《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_mullite whisker2 Mullite whiskers derived from coal fly ash

MATERIALS HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 454-455(2007)518-522 www.elsevier.com/locate/msea Mullite whiskers derived from coal fly ash Y.M. Park, T.Y. Yang a, S.Y. Yoon, R. Stevens, H.C. Park a, K Department of Materials Science and Engineering, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Pusan 609-735, South Kore b Department of Engineering and Applied Science, University of Bath, Bath BA2 7AY UK Received 2 August 2006: received in revised form 9 November 2006: accepted 23 November 2006 Abstract Alumina-deficient(Al2O3/SiO2= 1. 12, molar ratio), orthorhombic mullite whiskers with a diameter of 0.6-1. 8 um(aspect ratio >30) have been manufactured by firing compacts of coal fly ash and NHAl(SO4)2. 12H2O powders, with a small addition of NaH2PO42H=O, at 1300C for 10h. The manufacturing process, the morphology, and structure of the whiskers are described 2006 Elsevier B. v. All rights reserved Keywords: Mullite whiskers; Coal fly ash 1. Introduction under a flow of H2/CF4 at 1450C for 3h Liet al. [5] prepared mullite whiskers from commercial kaolin with the addition of The stable intermediate compound mullite(3Al2O32SiO2), foaming agents and discussed the effect of foaming which is formed in the Al2O3-SiO2 system [1] is a potential the development of mullite whiskers and the dissolution of the candidate material for advanced structural applications because glass matrix in hydrofluoric acid. it has a high melting point, low coefficient of thermal expan The recycling of industrial waste to produce valuable materi- sion, excellent creep resistance, good chemical stability and als attracts a great deal interest from society and scientists alike high strength at high temperature. The stable crystal structure Resource recovery from the huge amounts of fly ash generated of mullite is orthorhombic with lattice constants a=7.545A, by combustion of coal in a thermoelectric power plant is one b=7.689A and c=2.884A (CPDS Card #15-776), and it con- of the important problems that require urgent attention. Coal fly sists of edge-shared AlO6 octahedral chains aligned in the ash consists of fine inorganic particles having Sio2 and Al2O c-direction and crosslinked by corner-shared (Si, Al)O4 tetrahe- as the main components, often in the form of cenospheres. The dra[2]. Thus, the crystal growth may be fasterin crystallographic chemical composition allows the synthesis of high performance direction parallel to the c-axis than in any other, resulting in a mullite whiskers from the coal fly ash and this is of commercial high degree of orientation. interest. The objective of the present work is to facilitate the Whisker-shaped mullite has attracted attention because of its recycling of coal fly ash and to produce mullite whiskers positive consideration as a reinforcement phase fo high temperature composites. Various processing routes have bee reported for the preparation of mullite whiskers. Hashimoto and 2. Experimental procedure Yamaguchi [3] synthesized mullite whiskers with a diameter of 0.5-2 um(aspect ratio 15-20), by firing a powder mixture of The morph phology and characteristics of the as-received coal Al(SO4)3, amorphous Sio2, and Na2SO4 in an alumina cru- fly ash are shown in Fig. I and Table 1, respectively. The cible at 1000C for 2 h Choi and Lee [4] obtained very large coal fly ash contained 23.29 wt% Al2O3 and 53.83 wt% SiO2 mullite whiskers(15 um in diameter, >300 um in length) by (Al2O3/SiO2=0. 25, molar ratio); it consisted mainly of silicate heating a mixture of SiO2 and silicon in an alumina tube reactor, minerals, being a mixture of ake -llke and nearly sphenic shaped particles, with citic surface area of 3.82 m-g and an agglomerate size of 42. 1 um(<90%). The coal fly ash was Corresponding author. Tel. +82 51 510 2392: fax: +8251 512 0528 calcined in air at 600oC for 2 h to remove the residual carbon E-mail address: hcparkl@ pusan. ac kr(HC. Park nd subsequently ball-milled for 24 h in ethanol. After rotary 0921-5093/S-see front matter 2006 Elsevier B v. All rights reserved doi:10.1016/1msea.2006.11.114

