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Freeze casting of porous Ni–YSZ cermets
Materials Letters 61 (2007) 1283 – 1287 www.elsevier.com/locate/matlet
Freeze casting of porous Ni–YSZ cermets Young-Hag Koh ⁎, Jong-Jae Sun, Hyoun-Ee Kim School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Republic of Korea Received 7 March 2006; accepted 4 July 2006 Available online 24 July 2006
Abstract Dual-phase porous Ni–YSZ cermets were fabricated via the freeze casting of a ceramic/camphene slurry. After removing the frozen camphene via sublimation at room temperature, the green samples were sintered for 3 h in air at various temperatures, ranging from 1100 to 1350 °C, and then reduced in an Ar–5% H2 atmosphere at 700 °C for 3 h. The fabricated Ni–YSZ cermets showed 3-D pore channels formed by the replication of the entangled camphene dendrite network and small pores in the Ni–YSZ walls produced by partial sintering of the NiO–YSZ composite. Furthermore, the fabricated samples were found to possess reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs). © 2006 Elsevier B.V. All rights reserved. Keywords: Ni–YSZ; Cermets; Porous; Anode; Fuel cells
1. Introduction Solid oxide fuel cells (SOFCs) are energy conversion devices that convert the chemical energy of gaseous fuels directly into an electrical energy [1,2]. In principle, SOFCs are comprised of three key components, namely a fuel electrode, an air electrode and an electrolyte. Among the possible choices of anode material, Ni–yttria-stabilized zirconia (YSZ) cermets have been widely used because of the high electrical conductivity provided by the metallic Ni and the good ionic conductivity afforded by the YSZ [3]. In addition, these materials are often used as a support to reduce the thickness of the electrolyte layer, in order to lower the operating temperature [4]. The anode material should be porous, in order to allow the fuel to pass and to provide appropriate reaction sites for its anodic oxidation. The properties of such anode materials are mostly dependent on their microstructure and pore structure, as well as on the distribution of the conducting phases [5–8]. In general, the reaction is limited by the length of the so called triple-phase boundary (TPB), where the Ni, YSZ, and fuel gas are in contact with each other [9]. A greater TPB length can be ⁎ Corresponding author. Tel.: +82 2 880 9046. E-mail address: [email protected] (Y.-H. Koh). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.009
achieved by using smaller pore and particle sizes, but this interrupts the gas transport [10]. Recently, in an attempt to solve this problem, a novel structure was proposed, which contained large pores to provide rapid gas transport and small pores to allow for fast electrochemical reactions [11]. In this method, a foamy poly(methyl methacrylate) (PMMA), consisting of closed-packed PMMA spheres, is infiltrated with an electrodecontaining slurry, followed by calcination. This method produces macropores by removing the PMMA template and, at the same time, mesopores in electrode walls by heat-treating nanoparticles. To date, a variety of manufacturing techniques have been developed to fabricate porous anode materials [12–16]. Amongst these different techniques, tape casting with pore-forming fugitive phases (e.g., organic phases and graphite) is one of the most commonly used methods, owing to its simple processability [12–14]. However, it is difficult to optimize the pore structure using this method. On the other hand, freeze casting methods are known to be capable of producing porous materials with a controlled pore structure [17–20]. These methods basically make full use of the dendritic growth of ice [17,18] or camphene [19,20], with these materials being used as vehicles in ceramic powder slurries. Therefore, in this study, we fabricated dual-phase porous Ni–YSZ cermets, which had large pore channels to provide
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rapid gas transport and small pores to allow for fast electrochemical reactions, using the freeze casting of an NiO–YSZ/camphene slurry. This method enabled the Ni–YSZ cermets to have threedimensionally connected large pore channels formed as a replication of the camphene dendrite network and small pores in the Ni–YSZ walls formed by controlling the sintering temperature of the NiO–YSZ composites. The fabricated samples were characterized in terms of their microstructures and electrical conductivity. 2. Experimental procedure Commercially available nickel oxide (NiO) (Wako Pure Chemicals, Osaka, Japan) and yttria-stabilized zirconia (YSZ) doped with 8 mol% Y2O3 (TZ-8Y; Tosoh Co., Tokyo, Japan) were used as the ceramic materials. Camphene (C10H16, Alfa Aesar/Avocado Organics, Ward Hill, MA, USA) with a melting temperature of 44–48 °C was used without further purification as the vehicle for freeze casting. A 1:1 wt.% mixture of NiO and YSZ was ball-milled for 3 days using zirconia balls as the media, followed by drying and sieving, in order to ensure homogeneous mixing and reduce the particle size. It was observed that the ball-milled powders were very fine (particle size b 100 nm), which was suitable for preparing the ceramic/ camphene slurry. A 20 vol.% of the NiO–YSZ powder was ball-milled with molten camphene containing 3 wt.% of oligomeric polyester (Hypermer KD-4; UniQema, Everburg, Belgium) as a dispersant at 60 °C for 24 h using zirconia balls as the media. The warm slurry was then cast into a 20 × 20 × 5 mm mold at a constant temperature of 20 °C in order to control the solidification rate of the slurry. After 30 min, the green samples were carefully removed from the molds and kept in a hood with an air flow of ∼ 0.02 m/s at room temperature for 24 h to completely remove the camphene via sublimation. The camphene-free samples were then sintered at various temperatures ranging from 1100 to 1400 °C for 3 h in air, in order to control the porosity in the NiO–YSZ walls. Thereafter, the sintered samples were reduced in an Ar–5% H2 atmosphere for 3 h at temperatures ranging from 600 to 900 °C, to produce the dualphase porous Ni–YSZ cermets. The microstructures of the sintered and reduced samples were characterized using scanning electron microscopy (SEM, JSM-6330, JEOL Techniques, Tokyo, Japan). The crystalline phases of the samples were characterized using X-ray diffraction (XRD, MXP18A-HF, MAC Science, Tokyo, Japan). The electrical conductivities of the Ni–YSZ cermets were measured using DC four-point probe techniques (model CMT-SR 1000, Chang-Min, Seoul, Korea) at room temperature after grinding with successively smoother grades of sandpaper down to 2000 grit. In order to evaluate the porosity, pore size, and degree of interconnection, the samples were infiltrated with an epoxy resin (Spurrs epoxy, Polysciences Inc., Warrington, PA) and then cured at 70 °C for 24 h. Thereafter, the samples were ground and analyzed using SEM. The porosity was calculated from the digitally colored SEM images of the epoxy filled sample by measuring the area occupied by the epoxy.
3. Results and discussion We employed the freeze casting of the ceramic/camphene slurry to produce dual-phase porous Ni–YSZ cermets. This method exploits the fact that camphene forms dendrites when solidified under an appropriate temperature gradient or solute gradient in an alloy system [21,22]. When ceramic powders are present in the system, they are repelled by the growing camphene dendrites and become concentrated between the dendrite arms or neighboring dendrites [19,20]. Such solidification of the ceramic/camphene slurry results in the production of a 3-dimensionally interconnected camphene network, surrounded by concentrated ceramic powder walls, as shown in Fig. 1(A). The large pore channels are produced by removing the solidified camphene network, while the small pores in the NiO–YSZ walls are produced by adjusting the sintering temperature. A typical micrograph of the fabricated NiO–YSZ composites after sintering at 1250 °C for 3 h is shown in Fig. 1(B). All of the samples exhibited highly porous structures formed as a replica of the entangled dendritic growth of the frozen camphene, regardless of the sintering temperature. The measured porosity associated with the 3-D large pore channels was as high as 56%. The pores produced in the NiO–YSZ walls sintered at various temperatures, ranging from 1100 to 1300 °C, are shown in Fig. 2(A)–(D). When sintered at a relatively low temperature of 1100 °C, the sample exhibited a high degree of porosity, owing to the relatively poor densification, as shown in Fig. 2(A). The densification of the NiO–YSZ walls was notably enhanced when the sintering temperature was increased to 1200 °C and 1250 °C, while the engineered pores were still preserved, as shown in Fig. 2(B) and (C), respectively. At the higher sintering temperature of 1300 °C, the NiO–YSZ walls were almost fully dense, as shown in Fig. 2(D). These observations suggest that it is possible to introduce small pores into the sintered NiO–YSZ, without disturbing the large pore channels, by carefully controlling the sintering temperature,
Fig. 1. (A) Schematic illustration showing the solidified camphene dendrites and the concentrated NiO–YSZ powder walls and (B) typical SEM micrograph of the NiO–YSZ composite sintered at 1200 °C for 3 h in air.
