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Synthesis and growth of boron nitride nanotubes by a ball milling–annealing process
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Acta Materialia 59 (2011) 2807–2813 www.elsevier.com/locate/actamat
Synthesis and growth of boron nitride nanotubes by a ball milling–annealing process Jaewoo Kim a,⇑, Sol Lee a,b, Young Rang Uhm a, Jiheon Jun a, Chang Kyu Rhee a, Gil Moo Kim b a
Nuclear Materials Research Division, Korea Atomic Energy Research Institute, 1045 Daedukdaero, Yuseong-Gu, Daejeon 305-353, Republic of Korea b Materials Science and Engineering Department, Chungnam National University, Gung-Dong, Yuseong-Gu, Daejeon 305-764, Republic of Korea Received 1 November 2010; received in revised form 5 January 2011; accepted 6 January 2011
Abstract The synthesis and growth of boron nitride nanotubes (BNNTs) based on ball milling of crystalline boron powder followed by heat treatment were investigated. Fe-based stainless steel (STS) balls and milling vessels were used for milling, and the Fe impurity produced during milling acts as a catalyst for the generation of BNNTs during annealing under a nitrogen environment. Structural deformation of crystalline boron was observed for milled boron powder based on X-ray diffraction spectra and electron microscopy images. No chemical reactions of boron with nitrogen occurred during milling, and BNNTs were only synthesized during annealing. The BNNTs produced are basically multi-walled cylindrical- or bamboo-types mixed into nanotube clusters. The diameters of BNNTs are in the range of 50–150 nm, and numbers of the walls are 30–100 with a 0.3 nm gap on average. It was observed that BN was synthesized from amorphous boron coated onto the surface of the Fe particles. In addition, the types of grown nanotubes could be determined by the initial shapes of BN clusters on a Fe catalyst particle, which are nanoshells or opened nanocylinders. Yields of BNNTs were strongly dependent on the amorphous structure of the boron particles rather than on the residual crystalline boron particles in the milled samples. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: BN; BNNTs; Nanotube growth; Ball milling; Annealing
1. Introduction Boron is a very important material in nuclear engineering due to its excellent thermal neutron absorbing ability (the thermal neutron absorption cross-section of boron is 760b, b = 10 24 cm2). It is also important in space engineering due to its superior material properties. Crystalline boron is one of the hardest materials known, with a Vicker’s hardness (Hv) of 50 GPa, slightly lower than that of diamond (Hv 110 GPa) [1]. The density of boron is relatively low (2.5 g cm 3), while its melting point is very high (2000 °C). Moreover, boron nitride nanotubes (BNNTs), which are tubular structured compounds of boron and nitrogen, possess more chemically inert, ther⇑ Corresponding author.
E-mail address: [email protected] (J. Kim).
mally conductive and mechanically reliable characteristics than the general hexagonal structured BN compound. Hence, it is highly likely that BNNTs will be used in efficient neutron shielding and structural materials in nuclear or space engineering. BNNTs were first synthesized using a carbon-free plasma discharge between a BN powder-packed hollow tungsten anode and a copper electrode [2]. Various methods have since been developed for syntheses of BNNTs including laser ablation or melting [3–7], chemical vapor deposition (CVD) [8–10], and ball milling followed by annealing [11–15]. All of these methods are basically identical or similar to those used for carbon nanotube (CNT) syntheses. Syntheses and characteristics of BNNTs have been increasingly explored due to their superior chemical stability and competitive thermal conductivity properties compared to CNTs, while their electrical properties are somewhat different from CNTs [16].
1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.01.019
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BNNTs are known to be chemically stable at temperatures as high as 800 °C unlike CNTs which are easily oxidized at 400 °C [17]. Theoretical estimation also shows that BNNTs have thermal conductivities comparable to, or higher than, those of CNTs [18]. An experiment performed using 11B-enriched BNNTs also indicates that these have similar thermal conductivities to those of CNTs [19]. The electrical characteristics of CNTs, which exhibit conducting or semiconducting (band gap 2 eV) behavior depending on their tube shape or chirality [20], are one of the most critical obstacles to their electronic applications. On the other hand, BNNTs always possess a higher-energy band gap (5 eV) regardless of their shape and chirality [16]. This high-energy band gap and the thermal conducting behavior of BNNTs might also make this material useful as an electrical insulator with very high thermal conductivity. For applications of BNNTs in electronics as well as in the nuclear or space industries, it is important to develop a relatively simple and economic production process. In this investigation, we analyzed the synthesis and growth of BNNTs produced using a ball milling–annealing process which was particularly dependent on the milling conditions and the catalysts used. It was observed that the nanotubes were basically multi-walled with cylindricalor bamboo-types mixed in nanotube clusters. The synthesis of BN was assumed to be more dependent on the milling conditions rather than on the annealing conditions, i.e. the structural status of boron powder might be important for its chemical reactions with nitrogen. The structures, compositions and yields of BNNTs were analyzed by means of various analytical tools, and models for the synthesis and growth of BNNTs were proposed. 2. Experimental Crystalline boron powder (SCAF Global, 60 mesh, >99% purity) was used as the raw material and a planetary mill (P-100, Tae Myung Sci., Korea) was used to produce meso- or nano-scale boron particles. Pulverization of powder was performed at 300 rpm or higher disk revolution under 2 atm of N2 gas in sealed milling vessels. Fe-based stainless steel (STS) balls (5 mm diameter) and milling vessels were used. The ball-to-powder ratio (BPR) was set at 10:1, and 4 g of boron powder was pulverized during each run. The milling times were set at 12, 24 and 100 h, while the temperature of the milling vessels was maintained at 30 °C or less for all experiments by water cooling. Pulverized meso- and nano-particles were then annealed at 1100 °C under a N2 gas environment for 6 h. An alumina boat with a surface area of 5 cm2 was used as a powder holder. The quantity of the annealed powder was varied from 0.5 to 1.0 g. Fig. 1 illustrates the experimental steps used for the synthesis and growth of BNNTs using a ball milling–annealing process. Samples collected at the end of each milling–annealing step were ultrasonicated for more than 20 min in an ethanol solution prior to the analyses. The samples were then
