Journal of Materials Science & Technology  2019 , 35 (12): 2809-2813 https://doi.org/10.1016/j.jmst.2019.07.002

Orginal Article

Mechanical and electromagnetic wave absorption properties of Cf-Si3N4 ceramics with PyC/SiC interphases

Wei Zhoua*, Lan Longab, Yang Lib*

a.College of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou 412008, China
b.State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Corresponding authors:   *Corresponding authors.E-mail addresses: zhouwei@hut.edu.cn (W. Zhou), liyang csu@126.com (Y. Li).

Received: 2019-07-9

Online:  2019-12-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Aim

ing to obtain microwave absorbing materials with excellent mechanical and microwave absorption properties, carbon fiber reinforced Si3N4 ceramics (Cf-Si3N4) with pyrolytic carbon (PyC)/SiC interphases were fabricated by gel casting. The influences of carbon fibers content on mechanical and microwave absorption properties of as-prepared Si3N4 based ceramics were investigated.

Results

show that chemical compatibility between carbon fibers and Si3N4 matrix in high temperature environment can be significantly improved after introduction of PyC/SiC interphases. As carbon fibers content increases from 0 to 4 wt%, flexural strength of Si3N4 based ceramics decreases slightly while fracture toughness obviously increases. Moreover, both the real and imaginary parts of complex permittivity increase with the rising of carbon fibers content within the frequency range of 8.2-12.4 GHz. Investigation of microwave absorption shows that the microwave attenuation ability of Cf-Si3N4 ceramics with PyC/SiC interphases is remarkably enhanced compared with pure Si3N4 ceramics. Effective absorption bandwidth (<-10 dB) of 10.17-12.4 GHz and the minimum reflection less of -19.6 dB are obtained for Si3N4 ceramics with 4 wt% carbon fibers in 2.0 mm thickness. Cf-Si3N4 ceramics with PyC/SiC interphases are promising candidates for microwave absorbing materials with favorable mechanical property.

Keywords: Silicon nitride ; Carbon fibers ; PyC/SiC interphases ; Mechanical properties ; Microwave absorption

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Wei Zhou, Lan Long, Yang Li. Mechanical and electromagnetic wave absorption properties of Cf-Si3N4 ceramics with PyC/SiC interphases[J]. Journal of Materials Science & Technology, 2019, 35(12): 2809-2813 https://doi.org/10.1016/j.jmst.2019.07.002

1. Introduction

With the rapid development in wireless communications and digital devices, electromagnetic absorbing materials have attracted tremendous attention in the field of microwave frequency application for solving more and more serious problems like microwave radiation and pollution [[1], [2], [3]]. In the past decades, carbonyl iron [[4], [5], [6]], carbon materials [[7], [8], [9]] and conductive polymers [[10], [11], [12]] have been demonstrated to be excellent microwave absorbing materials and achieved widely applications. However, due to the poor oxidation resistance or demagnetization features, the applications in high-temperature environments of these above mentioned microwave absorbers are limited.

Si3N4 ceramics, well known for relatively low cost, lightweight, high strength and hardness, favorable dielectric properties and excellent environmental stability, have been proven as not only ideal thermal structure materials but also ideal candidate matrix for high-temperature microwave absorbing materials [[13], [14], [15], [16]]. Carbon fibers with light weight, high strength and modulus, and excellent electrical conductivity, are often used as reinforcement and absorbers [[17], [18], [19]]. Many investigations indicated that the composites containing carbon fibers as fillers for microwave absorbing possess attractive microwave absorbing property and potentially practical applications especially in high temperature microwave absorption [[20], [21], [22], [23]]. Therefore, it is now generally believed that Si3N4 ceramic with carbon fibers filler (Cf-Si3N4) are promising microwave absorbing materials with favorable mechanical properties to be used for microwave absorption at both room and high temperature. However, it is notable that carbon fiber and Si3N4 matrix were chemically incompatible with each other at elevated temperatures [24,25]. Several studies demonstrated that using of interphase [[26], [27], [28]], for instance, SiC interphase prepared by chemical vapor deposition (CVD), is an effective approach for improving chemical compatibility between carbon fiber and Si3N4 at high-temperature [29]. Nevertheless, carbon fiber would be damaged during the preparation of CVD-based SiC interphase due to the chemical reaction between C atom and Si atom at process temperature, leading to significant degradation of mechanical properties. In this study, pyrolytic carbon (PyC) coating was firstly deposited on carbon fibers and then SiC coating was deposited on the PyC-coated carbon fibers, to solve the above mentioned damage and chemical incompatible problems. Microstructure, mechanical and microwave absorbing properties of as-prepared Si3N4 based ceramics with different mass ratio of PyC/SiC coated carbon fibers were investigated. Moreover, the related mechanical, dielectric and microwave absorption mechanism for Cf-Si3N4 ceramics with PyC/SiC interphases were discussed in detail.

