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Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics Zheng Miaon,1, Nan Li, Wen Yan The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, China Received 1 July 2014; accepted 20 July 2014
Abstract The effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics prepared from kaolinite gangue, Al(OH)3 and CaCO3 were studied by SEM, XRD and Factsage 6.2 thermalchemical software calculation. It was found that the contents of anorthite, mullite and corundum in porous ceramics depended little on the sintering temperature, but depended mostly on the chemical composition of specimens. Anorthite and mullite existed in the pseudomorphs of kaolinite gangue which were dense and corundum existed in the Al(OH)3 pseudomorphs with micropores. With elevating sintering temperature from 1300 1C to 1450 1C, the apparent porosity of the specimens changed a little but the size of secondary pores increased because of increase of liquid content, resulting in improvement of sintering and elevation of cold crushing strength of specimens. The size distributions of primary pores were almost the same in the specimens sintered at 1300 1C and 1400 1C. In specimens sintered at 1450 1C, the pores in Al(OH)3 pseudomorphs had a considerable growth because sintering of the Al2O3 crystallites was improved. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Anorthite; Porous ceramics; Microstructure; Pore size distribution
1. Introduction Porous ceramics have been widely used in multifunction applications: infiltration, biomaterials and insulating materials. Anorthite processes low thermal conductivity, low thermal expansion coefficient and high thermal shock resistance. There have been several papers on porous anorthite ceramics. Han [1] prepared an ultra low thermal conductivity of porous anorthite ceramics by hydrous foam-gelcasting process. Liu [2] prepared anorthite lightweight thermal insulating bricks and studied the formation process of anorthite. Primachenko [3] reported that an anorthite lightweight material with micropores which had very low thermal conductivity and they also used this n
Corresponding author. Tel.: þ86 027 68862511; fax: þ 86 027 68862121. E-mail addresses: [email protected] (Z. Miao), [email protected] (N. Li), [email protected] (W. Yan). 1 Postal address: Mail box 181, Wuhan University of Science and Technology, Heping Road 947#, Qingshan District, Wuhan City, Hubei Province, 430081, China.
microporous anorthite as aggregate to prepare high performance heat insulating castable [4]. However, lower melting temperature (about 1550 1C), lower refractoriness and refractoriness under load of anorthite result in lower serving temperature. In order to improve the properties at elevated temperature and strength of porous anorthite ceramics, mullite was introduced in anorthite ceramics. Harabi [5] reported an economic approach to fabricate the mullite and anorthite based ceramics. Sutcu [6,7] used recycled paper processing residues and clay to produce porous anorthite ceramics with mullite as a secondary phase. Bao [8] prepared an anorthite/mullite composite to improve the strength of composite. In CaO–Al2O3– SiO2 system, besides anorthite and mullite, corundum is an important crystal phase. However, there are few papers dealing with anorthite–mullite–corundum porous ceramics. The phase composition and microstructure of the CaO–Al2O3–SiO2 porous ceramics depend on the formula of the matrix and sintering temperature. In the present study, the effect of sintering temperature on the phase composition and microstructure of
http://dx.doi.org/10.1016/j.ceramint.2014.07.105 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
2
anorthite–mullite–corundum porous ceramics which is prepared from kaolinite gangue, Al(OH)3 and CaCO3 is investigated. 2. Experimental procedure Al(OH)3 (d50, 16 μm), kaolinite gangue (d50, 16 μm) and CaCO3 (d50, 10 μm) were used as raw materials. The chemical compositions of raw materials are listed in Table 1. The powder mixture consisting of 61.57 wt% kaolinite gangue, 21.08 wt% Al(OH)3 and 17.35 wt% CaCO3 were mixed for 4 h. Then the powder was pressed into rectangular samples of 125 mm length 25 mm width 25 mm thickness at a pressure of about 30 MPa. The green compacts after drying at 110 1C were heated at 1300 1C, 1400 1C, 1450 1C for 180 min in an electric furnace, then furnace-cooled. Apparent porosity was measured by Archimedes' principle with water as the medium. The linear change of samples was determined by the length measurements before and after the sintering process. The cold crushing strength of the specimens was obtained according to Chinese standard GB/T 3997.21998. The microstructure of the samples was obtained using scanning electron microscopy with EDS (SEM, Quanta 400, FEI Company, USA). Pore size distribution and median pore size were obtained by a microscopy measurement method through an optical microscope (Axioskop40). Phase analysis was performed using X-ray diffraction and Cu Kα radiation with a scanning speed of 21 per minute (X'pert PRO MPD, Philips, Eindhoven, Netherlands). The relative content of identified phases was calculated by the semi-quantitative
analysis in HighScore works on basis of the RIR ( ¼ Reference Intensity Ratio) values. Simultaneously, phase contents of specimens were also calculated by factsage 6.2 thermalchemical software. In this calculation, the database of Fact, Fact53, FToxid, Ftsalt were used. 3. Results and discussion Fig. 1 shows change of apparent porosity and linear change with sintering temperature, and Fig. 2 shows change of cold crushing strength with sintering temperature. With the temperature
Fig. 2. Cold crushing strength of specimens as a function of temperature.
Table 1 Chemical composition of raw materials (wt%). Materials
Al2O3 SiO2 Fe2O3 CaO
Kaolinite gangue Al(OH)3 CaCO3
38.87 43.03 0.30 66.79 0.09
0.12 0.07 0.71 0.06
MgO K2O
Na2O TiO2 Ignition loss
0.64 0.42 0.087 0.26
0.48
15.91
0.49 0.33 0.01 0.13 0.019 32.04 53.66 1.64 0.005 0.015 0.007 43.81
Fig. 3. XRD patterns of samples sintered at various temperatures. ▲ – Mullite, ★ – Anorthite, ■ – Corundum, ● – Gehlenite, and ○ – Cristobalite.
Table 2 Relative contents of the crystalline phase of specimens sintered at different temperatures (wt%).
Fig. 1. Apparent porosity and linear change of specimens as a function of temperature.
1300 1C 1400 1C 1450 1C
Mullite
Anorthite
Corundum
Gehlenite
Cristobalite
16 15 15
62 70 67
11 15 18
8
3
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
increasing, apparent porosity and linear shrinkage changed a little. With the temperature increasing, cold crushing strength of specimens increased and when the sintering temperature increased from 1400 1C to 1450 1C strength of specimens increased sharply. XRD patterns of samples sintered at various temperatures are shown in Fig. 3. When the specimens were sintered at 1300 1C, the major phase was anorthite (PDF, 891462); and
3
minor phases were corundum (PDF, 711123), cristobalite (PDF, 850621), mullite (PDF, 791457), gehlenite (PDF, 732041). When the specimens were sintered at a temperature above 1400 1C, peaks of gehlenite and cristobalite disappeared. Table 2 lists the relative contents of the crystalline phase of specimens obtained by XRD. With increase of sintering temperature, the contents of anorthite and mullite changed a little, but corundum content increased slightly. The dependence of phase contents obtained by Factsage 6.2 on sintering temperature is shown in Fig. 4. The content of anorthite, mullite and corundum changed slightly, according with the results of XRD. It means that based on the chemical composition of the specimens in this study, the content of
Table 3 EDS analysis of points in Fig. 5 (At%).
Fig. 4. Phase content obtained by Factsage 6.2 as a function of sintering temperature.
Fig. 5. SEM micrographs of specimens sintered at 1300 1C and 1450 1C.
Points
Al (K)
Si (K)
Ca (K)
O (K)
1 2 3 4 5
16.21 15.72 42.26 13.57 15.52
18.68 16.59
0.45 3.37
13.53 16.48
6.44 3.98
64.66 66.04 57.60 66.07 64.02
Fig. 6. Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1450 1C.
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
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Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 7. Pore size distribution (a) and cumulative volume (b) of secondary pores of specimens sintered at various temperatures.
