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.