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Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate
Ceramics International xxx (xxxx) xxx–xxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate ⁎
Liangliang Wua, Feng Xianga, , Wenlong Liua, Rong Maa, Hong Wangb, a b
⁎
School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Hot pressing B. Composites C. Dielectric properties C. Thermal conductivity
High thermal conductivity and low dielectric constant are the more and more important properties for highfrequency substrate materials to enhance their heat radiation and reduce signal delay. In this work, a series of BN-SiO2 composite ceramics for high frequency application were successfully synthesized by hot-pressing sintering method. And their structures, thermal and dielectric properties were systematically studied. According to the results, the excellent thermal conductivity with low dielectric constant and low dielectric loss has been obtained in the BN-SiO2 ceramic. Compared to the pure SiO2, the sample with 50 wt% BN addition sintered at 1650 ℃ exhibited excellent physical properties, including a high thermal conductivity of 6.75 W/m K which is almost five times higher than that of pure SiO2 and a low dielectric constant of 3.73. The achieved high thermal conductivity and appropriate dielectric property of the BN-SiO2 composite ceramic make it a promising candidate for high-frequency substrate application.
1. Introduction With the rapid development of high speed circuits and power electronic devices used in high frequency, how to solve the issues of heat radiation and single delay in the substrate is becoming a bottleneck in practical applications. Therefore, the demand for high-frequency substrate materials with high thermal conductivity and low dielectric constant increases quickly in recent years [1–4]. However, many traditional ceramic substrate materials cannot meet these requirements. For example, Al2O3 and AlN are widely used as substrates because of their high thermal conductivity. However, the large signal propagation delay limits their applications at high frequency, resulting from their relatively large dielectric constants [5]. Besides, many ceramics with dielectric constant less than 6 have been reported, such as SiO2-AlN-BN, LZS-Al2O3, Si-N-Of/BN, CaF-AlF3-SiO2 and CaO-B2O3SiO2 [6–10], but they were lack of an adequate thermal conductivity when used for the heat conduction in the high frequency. So, it is urgent to find or synthesize some materials with both high thermal conductivity and low dielectric constant. BN is a potential substrate material with high thermal conductivity and low dielectric constant [11]. However, its high cost and tough process technology immensely hinder the wide application of BN substrate [12]. Meanwhile, although the thermal conductivity of SiO2 is insufficient to meet the requirement of heat radiation, SiO2 powder is a
⁎
kind of useful filler in the composite ceramics for its high performance such as low dielectric constant, low dielectric loss, high mechanical strength and low cost [13]. Therefore, if SiO2 and BN could be combined together, the processing difficulty of BN would be immensely reduced, and the dielectric constant of BN would be further decreased due to the smaller dielectric constant of SiO2. Nevertheless, it is difficult to calcine and fabricate dense BN-SiO2 composite ceramics by conventional pressureless sintering because BN powders are easily agglomerate to form large BN particles or platelets inside the sintered composite and the sintering temperature is very high [14]. According to some reports [15–18], the hot-pressing sintering is a better choice to fabricate dense BN-SiO2 composite ceramic. Although the BN-SiO2 composite ceramics with low dielectric constant of 2.5–3.78 and low dielectric loss were prepared in previous studies [19,20], their thermal properties were hardly analyzed. In this work, a series of BN-SiO2 composite ceramics sintered by hotpressing were fabricated. The phases and microstructures of as-prepared products were characterized. Their thermal properties and the dielectric properties were also measured. The optimized sample with addition of 50 wt% BN sintered at 1650 ℃ exhibits excellent physical properties, including a high thermal conductivity of 6.75 W/m K and a low dielectric constant of 3.73, which make it a promising candidate material as high-frequency substrate.
Corresponding authors. E-mail addresses: [email protected] (F. Xiang), [email protected] (H. Wang).
