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Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss
Journal Pre-proof Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss Pei Yang, Lixiang Wang, Weihua Zhao, Laixin Cai, Yongbao Feng PII:
S0272-8842(19)33566-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.077
Reference:
CERI 23730
To appear in:
Ceramics International
Received Date: 1 November 2019 Revised Date:
26 November 2019
Accepted Date: 6 December 2019
Please cite this article as: P. Yang, L. Wang, W. Zhao, L. Cai, Y. Feng, Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.077. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
[Title Page]
Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss
Pei Yang, Lixiang Wang, Weihua Zhao, Laixin Cai, and Yongbao Feng
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China
Correspondence information: Yongbao Feng, affiliation: College of Materials Science and Engineering, Nanjing Tech University, detailed permanent address:No. 5 Xinmofan Road, Nanjing , China, email address:[email protected] telephone number:(+86)13951623119
Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss
Pei Yang, Lixiang Wang, Weihua Zhao, Laixin Cai, and Yongbao Feng*
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Abstract In this paper, AlN-BN ceramics with high thermal conductivity and low tan δ were prepared by hot-pressing sintering for 2 h using YF3 as sintering aids. Effect of sintering temperature and YF3 content on microstructure and properties of samples were investigated. The results show that the apparent porosity of AlN-BN ceramics with 3 wt.% YF3 reached 0.453%, and the thermal conductivity of AlN-BN ceramics reached up to 112 W·m-1·K-1. And the flexural strength and fracture toughness of the sample were 286±9 MPa and 4±0.1 MPa·m1/2, respectively. The dielectric constant of AlN-BN ceramics increased and the tan δ decreased firstly and then increased with the increase of YF3. The dielectric loss of AlN-BN with 3 wt.% YF3 reached 5.2×10-4, and the dielectric constant was 7.82 at 1 MHz. Key words: hot-pressing sintering, AlN-BN composite ceramics, YF3, thermal conductivity, dielectric properties, mechanical properties
1. Introduction AlN ceramics have a series of excellent properties, such as high thermal conductivity, high flexural strength, and high corrosion resistance and so on, so the researchers at home and abroad have conducted extensive research and attention. [1-7] But the dielectric constant and dielectric loss of AlN ceramics is slightly higher. The h-BN not only has unique advantages in high temperature resistance, dielectric properties, but also has excellent mechanical properties. But the thermal conductivity is slightly lower. It is also a solid lubricating material, which has good antifriction to a certain extent. [8] Therefore, AlN-BN ceramics with higher thermal conductivity, good insulation, mechanical processing can be obtained by compounding AlN and h-BN. [9-11] Thus, AlN-BN can be applied to substrate materials and electronic packaging materials of integrated circuits on a large scale. However, since aluminum nitride and hexagonal boron nitride are covalent bond compounds, it is difficult to get dense multiphase ceramics by pressureless sintering. Without high density, the composite ceramics will not have excellent properties. In order to prepare dense AlN-BN ceramics, the researchers used different sintering aids and sintering methods to prepare the splendid AlN-BN ceramics. Using CaC2 as sintering aid, Takao Kanai et al. [12] researched the influence of AlN:BN on the performance of composite ceramics, but the thermal conductivity and flexural strength of the samples were lower. M. S. Koval’chenko et al. [13] explored the influence of the particle size of the original powder on the density and flexural strength of the composite ceramics. The results show that the density and mechanical strength of the samples increase with the
decrease of powder size. Jin Haiyun et al. [14] synthesized AlN particles which were coated with nano h-BN, and prepared the hot-pressing sintered AlN-BN composite ceramic. With the increase of the content of h-BN, the flexural strength of the composite ceramics decreases slowly, and the workability increased obviously. Li Meijuan et al. [15] used Sm2O3 as sintering aid to prepare composite ceramics by SPS and GPS, while the thermal conductivity of the ceramics was generally lower than 65 W·m-1·K-1. Zhao Haiyang et al. [16] used CaF2 as sintering aid to prepare composite ceramics by SPS, and the thermal conductivity of samples are 78.6 W·m-1·K-1. Xu Jingwen et al. [17] studied the influence of h-BN content on the mechanical properties and microstructure of the ceramics prepared by SPS. He Xiulan et al. [18] researched the effect of Sm2O3-CaF2 and h-BN content on the different properties of AlN-BN ceramics by plasma sintering, but the thermal conductivity was low and the dielectric loss was high. Peng Xu et al. [19] used hot isostatic pressing sintering to study the effect of different raw material ratio on the microstructures and properties of samples. Jin Cancan et al. [20] prepared the hot-pressing sintered AlN-BN ceramics without sintering additives, but the thermal conductivity was low. Grigor’ev, O. M et al. [21] discussed the influence of Y2O3 and SiC on the phase composition and properties of hot-pressing sintered AlN-BN composite ceramics. Woo-Jin Cho et al. [22] studied the influence of h-BN on the mechanical properties and microstructures of AlN-BN ceramics in detail. For AlN-BN system, Sm2O3, Y2O3 and CaF2 are generally used as sintering aids. But the oxide sintering aids may introduce oxygen in AlN-BN composite ceramics, which is not conducive to performance AlN-BN composite
ceramics. In consideration of the low thermal conductivity and high tan δ of AlN-BN ceramics, the hot-pressing sintered AlN-BN ceramics using YF3 as sintering aid were prepared in the paper. The influence of sintering temperature and yttrium fluoride content on the microstructure, dielectric and thermal properties of the composite were investigated. It is hoped that AlN-BN ceramics with high thermal conductivity and low loss can be prepared.
2. Experimental Section 2.1 Experimental Procedure The commercial AlN powder （purity＞97%, Tokuyama Co., Tokyo, Japan), h-BN powder （purity＞98%, average size 1.5 µm, Eno Material Company, China）and YF3 (purity＞99%, average size 2 µm, Xiamen wuye Company, China)were weighed in a certain proportion. The raw materials including 85:15(vol.%) of AlN:BN and 0-7 wt.% YF3 were put in nylon tank and milled on ball mill for 6 h at 180 r/min. The raw materials were dried in an oven. The raw materials were cold pressed in graphite mould coated with h-BN on the inner wall and then sintered at 1850℃ for 2 h in a flowing nitrogen atmosphere in vacuum hot-pressing furnace.
