Influence of Thermal Oxidation on Complex Permittivity and Microwave Absorbing Potential of KD-I SiC Fiber Fabrics

Transcription

1 Influence of Thermal Oxidation on Complex Permittivity and Microwave Absorbing Potential of KD-I SiC Fiber Fabrics Donghai Ding 1, Fa Luo 2, Yimin Shi 2, Jin Chen 1 1 College of Materials and Mineral Resources, Xi an University of Architecture and Technology, Xi'an, Shaanxi CHINA 2 State Key Laboratory of Solidification Processing, Northwester Polytechnical University, Xi'an, Shaanxi CHINA Correspondence to: Donghai Ding @qq.com ABSTRACT The complex permittivity and microwave absorbing potential of KD-I SiC fiber fabrics at X band as functions of thermal oxidation time and temperature were investigated. The values of ε (real part) and ε (imaginary part) of the complex permittivity of the fabrics decreased with increasing thermal oxidation time and temperature. It was found that the microwave absorbing potential of fabrics can be optimized by thermal oxidation. The best reflection loss was obtained for the sample oxidized for 30 min at 500. Its lowest reflection loss is about db at 10.3 GHz, and its absorbing frequency band is about 4 GHz. The oxidized KD-I SiC fiber fabric is a good microwave absorbing material with low density, high temperature resistance, and flexibility. Keywords: Complex permittivity; SiC fiber fabric; Free carbon INTRODUCTION Increasing attention has been focused on polycarbosilane derived SiC fibers due to its excellent properties such as thermal resistance, chemical stabilization, high tensile strength, and low density [1]. Because it can be woven into fabric with different structures including 2D, 2.5D and 3D, SiC fibers are usually used as reinforcement for high temperature ceramic matrix composites [2,3]. It has been recognized that the micro-structure and mechanical properties of SiC fibers have been investigated well in the past several decades [4,5]. Nowadays, the electric and microwave absorbing properties of SiC fibers are attracting more and more attention [6-12]. Shimoo et al. researched the effect of heat-treatment in hydrogen on C/Si ratio and specific resistivity of Hi-Nicalon SiC fibers [6]. Tan et. al. studied the electromagnetic wave absorption potential of Tyranno SiC fiber woven fabrics in the GHz frequency range [7]. As microwave absorbing materials, the SiC fiber fabrics have some advantages such as low density, adjustable electrical property, and resistance to high temperature. As proposed in our previous investigation [12], the rich carbon outer layer plays a key role in the complex permittivity of KD-I SiC fiber fabrics and the imaginary part is too high to absorb microwave effectively. Haitao Liu studied the effects of the oxidation parameters on the flexural strength of KD-I fibers and found that the strength retention ratio of the fibers was 89.6% after 60 min oxidation at 600 C [13]. Therefore, the thermal oxidation could be used to improve the microwave absorbing properties of KD-I SiC fiber fabrics without obvious degradation of strength. In this paper, the influence of thermal oxidation on complex permittivity and microwave absorbing potential of KD-I fabrics was studied within the frequency range of GHz (X band) at room temperature. Due to the intimate relation between complex permittivity and DC electrical conductivity, we studied the effect of thermal oxidation time on DC electrical conductivity. In addition, the chemical compositions were characterized by EDS in order to gain a better understanding from the microscopic aspect. Journal of Engineered Fibers and Fabrics 99