Fig. 8. Pore size distribution (a) and cumulative volume (b) of primary pores in Al(OH)3 pseudomorphs.
anorthite, mullite and corundum of the specimens depends little on sintering temperature, but mostly depends on the chemical composition. However, the liquid phase content increased with sintering temperature. Fig. 5 gives the SEM microstructures of specimens sintered at 1300 1C and 1450 1C. The EDS results are given in Table 3. Some pseudomorphs of Al(OH)3 and kaolinite gangue existed. Kaolinite gangue pseudomorphs were dense because they were easy to be sintered at lower temperatures. A lot of micropores were in the Al(OH)3 pseudomorphs, showing as Fig. 6. We called them ‘primary pores’ and pores among the pseudomorphs were called ‘secondary pores’ [9,10]. Fig. 6 shows the microstructure of Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1450 1C. In the specimens sintered at 1300 1C, corundum crystallites sizes were small and a lot of micropores were located in the Al(OH)3 pseudomorphs; With the sintering temperature increasing to 1450 1C, size of corundum crystallites and pores increased a lot in the Al(OH)3 pseudomorphs. Fig. 7 shows the pore size distribution and cumulative volume of secondary pores and Fig. 8 shows the pores size distribution and cumulative volume of the primary pores in the pseudomorphs of Al(OH)3. In the three specimens, most of secondary pores were 40 μm in size. With increase of sintering temperature from 1300 1C to 1450 1C, the size of secondary
pores increased. The d50 of the specimens sintered at 1300 1C, 1400 1C and 1450 1C were 11.21 μm, 19.22 μm and 23.40 μm, respectively. For the specimens sintered at 1450 1C, the pores with size in the range from 20 μm to 30 μm increased, results in increase of the thickness of solid phase between pores. As a result, the cold crushing strength of specimens sintered at 1450 1C was higher than those sintered at 1300 1C and 1400 1C. The size distributions of primary pores in Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1400 1C were almost the same with the d50 of 0.56 μm. It means that the sintering of Al2O3 crystallites in Al(OH)3 pseudomorphs occurs slightly, because the impurities and liquid in the pseudomorphs are very low. In the specimens sintered at 1450 1C, the primary pores grow, because the sintering of Al2O3 crystallites occurred considerably. 4. Conclusions Based on the chemical composition in this study, the contents of anorthite, mullite and corundum in porous ceramics depend little on the sintering temperature, but depend mostly on the chemical composition of specimens. Anorthite and mullite exist in the kaolinite gangue pseudomorphs which are dense and corundum exists in the Al(OH)3 pseudomorphs which are porous. With elevating sintering
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
temperature from 1300 1C to 1450 1C, the apparent porosity of the specimens changes a little but the size of secondary pores increase because of increase of liquid content, resulting in improvement of sintering and elevation of cold crushing strength of specimens. The size distributions of primary pores are almost the same in the specimens sintered at 1300 1C and 1400 1C. In specimens sintered at 1450 1C, the pores in Al(OH)3 pseudomorphs have a considerable growth because sintering of the Al2O3 crystallites is improved. Acknowledgments The authors would like to thank the Educational Commission of Hubei Province of China (Grant no. Q20131111) and the 973 Program Earlier Research Project (Grant no. 2012CB722702) for financially supporting this work. References [1] Y. Han, C.W. Li, C. Bian, S.B. Li, C.A. Wang, Porous anorthite ceramics with ultra-low thermal conductivity, J. Eur. Ceram. Soc. 33 (2013) 2573–2578. [2] Q. Liu, Z.H. Pan, Q.B. Li, Y.H. Ruan, Preparation of anorthite lightweight thermal insulating brick and the formation process of anorthite, Bull. Chin. Ceram. Soc. 29 (2010) 1269–1274 (in Chinese).
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[3] V.V. Primachenko, V.V. Martynenko, L. Dergaputskaya, Anorthite lightweight material with microporous structure, in: Proceedings of the 7th UNITECR' 2001, 2001, pp. 1193–1195. [4] V.V. Primachenko, V.V. Martynenko, L. Dergaputskaya, High performance heat insulating castable with microporous anorthite aggregate, in: Proceedings of the 10th UNITECR' 2007, 2007, pp. 1125–1128. [5] A. Harabi, F. Zenikheri, B. Boudaira, F. Bouzerara, A. Guechi, L. Foughali, A new and economic approach to fabricate resistant porous membrane supports using kaolin and CaCO3, J. Eur. Ceram. Soc. 34 (2014) 1329–1340. [6] M. Sutcu, S. Akkurt, A. Bayram, U. Uluca, Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition, Ceram. Int. 38 (2012) 1033–1041. [7] M. Sutcu, S. Akkurt, Utilization of recycled paper processing residues and clay of different sources for the production of porous anorthite ceramics, J. Eur. Ceram. Soc. 30 (2010) 1785–1793. [8] Q.F. Bao, W.X. Dong, X.Y. Gu, K.Y. Hu, X.H. Zhou, Preparation on the in-situ growth of the anorthite/mullite composites, Bull. Chin. Ceram. Soc. 31 (2012) 809–812 (in Chinese). [9] S.J. Li, N. Li, Effects of composition and temperature on porosity and pore size distribution of porous ceramics prepared from Al(OH)3 and kaolinite gangue, Ceram. Int. 33 (2007) 551–556. [10] S.J. Li, N. Li, Y.W. Li, Processing and microstructure characterization of porous corundum-spinel ceramics prepared by in situ decomposition pore-forming technique, Ceram. Int. 34 (2008) 1241–1246.