https://doi.org/10.1016/j.ceramint.2018.06.084 Received 1 May 2018; Received in revised form 7 June 2018; Accepted 10 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Wu, L., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.06.084
Ceramics International xxx (xxxx) xxx–xxx
L. Wu et al.
2. Experimental procedure A series of BN-SiO2 composite ceramics sintered at different temperatures were synthesized by hot-pressing sintering. The boron nitride was purchased from Qingzhou Maiteke new materials Co. Ltd (Weifang, China) with an average particle size about 10 µm and a high purity (≥ 99%). Silicon dioxide (Sinopharm Chemical Reagent, Shanghai, China) was fully amorphous with an average particle size about 10 µm and a high purity (≥ 99%). SiO2 powders and BN powders were mixed according to the weight ratios of 1:3, 1:2, 1:1, 2:1, 3:1, respectively. The powders were ball-milled with ZrO2 balls as the media for 4 h with a rate of 400 rpm in ethyl alcohol by planetary ball mill (QM-3SP2, Nanjing Machine Factory, Nanjing, China). Subsequently, the mixtures were dried at 100 ℃ for 12 h. Then the powders were hot-pressed to Φ20mm discs at 1400 ℃~1700 ℃ with a pressure of 30 MPa for 1 h under air atmosphere. After that, the hot-pressed discs were cut into 10 × 10 × 2 mm3 standard bar specimens for thermal conductivity testing. The phases of prepared samples were identified by powder X-ray diffraction (XRD) using Cu-Ka radiation (D/MAX-2400, Rigaku, Tokyo, Japan). The morphologies and the elemental analysis of the ceramics were observed by scanning electron microscopy (FEI QUANTA FEG 250, FEI, Hillsboro, Oregon, USA) and energy dispersive spectrometer module, respectively. Thermal conductivities were measured by laser
Fig. 1. XRD patterns of the BN-SiO2 composite ceramic with 50 wt% BN at different temperatures.
Fig. 2. SEM of the cross-sectional morphology of the BN-SiO2 composite ceramic with 50 wt% BN sintered at (a) 1400 ℃ (b) 1500 ℃ (c) 1650 ℃ (d) 1700 ℃ and EDS analysis of the cross-sectional morphology of the ceramic sintered at 1650 ℃. 2
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Fig. 3. (a) Density and relative density of the BN-SiO2 composite ceramics change with the sintering temperature; (b) Volume fraction of the BN, SiO2 and air in the composite ceramics at different sintering temperature.
component for the BN-SiO2 composite ceramic. Based on this, the followed samples were sintered at different temperatures with this component. Fig. 1 presents the X-ray diffraction patterns of the raw materials and the BN-SiO2 composite ceramics with 50 wt% BN sintered at 1400–1700 ℃. It can be seen that SiO2 exhibits an amorphous pattern with a broad peak in the range of 2θ 15–30°, which is a typical pattern for an amorphous material. The X-ray diffraction pattern of BN shows well-developed diffraction peaks, which are characteristics of hexagonBN structure. Similar to h-BN, BN-SiO2 ceramics also exhibit the diffraction peaks of h-BN at different sintering temperatures. Moreover, there is a peak of α-cristobalite (low temperature cristobalite) around 22° at 1400 ℃ and 1500 ℃, which means partial SiO2 has been converted into tetragonal structure at both 1400 ℃ and 1500 ℃. In general, the XRD patters show that the crystal structure of BN is not changed with the increase of the sintering temperature and there is no other phases in the process, indicating good chemical stability of the two materials. Fig. 2 shows the scanning electron microscopic (SEM) and energy dispersive spectrometer (EDS) analysis of the cross-sectional morphologies of the composite ceramics sintered at different temperatures. The SEM images suggest that the BN is schistose and the SiO2 is difficult to be distinguished because most of the particles are amorphous. However, the EDS result respects all scanned elements including B, N, Si and O, which confirms the existence of SiO2. It can be seen from Fig. 2(a)–(c) that the BN flakes distribute uniformly throughout the SiO2 matrix and the whole structure is gradually densified from 1400 ℃ to 1650 ℃ with the increase of the sintering temperature. And the flakes become more obvious at 1700 ℃ in Fig. 2(d), indicating that the SiO2 powders melt at this temperature. Consequently, it is proved that the optimum sintering temperature for the composite ceramic is 1650 ℃ at 30 MPa.