2.2 Performance characterization The apparent porosity and bulk density of sintered body were tested by Archimedes drainage method. Phase analysis of AlN-BN composite was measured by the X ray diffraction (XRD, Rigaku, SmartLab, Japan). The diffraction source is Cu Kα ray, λ=0.15402 nm, the scanning speed is 10。/min, and the step length is 0.02。. The
microstructure of the samples was detected by scanning electron microscopy (SEM, JSM-6510, Japan) and elements were detected by EDS. In this paper, the flexural strength of specimens was tested by three point bending method (GB/T-6569-2006). The specimen size was 36 mm × 4 mm × 3 mm. The fracture toughness was tested by single-edge notched beam method and the specimen size was 30 mm × 4 mm × 2 mm. The AlN-BN composite ceramics was made into a wafer of φ12.7 mm × 2 mm and the thermal properties of the sintered body was tested by Netzsch LFA467 laser thermal conductivity tester. The thermal conductivity of the samples was calculated by the λ=α × Cp × ρ. (α-thermal diffusivity, Cp-heat capacity, ρ-bulk density) The dielectric constant and tan δ at 1 MHz were measured by the digital bridge (LCR, Tonghui, Changzhou).
3. Results and discussions 3.1 Sintering performance The bulk density and apparent porosity of the sintered body were shown in Table 1. It can be seen from Table 1 that with the increase of yttrium fluoride from 0 to 1 wt.%, the bulk density of the sample increases rapidly and the apparent porosity decreases rapidly. With the addition of yttrium fluoride, the bulk density of the sintered body increased slowly. The reason may be that the density of yttrium fluoride is much higher than that of aluminum nitride and hexagonal boron nitride. When excessive yttrium fluoride was added, a small amount of yttrium fluoride did not volatilize. This can be confirmed in the following XRD. At the same time, the apparent porosity of
the samples maintained at about 0.453%. This indicated that the addition of yttrium fluoride obviously boosted the densification of composite ceramics. It can also be seen from Table 1 that with the increase of sintering temperature, the bulk density of the sintered body increase slowly, and the apparent porosity decrease slowly. Because the increase of temperature will accelerate the mass flow and diffusion inside the material, the densification of the material will further increase. The XRD diffraction pattern of AlN-BN composite ceramics with different yttrium fluoride content is shown in Fig. 1. It can be seen from the above figure that when the yttrium fluoride content was 1 wt.% and 3 wt.%, the diffraction patterns were AlN and h-BN diffraction peaks. The diffraction peaks of AlN and h-BN were very sharp, which indicated that their crystallization properties are very good. There was no diffraction peak of aluminate in the figure, which may be due to its decomposition and volatilization at high temperature or very low content. This shows that yttrium fluoride can reduce the generation of grain boundary phase more than yttrium oxide. When the content of yttrium fluoride was 5 wt.% and 7 wt.%, there were not only diffraction peaks of AlN and h-BN, but also tiny diffraction peaks of yttrium fluoride. 3.2 Microstructure The influence of sintering temperature on the microstructure of composite ceramics is shown in Fig. 2. As temperature rise, the porosity of the sample decreases gradually, which is consistent with the apparent porosity data of the sample. When the sintering temperature was 1750℃, the AlN grain size was about 2 µm. With the further increase of sintering temperature, aluminum nitride grains grew further and the flaky crystal
structure of h-BN is gradually improved. When the sintering temperature was 1850℃, the grain size of AlN was about 4 µm, and the grains were closely connected, and h-BN grains were evenly distributed around AlN grains. The SEM of AlN-BN composite ceramics with different yttrium fluoride content is shown in Fig. 3. As shown in Fig. 3, the size of AlN grains with yttrium fluoride was obviously larger than that without yttrium fluoride. The h-BN grains were evenly distributed around the AlN grains. This indicated that yttrium fluoride can promote the sintering performance of the material. Fig. 4 is the back-scattered SEM of composite ceramics with different yttrium fluoride contents. When the content of yttrium fluoride was 1 wt.% and 3 wt.%, there was no obvious bright spot in the sample. Only a very narrow bright region appeared at the grain boundary, which is far lower than the width of the grain boundary phase produced by yttrium oxide as sintering aid. [23] Fig. 5(b) also indicated that the content of yttrium and fluorine is very low. This showed that Y-Al-O had substantially evaporated in nitrogen containing carbon, as mentioned by Qiao. [24] The phenomena illustrated that the ability of yttrium fluoride to weaken the grain boundary phase is better than that of yttrium oxide. Because of the content of grain boundary phase in AlN-BN composite ceramics will seriously affect its thermal and dielectric properties, one of the key technologies to prepare AlN-BN ceramics with higher thermal conductivity and low loss is to control the content of grain boundary phase. When the content of yttrium fluoride is 5 wt.% and 7 wt.%, there are a few white bright spots in the sample, which are the residual yttrium fluoride or small amount of unvolatile aluminate in the
sample(as shown in Fig.5(a)). This illustrated that the amount of yttrium fluoride should not be too much. 3.3 Mechanical properties The effect of temperature on the flexural strength and fracture toughness of the specimen is shown in Fig. 6. As temperature rise, the flexural strength and fracture toughness of the specimens increase firstly and then decrease slightly. When the temperature increased from 1750℃ to 1850℃, the bending strength of the sample increased from 260±11 MPa to 286±9 MPa, while the fracture toughness of the sample increased from3.3±0.2 MPa·m1/2 to 4.0±0.1 MPa·m1/2. The reason for the increase of flexural strength and fracture toughness is that the apparent porosity decreases with the increase of temperature. Apparent porosity is one of the main factors affecting the bearing capacity of materials. The existence of pores not only reduces the load area, but also causes stress concentration in the adjacent area, which weakens the load capacity of materials. The porosity of the material is inversely proportional to the elastic modulus and strength. The effect of yttrium fluoride content on the flexural strength and fracture toughness of the sample is shown in Fig. 7. As yttrium fluoride rise, the fracture toughness and flexural strength of the samples increased firstly and then decreased. When the yttrium fluoride is 3 wt.%, the flexural strength and fracture toughness were 286±9 MPa and 4±0.1 MPa·m1/2, respectively. This reason may be that the porosity is the lowest. The mechanical properties of AlN ceramics with 3 wt.% YF3 and prepared at 1850℃ for 2 h is shown in Table 2. Compared with AlN ceramics, the
