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Morphology-retaining synthesis of AlN particles by gas reduction–nitridation
December 2002
Materials Letters 57 (2002) 910 – 913 www.elsevier.com/locate/matlet
Morphology-retaining synthesis of AlN particles by gas reduction–nitridation T. Suehiro a,*, J. Tatami a, T. Meguro a, S. Matsuo b, K. Komeya a a
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai Hodogaya-ku, Yokohama 240-8501, Japan b SNT Co., Ltd., 599-15 Jisan-Dong, Pyungtaek, Kyunggi-Do, South Korea Received 7 January 2002; received in revised form 16 April 2002; accepted 30 April 2002
Abstract Al2O3 raw materials with characteristic morphologies were converted to AlN, utilizing an NH3 – C3H8 gas mixture as a reduction – nitridation agent. A high conversion to AlN was achieved at low temperatures of 1400 – 1500 jC in a reaction time of 0.5 h. Observation by SEM clearly indicated that the product AlN maintained the original particle morphology of the initial Al2O3 and, thus, a morphological control of the final product was found to be possible. Morphology-retaining syntheses of AlN fibres, spherical AlN particles and granules were demonstrated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: AlN; Al2O3; Spherical AlN particles; AlN fibres; Morphological control; Gas reduction – nitridation method
1. Introduction Aluminium nitride (AlN) has attracted much interest in recent years, particularly in the electronics industry as a substrate material, because of its high intrinsic thermal conductivity (f320 W m1 K1), high electrical insulation and thermal expansion coefficient close to that of silicon [1,2]. In addition, another application of AlN as a filler for polymers to improve heat-dissipating capability of electronic packaging is becoming increasingly important [3– 5]. Two main synthesis processes have been developed for commercial production of AlN powders: the direct nitridation of aluminium metal and the carbo*
Corresponding author. Present address: National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail address: [email protected] (T. Suehiro).
thermal reduction of alumina in the presence of nitrogen. Although various attempts have been made on the laboratory-scale production of AlN powder [6] in addition to these two methods, procedures capable of selective synthesis of desirable particle morphologies are rarely found among them. Recently, a new process has been reported by the present authors [7 –9] for synthesizing AlN by reduction – nitridation of Al2O3 using NH3 and C3H8 as reactant gases. The overall reaction of this system can be expressed as Al2 O3 þ 2NH3 ðgÞ þ C3 H8 ðgÞ ! 2AlN þ 3COðgÞ þ 7H2 ðgÞ: In comparison with existing synthesis processes, there are significant advantages in producing a fine powder by this method. First, no mixing step or postsynthesis
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 9 4 - 7
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
processing is necessary and, hence, high-purity products can be synthesized in a single-step process. Second, the original particle morphology of the starting alumina can be maintained, owing to the gas – solid reaction between the reactants. Third, starting materials possessing characteristic particle morphologies are commercially available. Finally and most importantly, the morphology of the product AlN can be tailored by specifying the morphology of the raw material. Herein will be reported fabrications of some morphologically characteristic AlN products by this promising synthesis technique, the gas reduction – nitridation method.
911
Phase analysis of the reaction products was performed by an X-ray diffractometer (XRD, RINT 2500, Rigaku) using CuKa radiation operated at 50 kV and 300 mA. For fibre samples, a powdered specimen for XRD was prepared by grinding the products to a suitable size. Mean crystallite dimensions of the raw materials and the nitrided samples were determined by Scherrer analysis of XRD line broadening. Morphological analysis was performed by a scanning electron microscope (SEM, JSM-5200, JEOL).
3. Results and discussion 3.1. Conversion of spherical Al2O3 particles to AlN
2. Experimental procedure Two types of raw alumina, a spherical particles (coded SP) and a wool-like short fibre (coded F) were used as starting materials; their main characteristics are given in Table 1. The transition temperatures to aAl2O3 were determined by the differential thermal analysis with a ramping rate of 8 jC/min. The weighed sample was placed in a high-purity alumina boat and set in an alumina tube furnace (inner diameter of 42 mm). After appropriate purging of the reactor, a gas mixture of NH3 (99.97% purity) and C3H8 (99.99%) at a molar ratio of C3H8/NH3= 5103 was introduced from the extremity of the reactor, at a flow rate of 4 l/min (STP). The furnace was subsequently heated to the experimental reaction temperature (1100 –1500 jC) at a rate of 8 jC/min. After the predetermined reaction time (0.5 h), the sample was cooled at approximately 6 jC/min in an ammonia atmosphere. The products were then reweighed, and the degree of conversion was calculated for each run from the weight loss.
