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Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting
Author’s Accepted Manuscript Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting Zhaoping Hou, Biao Zhang, Rui Zhang, Liangliang Liu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)30602-8 http://dx.doi.org/10.1016/j.ceramint.2017.04.012 CERI14992
To appear in: Ceramics International Received date: 6 February 2017 Accepted date: 3 April 2017 Cite this article as: Zhaoping Hou, Biao Zhang, Rui Zhang and Liangliang Liu, Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting Zhaoping Houa, *, Biao Zhangb, Rui Zhanga, Liangliang Liua a
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
Abstract
Porous mullite microspheres with a highly open porosity and average diameter of more than 800 µm were fabricated via an oil-drop molding method accompanied by a freeze casting process. After sintering, a highly porous structure was formed due to interlocking whisker-shaped mullite grains and formation of interconnected skeletons during the freeze-casting process. Additionally, it was found that a high porosity and large pore size in the microspheres green bodies are favorable for the synthesis of mullite whiskers with high aspect ratio.
Keywords: Porous microspheres; Mullite; Porous structure; Freeze casting
1.Introduction
Porous ceramic microspheres are widely used in a broad range of applications, such as catalyst supports, drug delivery, and sorbent materials, owing to their attractive
*
Corresponding author, Tel.:+86351 6111121 E-mail addresses: [email protected], [email protected] 1
properties, such as low density, high specific surface areas, low thermal conductivity, and excellent resistance against thermal shock and chemical attack [1-4].
All porous microsphere applications are sensitive to the functional composite structure and properties. When applied as catalyst supports in fluidized bed reactions, the fluidity and mechanical attrition resistance are strongly influenced by their spherical shape and structure [5]. As absorbents in water purification, high open porosities, tunable pore size distributions, and high specific surface areas are required for the extraction process [4]. Furthermore, interconnected channels are essential for the supply of nutrients when using porous microspheres for cell delivery [6].
To obtain porous ceramic microspheres with a tailorable porosity and tunable pore size and structure, several routes have been developed, including ice-templated spray drying [7], freeze-photocuring-casting [8], emulsification technique [9], and the droplet extrusion method combined with ionotropic gelation [10]. In the above methods, micropores are generated by introducing different porogens, such as ice [7,8,11], PE powder [12], camphene [13], polymer [10], or carbon powder [14]. The ice-templated assisted method is an especially convenient and efficient method to manufacture various porous structures in ceramic materials by varying the rate of freezing, suspension viscosity and solid loading [15]. However, it is also of significant interest to explore the effects of grains with an anisotropic morphology on the 2
microstructure of ice-templated porous ceramics [16]. Tailoring the composition and grain morphology of the pore walls can not only enhance the mechanical properties of porous ceramics but also lead to functional composite structures that are important for further applications.
Recently, porous mullite ceramic has attracted extensive attentions and has been applied to many fields, such as separation, catalysis, adsorption, and drug delivery [17,18]. In addition, the growth of mullite crystal paralleling to the c-axis is faster than in other directions, which is favorable for the formation of mullite whiskers [19]. Whisker-shaped mullite grains will fill up pores formed in the freeze-casting process and then divide them; thus, the pore size will decrease. Meanwhile, porous ceramics with smaller pore sizes have higher surface areas.
In this study, we present an alternative, versatile route for the fabrication of customizable porous mullite microspheres with a high porosity and interconnected pore structure based on freeze casting and the extrusion dripping method. In addition, the effects of solid loading on the grain morphology and porous structure is also investigated.
2. Experimental procedure
Kaolin clay (Xinzhou, Shanxi Province, China) and Al2O3 (Guangcheng Chemical 3
Reagent Ltd., Tianjing, China) were used as the starting materials to fabricate porous mullite ceramics microspheres. AlF3 (Guangfu Fine Chemical Reagent Ltd., Tianjin, China) was used as the crystallization catalyst. MoO3 (99.5% purity, Kermel Chemical Reagent Ltd., Tianjing, China) was used as the sintering aid.
According to our previous work [20], the mullite precursors were mixed in ethanol for 12 h with Al2O3 balls as the mixing media. After drying, suspensions were prepared by adding the mixed ceramic powders, 1 wt% binder (PVA, Shanghai chongjing Co. Ltd. China), and 0.5 wt% dispersant (PAA, Kelong Chemical Inc. China) in deionized water. Suspensions with different solid loadings, 10 vol%, 20 vol%, and 30 vol%, were prepared, followed by ball milling for 24 h to homogenize the slurries.
