Changes In Thermal Conductivity Research Articles

Heat dissipation performance research of battery modules based on composite phase change materials cooling and electrochemical thermal coupling model

In order to improve electric vehicle lithium-ion batteries (LIBs) performance and increase cycle life, it is crucial to design a reliable and efficient battery thermal management system (BTMS). Compared with other active battery cooling methods, passive phase change material (PCM) cooling has the advantages of simple structure, good temperature control uniformity, strong reliability, and no additional energy consumption. This article uses a series of experimental procedures to add expanded graphite to traditional paraffin wax PCM and obtain composite phase change materials (CPCM) with different mixing ratios. A control experiment is created to compare the cooling performance of the CPCM cooling with natural convection cooling, research shows that the cooling performance of CPCM can well meet the cooling demand which maximum temperature and maximum temperature difference of battery modules. In addition, to study the performance of thermal properties parameters related to CPCM, a COMSOL numerical calculation model of battery module based on CPCM cooling and electrochemical thermal coupling model is established. The effectiveness and accuracy of the model are verified through heat generation experiments at different rates (0.5 C, 1 C, 1.5 C), with a maximum temperature error of 1.6 K and an error accuracy of less than 0.6%. Subsequently, the performance of phase change temperature, thermal conductivity and phase change latent heat is studied and reasonable value ranges are obtained, which are 312 K∼318 K, 2.5 W /(m·K), and 175 J/g∼200 J/g, respectively.

Controllable thermal rectification design for buildings based on phase change composites

Phase-change material (PCM) is widely used in thermal management due to their unique thermal behavior. However, related research in thermal rectifier is mainly focused on exploring the principles at the fundamental device level, which results in a gap to real applications. Here, we propose a controllable thermal rectification design towards building applications through the direct adhesion of composite thermal rectification material (TRM) based on PCM and reduced graphene oxide (rGO) aerogel to ordinary concrete walls (CWs). The design is evaluated in detail by combining experiments and finite element analysis. It is found that, TRM can regulate the temperature difference on both sides of the TRM/CWs system by thermal rectification. The difference in two directions reaches to 13.8 K at the heat flow of 80 W/m2. In addition, the larger the change of thermal conductivity before and after phase change of TRM is, the more effective it is for regulating temperature difference in two directions. The stated technology has a wide range of applications for the thermal energy control in buildings with specific temperature requirements.

Glass Transition Temperatures and Thermal Conductivities of Polybutadiene Crosslinked with Randomly Distributed Sulfur Chains Using Molecular Dynamic Simulation.

The thermal conductivities and glass transition temperatures of polybutadiene crosslinked with randomly distributed sulfur chains having different lengths from mono-sulfur (S1) to octa-sulfur (S8) were investigated. The thermal conductivities of the related models as a function of the heat flux autocorrelation function, applying an equilibrium molecular dynamic (EMD) simulation and the Green-Kubo method, were studied for a wide range of temperatures. The influence of the length of sulfur chains, degree of crosslinking, and molar mass of the crosslinker on the glass transition temperature and final values of thermal conductivities were studied. First, the degree of crosslinking is considered constant for the eight simulation models, from mono-sulfur (S1) to octa-sulfur (S8), while the molar mass of the sulfur is increases. The results show that the thermal conductivities of the crosslinked structure decrease with increasing temperature for each model. Moreover, by increasing the lengths of the sulfur chains and the molar weight of the crosslinker, thermal conductivity increases at a constant temperature. The MD simulation demonstrates that the glass transition temperature and density of the crosslinked structure enhance as the length of the sulfur chains and molar mass of the sulfur increase. Second, the molar weight of sulfur is considered constant in these eight models; therefore, the degree of crosslinking decreases with the increase in the lengths of the sulfur chains. The results show that the thermal conductivities of the crosslinked structure decrease with the increase in the temperature for each model. Moreover, by increasing the lengths of sulfur chains and thus decreasing the degree of crosslinking, the trend in changes in thermal conductivities are almost the same for all of these models, so thermal conductivity is constant for a specific temperature. In addition, the glass transition temperature and density of the crosslinked structure decrease.

