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Synthesis of cubic boron nitride from amorphous boron nitride containing oxide impurity using Mg–Al alloy catalyst solvent
ARTICLE IN PRESS
Journal of Crystal Growth 260 (2004) 217–222
Synthesis of cubic boron nitride from amorphous boron nitride containing oxide impurity using Mg–Al alloy catalyst solvent S.K. Singhala,*, J.K. Parkb b
a Division of Engineering Materials, National Physical Laboratory, New Delhi 110012, India Division of Ceramics, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Received 30 July 2003; accepted 22 August 2003 Communicated by M. Schieber
Abstract Single crystals of cubic boron nitride (cBN) were synthesized from amorphous boron nitride (aBN) under static high pressures and temperatures (40–50 kb, 1200–1500 C) using Mg–Al alloy catalyst-solvent material. The weight percentage of magnesium in the alloy powder was about 40%. It was found that aBN containing small amount of B2O3 as an oxide impurity transforms easily into cBN (in the thermodynamically stable region of cBN) whereas aBN powder without B2O3 did not transform into cBN to the same extent under the similar P2T conditions. It appears therefore, that the presence of oxide impurity in aBN powder facilitates the transformation of aBN into cBN although it does not have any catalytic action for aBN–cBN phase transformation. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Uw Keywords: Amorphous boron nitride; Oxide impurity; Phase transformation; Cubic boron nitride; High pressure
1. Introduction For the past several decades synthesis of cubic boron nitride (a material next to diamond in hardness) has been extensively investigated due to its unique physical and chemical properties including its wide band gap (>6.4 eV), thermal stability up to about 1300 C, chemically inert towards ferrous metals and alloys and optical transparency over the wide range of wave length. As it can be *Corresponding author. Fax: +91-11-25726938. E-mail address: [email protected] (S.K. Singhal).
doped as a p-type (Be dopent) or an n-type (Si dopent) it is considered to be a promising wide band gap III–V semiconductor suitable for high power transistor and optoelectronic devices. It is well known that under the application of high pressures and temperatures the hexagonal modification of boron nitride (hBN) transforms into the cubic (cBN) and wurtzitic modifications (wBN) either directly or by the nucleation and growth process from a particular flux which is generated during the synthesis by the reaction of hexagonal boron nitride with the nitrides of alkali metals or alkaline earth metals. In recent years, efforts have been by a number of researchers to
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.068
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synthesize cBN from low ordered boron nitrides namely turbostratic boron nitride (tBN), amorphous boron nitride (aBN) and also from rhombohedral boron nitride (rBN) as the starting materials [1–9]. It has been found that hexagonal and amorphous boron nitride powders normally contain oxide impurity (B2O3) with different concentration depending upon its method of preparation. It has been reported by Endo et al. [9] that a hBN with lower concentration of oxygen impurity is preferred for cBN synthesis as higher concentration of oxygen reduces the chemical activity of the catalyst used and also the minimum temperature is also lower by about 300 C. However, not much information is available about the role of oxide impurity present in amorphous boron nitride in its transformation to cubic modification in the presence of a catalyst-solvent, although Choi et al. [6] studied some effects of B2O3 in the synthesis of cBN using AlN as a catalyst which is present in the solid state under the high P2T conditions. The present work was, therefore, carried out to know more in detail about the effect of B2O3 oxide impurity present in aBN in the synthesis of cBN. An alloy powder of Mg and Al was used as the catalyst-solvent because it had a strong catalytic effect in cBN synthesis [10] and the cell assembly need not be wrapped with refractory or inert metals as required in the case of conventional catalysts since most of these catalysts are hygroscopic in nature and easily react with air.
