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Growth of cubic and hexagonal BN particles by using BBr3, NH4Br and metallic Na as reactants
Diamond & Related Materials 18 (2009) 1421–1425
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Growth of cubic and hexagonal BN particles by using BBr3, NH4Br and metallic Na as reactants Menghua Li, Liqiang Xu ⁎, Lishan Yang, Zhongchao Bai, Yitai Qian ⁎ School of Chemistry and Chemical Engineering, Shandong University and Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2009 Received in revised form 31 July 2009 Accepted 24 August 2009 Available online 2 September 2009 Keywords: Nanocrystalline Boron nitride Synthesis Characterization
a b s t r a c t Hexagonal BN (h-BN) particles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves at 450 ºC for 24 h; When the target temperature was set in the range of 500–600 ºC, cubic BN (c-BN) particles that co-existed with h-BN were obtained. It is found that 600 ºC and a molar ratio of BBr3 to NH4Br of 1:3 are the optimal reaction parameters for the highest yield production of c-BN particles. c-BN powder with controllable content has been synthesized through a one-step method in the absence of solvents. The as-obtained samples that contain pure h-BN and the mixture of c-BN and h-BN have high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron nitride (BN) has several different phases: such as cubic (c-BN), hexagonal (h-BN), wurtzite (w-BN), and rhombohedral (r-BN) etc. [1-3]. Among them, c-BN has a similar diamond-like structure (the B atoms and N atoms are tetrahedrally coordinated), which exhibits many unique properties. For example, it is the second hardest material only inferior to diamond, while its thermal stability is superior to that of diamond [4,5]. The oxidation (1200 ºC) and graphitization temperatures (1500 ºC) of c-BN are distinctly higher than those of the diamond (600 ºC and 1400 ºC), respectively. In addition, c-BN is chemically inert against molten ferrous materials [6–9], it can also be doped for both nand p-type conductivity [10]. c-BN was first prepared by the high-temperature high-pressure synthesis method at 1800 ºC under 8.5 GPa in 1957 [11], Since then, it has been common for the synthesis of c-BN crystals by the conversion of h-BN into c-BN in the presence of catalysts including alkali metals, alkaline earth metals and their nitrides to reduce the temperature and pressure [12]. c-BN has also been obtained via low (or atmospheric) pressure and high temperature routes, such as molten salts [13,14] or chemical vapor deposition methods [15,16]. Recently, c-BN has been prepared in autoclaves at relative low temperature, for instance, small amount of c-BN that co-existed with hBN were obtained via the reaction of KBH4 and NH4Cl at 650 ºC under 22 MPa [17]. Benzene-thermal route by using BBr3 and Li3N as reactants at 480 ºC or hydrothermal process at 300 ºC was also applied to synthesize c-BN that co-existed with two other BN phases in the ⁎ Corresponding authors. Tel./fax: +86 531 8836 6280. E-mail address: [email protected] (L. Xu). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.08.011
autoclaves [18]. Almost pure c-BN was prepared by using H3BO3, NaN3, N2H4 · H2O and N(CH3)3 as reactants via hydrothermal routes [19–22]. In this study, pure h-BN particles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves at 450 ºC for 24 h; when the target temperature was set in the range of 500–600 ºC, c-BN particles (with an average diameter of ~100 nm) together with h-BN flakes were obtained. It is found that the optimal reaction parameters for the highest yield production of c-BN particles were at 600 ºC with the molar ratio of BBr3 to NH4Br of 1:3. The two samples display high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively. As element “Cr” that originates from the stainless steel autoclave has the ability to enhance nitrogen solubility [23], it is found that CrN frequently co-exists with the c-BN in the raw products. To avoid introducing the CrN impurity, a Cu or Fe tube was inserted into the same autoclave as a liner, while keeping other conditions unchanged, only h-BN was produced. 2. Experimental section 2.1. Synthesis All the manipulations were carried out in a dry glove box with flowing Ar. All the reagents used were of analytical grade purity: NH4Br (purchased from Sinopharm Chemical Reagent Co., Ltd.), BBr3 (Beijing Gaohuan Co., Ltd.), and Na (Tianjin Kaitong Chem. Co., Ltd.). In a typical process, NH4Br (0.03 mol), BBr3 (0.01 mol), and sodium (0.13 mol) were placed into a 20 ml stainless steel autoclave. The autoclave was sealed and heated from room temperature to 450–600 ºC at a rate of 10 ºC min− 1, then maintained at the target temperatures for 24 h in an electrical furnace. After the autoclave was cooled to room temperature,
