Abstract The present work in nanofluids is focusing into using the electro-kinetic phenomenal occurring around nanoparticles immersed in a base fluid as a method to stabilize a nanofluid and enhance its thermal conductivity. The electro-kinetic physic establishes, that when an electrolyte solution is in contact with a solid, an electric double layer (EDL) is produced on the solid surface. Due to the high concentration of ions with the same charge around of the particle surface, “it is possible to stabilize a nanofluid by the action of an electro repulsive force caused by ions over the nanoparticle surface and enhance its thermal conductivity as the concentration of the solutions increases”. The nanofluid samples were prepared by the two-step method and a continuous ultrasonication. 1wt% and 3wt% concentration (mass fraction) of Titanium oxide, Anatase (TiO 2 ) nanoparticles, is added in an electrolyte solution (base fluid) made of different concentration of Potassium Chloride (KCl), and deionized water. The pH of the base fluid is maintained constant adding HEPES as a buffering agent. To measure the different level of stability for the nanofluid we used the thermal conductivity enhancement of the base fluid by nanoparticles. The experimental results under controlled temperature condition show that an electrolyte solution with nanoparticles after 20 days of preparation, presents a higher thermal conductivity with respect to the base fluid with an improvement rate ranging from 0.43±0.12% to 0.72±0.12% for 1wt%, and 2.15±0.17% to 3.03±0.21% for 3wt% of nanoparticles added respectively. The higher improvement shows sign of a major level of homogeneity of the nanofluid, and this behavior seems to be directly proportional to the KCl concentration.

Abstract The in-plane graphene/hexagonal boron nitride (Gr/h-BN) heterostructures have received extensive attention in recent years due to their excellent physical properties and the development potential of next-generation nanoelectronic devices. Generally, different bonding types between Gr and h-BN are considered in different non-equilibrium molecular dynamics (NEMD) simulations studies. However, which type of bonding is most conducive to interface thermal transport is still very confusing. In this work, we investigate the interfacial thermal conductance (ITC) and the thermal rectification (TR) in five different bonding types of in-plane Gr/h-BN heterostructures by using NEMD simulations. It is found that the ITC depends strongly on the bonding strength and arrangement of different atoms across the boundary. Among the five different bonding types of heterostructures, the C-N bonded heterojunction exhibits the highest ITC due to its stronger interfacial bonding. The analyses on the strain distribution indicated that a low interfacial stress level at the interface junction, may facilitate the heat conduction, thus leading to a higher ITC. In addition, we found that TR occurs in all five bonded heterostructures, and the C-B bonded heterojunction possesses the highest TR factor. The present study is of significance for understanding the thermal transport behavior of Gr/h-BN heterostructures and promoting their future applications in thermal management and thermoelectric devices.

Abstract Thermal energy diffusion through two directions of a micro/nanoscale thin film is modeled by a dimensionless form of Boltzmann transport equations of phonon density distribution functions. With the model named a lattice Boltzmann method (LBM), the discrete Boltzmann transport equations are able to be solved directly. The present model applied is based on physic expression of the dimensionless phonon density distribution functions together with both physic based dimensionless relaxation time models and the physic based dimensionless form of boundary conditions. Effects due to the variations of film thickness, distribution of temperature, and phonon transport frequency are all included in the physic based model. Phonon energy and effective thermal conductivity distributions are shown in the two-dimensional (2D) space. The spatial distributions of temperatures and thermal conductivities are validated by comparing with previous studies. Effects of the longitude and transvers direction heat transfer patterns and their effective thermal conductivities under different size and geometry ratios are compared.

