- Email: [email protected]
Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives ⁎
Zhuo Zhanga,b, Xiaoming Duana,b,c, , Baofu Qiua,b, Zhihua Yanga,b,c, Delong Caia,b, Peigang Hea,b, ⁎ Dechang Jiaa,b,c, , Yu Zhoua,b a
Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin Institute of Technology, Harbin, 150001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Liquid phase sintering Texture Anisotropic properties
Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y2O35Al2O3 (mole ratio of 3:5) and 3Y2O3-5Al2O3-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y2O3-5Al2O3-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y2O3-5Al2O3 is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.
1. Introduction
significantly related with the following factors:
Hexagonal boron nitride (h-BN) has a graphite-like layered structure, where B and N atoms in the same layer are combined by covalent bonds with sp2 hybridization, while different layers are combined by van der Waals forces [1–3]. The physical properties of the h-BN crystal, such as elastic moduli, thermal conductivities, thermal expansion coefficients, electric conductivities and dielectric constants, show obvious anisotropy parallel and perpendicular to the c-axis [4–12]. For instance, the theoretical thermal conductivities parallel and perpendicular to the c-axis of bulk h-BN single crystal were calculated to be 4.1 W/(m⋅K) and 537 W/(m⋅K), respectively [13]. If h-BN grains can be preferentially aligned during ceramic preparation, textured bulk ceramics possessing anisotropic properties can be prepared [14,15]. The texturing of h-BN matrix ceramics can significantly improve their performances along specific direction and expand their application fields, for example, as heat sinks for semiconductor parts [16]. Textured h-BN matrix ceramics are usually prepared by hot pressing using liquid phase sintering additives [5,17]. The liquid phase formed at high temperature provides an environment for grain rotation. Generally, h-BN grains can be oriented with the c-axis parallel to the sintering pressure due to the lamellar structure. The texture degree is
(1) Morphology of raw powders. Compared with irregularly shaped raw powders, platelike h-BN raw powders are more beneficial to grain rotation under sintering pressure [18]. (2) Sintering pressure. High pressure is beneficial to grain rotation and texture formation, resulting in significantly anisotropic mechanical properties [15,19]. (3) Sintering additives selection. The good wettability of sintering additives to h-BN grains can promote grain rotation when the uniaxial pressing is applied, which is favorable to texture formation [14]. However, hot pressing does not necessarily lead to this kind of texture microstructures. When raw h-BN powders with a low degree of order and a broad particle size distribution are sintered at high temperature and moderate pressure without sintering additives, the moderate pressure isn’t high enough to rotate h-BN grains but can promote the contact of h-BN grains along the pressure, accelerating platelike grain growth along this direction and forming texture microstructures where the c-axis orientation is perpendicular to the pressure. But without grain rotation in liquid phase environment, the texture degree won’t be high [16].
⁎ Corresponding authors at: Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (X. Duan), [email protected] (D. Jia).
https://doi.org/10.1016/j.jeurceramsoc.2019.01.003 Received 17 October 2018; Received in revised form 27 December 2018; Accepted 3 January 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, Z., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.01.003
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
2. Materials and methods of preparation and characterization The raw materials used in our study are h-BN powders with platelike morphology (15 μm in diameter and 0.3 μm in thickness, purity > 99%, Qingzhou Fangyuan Boron Nitride Factory, Weifang, China), Al2O3 powders (˜50 nm, purity > 99.5%, Showa Denko K.K., Yokohama, Japan), Y2O3 powders (˜80 nm, purity > 99.9%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and MgO powders (˜100 nm, purity > 99.9%, Xuancheng Jingrui New Material Co., Ltd, Xuancheng, China). 3Y2O3-5Al2O3 (mole ratio of 3:5) and 3Y2O3-5Al2O3-4MgO (mole ratio of 3:5:4) were used as sintering additives, respectively. The mass ratio of h-BN to the sintering additive was 8:2. Raw powders were mixed by ball milling with Al2O3 balls in ethanol on a pot mill machine with a speed of 80 rpm for 24 h and then dried at 70 °C. The mixed powders were loaded into cylindrical graphite die and hot pressed at 1900 °C (with the heating and cooling rate of 20 °C/min) under the sintering pressure of 30 MPa in N2 atmosphere for 1 h. The specimen sintered using 3Y2O3-5Al2O3 as the sintering additive was marked as BN-Y3A5. The specimen sintered using 3Y2O3-5Al2O34MgO as the sintering additive was marked as BN-Y3A5M4. Apparent densities of the sintered specimens were measured by Archimedes method. The crystallographic orientation and phase compositions of the specimens were analyzed by X-ray diffraction (XRD; RTP 300, Rigaku, Japan). Pole figures were provided by X-ray diffraction (XRD; D8 discover, Bruker, Germany). Phase identification was realized by transmission electron microscopy (TEM; Talos F200X, FEI, USA). Mechanical properties including elastic moduli, flexural strength and fracture toughness were measured along three different loading directions by mechanical strength testing machine (Model 5569, Instron, USA). Flexural strength was measured on bars (3 mm × 4 mm × 36 mm) by three-point bending test with a span of 30 mm and a traverse feed of 0.5 mm/min. Elastic moduli were measured by strain gauges stuck on the tensile surfaces of these bars. Fracture toughness was measured on single-edge-notched beams (2 mm × 4 mm × 20 mm) by three-point bending test with a span of 16 mm and a traverse feed of 0.05 mm/min. Microstructures of fracture surfaces were observed by scanning electron microscopy (SEM; Quanta 200 FEG, FEI, USA). Thermal expansion coefficients were measured on bars (5 mm × 5 mm × 25 mm) by dilatometer (DTL 402C, Netzsch, Germany) with temperature ranging from 20 °C to 1205 °C and heating rate of 5 °C/min in Ar atmosphere. Thermal diffusivities were measured on discs (Φ 12.7 mm × 2˜3 mm) by laser flash analysis method (LFA 427, Netzsch, Germany) with laser voltage of 550 V and pulse width of 0.8 ms in Ar atmosphere. Specific heat was measured using differential scanning calorimetry (DSC STA449F3, Netzsch, Germany).
Fig. 1. XRD patterns for TS sample direction and SS sample direction of (a) BNY3A5 and (b) BN-Y3A5M4.
3Y2O3-5Al2O3 can form Y3Al5O12 (YAG) liquid phase at high temperature, which has been widely used as the sintering additive for SiC, Si3N4 and SiAlON ceramics [20–25]. Besides, MgO can also be introduced into Y2O3-Al2O3 to obtain materials with different properties by forming different liquid phases during sintering and controlling phase compositions [26–28]. But these two sintering additive systems have seldom been used in h-BN matrix ceramics. In this work, two textured h-BN matrix ceramic specimens were hot pressed using 3Y2O3-5Al2O3 and 3Y2O3-5Al2O3-4MgO as sintering additives, respectively. The phase compositions, texture characteristics of the two textured specimens, and their anisotropic mechanical and thermal properties were investigated.
