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Synthesis of uniform plate-like boron nitride nanoparticles from boron oxide by ball milling and annealing process
Materials Letters 108 (2013) 96–102
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of uniform plate-like boron nitride nanoparticles from boron oxide by ball milling and annealing process Yanjiao Li a,n, Youjun Wang a, Qiujuan Lv a, Zhongbao Qin a, Xinkuan Liu b,n a b
Xi'an Hi-Tech Research Institute, Hongqing Town, Xi'an 710025, China School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 February 2013 Accepted 16 June 2013 Available online 21 June 2013
A great many boron nitride (BN) nanoparticles were synthesized from boron oxide by high-energy ball milling and annealing process at 1200 1C. The sample was characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, selected area electron diffraction, high-resolution transmission electron microscopy and the X-Ray energy dispersive spectroscopy. The results showed that the morphology of most synthesized BN nanoparticles is elliptical plate-like, while the others are circular plate-like. The BN nanoparticles are uniform at the diameter from 20 nm to 100 nm, and most of them are around 70 nm in diameter. The thickness of them is about 10 nm estimated from SEM image. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ceramics Boron nitride Ball milling Plate-like
1. Introduction Since the discovery of boron nitride (BN) nanotubes [1], various types of BN nanostructured materials such as BN nanocage clusters [2], BN nanotubes [3–5], BN nanocapsules [6,7], BN nanohorns [8,9], BN nanoropes [10] and BN nanoparticles [11,12] have been reported for their advantageous properties such as high thermal conductivity, marked chemical inertness and good electrical insulation. BN nanoparticles, which can be used as catalytic supports [13], thermalconductive fillers [14] and composite additives [15] received considerable attention recently. For their high thermal conductivity and good insulating property, BN nanoparticles powder may be a suitable additive for synthesis of dielectric nanofluids [16]. Spherical and plate-like is the common morphology of boron nitride nanoparticles. Compared with spherical shape BN nanoparticles, the specific surface area of plate-like BN nanoparticles is more higher [13,15], which is good for thermal conductivity enhancement of nanofluids. Besides, the suspension stability of plate-like BN nanoparticles in base fluids will be better than spherical shape BN nanoparticles at the same size [16]. The ball milling-annealing method, which was first proposed by Chen et al. [17], is a widely used approach for producing nanomaterials [18]. The main merits of this method are available in low reaction temperature and high yield. In this paper, synthesis of uniform plate-like BN nanoparticles from boron oxide by ball millingannealing method will be reported and the micro morphology of the synthesized BN nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM) also. In this research, n
Corresponding authors. Tel.: +86 215 527 1682. E-mail addresses: [email protected].(Y. Li) [email protected], [email protected].(X. Liu) 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.06.043
boron oxide, which was inexpensive compared with boron or boron nitride, was used as raw material. 2. Experiments B2O3 powder with a purity of 99.9% was first milled in a highenergy ball mill with a stainless steel vessel and hardened chromium steel ball of 6 mm in diameter under high purity N2 atmosphere at a rotation speed of 450 rpm for 60 h. The weight ratio of the milling balls to B2O3 powder was 20:1. The milled B2O3 powder was then annealed in a GLS1600 tube furnace at 1200 1C for 6 h under an ammonia gas flow. The high purity ammonia gas flow was controlled at 150 ml/min. Crystalline structure of the product was investigated by means of X-ray diffraction analysis(XRD) using Cu Ka radiation (λ¼0.15418 nm) at room temperature. Morphology of the product was analyzed using a scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). The selected area electron diffraction (SAED) of the sample was performed on the TEM. High-resolution transmission electron microscopy (HRTEM) was employed to carefully characterize the product on a JEM-2100F microscope operated at 200 kV. The X-Ray energy dispersive spectroscopy (EDS) attached to high-resolution transmission electron microscopy was employed to determine chemical composition of the sample. Rize-2008 laser particle size analyzer was used to analyse the average granularity of the milled B2O3 powder.
3. Results and discussions The original B2O3 powder is white irregular crystal particles, the average size of them is about 200 μm. After milled for 60 h in high pure N2, the color of them changed from white into gray.
Y. Li et al. / Materials Letters 108 (2013) 96–102
97
1000 * B2O3 800
Intensity /cps
B2O3 milled for 60h 600
400
200 Original B2O3 0 10
20
30
40
50
60
70
2θ/degree Fig. 1. SEM image of B2O3 milled for 60 h (a) and XRD patterns of original B2O3 and B2O3 milled for 60 h (b).
Fig. 1(a) is SEM image of the original B2O3 powder milled for 60 h in high pure N2. It can be observed that the B2O3 powder transformed into uniform equiaxial particles. The average granularity of the milled B2O3 powder measured by laser particle size analyzer is about 20 μm. Some big particles are aggregation of small particles. Fig. 1(b) shows the XRD patterns of the original B2O3 powder and B2O3 powder milled in high pure N2 for 60 h. It can be observed that crystallization of the original B2O3 is not completely and some amorphous component exists. After milling for 60 h in high pure N2, the intensity of B2O3 peaks was weakened and most milled B2O3 changed into amorphous. The product obtained after annealing was white loose powder. Fig. 2(a) is SEM image of the product. It can be found that the
morphology of most BN nanoparticles is flaky. Most of them are approximately elliptical shape, and some of them are circular form, as shown by white arrows in Fig. 2(a). The size of the nanoparticles is uniform, their diameter is between 20 nm and 100 nm, and most of them are about 70 nm in diameter. The thickness of them is about 10 nm. Fig. 2(b) is XRD pattern of the products. Analysis from Fig. 2(b) reveals that the product after annealing was mainly composed of hexagonal BN (h-BN), Fe2N and Al18B4O33. In Fig. 3(a) and (b), the TEM image and corresponding SAED pattern of the product are shown. Uniform nanoparticles can be observed in the image and most of them are about 70 nm in diameter. The morphology of the nanoparticles is approximately ellipsoid and minority of them are circular. The SAED pattern
98
Y. Li et al. / Materials Letters 108 (2013) 96–102
600
500 ƹ
ƽ
Ʒ
H-BN
ƽ
Fe2N
ƹ
Al18B4O33
Intensity/cps
400 Ʒ
300
200 ƹ ƹ
100
ƹ
ƽ
ƽƷ
ƽ Ʒ
Ʒ
ƹ
ƹ
0 10
20
30
40
50
60
70
2θ/degree Fig. 2. SEM image (a) and XRD patterns (b) of the product.
identifies that the product is h-BN, which is in conformity with the XRD analysis. Fig. 4(a) is TEM image of the BN nanoparticles at higher magnification. It can be observed that the diameter of them is about 70 nm. Most of them are elliptical shape and some of them are circular form. The morphology and size of these nanoaprticles is similar to that synthesized by Lin [13] through a two stage synthetic process. In Lin's paper, the morphology of the BN nanoparticles was described as acetabuliform. Fig. 4(b) is EDS spectrum of the BN nanoparticles shown in Fig. 4(a). The atomic
ratio of BːN is 46.13:46.26 based on EDS analysis, which is in good agreement with the chemical stoichiometric ratio of BN. The signals of C and Cu arise from the preparation of TEM sample and the signals of O and Si come from impurity. So it can be concluded that the nanoparticles showed in Fig. 4(a) were BN nanoparticles. Fig. 5(a) is the TEM image of the synthesized BN nanoparticle. Fig. 5(b) is the HRTEM images of the nanoparticle pointed by arrow in Fig. 5(a). Regular lattice arrangement without any lattice distortion demonstrates that the nanoparticle is well ordered.
Y. Li et al. / Materials Letters 108 (2013) 96–102
99
Fig. 3. TEM image (a) and corresponding SAED pattern (b) of the product.
Furthermore, clear lattice fringes at 0.33 nm can be calculated, which is in good agreement with the interplanar distance of (110) and (002) in bulk h-BN [19]. As for the formation mechanism of plate-like BN nanoparticles, it may be ascribed to the layered crystal structure of h-BN, which is similar to graphite. As a result, the h-BN nanoparticle shows plate-like morphology.
4. Conclusions In summary, BN nanoparticles were synthesized from boron oxide by high-energy ball milling and annealing process. The morphology of most BN nanoparticles is elliptical plate-like, while the others are circular plate-like. The nanoparticles are uniform and their diameter is ranging from 20 nm to 100 nm. Most of them
Y. Li et al. / Materials Letters 108 (2013) 96–102
Intensity /arb.unit
100
Energy / KeV Fig. 4. TEM image (a) and EDS spectrum (b) of the product.