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the inner raw product was collected and washed with absolute ethanol to remove the residual sodium and the sodium bromide. Then, diluted hydrochloric acid and perchloric acid (HClO4) were added to remove the CrN. In the following steps, the product was washed with distilled water and absolute ethanol for several times to remove the residual impurities. Finally, the black powders were obtained after a drying process in a vacuum at 60 ºC for 12 h. 2.2. Characterization The phases of the products were measured using a Bruker D8 advanced X-ray powder diffractometer. The sizes, structure and chemical compositions of the products were examined by transmission electron microscopy (Hitachi-7000) and high-resolution TEM (JEM-2100). Fourier transform infrared (FTIR) spectra were recorded on a NICOLET 5700 FTIR spectrometer (Thermo Electron Corporation) in the transmission mode using a KBr wafer. The morphology of the products was studied using an Oxford Quanta 400 FEG/INC/HKL thermal FE scanning electron microscope (Oxford Scientific Co.U.K.). Thermal gravimetric analysis (TGA) was taken on a Mettler Toledo TGA/SDTA851 thermal analyzer apparatus under air and N2 flow, respectively. 3. Results Fig. 1 shows the XRD patterns of the BN samples obtained by the reaction of NH4Br, BBr3, and Na at 600 ºC for 24 h (defined as “Sample 1”) and the sample after the circumfluence treatment with perchloride acid at 150 ºC for 24 h (defined as “Sample 2”). The result of Fig. 1a indicates that Sample 1 is composed of c-BN, h-BN and CrN, while Sample 2 consists of c-BN and h-BN (Fig. 1b). All the diffraction peaks marked with hollow dots in Fig. 1a can be indexed to be c-BN with a calculated lattice constant a = 3.609 Å, which is close to the reported value (JCPDS card no. 35–1365, a = 3.616 Å) of the c-BN. The diffraction peaks marked with solid dots can be indexed as h-BN with calculated lattice constants a = 2.497 and c = 6.634 Å, which are near the reported values (JCPDS card no. 34-0421, a = 2.504 and c = 6.656 Å). The diffraction peaks marked with triangle can be indexed as the CrN, the calculated lattice constant a = 4.140 Å is near the reported value (JCPDS card no.65-2899, a = 4.149 Å) of CrN. The results of the XRD patterns indicate that the CrN impurity that formed during the experimental process has been removed by the perchloride acid. Fig. 2 presents the FTIR spectrum of Sample 2, in which the peak centered at 1110 cm− 1 can be attributed to the TO phonons of c-BN [24– 26]. The peaks located around 1383 and 813 cm− 1 belong to the
Fig. 1. Typical XRD patterns of the products obtained at 600 ºC, (a) before and (b) after post treatment by the HClO4.
Fig. 2. FT-IR spectrum of Sample 2.
adsorptions of h-BN, the former could be indexed as B–N stretching vibrations, while the latter should be identified with the B–N–B bonding vibrations of h-BN [27]. The weak peak at 2200–2300 cm− 1 might be attributed to the adsorption of carbon dioxide on their surfaces. Two broad absorption bands near 3433 and 1640 cm− 1 could be assigned to the O–H bonds that originated from the moisture absorbed on the surfaces of the sample. “The characteristic peaks of boron oxide [28] were not observed in this spectrum, this result together with that of XRD patterns reveals that the product has a relative high purity.” Fig. 3 displays the SEM and TEM images of Sample 2. The typical SEM image shown in Fig. 3a indicates that the product is mainly composed of c-BN particles (with diameters in the range of 80– 150 nm), and the rest are flakes. From the typical TEM image (Fig. 3b) we can see that the particles that co-existed with flakes were agglomerated to some extent. The morphology and detailed structure of the sample were further examined by HRTEM and selected area electron diffraction (SAED). The SAED pattern inset in Fig. 3c