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Preparation and characterization of CBN ternary compounds with nano-structure
ARTICLE IN PRESS
Physica B 382 (2006) 151–155 www.elsevier.com/locate/physb
Preparation and characterization of CBN ternary compounds with nano-structure Y.H. Xionga,b,, S. Yanga, C.S. Xionga, H.L. Pia, J. Zhanga, Z.M. Rena, Y.T. Maia, W. Xua, G.H. Daia, S.J. Songa, J. Xiongc, L. Zhanga, Z.C. Xiaa, S.L. Yuana a Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Department of Astronomy and Applied physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China c Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b
Received 1 August 2005; received in revised form 15 February 2006; accepted 19 February 2006
Abstract CBN ternary compounds with nano-structure have been prepared directly by a mechanical alloying technique at room temperature. The characteristic and formation mechanism of CBN are discussed. The nano-sheets and nano-layered rods of CBN are observed according to the morphology of scanning electron microscopy. It is substantiated that the microstructure of CBN was closely related to the ball milling time and the ball milling condition according to the results of X-ray diffraction of CBN with different ball milling time. After ball milling for 60 and 90 h, some new diffraction peaks are observed, which implies that some unknown microstructure and phase separation are induced in the reactive ball milling of CBN. The results of XRD are in accordance with that of X-ray photoelectron spectroscopy of CBN before ball milling and after ball milling for 90 h. r 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Ev; 81.05.Je; 61.10.Nz; 61.16.Bg Keywords: CBN ternary compounds; Nano structure; Mechanical alloying; SEM; XRD; XPS
1. Introduction CBN ternary compounds have kindled swingeing interest recently due to its various unusual properties, e.g., surper-hardness, high thermal conductivity, low dielectric constant, thermal stability at high temperature, excellent electronic structure, rich semiconductor properties and so on [1–5]. These properties can be controlled easily by changing their composition and morphologies [1,3]. CBN with different morphologies were prepared by different methods, including electric arc discharge, dual cathode magnetron sputtering, and chemical vapor deposition and so on [6–10]. However, it is usually difficult to control the elemental component of CyBxNz products using the Corresponding author. Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China. Tel.: +86 27 87556914; fax: +86 27 87556914. E-mail address: [email protected] (Y.H. Xiong).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.02.020
methods mentioned above. The structural changes and chemical reactions of the samples can be induced by highenergy ball milling method at room temperature. The process embraces a complex mixture of fracturing, grinding, high-speed plastic deformation, cold welding, thermal shock, intimate mixing, etc. [11–13]. It has been established that the mechanical alloying of metallic elements by highenergy ball milling may lead to a metastable amorphous phase of solid solutions. More recently, Yao et al. [14] reported the formation of cubic C-BN by ball milling and recrystallization, and the Lee’s group [5] reported the synthesis of Cx(BN)1x by ball milling and recrystallization (at 1200 1C for 24 h). They have observed that there are the characteristic amorphous structure in X-ray diffraction (XRD) of Cx(BN)1x before recrystallization and after recrystallization, and there are some minor diffraction peaks that are not indexedin addition to the (0 0 2), (1 0 0) and (0 0 4) peaks of hexagonal structure. But synthesized nanocrystalline CBN with nano-layer structure prepared
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directly by high-energy ball milling at room temperature was seldom reported. In this paper we report different morphologies and microstructure of CBN ternary compounds prepared directly by a reactive mechanical alloying with proper pressure of nitrogen at room temperature. Some new diffraction peaks of X-ray diffraction were observed, which implied that some unknown structure and phase separation were induced during the reactive ball milling of CBN. 2. Experimental
Fig. 1. SEM feature micrograph of CBN after ball milling for 30 h (a), 60 h (b) and 90 h (c).
0.3338 (b) BN 0 h
0.2166 0.1644 0.2058 0.1817 intensity (a. u.)
The graphite and hexagonal boron nitride powder with the purity higher than 99.9%, and a particle size less than 100 mm were used as the original materials. The powders, whose weights were 3.4 and 6.6 g for graphite and hexagonal boron nitride, respectively, were sealed in a stainless steel vial, in which the ratio of ball to powder weight was 40:1. To prevent contamination of oxygen, the stainless steel vial was vacuumed and purged with nitrogen gas several times, and kept at a proper pressure of nitrogen gas in the end. The ball milling time was 30, 60, 90, and 120 h, respectively. The rotating speed was set 220 rpm. The XRD patterns of the samples with different ball milling times were obtained from a Japan Rigaku D/max-g a rotating X-ray diffractometer with the Cu Ka radiation (l ¼ 0:15418 nm), an operating voltage of 40 kV and a current of 100 mA. The feature micrographs of CBN were determined by scanning electron microscopy (SEM, Hitachi X-650). The X-ray photoelectron spectroscopy (XPS) was performed in an ESCALAB MK II system with a spherical sector analyzer. The base pressure was 2 1010 mbar. The photoelectrons were collected at an emission angle of 151 with respect to the normal surface. An Mg Ka radiation source (hn ¼ 1253:6 eV) was used in XPS measurements.
0.1230 0.1253
0.3373 (a) C 0 h
3. Results and discussion Fig. 1(a), (b) and (c) show the typical SEM feature micrograph of CBN after ball milling for 30, 60 and 90 h. It is well known that the graphite belongs to a layeredstructure. In the layer plane, there is a strong covalent bond due to sp2 orbital, so the structure is very stable in the layer plane. But there are van der Waals’ forces between the layers, so the structure can be changed or destroyed easily. The h-BN has a similar structure to the graphite (see Fig. 2 and Table 1). Under the strong influence of external force, the slipping between layers can occur easily. During the process of high-energy ball milling, there is an instantaneous local pressure about 2–6 GPa [15]. This pressure will cause the change of the microstructures between layers of C and BN, which leads to slipping, stacking default, fracture and surface stripping between layers, and forms the hexagonal-like sheets. From Fig. 1 we can observe different morphology clearly in CBN ternary compounds. When the ball milling time is 30 h, the hexagonal-like sheets can be
0.1682 0.2130 0.2020 0.1231
5
15
25
35
45
55
65
75
2θ (°) Fig. 2. The XRD patterns of BN and C original powders before ball milling.
observed. The size and the thickness of these sheets are about 200–2800 nm and 101–102 nm, respectively (see Fig. 1(a)). After ball milling for 60 and 90 h, according to SEM feature micrograph of CBN ternary compounds, we can observe that there are different shapes of nano-grains, sheets, composite nano-layered-rods (see Fig. 1(b) and (c)). The diameter and the growth step thickness of composite
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153
Table 1 The parameters of experiments and calculated by a maths analyticmethod for CBN samples CBN
Exp. values
Exp. values
0h
0.3373 0.2130 0.2020 0.1682 0.1231
0h
0.3338 0.2166 0.2058 0.1817 0.1664 0.1253 0.1230
002 100 101 102 004 110 111
60 h
90 h
1.190 0.8358 0.5980 0.5701 0.5502 0.5360
1.187 0.8356 0.5980 0.5711 0.5460 0.5368 0.4741 0.4290 0.3930 0.3734 0.3620 0.3531 0.3398 0.3358 0.3045 0.2665 0.2562 0.2510 0.2323 0.2225
0.4290 0.3951 0.3735 0.3626 0.3531 0.3403 0.3370 0.3046 0.2665 0.2561 0.2513 0.2317 0.2225
Cal. values a ¼ 1.6204 (nm) c ¼ 1.7810 (nm) hkl 1.190 0.8102 0.5992 0.5728 0.5574 0.5401 0.4788 0.4293 0.3950 0.3734 0.3623 0.3516 0.3401 0.3361 0.3045 0.2664 0.2562 0.2513 0.2325 0.2225
101 200 202 220 103 300 203 104 401 331 420 224 115 314 502, 432 610 620 107 227 720
(c) 90 h
intensity (a.u.)
Graphite h-BN
(d) 120 h
(b) 60 h
(a) 30 h 20
30
40
50
60
70
Fig. 3. The XRD patterns of C samples with different ball milling time.
(d) 120 h
Note: Where d (nm) is the spacing of index (h k l) in the direct lattice.