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics Zheng Miaon,1, Nan Li, Wen Yan The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, China Received 1 July 2014; accepted 20 July 2014
Abstract The effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics prepared from kaolinite gangue, Al(OH)3 and CaCO3 were studied by SEM, XRD and Factsage 6.2 thermalchemical software calculation. It was found that the contents of anorthite, mullite and corundum in porous ceramics depended little on the sintering temperature, but depended mostly on the chemical composition of specimens. Anorthite and mullite existed in the pseudomorphs of kaolinite gangue which were dense and corundum existed in the Al(OH)3 pseudomorphs with micropores. With elevating sintering temperature from 1300 1C to 1450 1C, the apparent porosity of the specimens changed a little but the size of secondary pores increased because of increase of liquid content, resulting in improvement of sintering and elevation of cold crushing strength of specimens. The size distributions of primary pores were almost the same in the specimens sintered at 1300 1C and 1400 1C. In specimens sintered at 1450 1C, the pores in Al(OH)3 pseudomorphs had a considerable growth because sintering of the Al2O3 crystallites was improved. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Anorthite; Porous ceramics; Microstructure; Pore size distribution
1. Introduction Porous ceramics have been widely used in multifunction applications: infiltration, biomaterials and insulating materials. Anorthite processes low thermal conductivity, low thermal expansion coefficient and high thermal shock resistance. There have been several papers on porous anorthite ceramics. Han [1] prepared an ultra low thermal conductivity of porous anorthite ceramics by hydrous foam-gelcasting process. Liu [2] prepared anorthite lightweight thermal insulating bricks and studied the formation process of anorthite. Primachenko [3] reported that an anorthite lightweight material with micropores which had very low thermal conductivity and they also used this n
Corresponding author. Tel.: þ86 027 68862511; fax: þ 86 027 68862121. E-mail addresses: [email protected] (Z. Miao), [email protected] (N. Li), [email protected] (W. Yan). 1 Postal address: Mail box 181, Wuhan University of Science and Technology, Heping Road 947#, Qingshan District, Wuhan City, Hubei Province, 430081, China.
microporous anorthite as aggregate to prepare high performance heat insulating castable [4]. However, lower melting temperature (about 1550 1C), lower refractoriness and refractoriness under load of anorthite result in lower serving temperature. In order to improve the properties at elevated temperature and strength of porous anorthite ceramics, mullite was introduced in anorthite ceramics. Harabi [5] reported an economic approach to fabricate the mullite and anorthite based ceramics. Sutcu [6,7] used recycled paper processing residues and clay to produce porous anorthite ceramics with mullite as a secondary phase. Bao [8] prepared an anorthite/mullite composite to improve the strength of composite. In CaO–Al2O3– SiO2 system, besides anorthite and mullite, corundum is an important crystal phase. However, there are few papers dealing with anorthite–mullite–corundum porous ceramics. The phase composition and microstructure of the CaO–Al2O3–SiO2 porous ceramics depend on the formula of the matrix and sintering temperature. In the present study, the effect of sintering temperature on the phase composition and microstructure of
http://dx.doi.org/10.1016/j.ceramint.2014.07.105 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
2
