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Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation
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CERAMICS INTERNATIONAL
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Short communication
Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation Carlos R. Ramboa,n, Dachamir Hotzab Laboratory of Electrical Materials – LAMATE, Department of Electrical Engineering, Federal University of Santa Catarina, P.O. Box 476, 88040-900 Florianópolis, SC, Brazil b Group of Ceramic and Glass Materials– CERMAT, Department of Chemical Engineering, Federal University of Santa Catarina, P.O. Box 476, 88040-900 Florianópolis, SC, Brazil a
Received 1 March 2014; received in revised form 2 April 2014; accepted 2 April 2014
Abstract Microporous Al2O3/AlN composites with aligned channels were fabricated by pressureless infiltration–nitridation of Al–Mg alloy into biomorphous Al2O3 preforms. The biomorphous Al2O3 was produced by Al-vapor infiltration in pyrolyzed rattan templates with subsequent oxidation. Infiltration–nitridation was performed using an AlMg3 alloy infiltrated into the porous Al2O3 under flowing N2. After infiltration– nitridation into the porous Al2O3 templates a composite consisting of AlN and Al2O3 as major phases was obtained. Nitridation of Al in these conditions led to hollow hexagonal AlN crystals, which coated uniformly the Al2O3 channels surface. An increase of the BET specific surface area after infiltration–nitridation of Al into the biomorphous Al2O3 was observed. Microchanneled Al2O3 coated with AlN may be used as substrates in microelectronic applications that require high thermal conductivity and low dielectric constant. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Aluminum nitride; Biomorphic ceramics; Biotemplating; Pressureless infiltration; Nitridation
1. Introduction Aluminum nitride is a wide band gap semiconductor of the III–V family with a hexagonal close-packed wurtzite structure. AlN is resistant to attack by most molten metals. It is resistant to attack from most molten salts including chlorides and cryolite. It exhibits high thermal conductivity for a ceramic material (only lower than beryllia), high volume resistivity and high dielectric strength [1]. Due to these superior properties, aluminum nitride has been employed in many applications, e.g., as passivation layer for optical devices, corrosion-resistant coatings for heating elements, integrated circuit packaging, and sensors [2–4]. In addition, AlN is considered to be a potential candidate material for high frequency surface acoustic wave (SAW) devices due to its unique piezoelectric properties with high acoustic velocity and fairly large piezoelectric n
Corresponding author. Tel.: þ55 48 37212301; fax: þ 55 48 37219280. E-mail address: [email protected] (C.R. Rambo).
coupling factor [5,6]. Micro and nanoporous AlN-based materials are specially desired in advanced microelectronics and applications such as gas sensing [7]. A successful approach for processing porous, microchanneled advanced ceramics is the reproduction of plant morphologies by biotemplating, where the anatomical features of the native wood are maintained in the ceramic product [8–10]. The main innovative feature of this technology, developed in the last two decades, is the possibility to design macro and microporous devices, which cannot be manufactured by conventional forming techniques. Several biotemplating techniques have been reported including molten metal infiltration, metal vapor infiltration-reaction and sol–gel process leading to a large variety of ceramic products, e.g. carbides, oxides and nitrides [11–14]. In addition, new applications have been investigated for biomorphous ceramics, mainly focused on coating processes with semiconductor materials like photoluminescent ZnO tetrapods, which could provide desired surface features, allied
http://dx.doi.org/10.1016/j.ceramint.2014.04.015 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
2
to structural properties, and unique microcellular morphology of biomorphous ceramics [15]. In situ coatings with functional materials can be achieved by using different methods. Metal melt infiltration, for instance, offers the possibility to produce multiphase ceramics by homogeneously coating a porous ceramic preform with a ceramic layer or an active metal. In the last case, the metal is submitted to another process to be converted into a ceramic. This work reports the production and characterization of microchanneled AlN/Al2O3 composites with aligned channels by pressureless infiltration–nitridation of molten Al–Mg alloy into biomorphous Al2O3 rattan-derived preforms.