Y. Li et al. / Materials Letters 108 (2013) 96–102
Fig. 5. TEM image (a) and HRTEM image (b) of the BN nanoparticles.
101
102
Y. Li et al. / Materials Letters 108 (2013) 96–102
are about 70 nm in diameter. The thickness of them is about 10 nm estimated from SEM image. HRTEM image of single nanoparticle indicate that the nanoparticle is well ordered and the calculated lattice fringes is 0.33 nm, which is in good agreement with the interplanar distance of (110) and (002) in bulk h-BN. Aknowledgement The authors acknowledge the financial support from the GM Corporation for this work. References [1] Chopra NG, Luyken RJ, Cherrey K, Crespi VH, Cohen ML, Louie SG, et al. Boron nitride nanotubes. Science 1995;269:966–7. [2] Golberg D, Bando Y, Stéphan O, Kurashima K. Octahedral boron nitride fullerenes formed by electron beam irradiation. Appl Phys Lett 1998;73: 2441–3. [3] Zhi CY, Yoshio B, Tang CC, Golberg D. Boron nitride nanotubes. Mater Sci Eng R 2010;70:92–111. [4] Zhang LP, Gu YL, Wang JL, Zhao GW, Qian QL, Li J, et al. Catalytic synthesis of bamboo-like multiwall BN nanotubes via SHS-annealing process. J Solid State Chem 2011;184:633–6. [5] Velazquea-Salazar JJ, Munoz-Sandoval E, Romo-Herrera JM, Lupo F, Ruhle M, Terrones H. Synthesis and state of art characterization of BN bamboo-like nanotubes: evidence of a root growth mechanism catalyzed by Fe. Chem Phys Lett 2005;416:342–8. [6] Oku T, Narita I, Tokoro H. Synthesis and magnetic property of boron nitride nanocapsules encaging iron and cobalt nanoparticles. J Phys Chem Solids 2006;67:1152–6.
[7] Hirano T, Oku T, Suganuma K. Fabrication and magnetic properties of boron nitride nanocapsules encaging iron oxide nanoparticles. Diamond Relat Mater 2000;9:476–9. [8] Nishiwaki A, Oku T. Atomic and electronic structures of boron nitride nanohorns studied by high-resolution electron microscopy and molecular orbital calculations. Diamond Relat Mater 2005;14:1183–9. [9] Nishiwaki A, Oku T. Atomic structures and formation mechanism of boron nitride nanotubes and nanohorns synthesized by arc-melting LaB6 powders. J Eur Ceram Soc 2006;26:435–41. [10] Zheng MT, Gu YL, Xu ZL, Liu YL. Synthesis and characterization of boron nitride nanoropes. Mater Lett 2007;61:1943–5. [11] Oku T, Hiraga K, Matsuda T, Hirai T, Hirabayashi M. Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride. Diamond Relat Mater 2003;12:1918–26. [12] Emily A, Rudolph ME, Trevor ED. Formation of c-BN nanoparticles by helium, lithium and boron ion implantation. Nucl Instrum Methods Phys Res B 2012;272:57–60. [13] Lin LX, Zheng Y, Li ZH, Wei KM. Synthesis of novel acetabuliform boron nitride nanoparticles with high surface area. Scripta Mater 2008;59:1151–4. [14] Zhi CY, Xu YB, Bando Y. Highly thermal-conductivity fluid with boron nitride nanofillers. Am Chem Soc 2011;5(8):6571–7. [15] Tang CC, Bando Y, Huang Y, Zhi CY, Golberg D. Synthetic Routes and Formation Mechanisms of Spherical Boron Nitride Nanoparticles. Adv Funct Mater 2008;18:3653–61. [16] Li YJ, Zhou JE, Luo ZF, Tung S, Schneider E, Wu JT, et al. Investigation on two abnormal phenomena about thermal conductivity enhancement of BN/EG nanofluids. Nanoscale Res Lett 2011;6:443. [17] Chen Y, Gerald JF, Williams JS, Bulcock S. Synthesis of boron nitride nanotubes at low temperatures using reactive ball milling. Chem Phys Lett 1999;299: 260–4. [18] Chen Y, Li CP, Chen H, Chen YJ. One-dimensional nanomaterials synthesized using high-energy ball milling and annealing process. Sci Technol Adv Mat 2006;7:839–46. [19] Pease RS. An X-ray study of boron nitride. Acta Crystallogr 1952;5:356–61.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of uniform plate-like boron nitride nanoparticles from boron oxide by ball milling and annealing process Yanjiao Li a,n, Youjun Wang a, Qiujuan Lv a, Zhongbao Qin a, Xinkuan Liu b,n a b
Xi'an Hi-Tech Research Institute, Hongqing Town, Xi'an 710025, China School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 February 2013 Accepted 16 June 2013 Available online 21 June 2013
