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1 Interceram 0102/ K. Das*, S. Das*, S. Mondal*, P.K. Maiti** Effect of Calcium Fluoride Addition on Densification and Microstructure of Reaction-Sintered Indian Clay-Alumina Mixtures The author abstract Keywords The corresponding author, Dr. Kaberi Das is working as Assistant Professor in Ceramic Technology at Government College of Engineering & Ceramic Technology, Kolkata, and India. She obtained her M.Tech. degree in Ceramic Technology and Ph.D. (tech) degree from the University of Calcutta in 1990 and 1996 respectively. Her main research interest is the development of synthetic refractory aggregates from Indian natural raw materials. A mullite compound was prepared using Indian clay and reactive alumina following a reaction-sintering route in the presence of a calcium fluoride (CaF 2 ) additive. 0.25, 0.50, and 1.00 mass-% CaF 2 was added to the mullite (stoichiometric) forming batch and the batches were sintered at temperatures of 1550, 1600, and 1650 C for 2 h. The batch without CaF 2 completed the reaction at 1600 C whilst a relative density of 97 % of the theoretical density of mullite was attained at 1650 C/2 h, whereas with 0.25 mass-% CaF 2 the batch completed the reaction at 1550 C and achieved the same relative density at 1600 C/2 h. A bimodal distribution of elongated lath-shaped primary mullite and equiaxed secondary mullite was found in the secondary electron image of the 0.25 % CaF 2 -containing batch sintered at 1650 C/2 h. The microstructures of the CaF 2 -containing batches are discussed. kaolinite, calcium fluoride, reaction sintering, primary mullite, secondary mullite Interceram 63 (2014) [12] 1 Introduction Mullite (3Al 2 O 3 2SiO 2 ), the most stable phase in the Al 2 O 3 SiO 2 system, is rarely present in nature. It is an important crystalline phase not only in conventional ceramics but also in advanced structural ceramics for its outstanding properties at room temperature as well as at elevated temperature viz. low density (3.17 g/cm 3 ) low thermal conductivity (2 W m 1 K 1 ), low thermal coefficient (α 20/200 C = K 1 ), low dielectric constant (K = 6.5 at 1 MHz), and excellent mechanical properties at high temperature (strength in excess of 400 MPa at 1200 C) and low creep rate [15]. A very high sintering temperature (1700 C) for 12 h is required to obtain dense mullite compacts from pure alumina and silica mixtures because of the high activation energy value (170 kcal/mol) for bulk diffusion of Al and Si in the mullite lattice [3, 6]. Several alternative routes were proposed in the literature: coprecipitation [7], sol-gel [89], where virtually atomic-level silicaalumina mixtures were obtained. In these cases, * Government College of Engineering and Ceramic Technology, 73, A.C. Banerjee Lane, Kolkata (India) ** Department of Chemical Technology, University of Calcutta, 92, A. P. C. Road, Kolkata (India) dense mullite is obtained at less than 1650 C in the absence of a liquid phase. However, because of the extremely high cost of the starting powders, these routes are not appropriate for large-scale production. A technologically more feasible route for largescale production of mullite can be obtained by using a conventional raw material such as a mixture of kaolinite and alumina according to the following reaction [5, 1013]: Al 2 O 3 2SiO 2 2H 2 O + 2Al 2 O 3 3Al 2 O 3 2SiO 2 + 2H 2 O (1) Many researchers have already investigated the effect of different additives on the mullitization and densification behavior of reaction-sintered kaolinalumina mixtures. The densification was enhanced with small additions of B 2 O 3. It was observed that 0.5 mass-% B 2 O 3 increased the content and growth of mullite particles [14]. The effect of MgO, Y 2 O 3, and boehmite additives on the sintering behavior of mullite formed from kaolinite-reactive alumina mixtures was studied by V. Viswabaskaran et al. [15]. Additions of 3 mass-% MgO improved density up to 2.9 g/cm 3 with a maximum strength of 125 MPa. The addition of Y 2 O 3 favored the development of equiaxed mullite. The effect of TiO 2 on the interaction of dehydroxylated English China clay with Al(OH) 3 gel was studied by Mitra et al. [16]. TiO 2 played a positive role in the formation of mullite. It formed a liquid phase, which promoted densification. It was observed that the presence of small amounts of fluoride (AlF 3 /CaF 2 ) accelerated the sintering of both the MgOAl 2 O 3 system [17] and the hydroxyapatite-zirconia composite [18]. The objective of the present work is to study the effect of a small amount of CaF 2 additive on the mullitization and microstructural development of mullite during reaction sintering of Indian clay and reactive alumina. 2 Experimental 2.1 Starting materials Indian clay: In the present work, Indian clay (from the Birbhum district of West Bengal, India) was used as one of the principal raw materials. It was crushed and purified as per standard methods for removal of any impurities adhering to the clay [19]. The purified clay was characterized by chemical analysis, X-ray diffraction, differential thermal analysis (DTA), particle size, and true density measurement. Reactive alumina: The alumina starting powder was prepared after heat treatment of aluminum hydroxide. First, aluminum hydroxide was precipitated by adding ammonium hydroxide (1 : 1) (ammonia solution, 25 vol.-% pure, E. Merck India Ltd.)

