PREPARATION OF DENSE CORDIERITE CERAMICS FROM ULTRAFINE PARTICLES OF MAGNESIUM-HYDROXIDE AND KAOLIN

Thermal reactions and sintering characteristics were investigated to o btain dense cordierite ceramics using kaolin minerals (Al2O3. 2SiO(2). nH(2)O), such as kaolinite or halloysite, and chemically synthesized magnesium hydroxide in the form of ultrafine hexagonal tabular particl es, Mixtures of magnesium hydroxide and submicron tabular-kaolinite pa rticles in composition of MgO . Al2O3. 2SiO(2) resulted in the formati on of amorphous state at about 800-900 degrees C after a reaction of t hermal decomposition. The amorphous materials abruptly shrank at about 900 degrees C up to a relative density of about 90%, above which crys tallization of a mu-cordierite phase restrained further densification. Firing the mixtures to about 1300-1350 degrees C yielded dense alpha- cordierite ceramics with a relative density higher than 95% and a negl igible apparent porosity and showing a linear thermal expansion coeffi cient of 2.2 x 10(-6)/K. Or. the other hand, mixtures of magnesium hyd roxide and submicron needlelike-halloysite particles resulted in insuf ficient thermal reaction at about 800-900 degrees C because of the les s contact area of mixed particles which, affecting sintering and cryst allization characteristics, produced several percents of apparent poro sity by firing at 1350 degrees C for Ih.