Fig. 1 shows a schematic diagram of the procedures for preparing porous mullite microspheres. The slurry was dropped using a syringe (inner needle diameter of 0.55 mm) into a corn oil-kerosene mixture at -25 ℃ where the microspherical droplets freeze. The frozen microspheres were separated from the oil and washed repeatedly with Ethyl acetate at -35 ℃until the oil residue was removed. The resulting microspheres were freeze-dried for 24 h and then sintered in a furnace at 1400 ℃ for 2 h with a heating rate of 5 ℃/min.
X-ray diffraction (XRD, Philips Co. Ltd, Holland) was applied to detect the phase composition of the sintered microspheres. Open porosity was estimated by 4
weighting the dried microspheres and wet microspheres immersed in distilled water. The porosity and pore size distribution were measured by mercury intrusion porosimetry (Quantachrome instruments, America). The surface morphology of the sintered microspheres was characterized by a field emission scanning electron microscope (FESEM, S-4800).
3.Results and discussion
Porous mullite microspheres produced by extrusion dripping, freezing, washing, and freeze drying, followed by sintering at 1400 ℃ were uniform in size and less than 800 μm in diameter (Fig. 1(d)). The XRD patterns of samples with different solid loading contents (Fig. 2) after sintering at 1400 ℃ for 2 h revealed that pure mullites were formed from kaolin and Al2O3 in the presence of MoO3 as a sintering aid, which is consistent with our previous work [20].
SEMs of the porous mullite microspheres with different solid loading contents show that they had a perfect spherical shape (Fig. 3(a-c)) due to surface tension where drops of the mullite precursor suspensions dropped into the cool oil [21]. Hence, the droplet regained its spherical shape prior to the freezing process.
Higher magnification images of the surfaces of the microspheres are presented in Fig. 3(d-f). The image of the microsphere with 10 vol% solid loading (Fig. 3(d)) shows 5
orientated pore walls with a thickness of 2-3 μm, where mullite whiskers with a high aspect ratio grow from the walls to the pore channels. The random lapping of the mullite whiskers protruding from the internal walls of the aligned pores lead to the disappearance of micro-pores, either partially or completely. Microsphere with a 20 % solid loading content shows many large elliptic pores (Fig. 3(e)). When the solid loading content was increased to 30 vol% (Fig. 3(f)), only fine pores with a rigid framework were formed through the bonding of the smooth whiskers.
In general, freezing occurs when the suspension droplet falls into cool corn oil. The droplet is frozen, which induces its separation into two phases, the ice and particle phases. These pores were created by the sublimation of ice from the frozen droplet during the freeze-drying stage. The pore structures varied with the solid loading content. During the sintering process, the green pore structure was remodeled by the growth of mullite whiskers during the sintering stage. Additionally, it should be noted that a higher aspect ratio and lower grain diameter of the mullite whiskers were obtained in the sample with the lower solid loading content (Fig. 4), which might be due to its higher porosity and larger pores in the green body. These pores are favorable for gas diffusion and whisker synthesis because the synthesis process is a gas-solid reaction with the aid of the volatilization of AlF3 [22]. On the other hand, these pores restrict steric hindrance and provide an unconstrained environment for the growth of mullite grains. Otherwise, mullite whiskers can easily impinge and coalesce into each other [23]. 6
The above results demonstrate that the formation of pore structures of porous mullite microspheres resulted from an interlocking mullite whisker framework and interconnected skeletons created by the replica of ice.
The open porosities of porous mullite microspheres with solid loading contents of 10 vol%, 20 vol%, 30 vol% were 72.35 %, 66.73 %, 58.52 %, respectively. Overall, the products were highly porous, and their porosity depended on the initial solid loading content of the slurry in the formulation. The same relationship was observed in the production of porous ceramics by freeze-casting [15].
The pore size distributions of sintered porous mullite microspheres are shown in Fig. 5. A distinct multimodel pore size distribution was found for all porous mullite microspheres. Macropores in the range of 0.3-11 μm were found. The first maximum appeared at ≈6 μm and decreased with the solid loading content. For samples with 30 vol%, the peak disappeared. The second peak (0.5-2 μm) shifted toward lower values with increasing solid loading content. Since these pores were created by the sublimation of ice, the size of these pores decreased as the solid loading increased. However, the distinction of pore sizes for samples with different solid loadings was not obvious. This was attributed to the formation of mullite whiskers protruding from the walls, dividing the green pore into smaller pores. Mesopores (6 nm) were observed in addition to macropores. Their pore size may be due to spaces of the 7
whiskers in the framework. The pore size distribution was confirmed by SEM observation.
