Suppressing the thermal conduction in glass–ceramic foams by controlling crystallization

Abstract

Glass-based insulating materials have attracted considerable attention owing to their tailorable properties. It is known that the thermal conductivity of glass ceramics can be greatly influenced by varying their crystallinity. However, the mechanism of such influence in glass–ceramic foams remains poorly understood. In this study, we demonstrate our new findings regarding the correlation between thermal conductivity and crystallinity in silicate glass–ceramic foams. The foams were produced by mixing ZrO₂-containing soda-lime glass powder with CaCO₃ as foaming agent and foam them using a thermochemical approach. ZrO₂ was introduced as a nucleation agent. The crystallinity of the foams was varied by adjusting the heating protocol, i.e., by varying temperature, time, and number of heating cycles. The glass–ceramic foams exhibited relative crystallinities of <30%. The identity of the crystalline phases in the glass–ceramic foams varies with crystallinity. Specifically, cristobalite diminished, but devitrite grew with increasing crystallinity. It was observed that the crystallinity had a nonmonotonic impact on the thermal conductivity of the glass–ceramic foams. The optimum crystallinity for achieving the lowest thermal conductivity was 8–10%, resulting in an approximately 20% lower thermal conductivity compared to noncrystalline. Our findings have implications for the future design of glass–ceramic foams.