Effect of different kinds of SiC fibers on microwave absorption and mechanical properties of SiCf/SiC composites

The SiC fibers are essential for designing microwave absorption and mechanical properties of multifunctional composites. In this study, SiCf/SiC composites were fabricated by SLF, KD-II, and KD-S SiC fibers. The SLF fibers are composed by amorphous SiOC. The KD-II and KD-S fibers reveal higher crystallizations. The conductivity of SLF, KD-II, and KD-S SiC fibers are 0.0127, 1.184, and 0.1316 S/cm, respectively. The flexural strength of SLF, KD-II, and KD-S SiCf/SiC composites are 147.77, 322.57, and 248.16 MPa, respectively. With the thickness of 2.3 mm, the microwave absorption property of SLF SiCf/SiC composites is over − 25 dB in X band. Additionally, the effective absorption bandwidth (EAB) of SLF SiCf/SiC composites below − 10 dB reaches 3.72 GHz with the thickness of 2.7 mm. In contrast, the KD-II SiCf/SiC composites only reach − 3.6 dB in X band when the thickness varies from 2 to 2.9 mm. With the thickness of 2 mm, the microwave absorption property of KD-S SiCf/SiC composites is over − 9 dB. With the thickness of 2.2 mm, the EAB of KD-S SiCf/SiC composites below − 7 dB reaches 4.12 GHz. The mechanisms of mechanical, microwave absorption, and penitential applications for SiCf/SiC composites are also discussed.


Introduction
Nowadays, as the development of microwave technology, electromagnetic pollution usually appears in daily life. Numerous researches have been devoted to designing and implementing excellent electromagnetic absorption materials [1][2][3][4][5][6][7]. In recent years, ceramic composites with excellent microwave absorption properties have increasingly attracted attentions, such as SiC nanowires-reinforced SiOC ceramic, Si 3 N 4 -SiCN ceramics composites [8], and Fe 3 O 4 nanoparticles on MXenes [9]. Continuous fiber-reinforced ceramics matrix composites (CFRCMCs) were believed to be the most prospective multifunctional composites due to their high mechanical strength, stability under high temperature, low densities, as well as oxidation resistance properties [10][11][12]. Therefore, SiC f /SiC composites can be a kind of multifunctional materials with microwave absorption and mechanical properties.
The abovementioned properties of SiC CFRCMCs depend on complex permittivity, electrical conductivity, and free carbon content of SiC fibers, as well as interphase and matrix. The SiC ceramics matrix can be fabricated by chemical vapor infiltration (CVI) [13], precursor impregnation and pyrolysis (PIP) [14], liquid silicon infiltration (LSI) [15], and melt infiltration (MI) [16]. Among these techniques, the PIP method has many advantages, for instance, the designable molecular precursor of polymer-derived ceramics (PDC), safe in operation, and the feasibility of manufacture in large complex components. Thus, the PIP method should be a better way to fabricate the multifunctional SiC f /SiC composites.
The properties of SiC f /SiC composites are subject to a good many of factors, such as the types of SiC fibers. There are three generations of SiC fibers that have been studied in recent years [17][18][19]. The Nicalon 200 from Nippon Carbon and Tyranno LOX-M from UBE industries can represent the first generation of SiC fibers. The Hi-Nicalon from Nippon Carbon and Tyranno ZE from UBE Industries can represent the second generation of SiC fibers. The Tyranno SA 1 from UBE Industries and Sylramic from COI ceramics can represent the third generation of SiC fibers. With the generation developed, the oxygen contents in fibers came 1 3 down. The oxygen content of first generation could go as high as 12 wt% with the maximum limit temperature of only 1200 °C. The excessive oxygen content of SiC fibers was harmful. Since the active oxidation could cause coarsening of β-SiC crystals at high temperature [20], the oxygen content of the second generation is lower than 2 wt% with enhanced limit temperature of about 1300 °C. However, the second generation also has poor oxidation resistance property due to the high C/Si ratio of 1.5. The oxygen content of the third generation is lower than 1 wt% with the maximum temperature over 1700 °C and the C/Si ratio only about 1. Moreover, the third generation has higher crystallinity and density. The polycrystalline SiC fibers can give rise to better elastic modulus and tensile strength of fibers [21]. All of the abovementioned SiC fibers are from Japan, America, and Germany. However, with the efforts of Chinese researchers, SiC fibers can be fabricated in China at present.
As mentioned above, this research studied the SLF, KD-II, and KD-S SiC fibers that are all produced in China and represent first, second, and third generation of SiC fibers, respectively. These SiC fibers were used as reinforcements. The matrix is the SiC ceramics which were pyrolyzed from precursor polycarbosilane (PCS). For avoiding the effect of interphase on SiC f /SiC composites, it was not fabricated in this work. This paper systematically researched the properties of SiC fibers and the influence of different SiC fibers on microwave absorption and mechanical properties of SiC f /SiC composites.