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Fig. 2. SEM micrographs of the NiO–YSZ walls sintered for 3 h in air at (A) 1100 °C, (B) 1200 °C, (C) 1250 °C, and (D) 1300 °C.
which in turn allows the fraction of small pores that develop in the Ni–YSZ cermets to be tailored. It should be noted that the grain size increased with increasing sintering temperature. Approximately 17% linear shrinkage was observed after sintering. In order to produce the Ni–YSZ cermets, the porous NiO–YSZ composites were reduced in an Ar–5% H2 atmosphere at temperatures ranging from 600 to 900 °C for 3 h. The typical XRD patterns of the samples after reduction are shown in Fig. 3(A)–(D). Before reduction, the samples showed only crystalline peaks corresponding to the YSZ and NiO phases (not shown). However, after reduction at 600 °C, the sample revealed peaks corresponding to Ni, as well as NiO phases, indicating that this temperature was insufficient to completely reduce the NiO phases (Fig. 3(A)). On the other hand, as the reducing temperature was increased to 700 °C, the NiO peaks disappeared and only Ni peaks were observed along with the YSZ peaks (Fig. 3(B)). Further increase in the reducing temperature did not result in any changes in the crystalline phases (Fig 3(C) and (D)). Considering that smaller pore and particle sizes lead to a greater TPB length, the lowest reducing temperature of 700 °C was selected for the subsequent investigations. The SEM micrographs of the fabricated dual-phase porous Ni–YSZ cermets produced using the NiO–YSZ composites sintered at various temperatures ranging from 1100 to 1300 °C, are shown in Fig. 4(A) – (D). The highly porous structure of the composites, comprised of 3-D large pore channels with a size of several tens of microns, was retained in all of the Ni–YSZ cermets, while the degree of pore development in the Ni–YSZ walls was significantly increased compared to that of the NiO–YSZ walls, owing to the shrinkage caused by the reduction of NiO to Ni phase. The Ni–YSZ cermet produced using the NiO–YSZ composite sintered at the relatively low temperature of 1100 °C exhibited a very high degree of porosity in the walls (Fig. 4(A)). The degree of porosity was decreased by using the NiO–YSZ composites sintered at higher temperatures. However, the samples sintered at 1250 °C and 1300 °C still had highly porous Ni–YSZ walls with a pore size of several hundreds of nanometers, as shown in Fig. 4(C) and (D), respectively.
In order to verify the possibility of using the dual-phase porous Ni–YSZ cermets as anode substrates, their electrical conductivities were measured at room temperature using a four-point fixture. The electrical conductivity of the Ni–YSZ cermet as a function of the sintering temperature is shown in Fig. 5. The electrical conductivity of the Ni–YSZ cermet was strongly influenced by the sintering temperature, due to its effect on the porosity that developed in the Ni–YSZ walls. The Ni–YSZ cermet produced using the NiO–YSZ composite sintered at the relatively low temperature of 1100 °C showed negligible electrical conductivity. This insulating behavior was due to the insufficient percolation of the conducting Ni phases, which was associated with not only the poor coarsening of Ni, but also the presence of excess pores. On the other hand, the Ni–YSZ cermets produced using the
Fig. 3. XRD patterns of the sample sintered at 1250 °C for 3 h (A) before reduction and after reduction in a flowing Ar–5% H2 atmosphere for 3 h at (B) 600 °C, (C) 700 °C, (D) 800 °C, and (E) 900 °C. *YSZ (❍), NiO (❖), Ni ( ).