Improving Dynamic Thermal Characteristics of Power Transformers Using an Earth-Oil Heat Exchanger

A significant aspect affecting the power transformer’s performance and lifetime is the maximum temperature within the machine. This research estimates the top-oil temperature (TOT), bottom-oil temperature (BOT), and hot-spot temperature (HST) for a 2500 kVA oil natural air natural (ONAN) distribution transformer with or without an earth-oil heat exchanger (EOHE). The differential equations of the thermal models are solved using the Runge-Kutta fourth-order method. MATLAB code is used to simulate the created models. The present model takes into account how oil properties such as dynamic viscosity, thermal conductivity, and density change with temperature. The IEEE model is used for model validation. The estimated findings are of high accuracy compared with the others. Results show that the maximum reductions in the HST, TOT, and BOT due to EOHE are 48.82%, 41.93%, and 29.59%, respectively. The use of EOHE decreases the temperature of the transformer oil as well as increases the transformer’s lifetime.

Analysis of mixed convective stagnation point flow of hybrid nanofluid over sheet with variable thermal conductivity and slip Conditions: A Model-Based study

This research looks at the stagnation point flow of MHD Al2O3-Cu/H2O hybrid nanofluid against a permeable, vertically extending surface that uses thermal radiation and how different models of thermal conductivity affect it. This study is unique because it looks at how changes in thermal conductivity, mixed convection, and the slip velocity of hybrid nanofluids affect the stretching surface. It also looks at how changes in temperature, skin-friction coefficient, Nusselt number, and velocity affect convective thermal boundary conditions. With the use of boundary layer approximations, the complicated system of PDEs is simplified. The dimensionality of these PDEs and the boundary conditions they include are eliminated by applying certain modifications. Combining a local non-similarity approach up to the second truncation level with MATLAB's built-in finite difference code (bvp4c), one can obtain the results of the updated model. The research demonstrates and analyzes the impact of different factors on fluid flow and heat transfer characteristics in the studied flow situations. This is done by comparing the computed data with available literature and presenting the findings in graphical form. Tables are generated to present the numerical fluctuations of the drag coefficient and Nusselt number. This study shows how important thermal conductivity is in the mixed convection of hybrid nanofluids and looks into what happens when the thermal conductivity parameter is changed. Significantly, an augmentation in this parameter results in an elevation in the temperature distribution of the hybrid nanofluid while simultaneously reducing the rate of heat transfer across various models. Furthermore, increasing the nanoparticle volume fraction parameter leads to higher temperature and Nusselt number profiles while reducing skin friction. The mixed convection parameter has a notable impact on increasing the friction coefficient on the stretched vertical surface. However, because these elements function as regulating variables, it decreases when the magnetic, mass suction, and velocity slip parameters are present. The results also demonstrate notable discrepancies in the mean Nusselt values generated by various thermal conductivity models. According to the analysis, the Hamilton-Crosser model has the lowest average Nusselt numbers, followed by the Yamada-Ota model and the Xue model, in that order.

Enhanced thermoelectric performance of graphene at room temperature by cut-off phonon frequency

Based on the full electron-phonon interactions and temperature-induced phonon renormalization, both electrical transport and thermal transport of graphene are systematically studied under different cut-off phonon frequencies. When the electron concentration is less than 1013 cm−2, the electrical transport properties increase with the cut-off phonon frequency. Otherwise, the electrical transport properties are nearly the same between 1013 cm−2 and 1015 cm−2, indicating that the contribution of cut-off phonon frequency decreases with the electron concentration. Considering the three-phonon, four-phonon, and phonon-electron scattering processes, the lattice thermal conductivity is studied, showing a value of 1357 W/mK at room temperature. At Van Hove singularity (5 × 1014 cm−2), the lattice thermal conductivity changes from 1357 W/mK to 557 W/mK. The effect of cut-off phonon frequency on thermal transport is nearly negligible. The electrical/phonon transport property accumulation concerning the mean free path shows the quantitative relationship between the mean free path and transport property, indicating a key mean free path of 186 nm. With a grain boundary of 186 nm, the ZT changes from 0.22 to 0.56 under a cut-off phonon frequency of 0 cm−1, while increasing from 0.64 to 1.23 under a cut-off phonon frequency of 30 cm−1. This work provides a strategy to enhance the thermoelectric property by regulating the electron-phonon coupling strength through cut-off phonon frequency.

