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Synthesis of nano-sized amorphous boron powders through active dilution self-propagating high-temperature synthesis method
Materials Research Bulletin 48 (2013) 2018–2022
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of nano-sized amorphous boron powders through active dilution self-propagating high-temperature synthesis method Jilin Wang a, Yunle Gu b, Zili Li a, Weimin Wang a,*, Zhengyi Fu a a b
The State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 May 2012 Received in revised form 25 January 2013 Accepted 27 January 2013 Available online 27 February 2013
Nano-sized amorphous boron powders were synthesized by active dilution self-propagating hightemperature synthesis (SHS) method at temperatures ranging from 700 8C to 850 8C in a SHS furnace using Mg, B2O3 and KBH4 as raw materials. Samples were characterized by X-ray powder diffraction (XRD), Laser particle size analyzer, Fourier transform infrared spectra (FTIR), X-ray energy dispersive spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission TEM (HRTEM). The boron powders demonstrated an average particle size of 50 nm with a purity of 95.64 wt.%. The diluter KBH4 played an important role in the active dilution synthesis of amorphous nano-sized boron powders. The effects of endothermic reaction rate, the possible chemical reaction mechanism and active dilution model for synthesis of the product were also discussed. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials A. Nanostructures B. Chemical synthesis C. Electron diffraction
1. Introduction Amorphous boron powders have a higher volumetric energy density than the traditional fuels like magnesium, aluminum, carbon and hydrocarbon [1–4]. It also reveals a high melting point, high hardness, low density, good chemical inertness and thermal stability, interesting electrical and optical properties [5–8]. These unique properties make amorphous boron powders an attractive prospect for practical applications in many fields such as nuclear reactor control, rocket igniter, thermo-electric energy conversion, high-temperature devices, refractory materials and semiconductors [4,6,9–12]. Recent studies about the element boron tend to pay more attention to the preparation, characterization and application of crystalline boron but seldom of amorphous boron [5,13–16]. As to amorphous boron, various synthetic methods have been employed to prepare amorphous boron powders, such as diborane pyrolysis [17], laser ablation [18,19], molten-salt electrolysis [20], magnesiothermic reduction [21–23]. In these methods, the magnesiothermic reduction also belongs to self-propagating hightemperature synthesis (SHS), which is a novel technology for synthesis of various of materials. Once the reaction starts, it will
* Corresponding author. Tel.: +86 27 87215421; fax: +86 27 87215421. E-mail addresses: [email protected], [email protected] (W. Wang). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.053
continue with the help of the energy provided by itself, rather than from an outside system. The amorphous boron powders prepared through this method present a small particle size and high activity, which are appropriate for the popularization and application. However, it is hard to control the SHS reaction process due to the high exothermic effect in a few seconds. Excessive temperature will cause many kinds of impurities, such as MgxB2O(x+1) (x = 2, 3), MgBx (x = 2, 4, 6), BxO [24–26], and lead to low purity (88 wt.%) as well as large particle size (about 0.5–5 mm). Generally, two dilution methods were used to adjust the SHS reaction process, excess B2O3 or Mg, and inert diluents (NaCl, KCl, MgCl2 and MgO). Nevertheless, these control methods were demonstrated limited effect and consumptive of reactants or inert reagents. Moreover, additional three leaching and purifying processes were needed in order to obtain higher purity (90 wt.%) products [27]. The present work introduces a new way so-called active dilution method to synthesize nano-sized amorphous boron powders. Higher purity products could be prepared after only one leaching and purifying processes. The exothermic reaction could be effectively controlled through changing the endothermic reaction rate, which promotes the preparation of amorphous boron powders and inhibits the formation of by-products. The effects of endothermic reaction rate, the possible chemical reaction mechanism and active dilution model for synthesis of the product were also discussed. In addition, synthesized high active nano-sized amorphous boron powders have been applied successfully to the large scale preparation of boron nitride nanotubes [12,28,29].
J. Wang et al. / Materials Research Bulletin 48 (2013) 2018–2022
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powders could be controlled via changing the endothermic reaction rate.
2. Experimental 2.1. Materials
2.3. Synthesis techniques The reactants, magnesium (Mg), boron oxide (B2O3) and potassium borohydride (KBH4) were of analytical pure grade and about 150–300 mesh (50–100 mm) powders. 2.2. Theoretical calculations The chemical reaction equations of the SHS reaction system B2O3/Mg/KBH4 are presented as follows: B2 O3 þ 3Mg ! 2B þ 3MgO
(1)
B2 O3 þ 2KBH4 ! 4B þ 2KOH þ H2 O þ 2H2
(2)
In a typical procedure, 27.38 g B2O3, 23.64 g Mg and 7.52 g KBH4, where the endothermic rate was 30%, were mixed by a blender mixer for 6–12 min. Then the mixture was pressed to be a cylinder with a diameter of 30 mm. The cylinder was heated under argon atmosphere at 830 8C for 13 min in a SHS-furnace. After cooled to room temperature naturally, the furnace was opened, and the crude product was collected and washed with 5 M hydrochloric acid, ethanol and distilled water. Finally, 8.68 g brown boron powders were collected by drying in vacuum at 80 8C for 12 h. The yield of amorphous boron powders based on boron was about 86.8%. 2.4. Characterization
The standard molar enthalpies of Eqs. (1) and (2) were calculated as 24.57 kJ/(gram B) and +14.73 kJ/(gram B). The reaction system B2O3/Mg is extremely exothermic while the other reaction system B2O3/KBH4 is highly endothermic. The SHS reaction process could be effectively controlled through properly changing the additional rate of the diluter KBH4. The relationship between endothermic rate and thermal effect was illustrated in Fig. 1(a). The abscissa endothermic rate represents the percentage of B prepared through endothermic reaction Eq. (2) in the total B. When the heat effect of the whole reaction system achieves equilibrium, the endothermic rate will reach 62.52%. In addition, we suggested that the proper endothermic rate is ranging from 10% to 70%. Moreover, it is likely that the SHS intensity, particle size, purity and morphology of the boron
The samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/MAX-LLIA X-ray diffractometer (radiation wavelength l = 1.5406 A˚). Mastersizer 2000 laser particle size analyzer was used for analysis of the particle size distribution of the sample. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 6700 FTIR spectrometer. The scanning electron microscopy (SEM) and field emission scanning electron microscopy (FSEM) micrographs were obtained on Hitachi S3400N SEM and S-4800 FSEM respectively. The X-ray energy dispersive spectroscopy (EDX) attached to S-4800 FSEM was employed to determine chemical contents. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL JEM-2100F TEM.
