CRYSTAL-GROWTH KINETICS IN BAOAL(2)O(3)2SIO(2) AND SROAL(2)O(3)2SIO(2) GLASSES

Crystallization kinetics of BaOAl(2)O(3)2SiO(2) (BAS) and SrOAl(2)O(3) 2SiO(2) (SAS) glasses in bulk and powder forms have been studied by no n-isothermal differential scanning calorimetry (DSC). The crystal grow th activation energies were evaluated to be 473 and 451 kJ mol(-1) for bulk samples and 560 and 534 kJ mol(-1) for powder specimens in BAS a nd SAS glasses, respectively. Development of crystalline phases on the rmal treatments of glasses at various temperatures has been followed b y powder X-ray diffraction. Powder samples crystallized at lower tempe ratures than the bulk and the crystallization temperature was lower fo r SAS glass than BAS. Crystallization in both glasses appeared to be s urface nucleated. The high temperature phase hexacelsian, MAl(2)Si(2)O (8) (M=Ba or Sr), crystallized first by nucleating preferentially on t he glass surface. Also, monoclinic celsian does not nucleate directly in the glass, but is formed at higher temperatures from the transforma tion of the metastable hexagonal phase. In SAS the transformation to m onoclinic celsian occurred rapidly after 1 h at 1100 degrees C. In con trast, in BAS this transformation is sluggish and difficult and did no t go to completion even after 10 h heat treatment at 1400 degrees C. T he crystal growth morphologies in the glasses have been observed by op tical microscopy. Some of the physical properties of the two glasses a re also reported.