Сравнительный анализ теоретических моделей определения теплопроводности наножидкости

This article is devoted to the study of thermal conductivity of nanofluid. A nanofluid is a liquid in which nanometer-sized solid particles are dispersed. These particles are called nanoparticles. Nanofluids have new promising thermophysical properties compared to conventional heat transfer fluids. Thermal conductivity is one of the main thermophysical properties of a liquid. Thermal conductivity is of great importance in processes where heat transfer and fluid flow occur. The article presents well-known theoretical models for determining the thermal conductivity of nanofluid. A brief description of these models is given. Some experimental work on determining the thermal conductivity of various nanofluids is considered. A computational study of the effect of aluminum oxide (Al2O3) and silicon dioxide (SiO2) nanoparticles on the change in thermal conductivity of a nanofluid has been performed. A comparative analysis of known computational models and experimental data is carried out. The accuracy of the calculated models is determined by determining the thermal conductivity of the nanofluid.

Stochastic wave propagation in magneto-thermoelastic materials subjected to the change in electrical and thermal conductivity

This paper presents the results of an investigation that focuses on the stochastic wave propagation that occurs within an elastic medium when the thermoelectric properties are taken into consideration. It has been found that there is a clear association between the characteristics of electrical conductivity and thermal conductivity. As a result, the current topic focuses on the influence a magnetic field has on the variable thermal conductivity and electrical conductivity that exists within a material. This is done within the context of dual phase lag thermoelasticity. Several parameters are incorporated into the governing equations, which exhibit coupling. Thermal shock has been applied to the boundary of the medium that represents the non-traction barrier. At the border of the problem, stochasticity is imposed in order to make the problem appear more realistic. Within the boundary conditions, it is presumed that the white-noise function is present. Utilizing the Laplace transform method is the approach that is taken to address the issue. It is possible to determine the inverse transformations using numerical approximation approaches. Both graphical and visual analyses are performed on the dataset. In conclusion, graphical representations are utilized in order to make comparisons between the deterministic and stochastic solutions of all physical fields.

Mechanical regulation to interfacial thermal transport in GaN/diamond heterostructures for thermal switch.

Gallium nitride offers an ideal material platform for next-generation high-power electronics devices, which enable a spectrum of applications. The thermal management of the ever-growing power density has become a major bottleneck in the performance, reliability, and lifetime of the devices. GaN/diamond heterostructures are usually adopted to facilitate heat dissipation, given the extraordinary thermal conduction properties of diamonds. However, thermal transport is limited by the interfacial conductance at the material interface between GaN and diamond, which is associated with significant mechanical stress at the atomic level. In this work, we investigate the effect of mechanical strain perpendicular to the GaN/diamond interface on the interfacial thermal conductance of heterostructures using full-atom non-equilibrium molecular dynamics simulations. We found that the heterostructure exhibits severe mechanical stress at the interface in the absence of loading, which is due to lattice mismatch. Upon tensile/compressive loading, the interfacial stress is more pronounced, and the strain is not identical across the interface owing to the contrasting elastic moduli of GaN and diamond. In addition, the interfacial thermal conductance can be notably enhanced and suppressed by tensile and compressive strains, respectively, leading to a 400% variation in thermal conductance. More detailed analyses reveal that the change in interfacial thermal conductance is related to the surface roughness and interfacial bonding strength, as described by a generalized relationship. Moreover, phonon analyses suggest that the unequal mechanical deformation under compressive strain in GaN and diamond induces different frequency shifts in the phonon spectra, leading to an enhancement in phonon overlapping energy, which promotes phonon transport at the interface and elevates the thermal conductance and vice versa for tensile strain. The effect of strain on interface thermal conductance was investigated at various temperatures. Based on the mechanical tunability of thermal conductance, we propose a conceptual design for a mechanical thermal switch that regulates thermal conductance with excellent sensitivity and high responsiveness. This study offers a fundamental understanding of how mechanical strain can adjust interface thermal conductance in GaN/diamond heterostructures with respect to mechanical stress, deformation, and phonon properties. These results and findings lay the theoretical foundation for designing thermal management devices in a strain environment and shed light on developing intelligent thermal devices by leveraging the interplay between mechanics and thermal transport.