anorthite–mullite–corundum porous ceramics which is prepared from kaolinite gangue, Al(OH)3 and CaCO3 is investigated. 2. Experimental procedure Al(OH)3 (d50, 16 μm), kaolinite gangue (d50, 16 μm) and CaCO3 (d50, 10 μm) were used as raw materials. The chemical compositions of raw materials are listed in Table 1. The powder mixture consisting of 61.57 wt% kaolinite gangue, 21.08 wt% Al(OH)3 and 17.35 wt% CaCO3 were mixed for 4 h. Then the powder was pressed into rectangular samples of 125 mm length 25 mm width 25 mm thickness at a pressure of about 30 MPa. The green compacts after drying at 110 1C were heated at 1300 1C, 1400 1C, 1450 1C for 180 min in an electric furnace, then furnace-cooled. Apparent porosity was measured by Archimedes' principle with water as the medium. The linear change of samples was determined by the length measurements before and after the sintering process. The cold crushing strength of the specimens was obtained according to Chinese standard GB/T 3997.21998. The microstructure of the samples was obtained using scanning electron microscopy with EDS (SEM, Quanta 400, FEI Company, USA). Pore size distribution and median pore size were obtained by a microscopy measurement method through an optical microscope (Axioskop40). Phase analysis was performed using X-ray diffraction and Cu Kα radiation with a scanning speed of 21 per minute (X'pert PRO MPD, Philips, Eindhoven, Netherlands). The relative content of identified phases was calculated by the semi-quantitative
analysis in HighScore works on basis of the RIR ( ¼ Reference Intensity Ratio) values. Simultaneously, phase contents of specimens were also calculated by factsage 6.2 thermalchemical software. In this calculation, the database of Fact, Fact53, FToxid, Ftsalt were used. 3. Results and discussion Fig. 1 shows change of apparent porosity and linear change with sintering temperature, and Fig. 2 shows change of cold crushing strength with sintering temperature. With the temperature
Fig. 2. Cold crushing strength of specimens as a function of temperature.
Table 1 Chemical composition of raw materials (wt%). Materials
Al2O3 SiO2 Fe2O3 CaO
Kaolinite gangue Al(OH)3 CaCO3
38.87 43.03 0.30 66.79 0.09
0.12 0.07 0.71 0.06
MgO K2O
Na2O TiO2 Ignition loss
0.64 0.42 0.087 0.26
0.48
15.91
0.49 0.33 0.01 0.13 0.019 32.04 53.66 1.64 0.005 0.015 0.007 43.81
Fig. 3. XRD patterns of samples sintered at various temperatures. ▲ – Mullite, ★ – Anorthite, ■ – Corundum, ● – Gehlenite, and ○ – Cristobalite.
Table 2 Relative contents of the crystalline phase of specimens sintered at different temperatures (wt%).
Fig. 1. Apparent porosity and linear change of specimens as a function of temperature.
1300 1C 1400 1C 1450 1C
Mullite
Anorthite
Corundum
Gehlenite
Cristobalite
16 15 15
62 70 67
11 15 18
8
3
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
increasing, apparent porosity and linear shrinkage changed a little. With the temperature increasing, cold crushing strength of specimens increased and when the sintering temperature increased from 1400 1C to 1450 1C strength of specimens increased sharply. XRD patterns of samples sintered at various temperatures are shown in Fig. 3. When the specimens were sintered at 1300 1C, the major phase was anorthite (PDF, 891462); and
3
minor phases were corundum (PDF, 711123), cristobalite (PDF, 850621), mullite (PDF, 791457), gehlenite (PDF, 732041). When the specimens were sintered at a temperature above 1400 1C, peaks of gehlenite and cristobalite disappeared. Table 2 lists the relative contents of the crystalline phase of specimens obtained by XRD. With increase of sintering temperature, the contents of anorthite and mullite changed a little, but corundum content increased slightly. The dependence of phase contents obtained by Factsage 6.2 on sintering temperature is shown in Fig. 4. The content of anorthite, mullite and corundum changed slightly, according with the results of XRD. It means that based on the chemical composition of the specimens in this study, the content of
Table 3 EDS analysis of points in Fig. 5 (At%).
Fig. 4. Phase content obtained by Factsage 6.2 as a function of sintering temperature.
Fig. 5. SEM micrographs of specimens sintered at 1300 1C and 1450 1C.
Points
Al (K)
Si (K)
Ca (K)
O (K)
1 2 3 4 5
16.21 15.72 42.26 13.57 15.52
18.68 16.59
0.45 3.37
13.53 16.48
6.44 3.98
64.66 66.04 57.60 66.07 64.02
Fig. 6. Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1450 1C.
