Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 8253–8259 www.elsevier.com/locate/ceramint

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting Heng Luo, Peng Xiaon, Long Huang, Wen Hong State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR China Received 22 December 2013; received in revised form 3 January 2014; accepted 6 January 2014 Available online 25 January 2014

Abstract Cf–Si3N4 sandwich composites were prepared by gelcasting using α-Si3N4 powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of Cf–Si3N4 sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 1C. The investigation of microwave absorbing property reveals that, compared with the Si3N4 ceramics, both the real (ε') and imaginary (ε'') permittivity of Cf–Si3N4 sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the Cf–Si3N4 sandwich composites are significantly enhanced compared with pure Si3N4 ceramic, and the reflection loss gradually decreases from
3.5 dB to
14.4 dB with the increase of frequency, while the pure Si3N4 ceramic keeps at
0.1 dB. Particularly, the relationship between permittivity of Cf–Si3N4 sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε' and ωε'' are inversely proportional to the frequency square ω2 , and the predicted results agree quite well with the measured data. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Si3N4 sandwich composites; Gelcasting; Dielectric; Equivalent circuit model

1. Introduction Recently, the electromagnetic absorbing materials that show outstanding microwave absorbing properties not only at room temperature but also at high temperature are widely used in commercial and military applications [1–10]. Silicon nitride (Si3N4) ceramics are well known for not only high-temperature strength, good oxidation, resistance, thermal–chemical corrosion resistance, thermal shock resistance and low thermal expansion coefficient in the field of thermal structure materials, but also its excellent electrical insulation properties, low dielectric constant and loss tangent in the field of microwave transmission materials [11–12]. Therefore, the preparation and microstructure on the mechanical and microwave properties of Si3N4 ceramics has been widely studied [13–22]. Among the numerous preparation technologies of the Si3N4, gelcasting is a well-established colloidal processing method with a short n

Corresponding author. Fax: þ86 73188830131. E-mail address: [email protected] (P. Xiao).

forming time, high yields, high green capacity and low-cost machining, and has been used to prepare high-quality and complex-shaped ceramics parts [23]. The carbon fiber with light weight, high strength and excellent thermal stability, has been found to be fascinating candidate for microwave absorption materials, especially in high temperature applications [19,24–27]. However, it has been demonstrated that carbon fiber and Si3N4 matrix were incompatible with each other both physically and chemically in the high temperature environment [19,28,29]. In order to overcome this incompatibility, various means have been tried already, including the addition of a small amount of zirconia [28] or carbon powder [19] to the Si3N4 matrix, and decrease of sintering temperature during the fabrication process [29]. Nevertheless, the mentioned efforts could not solve the above incompatible problems fundamentally. In spite of the fact that the using of interphase to protect carbon fiber has been investigated in several studies [30–36], no study on the application of chemical vapor deposition (CVD) SiCinterphase to overcome incompatibility between carbon fiber and Si3N4 matrix has been reported yet.

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H. Luo et al. / Ceramics International 40 (2014) 8253–8259

Both theoretical [37,38] and experimental investigations [39–43] have showed that double layered or multilayered structure with matching layer and absorption layers can lower the reflection loss. More importantly, gelcasting provides a method to acquire multilayered structure of absorbing material. Therefore in this paper, taking multilayered structure and SiC-interphase into consideration, Cf–Si3N4 sandwich composites with Y2O3–Al2O3 as sintering additives were fabricated by gelcasing. The microstructure and composition, dielectric properties of Cf–Si3N4 sandwich composites were investigated. In particular, we put forward the corresponding theoretical model of dielectric properties of Cf–Si3N4 sandwich composites. 2. Experimental procedure 2.1. Raw materials As starting materials, the α-Si3N4 powder (purity 493%, d50=0.5 μm, Beijing Unisplendor Founder High Technology Ceramics Co., Ltd. China.), sintering additives Al2O3 (Xilong Chemical Co., Ltd. China) and Y2O3 (Changsha Deli Rare Earth Chemical Co., Ltd. China) for the mixed AM-MBAM system were listed in Table 1. As for the carbon fibers (T700, 12 K, TohoTenax, Inc.), Methyltrichloresilan (MTS)–H2–Ar system was adopted to modify fibers and the deposition temperature was 1150 1C. 2.2. Preparation of suspensions The schematic forming process of gelcasting was described in Fig. 1. First, the mixture powder of 85 wt% α-Si3N4, 10 wt% Al2O3 and 5 wt% Y2O3 were added to premix solution of organic monomer by ball-milling till solids loading up to 45 vol%. Afterwards, the mixtures were milled for 12 h in a zirconia jar using zirconia ball-milling media to break down agglomerates and achieve good homogeneity. After degassing for 10–15 min in a rotary-vane pump under vacuum, the initiator and catalyst were applied to the slurry. Then it was cast into silicone rubber mold at room temperature. During the solidification of slurry, chopped SiC-coated carbon fibers were evenly sprinkled on the surface of slurry. After semi-solidifying for 10–15 min, the gelcasting and sprinkle process were repeated until three layers of fiber and four layers of slurry were arranged alternatively, and the fiber content ratio of three layers was 1:2:4. The consolidation of suspension formed a green body and all processes above were in the nitrogen gas environment. After consolidation, the green body were dried