2. Experimental procedures

2.1. Preparation of samples

The raw material of α-Si3N4 powders were purchased from Beijing Unisplendor Founder High Technology Ceramics Co., Ltd. China. The diameter of α-Si3N4 powders is about d50 = 0.5 μm and the purity is above 93%. PAN based carbon fibers (T700, 12 K) were purchased from Tianniao Company, Jiangsu, China. The diameter of carbon fiber is about 7 μm. Al2O3 (AR, Sinopharm Chemical Reagent Co., Ltd., China) and Y2O3 (AR, Sinopharm Chemical Reagent Co., Ltd., China) as sintering additives with mass fraction of 10% and 5%, respectively, were introduced into Si3N4 powders to promote densification of Si3N4 ceramics.

Cf-Si3N4 ceramics with PyC/SiC interphases and different contents of carbon fibers (0, 2 and 4 wt%) were prepared by combination of chemical vapor deposition (CVD) and gel casting. Firstly, carbon fibers were put into a CVD reactor to deposit PyC coating using butane (C3H6) precursor at 900 °C and a reduced pressure of 500-700 Pa with dwell time of 2 h, and then deposited SiC coating on the PyC-coated carbon fibers using methyltrichloresilan (MTS)-H2-Ar system at 1100 °C and a reduced pressure of 500-700 Pa with dwell time of 4 h. The powder mixture of 85 wt% Si3N4, 10 wt% Al2O3 and 5 wt% Y2O3 was mixed with solvent-based Acrylamide-N,N’-methylenebisacrylamide (AM-MBAM) system to obtain Si3N4 slurry with solids loading of 45 vol.% by ball-milling for 4 h. After adding PyC/SiC coated carbon fibers with an average length of 2 ± 0.3 mm into Si3N4 slurry by mechanically stirring for 30 min, the final mixture slurry was vacuum-deformed and casted into silicon rubber mold with the help of initiator (Ammonium per-sulphate) and catalyst (N,N,N’,N’-tetramethylethylenediamine). Shaped green bodies were formed after the consolidation of suspension. Finally, the dried green bodies were embedded in BN powder in a graphite crucible and sintered by pressureless sintering at 1700 °C for 1.5 h with heating rate of 5 °C /min.

2.2. Characterizations

Bulk density and open porosity of as-prepared ceramics after sintering were determined by means of Archimedes method using distilled water as medium according to DIN EN 1389. Phase analysis was conducted by X-ray diffraction (XRD, D/max2550, Rigaku) with Cu Kα radiation at 35 kV and 20 mA. Samples were mechanically milled into fine powders for XRD characterization. Cross-section morphology of as-prepared ceramics was observed by scanning electron microscopy (SEM, Nova Nano SEM 230). Flexural strength was measured by three-point bending test on an electronic universal testing machine (Instron-3369, USA) with a span of 30 mm and a crosshead speed of 0.5 mm/min, according to ASTM standard C1341. Five specimens were machined into standard bar with the dimension of 3 mm × 4 mm × 40 mm for strength test to obtain the average value. Fracture toughness, KIC, was measured by using Single Edge Notched Beam (SENB) method with a loading speed of 0.05 mm/min, according to ASTM standard C1421-10. Edge notch was made in the middle of each sample by a diamond cutting wheel with a thickness of 0.1 mm. The depth and width of notch were about 2.5 mm and 0.10 to 0.15 mm, respectively. Five specimens with geometry of 5 mm × 5 mm × 25 mm were tested to obtain the average toughness. Complex permittivity (ε', ε”) of samples with a size of 22.86 mm (length) × 10.16 mm (width) × 2 mm (thickness) was measured by using a vector network analyzer (Agilent N5230A) in the frequency range from 8.2 to 12.4 GHz (X-band), according to rectangle waveguide method.

