- Email: [email protected]
Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freeze-casting
Available online at www.sciencedirect.com
Scripta Materialia 60 (2009) 563–566 www.elsevier.com/locate/scriptamat
Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freeze-casting Changqing Hong,* Xinghong Zhang, Jiecai Han, Jiancong Du and Wenbo Han Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150001, China Received 27 October 2008; revised 4 December 2008; accepted 4 December 2008 Available online 25 December 2008
Ultra-high-porosity zirconia ceramics with interconnected pores were prepared by freezing zirconia/camphene slurries with solid loading ranging from 10 to 20 vol.%, followed by freeze-drying in air and subsequent heat treatment at 1500 °C for 2 h. All of the fabricated samples showed highly porous microstructures with moderate pores ranging from 20 to 50 lm in size. The porosity varied between 82% and 67%, while the compressive strength was significantly improved from 41 to 63 MPa. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Freeze-casting; Porous material; Dendritic growth; Microstructure; Mechanical properties
Materials containing tailored porosity exhibit special properties and features that usually cannot be achieved by their conventional dense counterparts. The possibilities offered by highly porous ceramics are attracting considerably more attention today than just a few years ago [1]. Ceramics with low density (high porosity) can be engineered to combine several advantages inherent to their architecture, e.g. as supports for catalysts, artificial bones, ceramic filters and lightweight parts for use at high temperature [2,3]. To meet these requirements, various manufacturing methods are adopted to produce highly porous ceramics. These methods include replication of polymer foams by ceramic dip coating, foaming of aqueous ceramic powder suspensions, pyrolysis of preceramic precursors, and firing of ceramic powder compacts with pore-forming fugitive phases [4–6]. However, it is still challenging to develop novel methods which can precisely control the pore structure (e.g. porosity, pore size and shape, interconnection between pores, and pore alignment) in a cost-effective way, to achieve improved mechanical properties and thermal performance [7,8]. Recently, the freeze-casting process has emerged as a good candidate method, as it can produce interconnected pore channels in a tailored manner, e.g. aligned pore channels on a scale of several microns, which offers superior mechanical properties and functions. This method makes full use of a three-dimensionally inter* Corresponding author. E-mail: [email protected]
connected frozen vehicle network, which sublimes and in turn leaves pore channels in the ceramic body [9,10]. In this method, the ceramic slurry is normally prepared by ball-milling and then cast into the mold at a temperature below its freezing temperature (i.e. the solidification temperature of the molten vehicle). To date, water [1,8], camphene [11,12] or tert-butyl alcohol [13] have been successfully used as freezing vehicles. Of these, camphene can be frozen and easily sublimed near room temperature, offering more flexibility in the process. Moreover, it was found that the camphene-based freeze-casting could be used to freeze very dilute ceramic slurries with low solid loadings, which, accordingly, allowed ultra-high-porosity ceramics with completely interconnected pore channels to be produced. Here, we report a beneficial use of the freeze-casting technique to fabricate high-porosity zirconia ceramics for thermal insulation application. The fabricated samples were characterized by evaluating the development of the pore structure and the mechanical properties. Commercially available yttria-stabilized zirconia doped with 5 mol.% Y2O3 (0.4 lm, Fanmeiya Powders Co. Ltd., Jiangxi, China) was used as the ceramic framework and camphene (C10H16, Guangzhou Huangpu Chemical Factory, Guangzhou, China) was used as the freezing vehicle. To produce stable ZrO2 suspensions in liquid camphene, we used an amine derivative of a fatty acid condensation polymer (Beijing Huadong Polymer Co. Ltd., Beijing, China) as a dispersant. Zirconia/ camphene slurries with various zirconia contents (10, 15 and 20 vol.%) were prepared by ball-milling at 60 oC for
