The dielectric and microwave absorption properties of polymer-derived SiCN ceramics

Accepted Manuscript Title: The dielectric and microwave absorption properties of polymer-derived SiCN ceramics Authors: Xue Guo, Yurun Feng, Xiao Lin, Yu Liu, Hongyu Gong, Yujun Zhang PII: DOI: Reference:

S0955-2219(17)30709-4 https://doi.org/10.1016/j.jeurceramsoc.2017.10.031 JECS 11518

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Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

12-9-2017 9-10-2017 16-10-2017

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The dielectric and microwave absorption properties of polymer-derived SiCN ceramics Xue Guo1,2, Yurun Feng1,2, Xiao Lin1,2, Yu Liu1,2, Hongyu Gong1,2*, Yujun Zhang1, 2* 1

Key Laboratory for liquid-solid Structural Evolution & Processing of Materials of Ministry of

Education, Shandong University, Jinan 250061, P. R. China 2

Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong

University, Jinan 250061, P. R. China. Abstract. The polymer-derived SiCN ceramics were synthesized at different annealing temperature (900~1400 C). The XRD, SEM, FT-IR, Raman and XPS were used to analyze the phase composition and microstructure. The result indicated that the crystallization degree and content of free carbon gradually improved with the increase of annealing temperature. The resistivity, dielectric and microwave absorption properties of the samples were studied at 2~18 GHz. The resistivity decreased gradually as the annealing temperature rose. The dielectric constant of sample decreased with the increase of frequency in 1~5 MHz. The existence of free carbon could improve the dielectric properties of polymer-derived SiCN ceramics at high frequency. The reflectance of the sample synthesized at 1100 C was below -10 dB (>90% absorption) in a wide frequency range of 6～16 GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16 GHz. Keywords: polymer-derived; SiCN ceramics; dielectric; Microwave absorption. 1. Introduction In recent years, polymer-derived method has attracted much attention in the preparation of ceramics [1-4]. Polymer-derived ceramics have many excellent physical and chemical properties, such as

* Corresponding author. Tel / Fax: +86-531-88399760; E-mail addresses: [email protected] [email protected] 1

oxidation resistance, creep resistance and thermal stability, etc [5]. Therefore, polymer-derived ceramics are widely used in the coating, fiber, thin film and ceramic composite materials. Among these, polymer-derived SiCN ceramics have been researched extensively as microwave absorption materials due to their unique chemical structure and excellent electrical properties [6]. Li et al. [7] synthesized PDCs-SiCN ceramics at different annealing temperature (1350~1700 C) and the result indicated that the sample possessed general wave absorption property within narrow (9~11 GHz) effective absorbing microwave range. However, the wide band absorption has served as the development mainstream direction of current absorbing materials. Meanwhile, the dielectric and microwave absorption properties of polymer-derived SiCN ceramics at 900~1400 C have not been investigated systematically yet. In this paper, the polymer-derived SiCN ceramics were synthesized at different annealing temperature (900~1400 C). The phase composition and microstructure were analyzed by XRD, SEM, FT-IR, Raman and XPS. In addition, the dielectric and microwave absorption properties of the samples were studied at 2~18 GHz. 2. Experimental The polysilazane (PSZ) (HTT1800, Haiyi Technology and Trading Co. Ltd) and dicumyl peroxide (DCP, 99%, Aladdin) were mixed at 70°C and cross-linked at 600 °C under N2 atmosphere. The obtained sample was crushed and ball-milled for 1 h. Then the powders were pressed at 200 MPa to obtain the green bodies. Finally, the samples were annealed at 900~1400 C under N2 atmosphere. The phase composition and microstructure were analyzed by X-ray diffraction (XRD, EVO-18, CARL ZEISS SMT Ltd), Fourier transform infrared spectroscopy (FT-IR, TENSOR 37), Raman spectroscopy (Lab Ram-1 B) and X-ray photoelectron spectroscopy (XPS, PHI1600EXCA). The electrical resistivity was measured by a Four-probe resistivity/resistance tester (KDY-1). The low frequency (20 Hz~5 MHz) dielectric properties of the sample were tested by LCR meter (TH2826). And 2

the high frequency (2 GHz~18 GHz) electromagnetic properties were analyzed by coaxial line reflection using vector network analyzer (VNA, 5244A). 3. Results and discussion 3.1. Characterization of samples 3.1.1 XRD analysis Fig. 1 shows the XRD patterns of PDCs-SiCN ceramics at different annealing temperature. The diffraction peak around 20 belonged to amorphous carbon. With the increase of annealing temperature, the peak strength and shape of the amorphous carbon became strong and sharp gradually. It demonstrated that the content and crystallinity degree of free carbon increased with the improvement of annealing temperature.

Fig. 1 XRD patterns of PDCs-SiCN ceramics at different annealing temperature 3.1.2 FT-IR analysis

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Fig. 2 FTIR spectra of PDCs-SiCN precursor at different annealing temperature Fig. 2 showed the FT-IR spectra of PDCs-SiCN precursor at different annealing temperature. The wide diffraction peak at 800~1200 cm-1 was attributed to the superposition of Si-C and Si-N bonds, indicating that the main structure of PDCs-SiCN ceramics were Si-C-N framework. In addition, the intensity of the diffraction peak weakened gradually with the increase of annealing temperature, illustrating the growth of inorganic degree. 3.1.3 SEM analysis The SEM images of PDCs-SiCN ceramics at different annealing temperature were shown in Fig. 3. The samples were composed of large size particles with no obvious crystalline structure. It contained large number of pores, which might be caused by the generation of CO, NH3 and etc [8]. Fig. 4 showed the open porosities and volume density of PDCs-SiCN ceramics at different annealing temperature. With the increase of annealing temperature, the volume density and open porosity of samples increased gradually. The lighter elements overflowing from the gas of small molecules resulted in an increase in the density of the sample.

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Fig. 3 SEM images of PDCs-SiCN ceramics at different annealing temperature

Fig.4 The open porosities and volume density of PDCs-SiCN ceramics at different annealing temperature 5

3.1.4 Raman analysis

Fig. 5 The Raman spectra of PDCs-SiCN ceramics at different annealing temperature

The Raman spectrum analysis can effectively detect the free carbon in the samples and its heat treatment state [9]. In general, the G peak represents the stretching vibration of graphite lattice plane and the D peak represents the graphite lattice defects and the change of the structure disorder. The Raman spectra of SiCN precursor at different annealing temperature was shown in Fig. 5. The peaks at 1575 cm-1 and 1330 cm-1 represented the carbon sp2 hybrid (G) and carbon sp3 hybrid (D), respectively, illustrating the existence of free carbons.

The Raman spectra were studied by Gaussian curve fitting to analyze the changes of carbon structure. The Raman spectra and parameters of SiCN precursor at different annealing temperature were shown in Fig. 6 and Table 1. With the growth of annealing temperature from 900 C to 1400 C, the overlap degree and intensity of D and G peaks gradually decreased, suggesting the increase of crystallization degree of free carbon. As shown in Table 1, the position of G peak (W D) moved to the high wave number (blue shift) with the increase of annealing temperature，which might be caused by the change and migration of Si-C and Si-N bonds. Meanwhile, the increment of carbon crystallisation degree resulted in the decrease of full width half maximum (FWHH) of D and G peaks. Generally, ID/IG represents the order degree of free carbon in the sample. The values of I D/IG increased when the annealing temperature 6

rose, which illustrated the order degree of free carbon decreased gradually.

Fig. 6 The Raman spectra of SiCN precursor at different annealing temperature Table 1 The Raman parameters of SiCN precursor at different annealing temperature Temperature

WD

FWHHD

WG

FWHHG

(°C)

（cm ）

（cm ）

（cm ）

（cm-1）

900

1326.93

110.08

1576.93

133.79

1.23

1000

1330.07

111.63

1574.66

106.22

1.27

1100

1333.21

111.48

1569.63

113.70

1.28

1200

1328.58

112.22

1574.67

109.87

1.42

1300

1333.66

99.19

1578.09

113.97

1.38

1400

1330.53

107.22

1582.48

77.42

2.30

-1

-1

-1

7

ID/IG

3.1.6 XPS analysis

Fig. 7 XPS survey spectrum of polymer-derived SiCN ceramics at 1100°C

Fig. 8 XPS spectrum of Si 2p, C 1s and N 1s of polymer-derived SiCN ceramics at 1100 °C The survey XPS spectrum in Fig. 7 indicated the main elements in polymer-derived SiCN ceramics were Si, C, N and O. The O element might be introduced during the preparation and measurement process. In order to investigate the chemical bonding type of the sample, the XPS spectra was fitted by

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Gaussian-Lorentzian function. As shown in Fig. 8, the main chemical bonds including Si-C, Si-N and C-C [10-12], illustrated the generation of Si-C-N network structure. Meanwhile, the C 1s spectrum illustrated that free carbon was generated during the annealing process. 3.2. Dielectric properties of polymer-derived SiCN ceramics 3.2.1 Resistivity

Fig. 9 Resistivity of polymer-derived SiCN ceramics at different annealing temperature

Fig. 9 shows the resistivity of polymer-derived SiCN ceramics at different annealing temperature. The resistivity of the sample decreased gradually as the annealing temperature rose. And the resistivity was above 10^5 Ω·m at 900~1200 C, indicating seldom conductive phases in the sample. When the annealing temperature was more than 1300 C, the resistivity of polymer-derived SiCN ceramics decreased sharply. It was mainly caused by the generation of free carbon during the annealing process.

3.2.2 Dielectric properties

The real part of dielectric constant (′) represents as relatively stable constant under the static electric field and it gradually reduces with the increase of frequency under the alternating electric field. Fig. 10 shows the real part of dielectric constant of polymer-derived SiCN ceramics under different annealing

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temperature at 1~5 MHz. The decreasing of dielectric constant was related to the polarization mechanism of dipole in the alternating electric field. The dipole could generate displacement polarization or steering polarization under the alternating electric field. When the frequency increased instantly, the dipole polarization gradually lagged behind the changes. Especially for the steering polarization (long relaxation time), thereby reducing the dielectric constant of polymer-derived SiCN ceramics.

Fig. 10 The real part of dielectric constant of polymer-derived SiCN ceramics at different annealing temperature

The complex dielectric permittivity (′ and ″) can be calculated by the Debye relaxation equation [13]. The equation was presented as follows:

 '  

 ''

s   2 1   2 T 

s    (T )  (T )    p ' ' c ' ' 2 2  0 1    T 

(1)

(2)

Where τ (T) is the polarization relaxation time, σ (T) is the electrical conductivity; ω is the angular frequency, εs is the static electric constant, ε0 is the dielectric constant in vacuum (8.854×10-12 F m-1), εp″ 10

and εc″ represent polarization loss and conduction loss of the sample, respectively. The εp″ and εc″ of polymer-derived SiCN ceramics at different annealing temperature at 1-5 MHz are shown in Fig. 11. The value of polarization loss (0.2~3.8) was more than the conduction loss (0~0.4), which might be related to the high resistivity (low leakage) of polymer-derived SiCN ceramics. Therefore, the main loss mechanism of polymer-derived SiCN ceramics was polarization loss. In addition, the major reason for the polarization loss of the sample was the existence of free carbon.

Fig. 11 The polarization loss and conduction loss of polymer-derived SiCN ceramics at different annealing temperature

The dielectric performance and potential absorption properties of the sample can be expressed by complex dielectric constant (εr=ε′-jε″). The real part of dielectric constant (ε′) represents the polarizing capability. The imaginary part of dielectric constant (ε″) refers to electromagnetic wave loss ability. The dielectric loss angle tangent (tanδ=ε″/ε′) is the transformation ability of electromagnetic waves energy for the sample. Fig. 12 shows the dielectric properties of polymer-derived SiCN ceramics at different annealing temperature. The value of ε′ fluctuated in 3.5~4 at 2~10 GHz and then emerged several vibration peaks at 10~18 GHz. It might be caused by the dielectric relaxation phenomenon of the sample in the electric field. The orientation polarization and displacement polarization of the free carbon lagged

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behind the relaxation time, and even part of dipole stopped inversion in high frequency. As shown in Fig. 12(b), the vibration peaks of dielectric loss angle tangent in 10~18 GHz were created by the withdrawal and transformation of polarization mechanism [14]. The free carbon (generated during the annealing process) led to the orientation polarization transformation and displacement polarization in the high frequency. When the annealing temperature was 1100 C, the maximum values of ε′ and tan δ in 15~16 GHz were 4.5 and 0.6, respectively.

