  In-plane thermal conductivities of small-scale samples are hard to measure,
especially for the lowly conductive ones and those lacking in-plane symmetry
(i.e., transversely anisotropic materials). State-of-the-art pump-probe
techniques including both the time-domain and the frequency-domain
thermoreflectance (TDTR and FDTR) are advantageous in measuring the thermal
conductivity of small-scale samples, and various advanced TDTR and FDTR
techniques have been developed to measure transversely anisotropic materials.
However, the measurable in-plane thermal conductivity (k_in) is usually limited
to be >10 W/(m K). In this work, a new spatial-scan thermoreflectance (SSTR)
method has been developed to measure a broad range of k_in of millimeter-scale
small samples, including those lacking in-plane symmetry, extending the current
limit of the measurable k_in to as low as 1 W/(m K). This SSTR method
establishes a new scheme of measurements using the optimized laser spot size
and modulation frequency and a new scheme of data processing, enabling
measurements of in-plane thermal conductivity tensors of a broad range of k_in
values with both high accuracy and ease of operation. Some details such as the
requirement on the sample geometry, the effect of the transducer layer, and the
effect of heat loss are also discussed. As a verification, the k_in of some
transversely isotropic reference samples with a wide range of k_in values
including fused silica, sapphire, silicon, and highly ordered pyrolytic
graphite (HOPG) have been measured using this new SSTR method. The measured
k_in agree perfectly well with the literature values with a typical uncertainty
of 5%. As a demonstration of the unique capability of this method, the in-plane
thermal conductivity tensor of x-cut quartz, an in-plane anisotropic material,
has also been measured.

Materials Science;Applied Physics