Effect of particle-size distribution and pressure-induced densification on the microstructure and properties of printable thermoelectric composites and high energy density flexible devices

Abstract

To address the global demand for low-cost, flexible thermoelectric generators (TEGs), this work describes a novel, energy-efficient method of controlling the composite microstructure and resulting thermoelectric (TE) properties of p-type composite films. Combining (1) a small amount of naturally occurring binder that is sufficient to hold TE particles together without significantly decreasing electrical conductivity and (2) a wide distribution of heterogeneous (micro and nano) particles tightly packed on (3) application of mechanical pressure at (4) low-temperature curing for a short duration, this method yields big grains larger than the mean free path for charge carriers (for high electrical conductivity) and nano features smaller than the mean free path for phonons (for low thermal conductivity). The best properties were achieved with 0.05 wt% of chitosan binder, heterogeneous-sized TE particles (100-mesh Bi0₀.₅Sb₁.₅Te₃), 200 MPa applied uniaxial pressure, and curing at 150 °C for 30 min to densify the as-deposited films. The 100-mesh chitosan-BST composite film achieved a ZT of 0.7, comparable to the best reported ZT of printed p-type TE films, but without using high-temperature and long-duration curing. The power output of our best-reported scalable 3-leg-BST TEG was 58 μW. The corresponding power density was 5.72 mW/cm² at a temperature difference of 38 K, significantly higher than the best reported values for single-leg TEGs. Furthermore, the flexibility of this prototype was performed by successfully bending at a 3-cm radius for 1000 cycles, thus demonstrating a high potential to be used as self-sufficient batteries for next-generation wearable devices and wireless sensor network applications.