Fig. 4. Thermal conductivity of the BN-SiO2 composite ceramic changes with the sintering temperature.
thermal conductivity analyzer (LFA447, NETZSCH, Germany). The densities of the samples were determined by the Archimede's displacement method using electronic balance (XS204, METTLER TOLEDO, Greifensee, Switzerland). The dielectric properties were measured using the impedance analyzer (4990 A, Agilent, Palo Alto, USA). Silver paste was brushed on the top and bottom of pressed samples then fired at 600 ℃ for 30 min as electrodes. 3. Results and discussion A series of samples with different BN contents were sintered at 1650 ℃ to determine the optimum BN content for the composite. The densities of the samples increase with the decrease of BN contents from 75 wt% to 50 wt%. Then the samples melt with the continuing decreasing of BN content. Therefore, the 50 wt% BN content is the best
Fig. 5. (a) Dielectric constants of the BN-SiO2 composite ceramics from 10 kHz to 1 MHz; (b) Dielectric losses of the BN-SiO2 composite ceramics from 10 kHz to 1 MHz; (c) The dielectric constant at 1 MHz of the BN-SiO2 composite ceramics sintered at different temperature. 3
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The dielectric constants of different BN-SiO2 composite ceramic samples are shown in Fig. 5(a) in the frequency range from 10 kHz to 1 MHz. The dielectric constants remain stable with the change of the frequency due to the excellent frequency stability of SiO2 [13]. At 1 MHz, the relative dielectric permitivities increase first and then decrease with the increasing of the sintering temperature in Fig. 5(c). The peak of the dielectric constant is about 3.73 at 1650 ℃, which is a quite low value and can effectively prevent large signal propagation delay. Compared with the density versus temperature curve in Fig. 3(a), the trend of the dielectric constant at 1 MHz is same with the trend of the density, which probably is due to that the content of the pores influences the dielectric constant of the BN-SiO2 composite ceramic [24]. The dielectric losses of these samples decrease with the frequency range from 10 kHz to 100 kHz, and the dielectric losses are almost unchanged from 100 kHz to 1 MHz in Fig. 5(b). It indicates that the dielectric losses remain steady from 100 kHz to 1 MHz. Meanwhile, the dielectric losses are generally less than 0.01 at 1 MHz at different sintering temperatures, which is relatively low and can be practically used. To further verify the effect of air content on dielectric properties of the composite, the mixture logarithmic law was used to calculate the theoretical value of the dielectric constant,
Fig. 3(a) presents the change of the density and the relative density of the BN-SiO2 composite ceramics with the increase of the sintering temperature. As the sintering temperature increases from 1400 ℃ to 1700 ℃, the densities of the BN-SiO2 ceramic are in the range of 2.08–2.17 g/cm3 and the maximum value of the density is 2.17 g/cm3 (relative density 97%) at 1650 ℃. Comparing with the SEM results, the decreasing of the density at 1700 ℃ may be due to the fact of melted SiO2. All these results further demonstrate that 1650 ℃ is the best sintering temperature for BN-SiO2 composite ceramics. Because the density and air content of the composite have great influence on the properties of the ceramic, the theoretical volume contents of air in composite ceramics were calculated for the later analysis by a fitting formula of composite density as following:
ρ = V1 ρ1 + V2 ρ2 + V3 ρ3
(1)
where, ρ is the density of the composite ceramic; V1, V2 and V3 are the volume fractions of air, SiO2 and BN, respectively; ρ1 (1.29 ×10−3 g/ cm3), ρ2 (2.2 g/cm3) andρ3 (2.25 g/cm3) are the densities of air, SiO2 and BN, respectively. Since the SiO2 and BN were mixed with the weight ratio of 1:1, the volume ratio of SiO2 and BN can be calculated through their densities. And the volume fraction of the air can be calculated by formula (1). The results are shown in Fig. 3(b). The changes of the volume content of BN and SiO2 with the sintering temperature are consistent with the trend of the density shown in Fig. 3(a), while the change of the volume fraction of the air is opposite to that. The thermal conductivity curves of BN-SiO2 composite ceramic are presented in Fig. 4. From the experimental data, it can be seen that the thermal conductivity curve is initially increased and then declined with the increase of sintering temperature. It reaches the maximum peak of 6.75 W/m K in the sample sintered at 1650 ℃, which is nearly five times higher than that of the pure SiO2 (~1.4 W/m K). Because the primary heat transport mechanism of BN is phonon propagation, the thermal conductivity (λP) can be approximately expressed by the Debye heat conduction formula [21]:
λp =
1 Clp Vp 3
lnεr = V1lnε1 + V2lnε2 + V3lnε3
where,ε1 (1), ε2 (3.8) andε3 (4.2) are the relative dielectric permittivity of air, SiO2 and BN, respectively. V1, V2 and V3 are the volume fractions of air, SiO2 and BN, respectively. And εr is the relative dielectric permittivity of the composite ceramic. The variation tendency of the calculated dielectric constant is similar with the tendency of the experimental result obtained at 1 MHz as shown in Fig. 5(c). It confirms that the mixture logarithmic rule is a useful method to predict the relative dielectric constant of the composite. Moreover, it also can prove that the dielectric constant is immensely affected by the air content in the composite. 4. Conclusion
(2)
In summary, the BN-SiO2 composite ceramics were successfully synthesized by the hot-pressing sintering. In the resultant BN-SiO2 composite ceramics, BN with hexagonal structure distributed in the SiO2 matrix uniformly, which promotes the thermal conductivity of the composite ceramic and maintains a low dielectric constant. Moreover, the measured thermal conductivities and dielectric constants of the BNSiO2 composite ceramics are consistent with the calculated values, indicating that the properties of the material are related to the content of the air. Therefore, it is shown that the performance of the BN-SiO2 can be improved by increasing the density of the composite. In general, the densest sample sintered at 1650 ℃ with 50 wt% BN exhibits a relatively high thermal conductivity of 6.75 W/m K, a low dielectric constant of 3.73 and a low dielectric loss of 3.18 × 10−3, suggesting it is a good candidate for high frequency substrates applications.
