EFFECT OF HEAT ON AL2O3-SIO2 GEL CO-PRECIPITATED AT DIFFERENT PH

In the present studies Al2O3—SiO2 mullite gels have been synthesized in close range of pH between 6 and 9 by co-precipitating technique using dilute NH4OH solution. Both qualitative and semi-quantitative mullite formation studies have been done. Results show that a spinel to mullite formation occurs during heating the co-precipitated gels in the temperature range between 1100° and 1200°C with 2 hours soaking in each case. The quantity of mullite in all the cases increases with the continued increase in temperature. With the increase in pH, during co-precipitation, the approximate yield of mullite decreases from pH 6 to 9. It is concluded that co-precipitated Al2O3—SiO2 gel becomes inhomogeneous with increase in pH during precipitation. The appearance of boehmite at higher pH range is responsible for low mullitization growth during heat treatment.