Raw materials of SiC f /SiC composites
The SLF fibers and PCS powders were bought from Suzhou CeraFil Ceramic Fiber Co., Ltd. KD-II and KD-S fibers were purchased from the National University of Defense and Technology (NUDT). General properties of three kinds of SiC fibers are listed in Table 1 [11,22]. These SiC fibers were used as reinforcement and fabricated into 2.5D with a volume fraction of 35%. Additionally, the PCS/xylene solution was prepared at 50 wt%.

Preparation of SiC f /SiC composites with different SiC fibers
Firstly, these three kinds of 2.5D SiC fibers were ultrasonically cleared in alcohol and acetone solution for 15 min. Afterwards, PCS and xylene solution were infiltrated into these 2.5D SiC fibers for 30 min in the vacuum infiltration oven and then dried at 80 °C for 3 h in the vacuum drying oven. After drying, these composites were pyrolyzed in a vacuum-sintering furnace at 1000 °C for 2 h. The PIP procedure was used as a method to fabricate SiC f /SiC composites. In addition, this PIP procedure should repeat until the weight gain is < 1% per cycle. where B is the full width at half maximum (FWHM) of the (1 1 1) peak, θ is the Bragg's diffraction angle, K is the Scherrer's constant, and λ is the X-ray wavelength. Additionally, crystallinity of SiC fibers can be calculated by the following equation with XRD spectra:

Characterization of SiC f /SiC composites
where I c represents crystal diffraction peak area and I a represents amorphous diffraction peak area. The judging standard of crystal diffraction peak is FWHM < 3.0.
The flexural strength of SiC f /SiC composites with different kinds of SiC fibers was characterized by three-point bending test using the universal testing machine (Haida Qualitative Analysis, HD-609B) at room temperature. Mechanical samples were cut into 40 mm × 4 mm × 3 mm. The cross head speed of this universal testing machine is 0.5 mm/min with a span of 30 mm. The scanning electron microscope (Model VEGA3 SBH, TESCAN, Brno, Czech) with energy dispersion spectroscopy (EDS) can be used in observing the surface of SiC fibers and fracture surfaces of SiC f /SiC composites. The conductivity of SiC fibers can be calculated by the equation: where L is the length and S is the cross-sectional area of a bunch of SiC fibers, respectively. R is the resistance of SiC fibers, which was measured by a resistant tester. The complex permittivity measurement of SiC f /SiC composites with the X band (8.2 and 12.4 GHz) was done using the vector network analyzer (Agilent Technologies E8362B). The dimension of complex permittivity samples was 22.86 mm × 10.16 mm × 2.0 mm.

