Availableonlineatwww.sciencedirect.com ° ScienceDirect CERAMICS INTERNATIONAL ELSEVIER Ceramics International 35(2009)579-583 www.elsevier.com/locate/ceramint Mullite whiskers derived from kaolin B.M. Kim"YK Cho.SY Yoon.R. Stevens h.c. park Department of Materials Science and Engineering Pusan National University, Pusan 609-735., South Korea Department of Mechanical Engineering. University of Bath, Bath BA2 97AY, UK Received 19 September 2007; received in revised form 30 October 2007; accepted 10 January 2008 vailable online 24 April 2008 Abstract Short mullite whiskers prepared by firing compacts of kaolin and NH,Al(SO4)2 12H20 powders, with a small addition (0.8, 1.5 wt%)of NaH2PO42H20, in air 1300 and 1400C for 15 h have been characterized in terms of whisker morphology, composition and structure. Relatively uniform whisker shaped crystals grew within the silicate glass matrix. After chemically leaching the glass matrix with HF solution using a microwave heating source, the resulting whiskers were exposed as isolated crystals and exhibited an aspect ratio of >17(0.5 um in diameter) The mullite whiskers had a composition of 51.06 mol% Al2O3 and 48.94 mol% SiO2, with an orthorhombic crystallographic structure. C 2008 Elsevier Ltd and Techna Group S.r. l. All rights reserved. Keywords: A. Calcination; B. Whiskers; D. Mullite: E Structural applications 1. Introduction tube reactor, under a flow of H2/CF4 at 1450C for 3 h. Moyer and Hughes[4] formed fluorotopaz by reacting a mixture of Mullite(3Al2O32SiO2)is an attractive potential engineer- alumina and silica with SiF4 at >600C and then produced the ng ceramic because it has high strength and high creep large interlocked mullite whiskers(diameter 100 um and resistance at both low and high temperatures, a low thermal aspect ratio 10-15) by its subsequent decomposition below expansion coefficient, and good chemical and thermal stability. 1000C. Peng and Sorrell [5] introduced an inexpensive, Whisker shaped mullite has attracted attention as a possible simple method for preparation of mullite whiskers with an reinforcement for high temperature structural materials. The average length of 100 um; the thermal decomposition of stable crystal structure of mullite is orthorhombic with lattice natural topaz with the addition of 0.5 wt% AlF3 was done at constants a=7.545 A, b=7.689A and c=2884 A(JCPDS 1300C for 4 h with and without flowing air Hong et al. [6] Card 15-776), and it consists of edge-shared AlO6 octahedral investigated anisotropic grain growth of B2O3-doped mullite chains aligned in the c-direction and cross-linked by corner- which was seeded with mullite whiskers; after firing at 1650C shared (Si, Al)O4 tetrahedra [1]. Thus, the crystal growth may for 5 h, a system seeded with 2 wt %o mullite whiskers and doped be faster in crystallographic direction parallel to the c-axis than with 2 wt% B2O3, exhibited the largest anisotropic grains with in any other, resulting in a high degree of orientation an aspect ratio of >10 and a length of >30 um. Perera and Several processing routes have been reported for the Otsuka [7] synthesized mullite whiskers with an aspect ratio of preparation of mullite whiskers. Hashimoto and Yamaguchi 3-8(0.5-5 um in length) by the simple process of firing kaolin [2] synthesized mullite whiskers with an aspect ratio of 15-20 minerals at 1400-1600C Al2(SO4)3 and amorphous SiOz together with Na2 SO4 flux in an il More recently, Li et al. [8] prepared short mullite fibers by and a diameter of 0.5-2 um, by firing a powder mixture of apting the kneading-drying-calcination(KDC) process and alumina crucible at 1000C for 2 h Choi and Lee [3]obtained examined the effect of foaming agent on the formation of very large mullite whiskers(>15 um in diameter, >300 um in mullite fiber from kaolin. They reported that the addition of length) by heating a mixture of Sio2 and silicon in an alumina 10 wt% sodium dihydrogen phosphate followed by calcination at 1500C for 10 h, promoted the growth of mullite fiber c1.0 um)and Eo moil addres bcpoarklont 5102392;fax:+82515120528. treatment time (3 h) required for dissolution of the glass an ac kr(H C. Park) matrix in 20 wt HF solution. They also noted that the addition 2-8842/34.00@ 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved 10.1016/j-cera 008.01017