flexural strength and fracture toughness of AlN-BN ceramics decreased. The addition of h-BN makes the sintering of AlN-BN ceramics become more difficult. The existence of pores and defects may lead to the decrease of mechanical properties of the AlN-BN ceramics. On the other hand, the mechanical properties of h-BN ceramics are lower than that of AlN ceramics. [25] 3.4 Dielectric properties The influence of sintering temperature on the dielectric properties of the sintered body is shown in Fig. 8. As temperature rise, the dielectric constant increases firstly and then increased slightly, while the tan δ decreases firstly and then becomes stable. The reason may be that the porosity of the material is higher when the temperature is lower than 1850℃(as shown in Table 1). The porosity is an essential factor affecting the dielectric properties of the material. At 1850℃, the dielectric property of the material is the best, and the dielectric constant and tan δ were 7.82 and 5.04×10-4, respectively. The tan δ is apparently lower than that of Takao et al. [12]. The effect of yttrium fluoride content on the dielectric constant and tan δ of the sample is shown in Fig. 9. As yttrium fluoride rise, the dielectric constant of the sample increases and the tan δ decreases firstly and then increases. The reason is that the dielectric properties of the material are not only determined by the material itself, but also affected by impurities, grain boundary phase and porosity. [26] When the content of yttrium fluoride is less than 3 wt.%, the dielectric properties of the sintered body are greatly influenced by the apparent porosity. When the content of yttrium fluoride is higher than 3 wt.%, the dielectric properties of the sintered body are greatly
influenced by the grain boundary phase. When the content of yttrium fluoride is 3 wt.%, the apparent porosity is the lowest and the microstructure is the purest (as shown in Table 1 and Fig 4(2)). 3.5 Thermal conductivity The influence of sintering temperature on the thermal conductivity of the sintered body is shown in Fig. 10. As temperature rise, the thermal conductivity of the sample increases firstly and then decreases slightly. When the sintering temperature is 1850℃, the thermal conductivity of the sample reached up to 112 W·m-1·K-1. For AlN and h-BN, the main heat conduction mechanism is phonon propagation. The thermal conductivity is mainly determined by the mean free path of phonon, and the main factors affecting the free path of phonon are phonon collision and scattering. The existence of grain boundary, impurity, second phase and lattice defect in the crystal will lead to phonon scattering, decrease the mean free path of phonon and the thermal conductivity of the material. [27, 28] As temperature rise, the density of the sample increases, the grain growth, the porosity, grain boundary and lattice defects in the sample decrease, and the scattering effect on phonon weakens, so the thermal conductivity increases. The influence of yttrium fluoride content on the thermal conductivity of the sintered body is shown in Fig. 11. When yttrium fluoride increases from 0 to 3 wt.%, the thermal conductivity increases from 90 W·m-1·K-1 to 112 W·m-1·K-1. The value is higher than previous reports. [15, 18, 19] When the content of yttrium fluoride exceeded 3 wt.%, the thermal conductivity of the sample decreased because of the
nonvolatile grain boundary phase in the sample. The thermal conductivity of AlN ceramics with 3 wt.% YF3 and prepared at 1850℃ for 2 h is shown in Table 2. Compared with AlN ceramics, the thermal conductivity of AlN-BN ceramics decreased. The addition of h-BN made the sintering of AlN-BN ceramics become more difficult. The existence of pores and defects intensifies the phonon scattering. On the other hand, the addition of h-BN hinders the vibration of AlN lattice, which intensifies the phonon scattering. The above factors may lead to the decrease of thermal conductivity of the AlN-BN ceramics.
4. Conclusion
1、The hot-pressing sintered AlN-BN ceramics were prepared in 1850℃ for 2 h. The apparent porosity of the samples with 3 wt.% YF3 reached 0.453%, and the flexural strength and fracture toughness of the sample were 286 ±9 MPa and 4 ±0.1 MPa·m1/2 , respectively. 2、The thermal conductivity of AlN-BN composite ceramics increased first and then decreased with the addition of YF3. When YF3 content was 3 wt.%, the thermal conductivity of AlN-BN composite ceramics reached up to 112 W·m-1·K-1. 3、The dielectric loss of AlN-BN ceramics with 3 wt.% YF3 reached 5.2×10-4, and the dielectric constant was 7.82 at 1 MHz.
Acknowledgments: This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and National Natural Science
Foundation of China (No. 51307079).
Reference [1] Mashhadi H A , Manikandan P , Suetsugu R , et al. Synthesis of AlN–TiN nanostructured composite powder by reactive ball milling and subsequent thermal treatment[J]. Journal of Alloys and Compounds, 2010, 506(2):653-660. [2] Watari K , Ishizaki K , Fujikawa T . Thermal conduction mechanism of aluminium nitride ceramics[J]. Journal of Materials Science, 1992, 27(10):2627-2630. [3] Huang, D, Tian Z, Cui W, et al. Effects of Y2O3 and yttrium aluminates as sintering additives on the thermal conductivity of AlN ceramic substrates. Ceramics International, 2018. 44(16):20556-20559. [4] Li, F, Qiao L, Zheng J, et al. Phase, microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition. Journal of the European Ceramic Society, 2018. 38(4):1170-1178. [5] Hai Jiang, Wang X, Fan G, et al. Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities. Ceramics International, 2019. 45(10):13019-13023. [6] Wang D , Wang C , Li M , et al. Effect of NH4F additive on purification of AlN ceramics[J]. Journal of Materials Science: Materials in Electronics, 2017, 28(9):6731-6736. [7] Jarrige, J., et al., Effect of oxygen on the thermal conductivity of aluminium nitride ceramics. Journal of the European Ceramic Society, 1997. 17(15):1891-1895. [8] Xiang L , Cheng L , Shi L , et al. Mechanical and ablation properties of laminated ZrB2– SiC/BN ceramics[J]. Journal of Alloys and Compounds, 2015, 638:261-266. [9] G. N. Makarenko, L. A. Krushinskaya, V. B. Fedorus, et al. Production and properties of AlN– BN composite[J]. Powder Metallurgy and Metal Ceramics, 2011, 49(11-12):670-674. [10] Cho W S , Cho M W , Lee J H , et al. Effects of h-BN additive on the microstructure and mechanical properties of AlN-based machinable ceramics[J]. Materials Science & Engineering A (Structural Materials:, Properties, Microstructure and Processing), 2006, 418(1-2):61-67. [11] Zhao H , Wang W , Fu Z , et al. Thermal conductivity and dielectric property of hot-pressing sintered AlN–BN ceramic composites[J]. Ceramics International, 2009, 35(1):105-109. [12] K. Takao, T. Kei, K. Hiroshi, Hot-pressed BN–AlN ceramic composites of high thermal conductivity, J. Jpn. J. Appl. Phys. 29 (4) (1990) 683–687. [13] M. S. Koval’chenko, Dubovik T V , Rogozinskaya A A , et al. Effects of initial powder vibrogrinding on the properties of AlN-BN composites[J]. Powder Metallurgy and Metal Ceramics, 2004, 43(9-10):443-446. [14] Jin H, Wang W, Gao J, et al. Study of machinable AlN/BN ceramic composites[J]. Materials Letters, 2006,60(2):190-193. [15] Li Meijuan, Zhang Lianmeng, Shen Qiang et al. Effects of Microstructure Modification on Properties of AIN/BN Composites[J]. Journal of Wuhan University of Technology-Mater, 2008, 23(1):121-124.