2 EXPERIMENTAL Fabrics Preparation and Thermal Oxidation The KD-I fibers were provided by National University of Defense Technology (China). Table I shows the parameters of SiC fiber. The fabrics were braided by Nanjing Glass Fiber Institute (China). The fabrics structure is the 2.5D shallow bending joint shown by Figure 1. The fiber volume fraction of the fabrics is 40%. The density of fabrics is about 1.0 g/cm -3 calculated on the basis of fiber volume fraction and density of SiC fiber. Fiber type TABLE I. Parameters of KD-I SiC fiber [8]. Diameter (μm) Density (g/cm3) Tension strength (MPa) KD-I 14~ ~2200 Because of the low complex permittivity of paraffin wax, melted paraffin was cast into the fabrics to obtain fabric/wax composites so that the fiber bundles in fabric were fixed to facilitate the measurement of permittivity. The measured samples were cut into mm mm 2.0 mm. The wax is insulator, and the ε and ε are 2.26 and 0 at X band. So the fabric mostly determines the complex permittivity of the fabric/wax composites. Given the convenience of discussions later, the influence of wax on the complex permittivity was ignored. Based on the measured complex permittivity and permeability, the microwave absorbing properties can be calculated by the following equations [14]: RL Zin Z0 (1) = 20log Zin + Z0 where RL denotes the reflection loss in db unit. Z 0 is the characteristic impedance of free space. Z in is the input characteristic impedance at the absorber/free space interface, which can be expressed as: Z Z µ r 2πft tanh j µ rε ε r c in = 0 r (2) FIGURE 1. Schematic showing of KD-i SiC fibers fabric structure. The fabrics were oxidized in air within the temperature range of C in a closed box-type furnace. Analysis and Characterization Techniques The ε and ε of complex permittivity are macroscopical parameters that correlate with polarization and dielectric loss of materials, respectively; the real part (μ ) of complex permeability represents the amount of energy stored in the material from ac magnetic field, while the imaginary part (μ ) is the energy loss to the magnetic field. The complex permittivity of fabrics was measured by the method of rectangle waveguide using vector network analyzer (E8362B). where c is the velocity of light, t is the thickness of an absorber. In this paper, all t values are 3 mm. ε r and μ r are the measured relative complex permittivity and permeability, respectively. The DC electric conductivity (σ) of SiC filaments was calculated from the equation: L σ = 1 / ρ = (3) RS where σ is special conductivity, R is the resistance, S is the cross section area of filament, and L is the length of filament. The electrical resistance of SiC filaments at room temperature was measured by two probes direct current method. The filaments were tin-lead bonded on alumina substrate. The length of filaments was 10 mm. The average diameter of KD-I filaments was both considered as 14 μm. Journal of Engineered Fibers and Fabrics 100

3 RESULTS AND DISCUSSION Microstructure Characterization Figure 2 shows the chemical element intensity profile of the KD-I fibers as received and oxidized KD-I fiber diameter was characterized by EDS. It can be observed that there is an excess carbon outer layer on as received KD-I fibers while the carbon element density along the oxidized fiber diameter is approximately uniform. It can be concluded that the carbon outer layer was vaporized after thermal oxidation. Table II displays the chemical composition of KD-I SiC fibers dependence on thermal oxidation temperature for 30 min. Because of the lower oxidation temperature, the oxidation of SiC cannot occur. The decreasing of C/Si atomic ratio after oxidation should be due to the oxidation of free carbon. After oxidation at 500 C for 30 min, the C/Si decreased to 1.09 %. In addition, the decreased weight in the process of thermal oxidation is shown in Table III which should be attributed to the oxidation of free carbon. Based on the above results, it was deduced that the free carbon is obviously oxidized from about 400. Besides, the weight loss also improved the oxidation of free carbon. TABLE II. The chemical components of KD-1 SiC fiber after thermal oxidation at different temperature. Chemical elements (at %) C Si O C/Si as received Electrical Conductivity In the present study, the electrical conductivity of KD-I SiC fiber was measured, and the results were shown in Table IV. The obvious electrical conductivity decreased from 0.05 to 10-6 S/cm after thermal oxidation at 500 C. Based on discussions of previous investigation [12], a conceptual microscopic explanation for the conductivity drop of KD-I SiC fiber is the idea that the conductive network formed by free carbon has been damaged by vaporization of free carbon through thermal oxidation. In the present investigation, the electrical conductivity of SiC fiber decreased sharply when the thermal oxidation time is in the range of 0-60 min shown in Table IV, which is agreement with the data of weight loss of SiC fibers oxidation at 500 C for different time shown in Table III. TABLE III. Weight loss of SiC fiber oxidation at 500 C for different time. Time/minute Weight loss/% TABLE IV. Conductivity of SiC filament after oxidation at 500 for different time. Time/minute Conductivity/ (S/cm) FIGURE 2. Chemical element intensity profile along fiber diameter. (a) as received fiber; (b) after oxidized fiber. Complex Permittivity As our next step, we studied the influence of thermal oxidation on the complex permittivity of fabrics. Figure 3 shows the variation of complex permittivity of KD-I SiC fiber fabrics after thermal oxidation for different time at 500 C at X band. Journal of Engineered Fibers and Fabrics 101

