- Email: [email protected]
Effect of cooling rate and polyvinyl alcohol on the morphology of porous hydroxyapatite ceramics
Materials and Design 31 (2010) 3090–3094
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of cooling rate and polyvinyl alcohol on the morphology of porous hydroxyapatite ceramics Kai Hui Zuo, Yuping Zeng *, Dongliang Jiang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 5 November 2007 Accepted 28 December 2009 Available online 7 January 2010
a b s t r a c t Hydroxyapatite (HAP) ceramics with unidirectional lamellar pores and ceramic walls were fabricated by the unidirectional freezing process and freeze drying method. The composition of HAP aqueous slurry and the cooling rate both affected the shape and size of pores. The size of lamellar pores in HAP ceramic was longer than that in HAP ceramic with polyvinyl alcohol (PVA) additive. When the PVA additive was added into the slurries, there were smaller dendrite structures on the ceramic walls frozen with higher cooling rate, while the lower cooling rate resulted in the smooth surfaces. However, there was no obvious distinction between the ceramics without additive fabricated with different cooling rates. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Ceramics with unidirectional pores are now expected to be used as separation filters, catalyst supports, sensor and biomaterials, etc. For such applications, it is important to control the pore morphology, orientation and porosity. Therefore, the fabrication method is the key issue. Porous ceramics with aligned pores can be produced by several methods including ceramic foaming technique [1], polymeric sponge method [2,3], gel casting [4,5], freeze drying processing [6], and so on. Among these methods, the freeze drying method is a simple and economic technology for small forming shrinkage, widely controllable porosity, and relatively good mechanical strength [6–8]. Research indicates that the slurry concentration, the cooling rate, the kind and amount of organic additive all affect the morphologies of pores and mechanical properties of ceramics fabricated by the freeze drying process [8–11]. Porous HAP ceramics, a promising material in biomedical application, have been used as bone defects, artificial bone graft material, and prosthesis revision surgery. Porous HAP ceramics fabricated by the freeze drying without using organic additive almost cannot be used in anyplace for the big lamellar pores and low strength [8,12]. By increasing the cooling rate and slurry concentration, the morphologies change and the strength increases. Deville has fabricated unidirectional porous HAP ceramics by controlling the slurry concentration and the cooling rate tuned by a cooling machine [8]. In fact, not only the cooling machine, but also the cooling molds both affect the cooling rate.
* Corresponding author. Tel.: +86 021 52415232. E-mail address: [email protected] (Y. Zeng). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.12.044
In this work, porous HAP ceramics with dissimilar morphologies were obtained by adding an organic additive: polyvinyl alcohol (PVA) into HAP slurries and changing the cooling rate using different molds at the same time. The effects of PVA and the cooling rate on the morphology of porous HAP ceramics were discussed. In addition, the phase composition and open porosity were also investigated.
2. Materials and methods HAP powder (d50 = 0.4 lm) was prepared by wet mechanochemical method with Ca(OH)2, (NH4)2HPO4 (both are analyticalgrade, Shanghai Chemical Reagent Corp., China) and deionized water. Aqueous HAP slurry with initial solid loading of 50 wt.% was prepared by mixing powder with a dispersant agent (ammonium polyacrylate, Bk Giulini, Germany) in deionized water. The slurry was ball-milled for 24 h. The initial HAP slurry was named 50H slurry. Then, prepared PVA water solution (Shanghai Chemical Reagent Corp., China) was added in the 50H slurry. The ratio of PVA to HAP powder is 2 wt.% and the concentration of PVA water solution is 12 wt.%. The mixed slurry was further stirred by magnetic force for 0.5 h. The final solid loading of the mixed slurry was changed to 46.1 wt.%, which was named as 50HP slurry. Slurry with the final solid loading of 40 wt.% was also prepared by adding 6 wt.% PVA additive, named as 50HP2 slurry. For comparison, the slurry without PVA additive was prepared. The resultant slurries were poured into silicone rubber mold and steel mold, respectively. These molds had not been frozen before using; otherwise, the temperature gradient between the upside and downside of molds, which is a key point to produce
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
Fig. 1. XRD patterns of HAP powder and sintered porous HAP ceramics.