2. Experimental procedures The starting material used in the present study was aBN powder containing oxide impurity and aBN powder with almost no oxide impurity. The aBN powder was prepared in accordance with the method by Thomas et al. [11]. A 1:1 powdered mixture of urea and boric acid was heated very slowly to about 850 C for about 3 h in nitrogen atmosphere. Chemically this reaction is represented by 2H3 BO3 þ ðNH2 Þ2 CO-2BN þ 5H2 O þ CO2 : This material was found to contain much oxide impurity as shown in the X-ray diffraction pattern
represented by Fig. 1(a). Some peaks of Al2O3 also appear in this XRD as the aBN powder was prepared and grounded in alumina boat and mortar, respectively. It is clear from this figure that most of the material was in the amorphous state. In order to know the effect of oxide impurity in aBN on the synthesis of cBN some of the aBN powder prepared as above, was treated with dilute nitric acid followed by washing with doubled distilled water and finally by drying in vacuum at about 1000 C for about 10 h. An XRD pattern of this material is shown in Fig. 1(b). Both these types of boron nitrides were used as the starting material for the synthesis of cBN in the present study. An alloy powder of Mg and Al containing 40 wt% of magnesium was prepared by induction melting in air and then crushing the powder to about 50 mm. The starting material and the solvent powder were homogeneously mixed in atmospheric conditions. The amount of solvent was about 5 wt% of the starting material. The cBN synthesis experiments were carried out on a belt type apparatus with a 20 mm bore diameter. The pressure was calibrated at room temperature by observing the well known phase transitions in Bi (I–II) at 25.5 kb, Tl (II–III) at 37.6 kb and Ba (I–II) at 55 kb. The temperature calibration was done by inserting a Pt 13% Rh– Pt 20% Rh thermocouple at the centre of the reaction cell. No correction was made for the pressure effect on the e.m.f. of the thermocouple. The powdered mixture of aBN and Mg–Al alloy was compacted into a cylindrical shape with a diameter of 7.0 mm and a height of 8.0 mm and inserted into the high pressure reaction cell as shown in Fig. 2. The specimen was then subjected to the desired pressures and temperatures for a 10 min duration.
3. Results and discussion Fig. 3(a) shows the pressure and temperature region of cBN formation from aBN powder containing B2O3 oxide impurity and 40 Mg–60 Al alloy catalyst solvent and a reference hBN–cBN equilibrium line as calculated by Nakano and Fukunaga [12] and supported by thermodynamic calculations by Solozhenko [13]. The shaded
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Fig. 1. (a) X-ray diffraction pattern of aBN powder with oxide impurity, (b) X-ray diffraction pattern of aBN powder with no oxide impurity.
Fig. 2. Reaction cell assembly used for cBN synthesis.
circles indicate the P2T conditions under which aBN powder was transformed into cBN and the open circles indicate the conditions where no transformation from aBN to cBN was observed.
The minimum pressure and temperature required for any cBN formation using aBN with B2O3 oxide impurity and Mg–Al alloy solvent were found to be 44 kb and 1200 C. The yield of cBN increased sharply as the value of pressure was increased from 44 to 46 kb. A maximum conversion of about 80% was obtained at 46 kb and 1300 C. Still higher conversion could be achieved if the value of pressure is further increased as the P2T conditions are moved deep into the thermodynamical stable region of cBN where the supersaturation of the solution is continued to increase. Thus, although the minimum pressure value for cBN synthesis from aBN containing B2O3 has been shifted to the higher side than reported earlier (o40 kb) [7,8,14], the minimum temperature (1200 C) is comparable. According to Endo et al. [9] the presence of large amount of oxide impurity
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Fig. 3. (a) P2T region and yield of cBN formation from aBN powder containing B2O3 impurity, (b) Yield of cBN formed from aBN powder free from oxide impurity at different P2T conditions.