where, C is the heat capacity, Vp is the phonon velocity and Ɩp is the phonon MFP (mean free path). When the composition and structure of the phases remain unchanged, the thermal conductivity of the composite ceramic is affected mainly by porosity via enhanced phonon scattering [22]. The increase of the pores in the ceramic blocks the connection of thermal conductive network of BN in the SiO2 matrix, resulting the decrease of the thermal conductivity. Thus, the thermal conductivity achieves the best value when its density is largest. In order to verify the above result, the thermal conductivity formula of the Bruggeman model for the composite material was used to fit the experimental data, as shown in formula (3):
(1−V )3 =
λ1 λ − λ2 λ ( ) λ λ1 − λ2
(4)
(3)
where, λ1 is the matrix thermal conductivity, λ2 is the thermal conductivity of filler, λ is the thermal conductivity of the composite, V is the volume fraction of the filler. In this research, the thermal conductivity of SiO2, BN and air are 1.4 W/m K, 57 W/m K and 0.023 W/ m K, respectively. The thermal conductivity of the composite is calculated by using the SiO2 as the matrix, the BN and the air as the filler with the volume fraction shown in Fig. 3(b). The results of calculation are displayed in Fig. 4. The calculated thermal conductivities are slightly lower than the experimental values, which may be due to that the effect of interaction between fillers is not taken into consider in the Bruggeman model [23]. Furthermore, the curve shows that the calculated thermal conductivity is 6.01 W/m K at 1650 ℃, which is quite close to the experimental result. These results are expected, and it indicates the thermal conductivity is affected by the content of pores in the composite.
Acknowledgment This work was supported by the National Key Research Program of China (No: 2017YFB0406303). The SEM work was done at International Center for Dielectric research, Xi’an Jiaotong University, Xi’an, China. References [1] S. Ghaffari Mosanenzadeh, M.W. Liu, H.G. Palhares, H.E. Naguib, Design of thermal hybrid composites based on liquid crystal polymer and hexagonal boron nitride fiber network in polylactide matrix, J. Polym. Sci. Part B: Polym. Phys. 54 (2016) 457–464. [2] J.E.Q. Quinsaat, M. Alexandru, F.A. Nüesch, H. Hofmann, A. Borgschulte, D.M. Opris, Highly stretchable dielectric elastomer composites containing high volume fractions of silver nanoparticles, J. Mater. Chem. A 3 (2015) 14675–14685.
