Thermal Transport Characteristics Research Articles

Influence of exfoliated graphite inclusion on the thermal, mechanical, dielectric and solvent transport characteristics of fluoroelastomer nanocomposites

Nanocomposites with multifunctional properties are of great interest as they meet the needs of the developing industrial and engineering fields. The present study focus on the development of multifunctional elastomeric nanocomposites with exfoliated graphite (EG) as the reinforcing content and the speciality material fluoroelastomer as the polymer matrix. Morphological analysis of the nanocomposites in terms of TEM, AFM and SEM were done to look at the polymer-filler interactions which in turn lead to the property enhancement. The dispersion of the EG in the nanoscale which resulted in the property enhancement is clear from the TEM images. Nanocomposites exhibited improved thermal stability and higher mechanical strength by the addition of 12 phr of EG nanofiller. Moreover the dielectric performance of the nanocomposites was also enhanced by the addition of conductive EG nanofiller and the nanocomposites showed higher dielectric constant value and AC conductivity with lower dielectric loss. In addition to this the solvent transport features of the nanocomposites were also analyzed to reveal the reinforcement effect of the nanofiller and the effective polymer-filler interactions.

Effects of undulated wall on the hydrodynamic and thermal transport characteristics of turbulent jet

The present study investigates numerically the effect of wall undulation on the thermo-hydraulic transport characteristics of turbulent jet. In order to explore the influence of position of the nozzle exit from the wall, both wall jet and offset jet are considered. The theoretical model is numerically solved using shear stress transport (SST) model to predict the effects of height of undulation of the wall and the offset jet ratio on the re-attachment length, the velocity profile at different downstream locations, the coefficient of friction and the Nusselt number. The results indicate that the undulation of wall has a strong influence in altering the friction coefficient, Nusselt number and re-attachment length for offset jet. It further reveals that there is an intricate interplay between undulation height and offset ratio in dictating the flow and heat transfer characteristics.

Application of the inverse method in the 3 ω method for measuring the thermal conductivity of thin films

Planar nano-materials generally refer to samples that are extremely thin in the direction of thickness, such as nanofilms, graphene, and superlattice. They are of great importance in various fields, including transistors, thermoelectrics, and optoelectronics, etc. In practice, materials’ thermal transport characteristics often play a crucial role in applications. More importantly, due to the confinement in the direction of thickness, the mechanisms of thermal transport process in planar nano-materials become very different from those at the macroscale. For example, the effective lattice thermal conductivities of nanofilms are always highly dependent on their thickness, since phonon coherence, ballistic transport, and boundary scattering effects could occur in this case. Therefore, probing thermal transport properties of planar materials has been a research hotspot in both the research and industrial communities. The 3 ω method has been proven to be a powerful tool that can measure the thermal conductivities of planar materials effectively. Particularly for a thin film on a substrate, the differential 3 ω method was developed. In this scheme, it is necessary to determine the thermal properties of substrate materials in the first step. Thus, a reference sample should be measured in addition to the target sample, which not only increases the complexity of measurement but also reduces the measurement accuracy of the thin film’s thermal transport properties. In the present work, we used the numerical simulations to demonstrate that in the 3 ω method the combination of finite element method (FEM) and inverse problem method can derive the thermal properties of thin film and substrate simultaneously. It was found that the thermal conductivities could be determined accurately despite the cutoff frequency of measurement, whereas the thermal diffusivities’ accuracy could be improved with increasing cutoff frequency of measurement. Furthermore, as a test, the thermal conductivities of silicon dioxide (SiO2) thin film on the silicon (Si) substrate were measured using the method above. In our experiments, the thickness of Si substrate was 450 μm, and the thickness of SiO2 thin film was 950 nm. The frequency of driving current was set from 100 to 5000 Hz (the cutoff frequency). The trust region reflective method was adopted in the fitting process. Moreover, it was found that the measurement uncertainty of temperature rise was less than 1% and thus its influence on the uncertainties of measured thermal properties could be ignored. As a result, the major uncertainties of thermal conductivity measurement came from measuring the thickness of SiO2 thin film. The thermal conductivity of Si substrate was 142.1 W/(m K) with a relative error equal to approximately 2%, and the thermal conductivity of SiO2 thin film was 1.01 W/(m K) with a relative error equal to approximately 2%, which could agree well with the reported value in literature. In summary, the present work demonstrated that the combination of FEM and inverse method could be employed for thermal properties measurement, and thus analytical models are not always necessary. For future work, the methodology presented in this paper can be extended to deal with the measurement of thermal transport properties of irregular-shape thin flakes on a substrate.