2 32 Interceram 0102/ Relative density Sintering temperature / C Fig. 1 Relative density as a function of sintering temperature of a clay-reactive alumina batch Table 1 Batch composition and sample codes Batch code Clay / g Calcined alumina / g Additive (CaF 2 ) / g CA CA CA CA Table 2 Physical and chemical properties of the main raw materials Oxide / mass-% Clay Reactive alumina (Lab. developed) SiO Al 2 O TiO Fe 2 O MgO Trace CaO 0.53 Na 2 O 0.67 K 2 O 0.75 LOI / mass-% Crystalline phases present: Major Minor DTA Peaks Endothermic / C Exothermic / C Kaolinite 120 and γ-alumina 150 and 296 Mean particle size / μm True density / g/cm to a 0.5 (M) solution of aluminum nitrate (95 mass-% Al 2 (NO 3 ) 3 9H 2 O, E. Merck India Ltd.) at ph 9. The precipitate was thoroughly washed to remove any adhering electrolyte. The precipitate was dried at 90 C till a constant mass was obtained. The dried precipitate was characterized by differential thermal analysis (DTA-1500, OKAY, Bysakh & Co., India) to identify the calcination temperature. The dried mass was calcined at 400 C/1 h to obtain reactive alumina. The calcined alumina powder was characterized by particle size measurement (Nano S 1600, Malvern, USA), phase analysis by X-ray diffraction, and true density. 2.2 Preparation of stoichiometric mullite The weight of the each batch was 100 g. Batch composition was selected so as to obtain stoichiometric mullite (3Al 2 O 3 2SiO 2 ) in the final sintered samples. Calcium fluoride (LOBA Chemie Pvt. Ltd., India) which was used as a sintering aid was in the range 01 mass-%. The batch composition and the sample codes are presented in Table 1. Each batch was mixed thoroughly in a rubber-lined 500-ml pot mill with alumina grinding media in water for 4 h. The mass ratio of grinding media:powder was maintained at 10 : 1. The slurry was dried at 110 C till a constant mass was obtained and granulated after mixing with 5 mass-% moisture and passing through a BS 30 sieve. The granules were uniaxially pressed into discs of 25 mm diameter at 100 MPa pressures. After drying at 110 C overnight, the samples were sintered at 1550, 1600, and 1650 C for 2 h with a heating rate of 8 C/ min up to 1000 C followed by 5 C/min up to the specific sintering temperature. 2.3 Characterization techniques The sintered body was characterized by measuring bulk density and apparent porosity following Archimedes principle. The crystalline phases developed during sintering were identified by X-ray diffraction (Rigaku, Minisles) of the powdered sample using Ni-filtered Cu-K α radiation with a scanning speed of 2 /min. The powder for the powder X-ray diffraction study was prepared as follows. A sintered disc was crushed in a percussion mortar followed by magnetic separation and sieving. The -60 BS mesh particles were ground in an agate mortar to pass through the 200 mesh BS sieve and the resulting powder was used for the powder X-ray diffraction study. The true density was measured using a pycnometer on -100 BS sieved powder. The microstructural study was performed by scanning electron microscopy (HITACHI, S 3400N Model) on the chemically etched polished and fractured surfaces. The grinding was carried out with abrasive SiC powder down to grit 1000 and fine polishing using 3-, 1-, and 0.25-µm diamond paste. The chemical etching was performed in a 1 : 1 HFwater solution for 30 s followed by ultrasonic cleaning with a 1 : 1 ethyl alcohol-water mixture. 3 Results and discussion The chemical and physical analyses of the clay (Birbhum district of West Bengal, India) and reactive alumina are presented in Table 2 [4]. The molar ratio of SiO 2 : Al 2 O 3 is 2.14, which is close to the theoretical value of TiO 2 and Fe 2 O 3 are present as impurities. The true density of the clay is 2.59 g/cm 3, very close to that of pure kaolinite (2.61 g/cm 3 ) [19]. The X-ray diffraction of the clay also confirms the presence of kaoli nite as a major phase. The differential thermal analysis of the clay shows two endothermic peaks, one at 120 C due to expulsion of physically absorbed water and another at 530 C representing the removal of chemically combined water to form metakaolin from kaolinite. Several authors have reported that after heating at 950 C, SiO 4 groups combine with the AlO 6 group to form an alumina-silica spinel phase which is a short-range ordered structure [2021]. The exothermic peak at 973 C is therefore believed to be due to either alumina-silica

