Hiroki Matsubara

The shrinking size of integrated chips poses thermal management challenges. Understanding the size effect of chemical heterogeneity on solid–liquid interfacial thermal transfer is essential for heterogeneous chip design, yet the underlying mechanisms remain lacking. The present work used the liquid n-alkanes as the thermal interface material between solid platinum substrates. To characterize chemical heterogeneity, periodic solid surface patterns composed of patches with alternating solid–liquid affinities were constructed. By using non-equilibrium molecular dynamics simulations, we investigated the size effect of chemically heterogeneous patterns on interfacial thermal resistance (ITR) at the nanoscale. At larger heterogeneity sizes, i.e., larger patch sizes, most alkane molecules directly in contact weak interaction patches cannot interact with strong interaction patches due to long atomic distances. In the case of alkanes in contact a cold substrate, alkanes in contact weak interaction patches transferred thermal energy to the substrate at a lower rate than those in contact strong interaction patches. The different rates resulted in the higher temperature of alkanes in contact weak interaction patches than those in contact strong interaction patches and, therefore, a larger disparity between temperature jump at the strong interaction areas and that at the weak interaction areas. The non-uniformity of temperature jump distribution increased ITR when compared to the heterogeneous surface system characterized by a smaller patch size with a more uniform temperature distribution in the plane perpendicular to the heat flux direction. In addition, the classical parallel thermal resistance model predicted ITR accurately for the heterogeneous surface systems with small size patches but overestimated overall thermal resistance.

Understanding the atomistic mechanism of interfacial thermal transport at solid-liquid interfaces is a key challenge in thermal management at the nanoscale. A recent molecular-dynamics study demonstrated that interfacial thermal resistance (ITR) at the interface between a solid and a surfactant solution can be minimized by adjusting the molecular mass of the surfactant. In the present study, we explain the mechanism of this ITR minimization in view of vibration-mode matching using a one-dimensional (1D) harmonic chain model of a solid-liquid interface having an interfacial adsorption layer of surfactant molecules. The equation of motion for the 1D chain is described by a classical Langevin equation and is analytically solved by the nonequilibrium Green's function (NEGF) method. The resultant ITR is expressed in a form of vibrational matching, and its relationship to the overlap of the vibrational density of states is also discussed. The analysis leads to a conclusion that the damping coefficient η in the Langevin equation should be a finite and sufficiently large value to represent the rapid damping of vibration modes at solid-liquid interfaces. This conclusion provides a clue to seamlessly extend the conventional NEGF-phonon transmission picture of solid-solid interfacial thermal transport, which assumes η to be infinitesimal, to solid-liquid interfaces.

The thermal conductivity of a single polymer chain, which is an important factor in the rational design of polymer-based thermal management materials, is strongly affected by the strain state of the chain. In the present study, using non-equilibrium molecular dynamics simulations, the thermal conductivity of a single polyethylene chain, representing a typical polymer chain, was calculated as a function of strain. To investigate the effect of different modeling of covalent bonds, the results were compared for reactive and non-reactive potential models, the AIREBO and NERD potentials, respectively. When the strain ε was as small as ε < −0.03, i.e., under slight compression, the thermal conductivity values were similar regardless of the potential model and increased with increasing strain. However, the two potential models showed qualitatively different behaviors for larger strains up to ε < 0.15: the thermal conductivity calculated by the non-reactive potential continually grows with increasing strain, whereas that by the reactive potential model is saturated. The analysis of internal stress and vibrational density of states suggested that the saturation behavior is due to the weakening of the covalent bond force as the C–C bond elongates, and thus, the result of the reactive model is likely more realistic. However, for ε > 0.1, the reactive potential also produced unphysical results due to the effect of the switching function, describing the formation and breaking of covalent bonds. The present results indicate that careful selection of the potential model and deformation range is necessary when investigating the properties of polymers under tensile strain.