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.011
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
20 h. The prepared warm slurries were poured into polyethylene (PE) molds in cool EtOH environment (about 5 °C) to produce a disc 30 mm in diameter and 5 mm thick. After casting, the slurries quickly started to solidify and, typically within about 4 min, the solidification was complete, yielding a solid green body. After demolding, these green bodies were placed in an ambient atmosphere with an airflow rate of 0.02–0.05 m s 1 to sublime the frozen camphene from the green bodies. Judging from the weight change, the sublimation process typically finished in 24 h. During this freeze-drying process, negligible shrinkage was observed. After the freeze-drying process, the cast bodies were sintered at 1500 oC (ramping rate of 2 oC min 1) for 2 h. The bulk density, open porosity and closed porosity of sintered bodies were measured using the water displacement method based on the Archimedes principle. Microstructure of the sintered bodies was observed with scanning electron microscopy (SEM; FEI Sirion, Philips, Holland). To determine the unidirectional compression strength of ordered porous zirconia sintered bulks, cylinders 12 mm in diameter and 25–30 mm high were tested (Instron-5569, UK). Camphene is a cyclic hydrocarbon and a crystalline plastic solid at room temperature. It has a melting temperature around 44–48 oC and a liquid viscosity of 1.4 mPa s at 47 oC. Since its vapor pressure of 1.3 kPa (just below the melting temperature) is high enough to allow sublimation at room temperature [14], solid camphene can be easily converted into the gaseous state. Porosity is created where the solvent camphene solvent crystals were, so that a green porous structure is obtained and the porosity is a direct replica of the solidified solvent structure. Figure 1 shows the mass loss results for ultra-lowdensity green bodies that had been dried at room
20vol% ZrO2/Camphene green body 15vol% ZrO2/Camphene green body 10vol% ZrO2/Camphene green body
0 -10 -20
Mass (%)
564
-30 -40 -50 -60 0
5
10
15
20
Drying time (hours) Figure 1. Mass loss during freeze-drying of camphene-based green bodies with solid loadings of 10, 15 and 20 vol.%.
temperature in air. It can be seen that camphene-based green bodies can be completely dried at this low temperature in a much shorter time (about 18 h), resulting in no shrinkage of green bodies with ultra-low-density. Moreover, the camphene-based green body with higher solid content (for 20 vol.% ZrO2) shows a relatively faster sublimated velocity compared with the lower solid content (for 10 vol.% ZrO2). The sintered sample showed no noticeable macroscopic defects (as shown in Fig. 2a), such as cracking or distortion. This good shape tolerance was related to the use of the highly concentrated zirconia powder walls formed during the freezing of the zirconia/C10H16 slurry. SEM observations revealed that the sintered samples preserved their highly porous structures with completely interconnected pore channels having diameters of several tens of micrometers, as shown in Figure 2b and c. During the freezing process, the zirconia powders are repelled by the growing camphene dendrites and become
Figure 2. Optical and microstructural photographs of the porous zirconia ceramics with 15 vol.% solid content. (a) Optical photograph (diameter 30 mm, height 45 mm); (b) the uniform pore distribution vertical to the solidification direction (low magnification); (c) the pore channel formed in the porous core (high magnification); (d) the ceramic walls between adjacent pores.
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
Figure 3. (a) Schematic illustrations showing freezing phenomenon of the ZrO2/camphene slurry. (b) Optical micrographs of incomplete camphene dendritic growth and (c) the regular continuous structure after freezing is completed for 10 vol.% ZrO2 slurry.
concentrated between the dendrite arms or neighboring dendrites [8,9]. Such solidification of the ceramic/ camphene slurry results in the formation of a threedimensional interconnected camphene network, surrounded by concentrated zirconia powder walls (shown in Fig. 2d). The solidification phenomenon of camphene has been investigated by many researchers, and it has been found that camphene forms dendrites when solidified under a certain temperature gradient. The dendrite morphology and growth kinetics of highly purified camphene have been discussed in detail by Rubinstein [15] and Cadirli et al. [16]. Even in the nondirectional solidification used in the present work where a polyurethane mold was used as a thermal insulator, a temperature gradient in the slurry is inevitable. Moreover, since the liquid phase in the slurry is a multicomponent system, i.e. a mixture of molten commercial (±)-camphene and a liquid dispersant, a solute gradient is likely to occur in the liquid phase during solidification [14]. Therefore, it is certain that dendrite formation of camphene occurs from the mold wall to the center of the cast body during freeze-casting. During solidification, the ceramic particles are rejected by the solid–liquid interface and pushed along ahead of the advancing interface when the interface velocity is below a certain value. The particles are not only pushed along ahead of the advancing macroscopic solidification front composed of tips of growing dendrites, but are also concentrated between dendrite arms or neighboring dendrites on the spot after being rejected by dendrite arms (shown in Fig. 3a).
565
To investigate the formation mechanism of camphene-based solution during solidification, the solidification behavior of the slurries was directly observed by optical microscopy. The warm slurry (about 60 °C) with 10 vol.% ZrO2 was prepared and then dropped onto a pre-warmed slide glass having about the same temperature as the dropped slurry, then covered rapidly by a pre-warmed cover glass, and allowed to solidify in a freeze drier (5 °C). The camphene-based slurry begins to grow until the sample ends. Incomplete growth of dendrites from outward to inward (shown in Fig. 3b) is observed, since this model experiment is similar to two-dimensional solidification, which is different from three-dimensional solidification in the actual casting bodies [14]. It is noticeable that a replica of such incomplete dendritic growth was not found in the sintered bodies. After the freezing was completed, a unique phase-separated structure was produced, in which aligned camphene dendrites with a well-defined morphology surrounded by zirconia networks were formed, as shown in Figure 3c. The microstructures of zirconia ceramics with low density were significantly influenced by the initial solid loading sintered at a given temperature, which determines the pore size, porosity and mechanical properties of the final products.Figure 4a–c shows the pore structures after sintering at 1500 °C with the solid loading varying from 10 to 20 vol.%. It is clear that lower solid loading results in higher porosity and larger pore sizes, while the sintered zirconia walls became thinner. During freezing, the camphene dendrites can grow until the force created by the particle concentration exceeds the capillary drag force
Figure 4. Microstructures of sintered (1500 °C) zirconia acquired from slurries with different solid loadings: (a) 10 vol.%, (b) 15 vol.% and (c) 20 vol.%.
566
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
Table 1. Properties of ZrO2 with different initial densities after sintering at 1500 °C for 2 h. Porosity Slurry solid Bulk density after Compression loading (vol.%) sintering (g cm 3) strength (MPa) content (%) 10 15 20
0.92–0.97 1.24–1.30 1.71–1.76
41.4 ± 1.8 55.4 ± 2.3 62.8 ± 3.1
82 75 67