examined by: (i) X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) to analyze their compositions (or decompositions); (ii) scanning electron microscopy (SEM, Sirion, FEI, the Netherlands) and transmission electron microscopy (TEM, JEM-2100F, Japan) for structural and morphological analyses; and (iii) Brunauer–Emmett–Teller (BET, BEL 30RP-mini, Japan) analysis for specific surface area measurements. Generation of BNNTs was confirmed by comprehensive examinations of the SEM/TEM-EDX images and XRD spectra. Synthesis and growth of BNNTs, which are dependent on milling time, were also analyzed by means of the XRD spectra and BET measurements. 3. Results and discussion 3.1. Pulverization of boron powders As an initial step for synthesis of BNNTs, crystalline elemental boron was first milled using a planetary mill under 2 atm of N2. Fig. 2 shows the SEM images of boron powder pulverized at 300 rpm using the STS milling balls and STS milling vessels for 12, 24 and 100 h, respectively. Fig. 1 shows an optically magnified image of the initial boron powder. As shown in the optical and SEM images, the structures of the particles appear to be changed into semicrystalline or amorphous meso- and nano-sized particles by the pulverization. The structural changes of the particles are due to mechanically activated energy transfer between the milling balls and powder particles during collisions. The degree of structure disorder of milled powder particles appears higher as the milling time increases. Here, the milled powder samples may include the metallic impurities such as Fe and Cr as well as B due to collisions between the balls themselves. Generation of the metallic impurities in the sample milled for 12 h was measured using a TEM-EDX image as shown in Fig. 3. Particulate STS spallation debris several hundred nanometers in size was constituted of mostly Fe and Cr as well as limited amounts of Si and Ca. In general, it can be assumed that a Fe impurity particle in milled powder may act as a catalyst, enhancing the synthesis and growth of BNNTs [11]. Based on close examination of the image, it can be assumed that 4.5 wt.% boron is coated onto the surface of the Fe particle, while the structure of boron appears to be amorphous. More importantly, in Fig. 3, nitrogen was not detected in the milled particle, indicating that no chemical reactions of boron with nitrogen or with the other elements occurred during milling. The effects of milling on the deformation of the elemental structures of boron can be observed from the XRD spectra shown in Fig. 4. In addition, the types of impurity added to the milled powder can be estimated for each case. The spectra show a decrease in the boron peaks for 100 h milled powder, which indicates the structural deformation of boron powder particles, while the spectra are similar for 12 and 24 h milled powder. The impurity particles as seen in the TEM image in Fig. 3 can be
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Fig. 1. Experimental procedure for synthesis and growth of BNNTs.
Fig. 3. TEM-EDX image and atomic composition for a particle milled for 12 h milled particle using STS balls and milling vessels.
100 h milled boron
1400 1200
24 h milled boron
- B (443)
- B (063)
- B (24 10) - B (615)
- B (238)
- B (131) - B (217) - B (401)
- B (324)
200
- B (125)
400
B (104)
600
- B (003) - B (012)
800
B (021) - B (024) - B (211)
1000
- B (110)
Relative Intensity (a.u.)
1600
- Fe (211)
1800
- Fe (200)
- Fe (110)
2000
12 h milled boron
initial boron
0 20
40
60
80
2θ
Fig. 4. Milling time dependent XRD spectra of milled boron powder.
Fig. 2. Milling time dependent SEM images of boron powder milled using STS balls and milling vessels.
confirmed by newly produced Fe peaks, while other impurities are barely distinguishable. According to the analyses
of the peak intensities and the shapes for each milled powder, it is noticeable that the Fe peaks are broadened compared to the boron peaks. It is assumed that ball milling of boron powder produces a mixture of boron mesoparticles with mostly an amorphous phase of boron on the surface of the Fe particles as the milling time increases, while nanoscale Fe particles due to the high hardness of crystalline boron are expected in the milled samples. The peak intensities of the Fe impurities were also lowered as the milling time increased. This indicates that the Fe particles are mostly nanoscale in size, while the lattice structure of
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3.2. Synthesis of BN Milled powders, including boron as well as the Fe metallic impurity, were then annealed in a furnace at 1000 °C under a N2 gas flow for 6 h. It is known that the annealing temperature may not be critical for tube synthesis itself, but rather is related to the characteristics of the nanotubes, such as the diameter [11,12]. After annealing of the milled powders, synthesis of BN was clearly confirmed by the XRD spectra in Fig. 5. The three XRD spectra are from the samples milled for 12, 24 and 100 h, and show the generation of a main h-BN(0 0 2) peak. In the spectra, the BN(0 0 2) peak intensity for 100 h milled powder is higher than for the powders milled for 12 and 24 h, while the BN peak for 24 h milled powder is the smallest. As a result, it is observed that more residual boron remained in the 24 h milled powder than in the 12 h milled powder. This is somewhat unexpected if the milling time is dependent on the mechanical activation energy transfer between the balls and powder particles, and if it is considered as an important factor in the synthesis of BN. According to Fig. 4, powders milled for 12 and 24 h produced similar intensities and peak shapes for boron and Fe. It might be assumed that the reagglomeration of boron particles for the 24 h milled powder is more significant, even though the
2500
h-BN (002)
- Fe (211)
- h-BN (101)
- Fe (200)
- h-BN (102)
1500
- h-BN (004)
2000 - h-BN (100) - h-BN (101) - Fe (110)
the particles is generally maintained during milling, according to the SEM images and XRD spectra. This type of structural change of boron based on collisions with milling balls and milling vessels is commonly observed in the milling process [13,21,22]. Particularly for the samples milled for 100 h, more structural changes for both boron and the impurities (mainly Fe) are expected. The disordered particulate structures may enhance the chemical reactivity due to the unstable and excessive energy of the surface of the milled particles [23–25]. During milling, a mechanochemical or thermochemical reaction of boron with nitrogen is not expected in this investigation as the temperature of the milling vessels is maintained at <30 °C through water cooling. On the other hand, it was reported that the NH3 gas environment during milling causes a nitration reaction of boron, which was explained by changes in the gaseous pressure [11]. However, this might be disputable since no BN peaks in the XRD spectra of milled powder were observed in the paper. A chemical reaction of boron with nitrogen generally requires a temperature and pressure higher than 1500 °C and 2 GPa, respectively [26]. There are no noticeable peaks for BN(0 0 2) found at 2h 26.63° for ˚ in Fig. 4, as mentioned. In addition, no chemik = 1.54 A cal reactions of the metallic impurities Fe with B or N occurred during milling, as only the peaks for the initial materials are seen. Although the mechanisms of nanotube synthesis and growth are still ambiguous, it is clear that the effects of chemical reactions of boron with nitrogen on the synthesis of BNNTs can be initiated from the Fe surface coated amorphous boron.