in hBN powder (if used as a starting material for cBN synthesis) shifts the lower value of temperature to a much higher side than that required with low oxide content. In his experiments Endo et al. used two types of hBN as the starting materials, one with 1.9 wt% of oxygen called R-type and another with 7.9 wt% of oxygen and is called N1type. The lower temperature limit for cBN formation using R-type hBN was about 1380 C at 60–80 kb whereas in case of N1-type hBN it was around 1700 C (about 300 C higher that that required for R-type). The high concentration of oxygen impurity present in N1-type hBN reacted with the catalyst used (Mg3B2N4), thereby reducing its catalytic activity and the growth of cBN diminishes or halts completely. At sufficiently high temperatures above the eutectic point of BN– Mg3B2N4 the amount of liquid increases and the residual liquid which does not suffer from the effect of oxygen can act as a catalyst-solvent. That is why Endo et al. have observed cBN formation at higher temperatures in a high oxygen containing system as in the case of hBN of N1-type. The effect of oxygen impurity on hBN–cBN transformation has also been studied by Bindal et al. [15]. They reported that after a certain temperature (eutectic temperature) its effect on hBN–cBN transformation is very small as the cBN growth formation proceeds via the dissolution of hBN in the eutectic liquid which become dominant at higher tempera-
tures followed by the precipitation of cBN in its thermodynamical stable region. However, the transformation results were greatly affected if instead of hBN, aBN powder was used as a starting material for cBN synthesis. In order to know the role of B2O3 in aBN in its conversion to cBN some of the aBN powder prepared as described earlier was washed thoroughly with dilute nitric acid and then with ethanol. Ethanol was first removed by heating the powder at about 50 C in an oven and was finally dried in a vacuum of about 102 Torr at 1000 C for about 10 h. By this treatment most of the oxide impurity from aBN powder was removed as shown in Fig. 1(b). Further, this aBN powder was partly crystallized but still remained mostly in the amorphous state as the separation of {1 0 0} and {1 0 1} peaks of hBN is not distinct and {1 0 2} peak does not appear. This material was then subjected to the same P2T conditions as those for aBN powder containing oxide impurity. It was found that the specimens of aBN powder although with almost no oxide impurity transform into cBN but the yield of cBN was comparatively much less as can be seen in Fig. 3(b). It is clear therefore, that although the presence of oxide impurity in amorphous boron nitride shifts the lower limit of pressure to higher side as compared to those of hexagonal boron nitride it does not have adverse effect on the transformation of aBN into cBN rather it
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Fig. 4. (a) XRD pattern of the reaction product obtained at 44 kb, 1300 C from aBN powder containing oxide impurity, (b) XRD pattern of the reaction product obtained at 44 kb, 1300 C from aBN powder free from oxide impurity.
facilitated the growth process as revealed by the high yield of cBN from this powder. An experiment, for example conducted at the same P2T condition (44 kb, 1300 C) gave higher yield of cBN from aBN powder containing oxide impurity as shown by the relative peak intensity of cBN in both the cases as shown by Fig. 4(a) and (b). Further, the XRD pattern in both the cases showed the presence of hBN phase along with cBN. This indicated that the cBN formation from aBN proceeded via first crystallization to hBN and that the oxide impurity facilitated the crystallization of aBN as well as its transformation into cBN. Thus, it appears that the presence of B2O3 impurity in aBN plays a very important role in its
transformation into cBN, although it does not have any catalytic action for aBN–cBN phase transformation. To confirm this an experiment was conducted by subjecting only aBN powder containing oxide impurity at 46 kb, 1400 C without using Mg–Al alloy catalyst. The XRD pattern of the recovered specimen as shown in Fig. 5 did not show the presence of any cBN in the product. The peaks were mainly due to hBN, B2O3 and Al2O3. The presence of hBN in the specimens recovered after high pressure–high temperature treatment also indicated the most probable path for cBN growth as aBN–hBN–cBN although the direct transformation of aBN into cBN is not completely ruled out. Although no quantitative
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containing oxide impurity as well as free from oxide impurity and Mg–Al alloy solvent catalyst. However, the transformation to cBN was comparatively low from oxide free aBN powder. The presence of small amount of B2O3 in aBN powder although shifts the lower value pressure to higher side as compared to those from hBN powder, the aBN–cBN phase transformation was not found to be affected adversely by its presence rather it facilitated the conversion of aBN into cBN through the liquid phase.
Fig. 5. XRD pattern of the reaction product obtained at 46 kb, 1400 C from aBN powder containing oxide impurity without using Mg–Al alloy catalyst.
measurement of oxide impurity in the starting aBN powder used was carried out it is almost certain that its concentration was even much lower than those found in R-type hBN powder used by Endo et al. which contained about 1.9 wt% oxygen impurity. Therefore, in the present experiments the lower temperature limit for cBN formation was only about 1200 C at 42 kb as compared to about 1400 C at 60–80 kb for R-type hBN (lower by another 200 C). The cBN crystals synthesized from aBN powder containing B2O3 impurity and 40 Mg–60 Al alloy catalyst-solvent were irregular in shape, yellow to brown in colour depending on the synthesis temperature and fall in the 10–50 mm size range and are very useful for various cutting tool applications.