firstly in absolute ethyl alcohol for 12 h to avoid cracking and non-uniform shrinkage due to rapid drying, and then moved into an oven at temperature of 50–80 1C for 5 h. Samples were embedded in BN powder in a graphite crucible and firstly operated at 550 1C in 0.05 MPa nitrogen atmosphere with a heating rate of 1 1C/min for 1.5 h to burn out the PMMA, followed by then sintered at 1700 1C in 0.05 MPa nitrogen atmosphere for 1.5 h at a heating and cooling rates of 5 1C/min. The micro-morphologies of the fiber and Si3N4 matrix were investigated using scanning electron microscopy (SEM, Nova NanoSEM 230). Phase analysis was conducted by X-ray diffraction (XRD, D/max 2550).The porosity and bulk density was determined by the Archimedes method using distilled water as medium. The complex permittivity (ε'; ε'') and complex permeability (μ'; μ'') of the samples with size of 22.86  10.16  5 mm3 were measured in the frequency range of 8.2–12.4 GHz at room temperature using a network analyzer (Agilent N5230A). The reflection loss was calculated by using the measured values of the complex permittivity (ε'; ε'') and the complex permeability (μ'; μ'') of pure Si3N4 ceramics and Cf–Si3N4 sandwich composites based on the transmission line theory [44–46]
Z
1

in
R ðdBÞ ¼ 20 log
ð1Þ
Z in þ 1    rffiffiffiffiffi μr 2π pffiffiffiffiffiffiffiffi μ r εr f d ð2Þ tanh j Z in ¼ c εr where c is the velocity of electromagnetic waves in free space, f is the microwave frequency, and d is the thickness of the absorbing material. 3. Results and discussion 3.1. Morphology and chemical composition of Cf–Si3N4 sandwich composites Table 2 lists the density and porosity of pure Si3N4 ceramic and Cf–Si3N4 sandwich composites. As can be seen from Table 2, the pores are mainly distributed inside of composites. The SEM micrographs of the Si3N4 matrix and SiC coated carbon fibers are shown in Fig. 2. It can clearly be seen that the microstructure of Si3N4 ceramic was formed by rod-like particles, which are evenly distributed and intercross with each other to form the main pores. And XRD pattern (see Fig. 3) can further prove that the rod-like particles are β-Si3N4. It also can be seen from Fig. 3 that the phase transformation from α-Si3N4 to β-Si3N4

Table 1 Raw materials for the AM-MBAM system. Function

Raw material

Manufacturer

Monomer Crosslinker Dispersant Initiator Catalyst Solvent

Acrylamide (AM) N,N'-methylenebisacrylamide (MBAM) Tetramethylammonium hydroxide solution Ammonium persulphate N,N,N',N'-tetramethylethylenediamine (TEMED) Deionized water

Sinopharm Chemical Reagent Co., Sinopharm Chemical Reagent Co., Sinopharm Chemical Reagent Co., Xilong Chemical Co., Ltd., China Sinopharm Chemical Reagent Co.,

Ltd., China Ltd., China Ltd., China Ltd., China

H. Luo et al. / Ceramics International 40 (2014) 8253–8259

Ceramic powder

Monomer

Deionized water

Dispersant

8255

Additive

Slurry

Vacuum defoaming Initiator

Catalyst

Casting Modified carbon fiber Semi-solidified

3 cycles

Drying

Binder burnout

Sintering Fig. 1. The gelcasting process flow chart. Table 2 The density and porosity of pure Si3N4 ceramic and Cf–Si3N4 sandwich composites. Samples

Si3N4 Cf–Si3N4

Density (g/cm3)

