Effect of stirring rate on microstructure and properties of microporous mullite ceramics

Abstract

When suspensions are stirred at low stirring rates (400–800 rpm), microporous mullite ceramics with high bulk density and low apparent porosity can be achieved because the content of bubbles in suspensions is scarce. With the stirring rate increases from 1200 rpm to 2000 rpm, more and larger bubbles are generated in suspensions. Under actions of the force from spinning blades and the shear stress from adjacent flow layers, these larger bubbles are divided into smaller ones and dispersed evenly, which is helpful to reduce the bulk density and thermal conductivity of as-prepared ceramics. When the stirring rate increases up to 2400 rpm, bubble structures are easily destroyed because some suspensions are thrown out of the mixer by the excessive force of blades, which lead to increases in the bulk density, crushing strength and thermal conductivity.