Abstract This work provides a molecular dynamics simulation of the thermal conductivity and viscosity of thin water film. The results show that the average normal thermal conductivity and viscosity of thin water film is about an order of magnitude lower than those of bulk water, and they increase nonlinearly with the increase of thin film thickness. However, the viscosities at different sub-layers of a thin water film are apparently different. The density profiles at different positions of the thin water films are given to indicate their influence on the normal thermal conductivity and viscosity. The large increase of the viscosity and substantial decrease of the normal thermal conductivity in the near wall region of the substrate is influenced by the structural transition of thin water film because of its high density, which has prominent impact on the mean free path at the nanoscale. It provides a viable guidance for the mechanism study on the heat and mass transfer of an evaporating thin liquid film near the triple line.

Abstract It is known that the interlayer van der Waals (vdW) interactions will decrease the thermal conductivity of graphene. Single layer graphene (SLG) has the highest thermal conductivities, double layer graphene (DLG) would decrease to about half of the thermal conductivity of SLG. The graphite was measured to have a thermal conductivity of about 2000 W/m-K. Some research shows that graphite differs from SLG within a factor of 2, and DLG has almost the same thermal conductivity with graphite. In theoretical aspect, how to simulate the vdW interaction between graphene layers is a long existing problem. It is only until recently that the vdW interaction is still an active topic in first principle calculations. The popular methods include the Grimme’s DFT-D, vdW-DF and vdW-DFT-R methods. The vdW-DFT-R method was further optimized to increase accuracy by Hamada and was found to predict the most accurate interlayer distance between AB-stacked graphene in our recent study. The motivation of this work is to investigate the effect of vdW interaction on the thermal conductivity of multiple layer graphene from principles. We will calculate firstly the phonon dispersion relations of multiple layer graphene with the vdW interaction included. The obtained phonon properties and force constants will be combined with the ShengBTE method to calculate the thermal conductivity. The results show how vdW interaction causes the dimensional crossover of graphene thermal conductivity.

Abstract Due to the small size of low-dimensional materials, traditional experimental methods can hardly meet the requirements of accurate measurement. This paper presented a method for measuring the thermal conductivity of low-dimensional materials based on DC heating. This method adopted a micro-machining process to prepare a measuring electrode in advance, and only needed to suspend the object (one-dimensional wire or two-dimensional film) on the electrodes and maintain close contact. Finally, a standard diameter of 20 μm platinum wire was used to verify the measurement accuracy of this method. The application and future development of thermal conductivity testing structures for low-dimensional materials were also prospected.

Abstract Thermoelectric materials can convert thermal energy into electrical energy without any moving part which leads its path of application to the era of printed and flexible electronics. CsSnI 3 perovskite can be a promising thermoelectric material for the next-generation energy conversion due to its intrinsic ultra-low thermal conductivity and large Seebeck coefficient but enhancement of electrical conductivity is still required. CsSnI 3 can be prepared by wet process which can reduce the cost of flexible thermoelectric module. In this work, CsSnI 3 thin films were fabricated by spin coating wet process. Thin films were structurally and chemically characterized using XRD and SEM. Thermoelectric properties such as electrical conductivity, Seebeck coefficient, and thermal conductivity were measured at 300 K. Uni-leg thermoelectric modules were fabricated on a glass substrate using CsSnI 3 thin films. The maximum output is about 0.8 nW for 5 legs (25 mm × 3 mm × 600 nm) modules for the temperature difference of about 5°C. These results will open a new pathway to thermoelectric modules for flexible electronics in spite of low output power.

Abstract The high-purity electron-doped manganites Sm 1-x Ca x MnO 3 nanopowder were prepared by the solid-state reaction method, then the bulk material were obtained through granulation, molding, calcining, grinding and polishing. SCMO nanoparticles with 200 nm were obtained by the sol-gal process. The phase and surface morphology of these materials were characterized by X-ray diffraction and Scanning electron microscope and other experiments. The variable resistivity of the bulk materials were measured by two-wire method in the temperature range of 100–420K. The thermal conductivity was measured by the Laser Flash method. The results show that different doping ratios can change the phase transition temperature of the metal-insulation state. The temperature changed from 0 to 50 °C. The T MI could be regulated to room temperature. When the temperature is high than the T MI , it performs as metal state, on the contrary, it performs as an insulating state.