A molecular dynamics study of thermal boundary resistance over solid interfaces with an extremely thin liquid film

We investigated the characteristics of thermal energy transport over two solid surfaces joined via an extremely thin liquid film where the liquid molecules are under the influence of both solid surfaces simultaneously. Using non-equilibrium molecular dynamics simulations, the thermal resistance between the two solid surfaces was examined for different thickness of liquid film and different alignment (in-plane orientation) of the two solid surfaces. Both solid surfaces were the (1 1 0) plane of face centered cubic lattice, and two different combinations of alignment, i.e., either parallel or crossed to each other, were examined. The thermal resistance between the solid surfaces was decomposed into the thermal boundary resistance at the solid-liquid interfaces and the thermal resistance of the liquid film, which were analyzed separately. The results showed that when the liquid film thickness is equal or less to four molecular dimensions, both the film thickness and the surface alignment have significant influence on the thermal resistance. Specifically, when the liquid film is a single layer of liquid molecules (LLM), the thermal resistance between solid surfaces is extremely low when compared with the cases of more LLM, and increases with increasing liquid density. In contrast, when the film is composed of two or three LLM, the solid-liquid interfacial thermal resistance decreases with increasing liquid density, and a discontinuous increase occurs as the number of LLM changes from two to three. As for the effect of surface alignment, it was found that the parallel surface alignment gives a lower thermal resistance than the crossed surface alignment.

Hybrid Thermal Transport Characteristics of Doped Organic Semiconductor Poly(3,4-ethylenedioxythiophene):Tosylate

Understanding thermal transport in doped organic semiconductors would promote the development of flexible electronics and organic thermoelectrics. Temperature-dependent thermal conductivity of poly...

Solidification of Graphene-Assisted Phase Change Nanocomposites inside a Sphere for Cold Storage Applications

In this work, we experimentally investigated the solidification behavior of functionalized graphene-based phase change nanocomposites inside a sphere. The influence of graphene nanoplatelets on thermal transport and rheological characteristics of the such nanocomposites were also discussed. We adopted the covalent functionalization method to prepare highly stable phase change nanocomposites using commercially available phase change material (PCM) OM08 as the host matrix and graphene nanoplatelets (GnPs) with 0.1, 0.3, and 0.5 volume percentage as the nano inclusions. We report a maximum thermal conductivity enhancement of ~102 and ~46% with 0.5 vol% in the solid and liquid states, respectively. Rheological measurements show that the pure PCM shows Newtonian behavior, whereas the inclusion of GnPs leads to the transition to non-Newtonian behavior, especially at lower shear rates. Viscosity of the nanocomposite increases with an increase in the volume fraction of GnP. For 0.5 vol% of GnPs, maximum increase in viscosity was found to be ~37% at a shear rate of 1000 s−1. Time required for complete solidification decreases with the loading of GnPs. Maximum reduction in solidification time with 0.5 vol% of GnPs was ~40% for bath temperature of −10°C.