Mullite whiskers derived from kaolin B.M. Kim a , Y.K. Cho a , S.Y. Yoon a , R. Stevens b , H.C. Park a, * a Department of Materials Science and Engineering, Pusan National University, Pusan 609-735, South Korea b Department of Mechanical Engineering, University of Bath, Bath BA2 97AY, UK Received 19 September 2007; received in revised form 30 October 2007; accepted 10 January 2008 Available online 24 April 2008 Abstract Short mullite whiskers prepared by firing compacts of kaolin and NH4Al(SO4)212H2O powders, with a small addition (0.8, 1.5 wt%) of NaH2PO42H2O, in air 1300 and 1400 8C for 15 h have been characterized in terms of whisker morphology, composition and structure. Relatively uniform whisker shaped crystals grew within the silicate glass matrix. After chemically leaching the glass matrix with HF solution using a microwave heating source, the resulting whiskers were exposed as isolated crystals and exhibited an aspect ratio of >17 (0.5 mm in diameter). The mullite whiskers had a composition of 51.06 mol% Al2O3 and 48.94 mol% SiO2, with an orthorhombic crystallographic structure. # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Calcination; B. Whiskers; D. Mullite; E. Structural applications 1. Introduction Mullite (3Al2O32SiO2) is an attractive potential engineering ceramic because it has high strength and high creep resistance at both low and high temperatures, a low thermal expansion coefficient, and good chemical and thermal stability. Whisker shaped mullite has attracted attention as a possible reinforcement for high temperature structural materials. The stable crystal structure of mullite is orthorhombic with lattice constants a = 7.545 A˚ , b = 7.689 A˚ and c = 2.884 A˚ (JCPDS Card # 15-776), and it consists of edge-shared AlO6 octahedral chains aligned in the c-direction and cross-linked by cornershared (Si,Al)O4 tetrahedra [1]. Thus, the crystal growth may be faster in crystallographic direction parallel to the c-axis than in any other, resulting in a high degree of orientation. Several processing routes have been reported for the preparation of mullite whiskers. Hashimoto and Yamaguchi [2] synthesized mullite whiskers with an aspect ratio of 15–20 and a diameter of 0.5–2 mm, by firing a powder mixture of Al2(SO4)3 and amorphous SiO2 together with Na2SO4 flux in an alumina crucible at 1000 8C for 2 h. Choi and Lee [3] obtained very large mullite whiskers (>15 mm in diameter, >300 mm in length) by heating a mixture of SiO2 and silicon in an alumina tube reactor, under a flow of H2/CF4 at 1450 8C for 3 h. Moyer and Hughes [4] formed fluorotopaz by reacting a mixture of alumina and silica with SiF4 at >600 8C and then produced the large interlocked mullite whiskers (diameter 100 mm and aspect ratio 10–15) by its subsequent decomposition below 1000 8C. Peng and Sorrell [5] introduced an inexpensive, simple method for preparation of mullite whiskers with an average length of 100 mm; the thermal decomposition of natural topaz with the addition of 0.5 wt% AlF3 was done at 1300 8C for 4 h with and without flowing air. Hong et al. [6] investigated anisotropic grain growth of B2O3-doped mullite which was seeded with mullite whiskers; after firing at 1650 8C for 5 h, a system seeded with 2 wt% mullite whiskers and doped with 2 wt% B2O3, exhibited the largest anisotropic grains with an aspect ratio of >10 and a length of >30 mm. Perera and Otsuka [7] synthesized mullite whiskers with an aspect ratio of 3–8 (0.5–5 mm in length) by the simple process of firing kaolin minerals at 1400–1600 8C. More recently, Li et al. [8] prepared short mullite fibers by adapting the kneading–drying–calcination (KDC) process and examined the effect of foaming agent on the formation of mullite fiber from kaolin. They reported that the addition of 10 wt% sodium dihydrogen phosphate followed by calcination at 1500 8C for 10 h, promoted the growth of mullite fiber (aspect ratio 28, diameter 1.0 mm) and reduced the treatment time (3 h) required for dissolution of the glass matrix in 20 wt% HF solution. They also noted that the addition www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 579–583 * Corresponding author. Tel.: +82 51 510 2392; fax: +82 51 512 0528. E-mail address: hcpark1@pusan.ac.kr (H.C. Park). 0272-8842/$34.00 # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2008.01.017
580 B.M. Kim et al/Ceramics International 35(2009)579-583 of 10 wt% sodium dihydrogen phosphate caused a sudden reduction(41%)in yield of mullite, compared with 2 wt% However, except for noting the effect of an addition of sodium phosphate to kaolin to aid the fabrication of short mullite fibers the authors gave no detailed data concerning the composi structure of their mullite fibers In the fabrication of mullite whiskers by solid-state reaction in the presence of a liquid phase, it is difficult to dissolve effectively the glass matrix which forms around the whiskers, even though a solution of hf in water is used. In such cases, a highly concentrated leaching solution and/or a prolonged leaching time can also dissolve the whisker pha 山M出 dse formed in the reaction. Microwave-assisted leaching techni- 20. CuKo hues can be used to remove the glass matrix rather more asily due to the Fig. 1. XRD e inherent a dvantages of microwave energy NaH, PO, 2H, O, fired at 1300oC for 15 h; they being selective [9 mullite (JCPDS Card #15-776) In the present work, a similar method to that of li