(c) 90 h
intensity (a. u.)
nano-layered-rods are about 200–900 nm and 15–40 nm, respectively. The XRD patterns of BN and C original powders are shown in Fig. 2. From it we can see the graphite hexagonal structure (a ¼ 0:2463 nm, c ¼ 0:6714 nm) in the original powders of C and hexagonal structure (a ¼ 0:2504 nm, c ¼ 0:6661 nm) of BN, respectively. Fig. 3 shows the XRD patterns of C samples with different ball milling time. From Fig. 3, we can see there are no new phases except that the diffraction peaks are broadened and weaken with the increase of ball milling time, which indicates that with the increase of ball milling time, the crystalline grains of C become smaller and nanocrystal and amorphous grains are formed in the end. Because C powder is a single phase powder and there is no reaction among C powder, the stainless steel vial and the balls, there are no new compounds formed during the process of the ball milling. We can observe the similar results of BN original powders with different ball milling time too. However, we can observe some new diffraction peaks in the mixed powder of graphite and hexagonal boron nitride with increase of ball milling time. The XRD patterns of CBN samples with different ball milling times (30, 60, 90 and 120 h) are shown in Fig. 4. After ball milling for 30 h, the intensities of the diffraction peaks of the C and BN are weakened and some XRD peaks
80
2θ (°)
(b) 60 h
(a) 30 h
5
15
25
35
45 2θ (°)
55
65
75
Fig. 4. The XRD patterns of CBN samples with different ball milling time.
at high diffraction angle almost disappeared, but two new XRD peaks appeared (see Fig. 4(a)). After ball milling for 60 h, in the range of low diffraction angle, more than ten new XRD peaks appeared (see Fig. 4(b)). With further increase of ball milling time, these new XRD peaks become sharper, which indicates that some new phase of CBN was formed. In order to study these new diffraction peaks further, we try to fit and index these new diffraction peaks
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using a mathematical analytic method (It is the well-known Cohen’s method). It is more interesting that all peaks can be indexed as tetragonal crystal system. The calculated values are shown in Table 1, from which we can see that there may exist a tetragonal system in CBN with ball milling time of 60 h and 90 h and its lattice parameters are a ¼ 1:6204 nm, c ¼ 1:7810 nm, respectively. The calculated values are in good agreement with the results of our experiment. Of course, this is not the only explanation and there is a possibility to form another phase structure or compounds. According to the further analyses of Fig. 4(b) and (c), we can see that the diffraction peaks of these tetragonal phase are superposed upon a few weakened and broadened diffraction peaks of hexagonal boron nitride and graphite. After ball milling for 120 h, these tetragonal phase diffraction peaks almost vanish and there are a few very weakened and broadened diffraction peaks, which implies that CBN amorphous and nancrystalline are central phases here (see (d) in Fig. 4). These calculated results are in accordance with the data of PDF cards (090012 and 23-0064) and another group’s results. In 1965, Batsanov et al. [16] have observed that the action of powerful shock wave brought about the crystallization of amorphous BN, its hexagonal lattice being retained, while the very high compressive forces produced a small amount of a new modification, which was named the e-form (from the word ‘‘explosion’’). Recently the Lee’ group [5] has observed some minor XRD peaks of unknown crystal system in the recrystallized Cx(BN)1x sample (after ball milling for 72 h, heated at 1200 1C for 24 h). We suggest that the reasons inducing the increase of tetragonal CBN crystal cell or the formation of some unknown microstructure and phase separation may be as follows: firstly, there are many defects, dislocation, serious deformation of lattice due to strong shock and grinding during the process of high energy ball milling; secondly, because the crystalline grain became smaller, which will increase the interdiffusion of the atoms that are located in the surface and interface of the C and BN crystalline grain, the lattice constants of prime hexagonal graphite and hexagonal boron nitride will be changed; finally the interdiffusion between atoms will give rise to a local solid state reaction and the formation of new phase structure or a new compounds. In this case, both the valence state and the bond length of CBN are changed. In order to understand the varied bonding state and microstructure characterization of CBN with abundant phase structure and nanostructure further, we have discussed the XPS of CBN. Fig. 5 shows XPS analysis of the C1s, N1s and B1s spectrum with ball milling 90 h and the inset is XPS analysis of the C1s, N1s and B1s spectrum before ball milling. From Table 2 and Fig. 5, we can see that there is an obvious difference between XPS results before ball milling and that after ball milling for 90 h. Before ball milling, for B1s, the bonding energy is 190.52 eV, which corresponds to B–N bonds of h-BN; for C1s, the bonding energy is 284.35 eV, which corresponds to
500 400 300 200 100 0
600 C-90 h 400 200 0
280
600
284
C-0 h
280
400
288
292
288 600 450 300 150 0 394
N-90 h
284
292
N-0 h
398
402
406
200 0
600
394
398 500 400 300 200 100 0
B-90 h
400 200 0
186
402
190
406
B-0 h
186 190 194 198 Bonding energy (eV)
194
198
Bonding energy (eV) Fig. 5. XPS analysis of C1s, N1s and B1s spectrum after ball milling 90 h. The inset is XPS analysis of C1s, N1s and B1s spectrum before ball milling.
