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Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine ⁎
Shitao Gao, Bin Li , Duan Li, Changrui Zhang, Rongjun Liu, Siqing Wang Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical vapor deposition Pyrolytic boron nitride Morphology Structure Mechanism
Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900 °C and total pressures of 50–1000 Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The asdeposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900 °C with relative low pressures of 50–200 Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100 °C with higher pressures above 250 Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d(002)) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d(002). The crystal growth mechanism of the three types of PBN was discussed.
1. Introduction Pyrolytic boron nitride (PBN) possesses plenty of remarkable performances such as extreme chemical inertness, high dielectric strength, excellent thermal shock resistance, non-wetting, non-toxic, oxidation resistance and negligible outgassing [1,2]. It has highly ordered planar texture analogous to pyrolytic graphite (PG) which exhibits evident anisotropic properties such as lower dielectric constant vertical to the crystal plane and higher bending strength along the crystal plane [3,4]. Therefore, PBN material has been widely manufactured as crucibles of compound semiconductor crystals, output windows and dielectric rods of traveling-wave tubes, high-temperature jigs and insulators [5–7]. Furthermore, in the incoming space exploration of Solar Probe Plus (SPP, NASA), PBN has been tested as the candidate material for the protective coating of the heat shield on the solar probe, not only for its excellent thermal and chemical stability in vacuum, but also for its high emissivity and reflectivity in the solar spectrum which would reduce the surface temperature of the spacecraft [8]. The synthesis of PBN has been carried out by a number of researchers using various reaction systems by chemical vapor deposition (CVD) [9–11]. The commercial production of PBN is based on the pyrolysis of BCl3 and NH3 at temperatures of 1900–2100 °C and pressures below 1 Torr in a hot-wall thermally insulated furnace [12,13]. In order to obtain stoichiometric PBN, the flow rate and relative ratio of
⁎
the reactants must be controlled precisely. Moreover, complicated simulation and verification of the gas flow field are need in order to realize the intensive mixing of the reactants gases and the obtaining of homogeneous products. Therefore, this process had a high production cost and several disadvantages such as low yield, long deposition duration, and high deposition temperature and energy consumption. There are also problems of the corrosive and toxic substances both in the precursors and by-products which may corrode the device and pollute the environment [14]. Borazine (B3N3H6), which possesses the same stoichiometric proportion of B/N with hexagonal boron nitride (h-BN), provides a potential alternative to the conventional CVD precursors [15]. It is isostructural and isoelectronic with benzene and has no impurity element except hydrogen [16,17]. Moreover, borazine is liquid at room temperature and has a relatively high vapor pressure [18]. Lots of investigations have been reported on the CVD of h-BN monolayer and nanosheet by using borazine as precursor in the last decade [19–21]. Borazine is also reported to prepare thin films about hundreds of nanometers thick by plasma enhanced (PE)CVD [22,23]. However, there has been little study on the preparation of PBN with the thickness of millimeter scale using borazine as the precursor. There must be several issues generated when extending the two-dimensional h-BN monolayer or film to the three-dimensional PBN [18]. In this work, we report the CVD of PBN using borazine as the precursor in temperatures of
Corresponding author. E-mail address: [email protected] (B. Li).
https://doi.org/10.1016/j.ceramint.2018.03.201 Received 2 March 2018; Accepted 22 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Gao, S., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.201
Ceramics International xxx (xxxx) xxx–xxx
S. Gao et al.
own axis during the reaction. The borazine used in this work was produced and purified with trap to trap distillation according to Li's report [24]. Nitrogen was passed into a bubbler which contained the precursor to carry the borazine vapor into the reaction chamber. The temperature of bubbler was kept at 0 °C at which the borazine had a vapor pressure of 11.2kPa during the deposition step. At the same time, the total pressure of the bubbler was conducted at a constant value in order to keep the borazine concentration of the gas mixture controllable. Then the gas mixture was transported into the reaction chamber with 1slm measured by gas mass flowmeter. The total gas pressure of the reactor was maintained at a constant pressure with an automatic pressure controller valve attached to the exhaust side. The deposition pressure was varied in the range of 50–1000 Pa, and the deposition temperature was performed in the range of 900–1900 °C. The as-deposited PBN was peel off from the graphite disc with thickness about 0.2–1 mm. Fig. 1. Effect of deposition temperature and pressure on the morphology of the PBN. Type A: product with dense and obviously laminated structure; Type B: product with dense and glass-like structure; Type C: product with porous and isotropic structure.
2.2. Characterization of the PBN The micromorphology of the PBN were observed by field emission scanning electron microscopy (FESEM, Hitachi S4800-Ⅱ). Due to the insulating property of h-BN, the sample surface was sputtered with a gold film previous to the SEM imaging. The crystallization state of PBN were characterized by X-ray diffraction (XRD, Bruker ADVANCED D8 diffractometer, Germany) at a wavelength of 1.5418 Å (Cu Kα radiation). The microstructure of the PBN was examined by transmission electron microscopy (TEM, FEI Tecnai G2 F20) analyses. The simples were prepared according to a simple manner by mechanical grinding and ultrasonic dispersion.