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[14] A. Lipp, K.A. Schwetz, K. Hunold, Hexagonal boron nitride: fabrication, properties and applications, J. Eur. Ceram. Soc. 5 (1989) 3–9. [15] T. Burton, A.M. Schinder, G. Capuano, J.J. Rimoli, M.L.R. Walker, G.B. Thompson, Plasma-Induced erosion on ceramic wall structures in Hall-effect thrusters, J. Propuls. Power 30 (2014) 690–695. [16] X. Duan, D. Jia, Q. Meng, Z. Yang, Y. Yu, Y. Zhou, D. Yu, Y. Ding, Study on the plasma erosion resistance of ZrO2p(3Y)/BN–SiO2 composite ceramics, Compos. Part B: Eng. 46 (2013) 130–134. [17] G. Sun, J. Bi, W. Wang, J. Zhang, Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites, J. Eur. Ceram. Soc. 37 (2017) 3195–3202. [18] Z. Tian, X. Duan, Z. Yang, S. Ye, D. Jia, Y. Zhou, Microstructure and erosion resistance of in-situ SiAlON reinforced BN-SiO2 composite ceramics, J. Wuhan. Univ. Technol.-Mater. Sci. Ed. 31 (2016) 315–320. [19] Z. Huazhang, C. Hongnian, Y. Xiaozhang, L. Jianbao, G. Gangfeng, C. Cao, Preparation and properties of BN-SiO2 composite ceramics, 2007. [20] J. Wang, G.W. Wen, Q.C. Meng, Preparation of BN/SiO2 ceramics by PIP method, J. Cent. South Univ. 12 (2005) 31–34. [21] A. Kalemtas, G. Topates, O. Bahadir, P. Kaya Isci, H. Mandal, Thermal properties of pressureless melt infiltrated AlN–Si–Al composites, Trans. Nonferrous Met. Soc. China 23 (2013) 1304–1313. [22] Y. Sun, Z. Yang, D. Cai, Q. Li, H. Li, S. Wang, D. Jia, Y. Zhou, Mechanical, dielectric and thermal properties of porous boron nitride/silicon oxynitride ceramic composites prepared by pressureless sintering, Ceram. Int. 43 (2017) 8230–8235. [23] K. Soga, T. Saito, T. Kawaguchi, I. Satoh, Percolation effect on thermal conductivity of filler-dispersed polymer composites, J. Therm. Sci. Technol. 12 (2017) (JTST0013-JTST0013). [24] J.-q. Li, F. Luo, D.-m. Zhu, W.-c. Zhou, Preparation and dielectric properties of porous silicon nitride ceramics, Trans. Nonferrous Met. Soc. China 16 (2006) s487–s489.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate ⁎
Liangliang Wua, Feng Xianga, , Wenlong Liua, Rong Maa, Hong Wangb, a b
⁎
School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Hot pressing B. Composites C. Dielectric properties C. Thermal conductivity
High thermal conductivity and low dielectric constant are the more and more important properties for highfrequency substrate materials to enhance their heat radiation and reduce signal delay. In this work, a series of BN-SiO2 composite ceramics for high frequency application were successfully synthesized by hot-pressing sintering method. And their structures, thermal and dielectric properties were systematically studied. According to the results, the excellent thermal conductivity with low dielectric constant and low dielectric loss has been obtained in the BN-SiO2 ceramic. Compared to the pure SiO2, the sample with 50 wt% BN addition sintered at 1650 ℃ exhibited excellent physical properties, including a high thermal conductivity of 6.75 W/m K which is almost five times higher than that of pure SiO2 and a low dielectric constant of 3.73. The achieved high thermal conductivity and appropriate dielectric property of the BN-SiO2 composite ceramic make it a promising candidate for high-frequency substrate application.
1. Introduction With the rapid development of high speed circuits and power electronic devices used in high frequency, how to solve the issues of heat radiation and single delay in the substrate is becoming a bottleneck in practical applications. Therefore, the demand for high-frequency substrate materials with high thermal conductivity and low dielectric constant increases quickly in recent years [1–4]. However, many traditional ceramic substrate materials cannot meet these requirements. For example, Al2O3 and AlN are widely used as substrates because of their high thermal conductivity. However, the large signal propagation delay limits their applications at high frequency, resulting from their relatively large dielectric constants [5]. Besides, many ceramics with dielectric constant less than 6 have been reported, such as SiO2-AlN-BN, LZS-Al2O3, Si-N-Of/BN, CaF-AlF3-SiO2 and CaO-B2O3SiO2 [6–10], but they were lack of an adequate thermal conductivity when used for the heat conduction in the high frequency. So, it is urgent to find or synthesize some materials with both high thermal conductivity and low dielectric constant. BN is a potential substrate material with high thermal conductivity and low dielectric constant [11]. However, its high cost and tough process technology immensely hinder the wide application of BN substrate [12]. Meanwhile, although the thermal conductivity of SiO2 is insufficient to meet the requirement of heat radiation, SiO2 powder is a
⁎
kind of useful filler in the composite ceramics for its high performance such as low dielectric constant, low dielectric loss, high mechanical strength and low cost [13]. Therefore, if SiO2 and BN could be combined together, the processing difficulty of BN would be immensely reduced, and the dielectric constant of BN would be further decreased due to the smaller dielectric constant of SiO2. Nevertheless, it is difficult to calcine and fabricate dense BN-SiO2 composite ceramics by conventional pressureless sintering because BN powders are easily agglomerate to form large BN particles or platelets inside the sintered composite and the sintering temperature is very high [14]. According to some reports [15–18], the hot-pressing sintering is a better choice to fabricate dense BN-SiO2 composite ceramic. Although the BN-SiO2 composite ceramics with low dielectric constant of 2.5–3.78 and low dielectric loss were prepared in previous studies [19,20], their thermal properties were hardly analyzed. In this work, a series of BN-SiO2 composite ceramics sintered by hotpressing were fabricated. The phases and microstructures of as-prepared products were characterized. Their thermal properties and the dielectric properties were also measured. The optimized sample with addition of 50 wt% BN sintered at 1650 ℃ exhibits excellent physical properties, including a high thermal conductivity of 6.75 W/m K and a low dielectric constant of 3.73, which make it a promising candidate material as high-frequency substrate.
