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Influence of silica sources on morphology of mullite whiskers in Na2SO4 flux
Journal of Alloys and Compounds 484 (2009) 580–584
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Influence of silica sources on morphology of mullite whiskers in Na2 SO4 flux Pengyu Zhang ∗ , Jiachen Liu, Haiyan Du, Zhongqiu Li, Shun Li, Chao Chen Key Lab of Advanced Ceramics and Machining Technology, College of Materials Science & Engineering, Tianjin University, Tianjin 300072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 29 April 2009 Accepted 29 April 2009 Available online 6 May 2009
a b s t r a c t The influence of crystalline form of silica on the morphology of mullite whiskers in Na2 SO4 flux was studied. Isolated mullite whiskers with 5 m in average length were observed in the sample with amorphous and fused silica while clusters of mullite whiskers were obtained in the sample with quartz. Two distinct nucleation mechanisms were proposed to explain the morphology of the mullite whiskers. © 2009 Elsevier B.V. All rights reserved.
Keywords: Ceramics Crystal growth Anisotropy Scanning electron microscopy X-ray diffraction
1. Introduction Mullite ceramics are known as superior engineering materials due to their excellent mechanical property [1–3]. Mullite whiskers, an important component in making reinforced composites, were used at high-temperature owing to their stable flexural strength which does not decrease even at temperatures of as high as approximately 1300 ◦ C [4]. Recently, porous mullite ceramics prepared with mullite whiskers were selected as membrane reactor and hightemperature particulate filter due to its specific pore distribution [5,6]. Various processing routes have been reported for their preparation such as vapor-phase-reaction [7], thermal decomposition of minerals [8] and rare earth oxide doping [9,10]. As a new ceramic powder synthesis method, molten salts method was widely employed to synthesize functional and structural ceramics [11,12]. Molten salts provide liquid environment in which the growth of whiskers was facilitated. Nucleation and growth of the grains are dependent on dissolution of chemical reagents in the molten flux. Hence, the morphology and characteristics of grains can be varied by adjusting raw materials, temperature, sorts and amount of molten salts. Hashimoto and Yamaguchi synthesized acicular mullite powders by reaction of aluminum sulfate and amorphous silica in various molten salts [13,14]. However, the morphology of the whiskers was observed to be different in K2 SO4 and Na2 SO4 flux. Ouatib et al. prepared mullite whiskers by reaction of aluminum sulfate and amorphous silica in molten eutectic
∗ Corresponding author. E-mail address: [email protected] (P. Zhang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.138
(Na0.8 ,K0.2 )2 SO4 [15]. While the effect of molten salts on morphology of mullite whiskers has been demonstrated, the influence of raw materials was scarcely reported. The present paper is concerned with the influence of crystalline form of silica raw materials on the morphology of mullite whiskers. The dissolution capability of various silica raw materials was investigated for the reason that it could change the characters of melt and further affect the morphology of mullite whiskers. Furthermore, two distinct nucleation
Fig. 1. Phase diagram in the system Na2 SO4 –Al2 (SO4 )3 [17].
P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
Fig. 2. XRD patterns of the samples [Al2 (SO4 )3 :amorphous SiO2 :Na2 SO4 = 2.5:1:X (wt%), X = 1.75, 3.5, 7, 14] heated at 1000 ◦ C for 3 h. M; mullite, A; alumina, N; NaAlSiO4 , C; cristobalite.
mechanisms were proposed to explain the observed morphology of mullite whiskers. 2. Experimental procedure As raw materials, various silica sources including amorphous silica, quartz and fused silica (>99% purity), aluminum sulfate hydrate [Al2 (SO4 )3 ·18H2 O] (>99% purity) and sodium sulfate (Na2 SO4 ) (>99% purity) (Jiangtian Chemical Co., Tianjin, China) were used. Particle size of the amorphous silica, quartz and fused silica powders was 20–30 m. First, Al2 (SO4 )3 ·18H2 O were calcined at 300 ◦ C for 3 h to obtain Al2 (SO4 )3 . According to the former research results that the Si-rich composition is beneficial for needlelike mullite grains formation [15,16], the constant
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Fig. 3. XRD patterns of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h. M; mullite, Q; quartz, AS; aluminum oxonium sulfate hydroxide.