Materials Science and Engineering A 454–455 (2007) 518–522 Mullite whiskers derived from coal fly ash Y.M. Park a, T.Y. Yang a, S.Y. Yoon a, R. Stevens b, H.C. Park a,∗ a Department of Materials Science and Engineering, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Pusan 609-735, South Korea b Department of Engineering and Applied Science, University of Bath, Bath BA2 7AY, UK Received 2 August 2006; received in revised form 9 November 2006; accepted 23 November 2006 Abstract Alumina-deficient (Al2O3/SiO2 = 1.12, molar ratio), orthorhombic mullite whiskers with a diameter of 0.6–1.8 m (aspect ratio >30) have been manufactured by firing compacts of coal fly ash and NH4Al(SO4)2·12H2O powders, with a small addition of NaH2PO4·2H2O, at 1300 ◦C for 10 h. The manufacturing process, the morphology, and structure of the whiskers are described. © 2006 Elsevier B.V. All rights reserved. Keywords: Mullite whiskers; Coal fly ash 1. Introduction The stable intermediate compound mullite (3Al2O3·2SiO2), which is formed in the Al2O3–SiO2 system [1] is a potential candidate material for advanced structural applications because it has a high melting point, low coefficient of thermal expan￾sion, excellent creep resistance, good chemical stability and high strength at high temperature. 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 con- ˚ sists of edge-shared AlO6 octahedral chains aligned in the c-direction and crosslinked by corner-shared (Si,Al)O4 tetrahe￾dra [2]. 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. Whisker-shaped mullite has attracted attention because of its positive consideration as a reinforcement phase for use in high temperature composites. Various processing routes have been reported for the preparation of mullite whiskers. Hashimoto and Yamaguchi [3] synthesized mullite whiskers with a diameter of 0.5–2m (aspect ratio 15–20), by firing a powder mixture of Al2(SO4)3, amorphous SiO2, and Na2SO4 in an alumina cru￾cible at 1000 ◦C for 2 h. Choi and Lee [4] obtained very large mullite whiskers (>15 m in diameter, >300 m in length) by heating a mixture of SiO2 and silicon in an alumina tube reactor, ∗ Corresponding author. Tel.: +82 51 510 2392; fax: +82 51 512 0528. E-mail address: hcpark1@pusan.ac.kr (H.C. Park). under a flow of H2/CF4 at 1450 ◦C for 3 h. Li et al. [5] prepared mullite whiskers from commercial kaolin with the addition of foaming agents and discussed the effect of foaming agents on the development of mullite whiskers and the dissolution of the glass matrix in hydrofluoric acid. The recycling of industrial waste to produce valuable materi￾als attracts a great deal interest from society and scientists alike. Resource recovery from the huge amounts of fly ash generated by combustion of coal in a thermoelectric power plant is one of the important problems that require urgent attention. Coal fly ash consists of fine inorganic particles having SiO2 and Al2O3 as the main components, often in the form of cenospheres. The chemical composition allows the synthesis of high performance mullite whiskers from the coal fly ash and this is of commercial interest. The objective of the present work is to facilitate the recycling of coal fly ash and to produce mullite whiskers. 2. Experimental procedure The morphology and characteristics of the as-received coal fly ash are shown in Fig. 1 and Table 1, respectively. The coal fly ash contained 23.29 wt% Al2O3 and 53.83 wt% SiO2 (Al2O3/SiO2 = 0.25, molar ratio); it consisted mainly of silicate minerals, being a mixture of flake-like and nearly spherical￾shaped particles, with a specific surface area of 3.82 m2/g and an agglomerate size of 42.1 m (<90%). The coal fly ash was calcined in air at 600 ◦C for 2 h to remove the residual carbon and subsequently ball-milled for 24 h in ethanol. After rotary 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.114