Fig. 2. SEM micrographs of HAP ceramics: (a) 50H-Si ceramic, (b) 50H-St ceramic, (c, d) 50HP-Si ceramic, (e, f) 50HP-St ceramic.
3091
3092
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
unidirectional ice crystals, cannot be obtained. Then, molds were frozen for 4 h in a refrigerant with temperature of 18 °C. After completely solidification, samples were moved into a lyophilizer (TLG-A, Zhongke Biologic Co. Ltd., China) to dry. The drying time, temperature and degree of vacuum are 8 h, 60 °C and 4 Pa, respectively. Then, the greens were heated up to 550 °C to remove organic additives and sintered at 1300 °C for 1 h in air. The obtained porous HAP ceramics were named as 50H, 50HP and 50HP2 ceramics, respectively. Phase analysis of the sintered porous HAP ceramics was conducted by X-ray diffraction (XRD, D/max 2550 V, Rigaku, CuKa, k = 0.15406 nm). Morphologies were observed by scanning elec-
tron microscopy (SEM, JSM-6700F, JEOL, Akishima, Japan). Pore size distributions were characterized by the mercury porosimetry (Model Pore Sizer 9320, Micromeritics, Norcross, GA, USA). The open porosities of sintered HAP ceramics were measured by the Archimedes method.
3. Results and discussion Fig. 1 shows the XRD patterns of the original HAP powder, the sintered 50H and 50HP porous ceramics. The visible peaks in all samples can be indexed to HAP phase. No processing residue phase
Fig. 3. SEM micrographs of 50HP2 ceramics: (a, b) 50HP2-Si ceramic, (c, d) 50HP2-St ceramic.
Fig. 4. Pore size distribution of the 50HP ceramics: (a) 50HP-Si, (b) 50HP-St.
3093
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094 Table 1 Open porosity of HAP ceramics. Sample
50H-Si
50H-St
50HP-Si
50HP-St
50HP2-Si
50HP2-St
Open porosity (%)
19.8
41.48
60.1
49.19
62.1
48.1
is found in sintered samples. The result indicates the PVA additive has no effect on the phase composition of HAP ceramics. Fig. 2 shows the SEM micrographs of porous HAP and HAP with PVA additive ceramics fabricated with silicone rubber and steel molds. All these samples are composed of unidirectional lamellar pores and porous ceramic walls. However, 50H and 50HP ceramics have obvious different morphologies. The size of lamellar pores in 50HP ceramic is smaller than that in 50H ceramic. The surfaces of 50HP ceramic walls fabricated in silicon rubber mold (named 50HP-Si ceramic) are smooth, and the surfaces of 50HP ceramic walls fabricated in steel mold (named 50HP-St ceramic) are rough. There are smaller dendrite structures on the surfaces of 50HP-St ceramic walls. But, there are no obvious distinctions between the 50H ceramics fabricated in the two molds (named 50H-Si and 50H-St ceramics, respectively). It is known that the ice crystals easily grow along the temperature gradient [6]. During the unidirectional freezing process, the ice crystals grow from the upside of slurry to the downside and form unidirectional lamellar ice crystals. Simultaneously, HAP particles expelled from the ice form the particles walls, which block the longitudinal and transverse growth of ice crystals. The nature of the resistance is the opposite force of particles agglomeration. When adding PVA in HAP slurries, the PVA solution gelatinizes before the ice crystals nucleate and grow [13]. Thus, phase separations will occur and there are two phases: one is ice and another is the powder wall contained by the gelled PVA. The resistance of the gelled PVA and HAP particles to the ice growth is bigger than that of pure HAP particles walls, thus the size of lamellar pores in 50HP ceramic is smaller than that of 50H ceramic after sublimation and sintering. For the higher thermal conductivity coefficient of the steel than that of silicon rubber, the slurry in the steel mold has higher cooling rate. Thus, PVA in the steel mold will gelatinize incompletely before the ice crystals grow. So the formed particle walls contained by the PVA gel in the steel mold have more water. After forming the primal lamellar ices and particle walls, the water in the particle walls will be expelled and form the secondary ice crystals on the particle walls. This results in the rough surfaces of 50HP-St ceramic wall. When the PVA additive has not been added in the HAP slurry, there only exist the primal lamellar ices and particle walls, so the surfaces are both smooth using the two