Reliably obtaining thermal properties of complex systems, which often involves computing heat flux to obtain thermal conductivity via either Fourier’s law or the Green–Kubo relation, is an important task in modern molecular dynamics simulations. In our previous work [Surblys et al., Phys. Rev. E 99, 051301(R) (2019)], we have demonstrated that atomic stress could be used to efficiently compute heat flux for molecules with angle, dihedral, or improper many-body interactions, provided a newly derived “centroid” form was used. This was later successfully implemented in the LAMMPS simulation package. On the other hand, small rigid molecules, like water and partial constraints in semi-flexible molecules, are often implemented via constraint force algorithms. There has been a lack of clarification if the constraint forces that maintain geometric constraints and can also be considered as many-body forces contribute to the overall heat flux and how to compute them correctly and efficiently. To address this, we investigate how to apply the centroid atomic stress form to reliably compute heat flux for systems with constraint or rigid body dynamics. We successfully apply the centroid atomic stress form to flexible, semi-flexible, and rigid water models; decompose the computed thermal conductivity into separate components; and demonstrate that the contribution from constraint forces to the overall heat flux and thermal conductivity is small but non-negligible. We also show that while the centroid formulation produces correct heat flux values, the original “group” formulation produces incorrect and sometimes unphysical results. Finally, we provide insight into the meaning of constraint force contribution.

The analysis of molecular-scale heat transfer in paraffin/graphene composite based on molecular dynamics simulation suggests an important effect of the in-plane aspect ratio of a graphene filler on the effective thermal conductivity of the composite.

The addition of surfactants to polymer-based thermal interface materials applied to improve the heat dissipation efficiency of chip surfaces in contact has attracted attention for the microelectronic processing technology. In the present study, the mechanism by which a long-chain surfactant affects heat transfer across the interface between solid surface and polymer liquid was investigated by non-equilibrium molecular dynamics simulation. We constructed a system where tetracosane was used as a solvent and contained alcohol molecules as a surfactant, and they were placed between two flat silica surfaces under a thermal gradient. The effect of the hydrophilicity of silica surface, the concentration of the surfactant, and chain length of the surfactant on silica–liquid interfacial thermal resistance R_b were examined. Alcohol surfactant molecules preferred to adsorb onto the hydrophilic silica (Si–OH) surface due to hydrogen bonding between alcohol and silanol hydroxyl groups. It was found that R_b reduced not only with the adsorption amount of alcohol molecules but also with the chain length of alcohol. The van der Waals interaction contribution was dominant for solid–liquid and liquid–liquid heat conduction near the interface. The hydroxyl terminals of alcohol molecules were vertically adsorbed onto the Si–OH surface due to hydrogen bonds, which produced a heat path from silanols to the hydroxyl groups of alcohol. Furthermore, heat was also exchanged between alcohol hydroxyl and alkyl groups via intramolecular interaction and between the alcohol alkyl groups and nearby solvent molecules via van der Waals (vdW) intermolecular interaction. This resulted in an efficient heat path from solid surface silanols to liquid bulk. As the alcohol chain length increased without changing the number of adsorbed alcohol molecules, the heat transfer through this heat path increased, which led to a decrease in R_b. These results provided insight toward the guiding principle for the molecular design of complex surfactants to enhance the interfacial heat transfer.

A material with anisotropic heat conduction characteristics, which is determined by molecular scale structure, provides a way of controlling heat flow in nanoscale spaces. As such, here we consider layer-by-layer (LbL) membranes, which are an electrostatic assembly of polyelectrolyte multilayers and are expected to have different heat conduction characteristics between cross-plane and in-plane directions. We constructed models of a polyacrylic acid/polyethylenimine (PAA/PEI) LbL membrane sandwiched by charged solid walls and investigated their anisotropic heat conduction using molecular dynamics simulations. In the cross-plane direction, the thermal boundary resistance between the solid wall and the LbL membrane and that between the constituent PAA and PEI layers decrease with increasing degree of ionization (solid surface charge density and the number of electric charges per PAA/PEI molecule). When the degree of ionization is low, the cross-plane thermal conductivity of a constituent layer is higher than that of bulk state. As the degree of ionization increases, however, the cross-plane thermal conductivity of PAA, a linear polymer, decreases because of the increase in the number of in-plane oriented polymer chains. In the in-plane direction, we investigated heat conduction of each layer, and found the enhancement of effective in-plane thermal conductivity again due to the in-plane oriented chain alignment. The heat conduction in the LbL membrane is three-dimensionally enhanced when compared with those in the bulk states of the constituent polymers, because of the electrostatic interactions in the cross-plane direction and the molecular alignment in the in-plane direction.