CRYSTALLIZATION OF SILICA STUDIED BY POSITRON-ANNIHILATION

Crystallization of SiO2 glass was studied by positron lifetime spectro scopy and Doppler broadening. Heat treatments were performed in the te mperature range between 700 and 1600 degrees C to investigate the crys tallization process isothermal and isochronal. Lifetime spectra were a nalyzed by two lifetime components. The decrease of the short lifetime (300 ps) was attributed to the increasing volume fraction of the crys talline phase. The long lifetime (1500 ps) was related with the pick-o ff annihilation of the ortho-positronium state in the free volume. Lon g lifetime increase, with the onset of crystallization, has been expla ined by expanding cavities at the interface between the crystalline ph ase and amorphous matrix. Positronium formation decreases with devitri fication, which is in agreement with measurements of Doppler broadenin g. Positron annihilation was shown to be especially suited to investig ate surface crystallization. (C) 1997 Elsevier Science B.V.