was obtained by directing the electron beam perpendicular to a single particle. It exhibits a symmetric character, which could be indexed to be c-BN with a zone axis of [110], indicating its single-crystalline nature. The lattice fringes with an average spacing of 0.180 nm correspond well to the (200) lattice parameter of the c-BN (JCPDS card no. 35-1365). HRTEM and SEAD examinations of other particles show similar results. In Fig. 3d, the clear lattice fringes with an average spacing of 0.330 nm of a randomly selected nanoplate could be indexed to the (002) planes of h-BN (JCPDS card no 34-0421). The results of HRTEM and SEAD patterns of the products are consonant with that of the XRD patterns, evidently proving the high crystallinity and purity of Sample 2. To investigate the thermal stabilities of the products, TGA analyses were carried out with temperatures ranging from room temperature to 1200 ºC in ambient atmosphere. Fig. 4 shows the typical TGA curve of Sample 2 (curve a) and Sample 3 (curve b, the product with pure h-BN was prepared with a target temperature of 450 ºC), respectively. A slight weight loss of the two samples that occurred below 920 ºC might be attributed to the loss of small molecules that adsorbed on their surfaces, which is consistent with the previous report [29]. While distinct characteristic differences of the two curves were observed after the temperature exceeded 920 ºC, it is found that curve b (Sample 3, containing pure h-BN) owns a dramatic weight gain velocity indicating that a faster oxidation process has occurred, this result is similar with the previous report [30]; but curve a only has a mild weight gain tendency compared with curve b revealing that Sample 2 (containing mixtures of c-BN and h-BN) has a higher thermal stability than that of
M. Li et al. / Diamond & Related Materials 18 (2009) 1421–1425
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Fig. 3. Representative SEM and TEM images of Sample 2: (a) SEM image. (b) TEM image. (c–d) HRTEM images and SAED patterns.
Sample 3. Therefore, the as-obtained c-BN product (Sample 2) has a superior thermal stability than that of pure h-BN, which might be utilized in high temperature environments [31]. 4. Discussion According to the thermodynamic dates referenced from the Lange's Handbook of Chemistry (all the values are referred to the standard-state, that is to say at 298 K), the free energy (ΔG0) and enthalpy (ΔH0) of the
reaction (4) calculated by Hess's Law are −1961.5 and −1992.46 kJ/ mol, respectively [32], revealing that the reaction is spontaneous and exothermic. So the maximum temperature during the reaction (4) in the autoclave might be higher than the temperature in the stove. Therefore, in this experiment, along with the increasing temperature, BBr3 and NH4Br would decompose easily, the intermediates that contain both B and N elements, such as B2 (NH)3, might tend to form cubic boron nitride, this feasibility has also been proved by the reported theoretical calculations [33,34]. The reactions that possibly occurred could be written as follows:
BBr3 þ 3Na→B þ 3NaBr
ð1Þ
2Na þ 2NH4 Br→2NaBr þ 2NH3 þ H2
ð2Þ
2B þ2NH3 →2BN þ 3H2
ð3Þ
[35,36] The overall reaction involved in this experiment could be formulated as follows: BBr3 þ 3NH4 Br þ 6Na ¼ BN þ 6NaBr þ 3H2 þ 2NH3
Fig. 4. The TGA curves of the products: (a) Sample 2 produced at 600 ºC. (b) Sample 3 prepared at 450 ºC.
ð4Þ
The effects of the reaction temperature, molar ratios of reactants and other parameters on the formation and yield of c-BN were systematically studied. First, the effects of reaction temperatures on the yield of the c-BN were studied through XRD patterns (see Fig. 5) and FTIR spectra (Fig. 6). If the reaction temperature was set below
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M. Li et al. / Diamond & Related Materials 18 (2009) 1421–1425 Table 1 The yield of the c-BN obtained through changing boron or nitrogen source but with a fixed molar ratio of 1:3. Group number
Boron source
Nitrogen source
The yield of c-BN
1
B power B2O3 H3BO3 NaBF4 NaBH4 BBr3 BBr3 BBr3 BBr3 BBr3
NH4Br NH4Br NH4Br NH4Br NH4Br NH4F NH4Cl NH4I (NH2)2CO NH4HCO3
None None None None None Little amount (~1%) Little amount (~5%) Little amount (~3%) None None
2
Fig. 5. XRD patterns of the products obtained at (a) 450 º, (b) 500 ºC, (c) 550 ºC, and (d) 650 ºC.