A great many boron nitride (BN) nanoparticles were synthesized from boron oxide by high-energy ball milling and annealing process at 1200 1C. The sample was characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, selected area electron diffraction, high-resolution transmission electron microscopy and the X-Ray energy dispersive spectroscopy. The results showed that the morphology of most synthesized BN nanoparticles is elliptical plate-like, while the others are circular plate-like. The BN nanoparticles are uniform at the diameter from 20 nm to 100 nm, and most of them are around 70 nm in diameter. The thickness of them is about 10 nm estimated from SEM image. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ceramics Boron nitride Ball milling Plate-like
1. Introduction Since the discovery of boron nitride (BN) nanotubes [1], various types of BN nanostructured materials such as BN nanocage clusters [2], BN nanotubes [3–5], BN nanocapsules [6,7], BN nanohorns [8,9], BN nanoropes [10] and BN nanoparticles [11,12] have been reported for their advantageous properties such as high thermal conductivity, marked chemical inertness and good electrical insulation. BN nanoparticles, which can be used as catalytic supports [13], thermalconductive fillers [14] and composite additives [15] received considerable attention recently. For their high thermal conductivity and good insulating property, BN nanoparticles powder may be a suitable additive for synthesis of dielectric nanofluids [16]. Spherical and plate-like is the common morphology of boron nitride nanoparticles. Compared with spherical shape BN nanoparticles, the specific surface area of plate-like BN nanoparticles is more higher [13,15], which is good for thermal conductivity enhancement of nanofluids. Besides, the suspension stability of plate-like BN nanoparticles in base fluids will be better than spherical shape BN nanoparticles at the same size [16]. The ball milling-annealing method, which was first proposed by Chen et al. [17], is a widely used approach for producing nanomaterials [18]. The main merits of this method are available in low reaction temperature and high yield. In this paper, synthesis of uniform plate-like BN nanoparticles from boron oxide by ball millingannealing method will be reported and the micro morphology of the synthesized BN nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM) also. In this research, n
Corresponding authors. Tel.: +86 215 527 1682. E-mail addresses: [email protected].(Y. Li) [email protected], [email protected].(X. Liu) 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.06.043
boron oxide, which was inexpensive compared with boron or boron nitride, was used as raw material. 2. Experiments B2O3 powder with a purity of 99.9% was first milled in a highenergy ball mill with a stainless steel vessel and hardened chromium steel ball of 6 mm in diameter under high purity N2 atmosphere at a rotation speed of 450 rpm for 60 h. The weight ratio of the milling balls to B2O3 powder was 20:1. The milled B2O3 powder was then annealed in a GLS1600 tube furnace at 1200 1C for 6 h under an ammonia gas flow. The high purity ammonia gas flow was controlled at 150 ml/min. Crystalline structure of the product was investigated by means of X-ray diffraction analysis(XRD) using Cu Ka radiation (λ¼0.15418 nm) at room temperature. Morphology of the product was analyzed using a scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). The selected area electron diffraction (SAED) of the sample was performed on the TEM. High-resolution transmission electron microscopy (HRTEM) was employed to carefully characterize the product on a JEM-2100F microscope operated at 200 kV. The X-Ray energy dispersive spectroscopy (EDS) attached to high-resolution transmission electron microscopy was employed to determine chemical composition of the sample. Rize-2008 laser particle size analyzer was used to analyse the average granularity of the milled B2O3 powder.
3. Results and discussions The original B2O3 powder is white irregular crystal particles, the average size of them is about 200 μm. After milled for 60 h in high pure N2, the color of them changed from white into gray.
Y. Li et al. / Materials Letters 108 (2013) 96–102
97
1000 * B2O3 800
Intensity /cps
B2O3 milled for 60h 600
400
200 Original B2O3 0 10
20
30
40
50
60
70
2θ/degree Fig. 1. SEM image of B2O3 milled for 60 h (a) and XRD patterns of original B2O3 and B2O3 milled for 60 h (b).