displayed in detail in Fig. 1b. The microstructural features of the biomorphous Al2O3 rattan sintered at 1550 1C can be seen in Fig. 1c. The micrographs were taken from axial cuts (oriented along with the rattan vessels). The microarchitecture of the original carbonized rattan samples was well reproduced after biotemplating into Al2O3, including the small and middlesized phloem cells (Fig. 1d). The X-ray diffractogram after nitridation (Fig. 2) revealed that the composite consists of AlN, Al2O3 and MgAl2O4 (spinel). The formation of AlN by direct nitridation of Al by pure N2 is expected to occur above the melt temperature of Al according to the reaction [18,19]:
2. Experimental
AlðlÞ þ 1=2N2ðgÞ -AlNðsÞ
Rattan palm (Calamoideae subfamily of the Arecaceae family) was used as bio-template. In natura samples were cut into discs of approximately 2 cm diameter and 1 cm height, dried (130 1C/2 h in air) and pyrolyzed at 800 1C for 1 h in N2-atmosphere in order to decompose the polyaromatic hydrocarbon polymers into carbon. Subsequently, the carbonized templates were disposed above an Al-powder bed (Alfa Aesar, 325 mesh, purity 99.5%) in an Al2O3-crucible without contact to the powder. The system was placed in a conventional tube furnace and submitted to an Al-vapor phase infiltration process at 1600 1C for 1 h under vacuum (1 0.1 Pa) for reacting the carbonized specimens into Al4C3. After pyrolysis, the samples were oxidized and sintered at 1550 1C in air for 3 h for converting the Al4C3 into Al2O3. Details of the Al infiltration–oxidation process are described in previous publication [16]. The biomorphous substrates were then submitted to a pressureless infiltration–nitridation process to form an AlN coating layer. Pieces of AlMg3 alloy (15 15 3 mm3) were placed above the biomorphous samples and infiltrated into the Al2O3 channels at 1075 1C for 2 h under flowing N2 (0.1 L/min). The phase composition of the ceramic products was determined by X-ray diffractometry, XRD (Philips, X´Pert) working with monochromatic CuKα radiation. The microstructure was characterized by scanning electron microscopy, SEM (Philips, XL-30) equipped with X-ray energy dispersive spectroscopy (EDS). The skeleton density was measured by helium pycnometry (Micromeritics, Accu Pyk 1330). The open porosity was estimated by the relation between the skeleton and the geometrical densities. Specific surface area was determined by N2 adsorption isotherms using the B.E.T. method (Quantachrome, Autosorb). 3. Results and discussion Fig. 1 shows the SEM micrographs of a pyrolyzed rattan and respective biomorphous Al2O3 ceramic evidencing their characteristic cell morphology. In contrast to wood, rattan is a tropical climbing palm that exhibits no branches or seasonal rings [17]. It is characterized by a homogeneous profile and vessel distribution. In Fig. 1a the large vessels (200–330 μm) from the metaxylem characteristic from this plant is shown. The middle-sized cells of around 90 μm from the phloem are
ð1Þ
Direct nitridation, however, is strongly sensitive to the atmosphere and alloy composition. Therefore, control over these parameters would change the nitridation mechanism. Mg can reduce the partial pressure of residual O2 to less than 10 40 bar at 800 1C. In the first stages Mg is oxidized by the oxygen present in the N2; spinel is then formed at the interface according to the reactions: MgðlÞ þ 1=2O2ðgÞ -MgOðsÞ
ð2Þ
4MgOðsÞ þ 2AlðlÞ -MgAl2 O4ðsÞ þ 3MgðlÞ
ð3Þ
