Thermal Conduction Mechanism Research Articles

Thermal conductivity calculations of binary liquid organic mixtures by molecular dynamics simulation and its interpretation of microscopic heat transfer mechanism

ABSTRACT Heat transfer is an important transmission phenomenon in the nature, and thermal conductivity is a basic thermodynamic data. In the process of chemical production, the reactants and products are mostly mixed liquid systems, but their thermal conductivity data are missing and difficult to define. The mechanism of thermal conductivity of liquid organics is still not sufficiently clarified at present. In this paper, the heat conduction of binary liquid organic mixtures: 3-pentanone + butanol, butanol + decane and heptane + n-undecane was simulated by nonequilibrium molecular dynamics, and the thermal conductivity at different weight fraction was calculated. The deviation between the calculated value and the experimental value of thermal conductivity is less than 9.3%. The results of heat flux decomposition and atomic heat path analysis show that thermal energy is mainly transferred through Coulomb interaction, kinetic energy and torsion angle. In addition, the molecular structure of the mixture and the weight fraction of each component are significantly correlated with the heat transfer mechanism. This study provides a preliminary insight into the heat transfer mechanism of binary liquid organic mixtures.

“I got to see, and I got to be a part of it”: How cued gesturing facilitates middle‐school students' explanatory modeling of thermal conduction

AbstractConstructing causal mechanistic explanations of observable phenomena is a key science practice that is often challenging for students as most mechanisms involve interactions of unobservable entities and activities. In this study, we examined how gesturing with a computer simulation that depicts the molecular mechanism of thermal conduction supported middle‐school students in constructing causal mechanistic explanations. We designed a gesture‐augmented computer simulation in which students were cued to use hand gestures to control the simulation. These cued gestures represent core causal interactions of conduction and they prompt students to physically engage with the simulation in conceptually meaningful ways. In this study, we examined how 21 students used the simulation and explained thermal conduction in a semi‐structured interview, followed by a mixed‐methods analysis. Quantitative analysis shows that students moved toward articulating the canonical causal mechanistic explanation of thermal conduction using the simulation. Three representative cases were identified to explore how students' explanations were facilitated by cued gestures. The analysis shows two main ways the cued gestures supported all students in the study: (a) by helping them attribute causal agency to molecules rather than an entity called Heat, and (b) by reifying the core mechanism of molecular collisions in conduction. Furthermore, the case studies show how each student's unique ways of sensemaking impacted their gesture use. Implications for instruction with gestures and design of augmented environments are discussed.

Preparation and properties of nanopore-rich cement-based porous materials based on colloidal silica sol

Cement-based porous materials (CPMs) with different dry densities were successfully prepared by using silica sol as pore-forming agent. The rheological behavior of the fresh CPMs was discussed, then pore structure, pore-forming mechanism of silica sol, dry density, compressive strength and thermal conductivity of CPMs were investigated and presented. The results indicate that nanopores in cement paste were formed in-situ by adding silica sol, which led to the optimization of pore structure of CPMs, thereby improving the properties of CPMs. When the substitution of cement matrix by silica sol reached to 95 vol%, CPM with a nanopore volume of 0.675 cc/g, dry density of 488 kg/m3, thermal conductivity of 0.081 W/(m·K) and compressive strength of 0.5 MPa was successfully obtained, showing excellent insulation performance. CPMs with achieved strengths and low thermal conductivities show great potential in the application of semi-structural and structural thermal insulation systems of buildings.

A reconstruction of Hamilton-Crosser model for effective thermal conductivity of nanofluids based on particle clustering and nanolayer formation

Although many models have been proposed to estimate the effective thermal conductivity (ETC) of nanofluids, the thermal conduction mechanisms need to be further addressed to improve the prediction accuracy of ETC model. In this paper, by fully considering the effects of particle clustering, Brownian motion, Kapitza resistance and nanolayer of particle, Hamilton-Crosser model is reconstructed to establish an improved model for the ETC of nanofluids. To develop this model, the particle clustering is characterized by the particle size distribution analysis, and the thermal conductivity distribution in the nanolayer is represented as a specific function of the distance from the nanoparticle. The influences of temperature, viscosity, particle size and other factors on the ETC of nanofluids are also included in this model. The results show that the accuracy of this model can be improved as compared to those considering only several of these factors, and the maximum error is 3% against the available experimental data. With this model, the inconsistency phenomena in ETC data of nanofluids can be explained in the view of the agglomeration and Brownian motion with the system conditions.