[16] Zhao H , Wang W , Wang H , et al. Spark Plasma Sintered AIN-BN Composites and Its Thermal Conductivity[J]. Journal of Wuhan University of Technology-Mater, 2009, 23(6):866-869. [17] Xu J, Lee K, Beck S, et al. Mechanical properties and machinability of AlN-hBN ceramics prepared by spark plasma sintering[J]. Journal of the Ceramic Society of Japan, 2009,117(1369):1028-1031. [18] He X , Gong Q , Guo Y , et al. Microstructure and properties of BN/AlN composites prepared by sparking plasma sintering method[J]. Journal of Alloys & Compounds, 2016, 675:168-173. [19] Peng Xu, Zhu De-Gui, Li Yang-Xu, et al. Hot isostatic pressing preparation and properties of AlN-BN composite ceramics[J]. Journal of Inorganic Materials, 2016, 31(5). [20] Jin C , Wang T , Pan L , et al. Preparation and properties of sintering additive-free AlN–BN composite ceramics by hot-pressing sintering[J]. Journal of Materials Science: Materials in Electronics, 2015. [21] Grigor’ev, O. M, Dubovik T V , Bega N D , et al. Phase composition and properties of hot-pressed AlN–BN materials[J]. Powder Metallurgy and Metal Ceramics, 2013, 52(3-4):161-166. [22] Woo-Jin Cho, S.O.A.I., Effect of BN Addition on the Mechanical Properties of AlN. Korean J. Met. Mater., 2019. 57(5):310-315. [23] Li Y L , Zhang J , Zhang J X . Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering[J]. Ceramics International, 2009, 35(6):2219-2224. [24] Qiao Liang, Zhou Heping, Liu Yaocheng, Wang Yuqi. Liquid Phase Migration in Low Temperature Sintering of AlN Ceramics[J]. Journal of Materials Engineering (10): 7-10. [25] Gu Lide. Boron nitride ceramics [M]. Beijing: China Construction Industry Press, 1982. [26] Kume S, Yasuoka M, Omura N, et al. Effects of MgO addition on the density and dielectric loss of AlN ceramics sintered in presence of Y 2 O 3 [J]. Journal of the European Ceramic Society, 2005. 25(12):2791-2794. [27] Zheng Shui, Han Jiecai, Du Shanyi. Analysis of Combustion Synthetic Ceramics and Their Densification Mechanism[J]. Journal of Inorganic Materials, 2000,15(4):625~630 [28] H Harris, Youngman R A and Teller R G. On the nature of the oxide~related defect in aluminum nitride[J]. Mater. Res. 1990, 5(6):1763~1772
Table 1 Composition and properties of different samples Table 2 Performance comparisons between AlN-BN and AlN ceramics prepared at 1850
for 2 h
Sample
YF3 (wt.%)
Sintering temperature ( )
Bulk density/ g·cm-3
Apparent porosity/ %
1 2 3 4 5 6 7 8
0 1 3 5 7 3 3 3
1850 1850 1850 1850 1850 1750 1800 1900
3.053 3.109 3.126 3.131 3.139 3.112 3.121 3.125
0.951 0.601 0.453 0.456 0.457 0.521 0.485 0.453
Table 1 Composition and properties of different samples The flexural strength
The fracture toughness
(W·m ·K )
(MPa)
(MPa·m1/2)
AlN
159
334±12
4.3±0.12
AlN-BN
112
286±9
4.0±0.1
Sample
The thermal conductivity -1
-1
Table 2 Performance comparisons between AlN-BN and AlN ceramics prepared at 1850
for 2 h
Fig. 1 XRD diffraction patterns of the samples with different YF3 contents Fig. 2 Fracture surface of samples with 3 wt.% YF3 in different temperature(1)1750 , (2) 1800 , (3) 1850 , (4) 1900 Fig. 3 Fracture surface of samples with different YF3 (1)0, (2)1 wt.%, (3)3 wt.%, (4)5 wt.%, (5)7 wt.% Fig. 4 Back-Scattered SEM images of samples with different YF3 (1)1 wt.%, (2)3 wt.%, (3)5 wt.%, (4)7 wt.% Fig. 5 EDS spectrum of (a) α point of Fig 4(4); (b) Fig 4(2) Fig. 6 Effect of temperature on the flexural strength and fracture toughness of samples Fig. 7 Effect of YF3 content on the flexural strength and fracture toughness of samples Fig .8 Dielectric constant and tan δ of the sintered body with different temperature at 1 MHz Fig .9 Dielectric constant and tan δ of the sintered body with different YF3 contents at 1 MHz Fig. 10 Influence of temperature on thermal conductivity of samples Fig. 11 Influence of YF3 content on thermal conductivity of samples
Fig. 1 XRD diffraction patterns of the samples with different YF3 contents
Fig. 2 Fracture surface of samples with 3 wt.% YF3 in different temperature(1)1750 , (2) 1800 , (3) 1850 , (4) 1900
Fig. 3 SEM images of saples with different YF3 (1)0, (2)1 wt.%, (3)3 wt.%, (4)5 wt.%, (5)7 wt.%
Fig. 4 Back-Scattered SEM images of saples with different YF3 (1)1 wt.%, (2)3 wt.%, (3)5 wt.%, (4)7 wt.%
Fig. 5 EDS spectrum of (a) α point of Fig 4(4); (b) Fig 4(2)
Fig. 6 Effect of temperature on the flexural strength and fracture toughness of samples
Fig. 7 Effect of YF3 content on the flexural strength and fracture toughness of samples
Fig .8 Dielectric constant and tan δ of the sintered body with different temperature at 1 MHz
Fig .9 Dielectric constant and tan δ of the sintered body with different YF3 contents at 1 MHz
Fig. 10 Influence of temperature on thermal conductivity of samples
Fig. 11 Influence of YF3 content on thermal conductivity of samples
Declaration of interests statement It is stated that there are no competing or conflict of interests to declare by the authors of this manuscript. Corresponding Author Yongbao Feng
S0272-8842(19)33566-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.077
Reference:
CERI 23730
To appear in:
Ceramics International
Received Date: 1 November 2019 Revised Date:
26 November 2019
Accepted Date: 6 December 2019
Please cite this article as: P. Yang, L. Wang, W. Zhao, L. Cai, Y. Feng, Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.077. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
[Title Page]
Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss
Pei Yang, Lixiang Wang, Weihua Zhao, Laixin Cai, and Yongbao Feng
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China
Correspondence information: Yongbao Feng, affiliation: College of Materials Science and Engineering, Nanjing Tech University, detailed permanent address:No. 5 Xinmofan Road, Nanjing , China, email address:[email protected] telephone number:(+86)13951623119
