Measuring the Thermal Conductivity of Thin Films
The most widely researched thermoelectric materials are semiconductors and metal alloys, but alloys are not cost-effective because they either decompose or oxidize in an air atmosphere. Also, these materials are expensive and relatively difficult to fabricate in energy-intensive, high-temperature processing. However, organic and polymer thermoelectric materials are very attractive because they possess lower thermal conductivity, which is ideal for thermoelectric functions. Polymers are flexible and can therefore be easily integrated into unusual topologies to fit the geometrical requirements. They possess unique features and can be produced at a low cost too.
Researchers at Korea Advanced Institute of Science and Technology published their work aimed to fabricate highly efficient thermoelectric polymer thin films by exploiting the benefits of the enhanced electrical conductivity of the graphene and the lower thermal conductivity of the conducting polymer (PEDOT : PSS).
Figure 1. Room temperature measurement of thermal conductivity of PEDOT:PSS containing 1, 2, and 3 wt% graphene; (a) electronic thermal conductivity (ke), (b) lattice thermal conductivity (kl), and (c) total thermal conductivity.
Fig. 1 shows the thermal conductivity of the nanocomposite thin films in relation to the graphene content. The work highlights application of a C-Therm TCi Thermal Conductivity Analyzer in characterizing the thermal conductivity of the thin films after they were prepared with spin-coating and other procedures. The thermal conductivity in a thermoelectric material comes from two sources: the electron and hole transporting heat (ke); and the phonons that travel through the lattice (kl). The total thermal conductivity (k) comprises an electronic term (ke) and a lattice term (kl). The PEDOT:PSS thin film has a smaller level of thermal conductivity (0.24 W mK-1) than inorganic nanomaterials (1 W mK-1 to 10 W mK-1) due to its low lattice and electrical thermal conductivity. However, graphene generates a high level of thermal conductivity because of the fast electron mobility (200 000 cm2 V-1 s-1) and high level of lattice thermal conductivity (5.3 x 103 W mK-1), which is phonon-dominated.
When the graphene content was increased from 0 wt% to 3 wt%, the thermal conductivity increased from 0.24 to 0.30 (W mK-1); the electron thermal conductivity changed from 5.3 x 10-4 to 2.2 x 10-2 (W mK-1); and the lattice thermal conductivity changed from 2.4 x 10-1 to 2.7 x 10-1 (W mK-1). The thermal conductivity mainly depends on the lattice thermal conductivity (kl) because of the small value of the electron term (ke). The thermal conductivity increases as the graphene content is increased but is still as low as 0.30 W mK-1 for a graphene content of 3 wt%. Such a low level of thermal conductivity can be compared with the 0.4 W mK-1 level of a PEDOT: PSS thin film with a SWNTs content of 35 wt%. This behavior is attributed to well-stacked graphene multilayers which are covered with the PEDOT: PSS matrix; in this case, the surface porosity may act as effective scattering centers for phonons and decrease the lattice thermal conductivity. On the other hand, the electron mobility can be maintained in this structure because the thermal conductivity is phonon-dominated.
Information source: G.H. Kim, et al., Thermoelectric Properties of Nanocomposite Thin Films Prepared with Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate) and Graphene, Physical Chemistry Chemical Physics (2012)
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