Fabrication of dense celsian ceramics by reaction of kaolin with SrCO3 andBaCO3

Reaction and sintering characteristics of mixtures of kaolin (Al2O3. 2SiO(2 ). 2H(2)O) with BaCO3 or SrCO3 were investigated using kaolin and finely gr ound carbonate powders, Although the decomposition temperature of barium ca rbonate and strontium carbonate are higher than 1050 degrees C, both carbon ate powders decomposed at much lower temperature of about 800 degrees C and then reacted with decomposed kaolin to form amorphous materials. Thus form ed amorphous materials are extremely sinterable, and pressed specimens can be rapidly sintered at about 950 degrees C by viscous flow. Densification i s inhibited by the immediate crystallization of hexacelsian crystals (SrO . Al2O3. 2SiO(2), or BaO . Al2O3. 2SiO(2)) at about 1000 degrees C, Further densification was not observed at temperatures above 1000 degrees C, becaus e of completion of the crystallization of amorphous phases. Amount of SrO o r BaO for sufficient sintering were larger than n=1.2 and n=1.6, respective ly, with n being the mol ratio of SrO/Al2O3 and BaO/Al2O3. The reason of th e difference in the n values was attributed to the difference in viscosity between the amorphous phases, Dense polycrystals consisting of mainly monoc linic Sr-celsian were obtained at about 1200 degrees C.