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Reduction–boronation route to chromium boride (CrB) nanorods
Chemical Physics Letters 381 (2003) 194–198 www.elsevier.com/locate/cplett
Reduction–boronation route to chromium boride (CrB) nanorods Jianhua Ma a
a,b
, Yunle Gu a, Liang Shi a, Luyang Chen a, Zeheng Yang a, Yitai Qian a,*
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, East Campus, Building 323, Room 203, Hefei, Anhui 230026, PR China b Wenzhou University, Wenzhou, Zhejiang 325027, PR China Received 7 July 2003; in final form 22 September 2003
Published online:
Abstract Chromium boride (CrB) nanorods were synthesized via a reduction–boronation route at 650 °C in molten salt in an autoclave. The X-ray diffraction pattern of the product was indexed as the orthorhombic CrB. Transmission electron microscope images indicated that the sample consisted of single-crystalline nanorods with the diameter of 10–30 nm and a maximum length of 1.5 lm. The PL spectra showed the ultraviolet light emission at 340 nm. The thermogravimetric analysis showed that the product also had very good anti-oxidation properties below 630 °C. A possible growth mechanism was proposed. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction One-dimension (1D) nanostructures, such as nanorods, nanowires and nanotubes are attracting increasing interest due to their remarkable optical, electrical, and mechanical properties and their potential applications ranging from probe microscopy tips to interconnections in nanoelectrical devices [1,2]. Therefore, the synthesis and study of the nanoscale one-dimensional structures become
*
Corresponding author. Fax: +86-551-3607402. E-mail address: [email protected] (Y. Qian).
the focus of much current attention in mesoscopic physics and in nanodevices [3]. Generally, 1D structures were prepared by template method, such as synthesis of carbide nanorods (TiC, SiC, NbC, BCx ) [4] or nitride nanorods (GaN, Si3 N4 ) [5] at high reaction temperature by using carbon nanotubes, electrochemical processes of a variety of metal and semiconductor nanorods by using 1D nanochannel in porous anodic alumina [6], high pressure injection synthesis of nanorods by using 1D nanochannel in nuclear track polycarbonate membranes [7]. Other methods, such as solutionphase growth synthesis of nanowires in organic solvents [8] at lower temperatures and synthesis of
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.128
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
111 30
40
50
60
70
Fig. 1. XRD pattern of the CrB nanorods.
221 151
112
200 002
131 041
130 040
All the reagents were analytically pure. All of the manipulations were carried out in a dry glove box filled with N2 . Firstly, 0.015 mol of anhydrous chromium trichloride, 0.015 mol of amorphous boron powder and 0.045 mol of metallic sodium were placed into a stainless steel autoclave. Then about 30 g of anhydrous aluminum trichloride were added to the autoclave. Finally, the autoclave was sealed under argon atmosphere. The autoclave was heated at 650 °C for 10 h, followed by naturally cooling to the room temperature in the furnace. Absolute ethanol was used to dissolve the aluminum trichloride. Then the products were washed several times with distilled water, diluted acid, diluted sodium hydroxide solution and absolute ethanol in sequence to remove impurities. The final product was vacuum-dried at 60 °C for 12 h. Black powder product was obtained. The powder product was analyzed by powder X-ray diffraction (XRD) on an X-ray diffractometer (Rigaku rA) using Cu Ka radiation (wave), and transmission electron length k ¼ 1:54178 A microscopy on a Hitachi 800 transmission electron microscope (TEM). The product composition was analyzed by inductively coupled plasma atomic
Fig. 1 shows the XRD pattern of the as-prepared CrB nanorods. The results of the XRD pattern of the product are recorded in Table 1. All the peaks at different d-spacing can be indexed as orthorhombic structure of CrB. Refinement re, veals that the cell constants are a ¼ 2:963 A and c ¼ 2:933 A , which are close to b ¼ 7:838 A , b ¼ 7:810 A and the literature values (a ¼ 2:960 A ) [13]. No evidences of the impurities c ¼ 2:940 A such as Cr, B and Cr2 O3 etc., can be found in the XRD pattern. The TEM images and selected area transmission electron diffraction (ED) pattern of CrB nanorods are shown in Fig. 2. It can be seen that the sample consists of nanorods with the diameter
021
2. Experimental
3. Results and discussion
110
CrCl3 þ B þ 3Na ! CrB þ 3NaCl
emission spectrometry (ICPAES, Atomscan Advantage, Thermo Jarrell Ash Corporation). Photoluminescence spectra of the CrB nanorods were measured in a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. Thermogravimetric analysis (TGA) was carried on a Shimadzu TGA-50H thermogravimeter analyzer. Scans were run at a ramp rate of 10 °C/min from room temperature to 1000 °C under the flowing air atmosphere.