Physical insights on the low lattice thermal conductivity of AgInSe2

Uncovering the microscopic mechanism of low lattice thermal conductivity is essential for exploration and design of high-performance thermoelectrics. AgInSe2 exhibits high thermoelectric performance mainly due to its low thermal conductivity. Here, the origin of its intrinsic low lattice thermal conductivity is studied by temperature-dependent inelastic neutron scattering (INS), X-ray absorption fine structure (XAFS) spectra measurements, and first-principles calculations. A prominent “avoided crossing” feature and low-lying optical modes in the phonon dispersion of AgInSe2 are observed experimentally. These lattice dynamical features cause a local reduction of the phonon group velocity and strongly scatter heat-carrying acoustic phonons, contributing to its intrinsic low lattice thermal conductivity. In addition, both temperature-dependent phonon dispersions and phonon density-of-states measurements reveal strong anharmonicity or phonon-phonon interactions in AgInSe2. XAFS and phonon eigenvector analysis demonstrate the dominant role of Ag vibrations, which is closely associated with the “avoided crossing”, low-lying optical modes and large structural distortion, and thus dominates the reduction of lattice thermal conductivity of AgInSe2.

Influences of nanoparticle content on electrical and thermal performances of micro‐nano hybrid composites for packaging materials

AbstractThis paper presents electrical and thermal properties of a novel type of micro‐nano hybrid composites to be potential for packaging. The micro‐nano hybrid composites contain 20 wt% of micro‐aluminium nitride (AlN) with 1, 3, and 5 wt% of nano‐AlN, respectively. Electrical measurement and thermal analysis are used to analyze the performances. The results show that the nano‐AlN in the hybrid composites can suppress the space charge accumulation near the cathode and facilitate the conversion of hetero charge into the homo‐charge accumulations near the anode. No apparent electric field distortion can be observed in the M20N3 sample, whereas it exhibits a minimum electrical conductivity and highest thermal conductivity amongst all samples. Furthermore, the nano‐filler addition can enhance the DC breakdown strength and the thermal stability of the hybrid composites. The thermal and electrical conduction mechanism of the micro and nano fillers in the hybrid composites is also elucidated.

Analysis of enhancement mechanism for thermal conductivity of nanofluids by inelastic X-ray scattering

The inelastic X-ray scattering for the nanofluid composed of silicon dioxide (SiO2) nanoparticles (various sizes exceeding 100 nm) and ethylene glycol aqueous solution has revealed one of the mechanisms for the thermal conductivity enhancement of nanofluids, which the addition of a small amount of nanoparticles can improve the thermal conductivity of nanofluids beyond the theoretical value. The high-frequency sound velocity derived from the inelastic X-ray scattering spectra of nanofluids may be generally correlated with the structural relaxation of solvent molecules and corresponding thermal conductivity. It can also clarify the effect of nanoparticles on the thermal transport in nanofluids and the molecular dynamics of the solvent around nanoparticles. The size of the SiO2 particles in the nanofluid, which are 100, 300, 500, and 1000 nm, is approximately 10 times larger than the typical size of nanoparticles considered in research. Nevertheless, the experimental results indicate that SiO2 particles may increase the high-frequency sound velocity in the ethylene glycol aqueous solution because of the highly restricted movement of the solvent molecules around these nanoparticles. The high-frequency sound velocity in the SiO2 nanofluid is proportional to the thermal conductivity and that containing particles 300 nm in size is higher than those of other nanofluids containing particles 100, 500, and 1000 nm in size.