molds. The rough surfaces will also occur if changing the amount of PVA additive. Fig. 3 shows the SEM micrographs of 50HP2 ceramics. The surfaces of 50HP2-Si ceramic walls are smooth and those of 50HP2-St ceramic walls are rough, which are caused by the same reason referred above. Fig. 4 shows the pore size distributions of the 50HP ceramics fabricated with silicone rubber and steel molds. The total porosities of 50HP-Si and 50HP-St ceramics are 60.5% and 51.03%. The 50HP-Si ceramic has a bimodal pore size distribution, while the cure of 50HP-St ceramic shows the multimodal distribution. In the cure with the bimodal distribution, the former peak (pore diameter is about 5 lm) corresponds to the pores in the ceramic walls and the latter peak (pore diameter is about 17 lm) to the lamellar pores. The peaks 1 and 2 in the cure of 50HP-St, also correspond to the pores in the ceramic walls and the lamellar pores, respectively. The peaks in the range of 3 (Fig. 4b) probably correspond to the small lamellar pores on the ceramic wall surfaces (Fig. 2f). These results are in good agreement with the SEM pictures.
Table 1 shows the open porosities of HAP ceramics. The cooling rate and composition of ceramic slurry both affect the process of ice growth. The porosities of 50HP ceramics are higher than those of 50H ceramic, which may be attributed to two reasons. One is that the solid loading of 50HP slurry is lower than that of 50H slurry. Another is the burned out PVA increases the volume of micro pores. For the higher cooling rate, there are more and smaller ice crystals in 50H-St ceramics. So the porosity of 50H-St is higher than that of 50H-Si. However, the opposite results happen in 50HP ceramics. The reason is that there is more non-chilled water in formed particle walls contained by the PVA gel. The porosity of 50HP2-St is also lower that of 50HP2-Si. Results also indicate that the porosity of 50HP2-Si is higher 50HP-Si and that of 50HP2-St is lower. Commonly, more PVA cause higher porosity, but in steel mold, more PVA means that there is more non-chilled water after solidification. 4. Conclusions HAP ceramics composed of unidirectional lamellar pores and porous ceramic walls are fabricated by the unidirectional freezing and freeze drying method. The PVA additive in the HAP slurries and cooling rate both affect the microstructures of porous ceramics. The PVA additive decreases the size of lamellar pores and increases the porosity of HAP ceramics. The surfaces of 50HP-Si ceramic walls are smooth, and there are smaller dendritic ceramic structures on the surface of 50HP-St ceramic walls. But there is no obvious distinction between the 50H ceramics fabricated with different cooling rates. Using the cooling molds to control the cooling rate is a simple and economic route. Controlling the cooling rate combining adding PVA addition, porous HAP ceramics with different morphologies can be fabricated, which can be considered as medicine carrier and basal body of HAP ceramics in order to improve the strength. Acknowledgements The authors thank for the financial support from the National Natural Science Foundation of China (Project No. 50902140) and the support of Innovative Foundation of Shanghai Institute of Ceramics, Chinese Academy of Science. The comments of the reviewers are appreciated. References [1] Woyansky JS, Scott CE, Minnear WP. Processing of porous ceramics. Am Ceram Soc Bull 1992;71:1674–82. [2] Tampieri A, Celotti G, Sprio S, Delcogliano A, Franzese S. Porosity-graded hydroxyapatite ceramics to replace natural bone. Biomaterials 2001;22:1365–70. [3] Ramay HR, Zhang M. Preparation of porous hydroxyapatite scaffolds by combination of the gel-casting and polymer sponge methods. Biomaterials 2003;24:3293–302. [4] Sepulveda P, Binner JGP. Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers. J Eur Ceram Soc 1999;19:2059–66. [5] Sepulveda P, Binner JGP, Rogero SO, Higa OZ, Bressiani JC. Production of porous hydroxyapatite by the gel-casting of foams and cytotoxic evaluation. J Biomed Mater Res Part A 2000;50:27–34. [6] Deville S, Saiz E, Nalla RK, Tomsia AP. Freezing as a path to build complex composites. Science 2006;311:515–8. [7] Han J, Hu L, Zhang YM, Zhou YF. Fabrication of ceramics with complex porous structures by the impregnate–freeze-casting process. J Am Ceram Soc 2009:1–4.