Intensity / A.U.
semiconductor silicon nanowires or silica nanowires based on oven laser evaporation [9,10], were useful. Borides of transition metals have attracted considerable interest for their various unique properties. Among these borides, chromium boride (CrB) is currently of great technological importance because of its high melting point, high chemical stability, high hardness and strength. So it has found applications in structure materials. The literatures [11,12] reported on the synthesis of CrB crystals above the temperature of 900 °C. In this Letter, we have developed a molten salt (anhydrous aluminum trichloride) method to prepare CrB nanorods via a reduction–boronation route at 650 °C in an autoclave. It can be described as follows,
195
80
196
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
Table 1 The results of the XRD spectra of the CrB nanorods
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Experimental ) values d (A 2.773 2.343 2.015 1.966 1.966 1.630 1.630 1.482 1.469 – – – – 1.295 1.255 1.250
Reported values [13] hk l
) d (A
110 021 111 130 040 131 041 200 002 220 150 022 060 112 221 151
2.7678 2.3487 2.0152 1.9548 1.9548 1.6278 1.6278 1.4800 1.4700 1.3839 1.3814 1.3757 1.3016 1.2982 1.2521 1.2503
of 10–30 nm and the maximum length of 1.5 lm. The ED pattern is consistent with the high crystallinity of CrB. The CrB sample was also analyzed by ICPAES by dissolving CrB with molten sodium hydroxide and sodium peroxide. The result shows the content Cr and B in the sample are about 80.67 and 15.59 wt.%, respectively. The atomic ratio of Cr:B is 1.0759:1, which closely agrees with the formula of CrB. The yield of CrB nanorods is estimated to be 55–60% in the sample according the TEM view scope.
340 nm
Fluroscence / A.U.
No.
280
300
320
340
360
380
400
420
440
460
480
500
Fig. 3. Room temperature photoluminescence emission spectra of the CrB nanorods.
The PL emission spectra of the CrB nanorods dispersed in ethanol by sonication are showed in Fig. 3. The excitation wavelength is 220 nm and the filter wavelength is 290 nm. It is clear that there is a strong peak centered at 340 nm in the spectra. Since the PL spectra can reflect the optical effect of the material excited by the lights with different wavelength, we can conclude that the as-prepared CrB nanorods might be regarded as the luminescence material though there is not any literature reported on the optical property of CrB. The oxidation behavior of CrB nanorods was studied at the temperature below 1000 °C by TGA,
Fig. 2. TEM and ED images of the CrB nanorods.