3. Results and discussion

3.1. Microstructure

XRD patterns of as-prepared Si3N4 based ceramics are shown in Fig. 1. As shown in Fig. 1(a), the characteristic diffraction peaks presented in pure Si3N4 ceramics all are well indexed to β-Si3N4 (ICCD card no. 82-0697), indicating completely phase transformation from α-Si3N4 to β-Si3N4. After introducing PyC/SiC interphases, some peaks indexed to β-SiC (ICCD card no. 65-0360) appear in Cf-Si3N4 ceramics with 4 wt% PyC/SiC coated carbon fibers (Fig. 1(b)). However, no carbon diffraction peaks were detected due to their low degree of crystallization.

Fig. 1.   Phases of as-prepared Si3N4 based ceramics: (a) pure Si3N4 ceramics, (b) Cf-Si3N4 ceramics with PyC/SiC interphases.

Typical cross-section morphologies of as-prepared Si3N4 based ceramics after sintering are represented in Fig. 2. It can be seen from Fig. 2(a), Microstructure of pure Si3N4 ceramics is relatively dense and rod-like grains are well interconnected with each other. While a few of pores formed during preparation process are also observed. From Fig. 2(b), obvious holes left by completely reacted carbon fibers can be found, indicating pronounced chemical corrosion at sintering temperature in this work. However, within Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers (Fig. 2(c)), apparently, a large amount of carbon fibers dispersed uniformly in the matrix, presenting no obvious chemical corrosion during the sintering process, compared to those without interphase. Furthermore, it can be clearly seen that carbon fibers with smooth surface are well protected by PyC/SiC coatings (Fig. 2(d)), and the thickness of PyC coating and SiC coating are about 1.3 μm and 1.7 μm, respectively. It is reasonable to assume that this special microstructure of Cf-Si3N4 ceramics with PyC/SiC interphases would greatly affect mechanical and microwave absorbing properties. SEM results also confirmed that PyC/SiC interphases could significantly improve compatibility between carbon fibers and Si3N4 matrix at elevated temperatures.

Fig. 2.   SEM images of fracture surface of as-prepared Si3N4 ceramics after sintering: (a) pure Si3N4 ceramics, (b) Cf-Si3N4 ceramics without PyC/SiC interphases, (c) Si3N4 ceramics with 4 wt% PyC/SiC coated carbon fibers, (d) magnification of Fig. 2(c).

3.2. Mechanical properties

Density, open porosity and mechanical properties of as-prepared ceramics are presented in Table 1. It can be seen that density and flexural strength of as-prepared ceramics decrease with increasing carbon fiber content from 0 wt% to 4 wt%. This is closely related to the microstructure of the sintered ceramics [30]. It is inevitable that pores were introduced into Si3N4 ceramics during adding carbon fibers into matrix because of their poor wetting. Moreover, some new pores were generated during the preparation process due to the overlap of elongated carbon fibers. Thus, open porosity of as-prepared Si3N4 based ceramics increased from 1.2% to 4.1% with the increased carbon fibers content. It is well known that strength of ceramics decreases exponentially with increasing porosity owing to less solid cross-section, larger actual stress and more local stress concentration in porous matrix [31,32]. Therefore, increased pores with higher fiber content lead to the increase of open porosity but decrease of density and flexural strength of the Si3N4 ceramics. However, fracture toughness of as-prepared Si3N4 ceramics rises from 3.43 MPa m1/2 to 8.94 MPa m1/2 as carbon fibers content increases from 0 wt% to 4 wt%. The maximum value of fracture toughness of Cf-Si3N4 ceramics with PyC/SiC interphases containing 4 wt% carbon fibers is about 1.6 times higher than that of pure Si3N4 ceramics. The results indicate that these introduced carbon fibers play important role in enhancement of fracture toughness for the Si3N4 based ceramics. Resistance of crack propagation can be effectively improved by adding carbon fibers into Si3N4 matrix, which, as a result, promotes the fracture toughness. Furthermore, carbon fibers with good mechanical properties dispersed in matrix as reinforcement phase, which enables the loads effectively transfer from Si3N4 matrix to fibers. More energy would be consumed during pull-out of carbon fibers, leading to the enhancement of fracture toughness of ceramics. Therefore, the toughness fracture of Cf-Si3N4 ceramics with PyC/SiC interphases is improved. However, it is still very difficult to disperse carbon fibers uniformly in matrix and inevitably agglomeration of carbon fibers with the increasing of fibers content. Thus, Cf-Si3N4 ceramics with PyC/SiC interphases show comparatively low enhancement in fracture toughness compared with pure Si3N4 ceramics due to the significant influence of increased porosity.

