The reaction sequence in the synthesis of aluminium borate whiskers

The reaction sequences in the synthesis of aluminium borate whiskers from the raw material mixtures (Al2(SO4)3/H3BO3/K2SO4, K2SO4 as a flux) were investigated by differential thermal analysis-thermogravimetric analysis (DTA-TG), X-ray diffraction analysis (XRD), and a direct observation of the reacting mixture. DTA of a mixture with the optimum composition for obtaining well-grown whiskerswith a high yield, and XRD of the same mixtures quenched at various temperatures, showed that the reaction proceeds through formation and decomposition of double salts of aluminium: first tripotassium aluminium sulphate and then monopotassium aluminium sulphate. This reaction sequence and the DTA results of mixtures with different K2SO4/(Al-B) ratios were successfully explained by the phase diagram of the Al2(SO4)3 K2SO4 system. Direct observation of the reacting mixture confirmed the explanation. The compositions of reacting mixtures were then calculated from the TG data and traced on the phase diagram. The tracing showed that the decomposition of aluminium salts occurred in different physical states, depending on the K2SO4/(Al+B) ratio, in solid, solid and then liquid, or liquid phase. This difference in decomposition explained well the effect of the K2SO4/(Al+B) ratio on the morphology of the whiskers. The effect of the B/Al ratio in the mixture was similarly explained by the correlated change in the K2SO4/Al ratio.