The presence of small amount of O2 in the N2-atmosphere could inhibit the formation of AlN, due to its chemisorption at the interface N2/Al, which lowers the free Gibb's energy, favoring oxidation instead of nitridation of Al. The presence of Mg, however, is expected to avoid this effect. Moreover, Mg decreases the melting temperature of the alloy [20]. It is important to note that, despite its low eutectic point the alloy must wet appropriately the preform to infiltrate it. The presence of Mg in the alloy not only lowers the melting temperature but also significantly improves the wettability of Al on Al2O3 surface [20,21]. Additionally, with respect to oxidation, Mg is known to exert a catalytic effect on the nitridation of Al, avoiding the formation of a dense nitride layer and transferring surface reaction to a volume reaction. In contrast to the surface nitridation, the volume nitridation occurs internally in the bulk of the molten metal according to a solution–precipitation or (vapor–liquid–solid) VLS mechanism [21]. Fig. 3 illustrates the infiltration–nitridation mechanism with subsequent AlN grain growth. The microstructure of the biomorphous AlN/Al2O3 composite is shown in Fig. 4. The cellular anatomy of rattan was maintained after the coating process (Fig. 4a). The cell walls of the biomorphous Al2O3 are covered by AlN crystals (Fig. 4b). The AlN crystals exhibit a hollow structure, perpendicular to the vessel walls/melt surface, with a mean diameter of 5 μm typical from grains formed by Al-alloy melt nitridation [20]. The yield and the mechanism of AlN formation during nitridation of Al-alloys are strongly dependent on the alloy composition. For alloys containing more than 2 wt% Mg the formation of AlN layers occurs by outward growth of AlN towards the N2. Further N2 diffused through the crystal channels reacts with remaining Al. Aluminum was fully consumed, which is the reason for the absence of metallic
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 1. SEM micrographs of the microcellular templates: (a), (b) carbonized rattan and (c), (d) biomorphous Al2O3.
Fig. 3. Infiltration–nitridation reaction and AlN grain growth on Al2O3 channel walls. Fig. 2. X-ray diffractogram of the biomorphous Al2O3/AlN composite.
Al after nitridation. The EDS spectrum of the marked region in Fig. 4c confirmed the presence of N and Al as shown in Fig. 4d. Table 1 presents the porosity and B.E.T. specific surface area and porosity before and after the AlN coating. The coated biomorphous supports exhibit higher specific surface area than the respective biomorphous Al2O3, which is a result of AlN grain growth on the surface of alumina channels. The open porosity, however, was not substantially affected after AlN coating.
Microporous AlN-coated Al2O3 with aligned pore channels may be used as substrates in applications that require high thermal conductivity, low dielectric constant and high chemical resistance, allied to the mechanical strength and thermal stability of alumina such as catalyst supports, gas sensors or as substrates in microelectronic applications. 4. Conclusions Microcellular, biomorphous Al2O3 was successfully coated with AlN by pressureless infiltration of an Al–Mg alloy and subsequent nitridation of the melt. The biomorphous Al2O3 ceramics, as well
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 4. (a)–(c) SEM micrographs of the microchanneled, biomorphous Al2O3/AlN and (d) EDS spectrum of the region marked in (c). Table 1 B.E.T. specific surface area (SSA) and open porosity of the microchanneled samples before and after AlN coating.
Al2O3-suport AlN–Al2O3
Porosity (%)
SSA (m2/g)
897 1 847 1
1.757 0.05 2.417 0.05
as the final AlN/Al2O3 product maintained the anatomical features of the original native biostructure down to the micrometer level. The microstructure of the biomorphous composite is composed of hollow AlN crystals, which covered the rattan cell walls uniformly. B.E.T. surface area increased from 1.7 m2/g in the Al2O3 templates to 2.4 m2/g in the AlN/Al2O3 composites. Acknowledgments The authors thank the National Council for Scientific and Technological Development (CNPq-Brazil) and Volkswagen Foundation (Contract I/73 043) for their financial support. Central Laboratory of Electronic Microscopy (LCME-UFSC) is also acknowledged. References [1] C.C. Wang, C.J. Lu, M.H. Shiao, F.S. Shieu, Microstructural evolution of AlN coatings synthesized by unbalanced magnetron sputtering, J. Vac. Sci. Technol. A 23 (2005) 621.