composition [Al2 (SO4 )3 :SiO2 = 2.5:1(wt%)] was employed in all samples. For the samples with various Na2 SO4 contents, Al2 (SO4 )3 and Na2 SO4 by weight percentage (wt%) as 2.5:14, 2.5:7, 2.5:3.5, and 2.5:1.75 were used and present by solid circles in Al2 (SO4 )3 –Na2 SO4 phase diagram [17] (Fig. 1). For the samples with various silica sources, the constant ratio of raw materials was selected to observe the influence of crystalline form of silica on morphology of mullite whiskers. The powder mixtures were milled in a jar for 4 h. According to the former research [14], they were then heated at 1000 ◦ C for 0.5 h and 3 h in a conventional electric furnace. The heating rate was 300 ◦ C/h. Sodium sulfate in the reactant mass was removed by washing with 95 ◦ C water. Finally, white powders were obtained. Crystalline phase and morphology of the powders were examined by X-ray diffraction (XRD, D/Max-2500 Rigaku) and scanning electronic microscope (SEM, XL30 Philips), respectively.
Fig. 4. SEM photographs of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h.
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3. Results and discussion Fig. 2 shows phase compositions of the samples [Al2 (SO4 )3 :amorphous SiO2 :Na2 SO4 = 2.5:1:X (wt%), X = 1.75, 3.5, 7, 14] obtained after heating at 1000 ◦ C. It is found that at X = 1.75, NaAlSiO4 (PDF# 521342) was detected in sample (Fig. 2). This is similar to that observed when preparing mullite whiskers with K2 SO4 flux [13]. At X = 7 and 14, the peak intensities of mullite (3Al2 O3 ·2SiO2 , orthorhombic system, PDF# 791455) decreased, whereas those of corundum (Al2 O3 , hexagonal system, PDF# 421468) and cristobalite (-SiO2 , tetragonal system, PDF# 821409) increased. It seems that the formation of mullite and the polymorphic transformation of alumina and silica are in competition, and obviously excess of Na2 SO4 flux is not beneficial for the formation of mullite. At X = 3.5, the diffraction peaks of mullite were sharper than the other cases, it is therefore the most appropriated composition for the crystallization of mullite whiskers. As a result, Al2 (SO4 )3 :SiO2 :Na2 SO4 = 2.5:1:3.5 (wt%), was selected as the optimal ratio in the experiments to investigate the influence of crystalline form of the silica. Fig. 3 shows phase compositions of the samples obtained after heating at 1000 ◦ C for 0.5 h. It is found that mullite phase has been formed in all samples. In samples with quartz and fused silica, aluminum sulfate hydrate transformed to mullite. A small amount of aluminum oxonium sulfate hydroxide [(H3 O)Al3 (SO4 )2 (OH)6 , PDF# 160409] was detected in the sample with amorphous silica, which means that the reaction was incomplete. Fig. 4 shows SEM photographs of the samples. An aggregated structure was observed in the samples with amorphous silica and fused silica (Fig. 4a and b). On the other hand, individual silica was not observed in the sample with quartz. It seemed that quartz was enwrapped by clusters of needlelike grains (Fig. 4c). Both XRD patterns and SEM pho-
Fig. 5. XRD patterns of the samples with (a) amorphous silica, (b) fused silica, (c) quartz heated at 1000 ◦ C for 3 h. M; mullite, Q; quartz.
tographs indicated that the crystallization of mullite whiskers was incomplete. Based on the above experimental results, longer dwelling time was employed to observe further crystallization of the mullite whiskers at the final stage. Fig. 5 shows phase compositions of the samples obtained after heating at 1000 ◦ C for 3 h. When the dwelling time was increased from 0.5 h to 3 h, the diffraction peaks of mullite became sharper in all samples. The residual amorphous SiO2 (consistent diffraction peak at 14–25◦ ) and quartz were detected owing to the Si-rich composition. Fig. 6 shows SEM pho-
Fig. 6. SEM photographs of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 3 h.