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Fig. 4. SEM micrographs of the Ni–YSZ cermets produced using the NiO–YSZ composites sintered at (A) 1100 °C, (B) 1200 °C, (C) 1250 °C, and (D) 1300 °C, after reduction in a flowing Ar–5% H2 atmosphere.
NiO–YSZ composites sintered at higher temperatures (N 1200 °C) showed reasonable electrical conductivity ranging from 6.3 to 9.5 S/cm, which would allow them to be efficiently used as an anode substrate [23]. It should be noted that the electrical conductivity of the Ni–YSZ cermet could be increased by increasing the sintering temperature of the NiO–YSZ composite and thereby reducing the porosity in the Ni–YSZ walls; however, this would inevitably decrease the TPB lengths and in turn reduce the electrochemical activity of the anode. In general, the electrochemical activity of the anode material is dependent not only on the materials' properties, such as the TPB lengths, electrical conductivity, and pore size, but also on the various operating parameters [23]. Although we did not directly measure the electrochemical activity of the dual-phase porous Ni–YSZ cermets in this study, it is believed that these cermets would provide for the easy passage of the fuel, due to the large pore channels and long TPB length, owing to the small pores formed in the Ni–YSZ walls.
4. Conclusions We fabricated dual-phase porous Ni–YSZ cermets using the freeze casting of a ceramic/camphene slurry, in which 3-D pore channels were produced by the replication of the entangled camphene dendrite network and small pores were produced in the Ni–YSZ walls by controlling the sintering temperature of the NiO–YSZ composite. The fabricated NiO–YSZ composite exhibited a highly porous structure, while the porosity in the NiO–YSZ walls was increased by lowering the sintering temperature from 1300 to 1100 °C. Dual-phase porous Ni–YSZ cermets were fabricated by reducing the NiO–YSZ composites in an Ar–5% H2 atmosphere at 700 °C for 3 h. It was observed that the fabricated Ni–YSZ cermets had 3-D pore channels with the size of several tens of microns and small pores with the size of several hundreds of nanometers in the Ni–YSZ walls, as well as reasonable electrical conductivities, which would allow them to be efficiently used as an anode substrate. Acknowledgments This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2005-003-D00129). References
Fig. 5. Electrical conductivity of the Ni–YSZ cermet produced using the NiO–YSZ composites as a function of the sintering temperature.
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N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. R.M. Ormerod, Chem. Soc. Rev. 32 (2003) 17. B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. S.A. Barnett, Energy 15 (1990) 1. D.S. Lee, J.H. Lee, J. Kim, H.W. Lee, H.S. Song, Solid State Ionics 166 (2004) 13.
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R.M.C. Clemmer, S.F. Corbin, Solid State Ionics 166 (2004) 251. S.K. Pratihar, A. Dassharma, H.S. Maiti, Mater. Res. Bull. 40 (2005) 1936. S. Mclntosh, R.J. Gorte, Chem. Rev. 104 (2004) 4845. P. Holtappels, I.C. Vinke, L.G.J. de Haart, U. Stimming, J. Electrochem. Soc. 146 (1999) 2976. L.M. Van der Haar, H. Verweij, J. Membr. Sci. 180 (2000) 147. Y.L. Zhang, S.W. Zha, M.L. Liu, Adv. Mater. 17 (2005) 487. S.F. Corbin, P.S. Apte, J. Am. Ceram. Soc. 82 (1999) 1693. F. Corbin, J. Lee, X. Qiao, J. Am. Ceram. Soc. 84 (2001) 41. M. Boaro, J.M. Vohs, R.J. Gorte, J. Am. Ceram. Soc. 86 (2003) 395. R. Craciun, S. Park, R.J. Gorte, J.M. Vohs, C. Wang, W. Worrell, J. Electrochem. Soc. 146 (1999) 4019.