Fig. 1. Relationship between endothermic rate and thermal effect (a); typical XRD patterns (b and c) and FTIR spectrum (d) of the amorphous boron powders.
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3. Results and discussion 3.1. Results Fig. 1(b) and (c) give the typical XRD patterns of the obtained raw products before and after purification processes respectively (the endothermic rates of 30%). It is obvious that the presented peaks in Fig. 1(b) could be indexed to be MgO (JCPDF 65-0476), Mg3B2O6 (JCPDF 38-1475) and KOH (JCPDF 15-0890). No noticeable peaks of other impurities, such as BxO and MgBx, were detected in this pattern. The XRD patterns of boron powders (Fig. 1(c)) reveal a broad peak ascribed to an amorphous structure. As shown in the FTIR spectrum (Fig. 1(d)), the characteristic peaks (2843 cm1 and 2917 cm1) of crystalline boron could not be observed [5,30]. However, the characteristic absorption peaks of boric acid (712 cm1, 812 cm1, 902 cm1, 1108 cm1, 1380 cm1 and 1640 cm1) could be demonstrated because of surface oxidation and hydrolysis [31]. The absorption bands at about 1640 cm1 and 1380 cm1 are due to bending vibration of H–O–H bonds and asymmetric stretching vibration of trigonal B–O bonds respectively. The absorption bands at 1108 cm1 and 902 cm1 are assigned as B–O–H in plane bending vibration and symmetrical trigonal B–O stretching vibration respectively. The absorption bands at 812 cm1 and 712 cm1 are corresponding to B–O–H out of plane bending vibration and trigonal B–O bending vibration respectively [5,31]. Therefore, the prepared boron powders were amorphous. The typical SEM and FSEM images of the prepared amorphous boron powders with different endothermic rates were depicted in Fig. 2. When the endothermic rate was 0% (without diluter KBH4), the sample displayed a random structure with non-uniform particle sizes in the range of 0.2–3.0 mm and an average of about 0.5 mm as shown in Fig. 2(a). With the increase of KBH4 (30% and 60%), the morphology of amorphous boron powders changed from sphericity (Fig. 2(c)) to rod (Fig. 2(e) and (g)). Both of them demonstrate uniform particle sizes and the corresponding estimated average particle sizes were about 50 nm and 80 nm
respectively. It indicates that the morphology and particle size could be controlled effectively through properly changing the additional rate of the diluter KBH4. TEM and HRTEM observations were also carried out to investigate the structural features of the products. The typical TEM image of Fig. 2(h) indicates an amorphous rod-like structure with an average diameter of 20 nm and a length of 80 nm. As shown in HRTEM image (Fig. 2(j)), the lattice fringes are not orderly arranged, suggesting an amorphous structure of the boron powders. In fact, amorphous materials have a disordered atomic structure as compared to crystalline materials. That is, they have only short-range order rather than the long-range periodicity of crystals, which could also be observed from the insert image of partial enlargement (Fig. 2(i)) denoted by a frame in Fig. 2(j). The corresponding EDX spectrums with different endothermic rates (0%, 30% and 60%) presenting in Fig. 2(b), (d) and (f) demonstrate the signal of boron. The oxygen peak may be ascribed to impurity and the surface oxidation. And the Mg peak can be resulted from impurity. In addition, quantitative analysis indicates the contents of boron are 87.30 wt.%, 95.64 wt.% and 93.69 wt.% respectively. The particle size distribution of the prepared amorphous boron powders was also observed through the laser particle size analyzer, as shown in Fig. 3(a) and (b). When the endothermic rate was 0%, the obtained d0.5 = 2.31 mm, d0.1 = 1.22 mm and d0.9 = 3.89 mm (Fig. 3(a)). As the endothermic rate increased to 30%, the obtained d0.5 = 0.87 mm, d0.1 = 0.42 mm and d0.9 = 1.52 mm (Fig. 3(b)). There exists a significant difference between the results of the laser particle size analyzer and those of FSEM. We suggest that it will be caused by two reasons: (1) the intrinsic property effect of the prepared amorphous boron powders. The irregular amorphous boron powders present a large surface free energy and a strong surface adsorption. It is very easy that many single amorphous boron particles assemble together and form new larger congeries. So the obtained laser particle size results may display the actual particle size of the larger congeries but not that of the single amorphous boron particles [32]. (2) The effect of the test
Fig. 2. Typical SEM (e), FSEM (a, c and g), EDX (b, d and f), TEM (h) and HRTEM (i and j) images of the amorphous boron powders. Scale bars: (a) 3 mm, (c) 500 nm, (e) 5 mm, (g) 1 mm, (h) 50 nm and (j) 5 nm.