Table 1 Main characteristics of the Al2O3 raw materials Code used
Purity (%)
Particle diameter (Am)
Crystalline phase
Crystallite size (nm)
Transition to a-Al2O3 (jC)
SP
>99.8
0.91
42
1270
F
>99.9
5.8
y-Al2O3, u-Al2O3 g-Al2O3
6
1140
Table 2 summarizes the results of the nitridation of SP powder at various temperatures in the range of 1200– 1500 jC for a soaking time of 0.5 h. Measurements by XRD confirmed that the final reaction products consisted exclusively of AlN and unreacted alumina in all runs. Up to 70% of the starting alumina was converted to AlN at 1400 jC (sample SP3), and the increase of processing temperature raised the conversion up to 94% (sample SP4). SEM micrographs of the initial SP powder and the nitrided SP3 sample are shown in Fig. 1a and b, respectively. It is clear that the particle morphology of the nitrided powder is exactly the same as that of the alumina raw material and has retained the original spherical shape. The crystallite sizes of the SP3 and the SP4 powders, estimated from Table 2 Conversion of spherical Al2O3 particles to AlN by NH3 – C3H8 gas Sample
Reaction temperature (jC)
Conversion to AlN (%)
SP1
1200
7
SP2
1300
30
SP3
1400
70
SP4
1500
94
SP5
1500
93
Crystalline phases u-Al2O3, y-Al2O3, AlN u-Al2O3, y-Al2O3 AlN A1N, u-Al2O3, trace y-Al2O3 AlN, trace u-Al2O3 AlN, trace a-Al2O3, u-Al2O3
Crystallite size (nm)
35 44
912
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
Fig. 1. SEM micrographs of (a) raw SP powder before nitridation, (b) SP3 sample after nitridation at 1400 jC, (c) spray-dried granules of the SP powder and (d) SP5 sample after nitridation at 1500 jC.
the (110) peak width of AlN, were 35 and 44 nm, respectively, which are still comparable to the initial value of raw alumina (42 nm). Aqueous spray-dried granules of the raw SP powder (Fig. 1c) were also nitrided (sample SP5) by the same procedure and the morphology of the product is shown in Fig. 1d. The nitridation extent of the SP5 sample reached 93%, while the initial grain morphology was kept essentially intact. This new approach for producing AlN granules appears to be promising, and may have advantages over the conventional postnitridation spray drying, which unfavorably requires nonaqueous processing.
polymorphic transition temperature (1140 jC) suppresses the transformation of g-Al2O3 into a-Al2O3. This is consistent with the fact that the formation of AlN in the system Al2O3 –NH3 –C3H8 is thermodynamically possible at temperatures as low as 1200 K (927 jC) [7]. Similar results were observed also for the SP samples (see Table 2). Fig. 2 shows SEM micrographs of the initial alumina fibres and the nitrided F4 sample. It is quite evident that the morphology of the nitrided fibres was exactly the same as that of the alumina raw material, retaining the regular shape and the smooth surface of
3.2. Conversion of Al2O3 fibres to AlN fibres
Table 3 Conversion of Al2O3 fibres to AlN fibres by NH3 – C3H8 gas
The properties of the synthesized fibres are listed in Table 3. It was confirmed that the partly converted samples consisted exclusively of AlN and unreacted g-Al2O3 (samples F1 and F2). Only AlN was detected in the samples formed at temperatures above 1300 jC. These observations clearly indicated that in this reaction system, AlN was formed from g-Al2O3 without its transformation to a-Al2O3. A possible explanation for this is that formation of AlN prior to reaching the
Sample
Reaction temperature (jC)
Conversion to AlN (%)
Crystalline phases
Crystallite size (nm)
F1
1100
63
11
F2
1200
87
F3 F4
1300 1400
91 95
AlN, g-Al2O3 AlN, trace g-Al2O3 AlN AlN
20
24 31
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
913
4. Conclusion The conversions of various Al2O3 raw materials to AlN by the gas reduction –nitridation method were investigated. A high conversion to AlN (70 –95%) was achieved at reaction temperatures of 1400 – 1500 jC, and the spherical AlN particles, spherical AlN grains and the AlN fibres were successfully synthesized. The AlN particles produced consisted of very small crystallites, which were several tens of nanometers in size, and thus retained exactly the original particle morphology of the raw materials. These results suggest the potentiality of this novel reduction –nitridation process for producing highly pure and fine AlN particles, morphology of which can be controlled by specifying the morphology of the raw material.
