Self-organized nano-crystallisation of BaF_2 from Na_2O/K_2O/BaF_2/Al_2O_3/SiO_2 glasses

摘要

In glasses with the compositions (100 - x)(2Na_2O centre dot 16K_2O centre dot 8Al_2O_3 centre dot 74SiO_2)xBaF_2 (with x = 0 to 6), the glass transition temperature decreases with increasing BaF_2-concentration. Samples with x = 6 were thermally treated at temperatures in the range from 500 to 600 deg C for 5-160 h. This leads to the crystallisation of BaF_2. The quantity of crystalline BaF_2 increases with increasing time of thermal treatment, while the mean crystallite size remains constant within the limits of error. The glass transformation temperature of partially crystallised samples increases with increasing crystallisation time and approaches a value equal to the temperature, at which the samples were treated. This is explained by the formation of a highly viscous layer enriched in SiO_2 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth.