Key Engineering Materials Vols. 264-268 (2004) pp. 117-120 online at http://www.scientific.net © 2004 Trans Tech Publications, Switzerland Investigation of Parameters Affecting Formation of Mullite from Kaolin C. Aksel and A. Kalemtaş Department of Materials Science and Engineering, Anadolu University, Iki Eylül Campus, 26470 Eskişehir, TURKEY. Keywords: Kaolin; mullite; sintering; microstructure Abstract. This study aims to improve the understanding of formation of mullite from kaolin at low process temperature using different sintering regimes. The optimum parameters affecting mullite formation from kaolin such as sintering temperature (from 1000 to 1500 °C), heating rate (3 and 20 °C min-1) and dwell time (3 min, 1 and 5 h) were investigated. There was a marked increase in bulk density values with increasing sintering temperature up to ~1300 °C, resulting in a significant decline at the apparent porosity values. The lower heating rate and the longest dwell time appeared to be more effective to obtain a maximum density value at 1300 °C. Microstructural features were examined using SEM, and the nature of crystalline phases present was confirmed by XRD analysis. It was observed that formation of mullite grains started at ~1100 °C. Although there was a significant amount of mullite grains at 1200 °C, completely elongated mullite crystals were formed at 1300 °C. Introduction Kaolin as a raw material has been commonly used in ceramic industries. After sintering kaolin at high temperatures, the main product phase is mullite [1-3]. Because of its excellent physical properties, such as low dielectric constant, low thermal expansion, high melting point, high resistance to creep, high temperature mechanical stability, and high thermal shock and chemical corrosion resistance, mullite is an important constituent of refractories, whitewares and structural clay products, and they are also widely used as electric resistor and thermal insulator [1-5]. Previous researchers [1,3,6] have used many approaches to prepare mullite by solid-state reaction, coating and using sol-gel technique. In addition to those approaches, to prepare mullite using kaolin as starting material is also an important one for its economic potential. Many ceramic processes aim to shorten the sintering schedule. A high heating rate of ceramic bodies induces a large change of thermal transformations of raw materials and their mixtures. It is reported  that the amount of mullite formed from kaolin between 1050 and 1150 °C is strongly dependent on the heating rate, and faster heating rates increase the amount of mullite. Previous researchers [8,9] have also investigated the effect of additives on mullite formation in kaolinite. It is reported that Fe2O3, CaO and K2O have the most distinctive effect on increasing the crystal size of mullite and accelerate the growth rate of needle-like mullite crystals noticeably, where Fe2O3 has the highest solid solubility (6-12 wt.%) in mullite at ~1300 to 1400 °C, though CaO, K2O and Na2O have a limited solubility (<1 wt.%). Mullite crystallisation is a common phenomenon involved in the thermal transformation of clay minerals. The quantity and development of this phase control the sintering mechanisms; however, mullite formation depends on the type of raw material and can be modified using various sintering regimes [7,10]. The objective of this present study is therefore to investigate the densification behaviour in terms of the effects of sintering temperature, heating rate and dwell time on the formation of mullite phase from kaolin by means of SEM and XRD analysis. Experimental Procedures The chemical composition of commercial kaolin (CC31) used in this work is given in Table 1. 