collaboration between researchers at the university of washington and the shenzhen institute of advanced technology has led to a breakthrough in the mapping of microscopic thermoelectric properties, such as thermal conductivity, which are crucial to determining macroscopic figure of merit zt.

this image shows quantitative mapping of thermal conductivities.
(science china press)

although there are many properties of microscopic materials that have been mapped using atomic resolution, according to a report from science china press, thermal conductivity has only been mapped at the macroscopic level and been theorized at the microscopic.

researchers used scanning thermal microscopy (sthm) to study a three-phase thermoelectric material.

the article explained, “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.”

the tip radius is only 10 nanometers, which means that the team could determine spatial resolution orders of magnitude higher than standard techniques. the researchers also conducted finite element computation to simulate local heat transfer using reference samples of known thermal conductivities.

“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,” the article added.

of note, the mapping showed the thermal conductivity variation across the interface and the “thermal image showed no crosstalk with topography.”

the work was recently published in national science review. the abstract stated:

“in the last two decades, a nanostructuring paradigm has been successfully applied in a wide range of thermoelectric materials, resulting in significant reduction in thermal conductivity and superior thermoelectric performance. these advances, however, have been accomplished without directly investigating the local thermoelectric properties, even though local electric current can be mapped with high spatial resolution.

“in fact, there still lacks an effective method that links the macroscopic thermoelectric performance to the local microstructures and properties. here, we show that local thermal conductivity can be mapped quantitatively with good accuracy, nanometer resolution and one-to-one correspondence to the microstructure using a three-phase skutterudite as a model system.

“scanning thermal microscopy combined with finite element simulations demonstrate close correlation between sample conductivity and probe resistance, enabling us to distinguish thermal conductivities spanning orders of magnitude, yet resolving thermal variation across a phase interface with small contrast.

“the technique thus provides a powerful tool to correlate local thermal conductivities, microstructures and macroscopic properties for nanostructured materials in general and nanostructured thermoelectrics in particular.”