The Influence of Substitutional Defects of Transition Metal Elements on the Stability and Thermal Properties of Al at Finite Temperatures: A First-Principles Study

Based on first-principles calculations, the effects of substitutional defects of the 3d–5d transition metal elements TMAl on the stability and thermal conductivity of the aluminum matrix were investigated. The results show that with an increase in the atomic number of TM, the defect-forming energy Ef of TMAl exhibits a periodic change feature, which depends on the valence electron configuration of the TM elements. The thermodynamic property parameters calculated with the Debye theory show that the addition of TM atoms does not change the stability of an Al system and can effectively reduce the thermal expansion coefficient of the material. But the equilibrium lattice constant a0 of Al-TMAl supercells changes very little. As the temperature increases, the relaxation time τ decreases, and both the electronic thermal conductivity κe and the total thermal conductivity κ decrease at the temperature range of 100–200 K, followed by a small increase or decrease. Because the lattice thermal conductivity κl is very small in the whole temperature range, the changes in electronic thermal conductivity and total thermal conductivity are basically the same. Moreover, when 1 at.% TM was added at both 300 K and 600 K, it was found that the influence of TM solute atoms on the thermal conductivity κ of Al was much greater than that of the second-phase particles. For solid solution atoms, Pd and Pt atoms have the greatest influence on the thermal conductivity of pure Al. This work is helpful for designing high-performance, heat-resistant Al-based alloys.

(Digital Presentation) Porous Silicon Formed by Electrochemical, Chemical and Combined Etching