3094
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
[8] Deville S, Saiz E, Tomsia AP. Freeze casting of hydroxyapatite scaffolds for bone tissue engineering. Biomaterials 2006;27:5480–9. [9] Fukasawa T, Ando M. Synthesis of porous ceramics with complex pore structure by freeze-dry processing. J Am Ceram Soc 2001;84:230–2. [10] Fukasawa T, Deng ZY, Ando M. Synthesis of porous silicon nitride with unidirectionally aligned channels using freeze-drying process. J Am Ceram Soc 2002;85:2151–5. [11] Zuo KH, Zeng YP, Jiang DL. Properties of microstructure controllable porous YSZ ceramics fabricated by freeze casting. Inter J Appl Ceram Tech 2008;5:198–203.
[12] Lee J, Koh YH, Yoon BH, Kim HE, Kim HW. Highly porous hydroxyapatite bioceramics with interconnected pore channels using camphene-based freeze casting. Mater Lett 2007;61:2270–3. [13] Zuo KH, Zeng YP, Jiang DL. Effect of polyvinyl alcohol additive on the pore structure and morphology of the freeze-cast hydroxyapatite ceramics. Mater Sci Eng C 2010;30:283–7.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effect of cooling rate and polyvinyl alcohol on the morphology of porous hydroxyapatite ceramics Kai Hui Zuo, Yuping Zeng *, Dongliang Jiang Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 5 November 2007 Accepted 28 December 2009 Available online 7 January 2010
a b s t r a c t Hydroxyapatite (HAP) ceramics with unidirectional lamellar pores and ceramic walls were fabricated by the unidirectional freezing process and freeze drying method. The composition of HAP aqueous slurry and the cooling rate both affected the shape and size of pores. The size of lamellar pores in HAP ceramic was longer than that in HAP ceramic with polyvinyl alcohol (PVA) additive. When the PVA additive was added into the slurries, there were smaller dendrite structures on the ceramic walls frozen with higher cooling rate, while the lower cooling rate resulted in the smooth surfaces. However, there was no obvious distinction between the ceramics without additive fabricated with different cooling rates. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Ceramics with unidirectional pores are now expected to be used as separation filters, catalyst supports, sensor and biomaterials, etc. For such applications, it is important to control the pore morphology, orientation and porosity. Therefore, the fabrication method is the key issue. Porous ceramics with aligned pores can be produced by several methods including ceramic foaming technique [1], polymeric sponge method [2,3], gel casting [4,5], freeze drying processing [6], and so on. Among these methods, the freeze drying method is a simple and economic technology for small forming shrinkage, widely controllable porosity, and relatively good mechanical strength [6–8]. Research indicates that the slurry concentration, the cooling rate, the kind and amount of organic additive all affect the morphologies of pores and mechanical properties of ceramics fabricated by the freeze drying process [8–11]. Porous HAP ceramics, a promising material in biomedical application, have been used as bone defects, artificial bone graft material, and prosthesis revision surgery. Porous HAP ceramics fabricated by the freeze drying without using organic additive almost cannot be used in anyplace for the big lamellar pores and low strength [8,12]. By increasing the cooling rate and slurry concentration, the morphologies change and the strength increases. Deville has fabricated unidirectional porous HAP ceramics by controlling the slurry concentration and the cooling rate tuned by a cooling machine [8]. In fact, not only the cooling machine, but also the cooling molds both affect the cooling rate.