C–C bonds of graphite; for N1s, the bonding energy is 398.16 eV, which corresponds to N-B bonds of h-BN. After ball milling for 90 h, B1s spectrum was decomposed into four distinguishable peaks, the bonding energies are 284.60, 285.80, 286.80 and 288.70 eV, respectively; C1s spectrum was decomposed into four distinguishable peaks too; but N1s spectrum was decomposed into five distinguishable peaks. These new bonding states correspond to B–N, C–B–N, B–O, C–C, C–N (sp2), C–N (sp3), C–O, N–C (sp2) and N–O bonds. The formation of these new bonding states, especially the formation of the bonding states of C–B–N, C–N (sp2), C–N (sp3), N–C (sp2) implies that the solid state reaction is induced, C and BN are dissolved by each other, and the new phase is separated during the process of the ball milling. These results are in accordance with that of XRD. The detailed discussion of the XPS of CBN with different ball milling time will be presented elsewhere. 4. Conclusions CBN ternary compounds with nano-layer structure and multi-phase structure were prepared by mechanical
ARTICLE IN PRESS Y.H. Xiong et al. / Physica B 382 (2006) 151–155 Table 2 The relation between bonding state of CBN and different ball milling time Sample CBN Ball milling times (h) B1s Bonding Bonding Bonding Bonding
energy energy energy energy
(ev) (eV) (eV) (eV)
C1s Bonding Bonding Bonding Bonding Bonding
energy energy energy energy energy
(eV) (eV) (eV) (eV) (eV)
N1s Bonding Bonding Bonding Bonding Bonding
energy energy energy energy energy
(eV) (eV) (eV) (eV) (eV)
Bonding state
0
90
190.52
190.00 191.45 192.80 193.20
B–N B–N C–B–N or B–O B–O
284.60 285.80 286.8 288.70
C–C (nanocrystal) C–C C–N (sp2) C–N (sp3) C–O
397.50 398.30 399.55 400.55 402.10
N–C N–B, N–C (sp3) C–B–N N–C (sp2) N–O
284.35
398.16
alloying technique at room temperature. The results of XRD and SEM of CBN with different ball milling time showed that the microstructure transformation, the formation of new phase and the feature micrograph of CBN samples were closely related to the ball milling time and ball milling condition. After ball milling for 60 and 90 hrs, some new diffraction peaks were observed, which implied that the reactive ball milling may induce some multi-phase structure or phase separation and some unknown microstructures of CBN composite were formed. The new phase may be fitted and indexed using the mathematical analytic method, and all peaks can be indexed by a tetragonal crystal system. According to the results of XPS of CBN samples before ball milling and after ball milling for 90 h, it was substantiated that the bonding state and microstructure of CBN were closely related to the ball milling time too.
155
Acknowledgment This work was supported by the National Science Foundation of China (Grant no. 10274022) and School of Science Foundation of Huazhong University of Science and Technology.
