z[Φ@<English> 1. ±q»@E]Ώj[YΖTv 2. €Tv 3. π 4. XVπ
Yasumasa TAKAO and Mutsuo SANDO
Research Group on Tailored Solid Source Ceramic Research Institute National Institute of Advanced Industrial Science and Technology (AIST) Ministry of Economy, Trade and Industry iMETIj
Keywords Flame synthesis, Aluminum nitride, Filler-powder, Spherical morphology, Direct-nitridation
This short communication offered one of flame aerosol synthesis of aluminum nitride, AlN, as a new filler-powder supply-route for resin polymer composite system filled with ceramic particles, which would have a promising advantage in thermal conductivity. Flame AlN synthesis was based on the aerosol preparation of conventional LPG-O2-N2-NH3 system flame, and the particulate raw material. Effect of flame temperature and heat amount was studied on the direct-nitridation of Al through CH4, C3H8, C2H2 flame of LPG-O2-N2-NH3 system. As-prepared resultants were consisted of an Al-O-N system and a remained Al raw material, and their annealed powder was comprised of a AlN majority and a little alpfa-Al2O3. Heating amount increments resulted in the yield-increase of polyphasic intermediate system Al-O-N and that of AlN. Flame synthesized powder had a fairly large aerosol particle size, roughly 10 microns in average size, and a bimodal size distribution. Especially, the new AlN powder had a non-squarish shape rather than one of commercial one with angular morphology, which would be good for high-density packing. Feasible potential of this synthesis method was confirmed to provide the AlN powder having suitable properties as the filler-powder.
An attempt was executed to provide aluminum nitride, AlN, as the non-squarish filler-powder for the resin polymer composite system filled with ceramic particles. The resin polymer composite is one of the important material systems for several recent electrical devices; i.e., a powder-polymer conductive paint (Che et al., 1997); an insulating spacer for the gas-insulated switchgear, GIS (Shang et al., 1995; Homma, 2000); an electrorheological fluid (Otsubo et al., 1992); a chemical mechanical polishing (planarization), CMP buffing compound (Wayne-Lougher, 1999); a semiconductor packaging material for high-fidelity VLSI electric devices, such as the chip size package of flip chip solder joints (Hagiwara, 1998; Nakajima, 1998; Yoshida, 1998; Gupta et al., 1999; McMurray et al., 1999; Jang et al., 2000; Takao et al., 2000 and in press). Semiconductor packaging material encapsulates IC chips etc., and necessitates a high thermal conductivity, a low thermal expansion and a good moldability. The packaging material was initially made from ceramics in the 1960's, and eventually from resin polymer composite filled with silica, SiO2, filler-powder. Higher the packing content of filler, higher the thermal conductivity (lower the thermal expansion); but then, the moldability degrades. "New filler-powder" having a high thermal conductive coefficient and a very spherical morphology is one of highly promising approach to improve the semiconductor-packaging properties. Presently, a spherical SiO2 particle flame-fused natural quartz ingredient has provided for the most part of practical filler-powder, having a bimodal size distribution and a roughly 10 microns in average size (Ogawa et al., 1990; Otsuka, 1993; Unger, 1994; Abe et al., 1996; Kitano, 1998), e.g., as shown in Fig. 1 ( Abe et al., 1996). AlN, initially, has received much interest in the print-circuit board as a substitute of conventional ceramic substrate. It has following advantages; (1) theoretical thermal conductive coefficients of AlN, metal Si, Al2O3 and SiO2 are roughly 300, 90, 20 and 2 Wm-1K-1 at 400K; (2) the thermal expansion coefficient of AlN is close to that of Si substrate, 5~10-6 K-1 (Sheppard, 1990; Nakajima, 1998). The common methods for commercial AlN powder are; (1) the direct-nitridation of metallic Al (Weimer et al., 1994; Nagai et al., 1997) and (2) the reduction-nitridation of Al2O3 with carbon reductant in the presence of nitrogen (Kuramoto et al.,, 1989; Komeya et al., 1993). Whereas, the direct-nitridation necessitates a crushing/milling procedure, and the resultant powder has an angular shape mostly, such as shown in Fig. 2 ( Nagai et al., 1997). The main target of commercially-circulated powder via reduction-nitridation is for the raw powder of ceramic sintered body, which has a monomodal size distribution and far often smaller in mean diameter than the practical SiO2 filler-powder (e.g., submicron). The process, furthermore, is an endoergic reaction and needs a pretty high heating temperature in powder preparation, around 1700 . It is not certain to be possible to nitride the large-sized raw Al2O3 necessitated to get the large-sized filler-powder. Several attempts in new aerosol processes such as CVD, plasma and fluidized-bed, were suggested too as a new production route of AlN, but all in all, they were likely to be insufficient for filler-powder needs or economical powder supply (Hotta et al., 1994; Wakimura et al., 1995; Pratsinis et al., 1995). The SiO2 filler-powder, practically, is produced by flame fusion, and the flame aerosol synthesis is an established industrial process bringing sizable profits for large-scale manufacture of spherical-shaped particle, also