pushing the particles at the solid–liquid interface [17]. Therefore, it is reasonable to suppose that a lower solid loading will lead to the formation of larger camphene dendrites and thinner concentrated zirconia powder walls. Highly porous ceramics fabricated by conventional methods often contain defects, such as cracking and surface flaws. For example, reticulated porous ceramics produced using the polymer replication method have longitudinal cracks and surface flaws on the sintered ceramics struts during pyrolysis of the polymeric sponge. Although cellular structures prepared by direct foaming usually exhibit mechanical strengths higher than that of replica techniques, due mainly to the absence of flaws in the cell struts, compressive strengths of no more than 20 MPa have been obtained with porous ceramics produced from particle-stabilized foams [18]. However, all of the prepared samples produced using the present method showed well-constructed zirconia walls without any noticeable defects, which should enable them to have good mechanical properties. The density and mechanical properties of the sintered samples are summarized in Table 1. The results show that after being sintered with a lower solid content, the sample acquired from 10 vol.% slurry was so light that bulk densities were even lower than water (1 g cm 3). As the initial solid loading was increased, the compression strength increased from 41 to 63 MPa. Generally, the strength of a porous ceramic is strongly affected not only by the porosity, but also by the formation of sintering neck on the ceramic wall as well as the smaller pore size (several tens of microns) compared to conventional processing methods (>100 lm). In the present study, the pore sizes of the prepared sampled ranged from approximately 20 to 50 lm. Moreover remarkable sintering necks were formed (shown in Fig. 2d), resulting in improved mechanical properties. With controllable microstructures and properties, the freeze-casting technique for the fabrication of ceramics with high porosity can be expanded from the laboratory to actual industrial applications. This technique is simple and low cost, and hence can easily be adapted to existing factory processing methods with low capital investment. It is also suitable for fabricating highly porous ceramics parts with special geometry, and is applicable to many other ceramic systems. In summary, highly porous zirconia with threedimensional interconnected pore channels have been
fabricated by a novel camphene-based freeze-casting technique. The porosity and mechanical properties can be controlled by adjusting the initial zirconia content used in the preparation of the zirconia/camphene slurry. As the initial zirconia content was increased from 10 to 20 vol.%, the porosity decreased from 82% to 67%, while still having highly interconnected pore structures. This reduction in porosity resulted in an improvement in the compressive strength from 41 to 63 MPa. By further pursuing this strategy, we expect further optimized processing means to be developed in the near future. This work was funded by the program of National Natural Science Funds for Distinguished Young Scholar (Nos. 10725207) and the National Natural Science Foundation of China (Nos. 50702016 and 50602010). We are also grateful for the support of the China Postdoctoral Science Foundation and the Development Program for Outstanding Young Teachers in Harbin Institute of Technology. [1] S. Deville, Adv. Eng. Mater. 10 (10) (2008) 155. [2] J.E. Gough, D.C. Clupper, L.L. Hench, J. Biomed. Mater. Res. 69A (2004) 621. [3] H.P. Yuan, J.D. de Bruijn, X.D. Zhang, C.A. van Blitterswijk, K. de Groot, J. Biomed. Mater. Res. 58 (2001) 70. [4] D.C. Clupper, J.J. Mecholsky, G.P. LaTorre, D.C. Greenspan, Biomaterials 23 (2002) 2599. [5] M. Boaro, J.M. Vohs, R.J. Gorte, J. Am. Ceram. Soc. 86 (2003) 395. [6] H. Schmidt, D. Koch, G. Grathwohl, J. Am. Ceram. Soc. 84 (2001) 2252. [7] J.-W. Moon, H.-J. Hwang, M. Awano, K. Maeda, Mater. Lett. 57 (2003) 1428. [8] S. Deville, E. Saiz, R.K. Nalla, A. Tomsia, Science 311 (2006) 15. [9] Y.H. Koh, J.J. Sun, H.E. Kim, Mater. Lett. 61 (2007) 283. [10] S. Deville, E. Saiz, A.P. Tomsia, Acta Mater. 55 (2007) 1965. [11] I.K. Jun, Y.H. Koh, J.H. Song, S.H. Lee, H.E. Kim, Mater. Lett. 60 (2006) 2507. [12] B.H. Yoon, W.Y. Choi, H.E. Kim, J.H. Kim, Y.H. Koh, Scripta Mater. 58 (2008) 537. [13] R.F. Chen, Y. Huang, C.A. Wang, J.Q. Qi, J. Am. Ceram. Soc. 90 (2007) 3424. [14] K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 88 (2005) 1108. [15] E.R. Rubinstein, M.E. Glicksman, J. Cryst. Growth 112 (1991) 97. [16] E. Cadirli, N. Marasli, B. Bayender, Mater. Res. Bull. 35 (2000) 985. [17] D.R. Uhlmann, B. Chalmers, K.A. Jackson, J. Appl. Phys. 35 (1964) 2986. [18] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, J. Am. Ceram. Soc. 89 (2006) 1771.