4. Conclusions Single crystals of cubic boron nitride were synthesized from amorphous boron nitride powder
References [1] A. Onodera, K. Inoue, H. Yoshinara, H. Nakae, T. Matsuda, T. Hirai, J. Mater. Sci. 25 (1990) 4279. [2] H. Sumiya, T. Iseki, O. Onodera, Mater. Res. Bull. 18 (1983) 1203. [3] I.S. Gladkaya, G.N. Kremkova, V.N. Slesarev, J. LessCommon Met. 117 (1986) 241. [4] T. Sata, T. Ishii, N. Setaka, J. Am. Ceram. Soc. 65 (1982) C-162. [5] T. Sekine, T. Sato, J. Appl. Phys. 74 (1993) 2440. [6] J.Y. Choi, S.T. Park, O. Fukunaga, J.K. Park, K.Y. Eun, J. Am. Ceram. Soc. 76 (1993) 2525. [7] M.M. Bindal, B.P. Singh, S.K. Singhal, R.K. Nayar, R. Chopra, J. Crystal Growth 144 (1994) 97. [8] M.M. Bindal, B.P. Singh, S.K. Singhal, R.K. Nayar, R. Chopra, E.S.R. Gopal, G. Will, XV AIRAPT International Conference on High Pressure Science and Technology, Warsaw, Poland, Sept. 11–14, 1995. [9] T. Endo, O. Fukunaga, M. Iwata, J. Mater. Sci. 14 (1979) 1375. [10] J.K. Park, S.T. Park, K.Y. Eun, Int. J. Refract. Metals & Hard Mater. 12 (1994) 225. [11] J. Thomas, N.E. Weston, T.E. O’Conner, J. Am. Chem. Soc. 84 (1963) 4619. [12] S. Nakano, O. Fukunaga, Diamond & Relat. Mater. 2 (1993) 1409. [13] V.L. Solozhenko, Zh. Fiz. Khim. 61 (1988) 2851. [14] V.L. Solozhenko, A.B. Slutsky, Yu.A. Ignatiev, J. Superhard Mater. 14 (1992) 61. [15] M.M. Bindal, S.K. Singhal, B.P. Singh, R.K. Nayar, R. Chopra, A. Dhar, J. Crystal Growth 112 (1991) 386.
Journal of Crystal Growth 260 (2004) 217–222
Synthesis of cubic boron nitride from amorphous boron nitride containing oxide impurity using Mg–Al alloy catalyst solvent S.K. Singhala,*, J.K. Parkb b
a Division of Engineering Materials, National Physical Laboratory, New Delhi 110012, India Division of Ceramics, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Received 30 July 2003; accepted 22 August 2003 Communicated by M. Schieber
Abstract Single crystals of cubic boron nitride (cBN) were synthesized from amorphous boron nitride (aBN) under static high pressures and temperatures (40–50 kb, 1200–1500 C) using Mg–Al alloy catalyst-solvent material. The weight percentage of magnesium in the alloy powder was about 40%. It was found that aBN containing small amount of B2O3 as an oxide impurity transforms easily into cBN (in the thermodynamically stable region of cBN) whereas aBN powder without B2O3 did not transform into cBN to the same extent under the similar P2T conditions. It appears therefore, that the presence of oxide impurity in aBN powder facilitates the transformation of aBN into cBN although it does not have any catalytic action for aBN–cBN phase transformation. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Uw Keywords: Amorphous boron nitride; Oxide impurity; Phase transformation; Cubic boron nitride; High pressure
1. Introduction For the past several decades synthesis of cubic boron nitride (a material next to diamond in hardness) has been extensively investigated due to its unique physical and chemical properties including its wide band gap (>6.4 eV), thermal stability up to about 1300 C, chemically inert towards ferrous metals and alloys and optical transparency over the wide range of wave length. As it can be *Corresponding author. Fax: +91-11-25726938. E-mail address: [email protected] (S.K. Singhal).
