Thermal Conductivity Of System Research Articles

Efficacy of dispersing nano-powders on rate of cold storage during solidification considering transient conduction

This study conducts a simulation of the freezing through a container featuring finned sinusoidal geometry. The application of the Galerkin method effectively captures the intricate dynamics of unsteady processes, showcasing a significant enhancement by introducing nanoparticles to augment the system's thermal conductivity—an essential element for optimizing conduction efficiency. The numerical approach underscores the importance of employing an adaptive grid in unsteady scenarios, tailoring the grid based on the solid front's position due to heightened scalar gradients. Various powder diameters (dp) and fractions (φ) are systematically investigated, elucidating their distinct impacts on process duration. Results reveal that an initial increase in dp causes to a decrement in period by 19.93%, followed by a subsequent rise of 49.06%, specifically for scenarios with a 0.04 nanoparticle concentration. The incorporation of nanoparticles yields a noteworthy 41.13% decrease in solidification time when optimal powder sizes are utilized. In contrast, the solidification time without additives is 278.95s, reduced to 164.19s with the introduction of optimally sized powders.

Enhanced dielectric and thermal properties of SrTiO3/polyolefin composite substrate with dopamine-modified BNNS for microwave applications

The performance of microwave devices is significantly influenced by the substrates employed. In this study, we introduce a novel microwave composite substrate material exhibiting high dielectric constant and exceptional thermal conductivity, comprising dopamine-modified boron nitride nanosheets (BNNS@PDA) and SrTiO3/polyolefin. With an 8 wt% BNNS@PDA filler content, the composite material achieves a high dielectric constant of 17.7 (@10 GHz) and remarkable thermal conductivity of 1.604 W/(m·K), representing enhancements of 34% and 104% compared to SrTiO3/polyolefin, respectively. Additionally, a low dielectric loss of 2.95 × 10−3 and minimal water absorption of 0.15% are attained. The incorporation of BNNS@PDA augments the compatibility between the ceramic particles and resin matrix, contributing to increased dielectric constant, superior density and improved mechanical properties of composite materials. As an interval within the thermal conductive pathway, the PDA coating diminishes the system's thermal conductivity. And its interplay with polymer matrix influences the resin's curing process, subtly reducing the composite's thermal stability. In summary, BNNS@PDA/SrTiO3/polyolefin represents a promising microwave dielectric substrate material, offering high dielectric constant and excellent thermal conductivity.

Thermally conductive process and mechanism of TATB‐based polymer‐bonded explosives filled with graphene: Effects of interfacial thermal resistance and morphology/content of graphene

AbstractThe thermal conductivity of energy‐containing materials has a significant impact on their environmental suitability during manufacturing, transportation, and storage. In order to effectively regulate the thermal conductivity of polymer‐bonded explosives (PBXs), this study investigates the effects of different aspect ratios of graphene binder and interfacial thermal resistance on the thermal conductivity of TATB‐based PBXs using the finite element method. A model was constructed using Abaqus software to establish the constitutive relationship between graphene with varying aspect ratios and the thermal conductivity of PBXs. The results indicate that the effect of different graphene aspect ratios on the overall thermal conductivity of PBXs is small when the graphene content is lower than 3%. However, the thermal conductivity of PBXs increases nonlinearly with the increase in graphene aspect ratio when the graphene content is higher than 8%. Furthermore, the interfacial thermal resistance reduces the thermal conductivity of the PBX system by approximately 0.16 W/(m K), and its effect on the thermal conductivity of the system is relatively stable and does not change significantly with increasing graphene content and aspect ratio.Highlights Thermally conductive behavior of graphene‐filled PBX is revealed by simulation. The effects of content and aspect ratio of graphene are systematically investigated. The effect of interface thermal resistance is considered in the simulation.

