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Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method
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CERAMICS INTERNATIONAL
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Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method Xiaoguang Liu, Wendong Xuen, Cunlan Shi, Jialin Sun Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing, China Received 29 April 2015; received in revised form 29 May 2015; accepted 30 May 2015
Abstract A fast cooling freeze casting (FCFC) method was developed for preparation of porous Al2O3 scaffolds with improved interconnectivity, unidirectional pore channels and dendritic walls. As estimated using the Archimedes method and mercury porosimetry, total porosity 64.5–68.5 vol% and the closed porosity o0.6 vol% reveal that most pores are open. The bimodal pore size distribution shifted to larger diameters with increasing sodium-hexametaphosphate (SHMP) concentration from 0.1% to 0.5%. Scanning electronic microscope (SEM) images of the resultant porous Al2O3 showed clear transformation from lamellae to dendrite structures with increasing polyvinyl alcohol (PVA) concentration from 1% to 3%. To discover the mechanism, the freezing curve of the Al2O3 slip was achieved, showing its supercooling point at about 10 1C where most isolated water solidifies at an average ice front velocity of 0.42 μm/s. Together with the PVA effects on the microstructure transformation, the underlying mechanism could be ascribed to both the supercooling and PVA effects. The lamellae ice front was initially formed under this velocity, but with increasing PVA, the ceramic slurry got more viscous and the rearrangement of ceramic particles was hindered, hence the ice front growth in the preferred a direction was limited whereas the growth in the c direction was promoted. Meanwhile under the effects of the applied thermal gradient, the lamellae were transformed into dendrite structures. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Fast cooling freeze casting; B. Porosity; D. Porous Al2O3 scaffold; Mechanism; Freezing curve
1. Introduction Macroporous Al2O3 ceramic is one of the most important catalyst scaffolds for automotive emission control due to its excellent thermal-shock resistance, mechanical properties and chemical stability [1,2]. One way to prepare such scaffolds is through a novel freeze casting technique, which has been extensively explored in recent years [3–6]. The resultant structure usually varies under different process parameters. Nevertheless, most water-based freeze casting technique favors porous lamellae structures with unidirectional through pores [3,7]. The lamellae show either smooth or rough surfaces [7]. Abbreviations: SEM: scanning electronic microscope; FCFC: fast cooling freeze casting; SHMP : sodium-hexametaphosphate; PVA: polyvinyl alcohol n Corresponding author. Tel.: þ86 10 6233 2666. E-mail address: [email protected] (W. Xue).
Even with ceramic bridges linking between the lamellae, fully interconnected pores were seldom seen. The lack of interconnectivity limits the contact between the reactant and the catalysts and thus degrades the activities of catalysts to some degree [8–9]. Although camphene-based freeze casting has been successfully developed for creating dendritic structures and thus fully interconnected pores [10], it is too expensive to become commercially available. To further improve the interconnectivity and lower the cost, novel techniques are highly demanded. In this work, a fast cooling freeze casting (FCFC) method was proposed where a freeze drier was prefrozen to 55 1C in order to create a large cooling gradient of ca. 70 1C. Comparing with conventional freeze casting techniques, the FCFC does not need an external device to control the thermal gradient, but introduces in-situ fast freezing on the ceramic slips. Therefore, it facilitates to scale-up trials and continuous production in the future. On the other hand, most of
http://dx.doi.org/10.1016/j.ceramint.2015.05.160 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
X. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
2
the previous works focus on the morphology of the porous scaffolds [3], and only a few reports have provided the supercooling degree of solvent in alumina slurries [11]. For understanding the formation mechanism, an on-site record of ceramic slip temperature under freezing is critical for tracking its behavior. In this work, a novel FCFC technique was successfully developed for porous Al2O3 scaffold preparation, and the underlying formation mechanism was proposed from analyzing the freezing curve of ceramic slips.
plate inside a chamber of a freeze dryer (Model LGJ-10F, Beijing Song Yuan Hua Xing Technology development Co., Ltd., China) and subsequently subjected to o 1 Pa vacuum to sublime the ice for 36 h. The dried samples were fired at a constant heating rate of 2 1C/min to 300 1C for 2 h to remove PVA binders and subsequently to 1550 1C for 2 h to sinter. 2.3. Characterizations Apparent porosities and closed porosities were measured according to the Archimedes method (ASTMC-373). The pore size distribution was examined using mercury porosimetry (Micromeritics AutoPore IV 9500 V1.09). A scanning electronic microscopy (SEM) from JSM-6510A JEOL Japan was employed to observe the fracture surfaces of Al2O3 porous scaffold in parallel and perpendicular direction to the applied thermal gradient. To clearly observe the through pores, the prepared Al2O3 ceramic were resin-embedded, ground, polished and coated with conductive carbon before SEM. A temperature probe was inserted into the well-mixed ceramic slurry with 0.3% SHMP and 3% PVA in order to in-situ record its temperature changes and to identify its supercooling degree during freezing.
