Abstract
A lithium disilicate glass was heat treated at 454°C, the temperature of maximum nucleation rate, up to 25days. At this temperature the growth rate is negligible, and therefore these thermal treatments lead to a glassy matrix in which ellipsoidal crystals of about the same size were randomly dispersed. Crystallized volume fraction (α) of up to 20% was reached. Electrical conductivity of glassy, partially crystallized, and 100% crystallized lithium disilicate was then measured by impedance spectroscopy. The crystallized surface layer was eliminated by polishing in order to solely analyze the effect of bulk crystals. The complex plane plots of impedance data showed one or two semicircles depending on (α). The high frequency semicircle represents the vitreous matrix while the low frequency one was ascribed to the blocking effect imposed to the lithium ions by the existing crystals in the partially crystallized samples. This hypothesis was confirmed by the analysis of the relaxation frequency related to each one of the semicircles and by comparison to the relaxation frequency of lithium disilicate glass and crystal. Results show that the blocking effect produces a second semicircle in the impedance diagram starting from a crystallized volume fraction of 15%.
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