4.Conclusion
The oil-drop molding method accompanied by a freeze casting process was demonstrated to be feasible for preparing highly porous mullite microspheres with tailorable porosity and an average diameter of 800 μm. The open porosity of the microspheres decreased from 72.35 % to 58.52 % with the increase of the solid loading content from 10 vol% to 30 vol%. Macropores (in the range 0.3-11 μm) and mesopores (6 nm) were observed in the samples. The formation of a multimodal pore structure resulted from the sublimation of ice and the spaces of the whiskers in the framework. The obtained microspheres can be further adapted to their extensive applications by additional functionalization steps.
References
[1] H. Yang, Y. Xie, G. Hao, W. Cai, X. Guo, Preparation of porous alumina microspheres via an oil-in-water emulsion method accompanied by a sol–gel process, New J. Chem. 40 (2016) 589-595. [2] U. Hess, G. Mikolajczyk, L. Treccani, P. Streckbein, C. Heiss, S. Odenbach, K. Rezwan, Multi-loaded ceramic beads/matrix scaffolds obtained by combining 8
ionotropic and freeze gelation for sustained and tuneable vancomycin release, Mater. Sci. Eng., C 67 (2016) 542-553. [3] R. S. Byrne, P. B. Deasy, Use of porous aluminosilicate pellets for drug delivery, J. Microencapsulation 22 (2005) 423-437. [4] T. C. Schumacher, T. Y. Klein, L. Treccani, K. Rezwan, Rapid Sintering of Porous Monoliths Assembled from Microbeads with High Specific Surface Area and Multimodal Porosity, Adv. Eng. Mater. 16 (2014) 151-155. [5] J. Pype, B. Michielsen, E. M. Seftel, S. Mullens, V. Meynen, Development of alumina microspheres with controlled size and shape by vibrational droplet coagulation, J. Eur. Ceram. Soc. 37 (2017) 189-198. [6] R. Li, K. Chen, G. Li, S. Yu, J. Yao, Y. Cai, Structure design and fabrication of porous hydroxyapatite microspheres for cell delivery, J. Mol. Struct. 1120 (2016) 34-41. [7] M. Yu, K. Zhou, Y. Zhang, D. Zhang, Porous Al2O3 microspheres prepared by a novel ice-templated spray drying technique, Ceram. Int. 40 (2014) 1215-1219. [8] Y.
Hazan,
Porous
ceramics,
ceramic/polymer,
and
metal ‐ doped
ceramic/polymer nanocomposites via freeze casting of photo‐curable colloidal fluids, J. Am. Ceram. Soc. 95(2012) 177-187. [9] C. Takai, T. Hotta, S. Shiozaki, S. Matsumoto, T. Fukui, Key techniques to control porous microsphere morphology in S/O/W emulsion system, Colloids Surf., A 373 (2011) 152-157. [10] C. C. Ribeiro, C. C. Barrias, M. A. Barbosa, Preparation and characterization of calcium-phosphate porous microspheres with a uniform size for biomedical 9
applications, J. Mater. Sci. - Mater. Med. 17 (2006) 455-463. [11] M. H. Hong, J. S. Son, K. M. Kim, M. Han, D. S. Oh, Y. K. Lee, Drug-loaded porous spherical hydroxyapatite granules for bone regeneration, J. Mater. Sci. - Mater. Med. 22 (2011) 349-355. [12] Y. C. Fu, M. L. Ho, S. C. Wu, H. S. Hsieh, C. K. Wang, Porous bioceramic bead prepared by calcium phosphate with sodium alginate gel and PE powder, Mater. Sci. Eng., C 28 (2008) 1149-1158. [13] U. S. Shin, J. H. Park, S. J. Hong, J. E. Won, H. S. Yu, H. W. Kim, Porous biomedical composite microspheres developed for cell delivering scaffold in bone regeneration, Mater. Lett. 64 (2010) 2261-2264. [14] C. Wu, H. Zreiqat, Porous bioactive diopside (CaMgSi2O6) ceramic microspheres for drug delivery, Acta Biomater. 6 (2010) 820-829. [15] S. Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (2008) 155-169. [16] D. Ghosh, M. Banda, H. Kang, N. Dhavale, Platelets-induced stiffening and strengthening of ice-templated highly porous alumina scaffolds, Scripta Mater. 125 (2016) 29-33. [17] J. Bai, Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3, Ceram. Int. 36 (2010) 673-678. [18] D. J. Zeng, Y. M. Zhang, J. F. Yang, B. Wang, J. Matsushita, Fabrication and Property of Mullite Whiskers Frameworks with an Ultrahigh Porosity by Expandable Mesocarbon Microbeads, J. Am. Ceram. Soc. 99 (2016) 2226-2228. 10
