly into the xylene to obtain the PCS/xylene solution,where the three-dimensional Ni/C foam was dipped into under the vacuum condition for 2 h. Then, the Ni/C foam filled by the PCS/xylene solution was dried for 4 h at 120°C and subsequently, pyrolyzed at 1100°C under the Ar atmosphere for another 2 h. After the natural cooling, the obtained sample was named Ni/C@SiC-I.The whole material preparation process was displayed in Fig.1.For comparison purpose,the PIP course was repeated for two and three cycles respectively,and the corresponding samples were marked as Ni/C@SiC-II and Ni/C@SiC-III.

Fig.1. Preparation progress of three-dimensional Ni/C@SiC composite.

Fig. 2. (a) Physical photographs of all the samples; (b) and (f) SEM images of Ni/C; (c) and (g) Ni/C@SiC-I; (d) and (h) Ni/C@SiC-II; (e) and (i) Ni/C@SiC-III.

Fig. 3. XRD spectrums of pure Ni/C foam and Ni/C@SiC composites.

2.3. Characterization

The microstructures were tested by the SEM (Quanta-650, FEI,USA).The physical phase and crystallinities were performed by the X-ray diffraction patterns (XRD, Rigaku). Tested rings were obtained from a mixture of 50 wt% the samples and 50 wt% paraffin wax matrix,and shaped into the coaxial rings with a φinof 3.04 mm and φoutof 7.00 mm, and then the Agilent PNA N5244A vector network analyzer(VNA)was used to measure the electromagnetic parameters.

3. Results and discussion

3.1. Development of Ni/C@SiC composite

The evolution progress of the Ni/C@SiC composite was displayed in Fig.2,including the physical and microstructure photographs of the pure Ni/C foam and Ni/C@SiC composites. After the CVD process,the pure Ni/C foam inherited the original shape of the Ni foam while the density increased from ~0.24 g/cm3to ~0.47 g/cm3. Besides, the skeleton of the Ni/C foam could maintain the threedimensional network in the following PIP cycles as shown in Fig. 2 (b)-(e), which verified that the Ni/C foam was an ideal template during the impregnation-pyrolysis process. In the vacuum impregnation progress,the pores of the Ni/C foam were filled by the PCS/xylene solution, which was converted into the SiC fillers after the dry and pyrolysis processes. The network skeleton of the Ni/C foam not only suffered the vacuum stress,but the high temperature thermal stress.With the increase of the PIP cycles,much more PCS/xylene solution seeped through the crack of the SiC matrix or between the SiC matrix and Ni/C foam skeleton during the vacuum impregnation process,and secondary SiC fillers had been generated after the pyrolysis process.As a result,the densities of the samples increased from~0.74 g/cm3to~0.96 g/cm3,and finally~1.15 g/cm3.The detailed microstructure images of the network skeleton were listed in Fig. 2(f)-(i). After the pyrolysis treatment, some particles absorbed on the skeleton surface were considered as the reactiongenerated SiC particles and some other residual impurities. The reason could be that the pyrolysis temperature was not high enough to make it completely reactive,which might affect the final properties.

Fig. 3 displayed the XRD spectrums of the pure Ni/C foam and Ni/C@SiC composites.Three characteristic peaks at 44.5°,51.9°,and 76.3°were consistent with the (111), (200), and (220) peaks, indicating that the CVD process would not destroy the crystal structure of Ni[29].The feature diffraction peak at 26.6°indexed to the(002)crystal plane of the graphitic carbon was quite low,which might be attribute to the low proportion [30]. However, after the pyrolysis progress, the intensity of the graphitic carbon became much stronger due to the carbon phase derived from the PCS/xylene mixture, while the Ni peaks weakened because of the side reactions, accompanied by some diffraction peaks of Si. Besides, the detected (111) plane around 35.46°was found to match well with the SiC peak,which became much stronger with the increase of the infiltration-pyrolysis processes.However,the crystallinity of the SiC fillers was much lower due to the low presence, especially compared with the SiC coating made by the CVD process, which owned a higher stoichiometric ratio [31].

3.2. Microwave absorbing performance

The RL values were determined by the electromagnetic characteristics via the line transmission theory as follows:

Where ε’was the real permittivity,μ’was the real permeability,ε’’was the imaginary permittivity,μ’’was the imaginary permeability,εrwas the normalized complex permittivity,μrwas the normalized complex permeability,Zinwas the impedance of the input absorber,Z0was the impedance of free space, c, d, and f referred to the velocity of the electromagnetic wave in free space, the matching coating thickness, and the electromagnetic frequency, RL represented the reflection loss value [32-34].