3 Interceram 0102/ Intensity (a.u.) Sintering temperature / C Fig. 2 The apparent porosity in vol.-% (AP) is plotted against sintering temperature of a clay-alumina mixture 2θ / Fig. 3 The powder X-ray diffraction of a clay-alumina mixture sintered at 1550 C/2 h; a a-alumina/corundum, m mullite 4 Fig. 4 The secondary electron image of a chemically etched polished surface of a reactionsintered mullite fired at 1550 C/2 h; CAO, CA25 (for batch code see Table 1) spinel or γ-al 2 O 3 with silica in solid solution [3, 13, 2226]. The average particle size of the clay was 3 µm. DTA of the dried aluminum hydroxide powder showed two endothermic peaks. The first endothermic peak at 150 C was related to expulsion of physically combined water and the second peak at 296 C represented formation of γ-alumina from aluminum hydroxide [3, 5]. The X-ray diffraction pattern of powder calcined at 400 C/1 h showed the presence of poorly crystallized γ-alumina. The true density of the calcined alumina was 2.58 g/cm 3, which suggests poor crystallinity and high reactivity of the calcined alumina. The average 5 Fig. 5 The scanning electron microstructure of chemically etched fracture surface of clay-reactive alumina mixture sintered at 1600 C/2 h; CAO, CA100 (for batch code see Table 1) particle size of the calcined alumina was 0.59 µm. The average particle size of the pot-milled mixture of kaolin-reactive alumina was 0.40 µm. The relative density (fraction of theoretical density of mullite) as a function of sintering temperature of different batches is shown in Fig. 1. The highest density (relative) was obtained at 1550 C in the CA0 batch. Among the CaF 2 -containing batches the CA25 batch shows the maximum R density (relative density) at 1550 C. Figure 2 is a plot of apparent porosity (AP) versus sintering temperature of the different batches. All four batches show the same trend, i.e., AP decreases with increasing sintering temperature. The CA100 batch shows the minimum AP value for all sintering temperatures. Figure 3 shows the powder X-ray diffraction pattern of the CA0 and CA25 batches sintered at 1550 C. No cristoballite peak is detected in the diffraction pattern. The main difference is the presence of α-alumina (corundum) as an unreacted crystalline phase in the CA0 batch. Both samples contain mullite as the main crystalline phase. Scanning electron microscopy images of the chemically etched polished surface of CA0 and CA25 fired at 1550 C are shown in Fig. 4. Figure 4 a, the secondary electron image of CA0, shows mainly elongated mullite crystals and tiny bright white particles of unreacted alumina embedded in the glassy matrix, whereas the SEM of CA25 presented in Fig. 4 b shows elongated primary mullite and equiaxed/rounded secondary mullite. No unreacted alumina in a glassy phase is seen here. Upon increasing the sintering temperature to 1600 C, the microstructure changes as shown in Fig. 5. Figure 5 a shows the SEM of CA0 sintered at 1600 C. Elongated columnar primary mullite of high aspect ratio [8.6 (l/a)] are mainly observed. But in the CA100 batch (shown in Fig. 5 b), the mullite grains are rounded (approx. diameter of 1.5 µm)/equiaxed (approx. size of 4 µm) without any primary elongated mullite. Upon increasing the sintering temperature to 1650 C, the microstructure of all batches changed drastically (Fig. 6). Most mullite particles are secondary mullite with an aspect ratio of 1.53 in the CA0 batch (Fig. 6 a). A bimodal distribution of primary mullite with an aspect ratio of 3.1 and equiaxed secondary mullite with a 1.0 aspect ratio is seen in the CA25 sample (Fig. 6 b); whereas the

4 34 Interceram 0102/ (c) Fig. 6 The secondary electron images of chemically etched fractured surface of reaction sintered mullite sintered at 1650 C/2h; CAO, CA25, (c) CA100 (for batch code see Table 1) CA100 batch consists of a glassy phase with embedded mullite crystals (Fig. 6 c). The maximum R density with low densification (high AP) of the CA0 batch with respect to the CaF 2 -containing batches at 1550/1600 C may be due to the presence of unreacted alumina (corundum) as the density of corundum (3.99 g/cm 3 ) is much higher than that of mullite (3.16 g/cm 3 ). The presence of unreacted alumina was also detected in the powder X-ray diffraction pattern (Fig. 3) and the SEM of the CA0 batch. Only elongated primary mullite and unreacted alumina in a glassy phase are observed in the secondary electron image (Fig. 4). The R density decreased to 0.97 with a minimum apparent porosity (0.18 vol.