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 leachingof 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 techni￾ques 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) com￾pacts 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