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Preparation of Al2O3 poly-hollow microsphere (PHM) ceramics using Al2O3 PHMs coated with sintering additive via co-precipitation method
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ScienceDirect Journal of the European Ceramic Society 35 (2015) 2593–2598
Preparation of Al2O3 poly-hollow microsphere (PHM) ceramics using Al2O3 PHMs coated with sintering additive via co-precipitation method Jin-Long Yang a , Xing-Xing Xu b , Jia-Min Wu a,c,∗ , Xiu-Hui Wang b , Zhen-Guo Su a , Chen-Hui Li c,∗∗ a
State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China c State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Received 16 December 2014; received in revised form 20 February 2015; accepted 22 February 2015 Available online 1 April 2015
Abstract In this paper, the Al2 O3 poly-hollow microspheres (PHM) were used to prepare novel Al2 O3 PHM ceramics. In order to improve the mechanical properties of the Al2 O3 PHM ceramics, the Al2 O3 PHMs were coated with CaSiO3 sintering additive via co-precipitation method. The effect of CaCl2 solution content on the properties of the Al2 O3 PHM ceramics was investigated. With the increase of the CaCl2 solution content, the porosity of the Al2 O3 PHM ceramics decreases, while the shrinkage, flexural strength and fracture toughness increase. When the CaCl2 solution content is 8 mL, the flexural strength and fracture toughness of the Al2 O3 PHM ceramics are as high as 191.09 MPa and 3.74 MPa·m1/2 , respectively. It is found that coating the Al2 O3 PHMs with sintering additive via co-precipitation method could effectively improve the properties of the novel Al2 O3 PHM ceramics. © 2015 Elsevier Ltd. All rights reserved. Keywords: Porous Al2 O3 ceramics; Sintering additive; Al2 O3 poly-hollow microspheres; Mechanical properties; Co-precipitation method
1. Introduction Porous ceramics combine the advantages of porous materials and ceramic materials, which have attracted more and more attention in recent years. Porous ceramics have comprehensive application in many areas due to their unique properties such as low density, high specific surface area, low thermal conductivity, good chemical stability, etc.1–6 Generally, the porous ceramics are usually prepared by direct foaming,7 adding pore-forming agent,8,9 replication of polymer sponge,10,11 freeze casting12,13
∗ Corresponding author at: State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Tel.: +86 27 87558155; fax: +86 27 87558155. ∗∗ Corresponding author. Tel.: +86 27 87541846; fax: +86 27 87541846. E-mail addresses: [email protected] (J.-M. Wu), [email protected] (C.-H. Li).
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.027 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
and so on. In addition, using the ceramic hollow spheres to prepare porous ceramics is also a novel and interesting way. In recent years, some researchers have successfully used the ceramic hollow spheres to prepare porous ceramics. Thijs et al.14 used the hollow spheres to prepare porous ceramics, which were prepared via the pyrolysis of the sacrificial cores (seeds, nut, peas, etc.). Shao et al.15 prepared porous Si3 N4 ceramics using fly ash cenospheres. In their research, the fly ash cenospheres act as pore-forming agent and sintering additive. Recently, a kind of novel ceramic hollow spheres has been developed by the researchers in Hebei Yonglong Bangda New Materials Co. Ltd. There are many micro-pores in the prepared ceramic hollow spheres, which are called ceramic poly-hollow microspheres (PHMs). Various ceramic PHMs (such as Al2 O3 , Si3 N4 , ZrO2 , SiC, etc.) have been successfully fabricated via the combination of particle-stabilized foams and centrifugal atomizing technology.16,17 Due to the porous inner structures, the ceramic PHMs could be the promising materials to be used
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Fig. 1. SEM micrographs of (a) the calcined Al2 O3 PHMs and (b) their inner structures.
as pore-forming agent to prepare porous ceramics. In 2014, Wu et al.18 successfully prepared the porous Si3 N4 ceramics by aqueous gelcasting using different amount of the Si3 N4 PHMs. However, according to our previous research, when the ceramic PHMs content is high, the ceramic PHMs could contact with each other, the bonding strength among different ceramic PHMs is low. Therefore, the mechanical properties of the prepared porous ceramics are relatively low. In order to improve the mechanical properties of this kind of novel porous ceramics, the bonding strength among the ceramic PHMs should be adjusted. As is well know, the co-precipitation method could be used to prepare powders with nanosized particles, high reactivity and uniform chemical composition.19–21 In this paper, the coprecipitation method was used to coat the Al2 O3 PHMs with CaSiO3 sintering additive, and then the coated Al2 O3 PHMs were used to prepare Al2 O3 PHM ceramics. Different amounts of CaCl2 solution were added into the mixture of Na2 SiO3 solution and Al2 O3 PHMs. The effect of CaCl2 solution content on the properties of Al2 O3 PHM ceramics was investigated.