1 g discs of compacted kaolin powder were uniaxially pressed in a 13 mm diameter die at ~100 MPa for 30 s. The discs were then pressureless sintered between 1000 and 1500 °C, at 100 °C intervals, for 3 min, 1 and 5 h. In all cases, the heating rate varied from 3 to 20 °C min-1, and the Licensed to NihanTuncer ([email protected]) - ANKARA - Turkey All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 22.214.171.124-16/08/04,07:21:03) 118 Euro Ceramics VIII cooling rate was 10 °C min-1. Bulk density and apparent porosity of the sintered samples were measured using the standard water immersion method , where three specimens were normally measured to obtain a mean value. The samples were mounted by using an epoxy resin, and polished down to 1 µm finish. Chemical etching was then carried out using boiling phosphoric acid for 6 min. Microstructural features were examined using a CamScan-S4 Scanning Electron Microscopy (SEM). X-ray diffraction (XRD) measurements were also carried out to identify the crystalline phases present. Table 1: Chemical composition of CC31. Constituent Wt. % SiO2 50.73 Al2O3 35.41 K2O 2.44 Fe2O3 0.63 MgO 0.51 Na2O 0.47 CaO 0.31 Ignition loss 9.50 Results and Discussion 2,7 2,6 2,5 2,4 2,3 2,2 2,1 2,0 1,9 1,8 1000 1100 1200 3 °C/min - 3 min 3 °C/min - 1 h 3 °C/min - 5 h 20 °C/min - 3 min 20 °C/min - 1 h 20 °C/min - 5 h 1300 1400 1500 T (°C) Figure 1. Bulk density (ρb) as a function of temperature. Pa (%) -3 ρ b (g cm ) Bulk density and apparent porosity values of the samples as a function of sintering temperature, for various heating rate and dwell time, are given in the Figs. 1 and 2, respectively. Fig. 1 showed that there was in general a significant increase (~40%) in the density values up to 1300 °C, and subsequently a marked decline up to the maximum sintering temperature used, apart from the samples sintered with 3 min dwell time, for which a high temperature (1400 °C) is required to achieve a similar improvement. In general, the highest heating rate (20 °C min-1) appeared to be relatively more effective to attain higher density values up to 1200 °C, where there was a marked effect of increased dwell time. However, further increase in the temperature (from 1200 to 1500 °C) made the lowest heating rate (3 °C min-1) more useful to obtain greater density values. Maximum densification was achieved at 1300 °C using the heating rate, 3 °C min-1, and dwell time, 5 h, where the minimum porosity values were attained at this process conditions. The apparent porosity values as a function of sintering temperature in terms of heating rate and dwell time were in general good agreement with the bulk density values (Fig. 2). 30 27 24 21 18 15 12 9 6 3 0 1000 1100 1200 3 °C/min - 3 min 3 °C/min - 1 h 3 °C/min - 5 h 20 °C/min - 3 min 20 °C/min - 1 h 20 °C/min - 5 h 1300 1400 1500 T (°C) Figure 2. Apparent porosity (Pa) as a function of temperature. The mullite is formed from kaolin by exsolution of silica, and possibly retains some of the naturally occurring impurities of kaolin in solid solution in the lattice. It is stated [8,9] that the existing impurities in kaolin (Table 1) accelerate the growth rate of needle-like mullite crystals as the viscosity of the liquid is decreased and the diffusivities are increased at high temperatures (≥1300 °C), where Fe2O3, CaO and K2O have the most pronounced effect on increasing the crystal size and acceleration of the crystallisation. It is reported [2,3] that the Fe2O3 changes to Fe3O4 and generates O2 at elevated temperature (≥1400 °C), where the release of oxygen gas causes bloating. It is also possible that an increase in the gas pressure because of an increase in temperature or the gas evolved from within the glass can cause pore enlargement and a volume expansion . The presence of impurities in the starting powder therefore leads to a marked decline in density and a gradual increase in porosity values at ≥1400 °C. Key Engineering Materials Vols. 264-268 119 XRD patterns of the samples sintered at various temperatures for 1 h using the heating rates of 3 and 20 °C min-1 are given in Figs. 3 and 4, respectively. XRD analysis showed that the kaolin used at room temperature contained some free quartz and its quantity decreased to a large extent with increasing sintering temperature (at 1000-1100 °C), and subsequently mullite formation started at ≥1100 °C. As the sintering temperature increased up to 1500 °C, there was a significant amount of rise in the intensity of mullite peaks (Figs. 3 and 4). Mullite formation was also found to be dependent on heating rate. The main peaks of mullite phases