Synthesis and thermal characterization of sodium borosilicate/carbon nanofiber composites

Sodium borosilicate (NBS) glass/annealed pyrolytic graphite (APG) fiber composites have been fabricated through a sol-gel process. A glass composition of 80wt% SiO2, 15wt% B2O3, and 5wt% Na2O was chosen, similar to commercial NBS glasses such as Pyrex. A variety of gels with increasing weight percentages of APG were produced and subsequently heat treated and sintered in a uniaxial hot-press. Sol-gel derived NBS glass composites have been prepared with up to 40wt% of APG homogeneously distributed throughout the glassy matrix. Increased loading of the sol-gel derived sodium borosilicate glass with SiO2-coated APG fibers has shown to decrease its coefficient of thermal expansion (CTE) below that of traditional melt-quenched NBS glasses of similar composition while keeping the bulk density relatively constant. The high aspect ratio of APG fibers encourages the formation of percolating networks within the glassy matrix, increasing the thermal conductivity of the composite material beyond that of a conventional NBS glass. The resulting thermal conductivity of the composites has been found to be directional, with a much higher conductivity value in the direction perpendicular to the axis of the hot press ('in-plane') than in its parallel direction ('through-thickness'), indicating of orientation of the fiber axes perpendicular to the direction of applied pressure during processing.