References [1] [2] [3] [4] [5] Fig. 2. SEM micrographs of (a) raw Al2O3 fibres before nitridation and (b) after nitridation at 1400 jC (sample F4).
the original fibres. While the crystallite size of the F4 sample was estimated to be 31 nm, i.e. about five times larger than the initial g-Al2O3 (f6 nm), this value is still much smaller than the diameter of fibres themselves (f6 Am).
[6] [7] [8] [9]
L.M. Sheppard, Am. Ceram. Soc. Bull. 69 (11) (1990) 1801. Y. Baik, R.A.L. Drew, Key Eng. Mater. 122 – 124 (1996) 553. T.J. Mroz Jr., E.A. Groat, Ceram. Trans. 33 (1993) 333. C.P. Wong, R.S. Bollampally, J. Appl. Polym. Sci. 74 (1999) 3396. J.-W. Bae, W. Kim, S.-H. Cho, S.-H. Lee, J. Mater. Sci. 35 (2000) 5907. J.M. Haussonne, Mater. Manuf. Process. 10 (4) (1995) 717. T. Suehiro, J. Tatami, T. Meguro, S. Matsuo, K. Komeya, J. Eur. Ceram. Soc. 22 (2002) 521. T. Suehiro, J. Tatami, T. Meguro, K. Komeya, S. Matsuo, J. Am. Ceram. Soc. 85 (2002) 715. T. Suehiro, J. Tatami, T. Ide, T. Meguro, K. Komeya, S. Matsuo, J. Ceram. Soc. Jpn. 110 (2002) 67.
Materials Letters 57 (2002) 910 – 913 www.elsevier.com/locate/matlet
Morphology-retaining synthesis of AlN particles by gas reduction–nitridation T. Suehiro a,*, J. Tatami a, T. Meguro a, S. Matsuo b, K. Komeya a a
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai Hodogaya-ku, Yokohama 240-8501, Japan b SNT Co., Ltd., 599-15 Jisan-Dong, Pyungtaek, Kyunggi-Do, South Korea Received 7 January 2002; received in revised form 16 April 2002; accepted 30 April 2002
Abstract Al2O3 raw materials with characteristic morphologies were converted to AlN, utilizing an NH3 – C3H8 gas mixture as a reduction – nitridation agent. A high conversion to AlN was achieved at low temperatures of 1400 – 1500 jC in a reaction time of 0.5 h. Observation by SEM clearly indicated that the product AlN maintained the original particle morphology of the initial Al2O3 and, thus, a morphological control of the final product was found to be possible. Morphology-retaining syntheses of AlN fibres, spherical AlN particles and granules were demonstrated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: AlN; Al2O3; Spherical AlN particles; AlN fibres; Morphological control; Gas reduction – nitridation method
1. Introduction Aluminium nitride (AlN) has attracted much interest in recent years, particularly in the electronics industry as a substrate material, because of its high intrinsic thermal conductivity (f320 W m1 K1), high electrical insulation and thermal expansion coefficient close to that of silicon [1,2]. In addition, another application of AlN as a filler for polymers to improve heat-dissipating capability of electronic packaging is becoming increasingly important [3– 5]. Two main synthesis processes have been developed for commercial production of AlN powders: the direct nitridation of aluminium metal and the carbo*
Corresponding author. Present address: National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail address: [email protected] (T. Suehiro).