-%) in the CA0 sample which confirmed the completion of the reaction and densification at 1650 C/2 h. It was reported that the reaction of α-alumina with kaolinite to form secondary mullite is initiated at 1250 C, proceeds extensively above 1380 C and becomes extremely fast above 1600 C and completion could be reached after 40 min at 1600 C [11]. With the increase in sintering temperature, more and more of the unreacted alumina was consumed by the liquid phase to form secondary mullite. Some investigators have also observed that if the firing temperature is greater than 1595 C, the reaction between kaolinite and alumina would be substantially increased because of the higher liquid content. This equilibrium liquid favors the solution precipitation process of liquid phase sintering [6, 27]. The R density obtained was higher than that reported by others [3, 15]. The higher densification at 1650 C was due to the high reactivity of the gamma alumina. At 1600 C (Fig. 5 a) only primary mullite of high aspect ratio was observed whereas at 1650 C (Fig. 6 a) only equiaxed secondary mullite was observed. Several authors [3, 11] have reported that during heating of a kaolinite and α-alumina mixture, primary mullite is formed from the kaolinite at 980 C, whilst secondary mullite is formed at higher temperatures by a solution precipitation mechanism from the transitory liquid phase. In the CaF 2 -containing samples, the CA25 batch always shows the highest relative density. The presence of calcium fluoride favored the reaction between reactants as well as the sintering process. No unreacted alumina phase was detected in the powder X-ray diffraction pattern as shown in Fig. 3. CaF 2 substantially reduced the viscosity of the liquid phase formed and thereby enhanced the rate of diffusion to facilitate densification. It was reported that in silicate glasses fluorine forms SiF with a nonbridging bond, resulting in a decrease of viscosity and in some silicate glasses, fluorine probably promotes the phase separation leading to a decrease in the energy barrier necessary for crystallization [28]. Upon increasing the additive content the R density decreased along with apparent porosity. With higher amounts of CaF 2 (1.0 mass-% of CaF 2 ), the amount of reactive liquid phase with low density was remarkably increased which swallowed the crystalline phases leading to a lowering of density as well as apparent porosity at different firing temperatures [2930]. This can also be corroborated by the microstructural analyses, from which it is observed that because of the presence of an excess liquid phase even at 1600 C, primary elongated mullite is converted to equiaxed secondary mullite (Fig. 5 b). At 1650 C, the CA100 batch consists of a glassy phase with embedded mullite crystals (Fig. 6 c). 4 Conclusions Mullite aggregate could be prepared from low-iron-containing Indian kaolinitic clay and reactive alumina mixtures through the reaction-sintering route Completion of the reaction between clay and reactive alumina as well as densification without any additive occurred at 1650 C, achieving 97 % of the theoretical density of mullite The presence of a small amount of CaF 2 (0.25 mass-%) not only facilitated the completion of the reaction but also enhanced sintering by forming a low viscosity liquid phase to aid liquid phase sintering with a BD of 3.07 g/cm 3 (97 % of the theoretical density of mullite) at low temperature Two types of mullite particles were found at 1650 C with 0.25 mass-% CaF 2 : columnar primary mullite and equiaxed secondary mullite The presence of higher amounts of calcium fluoride (1.0 mass-%) produced greater amounts of the low viscosity and low-density glassy phase which aided the low density and low apparent porosity at 1650 C/2 h The optimal amount of CaF 2 to act as a liquid-phase sintering aid for densification and reaction sintering of the clay and reactive alumina mixture was determined to be 0.25 mass-%. This material can be used as a high alumina mullite refractory aggregate for high-alumina-based shaped and monolithic refractory production. Acknowledgment The authors gratefully acknowledge Prof. S. Maitra, Officer-in-charge of the Govt. College of Engineering and Ceramic Technology, Kolkata, for his kind permission to publish this paper. References [1] Ohnishi, H., Maeda, K., Nakamura, T., Kawanami, T.: High Temperature Mechanical Properties of Mullite Ceramics. Mullite and Mullite Matrix Composites, Ceramic Transactions, vol. 6, S. Somiya, R.F. Davis, J.A. Pask (Editors), The Amer. Ceram Soc. (1990) [2] Schneider, H., Eberhard, E.: Thermal Expansion of Mullite. J. Amer. Ceram. Soc. 73 (1990) [3] Rezaie, H.R., Rainforth, W.M., Lee, W.E.: Mullite evolution in ceramics derived from kaolinite, kaolinite with added a-alumina, and sol-gel precursors. Trans. Br. Ceram. Soc. 96 (1997) [4] Raut, N.S., Biswas, P., Bhattacharya, T.K., Das, K.: Effect of Bauxite addition on densification and mullitization behaviour of West Bengal clay. Bull of Mater. Sc. 31 (2008) [5] Viswabaskaran, V., Gnanam, F.D., Balasubramanian, M.: Mullitization behaviour of calcined clay-alumina mixtures. Ceramic Internat. 29 (2003)

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