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Fig. 3. Particle size distribution of the amorphous boron powders prepared at different endothermic rates: (a) 0% and (b) 30%.
instrument. As to a laser particle size analyzer, the acquiescent morphology of the tested object is spherical particle. However, the as-synthesized amorphous boron particles have not only an irregular morphology but also a morphological change from a sphere-like to rod-like. Although the results are not accurate, it is obvious that the particle size has become smaller after adding the diluter KBH4 by comparing the particle size distribution at different endothermic rates (0% and 30%). The related characteristics of amorphous boron powders prepared at different endothermic rates from 0% to 70% were shown in Table 1. The oxygen content was investigated using Leco TC-600 oxygen and nitrogen analyzer. All of the yields were higher than 80%. In addition, when the endothermic rate was 0%, the sample displayed a low yield and purity as well as a large particle size. Compared with the sample synthesized via a single magnesiothermic reduction process, adding the diluter KBH4 could effectively moderate the violence of the reaction and thus achieve high purity nano-sized amorphous boron powders finally. In this experiment, the possible reactions are likely to occur as follows: KBH4 ! KH þ BH3
(3)
B2 O3 þ 3KH ! 2B þ 1:5 K2 O þ 1:5H2 O
(4)
B2 O3 þ 2BH3 ! 4B þ 3H2 O
(5)
nB ! amorphousB
(6)
K2 O þ H2 O ! 2KOH
(7)
The reactions Eqs. (1) and (2) are the whole reactions of synthesis of amorphous boron powders. And the reactions Eqs. (3)– (7) are supposed important reactions on the base of the whole reactions during the growth processes of boron powders. The Gibbs energies for the reaction Eqs. (1) and (2) were calculated as
Fig. 4. Proposed common active dilution model suitable for preparing various of inorganic materials. (A1, B1, A2, B2)-reactants; C1-product; (X, Y)-byproducts.
456.64 kJ/mol and 251.42 kJ/mol at about 750 8C, respectively. It indicates that the Eq. (1) reacts easily but difficult as to Eq. (2). It is reasonable that the reaction Eq. (2) could be induced via using the large quantities of heat released by Eq. (1). During the active dilution processes, magnesiothermic reduction produced nascent active boron (B*) vapor (Eq. (1)) [22,23,27]. The reducing agent KBH4 was decomposed into KH and BH3 at about 500 8C (Eq. (3)) [33–36]. The new prepared high chemical reactive substances KH and BH3 also present strong reducibility, and it is easy to react with liquid or gaseous B2O3 under the microcosmic regional high temperature and pressure enviroment followed generated B*, K2O and gaseous H2O (Eqs. (4) and (5)) [37–41]. Meanwhile, amorphous B began to grow in a very short time (Eq. (6)). Finally, K2O combined with H2O and formed KOH (Eq. (7)). In the matter of fact, this active dilution method could be further popularized and become a common approach to prepare various inorganic materials. The proposed active dilution model of preparing inorganic materials is illustrated in Fig. 4. Generally, it is a key point to find two or more reactions not only exothermic but also endothermic. In addition, the desired products of these reactions could be obtained through each reaction. Moreover, maybe the morphology, particle size and purity of the product could also be controlled by these principles. Further work is still needed to study this subject in the future.
Table 1 Related characteristics of amorphous boron powders prepared with different endothermic rates. Endothermic rate
Purity (wt.%)
Average particle size (nm)
Oxygen content (wt.%)
Yield (%)
SHS intensity
0% 10% 20% 30% 40% 50% 60% 70%
87.33 92.45 93.72 95.64 95.11 94.27 93.69 92.77
500 90 55 50 65 75 80 95
4.04 3.44 2.98 1.60 2.46 3.01 2.73 3.57
82.5 86.4 85.1 86.8 86.7 87.5 88.9 87.4
$$$$ Explosive $$$$ Very severe $$$ Moderate $$$ Mild $$ Very mild $$ Weak $ Very weak $ Very weak
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J. Wang et al. / Materials Research Bulletin 48 (2013) 2018–2022
4. Conclusions In summary, nano-sized amorphous boron powders were prepared effectively through active dilution self-propagating high-temperature synthesis (SHS) method. After only one leaching and purifying process, the purity of amorphous boron powders could reach 95.64 wt.%. The morphology, particle size and purity of the products could also be controlled via changing the endothermic reaction rate. This method could be further popularized and become a common approach to prepare various inorganic materials. We hope that the present work is useful for the popularization and application of the amorphous boron powders as well as the active dilution method. Acknowledgments The authors acknowledge the financial support from the National Science Foundation of China (A3 Foresight Project No. 51161140399) and the Fundamental Research Funds for the Central Universities (No. 123201003). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
M. Chobotov, G. Purobit, J. Spacecraft Rockets 30 (1993) 34–39. T. Liu, Propellants Explos. Pyrotech. 16 (1991) 156–166. B. Devener, J. Perez, J. Jankovich, S. Anderson, Energy Fuels 23 (2009) 6111–6120. W. Pang, X. Fan, H. Xu, F. Zhao, Y. Li, J. Solid Rocket Technol. 33 (2010) 196–200. T. Xu, J. Zheng, N. Wu, A. Nicholls, J. Roth, D. Dikin, R. Ruoff, Nano Lett. 4 (2004) 963–968. L. Guo, R. Singh, H. Kleebe, Chem.Vap. Deposition 12 (2006) 448–452. A. Newkirk, in: R. Adams (Ed.), Boron, Metallo-Boron Compounds and Boranes, Interscience Publishers, New York, 1964, pp. 233–288. M. Setten, M. Uijttewaal, G. Wijs, R. Groot, J. Am. Chem. Soc. 129 (2007) 2458–2465. P. Si, M. Zhang, C. You, D. Geng, J. Du, X. Zhao, X. Ma, Z. Zhang, J. Mater. Sci. 38 (2003) 689–692. A. Jain, S. Anthonysamy, K. Ananthasivan, G. Gupta, Thermochim. Acta 500 (2010) 63–68. I. Vishnevetsky, M. Epstein, T. Abuhamed, J. Karni, J. Sol. Energy Eng. 130 (2008) 014506(1)–014506(5).