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[16] M.A. Laguna-Bercero, A. Larrea, J.L. Pena, R.I. Merino, V.M. Orera, J. Eur. Ceram. Soc. 25 (2005) 1455. [17] T. Fukasawa, M. Ando, T. Ohji, S. Kanzaki, J. Am. Ceram. Soc. 84 (2001) 230. [18] G.J. Zhang, J.F. Yang, T. Ohji, J. Am. Ceram. Soc. 84 (2001) 1395. [19] K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 88 (2005) 1108. [20] Y.H. Koh, I.K. Jun, J.J. Sun, H.E. Kim, J. Am. Ceram. Soc. 89 (2006) 763. [21] E.R. Rubinstein, M.E. Glicksman, J. Cryst. Growth 112 (1991) 97. [22] E. Çadirli, N. Marasli, B. Bayender, M. Gündüz, Mater. Res. Bull. 35 (2000) 985. [23] S.P. Hiang, S.H. Chan, J. Mater. Sci. 39 (2004) 4405.
Freeze casting of porous Ni–YSZ cermets Young-Hag Koh ⁎, Jong-Jae Sun, Hyoun-Ee Kim School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Republic of Korea Received 7 March 2006; accepted 4 July 2006 Available online 24 July 2006
Abstract Dual-phase porous Ni–YSZ cermets were fabricated via the freeze casting of a ceramic/camphene slurry. After removing the frozen camphene via sublimation at room temperature, the green samples were sintered for 3 h in air at various temperatures, ranging from 1100 to 1350 °C, and then reduced in an Ar–5% H2 atmosphere at 700 °C for 3 h. The fabricated Ni–YSZ cermets showed 3-D pore channels formed by the replication of the entangled camphene dendrite network and small pores in the Ni–YSZ walls produced by partial sintering of the NiO–YSZ composite. Furthermore, the fabricated samples were found to possess reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs). © 2006 Elsevier B.V. All rights reserved. Keywords: Ni–YSZ; Cermets; Porous; Anode; Fuel cells
1. Introduction Solid oxide fuel cells (SOFCs) are energy conversion devices that convert the chemical energy of gaseous fuels directly into an electrical energy [1,2]. In principle, SOFCs are comprised of three key components, namely a fuel electrode, an air electrode and an electrolyte. Among the possible choices of anode material, Ni–yttria-stabilized zirconia (YSZ) cermets have been widely used because of the high electrical conductivity provided by the metallic Ni and the good ionic conductivity afforded by the YSZ [3]. In addition, these materials are often used as a support to reduce the thickness of the electrolyte layer, in order to lower the operating temperature [4]. The anode material should be porous, in order to allow the fuel to pass and to provide appropriate reaction sites for its anodic oxidation. The properties of such anode materials are mostly dependent on their microstructure and pore structure, as well as on the distribution of the conducting phases [5–8]. In general, the reaction is limited by the length of the so called triple-phase boundary (TPB), where the Ni, YSZ, and fuel gas are in contact with each other [9]. A greater TPB length can be ⁎ Corresponding author. Tel.: +82 2 880 9046. E-mail address: [email protected] (Y.-H. Koh). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.009
achieved by using smaller pore and particle sizes, but this interrupts the gas transport [10]. Recently, in an attempt to solve this problem, a novel structure was proposed, which contained large pores to provide rapid gas transport and small pores to allow for fast electrochemical reactions [11]. In this method, a foamy poly(methyl methacrylate) (PMMA), consisting of closed-packed PMMA spheres, is infiltrated with an electrodecontaining slurry, followed by calcination. This method produces macropores by removing the PMMA template and, at the same time, mesopores in electrode walls by heat-treating nanoparticles. To date, a variety of manufacturing techniques have been developed to fabricate porous anode materials [12–16]. Amongst these different techniques, tape casting with