[2] V. Dimitrova, D. Manova, R. Djulgerova, Element composition and electrochemical behaviour of polycrystalline AlN thin films, Surf. Coat. Technol. 123 (2000) 12–16. [3] K.S. Kao, C.C. Cheng, Y.C. Chen, Synthesis of c-axis-oriented aluminum nitride films by reactive RF magnetron sputtering for surface acoustic wave, Jpn. J. Appl. Phys. 40 (2001) 4969–4973. [4] K. Vijayalakshmi, V.V. Pillay, Influence of deposition time on the microstructure and dielectric properties of AlN/Si thin films for enhanced hydrogen sensing application, Microelectron. Eng. 113 (2014) 29–34. [5] F. Engelmark, G.F. Iriarte, I.V. Katardjiev, M. Ottosson, P. Muralt, S. Berg, Structural and electroacoustical studies of AlN thin films during low temperature radio frequency sputtering deposition, J. Vac. Sci. Technol. A 19 (2001) 2664–2669. [6] F. Lloret, D. Araújo, M.P. Villar, J.G. Rodríguez-Madrid, G.F. Iriarte, O.A. Williams, F. Calle, Diamond underlayer microstructure effect on the orientation of AlN piezoelectric layers for high frequency SAW resonators by TEM, Microelectron. Eng. 112 (2013) 193–197. [7] G.J. Yan, X.L. Liu, G.D. Chen, B.L. Jiang, F. Li, H.C. Liu, Preparation of nano-porous AlN micro-rods, Sci. China Tech. Sci. 55 (2012) 1523–1526. [8] C.E. Byrne, D.E. Nagle, Cellulose derived composites – a new method for materials processing, Mat. Res. Innov. 1 (1997) 137–144. [9] P. Greil, Biomorphous ceramics from lignocellulosics, J. Eur. Ceram. Soc. 21 (2001) 105–118. [10] C.R. Rambo, J. Cao, O. Rusina, H. Sieber, Manufacturing of biomorphic (Si, Ti, Zr)-carbide ceramics by sol–gel processing, Carbon 43 (2005) 1174–1183. [11] C.R. Rambo, H. Sieber, Manufacturing of cellular β-SiAlON/β-SiC composite ceramics from cardboard, J. Mater. Sci. 41 (2006) 3315–3322. [12] P. Greil, T. Lifka, A. Kaindl, Biomorphic cellular silicon carbide ceramics from wood: I. Processing and microstructure, J. Eur. Ceram. Soc. 18 (1998) 1961–1973. [13] J. Cao, C.R. Rambo, H. Sieber, Manufacturing of microcellular, biomorphous oxide ceramics from native pine wood, Ceram. Int. 30 (2004) 1967–1970.
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]] [14] C.R. Rambo, J. Cao, H. Sieber, Preparation and properties of highly porous, biomorphic YSZ ceramics, Mater. Chem. Phys. 87 (2004) 345–352. [15] C.R. Rambo, D. Hotza, C.R. da Cunha, C. Zollfrank, Directed photoluminescent emission of ZnO tetrapods on biotemplated Al2O3, Opt. Mater. 36 (2013) 562–567. [16] C.R. Rambo, H. Sieber, Novel synthetic route to biomorphous Al2O3 ceramics, Adv. Mater. 17 (2005) 1088–1091. [17] N.W. Uhl, J. Dransfield, General Palmarum, a Classification of Palms Based on the Work of H.E. Moore Jr. Bailei, Hortorium and International Palm Society, Kansas, 1987.
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[18] Q. Zheng, R.G. Reddy, Mechanism of in situ formation of AlN in Al melt using nitrogen gas, J. Mater. Sci. 39 (2004) 141. [19] N. Nagendra, B.S. Rao, V. Jayaram, Microstructures and properties of Al2O3/Al–AlN composites by pressureless infiltration of Al-alloys, Mat. Sci. Eng. A 269 (1999) 26–37. [20] H. Scholtz, P. Greil, Nitridation reactions of molten Al–(Mg, Si) alloys, J. Mater. Sci. 26 (1991) 669–677. [21] B.S. Rao, V. Jayaram, A new technique for pressureless infiltration of aluminium alloys into alumina preforms, J. Mater. Res. 16 (2001) 2906–2913.