[12] J. Wang, L. Zhang, Y. Gu, X. Pan, G. Zhao, Z. Zhang, Mater. Res. Bull. 48 (2013) 943–947. [13] K. Kirihara, T. Sasaki, N. Koshizaki, K. Kimura, Appl. Phys. Express 4 (2011) 0412011–0412013. [14] X. Wang, J. Tian, T. Yang, L. Bao, C. Hui, F. Liu, C. Shen, C. Gu, N. Xu, H. Gao, Adv. Mater. 19 (2007) 4480–4485. [15] W. Ding, L. Calabri, X. Chen, K. Kohlhaas, R. Ruoff, Compos. Sci. Technol. 66 (2006) 1109–1121. [16] A. Chakrabarti, T. Xu, L. Paulson, K. Krise, J. Maguire, N. Hosmane, J. Nanomater. (2010), http://dx.doi.org/10.1155/2010/589372. [17] H. Johnston, H. Hersh, E. Kerr, J. Am. Chem. Soc. 73 (1951) 1112–1117. [18] J. Casey, J. Haggerty, J. Mater. Sci. 22 (1987) 737–744. [19] X. Gao, Z. Wu, G. Tan, F. Yang, J. Beijing Univ. Chem. Technol. 22 (1995) 58–64. [20] H. Wang, W. Zhang, X. Zhang, L. Jiang, Chem. Eng. 36 (2008) 59–61. [21] Y. Yi, J. Mater. Sci. 25 (1990) 1159–1168. [22] J. Wu, B. Yang, W. Ma, D. Liu, Y. Dai, G. Zhang, Y. Zhai, J. Funct. Mater. 38 (2007) 2073–2076. [23] J. Wu, W. Ma, Y. Dai, Y. Zhai, G. Zhang, Rare Metal Mat. Eng. 38 (2009) 112–116. [24] K. Tilekar, V. Gajbhiye, H. Prasanth, T. Soman, Def. Sci J. 55 (2005) 471–475. [25] Y. Wang, X. Duan, L. Cao, G. Li, W. Wang, J. Cryst. Growth 244 (2002) 123–128. [26] S. Shalamberidze, G. Kalandadze, D. Khulelidze, B.T. Surtsumia, J. Solid State Chem. 154 (2000) 199–203. [27] Z. Dou, T. Zhang, Y. Wang, J. Northeastern Univ. 26 (2005) 267–270. [28] L. Zhang, J. Wang, Y. Gu, G. Zhao, Q. Qian, J. Li, X. Pan, Z. Zhang, Mater. Lett. 67 (2012) 17–20. [29] J. Wang, L. Zhang, Y. Gu, X. Pan, G. Zhao, Z. Zhang, J. Exp. Nanosci. (2012), http:// dx.doi.org/10.1080/17458080.2011.616537. [30] S. Yu, Y. Ji, T. Li, M. Han, G. Wang, X. Yuan, Z. Li, P. Chen, Solid State Commun. 115 (2000) 307–311. [31] D. Peak, G. Lither, D. Sparks, Geochim. Cosmochim. Acta 67 (2003) 2251– 2260. [32] J. Wu, W. Ma, G. Zhang, Y. Zhai, B. Yang, Y. Dai, Chin. J. Nonferrous Met. 17 (2007) 2034–2039. [33] G. Sun, F. Gao, L. Hou, Can. J. Chem. 88 (2010) 1256–1261. [34] J. Hu, Q. Lu, K. Tang, S. Yu, Y. Qian, G. Zhou, X. Liu, J. Wu, J. Solid State Chem. 148 (1999) 325–328. [35] P. Vajeeston, P. Ravindran, H. Fjellvag, J. Nanosci. Nanotechnol. 10 (2010) 1–6. [36] Y. Wu, Y. Ni, Chem. Eng. J. 187 (2012) 328–333. [37] H. Diyabalange, T. Nakagawa, R. Shrestha, T. Semelsberger, B. Davis, B. Scott, A. Burrel, W. David, K. Ryan, M. Jones, P. Edwards, J. Am. Chem. Soc. 132 (2010) 11836–11837. [38] J. Medina, G. Cruz, C. Cabrera, J. Soderquist, J. Org. Chem. 68 (2003) 4631–4642. [39] R. Pi, T. Friedl, P. Schelyer, J. Org. Chem. 52 (1987) 4299–4303. [40] R. Gremaud, A. Borgschulte, O. Friedrichs, A. Zuttle, J. Phys. Chem. C 115 (2011) 2489–2496. [41] C. Lane, Chem. Rev. 76 (1976) 773–799.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of nano-sized amorphous boron powders through active dilution self-propagating high-temperature synthesis method Jilin Wang a, Yunle Gu b, Zili Li a, Weimin Wang a,*, Zhengyi Fu a a b
The State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 May 2012 Received in revised form 25 January 2013 Accepted 27 January 2013 Available online 27 February 2013
Nano-sized amorphous boron powders were synthesized by active dilution self-propagating hightemperature synthesis (SHS) method at temperatures ranging from 700 8C to 850 8C in a SHS furnace using Mg, B2O3 and KBH4 as raw materials. Samples were characterized by X-ray powder diffraction (XRD), Laser particle size analyzer, Fourier transform infrared spectra (FTIR), X-ray energy dispersive spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission TEM (HRTEM). The boron powders demonstrated an average particle size of 50 nm with a purity of 95.64 wt.%. The diluter KBH4 played an important role in the active dilution synthesis of amorphous nano-sized boron powders. The effects of endothermic reaction rate, the possible chemical reaction mechanism and active dilution model for synthesis of the product were also discussed. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials A. Nanostructures B. Chemical synthesis C. Electron diffraction
1. Introduction Amorphous boron powders have a higher volumetric energy density than the traditional fuels like magnesium, aluminum, carbon and hydrocarbon [1–4]. It also reveals a high melting point, high hardness, low density, good chemical inertness and thermal stability, interesting electrical and optical properties [5–8]. These unique properties make amorphous boron powders an attractive prospect for practical applications in many fields such as nuclear reactor control, rocket igniter, thermo-electric energy conversion, high-temperature devices, refractory materials and semiconductors [4,6,9–12]. Recent studies about the element boron tend to pay more attention to the preparation, characterization and application of crystalline boron but seldom of amorphous boron [5,13–16]. As to amorphous boron, various synthetic methods have been employed to prepare amorphous boron powders, such as diborane pyrolysis [17], laser ablation [18,19], molten-salt electrolysis [20], magnesiothermic reduction [21–23]. In these methods, the magnesiothermic reduction also belongs to self-propagating hightemperature synthesis (SHS), which is a novel technology for synthesis of various of materials. Once the reaction starts, it will