Role of dual doping in zinc oxide for optimizing thermoelectric performance

In this study, we have investigated the effect of dual doping on the thermoelectric performance of zinc oxide (ZnO). Pure ZnO is a wide band-gap semiconductor with a very low carrier concentration in its intrinsic form. Doping ZnO by trivalent impurity ions is considered a viable approach to improve its electrical conductivity by enhancing the concentration of charge carriers in the material. However, the above enhancement in the carrier concentration can also increase the electronic component of the system's thermal conductivity, thereby limiting its thermoelectric performance. It is expected that the dual doping of ZnO can decrease its thermal conductivity without adversely affecting its electrical conductivity. To test the above hypothesis, we doped ZnO with four different pairs of dopants (In/B, In/Al, In/Ga, and In/Bi) and compared the thermoelectric performance of the resulting materials over the temperature range of 350 K–773 K. For the above comparison, Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured, and corresponding power factors (PF) and figure of merits (ZT) were determined. Among all the samples investigated in the study, the In/Bi-doped ZnO exhibited the highest ZT value of 0.37 at 773 K.

Modern solar systems driven by nanoparticles-based fatty acids and paraffin wax phase change materials

Integrating a suitable thermal energy depository device is extremely necessary to enhance the performance of solar thermal energy systems by a reduction in the discrepancies between the supply and demand of electricity. The most effective and potential system for the conservation of solar thermal energy is the dependence on phase change materials (PCMs). Here, we focus on the preparation and characterization of nanoparticle (NP)-reinforced PCM-based composites. Fatty acids and paraffin wax PCM, reinforced with different NPs, are extensively discussed in this review. To describe the composition, morphology, and thermal characters of the arranged NPs and the composite PCM, scanning electron microscopy (SEM) and Fourier transmission infrared (FTIR) spectroscopy are employed. To define the chemical structure, chemical density, component size, and internal structure of composites and chemical characteristics, EDX/XPS is performed on a component PCM, respectively. Graphene incorporation improves the whole system's thermal conductivity. Both used in the TGA and DTG study were the thermal decomposition details and chemical absorption of the polymer PCM. This study, therefore, focuses on PCMs, their methods for the processing of the materials, materials, and thermal characterization during the last five years.

Transferability of neural network potentials for varying stoichiometry: Phonons and thermal conductivity of MnxGey compounds

Germanium manganese compounds exhibit a variety of stable and metastable phases with different stoichiometries. These materials entail interesting electronic, magnetic, and thermal properties both in their bulk form and as heterostructures. Here, we develop and validate a transferable machine learning potential, based on the high-dimensional neural network formalism, to enable the study of MnxGey materials over a wide range of compositions. We show that a neural network potential fitted on a minimal training set reproduces successfully the structural and vibrational properties and the thermal conductivity of systems with different local chemical environments, and it can be used to predict phononic effects in nanoscale heterostructures.

Thermal conductivity of systems with a gap in the phonon spectrum**Project supported by the Ural Branch of the Russian Academy of Sciences, Russia (Grant No. 18-2-2-12), the Russian Foundation for Basic Research, Russia (Grant Nos. 16-07-00529 and 18-07-00191), and the Financing Program, Russia (Grant No. AAAA-A16-116021010082-8).

An original theoretical model for describing the low-temperature thermal conductivity in systems with a region of forbidden values (a gap) in the phonon spectrum is proposed. The model is based on new experimental results on the temperature dependence of the phonon diffusion coefficient in nanoceramics and dielectric glasses which showed a similar anomalous behavior of the diffusion coefficient in these systems that may be described under the assumption of a gap in the phonon spectrum. In this paper, the role of the gap in low-temperature behavior of the thermal conductivity, κ(T), is analyzed. The plateau in the temperature dependence of the thermal conductivity is shown to correlate with the position and the width of the gap. The temperature dependence of thermal conductivity of such systems when changing the scattering parameters related to various mechanisms is studied. It is found that the umklapp process (U-processes) involving low-frequency short-wavelength phonons below the gap forms the behavior of the temperature dependence of thermal conductivity in the plateau region. A comparison of the calculated and experimental results shows considerable possibilities of the model in describing the low-temperature thermal conductivity in glass-like systems.

Thermal conductivity of nano-layered systems due to surface phonon-polaritons

The effective thermal conductivity of a layered system due to the propagation of surface phonon-polaritons is studied. We analytically demonstrate that the thermal conductivity of a set of nanolayers can be described as one of a single layer with an effective permittivity, which does not ordinarily appear in nature and depends on the permittivities and thicknesses of the individual components. For a two-layer system of SiO2 and BaF2 surrounded by air, it is shown that: (i) the propagation length of surfaces phonon-polaritons can be as high as 3.3 cm for a 200 nm-thick system. (ii) The thermal conductivity of the system with total thickness of 50 nm is 3.4 W/m·K, which is twice that of a single layer of SiO2, at 500 K. Higher values are found for higher temperatures and thinner layers. The results show that an ensemble of layers provides more channels than a single layer for the propagation of surface phonon-polaritons and therefore for the enhancement of the thermal conductivity of common polar materials.

The effect of the electron-phonon coupling on the effective thermal conductivity of metal-nonmetal multilayers

The metal-nonmetal interface plays a critical role in modern electronic and energy conversion devices. For example, metal-nonmetal multilayered structures have recently been proposed as promising materials for solid-state thermionic devices, which could potentially achieve an efficiency that might not be feasible for metals or semiconductors alone. In this work, the effective thermal conductivity of a metal-nonmetal multilayered system (superlattices) is studied using the two-temperature model of heat conduction. By defining the total interfacial thermal resistance, which strongly depends on the electron-phonon coupling factor, it is shown that the thermal conductivity of the system has a simple interpretation as the sum of thermal resistances in series. The role of the electron-phonon coupling and the phonon-phonon interfacial thermal resistance on the total interfacial thermal resistance is discussed. The derived analytical expressions show that the effective thermal conductivity of the multilayered structure is, remarkably, determined by the ratio between the thickness of the metal layers and their intrinsic electron-phonon coupling length. It is demonstrated that the effective thermal conductivity of the metal-nonmetal system can be smaller, equal to, or larger than the thermal conductivity of the nonmetal layer, depending on the length of the metal layer.

Modeling of influence of lithium vacancy on thermal conductivity in lithium aluminate

The effects of temperature and Li-vacancy concentration on the thermal conductivity of LiAlO 2 were studied using molecular dynamics simulation. The simulation results well reproduced the reported experimental observation that thermal conductivity decreases as temperature increases. However, the calculated values displayed some discrepancies from the experimental values obtained at high temperatures. Regarding the effect of Li vacancy, the thermal conductivities of systems containing 0.39 and 0.78 at.% of Li vacancies were evaluated. The thermal conductivity decreased a little as the Li-vacancy concentration increased.

Equilibrium calculations of viscosity and thermal conductivity across a solid-liquid interface using boundary fluctuations

We calculate viscosity and thermal conductivity in systems of Lennard-Jones particles consisting of coexisting solid and liquid with different interface wetting properties using the recently developed equilibrium boundary fluctuation theory. We compare the slip length and equivalent liquid length obtained from these calculations with those obtained from nonequilibrium molecular dynamics. The equilibrium and nonequilibrium calculations of the slip length and the sum of the thermal equivalent lengths are in good agreement. We conclude that for both interfacial properties, the nonequilibrium simulations were probing the linear response. The significant dependence of the intrinsic equivalence length on the interfacial temperature difference used to generate the thermal gradient is explained as a consequence of the different thermodynamic states of the two interfaces.

A new correlation for thermal conductivity of liquids

On the supposition that liquid is composed of a mixture of microcrystallites and vacancies, and on the basis of significant structure theory and analysis of the radial distribution function of liquids, a new two-parameter model is presented for correlating thermal conductivities of liquids and liquid mixtures. The thermal conductivities of inorganic compounds, paraffins, olefins, cycloparaffins, aromatic hydrocarbons, alcohols, ethers, aldehydes, ketones, esters, organic acids and some refrigerants over wide ranges of temperature are calculated using the new model and compared with data reported in the literature. The method is extended to mixtures by using simple mixing rules. Calculated results show that the new model can be used for the correlation of the thermal conductivities of systems consisting of nonpolar and polar compounds and also for those of aqueous solutions. Especially, for pure compounds such as inorganic compounds, nitrogen- or sulfur-containing materials and aldehydes, and for mixtures consisting of alcohols and aqueous systems, the proposed correlation shows better agreement with experimental data than the models recommended by Reid et al. (1987, The Properties of Gases and Liquids, 4th Edition. McGraw-Hill, New York).