Characteristics of thermal and momentum transport during the lifetime of Ural blocking highs

Ural blocking (UB) event is an important large‐scale weather system dominating cold waves in East Asia. This paper investigates the thermal‐dynamical variation characteristics associated with UB evolution via 32 UB events in winter during 1979–2016. The results show that transport of stationary heat and momentum changes prominently from 3 and 6 days before and after UB establishment. Specifically, on 3 days prior to the UB event establishment, an apparent convergence of stationary heat flux ({v*T*}), which can extend from lower‐middle troposphere to the tropopause, is observed at around 60°N in the Urals region. The diagnosis shows that the convergence of stationary heat flux weakens the meridional temperature gradient () and thermal wind, and thus the zonal westerly weakens. The convergence of ({v*T*}) also support formation of the warm centre structure of UB event. Meanwhile, convergence of momentum flux reverses to be a divergence in the Urals region (60°N), which contributes to decrease of the zonal wind. The heat and momentum flux reduce the intensity of the westerly and are beneficial for the establishment of UB event. Along with UB event establishment, the convergence of stationary heat flux weakens, and the divergence of momentum flux strengthens, accompanying with decrease of wind speed at middle latitudes and maintaining of the UB event. Six days later, the convergence of stationary heat flux disappears and divergence of momentum flux changes to be convergence and then the UB event collapses. Further analyses also suggest that the polar front jet (PFJ) enhances (weakens) coincidently before (after) the establishment of UB, which is attributed to the strengthening (weakening) of the convergence of ({u*v*}) around 70°N.

Outstanding strength, optical characteristics and thermal conductivity of graphene-like BC3 and BC6N semiconductors

Carbon based two-dimensional (2D) materials with honeycomb lattices, like graphene, polyaniline carbon-nitride (C3N) and boron-carbide (BC3) exhibit exceptional physical properties. On this basis, we propose two novel graphene-like materials with BC6N stoichiometry. We conducted first-principles calculations to explore the stability, mechanical response, electronic, optical and thermal transport characteristics of graphene-like BC3 and BC6N monolayers. The absence of imaginary frequencies in the phonon dispersions confirm dynamical stability of BC3 and BC6N monolayers. Our first principles results reveal that BC3 and BC6N present high elastic moduli of 256 and 305 N/m, and tensile strengths of 29.0 and 33.4 N/m, with room temperature lattice thermal conductivities of 410 and 1710 W/m.K, respectively. Notably, the thermal conductivity of BC6N is one of the highest among all 2D materials. According to electronic structure calculations, monolayers of BC3 and BC6N are indirect and direct bandgap semiconductors, respectively. The optical analysis illustrate that the first absorption peaks along the in-plane polarization for single-layer BC3 and BC6N occur in the visible range of the electromagnetic spectrum. Our results reveal outstandingly high mechanical properties and thermal conductivity along with attractive electronic and optical features of BC3 and BC6N nanosheets and present them as promising candidates to design novel nanodevices.

How interlayer twist angles affect in-plane and cross-plane thermal conduction of multilayer graphene: A non-equilibrium molecular dynamics study

Graphene, a kind of emerging low dimensional carbon material, has admirable thermal properties with potential applications in the thermal management of aerospace and microelectronics fields. In recent studies, thermal properties of both conventional single-layer and multilayer graphene were extensively studied. However, few studies focused on that of multilayer graphene with twist angles, whose thermal properties are different from that of conventional graphene because of the existence of interlayer twist angles. Such knowledge gap hinders the potential applications. Therefore, the thermal conductivity of bilayer, 4-layers and 6-layer zigzag graphene with various twist angles is investigated utilizing non-equilibrium molecular dynamics at the typical temperature region in this study, and both in- and cross-plane thermal conductivity are investigated. The size of the studied graphene is 10 nm × 22 nm in the x-y plane, and the size in the z-direction depends on the number of layers. Moreover, the phonon vibrational density of state is also analyzed for mechanisms behind the thermal transport. The result indicates that a local maximum value can be found with the twist angle of 30° for in-plane thermal conduction. The results also suggest that the highest thermal conductivity could be reached without any twist angles for both in and cross-plane conduction. The observation would provide an opportunity to study the characteristics of in-plane and cross-plane thermal transport of twisted multilayer graphene.