2.2. Sample preparation Fig. 2 shows the flow chart of the preparation for the Al2 O3 PHMs ceramics by dry pressing. Firstly, the Al2 O3 PHMs were coated with CaSiO3 sintering additive via co-precipitation method. The CaCl2 and Na2 SiO3 solutions (both 1.5 mol/L) were prepared. The calcined Al2 O3 PHMs (50 g) were added into the Na2 SiO3 solution (50 mL), then the CaCl2 solution was added into the mixture of Na2 SiO3 solution and Al2 O3 PHMs drop by drop. The mixture was stirred by agitator, and different CaCl2 solution contents (1 mL, 3 mL, 5 mL, 8 mL, based on 50 g Al2 O3 PHMs) were added into the mixture. In this process, the CaSiO3 sintering additive was coated on the surfaces of the Al2 O3 PHMs via co-precipitation method. The coated Al2 O3 PHMs were dried in the oven at 80 ◦ C for 12 h. Subsequently, the 10 wt.% PVA solution (5 wt.%, based on the Al2 O3 PHMs) were added into the coated Al2 O3 PHMs, and then they were uniaxially pressed to form samples at 6 MPa for 1 min. Finally, all the samples were sintered at 1500 ◦ C for 2 h in air to form ceramics.
2. Experimental procedure 2.1. Materials The Al2 O3 PHMs were commercially available materials (Hebei Yonglong Bangda New Materials Co. Ltd., China). In order to ensure the mechanical strength of Al2 O3 PHMs, the Al2 O3 PHMs were calcined at 1200 ◦ C for 1 h in air. The average diameter of the calcined Al2 O3 PHMs was 38 m. In addition, the CaCl2 (Analytical reagent, Sinopharm Chemical Reagent Co. Ltd., China) and Na2 SiO3 (Analytical reagent, Sinopharm Chemical Reagent Co. Ltd., China) were used as the raw materials to coat the Al2 O3 PHMs with CaSiO3 sintering additive via co-precipitation method. Fig. 1 shows the SEM micrographs of the calcined Al2 O3 PHMs and their inner structures. The surfaces of the Al2 O3 PHMs are coarse due to the appearance of large Al2 O3 grains in the calcining process. Meanwhile, lots of micro-pores exist in the Al2 O3 PHMs, which could form the pores in the sintered samples.
Fig. 2. The flow chart of the preparation for the Al2 O3 PHM ceramics by dry pressing.
J.-L. Yang et al. / Journal of the European Ceramic Society 35 (2015) 2593–2598
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Fig. 3. SEM micrographs of the coated Al2 O3 PHMs with different CaCl2 solution contents: (a) 1 mL, (b) 3 mL, (c) 5 mL, (d) 8 mL.
2.3. Characterization The compositions of the Al2 O3 PHMs were analyzed by Xray Fluorescence Spectrometer (XRF-1800, Shimadzu, Kyoto, Japan). The phases of the Al2 O3 PHM ceramics were analyzed by X-ray diffraction (XRD) method using Cu K␣ radiation (D8 ADVANCE, Bruker, Karlsruhe, Germany). The microstructures of the samples were observed by scanning electron microscope (SEM) (SSX-550, Shimadzu, Kyoto, Japan). The shrinkage of the samples was determined with the following equation: S=
L0 − L × 100% L0
(1)
where L0 is the diameter of the green sample before drying, and L is the diameter of the ceramics. The porosity of the Al2 O3 ceramics was measured by the Archimedes method. The flexural strength of the samples was measured via the three-point bending test using the mechanical testing machine (AG-2000A, Shimadzu, Kyoto, Japan). The dimension of the samples was 3 mm × 4 mm × 40 mm, and the loading rate was 0.5 mm/min. The fracture toughness of the samples was measured via single edge notched beam (SENB) method using the same testing machine mentioned above. The dimension of the samples was 2.5 mm × 5 mm × 30 mm and notch length of 2.5 mm, and the loading rate was 0.05 mm/min. 3. Results and discussion Table 1 shows the compositions of the Al2 O3 PHMs. With the increase of CaCl2 solution content, the Al2 O3 content in the coated Al2 O3 PHMs decreases gradually, while the SiO2
and CaO contents increase, which indicates that more CaSiO3 sintering additive has been successfully coated on the surfaces of the Al2 O3 PHMs. In the later sintering process, the CaSiO3 sintering additive could greatly improve the sinterability of the Al2 O3 PHMs. Fig. 3 shows the SEM micrographs of the coated Al2 O3 PHMs with different CaCl2 solution contents. With the increase of CaCl2 solution content, it is found that more and more CaSiO3 sintering additive is observed on the surfaces of Al2 O3 PHMs, thus the surfaces of the Al2 O3 PHMs become coarser and coarser. Fig. 4 shows the XRD patterns of the Al2 O3 PHM ceramics with different CaCl2 solution contents. It is found that there is only Al2 O3 phase in all the Al2 O3 PHM ceramics regardless of the CaCl2 solution content. Due to the relatively low CaCl2 solution content, the content of CaSiO3 sintering additive coated on the Al2 O3 PHMs is low, thus the phase compositions are not affected in the Al2 O3 PHM ceramics. Fig. 5 shows the SEM micrographs of the Al2 O3 PHM ceramics with different CaCl2 solution contents. With the increase of the CaCl2 solution content, the microstructures of the samples change obviously, the pores in the samples become fewer Table 1 The compositions of the Al2 O3 PHMs. CaCl2 solution content (mL)
Al2 O3 (wt.%)
SiO2 (wt.%)
CaO (wt.%)
0 1 3 5 8
99.63 97.87 92.82 91.04 89.68
0.05 1.12 4.15 4.78 5.78
0.04 0.62 2.18 3.60 4.11
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Fig. 4. XRD patterns of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) 0 mL, (b) 1 mL, (c) 3 mL, (d) 5 mL, (e) 8 mL.
and fewer, thus the strong connection among different Al2 O3 PHMs is acquired. Regarding the samples without CaCl2 solution, the fracture path would follow the Al2 O3 PHMs due to their low contact area, thus the intact Al2 O3 PHMs are observed in the samples. However, the samples with CaCl2 solution mainly fracture across the Al2 O3 PHMs, and the broken Al2 O3 PHMs are obvious in the samples. As is well known, when the proper liquid phase forms in the sintering process, the particle rearrangement and mass transfer could be greatly improved. Due to the nanosized particles, high reactivity and uniform chemical composition, the CaSiO3 sintering additive coated on the Al2 O3 PHMs via co-precipitation method could form the liquid phase at relatively low temperature to promote sintering. Therefore, the contact area among different Al2 O3 PHMs increases, and the bonding strength among different Al2 O3 PHMs could be greatly improved. Meanwhile, with the increase of the CaCl2 solution content, fewer pores are observed in the samples, the Al2 O3 PHM ceramics become denser. In this case, the crack in the samples would propagate directly through the Al2 O3 PHMs.
Fig. 5. SEM micrographs of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a, b) 0 mL, (c, d) 1 mL, (e, f) 8 mL.
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Fig. 6. Schematic illustration of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) 0 mL, (b) 1 mL, (c) 8 mL.
Fig. 6 shows the schematic illustration of the Al2 O3 PHM ceramics with different CaCl2 solution contents. It is found that the pores in the Al2 O3 PHM ceramics are completely different from those in the porous ceramics prepared by other conventional methods, such as replication of polymer sponge, adding pore-forming agent, direct foaming and freeze casting. With the increase of the CaCl2 solution content, the contact area among different Al2 O3 PHMs increases due to the promoted liquid phase sintering by the CaSiO3 sintering additive, thus the bonding strength among different Al2 O3 PHMs increases, and the total content of pores decreases. When the CaCl2 solution is not added or its content is low (such as 0 mL or 1 mL), the contact area among different Al2 O3 PHMs is low, thus the bonding strength among them is low as well. Under this condition, there are two kinds of pores in the Al2 O3 PHM ceramics: (1) pores in the Al2 O3 PHMs (labeled as ␣, see Figs. 5(d) and 6(b)); (2) pores among different Al2 O3 PHMs (labeled as , see Figs. 5(d) and 6(b)). When the CaCl2 solution content is high (such as 8 mL), the contact area among different Al2 O3 PHMs is so high that the -pores gradually disappear, and there are only ␣-pores in the Al2 O3 PHM ceramics, thus the bonding strength among different Al2 O3 PHMs is high. Under this condition, the main pores in the Al2 O3 PHM ceramics originate from the inner pores in the Al2 O3 PHMs (␣-pores). Different microstructures in the samples would result in different properties of the samples, which will be discussed in the following part. Fig. 7 shows the properties of the Al2 O3 PHM ceramics with different CaCl2 solution contents. As shown in Fig. 7(a), with the increase of CaCl2 solution content, the porosity of the Al2 O3 PHM ceramics decreases from 49.68% to 21.59%, while the shrinkage of the Al2 O3 PHM ceramics increases from 12.00% to 26.26%. Meanwhile, with the increase of CaCl2 solution content, the flexural strength increases from 31.74 MPa to 191.09 MPa, and the fracture toughness increases from 0.76 MPa·m1/2 to 3.74 MPa·m1/2 (see Fig. 7(b)). When the CaCl2 solution content is 8 mL, the flexural strength and fracture toughness of the Al2 O3 PHM ceramics are as high as 191.09 MPa and 3.74 MPa·m1/2 , respectively. With the increase of the CaCl2 solution content, more CaSiO3 sintering additive would be coated on the surfaces of the Al2 O3 PHMs. In the sintering process, the CaSiO3 sintering additive on the surfaces of the Al2 O3 PHMs could form the liquid phase and further improve the mass transfer, thus strong connection among different Al2 O3 PHMs gradually forms, and the total pores in the samples vary from both ␣-pores and -pores to only ␣-pores (see Fig. 5). Accordingly, the porosity of the samples decreases,
Fig. 7. The properties of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) porosity and shrinkage, (b) flexural strength and fracture toughness.
while their shrinkage, flexural strength and the fracture toughness increase. 4. Conclusions In this paper, the co-precipitation method was used to coat Al2 O3 PHMs with CaSiO3 sintering additive, and the Al2 O3 PHM ceramics with good properties were successfully prepared using the coated Al2 O3 PHMs. It is found that the CaSiO3 sintering additive plays an important role in affecting the properties of the Al2 O3 PHM ceramics. With the increase of the CaCl2 solution content, more CaSiO3 sintering additive is coated on the surfaces of the Al2 O3 PHMs, the microstructures in the Al2 O3 PHM ceramics greatly change and the bonding strength among different Al2 O3 PHMs is improved. Therefore, the porosity of the Al2 O3 PHM ceramics decreases, while the shrinkage, flexural strength and fracture toughness increase. Acknowledgements Our research work presented in this paper was supported by China Postdoctoral Science Foundation (2013M530618), National Natural Science Foundation of China (51172120,
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51372093) and the 02 Important and Special Project for the State Science and Technology (2013ZX2104-001-002). References 1. Wang M, Du H, Guo A, Hao R, Hou Z. Microstructure control in ceramic foams via mixed cationic/anionic surfactant. Mater Lett 2012;88:97–100. 2. Zhang H, Cooper AI. Aligned porous structures by directional freezing. Adv Mater 2007;19:1529–33. 3. Woodard JR, Hilldore AJ, Lan SK, Park CJ, Morgan AW, Eurell JAC, et al. The mechanical properties and osteoconductivity of hydroxyapatite bone scaffolds with multi-scale porosity. Biomaterials 2007;28:45–54. 4. Sundaram S, Colombo P, Katoh Y. Selected emerging opportunities for ceramics in energy, environment, and transportation. Int J Appl Ceram Technol 2013;10:731–9. 5. Acosta GFA, Castillejos EAH, Almanza RJM, Flores VA. Analysis of liquid flow through ceramic porous media used for molten metal filtration. Metall Mater Trans B 1995;26:159–71. 6. Roohani-Esfahani SI, Chen Y, Shi J, Zreiqat H. Fabrication and characterization of a new, strong and bioactive ceramic scaffold for bone regeneration. Mater Lett 2013;107:378–81. 7. Fukushima M, Colombo P. Silicon carbide-based foams from direct blowing of polycarbosilane. J Eur Ceram Soc 2012;32:503–10. 8. Isobe T, Tomita T, Kameshima Y, Nakajima A, Okada K. Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method. J Eur Ceram Soc 2006;26:957–60. 9. Zhou J, Wang CA. Porous yttria-stabilized zirconia ceramics fabricated by nonaqueous-based gelcasting process with PMMA microsphere as poreforming agent. J Am Ceram Soc 2013;96:266–71.