Relative Intensity (a.u.)
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100 h milled - 6 h annealed
1000
24 h milled - 6 h annealed
500
0
12 h milled - 6 h annealed 20
40
60
80
2θ
Fig. 5. XRD spectra of annealed powder with different milling times.
structural information for the 12 and 24 h milled powder particles is still ambiguous. However, the intensities for BN peaks are certainly related to the synthesis and yields of BN nanotubes, while it may not be confirmed if the peaks for BN indicate BN nanotubes in Fig. 5. 3.3. Growth of boron nitride nanotubes Fig. 6 is a TEM image with an EDX analysis showing the composition of the particles in the 12 h milled and 6 h annealed samples. Position S1 in the image indicates a pure Fe particle, while position S2 is constituted of 42.7 wt.% of boron and 57.3 wt.% of nitrogen. This weight per cent ratio for boron and nitrogen is exactly the same as the atomic ratio of boron and nitrogen (=1:1) for BN. Several BN nanoshells and an opened nanocylinder are generated around the spherical Fe particle. The structure of BN is assumed to be amorphous at the initial stage of BN
Fig. 6. TEM-EDX image of a mixture of amorphous nanoshells and opened nanocylinders on the surface of an Fe particle.
J. Kim et al. / Acta Materialia 59 (2011) 2807–2813
synthesis, as shown in Fig. 6. Amorphous nanoshells might be the initial stage for growing bamboo-type BN nanotubes by rearrangement of the shells. At the same time, opened nanocylinders might be the initial shape of cylindrical BN nanotubes. Fig. 7 shows images of various tubular shapes taken from 12 h milled and 6 h annealed powders. Synthesis of BNNTs can be clearly confirmed from these images together with the above-mentioned XRD spectra and EDX measurements. According to Fig. 7a, the outer form of most of the BNNTs appears to be cylindrical. However, the bamboo-type BNNTs shown in Fig. 7b and c might be numerous and mixed with the others. Some of the end tips
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of the nanotubes are open, though many of are closed by a particle or by the tube itself. The outer diameters of the nanotubes are in the range 50–150 nm. The lengths of the tubes vary from sub-micrometer to several micrometers. Each nanochamber in the bamboo-type nanotubes is several tens of nanometers long, while its width is 40 nm at the barrel and 35 nm at the nodes, as shown in Fig. 7b. The TEM images in Fig. 7b and c show a typical node of the nanotubes, and the structure of the BN layers. BNNTs are multi-walled with a thickness of 10–30 nm, while the space between the walls is 0.3 nm, i.e. a there are typically 30–100 nanowalls. It is noticeable that the wall diameters are strongly dependent on the size of the seed particle, as can be seen in Fig. 7b, which is relatively narrower in diameter than the others. More interestingly, Fig. 7b shows two different growths of the nanowalls, i.e. bamboo- and cylindrical-type tube generations. This might be explained by the image in Fig. 6 showing a seed particle with the surrounding BN nanoclusters. Several are closed nanoshells, while one is an open-walled nanocylinder. Amorphous BN might become layer structured as the time used for the heat treatment increases and an additional reaction of boron with nitrogen proceeds, resulting in growth of the nanotubes. Fig. 6 can explain how the two different types of tube growth shown in Fig. 7b are possible. After the initial nanochambers are rearranged, as shown in Fig. 7b, each node is produced by peeling off some of the inner BN layers from the main wall body as the wall grows with additional BN synthesis. On the other hand, a cylindrical tube wall is growing in the other direction, following the initial open-cylindrical shape. The nanotube in Fig. 7c has differently shaped nanonodes. It can be assumed that some of the inner BN layers peel off from the main body of multi-walls as the nanotube grows in the same manner as in Fig. 7b, while no nanochambers are generated in this case. The reason for the BN layers peeling off of the main body wall is assumed to be due to the stress accumulated on the wall as the nanotube grows [12], while the shape of nanonodes generated in different ways is still not clearly understood. 3.4. Yields of BNNTs
Fig. 7. BNNTs produced from milled powder using STS balls and milling vessels: (a) SEM image showing the typical shapes of BNNTs; (b) TEM image of cylindrical- and bamboo-type BNNTs; and (c) TEM image of the structure of the other types of BNNTs.