P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
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Fig. 7. SEM photographs of (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h.
tographs of the samples obtained at 1000 ◦ C for 3 h. It is found that the morphology of the mullite powders changed to fine needlelike form. In the samples with amorphous silica and fused silica, isolated mullite whiskers are observed (Fig. 6a and b) while clusters of mullite whiskers in the sample with quartz (Fig. 6c). The results indicated that the effect of crystalline form of silica raw materials on the morphology of mullite whiskers was significant in molten salts method. Similar effect was reported in the case of solid-state reaction [18]. However, reasons of the influence of raw materials on mullite are still unclear in the molten salts method. It is well known that, for molten salts method, dissolved chemical reagents react with each other and transform via precipitation [15]. In this case, according to Al2 (SO4 )3 –Na2 SO4 phase diagram
(Fig. 1), the melting temperature of Al2 (SO4 )3 –Na2 SO4 mixture in the sample is about 675 ◦ C and thus Al2 (SO4 )3 has been dissolved in a liquid phase below the mullite formation temperature. Therefore, it is concluded that the crystallization of mullite whiskers was controlled by the dissolution of silica. Fig. 7 shows morphologies of the amorphous silica, fused silica and quartz heated at 1000 ◦ C for 0.5 h in Na2 SO4 flux. Dissolved-recrystallized structure was observed for whole grains of amorphous silica (Fig. 7a) and fuse silica (Fig. 7b) but the edge of quartz (Fig. 7c). These results indicate that amorphous silica and fused silica were completely dissolved in Na2 SO4 flux at 1000 ◦ C for 0.5 h and thus homogeneous melt formed in the two samples at the initial stage. In contrast, only the surface layer of quartz was dissolved under the same conditions, which leaded to heterogeneous melt.
Fig. 8. A schematic illustration of nucleation in the samples with amorphous silica and fused silica.
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P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
Fig. 9. A schematic illustration of nucleation in the sample with quartz.
Nucleation theory indicates that the nuclei formation is affected by the degree of homogeneity of melt. According to the theory, nucleation mechanism of mullite in sample with quartz is different from that of the other two samples. A schematic illustration is shown in Fig. 8 to describe the crystallization of mullite at different stages in the samples with amorphous silica and fused silica. In these two samples, homogeneous nucleation occurred in melt due to the strong dissolution capability of amorphous silica and fused silica at the initial stage. It is described as homogeneous nucleation. However, nuclei were not formed in homogeneous mixture at the initial stage due to the energy barrier of nucleation at liquid–solid interface. This explains the fact that no needlelike grains were observed by SEM (Fig. 4a) and aluminum oxonium sulfate hydroxide phase was detected by XRD in the sample with amorphous silica heated at 1000 ◦ C for 0.5 h. With increasing dwelling time, the homogeneous nucleation was promoted, which leaded to the formation of isolated mullite whiskers, as shown in Fig. 6a and b. A schematic illustration of the crystallization in the sample with quartz is presented in Fig. 9. In this case, owing to the lower barrier of nucleation at the interface provided by the undissolved quartz, crystal was easier to nucleate and sited at the interface at the initial stage. This is described as heterogeneous nucleation. The heterogeneous nucleation explains the fact that clusters of needlelike grains were formed which enwrapped the undissolved quartz at the initial stage, as shown in