Fig. 2. TEM images of BN-Y3A5: (a) TEM bright field image, (b) and (c) diffraction patterns of h-BN and YAG in (a). 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 3. TEM images of BN-Y3A5: (a–f) HAADF-STEM image and EDS elemental maps of one triangular pore between h-BN grains; (g) EDS line scan results along the green arrow marked in (a); (h) HRTEM image of the interface between h-BN and YAG (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
3. Results and discussion
XRD patterns. BN-Y3A5 is composed of h-BN and YAG, as the mole ratio of Y2O3 to Al2O3 is 3:5. BN-Y3A5M4 is mainly composed of h-BN, Y4Al2O9 (YAM) and MgAl2O4. The phase compositions of BN-Y3A5M4 are more complex than that of BN-Y3A5 due to the introduction of MgO. The formation of MgAl2O4 leads to the increase of the mole ratio of Y2O3 to Al2O3. So YAM appears instead of YAG. According to the phase compositions, the theoretical densities of BN-Y3A5 and BNY3A5M4 were estimated to be 2.533 g/cm3 and 2.501 g/cm3, respectively, and the relative densities of BN-Y3A5 and BN-Y3A5M4 are 88.66% and 89.62%, respectively. Fig. 2 shows TEM bright field image and diffraction patterns of BNY3A5, which demonstrates the formation of YAG. The zone axes of diffraction patterns in Fig. 2(b) and (c) are [001] of h-BN and [110] of
3.1. Phase compositions and texture characteristics The apparent densities of BN-Y3A5 and BN-Y3A5M4 are 2.246 g/ cm3 and 2.241 g/cm3, respectively. The top surfaces of the sintered specimens are perpendicular to the applied pressure during hotpressing process and marked as TS, and the side surfaces parallel to the applied pressure during hot pressing process are marked as SS. Fig. 1 shows the XRD patterns for TS sample direction and SS sample direction of BN-Y3A5 and BN-Y3A5M4. The contrast between the diffraction intensities of (002) peak and (100) peak can be seen in the illustrations. There is no sign of reaction between BN and sintering additives from 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 4. TEM images of BN-Y3A5M4: (a) TEM bright field image; (b) HRTEM image of the interface between h-BN and sintering additive area; (c) HRTEM image of sintering additive area; (d) FFT of (c).
XRD patterns. For SS sample direction, (100) peaks are higher than (002) peaks. The noticeable (002) peaks for TS sample direction of both specimens indicate that h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure. For textured h-BN matrix ceramics, the preferred orientation of hBN grains leads to the change of intensities of diffraction peaks. So the texture degree of hot pressed h-BN matrix ceramics can be quantitively characterized by the index of orientation preference (IOP) values calculated from XRD patterns [16,29]:
YAG, respectively. Fig. 3(a–f) show high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and energy dispersion spectrum (EDS) elemental maps of one triangular pore between h-BN grains. Y, Al and O elements are distributed on h-BN grain boundaries, which illustrates the formation of liquid phase possessing good wettability to h-BN grains. Fig. 3(g) gives EDS line scan results along the green arrow marked in (a), where peaks of Y, Al and O elements appear at h-BN grain boundaries. Fig. 3(h) shows high resolution transmission electron microscopy (HRTEM) image of the interface between h-BN and YAG, where d(100) of h-BN and d(002) of YAG are marked. h-BN and YAG are tightly integrated at the interface. Fig. 4(a) shows TEM bright field image of BN-Y3A5M4, where the diffraction pattern of h-BN is shown in the illustration. There are many nanocrystals at sintering additive area. Fig. 4(b) shows HRTEM image of the interface between h-BN and sintering additive area, where d(100) of h-BN is marked. Fig. 4(c) and (d) shows HRTEM image of sintering additive area and the corresponding FFT, which demonstrates the existence In Fig. 1, the (002) peaks for TS sample direction of the two specimens are too high that they are not fully presented. The contrast between (002) peaks and other diffraction peaks for TS sample direction can be seen from the thumbnails shown on the top right side of the
(I
IOP =
/I
⎧ (I ′100 / I002 ′ 100
)TS
002)SS
, When(I100/ I002)TS > (I ′100/ I ′002)SS
⎨ (I ′100 / I ′002)SS , ⎩ (I100 / I002)TS
When(I100/ I002)TS < (I ′100/ I ′002)SS
(1)
Where I and I' are the intensities of corresponding diffraction peaks on TS sample direction and SS sample direction, respectively. For a sintered h-BN specimen composed of randomly oriented grains, IOP = ± 1. If h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure, IOP < -1. If h-BN grains are preferentially oriented with the c-axis perpendicular to the sintering pressure, IOP > 1. Obviously, the larger absolute value of IOP indicates the more significant preferred orientation. 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 5. Pole figures and ODF cross sections for BN-Y3A5 and BN-Y3A5M4: (a) and (b) h-BN (002) pole figures of TS sample direction for BN-Y3A5 and BN-Y3A5M4, respectively; (c) and (d) ODF cross sections with φ2 = 0°, 30° and 60° for BN-Y3A5 and BN-Y3A5M4, respectively.
The IOP values of BN-Y3A5 and BN-Y3A5M4 are calculated to be -530 and -976, respectively, indicating that BN-Y3A5M4 possesses stronger preferred orientation than BN-Y3A5. The eutectic point of the ternary sintering additive Y2O3-Al2O3-MgO was reported to be 80 °C lower than that of the binary sintering additive Y2O3-Al2O3 [30,31], so h-BN grains are more easily to be rotated by the sintering pressure in 3Y2O3-5Al2O3-4MgO sintering additive than in 3Y2O3-5Al2O3 sintering additive, resulting in higher absolute value of IOP for BN-Y3A5M4 than BN-Y3A5. Fig. 5 shows the h-BN (002) pole figures of TS sample direction and orientation distribution function (ODF) cross section with φ2 = 0°, 30° and 60° for BN-Y3A5 and BN-Y3A5M4. Both the two pole figures are composed of irregular concentric circles with different colors. The numbers corresponding to these colors represent multiples of a random distribution (MRD) defined as the volume fraction of grains with certain
Fig. 6. Mechanical properties of BN-Y3A5 and BN-Y3A5M4 along D1, D2 and D3.
5
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 7. Fracture morphologies along D1, D2, D3 of (a), (b), (c) BN-Y3A5 and (d), (e), (f) BN-Y3A5M4, respectively.
Fig. 8. Fracture lateral morphologies along D1, D2, D3 of (a), (b), (c) BN-Y3A5 and (d), (e), (f) BN-Y3A5M4, respectively.
contour lines, where φ represents the angle between the c-axis of h-BN grains and the normal of TS, φ1 represents the angle between projection of the c-axis of h-BN grains on TS and the rolling direction (For hot pressing, the rolling direction can be randomly selected on TS), φ2 represents the rotation angle of h-BN grains around the c-axis. MRD values increase as φ approaches 0°, and the MRD value near φ = 0° for BN-Y3A5M4 is higher than that for BN-Y3A5, which is consistent with the results reflected by pole figures.