thermal reduction of alumina in the presence of nitrogen. Although various attempts have been made on the laboratory-scale production of AlN powder [6] in addition to these two methods, procedures capable of selective synthesis of desirable particle morphologies are rarely found among them. Recently, a new process has been reported by the present authors [7 –9] for synthesizing AlN by reduction – nitridation of Al2O3 using NH3 and C3H8 as reactant gases. The overall reaction of this system can be expressed as Al2 O3 þ 2NH3 ðgÞ þ C3 H8 ðgÞ ! 2AlN þ 3COðgÞ þ 7H2 ðgÞ: In comparison with existing synthesis processes, there are significant advantages in producing a fine powder by this method. First, no mixing step or postsynthesis
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 9 4 - 7
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
processing is necessary and, hence, high-purity products can be synthesized in a single-step process. Second, the original particle morphology of the starting alumina can be maintained, owing to the gas – solid reaction between the reactants. Third, starting materials possessing characteristic particle morphologies are commercially available. Finally and most importantly, the morphology of the product AlN can be tailored by specifying the morphology of the raw material. Herein will be reported fabrications of some morphologically characteristic AlN products by this promising synthesis technique, the gas reduction – nitridation method.
911
Phase analysis of the reaction products was performed by an X-ray diffractometer (XRD, RINT 2500, Rigaku) using CuKa radiation operated at 50 kV and 300 mA. For fibre samples, a powdered specimen for XRD was prepared by grinding the products to a suitable size. Mean crystallite dimensions of the raw materials and the nitrided samples were determined by Scherrer analysis of XRD line broadening. Morphological analysis was performed by a scanning electron microscope (SEM, JSM-5200, JEOL).
3. Results and discussion 3.1. Conversion of spherical Al2O3 particles to AlN
2. Experimental procedure Two types of raw alumina, a spherical particles (coded SP) and a wool-like short fibre (coded F) were used as starting materials; their main characteristics are given in Table 1. The transition temperatures to aAl2O3 were determined by the differential thermal analysis with a ramping rate of 8 jC/min. The weighed sample was placed in a high-purity alumina boat and set in an alumina tube furnace (inner diameter of 42 mm). After appropriate purging of the reactor, a gas mixture of NH3 (99.97% purity) and C3H8 (99.99%) at a molar ratio of C3H8/NH3= 5103 was introduced from the extremity of the reactor, at a flow rate of 4 l/min (STP). The furnace was subsequently heated to the experimental reaction temperature (1100 –1500 jC) at a rate of 8 jC/min. After the predetermined reaction time (0.5 h), the sample was cooled at approximately 6 jC/min in an ammonia atmosphere. The products were then reweighed, and the degree of conversion was calculated for each run from the weight loss.
Table 1 Main characteristics of the Al2O3 raw materials Code used
Purity (%)
Particle diameter (Am)
Crystalline phase
Crystallite size (nm)
Transition to a-Al2O3 (jC)
SP
>99.8
0.91
42
1270
F
>99.9
5.8
y-Al2O3, u-Al2O3 g-Al2O3
6
1140
Table 2 summarizes the results of the nitridation of SP powder at various temperatures in the range of 1200– 1500 jC for a soaking time of 0.5 h. Measurements by XRD confirmed that the final reaction products consisted exclusively of AlN and unreacted alumina in all runs. Up to 70% of the starting alumina was converted to AlN at 1400 jC (sample SP3), and the increase of processing temperature raised the conversion up to 94% (sample SP4). SEM micrographs of the initial SP powder and the nitrided SP3 sample are shown in Fig. 1a and b, respectively. It is clear that the particle morphology of the nitrided powder is exactly the same as that of the alumina raw material and has retained the original spherical shape. The crystallite sizes of the SP3 and the SP4 powders, estimated from Table 2 Conversion of spherical Al2O3 particles to AlN by NH3 – C3H8 gas Sample
Reaction temperature (jC)
Conversion to AlN (%)
SP1
1200
7
SP2
1300
30
SP3
1400
70
SP4
1500
94
SP5
1500
93
Crystalline phases u-Al2O3, y-Al2O3, AlN u-Al2O3, y-Al2O3 AlN A1N, u-Al2O3, trace y-Al2O3 AlN, trace u-Al2O3 AlN, trace a-Al2O3, u-Al2O3
Crystallite size (nm)
35 44
912
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
Fig. 1. SEM micrographs of (a) raw SP powder before nitridation, (b) SP3 sample after nitridation at 1400 jC, (c) spray-dried granules of the SP powder and (d) SP5 sample after nitridation at 1500 jC.