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
70
Weight gain / %
60 50 40 30 20 10 0 -10 200
400
600
800
1000
Fig. 4. TGA curves heated under the flowing air for the CrB nanorods.
as shown in Fig. 4. Below the temperature of 500 °C, the weight gain almost has not changed. From 500 to 1000 °C, the weight gain of CrB nanorods experiences four different steps. Below the temperature of 630 °C, the weight gain increases gradually, which indicates that the sample is oxidized by oxygen slowly forming chromium oxide and boron oxides. So it has very good antioxidation properties below this temperature. This result is somewhat different from the literatures [14,15]. Since the as-prepared product has high surface energy due to the fine size in diameter, they might be oxidized at lower temperature than bulk materials and the thin films. When the temperature is within 630–660 °C, the weight gain increases significantly indicating that the sample is oxidized rapidly forming oxide layer gradually at the surface of the nanorods. As the temperature keeps on rising, the oxidation process is slowed down due to the oxide layer. But when the temperature reaches 960 °C, the oxidation process becomes strengthened again. This may be that at this temperature the formed protective layer is destroyed and the oxidation rate increases rapidly again [14]. The influences of the reaction temperature and time on the formation of the CrB nanorods were investigated. A suitable reaction temperature played an important role in the formation of CrB nanorods. If the temperature was lower than 600 °C, the CrB nanorods could not be obtained.
197
If the temperature was higher than 700 °C, the diameters of the CrB nanorods increased obviously. An optimum reaction temperature for the CrB nanorods was about 650 °C. The reaction time must be sufficient to ensure the process of boronation. It was found that 10 h was needed for the synthesis of nanorods. The growth of CrB nanorods might be ascribed to the vapor–liquid–solid (VLS) mechanism [16]. In this work, when the reaction temperature rises, the molten salt AlCl3 can melt providing liquid medium which can accelerate the kinetics by enhancing diffusion since the diffusion coefficients in the liquid state are higher than those in the solid state. The molten salt flux may have a large accommodation coefficient and may be a preferred site for the deposition of atoms from reduction of chromium trichloride. Since AlCl3 is a kind of compound with covalent bond, the pressure provided by the evaporated AlCl3 may be very high in the autoclave at the reaction temperature. And this may benefit the VLS nucleation for the CrB. Meanwhile, the molten salt flux may also play a role in facilitating the growth of the CrB nanorods. Since crystal growth involves the exchange of atoms at the growth front planes, the high temperature molten salt flux could keep the impinging atoms from being dislodged by thermal diffusion and thermal vibrations, and therefore enhance the nanorod growth.
4. Conclusion In summary, CrB nanorods were successfully prepared via a reduction–boronation route at 650 °C in the molten salt of anhydrous aluminum trichloride in an autoclave. The as-prepared CrB nanorods typically had the diameter of 10–30 nm and the maximum length of 1.5 lm. The molten salt, anhydrous aluminum trichloride, might play an important role in the formation of CrB nanorods. The PL spectra of the CrB nanorods showed the ultraviolet light emission at 340 nm, which was reported for the first time. It also had very good anti-oxidation properties below the temperature of 630 °C. So this CrB nanorods might be applied as high temperature luminescence materials.
198
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
Acknowledgements Financial support from the Chinese National Natural Science Foundation is gratefully acknowledged. References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [3] G. Fasol, Science 280 (1998) 545. [4] H.J. Dai, E.W. Wong, Y.Z. Lu, S.S. Fan, C.M. Lieber, Nature 375 (1995) 769. [5] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. [6] D. Moutkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037.