References [1] N. Kawaguchi, Adv. Mater. 9 (1997) 615. [2] S. Fayeulle, M. Nastasi, J. Appl. Phys. 81 (1997) 6703. [3] M.O. Watanabe, S. Itoh, K. Mizushima, T. Sasaki, J. Appl. Phys. 78 (1995) 2880. [4] S. Ulrich, H. Ehrhardt, T. Theel, J. Schwan, S. Westermeyr, M. Scheib, P. Becker, H. Oechsner, G. Dollinger, A. Bergnaier, Diamond Relat. Mater. 7 (1998) 839. [5] Y.F. Zhang, Y.H. Tang, C.S. Lee, I. Bello, S.T. Lee, Diamond Relat. Mater. 8 (1999) 610. [6] O. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlich, J.M. Lambert, P. Bernier, P. Lefin, Science 266 (1994) 1683; Ph. Kohler-Redlich, M. Terrones, C. Manteca-Diego, W.K. Hsu, H. Terrones, M. Ru¨hle, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 310 (1999) 459. [7] A. Loiseau, F. Willaime, N. Demoncy, G. Hug, H. Pascard, Phys. Rev. Lett. 76 (1996) 4737; Y. Kusano, J.E. Evetts, I.M. Hutchings, Thin Solid Films 343–344 (1999) 250. [8] M. Terrones, A.M. Benito, C. Manteca-Diego, W.K. Hsu, O.I. Osman, J.P. Hare, D.G. Reid, H. Terrones, A.K. Cheetham, K. Prassides, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 257 (1996) 576; M. Terrones, W.K. Hsu, H. Terrones, J.P. Zhang, S. Ramos, J.P. Hare, R. Castillo, K. Prassides, A.K. Cheetham, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 259 (1996) 568. [9] K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278 (1997) 653. [10] Ph. Redlich, J. Loeffler, P.M. Ajayan, J. Bill, F. Aldinger, M. Ru¨hle, Chem. Phys. Lett. 260 (1996) 465. [11] B.S. Murty, S. Ranganathan, Int. Mater. Rev. 43 (1998) 101. [12] C. Suryanarayana, Progress Mater. Sci. 46 (2001) 1. [13] Y. Chen, L.T. Chadderton, J.F. Gerald, Appl. Phys. Lett. 74 (1999) 2960. [14] B. Yao, L. Liu, W.H. Su, J. Appl. Phys. 86 (1999) 2464. [15] J.Y. Huang, H. Yasuda, H. Mori, Chem. Phys. Lett. 303 (1999) 130. [16] S.S. Batsanov, G.E. Blokhina, A.A. Deribas, translated from Zhurnal Strukturnoi Khimii 6 (1965) 227 (also see JCPDS-ICDD 18-251).
Physica B 382 (2006) 151–155 www.elsevier.com/locate/physb
Preparation and characterization of CBN ternary compounds with nano-structure Y.H. Xionga,b,, S. Yanga, C.S. Xionga, H.L. Pia, J. Zhanga, Z.M. Rena, Y.T. Maia, W. Xua, G.H. Daia, S.J. Songa, J. Xiongc, L. Zhanga, Z.C. Xiaa, S.L. Yuana a Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Department of Astronomy and Applied physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China c Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b
Received 1 August 2005; received in revised form 15 February 2006; accepted 19 February 2006
Abstract CBN ternary compounds with nano-structure have been prepared directly by a mechanical alloying technique at room temperature. The characteristic and formation mechanism of CBN are discussed. The nano-sheets and nano-layered rods of CBN are observed according to the morphology of scanning electron microscopy. It is substantiated that the microstructure of CBN was closely related to the ball milling time and the ball milling condition according to the results of X-ray diffraction of CBN with different ball milling time. After ball milling for 60 and 90 h, some new diffraction peaks are observed, which implies that some unknown microstructure and phase separation are induced in the reactive ball milling of CBN. The results of XRD are in accordance with that of X-ray photoelectron spectroscopy of CBN before ball milling and after ball milling for 90 h. r 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Ev; 81.05.Je; 61.10.Nz; 61.16.Bg Keywords: CBN ternary compounds; Nano structure; Mechanical alloying; SEM; XRD; XPS
1. Introduction CBN ternary compounds have kindled swingeing interest recently due to its various unusual properties, e.g., surper-hardness, high thermal conductivity, low dielectric constant, thermal stability at high temperature, excellent electronic structure, rich semiconductor properties and so on [1–5]. These properties can be controlled easily by changing their composition and morphologies [1,3]. CBN with different morphologies were prepared by different methods, including electric arc discharge, dual cathode magnetron sputtering, and chemical vapor deposition and so on [6–10]. However, it is usually difficult to control the elemental component of CyBxNz products using the Corresponding author. Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China. Tel.: +86 27 87556914; fax: +86 27 87556914. E-mail address: [email protected] (Y.H. Xiong).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.02.020
methods mentioned above. The structural changes and chemical reactions of the samples can be induced by highenergy ball milling method at room temperature. The process embraces a complex mixture of fracturing, grinding, high-speed plastic deformation, cold welding, thermal shock, intimate mixing, etc. [11–13]. It has been established that the mechanical alloying of metallic elements by highenergy ball milling may lead to a metastable amorphous phase of solid solutions. More recently, Yao et al. [14] reported the formation of cubic C-BN by ball milling and recrystallization, and the Lee’s group [5] reported the synthesis of Cx(BN)1x by ball milling and recrystallization (at 1200 1C for 24 h). They have observed that there are the characteristic amorphous structure in X-ray diffraction (XRD) of Cx(BN)1x before recrystallization and after recrystallization, and there are some minor diffraction peaks that are not indexedin addition to the (0 0 2), (1 0 0) and (0 0 4) peaks of hexagonal structure. But synthesized nanocrystalline CBN with nano-layer structure prepared
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directly by high-energy ball milling at room temperature was seldom reported. In this paper we report different morphologies and microstructure of CBN ternary compounds prepared directly by a reactive mechanical alloying with proper pressure of nitrogen at room temperature. Some new diffraction peaks of X-ray diffraction were observed, which implied that some unknown structure and phase separation were induced during the reactive ball milling of CBN. 2. Experimental
Fig. 1. SEM feature micrograph of CBN after ball milling for 30 h (a), 60 h (b) and 90 h (c).