at a pigmentary titania, a fumed silica for optical fiber and a super-paramagnetic particle (McMillin et al., 1996; Pratsinis et al., 1996). AlN filler-powder via flame synthesis route is reasonable because of its simplification, spherical-shaped particle, large-scale manufacture and cost-effectiveness. It could be also put the hopes to use the pre-existing manufacturing apparatus and scientific foundation built at the SiO2 filler-powder. However, there were very little concerns on it except SiO2 thus far, although only a few patents/articles were reported such as the flame-fused delta-Al2O3 particle (Hiragushi et al., 1982; Ogawa et al., 1986), or the diamond synthesis via the imperfect-combustion flame, which meant an insufficient O2 ratio to LPG (liquefied petroleum gas) (Hirose, 1996). Conceivable reason for the little concern in flame synthesis AlN might be its obscurity in melting point or its apt tendency to make angular shape. Al2O3 also has an angular shape based on their crystalline phases, but very spherical alpfa-Al2O3 was developed by one of gas-phase synthesis (Tanaka et al., 1997). Ease-to-make tendency of spherical shape, which was an essentiality of aerosol synthesis, was utilized in the preparation; AlN is just as anxious to do likewise. This short communication is concerned with a flame AlN aerosol synthesis and its effect of flame temperature, which offers a new filler-powder supply for resin polymer composite system filled with ceramic particles. It appears to be available the AlN powder having suitable properties as filler-powder, i.e., a spherical morphology and a fairly large aerosol particle size, roughly 10 Κm in average size.
1. Experimental
Figure 3 shows a scheme of AlN filler-powder flame process. It was based on the aerosol synthesis of conventional LPG and O2-N2 system (or LPG-O2-N2-NH3 system) flame. Particulate raw material was supplied in it, not a liquid or gas phase material, because it is most desirable for filler applications in resultant powder comprised of fairly large-size (around 10 microns), bimodal size distribution and manufacturing productivity. So, the reaction of aimed powder was based on the conventional route not on MOCVD etc., i.e., the direct-nitridation of metallic aluminum aerosol and/or the reduction-nitridation of alumina with carbon aerosol. In this paper, the effect of flame temperature and heat amount was studied on the direct-nitridation of Al through CH4, C3H8, C2H2 flame of LPG-O2-N2-NH3 system; i.e., their maximum temperatures were on increasing as CH4, C3H8 and C2H2, gradually. Typical commercialized metallic Al powder, around 10 microns in median diameters, was used as the particulate raw material. Figure 4 shows its scanning electron microphotograph. The powder was made by gas-atomized method, and had a very spherical shape (Kojundo Chemical Laboratory Co., Ltd.). The raw material was carried to a fluidized-bed aerosol generator by kneader. In the bed, glass-made beads with 150 microns in diameters (Toshiba Plastic Beads GB-AD) were fluidized simultaneously, and the media were effective to prevent the formation of stiff agglomeration of Al raw powder (Takao, 1996). Generated gas-phase Al powder was sent to a cyclone separator, and then very large-sized ones were cut. Flame reactor in this experiment was consisted of a series of concentric stainless steel, SUS 304, tubes (Fig. 3), although it would not optimal because the reactant gas mixing configurations for flame synthesis were crucial problem (Pratsinis, 1996). The system had an internal tube for reactant ingredients and an external one for fuel ('the diffusion burner system'). The diameter of the central tube is 5 mm ID and the spacing between the successive tubes is 5 mm. The Al aerosol was carried to the central tube by N2. The N2 carrier gas flow rate was set to 10 L/min. NH3 was used as a nitriding ingredient for Al nitridation, while O2 as oxidant; and they were pre-mixed with Al powder. The NH3 flow rate was set to the stoichiometric ratio of direct-nitridation. That of O2 was controlled from the value of stoichiometric fuel-O2 ratio to that of imperfect-combustion flame; which aimed to control O2 content in reaction, and besides, to use also carbon byproduct formed at the imperfect combustion as the nitriding ingredient for Al nitridation. Hydrocarbon fuel was supplied to the external tube of the reactor and its flow rate was 5 L/min. Commercially available C3H8 and C2H2 were supplied, while CH4 was subrogated as the 13A-type manufactured gas in Aichi Prefecture. Resultant powder was formed in the simple open-equipped diffusion flame configuration without any enclosing shield to prevent the heat radiation loss, and collected on PTFE membrane filter (90 mm in diameter) with 3 microns pores. The as-prepared precursor powder, although not always necessarily, was annealed at around 1000 for 1 hour, in N2 flow (0.5 L/min) and air, respectively; in order to promote the crystallization and to eliminate the carbon byproduct. The resultant powder was evaluated its crystallinity and composition of particulate-matter by X-ray powder diffraction analysis (XRD) at room temperature; Philips Co. Ltd. Model APD-1700. Its morphology was observed by scanning electron microscopy (SEM); JEOL Co. Ltd. Model JSM-5600N.