The yields of the BNNTs can be estimated roughly based on the analyses of the XRD spectra. However, it is difficult to distinguish BN and BNNTs in the spectra. Amorphous boron is the source for synthesis of BN during annealing, while generations of both h-BN disks and tubular h-BN are possible. In this regard, the yields of BNNTs can be estimated comprehensively by examination of various analytic means such as XRD, SEM and BET measurements for the total pore volume (TPV) or specific surface area of the samples. TPV and the specific surface area can be measured by adsorption of nitrogen on the surface of the species in 77 K using BET. Since there are currently no available references for high-quality BNNTs, we measured the TPV for
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Table 1 Specific surface areas and total pore volume (TPV) of NTs. Samples
Specific surface area (m2 g 1)
Total pore volume (cm3 g 1)
Milled powder (12 h) Milled powder (24 h) BNNTs (12 h milled–6 h annealed) BNNTs (24 h milled–6 h annealed) MWCNTs
2.0 2.1 44.8 8.0 200.1
0.0011 0.0013 0.030 0.005 0.118
commercially available MWCNTs (Hanwha Chemical Co.) to compare them with our samples. In general, the specific surface area and TPV for MWCNTs are known to be 300 m2 g 1 and 1–1.5 cm3 g 1 [27]. In our measurements, the specific surface area and TPV of MWCNTs are 200.1 m2 g 1 and 0.118 cm3 g 1, respectively. BET results for milled powders, milled–annealed powders and MWCNTs are listed in Table 1. Since the specific surface areas and TPVs are strongly dependent on the length and diameter of the walls, number of walls, type of tubes (cylindrical or bamboo), the openness of the end tips, etc., it is very difficult to estimate the yields of the NTs directly based on these values. We can only assume the relative yields between the samples compared with MWCNTs. The specific surface areas for the 12 h and 24 h milled powders are similar to the XRD spectra in Fig. 4. However, the specific surface area for the 12 h milled and 6 h annealed sample is about 6 times larger than that for the 24 h milled and 6 h annealed sample. This means that the BNNT yield for the former is higher than the latter, which agrees well with the XRD spectra in Fig. 5. This indicates that the yields for BNNTs are strongly dependent on the structures of the powder particles. In fact, crystalline structured boron particles are remained in the annealed sample for 24 h milled powder, producing a lower yield of BNNTs. Consequently, all experimental measurements, including the XRD spectra for milled and annealed powders together with BET measurements, agree well with the BNNT yields. It is important to deform or maintain the structure of boron in an amorphous state to increase the yields of BNNTs. In this regard, longer milling of boron powder might be preferred. However, shorter milling is also acceptable as long as amorphization of boron is properly maintained. Hence, it will be helpful to further explore the mechanically activated, energy-dependent structure of the milled boron particles to understand the synthesis and growth of BNNTs. 4. Conclusion The synthesis and growth of BNNTs were investigated using milling of crystalline boron powder followed by a heat treatment. No chemical reactions producing BN were observed during milling, and only a structural disorder of boron powder particles occurred. The Fe impurity produced during milling acts as a catalytic particle for BN synthesis during annealing under an N2 environment.
Amorphous boron produced during milling on the surface of the Fe particle might be a source for synthesis of BN. It was observed that the nanoshells or opened nanocylinders of amorphous BN coated on the seed particle at the initial stage of nanotube growth determines whether BNNTs are of a cylindrical- or a bamboo-type. Closed nanoshells might developed into bamboo-type nanotube by their rearrangement, while opened nanocylinders will grow into cylindrical nanotubes. The synthesis and growth of BNNTs are dependent on the milling conditions rather than the annealing conditions. Consequently, the yield of BNNTs is strongly dependent on the amorphous structure of the boron particles rather than on the residual crystalline boron particles in the milled samples. Acknowledgements This work was performed under the financial support from the Creative Research Program of Korea Atomic Energy Research Institute. References [1] Solozhenko VL, Kurakevych OO, Oranov AR. J Superhard Mater 2008;30:428. [2] Chopra NG, Luyken RJ, Cherry K, Crespi VH, Cohen ML, Louie SG, et al. Science 1995;269:966. [3] Yu DP, Sun XS, Lee CS, Bello I, Lee ST, Gu HD, et al. Appl Phys Lett 1998;72:1966. [4] Golberg D, Bando Y, Eremets M, Takemura K, Kurashima K, Yusa H. Appl Phys Lett 1996;69:2045. [5] Laude T, Matsui Y, Marraud A, Jouffrey B. Appl Phys Lett 2000;76:3239. [6] Lee RS, Gavillet J, de la Lamy CM, Loiseau A, Cochon JL, Pigache D, et al. Phys Rev B 2001;64:1405. [7] Enouz S, Stephan O, Cochon JL, Colliex C, Loiseau A. Nano Lett 2007;7:1856. [8] Lourie OR, Jones CR, Bartlett BM, Gibbons PC, Rouff RS, Buhro WE. Chem Mater 2000;12:1808. [9] Ma R, Bando Y, Sato T. Chem Phys Lett 2001;337:61. [10] Tang C, Bando Y, Sato T, Kurashima K. ChemCommun 2002:1290. [11] Chen Y, Gerald JF, Williams JS, Bulcock S. Chem Phys Lett 1999;299:260. [12] Bae SY, Seo HW, Park J, Choi YS, Park JC, Lee SY. Chem Phys Lett 2003;374:534. [13] Singhal SK, Srivastava AK, Pant RP, Halder SK, Singh BP, Gupta AK. J Mater Sci 2008;43:5243. [14] Li Y, Zhou J, Zhao K, Tung S, Schneider E. Mater Lett 2009;63:1733. [15] Wen G, Zhang T, Huang XX, Zhong B, Zhang XD, Yu HM. Scr Mater 2010;62:25. [16] Golberg D, Bando Y, Tang C, Zhi C. Adv Mater 2007;19:2413. [17] Golberg D, Bando Y, Kurashima K, Sato T. Scr Mater 2001;44:1561. [18] Xiao Y, Yan XH, Cao JX, Ding JW, Mao YL, Xiang J. Phy Rev B 2004;69:205415. [19] Chang CW, Fennimore AM, Afanasiev A, Okawa D, Ikuno T, Garcia H, et al. Phy Rev Lett 2006;97:085901. [20] Rao CNR, Satiskumar BC, Govindaraj A, Nath M. CHEMPHYSCHEM 2001;2:78. [21] Jung J, Kim J, Uhm YR, Jeon JK, Lee S, Lee HM, et al. Thermochim Acta 2010;499:8.