2.63 2.61

Porosity (%) Total

Open

Close

17.63 18.13

2.44 3.81

15.19 14.32

is completely accomplished. In addition, an obvious peak of intergranular glass phase Y–N-apatite (Y20N4Si12O48) was detected. This can be explained that additive Y2O3 reacts with the SiO2–Si3N4, and SiO2 comes from the surface of Si3N4 particle. Fig. 2(b) shows the surface morphology of SiC coated carbon fiber embedded in the Si3N4 matrix. It can be seen that the fiber surface is smooth after sintered under the protection of SiC coating with a thickness of about 1.7 μm. Both SEM (Fig. 2(b)) and energy dispersive spectroscope (EDS) analysis (Fig. 2(c) and (d)) results indicated that SiC-interphase could overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 1C. 3.2. Dielectric properties of Cf–Si3N4 sandwich composites Complex permittivity (ε ¼ ε'
jε'') is an important parameter to characterize the dielectric properties of absorbing materials. Fig. 4(a) shows the real and imaginary permittivity of pure Si3N4 ceramic and Cf–Si3N4 sandwich composites at X-band. It can be

seen that the dielectric constant of the pure Si3N4 is less dependent on the frequency. The mean real part and imaginary part of permittivity and dielectric loss ( tan δ ¼ ε''=ε') of pure Si3N4 ceramic were 7.7, 0.04 and 5.3  10
3, respectively. Owing to relatively low dielectric constant, impedance match characteristic of materials can also be achieved, which tends to cause the less reflection to electromagnetic wave from the surface of material and the most energy propagating in the material. However, both the real and imaginary permittivity of Cf–Si3N4 sandwich composites decrease gradually as frequency increases at X-band, varying from 12.3 and 5.1 to 7.9 and 1.2, respectively. This phenomenon is usually called frequency dispersion characteristics, which is beneficial to broaden the microwave absorption bandwidth [47]. On one hand, the chopped carbon fibers play a role of dipoles, which polarized by electromagnetic wave. With the increase of frequency, the orientation of these dipoles could not keep up with change of electric field gradually, resulting in the real part of permittivity (ε') of Cf–Si3N4 sandwich composites decrease gradually. On the other hand, the imaginary part (ε'') represents the dielectric loss in Cf–Si3N4 sandwich composites, which consists of two parts: Joule heat loss resulted from leakage current and friction heat loss resulted from the orientation of these dipoles. With the increase of frequency, long-range migration of the conduction electrons and orientation polarization effect weakened, which resulted in ε'' decreased. Fig. 4(b) shows the reflection loss of Si3N4 and Cf–Si3N4 sandwich composites in the frequency range of 8.2–12.4 GHz according to Eqs. (1) and (2). In which, both the real part of the complex permeability μ' and the imaginary part of permeability μ'' of pure Si3N4 ceramics and Cf–Si3N4 sandwich composites

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Intensity (a.u.)

Fig. 2. (a) SEM micrograph of Si3N4 matrix, (b) SEM micrograph of SiC coated carbon fiber, (c) EDS analysis at P1 and (d) EDS analysis at P2.

10

20

30

40

50

60

70

80

2θ /(°) Fig. 3. XRD pattern of Si3N4 matrix.

are approximately equal to 1 and 0, respectively. It can be found that the microwave absorption ability of the Cf–Si3N4 sandwich composites was significantly enhanced compared with pure Si3N4 ceramic. The reflection loss of the Cf–Si3N4 sandwich composites gradually decreases from
3.5 dB to
14.4 dB with the increase of frequency, while the pure Si3N4 ceramic keeps at
0.1 dB.

In order to further explore the frequency dispersion characteristics of Cf–Si3N4 sandwich composites, here we proposed an idea that the Cf–Si3N4 sandwich composites (Fig. 5(a)) in the alternating current (AC) electric field can be described by equivalent RC circuit model, where each layer of carbon fiber plays a role of one electrode in a plane-parallel capacitor, while each layer of Si3N4 ceramic plays the role of the dielectric (Fig. 5(b)). Considering the existence of leakage current, leakage resistances were applied in equivalent circuit (Fig. 5(c)). According to the circuit theory knowledge, the complex impedance Z of Cf–Si3N4 sandwich composites can be calculated as follows:  1 1 R1 jωC þ jωC 1 2 Z ¼ R2 þ 1 1 R1 þ jωC þ jωC 2 1 jωR1 ðC 1 þ C 2 Þ jωðC 1 þ C 2 Þ
ω2 R1 C1 C2 R1 ðC 1 þ C2 Þ2 ¼ R2 þ ðC 1 þ C 2 Þ2 þ ðωR1 C1 C2 Þ2 ¼ R2 þ

H. Luo et al. / Ceramics International 40 (2014) 8253–8259

0

12

-2

10

-4

8

-6

R /dB

ε',, εε''

14

8257

6

-8 -10

4

-12

2

-14

0 8.0

8.8

9.6

10.4

11.2

12.0

-16

12.8

F /GHz

8.0

8.8

9.6

10.4

11.2

12.0

12.8

F /GHz

Fig. 4. (a) The permittivity and (b) reflection loss curves of Si3N4 and Cf–Si3N4 sandwich composites over X-band.