0.3338 (b) BN 0 h
0.2166 0.1644 0.2058 0.1817 intensity (a. u.)
The graphite and hexagonal boron nitride powder with the purity higher than 99.9%, and a particle size less than 100 mm were used as the original materials. The powders, whose weights were 3.4 and 6.6 g for graphite and hexagonal boron nitride, respectively, were sealed in a stainless steel vial, in which the ratio of ball to powder weight was 40:1. To prevent contamination of oxygen, the stainless steel vial was vacuumed and purged with nitrogen gas several times, and kept at a proper pressure of nitrogen gas in the end. The ball milling time was 30, 60, 90, and 120 h, respectively. The rotating speed was set 220 rpm. The XRD patterns of the samples with different ball milling times were obtained from a Japan Rigaku D/max-g a rotating X-ray diffractometer with the Cu Ka radiation (l ¼ 0:15418 nm), an operating voltage of 40 kV and a current of 100 mA. The feature micrographs of CBN were determined by scanning electron microscopy (SEM, Hitachi X-650). The X-ray photoelectron spectroscopy (XPS) was performed in an ESCALAB MK II system with a spherical sector analyzer. The base pressure was 2 1010 mbar. The photoelectrons were collected at an emission angle of 151 with respect to the normal surface. An Mg Ka radiation source (hn ¼ 1253:6 eV) was used in XPS measurements.
0.1230 0.1253
0.3373 (a) C 0 h
3. Results and discussion Fig. 1(a), (b) and (c) show the typical SEM feature micrograph of CBN after ball milling for 30, 60 and 90 h. It is well known that the graphite belongs to a layeredstructure. In the layer plane, there is a strong covalent bond due to sp2 orbital, so the structure is very stable in the layer plane. But there are van der Waals’ forces between the layers, so the structure can be changed or destroyed easily. The h-BN has a similar structure to the graphite (see Fig. 2 and Table 1). Under the strong influence of external force, the slipping between layers can occur easily. During the process of high-energy ball milling, there is an instantaneous local pressure about 2–6 GPa [15]. This pressure will cause the change of the microstructures between layers of C and BN, which leads to slipping, stacking default, fracture and surface stripping between layers, and forms the hexagonal-like sheets. From Fig. 1 we can observe different morphology clearly in CBN ternary compounds. When the ball milling time is 30 h, the hexagonal-like sheets can be
0.1682 0.2130 0.2020 0.1231
5
15
25
35
45
55
65
75
2θ (°) Fig. 2. The XRD patterns of BN and C original powders before ball milling.
observed. The size and the thickness of these sheets are about 200–2800 nm and 101–102 nm, respectively (see Fig. 1(a)). After ball milling for 60 and 90 h, according to SEM feature micrograph of CBN ternary compounds, we can observe that there are different shapes of nano-grains, sheets, composite nano-layered-rods (see Fig. 1(b) and (c)). The diameter and the growth step thickness of composite
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153
Table 1 The parameters of experiments and calculated by a maths analyticmethod for CBN samples CBN
Exp. values
Exp. values
0h
0.3373 0.2130 0.2020 0.1682 0.1231
0h
0.3338 0.2166 0.2058 0.1817 0.1664 0.1253 0.1230
002 100 101 102 004 110 111
60 h
90 h
1.190 0.8358 0.5980 0.5701 0.5502 0.5360
1.187 0.8356 0.5980 0.5711 0.5460 0.5368 0.4741 0.4290 0.3930 0.3734 0.3620 0.3531 0.3398 0.3358 0.3045 0.2665 0.2562 0.2510 0.2323 0.2225
0.4290 0.3951 0.3735 0.3626 0.3531 0.3403 0.3370 0.3046 0.2665 0.2561 0.2513 0.2317 0.2225
Cal. values a ¼ 1.6204 (nm) c ¼ 1.7810 (nm) hkl 1.190 0.8102 0.5992 0.5728 0.5574 0.5401 0.4788 0.4293 0.3950 0.3734 0.3623 0.3516 0.3401 0.3361 0.3045 0.2664 0.2562 0.2513 0.2325 0.2225
101 200 202 220 103 300 203 104 401 331 420 224 115 314 502, 432 610 620 107 227 720
(c) 90 h
intensity (a.u.)
