  Grain boundaries are commonly observed in carbon nanostructures, but their
influence on thermal and electric properties are still not completely
understood. Using a combined approach of density functional tight-binding
theory and non-equilibrium Green functions we investigate electron and phonon
transport in carbon based systems. In this work, quantum transport and
thermoelectric properties are summarized for graphene sheets, graphene
nanoribbons and carbon nanotubes with a variety of grain boundary types in a
wide temperature range. Motivated by previous findings that disorder scatters
phonons more effectively than electrons, a significant improvement in the
thermoelectric performance for polycrystalline systems is expected. As the
effect is marginally sensitive to the grain boundary type, we demonstrate that
grain boundaries are a viable tool to greatly enhance the figure of merit,
paving the way for the design of new thermoelectric materials.