Scripta Materialia 60 (2009) 563–566 www.elsevier.com/locate/scriptamat
Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freeze-casting Changqing Hong,* Xinghong Zhang, Jiecai Han, Jiancong Du and Wenbo Han Center for Composite Materials and Structure, Harbin Institute of Technology, Harbin 150001, China Received 27 October 2008; revised 4 December 2008; accepted 4 December 2008 Available online 25 December 2008
Ultra-high-porosity zirconia ceramics with interconnected pores were prepared by freezing zirconia/camphene slurries with solid loading ranging from 10 to 20 vol.%, followed by freeze-drying in air and subsequent heat treatment at 1500 °C for 2 h. All of the fabricated samples showed highly porous microstructures with moderate pores ranging from 20 to 50 lm in size. The porosity varied between 82% and 67%, while the compressive strength was significantly improved from 41 to 63 MPa. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Freeze-casting; Porous material; Dendritic growth; Microstructure; Mechanical properties
Materials containing tailored porosity exhibit special properties and features that usually cannot be achieved by their conventional dense counterparts. The possibilities offered by highly porous ceramics are attracting considerably more attention today than just a few years ago [1]. Ceramics with low density (high porosity) can be engineered to combine several advantages inherent to their architecture, e.g. as supports for catalysts, artificial bones, ceramic filters and lightweight parts for use at high temperature [2,3]. To meet these requirements, various manufacturing methods are adopted to produce highly porous ceramics. These methods include replication of polymer foams by ceramic dip coating, foaming of aqueous ceramic powder suspensions, pyrolysis of preceramic precursors, and firing of ceramic powder compacts with pore-forming fugitive phases [4–6]. However, it is still challenging to develop novel methods which can precisely control the pore structure (e.g. porosity, pore size and shape, interconnection between pores, and pore alignment) in a cost-effective way, to achieve improved mechanical properties and thermal performance [7,8]. Recently, the freeze-casting process has emerged as a good candidate method, as it can produce interconnected pore channels in a tailored manner, e.g. aligned pore channels on a scale of several microns, which offers superior mechanical properties and functions. This method makes full use of a three-dimensionally inter* Corresponding author. E-mail: [email protected]
connected frozen vehicle network, which sublimes and in turn leaves pore channels in the ceramic body [9,10]. In this method, the ceramic slurry is normally prepared by ball-milling and then cast into the mold at a temperature below its freezing temperature (i.e. the solidification temperature of the molten vehicle). To date, water [1,8], camphene [11,12] or tert-butyl alcohol [13] have been successfully used as freezing vehicles. Of these, camphene can be frozen and easily sublimed near room temperature, offering more flexibility in the process. Moreover, it was found that the camphene-based freeze-casting could be used to freeze very dilute ceramic slurries with low solid loadings, which, accordingly, allowed ultra-high-porosity ceramics with completely interconnected pore channels to be produced. Here, we report a beneficial use of the freeze-casting technique to fabricate high-porosity zirconia ceramics for thermal insulation application. The fabricated samples were characterized by evaluating the development of the pore structure and the mechanical properties. Commercially available yttria-stabilized zirconia doped with 5 mol.% Y2O3 (0.4 lm, Fanmeiya Powders Co. Ltd., Jiangxi, China) was used as the ceramic framework and camphene (C10H16, Guangzhou Huangpu Chemical Factory, Guangzhou, China) was used as the freezing vehicle. To produce stable ZrO2 suspensions in liquid camphene, we used an amine derivative of a fatty acid condensation polymer (Beijing Huadong Polymer Co. Ltd., Beijing, China) as a dispersant. Zirconia/ camphene slurries with various zirconia contents (10, 15 and 20 vol.%) were prepared by ball-milling at 60 oC for
