In the last two decades, engineering the microstructures and compositions at the nanoscale has resulted in substantial advances in materials properties, and this is particularly so for thermoelectric materials, which has seen their figure of merit ZT more than doubled since 2000 largely due to nanostructuring.

These materials are promising for recovery of tremendous waste heat produced during industrial production and in our daily life, and they can enable effective solid state thermal management such as heating and cooling as well.

Thermoelectric figure of merit ZT, which governs the conversion efficiency, thus is critical, yet very little is known about the local thermoelectric properties such as thermal conductivity that ultimately determine the macroscopic ZT, other than theoretical analysis. While the composition, phase, and microstructure of a material can now be mapped with atomic resolution, the properties such as thermal conductivity are usually measured at the macroscopic scale.

Now this missing link between microstructure and macroscopic property of material is connected by Nasr Esfahani and his colleagues from the University of Washington and Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences. This work, entitled Quantitative Nanoscale Mapping of Three-Phase Thermal Conductivities in Filled Skutterudites via Scanning Thermal Microscopy, was published in National Science Review.

The team used a technique known as scanning thermal microscopy (SThM) to study a three-phase thermoelectric material. A cantilever equipped with a microfabricated heater and a sharp tip was used to probe the sample, very much in a similar way as human finger touching the surface.

The heat dissipation through the sample reduces the temperature of the heater and changes its resistance, which can be accurately measured. Regions with higher thermal conductivity results in higher temperature drop, making it possible to differentiate materials with different thermal conductivities.

Since the tip radius is as small as 10 nanometers, spatial resolution orders of magnitude higher than conventional technique can be realized. The team then carried out detailed finite element computation to simulate the local heat transfer process, calibrated by a range of reference samples with known thermal conductivities. This make it possible for them to determine the local thermal conductivity quantitatively, with a spatial resolution better than 100 nanometer.

As seen in Figure 1 in their results, there is good agreement between finite element simulation and experimental measurement of resistance change of the probe upon touching samples with different thermal conductivities, and thus the spatial mapping of thermal conductivity can be derived from the experimentally measured resistance mapping.

It is particularly interesting to note that thermal conductivity variation across the interface is nicely captured, and unlike previous SThM studies, the thermal image show no crosstalk with topography, but nicely correlate with the microstructural composition (Figure 2).

As noted by Prof. Lidong Zhao of Beihang University and Prof. Mercouri G. Kanatzidis of Northwestern University, two leading material scientists in thermoelectrics, "This method reported by Esfahani et al. is a valuable advance in thermoelectric materials characterization, and if can be widely adopted it will add to the toolbox of characterization techniques used in searching for ever higher performance thermoelectric materials."

Research Report: Quantitative nanoscale mapping of three-phase thermal conductivities in filled skutterudites via scanning thermal microscopy

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