doped as a p-type (Be dopent) or an n-type (Si dopent) it is considered to be a promising wide band gap III–V semiconductor suitable for high power transistor and optoelectronic devices. It is well known that under the application of high pressures and temperatures the hexagonal modification of boron nitride (hBN) transforms into the cubic (cBN) and wurtzitic modifications (wBN) either directly or by the nucleation and growth process from a particular flux which is generated during the synthesis by the reaction of hexagonal boron nitride with the nitrides of alkali metals or alkaline earth metals. In recent years, efforts have been by a number of researchers to
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.068
ARTICLE IN PRESS 218
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synthesize cBN from low ordered boron nitrides namely turbostratic boron nitride (tBN), amorphous boron nitride (aBN) and also from rhombohedral boron nitride (rBN) as the starting materials [1–9]. It has been found that hexagonal and amorphous boron nitride powders normally contain oxide impurity (B2O3) with different concentration depending upon its method of preparation. It has been reported by Endo et al. [9] that a hBN with lower concentration of oxygen impurity is preferred for cBN synthesis as higher concentration of oxygen reduces the chemical activity of the catalyst used and also the minimum temperature is also lower by about 300 C. However, not much information is available about the role of oxide impurity present in amorphous boron nitride in its transformation to cubic modification in the presence of a catalyst-solvent, although Choi et al. [6] studied some effects of B2O3 in the synthesis of cBN using AlN as a catalyst which is present in the solid state under the high P2T conditions. The present work was, therefore, carried out to know more in detail about the effect of B2O3 oxide impurity present in aBN in the synthesis of cBN. An alloy powder of Mg and Al was used as the catalyst-solvent because it had a strong catalytic effect in cBN synthesis [10] and the cell assembly need not be wrapped with refractory or inert metals as required in the case of conventional catalysts since most of these catalysts are hygroscopic in nature and easily react with air.
2. Experimental procedures The starting material used in the present study was aBN powder containing oxide impurity and aBN powder with almost no oxide impurity. The aBN powder was prepared in accordance with the method by Thomas et al. [11]. A 1:1 powdered mixture of urea and boric acid was heated very slowly to about 850 C for about 3 h in nitrogen atmosphere. Chemically this reaction is represented by 2H3 BO3 þ ðNH2 Þ2 CO-2BN þ 5H2 O þ CO2 : This material was found to contain much oxide impurity as shown in the X-ray diffraction pattern
represented by Fig. 1(a). Some peaks of Al2O3 also appear in this XRD as the aBN powder was prepared and grounded in alumina boat and mortar, respectively. It is clear from this figure that most of the material was in the amorphous state. In order to know the effect of oxide impurity in aBN on the synthesis of cBN some of the aBN powder prepared as above, was treated with dilute nitric acid followed by washing with doubled distilled water and finally by drying in vacuum at about 1000 C for about 10 h. An XRD pattern of this material is shown in Fig. 1(b). Both these types of boron nitrides were used as the starting material for the synthesis of cBN in the present study. An alloy powder of Mg and Al containing 40 wt% of magnesium was prepared by induction melting in air and then crushing the powder to about 50 mm. The starting material and the solvent powder were homogeneously mixed in atmospheric conditions. The amount of solvent was about 5 wt% of the starting material. The cBN synthesis experiments were carried out on a belt type apparatus with a 20 mm bore diameter. The pressure was calibrated at room temperature by observing the well known phase transitions in Bi (I–II) at 25.5 kb, Tl (II–III) at 37.6 kb and Ba (I–II) at 55 kb. The temperature calibration was done by inserting a Pt 13% Rh– Pt 20% Rh thermocouple at the centre of the reaction cell. No correction was made for the pressure effect on the e.m.f. of the thermocouple. The powdered mixture of aBN and Mg–Al alloy was compacted into a cylindrical shape with a diameter of 7.0 mm and a height of 8.0 mm and inserted into the high pressure reaction cell as shown in Fig. 2. The specimen was then subjected to the desired pressures and temperatures for a 10 min duration.
3. Results and discussion Fig. 3(a) shows the pressure and temperature region of cBN formation from aBN powder containing B2O3 oxide impurity and 40 Mg–60 Al alloy catalyst solvent and a reference hBN–cBN equilibrium line as calculated by Nakano and Fukunaga [12] and supported by thermodynamic calculations by Solozhenko [13]. The shaded
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Fig. 1. (a) X-ray diffraction pattern of aBN powder with oxide impurity, (b) X-ray diffraction pattern of aBN powder with no oxide impurity.
Fig. 2. Reaction cell assembly used for cBN synthesis.
circles indicate the P2T conditions under which aBN powder was transformed into cBN and the open circles indicate the conditions where no transformation from aBN to cBN was observed.