900–1900 °C and total pressures of 50–1000 Pa. The micromorphological and crystal structural characterizations of the PBN are presented. 2. Material and methods 2.1. Synthesis of the PBN The synthesis of PBN was carried out in a vertical hot-wall CVD system. The graphite disc (Φ = 100 mm) polished as substrate was fixed to the mandrel in the deposition chamber which would revolve on its
Fig. 2. The deposition surfaces of PBN synthesized at 900 °C with different total pressures: (a), (b) 200 Pa; (c), (d) 1000 Pa. 2
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Fig. 3. The deposition surfaces of PBN synthesized at 1600 °C with different pressures: (a) 50 Pa; (b) 100 Pa; (c) 200 Pa;(d) 300 Pa.
3. Results and discussion
that the grains tend to be smaller with higher deposition pressure. Simultaneously, the PBN presented smoother surface with a higher pressure of 1000 Pa as shown in Fig. 2(c) and (d). The morphologies of PBN deposited at relativity higher temperatures above 1150 °C were eventually separated into three types of structure from A to C with the increase of pressure as shown in Fig. 1. The deposition surface of the PBN obtained at temperature of 1600 °C and pressures ranging from 50 to 300 Pa are presented in the SEM images of Fig. 3. Fig. (a) and (b) both are the PBN of Type A, while Fig. 3(c) and (d) belong to Type B and Type C, respectively. Similar with the result in Fig. 2, it was observed that the PBN was composed of pebble-like particles on all the deposition surfaces in Fig. 3. The particles of PBN deposited at 50 Pa and 100 Pa had a relative large size of more than 5 µm in diameter, and revealed relatively flat surfaces as shown in Fig. 3(a) and (b). However, the surface became rougher and the thickness of PBN plates tended to be non-uniform as deposition pressure increased. The size of the pebble-like particles became irregular and smaller of 1–3 µm in diameter with the increase of pressure as shown in Fig. 3(c) and (d). Meanwhile, the boundary of the fine spherical grains which composed the pebble-like particles looked gradually clear. The grains had an apparent diameter of 500 nm, 300 nm and 100 nm approximately at deposition pressure of 100 Pa, 200 Pa and 300 Pa respectively, which tended to be smaller in size with the increase of pressure as shown in Fig. 3(b) to (d). Moreover, the grains even showed a similar size order with the pebble-like particles deposited at 900 °C as presented in Fig. 2. The SEM photographs of Fig. 4(a) to (d) are the fracture surfaces in corresponding with the deposition surfaces in Fig. 3(a) to (d), respectively. The PBN of Type A revealed an layer structure and the layers aligned parallel to the deposition surface as shown in Fig. 4(a) and (b). Simultaneously, some flaky fragments and gaps were observed on the fracture surface in Fig. 4(a) and (b) due to the pulling out of the laminar
The characteristics of the h-BN films and coatings synthesized by CVD were depended on several process parameters including deposition temperature, pressure, proportion of the precursor, gas flow rate and the species of the substrate. The deposition temperature and pressure are two key parameters during the reaction process of CVD which determine the deposition mechanism and affect the morphology and structure of the product. It has been reported by many researchers with sorts of precursor such as BCl3/NH3 [25], BF3/NH3 [9] and BBr3/NH3 [26], ammonia borane (H3NBH3) [27] and tris(dimethylamino)borane (TDMAB) [28], but there has been few related report by using borazine as the precursor. Therefore, we focused on the effects of temperature and pressure on the micromorphology and crystal structure of PBN by using borazine as the precursor in the present study. 3.1. Micromorphology of the PBN The as-deposited PBN were classified into three Types (A to C) as shown in Fig. 1 according to their micromorphologies observed by SEM. The PBN synthesized at deposition temperatures below 1100 °C independent of pressure all belonged to Type B, which had dense body, glass-like structure and transparent appearance. The deposition surfaces of PBN are shown in Fig. 2 which was prepared respectively at 200 Pa and 1000 Pa with deposition temperature of 900 °C. A nonporous and pebble-like deposition surface was observed by SEM at low magnification and the pebble-like particles had similar diameters about 300–400 nm on both the deposition surfaces as shown in Fig. 2(a) and (c). SEM observation at high magnification in Fig. 2(b) and (d) clarified that the surface of these pebbles were swarmed by fine grains. The apparent diameter of the grains was approximately 40 nm and less than 10 nm respectively at pressures of 200 Pa and 1000 Pa, which reveals 3
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Fig. 4. The fracture surfaces of PBN synthesized at 1600 °C with different pressures: (a) 50 Pa; (b) 100 Pa; (c) 200 Pa;(d) 300 Pa.