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Short communication
Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation Carlos R. Ramboa,n, Dachamir Hotzab Laboratory of Electrical Materials – LAMATE, Department of Electrical Engineering, Federal University of Santa Catarina, P.O. Box 476, 88040-900 Florianópolis, SC, Brazil b Group of Ceramic and Glass Materials– CERMAT, Department of Chemical Engineering, Federal University of Santa Catarina, P.O. Box 476, 88040-900 Florianópolis, SC, Brazil a
Received 1 March 2014; received in revised form 2 April 2014; accepted 2 April 2014
Abstract Microporous Al2O3/AlN composites with aligned channels were fabricated by pressureless infiltration–nitridation of Al–Mg alloy into biomorphous Al2O3 preforms. The biomorphous Al2O3 was produced by Al-vapor infiltration in pyrolyzed rattan templates with subsequent oxidation. Infiltration–nitridation was performed using an AlMg3 alloy infiltrated into the porous Al2O3 under flowing N2. After infiltration– nitridation into the porous Al2O3 templates a composite consisting of AlN and Al2O3 as major phases was obtained. Nitridation of Al in these conditions led to hollow hexagonal AlN crystals, which coated uniformly the Al2O3 channels surface. An increase of the BET specific surface area after infiltration–nitridation of Al into the biomorphous Al2O3 was observed. Microchanneled Al2O3 coated with AlN may be used as substrates in microelectronic applications that require high thermal conductivity and low dielectric constant. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Aluminum nitride; Biomorphic ceramics; Biotemplating; Pressureless infiltration; Nitridation
1. Introduction Aluminum nitride is a wide band gap semiconductor of the III–V family with a hexagonal close-packed wurtzite structure. AlN is resistant to attack by most molten metals. It is resistant to attack from most molten salts including chlorides and cryolite. It exhibits high thermal conductivity for a ceramic material (only lower than beryllia), high volume resistivity and high dielectric strength [1]. Due to these superior properties, aluminum nitride has been employed in many applications, e.g., as passivation layer for optical devices, corrosion-resistant coatings for heating elements, integrated circuit packaging, and sensors [2–4]. In addition, AlN is considered to be a potential candidate material for high frequency surface acoustic wave (SAW) devices due to its unique piezoelectric properties with high acoustic velocity and fairly large piezoelectric n
Corresponding author. Tel.: þ55 48 37212301; fax: þ 55 48 37219280. E-mail address: [email protected] (C.R. Rambo).
coupling factor [5,6]. Micro and nanoporous AlN-based materials are specially desired in advanced microelectronics and applications such as gas sensing [7]. A successful approach for processing porous, microchanneled advanced ceramics is the reproduction of plant morphologies by biotemplating, where the anatomical features of the native wood are maintained in the ceramic product [8–10]. The main innovative feature of this technology, developed in the last two decades, is the possibility to design macro and microporous devices, which cannot be manufactured by conventional forming techniques. Several biotemplating techniques have been reported including molten metal infiltration, metal vapor infiltration-reaction and sol–gel process leading to a large variety of ceramic products, e.g. carbides, oxides and nitrides [11–14]. In addition, new applications have been investigated for biomorphous ceramics, mainly focused on coating processes with semiconductor materials like photoluminescent ZnO tetrapods, which could provide desired surface features, allied
http://dx.doi.org/10.1016/j.ceramint.2014.04.015 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
2
to structural properties, and unique microcellular morphology of biomorphous ceramics [15]. In situ coatings with functional materials can be achieved by using different methods. Metal melt infiltration, for instance, offers the possibility to produce multiphase ceramics by homogeneously coating a porous ceramic preform with a ceramic layer or an active metal. In the last case, the metal is submitted to another process to be converted into a ceramic. This work reports the production and characterization of microchanneled AlN/Al2O3 composites with aligned channels by pressureless infiltration–nitridation of molten Al–Mg alloy into biomorphous Al2O3 rattan-derived preforms.