* Corresponding author. Tel.: +86 27 87215421; fax: +86 27 87215421. E-mail addresses: [email protected], [email protected] (W. Wang). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.053
continue with the help of the energy provided by itself, rather than from an outside system. The amorphous boron powders prepared through this method present a small particle size and high activity, which are appropriate for the popularization and application. However, it is hard to control the SHS reaction process due to the high exothermic effect in a few seconds. Excessive temperature will cause many kinds of impurities, such as MgxB2O(x+1) (x = 2, 3), MgBx (x = 2, 4, 6), BxO [24–26], and lead to low purity (88 wt.%) as well as large particle size (about 0.5–5 mm). Generally, two dilution methods were used to adjust the SHS reaction process, excess B2O3 or Mg, and inert diluents (NaCl, KCl, MgCl2 and MgO). Nevertheless, these control methods were demonstrated limited effect and consumptive of reactants or inert reagents. Moreover, additional three leaching and purifying processes were needed in order to obtain higher purity (90 wt.%) products [27]. The present work introduces a new way so-called active dilution method to synthesize nano-sized amorphous boron powders. Higher purity products could be prepared after only one leaching and purifying processes. The exothermic reaction could be effectively controlled through changing the endothermic reaction rate, which promotes the preparation of amorphous boron powders and inhibits the formation of by-products. The effects of endothermic reaction rate, the possible chemical reaction mechanism and active dilution model for synthesis of the product were also discussed. In addition, synthesized high active nano-sized amorphous boron powders have been applied successfully to the large scale preparation of boron nitride nanotubes [12,28,29].
J. Wang et al. / Materials Research Bulletin 48 (2013) 2018–2022
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powders could be controlled via changing the endothermic reaction rate.
2. Experimental 2.1. Materials
2.3. Synthesis techniques The reactants, magnesium (Mg), boron oxide (B2O3) and potassium borohydride (KBH4) were of analytical pure grade and about 150–300 mesh (50–100 mm) powders. 2.2. Theoretical calculations The chemical reaction equations of the SHS reaction system B2O3/Mg/KBH4 are presented as follows: B2 O3 þ 3Mg ! 2B þ 3MgO
(1)
B2 O3 þ 2KBH4 ! 4B þ 2KOH þ H2 O þ 2H2
(2)
In a typical procedure, 27.38 g B2O3, 23.64 g Mg and 7.52 g KBH4, where the endothermic rate was 30%, were mixed by a blender mixer for 6–12 min. Then the mixture was pressed to be a cylinder with a diameter of 30 mm. The cylinder was heated under argon atmosphere at 830 8C for 13 min in a SHS-furnace. After cooled to room temperature naturally, the furnace was opened, and the crude product was collected and washed with 5 M hydrochloric acid, ethanol and distilled water. Finally, 8.68 g brown boron powders were collected by drying in vacuum at 80 8C for 12 h. The yield of amorphous boron powders based on boron was about 86.8%. 2.4. Characterization
The standard molar enthalpies of Eqs. (1) and (2) were calculated as 24.57 kJ/(gram B) and +14.73 kJ/(gram B). The reaction system B2O3/Mg is extremely exothermic while the other reaction system B2O3/KBH4 is highly endothermic. The SHS reaction process could be effectively controlled through properly changing the additional rate of the diluter KBH4. The relationship between endothermic rate and thermal effect was illustrated in Fig. 1(a). The abscissa endothermic rate represents the percentage of B prepared through endothermic reaction Eq. (2) in the total B. When the heat effect of the whole reaction system achieves equilibrium, the endothermic rate will reach 62.52%. In addition, we suggested that the proper endothermic rate is ranging from 10% to 70%. Moreover, it is likely that the SHS intensity, particle size, purity and morphology of the boron
The samples were characterized by X-ray powder diffraction (XRD) using a Rigaku D/MAX-LLIA X-ray diffractometer (radiation wavelength l = 1.5406 A˚). Mastersizer 2000 laser particle size analyzer was used for analysis of the particle size distribution of the sample. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 6700 FTIR spectrometer. The scanning electron microscopy (SEM) and field emission scanning electron microscopy (FSEM) micrographs were obtained on Hitachi S3400N SEM and S-4800 FSEM respectively. The X-ray energy dispersive spectroscopy (EDX) attached to S-4800 FSEM was employed to determine chemical contents. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL JEM-2100F TEM.
