toughening might be obtained by the generation of ZrO2 in tends to be more pure in carbon content. Hence these two addition to Al2 O3 and SiCw in this process sources were used to study its effect on the overall product Kaolin(M/s English India Clays, India), Sillimanite and 2. Experimental Zircon(M/s Indian Rare Earths, India) were taken as the 2.. Ranw materials given in Table 1 whilst particle size distributions of the various precursor powders(measured using a sedigraph The basic ingredients used for the carbothermic reac- 5100-Ms Micromeritcs, USA) are presented in Fig. 1. AC tion to produce the composite precursor powder mixtures (M/s Emerck, India)and CB (M/s Degussa, Germany)with are the silicate minerals(containing alumina and zirconia) a BET surface area of 1000 and 300 m-gm- were used and finely divided carbon Silicate minerals were chosen for SEM photographs of these carbon sources are presented in their low cost and their availability. The action of carbon Fig. 2 is twofold viz, as a reducer of the silicate and also a part goes into the product being formed. Carbon was taken in 2.2. Experimental procedt two forms, activated charcoal (AC) and carbon black(CB) AC being obtained from natural sources usually does have a The required amounts of AC or CB, Kaolin, Silliman- small percentage of impurities On the other hand, CB which ite and Zircon were taken to contain C and Sioz con- is obtained from the distillation of crude oil or lamp black tent(in silicates) in the molar ratio of 5.5: 1. The starting Table 1 Chemical composition of the silicate starting materials used for carbothermic synthesis Compounds Mineral Zircon (%, M/s Sillimanite(%, M/s Kaolin(%, M/ Indian Rare Earths) Indian Rare Earths) glish India Clays) 39.00 36.50 TIO, 00.03 Loss on ignition 14.30 —c b60 Fig. 1. Particle size distribution of the starting precursors used for the reaction:(a) Kaolin,(b) Zircon;(c) SillimaniteL. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 285 toughening might be obtained by the generation of ZrO2 in addition to Al2O3 and SiCw in this process. 2. Experimental 2.1. Raw materials The basic ingredients used for the carbothermic reac￾tion to produce the composite precursor powder mixtures are the silicate minerals (containing alumina and zirconia) and finely divided carbon. Silicate minerals were chosen for their low cost and their availability. The action of carbon is twofold viz, as a reducer of the silicate and also a part goes into the product being formed. Carbon was taken in two forms, activated charcoal (AC) and carbon black (CB). AC being obtained from natural sources usually does have a small percentage of impurities. On the other hand, CB which is obtained from the distillation of crude oil or lamp black Table 1 Chemical composition of the silicate starting materials used for carbothermic synthesis Compounds present Mineral Zircon (%, M/s Indian Rare Earths) Sillimanite (%, M/s Indian Rare Earths) Kaolin (%, M/s English India Clays) Al2O3 – 60.00 39.00 ZrO2 65.60 – – SiO2 31.10 36.50 46.00 TiO2 00.60 00.20 00.50 Fe2O3 00.08 00.50 00.52 CaO – – 00.06 MgO – – 00.06 K2O – – 00.03 Na2O – – 00.08 Loss on ignition – – 14.30 Fig. 1. Particle size distribution of the starting precursors used for the reaction: (a) Kaolin; (b) Zircon; (c) Sillimanite. tends to be more pure in carbon content. Hence these two sources were used to study its effect on the overall product formation. Kaolin (M/s English India Clays, India), Sillimanite and Zircon (M/s Indian Rare Earths, India) were taken as the silicate precursors. Chemical analysis of the precursors is given in Table 1 whilst particle size distributions of the various precursor powders (measured using a sedigraph 5100-M/s Micromeritcs, USA) are presented in Fig. 1. AC (M/s Emerck, India) and CB (M/s Degussa, Germany) with a BET surface area of 1000 and 300 m2 gm−1 were used. SEM photographs of these carbon sources are presented in Fig. 2. 2.2. Experimental procedure The required amounts of AC or CB, Kaolin, Silliman￾ite and Zircon were taken to contain C and SiO2 con￾tent (in silicates) in the molar ratio of 5.5:1. The starting