Y.M. Park et al. Materials Science and Engineering A 454-455 (2007)518-522 Table I Characteristics of coal fly ash Crystalline phase SBET(m-ig Agglomerate size distribution(um) SiO2(53.83),Al2O3(23.29),Fe2O3(5.96 Mullite, sillimanite, quartz 3.82 MnO(0.10,CaO(7.87,MgO(0.83), K2O(099),Na2O(0.70),P2Os(0.65) TO2(0.85, Ig. loss(10.33) vacuum evaporation, the dried powder was ground and passed(NaH2PO4 2H2O, Junsei Chemical Co., Tokyo, Japan)was through a 200 mesh sieve. Ammonium aluminum sulfate hydrate added to the mixture of ammonium alum and coal fly ash. The H4Al(SO4)2. 12H2O, ammonium alum)used in this study was batch powders were mixed and homogenized by ball milling obtained from coal fly ash [6]. The purity of the synthesized in ethanol for h using a high density polyethylene bottle with ammonium alum was >99.9%0; it consisted of nearly spherically alumina ball media. After drying, the mixed powders were shaped particles(Fig. 2)in the size range 100-200 um crushed in an agate mortar and passed through a 200 mesh A measured amount of ammonium alum was added to the sieve. Cylindrical (10 mm diameter x 5 mm) compacts were coal fly ash in order to increase the Al2O3/SiOz molar ratio up prepared by die pressing at 70 MPa. The compacts were placed to 0.32. In addition, 2 wt% of sodium dihydrogen phosphate in an alumina crucible, and not covered. After heating in air at 1300C for 10h, the compacts were treated with 20 wt% HF in water( Guaranteed Reagent, Junsei Chemical Co., Tokyo Japan ); the product was filtered, washed with water, and final dried. In this case, in order to effectively dissolve the glass matrix from the whiskers. the hf solution was heated to 50oC for 5 h using a microwave heating source The resulting whiskers were characterized using X-ray diffractometry(XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy(EDS), and transmission elec tron microscopy (TEM) 3. Results and discussion After chemically leaching the glass matrix with a 20 wt % HF solution using microwave heating (50C, 5h), the microstruc SKBS Oky x6.oaks: beri ture of the product is shown in Fig 3 using SEM. A mass Fig. 1. SEM micrograph of the as-received coal fly ash KBSIA15 5,日gK Fig. 3. SEM micrograph of whiskers obtained by firing compact 14,gmm15.0kv805。0um (Al2O3/SiO2=0.32, molar ratio) of coal fly ash and ammonium alur powders, with an addition of 2 wt% NaH2PO4 2H2O, at 1300C for 10h; etched with 20 wt% HF solution at 50C for 5h using microwave heating Fig. 2. SEM micrograph of ammonium alum obtained from coal fly ash

Y.M. Park et al. / Materials Science and Engineering A 454–455 (2007) 518–522 519 Table 1 Characteristics of coal fly ash Chemical component (wt%) Crystalline phase SBET (m2/g) Agglomerate size distribution (m) 99.9%; it consisted of nearly spherically shaped particles (Fig. 2) in the size range 100–200 m. A measured amount of ammonium alum was added to the coal fly ash in order to increase the Al2O3/SiO2 molar ratio up to 0.32. In addition, 2 wt% of sodium dihydrogen phosphate Fig. 1. SEM micrograph of the as-received coal fly ash. Fig. 2. SEM micrograph of ammonium alum obtained from coal fly ash. (NaH2PO4·2H2O, Junsei Chemical Co., Tokyo, Japan) was added to the mixture of ammonium alum and coal fly ash. 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 not covered. After heating in air at 1300 ◦C for 10 h, the compacts were treated with 20 wt% HF in water (Guaranteed Reagent, Junsei Chemical Co., Tokyo, Japan); 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 to 50 ◦C for 5 h using a microwave heating source. The resulting whiskers were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission elec￾tron microscopy (TEM). 3. Results and discussion After chemically leaching the glass matrix with a 20 wt% HF solution using microwave heating (50 ◦C, 5 h), the microstruc￾ture of the product is shown in Fig. 3 using SEM. A mass Fig. 3. SEM micrograph of whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of 2 wt% NaH2PO4·2H2O, at 1300 ◦C for 10 h; etched with 20 wt% HF solution at 50 ◦C for 5 h using microwave heating source.