500 ºC (such as 450 ºC), only h-BN could be obtained (Figs. 5a and 6a); At 500 ºC, little amount of c-BN was observed besides the dominant hBN (Figs. 5b and 6b). It is found that the higher reaction temperature usually leads to the higher yield of c-BN, and its yield was dramatically increased in the whole product if the target temperature was set at 600 ºC (Figs. 5c and 6c). If the reaction temperature was further raised, such as at 650 ºC or 700 ºC, the yield of cubic boron nitride increased slowly but impurity peaks simultaneously appeared (see Figs. 5d and 6d). The above results of the XRD patterns (Fig. 5) and FTIR spectra (see Fig. 6) reveal that the optimal reaction temperature for the highest yield synthesis of c-BN is at 600 ºC. Besides the reaction temperature, the molar ratios of BBr3 to NH4Br were found to play an important role for the formation of c-BN. For instance, amorphous boron would be produced along with h-BN in the raw product when the molar ratio is larger than 1:1 (such as 1.5:1 or 2:1). Only little c-BN that co-existed with h-BN were produced when the ratio was approaching 1:1, and its yield increased along with the elevated ratio. The optimal ratio for the highest yield production of c-BN was found to be 1:3. It is also observed that the longer reaction time usually leads to the production of c-BN with higher crystallinity, but has no obvious influence on its yield augment. Finally, a series of contrast experiments was carried out (as shown in Table 1) to investigate whether the result was similar or not if the reagents were substituted by others. For example, when the BBr3 was substituted by B powder, B2O3, H3BO3, NaBF4, NaBH4, or NH4HCO3, (NH2)2CO were used instead of NH4Br, no c-BN was obtained. However, a small
quantity of c-BN could be obtained if NH4Br was replaced by other ammonium salts such as NH4X (X = F, Cl, I). Table 1 lists the contrast experiments when changing boron source, nitrogen source but keeping the reaction temperature and time unchanged. The molar ratio of boron to nitrogen source is always 1:3 among 1–2 groups to avoid the boron impurity and to supply high pressure. The experiment results indicate that the best results were obtained when BBr3, NH4Br and Na were used simultaneously. It is worth noting that CrN impurity (which could be removed by HClO4 at 150 ºC) was frequently produced along with c-BN and h-BN (as element “Cr” which originates from the stainless steel autoclave has the ability to enhance nitrogen solubility) [23]. However, in the contrast experiments, only h-BN could be generated if a Cu or Fe tube was put into the same autoclave and acted as a liner while keeping other conditions unchanged. Therefore, it is obvious that the production of CrN was favorable for the formation of c-BN, but the specific reason is presently unclear which still needs further research. 5. Conclusions In this study, pure hexagonal BN particles were prepared by using BBr3, NH4Br and Na as reactants in stainless steel autoclaves at 450 ºC for 24 h. When the target temperature was set in the range of 500– 600 ºC, c-BN particles that co-existed with minor h-BN were obtained. It is found that at 600 ºC for 24 h and with BBr3 to NH4Br the molar ratio of 1:3 are the optimal reaction parameters for the highest yield production of c-BN. The as-obtained samples that contain pure h-BN and the mixture of c-BN and h-BN have high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively, which might be utilized in high temperature environments. Acknowledgements This work was supported by the National Nature Science Found of China (Grant Nos. 20701026, 20971079, 20871075), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20070422046), the 973 Project of China (No. 2005CB623601) and the Natural Science Foundation (no. 11190004010664) of Shandong Province. References
Fig. 6. FTIR spectra of the products obtained with a target temperature of (a) 450 ºC, (b) 500 ºC, (c) 550 ºC, and (d) 650 ºC.