Graphite h-BN
(d) 120 h
(b) 60 h
(a) 30 h 20
30
40
50
60
70
Fig. 3. The XRD patterns of C samples with different ball milling time.
(d) 120 h
Note: Where d (nm) is the spacing of index (h k l) in the direct lattice.
(c) 90 h
intensity (a. u.)
nano-layered-rods are about 200–900 nm and 15–40 nm, respectively. The XRD patterns of BN and C original powders are shown in Fig. 2. From it we can see the graphite hexagonal structure (a ¼ 0:2463 nm, c ¼ 0:6714 nm) in the original powders of C and hexagonal structure (a ¼ 0:2504 nm, c ¼ 0:6661 nm) of BN, respectively. Fig. 3 shows the XRD patterns of C samples with different ball milling time. From Fig. 3, we can see there are no new phases except that the diffraction peaks are broadened and weaken with the increase of ball milling time, which indicates that with the increase of ball milling time, the crystalline grains of C become smaller and nanocrystal and amorphous grains are formed in the end. Because C powder is a single phase powder and there is no reaction among C powder, the stainless steel vial and the balls, there are no new compounds formed during the process of the ball milling. We can observe the similar results of BN original powders with different ball milling time too. However, we can observe some new diffraction peaks in the mixed powder of graphite and hexagonal boron nitride with increase of ball milling time. The XRD patterns of CBN samples with different ball milling times (30, 60, 90 and 120 h) are shown in Fig. 4. After ball milling for 30 h, the intensities of the diffraction peaks of the C and BN are weakened and some XRD peaks
80
2θ (°)
(b) 60 h
(a) 30 h
5
15
25
35
45 2θ (°)
55
65
75
Fig. 4. The XRD patterns of CBN samples with different ball milling time.
at high diffraction angle almost disappeared, but two new XRD peaks appeared (see Fig. 4(a)). After ball milling for 60 h, in the range of low diffraction angle, more than ten new XRD peaks appeared (see Fig. 4(b)). With further increase of ball milling time, these new XRD peaks become sharper, which indicates that some new phase of CBN was formed. In order to study these new diffraction peaks further, we try to fit and index these new diffraction peaks
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using a mathematical analytic method (It is the well-known Cohen’s method). It is more interesting that all peaks can be indexed as tetragonal crystal system. The calculated values are shown in Table 1, from which we can see that there may exist a tetragonal system in CBN with ball milling time of 60 h and 90 h and its lattice parameters are a ¼ 1:6204 nm, c ¼ 1:7810 nm, respectively. The calculated values are in good agreement with the results of our experiment. Of course, this is not the only explanation and there is a possibility to form another phase structure or compounds. According to the further analyses of Fig. 4(b) and (c), we can see that the diffraction peaks of these tetragonal phase are superposed upon a few weakened and broadened diffraction peaks of hexagonal boron nitride and graphite. After ball milling for 120 h, these tetragonal phase diffraction peaks almost vanish and there are a few very weakened and broadened diffraction peaks, which implies that CBN amorphous and nancrystalline are central phases here (see (d) in Fig. 4). These calculated results are in accordance with the data of PDF cards (090012 and 23-0064) and another group’s results. In 1965, Batsanov et al. [16] have observed that the action of powerful shock wave brought about the crystallization of amorphous BN, its hexagonal lattice being retained, while the very high compressive forces produced a small amount of a new modification, which was named the e-form (from the word ‘‘explosion’’). Recently the Lee’ group [5] has observed some minor XRD peaks of unknown crystal system in the recrystallized Cx(BN)1x sample (after ball milling for 72 h, heated at 1200 1C for 24 h). We suggest that the reasons inducing the increase of tetragonal CBN crystal cell or the formation of some unknown microstructure and phase separation may be as follows: firstly, there are many defects, dislocation, serious deformation of lattice due to strong shock and grinding during the process of high energy ball milling; secondly, because the crystalline grain became smaller, which will increase the interdiffusion of the atoms that are located in the surface and interface of the