orientation in the textured specimen divided by that in the sample composed of randomly oriented grains [32]. In the h-BN (002) pole figures of TS sample direction for BN-Y3A5 and BN-Y3A5M4, the closer to the center of the map, the greater the MRD values, indicating that MRD values increase as the angle between the c-axis of h-BN grains and the normal of TS (the sintering pressure) approaches 0°. So h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure. The h-BN (002) pole figure of TS sample direction for BNY3A5M4 has higher MRD value in the center than that for BN-Y3A5, indicating that BN-Y3A5M4 has higher texture degree than BN-Y3A5. ODF cross sections also reflect the same results. These ODF sections are calculated from several pole figures and composed of a series of MRD
3.2. Anisotropic properties Due to the preferred orientation of textured h-BN matrix ceramics, 6
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
properties along D2 and D3 are higher than that along D1. The mechanical properties of BN-Y3A5M4 are slightly better than that of BN-Y3A5, especially along D2 and D3. The lower liquid phase formation temperature of 3Y2O3-5Al2O3-4MgO than 3Y2O3-5Al2O3 results in the more significant preferred orientation of BN-Y3A5M4, which leads to its more effective crack deflection along D2 and D3, resulting in its better mechanical properties than BN-Y3A5. Fig. 9 gives the engineering thermal expansion coefficients (Tref = 25 °C) parallel and perpendicular to the c-axis orientation of BNY3A5 and BN-Y3A5M4. The thermal expansion coefficients (25 °C ˜ 1200 °C) parallel and perpendicular to the c-axis orientation of BNY3A5 are 10.35 × 10−6/K and 2.49 × 10−6/K, respectively. That of BN-Y3A5M4 are 11.27 × 10−6/K and 2.40 × 10−6/K, respectively. Similar to graphite, h-BN possesses small negative thermal expansion coefficient perpendicular to the c-axis at low temperature [33,34]. It has been reported that the thermal expansion coefficient perpendicular to the c-axis is -2.72 × 10−6/K at room temperature and approaches zero at high temperature, while that along the c-axis is 37.7 × 10−6/K at room temperature and tends to be invariable at high temperature [7]. So textured h-BN matrix ceramics possess higher thermal expansion coefficient parallel to the c-axis orientation than perpendicular to the caxis orientation. The anisotropy of thermal expansion coefficients of BN-Y3A5M4 is more significant than BN-Y3A5. This is related to their texture microstructures. BN-Y3A5M4 has higher texture degree, leading to its more significantly anisotropic thermal expansion coefficients. Fig. 10 gives the anisotropic thermal diffusivities and thermal conductivities of BN-Y3A5 and BN-Y3A5M4 at room temperature. Although BN-Y3A5M4 has higher texture degree, BN-Y3A5 has more significantly anisotropic thermal diffusivities and thermal conductivities. The thermal conductivities parallel and perpendicular to the c-axis orientation of BN-Y3A5 are 17.62 W/(m⋅K) and 154.62 W/(m⋅K), respectively, with a difference of 8.78 times. That of BN-Y3A5M4 are 22.74 W/(m⋅K) and 137.01 W/(m⋅K), respectively, with a difference of 6.03 times. The phase compositions of BN-Y3A5M4 are more complex than that of BN-Y3A5 due to the introduction of MgO, resulting in more complex microstructure that increases phonon scattering. These nanocrystals in BN-Y3A5M4 also have an adverse effect on phonon thermal conduction along the layer. So the thermal diffusivity and thermal conductivity of BN-Y3A5M4 perpendicular to the c-axis orientation are lower than that of BN-Y3A5. But BN-Y3A5M4 has higher thermal diffusivity and thermal conductivity parallel to the c-axis orientation. This is probably due to its higher relative density. The ternary sintering additive 3Y2O3-5Al2O3-4MgO is more beneficial to grain rotation than 3Y2O3-5Al2O3, making h-BN grains interconnected more closely with each other along the c-axis orientation and resulting in relatively higher thermal conductivity. In summary, BN-Y3A5 possesses better directional thermal conduction performance. Compared with our textured h-BN matrix ceramics prepared using mullite as sintering additive under the same condition [5,15], textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives possess slightly lower mechanical properties but higher thermal conductivities.
Fig. 9. Engineering thermal expansion coefficients (Tref = 25 °C) parallel and perpendicular to the c-axis orientation of BN-Y3A5 and BN-Y3A5M4.
Fig. 10. Thermal diffusivities and thermal conductivities parallel and perpendicular to the c-axis orientation of BN-Y3A5 and BN-Y3A5M4 at room temperature.
the elastic moduli, flexural strength and fracture toughness should be various along different loading directions. Fig. 6 gives the mechanical properties of BN-Y3A5 and BN-Y3A5M4 along three different loading directions D1, D2 and D3. The embedded schematic in Fig. 6 shows the specimen cutting method of preparing bars to measure mechanical properties along D1, D2 and D3, where the arrows indicate loading directions. For D1, the specimen bar is parallel to the sintering pressure and the loading is perpendicular to the sintering pressure. As the h-BN grains are oriented with the c-axis parallel to the sintering pressure, the loading direction of D1 is parallel to the h-BN layers. Crack initiation and propagation only need to break the van der Waals forces between layers, leading to the lowest mechanical properties. For D2, the loading direction is parallel to the sintering pressure and perpendicular to the hBN layers. Crack initiation needs to destroy strong sp2 covalent bonds in the layer. Crack propagation needs to go through layers and employ the crack deflection toughening mechanism. For D3, although the loading direction is parallel to the layers, the initial crack is meandrate and the crack propagation needs to break strong sp2 covalent bonds in the layer. So the mechanical properties are much better along D2 and D3 than D1. Fig. 7 shows the fracture morphologies along D1, D2 and D3 of BNY3A5 and BN-Y3A5M4. Fracture morphologies along D1 are more flat than that along D2 and D3, and there are many platelike grains. While fracture morphologies along D2 and D3 are both tortuous. Fig. 8 shows the fracture lateral morphologies along D1, D2 and D3 of BN-Y3A5 and BN-Y3A5M4, where the vertical direction corresponds to the loading direction. The crack propagation paths along D1 and D3 are much straighter than that along D2. But there are many grain pull-out phenomena along D3. There are many crack deflection phenomena along D2, causing the tortuous crack propagation paths. So the mechanical
4. Conclusions (1) Textured h-BN matrix ceramics with anisotropic properties were prepared by hot pressing with 3Y2O3-5Al2O3(-4MgO) as sintering additives. (2) The mechanical properties of textured h-BN matrix ceramics vary greatly in different directions due to the different fracture mechanisms. For BN-Y3A5 and BN-Y3A5M4, the flexural strength along D3 (77.92 ± 5.64 MPa and 84.03 ± 10.28 MPa) is 4.73 times and 4.79 times that along D1 (16.46 ± 0.17 MPa and 17.53 ± 1.33 MPa), respectively. (3) The introduction of MgO lowers the liquid phase formation temperature, facilitates grain rotation and densification, and slightly 7
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
improves the mechanical properties of the composite ceramics. (4) The introduction of 4MgO to 3Y2O3-5Al2O3 promotes the formation of MgAl2O4, resulting in the increase of the mole ratio of Y2O3 to Al2O3 and leading to the formation of YAM instead of YAG and the residue of Y2O3. The more complicated phase compositions and microstructure enhance the scattering effect on the phonons, which is not beneficial to the directional thermal conduction of composite ceramics.
