Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal. CeramicsHigh-entropy alloys (HEAs), consisting of five or more approximately equimolar compositions of elements, [1,2] have proven to exhibit unique physical properties such as high hardness, [3] thermal stability, [4] structural stability, [5] as well as corrosion, oxidation, and wear resistance. [6][7][8] While microstructure and mechanical properties have been extensively studied, thermal
We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 -1636 nm grown via sputter deposition. While exhibiting a constant value up to ∼100 nm, the thermal conductivity increases with film thickness thereafter. This trend is in stark contrast with previous thermal conductivity measurements of amorphous systems, which have shown thickness-independent thermal conductivities. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of ∼1.8 THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.
The modal contributions to interfacial heat flow across Lennard-Jones based solid/solid, solid/liquid, and solid/gas interfaces are predicted via molecular dynamics simulations. It is found that the spectral contributions to the total heat flux from the solid that comprises the interface are highly dependent on the phase of the adjoining matter and the interfacial bond driving the interaction between the solid and the adjacent matter. For solid/solid interfaces, along with low temperatures, weak cross-species interaction strength can severely limit the conductance owing to the inhibition of inelastic channels that otherwise facilitate heat flow across the interface via anharmonic interactions. The increase in the crossspecies interaction strength is shown to shift the modal contributions to higher frequencies, and most of the inelastic energy exchange is due to the longitudinal vibrational coupling across the interface. For solid/liquid interfaces, the increase in the crossspecies interaction enhances the coupling of transverse vibrational frequencies in the interfacial solid region, which leads to an increase in the total heat current across the interface. Our modal analysis suggests that very high frequency vibrations (with frequencies greater than 80% of the maximum frequency in the bulk of the solid) have negligible contribution to heat flow across solid/liquid interfaces, even for a strongly bonded interface. In the limit of weakly interacting solid/gas interfaces, the modes coupling in the solid to the gas have signatures of reduced dimensionality, as evident by the surface-like density-of-states in the solid. Increasing the interfacial interaction shows similar trends to the solid/liquid case up to the limit in which gas atoms adsorb to the surface, enhancing the contribution of transverse phonons coupling at the solid interface. Our work elucidates general similarities in the influence of interfacial bond strength to thermal boundary conductance across solid/solid, solid/liquid, and solid/gas interfaces. In general, we find that the mode softening with a decrease in interfacial bond strength is more pronounced in the longitudinal modes as compared to transverse modes, and we consistently observe a decrease in the transverse mode contribution from the solid across the interface as the interfacial bond strength is decreased, regardless of the phase of matter on the other side.
In this study, we present a method to calculate the temperature and heat flux profiles as a function of depth and radius for bulk, homogeneous materials and samples with layered thin-film structures, including geometries supporting bidirectional heat fluxes, during pulsed and continuous wave (CW) laser heating. We calculate the temperature profiles for both modulated and unmodulated heating events to reveal that the thermal penetration depth (defined as the depth at which temperature decays to 1/e of the surface temperature) for a pulsed laser is highly dependent on time and repetition rate. In the high repetition rate limit, the temperature profile relaxes to that of a CW source profile, while in the opposite extreme, a single pulse response is observed such that the concept of the thermal penetration depth loses any practical meaning. For modulated heating events such as those used in time-and frequency-domain thermoreflectance, we show that there is a limit to the thermal penetration depth obtainable in an experiment, such that simple analytical expressions commonly used to determine thermal penetration depth break down. This effect is further compounded in samples with multiple layers, including the case when a $100 nm metallic transducer is deposited onto a bulk substrate, revealing that many recent studies relying on this estimation significantly over-predict the thermal penetration depth. Considering a bidirectional heat flow geometry (e.g., substrate/metal film/liquid), we find that heating from an unmodulated source results in an asymmetric heat flux about the plane of laser absorption to preserve a symmetric temperature profile when interfacial thermal resistance is negligible. However, the modulated case reveals a temperature asymmetry such that the thermal penetration depths in each side fall in line with those resulting from an insulated boundary condition. Published by AIP Publishing.
The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC). However, recent computational studies have suggested that interfacial defects can enhance this thermal boundary conductance through the emergence of unique vibrational modes intrinsic to the material interface and defect atoms, a finding that contradicts traditional theory and conventional understanding. By manipulating the local heat flux of atomic vibrations that comprise these interfacial modes, in principle, the TBC can be increased. In this work, experimental evidence is provided that interfacial defects can enhance the TBC across interfaces through the emergence of unique high-frequency vibrational modes that arise from atomic mass defects at the interface with relatively small masses. Ultrahigh TBC is demonstrated at amorphous SiOC:H/SiC:H interfaces, approaching 1 GW m K and are further increased through the introduction of nitrogen defects. The fact that disordered interfaces can exhibit such high conductances, which can be further increased with additional defects, offers a unique direction to manipulate heat transfer across materials with high densities of interfaces by controlling and enhancing interfacial thermal transport.
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