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Acta Materialia 59 (2011) 2807–2813 www.elsevier.com/locate/actamat
Synthesis and growth of boron nitride nanotubes by a ball milling–annealing process Jaewoo Kim a,⇑, Sol Lee a,b, Young Rang Uhm a, Jiheon Jun a, Chang Kyu Rhee a, Gil Moo Kim b a
Nuclear Materials Research Division, Korea Atomic Energy Research Institute, 1045 Daedukdaero, Yuseong-Gu, Daejeon 305-353, Republic of Korea b Materials Science and Engineering Department, Chungnam National University, Gung-Dong, Yuseong-Gu, Daejeon 305-764, Republic of Korea Received 1 November 2010; received in revised form 5 January 2011; accepted 6 January 2011
Abstract The synthesis and growth of boron nitride nanotubes (BNNTs) based on ball milling of crystalline boron powder followed by heat treatment were investigated. Fe-based stainless steel (STS) balls and milling vessels were used for milling, and the Fe impurity produced during milling acts as a catalyst for the generation of BNNTs during annealing under a nitrogen environment. Structural deformation of crystalline boron was observed for milled boron powder based on X-ray diffraction spectra and electron microscopy images. No chemical reactions of boron with nitrogen occurred during milling, and BNNTs were only synthesized during annealing. The BNNTs produced are basically multi-walled cylindrical- or bamboo-types mixed into nanotube clusters. The diameters of BNNTs are in the range of 50–150 nm, and numbers of the walls are 30–100 with a 0.3 nm gap on average. It was observed that BN was synthesized from amorphous boron coated onto the surface of the Fe particles. In addition, the types of grown nanotubes could be determined by the initial shapes of BN clusters on a Fe catalyst particle, which are nanoshells or opened nanocylinders. Yields of BNNTs were strongly dependent on the amorphous structure of the boron particles rather than on the residual crystalline boron particles in the milled samples. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: BN; BNNTs; Nanotube growth; Ball milling; Annealing
1. Introduction Boron is a very important material in nuclear engineering due to its excellent thermal neutron absorbing ability (the thermal neutron absorption cross-section of boron is 760b, b = 10 24 cm2). It is also important in space engineering due to its superior material properties. Crystalline boron is one of the hardest materials known, with a Vicker’s hardness (Hv) of 50 GPa, slightly lower than that of diamond (Hv 110 GPa) [1]. The density of boron is relatively low (2.5 g cm 3), while its melting point is very high (2000 °C). Moreover, boron nitride nanotubes (BNNTs), which are tubular structured compounds of boron and nitrogen, possess more chemically inert, ther⇑ Corresponding author.
E-mail address: [email protected] (J. Kim).
mally conductive and mechanically reliable characteristics than the general hexagonal structured BN compound. Hence, it is highly likely that BNNTs will be used in efficient neutron shielding and structural materials in nuclear or space engineering. BNNTs were first synthesized using a carbon-free plasma discharge between a BN powder-packed hollow tungsten anode and a copper electrode [2]. Various methods have since been developed for syntheses of BNNTs including laser ablation or melting [3–7], chemical vapor deposition (CVD) [8–10], and ball milling followed by annealing [11–15]. All of these methods are basically identical or similar to those used for carbon nanotube (CNT) syntheses. Syntheses and characteristics of BNNTs have been increasingly explored due to their superior chemical stability and competitive thermal conductivity properties compared to CNTs, while their electrical properties are somewhat different from CNTs [16].
1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.01.019
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BNNTs are known to be chemically stable at temperatures as high as 800 °C unlike CNTs which are easily oxidized at 400 °C [17]. Theoretical estimation also shows that BNNTs have thermal conductivities comparable to, or higher than, those of CNTs [18]. An experiment performed using 11B-enriched BNNTs also indicates that these have similar thermal conductivities to those of CNTs [19]. The electrical characteristics of CNTs, which exhibit conducting or semiconducting (band gap 2 eV) behavior depending on their tube shape or chirality [20], are one of the most critical obstacles to their electronic applications. On the other hand, BNNTs always possess a higher-energy band gap (5 eV) regardless of their shape and chirality [16]. This high-energy band gap and the thermal conducting behavior of BNNTs might also make this material useful as an electrical insulator with very high thermal conductivity. For applications of BNNTs in electronics as well as in the nuclear or space industries, it is important to develop a relatively simple and economic production process. In this investigation, we analyzed the synthesis and growth of BNNTs produced using a ball milling–annealing process which was particularly dependent on the milling conditions and the catalysts used. It was observed that the nanotubes were basically multi-walled with cylindricalor bamboo-types mixed in nanotube clusters. The synthesis of BN was assumed to be more dependent on the milling conditions rather than on the annealing conditions, i.e. the structural status of boron powder might be important for its chemical reactions with nitrogen. The structures, compositions and yields of BNNTs were analyzed by means of various analytical tools, and models for the synthesis and growth of BNNTs were proposed. 2. Experimental Crystalline boron powder (SCAF Global, 60 mesh, >99% purity) was used as the raw material and a planetary mill (P-100, Tae Myung Sci., Korea) was used to produce meso- or nano-scale boron particles. Pulverization of powder was performed at 300 rpm or higher disk revolution under 2 atm of N2 gas in sealed milling vessels. Fe-based stainless steel (STS) balls (5 mm diameter) and milling vessels were used. The ball-to-powder ratio (BPR) was set at 10:1, and 4 g of boron powder was pulverized during each run. The milling times were set at 12, 24 and 100 h, while the temperature of the milling vessels was maintained at 30 °C or less for all experiments by water cooling. Pulverized meso- and nano-particles were then annealed at 1100 °C under a N2 gas environment for 6 h. An alumina boat with a surface area of 5 cm2 was used as a powder holder. The quantity of the annealed powder was varied from 0.5 to 1.0 g. Fig. 1 illustrates the experimental steps used for the synthesis and growth of BNNTs using a ball milling–annealing process. Samples collected at the end of each milling–annealing step were ultrasonicated for more than 20 min in an ethanol solution prior to the analyses. The samples were then