(10) (2013) 4374–4382. [13] P. Jiang, X. Qian, R. Yang, L. Lindsay, Anisotropic thermal transport in bulk hexagonal boron nitride, Phys Rev Mater 2 (6) (2018). [14] T. Kusunose, T. Sekino, Thermal conductivity of hot-pressed hexagonal boron nitride, Scripta Mater. 124 (2016) 138–141. [15] X.M. Duan, M.R. Wang, D.C. Jia, N. Jing, Z.L. Wu, Z.H. Yang, Z. Tian, S.J. Wang, P.G. He, Y.J. Wang, Y. Zhou, Anisotropic mechanical properties and fracture mechanisms of textured h-BN composite ceramics, Mater. Sci. Eng. A 607 (2014) 38–43. [16] J.X. Xue, J.X. Liu, B.H. Xie, G.J. Zhang, Pressure-induced preferential grain growth, texture development and anisotropic properties of hot pressed hexagonal boron nitride ceramics, Scripta Mater. 65 (11) (2011) 966–969. [17] L.R. Vishnyakov, A.V. Maznaya, L.N. Pereselentseva, B.N. Sinaiskii, Structure and high-temperature strength of composite materials based on boron nitride, Powder Metall. Met. Ceram. 45 (5) (2006) 239–243. [18] D.W. Ni, G.J. Zhang, Y.M. Kan, Y. Sakka, Textured h-BN ceramics prepared by slip casting, J. Am. Ceram. Soc. 94 (5) (2011) 1397–1404. [19] X.M. Duan, D.C. Jia, Z.L. Wu, Z. Tian, Z.H. Yang, S.J. Wang, Y. Zhou, Effect of sintering pressure on the texture of hot-press sintered hexagonal boron nitride composite ceramics, Scripta Mater. 68 (2) (2013) 104–107. [20] G. Magnani, G.L. Minoccari, L. Pilotti, Flexural strength and toughness of liquid phase sintered silicon carbide, Ceram. Int. 26 (5) (2000) 495–500. [21] B. Zhang, H. Zhuang, W. Li, Phase evolution of Y-α/β-sialon ceramic with YAG as a sintering additive, J. Mater. Sci. Lett. 20 (2) (2001) 101–103. [22] H.C. Ewing, S. Yang, The effect of precursor composition and sintering additives on the formation of β-sialon from Al, Si and Al2O3 powders, Ceram. Int. 37 (5) (2011) 1667–1673. [23] Z.H. Huang, D.C. Jia, Y. Zhou, Y.G. Liu, A new sintering additive for silicon carbide ceramic, Ceram. Int. 29 (1) (2003) 13–17. [24] X. Zhu, Y. Sakka, Textured silicon nitride: processing and anisotropic properties, Sci. Technol. Adv. Mater. 9 (3) (2008) 033001. [25] A. Motealleh, A.L. Ortiz, O. Borrero-López, F. Guiberteau, Effect of hexagonal-BN additions on the sliding-wear resistance of fine-grained α-SiC densified with Y3Al5O12 liquid phase by spark-plasma sintering, J. Eur. Ceram. Soc. 34 (3) (2014) 565–574. [26] X. He, Y. Guo, Y. Zhou, D. Jia, Microstructures of short-carbon-fiber-reinforced SiC composites prepared by hot-pressing, Mater. Charact. 59 (12) (2008) 1771–1775. [27] M.I. Jones, M.-C. Valecillos, K. Hirao, Y. Yamauchi, Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders, J. Eur. Ceram. Soc. 22 (16) (2002) 2981–2988. [28] A. Noviyanto, D.-H. Yoon, Metal oxide additives for the sintering of silicon carbide: reactivity and densification, Curr. Appl. Phys. 13 (1) (2013) 287–292. [29] M. Hubäĉek, M. Ueki, T. Sató, Orientation and growth of grains in copper‐activated hot-pressed hexagonal boron nitride, J. Am. Ceram. Soc. 79 (1) (1996) 283–285. [30] G.T. Adylov, G.V. Voronov, E.P. Mansurova, L.M. Sigalov, E.M. Urazaeva, Zh. Neorg. Khim. 33 (7) (1988) 1867–1869. [31] G.T. Adylov, E.P. Mansurova, L.M. Sigalov, Dokl. Akad. Nauk UzSSR 4 (1988) 29–31. [32] M.M. Seabaugh, M.D. Vaudin, J.P. Cline, G.L. Messing, Comparison of texture analysis techniques for highly oriented α-Al2O3, J. Am. Ceram. Soc. 83 (8) (2000) 2049–2054. [33] C. Sevik, Assessment on lattice thermal properties of two-dimensional honeycomb structures: graphene, h-BN, h-MoS2, and h-MoSe2, Phys. Rev. B 89 (3) (2014). [34] P. Anees, M.C. Valsakumar, B.K. Panigrahi, Delineating the role of ripples on the thermal expansion of 2D honeycomb materials: graphene, 2D h-BN and monolayer (ML)-MoS2, Phys. Chem. Chem. Phys. 19 (2016) 10518–10526.
Acknowledgement This work was supported by National Natural Science Foundation of China (51672060, 51621091, 51372050). References [1] L. Song, L. Ci, H. Lu, P.B. Sorokin, C. Jin, J. Ni, A.G. Kvashnin, D.G. Kvashnin, J. Lou, B.I. Yakobson, Large scale growth and characterization of atomic hexagonal boron nitride layers, Nano Lett. 10 (8) (2010) 3209–3215. [2] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, Boron nitride nanotubes and nanosheets, ACS Nano 4 (6) (2010) 2979–2993. [3] E.K. Sichel, R.E. Miller, M.S. Abrahams, C.J. Buiocchi, Heat capacity and thermal conductivity of hexagonal pyrolytic boron nitride, Phys. Rev. B 13 (10) (1976) 4607–4611. [4] L. Xiao, W.J. He, Y.S. Yin, First-principles calculations of structural and elastic properties of hexagonal boron nitride, Adv. Mater. Res. 79-82 (2009) 1337–1340. [5] X.M. Duan, Z.H. Yang, L. Chen, Z. Tian, D.L. Cai, Y.J. Wang, D.C. Jia, Y. Zhou, Review on the properties of hexagonal boron nitride matrix composite ceramics, J. Eur. Ceram. Soc. 36 (15) (2016) 3725–3737. [6] A. Wilk, P. Rutkowski, D. Zientara, M.M. Bucko, Aluminium oxynitride-hexagonal boron nitride composites with anisotropic properties, J. Eur. Ceram. Soc. 36 (8) (2016) 2087–2092. [7] W. Paszkowicz, J.B. Pelka, M. Knapp, T. Szyszko, S. Podsiadlo, Lattice parameters and anisotropic thermal expansion of hexagonal boron nitride in the 10-297.5 K temperature range, Appl. Phys. A 75 (3) (2002) 431–435. [8] Z. Su, H. Wang, X. Ye, K. Tian, W. Huang, J. He, Y. Guo, X. Tian, Anisotropic thermally conductive flexible polymer composites filled with hexagonal boron nitride (h-BN) platelets and ammine carbon nanotubes (CNT-NH2): effects of the filler distribution and orientation, Compos Part A: Appl. Sci. Manuf. 109 (2018) 402–412. [9] N. Hamasaki, S. Yamaguchi, S. Use, T. Kawashima, H. Muto, M. Nagao, N. Hozumi, Y. Murakami, Influence of filler orientation and molding temperature on electrical and thermal properties of PMMA/h-BN composite material produced by electrostatic adsorption method, International Symposium on Electrical Insulating Materials, (2017), pp. 485–488. [10] S. Takahashi, Y. Imai, A. Kan, Y. Hotta, H. Ogawa, Dielectric and thermal properties of isotactic polypropylene/hexagonal boron nitride composites for high-frequency applications, J. Alloys. Compd. 615 (2014) 141–145. [11] Y. Hattori, T. Taniguchi, K. Watanabe, K. Nagashio, Anisotropic dielectric breakdown strength of single crystal hexagonal boron nitride, ACS Appl. Mater. Interfaces 8 (41) (2016). [12] M. Tanimoto, T. Yamagata, K. Miyata, S. Ando, Anisotropic thermal diffusivity of hexagonal boron nitride-filled polyimide films: effects of filler particle size, aggregation, orientation, and polymer chain rigidity, ACS Appl. Mater. Interfaces 5
8
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives ⁎
Zhuo Zhanga,b, Xiaoming Duana,b,c, , Baofu Qiua,b, Zhihua Yanga,b,c, Delong Caia,b, Peigang Hea,b, ⁎ Dechang Jiaa,b,c, , Yu Zhoua,b a
Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin Institute of Technology, Harbin, 150001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hexagonal boron nitride Liquid phase sintering Texture Anisotropic properties
Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y2O35Al2O3 (mole ratio of 3:5) and 3Y2O3-5Al2O3-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y2O3-5Al2O3-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y2O3-5Al2O3 is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.