Table 1   Density, open porosity and mechanical properties of the as-prepared ceramics.

SamplesFiber content
(wt%)
Density (g/cm3)Open Porosity (%)Flexure strength (MPa)Fracture toughness
(MPa·m1/2)
C003.25 ± 0.031.2 ± 0.2512 ± 233.43 ± 0.7
C223.18 ± 0.032.3 ± 0.3368 ± 186.98 ± 0.6
C443.03 ± 0.044.1 ± 0.3326 ± 218.94 ± 1.1

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3.3. Dielectric properties

The complex permittivity of as-prepared Si3N4 based ceramics is showed in Fig. 3. After introduction of carbon fibers into matrix, the complex permittivity of Cf-Si3N4 ceramics at X band is significantly enhanced. The values of real permittivity (ε′) and imaginary permittivity (ε″) increase from 5.09-5.21 to 11.80-12.69 and 0-0.01 to 1.75-4.18, respectively, as carbon fibers content increased from 0 wt% to 4 wt%. The ε′, in general, indicating polarization ability of the material, is mainly related to electronic relax polarization, especially for the material with conductive carbon fibers as absorbing fillers. Electronic relax polarization were generated due to the rapid response of free electrons in carbon fibers under an alternating electromagnetic field [33]. As carbon fibers content increases, electronic relax polarization is enhanced because more free electrons are introduced into the composites, leading to increase of ε′. Furthermore, much more boundaries and interfaces including Cf/PyC, PyC/SiC and SiC/Si3N4 will be generated in matrix after introduction of carbon fibers, resulting in stronger polarization for the composites [34]. In addition, the higher ε′ can be attributed to the enhanced Maxwell-Wagner-Sillars effect with increasing carbon fiber content, which is confirmed in previous investigations of other percolating composites [[35], [36], [37]]. Finally, according to the capacitance model proposed by Saib [38], the decreased distance between carbon fibers which are electrically conductive and Si3N4 matrix which is electrically insulating leads to larger capacitance and a higher ε′, as carbon fiber weight fraction increases. Therefore, the ε′ increases with the rising of carbon fibers content. The ε″ generally represents the dielectric of the material, which is mainly dependent on electrical conductivity and polarization relaxation [[39], [40], [41]]. It can be calculated according to the following equation [42]:

$ε"≈ε"_{relax}+\frac{σ}{ωε_{0}}$

where εrelax is relaxation polarization, σ is the electrical conductivity, ω is the angular frequency, and ε0 is the dielectric constant in vacuum. As mentioned above, more electronic relax polarization would be generated with the increase of carbon fibers content. Moreover, the increased amount of carbon fibers in matrix can provide more transport channels for mobile charge carriers, leading to higher conductivity of the composites. Additionally, it is well know that the conductivity of the composite containing carbon fibers can significantly increase with the rising of fiber content, owing to the excellent electrical conductivity of carbon fibers [43]. Therefore, according to Eq. (1), the ε″ increases with the higher fiber content due to the increased relaxation polarization and electrical conductivity.

Fig. 3.   Complex permittivity of Si3N4 based ceramics with different PyC/SiC coated carbon fibers contents: (a) the real permittivity (ε′), (b) the imaginary permittivity (ε″).

3.4. Microwave absorption properties

To further reveal the microwave absorption properties of Cf-Si3N4 ceramics with PyC/SiC interphases, the reflection loss (RL) is calculated by using the measured values of complex permittivity and permeability according to transmission line theory, which can be illustrated by the following equations [44]:

$RL(dB)=20log\lvert \frac{Z_{in}-1}{Z_{in}+1} \rvert$

$Z_{in}=(\frac{μ_{r}}{ε_{r}})^{1/2}tanh[j(\frac{2πfd}{c})(μ_{r}ε_{r})^{1/2}]$

where Zin is the normalized input impedance of the absorber, εr=ε′-″ and μr=μ′-″ are the complex permittivity and permeability of the material, and μr = 1 for Si3N4 ceramics which are nonmagnetic in this work. d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave in free space, respectively.