Electrode

Carbon fiber layer

Dielectric

Fig. 5. (a) Cross section morphology, (b) structural schematic diagram and (c) equivalent circuit diagram of Cf–Si3N4 sandwich composites.

Experimental data Predicted curve

Experimental data Predicted curve

y=a+b/x R2=0.9418

ε'

ω *ε''

y=(b+c*x)/(1+a*x) R2=0.9060

ω2

ω2

Fig. 6. Experimental data and predicted curves of (a) ε' and (b) ωε'' versus ω2 .


j

ωR1 2 C1 C2 ðC 1 þ C 2 Þ ðC1 þ C 2 Þ2 þ ðωR1 C1 C2 Þ2

ð3Þ

where ω is the angular frequency. Thus, the real and imaginary part of complex impedance Z is Z' ¼ R2 þ Z'' ¼

R1 ðC 1 þ C2 Þ2 ðC 1 þ C 2 Þ2 þ ðωR1 C1 C2 Þ2

ωR1 2 C 1 C 2 ðC1 þ C2 Þ ðC 1 þ C2 Þ2 þ ðωR1 C 1 C 2 Þ2

ð4Þ

in addition, the real part of complex impedance represents resistance, which is inversely proportional to ωε''. While the

imaginary part of complex impedance represents the reactance of the capacitor, which is inversely proportional to ωε', that is P P ¼ s ωε'' 1 Q ¼ ð5Þ Z'' ¼ ωC ωε' where P and Q are constants, C is capacitance. Substituting Eq. (4) into Eq. (5), results in

Z' ¼

ε' ¼

QðC 1 þ C 2 Þ2 1 QR1 2 C1 2 C 2 2 þ R1 2 C 1 C 2 ðC 1 þ C2 Þ ω2 R1 2 C1 C2 ðC 1 þ C 2 Þ

8258

ωε'' ¼ P

H. Luo et al. / Ceramics International 40 (2014) 8253–8259

ðC 1 þ C2 Þ2 þ R1 2 C1 2 C 2 2 ω2 ðR1 þ R2 ÞðC 1 þ C 2 Þ2 þ R1 2 R2 C1 2 C 2 2 ω2

ð6Þ

Fig. 6 shows the experimental data and curves of ε' and ωε'' versus ω2 based on Eq. (6). The points in Fig. 6 indicate the experimental data, while the results predicted by equivalent circuit model are given as solid line. As can be seen from Fig. 6, for Cf–Si3N4 sandwich composites, both ε' and (ωε'') are inversely proportional to the frequency square ω2 , and the predicted results agree quite well with the measured data. Additionally, the experimental data shows oscillation phenomena in high frequency, which may results from charge and discharge process between C1 and C2 (Fig. 5) with the increase of frequency. 4. Conclusions In this study, Cf–Si3N4 sandwich composites were successfully prepared by gelcasting and excellent dielectric properties in the frequency range of 8.2–12.4 GHz were obtained. The asprepared CVD SiC coating on surface of carbon fiber could effectively overcome incompatibility between carbon fiber and Si3N4 matrix under high temperature environment. A strong frequency dependence of the real and imaginary parts of permittivity was observed, varying from 12.3 and 5.1 to 7.9 and 1.2, respectively. Microwave absorption ability of the Cf–Si3N4 sandwich composites is significantly enhanced compared with pure Si3N4 ceramic. The reflection loss of the Cf–Si3N4 sandwich composites gradually decreases from
3.5 dB to
14.4 dB with the increase of frequency. Particularly, the relationship between permittivity of Cf–Si3N4 sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. The predicted results reveal that both ε' and (ωε'') are inversely proportional to the frequency square ω2 , and agree quite well with the measured data. Acknowledgments This work was supported by the State Key Development Program for Basic Research of China (Grant no. 2011CB605804) and the Hunan Provincial Innovation Foundation for Post graduate. The authors also gratefully thank Mr. Lianwen Deng for his help in technical and language checking. References [1] F. Qin, C. Brosseau, A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles, J. Appl. Phys. 111 (2012) 061301. [2] M.A. Alves, L.C. Folgueras, M.C. Rezende, Reduction of the radar cross section of a wind turbine using a microwave absorbing material, Microwave & Optoelectronics Conference (IMOC), 2011, pp. 551–555. [3] Chao Wang, Xijiang Han, Ping Xu, Xiaolin Zhang, Yunchen Du, Surong Hu, Jingyu Wang, Xiaohong Wang, The electromagnetic property of chemically reduced graphene oxide and its application as microwave absorbing material, Appl. Phys. Lett. 98 (2011) 072906.

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