1. Introduction
significantly related with the following factors:
Hexagonal boron nitride (h-BN) has a graphite-like layered structure, where B and N atoms in the same layer are combined by covalent bonds with sp2 hybridization, while different layers are combined by van der Waals forces [1–3]. The physical properties of the h-BN crystal, such as elastic moduli, thermal conductivities, thermal expansion coefficients, electric conductivities and dielectric constants, show obvious anisotropy parallel and perpendicular to the c-axis [4–12]. For instance, the theoretical thermal conductivities parallel and perpendicular to the c-axis of bulk h-BN single crystal were calculated to be 4.1 W/(m⋅K) and 537 W/(m⋅K), respectively [13]. If h-BN grains can be preferentially aligned during ceramic preparation, textured bulk ceramics possessing anisotropic properties can be prepared [14,15]. The texturing of h-BN matrix ceramics can significantly improve their performances along specific direction and expand their application fields, for example, as heat sinks for semiconductor parts [16]. Textured h-BN matrix ceramics are usually prepared by hot pressing using liquid phase sintering additives [5,17]. The liquid phase formed at high temperature provides an environment for grain rotation. Generally, h-BN grains can be oriented with the c-axis parallel to the sintering pressure due to the lamellar structure. The texture degree is
(1) Morphology of raw powders. Compared with irregularly shaped raw powders, platelike h-BN raw powders are more beneficial to grain rotation under sintering pressure [18]. (2) Sintering pressure. High pressure is beneficial to grain rotation and texture formation, resulting in significantly anisotropic mechanical properties [15,19]. (3) Sintering additives selection. The good wettability of sintering additives to h-BN grains can promote grain rotation when the uniaxial pressing is applied, which is favorable to texture formation [14]. However, hot pressing does not necessarily lead to this kind of texture microstructures. When raw h-BN powders with a low degree of order and a broad particle size distribution are sintered at high temperature and moderate pressure without sintering additives, the moderate pressure isn’t high enough to rotate h-BN grains but can promote the contact of h-BN grains along the pressure, accelerating platelike grain growth along this direction and forming texture microstructures where the c-axis orientation is perpendicular to the pressure. But without grain rotation in liquid phase environment, the texture degree won’t be high [16].
⁎ Corresponding authors at: Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (X. Duan), [email protected] (D. Jia).
https://doi.org/10.1016/j.jeurceramsoc.2019.01.003 Received 17 October 2018; Received in revised form 27 December 2018; Accepted 3 January 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, Z., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.01.003
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
2. Materials and methods of preparation and characterization The raw materials used in our study are h-BN powders with platelike morphology (15 μm in diameter and 0.3 μm in thickness, purity > 99%, Qingzhou Fangyuan Boron Nitride Factory, Weifang, China), Al2O3 powders (˜50 nm, purity > 99.5%, Showa Denko K.K., Yokohama, Japan), Y2O3 powders (˜80 nm, purity > 99.9%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and MgO powders (˜100 nm, purity > 99.9%, Xuancheng Jingrui New Material Co., Ltd, Xuancheng, China). 3Y2O3-5Al2O3 (mole ratio of 3:5) and 3Y2O3-5Al2O3-4MgO (mole ratio of 3:5:4) were used as sintering additives, respectively. The mass ratio of h-BN to the sintering additive was 8:2. Raw powders were mixed by ball milling with Al2O3 balls in ethanol on a pot mill machine with a speed of 80 rpm for 24 h and then dried at 70 °C. The mixed powders were loaded into cylindrical graphite die and hot pressed at 1900 °C (with the heating and cooling rate of 20 °C/min) under the sintering pressure of 30 MPa in N2 atmosphere for 1 h. The specimen sintered using 3Y2O3-5Al2O3 as the sintering additive was marked as BN-Y3A5. The specimen sintered using 3Y2O3-5Al2O34MgO as the sintering additive was marked as BN-Y3A5M4. Apparent densities of the sintered specimens were measured by Archimedes method. The crystallographic orientation and phase compositions of the specimens were analyzed by X-ray diffraction (XRD; RTP 300, Rigaku, Japan). Pole figures were provided by X-ray diffraction (XRD; D8 discover, Bruker, Germany). Phase identification was realized by transmission electron microscopy (TEM; Talos F200X, FEI, USA). Mechanical properties including elastic moduli, flexural strength and fracture toughness were measured along three different loading directions by mechanical strength testing machine (Model 5569, Instron, USA). Flexural strength was measured on bars (3 mm × 4 mm × 36 mm) by three-point bending test with a span of 30 mm and a traverse feed of 0.5 mm/min. Elastic moduli were measured by strain gauges stuck on the tensile surfaces of these bars. Fracture toughness was measured on single-edge-notched beams (2 mm × 4 mm × 20 mm) by three-point bending test with a span of 16 mm and a traverse feed of 0.05 mm/min. Microstructures of fracture surfaces were observed by scanning electron microscopy (SEM; Quanta 200 FEG, FEI, USA). Thermal expansion coefficients were measured on bars (5 mm × 5 mm × 25 mm) by dilatometer (DTL 402C, Netzsch, Germany) with temperature ranging from 20 °C to 1205 °C and heating rate of 5 °C/min in Ar atmosphere. Thermal diffusivities were measured on discs (Φ 12.7 mm × 2˜3 mm) by laser flash analysis method (LFA 427, Netzsch, Germany) with laser voltage of 550 V and pulse width of 0.8 ms in Ar atmosphere. Specific heat was measured using differential scanning calorimetry (DSC STA449F3, Netzsch, Germany).
Fig. 1. XRD patterns for TS sample direction and SS sample direction of (a) BNY3A5 and (b) BN-Y3A5M4.
3Y2O3-5Al2O3 can form Y3Al5O12 (YAG) liquid phase at high temperature, which has been widely used as the sintering additive for SiC, Si3N4 and SiAlON ceramics [20–25]. Besides, MgO can also be introduced into Y2O3-Al2O3 to obtain materials with different properties by forming different liquid phases during sintering and controlling phase compositions [26–28]. But these two sintering additive systems have seldom been used in h-BN matrix ceramics. In this work, two textured h-BN matrix ceramic specimens were hot pressed using 3Y2O3-5Al2O3 and 3Y2O3-5Al2O3-4MgO as sintering additives, respectively. The phase compositions, texture characteristics of the two textured specimens, and their anisotropic mechanical and thermal properties were investigated.