2. Experimental 2.1. Starting materials As-received Al2O3 powder (D50: 0.40–0.80 mm, RG4000, Almatis, Pittsburgh, PA) was used during preparation. Aqueous freeze-casting slurries were prepared using a sodiumhexametaphosphate (SHMP) as a dispersant and polyvinyl alcohol (PVA) as a binder to improve green strength for easyhandling. 2.2. Preparation of porous alumina Al2O3 slurries were prepared at 23.5 vol% for all the experiments. Prior to Al2O3 slurry preparation, PVA were premixed with deioned water in a concentration of 1–3 wt% using a magnetic-stirrer at 85 1C for 4 h. The PVA solution was then mixed with Al2O3 powders and 0.1–0.5 wt% SHMP based on the total weight of Al2O3 powder. The above slurries were homogenized via magnetic stirring for 48 h and then deaired under vacuum for 15 min before freezing. All samples were frozen for 5 h at 55 1C in-situ on a sample supporting
3. Results 3.1. Apparent porosity and bulk density Apparent porosities of the prepared porous Al2O3 ceramics are 67–68.5%, which agree well with their mercury porosity measurements 64.5–68.4%. Their relative bulk densities are around 1.24–1.31 (Table 1). According to the calculations the closed porosities are about 0–0.6%, indicating that almost all the pores are open. In addition, with varying the concentration of SHMP from 0.1 to 0.5%, the porosities of the resultant porous ceramics remained almost the same, suggesting that the SHMP concentration did not affect the total porosity much.
Table 1 Apparent porosities and bulk density of porous Al2O3 scaffold (PVA 3%). SHMP concentration (%)
1.27 1.31 1.24
Apparent porosity (%) 67.0 67.0 68.5
True porosity (%) 67.0 66.4 68.3
Closed porosity (%) 0 0.6 0.2
Mercury porosity (%)
3.2. Mercury porosimetry measurement
68.4 64.5 67.8
As shown in Fig. 1b, for SHMP 0.1% and 0.3% samples the pore size distributions are both bimodal with larger pores
dV/dlogD (a.u.)
0.1 0.3 0.5
Relative bulk density
100
0.1%SHMP 0.3%SHMP 0.5%SHMP
1000
10000
100000
Pore Diameter (nm)
Fig. 1. (a) SEM of porous Al2O3 ceramics with SHMP 0.3% and (b) mercury porosimetry measurement of samples with various SHMP concentration from 0.1% to 0.5%. Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
X. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
(meanwhile the most probable ones) at ca. 10 μm and smaller ones at ca. 480 nm. The most probable pores (10 μm) are very close to the average pores (11 μm) in the sizes, revealing a narrow pore size distribution. With increasing SHMP concentration from 0.3% to 0.5%, the pore sizes distribution slightly shifted to larger diameters ( 14 μm for the larger and 2 μm for the smaller pores of 0.5% SHMP sample). This suggested that the SHMP had affected the pore size distribution. Take SHMP 0.3% sample for example, the SEM (Fig. 1a) further confirms the pore sizes and morphology that the 10 μm pores are surrounded by as-sintered Al2O3 walls. The origin of the larger pores is more likely to be the replica of the ice crystals because almost no purities can exist in the solidified water, whereas the smaller ones are more like voids due largely to the burn-out of PVA binders and the partial sintering of Al2O3 ceramics at 1550 1C.
3.3. Microscopic observations In both the perpendicular and parallel directions to the thermal gradient, adjacent primary dendrite trunks were relatively parallel to each other as indicated by the arrows (Fig. 2a and c), where secondary ceramic arms bridge them together (Fig. 2b). Their main trunks aligned unidirectional (Fig. 2c and arrow 1 in d) with one of secondary arms (arrow 2 in Fig. 2d) split into additional tips (arrow 3 in Fig. 2d), thus the interconnectivity was further improved. The large pores were around 10 μm in diameter (Fig. 2b and d), showing good agreements with the mercury porosity measurements (Fig. 1).