examined by: (i) X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) to analyze their compositions (or decompositions); (ii) scanning electron microscopy (SEM, Sirion, FEI, the Netherlands) and transmission electron microscopy (TEM, JEM-2100F, Japan) for structural and morphological analyses; and (iii) Brunauer–Emmett–Teller (BET, BEL 30RP-mini, Japan) analysis for specific surface area measurements. Generation of BNNTs was confirmed by comprehensive examinations of the SEM/TEM-EDX images and XRD spectra. Synthesis and growth of BNNTs, which are dependent on milling time, were also analyzed by means of the XRD spectra and BET measurements. 3. Results and discussion 3.1. Pulverization of boron powders As an initial step for synthesis of BNNTs, crystalline elemental boron was first milled using a planetary mill under 2 atm of N2. Fig. 2 shows the SEM images of boron powder pulverized at 300 rpm using the STS milling balls and STS milling vessels for 12, 24 and 100 h, respectively. Fig. 1 shows an optically magnified image of the initial boron powder. As shown in the optical and SEM images, the structures of the particles appear to be changed into semicrystalline or amorphous meso- and nano-sized particles by the pulverization. The structural changes of the particles are due to mechanically activated energy transfer between the milling balls and powder particles during collisions. The degree of structure disorder of milled powder particles appears higher as the milling time increases. Here, the milled powder samples may include the metallic impurities such as Fe and Cr as well as B due to collisions between the balls themselves. Generation of the metallic impurities in the sample milled for 12 h was measured using a TEM-EDX image as shown in Fig. 3. Particulate STS spallation debris several hundred nanometers in size was constituted of mostly Fe and Cr as well as limited amounts of Si and Ca. In general, it can be assumed that a Fe impurity particle in milled powder may act as a catalyst, enhancing the synthesis and growth of BNNTs [11]. Based on close examination of the image, it can be assumed that 4.5 wt.% boron is coated onto the surface of the Fe particle, while the structure of boron appears to be amorphous. More importantly, in Fig. 3, nitrogen was not detected in the milled particle, indicating that no chemical reactions of boron with nitrogen or with the other elements occurred during milling. The effects of milling on the deformation of the elemental structures of boron can be observed from the XRD spectra shown in Fig. 4. In addition, the types of impurity added to the milled powder can be estimated for each case. The spectra show a decrease in the boron peaks for 100 h milled powder, which indicates the structural deformation of boron powder particles, while the spectra are similar for 12 and 24 h milled powder. The impurity particles as seen in the TEM image in Fig. 3 can be
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Fig. 1. Experimental procedure for synthesis and growth of BNNTs.
Fig. 3. TEM-EDX image and atomic composition for a particle milled for 12 h milled particle using STS balls and milling vessels.
100 h milled boron
1400 1200
24 h milled boron
- B (443)
- B (063)
- B (24 10) - B (615)
- B (238)
- B (131) - B (217) - B (401)
- B (324)
200
- B (125)
400
B (104)
600
- B (003) - B (012)
800
B (021) - B (024) - B (211)
1000
- B (110)
Relative Intensity (a.u.)
1600
- Fe (211)
1800
- Fe (200)
- Fe (110)
2000
12 h milled boron
initial boron
0 20
40
60
80
2θ
Fig. 4. Milling time dependent XRD spectra of milled boron powder.
Fig. 2. Milling time dependent SEM images of boron powder milled using STS balls and milling vessels.
confirmed by newly produced Fe peaks, while other impurities are barely distinguishable. According to the analyses
of the peak intensities and the shapes for each milled powder, it is noticeable that the Fe peaks are broadened compared to the boron peaks. It is assumed that ball milling of boron powder produces a mixture of boron mesoparticles with mostly an amorphous phase of boron on the surface of the Fe particles as the milling time increases, while nanoscale Fe particles due to the high hardness of crystalline boron are expected in the milled samples. The peak intensities of the Fe impurities were also lowered as the milling time increased. This indicates that the Fe particles are mostly nanoscale in size, while the lattice structure of
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3.2. Synthesis of BN Milled powders, including boron as well as the Fe metallic impurity, were then annealed in a furnace at 1000 °C under a N2 gas flow for 6 h. It is known that the annealing temperature may not be critical for tube synthesis itself, but rather is related to the characteristics of the nanotubes, such as the diameter [11,12]. After annealing of the milled powders, synthesis of BN was clearly confirmed by the XRD spectra in Fig. 5. The three XRD spectra are from the samples milled for 12, 24 and 100 h, and show the generation of a main h-BN(0 0 2) peak. In the spectra, the BN(0 0 2) peak intensity for 100 h milled powder is higher than for the powders milled for 12 and 24 h, while the BN peak for 24 h milled powder is the smallest. As a result, it is observed that more residual boron remained in the 24 h milled powder than in the 12 h milled powder. This is somewhat unexpected if the milling time is dependent on the mechanical activation energy transfer between the balls and powder particles, and if it is considered as an important factor in the synthesis of BN. According to Fig. 4, powders milled for 12 and 24 h produced similar intensities and peak shapes for boron and Fe. It might be assumed that the reagglomeration of boron particles for the 24 h milled powder is more significant, even though the
2500
h-BN (002)
- Fe (211)
- h-BN (101)
- Fe (200)
- h-BN (102)
1500
- h-BN (004)
2000 - h-BN (100) - h-BN (101) - Fe (110)
the particles is generally maintained during milling, according to the SEM images and XRD spectra. This type of structural change of boron based on collisions with milling balls and milling vessels is commonly observed in the milling process [13,21,22]. Particularly for the samples milled for 100 h, more structural changes for both boron and the impurities (mainly Fe) are expected. The disordered particulate structures may enhance the chemical reactivity due to the unstable and excessive energy of the surface of the milled particles [23–25]. During milling, a mechanochemical or thermochemical reaction of boron with nitrogen is not expected in this investigation as the temperature of the milling vessels is maintained at <30 °C through water cooling. On the other hand, it was reported that the NH3 gas environment during milling causes a nitration reaction of boron, which was explained by changes in the gaseous pressure [11]. However, this might be disputable since no BN peaks in the XRD spectra of milled powder were observed in the paper. A chemical reaction of boron with nitrogen generally requires a temperature and pressure higher than 1500 °C and 2 GPa, respectively [26]. There are no noticeable peaks for BN(0 0 2) found at 2h 26.63° for ˚ in Fig. 4, as mentioned. In addition, no chemik = 1.54 A cal reactions of the metallic impurities Fe with B or N occurred during milling, as only the peaks for the initial materials are seen. Although the mechanisms of nanotube synthesis and growth are still ambiguous, it is clear that the effects of chemical reactions of boron with nitrogen on the synthesis of BNNTs can be initiated from the Fe surface coated amorphous boron.