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[24] W.J. Zhang, Y.M. Chong, I. Bello, S.T. Lee, J. Phys., D, Appl. Phys. 40 (2007) 6159. [25] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. R21 (1997) 47. [26] P.B. Mirkarimi, K.F. MeCarty, D.L. Medlin, W.G. Wolf, T.A. Friedmann, E.J. Klaus, G.F. Cardinale, D.G. Howitt, J. Mater. Res. 9 (1994) 2925. [27] R. Geick, C.H. Perry, G. Rupprecht, Phys. Rev. 146 (1966) 543. [28] A. Oki, L. Adams, Z.P. Luo, Inorg. Chem. Commun. 11 (2008) 275. [29] J.H. Ma, J. Li, G.X. Li, Y.G. Tian, J. Zhang, J.F. Wu, J.Y. Zheng, H.M. Zhuang, T.H. Pan, Mater. Res. Bull. 42 (2007) 982. [30] S.Y. Dong, M.Y. Yu, X.P. Hao, D.L. Cui, Q.L. Wang, K. Li, M.H. Jiang, J. Cryst. Growth 254 (2003) 229. [31] D. Golberg, Y. Bando, C.C. Tang, C.Y. Zhi, Adv. Mater. 19 (2007) 2413. [32] J.G. Speight, Lange's Handbook of Chemistry, 16th ed. McGraw-Hill, New York, 2004. [33] J. Olander, K. Larsson, Diam. Relat. Mater. 11 (2002) 1286. [34] V.Z. Turkevich, J. Phys., Condens. Matter 14 (2002) 10963. [35] A. Denis, G. Goglio, G. Demazeau, Mater. Sci. Eng. R 50 (2006) 167. [36] S. Prouhet, F. Langlais, A. Guette, R. Naslain, J. Rey, Inorg. Chem. 30 (1993) 953.
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Growth of cubic and hexagonal BN particles by using BBr3, NH4Br and metallic Na as reactants Menghua Li, Liqiang Xu ⁎, Lishan Yang, Zhongchao Bai, Yitai Qian ⁎ School of Chemistry and Chemical Engineering, Shandong University and Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, PR China
a r t i c l e
i n f o
Article history: Received 31 January 2009 Received in revised form 31 July 2009 Accepted 24 August 2009 Available online 2 September 2009 Keywords: Nanocrystalline Boron nitride Synthesis Characterization
a b s t r a c t Hexagonal BN (h-BN) particles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves at 450 ºC for 24 h; When the target temperature was set in the range of 500–600 ºC, cubic BN (c-BN) particles that co-existed with h-BN were obtained. It is found that 600 ºC and a molar ratio of BBr3 to NH4Br of 1:3 are the optimal reaction parameters for the highest yield production of c-BN particles. c-BN powder with controllable content has been synthesized through a one-step method in the absence of solvents. The as-obtained samples that contain pure h-BN and the mixture of c-BN and h-BN have high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron nitride (BN) has several different phases: such as cubic (c-BN), hexagonal (h-BN), wurtzite (w-BN), and rhombohedral (r-BN) etc. [1-3]. Among them, c-BN has a similar diamond-like structure (the B atoms and N atoms are tetrahedrally coordinated), which exhibits many unique properties. For example, it is the second hardest material only inferior to diamond, while its thermal stability is superior to that of diamond [4,5]. The oxidation (1200 ºC) and graphitization temperatures (1500 ºC) of c-BN are distinctly higher than those of the diamond (600 ºC and 1400 ºC), respectively. In addition, c-BN is chemically inert against molten ferrous materials [6–9], it can also be doped for both nand p-type conductivity [10]. c-BN was first prepared by the high-temperature high-pressure synthesis method at 1800 ºC under 8.5 GPa in 1957 [11], Since then, it has been common for the synthesis of c-BN crystals by the conversion of h-BN into c-BN in the presence of catalysts including alkali metals, alkaline earth metals and their nitrides to reduce the temperature and pressure [12]. c-BN has also been obtained via low (or atmospheric) pressure and high temperature routes, such as molten salts [13,14] or chemical vapor deposition methods [15,16]. Recently, c-BN has been prepared in autoclaves at relative low temperature, for instance, small amount of c-BN that co-existed with hBN were obtained via the reaction of KBH4 and NH4Cl at 650 ºC under 22 MPa [17]. Benzene-thermal route by using BBr3 and Li3N as reactants at 480 ºC or hydrothermal process at 300 ºC was also applied to synthesize c-BN that co-existed with two other BN phases in the ⁎ Corresponding authors. Tel./fax: +86 531 8836 6280. E-mail address: [email protected] (L. Xu). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.08.011