et al. [8] has been applied to obtain mullite whiskers from kaolin; however, different processing conditions designed to be more energy dispersive spectroscopy (EDS), and transmission economical and less destructive to the natural environment have electron microscopy(TEM) been employed. At the same time the key objective of this study has been to further understand the effect of fundamental factors 3. Results underlying the development of mullite whiskers from kaolin, and to characterize the whiskers formed XRD analysis showed the product obtained at 1300C for 15 h to generate characteristic XRD peaks nearly all which 2. Experimental procedure corresponded to mullite(Fig. 1). The product had a relatively uniform microstructure(Fig. 2); it consisted of whiskers seen to A commercial-grade kaolin with a Al2O3/SiO2 molar ratio of have grown within the glass matrix. After chemically leaching 0. 13, NHAl(SO4)2 12H2O(purity >99.9%)obtained from the glass matrix with a 5 wt% HF solution using microwave coal fly ash [10] and reagent-grade NaH_. 2H_O (Junsei heating(50C, 1 h), short mullite whiskers with an aspect ratio Chemical Co., Tokyo, Japan) have been used as starting of <7(0.15 um in diameter)(Fig 3(a)were obtained. They materials. The kaolin was calcined in air at 800C for 2 h to were oriented to a favorable growth direction in specific ethanol for 24 h using a polyethylene bottle with alumina ball mullite into whiskers increased further (<10 in aspect ratio, media. After rotary vacuum evaporation (R-114, Buchi, 0.5-0.7 um in diameter) on firing at 1400C. At this higher Switzerland), the dried powder was ground in an agate mortar temperature, the preferential orientation of the whiskers was and passed through a 200 mesh nylon sieve. A measured not as apparent. The subsequent leaching, using 10 wt% H in order to increase the Al2O3/SiO2 molar ratio to 0.51. In whiskers compared with 5 wt% HF solution. The EDS analysis ddition, 0.8 and 1.5 wt NaH2PO4 2H20 were added to the spectrum of the whiskers confirmed that they consisted of mixture of kaolin and NH,Al(SO4)2. 12H2O. The starting batch composition was selected with reference to the literature of Li et al. [8]. The batch powders were mixed and homogenized by ball milling in ethanol for 8 h using a high density polyethylene bottle with alumina ball media. After drying, the mixed powders were crushed in an agate mortar and passed through a 00 mesh sieve. Cylindrical (10 mm diameter x 5 mm)com- pacts were prepared by die pressing at 70 MPa. The compacts were placed in an alumina crucible and calcined at 1300 and 1400C for 15 h The calcined compacts were treated with 5- 15 wt% HF solution in water; the product was filtered, washed with water, and finally dried. In this case, in order to effectively dissolve the glass matrix from the whiskers, the HF solution was heated at 50C for 1-3 h using a ave heating source gym (2.45 GHz, 3 kW, Hankuk Microwave Co Korea) The resulting whiskers were characterized using X-ray Fig. 2. SEM micrograph of the product obtained with an addition of 0.8 wt% diffractometry (XRD), scanning electron microscopy (SEM), NaH2PO4 2H20; fired at 1300C for 15 h; without chemically leaching
of 10 wt% sodium dihydrogen phosphate caused a sudden reduction (41%) in yield of mullite, compared with 2 wt%. However, except for noting the effect of an addition of sodium phosphate to kaolin to aid the fabrication of short mullite fibers, the authors gave no detailed data concerning the composition or structure of their mullite fibers. In the fabrication of mullite whiskers by solid-state reaction in the presence of a liquid phase, it is difficult to dissolve effectively the glass matrix which forms around the whiskers, even though a solution of HF in water is used. In such cases, a highly concentrated leaching solution and/or a prolonged leaching time can also dissolve the whisker phase formed in the reaction. Microwave-assisted leaching techniques can be used to remove the glass matrix rather more easily due to the inherent advantages of microwave energy being selective [9]. In the present work, a similar method to that of Li et al. [8] has been applied to obtain mullite whiskers from kaolin; however, different processing conditions designed to be more economical and less destructive to the natural environment have been employed. At the same time the key objective of this study has been to further understand the effect of fundamental factors underlying the development of mullite whiskers from kaolin, and to characterize the whiskers formed. 