3
The resultant dendrite structures may change from the lamellae counterparts, as illustrated in Fig. 3. At PVA concentration as low as 1%, the lamellae features were largely retained as indicated by the arrows, between which a few ceramic bridges were found loosely distributed. With PVA concentration increased up to 2%, dendrite structures started to form with parallel aligned trunks and homogeneously distributed dendrite arms. Slightly more homogeneous patterns were observed for the prepared porous ceramics containing 2% PVA than those containing 3% PVA. To clearly observe the through pore structures under SEM, resin-embedded porous ceramics (SHMP 0.3% and PVA 3%) were studied. Parallel pore channels tilt to the thermal gradient with an angle of ca. 201, suggesting that the real growth direction of ice crystals is closer to the large macroscopic gradient than the preferred growth direction [7]. In the perpendicular direction to the gradient (Fig. 4a), the pore spacing between ceramic walls is around 100 μm (arrow 1 in Fig. 4a) and the wall thickness is around 50–100 μm (arrow 2 in Fig. 4a). 3.4. Discussion The mercury porosity (64.5–68.4 vol%) and the Archimedes value (67.0–68.5 vol%), together with the low closed porosity (o 0.6 vol%), suggest that most of resultant pores are open. The results also showed that SHMP concentration had not influenced the total porosity, but it had affected the pore size distribution. The bi-modal pore size distribution was shifted to larger diameters with increasing SHMP concentration.
3
1 2
Fig. 2. SEM of Al2O3 scaffold (a and b) perpendicular and (c and d) parallel to thermal gradient, SHMP¼ 0.3%, PVA¼ 3%.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
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4
Low PVA T2 front
Al2O 3
T1 <
Ice growth
High PVA
•Increasing viscosity & hindering Al2O3 particle rearrangement
a a c
•Limiting ice growth in axis a
T1
a
Fig. 3. SEM of Al2O3 scaffolds with different PVA concentrations (a) 1%, (b) 2% and (c) 3%. (d) Scheme of transformation from lamellae to dendrite structures, illustrating the formation mechanism of porous Al2O3 ceramics. Axis a is the preferred growth direction of ice and c the limited direction. Axis a is parallel and c perpendicular to the thermal gradient from T1 to T2.
30
3
4
2 1
Temperature / o C
20
Sample Temperature 1
10
Support Plate Temperature
0 2
-10 -20 -30
3
-40 -50 -60 0
1
2
3
Time / h Fig. 4. SEM of Al2O3 scaffold (a) perpendicular and (b) parallel to thermal gradient, scale bar¼100 μm; 1¼ pore spacing; 2 ¼wall thickness; SHMP¼0.3%; PVA¼3%.
The larger through pores at ca. 10 μm ( 14 μm for 0.5% SHMP ) would greatly facilitate the mass and heat diffusion, lower the pressure drop and hinder the pores' plugging by dust. Meanwhile the 480 nm ( 2 μm for 0.5% SHMP) mesopores enhance the contact between reactants and catalysts and thus improve the catalyst activity. Such bi-modal pore size distribution is desirable by many automotive and petrochemical catalysis processes. The dendrite structures were predominantly observed under SEM. Its formation mechanism was studied by in-situ recording the freezing curve of the ceramic slips. Specifically, when placing the ceramic slips inside the pre-cooled freeze drier, the liquid water in the slurry cools down below ca. 0 1C sharply
Fig. 5. Freezing curve of sample and support plate.
(1 in Fig. 5). Subsequently the curve levels out at ca. 10 1C supercooling point (2 in Fig. 5) and lasts for about 40 min, during which isolated water must have solidified into ice at the presence of ceramic particles and other additives [12]. Considering the height of the ceramic slips is 1000 μm, the average ice front velocity can be calculated as 0.42 μm/s and the pore spacing is calculated as o 100 μm (Fig. 4) using the following equation [3, 13]: Porespacing ¼ 161nvelocitynexpð 0:8Þ
ð1Þ
Under this velocity, lamella structures should form as predominant structures initially [7]. However by varying the concentration of PVA from 1 to 3%, clear transformation from lamellae to dendrite structures was observed (Fig. 3a–c) with
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
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increasing PVA concentration. The mismatch suggests that there must exist a mechanism for the transformation from lamellae to dendrites to take place. The underlying mechanism may be that with increasing PVA concentration the ceramic slips normally get more viscous and thus the ice growth in its preferred direction a was limited (Fig. 3d). As an alternative, the ice might grow towards the c direction. Meanwhile, the direction c is perpendicular to the thermal gradient, therefore the ice grows in a direction between c and the thermal gradient, leaving dendrite ice structures behind [7,14–16], as illustrated in Fig. 3d. As above indicated, the plateau of the freezing curve is one of the most critical stages for pore structure evolution. After that, the freezing curve further decreases to ca. 40 1C (3 in Fig. 5) gradually where the remaining absorbed and/or bonding water may solidify until the whole freezing process finishes. 