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The thermal conductivities of (100) γ-Ga2O3 films deposited on (100) MgAl2O4 substrates with various thicknesses were measured using frequency-domain thermoreflectance. The measured thermal conductivities of γ-Ga2O3 films are lower than the thermal conductivities of (2¯ 01) β-Ga2O3 films of comparable thickness, which suggests that γ-phase inclusions in the doped or alloyed β-phase may affect its thermal conductivity. The thermal conductivity of γ-Ga2O3 increases from 2.3−0.5+0.9 to 3.5±0.7 W/m K for films with thicknesses of 75–404 nm, which demonstrates a prominent size effect on thermal conductivity. The thermal conductivity of γ-Ga2O3 also shows a slight increase as temperature increases from 293 to 400 K. This increase in thermal conductivity occurs when defect and boundary scattering suppress signatures of temperature-dependent Umklapp scattering. γ-Ga2O3 has a cation-defective spinel structure with at least two gallium vacancies in every unit cell, which are the likely source of defect scattering. 

An analytical model using three-directional anisotropic (TDA) dispersion and a novel anisotropic relaxation time (RT) relation for modeling the thermal conductivity ( k ) of intercalated layered materials is developed. The TDA dispersion eliminates the restriction of in-plane isotropy and is suitable for TDA materials such as black phosphorous. We compare calculations of k of bulk intercalated layered materials using the isotropic Debye dispersion and BvK dispersion with our TDA dispersion model, paired with both isotropic and anisotropic RTs. We find that calculated k values by the TDA dispersion model agree best with the experimental data. Furthermore, anisotropic RTs largely improve the performance of the Debye and BvK dispersion models whose average relative deviations for the in-plane k are reduced from 17.3% and 23.0% to 4.4% and 8.5%, respectively. Finally, thermal conductivity accumulation functions of intercalated MoS 2 and graphite are numerically calculated based on the TDA dispersion with anisotropic RTs. These models predict that intercalants cause increased contributions from phonons with shorter mean free paths, especially for in-plane thermal conductivity. 

To identify superior thermal contacts to graphene, we implement a high-throughput methodology that systematically explores the Ni−Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on thermal interface conductance with monolayer graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni−Pd/Cr/graphene/ SiO2 samples identify a maximum metal/graphene/SiO2 junction thermal interface conductance of 114 ± (39, 25) MW/m2 K and 113 ± (33, 22) MW/m2 K at ∼10 at. % Pd in Ninearly double the highest reported value for pure metals and 3 times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates with a region of the Ni−Pd phase diagram that exhibits a miscibility gap. Cross-sectional imaging by high-resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to two-dimensional materials for next-generation devices. 

To identify superior thermal contacts to graphene we implement a high throughput methodology that systematically explores the Ni-Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on the thermal interface conductance with monolayer CVD graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni- Pd/Cr/graphene/SiO2 samples both identify a maximum in the metal/graphene/SiO2 junction thermal interface conductance of 114± (39, 25) MW/m2K and 113± (33, 22) MW/m2K at ~10 atomic percent Pd in Ni—nearly double the highest reported value for pure metals and three times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates to a region of the Ni-Pd phase diagram that exhibits a miscibility gap. Cross sectional imaging by high resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to 2D materials for next generation devices. 

Abstract Whether the presence of adsorbates increases or decreases thermal conductivity in metal-organic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40 – 80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of low-frequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption.