Kohji Sakane

Effect of mixing state of raw materials in the preparation of Mg2B2O5 whiskers

We have studied various ways of preparing acicular magnesium pyroborate (Mg2B2O5). We formed acicular Mg2B2O5 from the reaction of an MgO-H3BO3-KCl mixture (2:3:5 mole ratio) at 850°C. The product, however, contained agglomerates derived from large MgO particles. In a variation, finely mixed raw material was prepared by the spray drying of slurry. The slurry was made by dissolving MgCl2, H3BO3 and KCl in hot water and neutralizing it with KOH. The reaction was again driven at 850°C. The product was acicular Mg2B2O5 without agglomerate, but the crystal length did not exceed 10μm. Finally, Mg2B2O5 crystals 20-40 μm long and 1-3 μm thick were obtained by seeding the slurry with small Mg2B2O5 crystals.

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.