Thermal Conduction Mechanism Research Articles

New insights into thermal conductivity of uniaxially stretched high density polyethylene films

Thermal conductivity of drawn polyethylene (PE) fibers has been extensively studied, while thermal conductivity for the stretched PE sheets still remains unknown. In addition, no consensus conclusions have been reached on the thermal conduction mechanisms for drawn PE. In this work, melt processed PE sheets are subjected to uniaxial stretching under different stretch ratios. The in-plane and out-of-plane thermal conductivity of stretched PE has been measured and the crystalline structure has been analyzed through 2-dimensional wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). Results show that both in-plane and out-of-plane thermal conductivities of stretched PE increase with increasing stretch ratios. The enhancement of thermal conductivity has been ascribed to the formation of interlocking shish-kebab superstructures induced by the stretching process. A new model is proposed to demonstrate the thermal conductivity improvement seen for the uniaxial-stretched PE. To the best of our knowledge, it is the first time that the mechanisms of thermal conductive enhancement were revealed for uniaxially stretched semi-crystalline PE sheet along with the understanding of the associated process-structure-property relationship. This work may provide key information towards design and manufacturing of polymer-based materials for thermal management applications.

Principles and Mechanisms of Strain-Dependent Thermal Conductivity of Polycrystalline Graphene with Varying Grain Sizes and Surface Hydrogenation

In this paper, the thermal conductivities (κ) of polycrystalline graphene (PG) with varying average grain size are investigated using reverse nonequilibrium molecular dynamics method. Due to the pr...

Full substitution of petroleum-based polyols by phosphorus-containing soy-based polyols for fabricating highly flame-retardant polyisocyanurate foams

To obtain a substitute for petroleum-based polyols and simultaneously solve the flammability problem of polyisocyanurate (PIR) foams, a novel phosphorus-containing soy-based polyol (PCSO) was firstly synthesized. Then, PCSO was used to fabricate biomass flame-retardant PIR foam (P-PIR) by fully replacing petroleum-based polyol. The morphology, compressive strength, thermal conductivity, thermal stability, flammability, and flame-retardant mechanism were studied for the developed PIR. The incorporation of PCSO results in a slight decrease in the compressive strength and an increase in the flame retardancy of PIR foam. To further improve the flame retardancy, a commercial phosphorus-containing polyol (BY30) and an expandable graphite (EG) were added in the P-PIR system. The test results show that the decompositon of PIR was hindered after 330 °C and the char residue was significantly improved. In addtion, the limiting oxygen index of PIR was significantly increased to 35%, and the peak of heat release rate and total heat release were dramatically decreased by 65% and 87%, respectively. A synergistic flame-retardant effect was achieved by combining the gas phase action of PCSO and BY30 with the condensed phase action of EG.

First-principles study of lattice thermal conductivity in ZrTe5 and HfTe5

Recently, the layered transition-metal pentatellurides ZrTe5 and HfTe5 have attracted increasing attention because of their interesting topological electronic properties. Nevertheless, some of their other good physical properties seem to be ignored now. Actually, both ZrTe5 and HfTe5 have high electric conductivities (>105 Ω−1 m−1) and Seebeck coefficients (> 100 μV/K) at room temperature, thus making them promising thermoelectric materials. However, the disadvantage is that the thermal conductivities of the two materials are relatively high according to the few available experiments; meanwhile, the detailed mechanism of the intrinsic thermal conductivity has not been studied yet. Based on the density functional theory and the Boltzmann transport theory, we present here the theoretical study of the intrinsic lattice thermal conductivities of ZrTe5 and HfTe5, which are found to be in the range of 5–8 W/m⋅K at room temperature and well consistent with the experimental results. We also find that the thermal conductivities of the two materials are anisotropic, which are mainly caused by their anisotropic crystal structures. Based on the detailed analysis, we proposed that the thermal conductivities of the two materials could possibly be reduced by different kinds of structural engineering at the atomic and mesoscopic scales, such as alloying, doping, nano-structuring, and polycrystalline structuring, which could make ZrTe5 and HfTe5 good thermoelectric materials for room temperature thermoelectric applications.