Fig. 1(a) is SEM image of the original B2O3 powder milled for 60 h in high pure N2. It can be observed that the B2O3 powder transformed into uniform equiaxial particles. The average granularity of the milled B2O3 powder measured by laser particle size analyzer is about 20 μm. Some big particles are aggregation of small particles. Fig. 1(b) shows the XRD patterns of the original B2O3 powder and B2O3 powder milled in high pure N2 for 60 h. It can be observed that crystallization of the original B2O3 is not completely and some amorphous component exists. After milling for 60 h in high pure N2, the intensity of B2O3 peaks was weakened and most milled B2O3 changed into amorphous. The product obtained after annealing was white loose powder. Fig. 2(a) is SEM image of the product. It can be found that the
morphology of most BN nanoparticles is flaky. Most of them are approximately elliptical shape, and some of them are circular form, as shown by white arrows in Fig. 2(a). The size of the nanoparticles is uniform, their diameter is between 20 nm and 100 nm, and most of them are about 70 nm in diameter. The thickness of them is about 10 nm. Fig. 2(b) is XRD pattern of the products. Analysis from Fig. 2(b) reveals that the product after annealing was mainly composed of hexagonal BN (h-BN), Fe2N and Al18B4O33. In Fig. 3(a) and (b), the TEM image and corresponding SAED pattern of the product are shown. Uniform nanoparticles can be observed in the image and most of them are about 70 nm in diameter. The morphology of the nanoparticles is approximately ellipsoid and minority of them are circular. The SAED pattern
98
Y. Li et al. / Materials Letters 108 (2013) 96–102
600
500 ƹ
ƽ
Ʒ
H-BN
ƽ
Fe2N
ƹ
Al18B4O33
Intensity/cps
400 Ʒ
300
200 ƹ ƹ
100
ƹ
ƽ
ƽƷ
ƽ Ʒ
Ʒ
ƹ
ƹ
0 10
20
30
40
50
60
70
2θ/degree Fig. 2. SEM image (a) and XRD patterns (b) of the product.
identifies that the product is h-BN, which is in conformity with the XRD analysis. Fig. 4(a) is TEM image of the BN nanoparticles at higher magnification. It can be observed that the diameter of them is about 70 nm. Most of them are elliptical shape and some of them are circular form. The morphology and size of these nanoaprticles is similar to that synthesized by Lin [13] through a two stage synthetic process. In Lin's paper, the morphology of the BN nanoparticles was described as acetabuliform. Fig. 4(b) is EDS spectrum of the BN nanoparticles shown in Fig. 4(a). The atomic
ratio of BːN is 46.13:46.26 based on EDS analysis, which is in good agreement with the chemical stoichiometric ratio of BN. The signals of C and Cu arise from the preparation of TEM sample and the signals of O and Si come from impurity. So it can be concluded that the nanoparticles showed in Fig. 4(a) were BN nanoparticles. Fig. 5(a) is the TEM image of the synthesized BN nanoparticle. Fig. 5(b) is the HRTEM images of the nanoparticle pointed by arrow in Fig. 5(a). Regular lattice arrangement without any lattice distortion demonstrates that the nanoparticle is well ordered.
Y. Li et al. / Materials Letters 108 (2013) 96–102
99
Fig. 3. TEM image (a) and corresponding SAED pattern (b) of the product.
Furthermore, clear lattice fringes at 0.33 nm can be calculated, which is in good agreement with the interplanar distance of (110) and (002) in bulk h-BN [19]. As for the formation mechanism of plate-like BN nanoparticles, it may be ascribed to the layered crystal structure of h-BN, which is similar to graphite. As a result, the h-BN nanoparticle shows plate-like morphology.
4. Conclusions In summary, BN nanoparticles were synthesized from boron oxide by high-energy ball milling and annealing process. The morphology of most BN nanoparticles is elliptical plate-like, while the others are circular plate-like. The nanoparticles are uniform and their diameter is ranging from 20 nm to 100 nm. Most of them
Y. Li et al. / Materials Letters 108 (2013) 96–102
Intensity /arb.unit
100
Energy / KeV Fig. 4. TEM image (a) and EDS spectrum (b) of the product.