4. Conclusion The FCFC method has been developed to produce unidirectional porous Al2O3 scaffold with improved interconnectivity. The pores are mostly open and the total porosity is 64.5–68.5 vol% from mercury porosimetry and Archimedes measurements. Bi-modal pore sizes shifted to larger diameters with increasing SHMP concentration from 0.1% to 0.5%, distributing at ca. 10 μm ( 14 μm for 0.5% SHMP) and 480 nm ( 2 μm for 0.5% SHMP). The transformation of lamellae to dendrite-like structures was clearly observed with the increasing PVA concentration from 1 to 3%. To reveal the transformation mechanism, the freezing curve and the supercooling degree of about 10 1C were obtained. It shows that the water solidifies at an ice front average velocity of 0.42 μm/ s. Lamellae structure was initially formed, but with increasing PVA, the viscosity of the ceramic slurry will be increased, the ceramic particle rearrangement during water solidification will be limited and the ice growth in the axis a may be hindered whereas that in the c direction would be promoted. Therefore, together under the influences of the applied thermal gradient, the lamellae are transformed into dendrites. Acknowledgment This work was supported by Beijing Natural Science Foundation of China (Grant no. 2154052), China Postdoctoral
5
Science Foundation funded project (2014M560044) and the Fundamental Research Funds for the Central Universities (FRF-TP-14-004A1).
References [1] W. Oelerich, T. Klassen, R. Bormann, Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials, J. Alloy. Compd. 315 (2001) 237–242. [2] H.S. Gandhi, G.W. Graham, R.W. McCabe, Automotive exhaust catalysis, J. Catal. 216 (2003) 433–442. [3] Sylvain Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (3) (2008) 155–169. [4] Christopher M.A. Parlett, Karen Wilson, Adam F. Lee, Hierarchical porous materials: catalytic applications, Chem. Soc. Rev. 42 (2013) 3876–3893. [5] Stephen W. Sofie, Fatih Dogan, Freeze casting of aqueous alumina slurries with glycerol, J. Am. Ceram. Soc. 84 (7) (2001) 1459–1464. [6] Christopher Pekor, Ian Nettleship, The effect of the molecular weight of polyethylene glycol on the microstructure of freeze-cast alumina, Ceram. Int. 40 (2014) 9171–9177. [7] Sylvain Deville, Eduardo Saiz, Antoni P. Tomsia, Ice-templated porous alumina structures, Acta Mater. 55 (2007) 1965–1974. [8] Shuangyin Wang, San Ping Jiang, T.J. White, Jun Guo, Xin Wang, Electrocatalytic activity and interconnectivity of Pt nanoparticles on multiwalled carbon nanotubes for fuel cells, J. Phys. Chem. C 113 (2009) 18935–18945. [9] Tanya Tsoncheva, Ljubomira Ivanova, Jessica Rosenholm, Mika Linden, Cobalt oxide species supported on SBA-15, KIT-5 and KIT-6 mesoporous silicas for ethyl acetate total oxidation, Appl. Catal. B Environ. 89 (2009) 365–374. [10] Kiyoshi Arakiand, John W. Halloran, Porous ceramic bodies with interconnected pore channels by a novel freeze casting technique, J. Am. Ceram. Soc. 88 (5) (2005) 1108–1114. [11] Sujith Vijayan, R. Narasimman, K. Prabhakaran, Freeze gelcasting of naphthalene-in-aqueous alumina slurry emulsions for the preparation of macroporous alumina ceramics, Ceram. Int. 41 (2015) 1487–1494. [12] Kenneth G. Libbrecht, The physics of snow crystals, Rep. Prog. Phys. 68 (2005) 855–895. [13] S. Deville, E. Saiz, R.K. Nalla, A. Tomsia, Freezing as a path to build complex composites, Science 311 (2006) 515–518. [14] Christopher Pekor, Ben Groth, Ian Nettleship, The effect of polyvinyl alcohol on the microstructure and permeability of freeze-cast alumina, J. Am. Ceram. Soc. 93 (1) (2010) 115–120. [15] Christopher M. Pekor, Predrag Kisa, Ian Nettleship, Effect of polyethylene glycol on the microstructure of freeze-cast alumina, J. Am. Ceram. Soc. 91 (10) (2008) 3185–3190. [16] S. Deville, J. Adrien, E. Maire, M. Scheel, M. Di Michiel, Time-lapse, three-dimensional in situ imaging of ice crystal growth in a colloidal silica suspension, Acta Mater. 61 (2013) 2077–2086.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method Xiaoguang Liu, Wendong Xuen, Cunlan Shi, Jialin Sun Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing, China Received 29 April 2015; received in revised form 29 May 2015; accepted 30 May 2015