Fig. 2. TEM images of BN-Y3A5: (a) TEM bright field image, (b) and (c) diffraction patterns of h-BN and YAG in (a). 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 3. TEM images of BN-Y3A5: (a–f) HAADF-STEM image and EDS elemental maps of one triangular pore between h-BN grains; (g) EDS line scan results along the green arrow marked in (a); (h) HRTEM image of the interface between h-BN and YAG (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
3. Results and discussion
XRD patterns. BN-Y3A5 is composed of h-BN and YAG, as the mole ratio of Y2O3 to Al2O3 is 3:5. BN-Y3A5M4 is mainly composed of h-BN, Y4Al2O9 (YAM) and MgAl2O4. The phase compositions of BN-Y3A5M4 are more complex than that of BN-Y3A5 due to the introduction of MgO. The formation of MgAl2O4 leads to the increase of the mole ratio of Y2O3 to Al2O3. So YAM appears instead of YAG. According to the phase compositions, the theoretical densities of BN-Y3A5 and BNY3A5M4 were estimated to be 2.533 g/cm3 and 2.501 g/cm3, respectively, and the relative densities of BN-Y3A5 and BN-Y3A5M4 are 88.66% and 89.62%, respectively. Fig. 2 shows TEM bright field image and diffraction patterns of BNY3A5, which demonstrates the formation of YAG. The zone axes of diffraction patterns in Fig. 2(b) and (c) are [001] of h-BN and [110] of
3.1. Phase compositions and texture characteristics The apparent densities of BN-Y3A5 and BN-Y3A5M4 are 2.246 g/ cm3 and 2.241 g/cm3, respectively. The top surfaces of the sintered specimens are perpendicular to the applied pressure during hotpressing process and marked as TS, and the side surfaces parallel to the applied pressure during hot pressing process are marked as SS. Fig. 1 shows the XRD patterns for TS sample direction and SS sample direction of BN-Y3A5 and BN-Y3A5M4. The contrast between the diffraction intensities of (002) peak and (100) peak can be seen in the illustrations. There is no sign of reaction between BN and sintering additives from 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 4. TEM images of BN-Y3A5M4: (a) TEM bright field image; (b) HRTEM image of the interface between h-BN and sintering additive area; (c) HRTEM image of sintering additive area; (d) FFT of (c).
XRD patterns. For SS sample direction, (100) peaks are higher than (002) peaks. The noticeable (002) peaks for TS sample direction of both specimens indicate that h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure. For textured h-BN matrix ceramics, the preferred orientation of hBN grains leads to the change of intensities of diffraction peaks. So the texture degree of hot pressed h-BN matrix ceramics can be quantitively characterized by the index of orientation preference (IOP) values calculated from XRD patterns [16,29]:
YAG, respectively. Fig. 3(a–f) show high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and energy dispersion spectrum (EDS) elemental maps of one triangular pore between h-BN grains. Y, Al and O elements are distributed on h-BN grain boundaries, which illustrates the formation of liquid phase possessing good wettability to h-BN grains. Fig. 3(g) gives EDS line scan results along the green arrow marked in (a), where peaks of Y, Al and O elements appear at h-BN grain boundaries. Fig. 3(h) shows high resolution transmission electron microscopy (HRTEM) image of the interface between h-BN and YAG, where d(100) of h-BN and d(002) of YAG are marked. h-BN and YAG are tightly integrated at the interface. Fig. 4(a) shows TEM bright field image of BN-Y3A5M4, where the diffraction pattern of h-BN is shown in the illustration. There are many nanocrystals at sintering additive area. Fig. 4(b) shows HRTEM image of the interface between h-BN and sintering additive area, where d(100) of h-BN is marked. Fig. 4(c) and (d) shows HRTEM image of sintering additive area and the corresponding FFT, which demonstrates the existence In Fig. 1, the (002) peaks for TS sample direction of the two specimens are too high that they are not fully presented. The contrast between (002) peaks and other diffraction peaks for TS sample direction can be seen from the thumbnails shown on the top right side of the
(I
IOP =
/I
⎧ (I ′100 / I002 ′ 100
)TS
002)SS
, When(I100/ I002)TS > (I ′100/ I ′002)SS
⎨ (I ′100 / I ′002)SS , ⎩ (I100 / I002)TS
When(I100/ I002)TS < (I ′100/ I ′002)SS
(1)
Where I and I' are the intensities of corresponding diffraction peaks on TS sample direction and SS sample direction, respectively. For a sintered h-BN specimen composed of randomly oriented grains, IOP = ± 1. If h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure, IOP < -1. If h-BN grains are preferentially oriented with the c-axis perpendicular to the sintering pressure, IOP > 1. Obviously, the larger absolute value of IOP indicates the more significant preferred orientation. 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 5. Pole figures and ODF cross sections for BN-Y3A5 and BN-Y3A5M4: (a) and (b) h-BN (002) pole figures of TS sample direction for BN-Y3A5 and BN-Y3A5M4, respectively; (c) and (d) ODF cross sections with φ2 = 0°, 30° and 60° for BN-Y3A5 and BN-Y3A5M4, respectively.
The IOP values of BN-Y3A5 and BN-Y3A5M4 are calculated to be -530 and -976, respectively, indicating that BN-Y3A5M4 possesses stronger preferred orientation than BN-Y3A5. The eutectic point of the ternary sintering additive Y2O3-Al2O3-MgO was reported to be 80 °C lower than that of the binary sintering additive Y2O3-Al2O3 [30,31], so h-BN grains are more easily to be rotated by the sintering pressure in 3Y2O3-5Al2O3-4MgO sintering additive than in 3Y2O3-5Al2O3 sintering additive, resulting in higher absolute value of IOP for BN-Y3A5M4 than BN-Y3A5. Fig. 5 shows the h-BN (002) pole figures of TS sample direction and orientation distribution function (ODF) cross section with φ2 = 0°, 30° and 60° for BN-Y3A5 and BN-Y3A5M4. Both the two pole figures are composed of irregular concentric circles with different colors. The numbers corresponding to these colors represent multiples of a random distribution (MRD) defined as the volume fraction of grains with certain
Fig. 6. Mechanical properties of BN-Y3A5 and BN-Y3A5M4 along D1, D2 and D3.
5
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
Fig. 7. Fracture morphologies along D1, D2, D3 of (a), (b), (c) BN-Y3A5 and (d), (e), (f) BN-Y3A5M4, respectively.
Fig. 8. Fracture lateral morphologies along D1, D2, D3 of (a), (b), (c) BN-Y3A5 and (d), (e), (f) BN-Y3A5M4, respectively.
contour lines, where φ represents the angle between the c-axis of h-BN grains and the normal of TS, φ1 represents the angle between projection of the c-axis of h-BN grains on TS and the rolling direction (For hot pressing, the rolling direction can be randomly selected on TS), φ2 represents the rotation angle of h-BN grains around the c-axis. MRD values increase as φ approaches 0°, and the MRD value near φ = 0° for BN-Y3A5M4 is higher than that for BN-Y3A5, which is consistent with the results reflected by pole figures.