Y.M. Park et al. /Materials Science and Engineering A 454-455(2007)518-522 Fig 4. XRD patterns of (a) JCPDS Card #15-776, mullite, and the products pained by firing compact (Al2O3/SiO2=0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of (b)2 wt% and (c)4 wt%o NaH POA2H,O, at 1300C for 10h 200nm of relatively well-developed and interlocked whisker-shaped TEM micrograph and microbeam diffraction of mullite whiskers crystals having a high aspect ratio of >30(about 0.6-1. um by firing compact (Al2O3/SiO2=0.32, molar ratio) of coal fly ash in diameter) was observed. The product generated XRD pat onium alum powders, with an addition of 2 wt% NaH2PO4 2H2O, at terns consisting of the mullite characteristic peaks with Miller for 10h indices of 120, 210, and 220(Fig 4); the XRD peak lines were not nearly shifted compared with JCPDS Card #15-776. the reaction sintering(1200C). the presence of a liquid phase As shown in Fig. 5, the EDS spectrum of the whiskers con- allows sintering of low-Al2O3 compositions and produces rel- firmed that they consisted of 65. wt% Al2O3 and 34.71 wt% atively large prismatic mullite crystals [7). On a similar basis, SiO2(Al2O3/SiO2=1. 12, molar ratio), this result indicating an in this study, it was concluded that the mullite developed into alumina deficient composition with respect to stoichiometric whiskers in the presence a considerable amount of liquid phase mullite(Al2O3/SiO2=1.5, molar ratio). The crystal structure with a low-Al203 composition because of the relatively high of the whiskers with a nanometer-sized diameter(<400 nm) iron oxide content(5.96 wt%)of the precursor materials. Minor as shown by observation of TEM and microbeam diffraction alkali impurities and/or other oxide components, which help for- ig. 6), to be orthorhombic. The EDS analysis also takes in the mation of low melting point liquids during firing, are also present background materials which could derive from material other in the coal fly ash(Table 1). The presence of such impurities will than the whiskers. Such material would only be present in small lower the softening point of the glass and indeed enhance the amounts due to the hf leaching volume fraction of glassy phase present at any given tempera With optimized additions of NHAI(SO4)2. 12H20 and ture Na20, K2O and Li2O are increasingly effective in reducing NaH2PO42H2O, mullite whiskers were prepared by firing com- the melting point of silica binaries to below 800C. The ternary pacts of coal fly ash at 1300C for 10h. In the later stages of of Na20-K20-Sio2 has a lowest melting point of 540Cand the introduction of iron oxide can further reduce this critical tem perature [8]. Thus, the richer the glass is in low melting point components, the lower will be its melting temperature and for any given temperature will lower the viscosity. This is accompa- nied by higher reaction rates with other phases at the glass solie phase boundary. The growth of the whiskers can be explained on the enhanced formation and lower melting point of a sec- ondary glass phase, allowing enhanced solution-precipitation in the glass The microstructure of compact without an addition of NaH?PO42H20, fired at 1300C for 10h is shown in Fig. 7 The mullite whiskers which grew at both the internal and exter nal surfaces were observed. The whiskers existing at external (Al12O3/SiO2=0. ratio)of coal fly ash and ammonium alum ompact surface were mainly embedded in the glassy phase and the powders, internal whiskers generally were random and interlocked. The with an addition NaH, PO: 2H,0. at 1300.C for 10h: this showing addition of 2 wt% NaH2PO4 2H20(Fig. 3)assisted the growth 39.36O.41.67 Al and1897Si of whiskers and the dissolution of glass matrix compared with

520 Y.M. Park et al. / Materials Science and Engineering A 454–455 (2007) 518–522 Fig. 4. XRD patterns of (a) JCPDS Card #15-776, mullite, and the products obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of (b) 2 wt% and (c) 4 wt% NaH2PO4·2H2O, at 1300 ◦C for 10 h. of relatively well-developed and interlocked whisker-shaped crystals having a high aspect ratio of >30 (about 0.6–1.8 m in diameter) was observed. The product generated XRD pat￾terns consisting of the mullite characteristic peaks with Miller indices of 1 2 0, 2 1 0, and 2 2 0 (Fig. 4); the XRD peak lines were not nearly shifted compared with JCPDS Card #15-776. As shown in Fig. 5, the EDS spectrum of the whiskers con- firmed that they consisted of 65.29 wt% Al2O3 and 34.71 wt% SiO2 (Al2O3/SiO2 = 1.12, molar