[19] K. Okada, N. Ōtuska, Synthesis of mullite whiskers and their application in composites, J. Am. Ceram. Soc. 74 (1991) 2414-2418. [20] Z. Hou, B. Cui, L. Liu, Q. Liu, Effect of the different additives on the fabrication of porous kaolin-based mullite ceramics, Ceram. Int. 42 (2016) 17254-17258. [21] B. B. Lee, P. Ravindra, E. S. Chan, Size and shape of calcium alginate beads produced by extrusion dripping, Chem. Eng. Technol. 36 (2013) 1627-1642. [22] B. Meng, J. Peng, Effects of in situ synthesized mullite whiskers on flexural strength and fracture toughness of corundum-mullite refractory materials, Ceram. Int. 39 (2013) 1525-1531. [23] C. Y. Chen, W. H. Tuan, Evolution of Mullite Texture on Firing Tape‐Cast Kaolin Bodies, J. Am. Ceram. Soc. 85 (2002) 1121-1126.
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Figure captions
Fig. 1 Schematic diagram of the procedures for preparing porous mullite microspheres.
Fig. 2 XRD patterns of the sintered porous mullite microspheres with different solid loading contents.
Fig. 3 Surface morphology of the porous mullite microspheres with different solid loading contents: (a), (d) 10 vol%; (b), (e) 20 vol%; (c), (f) 30 vol%.
Fig. 4 Higher resolution images of surfaces of the porous mullite microspheres with different solid loading contents: (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
Fig. 5 Pore size distribution of the sintered porous mullite microspheres.
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PII: DOI: Reference:
S0272-8842(17)30602-8 http://dx.doi.org/10.1016/j.ceramint.2017.04.012 CERI14992
To appear in: Ceramics International Received date: 6 February 2017 Accepted date: 3 April 2017 Cite this article as: Zhaoping Hou, Biao Zhang, Rui Zhang and Liangliang Liu, Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting Zhaoping Houa, *, Biao Zhangb, Rui Zhanga, Liangliang Liua a
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
Abstract
Porous mullite microspheres with a highly open porosity and average diameter of more than 800 µm were fabricated via an oil-drop molding method accompanied by a freeze casting process. After sintering, a highly porous structure was formed due to interlocking whisker-shaped mullite grains and formation of interconnected skeletons during the freeze-casting process. Additionally, it was found that a high porosity and large pore size in the microspheres green bodies are favorable for the synthesis of mullite whiskers with high aspect ratio.
Keywords: Porous microspheres; Mullite; Porous structure; Freeze casting
1.Introduction
Porous ceramic microspheres are widely used in a broad range of applications, such as catalyst supports, drug delivery, and sorbent materials, owing to their attractive
*
Corresponding author, Tel.:+86351 6111121 E-mail addresses: [email protected], [email protected] 1
properties, such as low density, high specific surface areas, low thermal conductivity, and excellent resistance against thermal shock and chemical attack [1-4].
All porous microsphere applications are sensitive to the functional composite structure and properties. When applied as catalyst supports in fluidized bed reactions, the fluidity and mechanical attrition resistance are strongly influenced by their spherical shape and structure [5]. As absorbents in water purification, high open porosities, tunable pore size distributions, and high specific surface areas are required for the extraction process [4]. Furthermore, interconnected channels are essential for the supply of nutrients when using porous microspheres for cell delivery [6].