[7] C.R. Martin, Science 266 (1994) 1961. [8] T.J. Trenteler, K.M. Hickman, S.B. Goel, A.M. Viano, P.C. Gobons, W.E. Buhro, Science 270 (1995) 191. [9] A.M. Morales, G.M. Lieber, Science 279 (1998) 208. [10] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [11] K. Iizumi, K. Kudaka, D. Maezawa, T. Sasaki, J. Ceram. Soc. Jpn. 107 (1999) 491. [12] K. Iizumi, N. Yoshikawa, S. Okada, K. Kudaka, T. Lundstorm, Nippon Kagaku Kaishi 11 (1992) 1320. [13] Joint Committee on Powder Diffraction Standards (JCPDS), pp. 75–1159. [14] V.I. Matkovich, Boron and Refractory Borides, Springer, New York, 1977. [15] M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 343–344 (1999) 234. [16] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
Reduction–boronation route to chromium boride (CrB) nanorods Jianhua Ma a
a,b
, Yunle Gu a, Liang Shi a, Luyang Chen a, Zeheng Yang a, Yitai Qian a,*
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, East Campus, Building 323, Room 203, Hefei, Anhui 230026, PR China b Wenzhou University, Wenzhou, Zhejiang 325027, PR China Received 7 July 2003; in final form 22 September 2003
Published online:
Abstract Chromium boride (CrB) nanorods were synthesized via a reduction–boronation route at 650 °C in molten salt in an autoclave. The X-ray diffraction pattern of the product was indexed as the orthorhombic CrB. Transmission electron microscope images indicated that the sample consisted of single-crystalline nanorods with the diameter of 10–30 nm and a maximum length of 1.5 lm. The PL spectra showed the ultraviolet light emission at 340 nm. The thermogravimetric analysis showed that the product also had very good anti-oxidation properties below 630 °C. A possible growth mechanism was proposed. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction One-dimension (1D) nanostructures, such as nanorods, nanowires and nanotubes are attracting increasing interest due to their remarkable optical, electrical, and mechanical properties and their potential applications ranging from probe microscopy tips to interconnections in nanoelectrical devices [1,2]. Therefore, the synthesis and study of the nanoscale one-dimensional structures become
*
Corresponding author. Fax: +86-551-3607402. E-mail address: [email protected] (Y. Qian).
the focus of much current attention in mesoscopic physics and in nanodevices [3]. Generally, 1D structures were prepared by template method, such as synthesis of carbide nanorods (TiC, SiC, NbC, BCx ) [4] or nitride nanorods (GaN, Si3 N4 ) [5] at high reaction temperature by using carbon nanotubes, electrochemical processes of a variety of metal and semiconductor nanorods by using 1D nanochannel in porous anodic alumina [6], high pressure injection synthesis of nanorods by using 1D nanochannel in nuclear track polycarbonate membranes [7]. Other methods, such as solutionphase growth synthesis of nanowires in organic solvents [8] at lower temperatures and synthesis of
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.128
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
111 30
40
50
60
70
Fig. 1. XRD pattern of the CrB nanorods.
221 151
112
200 002
131 041
130 040
All the reagents were analytically pure. All of the manipulations were carried out in a dry glove box filled with N2 . Firstly, 0.015 mol of anhydrous chromium trichloride, 0.015 mol of amorphous boron powder and 0.045 mol of metallic sodium were placed into a stainless steel autoclave. Then about 30 g of anhydrous aluminum trichloride were added to the autoclave. Finally, the autoclave was sealed under argon atmosphere. The autoclave was heated at 650 °C for 10 h, followed by naturally cooling to the room temperature in the furnace. Absolute ethanol was used to dissolve the aluminum trichloride. Then the products were washed several times with distilled water, diluted acid, diluted sodium hydroxide solution and absolute ethanol in sequence to remove impurities. The final product was vacuum-dried at 60 °C for 12 h. Black powder product was obtained. The powder product was analyzed by powder X-ray diffraction (XRD) on an X-ray diffractometer (Rigaku rA) using Cu Ka radiation (wave), and transmission electron length k ¼ 1:54178 A microscopy on a Hitachi 800 transmission electron microscope (TEM). The product composition was analyzed by inductively coupled plasma atomic
Fig. 1 shows the XRD pattern of the as-prepared CrB nanorods. The results of the XRD pattern of the product are recorded in Table 1. All the peaks at different d-spacing can be indexed as orthorhombic structure of CrB. Refinement re, veals that the cell constants are a ¼ 2:963 A and c ¼ 2:933 A , which are close to b ¼ 7:838 A , b ¼ 7:810 A and the literature values (a ¼ 2:960 A ) [13]. No evidences of the impurities c ¼ 2:940 A such as Cr, B and Cr2 O3 etc., can be found in the XRD pattern. The TEM images and selected area transmission electron diffraction (ED) pattern of CrB nanorods are shown in Fig. 2. It can be seen that the sample consists of nanorods with the diameter
021
2. Experimental
3. Results and discussion
110
CrCl3 þ B þ 3Na ! CrB þ 3NaCl
emission spectrometry (ICPAES, Atomscan Advantage, Thermo Jarrell Ash Corporation). Photoluminescence spectra of the CrB nanorods were measured in a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. Thermogravimetric analysis (TGA) was carried on a Shimadzu TGA-50H thermogravimeter analyzer. Scans were run at a ramp rate of 10 °C/min from room temperature to 1000 °C under the flowing air atmosphere.
