  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.