Fig. 4c. Local heterogeneities in the molten salt acted as nucleation sites for the grains [19]. With increasing dwelling time, growth of the mullite whiskers on surface of the undissolved quartz prevented its further dissolution. The undissolved quartz remained as the core of clusters of the mullite whiskers, which was detected by XRD in the sample with quartz heated at 1000 ◦ C for 3 h. 4. Conclusions Fine grained mullite whiskers were prepared via the reaction of powder mixtures [Al2 (SO4 )3 :SiO2 :Na2 SO4 = 2.5:1:3.5 (wt%)] at 1000 ◦ C for 3 h. Isolated mullite whiskers were observed in the
samples with amorphous silica and fused silica, while clusters of mullite whiskers were observed in the sample with quartz. It is concluded that dissolution capability of various silica raw materials changed the degree of homogeneity of melt and further affected the nucleation of mullite. The variation in the mullite nucleation from homogeneous nucleation to heterogeneous nucleation resulted in an essential influence on the morphology of mullite whiskers. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 program) under grant No. 2006AA03Z540, and the National Natural Science Foundation of China under grant No. 50772073. References [1] S. Kanzaki, H. Tabata, T. Kumazawa, S. Ohta, J. Am. Ceram. Soc. 68 (1985) 6–7. [2] M. Schmucker, W. Albers, H. Schneider, J. Eur. Ceram. Soc. 14 (1994) 511–515. [3] Y.M. Park, T.Y. Yang, S.Y. Yoon, R. Stevens, H.C. Park, Mater. Sci. Eng. A 454–455 (2007) 518–522. [4] M.G.M.U. Ismail, Z. Nakai, S. Somiya, J. Am. Ceram. Soc. 70 (1987) C7–C8. [5] A.J. Pyzik, C.S. Todd, C. Han, J. Eur. Ceram. Soc. 28 (2008) 383–391. [6] G.L. Chen, H. Qi, W.H. Xing, N.P. Xu, J. Membr. Sci. 318 (2008) 38–44. [7] K. Okada, N. Otuska, J. Am. Ceram. Soc. 74 (1991) 2414–2418. [8] Y.C. Dong, X.Y. Feng, X.F. Feng, Y.W. Ding, X.Q. Liu, G.Y. Meng, J. Alloys Compd. 460 (2008) 599–606. [9] L.B. Kong, T.S. Zhang, J. Ma, F. Boey, R.F. Zhang, J. Alloys Compd. 373 (2004) 290–299. [10] J. Paementier, S. Vilminot, J. Alloys Compd. 264 (1998) 136–141. [11] S. Okada, T. Shishido, T. Mori, K. Iizumi, K. Kudou, K. Nakajima, J. Alloys Compd. 458 (2008) 297–301. [12] B. Roy, P.A. Fuierer, J. Am. Ceram. Soc. 92 (2009) 520–523. [13] S. Hashimoto, A. Yamaguchi, J. Eur. Ceram. Soc. 20 (2000) 397–402. [14] S. Hashimoto, A. Yamaguchi, J. Ceram. Soc. Jpn. 112 (2004) 104–109. [15] R.E. Ouatib, S. Guillemet, B. Durand, A. Samdi, L.E. Rakho, R. Moussa, J. Eur. Ceram. Soc. 25 (2005) 73–80. [16] M.F. de Souza, J. Yamamoto, I. Regiani, C.O. Paiva-Santos, D.P.F. de Souza, J. Am. Ceram. Soc. 83 (2000) 60–64. [17] E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists 1975 Supplement, Fig. 4708, Am. Ceram. Soc., Westerville, OH, 1975, pp. 318–1318. [18] B. Saruhan, W. Albers, H. Schneider, W.A. Kaysser, J. Eur. Ceram. Soc. 16 (1996) 1075–1081. [19] B. Roy, S.P. Ahrenkiel, P.A. Fuierer, J. Am. Ceram. Soc. 91 (2008) 2455–2463.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Influence of silica sources on morphology of mullite whiskers in Na2 SO4 flux Pengyu Zhang ∗ , Jiachen Liu, Haiyan Du, Zhongqiu Li, Shun Li, Chao Chen Key Lab of Advanced Ceramics and Machining Technology, College of Materials Science & Engineering, Tianjin University, Tianjin 300072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 29 April 2009 Accepted 29 April 2009 Available online 6 May 2009
a b s t r a c t The influence of crystalline form of silica on the morphology of mullite whiskers in Na2 SO4 flux was studied. Isolated mullite whiskers with 5 m in average length were observed in the sample with amorphous and fused silica while clusters of mullite whiskers were obtained in the sample with quartz. Two distinct nucleation mechanisms were proposed to explain the morphology of the mullite whiskers. © 2009 Elsevier B.V. All rights reserved.
