Study On Thermal Transportation Mechanism Of Solid-Liquid Interfaces By Femtosecond Laser PUMP And Probe Method

In this work, an improved two-color femto-second laser pump-probe time-domain ther-moreflectance (TDTR) system is established for the study of thermal transport behavior within nano-scale materials and through interfaces from low temperature to high tem-perature and from solid materials to liquid materials. The experimental system employs an800nm femto-second laser pulses as the probe source and a400nm laser generated from a BIBO crystal as the pump source. This design ensures the system works at the most efficient wavelength due to the aluminum transducer has the highest thermore-flectance at800nm and thus the system has the highest temperature sensitivity. Mean-while, the edgepass color filter can cut off the pump laser at OD7which means the impact of the pump laser is reduced as much as possible. Moreover, sine wave modu-lation and low-pass filter reduce the high frequency noises of the system, and a CCD camera microscopy is used to detect any specific location on the samples. These modi-fications improve the sensitivity and accuracy of the TDTR system, and make the study of the complicated thermal transport in nano-scale materials possible.The system is first used to study the thermal transport in multi-layered nano-struc-tures, and the theoretical model for data analysis is also built. Thermal properties such as thermal conductivities, heat capacities and interfacial thermal conductances of bulk and nano-scale materials are measured. The measuring and data processing method for nano-film structures is proposed, and three simplified heat transfer models are investi-gated. The result shows that a separation failure of the thermal resistance between nano-film and interfaces will lead to an apparent size effect of the thermal conductivity. Moreover, a single modulation frequency method is developed for simultaneous deter-mination of thermal conductivity and heat capacity.A cryostat system is built based on the TDTR system, which is then used to study the temperature dependence of the thermal conductivies of isotropic and anisotropic materials. The result shows the interfacial thermal conductance is nearly independent to the temperature, while the thermal conductivity decreases with the raise of the tem-perature, and the temperature dependance shows an inverse proportion when above room temperature. For anisotropic material, the thermal conductivity is highest in the most compact crystal orientation, while the thermal conductivity difference between orientations decreases with the raise of the temperature, and it is expected that the dif-ference will disappear as the temperature is high enough to break the crystal structure.A platform for liquid thermal property determination is set up based on the TDTR system, and a bi-direction heat transfer model is also built for data processing. The thermal conductivities of liquids and hard-soft interfacial thermal conductances are measured, and then the study is focused on the thermal transport behavior through hard-soft interfaces. Self-assembled monolayers (SAMs) are fabricated on gold surface to modify the wettability and vibrational properties, and the result shows both wettability and vibrational mismatch (acoustic mismatch) will impact the interfacial thermal con-ductance. It is remarkable that SAMs can act as media to bridge the vibrational mis-match between gold and polymer and thus enable efficient resonance-like thermal transport. Such a strategy can be generalized to any hard-soft material interfaces to pro-vide a design principle that can be used to synthesize nanocomposites for heat transfer applications.