Microstructure of SiC fibers
As shown in Fig. 1, there are surface and cross-section morphologies of different kinds of SiC fibers. The surface of SiC fibers is smooth and homogeneous. The diameters of SiC fibers are all about 12 μm. The cross-sections of SiC fibers were cut by scissors, which were shown to have a dense morphology. Many particles with a small size can be seen on the cross-section of fibers.
The different SiC fibers' oxygen elements analysis of surface and cross-section at selected regions in Fig. 1 were measured by EDS, as seen in Fig. 2. By comparing SLF, KD-II, and KD-S SiC fibers, Fig. 2 shows that the content of oxygen element in SLF fibers is 17.21% on the surface and 10.36% in the cross-section. Therefore, SLF SiC fibers satisfy the characteristic of high oxygen content of first generational and process a large amount of SiOC phase [24]. In contrast, the oxygen content of KD-S SiC fibers is only 3.03% on surface and 2.34% in cross-section.
The different properties of these three types of SiC fibers can be reflected by polycrystalline microstructure [21]. As shown in Fig. 3, the XRD revealed the crystalline structure of different SiC fibers. The crystallinity and crystallite size of β-SiC crystallite for SLF, KD-II, and KD-S SiC fibers are shown in Table 2. The XRD patterns illustrate that the β-SiC is the main constituent of SiC fibers at peaks of 35.74°, 41.50°, 60.14°, 71.97°, and 75.71°, which are indexed to (1  1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattices, respectively. From Fig. 3, it can be concluded that the SLF SiC fibers present three main diffraction peaks at 35.74°, 60.14°, as well as 71.97°. These peaks represented the (1 1 1), (2 2 0), and (3 1 1) planes of the amorphous β-SiC phase. The sharp XRD peaks observed for the KD-II and KD-S SiC fibers clearly confirm larger crystallite size (4.4 and 4.1 nm) of β-SiC than that of the SLF SiC fibers (1.4 nm). Moreover, the (2 0 0) and (2 2 2) planes of the β-SiC are revealed in KD-II and KD-S SiC fibers. As same as the first generation of SiC fibers, the SLF fibers are almost amorphous, broad minor peaks can certify [25]. In contrast, KD-II and KD-S SiC fibers have high crystallinity with 83.92 and 87.76%, respectively.
Thanks to the amorphous phase and high oxygen content, the free carbon conductive network in the SLF fibers will be separated by Si-C-O phase [18]. Therefore, as shown in Table 3, the conductivity of SLF fibers is only 0.0127 S/cm. The conductivity of KD-II fibers is 1.1811 S/cm owing to the larger C/Si atomic ratio [21] and the conductive network of carbon-rich layers. The conductivity of KD-S fibers is 0.1316 S/cm due to the lower C and O content.

Mechanical properties and microstructure of SiC f /SiC composites
Mechanical properties of different kinds of SiC f /SiC composites can be seen in Table 4.   Fig. 4, the stress-displacement curves can be obtained from bending test for SiC f /SiC composites with different kinds of SiC fibers. The load on the different kinds of SiC f /SiC composites declines abruptly after reaching the peak value. It is revealed that all kinds of SiC fiber-reinforced SiC f /SiC composites appear to have a typical brittle fracture behavior. It can be concluded that the bonding strength between these SiC fibers and the matrix was strong.
Fracture morphologies of SiC f /SiC composites with SLF, KD-II, and KD-S fibers were shown in Fig. 5. As illustrated in Fig. 5a, fracture surface of SLF SiC f /SiC composites was plane and there are no fibers pull-out phenomenon appearance due to the strong bonding between matrix and the surface of SLF SiC fibers. Fracture surface of KD-II SiC f / SiC composites exhibits step-shaped appearance, as shown in Fig. 5b. In Fig. 5c, the step-shaped fracture and fibers pull-out can be discovered in the fracture surface of KD-S. According to the fracture morphologies, the matrix densification of SLF, KD-II, and KD-S SiC f /SiC composites are similar. With the same content of matrix and SiC fibers in SiC f /SiC composites, different mechanical properties should be effected by different kinds of SiC fibers.

Dielectric properties of SiC f /SiC composites with different SiC fibers
The complex permittivity (ε = ε′ − jε″) of SiC f /SiC composites with various SiC fibers was measured in the X band (8.2-12.4 GHz). Real part of permittivity (ε′) is correlated with polarization and the imaginary part of permittivity (ε″) in reference to the ability of dielectric loss and electrical conductivity [26]. The SiC f /SiC composites loss tangent (tanδ) can be calculated by tanδ = ε″/ε′ [27], which represent the microwave absorption ability [28]. As shown in Fig. 6, both real and imaginary permittivity of KD-II SiC f /SiC composites reveal a decreasing tendency as frequency increases in X band. This tendency can be called frequency dispersion effect. As shown in Fig. 8c, reflection loss peak of KD-II SiC f /SiC composites do not appear in the X band. Additionally, the reflection loss values of KD-II SiC f /SiC composites were only below − 4 dB and possessed few changes with every thickness from 2.0 to 2.9 mm in the whole X band. Consequently, it can be indicated that KD-II SiC f /SiC composites possess frequency dispersion effect and a broad microwave absorption band in X band. The complex permittivity of SLF and KD-S SiC f / SiC composites almost keeps constant in the measured range, in which the values are 7.9-4.0j and 11.6-9.2j at the frequency of 10 GHz, respectively. Compared with SLF and KD-S SiC f /SiC composites, KD-II SiC f /SiC composites possess much higher imaginary permittivity and loss tangent. The real and imaginary part of permittivity of KD-II SiC f / SiC composites are 12.6 and 25.0 at 10 GHz, respectively, and the loss tangent of KD-II SiC f /SiC composites is around 1.7-2.2 in the whole X band, as shown in Fig. 6c. It suggests that the KD-II SiC f /SiC composites have the strongest microwave absorption ability. Furthermore, the attenuation constant (α) is also a significant factor for the microwave absorption property. The higher α values mean better microwave absorption property. The attenuation constant can be calculated using the following equation [29]: The variation of attenuation constant of SLF, KD-II, and KD-S SiC f /SiC composites in the X band is shown in Fig. 6d. The KD-II SiC f /SiC composite has the highest value of attenuation constant ranging from 493 to 680, indicating the highest energy consumption ability of the microwave. It is corresponding to the dielectric tangent loss analysis.

