Sol-gel synthesis and characterization of annealed pyrolytic graphite/sodium borosilicate glass composites

Sodium borosilicate glass composites with up to 40 wt.% of annealed pyrolytic graphite (APG) have been fabricated through a sol-gel process. The APG was first encapsulated in a layer of mesoporous silica with a tunable thickness, found to substantially aid in dispersion and stability of the APG in a variety of alcohols. The silica coating on the APG provides an ideal interface between APG and a silica-based glass, achievable via a sol-gel process, as the aqueous precursors afford the opportunity to establish a uniform dispersion of the silica-coated APG and subsequently form a network around the dispersion. Multi-component gels of SiO2, B2O3, and Na2O with varying weight percentages of APG were produced. Gels were aged in sealed containers at an elevated temperature, significantly reducing the required gelation and aging times. Aged gels were heat treated and hot pressed under vacuum to yield fully dense, homogeneous glass free of phase separation or devitrification. Both unloaded and loaded sol-gel derived glasses have shown similar values for density and the coefficient of thermal expansion to that of a traditional melt-quenched glass of the same composition. Furthermore, both X-ray diffraction patterns and infrared spectra have shown similarities between the unloaded sol-gel derived glass and its melt-quenched counterpart. Loading of the sodium borosilicate glasses with silica-coated APG holds promise for enhanced thermal and electrical conductivity of the bulk material, as the high aspect ratio of the APG encourages formation of percolating networks within the glass matrix. Encapsulation of nanoparticles with silica provides a route for facile surface manipulation and obtaining stable, homogeneous dispersions in solution, while a multi-component sol-gel is attractive for other nanocomposite systems in which particle settling or agglomeration is a concern.