Hot-pressing sintered AlN-BN ceramics with high thermal conductivity and low dielectric loss
Pei Yang, Lixiang Wang, Weihua Zhao, Laixin Cai, and Yongbao Feng*
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Abstract In this paper, AlN-BN ceramics with high thermal conductivity and low tan δ were prepared by hot-pressing sintering for 2 h using YF3 as sintering aids. Effect of sintering temperature and YF3 content on microstructure and properties of samples were investigated. The results show that the apparent porosity of AlN-BN ceramics with 3 wt.% YF3 reached 0.453%, and the thermal conductivity of AlN-BN ceramics reached up to 112 W·m-1·K-1. And the flexural strength and fracture toughness of the sample were 286±9 MPa and 4±0.1 MPa·m1/2, respectively. The dielectric constant of AlN-BN ceramics increased and the tan δ decreased firstly and then increased with the increase of YF3. The dielectric loss of AlN-BN with 3 wt.% YF3 reached 5.2×10-4, and the dielectric constant was 7.82 at 1 MHz. Key words: hot-pressing sintering, AlN-BN composite ceramics, YF3, thermal conductivity, dielectric properties, mechanical properties
1. Introduction AlN ceramics have a series of excellent properties, such as high thermal conductivity, high flexural strength, and high corrosion resistance and so on, so the researchers at home and abroad have conducted extensive research and attention. [1-7] But the dielectric constant and dielectric loss of AlN ceramics is slightly higher. The h-BN not only has unique advantages in high temperature resistance, dielectric properties, but also has excellent mechanical properties. But the thermal conductivity is slightly lower. It is also a solid lubricating material, which has good antifriction to a certain extent. [8] Therefore, AlN-BN ceramics with higher thermal conductivity, good insulation, mechanical processing can be obtained by compounding AlN and h-BN. [9-11] Thus, AlN-BN can be applied to substrate materials and electronic packaging materials of integrated circuits on a large scale. However, since aluminum nitride and hexagonal boron nitride are covalent bond compounds, it is difficult to get dense multiphase ceramics by pressureless sintering. Without high density, the composite ceramics will not have excellent properties. In order to prepare dense AlN-BN ceramics, the researchers used different sintering aids and sintering methods to prepare the splendid AlN-BN ceramics. Using CaC2 as sintering aid, Takao Kanai et al. [12] researched the influence of AlN:BN on the performance of composite ceramics, but the thermal conductivity and flexural strength of the samples were lower. M. S. Koval’chenko et al. [13] explored the influence of the particle size of the original powder on the density and flexural strength of the composite ceramics. The results show that the density and mechanical strength of the samples increase with the
decrease of powder size. Jin Haiyun et al. [14] synthesized AlN particles which were coated with nano h-BN, and prepared the hot-pressing sintered AlN-BN composite ceramic. With the increase of the content of h-BN, the flexural strength of the composite ceramics decreases slowly, and the workability increased obviously. Li Meijuan et al. [15] used Sm2O3 as sintering aid to prepare composite ceramics by SPS and GPS, while the thermal conductivity of the ceramics was generally lower than 65 W·m-1·K-1. Zhao Haiyang et al. [16] used CaF2 as sintering aid to prepare composite ceramics by SPS, and the thermal conductivity of samples are 78.6 W·m-1·K-1. Xu Jingwen et al. [17] studied the influence of h-BN content on the mechanical properties and microstructure of the ceramics prepared by SPS. He Xiulan et al. [18] researched the effect of Sm2O3-CaF2 and h-BN content on the different properties of AlN-BN ceramics by plasma sintering, but the thermal conductivity was low and the dielectric loss was high. Peng Xu et al. [19] used hot isostatic pressing sintering to study the effect of different raw material ratio on the microstructures and properties of samples. Jin Cancan et al. [20] prepared the hot-pressing sintered AlN-BN ceramics without sintering additives, but the thermal conductivity was low. Grigor’ev, O. M et al. [21] discussed the influence of Y2O3 and SiC on the phase composition and properties of hot-pressing sintered AlN-BN composite ceramics. Woo-Jin Cho et al. [22] studied the influence of h-BN on the mechanical properties and microstructures of AlN-BN ceramics in detail. For AlN-BN system, Sm2O3, Y2O3 and CaF2 are generally used as sintering aids. But the oxide sintering aids may introduce oxygen in AlN-BN composite ceramics, which is not conducive to performance AlN-BN composite
ceramics. In consideration of the low thermal conductivity and high tan δ of AlN-BN ceramics, the hot-pressing sintered AlN-BN ceramics using YF3 as sintering aid were prepared in the paper. The influence of sintering temperature and yttrium fluoride content on the microstructure, dielectric and thermal properties of the composite were investigated. It is hoped that AlN-BN ceramics with high thermal conductivity and low loss can be prepared.
2. Experimental Section 2.1 Experimental Procedure The commercial AlN powder （purity＞97%, Tokuyama Co., Tokyo, Japan), h-BN powder （purity＞98%, average size 1.5 µm, Eno Material Company, China）and YF3 (purity＞99%, average size 2 µm, Xiamen wuye Company, China)were weighed in a certain proportion. The raw materials including 85:15(vol.%) of AlN:BN and 0-7 wt.% YF3 were put in nylon tank and milled on ball mill for 6 h at 180 r/min. The raw materials were dried in an oven. The raw materials were cold pressed in graphite mould coated with h-BN on the inner wall and then sintered at 1850℃ for 2 h in a flowing nitrogen atmosphere in vacuum hot-pressing furnace.
