Thermal Properties of SiOC Glasses and Glass Ceramics at Elevated Temperatures.

Christina Stabler,Ralf Riedel,Peter Stein,Emanuel Ionescu,Andreas Reitz,Barbara Albert

doi:10.3390/ma11020279

Abstract

In the present study, the effect of the chemical and phase composition on the thermal properties of silicon oxide carbides (SiOC) has been investigated. Dense monolithic SiOC materials with various carbon contents were prepared and characterized with respect to their thermal expansion, as well as thermal conductivity. SiOC glass has been shown to exhibit low thermal expansion (e.g., ca. 3.2 × 10−6 K−1 for a SiOC sample free of segregated carbon) and thermal conductivity (ca. 1.5 W/(m∙K)). Furthermore, it has been observed that the phase separation, which typically occurs in SiOC exposed to temperatures beyond 1000–1200 °C, leads to a decrease of the thermal expansion (i.e., to 1.83 × 10−6 K−1 for the sample above); whereas the thermal conductivity increases upon phase separation (i.e., to ca. 1.7 W/(m∙K) for the sample mentioned above). Upon adjusting the amount of segregated carbon content in SiOC, its thermal expansion can be tuned; thus, SiOC glass ceramics with carbon contents larger than 10–15 vol % exhibit similar coefficients of thermal expansion to that of the SiOC glass. Increasing the carbon and SiC content in the studied SiOC glass ceramics leads to an increase in their thermal conductivity: SiOC with relatively large carbon and silicon carbides (SiC) volume fractions (i.e., 12–15 and 20–30 vol %, respectively) were shown to possess thermal conductivities in the range from 1.8 to 2.7 W/(m∙K).

Highlights

Silicon oxide carbide glasses and glass ceramics belong to the group of polymer-derived ceramics (PDCs)
Values of the heat capacity, thermal diffusivity, and thermal conductivity of glassy silicon oxide carbides (SiOC) with no segregated carbon are similar to those reported for vitreous silica, whereas the thermal expansion in SiOC was slightly larger than that of silica
It is shown that the phase separation of SiOC glass leads to a rather significant decrease of the coefficient of thermal expansion, and a slight increase of the thermal conductivity (i.e., from 1.5 to 1.7 W/(m·K))

Summary

Introduction

Silicon oxide carbide (or silicon oxycarbide, SiOC) glasses and glass ceramics belong to the group of polymer-derived ceramics (PDCs). They consist of corner-sharing SiO4−x Cx tetrahedra (x = 0–4) [1], and can be described as vitreous silica glasses, with oxygen being partly replaced by carbon within the glass network. The main synthesis approach, which incorporates significant amounts of carbon into silica, relies on the thermal conversion of sol–gel derived precursors based on organo-substituted alkoxysilanes or of polyorganosiloxanes [2]. A significant amount of Si–O and Si–C bonds were preserved upon the thermal treatment, yielding X-ray amorphous SiOC at 1000 ◦ C. The final composition of the SiOC glasses can be tuned upon choosing appropriate preceramic precursors [2,4,5]

Methods

Results

Conclusion