Keywords: Ceramics Crystal growth Anisotropy Scanning electron microscopy X-ray diffraction
1. Introduction Mullite ceramics are known as superior engineering materials due to their excellent mechanical property [1–3]. Mullite whiskers, an important component in making reinforced composites, were used at high-temperature owing to their stable flexural strength which does not decrease even at temperatures of as high as approximately 1300 ◦ C [4]. Recently, porous mullite ceramics prepared with mullite whiskers were selected as membrane reactor and hightemperature particulate filter due to its specific pore distribution [5,6]. Various processing routes have been reported for their preparation such as vapor-phase-reaction [7], thermal decomposition of minerals [8] and rare earth oxide doping [9,10]. As a new ceramic powder synthesis method, molten salts method was widely employed to synthesize functional and structural ceramics [11,12]. Molten salts provide liquid environment in which the growth of whiskers was facilitated. Nucleation and growth of the grains are dependent on dissolution of chemical reagents in the molten flux. Hence, the morphology and characteristics of grains can be varied by adjusting raw materials, temperature, sorts and amount of molten salts. Hashimoto and Yamaguchi synthesized acicular mullite powders by reaction of aluminum sulfate and amorphous silica in various molten salts [13,14]. However, the morphology of the whiskers was observed to be different in K2 SO4 and Na2 SO4 flux. Ouatib et al. prepared mullite whiskers by reaction of aluminum sulfate and amorphous silica in molten eutectic
∗ Corresponding author. E-mail address: [email protected] (P. Zhang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.138
(Na0.8 ,K0.2 )2 SO4 [15]. While the effect of molten salts on morphology of mullite whiskers has been demonstrated, the influence of raw materials was scarcely reported. The present paper is concerned with the influence of crystalline form of silica raw materials on the morphology of mullite whiskers. The dissolution capability of various silica raw materials was investigated for the reason that it could change the characters of melt and further affect the morphology of mullite whiskers. Furthermore, two distinct nucleation
Fig. 1. Phase diagram in the system Na2 SO4 –Al2 (SO4 )3 [17].
P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
Fig. 2. XRD patterns of the samples [Al2 (SO4 )3 :amorphous SiO2 :Na2 SO4 = 2.5:1:X (wt%), X = 1.75, 3.5, 7, 14] heated at 1000 ◦ C for 3 h. M; mullite, A; alumina, N; NaAlSiO4 , C; cristobalite.
mechanisms were proposed to explain the observed morphology of mullite whiskers. 2. Experimental procedure As raw materials, various silica sources including amorphous silica, quartz and fused silica (>99% purity), aluminum sulfate hydrate [Al2 (SO4 )3 ·18H2 O] (>99% purity) and sodium sulfate (Na2 SO4 ) (>99% purity) (Jiangtian Chemical Co., Tianjin, China) were used. Particle size of the amorphous silica, quartz and fused silica powders was 20–30 m. First, Al2 (SO4 )3 ·18H2 O were calcined at 300 ◦ C for 3 h to obtain Al2 (SO4 )3 . According to the former research results that the Si-rich composition is beneficial for needlelike mullite grains formation [15,16], the constant
581
Fig. 3. XRD patterns of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h. M; mullite, Q; quartz, AS; aluminum oxonium sulfate hydroxide.