4 Overall, the ε and ε decreased with increasing of the time. From the value of ε =9.8 at 10 GHz for as received sample, it drops to 7.5 for the sample after oxidation for 30 min and followed by three obvious decreases for the samples after oxidation for 60, 90 and 120 min. As to the ε, the value decreased significantly from 17.3 to 3.4 at 10 GHz after oxidation for 30 min, but the decreases are much more slight for the samples after oxidation for 60, 90 and 120 min. In addition, the frequency response effect of ε and ε can be observed obviously only for as received sample. In addition, it can be observed from Figure 3 and Figure 4 that the decreasing of the values of ε is more obvious than that of ε. FIGURE 4. Complex permittivity of KD-1 SiC fiber fabrics after thermal oxidation at different temperature for 30 min. (a) real part; (b) imaginary part. FIGURE 3. Complex permittivity of KD-I SiC fiber fabrics after thermal oxidation at 500 for different time. (a) real part; (b) imaginary part. Figure 4 shows the complex permittivity of KD-I SiC fiber fabrics after thermal oxidation at different temperature for 30 min. The values of the complex permittivity decreased with the rising of temperature. It is worth mentioning that the ε begins to decrease obviously at 435 C while the ε decreased obviously at 400 C. It can be concluded that the decreasing rate of ε and ε increase as the increasing temperature in the range of C. At X band, the main mechanisms for the polarization in a dielectric material are relaxation polarization and interfacial polarization [15, 16]. The weakly bond electron in dielectric materials can induce the formation of electron relaxation polarization, which is an energy consumption process. This polarization form can increase the ε and ε of complex permittivity, which can be expressed as follows: ε s ε ε I ' = ε + 2 (4) 1+ ( ωτ ) ε σ ε σ ε I '' = ωτ + 2 (5) 1+ ( ωτ ) ε ω 0 Journal of Engineered Fibers and Fabrics 102

5 where ε I and ε I are the increased parts of ε and ε induced by electron relaxation polarization, σ is conductivity, ε 0 is the dielectric constant in vacuum, ε s is the static permittivity, ε is the relative dielectric permittivity at the high frequency limit, ω is the angular frequency and τ is the relaxation polarization time [15]. The excess carbon should play an important role in the higher complex permittivity of KD-I fabrics at X band. The free carbon with free electron can induce the electron relaxation polarization. Therefore, the values of ε and ε of as-received sample are both high. In the process of thermal oxidation, the free carbon should be gradually vaporized, which was indicated by weight loss and decreased C/Si ratio. Consequently, the relaxation polarization induced by the free electron weakens gradually. The conductivity decreasing approved this on the other hand. As a result, the complex permittivity decreased after thermal oxidation. Microwave Absorbing Potential FIGURE 5. Reflection loss of KD-I SiC fiber fabrics after thermal oxidation for different time at 500 C. To evaluate the microwave absorbing potential, the reflection loss (RL) in db unit can be calculated. If the RL values of an absorber are less than -10 db (90% absorption), then we can say that the absorber works very well. And, the microwave absorbing frequency band (AFB) is defined as the frequency range, in which the RL values are lower than -10 db. Figure 5 shows the calculated RL values of KD-I fabrics after thermal oxidation for different time at 500. It was observed that the microwave absorbing potential of KD-I fabrics can be improved by thermal oxidation. The best result is obtained for sample oxidized for 30 min. It shows the lowest RL value of about db at 10.3 GHz, and its AFB is 4 GHz. It can be seen from Figure 5 that the microwave absorbing potential of as received, 90 min and 120 min samples are all poor. The reason is that the complex permittivity for as received sample are too much higher, which cannot match the characteristic impedance of free space; and the complex permittivity for 90 min and 120 min samples are too much lower to dissipate electromagnetic energy. CONCLUSION The influence of thermal oxidation on complex permittivity and microwave absorbing potential of KD-I SiC fibers fabric within the frequency range of GHz were investigated. The DC electrical conductivity of KD-I SiC fiber decreased in the process of thermal oxidation at 500 C. Both the values of ε and ε of the fabrics decreased with increasing thermal oxidation time and temperature. The damage of conductive network due to vaporization of free carbon through thermal oxidation should mainly contribute to the decreasing of complex permittivity of KD-I SiC fiber fabrics. The best reflection loss was obtained for sample oxidized for 30 min at 500 C. Its lowest reflection loss is about db at about 10.3 GHz, and its absorbing frequency band is 4 GHz. Thermal oxidation in air within the temperature range of C could be one useful method to improve the microwave absorbing potential of KD-I fabrics. It was revealed that KD-I SiC fiber fabric has potential as light microwave absorbing material working at high temperature. In order to work at higher temperature in air, KD-I fabrics after proper thermal oxidation need anti-oxidation coatings to avoid deterioration of their microwave absorbing potential. In our next step, the anti-oxidation coatings will be fabricated on fabrics oxidized for 30 min at 500 C, and the high temperature microwave absorbing property will be evaluated. ACKNOWLEDGMENTS This work was supported by the Scientific Research Program Funded by Shaanxi Provincial Education Department (Grant No. 2013JK0922 and Grant No. 2013JK0921), by the fund of the State Key Laboratory of Solidification Processing in NWPUC (SKLSP201305)and by the basic research fund of Xi an University of Architecture and Technology (JC1310,RC1210). Journal of Engineered Fibers and Fabrics 103