Corresponding authors. E-mail addresses: [email protected] (F. Xiang), [email protected] (H. Wang).
https://doi.org/10.1016/j.ceramint.2018.06.084 Received 1 May 2018; Received in revised form 7 June 2018; Accepted 10 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Wu, L., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.06.084
Ceramics International xxx (xxxx) xxx–xxx
L. Wu et al.
2. Experimental procedure A series of BN-SiO2 composite ceramics sintered at different temperatures were synthesized by hot-pressing sintering. The boron nitride was purchased from Qingzhou Maiteke new materials Co. Ltd (Weifang, China) with an average particle size about 10 µm and a high purity (≥ 99%). Silicon dioxide (Sinopharm Chemical Reagent, Shanghai, China) was fully amorphous with an average particle size about 10 µm and a high purity (≥ 99%). SiO2 powders and BN powders were mixed according to the weight ratios of 1:3, 1:2, 1:1, 2:1, 3:1, respectively. The powders were ball-milled with ZrO2 balls as the media for 4 h with a rate of 400 rpm in ethyl alcohol by planetary ball mill (QM-3SP2, Nanjing Machine Factory, Nanjing, China). Subsequently, the mixtures were dried at 100 ℃ for 12 h. Then the powders were hot-pressed to Φ20mm discs at 1400 ℃~1700 ℃ with a pressure of 30 MPa for 1 h under air atmosphere. After that, the hot-pressed discs were cut into 10 × 10 × 2 mm3 standard bar specimens for thermal conductivity testing. The phases of prepared samples were identified by powder X-ray diffraction (XRD) using Cu-Ka radiation (D/MAX-2400, Rigaku, Tokyo, Japan). The morphologies and the elemental analysis of the ceramics were observed by scanning electron microscopy (FEI QUANTA FEG 250, FEI, Hillsboro, Oregon, USA) and energy dispersive spectrometer module, respectively. Thermal conductivities were measured by laser
Fig. 1. XRD patterns of the BN-SiO2 composite ceramic with 50 wt% BN at different temperatures.
Fig. 2. SEM of the cross-sectional morphology of the BN-SiO2 composite ceramic with 50 wt% BN sintered at (a) 1400 ℃ (b) 1500 ℃ (c) 1650 ℃ (d) 1700 ℃ and EDS analysis of the cross-sectional morphology of the ceramic sintered at 1650 ℃. 2
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Fig. 3. (a) Density and relative density of the BN-SiO2 composite ceramics change with the sintering temperature; (b) Volume fraction of the BN, SiO2 and air in the composite ceramics at different sintering temperature.