composition [Al2 (SO4 )3 :SiO2 = 2.5:1(wt%)] was employed in all samples. For the samples with various Na2 SO4 contents, Al2 (SO4 )3 and Na2 SO4 by weight percentage (wt%) as 2.5:14, 2.5:7, 2.5:3.5, and 2.5:1.75 were used and present by solid circles in Al2 (SO4 )3 –Na2 SO4 phase diagram [17] (Fig. 1). For the samples with various silica sources, the constant ratio of raw materials was selected to observe the influence of crystalline form of silica on morphology of mullite whiskers. The powder mixtures were milled in a jar for 4 h. According to the former research [14], they were then heated at 1000 ◦ C for 0.5 h and 3 h in a conventional electric furnace. The heating rate was 300 ◦ C/h. Sodium sulfate in the reactant mass was removed by washing with 95 ◦ C water. Finally, white powders were obtained. Crystalline phase and morphology of the powders were examined by X-ray diffraction (XRD, D/Max-2500 Rigaku) and scanning electronic microscope (SEM, XL30 Philips), respectively.
Fig. 4. SEM photographs of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h.
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P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
3. Results and discussion Fig. 2 shows phase compositions of the samples [Al2 (SO4 )3 :amorphous SiO2 :Na2 SO4 = 2.5:1:X (wt%), X = 1.75, 3.5, 7, 14] obtained after heating at 1000 ◦ C. It is found that at X = 1.75, NaAlSiO4 (PDF# 521342) was detected in sample (Fig. 2). This is similar to that observed when preparing mullite whiskers with K2 SO4 flux [13]. At X = 7 and 14, the peak intensities of mullite (3Al2 O3 ·2SiO2 , orthorhombic system, PDF# 791455) decreased, whereas those of corundum (Al2 O3 , hexagonal system, PDF# 421468) and cristobalite (-SiO2 , tetragonal system, PDF# 821409) increased. It seems that the formation of mullite and the polymorphic transformation of alumina and silica are in competition, and obviously excess of Na2 SO4 flux is not beneficial for the formation of mullite. At X = 3.5, the diffraction peaks of mullite were sharper than the other cases, it is therefore the most appropriated composition for the crystallization of mullite whiskers. As a result, Al2 (SO4 )3 :SiO2 :Na2 SO4 = 2.5:1:3.5 (wt%), was selected as the optimal ratio in the experiments to investigate the influence of crystalline form of the silica. Fig. 3 shows phase compositions of the samples obtained after heating at 1000 ◦ C for 0.5 h. It is found that mullite phase has been formed in all samples. In samples with quartz and fused silica, aluminum sulfate hydrate transformed to mullite. A small amount of aluminum oxonium sulfate hydroxide [(H3 O)Al3 (SO4 )2 (OH)6 , PDF# 160409] was detected in the sample with amorphous silica, which means that the reaction was incomplete. Fig. 4 shows SEM photographs of the samples. An aggregated structure was observed in the samples with amorphous silica and fused silica (Fig. 4a and b). On the other hand, individual silica was not observed in the sample with quartz. It seemed that quartz was enwrapped by clusters of needlelike grains (Fig. 4c). Both XRD patterns and SEM pho-
Fig. 5. XRD patterns of the samples with (a) amorphous silica, (b) fused silica, (c) quartz heated at 1000 ◦ C for 3 h. M; mullite, Q; quartz.
tographs indicated that the crystallization of mullite whiskers was incomplete. Based on the above experimental results, longer dwelling time was employed to observe further crystallization of the mullite whiskers at the final stage. Fig. 5 shows phase compositions of the samples obtained after heating at 1000 ◦ C for 3 h. When the dwelling time was increased from 0.5 h to 3 h, the diffraction peaks of mullite became sharper in all samples. The residual amorphous SiO2 (consistent diffraction peak at 14–25◦ ) and quartz were detected owing to the Si-rich composition. Fig. 6 shows SEM pho-
Fig. 6. SEM photographs of the samples with (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 3 h.
