The
development of high-performance and transferable thin-film
thermoelectric materials is important for low-power applications,
e.g., to power wearable electronics, and for on-chip cooling. Nanoporous
films offer an opportunity to improve thermoelectric performance by
selectively scattering phonons without affecting electronic transport.
Here, we report the growth of nanoporous Ca3Co4O9 thin films by a sequential sputtering-annealing method.
Ca3Co4O9 is promising for its high
Seebeck coefficient and good electrical conductivity and important
for its nontoxicity, low cost, and abundance of its constituent raw
materials. To grow nanoporous films, multilayered CaO/CoO films were
deposited on sapphire and mica substrates by rf-magnetron reactive
sputtering from elemental Ca and Co targets, followed by annealing
at 700 °C to form the final phase of Ca3Co4O9. This phase transformation is accompanied by a volume
contraction causing formation of nanopores in the film. The thermoelectric
propoperties of the nanoporous Ca3Co4O9 films can be altered by controlling the porosity. The lowest electrical
resistivity is ∼7 mΩ cm, yielding a power factor of 2.32
× 10–4 Wm–1K–2 near room temperature. Furthermore, the films are transferable from
the primary mica substrates to other arbitrary polymer platforms by
simple dry transfer, which opens an opportunity of low-temperature
use these materials.
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