Influence of ZrO2 alloying effect on the thermophysical properties of fluorite-type Eu3TaO7 ceramics

ZrO2 alloying effect is applied in fluorite-type Eu3TaO7 ceramics to decrease thermal conductivity and improve thermal expansion performance. The thermal conduction mechanism of Eu3TaO7 ceramics can be adjusted by ZrO2 alloying effect and the lowest thermal conductivity reaches 1.37 W·m−1·K−1 at 900 °C. The Young's modulus of x mol% ZrO2-Eu3TaO7 ceramics decreases with the increasing ZrO2 content, as ZrO2 alloying effect can weaken the bonding strength of Eu3TaO7 and then increase the thermal expansion coefficients (10.7 × 10−6 K−1, 1200 °C). Due to the excellent thermophysical properties, ZrO2-Eu3TaO7 ceramics exhibit great potentiality as high-temperature thermal insulation materials.

Intriguingly high thermal conductivity increment for CuO nanowires contained nanofluids with low viscosity

Nanofluids offer the exciting new possibilities to enhance heat transfer performance. In this paper, experimental and theoretical investigations have been conducted to determine the effect of CuO nanowires on the thermal conductivity and viscosity of dimethicone based nanofluids. The CuO nanowires were prepared through a thermal oxidation method, and the analysis indicated that the as-prepared CuO nanowires had high purity, monocrystalline with a monoclinic structure and large aspect ratio compared to CuO nanospheres. The experimental data show that the thermal conductivity of the nanofluids increases with the volume fraction of CuO nanowires or nanospheres, with a nearly linear relationship. For the nanofluid with the addition of 0.75 vol.% CuO nanowires, the thermal conductivity enhancement is up to 60.78%, which is much higher than that with spherical CuO nanoparticles. The nanofluids exhibit typical Newtonian behavior, and the measured viscosity of CuO nanowires contained nanofluids were found only 6.41% increment at the volume fraction of 0.75%. It is attractive in enhanced heat transfer for application. The thermal conductivity and viscosity of CuO nanofluids were further calculated and discussed by comparing our experimental results with the classic theoretical models. The mechanisms of thermal conductivity and viscosity about nanofluids were also discussed in detail.

Pore-size effects on thermal conductivity of SiO2 quartz using non-equilibrium molecular dynamics simulations

Porous materials are commonly used to decrease or regulate thermal transport in insulating materials due to the very low thermal conductivity of the pores. More pores can achieve a better insulating effect; however, this can also be a shortcoming when the materials are directly involved in physical movement with friction, such as an interior wall of a cylinder in an automobile engine intended to prevent heat loss. Thus, it is important to clarify the mechanisms of the pore-dependent thermal properties of porous materials. In this study, the effects of the number and size of pores in silica quartz on thermal conductivity were investigated using nonequilibrium molecular dynamics simulations. The results revealed that the thermal conductivity of porous silica quartz decreased in all cases with an increase in the volume fraction of pores. A lower number of pores in the system showed higher thermal conductivity in the range of low volume fractions. We also showed that the interstitial distance between pores mostly governs thermal conduction at low volume fractions [Formula: see text] vol.%. Additionally, the mechanism of thermal conduction was assessed with respect to several theoretical models.

Dimensional crossover of heat conduction in amorphous polyimide nanofibers

ABSTRACTThe mechanism of thermal conductivity in amorphous polymers, especially polymer fibers, is unclear in comparison with that in inorganic materials. Here, we report the observation of a crossover of heat conduction behavior from three dimensions to quasi-one dimension in polyimide nanofibers at a given temperature. A theoretical model based on the random walk theory has been proposed to quantitatively describe the interplay between the inter-chain hopping and the intra-chain hopping in nanofibers. This model explains well the diameter dependence of thermal conductivity and also speculates on the upper limit of thermal conductivity of amorphous polymers in the quasi-1D limit.

An investigation into application of the Green-Kubo method in molecular simulation to help understand the mechanisms of thermal conductivity of alcohols.

An investigation into application of the Green-Kubo method in molecular simulation to help understand the mechanisms of thermal conductivity of alcohols.

Electrical and Thermal Conductivity and Conduction Mechanism of Ge2Sb2Te5 Alloy

Ge2Sb2Te5 alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge2Sb2Te5 alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge2Sb2Te5 alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge2Sb2Te5 alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge2Sb2Te5 alloy accounts for the thermal conduction mechanism.