the (110) peak width of AlN, were 35 and 44 nm, respectively, which are still comparable to the initial value of raw alumina (42 nm). Aqueous spray-dried granules of the raw SP powder (Fig. 1c) were also nitrided (sample SP5) by the same procedure and the morphology of the product is shown in Fig. 1d. The nitridation extent of the SP5 sample reached 93%, while the initial grain morphology was kept essentially intact. This new approach for producing AlN granules appears to be promising, and may have advantages over the conventional postnitridation spray drying, which unfavorably requires nonaqueous processing.
polymorphic transition temperature (1140 jC) suppresses the transformation of g-Al2O3 into a-Al2O3. This is consistent with the fact that the formation of AlN in the system Al2O3 –NH3 –C3H8 is thermodynamically possible at temperatures as low as 1200 K (927 jC) [7]. Similar results were observed also for the SP samples (see Table 2). Fig. 2 shows SEM micrographs of the initial alumina fibres and the nitrided F4 sample. It is quite evident that the morphology of the nitrided fibres was exactly the same as that of the alumina raw material, retaining the regular shape and the smooth surface of
3.2. Conversion of Al2O3 fibres to AlN fibres
Table 3 Conversion of Al2O3 fibres to AlN fibres by NH3 – C3H8 gas
The properties of the synthesized fibres are listed in Table 3. It was confirmed that the partly converted samples consisted exclusively of AlN and unreacted g-Al2O3 (samples F1 and F2). Only AlN was detected in the samples formed at temperatures above 1300 jC. These observations clearly indicated that in this reaction system, AlN was formed from g-Al2O3 without its transformation to a-Al2O3. A possible explanation for this is that formation of AlN prior to reaching the
Sample
Reaction temperature (jC)
Conversion to AlN (%)
Crystalline phases
Crystallite size (nm)
F1
1100
63
11
F2
1200
87
F3 F4
1300 1400
91 95
AlN, g-Al2O3 AlN, trace g-Al2O3 AlN AlN
20
24 31
T. Suehiro et al. / Materials Letters 57 (2002) 910–913
913
4. Conclusion The conversions of various Al2O3 raw materials to AlN by the gas reduction –nitridation method were investigated. A high conversion to AlN (70 –95%) was achieved at reaction temperatures of 1400 – 1500 jC, and the spherical AlN particles, spherical AlN grains and the AlN fibres were successfully synthesized. The AlN particles produced consisted of very small crystallites, which were several tens of nanometers in size, and thus retained exactly the original particle morphology of the raw materials. These results suggest the potentiality of this novel reduction –nitridation process for producing highly pure and fine AlN particles, morphology of which can be controlled by specifying the morphology of the raw material.
References [1] [2] [3] [4] [5] Fig. 2. SEM micrographs of (a) raw Al2O3 fibres before nitridation and (b) after nitridation at 1400 jC (sample F4).
the original fibres. While the crystallite size of the F4 sample was estimated to be 31 nm, i.e. about five times larger than the initial g-Al2O3 (f6 nm), this value is still much smaller than the diameter of fibres themselves (f6 Am).
[6] [7] [8] [9]
L.M. Sheppard, Am. Ceram. Soc. Bull. 69 (11) (1990) 1801. Y. Baik, R.A.L. Drew, Key Eng. Mater. 122 – 124 (1996) 553. T.J. Mroz Jr., E.A. Groat, Ceram. Trans. 33 (1993) 333. C.P. Wong, R.S. Bollampally, J. Appl. Polym. Sci. 74 (1999) 3396. J.-W. Bae, W. Kim, S.-H. Cho, S.-H. Lee, J. Mater. Sci. 35 (2000) 5907. J.M. Haussonne, Mater. Manuf. Process. 10 (4) (1995) 717. T. Suehiro, J. Tatami, T. Meguro, S. Matsuo, K. Komeya, J. Eur. Ceram. Soc. 22 (2002) 521. T. Suehiro, J. Tatami, T. Meguro, K. Komeya, S. Matsuo, J. Am. Ceram. Soc. 85 (2002) 715. T. Suehiro, J. Tatami, T. Ide, T. Meguro, K. Komeya, S. Matsuo, J. Ceram. Soc. Jpn. 110 (2002) 67.