pore-forming fugitive phases (e.g., organic phases and graphite) is one of the most commonly used methods, owing to its simple processability [12–14]. However, it is difficult to optimize the pore structure using this method. On the other hand, freeze casting methods are known to be capable of producing porous materials with a controlled pore structure [17–20]. These methods basically make full use of the dendritic growth of ice [17,18] or camphene [19,20], with these materials being used as vehicles in ceramic powder slurries. Therefore, in this study, we fabricated dual-phase porous Ni–YSZ cermets, which had large pore channels to provide
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rapid gas transport and small pores to allow for fast electrochemical reactions, using the freeze casting of an NiO–YSZ/camphene slurry. This method enabled the Ni–YSZ cermets to have threedimensionally connected large pore channels formed as a replication of the camphene dendrite network and small pores in the Ni–YSZ walls formed by controlling the sintering temperature of the NiO–YSZ composites. The fabricated samples were characterized in terms of their microstructures and electrical conductivity. 2. Experimental procedure Commercially available nickel oxide (NiO) (Wako Pure Chemicals, Osaka, Japan) and yttria-stabilized zirconia (YSZ) doped with 8 mol% Y2O3 (TZ-8Y; Tosoh Co., Tokyo, Japan) were used as the ceramic materials. Camphene (C10H16, Alfa Aesar/Avocado Organics, Ward Hill, MA, USA) with a melting temperature of 44–48 °C was used without further purification as the vehicle for freeze casting. A 1:1 wt.% mixture of NiO and YSZ was ball-milled for 3 days using zirconia balls as the media, followed by drying and sieving, in order to ensure homogeneous mixing and reduce the particle size. It was observed that the ball-milled powders were very fine (particle size b 100 nm), which was suitable for preparing the ceramic/ camphene slurry. A 20 vol.% of the NiO–YSZ powder was ball-milled with molten camphene containing 3 wt.% of oligomeric polyester (Hypermer KD-4; UniQema, Everburg, Belgium) as a dispersant at 60 °C for 24 h using zirconia balls as the media. The warm slurry was then cast into a 20 × 20 × 5 mm mold at a constant temperature of 20 °C in order to control the solidification rate of the slurry. After 30 min, the green samples were carefully removed from the molds and kept in a hood with an air flow of ∼ 0.02 m/s at room temperature for 24 h to completely remove the camphene via sublimation. The camphene-free samples were then sintered at various temperatures ranging from 1100 to 1400 °C for 3 h in air, in order to control the porosity in the NiO–YSZ walls. Thereafter, the sintered samples were reduced in an Ar–5% H2 atmosphere for 3 h at temperatures ranging from 600 to 900 °C, to produce the dualphase porous Ni–YSZ cermets. The microstructures of the sintered and reduced samples were characterized using scanning electron microscopy (SEM, JSM-6330, JEOL Techniques, Tokyo, Japan). The crystalline phases of the samples were characterized using X-ray diffraction (XRD, MXP18A-HF, MAC Science, Tokyo, Japan). The electrical conductivities of the Ni–YSZ cermets were measured using DC four-point probe techniques (model CMT-SR 1000, Chang-Min, Seoul, Korea) at room temperature after grinding with successively smoother grades of sandpaper down to 2000 grit. In order to evaluate the porosity, pore size, and degree of interconnection, the samples were infiltrated with an epoxy resin (Spurrs epoxy, Polysciences Inc., Warrington, PA) and then cured at 70 °C for 24 h. Thereafter, the samples were ground and analyzed using SEM. The porosity was calculated from the digitally colored SEM images of the epoxy filled sample by measuring the area occupied by the epoxy.