Effect of nanoparticle dispersion and inclination angle on melting of PCM in a shell and tube heat exchanger

Abstract This paper presents an experimental numerical study on melting behavior of a phase change material in a shell and tube heat exchanger. Thus the effects of parameters including adding Nano-Enhanced Phase Change Material (NEPCM) and inclination angle are considered and investigated. Experimental studies are designed to characterize PCM/NEPCM properties. The details of the process progression are presented via temperature contours, streamlines and phase front displacement. Enthalpy porosity method is used for modeling of the phase change process in which pure conduction and natural convection are considered in the simulation. Results reveal that thermal conduction mechanism is dominated at initial and final steps of melting that slows down the process. As the process advances, the buoyancy effect becomes dominant in the melting process. By increasing mass fractions of nanoparticle, thermal conductivity coefficient increases that leads to faster melting rate at the bottom of the heat exchanger. Also results indicate that increasing inclination angle leads to faster pace of melting at higher attitudes that results in shorter total melting time. Contemporaneous adding nanoparticle and increasing inclination angle reveals that both natural convection and thermal conductivity improves that leads to better melting of NEPCM.

Enhanced anti‐deliquescent property and ultralow thermal conductivity of magnetoplumbite‐type LnMeAl11O19 materials for thermal barrier coating

AbstractMagnetoplumbite‐type LaMgAl11O19 ceramic has been proposed as one of promising candidates for the next generation thermal barrier coatings (TBCs) due to its low thermal conductivity. However, LaMgAl11O19 shows poor water‐resistance with significant weight loss at elevated temperatures in water‐containing atmosphere. In this work, we revealed that the essential reason for the poor water‐resistance of magnetoplumbite‐type LaMgAl11O19 ceramic is Mg2+ migration from the intrinsic site under moisture environment. And then an effective approach was proposed to improve its anti‐deliquescent property by completely substituting divalent alkaline earth ions Mg2+ with Zn2+. Finally, a panoscopic strategy was proposed to further lower thermal conductivity through co‐substituting La and Zn sites in LaZnAl11O19 with trivalent and divalent transition metal ions. The mechanism for the lowered thermal conductivity is due to the panoscopic approach, which providing all‐scale hierarchical architectures of phonon scattering mechanisms. The excellent anti‐moisture performance and ultralow thermal conductivity endow the LaZnAl11O19 based ceramics as a kind of promising candidates for advanced thermal barrier coatings.

Temperature dependent thermal conductivity and transition mechanism in amorphous and crystalline Sb2Te3 thin films

Sb2Te3 thin films are widely used in high density optical and electronic storage, high-resolution greyscale image recording, and laser thermal lithography. Thermal conductivity and its temperature dependence are critical factors that affect the application performance of thin films. This work aims to evaluate the temperature dependence of thermal conductivity of crystalline and amorphous Sb2Te3 thin films experimentally and theoretically, and explores into the corresponding mechanism of heat transport. For crystalline Sb2Te3 thin films, the thermal conductivity was found to be 0.35 ± 0.035 W m−1 K−1 and showed weak temperature dependence. The thermal conductivity of amorphous Sb2Te3 thin films at temperatures below ~450 K is about 0.23 ± 0.023 W m−1K−1, mainly arising from the lattice as the electronic contribution is negligible; at temperatures above 450 K, the thermal conductivity experiences an abrupt increase owing to the structural change from amorphous to crystalline state. The work can provide an important guide and reference to the real applications of Sb2Te3 thin films.

Understanding Stability, Measurements, and Mechanisms of Thermal Conductivity of Nanofluids

Understanding Stability, Measurements, and Mechanisms of Thermal Conductivity of Nanofluids

Thermal Conductivity of Graphene-Polymer Composites: Mechanisms, Properties, and Applications.

With the integration and miniaturization of electronic devices, thermal management has become a crucial issue that strongly affects their performance, reliability, and lifetime. One of the current interests in polymer-based composites is thermal conductive composites that dissipate the thermal energy produced by electronic, optoelectronic, and photonic devices and systems. Ultrahigh thermal conductivity makes graphene the most promising filler for thermal conductive composites. This article reviews the mechanisms of thermal conduction, the recent advances, and the influencing factors on graphene-polymer composites (GPC). In the end, we also discuss the applications of GPC in thermal engineering. This article summarizes the research on graphene-polymer thermal conductive composites in recent years and provides guidance on the preparation of composites with high thermal conductivity.