autoclaves [18]. Almost pure c-BN was prepared by using H3BO3, NaN3, N2H4 · H2O and N(CH3)3 as reactants via hydrothermal routes [19–22]. In this study, pure h-BN particles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves at 450 ºC for 24 h; when the target temperature was set in the range of 500–600 ºC, c-BN particles (with an average diameter of ~100 nm) together with h-BN flakes were obtained. It is found that the optimal reaction parameters for the highest yield production of c-BN particles were at 600 ºC with the molar ratio of BBr3 to NH4Br of 1:3. The two samples display high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively. As element “Cr” that originates from the stainless steel autoclave has the ability to enhance nitrogen solubility [23], it is found that CrN frequently co-exists with the c-BN in the raw products. To avoid introducing the CrN impurity, a Cu or Fe tube was inserted into the same autoclave as a liner, while keeping other conditions unchanged, only h-BN was produced. 2. Experimental section 2.1. Synthesis All the manipulations were carried out in a dry glove box with flowing Ar. All the reagents used were of analytical grade purity: NH4Br (purchased from Sinopharm Chemical Reagent Co., Ltd.), BBr3 (Beijing Gaohuan Co., Ltd.), and Na (Tianjin Kaitong Chem. Co., Ltd.). In a typical process, NH4Br (0.03 mol), BBr3 (0.01 mol), and sodium (0.13 mol) were placed into a 20 ml stainless steel autoclave. The autoclave was sealed and heated from room temperature to 450–600 ºC at a rate of 10 ºC min− 1, then maintained at the target temperatures for 24 h in an electrical furnace. After the autoclave was cooled to room temperature,
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the inner raw product was collected and washed with absolute ethanol to remove the residual sodium and the sodium bromide. Then, diluted hydrochloric acid and perchloric acid (HClO4) were added to remove the CrN. In the following steps, the product was washed with distilled water and absolute ethanol for several times to remove the residual impurities. Finally, the black powders were obtained after a drying process in a vacuum at 60 ºC for 12 h. 2.2. Characterization The phases of the products were measured using a Bruker D8 advanced X-ray powder diffractometer. The sizes, structure and chemical compositions of the products were examined by transmission electron microscopy (Hitachi-7000) and high-resolution TEM (JEM-2100). Fourier transform infrared (FTIR) spectra were recorded on a NICOLET 5700 FTIR spectrometer (Thermo Electron Corporation) in the transmission mode using a KBr wafer. The morphology of the products was studied using an Oxford Quanta 400 FEG/INC/HKL thermal FE scanning electron microscope (Oxford Scientific Co.U.K.). Thermal gravimetric analysis (TGA) was taken on a Mettler Toledo TGA/SDTA851 thermal analyzer apparatus under air and N2 flow, respectively. 3. Results Fig. 1 shows the XRD patterns of the BN samples obtained by the reaction of NH4Br, BBr3, and Na at 600 ºC for 24 h (defined as “Sample 1”) and the sample after the circumfluence treatment with perchloride acid at 150 ºC for 24 h (defined as “Sample 2”). The result of Fig. 1a indicates that Sample 1 is composed of c-BN, h-BN and CrN, while Sample 2 consists of c-BN and h-BN (Fig. 1b). All the diffraction peaks marked with hollow dots in Fig. 1a can be indexed to be c-BN with a calculated lattice constant a = 3.609 Å, which is close to the reported value (JCPDS card no. 35–1365, a = 3.616 Å) of the c-BN. The diffraction peaks marked with solid dots can be indexed as h-BN with calculated lattice constants a = 2.497 and c = 6.634 Å, which are near the reported values (JCPDS card no. 34-0421, a = 2.504 and c = 6.656 Å). The diffraction peaks marked with triangle can be indexed as the CrN, the calculated lattice constant a = 4.140 Å is near the reported value (JCPDS card no.65-2899, a = 4.149 Å) of CrN. The results of the XRD patterns indicate that the CrN impurity that formed during the experimental process has been removed by the perchloride acid. Fig. 2 presents the FTIR spectrum of Sample 2, in which the peak centered at 1110 cm− 1 can be attributed to the TO phonons of c-BN [24– 26]. The peaks located around 1383 and 813 cm− 1 belong to the
Fig. 1. Typical XRD patterns of the products obtained at 600 ºC, (a) before and (b) after post treatment by the HClO4.
