  Carbon nanotube (CNTs) networks embedded in polymer matrix have been
extensively studied over the recent years as a flexible thermoelectric (TE)
transport medium. However, their power factor has been largely limited by the
relatively inefficient tunneling transport at junctions between CNTs and the
low density of conducting channels through the networks. In this work, we
demonstrate that significant enhancement of power factor is possible by adding
electrically insulating microscale particles in CNT networks. When silica
particles of a few micrometer diameters were co-embedded in single-walled
CNT-polydimethylsiloxane (PDMS) composites, both the electrical conductivity
and the Seebeck coefficient were simultaneously enhanced, thereby boosting the
power factor by more than a factor of six. We find that the silica
microparticles excluded a large volume of the composite from the access of CNTs
and caused CNT networks to form around them using the polymer as a binder,
which in turn resulted in improved connectivity and alignment of CNTs. Our
theoretical calculations based on junction tunneling transport show that the
large power factor enhancement can be attributed to the enhanced tunneling with
reduced junction distance between CNTs and the increased geometric factor due
to better CNT alignment. Additional enhancement of power factor by more than a
factor of two was achieved by sample compression due to the further improvement
of CNT alignment.