Epoxy thermoset resins with high pristine thermal conductivity

Heat dissipation becomes a critical problem because of the miniaturisation and the increase of power density in electronic devices and electric equipment, which calls for electrical insulating materials with high thermal management capability. Epoxy thermosets have been widely used as electrical insulating materials, but suffer from their low thermal conductivity. This study reviewed the research progress on the development of epoxy thermosets with high pristine thermal conductivity. First, the thermal conduction mechanism of polymers was briefly introduced. Second, the approaches used to enhance the thermal conductivity of epoxy thermosets were summarised, which mainly dealt with the formation of microscopically anisotropic but macroscopically isotropic structure in the epoxy thermosets. Third, the applications of high thermal conductivity epoxy thermoset resins were reviewed. Finally, the review provided the existing challenges and the future directions for the development of epoxy thermosets with high pristine thermal conductivity.

Thermal conductivity of penta-graphene: The role of chemical functionalization

Penta-graphene (PG) has recently been identified as a novel two-dimensional carbon allotrope comprising entirely of pure pentagons and possessing many unique properties beyond graphene. Herein we investigate the thermal conductivity of hydrogenated and oxidized PGs at different levels of coverage by using non-equilibrium molecular dynamics simulations. Our simulation results show that the oxidized PG possesses a higher thermal conductivity than the hydrogenated counterpart at the same coverage. Interestingly, both functionalized PGs exhibit an unusual down-and-up trend in the thermal conductivity with the increase of coverage level, and the fully functionalized PGs (at 100% coverage) possess almost the same thermal conductivity as the pristine PG. We further explore the underlying mechanisms for the coverage-dependent thermal conduction. Our work provides important insights into the effect of chemical functionalization on the thermal conductivity of this new carbon allotrope.

Synthesis of various morphological Ca0.75Er0.25MnO3 powders by hydrothermal method and their electrical properties

The Ca0.75Er0.25MnO3 powders with better conductive properties and various morphologies have been prepared by using hydrothermal method with different concentrations of KOH and polyethylene glycol 400 (PEG-400). The resistivity of Ca0.75Er0.25MnO3 powders was decreased gradually with increase in KOH concentration. When it was 1000 g L−1, the resistivity obtained the smallest value and in this case, the electrical conduction mechanism was followed thermal activation conduction mechanism. Furthermore, the effect of PEG-400 concentration on the morphology and the mechanism for forming different morphologies were elaborated. When PEG-400 concentration was 1 g L−1, the particles were presented cube structure. And when it was 2 g L−1, the particles were presented rod structure. While the concentration was more than 2 g L−1, the powders were all presented planchet structure, but the particles were enlarged with increase in PEG-400. The resistivity of the powders with the rod structure reached the smallest value of 0.2705 kΩ m, because the rod structure is conducive to the formation of the conductivity pathway.

Molecular dynamics simulation of decomposition and thermal conductivity of methane hydrate in porous media**Project supported by the National Natural Science Foundation of Special Fund and Chinese Academy of Engineering (Grant No. L1322021).

The hydrate has characteristics of low thermal conductivity and temperature sensitivity. To further analysis the mechanism of thermal conductivity and provide method for the exploitation, transportation and utilization of hydrate, the effect of decomposition and thermal conductivity of methane hydrate in porous media has been studied by using the molecular dynamics simulation. In this study, the simulation is carried out under the condition of temperature 253.15 K–273.15 K and pressure 1 MPa. The results show that the thermal conductivity of methane hydrate increases with the increase of temperature and has a faster growth near freezing. With the addition of porous media, the thermal conductivity of the methane hydrate improves significantly. The methane hydrate-porous media system also has the characteristics of vitreous body. With the decrease of the pore size of the porous media, thermal conductivity of the system increases gradually at the same temperature. It can be ascertained that the porous media of different pore sizes have strengthened the role of the thermal conductivity of hydrates.

An evaluation of the melt crystallisation behaviour of injection-moulded high-density polyethylene (HDPE) based on a solidification kinetics analysis

ABSTRACTDynamical rheology, transient temperature measurement, and two-dimensional wide angle X-ray diffraction were performed in this work to investigate the effects of the solidification behaviour and crystallisation kinetics on the gradient structure formation of the injection-moulded high-density polyethylene (HDPE) samples across the thickness. The temperature profiles obtained via enthalpy transformation method were used to elucidate the effect of cooling rates on the solidification kinetics during the injection moulding process. A four-parameter model was proposed for the first time, based on the ‘three-parameter model’ of our previous research, and the former one is shown to have a better fitting effect. Heat transfer at locations χ ≤ 0.5 from the mould surface was found to be primarily dominated by a thermal conduction mechanism, unlike the situation when χ > 0.5. The degree of orientation of the HDPE samples was closely associated with the shear distributions and temperature fields.