This work is devoted the thermophysical characteristics (TPC) of PS formed by Electrochemical, Metal-assisted chemical and combined etching methods. The TPC of PS/MACE+EC formed by the MACE+EC– method is higher than those of the PS/EC and PS/MACE samples formed by the EC and MACE methods. The energy activity of water splitting on the surface of PS/MACE+EC is higher than the energy activity of the PS/EC and PS/MACE samples. The combined (MACE+EC) method is the best method for obtaining PS, which has the highest energy activity in water splitting processes. The activation energy of water splitting processes Ea(PS/MACE+EC) is higher than the energy activity of the Ea(PS/EC) and Ea(PS/MACE) samples. The presence of Ni particles in the pores of PS<Ni> stabilizes the them TPC. The TPC of PS were studied in comparison with the TPC of crystalline (c-Si) and powdered silicon (powder-Si). It is known that PS has a high catalytic activity of hydrogen evolution during photoelectrochemical splitting of water. For example, the use of a surface enriched Ni layer on the surface of a PS photocathode PS<Ni> made it possible to increase its catalytic activity two times higher than the catalytic activity of the Pt- electrode in the reaction of hydrogen evolution [1]. The TPC of PS samples formed by EC, MACE and combined (MACE+EC)- etching methods are studied in comparison with the TPC of c-Si and powder-Si samples, Table. Method Electrolyte Composition Marked I Et1 HF:H2O2=1:1 EC II Et2 HF:H2O2: Ni(NO3)2 =1:1:1 МАСЕ III Et3 HF:H2O2: Ni(NO3)2 =1:1:1 EC+МАСЕ According of electron microscopy research the highest porosity is observed on the PS/MACE+EC samples, the smallest - on the PS/MACE sample formed by the MACE- method. The Raman spectra testify this, PS/MACE+EC(Et3) samples have a higher intensity of the Raman peak at a high-frequency shift due to electron-phonon scattering, the intensity and frequency of which depend on the concentration of scattering centers and, consequently, on the conditions for the formation of pores [2]. The TPC of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) spectra record changes in the mass Δm of the sample and heat flux ΔQ with an isobaric temperature change ΔT at the points of T-characteristic. An analysis of the calorimetric TPC and the TGA and DSC spectra shows that c-Si and powder have almost the same patterns of change in thermal conductivity, in contrast to the PS-spectra. The PS/EC, PS/MACE samples have the maximum change in heat flux Q(PS/EC)=187.6 mW, Q(PS/MACE)=231.6 mW, in contrast to powder-Si Q(powder-Si)=107.1 mW and c-Si ΔQ(c-Si)=65.02 mW. This dependence, which is responsible for the heat flux ΔQ change in the high T-range is associated with the maximum developed surface PS. Due to the fact that the presence of the Ni in the pores of PS<Ni>/MACE and PS<Ni>/MACE+EC stabilizes the TPC of PS, and thereby prevents significant changes in the TPC of Ni-containing PS<Ni>, in contrast to PS that does not Ni-contain in pores. Also have carried out experiments on the evaporation of water droplets at room temperature from the surface of c-Si and PS/EC, PS/MACE PS/MACE+EC samples. Experiments on the evaporation of water droplets at Troom from the surface of crystalline and PS samples obtained by various etching methods showed that the highest energy activity is observed in PS/MACE+EC samples, Ea(PS/MACE+EC) = 0.2087 eV, which is much higher than Ea(PS/EC) = 0.070 eV, Ea(PS/MACE) = 0.050 eV and Еа(c-Si) = 0.025 eV,The porous surface of the PS/MACE+EC sample has the highest energy activity compared to the PS/EC and PS/MACE samples, which is due to the high porosity of PS/MACE+EC. The PS/MACE and PS/MACE+EC samples showed high energy stability over 1 year in water evaporation experiments. The values of the heat capacities of various EC-, MACE-, EC+MACE - etching methods are established: - Cp(c -Si) = 20.7 KJ/gK, Cp(powder-Si) = 13.11 KJ/gK, Cp(PS/MACE) = 12.79 KJ/gK, Cp(PS/AE) = 5.62 KJ/gK.Combination of electrochemical etching with Ni-stimulated and chemical etching (EC+MACE) makes the possibility Ni-containing nanoporous silicon with high energy activity in water splitting processes: - Еа(PS/MACE+EC) = 0.2087 eV, Еа(PS/MACE) = 0.070 eV , Еа(PS/АЕ) =0.050 eV.The Ni- presence in the pores stabilizes the TPC of PS/MACE and PS/MACE+EC. The stability of Ni-containing samples PS/MACE+EC) and Ea(PS/MACE) is maintained for one year. [1] K.B. Tynyshtykbayev et al. Russian Microelectronics, 2016, Vol. 45, Nos. 8–9, pp. 603–612. https://doi.org/10.1134/S106373971608014X[2] K. Tynyshtykbayev et al. ECS Journal of Solid State Science and Technology, 2021 10 013009 DOI: 10.1149/2162-8777/abdd86

Moisture Transport in Loose Fibrous Insulations under Steady-State Boundary Conditions.

