Thermal Conductivity Values Research Articles

Whole-Core 3D Multiphysics Transient Modeling of a Prismatic Micro High-Temperature Gas-Cooled Reactor

A three-dimensional whole-core transient coupled thermal-hydraulic and neutronics code system for modeling prismatic high-temperature gas-cooled reactors (HTGRs) is presented. The discrete ordinates method code PHANTOM-SN was used to solve the multigroup neutron transport problem with cross sections generated with Serpent. The new finite element code OPERA was developed to solve the heat equation in the core and includes simplified subcodes for the coolant, reactor pressure vessel, and concrete containment building, as well as the power conversion cycle. Core graphite thermal conductivity degradation is included as a function of temperature and irradiation temperature. A 20-MW(thermal) HTGR design was modeled using the coupled multiphysics code to prove inherent safety. We simulated steady state, a depressurized loss of forced cooling (DLOFC), a partial blockage, and a reactivity insertion incident. We show that the DLOFC is not the most severe scenario for the fuel temperature in this prismatic micro HTGR. Upon a DLOFC, the peak fuel temperature remains well below the tri-structural isotropic (TRISO) fuel limits, even when the power is increased to 40 MW(thermal). However, during a partial blockage incident of one fuel assembly stack, the maximum fuel temperature reaches 2300°C, severely exceeding the limits. We furthermore contend that the graphite thermal conductivity values used in modeling should always be made explicit and that the temperature of irradiation should be included as a parameter since it can cause a sharp decrease (up to 97%) in the conductivity. We show that using unirradiated graphite parameters leads to an underestimation in peak temperature of 165°C while using a relatively low power density compared to other HTGRs. Finally, we argue that for prismatic HTGRs with a central reflector, bypass flow may lower the maximum fuel temperature.

Thermoelectrics in alkaline earth metal tellurides of ATe2 type - A first-principles study

Abstract We report an extensive study on the structural and thermoelectric performance of the novel semiconducting binary type ATe2 (A=Mg,Be,Ca,Sr,Ba) earth abundant materials using Density Functional Theory (DFT) and semi-classical Boltzmann transport theory. The PBE-GGA exchange correlation potential method was used for electronic structure calculations within the framework of density functional theory. The structural stability of the materials was confirmed by ground state energy, formation energy and mechanical responses. The calculated lattice constants, energy band structures and density of states explain the nature of the materials and are agreed with previous experimental and theoretical works. According to our calculated results, we revealed a high figure of merit materials are CaTe2 and SrTe2 due to high Seebeck and electrical conductivity and low thermal conductivity values. Additionally, Slack's equation was utilized to obtain low lattice thermal conductivity values. Our findings will serve as theoretical guidelines for future experimental and industrial applications both in cooling and heating

Acoustic, Mechanical, and Thermal Characterization of Polyvinyl Acetate (PVA)-Based Wood Composites Reinforced with Beech and Oak Wood Fibers.

Considering the growing need for developing ecological materials, this study investigates the acoustic, mechanical, and thermal properties of wood composites reinforced with beech or oak wood fibres. Scanning electron microscopy (SEM) revealed a complex network of interconnected pores within the composite materials, with varying pore sizes contributing to the material's overall properties. Acoustic characterization was conducted using a two-microphone impedance tube. The results revealed that the fibre size significantly impacts the sound absorption coefficient, demonstrating that the highest sound absorption coefficient of 0.96 corresponds to the composites reinforced with oak wood fibres with a size of 2 mm in the low-frequency range of 1000-2500 Hz. Mechanical testing revealed a significant reduction in compressive strength as fibre size increased from 0.4 mm to 2 mm, correlating with the observed changes in sound absorption and thermal properties. Thermal analysis indicated thermal conductivity (λ) values ranging from 0.14 to 0.2 W/m·K, with a notable increase in conductivity as fibre size decreased. It was shown that composites reinforced with beech or oak wood fibres with a size of 2 mm are recommendable for insulation materials due to the lowest thermal conductivity of 0.14 W/(m·K). Oak wood composites with a fibre size of 0.4 mm recorded the highest heat capacity, which is 54.4% higher than the one corresponding to the composites reinforced with the largest fibres. The results regarding heat diffusion rates are also reported. The findings about the effects of fibre size and pores on thermal, acoustic and mechanical properties provide valuable insights for designing sustainable materials, offering potential applications in industries where balanced performance across multiple properties is required.

Unlocking the mechanical, thermodynamic and thermoelectric properties of NaSbS2: A DFT scheme.