P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
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Fig. 7. SEM photographs of (a) amorphous silica, (b) fused silica and (c) quartz heated at 1000 ◦ C for 0.5 h.
tographs of the samples obtained at 1000 ◦ C for 3 h. It is found that the morphology of the mullite powders changed to fine needlelike form. In the samples with amorphous silica and fused silica, isolated mullite whiskers are observed (Fig. 6a and b) while clusters of mullite whiskers in the sample with quartz (Fig. 6c). The results indicated that the effect of crystalline form of silica raw materials on the morphology of mullite whiskers was significant in molten salts method. Similar effect was reported in the case of solid-state reaction [18]. However, reasons of the influence of raw materials on mullite are still unclear in the molten salts method. It is well known that, for molten salts method, dissolved chemical reagents react with each other and transform via precipitation [15]. In this case, according to Al2 (SO4 )3 –Na2 SO4 phase diagram
(Fig. 1), the melting temperature of Al2 (SO4 )3 –Na2 SO4 mixture in the sample is about 675 ◦ C and thus Al2 (SO4 )3 has been dissolved in a liquid phase below the mullite formation temperature. Therefore, it is concluded that the crystallization of mullite whiskers was controlled by the dissolution of silica. Fig. 7 shows morphologies of the amorphous silica, fused silica and quartz heated at 1000 ◦ C for 0.5 h in Na2 SO4 flux. Dissolved-recrystallized structure was observed for whole grains of amorphous silica (Fig. 7a) and fuse silica (Fig. 7b) but the edge of quartz (Fig. 7c). These results indicate that amorphous silica and fused silica were completely dissolved in Na2 SO4 flux at 1000 ◦ C for 0.5 h and thus homogeneous melt formed in the two samples at the initial stage. In contrast, only the surface layer of quartz was dissolved under the same conditions, which leaded to heterogeneous melt.
Fig. 8. A schematic illustration of nucleation in the samples with amorphous silica and fused silica.
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P. Zhang et al. / Journal of Alloys and Compounds 484 (2009) 580–584
Fig. 9. A schematic illustration of nucleation in the sample with quartz.
Nucleation theory indicates that the nuclei formation is affected by the degree of homogeneity of melt. According to the theory, nucleation mechanism of mullite in sample with quartz is different from that of the other two samples. A schematic illustration is shown in Fig. 8 to describe the crystallization of mullite at different stages in the samples with amorphous silica and fused silica. In these two samples, homogeneous nucleation occurred in melt due to the strong dissolution capability of amorphous silica and fused silica at the initial stage. It is described as homogeneous nucleation. However, nuclei were not formed in homogeneous mixture at the initial stage due to the energy barrier of nucleation at liquid–solid interface. This explains the fact that no needlelike grains were observed by SEM (Fig. 4a) and aluminum oxonium sulfate hydroxide phase was detected by XRD in the sample with amorphous silica heated at 1000 ◦ C for 0.5 h. With increasing dwelling time, the homogeneous nucleation was promoted, which leaded to the formation of isolated mullite whiskers, as shown in Fig. 6a and b. A schematic illustration