Generally speaking, the effective absorption bandwidth was zoned for the frequency where the corresponding RL was below -10.00 dB, which meant that the absorber could absorb 90.00% of the electromagnetic microwaves [35]. But in fact, a growing demand was that the actual absorber should own a low RL value and a wide bandwidth, especially at low frequency. Fig. 4 showed the RL contour maps of the pure Ni/C foam and Ni/C@SiC composites in the frequency range of 2.00-8.00 GHz, along with the corresponding thickness within 1.00 and 5.00 mm. It was interesting that the RL values of the pure Ni/C foam,Ni/C@SiC-II and Ni/C@SiC-III composites were all above -10.00 dB, meaning that only the Ni/C@SiC-I composite could meet the growing demand.Besides,the RL values of all the samples were relatively poor at the minimum frequency boundary, which proved that it was hard to achieve suitable microwave absorbing property at low frequency.

Fig. 4. (a) RL contour map of Ni/C; (b) Ni/C@SiC-I; (c) Ni/C@SiC-II; (d) Ni/C@SiC-III.

Fig. 5. (a) 3D RL plot of Ni/C@SiC-I; (b) RL plots with 5.00 mm thickness; (c) RL plots with 3.85 mm thickness.

For detailed describing the microwave absorption property of the Ni/C@SiC-I composite, the three-dimensional RL map was presented in Fig.5(a).The Ni/C@SiC-I sample owned the strongest microwave absorption performance at 5.28 GHz, and the corresponding RL value and matching thickness is -25.87 dB and 5.00 mm respectively (Fig. 5(b)). Besides, the wide absorption bandwidth directly corresponded with the frequency range when the RL value was below-10.00 dB.As displayed in Fig.5(c),the Ni/C@SiC-I sample exhibited the widest effective bandwidth of 1.86 GHz from 5.88 GHz to 7.74 GHz at 3.85 mm.

To dig the deep cause, the electromagnetic parameters of the pure Ni/C foam and Ni/C@SiC composites were analyzed in Fig. 6.The values of ε’and ε’’were on behalf of the electric dissipation and storage ability while those of μ’ and μ’’ meant the magnetic dissipation and storage ability [36,37]. In Fig. 6 (a) and 6(b), the other three samples showed a relatively flat trend except the Ni/C@SiC-I composite.After the first pyrolysis progress,the ε’and ε’’of the pure Ni/C foam rose to a high level in the 2.00-8.00 GHz range, which might be largely due to the effect of the graphitized carbon [38]. Besides, the ε’ curve of the Ni/C@SiC-I composite showed a large fluctuation while the ε’’ increased with the frequency,which was good for the dielectric loss.The crystallizations of the carbon and SiC fillers played a significant place in the increase of ε’ and ε’’, however, ε’ and ε’’ decreased to a low level as the PIP cycle increased, which might be caused by the residual byproducts[39].Fig.6(c)showed the dielectric loss of the pure Ni/C foam and Ni/C@SiC composites via the following formula(tanδε= ε’’/ε’). It was clear that the Ni/C@SiC-I composite exhibited the maximum values, meaning that the robust dielectric loss property in the overall frequency range. The induced of the SiC matrix accompanied by the graphene carbon enhanced the dielectric loss, which was of benefit to the microwave absorbing performance[40].However,the residual byproducts played a more important role in adjusting the dielectric loss.Fig.6(d)showed the corresponding μ’and μ’’values of the prepared composites,which were near 1 and 0 respectively.A certain degree of deviation might be ascribed to the base Ni foam, and the corresponding magnetic loss tangent values calculated by the formula (tanδμ= μ’’/μ’) was displayed in Fig. 6(e), which also showed that the magnetic loss ability was smaller than the dielectric loss, especially at relatively high frequency [41]. Fig. 7(f) displayed the eddy current values at the prescribed frequency range, which fell to 0 at 8.00 GHz, indicating the ignorable exchange resonance and natural resonance[32].

Fig.6. (a)Real and(b)imaginary parts of complex permittivity;(c)tanδε curves and(d)real and imaginary parts of complex permeability;(e)tanδμ curves and(f)the eddy current values of pure Ni/C foam and Ni/C@SiC composites.

It was generally believed that not only the attenuation constant(α) but the perfect impedance matching characteristic (Z) decided the remarkable performance of the microwave absorber [42]. The attenuation constant was gained via the listed formula, which represented the integrated microwave dissipated ability [43]:

As displayed in Fig. 7(a), relatively low attenuation constant values except the Ni/C@SiC-I composite were exhibited,indicating that the incident microwave could be mostly absorbed within the internal Ni/C@SiC-I composite compared with the other three,which exactly fit the corresponding electromagnetic features. It also confirmed that the loss of the electromagnetic waves was largely decided by the dielectric attenuation.

