Measurements of thermal properties of phonon bridge adhesion layers, nanogaps and metal-organic frameworks using frequency-domain thermoreflectance (FDTR)

2018-10-23T18:59:00Z (GMT) by Minyoung Jeong
To meet the continuing demand for smaller yet faster electronic devices, many of the<br>components are packed closer together while they produce a significant amount of heat. If the<br>generated heat keeps accumulating due to a lack of efficient thermal management, devastating<br>effects such as thermal breakdown could occur. To enhance heat dissipation, higher thermal<br>conductivity and thermal interface conductance are required. In Chapter 2, the effect of inserting<br>thin metal adhesion layers of Cu and Cr between the Au (gold) - Al2O3 (sapphire) interface on<br>thermal interface conductance for the heat-assisted magnetic recording (HAMR) application is<br>investigated. It is found that without any adhesion layers, thermal interface conductance between<br>Au and Al2O3 layers is approximately 65 ± 10 MW/m2K. With the increasing thicknesses of Cu<br>and Cr adhesion layers between Au and Al2O3, this value increases and saturates to 180 ± 20<br>MW/m2K and 390 ± 70 MW/m2K, respectively. A significant amount of enhancement in thermal<br>interface conductance is observed for both metal adhesion layers even when they are less than 1-<br>nm thick. This is beneficial in terms of reducing the material costs as well as preserving Au’s<br>original optical properties required for HAMR application. Because HAMR heats the magnetic media via a near-field transducer (NFT) which flies<br>above the media with a very short distance of 5 nanometers to locally heat the magnetic domains,<br>the effect of near-field thermal radiation on overall performance of the NFT system is important<br>to understand. Near-field thermal radiation is a phenomenon where the radiative thermal transfer<br>exceeds the predicted blackbody limit with large contributions from evanescent modes generated<br>either by total internal reflection and surface polaritons. The evanescent modes can participate in<br>heat transfer only if the two bodies exchanging thermal energy are separated equal to or less than<br>a given decay length. In Chapter 3, designs and fabrications of thermomechanically stable<br>nanostructured gaps are presented. We successfully fabricate 10 nm and 50nm gaps sandwiched<br>between SiO2 – SiO2 and Au-SiO2 layers via mechanical pressing approach. The samples are<br>heated with the modulated laser, and the heat transfer coefficients across the gap are measured.<br>Based on the clear phase lag differences between the heating pump and temperature-measuring<br>probe lasers in the pillar and the gap regions, it is concluded that the gap with the intended<br>thicknesses did not collapse. Moreover, the fitted heat transfer coefficient values match<br>reasonably well with the analytically predicted values; the 50 nm and 10 nm gaps sandwiched<br>between the Au and SiO2 layers yielded a value of 9.69 ± 10.92 × 10􀬸 W/m2K and 4.27 ± 9.12<br>× 10􀬸, respectively, in the ambient environment. When the 10 nm gap is placed between the two<br>matching SiO2 plates, the heat transfer coefficient increases to 1.43 ± 1.51 × 10􀬹 W/m2K in the<br>ambient environment, which clearly indicates the effect of near-field radiative heat transfer. The<br>issue of large uncertainties involved in each data set is resolved by performing differential<br>analysis for phase lags. Through this approach, we obtain 1.15 ± 0.34 × 105 W/m2K and 1.65 ±<br>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<br>relying on adsorption and desorption of molecules also require faster heat transfer for improved<br>performance because adsorption and desorption processes are exothermic and endothermic<br>respectively. Metal-organic frameworks (MOFs) have been actively considered for such<br>applications because they can hold many molecules inside of their porous structures, but their<br>thermal conductivities, which are important to induce enhanced heat transfer for rapid adsorption<br>/ desorption, have been experimentally measured only a few times. Moreover, there is an<br>ongoing debate on how the thermal conductivity of MOFs would change through adsorption /<br>desorption. In Chapter 4, accurate experimental measurements of thermal conductivity of<br>HKUST-1 MOF single crystals before and adsorption of different liquid molecules of ethanol,<br>methanol and distilled water are presented. The pristine HKUST thermal conductivity after<br>thermal activation is measured as 0.68 ± 0.25 W/m∙K which matches well with the simulation<br>predicted value. This decreased to approximately 0.29 ± 0.13 W/m∙K, 0.15 ± 0.04 W/m∙K and<br>0.2 ± 0.09 W/m∙K after full methanol, ethanol and water liquid adsorption, respectively, which<br>suggests that the heat-carrying phonons indeed are scattered more because of pore-occupying<br>liquid molecules. The largest drop in thermal conductivity can be attributed to the lowest thermal<br>conductivity of intrinsic ethanol liquid. Also, the largest kinetic diameter of the liquid ethanol<br>molecule can scatter heat-carrying phonons more effectively than other liquid molecules. <br>