Fig. 12 The dielectric properties of polymer-derived SiCN ceramics at different annealing temperature: (a) the real part of dielectric constant; (b) dielectric loss angle tangent 3.3. Microwave absorption properties of polymer-derived SiCN ceramics

Fig. 13 The reflectivity of polymer-derived SiCN ceramics at different annealing temperature (thickness 8 mm, powers/paraffin weight ratio 1:2)

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Fig. 13 shows the reflectivity of polymer-derived SiCN ceramics at different annealing temperature. The reflectivity of polymer-derived SiCN ceramics was almost below -8 dB (>84% absorption) in 4~16 GHz. Especially for the sample at 1100 C, the reflectance was below -10 dB (>90% absorption) in 6~16 GHz. Table 2 Comparison on the properties of polymer-derived SiCN ceramics with other composites from the literature Dielectric Samples

Microstructure

Resistivity

& Composition

(Ω·m)

constant

Reflectivity

Reflectivity

Tem

 -10 dB

in X-band （dB）

(C)

(ε′ and tan δ) Crystallize, Ref [7]

SiC, Si3N4,

10^7

12.8 and 0.46

9~11 GHz

-18

1700

1.210^5

4.5 and 0.6

6~16 GHz

-12.5

1100

free carbon Amorphous, This

Si-C-N

work

framework, free carbon

Table 2 shows the comparison on the properties of polymer-derived SiCN ceramics with other composites from the literature. The reflectivity of the samples was similar in X-band range, but the synthesized temperature in this study was much lower than Ref [7]. In addition, the reflectivity of the sample synthesized at 1100 C was below -10 dB (>90% absorption) in a wide frequency range of 6～16 GHz. Therefore, the polymer-derived SiCN ceramics synthesized in this study could greatly reduce the production cost and possess potential prospect in wave adsorbing materials. 4. Conclusions The polymer-derived SiCN ceramics were synthesized at different annealing temperature (900~1400 C). The crystallization degree and content of free carbon gradually improved with the increase of annealing temperature. The resistivity of the sample decreased gradually as the annealing temperature 13

rose. The dielectric constant decreased with the increase of frequency in 1~5 MHz. The existence of free carbon could improve the dielectric properties of polymer-derived SiCN ceramics in the high frequency. The reflectance of the sample synthesized at 1100 C was below -10 dB (>90% absorption) in a wide frequency range of 6～16 GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16 GHz. Therefore, the polymer-derived SiCN ceramics showed excellent potential absorbing performance. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No.51572154) and Shandong Nature Science Foundation of China (2015ZRE27348). References [1] D.D. Guan, X.B He, R. Zhang, X.H. Qu, Microstructure and tensile properties of in situ polymer-derived particles reinforced steel matrix composites produced by powder metallurgy method, Materials Science & Engineering A 705 (2017) 231-238. [2] N.M. Chelliaha, H. Singh, R. Rajc, M.K. Surappa, Processing, microstructural evolution and strength properties of in-situ magnesium matrix composites containing nano-sized polymer derived SiCNO particles, Materials Science & Engineering A 685 (2017) 429-438. [3] P. Colombo, G. Mera, R. Riedel, G.D. Sorarù, Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics, Journal of the American Ceramic Society 93 (7) (2010)1805-1837. [4] N. Soltani, U. Simon, A. Bahrami, X.F. Wang, S. Selve, J.D. Eppinge, M.I. Pech-Canul, M.F. Bekheet, A.r Gurlo, Macroporous polymer-derived SiO2/SiOC monoliths freeze-cast from polysiloxane and amorphous silica derived from rice husk, Journal of the European Ceramic Society 37 (2017) 4809-4820. [5] L. Bharadwaj, Y. Fan, L.G. Zhang, D.P. Jiang, L.N. An, Oxidation behavior of a fully dense 14

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