The various microscale structures of materials play an important role in determining and predicting the thermal transport performance of the materials. To have a better understanding of the structure effect, this work focuses on studying the effect of microscale structure on thermal transport behaviors of materials. To this end, the thermal conductivity of several materials with different microscale structure has been characterized using different methods in this work. The thermal transport behaviors have been further studied in detail by analyzing the phonon propagation mechanisms in the materials. Specifically, a Johnson noise electro-thermal technique has been firstly developed to directly measure the thermal conductivity (k) of microscale glass fiber. This new technique provides a good improvement for the thermal properties measurement by avoiding the requirement of the resistance temperature coefficient calibration.

Furthermore, three typical materials: ultra-high molecular weight polyethylene (UHMWPE) fiber, giant size graphene on PMMA, and lignin-based carbon fiber (CF), are chosen to study the effect of structure on thermal conductivity. By using the transient electro-thermal technique, the thermal diffusivity and thermal conductivity variations against temperature of PE fibers from room temperature down to 22 K are measured. By using the newly developed “thermal reffusivity” model, the residual phonon thermal reffusivity (Θ0) of PE is obtained, which gives significant information about the phonon scattering in the materials. The defect-induced low-momentum phonon mean free paths are determined by using Θ0, which are much smaller than the crystallite size determined by XRD. The results strongly demonstrate the dominating diffuse phonon scattering at the grain boundaries. The carbon chains in the crystallites are quiet along the fiber axial direction, which contributes to the high thermal conductivity of our samples [~25 W/(mà  à ·K)]. The grains interface thermal conductance in PE fibers is subsequently evaluated.

Thermal properties measurement of mm-scale graphene is critical for device/system-level thermal design since it reflects the effect of abundant grains in graphene. The thermal conductivity of giant size graphene supported by PMMA is determined for the first time by using a differential technique. This giant graphene measurement eliminates the thermal contact resistance problems and edge phonon scattering encountered in μm-scale graphene thermal conductivity measurement. The thermal conductivity of 1.33-layered, 1.53-layered, 2.74-layered and 5.2-layered supported graphene is measured as 365, 359, 273 and 33.5 W/(mà  à ·K), respectively. The existence of graphene oxide, disorder in sp2 domain and stratification leads to the thermal conductivity reduction in 5.2-layered graphene.

The microstructure of lignin-based CF is important to study the thermal transport. The structure domain size of the CF is investigated by x-ray scattering, Raman scattering, and phonon scattering at the 0 K limit. The 0 K-limit phonon scattering mean free path (~12 à  à  ) uncovers a characteristic structure size, and it agrees well with the crystallite size by x-ray scattering (9 and 13 à  à  ) and the cluster size by Raman spectroscopy (23 à  à  ). The thermal diffusivity and reffusivity of CFs show little change from room temperature down to 10 K, uncovering the existence of extensive defects and grain boundaries which dominate the phonon scattering. The thermal conductivity of CFs is significantly increased by more than ten-fold after being annealed at 2770 K, to a level of 24 W/(mà  à ·K). This is due to the improvement of graphitic structure in the annealed CFs. Our microscale Raman scanning from slightly annealed to strongly annealed regions shows one-fold increase of the cluster size: from 1.83 nm to 4 nm, directly uncovering structure improvement by annealing. The inverse of the thermal conductivity is found linearly proportional to the annealing temperature in the range of 1000-2770 K.