of the crystallization in the sample with quartz is presented in Fig. 9. In this case, owing to the lower barrier of nucleation at the interface provided by the undissolved quartz, crystal was easier to nucleate and sited at the interface at the initial stage. This is described as heterogeneous nucleation. The heterogeneous nucleation explains the fact that clusters of needlelike grains were formed which enwrapped the undissolved quartz at the initial stage, as shown in Fig. 4c. Local heterogeneities in the molten salt acted as nucleation sites for the grains [19]. With increasing dwelling time, growth of the mullite whiskers on surface of the undissolved quartz prevented its further dissolution. The undissolved quartz remained as the core of clusters of the mullite whiskers, which was detected by XRD in the sample with quartz heated at 1000 ◦ C for 3 h. 4. Conclusions Fine grained mullite whiskers were prepared via the reaction of powder mixtures [Al2 (SO4 )3 :SiO2 :Na2 SO4 = 2.5:1:3.5 (wt%)] at 1000 ◦ C for 3 h. Isolated mullite whiskers were observed in the
samples with amorphous silica and fused silica, while clusters of mullite whiskers were observed in the sample with quartz. It is concluded that dissolution capability of various silica raw materials changed the degree of homogeneity of melt and further affected the nucleation of mullite. The variation in the mullite nucleation from homogeneous nucleation to heterogeneous nucleation resulted in an essential influence on the morphology of mullite whiskers. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 program) under grant No. 2006AA03Z540, and the National Natural Science Foundation of China under grant No. 50772073. References [1] S. Kanzaki, H. Tabata, T. Kumazawa, S. Ohta, J. Am. Ceram. Soc. 68 (1985) 6–7. [2] M. Schmucker, W. Albers, H. Schneider, J. Eur. Ceram. Soc. 14 (1994) 511–515. [3] Y.M. Park, T.Y. Yang, S.Y. Yoon, R. Stevens, H.C. Park, Mater. Sci. Eng. A 454–455 (2007) 518–522. [4] M.G.M.U. Ismail, Z. Nakai, S. Somiya, J. Am. Ceram. Soc. 70 (1987) C7–C8. [5] A.J. Pyzik, C.S. Todd, C. Han, J. Eur. Ceram. Soc. 28 (2008) 383–391. [6] G.L. Chen, H. Qi, W.H. Xing, N.P. Xu, J. Membr. Sci. 318 (2008) 38–44. [7] K. Okada, N. Otuska, J. Am. Ceram. Soc. 74 (1991) 2414–2418. [8] Y.C. Dong, X.Y. Feng, X.F. Feng, Y.W. Ding, X.Q. Liu, G.Y. Meng, J. Alloys Compd. 460 (2008) 599–606. [9] L.B. Kong, T.S. Zhang, J. Ma, F. Boey, R.F. Zhang, J. Alloys Compd. 373 (2004) 290–299. [10] J. Paementier, S. Vilminot, J. Alloys Compd. 264 (1998) 136–141. [11] S. Okada, T. Shishido, T. Mori, K. Iizumi, K. Kudou, K. Nakajima, J. Alloys Compd. 458 (2008) 297–301. [12] B. Roy, P.A. Fuierer, J. Am. Ceram. Soc. 92 (2009) 520–523. [13] S. Hashimoto, A. Yamaguchi, J. Eur. Ceram. Soc. 20 (2000) 397–402. [14] S. Hashimoto, A. Yamaguchi, J. Ceram. Soc. Jpn. 112 (2004) 104–109. [15] R.E. Ouatib, S. Guillemet, B. Durand, A. Samdi, L.E. Rakho, R. Moussa, J. Eur. Ceram. Soc. 25 (2005) 73–80. [16] M.F. de Souza, J. Yamamoto, I. Regiani, C.O. Paiva-Santos, D.P.F. de Souza, J. Am. Ceram. Soc. 83 (2000) 60–64. [17] E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists 1975 Supplement, Fig. 4708, Am. Ceram. Soc., Westerville, OH, 1975, pp. 318–1318. [18] B. Saruhan, W. Albers, H. Schneider, W.A. Kaysser, J. Eur. Ceram. Soc. 16 (1996) 1075–1081. [19] B. Roy, S.P. Ahrenkiel, P.A. Fuierer, J. Am. Ceram. Soc. 91 (2008) 2455–2463.