This research aimed to compare the transport capacity of loose-fill mineral wool, cellulose fibers, and wood wool to transfer moisture under steady-state conditions. The tests were carried out in the heat flow meter apparatus, which created a constant thermal field, limiting samples of sorptive moistened materials. The thermal conductivity, stabilization time, and moisture content of the samples were measured. Based on the variation in the results, the dynamism of moisture transport in the materials was determined. Mineral wool samples showed the lowest sorption. As a result, the moisture transport in this material stopped the fastest. In the case of cellulose and wood fibers, moisture transport continued throughout the whole test procedure. It was noted that the amount of moisture transport is influenced primarily by the structure of the fibers, the moisture content, and the possible presence of air in the pores. The wetter the material, the faster the transport. The dynamism of moisture transport according to trends of thermal conductivity changes over time was analyzed. The greater the slope of the linear regression line, the greater the dynamics of change. The smallest dynamics of change were found for mineral wool, for which the measured slope was between -0.008 and -0.033. For cellulose and wood wool, the range of slope was from -0.141 to -0.210, and from -0.162 to -0.211, respectively. The results of this research may provide the basis for further work on buffering moisture in the adjacent internal layers of the frame walls.

A fine pore-preserved deep neural network for porosity analytics of a high burnup U-10Zr metallic fuel

U-10 wt.% Zr (U-10Zr) metallic fuel is the leading candidate for next-generation sodium-cooled fast reactors. Porosity is one of the most important factors that impacts the performance of U-10Zr metallic fuel. The pores generated by the fission gas accumulation can lead to changes in thermal conductivity, fuel swelling, Fuel-Cladding Chemical Interaction (FCCI) and Fuel-Cladding Mechanical Interaction (FCMI). Therefore, it is crucial to accurately segment and analyze porosity to understand the U-10Zr fuel system to design future fast reactors. To address the above issues, we introduce a workflow to process and analyze multi-source Scanning Electron Microscope (SEM) image data. Moreover, an encoder-decoder-based, deep fully convolutional network is proposed to segment pores accurately by integrating the residual unit and the densely-connected units. Two SEM 250 × field of view image datasets with different formats are utilized to evaluate the new proposed model’s performance. Sufficient comparison results demonstrate that our method quantitatively outperforms two popular deep fully convolutional networks. Furthermore, we conducted experiments on the third SEM 2500 × field of view image dataset, and the transfer learning results show the potential capability to transfer the knowledge from low-magnification images to high-magnification images. Finally, we use a pre-trained network to predict the pores of SEM images in the whole cross-sectional image and obtain quantitative porosity analysis. Our findings will guide the SEM microscopy data collection efficiently, provide a mechanistic understanding of the U-10Zr fuel system and bridge the gap between advanced characterization to fuel system design.

Role of boiling for the formation of vertical metal zoning and bonanza zone in low-sulfidation epithermal gold‑silver deposits: A geochemical modeling approach

Boiling and mixing are thought to be the most important processes for mineralization in the low-sulfidation epithermal gold‑silver deposits. Some low-sulfidation epithermal gold‑silver deposits show vertical zoning of metals including gold, silver, copper, lead, and zinc, but effect of boiling and mixing for the vertical zoning of metals through mineralization has been scarcely debated. Therefore, we applied geochemical modeling for an open-isenthalpic boiling system, with a hydrothermal fluid with different initial temperatures, carbon dioxide contents, salinities, and total sulfur contents to numerically evaluate the effect of boiling phenomenon for the vertical zoning in low-sulfidation epithermal gold‑silver deposits. Geochemical modeling of boiling phenomenon suggests that boiling is effective for depositing gold, silver, copper, lead, and zinc and that it could form a bonanza zone enriched in gold and silver in the shallow zone above the lead and zinc-rich zone. Temperature, carbon dioxide content, salinity, and hydrogen sulfide content affect the deposited amount of metals, vertical extent, depth of mineralization, and gold/silver ratio of the ore zone, although other factors such as permeability, gangue mineralogy, duration of mineralization, kinetics of mineral precipitation, change in porosity, thermal conductivity, and fractures of host rocks could affect mineralization as well. Our modeling of boiling phenomenon demonstrates similar characteristics with natural low-sulfidation epithermal gold‑silver deposits, suggesting that boiling is an important mechanism to cause vertical metal zoning in the low-sulfidation epithermal gold‑silver deposits, even though mixing could also be important to precipitate metals in some deposits.