component for the BN-SiO2 composite ceramic. Based on this, the followed samples were sintered at different temperatures with this component. Fig. 1 presents the X-ray diffraction patterns of the raw materials and the BN-SiO2 composite ceramics with 50 wt% BN sintered at 1400–1700 ℃. It can be seen that SiO2 exhibits an amorphous pattern with a broad peak in the range of 2θ 15–30°, which is a typical pattern for an amorphous material. The X-ray diffraction pattern of BN shows well-developed diffraction peaks, which are characteristics of hexagonBN structure. Similar to h-BN, BN-SiO2 ceramics also exhibit the diffraction peaks of h-BN at different sintering temperatures. Moreover, there is a peak of α-cristobalite (low temperature cristobalite) around 22° at 1400 ℃ and 1500 ℃, which means partial SiO2 has been converted into tetragonal structure at both 1400 ℃ and 1500 ℃. In general, the XRD patters show that the crystal structure of BN is not changed with the increase of the sintering temperature and there is no other phases in the process, indicating good chemical stability of the two materials. Fig. 2 shows the scanning electron microscopic (SEM) and energy dispersive spectrometer (EDS) analysis of the cross-sectional morphologies of the composite ceramics sintered at different temperatures. The SEM images suggest that the BN is schistose and the SiO2 is difficult to be distinguished because most of the particles are amorphous. However, the EDS result respects all scanned elements including B, N, Si and O, which confirms the existence of SiO2. It can be seen from Fig. 2(a)–(c) that the BN flakes distribute uniformly throughout the SiO2 matrix and the whole structure is gradually densified from 1400 ℃ to 1650 ℃ with the increase of the sintering temperature. And the flakes become more obvious at 1700 ℃ in Fig. 2(d), indicating that the SiO2 powders melt at this temperature. Consequently, it is proved that the optimum sintering temperature for the composite ceramic is 1650 ℃ at 30 MPa.
Fig. 4. Thermal conductivity of the BN-SiO2 composite ceramic changes with the sintering temperature.
thermal conductivity analyzer (LFA447, NETZSCH, Germany). The densities of the samples were determined by the Archimede's displacement method using electronic balance (XS204, METTLER TOLEDO, Greifensee, Switzerland). The dielectric properties were measured using the impedance analyzer (4990 A, Agilent, Palo Alto, USA). Silver paste was brushed on the top and bottom of pressed samples then fired at 600 ℃ for 30 min as electrodes. 3. Results and discussion A series of samples with different BN contents were sintered at 1650 ℃ to determine the optimum BN content for the composite. The densities of the samples increase with the decrease of BN contents from 75 wt% to 50 wt%. Then the samples melt with the continuing decreasing of BN content. Therefore, the 50 wt% BN content is the best
Fig. 5. (a) Dielectric constants of the BN-SiO2 composite ceramics from 10 kHz to 1 MHz; (b) Dielectric losses of the BN-SiO2 composite ceramics from 10 kHz to 1 MHz; (c) The dielectric constant at 1 MHz of the BN-SiO2 composite ceramics sintered at different temperature. 3
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The dielectric constants of different BN-SiO2 composite ceramic samples are shown in Fig. 5(a) in the frequency range from 10 kHz to 1 MHz. The dielectric constants remain stable with the change of the frequency due to the excellent frequency stability of SiO2 [13]. At 1 MHz, the relative dielectric permitivities increase first and then decrease with the increasing of the sintering temperature in Fig. 5(c). The peak of the dielectric constant is about 3.73 at 1650 ℃, which is a quite low value and can effectively prevent large signal propagation delay. Compared with the density versus temperature curve in Fig. 3(a), the trend of the dielectric constant at 1 MHz is same with the trend of the density, which probably is due to that the content of the pores influences the dielectric constant of the BN-SiO2 composite ceramic [24]. The dielectric losses of these samples decrease with the frequency range from 10 kHz to 100 kHz, and the dielectric losses are almost unchanged from 100 kHz to 1 MHz in Fig. 5(b). It indicates that the dielectric losses remain steady from 100 kHz to 1 MHz. Meanwhile, the dielectric losses are generally less than 0.01 at 1 MHz at different sintering temperatures, which is relatively low and can be practically used. To further verify the effect of air content on dielectric properties of the composite, the mixture logarithmic law was used to calculate the theoretical value of the dielectric constant,
Fig. 3(a) presents the change of the density and the relative density of the BN-SiO2 composite ceramics with the increase of the sintering temperature. As the sintering temperature increases from 1400 ℃ to 1700 ℃, the densities of the BN-SiO2 ceramic are in the range of 2.08–2.17 g/cm3 and the maximum value of the density is 2.17 g/cm3 (relative density 97%) at 1650 ℃. Comparing with the SEM results, the decreasing of the density at 1700 ℃ may be due to the fact of melted SiO2. All these results further demonstrate that 1650 ℃ is the best sintering temperature for BN-SiO2 composite ceramics. Because the density and air content of the composite have great influence on the properties of the ceramic, the theoretical volume contents of air in composite ceramics were calculated for the later analysis by a fitting formula of composite density as following:
ρ = V1 ρ1 + V2 ρ2 + V3 ρ3
(1)
where, ρ is the density of the composite ceramic; V1, V2 and V3 are the volume fractions of air, SiO2 and BN, respectively; ρ1 (1.29 ×10−3 g/ cm3), ρ2 (2.2 g/cm3) andρ3 (2.25 g/cm3) are the densities of air, SiO2 and BN, respectively. Since the SiO2 and BN were mixed with the weight ratio of 1:1, the volume ratio of SiO2 and BN can be calculated through their densities. And the volume fraction of the air can be calculated by formula (1). The results are shown in Fig. 3(b). The changes of the volume content of BN and SiO2 with the sintering temperature are consistent with the trend of the density shown in Fig. 3(a), while the change of the volume fraction of the air is opposite to that. The thermal conductivity curves of BN-SiO2 composite ceramic are presented in Fig. 4. From the experimental data, it can be seen that the thermal conductivity curve is initially increased and then declined with the increase of sintering temperature. It reaches the maximum peak of 6.75 W/m K in the sample sintered at 1650 ℃, which is nearly five times higher than that of the pure SiO2 (~1.4 W/m K). Because the primary heat transport mechanism of BN is phonon propagation, the thermal conductivity (λP) can be approximately expressed by the Debye heat conduction formula [21]:
λp =
1 Clp Vp 3
lnεr = V1lnε1 + V2lnε2 + V3lnε3
where,ε1 (1), ε2 (3.8) andε3 (4.2) are the relative dielectric permittivity of air, SiO2 and BN, respectively. V1, V2 and V3 are the volume fractions of air, SiO2 and BN, respectively. And εr is the relative dielectric permittivity of the composite ceramic. The variation tendency of the calculated dielectric constant is similar with the tendency of the experimental result obtained at 1 MHz as shown in Fig. 5(c). It confirms that the mixture logarithmic rule is a useful method to predict the relative dielectric constant of the composite. Moreover, it also can prove that the dielectric constant is immensely affected by the air content in the composite. 4. Conclusion
(2)
In summary, the BN-SiO2 composite ceramics were successfully synthesized by the hot-pressing sintering. In the resultant BN-SiO2 composite ceramics, BN with hexagonal structure distributed in the SiO2 matrix uniformly, which promotes the thermal conductivity of the composite ceramic and maintains a low dielectric constant. Moreover, the measured thermal conductivities and dielectric constants of the BNSiO2 composite ceramics are consistent with the calculated values, indicating that the properties of the material are related to the content of the air. Therefore, it is shown that the performance of the BN-SiO2 can be improved by increasing the density of the composite. In general, the densest sample sintered at 1650 ℃ with 50 wt% BN exhibits a relatively high thermal conductivity of 6.75 W/m K, a low dielectric constant of 3.73 and a low dielectric loss of 3.18 × 10−3, suggesting it is a good candidate for high frequency substrates applications.
where, C is the heat capacity, Vp is the phonon velocity and Ɩp is the phonon MFP (mean free path). When the composition and structure of the phases remain unchanged, the thermal conductivity of the composite ceramic is affected mainly by porosity via enhanced phonon scattering [22]. The increase of the pores in the ceramic blocks the connection of thermal conductive network of BN in the SiO2 matrix, resulting the decrease of the thermal conductivity. Thus, the thermal conductivity achieves the best value when its density is largest. In order to verify the above result, the thermal conductivity formula of the Bruggeman model for the composite material was used to fit the experimental data, as shown in formula (3):
(1−V )3 =
λ1 λ − λ2 λ ( ) λ λ1 − λ2
(4)
(3)
where, λ1 is the matrix thermal conductivity, λ2 is the thermal conductivity of filler, λ is the thermal conductivity of the composite, V is the volume fraction of the filler. In this research, the thermal conductivity of SiO2, BN and air are 1.4 W/m K, 57 W/m K and 0.023 W/ m K, respectively. The thermal conductivity of the composite is calculated by using the SiO2 as the matrix, the BN and the air as the filler with the volume fraction shown in Fig. 3(b). The results of calculation are displayed in Fig. 4. The calculated thermal conductivities are slightly lower than the experimental values, which may be due to that the effect of interaction between fillers is not taken into consider in the Bruggeman model [23]. Furthermore, the curve shows that the calculated thermal conductivity is 6.01 W/m K at 1650 ℃, which is quite close to the experimental result. These results are expected, and it indicates the thermal conductivity is affected by the content of pores in the composite.
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