presented in Fig. 6. At the pressures below 300 Pa, the c-plane of h-BN of the PBN plates tended more parallel to the growth surface with lower PO values as the pressure decreased, which was in agreement with the result observed on the fracture surface as shown in Fig. 4. Meanwhile, at the pressures less than or equal to 200 Pa, the value of PO decreased obviously which represented better preferred orientation of the crystallite with increase of the deposition temperature as shown in Fig. 6. Therefore, PO possessed the minimum value of 0.42 at the highest temperature of 1900 °C and lowest pressure of 50 Pa. As shown in Fig. 5, the PBN synthesized at this condition had a stronger (0 0 2) diffraction and weaker (0 1) diffraction comparing to the XRD pattern of the powder, which indicated a higher preferred orientation of the crystallite. At the pressures above or equal to 300 Pa at which condition the PBN belonged to Type C, PO was independent of the temperature and pressure and the values were all closed to 1 which indicated an isotropic structure of the PBN plates as shown in Fig. 4(d). From the discussion above, it is demonstrated that the deposition temperature and pressure both have a significant influence on the formation of layered crystal structure of h-BN. High temperature and low pressure are indispensable for the synthesis of PBN plates with fine preferred orientation of crystallite. The relation between d(002) and the deposition condition was presented in Fig. 7. At the pressures of 50 Pa, 200 Pa and 600 Pa, the change of d(002) against temperature is plotted respectively. With the increase of the temperature, the diffraction peak of (0 0 2) became narrower and shifted to higher angles as shown in Fig. 5. The value of d(002) tended to reduce continuously as shown in Fig. 7 suggesting the increasing crystallization degree of the PBN, which was in common with the result what had been reported in our previous study [29]. The minimal value of d(002) was 0.342 nm which was obtained at 1900 °C and 50 Pa. Simultaneously, the PBN of this condition exhibited distinct (1 0 0) and (1 0 1) diffractions instead of a (1 0) band around 43° which
planes at the fracture process. It reveals that the PBN of Type A which deposited at relative low pressures of 50–200 Pa and temperatures above 1150 °C possessed crystal structure with relatively high preferred orientation and weak interplanar bonding strength. The PBN of type B obtained at 200 Pa displayed a dense body and glass-like fracture surface as shown in Fig. 4(c), and large amounts of fine grains could be recognized on the fracture surface which was not observed at lower deposition pressures in Type A. However, the PBN of type C deposited at the pressure of 300 Pa exhibited a porous fracture surface with an obvious accumulation of spherical grains of approximately 100 nm in diameter as shown in Fig. 4(d). 3.2. Crystal structure of the PBN To investigate the effects of deposition temperature and pressure on the crystal structure of the PBN, XRD analyses were carried out. The plate and the powder of PBN were respectively characterized by XRD as shown in Fig. 5. The interplanar space (d(002)) and preferred orientation of the crystallite (PO) [10] were calculated from the XRD data. The plate sample had a thickness about 200 µm and was peeled off from the graphite disc to avoid the interference from the diffraction of the substrate. Part of the plate was ground into powder and characterized by XRD to obtain d(002) calculated by the full width at half maximum (FWHM) mid-point of the (0 0 2) reflection of h-BN. PO was estimated from the integrated ratio of the (1 0) to (0 0 2) reflection [I(1 0)/I(002)] of the plate sample [10] and that of the powder sample according to the following equation:
PO =
[I (10)/ I (002)]plate [I (10)/ I (002)]powder
The relation between PO and the deposition condition was 4
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Fig. 7. Effect of deposition condition on the interplanar space of the PBN plates.
pressure almost had no influence on the change of d(002) above 1700 °C and the d(002) possessed nearly the same value at different pressures with the temperatures of 1700 °C and 1900 °C respectively. It is demonstrated that the value of d(002) is mainly depended on the deposition temperature, and high temperature always leads to high crystallinity with small interplanar space. 3.3. Deposition mechanism analysis of the PBN According to Matsuda's report [10], the CVD process of the PBN proceeds in three steps: (1) cluster (aggregate of precursor) formation and growth in vapor phase; (2) adsorption of cluster on the substrate surface; (3) decomposition and rearrangement of cluster on the substrate. The rate of these three steps are significantly affected by the deposition temperature and pressure during CVD. At the deposition condition with relative low pressure, the rearrangement and growth of cluster on the substrate is the dominant step due to the low rate of vapor phase reaction process. In this case, it is prone to the formation of Type A which possessed a laminated structure with high preferred orientation of crystallite. As the TEM photograph of Fig. 8(a) shown, the PBN of Type A deposited at 1600 °C and 50 Pa still has a flaky appearance after the mechanical grinding procedure during the preparation of TEM samples owing to the relatively weak interplanar bonding strength of h-BN, and the lattice fringes of (002) plane reveals preferred orientation paralleled to the flake surface in the HRTEM imaging of Fig. 8(b) which indicates a highly ordered crystal structure of the PBN. Meanwhile, the deposition surface of PBN tends to contain large size of pebble-like particles and indistinct grain boundary due to the rearrangement and growth process on the substrate as shown in Fig. 3(a) and (b). Moreover, high temperature further promoted the reaction process on the substrate and give stronger orientation in the PBN plates, thus PBN with ordered structure could be obtained at higher pressures with the increase of temperature, as the sloping boundary line between Type A and Type B shown in Fig. 1. At the deposition condition of Type C which had relative high pressures, the vapor phase reaction became the dominant step to determine the morphology and structure of the PBN. The partial pressure of precursor in the chamber raised with the increase of total pressure, which would lead to large increased occurrence of cluster formation in the vapor phase [31]. In this case, the rate of the rearrangement and the growth of cluster on the substrate would be much slower than that of the cluster formation in vapor phase and the adsorption on the substrate surface. This implies that the PBN of Type C was almost formed by the accumulation of the cluster from the vapor phase, which exhibited an isotropic structure. As the TEM photograph of Fig. 8(c) shown, the PBN deposited at 1600 °C and 300 Pa exhibits an aggregation of spherical clusters (as-marked by short arrows in Fig. 8(c)). The clusters has a similar size of about 100 nm in diameter with the fine grains in Fig. 4(d)
Fig. 5. XRD patterns of the plate and the powder of PBN synthesized at 50 Pa with different temperatures.