displayed in detail in Fig. 1b. The microstructural features of the biomorphous Al2O3 rattan sintered at 1550 1C can be seen in Fig. 1c. The micrographs were taken from axial cuts (oriented along with the rattan vessels). The microarchitecture of the original carbonized rattan samples was well reproduced after biotemplating into Al2O3, including the small and middlesized phloem cells (Fig. 1d). The X-ray diffractogram after nitridation (Fig. 2) revealed that the composite consists of AlN, Al2O3 and MgAl2O4 (spinel). The formation of AlN by direct nitridation of Al by pure N2 is expected to occur above the melt temperature of Al according to the reaction [18,19]:
2. Experimental
AlðlÞ þ 1=2N2ðgÞ -AlNðsÞ
Rattan palm (Calamoideae subfamily of the Arecaceae family) was used as bio-template. In natura samples were cut into discs of approximately 2 cm diameter and 1 cm height, dried (130 1C/2 h in air) and pyrolyzed at 800 1C for 1 h in N2-atmosphere in order to decompose the polyaromatic hydrocarbon polymers into carbon. Subsequently, the carbonized templates were disposed above an Al-powder bed (Alfa Aesar, 325 mesh, purity 99.5%) in an Al2O3-crucible without contact to the powder. The system was placed in a conventional tube furnace and submitted to an Al-vapor phase infiltration process at 1600 1C for 1 h under vacuum (1 0.1 Pa) for reacting the carbonized specimens into Al4C3. After pyrolysis, the samples were oxidized and sintered at 1550 1C in air for 3 h for converting the Al4C3 into Al2O3. Details of the Al infiltration–oxidation process are described in previous publication [16]. The biomorphous substrates were then submitted to a pressureless infiltration–nitridation process to form an AlN coating layer. Pieces of AlMg3 alloy (15 15 3 mm3) were placed above the biomorphous samples and infiltrated into the Al2O3 channels at 1075 1C for 2 h under flowing N2 (0.1 L/min). The phase composition of the ceramic products was determined by X-ray diffractometry, XRD (Philips, X´Pert) working with monochromatic CuKα radiation. The microstructure was characterized by scanning electron microscopy, SEM (Philips, XL-30) equipped with X-ray energy dispersive spectroscopy (EDS). The skeleton density was measured by helium pycnometry (Micromeritics, Accu Pyk 1330). The open porosity was estimated by the relation between the skeleton and the geometrical densities. Specific surface area was determined by N2 adsorption isotherms using the B.E.T. method (Quantachrome, Autosorb). 3. Results and discussion Fig. 1 shows the SEM micrographs of a pyrolyzed rattan and respective biomorphous Al2O3 ceramic evidencing their characteristic cell morphology. In contrast to wood, rattan is a tropical climbing palm that exhibits no branches or seasonal rings [17]. It is characterized by a homogeneous profile and vessel distribution. In Fig. 1a the large vessels (200–330 μm) from the metaxylem characteristic from this plant is shown. The middle-sized cells of around 90 μm from the phloem are
ð1Þ
Direct nitridation, however, is strongly sensitive to the atmosphere and alloy composition. Therefore, control over these parameters would change the nitridation mechanism. Mg can reduce the partial pressure of residual O2 to less than 10 40 bar at 800 1C. In the first stages Mg is oxidized by the oxygen present in the N2; spinel is then formed at the interface according to the reactions: MgðlÞ þ 1=2O2ðgÞ -MgOðsÞ
ð2Þ
4MgOðsÞ þ 2AlðlÞ -MgAl2 O4ðsÞ þ 3MgðlÞ
ð3Þ
The presence of small amount of O2 in the N2-atmosphere could inhibit the formation of AlN, due to its chemisorption at the interface N2/Al, which lowers the free Gibb's energy, favoring oxidation instead of nitridation of Al. The presence of Mg, however, is expected to avoid this effect. Moreover, Mg decreases the melting temperature of the alloy [20]. It is important to note that, despite its low eutectic point the alloy must wet appropriately the preform to infiltrate it. The presence of Mg in the alloy not only lowers the melting temperature but also significantly improves the wettability of Al on Al2O3 surface [20,21]. Additionally, with respect to oxidation, Mg is known to exert a