orientation in the textured specimen divided by that in the sample composed of randomly oriented grains [32]. In the h-BN (002) pole figures of TS sample direction for BN-Y3A5 and BN-Y3A5M4, the closer to the center of the map, the greater the MRD values, indicating that MRD values increase as the angle between the c-axis of h-BN grains and the normal of TS (the sintering pressure) approaches 0°. So h-BN grains are preferentially oriented with the c-axis parallel to the sintering pressure. The h-BN (002) pole figure of TS sample direction for BNY3A5M4 has higher MRD value in the center than that for BN-Y3A5, indicating that BN-Y3A5M4 has higher texture degree than BN-Y3A5. ODF cross sections also reflect the same results. These ODF sections are calculated from several pole figures and composed of a series of MRD
3.2. Anisotropic properties Due to the preferred orientation of textured h-BN matrix ceramics, 6
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
properties along D2 and D3 are higher than that along D1. The mechanical properties of BN-Y3A5M4 are slightly better than that of BN-Y3A5, especially along D2 and D3. The lower liquid phase formation temperature of 3Y2O3-5Al2O3-4MgO than 3Y2O3-5Al2O3 results in the more significant preferred orientation of BN-Y3A5M4, which leads to its more effective crack deflection along D2 and D3, resulting in its better mechanical properties than BN-Y3A5. Fig. 9 gives the engineering thermal expansion coefficients (Tref = 25 °C) parallel and perpendicular to the c-axis orientation of BNY3A5 and BN-Y3A5M4. The thermal expansion coefficients (25 °C ˜ 1200 °C) parallel and perpendicular to the c-axis orientation of BNY3A5 are 10.35 × 10−6/K and 2.49 × 10−6/K, respectively. That of BN-Y3A5M4 are 11.27 × 10−6/K and 2.40 × 10−6/K, respectively. Similar to graphite, h-BN possesses small negative thermal expansion coefficient perpendicular to the c-axis at low temperature [33,34]. It has been reported that the thermal expansion coefficient perpendicular to the c-axis is -2.72 × 10−6/K at room temperature and approaches zero at high temperature, while that along the c-axis is 37.7 × 10−6/K at room temperature and tends to be invariable at high temperature [7]. So textured h-BN matrix ceramics possess higher thermal expansion coefficient parallel to the c-axis orientation than perpendicular to the caxis orientation. The anisotropy of thermal expansion coefficients of BN-Y3A5M4 is more significant than BN-Y3A5. This is related to their texture microstructures. BN-Y3A5M4 has higher texture degree, leading to its more significantly anisotropic thermal expansion coefficients. Fig. 10 gives the anisotropic thermal diffusivities and thermal conductivities of BN-Y3A5 and BN-Y3A5M4 at room temperature. Although BN-Y3A5M4 has higher texture degree, BN-Y3A5 has more significantly anisotropic thermal diffusivities and thermal conductivities. The thermal conductivities parallel and perpendicular to the c-axis orientation of BN-Y3A5 are 17.62 W/(m⋅K) and 154.62 W/(m⋅K), respectively, with a difference of 8.78 times. That of BN-Y3A5M4 are 22.74 W/(m⋅K) and 137.01 W/(m⋅K), respectively, with a difference of 6.03 times. The phase compositions of BN-Y3A5M4 are more complex than that of BN-Y3A5 due to the introduction of MgO, resulting in more complex microstructure that increases phonon scattering. These nanocrystals in BN-Y3A5M4 also have an adverse effect on phonon thermal conduction along the layer. So the thermal diffusivity and thermal conductivity of BN-Y3A5M4 perpendicular to the c-axis orientation are lower than that of BN-Y3A5. But BN-Y3A5M4 has higher thermal diffusivity and thermal conductivity parallel to the c-axis orientation. This is probably due to its higher relative density. The ternary sintering additive 3Y2O3-5Al2O3-4MgO is more beneficial to grain rotation than 3Y2O3-5Al2O3, making h-BN grains interconnected more closely with each other along the c-axis orientation and resulting in relatively higher thermal conductivity. In summary, BN-Y3A5 possesses better directional thermal conduction performance. Compared with our textured h-BN matrix ceramics prepared using mullite as sintering additive under the same condition [5,15], textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives possess slightly lower mechanical properties but higher thermal conductivities.
Fig. 9. Engineering thermal expansion coefficients (Tref = 25 °C) parallel and perpendicular to the c-axis orientation of BN-Y3A5 and BN-Y3A5M4.
Fig. 10. Thermal diffusivities and thermal conductivities parallel and perpendicular to the c-axis orientation of BN-Y3A5 and BN-Y3A5M4 at room temperature.
the elastic moduli, flexural strength and fracture toughness should be various along different loading directions. Fig. 6 gives the mechanical properties of BN-Y3A5 and BN-Y3A5M4 along three different loading directions D1, D2 and D3. The embedded schematic in Fig. 6 shows the specimen cutting method of preparing bars to measure mechanical properties along D1, D2 and D3, where the arrows indicate loading directions. For D1, the specimen bar is parallel to the sintering pressure and the loading is perpendicular to the sintering pressure. As the h-BN grains are oriented with the c-axis parallel to the sintering pressure, the loading direction of D1 is parallel to the h-BN layers. Crack initiation and propagation only need to break the van der Waals forces between layers, leading to the lowest mechanical properties. For D2, the loading direction is parallel to the sintering pressure and perpendicular to the hBN layers. Crack initiation needs to destroy strong sp2 covalent bonds in the layer. Crack propagation needs to go through layers and employ the crack deflection toughening mechanism. For D3, although the loading direction is parallel to the layers, the initial crack is meandrate and the crack propagation needs to break strong sp2 covalent bonds in the layer. So the mechanical properties are much better along D2 and D3 than D1. Fig. 7 shows the fracture morphologies along D1, D2 and D3 of BNY3A5 and BN-Y3A5M4. Fracture morphologies along D1 are more flat than that along D2 and D3, and there are many platelike grains. While fracture morphologies along D2 and D3 are both tortuous. Fig. 8 shows the fracture lateral morphologies along D1, D2 and D3 of BN-Y3A5 and BN-Y3A5M4, where the vertical direction corresponds to the loading direction. The crack propagation paths along D1 and D3 are much straighter than that along D2. But there are many grain pull-out phenomena along D3. There are many crack deflection phenomena along D2, causing the tortuous crack propagation paths. So the mechanical
4. Conclusions (1) Textured h-BN matrix ceramics with anisotropic properties were prepared by hot pressing with 3Y2O3-5Al2O3(-4MgO) as sintering additives. (2) The mechanical properties of textured h-BN matrix ceramics vary greatly in different directions due to the different fracture mechanisms. For BN-Y3A5 and BN-Y3A5M4, the flexural strength along D3 (77.92 ± 5.64 MPa and 84.03 ± 10.28 MPa) is 4.73 times and 4.79 times that along D1 (16.46 ± 0.17 MPa and 17.53 ± 1.33 MPa), respectively. (3) The introduction of MgO lowers the liquid phase formation temperature, facilitates grain rotation and densification, and slightly 7
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Z. Zhang et al.
improves the mechanical properties of the composite ceramics. (4) The introduction of 4MgO to 3Y2O3-5Al2O3 promotes the formation of MgAl2O4, resulting in the increase of the mole ratio of Y2O3 to Al2O3 and leading to the formation of YAM instead of YAG and the residue of Y2O3. The more complicated phase compositions and microstructure enhance the scattering effect on the phonons, which is not beneficial to the directional thermal conduction of composite ceramics.