Figures 5, 6 and 7 show XRD patterns of the as-prepared precursor powder via direct-nitridation of metallic Al aerosol at changing hydrocarbon fuel ingredients, CH4, C3H8 and C2H2. In the cases, O2 flow rate was tentatively controlled to the imperfect-combustion flame condition. The attempt aimed to restrain Al2O3 formation at the as-prepared stage. The resultant in CH4 flame was comprised of the main phase of unreacted Al raw material and a little Al-O-N system peak (Fig. 5). The Al-O-N system was a typical polyphasic intermediate at AlN gas-phase syntheses (Tsuchida et al., 1987; Asper et al., 1991), which were fairly easy to transform to AlN system. CH4 was the LPG of lowest temperature and calorific value, which appeared to be insufficient for the nitridation of Al raw material, at least in this experiment. Figure 6 shows the XRD pattern of the prepared powder via nitridation of C3H8 flame. The resultant was also consisted of Al-O-N system peaks and remained Al raw material, although the fair percentage of Al-O-N was formed rather than that of CH4 flame. Calorific values and flame temperatures at complete combustion of fuel are extracted from the literature as 890 kJ/mol and 2005 for CH4; 2210 kJ/mol and 2120 for C3H8; 1310 kJ/mol and 2630 for C2H2 (Hikida ed., 2000). As widely known, C2H2 is the most flammable gas having the high chemical reaction ability with oxygen. Molecular weight per mole of carbon for C3H8 is larger than that of C2H2. Then in this experiment, C3H8 was the LPG of highest calorific value, although its flame temperature was lower than C2H2. It should be an important subject which contribution would be crucial, calorific values or flame temperature. Anyway, it went on the nitridation of Al raw material presumably at C3H8 and C2H2, as compared to the CH4 flame. Figure 7 shows the pattern of C2H2 case. The pattern was comprised of most Al-O-N system peaks, a little remained Al raw material and carbon byproduct, i.e., acetylene black. The C2H2 flame had the highest flame temperature in the LPG of this experiment. It should contribute to the progress of nitridation of Al raw material and the Al-O-N system formation. Further, the volume of carbon byproduct was highest in C2H2. It depended on that the fuel had the highest C elementary percentage in C/H mollar ratio as compared to CH4 and C3H8. Figure 8 shows the XRD pattern of the annealed powder of as-prepared precursor via direct-nitridation of C2H2 flame (Fig. 7). The resultant AlN powder via flame synthesis route was consisted of most AlN peaks and a little alpfa-Al2O3 ones. It appeared to be available for making AlN powder via LPG-O2-N2-NH3 system flame, fundamentally. Though the latter secondary phase was still uncertain to be essential in this route or to be not, the phase might be formed during the elimination process of carbon byproduct; similarly as the reduction-nitridation of Al2O3 with carbon reductant in the presence of nitrogen (Kuramoto et al.,, 1989; Komeya et al., 1993). Figure 9 shows the scanning electron microphotographs of a resultant AlN powder via flame synthesis route. The flame-synthesized powder had a fairly large particle size for gas-phase one. It was consisted of roughly 10 microns in average size of the larger group, and also a lot of fine particles of around 100 nm, i.e., bimodal size distribution. Especially, the new AlN had a non-squarish shape rather than one of commercial powder with angular morphology (Fig. 2). They would be good for high-density packing, and were likely to be sufficient for filler-powder needs. Thus, the feasible potential of this synthesis method was confirmed to offer the AlN powder as a new filler-powder supply for resin polymer composite system filled with ceramic particles. This process was built up with laying the foundation on aerosol process. Comparing to the conventional direct-nitridation process, the firmly adhesion of raw Al or formed AlN was expected to be unlikely probable, because each raw material was isolated during the gas-phase reaction, comparatively. This structural constitution of synthesis was supposed to contribute to the above-mentioned spherical shape of prepared powder. The secondary alpfa-Al2O3 was not necessarily bad for filler-powder application. The carbon elimination process is reported as an advantage of reduction-nitridation for chemical stability in the reaction of AlN with airborne moisture (H2O), because the AlN via reduction-nitridation has an Al2O3 thin film on it during the elimination process (Kuramoto et al., 1989). Instability of AlN to H2O is one of crucial problems, so then, several encapsulation methods are considered, in which each AlN particle was capsulated by polymer, SiO2 or NaCl (Axellbaum et al., 1996; Shan et al., 1999). The flame synthesized AlN powder might have the chemical stability essentially, which was based on the carbon byproduct-control of hydrocarbon fuel reactions, also as the reduction-nitridation. As-prepared precursor, which was consisted of Al-O-N system (and remained carbon), was annealed in this experiment, and then, the crystallinity of AlN system was increased. The as-prepared powder, however, was made by the simple open-equipped diffusion flame without any enclosing shield to prevent heat radiation loss. So, the polyphasic intermediate system, Al-O-N, was supposed to be mainly formed in as-preparation stage on a par with other gas-phase synthesis of AlN. According as the CH4, C3H8, C2H2 and its annealing, which meant the increase of heating amounts, the Al-O-N yields and also AlN were improved, gradually. Surely, it would be possible to combine the annealing procedure with the main process, likely to the previous work (Weimer et al., 1994). Appropriate flame configuration, so as to been designed to progress heat efficiency, could enable to create AlN system powder, rapidly, e.g., at the as-prepared stage. Further analysis will clarify a commercial potential (rather than feasible potential) for filler-powder supply more precisely.
(1) Flame AlN aerosol synthesis and its effect of flame temperature were studied. It was based on the aerosol synthesis of conventional LPG-O2-N2-NH3 system flame, and the particulate raw material. The direct-nitridation of Al through CH4, C3H8, C2H2 flame of LPG-O2-N2-NH3 system was applied. The as-prepared resultants were consisted of an Al-O-N system and a remained Al raw material. The annealed powder of as-prepared precursor via C2H2 flame was comprised of a AlN majority and a little alpfa-Al2O3. The heating amount increment resulted in the increase of AlN yield. The flame synthesized AlN powder had a secondary alpfa-Al2O3, which was expected to have the stability of AlN to H2O based on the carbon byproduct of hydrocarbon fuel reactions, also as the reduction-nitridation.
(2) The flame synthesized powder had a fairly large particle size for gas-phase one. It was consisted of roughly 10 microns in average size of the larger group, and also a lot of fine particles of around 100 nm, i.e., bimodal size distribution. Especially, the new AlN had a non-squarish shape rather than one of commercial powder with angular morphology. They would be good for high-density packing, and were likely to be sufficient for filler-powder needs. The feasible potential of this synthesis method was confirmed to offer the AlN powder as a new filler-powder supply for resin polymer composite system filled with ceramic particles, which would have a promising advantage in thermal conductivity.
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Fig. 1 Scanning electron microphotographs of a commercialized silica, SiO2, filler-powder prepared by the flame process (Denki Kagaku Kogyo Co., Ltd., FB-6S)
Fig. 2 Scanning electron microphotographs of a commercialized aluminum nitride, AlN, filler-powder prepared by the direct-nitridation of atomized metallic aluminum, Al (Toyo Aluminium K.K., R-15)
Fig. 3 Scheme of the aluminum nitride flame process via a direct-nitridation of metallic aluminum aerosol and/or a reduction-nitridation of alumina with carbon aerosol
Fig. 4 Scanning electron microphotographs of a commercialized atomized metallic aluminum, Al, powder (Kojundo Chemical Laboratory Co., Ltd.)
Fig. 5 X-ray diffraction pattern of an as-prepared precursor via direct-nitridation of metallic Al aerosol through CH4 flame
Fig. 6 X-ray diffraction pattern of an as-prepared precursor via direct-nitridation of metallic Al aerosol through C3H8 flame
Fig. 7 X-ray diffraction pattern of an as-prepared precursor via direct-nitridation of metallic Al aerosol through C2H2 flame
Fig. 8 X-ray diffraction pattern of a synthesized aluminum nitride, AlN, powder; the annealed powder of as-prepared precursor via direct-nitridation of C2H2 flame (Fig. 7)
Fig. 9 Scanning electron microphotographs of a synthesized aluminum nitride, AlN, powder