(A) study on the properties of aluminum hydroxide gels prepared by sol-gel method

Abstract

Aluminum hydroxide gels were prepared from aluminum iso-propoxide under acidic and basic conditions by sol-gel method. The aluminum hydroxide gels exhibited different structures and properties in relation to their preparation conditins. The viscosity behavior of the solution peptized with excess HCl showed a stationary region during the sol-gel transition, which was ascribed to a phase transition of platelet boehmite particles from random aggregated state to ordered state with parallel orientation. This orientation was proved by the SEM observations of the dried gels. In the TEM test for investigating wet gel structures, the basic gel showed a loose network of the boehmite particles interconnected with one another, while in acidic gel, it could not be seen the network structure but several aggregates. The differences of the wet gel structures brought the different microstructures of dried gels in SEM pictures. The acidic gel exhibited the pores much finer and more evenly distributed throughout the gel, because the boehmite particles were evenly dispersed and did not form the network structure in the wet gel. On the other hand, the basic gel showed larger microstructural features and appeared as a coagulation of particles. The larger particles observed in this gel were ascribed to the network structure of the wet gel. Viscosities and yield stresses of aluminum hydroxide sols were measured to investigate the effect of acidity on the rheological properties during the sol-gel transition. The acidic solution obtained by peptizing with HCl showed a transition from Newtonian behavior to shear thinning. It showed thixotropic behavior with hysteresis loop and finally the disappearance of the hysteresis loop as the solid concentration was increased. The gelling point may be regarded as the particle concentration at which the loop disappeared. The solution prepared with 0.07 HCl/Alkoxide molar ratio gave smaller viscosity and lower yield stress than any other solution w...