(10) (2013) 4374–4382. [13] P. Jiang, X. Qian, R. Yang, L. Lindsay, Anisotropic thermal transport in bulk hexagonal boron nitride, Phys Rev Mater 2 (6) (2018). [14] T. Kusunose, T. Sekino, Thermal conductivity of hot-pressed hexagonal boron nitride, Scripta Mater. 124 (2016) 138–141. [15] X.M. Duan, M.R. Wang, D.C. Jia, N. Jing, Z.L. Wu, Z.H. Yang, Z. Tian, S.J. Wang, P.G. He, Y.J. Wang, Y. Zhou, Anisotropic mechanical properties and fracture mechanisms of textured h-BN composite ceramics, Mater. Sci. Eng. A 607 (2014) 38–43. [16] J.X. Xue, J.X. Liu, B.H. Xie, G.J. Zhang, Pressure-induced preferential grain growth, texture development and anisotropic properties of hot pressed hexagonal boron nitride ceramics, Scripta Mater. 65 (11) (2011) 966–969. [17] L.R. Vishnyakov, A.V. Maznaya, L.N. Pereselentseva, B.N. Sinaiskii, Structure and high-temperature strength of composite materials based on boron nitride, Powder Metall. Met. Ceram. 45 (5) (2006) 239–243. [18] D.W. Ni, G.J. Zhang, Y.M. Kan, Y. Sakka, Textured h-BN ceramics prepared by slip casting, J. Am. Ceram. Soc. 94 (5) (2011) 1397–1404. [19] X.M. Duan, D.C. Jia, Z.L. Wu, Z. Tian, Z.H. Yang, S.J. Wang, Y. Zhou, Effect of sintering pressure on the texture of hot-press sintered hexagonal boron nitride composite ceramics, Scripta Mater. 68 (2) (2013) 104–107. [20] G. Magnani, G.L. Minoccari, L. Pilotti, Flexural strength and toughness of liquid phase sintered silicon carbide, Ceram. Int. 26 (5) (2000) 495–500. [21] B. Zhang, H. Zhuang, W. Li, Phase evolution of Y-α/β-sialon ceramic with YAG as a sintering additive, J. Mater. Sci. Lett. 20 (2) (2001) 101–103. [22] H.C. Ewing, S. Yang, The effect of precursor composition and sintering additives on the formation of β-sialon from Al, Si and Al2O3 powders, Ceram. Int. 37 (5) (2011) 1667–1673. [23] Z.H. Huang, D.C. Jia, Y. Zhou, Y.G. Liu, A new sintering additive for silicon carbide ceramic, Ceram. Int. 29 (1) (2003) 13–17. [24] X. Zhu, Y. Sakka, Textured silicon nitride: processing and anisotropic properties, Sci. Technol. Adv. Mater. 9 (3) (2008) 033001. [25] A. Motealleh, A.L. Ortiz, O. Borrero-López, F. Guiberteau, Effect of hexagonal-BN additions on the sliding-wear resistance of fine-grained α-SiC densified with Y3Al5O12 liquid phase by spark-plasma sintering, J. Eur. Ceram. Soc. 34 (3) (2014) 565–574. [26] X. He, Y. Guo, Y. Zhou, D. Jia, Microstructures of short-carbon-fiber-reinforced SiC composites prepared by hot-pressing, Mater. Charact. 59 (12) (2008) 1771–1775. [27] M.I. Jones, M.-C. Valecillos, K. Hirao, Y. Yamauchi, Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders, J. Eur. Ceram. Soc. 22 (16) (2002) 2981–2988. [28] A. Noviyanto, D.-H. Yoon, Metal oxide additives for the sintering of silicon carbide: reactivity and densification, Curr. Appl. Phys. 13 (1) (2013) 287–292. [29] M. Hubäĉek, M. Ueki, T. Sató, Orientation and growth of grains in copper‐activated hot-pressed hexagonal boron nitride, J. Am. Ceram. Soc. 79 (1) (1996) 283–285. [30] G.T. Adylov, G.V. Voronov, E.P. Mansurova, L.M. Sigalov, E.M. Urazaeva, Zh. Neorg. Khim. 33 (7) (1988) 1867–1869. [31] G.T. Adylov, E.P. Mansurova, L.M. Sigalov, Dokl. Akad. Nauk UzSSR 4 (1988) 29–31. [32] M.M. Seabaugh, M.D. Vaudin, J.P. Cline, G.L. Messing, Comparison of texture analysis techniques for highly oriented α-Al2O3, J. Am. Ceram. Soc. 83 (8) (2000) 2049–2054. [33] C. Sevik, Assessment on lattice thermal properties of two-dimensional honeycomb structures: graphene, h-BN, h-MoS2, and h-MoSe2, Phys. Rev. B 89 (3) (2014). [34] P. Anees, M.C. Valsakumar, B.K. Panigrahi, Delineating the role of ripples on the thermal expansion of 2D honeycomb materials: graphene, 2D h-BN and monolayer (ML)-MoS2, Phys. Chem. Chem. Phys. 19 (2016) 10518–10526.
Acknowledgement This work was supported by National Natural Science Foundation of China (51672060, 51621091, 51372050). References [1] L. Song, L. Ci, H. Lu, P.B. Sorokin, C. Jin, J. Ni, A.G. Kvashnin, D.G. Kvashnin, J. Lou, B.I. Yakobson, Large scale growth and characterization of atomic hexagonal boron nitride layers, Nano Lett. 10 (8) (2010) 3209–3215. [2] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, Boron nitride nanotubes and nanosheets, ACS Nano 4 (6) (2010) 2979–2993. [3] E.K. Sichel, R.E. Miller, M.S. Abrahams, C.J. Buiocchi, Heat capacity and thermal conductivity of hexagonal pyrolytic boron nitride, Phys. Rev. B 13 (10) (1976) 4607–4611. [4] L. Xiao, W.J. He, Y.S. Yin, First-principles calculations of structural and elastic properties of hexagonal boron nitride, Adv. Mater. Res. 79-82 (2009) 1337–1340. [5] X.M. Duan, Z.H. Yang, L. Chen, Z. Tian, D.L. Cai, Y.J. Wang, D.C. Jia, Y. Zhou, Review on the properties of hexagonal boron nitride matrix composite ceramics, J. Eur. Ceram. Soc. 36 (15) (2016) 3725–3737. [6] A. Wilk, P. Rutkowski, D. Zientara, M.M. Bucko, Aluminium oxynitride-hexagonal boron nitride composites with anisotropic properties, J. Eur. Ceram. Soc. 36 (8) (2016) 2087–2092. [7] W. Paszkowicz, J.B. Pelka, M. Knapp, T. Szyszko, S. Podsiadlo, Lattice parameters and anisotropic thermal expansion of hexagonal boron nitride in the 10-297.5 K temperature range, Appl. Phys. A 75 (3) (2002) 431–435. [8] Z. Su, H. Wang, X. Ye, K. Tian, W. Huang, J. He, Y. Guo, X. Tian, Anisotropic thermally conductive flexible polymer composites filled with hexagonal boron nitride (h-BN) platelets and ammine carbon nanotubes (CNT-NH2): effects of the filler distribution and orientation, Compos Part A: Appl. Sci. Manuf. 109 (2018) 402–412. [9] N. Hamasaki, S. Yamaguchi, S. Use, T. Kawashima, H. Muto, M. Nagao, N. Hozumi, Y. Murakami, Influence of filler orientation and molding temperature on electrical and thermal properties of PMMA/h-BN composite material produced by electrostatic adsorption method, International Symposium on Electrical Insulating Materials, (2017), pp. 485–488. [10] S. Takahashi, Y. Imai, A. Kan, Y. Hotta, H. Ogawa, Dielectric and thermal properties of isotactic polypropylene/hexagonal boron nitride composites for high-frequency applications, J. Alloys. Compd. 615 (2014) 141–145. [11] Y. Hattori, T. Taniguchi, K. Watanabe, K. Nagashio, Anisotropic dielectric breakdown strength of single crystal hexagonal boron nitride, ACS Appl. Mater. Interfaces 8 (41) (2016). [12] M. Tanimoto, T. Yamagata, K. Miyata, S. Ando, Anisotropic thermal diffusivity of hexagonal boron nitride-filled polyimide films: effects of filler particle size, aggregation, orientation, and polymer chain rigidity, ACS Appl. Mater. Interfaces 5
8