Abstract Polymers have been widely used due to low cost, light weight, chemical inertness and easy of processing. However, bulk polymers are usually considered as thermal insulators owing to their low thermal conductivity. In this paper, the 3D polyethylene (PE) structure based on knitting method is proposed. We investigate the thermo-mechanical property of 3D PE and make a comparison with the amorphous PE. The results show that the 3D PE has a higher thermal conductivity, elastic modulus, and shear modulus than amorphous PE, which provides a novel perspective on designing high thermal conductivity polymers. In the past decades, although the strain effect on the thermal conductivity of PE has been intensively studied, little research has been focused on the impact of shear. In this work, the thermal conductivity of PE under different shear strain is calculated by molecular dynamics simulation. Unexpectedly, the impact of shear on the amorphous PE and 3D PE is different. For amorphous PE, the average thermal conductivity is insensitive to the shear strain. However, the thermal conductivity of 3D PE can be slightly enhanced when the shear strain is large enough. The underlying mechanism is related to the specific morphology. Our findings can deliver new insights on designing high thermal conductive polymers.

The heat dissipation of current busbur in power plant is one of the important issues in power transmission, usually through the cylinder slotted to strengthen heat dissipation. Natural convection in a cylinder with an internal slotted annulus is the computational model abstracted from it. Natural convection in a cylinder with an concentric slotted annulus is concerned. Attention is focused on the effects of different slotted sizes on natural convection. Numerical results showed that, the equivalent thermal conductivity increases with the increase of Rayleigh number. At high Ra, the system heat transfer exhibit rich nonlinear characteristics. When the slotted direction or the slotted degree changed, it would have an important impact on the flow and heat transfer in the system, and also influence the related nonlinear characteristics.

The successful exfoliation of atomically-thin bismuth telluride quintuple layer (QL) attracts tremendous interest in investigating the electron and phonon transport properties in this quasi-two-dimensional material. While experimental results show that thermal conductivity is significantly reduced in Bi 2 Te 3 QL compared to the bulk phase, the underlying mechanisms for the reduction is still unclear. Also in some measurements, the Bi 2 Te 3 QL is usually supported on the substrate and the effect of the substrate on heat transfer in Bi 2 Te 3 QL is unknown. In this work, we have performed molecular dynamics simulations and normal mode analysis to study the mode-wise phonon properties in freestanding and supported Bi 2 Te 3 QL. We found that the existing of substrate will decrease the phonon relaxation times in Bi 2 Te 3 QL in the full frequency range. Thermal conductivity accumulation function for both freestanding and supported Bi 2 Te 3 QL are constructed and compared. We found that half of heat transfer in freestanding Bi 2 Te 3 QL contributed from phonons with mean free paths larger than 16.5 nm, while in supported Bi 2 Te 3 QL this value is reduced to 11 nm. In both cases phonons with MFPs in the range of 10–30 nm are the dominate heat carriers, which contribute to 55% and 53% of thermal conductivity in freestanding and supported cases.

A sample-based stochastic model is presented to investigate the effects of uncertainties of various input parameters, including laser fluence, laser pulse duration, thermal conductivity constants for electron, and electron-lattice coupling factor, on solid-liquid phase change of gold film under nano- to femtosecond laser irradiation. Rapid melting and resolidification of a free standing gold film subject to nano- to femtosecond laser are simulated using a two-temperature model incorporated with the interfacial tracking method. The interfacial velocity and temperature are obtained by solving the energy equation in terms of volumetric enthalpy for control volume. The convergence of variance (COV) is used to characterize the variability of the input parameters, and the interquartile range (IQR) is used to calculate the uncertainty of the output parameters. The IQR analysis shows that the laser fluence and the electron-lattice coupling factor have the strongest influences on the interfacial location, velocity, and temperatures.

The thermal conductivity of bulk CNT materials based on three-dimensional randomly oriented CNT networks is rather low comparing with that of the individual carbon nanotube, and one of the important reasons has been considered to be the huge intertube thermal resistance. Though intertube thermal resistance can be reduced via welded junctions between the tubes, it is still unknown whether the formation of the welded junctions can enhance the thermal conductivity of random SWCNTs network. In this approach, the calculations of the average number of thermal contacts per SWCNT are compared by two different methods, and influencing factors of the thermal conductivity of random SWCNTs network are discussed; a hybrid macroscopic analytical model is proposed to predict the overall thermal conductivity of the partially sintered randomly oriented carbon nanotube (CNT) networks. Results show that choosing proper fraction of welded contacts can enhance the thermal conductivity of randomly oriented CNT networks effectively.

Transient heat transfer during constrained melting of graphite-based solid-liquid phase change nanofluids in a spherical capsule was investigated experimentally. Nanofluids filled with self-prepared graphite nanosheets (GNSs) were prepared at various loadings up to 1% by weight, using a straight-chain saturated fatty alcohol, i.e., 1-dodecanol (C 12 H 26 O), with a nominal melting point of 22 °C as the base fluid. In-house measured thermal properties were adopted for data reduction, including thermal conductivity, dynamic viscosity, latent heat of fusion, specific heat capacity and density. A proper experimental approach depended on volume expansion was figured out to monitor the melting process of nano-enhanced phase change fluid in a spherical capsule indirectly and qualitatively characterize the process. A variety of boundary temperatures were also adopted to vary the intensity of natural convection. It was shown that under low boundary temperatures, a monotonous melting acceleration came into being while increasing the loading due to the monotonously increased thermal conductivity of the nanofluids. While increasing the boundary temperature leads to more intensive natural convection that in turn slowed down melting under the influence of nanoparticles because the contribution by natural convection is significantly suppressed by the dramatically grown dynamic viscosity, e.g., more than 60-fold increase at the loading of 1 wt.%. The melting rate is determined by the competition between the enhanced heat conduction and deteriorated natural convection.

A three-dimensional numerical study has been carried out to understand the effect of axial wall conduction in a conjugate heat transfer situation in a wavy wall square cross section microchannel engraved on solid substrate whose thickness varying between 1.2–3.6 mm. The bottom of the substrate (1.8 × 30 mm 2 ) is subjected to constant wall heat flux while remaining faces exposed to ambient are assumed to be adiabatic. The vertical parallel walls are considered wavy such that the channel cross section at any axial location will be a square (0.6 × 0.6 mm 2 ) and length of the channel is 30 mm. Wavelength (λ) and amplitude (A) of the wavy channel wall are 12 mm and 0.2 mm respectively. Simulations has been carried out for substrate thickness to channel depth ratio (δ sf ∼ 1–5), substrate wall to fluid thermal conductivity ratio (k sf ∼ 0.34–646) and flow rate (Re ∼ 100 to 500). The results show that with increase in flow rate (Re), the hydrodynamic and thermal boundary layers are thinned due to wavy passage and they shifted from the centerline towards the peak which improves the local heat transfer coefficient at the solid-fluid interface. It is also found that after attaining maximum Nu avg at optimum k sf , the slope goes downward with increasing k sf for all set of δ sf and flow rate (Re) considered in this study.

The flow and heat transfer characteristics of nanofluids in the near-wall region were studied by non-equilibrium molecular dynamics simulation. The nanofluid model consisted of one spherical copper nanoparticle and argon atoms as base liquid. The effective thermal conductivity (ETC) of nanofluids and base fluid in shear flow fields were obtained. The ETC was increased with the increasing of shear velocity for both base fluid and nanofluids. The heat transfer enhancement of nanofluids in the shear flow field ( v ≠0) is better than that in the zero-shear flow field ( v =0). By analyzing the flow characteristics we proved that the micro-motions of nanoparticles were another mechanism responsible for the heat transfer enhancement of nanofluids in the flow field. Based on the model built in the paper, we found that the thermal properties accounted for 52%–65% heat transfer enhancement and the contribution of micro-motions is 35%–48%.