2. Experimental procedure A commercial-grade kaolin with a Al2O3/SiO2 molar ratio of 0.13, NH4Al(SO4)212H2O (purity >99.9%) obtained from coal fly ash [10] and reagent-grade NaH2PO42H2O (Junsei Chemical Co., Tokyo, Japan) have been used as starting materials. The kaolin was calcined in air at 800 8C for 2 h to increase its reactivity; it was subsequently ball-milled in ethanol for 24 h using a polyethylene bottle with alumina ball media. After rotary vacuum evaporation (R-114, Buchi, Switzerland), the dried powder was ground in an agate mortar and passed through a 200 mesh nylon sieve. A measured amount of NH4Al(SO4)212H2O was added to the kaolin in order to increase the Al2O3/SiO2 molar ratio to 0.51. In addition, 0.8 and 1.5 wt% NaH2PO42H2O were added to the mixture of kaolin and NH4Al(SO4)212H2O. The starting batch composition was selected with reference to the literature of Li et al. [8]. The batch powders were mixed and homogenized by ball milling in ethanol for 8 h using a high density polyethylene bottle with alumina ball media. After drying, the mixed powders were crushed in an agate mortar and passed through a 200 mesh sieve. Cylindrical (10 mm diameter 5 mm) compacts were prepared by die pressing at 70 MPa. The compacts were placed in an alumina crucible and calcined at 1300 and 1400 8C for 15 h. The calcined compacts were treated with 5– 15 wt% HF solution in water; the product was filtered, washed with water, and finally dried. In this case, in order to effectively dissolve the glass matrix from the whiskers, the HF solution was heated at 50 8C for 1–3 h using a microwave heating source (2.45 GHz, 3 kW, Hankuk Microwave Co. Korea). The resulting whiskers were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). 3. Results XRD analysis showed the product obtained at 1300 8C for 15 h to generate characteristic XRD peaks nearly all which corresponded to mullite (Fig. 1). The product had a relatively uniform microstructure (Fig. 2); it consisted of whiskers seen to have grown within the glass matrix. After chemically leaching the glass matrix with a 5 wt% HF solution using microwave heating (50 8C, 1 h), short mullite whiskers with an aspect ratio of <7 (0.15 mm in diameter) (Fig. 3(a)) were obtained. They were oriented to a favorable growth direction in specific domains (Fig. 3(b)). As shown in Fig. 4, the growth of the mullite into whiskers increased further (<10 in aspect ratio, 0.5–0.7 mm in diameter) on firing at 1400 8C. At this higher temperature, the preferential orientation of the whiskers was not as apparent. The subsequent leaching, using 10 wt% HF in water, resulted in more unattached and less aggregated whiskers compared with 5 wt% HF solution. The EDS analysis spectrum of the whiskers confirmed that they consisted of Fig. 1. XRD patterns of the product obtained with an addition of 0.8 wt% NaH2PO42H2O, fired at 1300 8C for 15 h; they coincided well with those of mullite (JCPDS Card #15-776). Fig. 2. SEM micrograph of the product obtained with an addition of 0.8 wt% NaH2PO42H2O; fired at 1300 8C for 15 h; without chemically leaching. 580 B.M. Kim et al. / Ceramics International 35 (2009) 579–583
B.M. Kim et al./Ceramics Intemational 35(2009)579-583 (a)/A (a BS Fig. 3. SEM micrographs of the product obtained with an addition of NaH2PO4 2H2O; fired at 1300C for 15 h; leached by 5 wt% HF solt heating source: showing(a)random orie olution at Fig. 4. SEM micrographs of the product obtained with an addition of 0.8 wt% NaH2PO4 2H20: fired 1400C for 15 h: leached by (a)5 wt%o and(b)10 wt% HF solution at 50oC for I h and(b) preferential orientation, of the whiskers. 0.136Na2O,0.154CaO,.0.014MgO.0.008P2Os,and 63.59 wt% Al2O3 and 36 41 wt% SiO2(Al2O3/SiO2=1.04, 0.104 wt% TiO2; such components, together with the addition molar ratio)(Fig. 5), this result indicating an alumina deficient of NaH2 PO4 2H20 contribute to the formation of low meltin composition with respect to stoichiometric mullite(Al2O3/ point in the Al2O3-SiO2 system and relatively low viscosity SiO2=1.5, molar ratio). The crystal structure of the whiskers liquids during firing. The presence of such impurity oxides with a nanometer-sized diameter(300 nm)was confirmed to especially K2O and Na2O, enhances the volume fraction of be orthorhombic(Fig. 6) lassy phase present at any given temperature. It is worth noting After conventional leaching at 50C for 5 h, the micro- that the ternary of Na20-K20-SiO2 has a lowest melting point structure of the product obtained by firing the compact with an of 540C [11] and the introduction of glass-forming oxides addition of 0.8 wt% NaH? PO42H,0, at 1400C for 15, is shown in Fig. 7. Relatively well developed mullite whiskers with an aspect ratio of >12(0.4-0.6 um in diameter)were most all the glass phase had been dissolved. The prolonged leaching time and/or using a high concentration HF solution also appeared to dissolve the fine whiskers, the relatively coarse ones remaining undissolved, regardless of leaching source(Figs. 4, 7 and 8) 4. Discussion The growth of mullite into whiskers occurs on firing above the eutectic temperature in the Al2O3-SiO2 system; in such case, it is favorable in the presence of a considerable amount Fig. 5. EDS spectrum of mullite whiskers obtained with an addition of 0. 8 wt% NaHPO42H,O and liquid phase with a high-SiO2 composition. The impurity have the following chemical composition(wt%): 44.140, 37.11 Al and 18.75 content of as-received kaolin measured by XRF was 1. 123 K,O, Si