C and BN crystalline grain, the lattice constants of prime hexagonal graphite and hexagonal boron nitride will be changed; finally the interdiffusion between atoms will give rise to a local solid state reaction and the formation of new phase structure or a new compounds. In this case, both the valence state and the bond length of CBN are changed. In order to understand the varied bonding state and microstructure characterization of CBN with abundant phase structure and nanostructure further, we have discussed the XPS of CBN. Fig. 5 shows XPS analysis of the C1s, N1s and B1s spectrum with ball milling 90 h and the inset is XPS analysis of the C1s, N1s and B1s spectrum before ball milling. From Table 2 and Fig. 5, we can see that there is an obvious difference between XPS results before ball milling and that after ball milling for 90 h. Before ball milling, for B1s, the bonding energy is 190.52 eV, which corresponds to B–N bonds of h-BN; for C1s, the bonding energy is 284.35 eV, which corresponds to
500 400 300 200 100 0
600 C-90 h 400 200 0
280
600
284
C-0 h
280
400
288
292
288 600 450 300 150 0 394
N-90 h
284
292
N-0 h
398
402
406
200 0
600
394
398 500 400 300 200 100 0
B-90 h
400 200 0
186
402
190
406
B-0 h
186 190 194 198 Bonding energy (eV)
194
198
Bonding energy (eV) Fig. 5. XPS analysis of C1s, N1s and B1s spectrum after ball milling 90 h. The inset is XPS analysis of C1s, N1s and B1s spectrum before ball milling.
C–C bonds of graphite; for N1s, the bonding energy is 398.16 eV, which corresponds to N-B bonds of h-BN. After ball milling for 90 h, B1s spectrum was decomposed into four distinguishable peaks, the bonding energies are 284.60, 285.80, 286.80 and 288.70 eV, respectively; C1s spectrum was decomposed into four distinguishable peaks too; but N1s spectrum was decomposed into five distinguishable peaks. These new bonding states correspond to B–N, C–B–N, B–O, C–C, C–N (sp2), C–N (sp3), C–O, N–C (sp2) and N–O bonds. The formation of these new bonding states, especially the formation of the bonding states of C–B–N, C–N (sp2), C–N (sp3), N–C (sp2) implies that the solid state reaction is induced, C and BN are dissolved by each other, and the new phase is separated during the process of the ball milling. These results are in accordance with that of XRD. The detailed discussion of the XPS of CBN with different ball milling time will be presented elsewhere. 4. Conclusions CBN ternary compounds with nano-layer structure and multi-phase structure were prepared by mechanical
ARTICLE IN PRESS Y.H. Xiong et al. / Physica B 382 (2006) 151–155 Table 2 The relation between bonding state of CBN and different ball milling time Sample CBN Ball milling times (h) B1s Bonding Bonding Bonding Bonding
energy energy energy energy
(ev) (eV) (eV) (eV)
C1s Bonding Bonding Bonding Bonding Bonding
energy energy energy energy energy
(eV) (eV) (eV) (eV) (eV)
N1s Bonding Bonding Bonding Bonding Bonding
energy energy energy energy energy
(eV) (eV) (eV) (eV) (eV)
Bonding state
0
90
190.52
190.00 191.45 192.80 193.20
B–N B–N C–B–N or B–O B–O
284.60 285.80 286.8 288.70
C–C (nanocrystal) C–C C–N (sp2) C–N (sp3) C–O
397.50 398.30 399.55 400.55 402.10
N–C N–B, N–C (sp3) C–B–N N–C (sp2) N–O
284.35
398.16
alloying technique at room temperature. The results of XRD and SEM of CBN with different ball milling time showed that the microstructure transformation, the formation of new phase and the feature micrograph of CBN samples were closely related to the ball milling time and ball milling condition. After ball milling for 60 and 90 hrs, some new diffraction peaks were observed, which implied that the reactive ball milling may induce some multi-phase structure or phase separation and some unknown microstructures of CBN composite were formed. The new phase may be fitted and indexed using the mathematical analytic method, and all peaks can be indexed by a tetragonal crystal system. According to the results of XPS of CBN samples before ball milling and after ball milling for 90 h, it was substantiated that the bonding state and microstructure of CBN were closely related to the ball milling time too.
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Acknowledgment This work was supported by the National Science Foundation of China (Grant no. 10274022) and School of Science Foundation of Huazhong University of Science and Technology.
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