Heat transfer characteristics and entropy generation of electroosmotic flow in a rotating rectangular microchannel

This study aims to investigate thermal transport characteristics and analyze entropy generation of thermally fully-developed electroosmotic flow (EOF) in a rotating rectangular microchannel subject to constant wall heat flux. In the two-dimensional rectangular domain, the solutions for electric potential and flow velocity are obtained in terms of eigenfunction series by means of separation of variables technique, and the temperature field, which takes into consideration the effects of heat convection, Joule heating and viscous dissipation, is numerically solved. Results are generated to show the influences due to various factors, including rotational frequency, depth/width ratio of the rectangular channel, distribution of zeta potentials, strength of Joule heating and magnitude of wall heat flux, on the thermal transport characteristics. In particular, how the Nusselt number and the entropy generation vary with these factors are explored in details. To pursue optimization in energy utilization, an optimal aspect ratio is found at which the minimum entropy generation can be achieved. We believe that the findings in the present study will be useful in the design of energy efficient micro-systems which utilize the dual electrokinetic and centrifugal pumping effects.

Limiting thermal characteristics for flow of non-Newtonian fluids between asymmetrically heated parallel plates: An analytical study

In this work, an analytical study on the limiting thermal transport characteristics for the flow of a non-Newtonian fluid namely, power law liquid, between asymmetrically heated parallel plates is presented. The transport equations for mass, momentum, and energy are analytically solved to obtain closed-form expressions for the temperature distributions in the flow field and the limiting Nusselt number in terms of relevant parameters. The effects of frictional heating due to viscous dissipation on the heat transfer characteristics are studied. Results are presented in terms of degree of asymmetric heating, Brinkman number, and power law index. The most important finding from the present work is the nontrivial interplay between degree of asymmetric heating and the rheological behavior of the fluid in dictating the thermal transport characteristics of heat.

Poly(phenylene) synthesized using diels-alder chemistry and its sulfonation: Sulfonate group complexation with metal counter-ions, physical properties, and gas transport

Sulfonated poly(phenylene) (sPP) thermal stability, physical properties, and gas transport characteristics were evaluated as a function of ion-exchange capacity (x = IEC), and counter-ion type (M = H+, Cs+, Na+, Mg2+, and Al3+). Counter-ion substitution with its sulfonated groups produced Δv(SO3-) vibrational shifts that were related to ion type and valence observed using FTIR. Increasing IEC and counter ion complexation strength within sPP led to greater density and reduced fractional free volume (FFV). sPPx-M exhibited a single thermal degradation step greater than 550 °C that decreased with increasing IEC. Unsulfonated poly(phenylene) (PP) had a glass transition at 391 °C, which shifted to 420 °C upon sulfonation. Thermal stability and mechanical properties increased with counter-ion type in the following order: sPP2.5-Cs+ < sPP2.5-Na+ < sPP2.5-Mg2+. However, decreased thermal stability and physical properties were observed using Al3+ as a counter-ion (sPP2.5-Al3+). He, H2, CO2, N2, O2 and CH4 permeability decreased within sPP2.5-M based upon counter-ion type, size, and valence. Larger CO2 and H2 molecules were more permeable than He, which revealed a more complex transport process was occurring within sPP2.5. Metal-ion complexation with sPP2.5 significantly reduced CO2 and H2 permeability. PP possessed high CO2 permeability (152 Barrers) with moderate CO2/CH4 selectivity (12.6). sPP2.5-Na+ (IEC = 2.5 meq/g) had a CO2/CH4 selectivity of 71.3, and counter-ion substitution to create sPP2.5-Cs+ increased CO2/CH4 selectivity by 23%. Gas solubility and diffusivity decreased with increasing IEC and metal-ion complexation strength. sPPx-M had tunable ideal gas permselectivity that was largely a function of sulfonate group concentration and counter-ion type.