2.2 Performance characterization The apparent porosity and bulk density of sintered body were tested by Archimedes drainage method. Phase analysis of AlN-BN composite was measured by the X ray diffraction (XRD, Rigaku, SmartLab, Japan). The diffraction source is Cu Kα ray, λ=0.15402 nm, the scanning speed is 10。/min, and the step length is 0.02。. The
microstructure of the samples was detected by scanning electron microscopy (SEM, JSM-6510, Japan) and elements were detected by EDS. In this paper, the flexural strength of specimens was tested by three point bending method (GB/T-6569-2006). The specimen size was 36 mm × 4 mm × 3 mm. The fracture toughness was tested by single-edge notched beam method and the specimen size was 30 mm × 4 mm × 2 mm. The AlN-BN composite ceramics was made into a wafer of φ12.7 mm × 2 mm and the thermal properties of the sintered body was tested by Netzsch LFA467 laser thermal conductivity tester. The thermal conductivity of the samples was calculated by the λ=α × Cp × ρ. (α-thermal diffusivity, Cp-heat capacity, ρ-bulk density) The dielectric constant and tan δ at 1 MHz were measured by the digital bridge (LCR, Tonghui, Changzhou).
3. Results and discussions 3.1 Sintering performance The bulk density and apparent porosity of the sintered body were shown in Table 1. It can be seen from Table 1 that with the increase of yttrium fluoride from 0 to 1 wt.%, the bulk density of the sample increases rapidly and the apparent porosity decreases rapidly. With the addition of yttrium fluoride, the bulk density of the sintered body increased slowly. The reason may be that the density of yttrium fluoride is much higher than that of aluminum nitride and hexagonal boron nitride. When excessive yttrium fluoride was added, a small amount of yttrium fluoride did not volatilize. This can be confirmed in the following XRD. At the same time, the apparent porosity of
the samples maintained at about 0.453%. This indicated that the addition of yttrium fluoride obviously boosted the densification of composite ceramics. It can also be seen from Table 1 that with the increase of sintering temperature, the bulk density of the sintered body increase slowly, and the apparent porosity decrease slowly. Because the increase of temperature will accelerate the mass flow and diffusion inside the material, the densification of the material will further increase. The XRD diffraction pattern of AlN-BN composite ceramics with different yttrium fluoride content is shown in Fig. 1. It can be seen from the above figure that when the yttrium fluoride content was 1 wt.% and 3 wt.%, the diffraction patterns were AlN and h-BN diffraction peaks. The diffraction peaks of AlN and h-BN were very sharp, which indicated that their crystallization properties are very good. There was no diffraction peak of aluminate in the figure, which may be due to its decomposition and volatilization at high temperature or very low content. This shows that yttrium fluoride can reduce the generation of grain boundary phase more than yttrium oxide. When the content of yttrium fluoride was 5 wt.% and 7 wt.%, there were not only diffraction peaks of AlN and h-BN, but also tiny diffraction peaks of yttrium fluoride. 3.2 Microstructure The influence of sintering temperature on the microstructure of composite ceramics is shown in Fig. 2. As temperature rise, the porosity of the sample decreases gradually, which is consistent with the apparent porosity data of the sample. When the sintering temperature was 1750℃, the AlN grain size was about 2 µm. With the further increase of sintering temperature, aluminum nitride grains grew further and the flaky crystal
structure of h-BN is gradually improved. When the sintering temperature was 1850℃, the grain size of AlN was about 4 µm, and the grains were closely connected, and h-BN grains were evenly distributed around AlN grains. The SEM of AlN-BN composite ceramics with different yttrium fluoride content is shown in Fig. 3. As shown in Fig. 3, the size of AlN grains with yttrium fluoride was obviously larger than that without yttrium fluoride. The h-BN grains were evenly distributed around the AlN grains. This indicated that yttrium fluoride can promote the sintering performance of the material. Fig. 4 is the back-scattered SEM of composite ceramics with different yttrium fluoride contents. When the content of yttrium fluoride was 1 wt.% and 3 wt.%, there was no obvious bright spot in the sample. Only a very narrow bright region appeared at the grain boundary, which is far lower than the width of the grain boundary phase produced by yttrium oxide as sintering aid. [23] Fig. 5(b) also indicated that the content of yttrium and fluorine is very low. This showed that Y-Al-O had substantially evaporated in nitrogen containing carbon, as mentioned by Qiao. [24] The phenomena illustrated that the ability of yttrium fluoride to weaken the grain boundary phase is better than that of yttrium oxide. Because of the content of grain boundary phase in AlN-BN composite ceramics will seriously affect its thermal and dielectric properties, one of the key technologies to prepare AlN-BN ceramics with higher thermal conductivity and low loss is to control the content of grain boundary phase. When the content of yttrium fluoride is 5 wt.% and 7 wt.%, there are a few white bright spots in the sample, which are the residual yttrium fluoride or small amount of unvolatile aluminate in the
sample(as shown in Fig.5(a)). This illustrated that the amount of yttrium fluoride should not be too much. 3.3 Mechanical properties The effect of temperature on the flexural strength and fracture toughness of the specimen is shown in Fig. 6. As temperature rise, the flexural strength and fracture toughness of the specimens increase firstly and then decrease slightly. When the temperature increased from 1750℃ to 1850℃, the bending strength of the sample increased from 260±11 MPa to 286±9 MPa, while the fracture toughness of the sample increased from3.3±0.2 MPa·m1/2 to 4.0±0.1 MPa·m1/2. The reason for the increase of flexural strength and fracture toughness is that the apparent porosity decreases with the increase of temperature. Apparent porosity is one of the main factors affecting the bearing capacity of materials. The existence of pores not only reduces the load area, but also causes stress concentration in the adjacent area, which weakens the load capacity of materials. The porosity of the material is inversely proportional to the elastic modulus and strength. The effect of yttrium fluoride content on the flexural strength and fracture toughness of the sample is shown in Fig. 7. As yttrium fluoride rise, the fracture toughness and flexural strength of the samples increased firstly and then decreased. When the yttrium fluoride is 3 wt.%, the flexural strength and fracture toughness were 286±9 MPa and 4±0.1 MPa·m1/2, respectively. This reason may be that the porosity is the lowest. The mechanical properties of AlN ceramics with 3 wt.% YF3 and prepared at 1850℃ for 2 h is shown in Table 2. Compared with AlN ceramics, the