63.59 wt% Al2O3 and 36.41 wt% SiO2 (Al2O3/SiO2 = 1.04, molar ratio) (Fig. 5), this result indicating an alumina deficient composition with respect to stoichiometric mullite (Al2O3/ SiO2 = 1.5, molar ratio). The crystal structure of the whiskers with a nanometer-sized diameter (300 nm) was confirmed to be orthorhombic (Fig. 6). After conventional leaching at 50 8C for 5 h, the microstructure of the product obtained by firing the compact with an addition of 0.8 wt% NaH2PO42H2O, at 1400 8C for 15, is shown in Fig. 7. Relatively well developed mullite whiskers with an aspect ratio of >12 (0.4–0.6 mm in diameter) were observed, and almost all the glass phase had been dissolved. The prolonged leaching time and/or using a high concentration HF solution also appeared to dissolve the fine whiskers, the relatively coarse ones remaining undissolved, regardless of leaching source (Figs. 4, 7 and 8). 4. Discussion The growth of mullite into whiskers occurs on firing above the eutectic temperature in the Al2O3–SiO2 system; in such case, it is favorable in the presence of a considerable amount of liquid phase with a high-SiO2 composition. The impurity content of as-received kaolin measured by XRF was 1.123 K2O, 0.136 Na2O, 0.154 CaO, 0.014 MgO, 0.008 P2O5, and 0.104 wt% TiO2; such components, together with the addition of NaH2PO42H2O contribute to the formation of low melting point in the Al2O3–SiO2 system and relatively low viscosity liquids during firing. The presence of such impurity oxides, especially K2O and Na2O, enhances the volume fraction of glassy phase present at any given temperature. It is worth noting that the ternary of Na2O–K2O–SiO2 has a lowest melting point of 540 8C [11] and the introduction of glass-forming oxides Fig. 3. SEM micrographs of the product obtained with an addition of 0.8 wt% NaH2PO42H2O; fired at 1300 8C for 15 h; leached by 5 wt% HF solution at 50 8C for 1 h using microwave heating source; showing (a) random orientation and (b) preferential orientation, of the whiskers. Fig. 4. SEM micrographs of the product obtained with an addition of 0.8 wt% NaH2PO42H2O; fired 1400 8C for 15 h; leached by (a) 5 wt% and (b) 10 wt% HF solution at 50 8C for 1 h using microwave heating. Fig. 5. EDS spectrum of mullite whiskers obtained with an addition of 0.8 wt% NaH2PO42H2O and heat treated at 1400 8C for 15 h. Accordingly, the whiskers have the following chemical composition (wt%): 44.14 O, 37.11 Al and 18.75 Si. B.M. Kim et al. / Ceramics International 35 (2009) 579–583 581
582 B.M. Kim et al/Ceramics International 35(2009)579-583 aK Fig. 8. SEM micrograph of the product obtained with an on of 1.5 wt% NaH2PO4 2H2O; fired at 1400C for 15 h: leached by 5 wt% HF solution at 50C for 3 h using microwave heating source. Fig. 6. TEM micrograph and microbeam diffraction of mullite whisker with an addition of 0.8 wt% NaH,PO4'2H,O: fired at 1400C for 15 h. anisotropic with increasing Na2O content. Li et al. [ 8] considered that the presence of Na2O caused more glass formation and that P2O5 was beneficial for the growth of mullite fibers On the other hand, Johnson and Pask [15] found that the addition of alkali oxides did not have a great influence on the growth rate of mullite in spite of their strong fluxing effect on Al2O3-SiO2 mixtures. The presence of certain oxides in the glass phase can have a distinctive effect on the morphology of the mullite. Kong et al [13 investigated the effect of additions of MgO, Cao, SrO, and Bao on the reaction and morphology of the product. Only in the case of MgO addition well developed mullite whiskers formed the other oxides aided the formation of more plate-like grains. As a consequence, it is assumed that the higher aspect ratio of the whiskers obtained by the addition of 1.5 wt% NaH2PO4 2H20 is due to the presence of relatively KBSI 15.k V high concentration of Na2O and P2O5 in the glassy melts, producing larger volume fractions of low viscosity glass which Fig.7. SEM micrograph of the product obtained with an addition of 0. 