Fig. 6. Effect of deposition condition on the preferred orientation of the crystallite of the PBN plates.
was observed at lower deposition temperatures represented t-BN as shown in Fig. 5. It indicated that this PBN contained a boron nitride phase with three dimensional ordering structure, which was identified as h-BN [30]. At the temperatures below 1700 °C, pressure had a slightly effect on the value of d(002). The PBN with a minor value of d(002) was formed at relatively low deposition pressures. However, the 5
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Fig. 8. TEM photographs of PBN deposited at 1600 °C with different pressures: (a)(b) 50 Pa; (c)(d) 300 Pa.
Type C was determined by the vapor phase reaction procedure. An equilibrium state of these steps would lead to the growth of Type B.
which implies that the grains observed on the facture surface may represent the clusters mentioned above. The HRTEM imaging of Fig. 8(d) reveals that the lattice fringes of (002) basal plane in the cluster exhibit a random orientation which demonstrates a low degree of the preferred orientation of the crystallite. It is in agreement with the result of PO value calculated from the XRD analysis. The PBN of Type B showed a dense and glass-like fracture structure. This can be explained by the combined action of these three deposition steps in an equilibrium state.
References [1] N.A. Novikova, E.G. Vlasov, L.B. Nepomnyashchii, Some properties of pyrolitic boron nitride, Refractories 12 (1971) 677–680. [2] P.L. Fortucci, V.D. Meyer, E.K. Pang, Mass spectrometric characterization of outgassing of anisotropic pyrolytic boron nitride, Anal. Chem. 57 (1985) 14. [3] A. Lipp, K.A. Schwetz, K. Hunold, Hexagonal boron nitride: fabrication, properties and applications, J. Eur. Ceram. Soc. 5 (1989) 3. [4] R.T. Paine, C.K. Narula, Synthetic routes to boron nitride, Chem. Rev. 90 (1990) 73–91. [5] A.W. Moore, Characterization of pyrolytic boron nitride for semiconductor materials processin, J. Cryst. Growth 106 (1990) 6–15. [6] R. Haubner, M. Wilhelm, R. Weissenbacher, B. Lux, Boron nitrides-properties, synthesis and applications, High Performance Non-oxide Ceramics II, Springer, Berlin Heidelberg, 2002. [7] B.V. Prokofiev, Pyrolytical boron nitride as a window material for high power microwave electron devices, in: Proceedings of the 8th International IEEE on Vac. Electron Sources Conference Nanocarbon (IVESC), 2010: pp. 205–206. [8] E. Brodu, M. Balat-Pichelin, D.D.S. Meneses, J.-L. Sans, Reducing the temperature of a C/C composite heat shield for solar probe missions with an optically selective semi-transparent pyrolytic boron nitride (pBN) coating, Carbon N. Y. 82 (2015) 39–50. [9] F. Rebillat, A. Guette, R. Nasiain, C.R. Brosse, Highly ordered pyrolytic BN obtained by LPCVD, J. Eur. Ceram. Soc. 17 (1997) 1403–1414. [10] T. Matsuda, N. Uno, H. Nakae, T. Hirai, Synthesis and structure of chemically vapour-deposited boron nitride, J. Mater. Sci. 21 (1986) 649–658. [11] H. Wu, M. Chen, X. Wei, M. Ge, W. Zhang, Deposition of BN interphase coatings from B-trichloroborazine and its effects on the mechanical properties of SiC/SiC composites, Appl. Surf. Sci. 257 (2010) 1276–1281. [12] M. Basche, D. Schiff, New pyrolytic boron nitride, Mater. Des. Eng. 59 (1964) 78–81. [13] J.T. Mariner, Suitability of pyrolytic boron nitride, hot pressed boron nitride, and pyrolytic graphite for CIGS processes, Materials Challenges in Alternative and Renewable Energy, 2011. [14] V.N. Demin, I.P. Asanov, Z.L. Akkerman, Chemical vapor deposition of pyrolytic boron nitride from borazine, J. Vac. Sci. Technol. A 18 (2010) 94–98. [15] J.E. Huheey, Inorganic Chemistry, 2nd ed, Harper and Row, New York, 1978.