Abstract A fast cooling freeze casting (FCFC) method was developed for preparation of porous Al2O3 scaffolds with improved interconnectivity, unidirectional pore channels and dendritic walls. As estimated using the Archimedes method and mercury porosimetry, total porosity 64.5–68.5 vol% and the closed porosity o0.6 vol% reveal that most pores are open. The bimodal pore size distribution shifted to larger diameters with increasing sodium-hexametaphosphate (SHMP) concentration from 0.1% to 0.5%. Scanning electronic microscope (SEM) images of the resultant porous Al2O3 showed clear transformation from lamellae to dendrite structures with increasing polyvinyl alcohol (PVA) concentration from 1% to 3%. To discover the mechanism, the freezing curve of the Al2O3 slip was achieved, showing its supercooling point at about 10 1C where most isolated water solidifies at an average ice front velocity of 0.42 μm/s. Together with the PVA effects on the microstructure transformation, the underlying mechanism could be ascribed to both the supercooling and PVA effects. The lamellae ice front was initially formed under this velocity, but with increasing PVA, the ceramic slurry got more viscous and the rearrangement of ceramic particles was hindered, hence the ice front growth in the preferred a direction was limited whereas the growth in the c direction was promoted. Meanwhile under the effects of the applied thermal gradient, the lamellae were transformed into dendrite structures. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Fast cooling freeze casting; B. Porosity; D. Porous Al2O3 scaffold; Mechanism; Freezing curve
1. Introduction Macroporous Al2O3 ceramic is one of the most important catalyst scaffolds for automotive emission control due to its excellent thermal-shock resistance, mechanical properties and chemical stability [1,2]. One way to prepare such scaffolds is through a novel freeze casting technique, which has been extensively explored in recent years [3–6]. The resultant structure usually varies under different process parameters. Nevertheless, most water-based freeze casting technique favors porous lamellae structures with unidirectional through pores [3,7]. The lamellae show either smooth or rough surfaces [7]. Abbreviations: SEM: scanning electronic microscope; FCFC: fast cooling freeze casting; SHMP : sodium-hexametaphosphate; PVA: polyvinyl alcohol n Corresponding author. Tel.: þ86 10 6233 2666. E-mail address: [email protected] (W. Xue).
Even with ceramic bridges linking between the lamellae, fully interconnected pores were seldom seen. The lack of interconnectivity limits the contact between the reactant and the catalysts and thus degrades the activities of catalysts to some degree [8–9]. Although camphene-based freeze casting has been successfully developed for creating dendritic structures and thus fully interconnected pores [10], it is too expensive to become commercially available. To further improve the interconnectivity and lower the cost, novel techniques are highly demanded. In this work, a fast cooling freeze casting (FCFC) method was proposed where a freeze drier was prefrozen to 55 1C in order to create a large cooling gradient of ca. 70 1C. Comparing with conventional freeze casting techniques, the FCFC does not need an external device to control the thermal gradient, but introduces in-situ fast freezing on the ceramic slips. Therefore, it facilitates to scale-up trials and continuous production in the future. On the other hand, most of
http://dx.doi.org/10.1016/j.ceramint.2015.05.160 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
X. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
2
the previous works focus on the morphology of the porous scaffolds [3], and only a few reports have provided the supercooling degree of solvent in alumina slurries [11]. For understanding the formation mechanism, an on-site record of ceramic slip temperature under freezing is critical for tracking its behavior. In this work, a novel FCFC technique was successfully developed for porous Al2O3 scaffold preparation, and the underlying formation mechanism was proposed from analyzing the freezing curve of ceramic slips.
plate inside a chamber of a freeze dryer (Model LGJ-10F, Beijing Song Yuan Hua Xing Technology development Co., Ltd., China) and subsequently subjected to o 1 Pa vacuum to sublime the ice for 36 h. The dried samples were fired at a constant heating rate of 2 1C/min to 300 1C for 2 h to remove PVA binders and subsequently to 1550 1C for 2 h to sinter. 2.3. Characterizations Apparent porosities and closed porosities were measured according to the Archimedes method (ASTMC-373). The pore size distribution was examined using mercury porosimetry (Micromeritics AutoPore IV 9500 V1.09). A scanning electronic microscopy (SEM) from JSM-6510A JEOL Japan was employed to observe the fracture surfaces of Al2O3 porous scaffold in parallel and perpendicular direction to the applied thermal gradient. To clearly observe the through pores, the prepared Al2O3 ceramic were resin-embedded, ground, polished and coated with conductive carbon before SEM. A temperature probe was inserted into the well-mixed ceramic slurry with 0.3% SHMP and 3% PVA in order to in-situ record its temperature changes and to identify its supercooling degree during freezing.
