Abstract
Because of their inherent rigidity and brittleness, inorganic materials have seen limited use in flexible thermoelectric applications. On the other hand, for high output power density and stability, the use of inorganic materials is required. Here, we demonstrate a concept of fully inorganic flexible thermoelectric thin films with Ca3Co4O9-on-mica. Ca3Co4O9 is promising not only because of its high Seebeck coefficient and good electrical conductivity but also because of the abundance, low cost, and nontoxicity of its constituent raw materials. We show a promising nanostructural tailoring approach to induce flexibility in inorganic thin-film materials, achieving flexibility in nanostructured Ca3Co4O9 thin films. The films were grown by thermally induced phase transformation from CaO–CoO thin films deposited by reactive rf-magnetron cosputtering from metallic targets of Ca and Co to the final phase of Ca3Co4O9 on a mica substrate. The pattern of nanostructural evolution during the solid-state phase transformation is determined by the surface energy and strain energy contributions, whereas different distributions of CaO and CoO phases in the as-deposited films promote different nanostructuring during the phase transformation. Another interesting fact is that the Ca3Co4O9 film is transferable onto an arbitrary flexible platform from the parent mica substrate by etch-free dry transfer. The highest thermoelectric power factor obtained is above 1 × 10–4 W m–1 K–2 in a wide temperature range, thus showing low-temperature applicability of this class of materials.
Highlights
Microscale electronic components tend to operate on battery power,[1] which has limitations on their lifetime and requirement for recharging
The as-deposited films consist of CaO−CoO phases, which is consistent with the observations on a sapphire substrate.[26]
We demonstrated the occurrence of three-stage phase transformation during annealing, leading to the formation of the final phase of Ca3Co4O9.27
Summary
Microscale electronic components tend to operate on battery power,[1] which has limitations on their lifetime and requirement for recharging. There have been some investigations on developing flexible TEC based on inorganic materials.[9,10] In these investigations, flexible platforms are used to hold the thermocouples of inorganic materials, and the legs of the thermocouples are subjected to temperature gradient in an outof-plane direction of the flexible platform. To reduce the grain boundary scattering of charge carriers, the thermocouple legs can be deposited by sputter-deposition on flexible substrates.[17,18] the mechanical flexibility of the leg materials is still a challenge, which needs to be addressed by inducing mechanical flexibility in inorganic thin films but with no deterioration of their electronic properties. The film is transferable from mica by dry transfer, that is, mica can act as a sacrificial layer for the transferable film
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