3. Results and discussion We employed the freeze casting of the ceramic/camphene slurry to produce dual-phase porous Ni–YSZ cermets. This method exploits the fact that camphene forms dendrites when solidified under an appropriate temperature gradient or solute gradient in an alloy system [21,22]. When ceramic powders are present in the system, they are repelled by the growing camphene dendrites and become concentrated between the dendrite arms or neighboring dendrites [19,20]. Such solidification of the ceramic/camphene slurry results in the production of a 3-dimensionally interconnected camphene network, surrounded by concentrated ceramic powder walls, as shown in Fig. 1(A). The large pore channels are produced by removing the solidified camphene network, while the small pores in the NiO–YSZ walls are produced by adjusting the sintering temperature. A typical micrograph of the fabricated NiO–YSZ composites after sintering at 1250 °C for 3 h is shown in Fig. 1(B). All of the samples exhibited highly porous structures formed as a replica of the entangled dendritic growth of the frozen camphene, regardless of the sintering temperature. The measured porosity associated with the 3-D large pore channels was as high as 56%. The pores produced in the NiO–YSZ walls sintered at various temperatures, ranging from 1100 to 1300 °C, are shown in Fig. 2(A)–(D). When sintered at a relatively low temperature of 1100 °C, the sample exhibited a high degree of porosity, owing to the relatively poor densification, as shown in Fig. 2(A). The densification of the NiO–YSZ walls was notably enhanced when the sintering temperature was increased to 1200 °C and 1250 °C, while the engineered pores were still preserved, as shown in Fig. 2(B) and (C), respectively. At the higher sintering temperature of 1300 °C, the NiO–YSZ walls were almost fully dense, as shown in Fig. 2(D). These observations suggest that it is possible to introduce small pores into the sintered NiO–YSZ, without disturbing the large pore channels, by carefully controlling the sintering temperature,
Fig. 1. (A) Schematic illustration showing the solidified camphene dendrites and the concentrated NiO–YSZ powder walls and (B) typical SEM micrograph of the NiO–YSZ composite sintered at 1200 °C for 3 h in air.
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Fig. 2. SEM micrographs of the NiO–YSZ walls sintered for 3 h in air at (A) 1100 °C, (B) 1200 °C, (C) 1250 °C, and (D) 1300 °C.
which in turn allows the fraction of small pores that develop in the Ni–YSZ cermets to be tailored. It should be noted that the grain size increased with increasing sintering temperature. Approximately 17% linear shrinkage was observed after sintering. In order to produce the Ni–YSZ cermets, the porous NiO–YSZ composites were reduced in an Ar–5% H2 atmosphere at temperatures ranging from 600 to 900 °C for 3 h. The typical XRD patterns of the samples after reduction are shown in Fig. 3(A)–(D). Before reduction, the samples showed only crystalline peaks corresponding to the YSZ and NiO phases (not shown). However, after reduction at 600 °C, the sample revealed peaks corresponding to Ni, as well as NiO phases, indicating that this temperature was insufficient to completely reduce the NiO phases (Fig. 3(A)). On the other hand, as the reducing temperature was increased to 700 °C, the NiO peaks disappeared and only Ni peaks were observed along with the YSZ peaks (Fig. 3(B)). Further increase in the reducing temperature did not result in any changes in the crystalline phases (Fig 3(C) and (D)). Considering that smaller pore and particle sizes lead to a greater TPB length, the lowest reducing temperature of 700 °C was selected for the subsequent investigations. The SEM micrographs of the fabricated dual-phase porous Ni–YSZ cermets produced using the NiO–YSZ composites sintered at various temperatures ranging from 1100 to 1300 °C, are shown in Fig. 4(A) – (D). The highly porous structure of the composites, comprised of 3-D large pore channels with a size of several tens of microns, was retained in all of the Ni–YSZ cermets, while the degree of pore development in the Ni–YSZ walls was significantly increased compared to that of the NiO–YSZ walls, owing to the shrinkage caused by the reduction of NiO to Ni phase. The Ni–YSZ cermet produced using the NiO–YSZ composite sintered at the relatively low temperature of 1100 °C exhibited a very high degree of porosity in the walls (Fig. 4(A)). The degree of porosity was decreased by using the NiO–YSZ composites sintered at higher temperatures. However, the samples sintered at 1250 °C and 1300 °C still had highly porous Ni–YSZ walls with a pore size of several hundreds of nanometers, as shown in Fig. 4(C) and (D), respectively.