Dependence of drying rate on interfacial thermal contact conductance in drum drying of thin materials

Drum dryers are widely employed in various industrial fields for their remarkable capability to efficiently dry thin porous materials such as textiles and paper. The heat transfer process in drum drying involves an interaction between the drum surface and the materials being dried. However, measuring interfacial thermal contact conductance during drum drying is very challenging, and little data has been reported in the literature. It is therefore unclear how changes in interfacial thermal contact conductance quantitatively affect drum drying in terms of drying time. We created an innovative laboratory-scale experimental setup designed to measure changes in moisture content of textiles at various contact pressures during drum drying. By combining our experiments with a simple yet effective heat transfer model, we were able to quantify the impact of interfacial thermal contact resistance on drying time. Specifically, in drying tests using thin cotton textiles, the experimental results revealed that when the contact pressure was increased from 120 Pa to 480 Pa, the thermal contact conductance increased by approximately 30% and the drying time decreased by approximately 20%. The outcomes of our study significantly advance the current understanding of drum drying processes and reveal vital insights into techniques for optimizing energy consumption.

Thermal conductivity switching for a Y–Mg alloy hydride thin film due to hydrogenation/dehydrogenation reactions using dilute H2 gas

Thermal switching requires a significant contrast in thermal conductivity between the on and off states. We focus on thermal conductivity switching performance and mechanism for switchable mirror materials, which changes reversible metallic and semiconductor states due to hydrogenation and dehydrogenation. A thin film of yttrium–magnesium (Y–Mg) alloy hydride covered with a Pd catalyst layer was fabricated on quartz glass substrates by dc magnetron sputtering using a 60 at. % Y and 40 at. % Mg alloy target and a mixture of 50% Ar and 50% H2 gases. The crystal structure, electrical conductivity, and thermal conductivity in each state were measured using in situ x-ray diffraction analysis, Hall effect measurement, and thermoreflectance apparatus, respectively. The Y–Mg alloy hydride film was hydrogenated and dehydrogenated on exposure to a mixture of 3% H2 in N2 gas and air, respectively. The structural change in Y hydrides due to hydrogenation and dehydrogenation was clarified, whereas Mg or Mg hydride in the film showed no apparent crystallization. The thermal conductivity of the on-state was 4.5 times larger than that of the off-state. The thermal conductivity change from hydrogenated to dehydrogenated state was ∼5.4 W m−1 K−1, and ∼2.5 W m−1 K−1 of thermal conductivity change could be attributed to electron contribution based on the estimation using Wiedemann–Franz law. The thermal conductivity changes of Y–Mg alloy hydrides due to hydrogenation/dehydrogenation resulted from both electrons and phonons.

A sensitivity analysis on thermal conductivity of Al2O3-H2O nanofluid: A case based on molecular dynamics and support vector regression method

The current study examines the sensitivity study of the thermal conductivity of water-based alumina nanofluids with variations in concentration, sphericity, and temperature. Thermal conductivity data of alumina nanofluids were obtained by simulation using the non-equilibrium molecular dynamics method with the parameters of nanoparticle volume fraction (1.24–6.2%), particle sphericity (0.6975–1.0), and temperature (290–360 K). Volume fraction and temperature have been discovered to have a major effect on the thermal conductivity of nanofluids, but sphericity should also be taken into consideration. Further, for parameter sensitivity analysis, a support vector machine regression model of thermal conductivity versus parameters was created with a coefficient of determination up to 0.98. The findings show that an Al2O3-H2O nanofluid's thermal conductivity increases with temperature and volume fraction while decreasing with sphericity. Temperature, sphericity, and volume fraction are the variables that are most sensitive to modification, according to a sensitivity analysis. A change in thermal conductivity of up to 4 % can occasionally be brought on by temperature differences of merely 1 %. The use of spherical particles makes it possible for the thermal conductivity of the nanofluid to be more stable, and the sensitive volume fraction region that can cause thermal conductivity to fluctuate across a large range is between 2.5 % and 5.5 %.