To obtain porous ceramic microspheres with a tailorable porosity and tunable pore size and structure, several routes have been developed, including ice-templated spray drying [7], freeze-photocuring-casting [8], emulsification technique [9], and the droplet extrusion method combined with ionotropic gelation [10]. In the above methods, micropores are generated by introducing different porogens, such as ice [7,8,11], PE powder [12], camphene [13], polymer [10], or carbon powder [14]. The ice-templated assisted method is an especially convenient and efficient method to manufacture various porous structures in ceramic materials by varying the rate of freezing, suspension viscosity and solid loading [15]. However, it is also of significant interest to explore the effects of grains with an anisotropic morphology on the 2
microstructure of ice-templated porous ceramics [16]. Tailoring the composition and grain morphology of the pore walls can not only enhance the mechanical properties of porous ceramics but also lead to functional composite structures that are important for further applications.
Recently, porous mullite ceramic has attracted extensive attentions and has been applied to many fields, such as separation, catalysis, adsorption, and drug delivery [17,18]. In addition, the growth of mullite crystal paralleling to the c-axis is faster than in other directions, which is favorable for the formation of mullite whiskers [19]. Whisker-shaped mullite grains will fill up pores formed in the freeze-casting process and then divide them; thus, the pore size will decrease. Meanwhile, porous ceramics with smaller pore sizes have higher surface areas.
In this study, we present an alternative, versatile route for the fabrication of customizable porous mullite microspheres with a high porosity and interconnected pore structure based on freeze casting and the extrusion dripping method. In addition, the effects of solid loading on the grain morphology and porous structure is also investigated.
2. Experimental procedure
Kaolin clay (Xinzhou, Shanxi Province, China) and Al2O3 (Guangcheng Chemical 3
Reagent Ltd., Tianjing, China) were used as the starting materials to fabricate porous mullite ceramics microspheres. AlF3 (Guangfu Fine Chemical Reagent Ltd., Tianjin, China) was used as the crystallization catalyst. MoO3 (99.5% purity, Kermel Chemical Reagent Ltd., Tianjing, China) was used as the sintering aid.
According to our previous work [20], the mullite precursors were mixed in ethanol for 12 h with Al2O3 balls as the mixing media. After drying, suspensions were prepared by adding the mixed ceramic powders, 1 wt% binder (PVA, Shanghai chongjing Co. Ltd. China), and 0.5 wt% dispersant (PAA, Kelong Chemical Inc. China) in deionized water. Suspensions with different solid loadings, 10 vol%, 20 vol%, and 30 vol%, were prepared, followed by ball milling for 24 h to homogenize the slurries.
Fig. 1 shows a schematic diagram of the procedures for preparing porous mullite microspheres. The slurry was dropped using a syringe (inner needle diameter of 0.55 mm) into a corn oil-kerosene mixture at -25 ℃ where the microspherical droplets freeze. The frozen microspheres were separated from the oil and washed repeatedly with Ethyl acetate at -35 ℃until the oil residue was removed. The resulting microspheres were freeze-dried for 24 h and then sintered in a furnace at 1400 ℃ for 2 h with a heating rate of 5 ℃/min.
X-ray diffraction (XRD, Philips Co. Ltd, Holland) was applied to detect the phase composition of the sintered microspheres. Open porosity was estimated by 4
weighting the dried microspheres and wet microspheres immersed in distilled water. The porosity and pore size distribution were measured by mercury intrusion porosimetry (Quantachrome instruments, America). The surface morphology of the sintered microspheres was characterized by a field emission scanning electron microscope (FESEM, S-4800).
3.Results and discussion
Porous mullite microspheres produced by extrusion dripping, freezing, washing, and freeze drying, followed by sintering at 1400 ℃ were uniform in size and less than 800 μm in diameter (Fig. 1(d)). The XRD patterns of samples with different solid loading contents (Fig. 2) after sintering at 1400 ℃ for 2 h revealed that pure mullites were formed from kaolin and Al2O3 in the presence of MoO3 as a sintering aid, which is consistent with our previous work [20].
SEMs of the porous mullite microspheres with different solid loading contents show that they had a perfect spherical shape (Fig. 3(a-c)) due to surface tension where drops of the mullite precursor suspensions dropped into the cool oil [21]. Hence, the droplet regained its spherical shape prior to the freezing process.
Higher magnification images of the surfaces of the microspheres are presented in Fig. 3(d-f). The image of the microsphere with 10 vol% solid loading (Fig. 3(d)) shows 5
orientated pore walls with a thickness of 2-3 μm, where mullite whiskers with a high aspect ratio grow from the walls to the pore channels. The random lapping of the mullite whiskers protruding from the internal walls of the aligned pores lead to the disappearance of micro-pores, either partially or completely. Microsphere with a 20 % solid loading content shows many large elliptic pores (Fig. 3(e)). When the solid loading content was increased to 30 vol% (Fig. 3(f)), only fine pores with a rigid framework were formed through the bonding of the smooth whiskers.