Fig. 1. Relationship between endothermic rate and thermal effect (a); typical XRD patterns (b and c) and FTIR spectrum (d) of the amorphous boron powders.
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3. Results and discussion 3.1. Results Fig. 1(b) and (c) give the typical XRD patterns of the obtained raw products before and after purification processes respectively (the endothermic rates of 30%). It is obvious that the presented peaks in Fig. 1(b) could be indexed to be MgO (JCPDF 65-0476), Mg3B2O6 (JCPDF 38-1475) and KOH (JCPDF 15-0890). No noticeable peaks of other impurities, such as BxO and MgBx, were detected in this pattern. The XRD patterns of boron powders (Fig. 1(c)) reveal a broad peak ascribed to an amorphous structure. As shown in the FTIR spectrum (Fig. 1(d)), the characteristic peaks (2843 cm1 and 2917 cm1) of crystalline boron could not be observed [5,30]. However, the characteristic absorption peaks of boric acid (712 cm1, 812 cm1, 902 cm1, 1108 cm1, 1380 cm1 and 1640 cm1) could be demonstrated because of surface oxidation and hydrolysis [31]. The absorption bands at about 1640 cm1 and 1380 cm1 are due to bending vibration of H–O–H bonds and asymmetric stretching vibration of trigonal B–O bonds respectively. The absorption bands at 1108 cm1 and 902 cm1 are assigned as B–O–H in plane bending vibration and symmetrical trigonal B–O stretching vibration respectively. The absorption bands at 812 cm1 and 712 cm1 are corresponding to B–O–H out of plane bending vibration and trigonal B–O bending vibration respectively [5,31]. Therefore, the prepared boron powders were amorphous. The typical SEM and FSEM images of the prepared amorphous boron powders with different endothermic rates were depicted in Fig. 2. When the endothermic rate was 0% (without diluter KBH4), the sample displayed a random structure with non-uniform particle sizes in the range of 0.2–3.0 mm and an average of about 0.5 mm as shown in Fig. 2(a). With the increase of KBH4 (30% and 60%), the morphology of amorphous boron powders changed from sphericity (Fig. 2(c)) to rod (Fig. 2(e) and (g)). Both of them demonstrate uniform particle sizes and the corresponding estimated average particle sizes were about 50 nm and 80 nm
respectively. It indicates that the morphology and particle size could be controlled effectively through properly changing the additional rate of the diluter KBH4. TEM and HRTEM observations were also carried out to investigate the structural features of the products. The typical TEM image of Fig. 2(h) indicates an amorphous rod-like structure with an average diameter of 20 nm and a length of 80 nm. As shown in HRTEM image (Fig. 2(j)), the lattice fringes are not orderly arranged, suggesting an amorphous structure of the boron powders. In fact, amorphous materials have a disordered atomic structure as compared to crystalline materials. That is, they have only short-range order rather than the long-range periodicity of crystals, which could also be observed from the insert image of partial enlargement (Fig. 2(i)) denoted by a frame in Fig. 2(j). The corresponding EDX spectrums with different endothermic rates (0%, 30% and 60%) presenting in Fig. 2(b), (d) and (f) demonstrate the signal of boron. The oxygen peak may be ascribed to impurity and the surface oxidation. And the Mg peak can be resulted from impurity. In addition, quantitative analysis indicates the contents of boron are 87.30 wt.%, 95.64 wt.% and 93.69 wt.% respectively. The particle size distribution of the prepared amorphous boron powders was also observed through the laser particle size analyzer, as shown in Fig. 3(a) and (b). When the endothermic rate was 0%, the obtained d0.5 = 2.31 mm, d0.1 = 1.22 mm and d0.9 = 3.89 mm (Fig. 3(a)). As the endothermic rate increased to 30%, the obtained d0.5 = 0.87 mm, d0.1 = 0.42 mm and d0.9 = 1.52 mm (Fig. 3(b)). There exists a significant difference between the results of the laser particle size analyzer and those of FSEM. We suggest that it will be caused by two reasons: (1) the intrinsic property effect of the prepared amorphous boron powders. The irregular amorphous boron powders present a large surface free energy and a strong surface adsorption. It is very easy that many single amorphous boron particles assemble together and form new larger congeries. So the obtained laser particle size results may display the actual particle size of the larger congeries but not that of the single amorphous boron particles [32]. (2) The effect of the test
Fig. 2. Typical SEM (e), FSEM (a, c and g), EDX (b, d and f), TEM (h) and HRTEM (i and j) images of the amorphous boron powders. Scale bars: (a) 3 mm, (c) 500 nm, (e) 5 mm, (g) 1 mm, (h) 50 nm and (j) 5 nm.