10. Silva SA, Brunelli DD, Melo FCL, Thim GP. Preparation of a reticulated ceramic using vegetal sponge as templating. Ceram Int 2009;35:1575–9. 11. Delbrücke T, Gouvêa RA, Moreira ML, Raubach CW, Varela JA, Longo E, et al. Sintering of porous alumina obtained by biotemplate fibers for low thermal conductivity applications. J Eur Ceram Soc 2013;33:1087–92. 12. Araki K, Halloran JW. Porous ceramic bodies with interconnected pore channels by a novel freeze casting technique. J Am Ceram Soc 2005;88:1108–14. 13. Deville S, Saiz E, Tomsia AP. Ice-templated porous alumina structures. Acta Mater 2007;55:1965–74. 14. Thijs I, Luyten J, Mullens S. Producing ceramic foams with hollow spheres. J Am Ceram Soc 2004;87:170–2. 15. Shao Y, Jia D, Liu B. Characterization of porous silicon nitride ceramics by pressureless sintering using fly ash cenosphere as pore-forming agent. J Eur Ceram Soc 2009;29:1529–34. 16. Su Z, Xi X, Hu Y, Qi F, Yu S, Li H, et al. A new Al2 O3 porous ceramic prepared by addition of hollow spheres. J Porous Mater 2014;21:601–9. 17. Qi F, Xu X, Xu J, Wang Y, Yang J. A novel way to prepare hollow sphere ceramics. J Am Ceram Soc 2014;97:3341–7. 18. Wu JM, Zhang XY, Yang JL. Novel porous Si3 N4 ceramics prepared by aqueous gelcasting using Si3 N4 poly-hollow microspheres as pore-forming agent. J Eur Ceram Soc 2014;34:1089–96. 19. Chen Q, Shi Y, An L, Wang S, Chen J, Shi J. A novel co-precipitation synthesis of a new phosphor Lu2 O3 :Eu3+ . J Eur Ceram Soc 2007;27: 191–7. 20. Gupta UN, Pradhan S, Muthurajan H, Kumar HH, Kharat DK, Ravi V. A co-precipitation technique of preparing LaNbO4 powders. J Am Ceram Soc 2007;90:2661–3. 21. Li HL, Liu XJ, Xie RJ, Zeng Y, Huang LP. Fabrication of transparent ceriumdoped lutetium aluminum garnet ceramics by co-precipitation routes. J Am Ceram Soc 2006;89:2356–8.
ScienceDirect Journal of the European Ceramic Society 35 (2015) 2593–2598
Preparation of Al2O3 poly-hollow microsphere (PHM) ceramics using Al2O3 PHMs coated with sintering additive via co-precipitation method Jin-Long Yang a , Xing-Xing Xu b , Jia-Min Wu a,c,∗ , Xiu-Hui Wang b , Zhen-Guo Su a , Chen-Hui Li c,∗∗ a
State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China c State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Received 16 December 2014; received in revised form 20 February 2015; accepted 22 February 2015 Available online 1 April 2015
Abstract In this paper, the Al2 O3 poly-hollow microspheres (PHM) were used to prepare novel Al2 O3 PHM ceramics. In order to improve the mechanical properties of the Al2 O3 PHM ceramics, the Al2 O3 PHMs were coated with CaSiO3 sintering additive via co-precipitation method. The effect of CaCl2 solution content on the properties of the Al2 O3 PHM ceramics was investigated. With the increase of the CaCl2 solution content, the porosity of the Al2 O3 PHM ceramics decreases, while the shrinkage, flexural strength and fracture toughness increase. When the CaCl2 solution content is 8 mL, the flexural strength and fracture toughness of the Al2 O3 PHM ceramics are as high as 191.09 MPa and 3.74 MPa·m1/2 , respectively. It is found that coating the Al2 O3 PHMs with sintering additive via co-precipitation method could effectively improve the properties of the novel Al2 O3 PHM ceramics. © 2015 Elsevier Ltd. All rights reserved. Keywords: Porous Al2 O3 ceramics; Sintering additive; Al2 O3 poly-hollow microspheres; Mechanical properties; Co-precipitation method
1. Introduction Porous ceramics combine the advantages of porous materials and ceramic materials, which have attracted more and more attention in recent years. Porous ceramics have comprehensive application in many areas due to their unique properties such as low density, high specific surface area, low thermal conductivity, good chemical stability, etc.1–6 Generally, the porous ceramics are usually prepared by direct foaming,7 adding pore-forming agent,8,9 replication of polymer sponge,10,11 freeze casting12,13
∗ Corresponding author at: State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Tel.: +86 27 87558155; fax: +86 27 87558155. ∗∗ Corresponding author. Tel.: +86 27 87541846; fax: +86 27 87541846. E-mail addresses: [email protected] (J.-M. Wu), [email protected] (C.-H. Li).