Simulation and flow physics of a shocked and reshocked high-energy-density mixing layer

Abstract

Carbon nano-onions as a nanofiller for enhancing thermal conductivity of epoxy composites

Carbon nano-onions (CNOs) are a relatively new member in zero-dimensional carbon nanomaterials. Each constituent unit of CNOs is a multi-layer concentric spherical structure composed of multiple fullerene-like carbon spheres. CNOs are widely used for electronic components applications due to their large specific surface area and high electric conductivity. But a few works of CNOs on thermal conduction property in electronic device applications were studied. Therefore, we prepared the CNOs/epoxy composites by blending and casting method, and investigated the thermal conduction mechanism of CNOs in the epoxy matrix. The results show that the thermal conductivity of epoxy composite with 30 wt% CNOs loading reaches 0.59 W m−1 K−1 due to the formation of an effective thermal network of CNOs in epoxy resin, increasing by 320% in comparison with neat epoxy. In view of excellent thermal conduction performance, CNOs may be enable used as thermal conductive filler for the thermal management of electronic components.

Atomic-level breakdown of Green\u2013Kubo relations provides new insight into the mechanisms of thermal conduction

Precise control of thermophysical properties of liquids through tailor-made design of the liquid molecular structure is a goal that, if achieved, could have significant positive impacts on machine design, performance and durability. In this work we show how the breakdown of the Green–Kubo relations down to the atomic level in molecular dynamics simulation can give useful insight into the mechanisms of thermal conduction. Using a group of five small alcohols as a case study, we demonstrate how combining this level of insight with differential-structure analysis reveals the competition for conduction between carbon and hydroxyl group atoms, and show how this competition contributes to the change in thermal conductivity observed in experiment. We hope that this method will become a useful tool in the quest for molecular-structure based thermal design.

Molecular Dynamics Simulations of the Effects of a Nanoparticle Surface Adsorption Layer on the Thermal Conductivity of a Cu–Ar Nanofluid

Nanofluids formed by adding a small volume fraction of solid nanoparticles to the conventional fluid can greatly enhance the thermal transport performance of the conventional fluid. This study reveals the mechanism of thermal conductivity by studying the microscopic effect of nanoparticles in a Cu–Ar nanofluid on the base fluid. Due to the presence of a trapped, adsorbed Ar layer on the surface of the nanoparticle, under certain volume fractions, the influence of the nanoparticles with different sizes on the thermal conductivity is analyzed. The equilibrium molecular dynamics and nonequilibrium molecular dynamics methods are used to calculate and verify the thermal conductivity of the nanofluid, and the abilities of the Lennard–Jones (LJ) and embedded atom method (EAM) potential functions to accurately describe interactions between Cu atoms are compared. By calculating the contribution of different components to the thermal conductivity of the nanofluid, it is found that the composition of the base liquid plays a leading role, while the nanoparticle composition and the solid–liquid cross-section contribute very little. Through decomposition of the Green–Kubo formula (contains potential energy term (V), the kinetic energy term (K), the collision term (V)), it is found that the PV, KV, and VV terms are related to collisions (V) when the LJ potential function is used, which play a major role in the nanofluid’s thermal conductivity. Partial enthalpy terms h, PP, and KK contribute little to the thermal conductivity, and they hinder any decrease of the thermal conductivity as the size of the nanoparticles increases; meanwhile, the KP term remains basically constant. In systems described by the EAM potential function, the contributions of KP and PP are relatively high. Therefore, this paper analyzes the mechanism of increasing the thermal conductivity of Ar-based nanofluid from the perspective of molecular dynamics simulation and obtains the results.

Nonlinear electrical behaviors and conductivity mechanisms in Ca1‐xYbxCeNbWO8 ceramics

AbstractIn this study, we investigated the nonlinear electrical behaviors and conductivity mechanisms of Ca1‐xYbxCeNbWO8 ceramics, which are prepared by the solid‐state sintering method. CeO2 and CeNbO4 phases occur when the Yb3+ doping amount is greater than 0.1 in the CaCeNbWO8‐based ceramics. With the continuous incorporation of Yb3+, the grain size of the ceramics first decreases and then increases. The Ce3+ concentration in the ceramics decreases with increasing Yb3+ content. In the temperature range of 298.15 to 1073.15K, the relationship between ln ρ and 1000/T shows nonlinear electrical behaviors, which is attributed to the different conductivity mechanisms at high and low temperatures (ρ and T are resistivity and temperature, respectively). In the temperature range of 298.15 to 773.15K, the electrical conduction in Ca1‐xYbxCeNbWO8 ceramics follows the Efros‐Shklovskii variable‐range hopping (ES‐VRH) conduction mechanism, thermal activation conduction (x < 0.2), and two‐dimensional Mott VRH conduction mechanism (x = 0.2). Furthermore, the electrical conduction obeys the thermal activation conduction mechanism in the temperature range of 773.15 to 1073.15K. The activation energy for the electrical conduction is calculated based on the different hopping conduction mechanisms.