Intensity / A.U.
semiconductor silicon nanowires or silica nanowires based on oven laser evaporation [9,10], were useful. Borides of transition metals have attracted considerable interest for their various unique properties. Among these borides, chromium boride (CrB) is currently of great technological importance because of its high melting point, high chemical stability, high hardness and strength. So it has found applications in structure materials. The literatures [11,12] reported on the synthesis of CrB crystals above the temperature of 900 °C. In this Letter, we have developed a molten salt (anhydrous aluminum trichloride) method to prepare CrB nanorods via a reduction–boronation route at 650 °C in an autoclave. It can be described as follows,
195
80
196
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
Table 1 The results of the XRD spectra of the CrB nanorods
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Experimental ) values d (A 2.773 2.343 2.015 1.966 1.966 1.630 1.630 1.482 1.469 – – – – 1.295 1.255 1.250
Reported values [13] hk l
) d (A
110 021 111 130 040 131 041 200 002 220 150 022 060 112 221 151
2.7678 2.3487 2.0152 1.9548 1.9548 1.6278 1.6278 1.4800 1.4700 1.3839 1.3814 1.3757 1.3016 1.2982 1.2521 1.2503
of 10–30 nm and the maximum length of 1.5 lm. The ED pattern is consistent with the high crystallinity of CrB. The CrB sample was also analyzed by ICPAES by dissolving CrB with molten sodium hydroxide and sodium peroxide. The result shows the content Cr and B in the sample are about 80.67 and 15.59 wt.%, respectively. The atomic ratio of Cr:B is 1.0759:1, which closely agrees with the formula of CrB. The yield of CrB nanorods is estimated to be 55–60% in the sample according the TEM view scope.
340 nm
Fluroscence / A.U.
No.
280
300
320
340
360
380
400
420
440
460
480
500
Fig. 3. Room temperature photoluminescence emission spectra of the CrB nanorods.
The PL emission spectra of the CrB nanorods dispersed in ethanol by sonication are showed in Fig. 3. The excitation wavelength is 220 nm and the filter wavelength is 290 nm. It is clear that there is a strong peak centered at 340 nm in the spectra. Since the PL spectra can reflect the optical effect of the material excited by the lights with different wavelength, we can conclude that the as-prepared CrB nanorods might be regarded as the luminescence material though there is not any literature reported on the optical property of CrB. The oxidation behavior of CrB nanorods was studied at the temperature below 1000 °C by TGA,
Fig. 2. TEM and ED images of the CrB nanorods.
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
70
Weight gain / %
60 50 40 30 20 10 0 -10 200
400
600
800
1000
Fig. 4. TGA curves heated under the flowing air for the CrB nanorods.