Fig. 4 Stress-displacement curves of SiCf/SiC composites with SLF, KD-II, and KD-S SiC fibers
Furthermore, the SiC f /SiC composites are usually composited by SiC fibers, interface, and SiC matrix. Therefore, real and imaginary permittivity of SiC f /SiC composites can be illustrated as Lichtencker's logarithmic equations [10,26]: where v f , v m , and v i are volume contents, ε′ f , ε′ m , and ε′ i are real part of permittivities, and ε″ f , ε″ m a,nd ε″ I are imaginary part of permittivities of SiC fiber, SiC matrix, and interface, respectively. In this experiment, the interface was not fabricated and the porosity and density of SiC matrix were similar as shown in Table 4. Additionally, the SLF, KD-II, and KD-S SiC fibers were all fabricated into 2.5D with volume fraction of 35% and the process of manufacture of SiC f / SiC composites was with the same experimental condition. Consequently, the real and imaginary part of permittivities of SiC f /SiC composites are mainly dependent on SiC fibers.
On the one hand, there are lots of interfaces around SiC nanocrystals, free carbons, and amorphous SiOC phase. Moreover, there are many dipoles from defect in SiC nanocrystals and free carbons in SiC fibers [26]. KD-II and KD-S SiC fibers with high crystallinity have more grain boundaries and free carbons. Therefore, the higher real permittivity of KD-II and KD-S SiC f /SiC composites owing to polarization, which formed by interfaces between the grain boundary and dipoles in the KD-II and KD-S SiC fibers under the X band. On the other hand, from Debye theory [30], the imaginary part of permittivity can be calculated by the following equation: where σ is the conductivity, ε 0 is vacuum dielectric constant, and f is electromagnetic wave frequency. According to the analysis, the imaginary part of permittivity of SiC f /SiC composites in this experiment is proportional to conductivity of SiC fibers. As shown in Table 3, the conductivity of KD-II SiC fibers 1.1811 S/cm is the highest. Therefore, the higher electrical conductivity of KD-II SiC fibers gives rise to larger imaginary part of KD-II SiC f /SiC composites. where Z 0 is the free space characteristic impedance (377 Ω), Z in is the input impedances between SiC f /SiC composites and free space, μ 0 and ε 0 are the permeability and permittivity of vacuum, and μ r and ε r are measured relative permeability and permittivity. Because of negligible magnetic properties of SiC, the value of μ r , in this experiment, is taken as 1.
Consequently, the RL values of SiC f /SiC composites mainly depend on complex permittivity and thickness in X band. Generally, the high loss tangent can give rise to better microwave absorption abilities. However, the optimum impedance matching between free space and microwave absorption material leads microwave reflection reduction and then enhances the energy loss of the incident microwave. For optimum matching impedance, it demands that the input impedance Z in is equal with free space impedance Z 0 as far as possible. The input impedance Z in of SLF, KD-II, and KD-S SiC f /SiC composites with the thickness of 2.9 mm are shown in Fig. 7.
Z in of the KD-II SiC f /SiC composites is around 63.15-75.12 Ω. The higher difference between Z in and Z 0 leads to the stronger reflection of microwave on the SiC f / SiC composites. Therefore, KD-II SiC f /SiC composites possess poor microwave absorbing properties, as shown in Figs. 8b and 9c, d. In X band, KD-II SiC f /SiC composites has only − 3.6 dB RL values from 2 to 2.9 mm thickness and there is no reflection loss peak appearance. On the contrary, the Z in value of SLF SiC f /SiC composites is the highest, in the range of 328.65-178.70 Ω, and processes the lowest deviation between Z in and Z 0 . As a consequence, most of the microwave incident into SLF SiC f /SiC composites. As That deviation between Z in and Z 0 is higher than that of SLF SiC f /SiC composites and lower than that of KD-II SiC f /SiC composites. As shown in Figs. 8c and 9e, f, the RL values of KD-S SiC f /SiC composites over − 9 dB with a thickness of 2 mm and the EAB below − 7 dB reach 4.12 GHz (from 8.28 to 12.40 GHz), illustrating that excess 84% of microwave have been absorbed [28].
With the same SiC matrix, the SLF SiC f /SiC composites possess the optimum microwave absorption property due to the amorphous Si-O-C structure and low conductivity of SLF fibers. Additionally, the interfacial polarization is generated by the massive amount of amorphous SiOC and free carbon in SLF fibers. When microwave propagates into the SLF SiC f /SiC composites, a large number of dipoles is generated from free carbon, amorphous SiOC, and amorphous SiC. The dipoles could cause displacement polarization or steering polarization with changed electromagnetic wave. The dipoles polarization gradually lagged behind the changes. At this time, energy of electromagnetic wave can be consumption [4,31]. Instead, a large amount of interface generated between free carbon in the KD-II and KD-S fibers leads displacement polarization or steering of dipoles easily and consume less electromagnetic wave energy under the alternating microwave.
For better illustrating the morphologies of SiC fibers and SiCf/SiC composites effect on microwave absorption property, the sketch of microwave absorption mechanism was painted. As shown in Fig. 10, when microwaves are incident on the surface of SiCf/SiC composites, SLF, KD-S, and KD-II SiCf/SiC composites have different microwaveintroduced situations. For SLF SiCf/SiC composites, the microwave hardly reflected into air directly due to multiple reflection between the massive small scale SiC crystal grains and free carbons in the SLF fibers. For KD-S SiCf/ SiC composites, incident microwave reflected between largescale SiC crystal grain and few free carbons. A few portions of incident microwave reflect into air directly. A large portion of incident microwave was reflected into air by KD-II SiCf/SiC composites, due to a large amount of free carbon and large-scale SiC crystal grain. The incident microwave easily reflected by free carbon and the large-scale SiC crystal grains cannot reflect microwave in to the composites. Thus, the SLF SiCf/SiC composite has the best microwave absorption property. Above mentioned is corresponding to the impedance matching analysis.