J. Wang et al. / Materials Research Bulletin 48 (2013) 2018–2022
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Fig. 3. Particle size distribution of the amorphous boron powders prepared at different endothermic rates: (a) 0% and (b) 30%.
instrument. As to a laser particle size analyzer, the acquiescent morphology of the tested object is spherical particle. However, the as-synthesized amorphous boron particles have not only an irregular morphology but also a morphological change from a sphere-like to rod-like. Although the results are not accurate, it is obvious that the particle size has become smaller after adding the diluter KBH4 by comparing the particle size distribution at different endothermic rates (0% and 30%). The related characteristics of amorphous boron powders prepared at different endothermic rates from 0% to 70% were shown in Table 1. The oxygen content was investigated using Leco TC-600 oxygen and nitrogen analyzer. All of the yields were higher than 80%. In addition, when the endothermic rate was 0%, the sample displayed a low yield and purity as well as a large particle size. Compared with the sample synthesized via a single magnesiothermic reduction process, adding the diluter KBH4 could effectively moderate the violence of the reaction and thus achieve high purity nano-sized amorphous boron powders finally. In this experiment, the possible reactions are likely to occur as follows: KBH4 ! KH þ BH3
(3)
B2 O3 þ 3KH ! 2B þ 1:5 K2 O þ 1:5H2 O
(4)
B2 O3 þ 2BH3 ! 4B þ 3H2 O
(5)
nB ! amorphousB
(6)
K2 O þ H2 O ! 2KOH
(7)
The reactions Eqs. (1) and (2) are the whole reactions of synthesis of amorphous boron powders. And the reactions Eqs. (3)– (7) are supposed important reactions on the base of the whole reactions during the growth processes of boron powders. The Gibbs energies for the reaction Eqs. (1) and (2) were calculated as
Fig. 4. Proposed common active dilution model suitable for preparing various of inorganic materials. (A1, B1, A2, B2)-reactants; C1-product; (X, Y)-byproducts.
456.64 kJ/mol and 251.42 kJ/mol at about 750 8C, respectively. It indicates that the Eq. (1) reacts easily but difficult as to Eq. (2). It is reasonable that the reaction Eq. (2) could be induced via using the large quantities of heat released by Eq. (1). During the active dilution processes, magnesiothermic reduction produced nascent active boron (B*) vapor (Eq. (1)) [22,23,27]. The reducing agent KBH4 was decomposed into KH and BH3 at about 500 8C (Eq. (3)) [33–36]. The new prepared high chemical reactive substances KH and BH3 also present strong reducibility, and it is easy to react with liquid or gaseous B2O3 under the microcosmic regional high temperature and pressure enviroment followed generated B*, K2O and gaseous H2O (Eqs. (4) and (5)) [37–41]. Meanwhile, amorphous B began to grow in a very short time (Eq. (6)). Finally, K2O combined with H2O and formed KOH (Eq. (7)). In the matter of fact, this active dilution method could be further popularized and become a common approach to prepare various inorganic materials. The proposed active dilution model of preparing inorganic materials is illustrated in Fig. 4. Generally, it is a key point to find two or more reactions not only exothermic but also endothermic. In addition, the desired products of these reactions could be obtained through each reaction. Moreover, maybe the morphology, particle size and purity of the product could also be controlled by these principles. Further work is still needed to study this subject in the future.
Table 1 Related characteristics of amorphous boron powders prepared with different endothermic rates. Endothermic rate
Purity (wt.%)
Average particle size (nm)
Oxygen content (wt.%)
Yield (%)
SHS intensity
0% 10% 20% 30% 40% 50% 60% 70%
87.33 92.45 93.72 95.64 95.11 94.27 93.69 92.77
500 90 55 50 65 75 80 95
4.04 3.44 2.98 1.60 2.46 3.01 2.73 3.57
82.5 86.4 85.1 86.8 86.7 87.5 88.9 87.4
$$$$ Explosive $$$$ Very severe $$$ Moderate $$$ Mild $$ Very mild $$ Weak $ Very weak $ Very weak
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4. Conclusions In summary, nano-sized amorphous boron powders were prepared effectively through active dilution self-propagating high-temperature synthesis (SHS) method. After only one leaching and purifying process, the purity of amorphous boron powders could reach 95.64 wt.%. The morphology, particle size and purity of the products could also be controlled via changing the endothermic reaction rate. This method could be further popularized and become a common approach to prepare various inorganic materials. We hope that the present work is useful for the popularization and application of the amorphous boron powders as well as the active dilution method. Acknowledgments The authors acknowledge the financial support from the National Science Foundation of China (A3 Foresight Project No. 51161140399) and the Fundamental Research Funds for the Central Universities (No. 123201003). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
M. Chobotov, G. Purobit, J. Spacecraft Rockets 30 (1993) 34–39. T. Liu, Propellants Explos. Pyrotech. 16 (1991) 156–166. B. Devener, J. Perez, J. Jankovich, S. Anderson, Energy Fuels 23 (2009) 6111–6120. W. Pang, X. Fan, H. Xu, F. Zhao, Y. Li, J. Solid Rocket Technol. 33 (2010) 196–200. T. Xu, J. Zheng, N. Wu, A. Nicholls, J. Roth, D. Dikin, R. Ruoff, Nano Lett. 4 (2004) 963–968. L. Guo, R. Singh, H. Kleebe, Chem.Vap. Deposition 12 (2006) 448–452. A. Newkirk, in: R. Adams (Ed.), Boron, Metallo-Boron Compounds and Boranes, Interscience Publishers, New York, 1964, pp. 233–288. M. Setten, M. Uijttewaal, G. Wijs, R. Groot, J. Am. Chem. Soc. 129 (2007) 2458–2465. P. Si, M. Zhang, C. You, D. Geng, J. Du, X. Zhao, X. Ma, Z. Zhang, J. Mater. Sci. 38 (2003) 689–692. A. Jain, S. Anthonysamy, K. Ananthasivan, G. Gupta, Thermochim. Acta 500 (2010) 63–68. I. Vishnevetsky, M. Epstein, T. Abuhamed, J. Karni, J. Sol. Energy Eng. 130 (2008) 014506(1)–014506(5).