Relative Intensity (a.u.)
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100 h milled - 6 h annealed
1000
24 h milled - 6 h annealed
500
0
12 h milled - 6 h annealed 20
40
60
80
2θ
Fig. 5. XRD spectra of annealed powder with different milling times.
structural information for the 12 and 24 h milled powder particles is still ambiguous. However, the intensities for BN peaks are certainly related to the synthesis and yields of BN nanotubes, while it may not be confirmed if the peaks for BN indicate BN nanotubes in Fig. 5. 3.3. Growth of boron nitride nanotubes Fig. 6 is a TEM image with an EDX analysis showing the composition of the particles in the 12 h milled and 6 h annealed samples. Position S1 in the image indicates a pure Fe particle, while position S2 is constituted of 42.7 wt.% of boron and 57.3 wt.% of nitrogen. This weight per cent ratio for boron and nitrogen is exactly the same as the atomic ratio of boron and nitrogen (=1:1) for BN. Several BN nanoshells and an opened nanocylinder are generated around the spherical Fe particle. The structure of BN is assumed to be amorphous at the initial stage of BN
Fig. 6. TEM-EDX image of a mixture of amorphous nanoshells and opened nanocylinders on the surface of an Fe particle.
J. Kim et al. / Acta Materialia 59 (2011) 2807–2813
synthesis, as shown in Fig. 6. Amorphous nanoshells might be the initial stage for growing bamboo-type BN nanotubes by rearrangement of the shells. At the same time, opened nanocylinders might be the initial shape of cylindrical BN nanotubes. Fig. 7 shows images of various tubular shapes taken from 12 h milled and 6 h annealed powders. Synthesis of BNNTs can be clearly confirmed from these images together with the above-mentioned XRD spectra and EDX measurements. According to Fig. 7a, the outer form of most of the BNNTs appears to be cylindrical. However, the bamboo-type BNNTs shown in Fig. 7b and c might be numerous and mixed with the others. Some of the end tips
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of the nanotubes are open, though many of are closed by a particle or by the tube itself. The outer diameters of the nanotubes are in the range 50–150 nm. The lengths of the tubes vary from sub-micrometer to several micrometers. Each nanochamber in the bamboo-type nanotubes is several tens of nanometers long, while its width is 40 nm at the barrel and 35 nm at the nodes, as shown in Fig. 7b. The TEM images in Fig. 7b and c show a typical node of the nanotubes, and the structure of the BN layers. BNNTs are multi-walled with a thickness of 10–30 nm, while the space between the walls is 0.3 nm, i.e. a there are typically 30–100 nanowalls. It is noticeable that the wall diameters are strongly dependent on the size of the seed particle, as can be seen in Fig. 7b, which is relatively narrower in diameter than the others. More interestingly, Fig. 7b shows two different growths of the nanowalls, i.e. bamboo- and cylindrical-type tube generations. This might be explained by the image in Fig. 6 showing a seed particle with the surrounding BN nanoclusters. Several are closed nanoshells, while one is an open-walled nanocylinder. Amorphous BN might become layer structured as the time used for the heat treatment increases and an additional reaction of boron with nitrogen proceeds, resulting in growth of the nanotubes. Fig. 6 can explain how the two different types of tube growth shown in Fig. 7b are possible. After the initial nanochambers are rearranged, as shown in Fig. 7b, each node is produced by peeling off some of the inner BN layers from the main wall body as the wall grows with additional BN synthesis. On the other hand, a cylindrical tube wall is growing in the other direction, following the initial open-cylindrical shape. The nanotube in Fig. 7c has differently shaped nanonodes. It can be assumed that some of the inner BN layers peel off from the main body of multi-walls as the nanotube grows in the same manner as in Fig. 7b, while no nanochambers are generated in this case. The reason for the BN layers peeling off of the main body wall is assumed to be due to the stress accumulated on the wall as the nanotube grows [12], while the shape of nanonodes generated in different ways is still not clearly understood. 3.4. Yields of BNNTs
Fig. 7. BNNTs produced from milled powder using STS balls and milling vessels: (a) SEM image showing the typical shapes of BNNTs; (b) TEM image of cylindrical- and bamboo-type BNNTs; and (c) TEM image of the structure of the other types of BNNTs.