In general, freezing occurs when the suspension droplet falls into cool corn oil. The droplet is frozen, which induces its separation into two phases, the ice and particle phases. These pores were created by the sublimation of ice from the frozen droplet during the freeze-drying stage. The pore structures varied with the solid loading content. During the sintering process, the green pore structure was remodeled by the growth of mullite whiskers during the sintering stage. Additionally, it should be noted that a higher aspect ratio and lower grain diameter of the mullite whiskers were obtained in the sample with the lower solid loading content (Fig. 4), which might be due to its higher porosity and larger pores in the green body. These pores are favorable for gas diffusion and whisker synthesis because the synthesis process is a gas-solid reaction with the aid of the volatilization of AlF3 [22]. On the other hand, these pores restrict steric hindrance and provide an unconstrained environment for the growth of mullite grains. Otherwise, mullite whiskers can easily impinge and coalesce into each other [23]. 6
The above results demonstrate that the formation of pore structures of porous mullite microspheres resulted from an interlocking mullite whisker framework and interconnected skeletons created by the replica of ice.
The open porosities of porous mullite microspheres with solid loading contents of 10 vol%, 20 vol%, 30 vol% were 72.35 %, 66.73 %, 58.52 %, respectively. Overall, the products were highly porous, and their porosity depended on the initial solid loading content of the slurry in the formulation. The same relationship was observed in the production of porous ceramics by freeze-casting [15].
The pore size distributions of sintered porous mullite microspheres are shown in Fig. 5. A distinct multimodel pore size distribution was found for all porous mullite microspheres. Macropores in the range of 0.3-11 μm were found. The first maximum appeared at ≈6 μm and decreased with the solid loading content. For samples with 30 vol%, the peak disappeared. The second peak (0.5-2 μm) shifted toward lower values with increasing solid loading content. Since these pores were created by the sublimation of ice, the size of these pores decreased as the solid loading increased. However, the distinction of pore sizes for samples with different solid loadings was not obvious. This was attributed to the formation of mullite whiskers protruding from the walls, dividing the green pore into smaller pores. Mesopores (6 nm) were observed in addition to macropores. Their pore size may be due to spaces of the 7
whiskers in the framework. The pore size distribution was confirmed by SEM observation.
4.Conclusion
The oil-drop molding method accompanied by a freeze casting process was demonstrated to be feasible for preparing highly porous mullite microspheres with tailorable porosity and an average diameter of 800 μm. The open porosity of the microspheres decreased from 72.35 % to 58.52 % with the increase of the solid loading content from 10 vol% to 30 vol%. Macropores (in the range 0.3-11 μm) and mesopores (6 nm) were observed in the samples. The formation of a multimodal pore structure resulted from the sublimation of ice and the spaces of the whiskers in the framework. The obtained microspheres can be further adapted to their extensive applications by additional functionalization steps.
References
[1] H. Yang, Y. Xie, G. Hao, W. Cai, X. Guo, Preparation of porous alumina microspheres via an oil-in-water emulsion method accompanied by a sol–gel process, New J. Chem. 40 (2016) 589-595. [2] U. Hess, G. Mikolajczyk, L. Treccani, P. Streckbein, C. Heiss, S. Odenbach, K. Rezwan, Multi-loaded ceramic beads/matrix scaffolds obtained by combining 8
ionotropic and freeze gelation for sustained and tuneable vancomycin release, Mater. Sci. Eng., C 67 (2016) 542-553. [3] R. S. Byrne, P. B. Deasy, Use of porous aluminosilicate pellets for drug delivery, J. Microencapsulation 22 (2005) 423-437. [4] T. C. Schumacher, T. Y. Klein, L. Treccani, K. Rezwan, Rapid Sintering of Porous Monoliths Assembled from Microbeads with High Specific Surface Area and Multimodal Porosity, Adv. Eng. Mater. 16 (2014) 151-155. [5] J. Pype, B. Michielsen, E. M. Seftel, S. Mullens, V. Meynen, Development of alumina microspheres with controlled size and shape by vibrational droplet coagulation, J. Eur. Ceram. Soc. 37 (2017) 189-198. [6] R. Li, K. Chen, G. Li, S. Yu, J. Yao, Y. Cai, Structure design and fabrication of porous hydroxyapatite microspheres for cell delivery, J. Mol. Struct. 1120 (2016) 34-41. [7] M. Yu, K. Zhou, Y. Zhang, D. Zhang, Porous Al2O3 microspheres prepared by a novel ice-templated spray drying technique, Ceram. Int. 40 (2014) 1215-1219. [8] Y.
