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We developed an advanced 2ω method for thermal conductivity (κ) measurements that is also applicable to samples with a wide range of thicknesses, to which the flash method cannot be applied. The conventional 2ω method, which features a simple setup combined with thermoreflectance, is a κ measurement method for thin films on substrates. However, it is difficult to apply this method to bulk substrate samples without films because of the interfacial thermal resistance between the transducer metal film and the substrate. In the advanced 2ω method, the interfacial thermal resistance becomes negligible due to the coating of a bulk sample with a high-thermal-resistance film. We measured the κ values of various bulk samples (Si, Ge, fused quartz, and TiO2). Moreover, the κ value of thin (0.3-mm) bulk Si substrates, which is difficult to measure using the conventional flash method for bulk materials because of the high thermal diffusivity, could be measured. This study demonstrates that the range of sample thickness for the 2ω method is extended from thin films to bulk materials. [ABSTRACT FROM AUTHOR]

The 2 ω method is a technique to measure the cross-plane thermal conductivity, κ, of a film sample based on sinusoidal Joule-heating of a metal film deposited on the sample surface and its thermoreflectance (TR) measurement of surface temperature cooling due to the heat dissipation. The 2 ω method is being paid attention because it is more cost-effective and easier to use than the conventional time domain thermoreflectance (TDTR) method or the 3 ω method. In some cases, however, it is difficult to apply the conventional 2 ω method to the κ measurement of high thermal resistance films such as general thermoelectric films due to its non-linear TR signal response to (2 ω). Here, we present a 2 ω method based on a versatile TR signal analysis which enables the κ measurement of high thermal resistance film more explicitly than the conventional analysis based on a linear TR signal response. This method determines explicitly the thermal conductivities of PbTe films and PbTe/GeS superlattices grown on BaF(111) substrates by hot wall epitaxy: κ = 2.1 ± 0.13 Wm K and κ = 0.71 ± 0.05 Wm K, respectively. Furthermore, a significant impact of PbTe film crystallinity on thermal conductivity is demonstrated by comparative measurements between polycrystalline PbTe film and epitaxial PbTe film grown on the BaF(111) substrates. These results demonstrate that our method can be a powerful tool to measure the thermal conductivity of thermoelectric films. [ABSTRACT FROM AUTHOR]

Abstract ZnO, a wide bandgap (3.3 eV) semiconductor has been expected to be a transparent thermoelectric material for the purpose of energy harvesting application because it is a low-cost and ubiquitous element material with a high optical transmittance and a high power factor. Bulk Ga-doped ZnO (GZO) is expected to have higher electrical conductivity and lower thermal conductivity than bulk Al-doped ZnO. However, because reports on their thermoelectric properties of GZO films have been scarce up to now, it has been unclear what film characters affect the thermoelectric properties effectively. In this work, GZO thin films with different characters (c -axis orientation, crystal domains and the domain interfaces, and carrier activation rate) were fabricated by two different methods, sol-gel method and pulsed laser deposition. All samples have optical transmittance over 80% in visible range. The highly-oriented GZO films exhibit the highest power factors up to 2.8 μWcm−1 K−2 in the reported GZO materials and low thermal conductivities of 8.4 Wm−1 K−1 (1/4 as high as that of bulk GZO). This enhanced thermoelectric performance is attributed to the high carrier activation rate, and the interfaces of highly-oriented crystal domains with the small carrier scattering effect. Highlights • We studied transparent thermoelectric Ga-doped ZnO films by PLD and sol-gel method. • Both GZO films had highly (0001)-oriented nanometric crystal domains. • They have higher power factors and lower thermal conductivities than GZO bulks. [ABSTRACT FROM AUTHOR]