As to the impedance matching characteristic (Z), it was calculated as follows:

In general,the values of the impedance matching characteristic fell between 0.8 and 1.2 meant that the surface electromagnetic microwaves could enter the material to a great extent[41].Fig.7(b)showed the impedance matching characteristic values of all the samples at specific thicknesses. Obviously, the Ni/C, Ni/C@SiC-II,and Ni/C@SiC-III samples possessed high impedance matching characteristics, and only a small portion could fit the requirement.It was shown that the Ni/C@SiC-I composite could trigger the valid impedance matching area,which meant that after the primary PIP progress, more microwaves could enter the surface of the Ni/C@SiC-I composite and fewer microwaves would be reflected.Based on this, the sample could acquire a sufficient loss ability to consume the entered microwaves. Hence, the Ni/C@SiC-I composite with suitable impedance matching characteristic and strong attenuation ability was expected to own excellent microwave absorption performance [44]. To estimate the results of the impedance matching characteristic and attenuation constant, Fig. 7(c)clearly plotted the relationships between the RL value, the attenuation constant, and the impedance matching characteristic with the frequency range at the optimal matching thickness of 5.00 mm.The RLminvalue of -25.87 dB was obtained at 5.28 GHz when the attenuation constant was 72.40 and the value of the impedance matching characteristic was 0.89. However, the impedance matching feature arrived at the highest point of 0.94 at 4.92 GHz while the RL value and the attenuation constant were -16.20 dB and 66.82, respectively. On the contrary, the attenuation constant reached the peak of 156.65 at 8.00 GHz while the RL and impedance matching values were -4.68 dB and 0.26, respectively. The above results demonstrated that the ideal RL was obtained where no optimal matching value occurred between the attenuation constant and impedance. What was more, it proved that the impedance matching characteristic acted a vital role as only if the microwaves could enter the absorber, as well as the high attenuation effect working [45].

Fig.7. (a)Attenuation constant and(b)impedance matching characteristic of the samples;(c)impedance matching characteristic,attenuation constant,and RL of Ni/C@SiC-I at the corresponding matching thickness.

Fig. 8. Schematic mechanisms of microwave absorption properties for Ni/C@SiC-I composite.

Fig. 8 discussed the relevant mechanisms for improved microwave absorption performance of the Ni/C@SiC-I composite.Firstly,the graphene coating as well as the Ni skeleton promoted the microcurrent conduction, resulting in the enhanced conversion from microwave energy into electron kinetic energy. Then, the three-dimensional porous structure could further accelerate the multiple reflection of microwaves and improve the electromagnetic damping performance.Besides,the dielectric loss was achieved not only by the interfaces between the Ni foam skeleton, graphene coating and SiC fillers, but also the dipole polarization caused by functional groups and defects,which was crucially important to the enhancement of the microwave attenuation effect [41,46,47].Table 1 listed the microwave absorption performances of some reported SiC composites,and the Ni/C@SiC-I composite displayed a much better microwave absorption performance, which had extremely important to the defence technology [48-54].

Table 1 Electromagnetic wave absorption property of some reported SiC composites.

4. Conclusion

In a word,an efficient microwave absorbing composite made by the 3D Ni/C foam and SiC fillers was successfully synthesized via the PIP processes. The as-fabricated Ni/C@SiC composite with a novel filling structure was believed to be a consummate microwave absorption material, and after the primary PIP progress, the Ni/C@SiC-I composite indicated the optimum microwave absorption property with an RLminvalue of -25.87 dB. However, as the PIP course increased, the microwaving absorbing performance of the Ni/C@SiC-II and Ni/C@SiC-III samples were not good as expected,which might be ascribed to the residual product. The profound reason was further investigated,and the result showed that the Ni/C@SiC-I sample owned a better attenuation constant and suitable impedance matching characteristic,which proved the possibility of the application of the microwave absorbing material at low frequency.

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

This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. D5000210522 and D5000200408),Jiangsu Planned Projects for Postdoctoral Research Funds,National Natural Science Foundation of China[grant number 51772151], Natural Science Foundation of Shaanxi Province (Grant No. 2021JQ-117), Basic Research Programs of Taicang (Grant No.TC2020JC10), and Natural Science Foundation of Shandong Province (Grant No.ZR2020QE180).