Enhancement of polymer thermal conductivity using nanographene fillers and clarification of its molecular-scale mechanisms are of great concern in the development of advanced thermal management ...materials. In the present study, molecular dynamics simulation was employed to theoretically show that the in-plane aspect ratio of a graphene filler can have a significant impact on the effective thermal conductivity of paraffin/graphene composites. Our simulation included multiple graphene fillers aggregated in a paraffin matrix. The effective thermal conductivity of a paraffin/graphene composite, described as a second-rank tensor in the framework of equilibrium molecular dynamics simulation, was calculated for two types of graphene fillers with the same surface area but in-plane aspect ratios of 1 and 10. The filler with the higher aspect ratio was found to exhibit a much higher thermal conductivity enhancement than the one with the lower aspect ratio. This is because a high in-plane aspect ratio strongly restricts the orientation of fillers when they aggregate and, consequently, highly ordered agglomerates are formed. On decomposing the effective thermal conductivity tensor into various molecular-scale contributions, it was identified that the thermal conductivity enhancement is due to the increased amount of heat transfer inside the graphene filler, particularly along the longer in-plane axis. The present result indicates a possibility of designing the heat conduction characteristics of a nanocomposite by customizing the filler shapes so as to control the aggregation structure of the fillers.
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.
In the present study, we investigated thermal conductivity and its structural dependence of a spherical nanodiamond with 2.5 nm in diameter using molecular dynamics simulation. We briefly discussed ...the difficulty of computing the thermal conductivity of a free nanoparticle using conventional methods and here we derived it from the non-equilibrium molecular dynamics simulation of a composite system where a nanodiamond is sandwiched between two solid blocks. The structural dependence was examined by applying this method based on a composite system to the 2.5 nm nanodiamonds having different ratios of 3- and 4-coordinate carbons (termed sp2-like and sp3-like carbons, respectively), which were obtained from annealing at different temperatures. The thermal conductivity of the nanodiamond decreased from 28 to 10 W/(m·K) with decreasing ratio of sp3-like carbons until the number of sp2-like bonds exceeded that of sp3-like bonds. When sp2-like bond became richer than sp3-like bond, the thermal conductivity was less sensitive to further increase of the ratio of sp2-like carbons. Based on the consideration of the heat transfer associated with a single CC bond, we interpreted that this structural dependence reflects the heat transfer characteristics of sp3- or sp2-like bond, whichever is more abundant. This interpretation, as well as the methodology, is helpful for understanding thermal conductivity of nanodiamonds and other carbon nanomaterials.
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•Thermal conductivity of a spherical nanodiamond (ND) of 2.5 nm diameter was studied using NEMD simulation.•A composite system was utilized to avoid direct temperature control inside the ND.•Thermal conductivity of the 2.5 nm ND was 10 to 28 W/(m·K) depending on the structure.•sp3-Like bond (defined as a bond of a 4-coordinate carbon) is an efficient heat carrier than sp2-like one.•The structural dependence originates from heat transfer characteristics of sp3- or sp2-like bonds, whichever is richer.
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•Mechanism of heat transfer with various alcohol surfactants at the solid surface.•Primary OH groups in surfactant enhance heat transfer more than secondary OH groups.•More primary OH ...groups in surfactant don’t reduce thermal boundary resistance much.•Butanol diols have reverse temperature-sensitive tendency due to side chain.
In the present study, we investigated the effect of the number and position of functional groups, and the length of the main chain and side chain in organic surfactant on adsorption behavior and interfacial heat transfer between silica surface and alkane solvent by non-equilibrium molecular dynamics simulation, where the surfactants were primary/secondary alcohol, monohydric/dihydric alcohol, and linear/branched alcohol. The results showed a similar adsorption behavior for all the surfactant types, where hydroxyl functional (OH) groups adsorbed onto the silica surface and alkyl chain was in contact with the solvent, which produced a heat path from silica via surfactant to solvent. The number of adsorbed OH groups did not directly translate to significantly decreased thermal boundary resistance due to the adsorption structure. Coulomb interaction enabled the closer distance between primary terminal OH groups in surfactant and silica surface, which enhanced the solid-surfactant intermolecular heat transfer. However, Coulomb interaction contributed less to shorten the molecular distances between secondary OH groups in surfactant and silica surface, which was connected to less efficient heat transfer from silica to surfactant and thereby did not enhance the interfacial heat transfer as much as surfactants with terminal OH. The increase in terminal OH groups in the surfactant molecules could not significantly reduce thermal boundary resistance, although the adsorption amount of OH was distinctly greater than that of surfactants with single OH. The side chain in surfactant enabled the efficient surfactant-solvent intermolecular heat transfer but related to the desorption of surfactant when decreasing the temperature. Thus branched-chain dihydric alcohol performed better than other surfactants on reducing thermal boundary resistance when the interfacial temperature was high enough to maintain the sufficient adsorption amount. We considered such reverse temperature-sensitive surfactant has a great potential application to fulfill multiple needs for heat dissipation of electronic devices, especially the high temperature operation. The new insights obtained in the present study were a step towards a molecular structure design of surfactant enhancing solid–liquid interfacial heat transfer.