Irreversibility analysis in a slip aided electroosmotic flow through an asymmetrically heated microchannel: The effects of joule heating and the conjugate heat transfer

In this article, we discuss about the entropy generation minimization in a slip-modulated electrically actuated transport through an asymmetrically heated microchannel. While investigating the underlying thermo-hydrodynamics towards minimizing the irreversibility of the system under present consideration, we take the combined effects of Joule heating and the conjugate transfer of heat into account in this analysis. We primarily focus to tune the relevant thermo-physical as well as geometrical parameters towards minimizing the global irreversibility of the system. We show that the cooperative-correlative effects of the temperature gradient (between walls and fluid) and viscous dissipation in the system, as modulated by the slipping hydrodynamics stemming from the interfacial electrochemistry and Joule heating effects originating from higher conduction currents, bring in a change in the underlying thermal transport characteristics of heat, leading to an alteration in thermodynamic irreversibility in the system. We unveil optimum values of geometrical and thermo-physical parameters for which a change in thermal transport of heat as triggered by the viscous dissipation and joule heating effect leads to a minimum entropy generation in the system. Moreover, we show that the ionic concentration of the electrolyte present in the fluid can fetch a reduction in the irreversibility as well. We believe that the insights gained from this analysis may be useful for constructing the well-optimized futuristic micro heat exchanging systems/devices, typically used in MEMS.

Effect of Graphene for Ablation Study of Advanced Composite Materials for Aerospace Applications

Graphene was incorporated into elastomeric Matrices using dispersion kneader and two roller mixing mill to fabricate ablative nanocomposites used in hyperthermal environment encountered by space vehicle or rocket motor. The addition of graphene in the host matrix has remarkably reduced the backface temperature elevation during the ablation testing of the ablatives. The linear and mass ablation resistances have been diminished while insulation indexes of the nanocomposites have been increased the graphene incorporation into the elastomeric matrix. Thermal stability and heat absorbance capability of the polymer nanocomposites were progressed with increasing the filler to matrix ratio. Thermal conductivity of the ablatives have been conducted according to the ASTM E1225-99 and D5470-03, respectively to execute the effect of graphene concentration on the thermal transport characteristics of the tested specimens. Tensile strength of the nanocomposite specimen was augmented with increasing graphene to polymer ratio. Scanning electron microscopy was used to scrutinize the evenly dispersed graphene in the polymer matrix, polymer pyrolysis, and voids formation in the ablated nanocomposites.

Temperature dependence of the electrical and thermal transport in FeCo/Cu/Ni80Fe20 spin valves

Creating spin current under temperature gradient in magnetic devices has resulted in a new emerging field of spin caloritronics, but extraction of material parameters i.e. Seebeck coefficient and interpretation of thermal transport characteristics are still great challenges due to the thermal contact effect, especially in a wide temperature range. Because the heat-driven voltages do not depend on the change of magnetic states in spin valve, this could be used to obtain the accurate temperature difference applied on sample and exclude the thermal contact effect. Based on this calibration, the electrical and thermal spin transport behaviors in the in-plane FeCo/Cu/FeNi spin valve were systematically studied with various temperature. We observed that the Seebeck coefficients of spin valve was negative and spin-dependent Seebeck coefficient was proportional to the asymmetry parameter in a wide temperature range from 100 to 300 K, indicating that the spin-dependent thermal transports are closely related with electrical transports in the in-plane spin valve.

Analysis of thermal transport and fluid flow in high-temperature porous media solar thermochemical reactor

As a key technical indicator of porous media solar thermochemical reactor, thermal transport and fluid flow characteristics have significant impacts on hydrogen production efficiency. These issues could be achieved more efficiently and at a lower cost by applying computational fluid dynamics (CFD). However, the choice of different thermal transport models might cause the variation of results. In this study, the thermal transport and fluid flow characteristics in high-temperature porous media solar thermochemical reactor were investigated with different thermophysical models using FLUENT software user-defined functions (UDFs). The results indicate that the local thermal non-equilibrium model (LTNE) and radiative transfer model are proved to be indispensable for the thermal performance analysis of high working temperature thermochemical reacting system. Besides, Wu model of momentum source is more suitable for pressure simulation, while Wu model and Vafai model of heat transfer show little difference in temperature distribution.