Lattice Thermal Conductivity of Mg3(Bi,Sb)2 Nanocomposites: A First-Principles Study.

Mg3(BixSb1-x)2 (0 ≤ x ≤ 1) nanocomposites are a highly appealing class of thermoelectric materials that hold great potential for solid-state cooling applications. Tuning of the lattice thermal conductivity is crucial for improving the thermoelectric properties of these materials. Hereby, we investigated the lattice thermal conductivity of Mg3(BixSb1-x)2 nanocomposites with varying Bi content (x = 0.0, 0.25, 0.5, 0.75, and 1.0) using first-principles calculations. This study reveals that the lattice thermal conductivity follows a classical inverse temperature-dependent relationship. There is a significant decrease in the lattice thermal conductivity when the Bi content increases from 0 to 0.25 or decreases from 1.0 to 0.75 at 300 K. In contrast, when the Bi content increases from 0.25 to 0.75, the lattice thermal conductivity experiences a gradual decrease and reaches a plateau. For the nanohybrids (x = 0.25, 0.5, and 0.75), the distribution patterns of the phonon group velocity and phonon lifetime are similar, with consistent distribution intervals. Consequently, the change in lattice thermal conductivity is not pronounced. However, the phonon group speed and phonon lifetime are generally lower compared to those of the pristine components with x = 0 and x = 1.0. Our results suggest that the lattice thermal conductivity is sensitive to impurities but not to concentrations. This research provides valuable theoretical insights for adjusting the lattice thermal conductivity of Mg3(BixSb1-x)2 nanocomposites.

A changeable thermal conductivity and optoelectronic-mechanical wave behavior in a microelongated, non-locally rotating semiconductor media

In this study, we investigate the effect of a rotation field on a homogeneous photo-thermoelastic nonlocal material and how its thermal conductivity changes as a result of a linearly distributed thermal load. The thermal conductivity of an interior particle is supposed to increase linearly with temperature. Microelastic, non-local semiconductors are used to model the problem in accordance with optoelectronic procedures, as proposed by the thermoelasticity theory. The micropolar-photo-thermoelasticity theory takes into account the medium’s microelongation properties in accordance with the microelement transport processes. This mathematical model is solved in two dimensions (2D) using harmonic wave analysis. Dimensionless components of displacement, temperature, microelongation, carrier density, and stresses are generated when the non-local semiconductor surface is subjected to the right boundary conditions. For silicon (Si) material, the wave propagation impact of the main physical fields is examined and graphically shown for various values of variable thermal conductivity, thermal relaxation durations, nonlocality, and rotation parameters.

Experimental and Theoretical Studies of the Thermal Contact Conductance for Bundles of Round Steel Bars.

The paper presents investigations devoted to the analysis of the thermal contact conduction in a bundle of round steel bars. The phenomenon can be expressed quantitatively with the use of thermal contact conductance (hct). The starting points for the presented analysis were the results of the experimental measurements of the effective thermal conductivity. The measurements were performed for samples of a medium in the form of flat packed beds of bars with three different arrangements: staggered, in-line, and crossed and four bar diameters: 10, 20, 30, and 40 mm. Next, a mathematical model was developed, thanks to which the values of the hct coefficient were calculated for the analyzed cases. This approach consists in analyzing thermal resistances in the medium model, which is defined with an elementary cell. It was established that the value of the hct coefficient in the temperature range of 50-600 °C changes within the range of 50-175 W/(m2·K), and it decreases with an increase in the bar diameter. The final effect of the present study was to develop generalized approximation equations describing changes in thermal contact conductance in the heated bar bundle simultaneously in the temperature and bar diameter function.