Conclusion
In conclusion, SiC f /SiC composites fabricated with SLF, KD-II, and KD-S SiC fibers by PIP method have analogous density and open porosity. SLF SiC fiber possessed a large amount of oxygen and amorphous phase. The KD-II and KD-S SiC fiber exhibit higher crystallizations and higher free carbon contents than SLF SiC fibers. KD-II SiC f /SiC composites have the highest flexural strength of 322.57 MPa and possess the worst microwave absorption property owing to the massive carbon in the fibers. SLF SiC f /SiC composites possess the best microwave absorption property due to the best impedance matching with air and the lowest flexural strength can be ascribed to the poor strength of SLF SiC fibers.
With a thickness of 2.3 mm, the microwave absorption property of the SLF SiC f /SiC composites can be obtained over − 25 dB and the effective absorption bandwidth (EAB) below − 10 dB reaches 3.72 GHz with 2.7 mm thickness, indicating its potential application in the structural and absorbing field. Above all, SLF SiC fiber can be used as reinforcement for microwave absorption structure composites. But the interphase on the SLF SiC fibers was necessary to the flexural strength enhancement. Because of the high loss tangent and mechanical property, KD-II SiC fibers are better used as the absorption layer in the multi-layered radar absorbing structures composites field. The KD-S SiC fibers possess medium levels of microwave absorption and mechanical properties. Therefore, further studies should focus on the matrix and interphase modification of KD-S composites.