Measurements of thermal properties of phonon bridge adhesion layers, nanogaps and metal-organic frameworks using frequency-domain thermoreflectance (FDTR)
To meet the continuing demand for smaller yet faster electronic devices, many of the components are packed closer together while they produce a significant amount of heat. If the generated heat keeps accumulating due to a lack of efficient thermal management, devastating effects such as thermal breakdown could occur. To enhance heat dissipation, higher thermal conductivity and thermal interface conductance are required. In Chapter 2, the effect of inserting thin metal adhesion layers of Cu and Cr between the Au (gold) - Al2O3 (sapphire) interface on thermal interface conductance for the heat-assisted magnetic recording (HAMR) application is investigated. It is found that without any adhesion layers, thermal interface conductance between Au and Al2O3 layers is approximately 65 ± 10 MW/m2K. With the increasing thicknesses of Cu and Cr adhesion layers between Au and Al2O3, this value increases and saturates to 180 ± 20 MW/m2K and 390 ± 70 MW/m2K, respectively. A significant amount of enhancement in thermal interface conductance is observed for both metal adhesion layers even when they are less than 1- nm thick. This is beneficial in terms of reducing the material costs as well as preserving Au’s original optical properties required for HAMR application. Because HAMR heats the magnetic media via a near-field transducer (NFT) which flies above the media with a very short distance of 5 nanometers to locally heat the magnetic domains, the effect of near-field thermal radiation on overall performance of the NFT system is important to understand. Near-field thermal radiation is a phenomenon where the radiative thermal transfer exceeds the predicted blackbody limit with large contributions from evanescent modes generated either by total internal reflection and surface polaritons. The evanescent modes can participate in heat transfer only if the two bodies exchanging thermal energy are separated equal to or less than a given decay length. In Chapter 3, designs and fabrications of thermomechanically stable nanostructured gaps are presented. We successfully fabricate 10 nm and 50nm gaps sandwiched between SiO2 – SiO2 and Au-SiO2 layers via mechanical pressing approach. The samples are heated with the modulated laser, and the heat transfer coefficients across the gap are measured. Based on the clear phase lag differences between the heating pump and temperature-measuring probe lasers in the pillar and the gap regions, it is concluded that the gap with the intended thicknesses did not collapse. Moreover, the fitted heat transfer coefficient values match reasonably well with the analytically predicted values; the 50 nm and 10 nm gaps sandwiched between the Au and SiO2 layers yielded a value of 9.69 ± 10.92 × 10 W/m2K and 4.27 ± 9.12 × 10, respectively, in the ambient environment. When the 10 nm gap is placed between the two matching SiO2 plates, the heat transfer coefficient increases to 1.43 ± 1.51 × 10 W/m2K in the ambient environment, which clearly indicates the effect of near-field radiative heat transfer. The issue of large uncertainties involved in each data set is resolved by performing differential analysis for phase lags. Through this approach, we obtain 1.15 ± 0.34 × 105 W/m2K and 1.65 ± 0.49 × 105 W/m2K for the 10 nm Au-SiO2 and 10 nm SiO2-SiO2 gap samples, respectively. Not only electronics applications, but also other biological and chemical applications relying on adsorption and desorption of molecules also require faster heat transfer for improved performance because adsorption and desorption processes are exothermic and endothermic respectively. Metal-organic frameworks (MOFs) have been actively considered for such applications because they can hold many molecules inside of their porous structures, but their thermal conductivities, which are important to induce enhanced heat transfer for rapid adsorption / desorption, have been experimentally measured only a few times. Moreover, there is an ongoing debate on how the thermal conductivity of MOFs would change through adsorption / desorption. In Chapter 4, accurate experimental measurements of thermal conductivity of HKUST-1 MOF single crystals before and adsorption of different liquid molecules of ethanol, methanol and distilled water are presented. The pristine HKUST thermal conductivity after thermal activation is measured as 0.68 ± 0.25 W/m∙K which matches well with the simulation predicted value. This decreased to approximately 0.29 ± 0.13 W/m∙K, 0.15 ± 0.04 W/m∙K and 0.2 ± 0.09 W/m∙K after full methanol, ethanol and water liquid adsorption, respectively, which suggests that the heat-carrying phonons indeed are scattered more because of pore-occupying liquid molecules. The largest drop in thermal conductivity can be attributed to the lowest thermal conductivity of intrinsic ethanol liquid. Also, the largest kinetic diameter of the liquid ethanol molecule can scatter heat-carrying phonons more effectively than other liquid molecules.