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Preparation and characterization of alumina powders and suspensions
This study involves the preparation of fine alumina powders derived from Bayer gibbsite and also aqueous alumina suspensions by using tri block copolymers. Preparation of alumina powders was performed by decomposition of gibbsite into transition alumina phase followed by controlled transformation to alpha phase. To increase transformation rate.. to a-alumina in transition phase hence influence the nucleation and growth rate of the solid-solid phase transformation ball milling and ultrasonication was applied. Gibbsite was thermally treated at 900°C to reach a transition form of alumina. In some cases a heat treatment at 350 °C was applied to create a network of submicroscopic cracks in the heated gibbsite that may help grinding. Ball milling and ultrasonic treatment before calcination at 1100, 1200 °C and 1450 °C followed these heat treatments. Characterizations of the powders were performed with XRD, FTIR, thermal analysis, density measurements and particle size determinations. According to the XRD patterns, complete transformation to alpha form occurred in powders previously heat treated at 900°C, mechanical treated and then cfllcined at 1200 °C in 8 hours and 1450 °C in 2 hours. Powders that were calcined at 1100 °C and 1200 °C in 1-2 hours contained considerable amount of kappa form together with alpha. The effect of the polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO/PPO/PEO) blockcopolymers on the dispersion behaviour of alumina powder suspensions in water were investigated at <1>.0.125,1.0,14 and 50 vol% solid loadings by rheological and turbidity measurements. To compare the effects of block copolymers with other type of dispersants, measurements of some other well known dispersants were also conducted at 10-7 to 10-3 M. The results indicated that type block copolymers with high EO percentage have a positive effect when they are used with ultrasonic treatment on the agglomerated alumina suspensions. But it was not able to create stable dispersions ll1 the absence of ultrasonic bath application. Turbidity measurements at <1>.0.5 wt% showed that some dispersants gave higher dispersion but the stability was reached after a time period. Ultrasonic treatment created stability but lowered the turbidity values.