The Freestanding Sensor-Based3ω Method For Studying Of Thermal Transportation Mechanisms Of Micro And Nano-Scale Materials

The principle of a freestanding linear sensor-based3ω method for the thermal conductivity determination is proposed. The reasonable frequency range is reported for the use of the new technique to determine the thermal conductivity. An optimization scheme of the designed parameters is proposed to improve the measurement accuracy. The possible influence of the asymmetric protective layer-heater interfaces on the accuracy of the fitted sample thermal conductivity is discussed. The fitting algorithm is studied to eliminate the effect of interface thermal resistance on fitted sample thermal conductivity. The sensitivity analysis results for specimen thermal conductivity, protective layer thermal conductivity and protective layer thickness show the new system can accurately determine highly thermal-conductive bulks and hundreds of microns thick wafers. Furthermore, by correcting the two-dimensional (2D) heat spreading effect in the protective layer, the exact determination of specimen thermal conductivity is also realized for lowly thermal-conductive materials.The principle of a freestanding planar sensor-based3ω method for the thermal effusivity determination is proposed. A2D analytical solution of the temperature rise for the freestanding planar sensor is derived, indicating the simplified one-dimensional solution is accurate enough for the thermal effusivity determination. An optimization scheme of the designed parameters of the freestanding planar sensor is proposed to improve the measurement accuracy.By utilizing proper design and advanced flexible-substrate-based MEMS/NEMS technology, two types of freestanding sensors are fabricated for the thermal conductivity and thermal effusivity measurements, respectively. And the measurement system is built based on these two freestandign sensors. Several familiar materials with standardized components and always as the standard specimen for thermophysical properties measurement,304stainless steel, silicon wafer, PMMA, vitreous silica, pure copper, ethylene glycol and DI water are used to calibrate the new system. The thermal conductivity measurement range of the new system is0.01-400W·m-1·K-1. The uncertainties in thermal conductivity and thermal effusivity measurements are estimated to be within8.9%and8.5%, respectively.The freestanding sensor-based3ω method can expand the application of the tranditional3ω method to thermophysical properties determination of porous surface materials, conductive materials and bad heat-tolerance materials, which also lay the foundation for the realization of industrialization of the3ω measurement apparatus.Using the freestanding sensor-based3ω method, the thermal conductivity of a novel high temperature resistant and thermal insulating material, SiOC series porous ceramics, is measured. A linear relationship between the thermal conductivity and the apparent density is found. By further studying the inner microscopic heat transport mechanism of SiOC seires porous ceramics, it is found that the intersecting cubic array model can explain the observed linear relationship.By utilizing the3co method, the thermal conductivities and thermal diffusivities of thermal barrier coatings (TBCs) with thickness below1mm deposited on stainless substrates are measured, which avoids the problem that coatings are easily fragmented while pealing from the substrates for the flash method. The temperature and density effects on the thermal conductivity of TBCs are studied.The effect of grain size on the lattice thermal conductivity of general purpose grade polyacrylonitrile (PAN)-based carbon fiber is studied by using an individual fiber as a freestanding sensor. It is found that the lattice thermal conductivities of the fibers are linearly dependent on the reciprocal of in-plane coherence length La. The phonon scattering theory can give explanation to this relationship. Theoretical and experimental results show it is the combined effect of the boundary scattering and the point-defect scattering that determines the heat transport in the PAN-based carbon fiber. The constant for point-defect scattering (A) can also be estimated from the in-plane coherence length and the lattice thermal conductivity.The freestanding sensor-based3ω method studied in this thesis provides a measurement tool as well as an assessment method for energy-saving materials, thermal protection materials and thermal management materials.The proposed analysis on heat transportation mechanisms of SiOC porous ceramics, nanostructured TBCs and PAN-based carbon fibers enriches our knowledge on the heat transfer mechanisms of micro and nano-scale materials.