Combined ab initio and empirical model of the thermal conductivity of uranium, uranium-zirconium, and uranium-molybdenum

In this work we developed a practical and general modeling approach for thermal conductivity of metals and metal alloys that integrates ab initio and semiempirical physics-based models to maximize the strengths of both techniques. The approach supports creation of highly accurate, mechanistic, and extensible thermal conductivity modeling of alloys. The model was demonstrated on {\alpha}-U and U-rich U-Zr and U-Mo alloys, which are potential fuels for advanced nuclear reactors. The safe use of U-based fuels requires quantitative understanding of thermal transport characteristics of the fuel. The model incorporated both phonon and electron contributions, displayed good agreement with experimental data over a wide temperature range, and provided insight into the different physical factors that govern the thermal conductivity under different temperatures. This model is general enough to incorporate more complex effects like additional alloying species, defects, transmutation products, and noble gas bubbles to predict the behavior of complex metallic alloys like U-alloy fuel systems under burnup.

Heat transfer correlation of viscoplastic fluid flow between two parallel plates and in a circular pipe with viscous dissipation

The present paper investigates the laminar forced convection heat transfer for a generalized Casson fluid flow through a horizontal circular pipe and between two parallel plates maintained at a uniform wall temperature. This study focuses on the effect of yield stress and flow index as well as Peclet number on thermal transport characteristics for both the entrance and the fully developed regions. Because of the viscous character of this kind of fluids, viscous dissipation is taken into account. The regularized Papanastasiou model is considered to avoid the discontinuous-viscosity behavior of the fluid. The governing equations are discretized by means of the finite volume method using the power law scheme. The numerical simulations are conducted using a source code based on the FORTRAN language. The results show that the increase in the Peclet number leads to the increase in the heat transfer rate at the inlet. Furthermore, the Nusselt number decreases when the flow index increases and increases downstream when the Casson number increases. Heat transfer is also significantly improved when viscous dissipation is taken into account. A correlation is proposed at the end of the study; it gives the asymptotic Nusselt number over a wide range of the Casson number (0 ≤ Ca ≤ 20), when viscous dissipation is neglected and taken into account. The comparison between both geometrical configurations shows that, from a thermal point of view, it is more interesting to use parallel plates than a pipe.

Heat transfer of the nanofluid in soft nanochannels under the effects of the electric and magnetic field

In this article, the thermal transport characteristics of nanofluid flow through a parallel plate soft nanochannel are researched in presence of the imposed electric field and magnetic field. By incorporating the assumptions of the thermal fully developed flow and the Debye–Hückel linearization, the analytical solutions of the velocity and temperature field for nanofluids are derived. The classical boundary condition of uniform wall heat flux is utilized in the analysis, and the influences of viscous dissipation and Joule heating are taken into account. Furthermore, the Nusselt number indicating intensity of the heat transfer is investigated. The results for relevant dimensionless parameters are presented graphically and discussed in briefly.

Optical and thermal filtering nanoporous materials for sub-ambient radiative cooling

Emitting heat to Space is a passive, dry, and solid-state cooling approach with potential applications in thermal management of buildings, solar cells, and vehicles. Emitting in the 8–13 μm atmospheric window while preventing heat from reaching the cold emitter is important to the performance. Here, we investigate the design of infrared-transparent thermally-insulating materials for radiative cooling at deep sub-ambient temperatures. Unlike surface-based designs that rely on selective emission, selective radiative heat transfer with Space is achieved via selective transmission through an insulating cover. We model the radiative and thermal transport characteristics of several nanoporous materials that are transparent in the 8–13 μm range. We also study the effects of physical morphology and material composition on the scattering and absorption properties of the insulating cover, and its radiative cooling performance. By tailoring the nanostructure and material composition, one can define regions for short-wavelength scattering and long-wavelength absorption, and in turn, block solar and atmospheric heat. The combination of low thermal conductivity and selectively high transparency in the atmospheric window enable high-performance radiative cooling down to 35 K below ambient temperature.