4. Conclusions PBN has been synthesized by CVD at temperatures of 900–1900 °C and pressures of 50–1000 Pa on the graphite substrate using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure were investigated. The conclusions are drawn as follows: (1) The PBN possessed three types of structure according to their micromorphologies depending on the deposition condition. Type A: PBN with dense and obviously laminated structure was obtained at temperatures of 1150–1900 °C and relatively low pressures of 50–200 Pa depending on the temperature. Type C: PBN with isotropic and porous structure was obtained at temperatures above 1100 °C and pressures above 200 Pa. Type B: PBN with dense and glass-like fracture structure was obtained at the other range of the deposition condition in this study. (2) PBN with fine preferred orientation of the crystallite was obtained at high temperatures and low pressures. The interplanar space of the PBN was mainly depended on the deposition temperature, and d(002) became smaller at higher temperatures. (3) The deposition of Type A was mainly determined by the rearrangement and growth steps of the cluster on the substrate, while 6
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[24] J. Li, C. Zhang, B. Li, F. Cao, S. Wang, An investigation on the synthesis of borazine, Inorg. Chim. Acta 366 (2011) 173–176. [25] A.S. Rozenberg, Y.A. Sinenko, N.V. Chukanov, Regularities of pyrolytic boron nitride coating formation on a graphite matrix, J. Mater. Sci. 28 (1993) 5528–5533. [26] B.J. Choi, D.W. Park, D.R. Kim, Preparation of hexagonal boron nitride by low pressure chemical vapour deposition in the BBr3+NH3+H2 system, J. Mater. Sci. Lett. 14 (1995) (452–254). [27] N. Sun, C. Wang, L. Jiao, J. Zhang, D. Zhang, Controllable coating of boron nitride on ceramic fibers by CVD at low temperature, Ceram. Int. 43 (2017) 1509–1516. [28] C. Lorrette, P. Weisbecker, S. Jacques, R. Pailler, J.M. Goyheneche, Deposition and characterization of hex-BN coating on carbon fibres using tris(dimethylamino) borane precursor, J. Eur. Ceram. Soc. 27 (2007) 2737–2743. [29] S. Gao, B. Li, C. Zhang, D. Li, R. Liu, S. Wang, Chemical vapor deposition of pyrolytic boron nitride ceramics from single source precursor, Ceram. Int. 43 (2017) 10020–10025. [30] J. Thomas, N.E. Weston, T.E. O’Connor, Turbostratic boron nitride, thermal transformation to ordered-layer-lattice boron nitride, J. Am. Chem. Soc. 84 (1962) 4619–4622. [31] N. Patibandla, K.L. Luthra, Chemical vapor deposition of boron nitride, J. Electrochem. Soc. 139 (1992) 457–462.
[16] V.V. Volkov, G.I. Bagryantsev, K.G. Myakishev, Borazine synthesis via reaction between sodium borohydride and ammonium chloride, Zh. Neorg. Khim. 11 (1970) 2902–2906. [17] F.I. Hurwitz, T.R. McCue, Y.L. Chen, P.V. Chayka, C. Xu, Cer. Eng. Sci. Proc. 2008. [18] J.S. Li, C.R. Zhang, B. Li, Preparation and characterization of boron nitride coatings on carbon fibers from borazine by chemical vapor deposition, Appl. Surf. Sci. 257 (2011) 7752–7757. [19] M. Corso, W. Auwärter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Boron nitride nanomesh, Science 303 (2004) 217–220. [20] Y. Song, C. Zhang, B. Li, D. Jiang, G. Ding, H. Wang, X. Xie, Triggering the atomic layers control of hexagonal boron nitride films, Appl. Surf. Sci. 313 (2014) 647–653. [21] Y. Song, C. Zhang, B. Li, G. Ding, D. Jiang, H. Wang, X. Xie, Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene, Nanoscale Res. Lett. 9 (2014) 367. [22] T.P. Smirnova, L.V. Jakovkina, I.L. Jashkin, N.P. Sysoeva, J.I. Amosov, Boron nitride films prepared by remote plasma-enhanced chemical vapour deposition from borazine (B3N3H6), Thin Solid Films 237 (1994) 32–37. [23] Z.L. Akkerman, M.L. Kosinova, N.I. Fainer, Y.M. Rumjantsev, N.P. Sysoeva, Chemical stability of hydrogen-containing boron nitride films obtained by plasma enhanced chemical vapour deposition, Thin Solid Films 260 (1995) 156–160.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine ⁎
Shitao Gao, Bin Li , Duan Li, Changrui Zhang, Rongjun Liu, Siqing Wang Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chemical vapor deposition Pyrolytic boron nitride Morphology Structure Mechanism
Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900 °C and total pressures of 50–1000 Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The asdeposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900 °C with relative low pressures of 50–200 Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100 °C with higher pressures above 250 Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d(002)) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d(002). The crystal growth mechanism of the three types of PBN was discussed.
1. Introduction Pyrolytic boron nitride (PBN) possesses plenty of remarkable performances such as extreme chemical inertness, high dielectric strength, excellent thermal shock resistance, non-wetting, non-toxic, oxidation resistance and negligible outgassing [1,2]. It has highly ordered planar texture analogous to pyrolytic graphite (PG) which exhibits evident anisotropic properties such as lower dielectric constant vertical to the crystal plane and higher bending strength along the crystal plane [3,4]. Therefore, PBN material has been widely manufactured as crucibles of compound semiconductor crystals, output windows and dielectric rods of traveling-wave tubes, high-temperature jigs and insulators [5–7]. Furthermore, in the incoming space exploration of Solar Probe Plus (SPP, NASA), PBN has been tested as the candidate material for the protective coating of the heat shield on the solar probe, not only for its excellent thermal and chemical stability in vacuum, but also for its high emissivity and reflectivity in the solar spectrum which would reduce the surface temperature of the spacecraft [8]. The synthesis of PBN has been carried out by a number of researchers using various reaction systems by chemical vapor deposition (CVD) [9–11]. The commercial production of PBN is based on the pyrolysis of BCl3 and NH3 at temperatures of 1900–2100 °C and pressures below 1 Torr in a hot-wall thermally insulated furnace [12,13]. In order to obtain stoichiometric PBN, the flow rate and relative ratio of
⁎
the reactants must be controlled precisely. Moreover, complicated simulation and verification of the gas flow field are need in order to realize the intensive mixing of the reactants gases and the obtaining of homogeneous products. Therefore, this process had a high production cost and several disadvantages such as low yield, long deposition duration, and high deposition temperature and energy consumption. There are also problems of the corrosive and toxic substances both in the precursors and by-products which may corrode the device and pollute the environment [14]. Borazine (B3N3H6), which possesses the same stoichiometric proportion of B/N with hexagonal boron nitride (h-BN), provides a potential alternative to the conventional CVD precursors [15]. It is isostructural and isoelectronic with benzene and has no impurity element except hydrogen [16,17]. Moreover, borazine is liquid at room temperature and has a relatively high vapor pressure [18]. Lots of investigations have been reported on the CVD of h-BN monolayer and nanosheet by using borazine as precursor in the last decade [19–21]. Borazine is also reported to prepare thin films about hundreds of nanometers thick by plasma enhanced (PE)CVD [22,23]. However, there has been little study on the preparation of PBN with the thickness of millimeter scale using borazine as the precursor. There must be several issues generated when extending the two-dimensional h-BN monolayer or film to the three-dimensional PBN [18]. In this work, we report the CVD of PBN using borazine as the precursor in temperatures of
Corresponding author. E-mail address: [email protected] (B. Li).