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.011
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
20 h. The prepared warm slurries were poured into polyethylene (PE) molds in cool EtOH environment (about 5 °C) to produce a disc 30 mm in diameter and 5 mm thick. After casting, the slurries quickly started to solidify and, typically within about 4 min, the solidification was complete, yielding a solid green body. After demolding, these green bodies were placed in an ambient atmosphere with an airflow rate of 0.02–0.05 m s 1 to sublime the frozen camphene from the green bodies. Judging from the weight change, the sublimation process typically finished in 24 h. During this freeze-drying process, negligible shrinkage was observed. After the freeze-drying process, the cast bodies were sintered at 1500 oC (ramping rate of 2 oC min 1) for 2 h. The bulk density, open porosity and closed porosity of sintered bodies were measured using the water displacement method based on the Archimedes principle. Microstructure of the sintered bodies was observed with scanning electron microscopy (SEM; FEI Sirion, Philips, Holland). To determine the unidirectional compression strength of ordered porous zirconia sintered bulks, cylinders 12 mm in diameter and 25–30 mm high were tested (Instron-5569, UK). Camphene is a cyclic hydrocarbon and a crystalline plastic solid at room temperature. It has a melting temperature around 44–48 oC and a liquid viscosity of 1.4 mPa s at 47 oC. Since its vapor pressure of 1.3 kPa (just below the melting temperature) is high enough to allow sublimation at room temperature [14], solid camphene can be easily converted into the gaseous state. Porosity is created where the solvent camphene solvent crystals were, so that a green porous structure is obtained and the porosity is a direct replica of the solidified solvent structure. Figure 1 shows the mass loss results for ultra-lowdensity green bodies that had been dried at room
20vol% ZrO2/Camphene green body 15vol% ZrO2/Camphene green body 10vol% ZrO2/Camphene green body
0 -10 -20
Mass (%)
564
-30 -40 -50 -60 0
5
10
15
20
Drying time (hours) Figure 1. Mass loss during freeze-drying of camphene-based green bodies with solid loadings of 10, 15 and 20 vol.%.
temperature in air. It can be seen that camphene-based green bodies can be completely dried at this low temperature in a much shorter time (about 18 h), resulting in no shrinkage of green bodies with ultra-low-density. Moreover, the camphene-based green body with higher solid content (for 20 vol.% ZrO2) shows a relatively faster sublimated velocity compared with the lower solid content (for 10 vol.% ZrO2). The sintered sample showed no noticeable macroscopic defects (as shown in Fig. 2a), such as cracking or distortion. This good shape tolerance was related to the use of the highly concentrated zirconia powder walls formed during the freezing of the zirconia/C10H16 slurry. SEM observations revealed that the sintered samples preserved their highly porous structures with completely interconnected pore channels having diameters of several tens of micrometers, as shown in Figure 2b and c. During the freezing process, the zirconia powders are repelled by the growing camphene dendrites and become
Figure 2. Optical and microstructural photographs of the porous zirconia ceramics with 15 vol.% solid content. (a) Optical photograph (diameter 30 mm, height 45 mm); (b) the uniform pore distribution vertical to the solidification direction (low magnification); (c) the pore channel formed in the porous core (high magnification); (d) the ceramic walls between adjacent pores.
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
Figure 3. (a) Schematic illustrations showing freezing phenomenon of the ZrO2/camphene slurry. (b) Optical micrographs of incomplete camphene dendritic growth and (c) the regular continuous structure after freezing is completed for 10 vol.% ZrO2 slurry.
concentrated between the dendrite arms or neighboring dendrites [8,9]. Such solidification of the ceramic/ camphene slurry results in the formation of a threedimensional interconnected camphene network, surrounded by concentrated zirconia powder walls (shown in Fig. 2d). The solidification phenomenon of camphene has been investigated by many researchers, and it has been found that camphene forms dendrites when solidified under a certain temperature gradient. The dendrite morphology and growth kinetics of highly purified camphene have been discussed in detail by Rubinstein [15] and Cadirli et al. [16]. Even in the nondirectional solidification used in the present work where a polyurethane mold was used as a thermal insulator, a temperature gradient in the slurry is inevitable. Moreover, since the liquid phase in the slurry is a multicomponent system, i.e. a mixture of molten commercial (±)-camphene and a liquid dispersant, a solute gradient is likely to occur in the liquid phase during solidification [14]. Therefore, it is certain that dendrite formation of camphene occurs from the mold wall to the center of the cast body during freeze-casting. During solidification, the ceramic particles are rejected by the solid–liquid interface and pushed along ahead of the advancing interface when the interface velocity is below a certain value. The particles are not only pushed along ahead of the advancing macroscopic solidification front composed of tips of growing dendrites, but are also concentrated between dendrite arms or neighboring dendrites on the spot after being rejected by dendrite arms (shown in Fig. 3a).
565
To investigate the formation mechanism of camphene-based solution during solidification, the solidification behavior of the slurries was directly observed by optical microscopy. The warm slurry (about 60 °C) with 10 vol.% ZrO2 was prepared and then dropped onto a pre-warmed slide glass having about the same temperature as the dropped slurry, then covered rapidly by a pre-warmed cover glass, and allowed to solidify in a freeze drier (5 °C). The camphene-based slurry begins to grow until the sample ends. Incomplete growth of dendrites from outward to inward (shown in Fig. 3b) is observed, since this model experiment is similar to two-dimensional solidification, which is different from three-dimensional solidification in the actual casting bodies [14]. It is noticeable that a replica of such incomplete dendritic growth was not found in the sintered bodies. After the freezing was completed, a unique phase-separated structure was produced, in which aligned camphene dendrites with a well-defined morphology surrounded by zirconia networks were formed, as shown in Figure 3c. The microstructures of zirconia ceramics with low density were significantly influenced by the initial solid loading sintered at a given temperature, which determines the pore size, porosity and mechanical properties of the final products.Figure 4a–c shows the pore structures after sintering at 1500 °C with the solid loading varying from 10 to 20 vol.%. It is clear that lower solid loading results in higher porosity and larger pore sizes, while the sintered zirconia walls became thinner. During freezing, the camphene dendrites can grow until the force created by the particle concentration exceeds the capillary drag force
Figure 4. Microstructures of sintered (1500 °C) zirconia acquired from slurries with different solid loadings: (a) 10 vol.%, (b) 15 vol.% and (c) 20 vol.%.
566
C. Hong et al. / Scripta Materialia 60 (2009) 563–566
Table 1. Properties of ZrO2 with different initial densities after sintering at 1500 °C for 2 h. Porosity Slurry solid Bulk density after Compression loading (vol.%) sintering (g cm 3) strength (MPa) content (%) 10 15 20
0.92–0.97 1.24–1.30 1.71–1.76
41.4 ± 1.8 55.4 ± 2.3 62.8 ± 3.1
82 75 67
