Thermoelectric characteristics in out-of plane direction of thick carbon nanotube-polystyrene composites fabricated by the solution process

- Anisotropy of thick-printed carbon nanotube-polystyrene thermoelectric composites.
- Method to measure in-plane and out-of-plane direction of an identical composite.
- In-plane direction of the composites showed higher figure of merit.
- The anisotropy may be caused by orientation of the carbon nanotubes (CNT).
- Controlling the orientation of the CNTs is required to realize high performance.

When sheet-like flexible devices are placed on heat sources, a thermal flow is induced along the out-of-plane direction in regards to the device surface. To obtain carbon nanotube-based (CNT-based) thermoelectric devices that work under thermal flows along the out-of-plane direction, it is indispensable to fabricate sufficiently thick CNT-based materials that will allow for an out-of-plane temperature difference to occur. Therefore, we investigated the out-of-plane thermoelectric characteristics of thick CNT-polymer composites (thickness = 0.74-0.84 mm), which were fabricated by over-coating a dense CNT-dispersed solution, and found that the electrical and thermal conductivities of the composites in the out-of-plane direction were significantly lower than those in the in-plane direction, respectively. We observed that the composites' figure of merit values in the out-of plane direction was approximately one order of magnitude lower than those in the in-plane direction. This anisotropy in the characteristics of thick CNT-polystyrene composites may be caused by the orientation of the CNTs along the in-plane direction. Therefore, the results indicate that controlling the orientation of CNTs in thick composites is a key factor to improve the performance of devices that work on the basis of temperature differences along the out-of plane direction.

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