catalytic effect on the nitridation of Al, avoiding the formation of a dense nitride layer and transferring surface reaction to a volume reaction. In contrast to the surface nitridation, the volume nitridation occurs internally in the bulk of the molten metal according to a solution–precipitation or (vapor–liquid–solid) VLS mechanism [21]. Fig. 3 illustrates the infiltration–nitridation mechanism with subsequent AlN grain growth. The microstructure of the biomorphous AlN/Al2O3 composite is shown in Fig. 4. The cellular anatomy of rattan was maintained after the coating process (Fig. 4a). The cell walls of the biomorphous Al2O3 are covered by AlN crystals (Fig. 4b). The AlN crystals exhibit a hollow structure, perpendicular to the vessel walls/melt surface, with a mean diameter of 5 μm typical from grains formed by Al-alloy melt nitridation [20]. The yield and the mechanism of AlN formation during nitridation of Al-alloys are strongly dependent on the alloy composition. For alloys containing more than 2 wt% Mg the formation of AlN layers occurs by outward growth of AlN towards the N2. Further N2 diffused through the crystal channels reacts with remaining Al. Aluminum was fully consumed, which is the reason for the absence of metallic
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
3
Fig. 1. SEM micrographs of the microcellular templates: (a), (b) carbonized rattan and (c), (d) biomorphous Al2O3.
Fig. 3. Infiltration–nitridation reaction and AlN grain growth on Al2O3 channel walls. Fig. 2. X-ray diffractogram of the biomorphous Al2O3/AlN composite.
Al after nitridation. The EDS spectrum of the marked region in Fig. 4c confirmed the presence of N and Al as shown in Fig. 4d. Table 1 presents the porosity and B.E.T. specific surface area and porosity before and after the AlN coating. The coated biomorphous supports exhibit higher specific surface area than the respective biomorphous Al2O3, which is a result of AlN grain growth on the surface of alumina channels. The open porosity, however, was not substantially affected after AlN coating.
Microporous AlN-coated Al2O3 with aligned pore channels may be used as substrates in applications that require high thermal conductivity, low dielectric constant and high chemical resistance, allied to the mechanical strength and thermal stability of alumina such as catalyst supports, gas sensors or as substrates in microelectronic applications. 4. Conclusions Microcellular, biomorphous Al2O3 was successfully coated with AlN by pressureless infiltration of an Al–Mg alloy and subsequent nitridation of the melt. The biomorphous Al2O3 ceramics, as well
Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
C.R. Rambo, D. Hotza / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 4. (a)–(c) SEM micrographs of the microchanneled, biomorphous Al2O3/AlN and (d) EDS spectrum of the region marked in (c). Table 1 B.E.T. specific surface area (SSA) and open porosity of the microchanneled samples before and after AlN coating.
Al2O3-suport AlN–Al2O3
Porosity (%)
SSA (m2/g)
897 1 847 1
1.757 0.05 2.417 0.05
as the final AlN/Al2O3 product maintained the anatomical features of the original native biostructure down to the micrometer level. The microstructure of the biomorphous composite is composed of hollow AlN crystals, which covered the rattan cell walls uniformly. B.E.T. surface area increased from 1.7 m2/g in the Al2O3 templates to 2.4 m2/g in the AlN/Al2O3 composites. Acknowledgments The authors thank the National Council for Scientific and Technological Development (CNPq-Brazil) and Volkswagen Foundation (Contract I/73 043) for their financial support. Central Laboratory of Electronic Microscopy (LCME-UFSC) is also acknowledged. References [1] C.C. Wang, C.J. Lu, M.H. Shiao, F.S. Shieu, Microstructural evolution of AlN coatings synthesized by unbalanced magnetron sputtering, J. Vac. Sci. Technol. A 23 (2005) 621.
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Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015
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Please cite this article as: C.R. Rambo, D. Hotza, Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.015