The yields of the BNNTs can be estimated roughly based on the analyses of the XRD spectra. However, it is difficult to distinguish BN and BNNTs in the spectra. Amorphous boron is the source for synthesis of BN during annealing, while generations of both h-BN disks and tubular h-BN are possible. In this regard, the yields of BNNTs can be estimated comprehensively by examination of various analytic means such as XRD, SEM and BET measurements for the total pore volume (TPV) or specific surface area of the samples. TPV and the specific surface area can be measured by adsorption of nitrogen on the surface of the species in 77 K using BET. Since there are currently no available references for high-quality BNNTs, we measured the TPV for
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Table 1 Specific surface areas and total pore volume (TPV) of NTs. Samples
Specific surface area (m2 g 1)
Total pore volume (cm3 g 1)
Milled powder (12 h) Milled powder (24 h) BNNTs (12 h milled–6 h annealed) BNNTs (24 h milled–6 h annealed) MWCNTs
2.0 2.1 44.8 8.0 200.1
0.0011 0.0013 0.030 0.005 0.118
commercially available MWCNTs (Hanwha Chemical Co.) to compare them with our samples. In general, the specific surface area and TPV for MWCNTs are known to be 300 m2 g 1 and 1–1.5 cm3 g 1 [27]. In our measurements, the specific surface area and TPV of MWCNTs are 200.1 m2 g 1 and 0.118 cm3 g 1, respectively. BET results for milled powders, milled–annealed powders and MWCNTs are listed in Table 1. Since the specific surface areas and TPVs are strongly dependent on the length and diameter of the walls, number of walls, type of tubes (cylindrical or bamboo), the openness of the end tips, etc., it is very difficult to estimate the yields of the NTs directly based on these values. We can only assume the relative yields between the samples compared with MWCNTs. The specific surface areas for the 12 h and 24 h milled powders are similar to the XRD spectra in Fig. 4. However, the specific surface area for the 12 h milled and 6 h annealed sample is about 6 times larger than that for the 24 h milled and 6 h annealed sample. This means that the BNNT yield for the former is higher than the latter, which agrees well with the XRD spectra in Fig. 5. This indicates that the yields for BNNTs are strongly dependent on the structures of the powder particles. In fact, crystalline structured boron particles are remained in the annealed sample for 24 h milled powder, producing a lower yield of BNNTs. Consequently, all experimental measurements, including the XRD spectra for milled and annealed powders together with BET measurements, agree well with the BNNT yields. It is important to deform or maintain the structure of boron in an amorphous state to increase the yields of BNNTs. In this regard, longer milling of boron powder might be preferred. However, shorter milling is also acceptable as long as amorphization of boron is properly maintained. Hence, it will be helpful to further explore the mechanically activated, energy-dependent structure of the milled boron particles to understand the synthesis and growth of BNNTs. 4. Conclusion The synthesis and growth of BNNTs were investigated using milling of crystalline boron powder followed by a heat treatment. No chemical reactions producing BN were observed during milling, and only a structural disorder of boron powder particles occurred. The Fe impurity produced during milling acts as a catalytic particle for BN synthesis during annealing under an N2 environment.
Amorphous boron produced during milling on the surface of the Fe particle might be a source for synthesis of BN. It was observed that the nanoshells or opened nanocylinders of amorphous BN coated on the seed particle at the initial stage of nanotube growth determines whether BNNTs are of a cylindrical- or a bamboo-type. Closed nanoshells might developed into bamboo-type nanotube by their rearrangement, while opened nanocylinders will grow into cylindrical nanotubes. The synthesis and growth of BNNTs are dependent on the milling conditions rather than the annealing conditions. Consequently, the yield of BNNTs is strongly dependent on the amorphous structure of the boron particles rather than on the residual crystalline boron particles in the milled samples. Acknowledgements This work was performed under the financial support from the Creative Research Program of Korea Atomic Energy Research Institute. References [1] Solozhenko VL, Kurakevych OO, Oranov AR. J Superhard Mater 2008;30:428. [2] Chopra NG, Luyken RJ, Cherry K, Crespi VH, Cohen ML, Louie SG, et al. Science 1995;269:966. [3] Yu DP, Sun XS, Lee CS, Bello I, Lee ST, Gu HD, et al. Appl Phys Lett 1998;72:1966. [4] Golberg D, Bando Y, Eremets M, Takemura K, Kurashima K, Yusa H. Appl Phys Lett 1996;69:2045. [5] Laude T, Matsui Y, Marraud A, Jouffrey B. Appl Phys Lett 2000;76:3239. [6] Lee RS, Gavillet J, de la Lamy CM, Loiseau A, Cochon JL, Pigache D, et al. Phys Rev B 2001;64:1405. [7] Enouz S, Stephan O, Cochon JL, Colliex C, Loiseau A. Nano Lett 2007;7:1856. [8] Lourie OR, Jones CR, Bartlett BM, Gibbons PC, Rouff RS, Buhro WE. Chem Mater 2000;12:1808. [9] Ma R, Bando Y, Sato T. Chem Phys Lett 2001;337:61. [10] Tang C, Bando Y, Sato T, Kurashima K. ChemCommun 2002:1290. [11] Chen Y, Gerald JF, Williams JS, Bulcock S. Chem Phys Lett 1999;299:260. [12] Bae SY, Seo HW, Park J, Choi YS, Park JC, Lee SY. Chem Phys Lett 2003;374:534. [13] Singhal SK, Srivastava AK, Pant RP, Halder SK, Singh BP, Gupta AK. J Mater Sci 2008;43:5243. [14] Li Y, Zhou J, Zhao K, Tung S, Schneider E. Mater Lett 2009;63:1733. [15] Wen G, Zhang T, Huang XX, Zhong B, Zhang XD, Yu HM. Scr Mater 2010;62:25. [16] Golberg D, Bando Y, Tang C, Zhi C. Adv Mater 2007;19:2413. [17] Golberg D, Bando Y, Kurashima K, Sato T. Scr Mater 2001;44:1561. [18] Xiao Y, Yan XH, Cao JX, Ding JW, Mao YL, Xiang J. Phy Rev B 2004;69:205415. [19] Chang CW, Fennimore AM, Afanasiev A, Okawa D, Ikuno T, Garcia H, et al. Phy Rev Lett 2006;97:085901. [20] Rao CNR, Satiskumar BC, Govindaraj A, Nath M. CHEMPHYSCHEM 2001;2:78. [21] Jung J, Kim J, Uhm YR, Jeon JK, Lee S, Lee HM, et al. Thermochim Acta 2010;499:8.
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