The Present investigation has been carried out to study the performance of nano enhanced phase change material (NEPCM) based heat sink for thermal management of electronic components. Enthalpy based finite volume method is used for the analysis of phase change process in NEPCM. To enhance the thermal conductivity of phase change material (PCM), copper oxide nano particles of volume fractions 1%, 2.5% and 5% are added to PCM. A heat flux of 2500 W/m 2 is taken as input to the heat sink. The thermal performance of the heat sink with PCM is compared with NEPCM for each volume fraction of nano particle for both finned and unfinned configurations. It is observed that the nano particle volume concentration plays a major role in removing the heat from the chip in case of unfinned heat sink configuration. However, for finned heat sink configuration, the volume concentration effect is not appreciable. In addition, the performance of NEPCM based finned heat sink is studied under cyclic loading in both natural and forced convection boundary conditions. It is observed that under forced convection the solidification time is reduced.

This article presents a theoretical and numerical study on the heat conduction of gas confined in a nanoscale cube. An effective thermal conductivity model of confined gas using a modified mean free path is proposed for the heat conduction in transition regime inside a closure. Excellent agreement of the present model with the results from our simulations by the method of direct simulation Monte Carlo (DSMC) has been achieved for different boundary conditions of side walls. The temperature jumps and the reduction of local heat flux near the side walls are observed from the DSMC results.

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.

Transparent insulating materials combine high visible light transmission and excellent thermal insulation, and have potential applications in solar energy utilization, building energy conservation and commercial freezers. As a medium of low absorption and low thermal conduction, introducing gas bubbles into transparent mediums such as glass and polycarbonate (PC) may improve simultaneously their light transmission and thermal insulation performances through decreasing the absorption and thermal conduction in the materials. However, gas bubbles can also enhance the scattering which is a competition to the effect of the absorption decrease. Moreover, the material design should also consider the balance between the visible light transmittance and effective thermal conductivity. Therefore, a radiative transfer model for the transparent medium containing large gas bubbles (with a diameter much larger than the wavelengths concerned) with the assumption of independent scattering and the Maxwell–Eucken thermal conduction model were adopted to calculate the transmittance, reflectance and effective thermal conductivity. Subsequently, the effects of the volume fraction of gas bubbles ( f v ) and bubble radius ( r ) were discussed, and the two balances mentioned above were analyzed. The results showed that the transmittance always decreases when f v increases with fixed r or when r decreases with fixed f v . The transmittance includes two components, named as the collimated transmittance and bulk transmittance due to the forward scattering. The collimated transmittance depends on the effects of absorption decrease and scattering increase, whereas in the weak absorption region, the effect of the scattering increase dominates, making the collimated transmittance decrease, and the decreasing rate is larger than the increasing rate of the bulk transmittance as only the forward scattering contributes to the bulk transmittance. Therefore, the transmittance decreases when f v increases with fixed r or when r decreases with fixed f v . In addition, as f v increases from 0 to 0.5, the effective thermal conductivity ( k e ) of the glass decreases from 1.4 to 0.58 W/(m·K), and k e of the PC decreases from 0.236 to 0.113 W/(m·K). At the same time, the transmittances of both materials at 0.55 μm can be kept larger than 50% for f v =0.5 as long as the bubble radius is larger than 0.7 mm. To elucidate the application performance, a heat transfer model of a freezer adopting glass or PC as a cover was analyzed. Although the decrease percentage of k e for glass is higher than that of PC, the effect of the energy saving is more significant for PC, as the cooling load can be saved by 9.6% when f v increases from 0 to 0.5, while the corresponding value for glass is only 2.7% because that the decreasing rate of the cooling load with k e is higher at a lower k e .