The calculated RL of Si3N4 ceramics with different weight fractions of PyC/SiC coated carbon fibers in thickness of 2.0 mm within the frequency range of 8.2-12.4 GHz is shown in Fig. 4(a). The reflection loss of as-prepared Si3N4 ceramics decreases with the increased carbon fibers weight fractions. The RL of pure Si3N4 ceramic is close to 0, indicating that electromagnetic waves are almost not degraded through pure Si3N4 ceramics. Apparently, the reflection loss of Cf-Si3N4 ceramics with PyC/SiC interphases decreases significantly and the effective absorption bandwidth (RL<-10 dB) becomes broader, as the content of carbon fibers increases from 0 wt% to 4 wt%. Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers present the optimized microwave absorption properties. The effective absorption bandwidth (90% microwave absorption) is 10.17-12.4 GHz and the minimum RL of -19.6 dB is achieved at the matching frequency of 10.87 GHz. Fig. 4(b) shows the reflection loss of Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers in different thicknesses. Obviously, microwave absorption of as-prepared ceramics is significantly affected by the thickness of the sample. As the thickness increases from 1.0 mm to 3.0 mm, the reflection loss firstly decreases then increases, meanwhile the matching frequency of RL peak shifts toward the higher frequency region, The RL value of the sample less than -10 dB in X band can only be obtained with the thickness from 2.0 mm to 2.5 mm in this work. Based on the above discussions, Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers at 2.0 mm thickness exhibit the optimal microwave absorption property. The materials for microwave absorption should meet two basic conditions as follows [23]. Firstly, normalized input impedance of the material should match free space impedance as far as possible, so microwave can enter into the materials then be attenuated. Secondly, entered microwave must be attenuated rapidly by the absorber in the materials. In this work, according to Eq. (3), the complex permittivity of as-prepared Si3N4 based ceramics increased with the increased carbon fibers content, leading to decrease of the input impedance (Zin). From Eq. (2), the decreased impedance deviates from the matching condition, resulting in more reflective incident waves on the surface. Therefore, the enhancement of microwave absorption of Cf-Si3N4 ceramics with PyC/SiC interphases mainly results from the absorbing filler of carbon fibers through transforming incident microwave energy into heat. The maximum microwave absorbing peaks will occur when the thickness meets the conditions of destructive interference according to resonant absorb theory [45]. Therefore the RL peaks of the absorber appear at different frequency with different thickness. In the basis of discussion above, it could be concluded that microwave absorption of Cf-Si3N4 ceramics with PyC/SiC interphases is mainly determined by carbon fibers content and the thickness of sample.

Fig. 4.   (a) Reflection loss of Si3N4 based ceramics with different PyC/SiC coated carbon fibers contents at 2.0 mm thickness, (b) Reflection loss of Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers in various thicknesses.

4. Conclusion

In this study, Cf-Si3N4 ceramics with PyC/SiC interphases with favorable mechanical and microwave absorption properties were prepared by gel casting. The introduction of PyC/SiC coating on the surface of carbon fibers can effectively improve chemical compatibility between carbon fibers and Si3N4 matrix at elevated temperature. Compared with pure Si3N4 ceramics, Cf-Si3N4 ceramics with PyC/SiC interphases show lower flexural strength but much higher fracture toughness. The variation in mechanical properties is mainly determined by the porosity and carbon fibers weight fraction. Moreover, both the real and imaginary part of complex permittivity increases with the rising carbon fibers weight fraction, which is attributed to the increased relaxation polarization and electrical conductivity, respectively. The calculation of reflection loss shows that microwave absorption of Cf-Si3N4 based ceramics with PyC/SiC interphases can be tailored by changing carbon fibers content and thickness of the ceramics. The Si3N4 based ceramics with 4 wt% PyC/SiC coated carbon fibers presenting the optimal microwave absorption property and excellent mechanical properties are potentially applied as high-performance microwave absorbing materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51604107), and the Natural Science Foundation of Hunan Province (Grant No. 2019JJ50115 and 2019JJ50768).


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