Fig. 2. FT-IR spectrum of Sample 2.
adsorptions of h-BN, the former could be indexed as B–N stretching vibrations, while the latter should be identified with the B–N–B bonding vibrations of h-BN [27]. The weak peak at 2200–2300 cm− 1 might be attributed to the adsorption of carbon dioxide on their surfaces. Two broad absorption bands near 3433 and 1640 cm− 1 could be assigned to the O–H bonds that originated from the moisture absorbed on the surfaces of the sample. “The characteristic peaks of boron oxide [28] were not observed in this spectrum, this result together with that of XRD patterns reveals that the product has a relative high purity.” Fig. 3 displays the SEM and TEM images of Sample 2. The typical SEM image shown in Fig. 3a indicates that the product is mainly composed of c-BN particles (with diameters in the range of 80– 150 nm), and the rest are flakes. From the typical TEM image (Fig. 3b) we can see that the particles that co-existed with flakes were agglomerated to some extent. The morphology and detailed structure of the sample were further examined by HRTEM and selected area electron diffraction (SAED). The SAED pattern inset in Fig. 3c was obtained by directing the electron beam perpendicular to a single particle. It exhibits a symmetric character, which could be indexed to be c-BN with a zone axis of [110], indicating its single-crystalline nature. The lattice fringes with an average spacing of 0.180 nm correspond well to the (200) lattice parameter of the c-BN (JCPDS card no. 35-1365). HRTEM and SEAD examinations of other particles show similar results. In Fig. 3d, the clear lattice fringes with an average spacing of 0.330 nm of a randomly selected nanoplate could be indexed to the (002) planes of h-BN (JCPDS card no 34-0421). The results of HRTEM and SEAD patterns of the products are consonant with that of the XRD patterns, evidently proving the high crystallinity and purity of Sample 2. To investigate the thermal stabilities of the products, TGA analyses were carried out with temperatures ranging from room temperature to 1200 ºC in ambient atmosphere. Fig. 4 shows the typical TGA curve of Sample 2 (curve a) and Sample 3 (curve b, the product with pure h-BN was prepared with a target temperature of 450 ºC), respectively. A slight weight loss of the two samples that occurred below 920 ºC might be attributed to the loss of small molecules that adsorbed on their surfaces, which is consistent with the previous report [29]. While distinct characteristic differences of the two curves were observed after the temperature exceeded 920 ºC, it is found that curve b (Sample 3, containing pure h-BN) owns a dramatic weight gain velocity indicating that a faster oxidation process has occurred, this result is similar with the previous report [30]; but curve a only has a mild weight gain tendency compared with curve b revealing that Sample 2 (containing mixtures of c-BN and h-BN) has a higher thermal stability than that of
M. Li et al. / Diamond & Related Materials 18 (2009) 1421–1425
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Fig. 3. Representative SEM and TEM images of Sample 2: (a) SEM image. (b) TEM image. (c–d) HRTEM images and SAED patterns.
Sample 3. Therefore, the as-obtained c-BN product (Sample 2) has a superior thermal stability than that of pure h-BN, which might be utilized in high temperature environments [31]. 4. Discussion According to the thermodynamic dates referenced from the Lange's Handbook of Chemistry (all the values are referred to the standard-state, that is to say at 298 K), the free energy (ΔG0) and enthalpy (ΔH0) of the
reaction (4) calculated by Hess's Law are −1961.5 and −1992.46 kJ/ mol, respectively [32], revealing that the reaction is spontaneous and exothermic. So the maximum temperature during the reaction (4) in the autoclave might be higher than the temperature in the stove. Therefore, in this experiment, along with the increasing temperature, BBr3 and NH4Br would decompose easily, the intermediates that contain both B and N elements, such as B2 (NH)3, might tend to form cubic boron nitride, this feasibility has also been proved by the reported theoretical calculations [33,34]. The reactions that possibly occurred could be written as follows:
BBr3 þ 3Na→B þ 3NaBr
ð1Þ
2Na þ 2NH4 Br→2NaBr þ 2NH3 þ H2
ð2Þ
2B þ2NH3 →2BN þ 3H2
ð3Þ
[35,36] The overall reaction involved in this experiment could be formulated as follows: BBr3 þ 3NH4 Br þ 6Na ¼ BN þ 6NaBr þ 3H2 þ 2NH3
Fig. 4. The TGA curves of the products: (a) Sample 2 produced at 600 ºC. (b) Sample 3 prepared at 450 ºC.