pushing the particles at the solid–liquid interface [17]. Therefore, it is reasonable to suppose that a lower solid loading will lead to the formation of larger camphene dendrites and thinner concentrated zirconia powder walls. Highly porous ceramics fabricated by conventional methods often contain defects, such as cracking and surface flaws. For example, reticulated porous ceramics produced using the polymer replication method have longitudinal cracks and surface flaws on the sintered ceramics struts during pyrolysis of the polymeric sponge. Although cellular structures prepared by direct foaming usually exhibit mechanical strengths higher than that of replica techniques, due mainly to the absence of flaws in the cell struts, compressive strengths of no more than 20 MPa have been obtained with porous ceramics produced from particle-stabilized foams [18]. However, all of the prepared samples produced using the present method showed well-constructed zirconia walls without any noticeable defects, which should enable them to have good mechanical properties. The density and mechanical properties of the sintered samples are summarized in Table 1. The results show that after being sintered with a lower solid content, the sample acquired from 10 vol.% slurry was so light that bulk densities were even lower than water (1 g cm 3). As the initial solid loading was increased, the compression strength increased from 41 to 63 MPa. Generally, the strength of a porous ceramic is strongly affected not only by the porosity, but also by the formation of sintering neck on the ceramic wall as well as the smaller pore size (several tens of microns) compared to conventional processing methods (>100 lm). In the present study, the pore sizes of the prepared sampled ranged from approximately 20 to 50 lm. Moreover remarkable sintering necks were formed (shown in Fig. 2d), resulting in improved mechanical properties. With controllable microstructures and properties, the freeze-casting technique for the fabrication of ceramics with high porosity can be expanded from the laboratory to actual industrial applications. This technique is simple and low cost, and hence can easily be adapted to existing factory processing methods with low capital investment. It is also suitable for fabricating highly porous ceramics parts with special geometry, and is applicable to many other ceramic systems. In summary, highly porous zirconia with threedimensional interconnected pore channels have been
fabricated by a novel camphene-based freeze-casting technique. The porosity and mechanical properties can be controlled by adjusting the initial zirconia content used in the preparation of the zirconia/camphene slurry. As the initial zirconia content was increased from 10 to 20 vol.%, the porosity decreased from 82% to 67%, while still having highly interconnected pore structures. This reduction in porosity resulted in an improvement in the compressive strength from 41 to 63 MPa. By further pursuing this strategy, we expect further optimized processing means to be developed in the near future. This work was funded by the program of National Natural Science Funds for Distinguished Young Scholar (Nos. 10725207) and the National Natural Science Foundation of China (Nos. 50702016 and 50602010). We are also grateful for the support of the China Postdoctoral Science Foundation and the Development Program for Outstanding Young Teachers in Harbin Institute of Technology. [1] S. Deville, Adv. Eng. Mater. 10 (10) (2008) 155. [2] J.E. Gough, D.C. Clupper, L.L. Hench, J. Biomed. Mater. Res. 69A (2004) 621. [3] H.P. Yuan, J.D. de Bruijn, X.D. Zhang, C.A. van Blitterswijk, K. de Groot, J. Biomed. Mater. Res. 58 (2001) 70. [4] D.C. Clupper, J.J. Mecholsky, G.P. LaTorre, D.C. Greenspan, Biomaterials 23 (2002) 2599. [5] M. Boaro, J.M. Vohs, R.J. Gorte, J. Am. Ceram. Soc. 86 (2003) 395. [6] H. Schmidt, D. Koch, G. Grathwohl, J. Am. Ceram. Soc. 84 (2001) 2252. [7] J.-W. Moon, H.-J. Hwang, M. Awano, K. Maeda, Mater. Lett. 57 (2003) 1428. [8] S. Deville, E. Saiz, R.K. Nalla, A. Tomsia, Science 311 (2006) 15. [9] Y.H. Koh, J.J. Sun, H.E. Kim, Mater. Lett. 61 (2007) 283. [10] S. Deville, E. Saiz, A.P. Tomsia, Acta Mater. 55 (2007) 1965. [11] I.K. Jun, Y.H. Koh, J.H. Song, S.H. Lee, H.E. Kim, Mater. Lett. 60 (2006) 2507. [12] B.H. Yoon, W.Y. Choi, H.E. Kim, J.H. Kim, Y.H. Koh, Scripta Mater. 58 (2008) 537. [13] R.F. Chen, Y. Huang, C.A. Wang, J.Q. Qi, J. Am. Ceram. Soc. 90 (2007) 3424. [14] K. Araki, J.W. Halloran, J. Am. Ceram. Soc. 88 (2005) 1108. [15] E.R. Rubinstein, M.E. Glicksman, J. Cryst. Growth 112 (1991) 97. [16] E. Cadirli, N. Marasli, B. Bayender, Mater. Res. Bull. 35 (2000) 985. [17] D.R. Uhlmann, B. Chalmers, K.A. Jackson, J. Appl. Phys. 35 (1964) 2986. [18] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, J. Am. Ceram. Soc. 89 (2006) 1771.