flexural strength and fracture toughness of AlN-BN ceramics decreased. The addition of h-BN makes the sintering of AlN-BN ceramics become more difficult. The existence of pores and defects may lead to the decrease of mechanical properties of the AlN-BN ceramics. On the other hand, the mechanical properties of h-BN ceramics are lower than that of AlN ceramics. [25] 3.4 Dielectric properties The influence of sintering temperature on the dielectric properties of the sintered body is shown in Fig. 8. As temperature rise, the dielectric constant increases firstly and then increased slightly, while the tan δ decreases firstly and then becomes stable. The reason may be that the porosity of the material is higher when the temperature is lower than 1850℃(as shown in Table 1). The porosity is an essential factor affecting the dielectric properties of the material. At 1850℃, the dielectric property of the material is the best, and the dielectric constant and tan δ were 7.82 and 5.04×10-4, respectively. The tan δ is apparently lower than that of Takao et al. [12]. The effect of yttrium fluoride content on the dielectric constant and tan δ of the sample is shown in Fig. 9. As yttrium fluoride rise, the dielectric constant of the sample increases and the tan δ decreases firstly and then increases. The reason is that the dielectric properties of the material are not only determined by the material itself, but also affected by impurities, grain boundary phase and porosity. [26] When the content of yttrium fluoride is less than 3 wt.%, the dielectric properties of the sintered body are greatly influenced by the apparent porosity. When the content of yttrium fluoride is higher than 3 wt.%, the dielectric properties of the sintered body are greatly
influenced by the grain boundary phase. When the content of yttrium fluoride is 3 wt.%, the apparent porosity is the lowest and the microstructure is the purest (as shown in Table 1 and Fig 4(2)). 3.5 Thermal conductivity The influence of sintering temperature on the thermal conductivity of the sintered body is shown in Fig. 10. As temperature rise, the thermal conductivity of the sample increases firstly and then decreases slightly. When the sintering temperature is 1850℃, the thermal conductivity of the sample reached up to 112 W·m-1·K-1. For AlN and h-BN, the main heat conduction mechanism is phonon propagation. The thermal conductivity is mainly determined by the mean free path of phonon, and the main factors affecting the free path of phonon are phonon collision and scattering. The existence of grain boundary, impurity, second phase and lattice defect in the crystal will lead to phonon scattering, decrease the mean free path of phonon and the thermal conductivity of the material. [27, 28] As temperature rise, the density of the sample increases, the grain growth, the porosity, grain boundary and lattice defects in the sample decrease, and the scattering effect on phonon weakens, so the thermal conductivity increases. The influence of yttrium fluoride content on the thermal conductivity of the sintered body is shown in Fig. 11. When yttrium fluoride increases from 0 to 3 wt.%, the thermal conductivity increases from 90 W·m-1·K-1 to 112 W·m-1·K-1. The value is higher than previous reports. [15, 18, 19] When the content of yttrium fluoride exceeded 3 wt.%, the thermal conductivity of the sample decreased because of the
nonvolatile grain boundary phase in the sample. The thermal conductivity of AlN ceramics with 3 wt.% YF3 and prepared at 1850℃ for 2 h is shown in Table 2. Compared with AlN ceramics, the thermal conductivity of AlN-BN ceramics decreased. The addition of h-BN made the sintering of AlN-BN ceramics become more difficult. The existence of pores and defects intensifies the phonon scattering. On the other hand, the addition of h-BN hinders the vibration of AlN lattice, which intensifies the phonon scattering. The above factors may lead to the decrease of thermal conductivity of the AlN-BN ceramics.
4. Conclusion
1、The hot-pressing sintered AlN-BN ceramics were prepared in 1850℃ for 2 h. The apparent porosity of the samples with 3 wt.% YF3 reached 0.453%, and the flexural strength and fracture toughness of the sample were 286 ±9 MPa and 4 ±0.1 MPa·m1/2 , respectively. 2、The thermal conductivity of AlN-BN composite ceramics increased first and then decreased with the addition of YF3. When YF3 content was 3 wt.%, the thermal conductivity of AlN-BN composite ceramics reached up to 112 W·m-1·K-1. 3、The dielectric loss of AlN-BN ceramics with 3 wt.% YF3 reached 5.2×10-4, and the dielectric constant was 7.82 at 1 MHz.
Acknowledgments: This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and National Natural Science
Foundation of China (No. 51307079).
Reference [1] Mashhadi H A , Manikandan P , Suetsugu R , et al. Synthesis of AlN–TiN nanostructured composite powder by reactive ball milling and subsequent thermal treatment[J]. Journal of Alloys and Compounds, 2010, 506(2):653-660. [2] Watari K , Ishizaki K , Fujikawa T . Thermal conduction mechanism of aluminium nitride ceramics[J]. Journal of Materials Science, 1992, 27(10):2627-2630. [3] Huang, D, Tian Z, Cui W, et al. Effects of Y2O3 and yttrium aluminates as sintering additives on the thermal conductivity of AlN ceramic substrates. Ceramics International, 2018. 44(16):20556-20559. [4] Li, F, Qiao L, Zheng J, et al. Phase, microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition. Journal of the European Ceramic Society, 2018. 38(4):1170-1178. [5] Hai Jiang, Wang X, Fan G, et al. Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities. Ceramics International, 2019. 45(10):13019-13023. [6] Wang D , Wang C , Li M , et al. Effect of NH4F additive on purification of AlN ceramics[J]. Journal of Materials Science: Materials in Electronics, 2017, 28(9):6731-6736. [7] Jarrige, J., et al., Effect of oxygen on the thermal conductivity of aluminium nitride ceramics. Journal of the European Ceramic Society, 1997. 17(15):1891-1895. [8] Xiang L , Cheng L , Shi L , et al. Mechanical and ablation properties of laminated ZrB2– SiC/BN ceramics[J]. Journal of Alloys and Compounds, 2015, 638:261-266. [9] G. N. Makarenko, L. A. Krushinskaya, V. B. Fedorus, et al. Production and properties of AlN– BN composite[J]. Powder Metallurgy and Metal Ceramics, 2011, 49(11-12):670-674. [10] Cho W S , Cho M W , Lee J H , et al. Effects of h-BN additive on the microstructure and mechanical properties of AlN-based machinable ceramics[J]. Materials Science & Engineering A (Structural Materials:, Properties, Microstructure and Processing), 2006, 418(1-2):61-67. [11] Zhao H , Wang W , Fu Z , et al. Thermal conductivity and dielectric property of hot-pressing sintered AlN–BN ceramic composites[J]. Ceramics International, 2009, 35(1):105-109. [12] K. Takao, T. Kei, K. Hiroshi, Hot-pressed BN–AlN ceramic composites of high thermal conductivity, J. Jpn. J. Appl. Phys. 29 (4) (1990) 683–687. [13] M. S. Koval’chenko, Dubovik T V , Rogozinskaya A A , et al. Effects of initial powder vibrogrinding on the properties of AlN-BN composites[J]. Powder Metallurgy and Metal Ceramics, 2004, 43(9-10):443-446. [14] Jin H, Wang W, Gao J, et al. Study of machinable AlN/BN ceramic composites[J]. Materials Letters, 2006,60(2):190-193. [15] Li Meijuan, Zhang Lianmeng, Shen Qiang et al. Effects of Microstructure Modification on Properties of AIN/BN Composites[J]. Journal of Wuhan University of Technology-Mater, 2008, 23(1):121-124.
