HYSTERESIS EFFECTS IN LOW EXPANSION GLASS-CERAMICS

Li2O-Al2O3-SiO2-based low expansion glass-ceramics with a h-quartz solid solution crystalline phase often show very small hysteresis effects in two different temperature intervals, one lying in the temperature range from -100 to +40-degrees-C, the other in the range from +130 to +320-degrees-C. It is assumed that the observed hysteresis effects are due to some yet unspecified relaxation phenomena. The effects in both temperature intervals have been investigated intensively by dilatometry and to a lesser extent by dielectric measurements. Investigations performed include experiments with constant heating and cooling rates, isothermal holding periods within the hysteresis intervals, and property changes at temperatures below the hysteresis intervals after different cooling rates from temperatures above the respective hysteresis intervals. While dilatometry is the more versatile method when investigating the glass-ceramic materials, it cannot be applied for the investigation of the glass-ceramic base glasses or partially crystallized glass-ceramics, since these materials have large CTE-values compared with the corresponding glass-ceramics; possible hysteresis effects are therefore masked by the large length changes with temperature. The hysteresis effects in these latter materials are therefore investigated by dielectric measurements.