  The use of oxide glasses is pervasive throughout everyday amenities and
commodities. Such glasses are typically electrical insulators, and endowing
them with electrical conductivity without changing their salutary mechanical
properties, weight, or thermoformability enables new applications in
multifunctional utensils, smart windows, and automotive parts. Previous
strategies to impart electrical conductivity include modifying the glass
composition or forming a solid-in-solid composite of the glass and a conductive
phase. Here we demonstrate using the latter strategy the highest reported
room-temperature electrical conductivity in a bulk oxide glass 1800 S/m
corresponding to the theoretical limit for the loading fraction of the
conductive phase. This is achieved through glass-sintering of a mixture of
carbon nanofibers and oxide flint F2 or soda lime glasses, with the bulk
conductivity further enhanced by a polyethylene-block-poly(ethylene glycol)
additive. A theoretical model provides predictions that are in excellent
agreement with the dependence of conductivity of these composites on the
carbon-loading fraction. Moreover, nanoscale electrical characterization of the
composite samples provides evidence for the existence of a connected network of
carbon nanofibers throughout the bulk. Our results establish a potentially
low-cost approach for producing large volumes of highly conductive glass
independently of the glass composition.