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.02.027 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
and so on. In addition, using the ceramic hollow spheres to prepare porous ceramics is also a novel and interesting way. In recent years, some researchers have successfully used the ceramic hollow spheres to prepare porous ceramics. Thijs et al.14 used the hollow spheres to prepare porous ceramics, which were prepared via the pyrolysis of the sacrificial cores (seeds, nut, peas, etc.). Shao et al.15 prepared porous Si3 N4 ceramics using fly ash cenospheres. In their research, the fly ash cenospheres act as pore-forming agent and sintering additive. Recently, a kind of novel ceramic hollow spheres has been developed by the researchers in Hebei Yonglong Bangda New Materials Co. Ltd. There are many micro-pores in the prepared ceramic hollow spheres, which are called ceramic poly-hollow microspheres (PHMs). Various ceramic PHMs (such as Al2 O3 , Si3 N4 , ZrO2 , SiC, etc.) have been successfully fabricated via the combination of particle-stabilized foams and centrifugal atomizing technology.16,17 Due to the porous inner structures, the ceramic PHMs could be the promising materials to be used
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Fig. 1. SEM micrographs of (a) the calcined Al2 O3 PHMs and (b) their inner structures.
as pore-forming agent to prepare porous ceramics. In 2014, Wu et al.18 successfully prepared the porous Si3 N4 ceramics by aqueous gelcasting using different amount of the Si3 N4 PHMs. However, according to our previous research, when the ceramic PHMs content is high, the ceramic PHMs could contact with each other, the bonding strength among different ceramic PHMs is low. Therefore, the mechanical properties of the prepared porous ceramics are relatively low. In order to improve the mechanical properties of this kind of novel porous ceramics, the bonding strength among the ceramic PHMs should be adjusted. As is well know, the co-precipitation method could be used to prepare powders with nanosized particles, high reactivity and uniform chemical composition.19–21 In this paper, the coprecipitation method was used to coat the Al2 O3 PHMs with CaSiO3 sintering additive, and then the coated Al2 O3 PHMs were used to prepare Al2 O3 PHM ceramics. Different amounts of CaCl2 solution were added into the mixture of Na2 SiO3 solution and Al2 O3 PHMs. The effect of CaCl2 solution content on the properties of Al2 O3 PHM ceramics was investigated.
2.2. Sample preparation Fig. 2 shows the flow chart of the preparation for the Al2 O3 PHMs ceramics by dry pressing. Firstly, the Al2 O3 PHMs were coated with CaSiO3 sintering additive via co-precipitation method. The CaCl2 and Na2 SiO3 solutions (both 1.5 mol/L) were prepared. The calcined Al2 O3 PHMs (50 g) were added into the Na2 SiO3 solution (50 mL), then the CaCl2 solution was added into the mixture of Na2 SiO3 solution and Al2 O3 PHMs drop by drop. The mixture was stirred by agitator, and different CaCl2 solution contents (1 mL, 3 mL, 5 mL, 8 mL, based on 50 g Al2 O3 PHMs) were added into the mixture. In this process, the CaSiO3 sintering additive was coated on the surfaces of the Al2 O3 PHMs via co-precipitation method. The coated Al2 O3 PHMs were dried in the oven at 80 ◦ C for 12 h. Subsequently, the 10 wt.% PVA solution (5 wt.%, based on the Al2 O3 PHMs) were added into the coated Al2 O3 PHMs, and then they were uniaxially pressed to form samples at 6 MPa for 1 min. Finally, all the samples were sintered at 1500 ◦ C for 2 h in air to form ceramics.
2. Experimental procedure 2.1. Materials The Al2 O3 PHMs were commercially available materials (Hebei Yonglong Bangda New Materials Co. Ltd., China). In order to ensure the mechanical strength of Al2 O3 PHMs, the Al2 O3 PHMs were calcined at 1200 ◦ C for 1 h in air. The average diameter of the calcined Al2 O3 PHMs was 38 m. In addition, the CaCl2 (Analytical reagent, Sinopharm Chemical Reagent Co. Ltd., China) and Na2 SiO3 (Analytical reagent, Sinopharm Chemical Reagent Co. Ltd., China) were used as the raw materials to coat the Al2 O3 PHMs with CaSiO3 sintering additive via co-precipitation method. Fig. 1 shows the SEM micrographs of the calcined Al2 O3 PHMs and their inner structures. The surfaces of the Al2 O3 PHMs are coarse due to the appearance of large Al2 O3 grains in the calcining process. Meanwhile, lots of micro-pores exist in the Al2 O3 PHMs, which could form the pores in the sintered samples.
Fig. 2. The flow chart of the preparation for the Al2 O3 PHM ceramics by dry pressing.
J.-L. Yang et al. / Journal of the European Ceramic Society 35 (2015) 2593–2598
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Fig. 3. SEM micrographs of the coated Al2 O3 PHMs with different CaCl2 solution contents: (a) 1 mL, (b) 3 mL, (c) 5 mL, (d) 8 mL.
2.3. Characterization The compositions of the Al2 O3 PHMs were analyzed by Xray Fluorescence Spectrometer (XRF-1800, Shimadzu, Kyoto, Japan). The phases of the Al2 O3 PHM ceramics were analyzed by X-ray diffraction (XRD) method using Cu K␣ radiation (D8 ADVANCE, Bruker, Karlsruhe, Germany). The microstructures of the samples were observed by scanning electron microscope (SEM) (SSX-550, Shimadzu, Kyoto, Japan). The shrinkage of the samples was determined with the following equation: S=
L0 − L × 100% L0
(1)
where L0 is the diameter of the green sample before drying, and L is the diameter of the ceramics. The porosity of the Al2 O3 ceramics was measured by the Archimedes method. The flexural strength of the samples was measured via the three-point bending test using the mechanical testing machine (AG-2000A, Shimadzu, Kyoto, Japan). The dimension of the samples was 3 mm × 4 mm × 40 mm, and the loading rate was 0.5 mm/min. The fracture toughness of the samples was measured via single edge notched beam (SENB) method using the same testing machine mentioned above. The dimension of the samples was 2.5 mm × 5 mm × 30 mm and notch length of 2.5 mm, and the loading rate was 0.05 mm/min. 3. Results and discussion Table 1 shows the compositions of the Al2 O3 PHMs. With the increase of CaCl2 solution content, the Al2 O3 content in the coated Al2 O3 PHMs decreases gradually, while the SiO2
and CaO contents increase, which indicates that more CaSiO3 sintering additive has been successfully coated on the surfaces of the Al2 O3 PHMs. In the later sintering process, the CaSiO3 sintering additive could greatly improve the sinterability of the Al2 O3 PHMs. Fig. 3 shows the SEM micrographs of the coated Al2 O3 PHMs with different CaCl2 solution contents. With the increase of CaCl2 solution content, it is found that more and more CaSiO3 sintering additive is observed on the surfaces of Al2 O3 PHMs, thus the surfaces of the Al2 O3 PHMs become coarser and coarser. Fig. 4 shows the XRD patterns of the Al2 O3 PHM ceramics with different CaCl2 solution contents. It is found that there is only Al2 O3 phase in all the Al2 O3 PHM ceramics regardless of the CaCl2 solution content. Due to the relatively low CaCl2 solution content, the content of CaSiO3 sintering additive coated on the Al2 O3 PHMs is low, thus the phase compositions are not affected in the Al2 O3 PHM ceramics. Fig. 5 shows the SEM micrographs of the Al2 O3 PHM ceramics with different CaCl2 solution contents. With the increase of the CaCl2 solution content, the microstructures of the samples change obviously, the pores in the samples become fewer Table 1 The compositions of the Al2 O3 PHMs. CaCl2 solution content (mL)
Al2 O3 (wt.%)
SiO2 (wt.%)
CaO (wt.%)
0 1 3 5 8
99.63 97.87 92.82 91.04 89.68
0.05 1.12 4.15 4.78 5.78
0.04 0.62 2.18 3.60 4.11
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Fig. 4. XRD patterns of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) 0 mL, (b) 1 mL, (c) 3 mL, (d) 5 mL, (e) 8 mL.
and fewer, thus the strong connection among different Al2 O3 PHMs is acquired. Regarding the samples without CaCl2 solution, the fracture path would follow the Al2 O3 PHMs due to their low contact area, thus the intact Al2 O3 PHMs are observed in the samples. However, the samples with CaCl2 solution mainly fracture across the Al2 O3 PHMs, and the broken Al2 O3 PHMs are obvious in the samples. As is well known, when the proper liquid phase forms in the sintering process, the particle rearrangement and mass transfer could be greatly improved. Due to the nanosized particles, high reactivity and uniform chemical composition, the CaSiO3 sintering additive coated on the Al2 O3 PHMs via co-precipitation method could form the liquid phase at relatively low temperature to promote sintering. Therefore, the contact area among different Al2 O3 PHMs increases, and the bonding strength among different Al2 O3 PHMs could be greatly improved. Meanwhile, with the increase of the CaCl2 solution content, fewer pores are observed in the samples, the Al2 O3 PHM ceramics become denser. In this case, the crack in the samples would propagate directly through the Al2 O3 PHMs.