https://doi.org/10.1016/j.ceramint.2018.03.201 Received 2 March 2018; Accepted 22 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Gao, S., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.201
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own axis during the reaction. The borazine used in this work was produced and purified with trap to trap distillation according to Li's report [24]. Nitrogen was passed into a bubbler which contained the precursor to carry the borazine vapor into the reaction chamber. The temperature of bubbler was kept at 0 °C at which the borazine had a vapor pressure of 11.2kPa during the deposition step. At the same time, the total pressure of the bubbler was conducted at a constant value in order to keep the borazine concentration of the gas mixture controllable. Then the gas mixture was transported into the reaction chamber with 1slm measured by gas mass flowmeter. The total gas pressure of the reactor was maintained at a constant pressure with an automatic pressure controller valve attached to the exhaust side. The deposition pressure was varied in the range of 50–1000 Pa, and the deposition temperature was performed in the range of 900–1900 °C. The as-deposited PBN was peel off from the graphite disc with thickness about 0.2–1 mm. Fig. 1. Effect of deposition temperature and pressure on the morphology of the PBN. Type A: product with dense and obviously laminated structure; Type B: product with dense and glass-like structure; Type C: product with porous and isotropic structure.
2.2. Characterization of the PBN The micromorphology of the PBN were observed by field emission scanning electron microscopy (FESEM, Hitachi S4800-Ⅱ). Due to the insulating property of h-BN, the sample surface was sputtered with a gold film previous to the SEM imaging. The crystallization state of PBN were characterized by X-ray diffraction (XRD, Bruker ADVANCED D8 diffractometer, Germany) at a wavelength of 1.5418 Å (Cu Kα radiation). The microstructure of the PBN was examined by transmission electron microscopy (TEM, FEI Tecnai G2 F20) analyses. The simples were prepared according to a simple manner by mechanical grinding and ultrasonic dispersion.
900–1900 °C and total pressures of 50–1000 Pa. The micromorphological and crystal structural characterizations of the PBN are presented. 2. Material and methods 2.1. Synthesis of the PBN The synthesis of PBN was carried out in a vertical hot-wall CVD system. The graphite disc (Φ = 100 mm) polished as substrate was fixed to the mandrel in the deposition chamber which would revolve on its
Fig. 2. The deposition surfaces of PBN synthesized at 900 °C with different total pressures: (a), (b) 200 Pa; (c), (d) 1000 Pa. 2
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Fig. 3. The deposition surfaces of PBN synthesized at 1600 °C with different pressures: (a) 50 Pa; (b) 100 Pa; (c) 200 Pa;(d) 300 Pa.
3. Results and discussion
that the grains tend to be smaller with higher deposition pressure. Simultaneously, the PBN presented smoother surface with a higher pressure of 1000 Pa as shown in Fig. 2(c) and (d). The morphologies of PBN deposited at relativity higher temperatures above 1150 °C were eventually separated into three types of structure from A to C with the increase of pressure as shown in Fig. 1. The deposition surface of the PBN obtained at temperature of 1600 °C and pressures ranging from 50 to 300 Pa are presented in the SEM images of Fig. 3. Fig. (a) and (b) both are the PBN of Type A, while Fig. 3(c) and (d) belong to Type B and Type C, respectively. Similar with the result in Fig. 2, it was observed that the PBN was composed of pebble-like particles on all the deposition surfaces in Fig. 3. The particles of PBN deposited at 50 Pa and 100 Pa had a relative large size of more than 5 µm in diameter, and revealed relatively flat surfaces as shown in Fig. 3(a) and (b). However, the surface became rougher and the thickness of PBN plates tended to be non-uniform as deposition pressure increased. The size of the pebble-like particles became irregular and smaller of 1–3 µm in diameter with the increase of pressure as shown in Fig. 3(c) and (d). Meanwhile, the boundary of the fine spherical grains which composed the pebble-like particles looked gradually clear. The grains had an apparent diameter of 500 nm, 300 nm and 100 nm approximately at deposition pressure of 100 Pa, 200 Pa and 300 Pa respectively, which tended to be smaller in size with the increase of pressure as shown in Fig. 3(b) to (d). Moreover, the grains even showed a similar size order with the pebble-like particles deposited at 900 °C as presented in Fig. 2. The SEM photographs of Fig. 4(a) to (d) are the fracture surfaces in corresponding with the deposition surfaces in Fig. 3(a) to (d), respectively. The PBN of Type A revealed an layer structure and the layers aligned parallel to the deposition surface as shown in Fig. 4(a) and (b). Simultaneously, some flaky fragments and gaps were observed on the fracture surface in Fig. 4(a) and (b) due to the pulling out of the laminar