* Corresponding author. Tel.: +86 021 52415232. E-mail address: [email protected] (Y. Zeng). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.12.044
In this work, porous HAP ceramics with dissimilar morphologies were obtained by adding an organic additive: polyvinyl alcohol (PVA) into HAP slurries and changing the cooling rate using different molds at the same time. The effects of PVA and the cooling rate on the morphology of porous HAP ceramics were discussed. In addition, the phase composition and open porosity were also investigated.
2. Materials and methods HAP powder (d50 = 0.4 lm) was prepared by wet mechanochemical method with Ca(OH)2, (NH4)2HPO4 (both are analyticalgrade, Shanghai Chemical Reagent Corp., China) and deionized water. Aqueous HAP slurry with initial solid loading of 50 wt.% was prepared by mixing powder with a dispersant agent (ammonium polyacrylate, Bk Giulini, Germany) in deionized water. The slurry was ball-milled for 24 h. The initial HAP slurry was named 50H slurry. Then, prepared PVA water solution (Shanghai Chemical Reagent Corp., China) was added in the 50H slurry. The ratio of PVA to HAP powder is 2 wt.% and the concentration of PVA water solution is 12 wt.%. The mixed slurry was further stirred by magnetic force for 0.5 h. The final solid loading of the mixed slurry was changed to 46.1 wt.%, which was named as 50HP slurry. Slurry with the final solid loading of 40 wt.% was also prepared by adding 6 wt.% PVA additive, named as 50HP2 slurry. For comparison, the slurry without PVA additive was prepared. The resultant slurries were poured into silicone rubber mold and steel mold, respectively. These molds had not been frozen before using; otherwise, the temperature gradient between the upside and downside of molds, which is a key point to produce
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
Fig. 1. XRD patterns of HAP powder and sintered porous HAP ceramics.
Fig. 2. SEM micrographs of HAP ceramics: (a) 50H-Si ceramic, (b) 50H-St ceramic, (c, d) 50HP-Si ceramic, (e, f) 50HP-St ceramic.
3091
3092
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
unidirectional ice crystals, cannot be obtained. Then, molds were frozen for 4 h in a refrigerant with temperature of 18 °C. After completely solidification, samples were moved into a lyophilizer (TLG-A, Zhongke Biologic Co. Ltd., China) to dry. The drying time, temperature and degree of vacuum are 8 h, 60 °C and 4 Pa, respectively. Then, the greens were heated up to 550 °C to remove organic additives and sintered at 1300 °C for 1 h in air. The obtained porous HAP ceramics were named as 50H, 50HP and 50HP2 ceramics, respectively. Phase analysis of the sintered porous HAP ceramics was conducted by X-ray diffraction (XRD, D/max 2550 V, Rigaku, CuKa, k = 0.15406 nm). Morphologies were observed by scanning elec-
tron microscopy (SEM, JSM-6700F, JEOL, Akishima, Japan). Pore size distributions were characterized by the mercury porosimetry (Model Pore Sizer 9320, Micromeritics, Norcross, GA, USA). The open porosities of sintered HAP ceramics were measured by the Archimedes method.
3. Results and discussion Fig. 1 shows the XRD patterns of the original HAP powder, the sintered 50H and 50HP porous ceramics. The visible peaks in all samples can be indexed to HAP phase. No processing residue phase
Fig. 3. SEM micrographs of 50HP2 ceramics: (a, b) 50HP2-Si ceramic, (c, d) 50HP2-St ceramic.
Fig. 4. Pore size distribution of the 50HP ceramics: (a) 50HP-Si, (b) 50HP-St.