The adsorbed film of cetyltrimethylammonium chloride (CTAC) at the tetradecane (C14) - water interface undergoes a first-order surface transition from two-dimensional liquid to solid states upon ...cooling. In this paper, we utilized this surface freezing transition to realize a spontaneous demulsification of Pickering emulsions stabilized by silica particles. In the temperature range above the surface freezing transition, the interfacial tension of silica laden oil-water interface was lower than CTAC adsorbed film, hence, stable Pickering emulsion was obtained by vortex mixing. However, the interfacial tension of CTAC adsorbed film decreased rapidly below the surface freezing temperature and became lower than the silica laden interface. The reversal of the interfacial tensions between silica laden and CTAC adsorbed films gave rise to Pickering emulsion demulsification by the desorption of silica particles from the oil-water interface. The exchange of silica particles and CTAC at the surface of emulsion droplets was also confirmed experimentally by using phase modulation ellipsometry at the oil-water interface.
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.
•The molecular mechanism of heat transfer with surfactants at the solid surface.•A dominant heat path from the solid to the solvent via surfactant molecules.•Per molecule contribution to interfacial ...thermal conductance.•Correlation of interfacial thermal conductance with interfacial potential energy.
Molecular dynamics simulations of a liquid layer between solid surfaces under a temperature gradient were performed to investigate the mechanism by which solid-liquid interfacial heat transfer is affected by adsorption of surfactant on solid surfaces with various concentrations of surfactant. The surfactant and solvent were chosen to be single-atom molecules with a contact angle of 0 and 180 degrees to the solid surface, respectively. Density distributions showed that the surfactant molecules formed a layer on the solid surface. The heat flux across the solid-liquid interface and between two adsorption layers closest to the surface was decomposed into energy transport terms based on molecular motions and inter-molecular interactions to examine the molecular mechanism of heat transfer. The interfacial thermal conductance (ITC) was also evaluated, and the molecular mechanism contributing to it was analyzed. It was found that the surfactant molecules that were adsorbed onto the solid surface decreased the interfacial thermal resistance, causing an increase in the heat flux, where the heat path from the solid to the solvent molecules via surfactant molecules became dominant as compared with the direct path from the solid to solvent molecules. It resulted in the temperature of surfactant being closer to the temperature of the solid than that of solvent in the vicinity of the solid surfaces. This indicated that in order to increase heat transfer via surfactants, not only the surfactant affinity with solid surface, but also the surfactant-solvent affinity must be considered. The contribution of each surfactant molecule to the ITC was greater than that of each solvent molecule, and both were proportional to their intermolecular potential with the solid atoms. Also, the contributions of a single surfactant and solvent molecule to the ITC were independent of their concentrations in the adsorption layer.
When liquid alkane droplets are placed on a surfactant solution surface having a proper surface density, alkane molecules penetrated into the surfactant-adsorbed film to form a mixed monolayer. Such ...a mixed monolayer undergoes a thermal phase transition from two-dimensional liquid to solid monolayers upon cooling when surfactant tail and alkane have similar chain lengths. We applied the total-reflection XAFS spectroscopy and surface quasi-elastic light scattering to the mixed adsorbed film of cetyltrimethylammonium bromide and hexadecane to elucidate the impact on the surface phase transition on the counterion distribution of the mixed monolayer. The EXAFS analysis verified that a higher percentage of counter Br– ions were localized in the Stern layer than in the diffuse double layer in the surface solid film compared to the surface liquid film, which resulted in a reduction in the surface elasticity measured by the SQELS. The finding that the surface phase transition accompanies the change in the counterion distribution will be important to consider the future applications of the colloidal systems, in which the coexistence of a surfactant and alkane molecules is essential, such as foams and emulsions.
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