ratio), 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 (1200 ◦C), the presence of a liquid phase allows sintering of low-Al2O3 compositions and produces rel￾atively large prismatic mullite crystals [7]. On a similar basis, in this study, it was concluded that the mullite developed into whiskers in the presence a considerable amount of liquid phase with a low-Al2O3 composition because of the relatively high iron oxide content (5.96 wt%) of the precursor materials. Minor alkali impurities and/or other oxide components, which help for￾mation of low melting point liquids during firing, are also present in the coal fly ash (Table 1). The presence of such impurities will lower the softening point of the glass and indeed enhance the volume fraction of glassy phase present at any given tempera￾ture. Na2O, K2O and Li2O are increasingly effective in reducing the melting point of silica binaries to below 800 ◦C. The ternary of Na2O–K2O–SiO2 has a lowest melting point of 540 ◦C and the introduction of iron oxide can further reduce this critical tem￾perature [8]. Thus, the richer the glass is in low melting point components, the lower will be its melting temperature and for any given temperature will lower the viscosity. This is accompa￾nied by higher reaction rates with other phases at the glass solid phase boundary. The growth of the whiskers can be explained on the enhanced formation and lower melting point of a sec￾ondary glass phase, allowing enhanced solution–precipitation in the glass. The microstructure of compact without an addition of NaH2PO4·2H2O, fired at 1300 ◦C for 10 h is shown in Fig. 7. The mullite whiskers which grew at both the internal and exter￾nal surfaces were observed. The whiskers existing at external surface were mainly embedded in the glassy phase and the internal whiskers generally were random and interlocked. The addition of 2 wt% NaH2PO4·2H2O (Fig. 3) assisted the growth of whiskers and the dissolution of glass matrix compared with

Y.M. Park et al. / Materials Science and Engineering A 454-455 (2007)518-522 N KBS witn barograph of mullite whiskers obtained by firing compact Fig. 8. SEM micrograph of mullite whiskers obtained by firing 32, molar ratio)of coal fly ash and ammonium alum powder (Al2O3/SiO2=0.32, molar ratio) of coal fly ash and ammonium alum on of NaH? PO4.,O, at 1300C for 10h with an addition of 2 wt NaH? PO42H,O, at 1000C for 10h no addition. Both sodium and phosphate, in this case, together increasing addition content of sodium phosphates, they stated with impurities such as iron oxide can lead to the presence of that the introduction of Na2o caused more glass formation by increasing amounts of liquid phase especially if the temperature dissolving mullite and P2O5 enhanced the growth of mullite is raised; this facilitating the growth of whisker-shaped grains. fibers. In such case, however, it is not clear that more glass for- In such a situation the whiskers will develop by selective precip- mation is attributed to the dissolution of mullite phase or itation on the larger needles in the presence of rich liquid phase, the strong fluxing effect of Na2O; if the former cause is rea- which will grow at the expense of the finer needles and pow- sonable, the Sioz content in the glass matrix is presumably der particle that are in turn preferentially dissolved. Thus the considered to increase with further addition of sodium pho growth of whiskers or fibers is considered due to the conditions phates. On the other hand, Johnson and Pask [10] found that pertaining in the liquid glassy phase at the firing temperature. the addition of alkalis does not have a great influence on the Selective precipitation is occurring at the crystal faces at the tip growth rate of mullite in spite of their strong fuxing effect on of the whisker whilst the longer planar surfaces of the whisker Al2O3-SiOz mixtures Fahrenholtz and Smith [11] reported that grow at a slower rate NaO does not enhance the crystallization kinetics of mullite but On the other hand, the nucleation of mullite can occur at increases the grain size and anisotropic growth. The presence of the favorable particle surface sites by a solid-state reaction [9] certain oxides in the glass phase can have a distinctive effect and with increasing firing temperature the mullite can possi- on the morphology of the mullite when formed. Kong et al bly develop fine whisker-shaped morphology by consuming [12] investigated the effects of additions of MgO, CaO, SrO, the glassy melt. As shown in Fig. 8, the initial reaction prod uct obtained at 1000C for 10 h exhibited fine chrysanthemum flower shaped precipitates in a large volume of the particle sur- faces At 1200C, the appearance of whisker-shaped precipitates owing into the melt became evident(Fig. 9). With further increasing firing temperature(1300C), 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, resulting in an orthorhombic structure As shown in Fig. 10. however, the addition of 4 wt% NaH2PO4. 