[16] Zhao H , Wang W , Wang H , et al. Spark Plasma Sintered AIN-BN Composites and Its Thermal Conductivity[J]. Journal of Wuhan University of Technology-Mater, 2009, 23(6):866-869. [17] Xu J, Lee K, Beck S, et al. Mechanical properties and machinability of AlN-hBN ceramics prepared by spark plasma sintering[J]. Journal of the Ceramic Society of Japan, 2009,117(1369):1028-1031. [18] He X , Gong Q , Guo Y , et al. Microstructure and properties of BN/AlN composites prepared by sparking plasma sintering method[J]. Journal of Alloys & Compounds, 2016, 675:168-173. [19] Peng Xu, Zhu De-Gui, Li Yang-Xu, et al. Hot isostatic pressing preparation and properties of AlN-BN composite ceramics[J]. Journal of Inorganic Materials, 2016, 31(5). [20] Jin C , Wang T , Pan L , et al. Preparation and properties of sintering additive-free AlN–BN composite ceramics by hot-pressing sintering[J]. Journal of Materials Science: Materials in Electronics, 2015. [21] Grigor’ev, O. M, Dubovik T V , Bega N D , et al. Phase composition and properties of hot-pressed AlN–BN materials[J]. Powder Metallurgy and Metal Ceramics, 2013, 52(3-4):161-166. [22] Woo-Jin Cho, S.O.A.I., Effect of BN Addition on the Mechanical Properties of AlN. Korean J. Met. Mater., 2019. 57(5):310-315. [23] Li Y L , Zhang J , Zhang J X . Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering[J]. Ceramics International, 2009, 35(6):2219-2224. [24] Qiao Liang, Zhou Heping, Liu Yaocheng, Wang Yuqi. Liquid Phase Migration in Low Temperature Sintering of AlN Ceramics[J]. Journal of Materials Engineering (10): 7-10. [25] Gu Lide. Boron nitride ceramics [M]. Beijing: China Construction Industry Press, 1982. [26] Kume S, Yasuoka M, Omura N, et al. Effects of MgO addition on the density and dielectric loss of AlN ceramics sintered in presence of Y 2 O 3 [J]. Journal of the European Ceramic Society, 2005. 25(12):2791-2794. [27] Zheng Shui, Han Jiecai, Du Shanyi. Analysis of Combustion Synthetic Ceramics and Their Densification Mechanism[J]. Journal of Inorganic Materials, 2000,15(4):625~630 [28] H Harris, Youngman R A and Teller R G. On the nature of the oxide~related defect in aluminum nitride[J]. Mater. Res. 1990, 5(6):1763~1772
Table 1 Composition and properties of different samples Table 2 Performance comparisons between AlN-BN and AlN ceramics prepared at 1850
for 2 h
Sample
YF3 (wt.%)
Sintering temperature ( )
Bulk density/ g·cm-3
Apparent porosity/ %
1 2 3 4 5 6 7 8
0 1 3 5 7 3 3 3
1850 1850 1850 1850 1850 1750 1800 1900
3.053 3.109 3.126 3.131 3.139 3.112 3.121 3.125
0.951 0.601 0.453 0.456 0.457 0.521 0.485 0.453
Table 1 Composition and properties of different samples The flexural strength
The fracture toughness
(W·m ·K )
(MPa)
(MPa·m1/2)
AlN
159
334±12
4.3±0.12
AlN-BN
112
286±9
4.0±0.1
Sample
The thermal conductivity -1
-1
Table 2 Performance comparisons between AlN-BN and AlN ceramics prepared at 1850
for 2 h
Fig. 1 XRD diffraction patterns of the samples with different YF3 contents Fig. 2 Fracture surface of samples with 3 wt.% YF3 in different temperature(1)1750 , (2) 1800 , (3) 1850 , (4) 1900 Fig. 3 Fracture surface of samples with different YF3 (1)0, (2)1 wt.%, (3)3 wt.%, (4)5 wt.%, (5)7 wt.% Fig. 4 Back-Scattered SEM images of samples with different YF3 (1)1 wt.%, (2)3 wt.%, (3)5 wt.%, (4)7 wt.% Fig. 5 EDS spectrum of (a) α point of Fig 4(4); (b) Fig 4(2) Fig. 6 Effect of temperature on the flexural strength and fracture toughness of samples Fig. 7 Effect of YF3 content on the flexural strength and fracture toughness of samples Fig .8 Dielectric constant and tan δ of the sintered body with different temperature at 1 MHz Fig .9 Dielectric constant and tan δ of the sintered body with different YF3 contents at 1 MHz Fig. 10 Influence of temperature on thermal conductivity of samples Fig. 11 Influence of YF3 content on thermal conductivity of samples
Fig. 1 XRD diffraction patterns of the samples with different YF3 contents
Fig. 2 Fracture surface of samples with 3 wt.% YF3 in different temperature(1)1750 , (2) 1800 , (3) 1850 , (4) 1900
Fig. 3 SEM images of saples with different YF3 (1)0, (2)1 wt.%, (3)3 wt.%, (4)5 wt.%, (5)7 wt.%
Fig. 4 Back-Scattered SEM images of saples with different YF3 (1)1 wt.%, (2)3 wt.%, (3)5 wt.%, (4)7 wt.%
Fig. 5 EDS spectrum of (a) α point of Fig 4(4); (b) Fig 4(2)
Fig. 6 Effect of temperature on the flexural strength and fracture toughness of samples
Fig. 7 Effect of YF3 content on the flexural strength and fracture toughness of samples
Fig .8 Dielectric constant and tan δ of the sintered body with different temperature at 1 MHz
Fig .9 Dielectric constant and tan δ of the sintered body with different YF3 contents at 1 MHz
Fig. 10 Influence of temperature on thermal conductivity of samples
Fig. 11 Influence of YF3 content on thermal conductivity of samples
Declaration of interests statement It is stated that there are no competing or conflict of interests to declare by the authors of this manuscript. Corresponding Author Yongbao Feng