Recent progress in thermally conductive polymer/boron nitride composites by constructing three-dimensional networks

As one of the critical components in thermal management system, high thermally conductive polymer composites is the key factor affecting the heat dissipation of electronic devices. During the past few decades, considerable researches have been done to enhance the thermal conductivity of polymer composites by adding fillers with high thermal conductivity, such as metals, carbon-based materials and ceramics. Boron nitride (BN) has been considered as an ideal thermally conductive filler for polymer composites. However, the large interfacial thermal resistance at the filler-matrix interfaces and the filler-filler interfaces has greatly hampered the heat conduction, resulting in the limited improvement in thermal conductivity. In this mini review, we attempt to present recent progress in fabricating high thermally conductive polymer/boron nitride composites by constructing three-dimensional networks. We started from introducing the thermal conduction mechanism in polymer composites, and then briefly discussed the factors influencing the thermal conductivity of polymer composites, followed by presenting different fabrication methods of constructing three-dimensional networks in polymer/boron nitride composites to increase the thermal conductivity. We hope this mini review would provide some beneficial inspiration for improving the thermal conductivity of polymer composites.

Enhanced the Thermal Conductivity of Polydimethylsiloxane via a Three-Dimensional Hybrid Boron Nitride@Silver Nanowires Thermal Network Filler.

In this work, polydimethylsiloxane (PDMS)-based composites with high thermal conductivity were fabricated via a three-dimensional hybrid boron nitride@silver nanowires (BN@AgNWs) filler thermal network, and their thermal conductivity was investigated. A new thermal conductive BN@AgNWs hybrid filler was prepared by an in situ growth method. Silver ions with the different concentrations were reduced, and AgNWs crystallized and grew on the surface of BN sheets. PDMS-based composites were fabricated by the BN@AgNWs hybrid filler added. SEM, XPS, and XRD were used to characterize the structure and morphology of BN@AgNWs hybrid fillers. The thermal conductivity performances of PDMS-based composites with different silver concentrates were investigated. The results showed that the thermal conductivity of PDMS-based composite filled with 20 vol% BN@15AgNWs hybrid filler is 0.914 W/(m·K), which is 5.05 times that of pure PDMS and 23% higher than the thermal conductivity of 20 vol% PDMS-based composite with BN filled. The enhanced thermal conductivity mechanism was provided based on the hybrid filler structure. This work offers a new way to design and fabricate the high thermal conductive hybrid filler for thermal management materials.

Preparation, Properties and Mechanisms of Carbon Fiber/Polymer Composites for Thermal Management Applications.

With the increasing integration and miniaturization of electronic devices, heat dissipation has become a major challenge. The traditional printed polymer circuit board can no longer meet the heat dissipation demands of microelectronic equipment. If the heat cannot be removed quickly and effectively, the efficiency of the devices will be decreased and their lifetime will be shortened. In addition, the development of the aerospace, automobiles, light emitting diode (LED{ TA \\1 “LED; lightemitting diode” \\s “LED” \\c 1 }) and energy harvesting and conversion has gradually increased the demand for low-density and high thermal conductive materials. In recent years, carbon fiber (CF{ TA \\1 “CF; carbon fiber” \\c 1 }) has been widely used for the preparation of polymer composites due to its good mechanical property and ultra-high thermal conductivity. CF materials easily form thermal conduction paths through polymer composites to improve the thermal conductivity. This paper describes the research progress, thermal conductivity mechanisms, preparation methods, factors influencing thermal conductivity and provides relevant suggestions for the development of CF composites for thermal management.