2H20 constrained the growth of mullite whiskers, making the crystals more"blocky"reducing their aspect ratio The presence of an increased amount of glass melt could well induce crystal growth on the lateral crystal faces where the free energy difference is not so large. Li et al. [5] investi gated the role of NaH PO4 2H2O and Na2 HPO42H2O added vx5: B0K 6 ogum to aid the formation of mullite fibers from kaolin. On the basis Fig. 9. SEM micrograph of mullite whiskers obtained by firing compact of decreased mullite production, and increased AlO3, Na2O, (Al203/SiO2=0.32, molar ratio) of coal fly ash and ammonium alum powders, P2O5 and decreased Sio2 components in the glass matrix, with with an addition of 2 wt% NaH2PO42H2O, at 1200C for 10h

Y.M. Park et al. / Materials Science and Engineering A 454–455 (2007) 518–522 521 Fig. 7. SEM micrograph of mullite whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with no addition of NaH2PO4·2H2O, at 1300 ◦C for 10 h. no addition. Both sodium and phosphate, in this case, together with impurities such as iron oxide can lead to the presence of increasing amounts of liquid phase especially if the temperature is raised; this facilitating the growth of whisker-shaped grains. In such a situation the whiskers will develop by selective precip￾itation on the larger needles in the presence of rich liquid phase, which will grow at the expense of the finer needles and pow￾der particle that are in turn preferentially dissolved. Thus the growth of whiskers or fibers is considered due to the conditions pertaining in the liquid glassy phase at the firing temperature. Selective precipitation is occurring at the crystal faces at the tip of the whisker whilst the longer planar surfaces of the whisker grow at a slower rate. On the other hand, the nucleation of mullite can occur at the favorable particle surface sites by a solid-state reaction [9] and with increasing firing temperature the mullite can possi￾bly develop fine whisker-shaped morphology by consuming the glassy melt. As shown in Fig. 8, the initial reaction prod￾uct obtained at 1000 ◦C for 10 h exhibited fine chrysanthemum flower shaped precipitates in a large volume of the particle sur￾faces. At 1200 ◦C, the appearance of whisker-shaped precipitates growing into the melt became evident (Fig. 9). With further increasing firing temperature (1300 ◦C), 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, resulting in an orthorhombic structure. As shown in Fig. 10, however, the addition of 4 wt% NaH2PO4·2H2O constrained the growth of mullite whiskers, making the crystals more “blocky” reducing their aspect ratio. The presence of an increased amount of glass melt could well induce crystal growth on the lateral crystal faces where the free energy difference is not so large. Li et al. [5] investi￾gated the role of NaH2PO4·2H2O and Na2HPO4·2H2O added to aid the formation of mullite fibers from kaolin. On the basis of decreased mullite production, and increased Al2O3, Na2O, P2O5 and decreased SiO2 components in the glass matrix, with Fig. 8. SEM micrograph of mullite whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of 2 wt% NaH2PO4·2H2O, at 1000 ◦C for 10 h. increasing addition content of sodium phosphates, they stated that the introduction of Na2O caused more glass formation by dissolving mullite and P2O5 enhanced the growth of mullite fibers. In such case, however, it is not clear that more glass for￾mation is attributed to the dissolution of mullite phase or to the strong fluxing effect of Na2O; if the former cause is rea￾sonable, the SiO2 content in the glass matrix is presumably considered to increase with further addition of sodium phos￾phates. On the other hand, Johnson and Pask [10] found that the addition of alkalis does not have a great influence on the growth rate of mullite in spite of their strong fluxing effect on Al2O3–SiO2 mixtures. Fahrenholtz and Smith [11] reported that Na2O does not enhance the crystallization kinetics of mullite but increases the grain size and anisotropic growth. The presence of certain oxides in the glass phase can have a distinctive effect on the morphology of the mullite when formed. Kong et al. [12] investigated the effects of additions of MgO, CaO, SrO, Fig. 9. SEM micrograph of mullite whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of 2 wt% NaH2PO4·2H2O, at 1200 ◦C for 10 h