2. Experimental 2.1. Starting materials As-received Al2O3 powder (D50: 0.40–0.80 mm, RG4000, Almatis, Pittsburgh, PA) was used during preparation. Aqueous freeze-casting slurries were prepared using a sodiumhexametaphosphate (SHMP) as a dispersant and polyvinyl alcohol (PVA) as a binder to improve green strength for easyhandling. 2.2. Preparation of porous alumina Al2O3 slurries were prepared at 23.5 vol% for all the experiments. Prior to Al2O3 slurry preparation, PVA were premixed with deioned water in a concentration of 1–3 wt% using a magnetic-stirrer at 85 1C for 4 h. The PVA solution was then mixed with Al2O3 powders and 0.1–0.5 wt% SHMP based on the total weight of Al2O3 powder. The above slurries were homogenized via magnetic stirring for 48 h and then deaired under vacuum for 15 min before freezing. All samples were frozen for 5 h at 55 1C in-situ on a sample supporting
3. Results 3.1. Apparent porosity and bulk density Apparent porosities of the prepared porous Al2O3 ceramics are 67–68.5%, which agree well with their mercury porosity measurements 64.5–68.4%. Their relative bulk densities are around 1.24–1.31 (Table 1). According to the calculations the closed porosities are about 0–0.6%, indicating that almost all the pores are open. In addition, with varying the concentration of SHMP from 0.1 to 0.5%, the porosities of the resultant porous ceramics remained almost the same, suggesting that the SHMP concentration did not affect the total porosity much.
Table 1 Apparent porosities and bulk density of porous Al2O3 scaffold (PVA 3%). SHMP concentration (%)
1.27 1.31 1.24
Apparent porosity (%) 67.0 67.0 68.5
True porosity (%) 67.0 66.4 68.3
Closed porosity (%) 0 0.6 0.2
Mercury porosity (%)
3.2. Mercury porosimetry measurement
68.4 64.5 67.8
As shown in Fig. 1b, for SHMP 0.1% and 0.3% samples the pore size distributions are both bimodal with larger pores
dV/dlogD (a.u.)
0.1 0.3 0.5
Relative bulk density
100
0.1%SHMP 0.3%SHMP 0.5%SHMP
1000
10000
100000
Pore Diameter (nm)
Fig. 1. (a) SEM of porous Al2O3 ceramics with SHMP 0.3% and (b) mercury porosimetry measurement of samples with various SHMP concentration from 0.1% to 0.5%. Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
X. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
(meanwhile the most probable ones) at ca. 10 μm and smaller ones at ca. 480 nm. The most probable pores (10 μm) are very close to the average pores (11 μm) in the sizes, revealing a narrow pore size distribution. With increasing SHMP concentration from 0.3% to 0.5%, the pore sizes distribution slightly shifted to larger diameters ( 14 μm for the larger and 2 μm for the smaller pores of 0.5% SHMP sample). This suggested that the SHMP had affected the pore size distribution. Take SHMP 0.3% sample for example, the SEM (Fig. 1a) further confirms the pore sizes and morphology that the 10 μm pores are surrounded by as-sintered Al2O3 walls. The origin of the larger pores is more likely to be the replica of the ice crystals because almost no purities can exist in the solidified water, whereas the smaller ones are more like voids due largely to the burn-out of PVA binders and the partial sintering of Al2O3 ceramics at 1550 1C.
3.3. Microscopic observations In both the perpendicular and parallel directions to the thermal gradient, adjacent primary dendrite trunks were relatively parallel to each other as indicated by the arrows (Fig. 2a and c), where secondary ceramic arms bridge them together (Fig. 2b). Their main trunks aligned unidirectional (Fig. 2c and arrow 1 in d) with one of secondary arms (arrow 2 in Fig. 2d) split into additional tips (arrow 3 in Fig. 2d), thus the interconnectivity was further improved. The large pores were around 10 μm in diameter (Fig. 2b and d), showing good agreements with the mercury porosity measurements (Fig. 1).