The minimum pressure and temperature required for any cBN formation using aBN with B2O3 oxide impurity and Mg–Al alloy solvent were found to be 44 kb and 1200 C. The yield of cBN increased sharply as the value of pressure was increased from 44 to 46 kb. A maximum conversion of about 80% was obtained at 46 kb and 1300 C. Still higher conversion could be achieved if the value of pressure is further increased as the P2T conditions are moved deep into the thermodynamical stable region of cBN where the supersaturation of the solution is continued to increase. Thus, although the minimum pressure value for cBN synthesis from aBN containing B2O3 has been shifted to the higher side than reported earlier (o40 kb) [7,8,14], the minimum temperature (1200 C) is comparable. According to Endo et al. [9] the presence of large amount of oxide impurity
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Fig. 3. (a) P2T region and yield of cBN formation from aBN powder containing B2O3 impurity, (b) Yield of cBN formed from aBN powder free from oxide impurity at different P2T conditions.
in hBN powder (if used as a starting material for cBN synthesis) shifts the lower value of temperature to a much higher side than that required with low oxide content. In his experiments Endo et al. used two types of hBN as the starting materials, one with 1.9 wt% of oxygen called R-type and another with 7.9 wt% of oxygen and is called N1type. The lower temperature limit for cBN formation using R-type hBN was about 1380 C at 60–80 kb whereas in case of N1-type hBN it was around 1700 C (about 300 C higher that that required for R-type). The high concentration of oxygen impurity present in N1-type hBN reacted with the catalyst used (Mg3B2N4), thereby reducing its catalytic activity and the growth of cBN diminishes or halts completely. At sufficiently high temperatures above the eutectic point of BN– Mg3B2N4 the amount of liquid increases and the residual liquid which does not suffer from the effect of oxygen can act as a catalyst-solvent. That is why Endo et al. have observed cBN formation at higher temperatures in a high oxygen containing system as in the case of hBN of N1-type. The effect of oxygen impurity on hBN–cBN transformation has also been studied by Bindal et al. [15]. They reported that after a certain temperature (eutectic temperature) its effect on hBN–cBN transformation is very small as the cBN growth formation proceeds via the dissolution of hBN in the eutectic liquid which become dominant at higher tempera-
tures followed by the precipitation of cBN in its thermodynamical stable region. However, the transformation results were greatly affected if instead of hBN, aBN powder was used as a starting material for cBN synthesis. In order to know the role of B2O3 in aBN in its conversion to cBN some of the aBN powder prepared as described earlier was washed thoroughly with dilute nitric acid and then with ethanol. Ethanol was first removed by heating the powder at about 50 C in an oven and was finally dried in a vacuum of about 102 Torr at 1000 C for about 10 h. By this treatment most of the oxide impurity from aBN powder was removed as shown in Fig. 1(b). Further, this aBN powder was partly crystallized but still remained mostly in the amorphous state as the separation of {1 0 0} and {1 0 1} peaks of hBN is not distinct and {1 0 2} peak does not appear. This material was then subjected to the same P2T conditions as those for aBN powder containing oxide impurity. It was found that the specimens of aBN powder although with almost no oxide impurity transform into cBN but the yield of cBN was comparatively much less as can be seen in Fig. 3(b). It is clear therefore, that although the presence of oxide impurity in amorphous boron nitride shifts the lower limit of pressure to higher side as compared to those of hexagonal boron nitride it does not have adverse effect on the transformation of aBN into cBN rather it
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Fig. 4. (a) XRD pattern of the reaction product obtained at 44 kb, 1300 C from aBN powder containing oxide impurity, (b) XRD pattern of the reaction product obtained at 44 kb, 1300 C from aBN powder free from oxide impurity.