Y.M. Park et al. /Materials Science and Engineering A 454-455(2007)518-522 of whiskers of different compositions, which would be average out in an EDS beam analysis in the SEM. Another possible rea- son could well be the low Al2 O3 content in the starting batch composition compared with 3/2-mullite, resulting in a SiO2 rich, Al2O3 deficient melt. de Souza et al. [13] prepared mullite whiskers by firing compacts of erbia-doped aluminum hydrox- ide and silica gel, at 1600C for 1-8 h: the average molar ratio of AlzO3/SiO was 131, regardless of the Al2O3/SiO=1.5 or 2, molar ratio of the starting composition; in this case a relatively high Al2O3/Sio ratio in the glass phase generated a low ratio value in the mullite grains. The free energies for the range of phases could well be dependent on the temperature and batch compositi The leaching of glass phase by 20 wt% HF in water,as s,Y 2. s0K mht shown in Figs. 3 and ll, seemed to be more effective using microwave heating due to the inherent advantages of microwave Fig. 10. SEM micrograph of mullite whiskers obtained by firing compact [14], yielding further smooth surface of whiskers compared with (Al2O3/SiO2=0.32, molar ratio)of coal fly ash and ammonium alum powders, leaching by conventional heating. Clearly the microwave radia with an addition of 4 wt% NaH PO42H,O. at 1300C for 10h tion is aiding the dissolution process, a factor which could be due to microwave energy absorption at the water/ceramic interface and Bao on the reaction and morphology of the product Only increasing the local temperature and the rate of dissolution in the case of MgO addition well developed mullite whiskers ormed, the other oxides aided the formation of more platelike 4.Conclusions grains. The authors explained their observation on the basis of a dissolution-precipitation mechanisn Well developed mullite whiskers have been prepared Synthesized mullite can have any composition between by the firing of appropriate mixtures of coal fly ash x=0(sillimanite) and x=l(alumina) in general formula and NH4AI(SO4)2-12H20 with the addition of 2 wt% Al4+2rSi2-2rO10-r, dependant on starting material and process- NaH2 PO42H20 at 1300.C for 10h. The resulting whiskers ing route[4, 7, 13]. As aresult of this study, the orthorhombic type exhibited an aspect ratio of >30(0.6-1. 8 um in diameter), and mullite whiskers, which have a composition 471I mol% Sioz had a composition of 47 11 mol% SiO? and 52.89 mol% Al2O3 and 52.89 mol% Al2O3 were fabricated using SiOz-rich starting with an orthorhombic-type structure. The fabrication of high composition(Al2O3/SiO2=0.32, molar ratio). The reason is not per erformance mullite whiskers by means of the recycling of coal obvious why the composition of the resulting mullite is restricted fly ash has been clearly demonstrated in this study. The leaching to a molar ratio of Al203/SiO2=1. 12, but could be due to limi- of the glass phase by 20 wt% HF in water was more effective tations of the crystal chemistry, or possibly be due to a mixture using microwave heating rather than conventional heating References [I] L.A. Akasy, J.A. Pask, J. Am. Ceram Soc. 58(1975)507-512 [2] J.Y. Jaaski. H U Nissen, Phys. Chem. Miner. 10(1983)47- [3] S. Hashimoto, A. Yamaguchi, J Ceram Soc. Jpn. 112 (2004)104-109 [4] H.J. Choi, J.G. Lee, J. Am. Ceram Soc. 85(2002)481-483 [5] K Li, T Shimizu, K Igarashi. J. Am. Ceram Soc. 84(2001)497-503 [6] H.C. Park, YJ. Park, R. Stevens, Mater. Sci. Eng. A 367(2004)166- [7 H. Schneider, K. Okada, J.A. Pask, Mullite and Mullite Ceramics, John Wiley and Sons. New York, 1994, pp 158-159 [8 E M.Levin, C R. Robbins, H F. McMurdie, Phase Diagrams For Ceramists. vol. 1. The American Ceramic Society, Columbus, OH, 1964 9]JS Jung, H.C. Park, R. Stevens, J. Mater. Sci. Lett. 20(2001)1089-1091 [10] S. Johnson, J.A. Pask, Am. Ceram Soc. Bull. 61(1982)838-842 [11] w.G. Fahrenholtz, D.M. Smith, J. Am. Ceram Soc. 77(1994)1377-1380 [12] L B. Kong, Y.Z. Chen, T.S. Zhang, J. Ma, F. Boey, H. Huang Ceram. Int 3002004)131913 [13] 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 (AlO3/SiO2=0.32, molar ratio)of coal fly ash and ammonium alum powders, [141 IJ. Chabinsky, in: W.H. Sutton, M.H. Brooks, IJ. Chambinsky(Eds ) ith an addition of 2 wt% NaH2PO4 2H20, at 1300C for 10h; chemically Microwave Processing of Materials, Materials Research Society, Pitts- etched using a 20% HF solution, conventionally heated at 50C for 24 h urgh,1988,pp.17-29