Hazan,
Porous
ceramics,
ceramic/polymer,
and
metal ‐ doped
ceramic/polymer nanocomposites via freeze casting of photo‐curable colloidal fluids, J. Am. Ceram. Soc. 95(2012) 177-187. [9] C. Takai, T. Hotta, S. Shiozaki, S. Matsumoto, T. Fukui, Key techniques to control porous microsphere morphology in S/O/W emulsion system, Colloids Surf., A 373 (2011) 152-157. [10] C. C. Ribeiro, C. C. Barrias, M. A. Barbosa, Preparation and characterization of calcium-phosphate porous microspheres with a uniform size for biomedical 9
applications, J. Mater. Sci. - Mater. Med. 17 (2006) 455-463. [11] M. H. Hong, J. S. Son, K. M. Kim, M. Han, D. S. Oh, Y. K. Lee, Drug-loaded porous spherical hydroxyapatite granules for bone regeneration, J. Mater. Sci. - Mater. Med. 22 (2011) 349-355. [12] Y. C. Fu, M. L. Ho, S. C. Wu, H. S. Hsieh, C. K. Wang, Porous bioceramic bead prepared by calcium phosphate with sodium alginate gel and PE powder, Mater. Sci. Eng., C 28 (2008) 1149-1158. [13] U. S. Shin, J. H. Park, S. J. Hong, J. E. Won, H. S. Yu, H. W. Kim, Porous biomedical composite microspheres developed for cell delivering scaffold in bone regeneration, Mater. Lett. 64 (2010) 2261-2264. [14] C. Wu, H. Zreiqat, Porous bioactive diopside (CaMgSi2O6) ceramic microspheres for drug delivery, Acta Biomater. 6 (2010) 820-829. [15] S. Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (2008) 155-169. [16] D. Ghosh, M. Banda, H. Kang, N. Dhavale, Platelets-induced stiffening and strengthening of ice-templated highly porous alumina scaffolds, Scripta Mater. 125 (2016) 29-33. [17] J. Bai, Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3, Ceram. Int. 36 (2010) 673-678. [18] D. J. Zeng, Y. M. Zhang, J. F. Yang, B. Wang, J. Matsushita, Fabrication and Property of Mullite Whiskers Frameworks with an Ultrahigh Porosity by Expandable Mesocarbon Microbeads, J. Am. Ceram. Soc. 99 (2016) 2226-2228. 10
[19] K. Okada, N. Ōtuska, Synthesis of mullite whiskers and their application in composites, J. Am. Ceram. Soc. 74 (1991) 2414-2418. [20] Z. Hou, B. Cui, L. Liu, Q. Liu, Effect of the different additives on the fabrication of porous kaolin-based mullite ceramics, Ceram. Int. 42 (2016) 17254-17258. [21] B. B. Lee, P. Ravindra, E. S. Chan, Size and shape of calcium alginate beads produced by extrusion dripping, Chem. Eng. Technol. 36 (2013) 1627-1642. [22] B. Meng, J. Peng, Effects of in situ synthesized mullite whiskers on flexural strength and fracture toughness of corundum-mullite refractory materials, Ceram. Int. 39 (2013) 1525-1531. [23] C. Y. Chen, W. H. Tuan, Evolution of Mullite Texture on Firing Tape‐Cast Kaolin Bodies, J. Am. Ceram. Soc. 85 (2002) 1121-1126.
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Figure captions
Fig. 1 Schematic diagram of the procedures for preparing porous mullite microspheres.
Fig. 2 XRD patterns of the sintered porous mullite microspheres with different solid loading contents.
Fig. 3 Surface morphology of the porous mullite microspheres with different solid loading contents: (a), (d) 10 vol%; (b), (e) 20 vol%; (c), (f) 30 vol%.
Fig. 4 Higher resolution images of surfaces of the porous mullite microspheres with different solid loading contents: (a) 10 vol%; (b) 20 vol%; (c) 30 vol%.
Fig. 5 Pore size distribution of the sintered porous mullite microspheres.
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