3
The resultant dendrite structures may change from the lamellae counterparts, as illustrated in Fig. 3. At PVA concentration as low as 1%, the lamellae features were largely retained as indicated by the arrows, between which a few ceramic bridges were found loosely distributed. With PVA concentration increased up to 2%, dendrite structures started to form with parallel aligned trunks and homogeneously distributed dendrite arms. Slightly more homogeneous patterns were observed for the prepared porous ceramics containing 2% PVA than those containing 3% PVA. To clearly observe the through pore structures under SEM, resin-embedded porous ceramics (SHMP 0.3% and PVA 3%) were studied. Parallel pore channels tilt to the thermal gradient with an angle of ca. 201, suggesting that the real growth direction of ice crystals is closer to the large macroscopic gradient than the preferred growth direction [7]. In the perpendicular direction to the gradient (Fig. 4a), the pore spacing between ceramic walls is around 100 μm (arrow 1 in Fig. 4a) and the wall thickness is around 50–100 μm (arrow 2 in Fig. 4a). 3.4. Discussion The mercury porosity (64.5–68.4 vol%) and the Archimedes value (67.0–68.5 vol%), together with the low closed porosity (o 0.6 vol%), suggest that most of resultant pores are open. The results also showed that SHMP concentration had not influenced the total porosity, but it had affected the pore size distribution. The bi-modal pore size distribution was shifted to larger diameters with increasing SHMP concentration.
3
1 2
Fig. 2. SEM of Al2O3 scaffold (a and b) perpendicular and (c and d) parallel to thermal gradient, SHMP¼ 0.3%, PVA¼ 3%.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
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4
Low PVA T2 front
Al2O 3
T1 <
Ice growth
High PVA
•Increasing viscosity & hindering Al2O3 particle rearrangement
a a c
•Limiting ice growth in axis a
T1
a
Fig. 3. SEM of Al2O3 scaffolds with different PVA concentrations (a) 1%, (b) 2% and (c) 3%. (d) Scheme of transformation from lamellae to dendrite structures, illustrating the formation mechanism of porous Al2O3 ceramics. Axis a is the preferred growth direction of ice and c the limited direction. Axis a is parallel and c perpendicular to the thermal gradient from T1 to T2.
30
3
4
2 1
Temperature / o C
20
Sample Temperature 1
10
Support Plate Temperature
0 2
-10 -20 -30
3
-40 -50 -60 0
1
2
3
Time / h Fig. 4. SEM of Al2O3 scaffold (a) perpendicular and (b) parallel to thermal gradient, scale bar¼100 μm; 1¼ pore spacing; 2 ¼wall thickness; SHMP¼0.3%; PVA¼3%.
The larger through pores at ca. 10 μm ( 14 μm for 0.5% SHMP ) would greatly facilitate the mass and heat diffusion, lower the pressure drop and hinder the pores' plugging by dust. Meanwhile the 480 nm ( 2 μm for 0.5% SHMP) mesopores enhance the contact between reactants and catalysts and thus improve the catalyst activity. Such bi-modal pore size distribution is desirable by many automotive and petrochemical catalysis processes. The dendrite structures were predominantly observed under SEM. Its formation mechanism was studied by in-situ recording the freezing curve of the ceramic slips. Specifically, when placing the ceramic slips inside the pre-cooled freeze drier, the liquid water in the slurry cools down below ca. 0 1C sharply
Fig. 5. Freezing curve of sample and support plate.
(1 in Fig. 5). Subsequently the curve levels out at ca. 10 1C supercooling point (2 in Fig. 5) and lasts for about 40 min, during which isolated water must have solidified into ice at the presence of ceramic particles and other additives [12]. Considering the height of the ceramic slips is 1000 μm, the average ice front velocity can be calculated as 0.42 μm/s and the pore spacing is calculated as o 100 μm (Fig. 4) using the following equation [3, 13]: Porespacing ¼ 161nvelocitynexpð 0:8Þ
ð1Þ
Under this velocity, lamella structures should form as predominant structures initially [7]. However by varying the concentration of PVA from 1 to 3%, clear transformation from lamellae to dendrite structures was observed (Fig. 3a–c) with
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160
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increasing PVA concentration. The mismatch suggests that there must exist a mechanism for the transformation from lamellae to dendrites to take place. The underlying mechanism may be that with increasing PVA concentration the ceramic slips normally get more viscous and thus the ice growth in its preferred direction a was limited (Fig. 3d). As an alternative, the ice might grow towards the c direction. Meanwhile, the direction c is perpendicular to the thermal gradient, therefore the ice grows in a direction between c and the thermal gradient, leaving dendrite ice structures behind [7,14–16], as illustrated in Fig. 3d. As above indicated, the plateau of the freezing curve is one of the most critical stages for pore structure evolution. After that, the freezing curve further decreases to ca. 40 1C (3 in Fig. 5) gradually where the remaining absorbed and/or bonding water may solidify until the whole freezing process finishes. 4. Conclusion The FCFC method has been developed to produce unidirectional porous Al2O3 scaffold with improved interconnectivity. The pores are mostly open and the total porosity is 64.5–68.5 vol% from mercury porosimetry and Archimedes measurements. Bi-modal pore sizes shifted to larger diameters with increasing SHMP concentration from 0.1% to 0.5%, distributing at ca. 10 μm ( 14 μm for 0.5% SHMP) and 480 nm ( 2 μm for 0.5% SHMP). The transformation of lamellae to dendrite-like structures was clearly observed with the increasing PVA concentration from 1 to 3%. To reveal the transformation mechanism, the freezing curve and the supercooling degree of about 10 1C were obtained. It shows that the water solidifies at an ice front average velocity of 0.42 μm/ s. Lamellae structure was initially formed, but with increasing PVA, the viscosity of the ceramic slurry will be increased, the ceramic particle rearrangement during water solidification will be limited and the ice growth in the axis a may be hindered whereas that in the c direction would be promoted. Therefore, together under the influences of the applied thermal gradient, the lamellae are transformed into dendrites. Acknowledgment This work was supported by Beijing Natural Science Foundation of China (Grant no. 2154052), China Postdoctoral
5
Science Foundation funded project (2014M560044) and the Fundamental Research Funds for the Central Universities (FRF-TP-14-004A1).