Fig. 5. SEM micrographs of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a, b) 0 mL, (c, d) 1 mL, (e, f) 8 mL.
J.-L. Yang et al. / Journal of the European Ceramic Society 35 (2015) 2593–2598
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Fig. 6. Schematic illustration of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) 0 mL, (b) 1 mL, (c) 8 mL.
Fig. 6 shows the schematic illustration of the Al2 O3 PHM ceramics with different CaCl2 solution contents. It is found that the pores in the Al2 O3 PHM ceramics are completely different from those in the porous ceramics prepared by other conventional methods, such as replication of polymer sponge, adding pore-forming agent, direct foaming and freeze casting. With the increase of the CaCl2 solution content, the contact area among different Al2 O3 PHMs increases due to the promoted liquid phase sintering by the CaSiO3 sintering additive, thus the bonding strength among different Al2 O3 PHMs increases, and the total content of pores decreases. When the CaCl2 solution is not added or its content is low (such as 0 mL or 1 mL), the contact area among different Al2 O3 PHMs is low, thus the bonding strength among them is low as well. Under this condition, there are two kinds of pores in the Al2 O3 PHM ceramics: (1) pores in the Al2 O3 PHMs (labeled as ␣, see Figs. 5(d) and 6(b)); (2) pores among different Al2 O3 PHMs (labeled as , see Figs. 5(d) and 6(b)). When the CaCl2 solution content is high (such as 8 mL), the contact area among different Al2 O3 PHMs is so high that the -pores gradually disappear, and there are only ␣-pores in the Al2 O3 PHM ceramics, thus the bonding strength among different Al2 O3 PHMs is high. Under this condition, the main pores in the Al2 O3 PHM ceramics originate from the inner pores in the Al2 O3 PHMs (␣-pores). Different microstructures in the samples would result in different properties of the samples, which will be discussed in the following part. Fig. 7 shows the properties of the Al2 O3 PHM ceramics with different CaCl2 solution contents. As shown in Fig. 7(a), with the increase of CaCl2 solution content, the porosity of the Al2 O3 PHM ceramics decreases from 49.68% to 21.59%, while the shrinkage of the Al2 O3 PHM ceramics increases from 12.00% to 26.26%. Meanwhile, with the increase of CaCl2 solution content, the flexural strength increases from 31.74 MPa to 191.09 MPa, and the fracture toughness increases from 0.76 MPa·m1/2 to 3.74 MPa·m1/2 (see Fig. 7(b)). When the CaCl2 solution content is 8 mL, the flexural strength and fracture toughness of the Al2 O3 PHM ceramics are as high as 191.09 MPa and 3.74 MPa·m1/2 , respectively. With the increase of the CaCl2 solution content, more CaSiO3 sintering additive would be coated on the surfaces of the Al2 O3 PHMs. In the sintering process, the CaSiO3 sintering additive on the surfaces of the Al2 O3 PHMs could form the liquid phase and further improve the mass transfer, thus strong connection among different Al2 O3 PHMs gradually forms, and the total pores in the samples vary from both ␣-pores and -pores to only ␣-pores (see Fig. 5). Accordingly, the porosity of the samples decreases,
Fig. 7. The properties of the Al2 O3 PHM ceramics with different CaCl2 solution contents: (a) porosity and shrinkage, (b) flexural strength and fracture toughness.
while their shrinkage, flexural strength and the fracture toughness increase. 4. Conclusions In this paper, the co-precipitation method was used to coat Al2 O3 PHMs with CaSiO3 sintering additive, and the Al2 O3 PHM ceramics with good properties were successfully prepared using the coated Al2 O3 PHMs. It is found that the CaSiO3 sintering additive plays an important role in affecting the properties of the Al2 O3 PHM ceramics. With the increase of the CaCl2 solution content, more CaSiO3 sintering additive is coated on the surfaces of the Al2 O3 PHMs, the microstructures in the Al2 O3 PHM ceramics greatly change and the bonding strength among different Al2 O3 PHMs is improved. Therefore, the porosity of the Al2 O3 PHM ceramics decreases, while the shrinkage, flexural strength and fracture toughness increase. Acknowledgements Our research work presented in this paper was supported by China Postdoctoral Science Foundation (2013M530618), National Natural Science Foundation of China (51172120,
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