as shown in Fig. 4. Below the temperature of 500 °C, the weight gain almost has not changed. From 500 to 1000 °C, the weight gain of CrB nanorods experiences four different steps. Below the temperature of 630 °C, the weight gain increases gradually, which indicates that the sample is oxidized by oxygen slowly forming chromium oxide and boron oxides. So it has very good antioxidation properties below this temperature. This result is somewhat different from the literatures [14,15]. Since the as-prepared product has high surface energy due to the fine size in diameter, they might be oxidized at lower temperature than bulk materials and the thin films. When the temperature is within 630–660 °C, the weight gain increases significantly indicating that the sample is oxidized rapidly forming oxide layer gradually at the surface of the nanorods. As the temperature keeps on rising, the oxidation process is slowed down due to the oxide layer. But when the temperature reaches 960 °C, the oxidation process becomes strengthened again. This may be that at this temperature the formed protective layer is destroyed and the oxidation rate increases rapidly again [14]. The influences of the reaction temperature and time on the formation of the CrB nanorods were investigated. A suitable reaction temperature played an important role in the formation of CrB nanorods. If the temperature was lower than 600 °C, the CrB nanorods could not be obtained.
197
If the temperature was higher than 700 °C, the diameters of the CrB nanorods increased obviously. An optimum reaction temperature for the CrB nanorods was about 650 °C. The reaction time must be sufficient to ensure the process of boronation. It was found that 10 h was needed for the synthesis of nanorods. The growth of CrB nanorods might be ascribed to the vapor–liquid–solid (VLS) mechanism [16]. In this work, when the reaction temperature rises, the molten salt AlCl3 can melt providing liquid medium which can accelerate the kinetics by enhancing diffusion since the diffusion coefficients in the liquid state are higher than those in the solid state. The molten salt flux may have a large accommodation coefficient and may be a preferred site for the deposition of atoms from reduction of chromium trichloride. Since AlCl3 is a kind of compound with covalent bond, the pressure provided by the evaporated AlCl3 may be very high in the autoclave at the reaction temperature. And this may benefit the VLS nucleation for the CrB. Meanwhile, the molten salt flux may also play a role in facilitating the growth of the CrB nanorods. Since crystal growth involves the exchange of atoms at the growth front planes, the high temperature molten salt flux could keep the impinging atoms from being dislodged by thermal diffusion and thermal vibrations, and therefore enhance the nanorod growth.
4. Conclusion In summary, CrB nanorods were successfully prepared via a reduction–boronation route at 650 °C in the molten salt of anhydrous aluminum trichloride in an autoclave. The as-prepared CrB nanorods typically had the diameter of 10–30 nm and the maximum length of 1.5 lm. The molten salt, anhydrous aluminum trichloride, might play an important role in the formation of CrB nanorods. The PL spectra of the CrB nanorods showed the ultraviolet light emission at 340 nm, which was reported for the first time. It also had very good anti-oxidation properties below the temperature of 630 °C. So this CrB nanorods might be applied as high temperature luminescence materials.
198
J. Ma et al. / Chemical Physics Letters 381 (2003) 194–198
Acknowledgements Financial support from the Chinese National Natural Science Foundation is gratefully acknowledged. References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [3] G. Fasol, Science 280 (1998) 545. [4] H.J. Dai, E.W. Wong, Y.Z. Lu, S.S. Fan, C.M. Lieber, Nature 375 (1995) 769. [5] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. [6] D. Moutkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037.
[7] C.R. Martin, Science 266 (1994) 1961. [8] T.J. Trenteler, K.M. Hickman, S.B. Goel, A.M. Viano, P.C. Gobons, W.E. Buhro, Science 270 (1995) 191. [9] A.M. Morales, G.M. Lieber, Science 279 (1998) 208. [10] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [11] K. Iizumi, K. Kudaka, D. Maezawa, T. Sasaki, J. Ceram. Soc. Jpn. 107 (1999) 491. [12] K. Iizumi, N. Yoshikawa, S. Okada, K. Kudaka, T. Lundstorm, Nippon Kagaku Kaishi 11 (1992) 1320. [13] Joint Committee on Powder Diffraction Standards (JCPDS), pp. 75–1159. [14] V.I. Matkovich, Boron and Refractory Borides, Springer, New York, 1977. [15] M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 343–344 (1999) 234. [16] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.