References [1] W. Oelerich, T. Klassen, R. Bormann, Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials, J. Alloy. Compd. 315 (2001) 237–242. [2] H.S. Gandhi, G.W. Graham, R.W. McCabe, Automotive exhaust catalysis, J. Catal. 216 (2003) 433–442. [3] Sylvain Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (3) (2008) 155–169. [4] Christopher M.A. Parlett, Karen Wilson, Adam F. Lee, Hierarchical porous materials: catalytic applications, Chem. Soc. Rev. 42 (2013) 3876–3893. [5] Stephen W. Sofie, Fatih Dogan, Freeze casting of aqueous alumina slurries with glycerol, J. Am. Ceram. Soc. 84 (7) (2001) 1459–1464. [6] Christopher Pekor, Ian Nettleship, The effect of the molecular weight of polyethylene glycol on the microstructure of freeze-cast alumina, Ceram. Int. 40 (2014) 9171–9177. [7] Sylvain Deville, Eduardo Saiz, Antoni P. Tomsia, Ice-templated porous alumina structures, Acta Mater. 55 (2007) 1965–1974. [8] Shuangyin Wang, San Ping Jiang, T.J. White, Jun Guo, Xin Wang, Electrocatalytic activity and interconnectivity of Pt nanoparticles on multiwalled carbon nanotubes for fuel cells, J. Phys. Chem. C 113 (2009) 18935–18945. [9] Tanya Tsoncheva, Ljubomira Ivanova, Jessica Rosenholm, Mika Linden, Cobalt oxide species supported on SBA-15, KIT-5 and KIT-6 mesoporous silicas for ethyl acetate total oxidation, Appl. Catal. B Environ. 89 (2009) 365–374. [10] Kiyoshi Arakiand, John W. Halloran, Porous ceramic bodies with interconnected pore channels by a novel freeze casting technique, J. Am. Ceram. Soc. 88 (5) (2005) 1108–1114. [11] Sujith Vijayan, R. Narasimman, K. Prabhakaran, Freeze gelcasting of naphthalene-in-aqueous alumina slurry emulsions for the preparation of macroporous alumina ceramics, Ceram. Int. 41 (2015) 1487–1494. [12] Kenneth G. Libbrecht, The physics of snow crystals, Rep. Prog. Phys. 68 (2005) 855–895. [13] S. Deville, E. Saiz, R.K. Nalla, A. Tomsia, Freezing as a path to build complex composites, Science 311 (2006) 515–518. [14] Christopher Pekor, Ben Groth, Ian Nettleship, The effect of polyvinyl alcohol on the microstructure and permeability of freeze-cast alumina, J. Am. Ceram. Soc. 93 (1) (2010) 115–120. [15] Christopher M. Pekor, Predrag Kisa, Ian Nettleship, Effect of polyethylene glycol on the microstructure of freeze-cast alumina, J. Am. Ceram. Soc. 91 (10) (2008) 3185–3190. [16] S. Deville, J. Adrien, E. Maire, M. Scheel, M. Di Michiel, Time-lapse, three-dimensional in situ imaging of ice crystal growth in a colloidal silica suspension, Acta Mater. 61 (2013) 2077–2086.
Please cite this article as: X. Liu, et al., Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.160