[12] J. Wang, L. Zhang, Y. Gu, X. Pan, G. Zhao, Z. Zhang, Mater. Res. Bull. 48 (2013) 943–947. [13] K. Kirihara, T. Sasaki, N. Koshizaki, K. Kimura, Appl. Phys. Express 4 (2011) 0412011–0412013. [14] X. Wang, J. Tian, T. Yang, L. Bao, C. Hui, F. Liu, C. Shen, C. Gu, N. Xu, H. Gao, Adv. Mater. 19 (2007) 4480–4485. [15] W. Ding, L. Calabri, X. Chen, K. Kohlhaas, R. Ruoff, Compos. Sci. Technol. 66 (2006) 1109–1121. [16] A. Chakrabarti, T. Xu, L. Paulson, K. Krise, J. Maguire, N. Hosmane, J. Nanomater. (2010), http://dx.doi.org/10.1155/2010/589372. [17] H. Johnston, H. Hersh, E. Kerr, J. Am. Chem. Soc. 73 (1951) 1112–1117. [18] J. Casey, J. Haggerty, J. Mater. Sci. 22 (1987) 737–744. [19] X. Gao, Z. Wu, G. Tan, F. Yang, J. Beijing Univ. Chem. Technol. 22 (1995) 58–64. [20] H. Wang, W. Zhang, X. Zhang, L. Jiang, Chem. Eng. 36 (2008) 59–61. [21] Y. Yi, J. Mater. Sci. 25 (1990) 1159–1168. [22] J. Wu, B. Yang, W. Ma, D. Liu, Y. Dai, G. Zhang, Y. Zhai, J. Funct. Mater. 38 (2007) 2073–2076. [23] J. Wu, W. Ma, Y. Dai, Y. Zhai, G. Zhang, Rare Metal Mat. Eng. 38 (2009) 112–116. [24] K. Tilekar, V. Gajbhiye, H. Prasanth, T. Soman, Def. Sci J. 55 (2005) 471–475. [25] Y. Wang, X. Duan, L. Cao, G. Li, W. Wang, J. Cryst. Growth 244 (2002) 123–128. [26] S. Shalamberidze, G. Kalandadze, D. Khulelidze, B.T. Surtsumia, J. Solid State Chem. 154 (2000) 199–203. [27] Z. Dou, T. Zhang, Y. Wang, J. Northeastern Univ. 26 (2005) 267–270. [28] L. Zhang, J. Wang, Y. Gu, G. Zhao, Q. Qian, J. Li, X. Pan, Z. Zhang, Mater. Lett. 67 (2012) 17–20. [29] J. Wang, L. Zhang, Y. Gu, X. Pan, G. Zhao, Z. Zhang, J. Exp. Nanosci. (2012), http:// dx.doi.org/10.1080/17458080.2011.616537. [30] S. Yu, Y. Ji, T. Li, M. Han, G. Wang, X. Yuan, Z. Li, P. Chen, Solid State Commun. 115 (2000) 307–311. [31] D. Peak, G. Lither, D. Sparks, Geochim. Cosmochim. Acta 67 (2003) 2251– 2260. [32] J. Wu, W. Ma, G. Zhang, Y. Zhai, B. Yang, Y. Dai, Chin. J. Nonferrous Met. 17 (2007) 2034–2039. [33] G. Sun, F. Gao, L. Hou, Can. J. Chem. 88 (2010) 1256–1261. [34] J. Hu, Q. Lu, K. Tang, S. Yu, Y. Qian, G. Zhou, X. Liu, J. Wu, J. Solid State Chem. 148 (1999) 325–328. [35] P. Vajeeston, P. Ravindran, H. Fjellvag, J. Nanosci. Nanotechnol. 10 (2010) 1–6. [36] Y. Wu, Y. Ni, Chem. Eng. J. 187 (2012) 328–333. [37] H. Diyabalange, T. Nakagawa, R. Shrestha, T. Semelsberger, B. Davis, B. Scott, A. Burrel, W. David, K. Ryan, M. Jones, P. Edwards, J. Am. Chem. Soc. 132 (2010) 11836–11837. [38] J. Medina, G. Cruz, C. Cabrera, J. Soderquist, J. Org. Chem. 68 (2003) 4631–4642. [39] R. Pi, T. Friedl, P. Schelyer, J. Org. Chem. 52 (1987) 4299–4303. [40] R. Gremaud, A. Borgschulte, O. Friedrichs, A. Zuttle, J. Phys. Chem. C 115 (2011) 2489–2496. [41] C. Lane, Chem. Rev. 76 (1976) 773–799.