facilitated the growth process as revealed by the high yield of cBN from this powder. An experiment, for example conducted at the same P2T condition (44 kb, 1300 C) gave higher yield of cBN from aBN powder containing oxide impurity as shown by the relative peak intensity of cBN in both the cases as shown by Fig. 4(a) and (b). Further, the XRD pattern in both the cases showed the presence of hBN phase along with cBN. This indicated that the cBN formation from aBN proceeded via first crystallization to hBN and that the oxide impurity facilitated the crystallization of aBN as well as its transformation into cBN. Thus, it appears that the presence of B2O3 impurity in aBN plays a very important role in its
transformation into cBN, although it does not have any catalytic action for aBN–cBN phase transformation. To confirm this an experiment was conducted by subjecting only aBN powder containing oxide impurity at 46 kb, 1400 C without using Mg–Al alloy catalyst. The XRD pattern of the recovered specimen as shown in Fig. 5 did not show the presence of any cBN in the product. The peaks were mainly due to hBN, B2O3 and Al2O3. The presence of hBN in the specimens recovered after high pressure–high temperature treatment also indicated the most probable path for cBN growth as aBN–hBN–cBN although the direct transformation of aBN into cBN is not completely ruled out. Although no quantitative
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containing oxide impurity as well as free from oxide impurity and Mg–Al alloy solvent catalyst. However, the transformation to cBN was comparatively low from oxide free aBN powder. The presence of small amount of B2O3 in aBN powder although shifts the lower value pressure to higher side as compared to those from hBN powder, the aBN–cBN phase transformation was not found to be affected adversely by its presence rather it facilitated the conversion of aBN into cBN through the liquid phase.
Fig. 5. XRD pattern of the reaction product obtained at 46 kb, 1400 C from aBN powder containing oxide impurity without using Mg–Al alloy catalyst.
measurement of oxide impurity in the starting aBN powder used was carried out it is almost certain that its concentration was even much lower than those found in R-type hBN powder used by Endo et al. which contained about 1.9 wt% oxygen impurity. Therefore, in the present experiments the lower temperature limit for cBN formation was only about 1200 C at 42 kb as compared to about 1400 C at 60–80 kb for R-type hBN (lower by another 200 C). The cBN crystals synthesized from aBN powder containing B2O3 impurity and 40 Mg–60 Al alloy catalyst-solvent were irregular in shape, yellow to brown in colour depending on the synthesis temperature and fall in the 10–50 mm size range and are very useful for various cutting tool applications.
4. Conclusions Single crystals of cubic boron nitride were synthesized from amorphous boron nitride powder
References [1] A. Onodera, K. Inoue, H. Yoshinara, H. Nakae, T. Matsuda, T. Hirai, J. Mater. Sci. 25 (1990) 4279. [2] H. Sumiya, T. Iseki, O. Onodera, Mater. Res. Bull. 18 (1983) 1203. [3] I.S. Gladkaya, G.N. Kremkova, V.N. Slesarev, J. LessCommon Met. 117 (1986) 241. [4] T. Sata, T. Ishii, N. Setaka, J. Am. Ceram. Soc. 65 (1982) C-162. [5] T. Sekine, T. Sato, J. Appl. Phys. 74 (1993) 2440. [6] J.Y. Choi, S.T. Park, O. Fukunaga, J.K. Park, K.Y. Eun, J. Am. Ceram. Soc. 76 (1993) 2525. [7] M.M. Bindal, B.P. Singh, S.K. Singhal, R.K. Nayar, R. Chopra, J. Crystal Growth 144 (1994) 97. [8] M.M. Bindal, B.P. Singh, S.K. Singhal, R.K. Nayar, R. Chopra, E.S.R. Gopal, G. Will, XV AIRAPT International Conference on High Pressure Science and Technology, Warsaw, Poland, Sept. 11–14, 1995. [9] T. Endo, O. Fukunaga, M. Iwata, J. Mater. Sci. 14 (1979) 1375. [10] J.K. Park, S.T. Park, K.Y. Eun, Int. J. Refract. Metals & Hard Mater. 12 (1994) 225. [11] J. Thomas, N.E. Weston, T.E. O’Conner, J. Am. Chem. Soc. 84 (1963) 4619. [12] S. Nakano, O. Fukunaga, Diamond & Relat. Mater. 2 (1993) 1409. [13] V.L. Solozhenko, Zh. Fiz. Khim. 61 (1988) 2851. [14] V.L. Solozhenko, A.B. Slutsky, Yu.A. Ignatiev, J. Superhard Mater. 14 (1992) 61. [15] M.M. Bindal, S.K. Singhal, B.P. Singh, R.K. Nayar, R. Chopra, A. Dhar, J. Crystal Growth 112 (1991) 386.