The present study focuses on the ground state mechanical, acoustic, thermodynamic and electronic transport properties of NaSbS2 polymorphs using the density functional theory (DFT) and semi-classical Boltzmann transport theory. The mechanical stability of the polymorphs is affirmed by the calculated elastic tensor. The calculated elastic properties asserted that all the polymorphs exhibit soft, brittle, anisotropic nature containing dominant covalent bonding. The 2D polar graphs are used to describe the anisotropic characteristic of the elastic parameters. The estimated value of Young's modulus and lattice thermal conductivity suggested that the polymorphs could be suitable for thermal barrier coating. Heat capacity, melting temperature, thermal conductivities, Grüneisen parameter, and thermal expansion coefficient of the polymorphs have also been studied to demonstrate thermodynamic behavior. The predicted lower values of lattice thermal conductivity declared that NaSbS2 polymorphs exhibit excellent electrical conductivity and transport properties. The estimated Seebeck coefficient (S), power factor (PF) and figure of merit (ZT) suggested that n-type triclinic and monoclinic, as well as p-type trigonal NaSbS2, are better for thermoelectric applications. The optimal carrier concentration for monoclinic structure is 1021cm-3 for T<750K, while it becomes 1020cm-3 for T>750K. It is also found that the optimal carrier concentration of the trigonal is 1021cm-3, whereas it is 1020cm-3 for triclinic structures. Therefore, it can be stated that NaSbS2 polymorphs possess excellent thermoelectric features, making them a promising choice for thermoelectric (TE) applications.

Harnessing Machine Learning for High-Throughput Screening of High Thermal Conductivity Polyimides: A Multiscale Feature Engineering Approach

Polyimides are known for their exceptional thermal stability and are widely utilized in high-temperature and electronic applications. To further explore and broaden their application in electronic packaging and thermal management, developing polyimide with high thermal conductivity remains a significant challenge. In this study, we present a machine learning technique with novel multiscale feature engineering approach to predict and identify high thermal conductivity polyimide efficiently. Our workflow involves digitally representing chemical structures using physical insights and refining these representations through the Statistical – Tree – Recursive feature engineering pipeline, which includes three steps--statistical selection, tree-based cascading feature selection, and recursive feature elimination. This process results in the creation of a comprehensive combinational feature set. Multiple machine learning models were trained and validated, demonstrating high predictive accuracy and generalizability. High-throughput screening identified polyimide candidates with thermal conductivity values exceeding 0.4W/(m⋅K), and these predictions were validated using non-equilibrium molecular dynamics simulation. This workflow provides valuable insights into structure-property relationships, offering a robust framework for designing polymer materials with improved thermal properties for applications in electronics packaging, flexible sensors, and other high-performance devices.

Frequency-domain thermoreflectance with beam offset without the spot distortion for accurate thermal conductivity measurement of anisotropic materials.

The measurement of thermal conductivities of anisotropic materials and atomically thin films is pivotal for the thermal design of next-generation electronic devices. Frequency-domain thermoreflectance (FDTR) is a pump-probe technique that is known for its accurate and straightforward approach to determining thermal conductivity and stands out as one of the most effective methodologies. Existing research has focused on advancing a measurement system that incorporates beam-offset FDTR. In this approach, the irradiation positions of the pump and probe lasers are spatially offset to enhance sensitivity to in-plane thermal conductivity. Previous implementations primarily adjusted the laser positions by modifying the mirror angle, which inadvertently distorted the laser spot. Such distortion significantly compromises measurement accuracy, which is especially critical in beam-offset FDTR, where the spot radius has a crucial impact on measured values. This study introduces an advanced FDTR measurement system that realizes probe laser offset without inducing spot distortion, utilizing a relay optical system. The system was applied to measure the thermal conductivities of both isotropic standard materials and anisotropic samples, including highly oriented pyrolytic graphite and graphene. The findings corroborate those of prior studies, validating the measurement's reliability in terms of sensitivity. This development of a beam-offset FDTR system without laser spot distortion establishes a robust basis for accurate thermal conductivity values of anisotropic materials via thermoreflectance methods.

Lattice thermal conductivity and phonon properties of polycrystalline graphene.

Using the spectral energy density method, we predict the phonon scattering mean lifetimes of polycrystalline graphene (PC-G) having polycrystallinity only along the x-axis with seven different misorientation (tilt) angles at room temperature. Contrary to other studies on PC-G samples, our results indicate a strong dependence of the thermal conductivity (TC) on the tilt angles which we attribute to careful preparation of our grain boundaries-based samples without introducing any local strains and ensuring periodic boundary conditions for the supercells along the x and y axes. We also show that the square of the group velocity components along x and y axes and the phonon lifetimes are uncorrelated and the phonon density of states are almost the same for all samples with different tilt angles. Further, a distribution of the group velocity component along x or y axis as function of normal frequency is found to be exponentially decaying whereas that of the phonon lifetime showed piecewise constant function behavior with respect to the frequency. We provide parameters for these distribution functions and suggest another measure of the TC based on these distributions. Finally, we perform a size-dependent analysis for two tilt angles, 21.78° and 32.20°, and find that bulk TC components decrease by around 34% to 62% in comparison to the bulk TC values of the pristine graphene. Our analysis reveals intriguing insights into the interplay between grain orientation, phonon scattering and thermal conductivity in graphene.