8 wt% acts as the solvent to the mullite and its precursor oxides. For NaH2PO42H2O; fired at 1400C for 15 h: leached by 5 wt% HF solution at the same reason. smaller amounts of whiskers tend to be formed 50°Cfor5h with the addition of 1.5 wt% NaH2PO4 2H20 compared with 0.8 wt%. In addition, there could well be a poisoning effect c such as B2O3 and P2Os can reduce the viscosity of silicate the certain crystallographic planes, inhibiting the growth on melts. Thus, the glass phase rich in low melting components these particular planes and allowing the preferential growth will have a lower eutectic temperature and a reduced viscosity. on planes perpendicular or close to perpendicular to the The low viscosity is accompanied by a higher reaction rate with long axis of the mullite whisker crystals. Such a process rapid removal and replenishment of the reaction species. Thus the present work, pec other phases at the glass/solid phase boundary, due to the more generates the high aspect whisker morphology observed in the development of the mullite whiskers can be explained on the The structure of mullite can have any composition enhanced formation and lower melting point of the secondary theoretically between x=0(disordered sillimanite) and glass phase, its presence allowing enhanced solution-precipita- x=1.00 (aluminum oxide) in general formula tion [12, 13]of the mullite crystals in the liquid glass matrix. Al4+2rSi2-2010-x [16], dependant on starting material and As shown in Fig 8, the addition of 1.5 wt% NaH2PO42H20 processing route. As a result of this study, the orthorhombic resulted in small amounts of enhanced the growth of whiskers type mullite whiskers, which have a composition 51.06 mol% with an aspect ratio of >17(0.5 um in diameter), possibly Al2O3 and 48.94 mol% SiOz were fabricated using SiO2-rich due to the formation of more liquid phase and the lower starting composition(Al2O3/SiO2=0.51, molar ratio). The scosity of the melt, compared with the addition of 0. 8 wt% reason is not obvious why the composition of the resulting (Fig 4). Fahrenholtz and Smith [14] reported that Na2O did not whiskers is limited to a molar ratio of Al2O3/Sio2=1.04, but it enhance the crystallization kinetics of mullite but increased the could be due to limitations of the crystal chemistry of mullite grain size; the grain morphology changed from equiaxed Another possible reason could well be the low Al2O3 content in
such as B2O3 and P2O5 can reduce the viscosity of silicate melts. Thus, the glass phase rich in low melting components will have a lower eutectic temperature and a reduced viscosity. The low viscosity is accompanied by a higher reaction rate with other phases at the glass/solid phase boundary, due to the more rapid removal and replenishment of the reaction species. Thus the development of the mullite whiskers can be explained on the enhanced formation and lower melting point of the secondary glass phase, its presence allowing enhanced solution-precipitation [12,13] of the mullite crystals in the liquid glass matrix. As shown in Fig. 8, the addition of 1.5 wt% NaH2PO42H2O resulted in small amounts of enhanced the growth of whiskers with an aspect ratio of >17 (0.5 mm in diameter), possibly due to the formation of more liquid phase and the lower viscosity of the melt, compared with the addition of 0.8 wt% (Fig. 4). Fahrenholtz and Smith [14] reported that Na2O did not enhance the crystallization kinetics of mullite but increased the grain size; the grain morphology changed from equiaxed to anisotropic with increasing Na2O content. Li et al. [8] considered that the presence of Na2O caused more glass formation and that P2O5 was beneficial for the growth of mullite fibers. On the other hand, Johnson and Pask [15] found that the addition of alkali oxides did not have a great influence on the growth rate of mullite in spite of their strong fluxing effect on Al2O3–SiO2 mixtures. The presence of certain oxides in the glass phase can have a distinctive effect on the morphology of the mullite. Kong et al. [13] investigated the effect of additions of MgO, CaO, SrO, and BaO on the reaction and morphology of the product. Only in the case of MgO addition well developed mullite whiskers formed, the other oxides aided the formation of more plate-like grains. As a consequence, it is assumed that the higher aspect ratio of the whiskers obtained by the addition of 1.5 wt% NaH2PO42H2O is due to the presence of relatively high concentration of Na2O and P2O5 in the glassy melts, producing larger volume fractions of low viscosity glass which acts as the solvent to the mullite and its precursor oxides. For the same reason, smaller amounts of whiskers tend to be formed with the addition of 1.5 wt% NaH2PO42H2O compared with 0.8 wt%. In addition, there could well be a poisoning effect on the certain crystallographic planes, inhibiting the growth on these particular planes and allowing the preferential growth on planes perpendicular or close to perpendicular to the long axis of the mullite whisker crystals. Such a process generates the high aspect whisker morphology observed in the present work. The structure of mullite can have any composition theoretically between x = 0 (disordered sillimanite) and x = 1.00 (aluminum oxide) in general formula Al4+2xSi22xO10x [16], dependant on starting material and processing route. As a result of this study, the orthorhombic type mullite whiskers, which have a composition 51.06 mol% Al2O3 and 48.94 mol% SiO2 were fabricated using SiO2-rich starting composition (Al2O3/SiO2 = 0.51, molar ratio). The reason is not obvious why the composition of the resulting whiskers is limited to a molar ratio of Al2O3/SiO2 = 1.04, but it could be due to limitations of the crystal chemistry of mullite. Another possible reason could well be the low Al2O3 content in Fig. 6. TEM micrograph and microbeam diffraction of mullite whisker with an addition of 0.8 wt% NaH2PO42H2O; fired at 1400 8C for 15 h. Fig. 7. SEM micrograph of the product obtained with an addition of 0.8 wt% NaH2PO42H2O; fired at 1400 8C for 15 h; leached by 5 wt% HF solution at 50 8C for 5 h using conventional heating. Fig. 8. SEM micrograph of the product obtained with an addition of 1.5 wt% NaH2PO42H2O; fired at 1400 8C for 15 h; leached by 5 wt% HF solution at 50 8C for 3 h using microwave heating source. 582 B.M. Kim et al. / Ceramics International 35 (2009) 579–583
B.M. Kim et al. /Ceramics International 35(2009)579-583 the starting batch composition compared with 3/2-mullite, References resulting in an Al,O3-unsaturated, SiO2-rich melt. de Souza et al. [17] study mullite whiskers and anisotropic grains [l] J.Y. Jaaski, H.U. Nissen, Investigation of superstructures in mullite by derived from 3 mol% erbia-doped aluminum hydroxide and high resolution microscopy and electron diffraction, Phys. Chem. Miner. silica gel; the compacts were fired at 1600C for 1-8 h. The 10(1983)47-54. average molar ratio of Al,O/SiO, in the whiskers was 1.31 [2]S Hashimoto, A Yamaguchi, Synthesis of mullite whiskers using Na2SO4 flux, J Ceram Soc. Jpn. 112(2004)104-109 regardless of the AlO3/SiO2=1. 5 or 2, molar ratio of the [3] H.J. Choi, J.G. Lee, Synthesis of mullite whiskers,J.Am. Ceram Soc.85 starting composition; in this case a relatively high Al2O3/SiO2 2002)481-483 ratioin the glass phase generated a low ratio value in the mullite 41 R. Moyer, N.N. Hughes, A catalytic for mullite whiskers, J. Am. Ceram raIns [5] P Peng, C. Sorrell, Preparation of mullite whiskers from topaz decom- position, Mater. Lett. 58(2004)1288-1291 5. Conclusions [6] S.H. Hong, w. Gemignani, G.L. Messing, Anisotropic grain growth in seeded and B2O3-doped diphasic mullite gels, J. Eur. Ceram. Soc. 16 Relatively well developed whisker-shaped mullite can be [7 D.S. Perera, N Otsuka, Mullite morphology in fired kaolinite /halloysit prepared by calcining at 1400C for 15 h in air of appropriate clays, J Mater. Sci. Lett. 4(1985)1270-1272 mixture of fine particle kaolin and NHAAl(SO4)2 12H2O [8] K Li, T. Shimizu, K. Igarashi, Preparation of short mullite fibers from powders, with the addition of 1.5 wt% NaH2PO42H20 kaolin via the addition of foaming agents, J. Am. Ceram. Soc. 84(2001) a solution-precipitation until they impinge; in such case, the (9/7=5303 oky, Application of microwave energy past, present and future During firing, the whiskers continue to grow into the melts by in: W.H. Sutton, M.H. Brooks, IJ. Chambinsky Microwave whiskers grow preferentially along the parallel direction to Processing of Materials, Materials Research Societ rgh,1988 the c-axis, this resulting in an orthorhombic-type crystal lographic structure. The presence of a considerable amount of [10 H.C. Park, Y.J. Park, R. Stevens, Synthesis of alum glass melt could well induce crystal growth on the lateral derived from coal fly ash, Mater Sci Eng. A 367 170 [11] E.M. Levin, C R. Robbins. H F. Mcmurdie, Phase for cera. crystal faces where the free energy difference is not so large The addition of NaH_. 2H2O in this respect appeared to be [12] M. Bartsch, B Saruhan, M. Schmucker, H Schneider, Novel low-tem- responsible for the development of whisker shaped morphol- perature processing route of dense mullite ceramics by reaction sintering ogy, possibly due to the further formation of low melting of amorphous SiO2-coated y-Al2O3 particle nanocomposites, J. Am. Ceran.Soc.82(1999)1388-1392. poInt liquids with low viscosity in the Al2O3-SiO2 system [13] L.B. Kong. Y.Z. Chen, T.S. Zhang, J. Ma. F Boey, H Huang, Effects of during firing. The leaching of the glass phase by 5 wt% HF in alkaline-earth oxides on phase formation and morphology development of ater is more effective using microwave heating source rather mullite ceramics, Ceram. Int. 30(2004)1319-1323 than conventional heating [14 w.G. Fahrenholtz, D.M. Smith, Densification and microstructure of sodium- doped colloidal mullite, J. Am. Ceram Soc. 77(1994)1377-1380 [15] S. Johnson, J.A. Pask, Role of impurities on formation of mullite from Acknowledgement kaolinite and Al2O3-SiO mixtures, Am. Ceram. Soc. Bull. 61(1982) 838-842 This work was supported financially by the Korea Energy [16] CW.Burnham, Composition limits of mullite and the sillimanite-mullite Management Corporation(Project number: 2005-R-NMO3- solid solution problem, Camegie Inst. Washington Year Book 63(1964) P02)and partially supported by grants-in-aid for the National 227-228 [171 M.F. de Souza, J. Yamamoto, I. Regiani, C O. Paiva-Santos, D P.F. Core Research Center Program from MOST/KOSEF (No R15 Souza, Mullite whiskers grown from erbia-doped aluminum hydroxide- 2006-022-01001-0) ilica gel, J. Am. Ceram. Soc. 83(2000)60-64