Y. Li et al. / Materials Letters 108 (2013) 96–102
Fig. 5. TEM image (a) and HRTEM image (b) of the BN nanoparticles.
101
102
Y. Li et al. / Materials Letters 108 (2013) 96–102
are about 70 nm in diameter. The thickness of them is about 10 nm estimated from SEM image. HRTEM image of single nanoparticle indicate that the nanoparticle is well ordered and the calculated lattice fringes is 0.33 nm, which is in good agreement with the interplanar distance of (110) and (002) in bulk h-BN. Aknowledgement The authors acknowledge the financial support from the GM Corporation for this work. References [1] Chopra NG, Luyken RJ, Cherrey K, Crespi VH, Cohen ML, Louie SG, et al. Boron nitride nanotubes. Science 1995;269:966–7. [2] Golberg D, Bando Y, Stéphan O, Kurashima K. Octahedral boron nitride fullerenes formed by electron beam irradiation. Appl Phys Lett 1998;73: 2441–3. [3] Zhi CY, Yoshio B, Tang CC, Golberg D. Boron nitride nanotubes. Mater Sci Eng R 2010;70:92–111. [4] Zhang LP, Gu YL, Wang JL, Zhao GW, Qian QL, Li J, et al. Catalytic synthesis of bamboo-like multiwall BN nanotubes via SHS-annealing process. J Solid State Chem 2011;184:633–6. [5] Velazquea-Salazar JJ, Munoz-Sandoval E, Romo-Herrera JM, Lupo F, Ruhle M, Terrones H. Synthesis and state of art characterization of BN bamboo-like nanotubes: evidence of a root growth mechanism catalyzed by Fe. Chem Phys Lett 2005;416:342–8. [6] Oku T, Narita I, Tokoro H. Synthesis and magnetic property of boron nitride nanocapsules encaging iron and cobalt nanoparticles. J Phys Chem Solids 2006;67:1152–6.
[7] Hirano T, Oku T, Suganuma K. Fabrication and magnetic properties of boron nitride nanocapsules encaging iron oxide nanoparticles. Diamond Relat Mater 2000;9:476–9. [8] Nishiwaki A, Oku T. Atomic and electronic structures of boron nitride nanohorns studied by high-resolution electron microscopy and molecular orbital calculations. Diamond Relat Mater 2005;14:1183–9. [9] Nishiwaki A, Oku T. Atomic structures and formation mechanism of boron nitride nanotubes and nanohorns synthesized by arc-melting LaB6 powders. J Eur Ceram Soc 2006;26:435–41. [10] Zheng MT, Gu YL, Xu ZL, Liu YL. Synthesis and characterization of boron nitride nanoropes. Mater Lett 2007;61:1943–5. [11] Oku T, Hiraga K, Matsuda T, Hirai T, Hirabayashi M. Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride. Diamond Relat Mater 2003;12:1918–26. [12] Emily A, Rudolph ME, Trevor ED. Formation of c-BN nanoparticles by helium, lithium and boron ion implantation. Nucl Instrum Methods Phys Res B 2012;272:57–60. [13] Lin LX, Zheng Y, Li ZH, Wei KM. Synthesis of novel acetabuliform boron nitride nanoparticles with high surface area. Scripta Mater 2008;59:1151–4. [14] Zhi CY, Xu YB, Bando Y. Highly thermal-conductivity fluid with boron nitride nanofillers. Am Chem Soc 2011;5(8):6571–7. [15] Tang CC, Bando Y, Huang Y, Zhi CY, Golberg D. Synthetic Routes and Formation Mechanisms of Spherical Boron Nitride Nanoparticles. Adv Funct Mater 2008;18:3653–61. [16] Li YJ, Zhou JE, Luo ZF, Tung S, Schneider E, Wu JT, et al. Investigation on two abnormal phenomena about thermal conductivity enhancement of BN/EG nanofluids. Nanoscale Res Lett 2011;6:443. [17] Chen Y, Gerald JF, Williams JS, Bulcock S. Synthesis of boron nitride nanotubes at low temperatures using reactive ball milling. Chem Phys Lett 1999;299: 260–4. [18] Chen Y, Li CP, Chen H, Chen YJ. One-dimensional nanomaterials synthesized using high-energy ball milling and annealing process. Sci Technol Adv Mat 2006;7:839–46. [19] Pease RS. An X-ray study of boron nitride. Acta Crystallogr 1952;5:356–61.