6 REFERENCES [1] Yajima S; Hayashi J; Omori M; Okamura K; Development of a silicon carbide fibre with high tensile strength, Nature, 261;1976; [2] Naslain R; Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview, Compos Sci Technol, 64; 2004; [3] Naslain R; SiC-matrix composites: nonbrittle ceramics for thermostructural application, Int J Appl Ceram Technol 2; 2005; [4] Ishikawa T; Advances in inorganic fibers, Adv Polym Sci, 178; 2005; [5] Bunsell A R; Plant A; A review of the development of three generations of small diameter silicon carbide fibres, J Mater Sci, 41; 2006; [6] Shimoo T; Okamura K; Mutoh W; Carbon elimination by heat-treatment in hydrogen and its effect on thermal stability of polycarbosilane-derived silicon carbide fibers, J Mater Sci, 38; 2008; [7] Tan E; Kagawa Y; Dericioglu A F; Electromagnetic wave absorption potential of SiC-based ceramic woven fabrics in the GHz range, J Mater Sci, 44; 2009; [8] Liu HT; Cheng HF; Wang J; Tang GP; Effects of the fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiC fiber reinforced SiC matrix composites, Mater Sci Eng A, 525; 2009; [9] Zhu YZ; Zhu SZ; Huang ZR; Dong SM; Jiang DL; Properties and microstructure of KD-I SiC/SiC composites by combined process of CVI/RB/PIP, Mater Sci Eng A, 477; 2008; [10] Yu XM; Zhou WC; Luo F; Zheng WJ; Zhu DM; Effect of fabrication atmosphere on dielectric properties of SiC/SiC composites, J Alloys Compd, 479; 2009; L1-L3. [11] Liu HT; Cheng HF; Wang J; Tang GP; Dielectric properties of the SiC fiber-reinforced SiC matrix composites with the CVD SiC interphases, J Alloys Compd, 49; 2010; [12] Ding DH; Zhou WC; Zhang B; Luo F; Zhu DM; Complex permittivity and microwave absorbing properties of SiC fiber woven fabrics, J Mater Sci, 46; 2011; [13] Liu HT. Designs, preparations and properties of the SiC/SiC absorbing materials with sandwich structures. National University of Defense Technology, [14] Meshram MR; Agrawal NK; Sinha B; Characterization of M-type barium hexagonal ferrite-based wide band microwave absorber, J Magn Mater, 271; 2004; [15] Cao MS; Song WL; Hou ZL; Yuan J; The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and microwave-absorption of short carbon fiber/silica composites, Carbon, 48; 2010; [16] Qing YC; Zhou WC; Luo F; Zhu DM; Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber, Carbon, 48; 2010; AUTHORS ADDRESSES Donghai Ding, PhD Fa Luo Yimin Shi Jin Chen College of Materials and Mineral Resources Xi an University of Architecture and Technology No. 13 Yanta Road Xi'an, Shaanxi CHINA Journal of Engineered Fibers and Fabrics 104