The characteristics of the h-BN films and coatings synthesized by CVD were depended on several process parameters including deposition temperature, pressure, proportion of the precursor, gas flow rate and the species of the substrate. The deposition temperature and pressure are two key parameters during the reaction process of CVD which determine the deposition mechanism and affect the morphology and structure of the product. It has been reported by many researchers with sorts of precursor such as BCl3/NH3 [25], BF3/NH3 [9] and BBr3/NH3 [26], ammonia borane (H3NBH3) [27] and tris(dimethylamino)borane (TDMAB) [28], but there has been few related report by using borazine as the precursor. Therefore, we focused on the effects of temperature and pressure on the micromorphology and crystal structure of PBN by using borazine as the precursor in the present study. 3.1. Micromorphology of the PBN The as-deposited PBN were classified into three Types (A to C) as shown in Fig. 1 according to their micromorphologies observed by SEM. The PBN synthesized at deposition temperatures below 1100 °C independent of pressure all belonged to Type B, which had dense body, glass-like structure and transparent appearance. The deposition surfaces of PBN are shown in Fig. 2 which was prepared respectively at 200 Pa and 1000 Pa with deposition temperature of 900 °C. A nonporous and pebble-like deposition surface was observed by SEM at low magnification and the pebble-like particles had similar diameters about 300–400 nm on both the deposition surfaces as shown in Fig. 2(a) and (c). SEM observation at high magnification in Fig. 2(b) and (d) clarified that the surface of these pebbles were swarmed by fine grains. The apparent diameter of the grains was approximately 40 nm and less than 10 nm respectively at pressures of 200 Pa and 1000 Pa, which reveals 3
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Fig. 4. The fracture surfaces of PBN synthesized at 1600 °C with different pressures: (a) 50 Pa; (b) 100 Pa; (c) 200 Pa;(d) 300 Pa.
presented in Fig. 6. At the pressures below 300 Pa, the c-plane of h-BN of the PBN plates tended more parallel to the growth surface with lower PO values as the pressure decreased, which was in agreement with the result observed on the fracture surface as shown in Fig. 4. Meanwhile, at the pressures less than or equal to 200 Pa, the value of PO decreased obviously which represented better preferred orientation of the crystallite with increase of the deposition temperature as shown in Fig. 6. Therefore, PO possessed the minimum value of 0.42 at the highest temperature of 1900 °C and lowest pressure of 50 Pa. As shown in Fig. 5, the PBN synthesized at this condition had a stronger (0 0 2) diffraction and weaker (0 1) diffraction comparing to the XRD pattern of the powder, which indicated a higher preferred orientation of the crystallite. At the pressures above or equal to 300 Pa at which condition the PBN belonged to Type C, PO was independent of the temperature and pressure and the values were all closed to 1 which indicated an isotropic structure of the PBN plates as shown in Fig. 4(d). From the discussion above, it is demonstrated that the deposition temperature and pressure both have a significant influence on the formation of layered crystal structure of h-BN. High temperature and low pressure are indispensable for the synthesis of PBN plates with fine preferred orientation of crystallite. The relation between d(002) and the deposition condition was presented in Fig. 7. At the pressures of 50 Pa, 200 Pa and 600 Pa, the change of d(002) against temperature is plotted respectively. With the increase of the temperature, the diffraction peak of (0 0 2) became narrower and shifted to higher angles as shown in Fig. 5. The value of d(002) tended to reduce continuously as shown in Fig. 7 suggesting the increasing crystallization degree of the PBN, which was in common with the result what had been reported in our previous study [29]. The minimal value of d(002) was 0.342 nm which was obtained at 1900 °C and 50 Pa. Simultaneously, the PBN of this condition exhibited distinct (1 0 0) and (1 0 1) diffractions instead of a (1 0) band around 43° which
planes at the fracture process. It reveals that the PBN of Type A which deposited at relative low pressures of 50–200 Pa and temperatures above 1150 °C possessed crystal structure with relatively high preferred orientation and weak interplanar bonding strength. The PBN of type B obtained at 200 Pa displayed a dense body and glass-like fracture surface as shown in Fig. 4(c), and large amounts of fine grains could be recognized on the fracture surface which was not observed at lower deposition pressures in Type A. However, the PBN of type C deposited at the pressure of 300 Pa exhibited a porous fracture surface with an obvious accumulation of spherical grains of approximately 100 nm in diameter as shown in Fig. 4(d). 3.2. Crystal structure of the PBN To investigate the effects of deposition temperature and pressure on the crystal structure of the PBN, XRD analyses were carried out. The plate and the powder of PBN were respectively characterized by XRD as shown in Fig. 5. The interplanar space (d(002)) and preferred orientation of the crystallite (PO) [10] were calculated from the XRD data. The plate sample had a thickness about 200 µm and was peeled off from the graphite disc to avoid the interference from the diffraction of the substrate. Part of the plate was ground into powder and characterized by XRD to obtain d(002) calculated by the full width at half maximum (FWHM) mid-point of the (0 0 2) reflection of h-BN. PO was estimated from the integrated ratio of the (1 0) to (0 0 2) reflection [I(1 0)/I(002)] of the plate sample [10] and that of the powder sample according to the following equation:
PO =
[I (10)/ I (002)]plate [I (10)/ I (002)]powder
The relation between PO and the deposition condition was 4
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Fig. 7. Effect of deposition condition on the interplanar space of the PBN plates.