occurred at the maximum sintering temperature (1500 °C) using the heating rate of 3 °C min-1 appeared to be relatively greater than that of 20 °C min-1. It is therefore considered that lower heating rate increased the mullite formation noticeably at high temperatures (Figs. 3 and 4). M M M M M M M M M M 1500 °C 1400 °C K 1300 °C 1200 °C 1100 °C K 15 KK 20 K K KK K Q 25 30 35 40 1000 °C K 45 Relative Intensity Relative Intensity M M M M M M M M 1500 ºC 1400 ºC 1300 ºC K 1200 ºC 1100 ºC K K KK 25 °C 50 M M 15 20 Q 25 30 1000 ºC KK K K K 25 ºC 35 45 40 50 2θ 2θ Figure 3. The XRD patterns of kaolin sintered at various temperatures for 1 h: heating rate = 3 °C min-1 (M: Mullite, K: Kaolin, Q: Quartz). Figure 4. The XRD patterns of kaolin sintered at various temperatures for 1 h: heating rate = 20 °C min-1 (M: Mullite, K: Kaolin, Q: Quartz). Fig. 5 showed that mullite formation was observed noticeably for the samples sintered at 1200 °C for 1 h using the heating rate of 3 °C min-1, where there was a marked amount of globular grains with a few elongated grains. The rise in dwell time to 5 h with the same sintering regime led to further extension in the length of elongated crystals, although there were some coarse grains (Fig. 6). It is therefore suggested that the use of an elevated dwell time was in general found to be favourable Figure 5. SEM picture of sample sintered at 1200 °C for 1 h, using heating rate of 3 °C min-1 (Scale bar: 50 µm). Figure 6. SEM picture of sample sintered at 1200 °C for 5 h, using heating rate of 3 °C min-1 (Scale bar: 50 µm). 120 Euro Ceramics VIII to obtain a high aspect ratio of mullite grains. Figs. 7 and 8 also illustrate the microstructural features of mullite crystals based on the variation in the heating rate and dwell time at 1300 °C. Using a higher heating rate and 1 h dwell time, the average length of elongated crystals is generally bigger than ~10 µm, but there are still some coarse grains observed in the microstructure (Fig. 7). Fig. 8 shows that using the heating rate of 3 °C min-1 and 5 h dwell time resulted in the formation of completely elongated mullite grains, which were randomly oriented. The lengths of needle-like mullite crystals were mostly greater than 10 µm and the aspect ratio increased significantly. The lower heating rate therefore came out to be more useful to attain the greatest length of crystals with the highest aspect ratio, at 1300 °C. Figure 7. SEM picture of sample sintered at 1300 °C for 1 h, using heating rate of 20 °C min-1 (Scale bar: 50 µm). Figure 8. SEM picture of sample sintered at 1300 °C for 5 h, using heating rate of 3 °C min-1 (Scale bar: 20 µm). Conclusions The densification behaviour of mullite obtained from kaolin was optimised using various sintering regimes. A significant increase was occurred in bulk density values up to ~1300 °C, leading to a marked decrease in the apparent porosity. However, there was a noticeable decline in density and a slow rise in the porosity values at ≥1400 °C. This can be attributed to the presence of impurities in kaolin, which leads to a volume expansion because of bloating. Microstructural examination showed that the formation of mullite grains started noticeably at 1200 °C with elongated morphology, but completely needle-like mullite crystals developed at 1300 °C. The lowest heating rate (3 °C min-1) and the longest dwell time (5 h) at 1300 °C were found to be the most suitable conditions for obtaining needle-like mullite crystals with a high aspect ratio. References             H. Schneider, K. Okada and J. Pask: Mullite and Mullite Ceramics (John Wiley&Sons Ltd., New York 1994). C.Y. Chen and W.H. Tuan: Ceramics International Vol. 27 (2001), p. 795. C.Y. Chen, G.S. Lan and W.H. Tuan: Ceramics International Vol. 26 (2000), p. 715. C. Aksel: Ceramics International Vol. 29(2) (2003), p. 183. C. Aksel: Materials Letters Vol. 57(3) (2002), p. 708. F. Kara and J.A. Little: J. Eur. Ceram. Soc., Vol. 16(6) (1996), p. 627. O. Castelein, B. Soulestin, J.P. Bonnet and P. Blanchart: Ceramics International Vol. 27 (2001), p. 517. S.M. Johnson, J.A. Pask and J.S. 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