3093
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094 Table 1 Open porosity of HAP ceramics. Sample
50H-Si
50H-St
50HP-Si
50HP-St
50HP2-Si
50HP2-St
Open porosity (%)
19.8
41.48
60.1
49.19
62.1
48.1
is found in sintered samples. The result indicates the PVA additive has no effect on the phase composition of HAP ceramics. Fig. 2 shows the SEM micrographs of porous HAP and HAP with PVA additive ceramics fabricated with silicone rubber and steel molds. All these samples are composed of unidirectional lamellar pores and porous ceramic walls. However, 50H and 50HP ceramics have obvious different morphologies. The size of lamellar pores in 50HP ceramic is smaller than that in 50H ceramic. The surfaces of 50HP ceramic walls fabricated in silicon rubber mold (named 50HP-Si ceramic) are smooth, and the surfaces of 50HP ceramic walls fabricated in steel mold (named 50HP-St ceramic) are rough. There are smaller dendrite structures on the surfaces of 50HP-St ceramic walls. But, there are no obvious distinctions between the 50H ceramics fabricated in the two molds (named 50H-Si and 50H-St ceramics, respectively). It is known that the ice crystals easily grow along the temperature gradient [6]. During the unidirectional freezing process, the ice crystals grow from the upside of slurry to the downside and form unidirectional lamellar ice crystals. Simultaneously, HAP particles expelled from the ice form the particles walls, which block the longitudinal and transverse growth of ice crystals. The nature of the resistance is the opposite force of particles agglomeration. When adding PVA in HAP slurries, the PVA solution gelatinizes before the ice crystals nucleate and grow [13]. Thus, phase separations will occur and there are two phases: one is ice and another is the powder wall contained by the gelled PVA. The resistance of the gelled PVA and HAP particles to the ice growth is bigger than that of pure HAP particles walls, thus the size of lamellar pores in 50HP ceramic is smaller than that of 50H ceramic after sublimation and sintering. For the higher thermal conductivity coefficient of the steel than that of silicon rubber, the slurry in the steel mold has higher cooling rate. Thus, PVA in the steel mold will gelatinize incompletely before the ice crystals grow. So the formed particle walls contained by the PVA gel in the steel mold have more water. After forming the primal lamellar ices and particle walls, the water in the particle walls will be expelled and form the secondary ice crystals on the particle walls. This results in the rough surfaces of 50HP-St ceramic wall. When the PVA additive has not been added in the HAP slurry, there only exist the primal lamellar ices and particle walls, so the surfaces are both smooth using the two molds. The rough surfaces will also occur if changing the amount of PVA additive. Fig. 3 shows the SEM micrographs of 50HP2 ceramics. The surfaces of 50HP2-Si ceramic walls are smooth and those of 50HP2-St ceramic walls are rough, which are caused by the same reason referred above. Fig. 4 shows the pore size distributions of the 50HP ceramics fabricated with silicone rubber and steel molds. The total porosities of 50HP-Si and 50HP-St ceramics are 60.5% and 51.03%. The 50HP-Si ceramic has a bimodal pore size distribution, while the cure of 50HP-St ceramic shows the multimodal distribution. In the cure with the bimodal distribution, the former peak (pore diameter is about 5 lm) corresponds to the pores in the ceramic walls and the latter peak (pore diameter is about 17 lm) to the lamellar pores. The peaks 1 and 2 in the cure of 50HP-St, also correspond to the pores in the ceramic walls and the lamellar pores, respectively. The peaks in the range of 3 (Fig. 4b) probably correspond to the small lamellar pores on the ceramic wall surfaces (Fig. 2f). These results are in good agreement with the SEM pictures.