Microscopic mechanism of unusual lattice thermal transport in TlInTe2

We investigate the microscopic mechanism of ultralow lattice thermal conductivity (κl) of TlInTe2 and its weak temperature dependence using a unified theory of lattice heat transport, that considers contributions arising from the particle-like propagation as well as wave-like tunneling of phonons. While we use the Peierls–Boltzmann transport equation (PBTE) to calculate the particle-like contributions (κl(PBTE)), we explicitly calculate the off-diagonal (OD) components of the heat-flux operator within a first-principles density functional theory framework to determine the contributions (κl(OD)) arising from the wave-like tunneling of phonons. At each temperature, T, we anharmonically renormalize the phonon frequencies using the self-consistent phonon theory including quartic anharmonicity, and utilize them to calculate κl(PBTE) and κl(OD). With the combined inclusion of κl(PBTE), κl(OD), and additional grain-boundary scatterings, our calculations successfully reproduce the experimental results. Our analysis shows that large quartic anharmonicity of TlInTe2 (a) strongly hardens the low-energy phonon branches, (b) diminishes the three-phonon scattering processes at finite T, and (c) recovers the weaker than T−1 decay of the measured κl.

Hierarchical thermal transport mechanisms in Yb2Si2O7 multifunctional thermal and environmental barrier coating (TEBC) material

Understanding the mechanism of thermal conduction and its modulation strategy is a threshold task in the search of advanced multifunctional thermal and environmental barrier coating materials (TEBCs). We herein disclose the hierarchical features in the thermal transport of a typical TEBC material, Yb2Si2O7, using the state-of-the-art dual-phonon theory. Large amounts of vibrational modes transport heat in a diffusive feature, different from the propagative feature for normal phonons, and make significant contribution to the lattice thermal conductivity (κL) at high temperatures (T). This mechanism drives a weak κL~T dependence at high temperatures, deviating from the κL~T−1 trend predicted by the phonon Boltzmann transport equation (BTE) theory. The present study indicates that modulating the κL of complex-structure low-κL TEBC materials requires wisdoms beyond the BTE theory and considering the joint effects from different thermal transport channels.

Mechanisms for Thermal Conduction in Molten Salt-Based Nanofluid

Mechanisms for Thermal Conduction in Molten Salt-Based Nanofluid

Modification of the three-dimensional graphene aerogel self-assembled network using a titanate coupling agent and its thermal conductivity mechanism with epoxy composites.

Three-dimensional thermally conductive graphene aerogels have become more and more significant in practical thermal management applications. However, the interface between the graphene aerogel and the polymer has a strong interface thermal resistance, and the compatibility between the interfaces is also poor. In this study, a simple and versatile method for grafting graphene aerogels with titanate coupling agents on the surface was developed so that the modified graphene aerogels exhibit excellent thermal conductivity and mechanical properties and reduce the interface thermal resistance and increase the interface compatibility between graphene aerogels and epoxy resin. A high thermal conductivity of 2.53 W m-1 K-1 was obtained under a low graphene load of 2.5 wt%, corresponding to a thermal conductivity enhancement of approximately 1388% compared with pure epoxy resin. It provides a facile new idea for the preparation of high-quality three-dimensional graphene epoxy composites.

Thermal conductivity and electrical resistivity of single copper nanowires.

Copper nano-interconnects are ubiquitous in semiconductor devices. The electrical and thermal properties of copper nanowires (CuNWs) profoundly affect the performance of electronics. In contrast to the intensively studied electrical properties of CuNWs, the thermal conductivities of CuNWs have seldom been examined. In this study, the electrical resistivity and thermal conductivity of single CuNWs were investigated. The Bloch-Grüneisen formula was introduced to determine the mechanisms responsible for the obtained electrical resistivity of the CuNWs. High residual resistivity was found, which indicated strong structural scattering on the electron transport resulting from defect scattering and boundary scatterings at the copper-copper oxide interface and grain boundaries. The mean structural scattering distance was employed to appreciate the degree of structural scattering in the CuNWs. The residual resistivity and electron-phonon coupling parameter were found to increase with the degree of structural scattering. Moreover, the unified thermal resistivity was introduced to illustrate the mechanisms responsible for the CuNWs' thermal conductivities. Similarly, large values of residual unified thermal resistivity and electron-phonon-induced unified thermal resistivity were found. The obtained unified thermal resistivities of the CuNWs could also be qualitatively explained by the degree of structural scattering in the CuNWs. The results suggested that structural scattering was predominant in the electrical current transport and heat transfer in the nanowires. This study revealed the mechanisms of electrical resistivity and thermal conductivity of CuNWs, and the insights could assist in improving the design of semiconductor architectures.