ð4Þ
The effects of the reaction temperature, molar ratios of reactants and other parameters on the formation and yield of c-BN were systematically studied. First, the effects of reaction temperatures on the yield of the c-BN were studied through XRD patterns (see Fig. 5) and FTIR spectra (Fig. 6). If the reaction temperature was set below
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M. Li et al. / Diamond & Related Materials 18 (2009) 1421–1425 Table 1 The yield of the c-BN obtained through changing boron or nitrogen source but with a fixed molar ratio of 1:3. Group number
Boron source
Nitrogen source
The yield of c-BN
1
B power B2O3 H3BO3 NaBF4 NaBH4 BBr3 BBr3 BBr3 BBr3 BBr3
NH4Br NH4Br NH4Br NH4Br NH4Br NH4F NH4Cl NH4I (NH2)2CO NH4HCO3
None None None None None Little amount (~1%) Little amount (~5%) Little amount (~3%) None None
2
Fig. 5. XRD patterns of the products obtained at (a) 450 º, (b) 500 ºC, (c) 550 ºC, and (d) 650 ºC.
500 ºC (such as 450 ºC), only h-BN could be obtained (Figs. 5a and 6a); At 500 ºC, little amount of c-BN was observed besides the dominant hBN (Figs. 5b and 6b). It is found that the higher reaction temperature usually leads to the higher yield of c-BN, and its yield was dramatically increased in the whole product if the target temperature was set at 600 ºC (Figs. 5c and 6c). If the reaction temperature was further raised, such as at 650 ºC or 700 ºC, the yield of cubic boron nitride increased slowly but impurity peaks simultaneously appeared (see Figs. 5d and 6d). The above results of the XRD patterns (Fig. 5) and FTIR spectra (see Fig. 6) reveal that the optimal reaction temperature for the highest yield synthesis of c-BN is at 600 ºC. Besides the reaction temperature, the molar ratios of BBr3 to NH4Br were found to play an important role for the formation of c-BN. For instance, amorphous boron would be produced along with h-BN in the raw product when the molar ratio is larger than 1:1 (such as 1.5:1 or 2:1). Only little c-BN that co-existed with h-BN were produced when the ratio was approaching 1:1, and its yield increased along with the elevated ratio. The optimal ratio for the highest yield production of c-BN was found to be 1:3. It is also observed that the longer reaction time usually leads to the production of c-BN with higher crystallinity, but has no obvious influence on its yield augment. Finally, a series of contrast experiments was carried out (as shown in Table 1) to investigate whether the result was similar or not if the reagents were substituted by others. For example, when the BBr3 was substituted by B powder, B2O3, H3BO3, NaBF4, NaBH4, or NH4HCO3, (NH2)2CO were used instead of NH4Br, no c-BN was obtained. However, a small
quantity of c-BN could be obtained if NH4Br was replaced by other ammonium salts such as NH4X (X = F, Cl, I). Table 1 lists the contrast experiments when changing boron source, nitrogen source but keeping the reaction temperature and time unchanged. The molar ratio of boron to nitrogen source is always 1:3 among 1–2 groups to avoid the boron impurity and to supply high pressure. The experiment results indicate that the best results were obtained when BBr3, NH4Br and Na were used simultaneously. It is worth noting that CrN impurity (which could be removed by HClO4 at 150 ºC) was frequently produced along with c-BN and h-BN (as element “Cr” which originates from the stainless steel autoclave has the ability to enhance nitrogen solubility) [23]. However, in the contrast experiments, only h-BN could be generated if a Cu or Fe tube was put into the same autoclave and acted as a liner while keeping other conditions unchanged. Therefore, it is obvious that the production of CrN was favorable for the formation of c-BN, but the specific reason is presently unclear which still needs further research. 5. Conclusions In this study, pure hexagonal BN particles were prepared by using BBr3, NH4Br and Na as reactants in stainless steel autoclaves at 450 ºC for 24 h. When the target temperature was set in the range of 500– 600 ºC, c-BN particles that co-existed with minor h-BN were obtained. It is found that at 600 ºC for 24 h and with BBr3 to NH4Br the molar ratio of 1:3 are the optimal reaction parameters for the highest yield production of c-BN. The as-obtained samples that contain pure h-BN and the mixture of c-BN and h-BN have high thermal stabilities in ambient atmosphere below 920 and 1100 ºC, respectively, which might be utilized in high temperature environments. Acknowledgements This work was supported by the National Nature Science Found of China (Grant Nos. 20701026, 20971079, 20871075), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20070422046), the 973 Project of China (No. 2005CB623601) and the Natural Science Foundation (no. 11190004010664) of Shandong Province. References
Fig. 6. FTIR spectra of the products obtained with a target temperature of (a) 450 ºC, (b) 500 ºC, (c) 550 ºC, and (d) 650 ºC.
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