pressure almost had no influence on the change of d(002) above 1700 °C and the d(002) possessed nearly the same value at different pressures with the temperatures of 1700 °C and 1900 °C respectively. It is demonstrated that the value of d(002) is mainly depended on the deposition temperature, and high temperature always leads to high crystallinity with small interplanar space. 3.3. Deposition mechanism analysis of the PBN According to Matsuda's report [10], the CVD process of the PBN proceeds in three steps: (1) cluster (aggregate of precursor) formation and growth in vapor phase; (2) adsorption of cluster on the substrate surface; (3) decomposition and rearrangement of cluster on the substrate. The rate of these three steps are significantly affected by the deposition temperature and pressure during CVD. At the deposition condition with relative low pressure, the rearrangement and growth of cluster on the substrate is the dominant step due to the low rate of vapor phase reaction process. In this case, it is prone to the formation of Type A which possessed a laminated structure with high preferred orientation of crystallite. As the TEM photograph of Fig. 8(a) shown, the PBN of Type A deposited at 1600 °C and 50 Pa still has a flaky appearance after the mechanical grinding procedure during the preparation of TEM samples owing to the relatively weak interplanar bonding strength of h-BN, and the lattice fringes of (002) plane reveals preferred orientation paralleled to the flake surface in the HRTEM imaging of Fig. 8(b) which indicates a highly ordered crystal structure of the PBN. Meanwhile, the deposition surface of PBN tends to contain large size of pebble-like particles and indistinct grain boundary due to the rearrangement and growth process on the substrate as shown in Fig. 3(a) and (b). Moreover, high temperature further promoted the reaction process on the substrate and give stronger orientation in the PBN plates, thus PBN with ordered structure could be obtained at higher pressures with the increase of temperature, as the sloping boundary line between Type A and Type B shown in Fig. 1. At the deposition condition of Type C which had relative high pressures, the vapor phase reaction became the dominant step to determine the morphology and structure of the PBN. The partial pressure of precursor in the chamber raised with the increase of total pressure, which would lead to large increased occurrence of cluster formation in the vapor phase [31]. In this case, the rate of the rearrangement and the growth of cluster on the substrate would be much slower than that of the cluster formation in vapor phase and the adsorption on the substrate surface. This implies that the PBN of Type C was almost formed by the accumulation of the cluster from the vapor phase, which exhibited an isotropic structure. As the TEM photograph of Fig. 8(c) shown, the PBN deposited at 1600 °C and 300 Pa exhibits an aggregation of spherical clusters (as-marked by short arrows in Fig. 8(c)). The clusters has a similar size of about 100 nm in diameter with the fine grains in Fig. 4(d)
Fig. 5. XRD patterns of the plate and the powder of PBN synthesized at 50 Pa with different temperatures.
Fig. 6. Effect of deposition condition on the preferred orientation of the crystallite of the PBN plates.
was observed at lower deposition temperatures represented t-BN as shown in Fig. 5. It indicated that this PBN contained a boron nitride phase with three dimensional ordering structure, which was identified as h-BN [30]. At the temperatures below 1700 °C, pressure had a slightly effect on the value of d(002). The PBN with a minor value of d(002) was formed at relatively low deposition pressures. However, the 5
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Fig. 8. TEM photographs of PBN deposited at 1600 °C with different pressures: (a)(b) 50 Pa; (c)(d) 300 Pa.
Type C was determined by the vapor phase reaction procedure. An equilibrium state of these steps would lead to the growth of Type B.
which implies that the grains observed on the facture surface may represent the clusters mentioned above. The HRTEM imaging of Fig. 8(d) reveals that the lattice fringes of (002) basal plane in the cluster exhibit a random orientation which demonstrates a low degree of the preferred orientation of the crystallite. It is in agreement with the result of PO value calculated from the XRD analysis. The PBN of Type B showed a dense and glass-like fracture structure. This can be explained by the combined action of these three deposition steps in an equilibrium state.
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4. Conclusions PBN has been synthesized by CVD at temperatures of 900–1900 °C and pressures of 50–1000 Pa on the graphite substrate using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure were investigated. The conclusions are drawn as follows: (1) The PBN possessed three types of structure according to their micromorphologies depending on the deposition condition. Type A: PBN with dense and obviously laminated structure was obtained at temperatures of 1150–1900 °C and relatively low pressures of 50–200 Pa depending on the temperature. Type C: PBN with isotropic and porous structure was obtained at temperatures above 1100 °C and pressures above 200 Pa. Type B: PBN with dense and glass-like fracture structure was obtained at the other range of the deposition condition in this study. (2) PBN with fine preferred orientation of the crystallite was obtained at high temperatures and low pressures. The interplanar space of the PBN was mainly depended on the deposition temperature, and d(002) became smaller at higher temperatures. (3) The deposition of Type A was mainly determined by the rearrangement and growth steps of the cluster on the substrate, while 6
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