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
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Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
Fig. 7. Pore size distribution (a) and cumulative volume (b) of secondary pores of specimens sintered at various temperatures.
Fig. 8. Pore size distribution (a) and cumulative volume (b) of primary pores in Al(OH)3 pseudomorphs.
anorthite, mullite and corundum of the specimens depends little on sintering temperature, but mostly depends on the chemical composition. However, the liquid phase content increased with sintering temperature. Fig. 5 gives the SEM microstructures of specimens sintered at 1300 1C and 1450 1C. The EDS results are given in Table 3. Some pseudomorphs of Al(OH)3 and kaolinite gangue existed. Kaolinite gangue pseudomorphs were dense because they were easy to be sintered at lower temperatures. A lot of micropores were in the Al(OH)3 pseudomorphs, showing as Fig. 6. We called them ‘primary pores’ and pores among the pseudomorphs were called ‘secondary pores’ [9,10]. Fig. 6 shows the microstructure of Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1450 1C. In the specimens sintered at 1300 1C, corundum crystallites sizes were small and a lot of micropores were located in the Al(OH)3 pseudomorphs; With the sintering temperature increasing to 1450 1C, size of corundum crystallites and pores increased a lot in the Al(OH)3 pseudomorphs. Fig. 7 shows the pore size distribution and cumulative volume of secondary pores and Fig. 8 shows the pores size distribution and cumulative volume of the primary pores in the pseudomorphs of Al(OH)3. In the three specimens, most of secondary pores were 40 μm in size. With increase of sintering temperature from 1300 1C to 1450 1C, the size of secondary
pores increased. The d50 of the specimens sintered at 1300 1C, 1400 1C and 1450 1C were 11.21 μm, 19.22 μm and 23.40 μm, respectively. For the specimens sintered at 1450 1C, the pores with size in the range from 20 μm to 30 μm increased, results in increase of the thickness of solid phase between pores. As a result, the cold crushing strength of specimens sintered at 1450 1C was higher than those sintered at 1300 1C and 1400 1C. The size distributions of primary pores in Al(OH)3 pseudomorphs of specimens sintered at 1300 1C and 1400 1C were almost the same with the d50 of 0.56 μm. It means that the sintering of Al2O3 crystallites in Al(OH)3 pseudomorphs occurs slightly, because the impurities and liquid in the pseudomorphs are very low. In the specimens sintered at 1450 1C, the primary pores grow, because the sintering of Al2O3 crystallites occurred considerably. 4. Conclusions Based on the chemical composition in this study, the contents of anorthite, mullite and corundum in porous ceramics depend little on the sintering temperature, but depend mostly on the chemical composition of specimens. Anorthite and mullite exist in the kaolinite gangue pseudomorphs which are dense and corundum exists in the Al(OH)3 pseudomorphs which are porous. With elevating sintering
Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105
Z. Miao et al. / Ceramics International ] (]]]]) ]]]–]]]
temperature from 1300 1C to 1450 1C, the apparent porosity of the specimens changes a little but the size of secondary pores increase because of increase of liquid content, resulting in improvement of sintering and elevation of cold crushing strength of specimens. The size distributions of primary pores are almost the same in the specimens sintered at 1300 1C and 1400 1C. In specimens sintered at 1450 1C, the pores in Al(OH)3 pseudomorphs have a considerable growth because sintering of the Al2O3 crystallites is improved. Acknowledgments The authors would like to thank the Educational Commission of Hubei Province of China (Grant no. Q20131111) and the 973 Program Earlier Research Project (Grant no. 2012CB722702) for financially supporting this work. References [1] Y. Han, C.W. Li, C. Bian, S.B. Li, C.A. Wang, Porous anorthite ceramics with ultra-low thermal conductivity, J. Eur. Ceram. Soc. 33 (2013) 2573–2578. [2] Q. Liu, Z.H. Pan, Q.B. Li, Y.H. Ruan, Preparation of anorthite lightweight thermal insulating brick and the formation process of anorthite, Bull. Chin. Ceram. Soc. 29 (2010) 1269–1274 (in Chinese).
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Please cite this article as: Z. Miao, et al., Effect of sintering temperature on the phase composition and microstructure of anorthite–mullite–corundum porous ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.105