the starting batch composition compared with 3/2-mullite, resulting in an Al2O3-unsaturated, SiO2-rich melt. de Souza et al. [17] study mullite whiskers and anisotropic grains derived from 3 mol% erbia-doped aluminum hydroxide and silica gel; the compacts were fired at 1600 8C for 1–8 h. The average molar ratio of Al2O3/SiO2 in the whiskers was 1.31, regardless of the Al2O3/SiO2 = 1.5 or 2, molar ratio of the starting composition; in this case a relatively high Al2O3/SiO2 ratio in the glass phase generated a low ratio value in the mullite grains. 5. Conclusions Relatively well developed whisker-shaped mullite can be prepared by calcining at 1400 8C for 15 h in air of appropriate mixture of fine particle kaolin and NH4Al(SO4)212H2O powders, with the addition of 1.5 wt% NaH2PO42H2O. During firing, the whiskers continue to grow into the melts by a solution-precipitation until they impinge; in such case, the whiskers grow preferentially along the parallel direction to the c-axis, this resulting in an orthorhombic-type crystallographic structure. The presence of a considerable amount of glass melt could well induce crystal growth on the lateral crystal faces where the free energy difference is not so large. The addition of NaH2PO42H2O in this respect appeared to be responsible for the development of whisker shaped morphology, possibly due to the further formation of low melting point liquids with low viscosity in the Al2O3–SiO2 system during firing. The leaching of the glass phase by 5 wt% HF in water is more effective using microwave heating source rather than conventional heating. Acknowledgement This work was supported financially by the Korea Energy Management Corporation (Project number: 2005-R-NM03- P02) and partially supported by grants-in-aid for the National Core Research Center Program from MOST/KOSEF (No. R15- 2006-022-01001-0). References [1] J.Y. Jaaski, H.U. Nissen, Investigation of superstructures in mullite by high resolution microscopy and electron diffraction, Phys. Chem. Miner. 10 (1983) 47–54. [2] S. Hashimoto, A. Yamaguchi, Synthesis of mullite whiskers using Na2SO4 flux, J. Ceram. Soc. Jpn. 112 (2004) 104–109. [3] H.J. Choi, J.G. Lee, Synthesis of mullite whiskers, J. Am. Ceram. Soc. 85 (2002) 481–483. [4] J.R. Moyer, N.N. Hughes, A catalytic for mullite whiskers, J. Am. Ceram. Soc. 77 (1994) 1083–1086. [5] P. Peng, C. Sorrell, Preparation of mullite whiskers from topaz decomposition, Mater. Lett. 58 (2004) 1288–1291. [6] S.H. Hong, W. Germignani, G.L. Messing, Anisotropic grain growth in seeded and B2O3-doped diphasic mullite gels, J. Eur. Ceram. Soc. 16 (1996) 133–141. [7] D.S. Perera, N. Otsuka, Mullite morphology in fired kaolinite/halloysite clays, J. Mater. Sci. Lett. 4 (1985) 1270–1272. [8] K. Li, T. Shimizu, K. Igarashi, Preparation of short mullite fibers from kaolin via the addition of foaming agents, J. Am. Ceram. Soc. 84 (2001) 497–503. [9] I.J. Chabinsky, Application of microwave energy past, present and future, in: W.H. Sutton, M.H. Brooks, I.J. Chambinsky (Eds.), Microwave Processing of Materials, Materials Research Society, Pittsburgh, 1988 , pp. 17–29. [10] H.C. Park, Y.J. Park, R. Stevens, Synthesis of alumina from high purity derived from coal fly ash, Mater. Sci. Eng. A 367 (2004) 166–170. [11] E.M. Levin, C.R. Robbins, H.F. Mcmurdie, Phase Diagrams for Ceramists, vol. 1, The American Ceramic Society, USA, 1964. [12] M. Bartsch, B. Saruhan, M. Schmucker, H. Schneider, Novel low-temperature processing route of dense mullite ceramics by reaction sintering of amorphous SiO2-coated g-Al2O3 particle nanocomposites, J. Am. Ceram. Soc. 82 (1999) 1388–1392. [13] L.B. Kong, Y.Z. Chen, T.S. Zhang, J. Ma, F. Boey, H. Huang, Effects of alkaline-earth oxides on phase formation and morphology development of mullite ceramics, Ceram. Int. 30 (2004) 1319–1323. [14] W.G. Fahrenholtz, D.M. Smith, Densification and microstructure of sodiumdoped colloidal mullite, J. Am. Ceram. Soc. 77 (1994) 1377–1380. [15] S. Johnson, J.A. Pask, Role of impurities on formation of mullite from kaolinite and Al2O3–SiO2 mixtures, Am. Ceram. Soc. Bull. 61 (1982) 838–842. [16] C.W. Burnham, Composition limits of mullite and the sillimanite-mullite solid solution problem, Carnegie Inst. Washington Year Book 63 (1964) 227–228. [17] M.F. de Souza, J. Yamamoto, I. Regiani, C.O. Paiva-Santos, D.P.F. de Souza, Mullite whiskers grown from erbia-doped aluminum hydroxidesilica gel, J. Am. Ceram. Soc. 83 (2000) 60–64. B.M. Kim et al. / Ceramics International 35 (2009) 579–583 583