Table 1 shows the open porosities of HAP ceramics. The cooling rate and composition of ceramic slurry both affect the process of ice growth. The porosities of 50HP ceramics are higher than those of 50H ceramic, which may be attributed to two reasons. One is that the solid loading of 50HP slurry is lower than that of 50H slurry. Another is the burned out PVA increases the volume of micro pores. For the higher cooling rate, there are more and smaller ice crystals in 50H-St ceramics. So the porosity of 50H-St is higher than that of 50H-Si. However, the opposite results happen in 50HP ceramics. The reason is that there is more non-chilled water in formed particle walls contained by the PVA gel. The porosity of 50HP2-St is also lower that of 50HP2-Si. Results also indicate that the porosity of 50HP2-Si is higher 50HP-Si and that of 50HP2-St is lower. Commonly, more PVA cause higher porosity, but in steel mold, more PVA means that there is more non-chilled water after solidification. 4. Conclusions HAP ceramics composed of unidirectional lamellar pores and porous ceramic walls are fabricated by the unidirectional freezing and freeze drying method. The PVA additive in the HAP slurries and cooling rate both affect the microstructures of porous ceramics. The PVA additive decreases the size of lamellar pores and increases the porosity of HAP ceramics. The surfaces of 50HP-Si ceramic walls are smooth, and there are smaller dendritic ceramic structures on the surface of 50HP-St ceramic walls. But there is no obvious distinction between the 50H ceramics fabricated with different cooling rates. Using the cooling molds to control the cooling rate is a simple and economic route. Controlling the cooling rate combining adding PVA addition, porous HAP ceramics with different morphologies can be fabricated, which can be considered as medicine carrier and basal body of HAP ceramics in order to improve the strength. Acknowledgements The authors thank for the financial support from the National Natural Science Foundation of China (Project No. 50902140) and the support of Innovative Foundation of Shanghai Institute of Ceramics, Chinese Academy of Science. The comments of the reviewers are appreciated. References [1] Woyansky JS, Scott CE, Minnear WP. Processing of porous ceramics. Am Ceram Soc Bull 1992;71:1674–82. [2] Tampieri A, Celotti G, Sprio S, Delcogliano A, Franzese S. Porosity-graded hydroxyapatite ceramics to replace natural bone. Biomaterials 2001;22:1365–70. [3] Ramay HR, Zhang M. Preparation of porous hydroxyapatite scaffolds by combination of the gel-casting and polymer sponge methods. Biomaterials 2003;24:3293–302. [4] Sepulveda P, Binner JGP. Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers. J Eur Ceram Soc 1999;19:2059–66. [5] Sepulveda P, Binner JGP, Rogero SO, Higa OZ, Bressiani JC. Production of porous hydroxyapatite by the gel-casting of foams and cytotoxic evaluation. J Biomed Mater Res Part A 2000;50:27–34. [6] Deville S, Saiz E, Nalla RK, Tomsia AP. Freezing as a path to build complex composites. Science 2006;311:515–8. [7] Han J, Hu L, Zhang YM, Zhou YF. Fabrication of ceramics with complex porous structures by the impregnate–freeze-casting process. J Am Ceram Soc 2009:1–4.
3094
K.H. Zuo et al. / Materials and Design 31 (2010) 3090–3094
[8] Deville S, Saiz E, Tomsia AP. Freeze casting of hydroxyapatite scaffolds for bone tissue engineering. Biomaterials 2006;27:5480–9. [9] Fukasawa T, Ando M. Synthesis of porous ceramics with complex pore structure by freeze-dry processing. J Am Ceram Soc 2001;84:230–2. [10] Fukasawa T, Deng ZY, Ando M. Synthesis of porous silicon nitride with unidirectionally aligned channels using freeze-drying process. J Am Ceram Soc 2002;85:2151–5. [11] Zuo KH, Zeng YP, Jiang DL. Properties of microstructure controllable porous YSZ ceramics fabricated by freeze casting. Inter J Appl Ceram Tech 2008;5:198–203.
[12] Lee J, Koh YH, Yoon BH, Kim HE, Kim HW. Highly porous hydroxyapatite bioceramics with interconnected pore channels using camphene-based freeze casting. Mater Lett 2007;61:2270–3. [13] Zuo KH, Zeng YP, Jiang DL. Effect of polyvinyl alcohol additive on the pore structure and morphology of the freeze-cast hydroxyapatite ceramics. Mater Sci Eng C 2010;30:283–7.