522 Y.M. Park et al. / Materials Science and Engineering A 454–455 (2007) 518–522 Fig. 10. SEM micrograph of mullite whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of 4 wt% NaH2PO4·2H2O, at 1300 ◦C for 10 h. 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 platelike grains. The authors explained their observation on the basis of a dissolution–precipitation mechanism. Synthesized mullite can have any composition between x = 0 (sillimanite) and x = 1 (alumina) in general formula Al4+2xSi2−2xO10−x, dependant on starting material and process￾ing route [4,7,13]. As a result of this study, the orthorhombic type mullite whiskers, which have a composition 47.11 mol% SiO2 and 52.89 mol% Al2O3 were fabricated using SiO2-rich starting composition (Al2O3/SiO2 = 0.32, molar ratio). The reason is not obvious why the composition of the resulting mullite is restricted to a molar ratio of Al2O3/SiO2 = 1.12, but could be due to limi￾tations of the crystal chemistry, or possibly be due to a mixture Fig. 11. SEM micrograph of mullite whiskers obtained by firing compact (Al2O3/SiO2 = 0.32, molar ratio) of coal fly ash and ammonium alum powders, with an addition of 2 wt% NaH2PO4·2H2O, at 1300 ◦C for 10 h; chemically etched using a 20% HF solution, conventionally heated at 50 ◦C for 24 h. of whiskers of different compositions, which would be averaged out in an EDS beam analysis in the SEM. Another possible rea￾son could well be the low Al2O3 content in the starting batch composition compared with 3/2-mullite, resulting in a SiO2- rich, Al2O3 deficient melt. de Souza et al. [13] prepared mullite whiskers by firing compacts of erbia-doped aluminum hydrox￾ide and silica gel, at 1600 ◦C for 1–8 h; the average molar ratio of Al2O3/SiO2 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. The free energies for the range of phases could well be dependent on the temperature and batch composition. The leaching of glass phase by 20 wt% HF in water, as shown in Figs. 3 and 11, seemed to be more effective using microwave heating due to the inherent advantages of microwave [14], yielding further smooth surface of whiskers compared with leaching by conventional heating. Clearly the microwave radia￾tion is aiding the dissolution process, a factor which could be due to microwave energy absorption at the water/ceramic interface increasing the local temperature and the rate of dissolution. 4. Conclusions Well developed mullite whiskers have been prepared by the firing of appropriate mixtures of coal fly ash and NH4Al(SO4)2·12H2O with the addition of 2 wt% NaH2PO4·2H2O at 1300 ◦C for 10 h. The resulting whiskers exhibited an aspect ratio of >30 (0.6–1.8 m in diameter), and had a composition of 47.11 mol% SiO2 and 52.89 mol% Al2O3, with an orthorhombic-type structure. The fabrication of high performance mullite whiskers by means of the recycling of coal fly ash has been clearly demonstrated in this study. The leaching of the glass phase by 20 wt% HF in water was more effective using microwave heating rather than conventional heating. References [1] I.A. Akasy, J.A. Pask, J. Am. Ceram. Soc. 58 (1975) 507–512. [2] J.Y. Jaaski, H.U. Nissen, Phys. Chem. Miner. 10 (1983) 47–54. [3] S. Hashimoto, A. Yamaguchi, J. Ceram. Soc. Jpn. 112 (2004) 104–109. [4] H.J. Choi, J.G. Lee, J. Am. Ceram. Soc. 85 (2002) 481–483. [5] K. Li, T. Shimizu, K. Igarashi, J. Am. Ceram. Soc. 84 (2001) 497–503. [6] H.C. Park, Y.J. Park, R. Stevens, Mater. Sci. Eng. A 367 (2004) 166– 670. [7] H. Schneider, K. Okada, J.A. Pask, Mullite and Mullite Ceramics, John Wiley and Sons, New York, 1994, pp. 158–159. [8] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase Diagrams For Ceramists, vol. 1, The American Ceramic Society, Columbus, OH, 1964. [9] J.S. Jung, H.C. Park, R. Stevens, J. Mater. Sci. Lett. 20 (2001) 1089–1091. [10] S. Johnson, J.A. Pask, Am. Ceram. Soc. Bull. 61 (1982) 838–842. [11] W.G. Fahrenholtz, D.M. Smith, J. Am. Ceram. Soc. 77 (1994) 1377–1380. [12] L.B. Kong, Y.Z. Chen, T.S. Zhang, J. Ma, F. Boey, H. Huang, Ceram. Int. 30 (2004) 1319–1323. [13] 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. [14] I.J. Chabinsky, in: W.H. Sutton, M.H. Brooks, I.J. Chambinsky (Eds.), Microwave Processing of Materials, Materials Research Society, Pitts￾burgh, 1988, pp. 17–29.