In order to verify the possibility of using the dual-phase porous Ni–YSZ cermets as anode substrates, their electrical conductivities were measured at room temperature using a four-point fixture. The electrical conductivity of the Ni–YSZ cermet as a function of the sintering temperature is shown in Fig. 5. The electrical conductivity of the Ni–YSZ cermet was strongly influenced by the sintering temperature, due to its effect on the porosity that developed in the Ni–YSZ walls. The Ni–YSZ cermet produced using the NiO–YSZ composite sintered at the relatively low temperature of 1100 °C showed negligible electrical conductivity. This insulating behavior was due to the insufficient percolation of the conducting Ni phases, which was associated with not only the poor coarsening of Ni, but also the presence of excess pores. On the other hand, the Ni–YSZ cermets produced using the
Fig. 3. XRD patterns of the sample sintered at 1250 °C for 3 h (A) before reduction and after reduction in a flowing Ar–5% H2 atmosphere for 3 h at (B) 600 °C, (C) 700 °C, (D) 800 °C, and (E) 900 °C. *YSZ (❍), NiO (❖), Ni ( ).
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Fig. 4. SEM micrographs of the Ni–YSZ cermets produced using the NiO–YSZ composites sintered at (A) 1100 °C, (B) 1200 °C, (C) 1250 °C, and (D) 1300 °C, after reduction in a flowing Ar–5% H2 atmosphere.
NiO–YSZ composites sintered at higher temperatures (N 1200 °C) showed reasonable electrical conductivity ranging from 6.3 to 9.5 S/cm, which would allow them to be efficiently used as an anode substrate [23]. It should be noted that the electrical conductivity of the Ni–YSZ cermet could be increased by increasing the sintering temperature of the NiO–YSZ composite and thereby reducing the porosity in the Ni–YSZ walls; however, this would inevitably decrease the TPB lengths and in turn reduce the electrochemical activity of the anode. In general, the electrochemical activity of the anode material is dependent not only on the materials' properties, such as the TPB lengths, electrical conductivity, and pore size, but also on the various operating parameters [23]. Although we did not directly measure the electrochemical activity of the dual-phase porous Ni–YSZ cermets in this study, it is believed that these cermets would provide for the easy passage of the fuel, due to the large pore channels and long TPB length, owing to the small pores formed in the Ni–YSZ walls.
4. Conclusions We fabricated dual-phase porous Ni–YSZ cermets using the freeze casting of a ceramic/camphene slurry, in which 3-D pore channels were produced by the replication of the entangled camphene dendrite network and small pores were produced in the Ni–YSZ walls by controlling the sintering temperature of the NiO–YSZ composite. The fabricated NiO–YSZ composite exhibited a highly porous structure, while the porosity in the NiO–YSZ walls was increased by lowering the sintering temperature from 1300 to 1100 °C. Dual-phase porous Ni–YSZ cermets were fabricated by reducing the NiO–YSZ composites in an Ar–5% H2 atmosphere at 700 °C for 3 h. It was observed that the fabricated Ni–YSZ cermets had 3-D pore channels with the size of several tens of microns and small pores with the size of several hundreds of nanometers in the Ni–YSZ walls, as well as reasonable electrical conductivities, which would allow them to be efficiently used as an anode substrate. Acknowledgments This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2005-003-D00129). References
Fig. 5. Electrical conductivity of the Ni–YSZ cermet produced using the NiO–YSZ composites as a function of the sintering temperature.
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