Ultra‐High in‐Plane Thermal Conductivity of Epoxy Composites Reinforced by Three‐Dimensional Carbon Fiber/Graphene Hybrid Felt

ABSTRACTThe heat dissipation problem brought about by high heat density components is a key bottleneck restricting the development of the new electronics industry. At present, traditional low thermal conductivity (TC) polymer composites can no longer meet the heat dissipation demand. The requirements for their thermal performance are also increasing. In this work, the epoxy resin/carbon fiber /graphene hybrid felt (Epoxy/CF/G) composites with high in‐plane TC were constructed through the synergistic interaction of one‐dimensional carbon fiber (CF) and two‐dimensional graphene (G). The synergistic effect of CF and G on thermal conductivity is explored. Graphene can bridge adjacent carbon fibers (CFs) to reduce phonon scattering and build multistage heat transfer paths, facilitating the thermal properties. The thermal conductivity value (K) of the prepared composites reaches 24.09 W/mK at a packing load of 35.25 wt%, which is superior to that of Epoxy/CF composites (12.73 W/mK). The heat transport mechanism is analyzed using infrared thermography. The heat dissipation effect is verified by light emitting diodes, further understanding the internal heat flow conduction process. This synergistic effect strategy provides a promising approach for further constructing thermal conduction networks with industrial application value.

Preparation and Characterization of Melamine Aniline Formaldehyde-Organo Clay Nanocomposite Foams (MAFOCF) as a Novel Thermal Insulation Material.

The main purpose of this study is to prepare a melamine aniline formaldehyde foam, an MAF copolymer, with lower water sensitivity and non-flammability properties obtained by the condensation reaction of melamine, aniline, and formaldehyde. In addition, the preparation of MAFF composites with organoclay reinforcement was determined as a secondary target in order to obtain better mechanical strength, heat, and sound insulation properties. For the synthesis of foams, the microwave irradiation technique, which offers advantages such as faster reactions, high yields and purities, and reduced curing times, was used together with the heating technique and the effect of organoclay content on the structural and textural properties of foams and both heat insulation and mechanical stability was investigated. Virgin melamine formaldehyde foam, MFF, melamine aniline formaldehyde foam, MAFFF, and melamine aniline formaldehyde-organoclay nanocomposite foams prepared with various organoclay contents, MAFOCFs, were characterized by HRTEM, FTIR, SEM, and XRD techniques. From spectroscopic and microscopic analyses, it was observed that organoclay flakes could be exfoliated without much change in the resin matrix with increasing clay content. In addition, it was determined that aniline formaldehyde, which is thought to enter the main polymer network as a bridge, caused textural changes in the polymeric matrix, and organoclay reinforcement also affected these changes. Although the highest compressive strength was obtained in MAFOCF5 foam with high organoclay content (0.40 MPa), it was determined that the compressive strengths in the nanocomposites were generally quite high despite their low bulk densities. In the prepared nanocomposite with 0.30% organoclay content (MAFOCF2), 0.33 MPa compressive strength and 0.051 thermal conductivity coefficient were measured. For virgin polymers and composites, bulk density, thermal conductivity, and compressive strength values were determined in the order of magnitude as MFF > MAFOCF1 > MAFOCF5 > MAFOCF6 > MAFF > MAFOCF3 > MAFOCF2 > MAFOCF4; MFF > MAFF > MAFOCF6 > MAFOCF5 > MAFOCF1 > MAFOCF4 > MAFOCF3 > MAFOCF2 and MAFOCF5 > MAFOCF4 > MAFOCF2 > MAFF > MAFOCF6 > MFF > MAFOCF1 > MAFOCF3. As a result, both compressive strength and thermal conductivity values indicate that nanocomposite foam with 0.20 wt% organoclay content can be a promising new insulation material.

Study of aerodynamic parameters of air distributors during interaction of round non-coaxial jets

Purpose of reseach. The aim of the work is to determine the parameters of the resulting flow in the interaction of counter-misaligned air flows to ensure an intensive decrease in air velocity in production and technological premises of small volume. To achieve this goal, it is necessary to solve such tasks as analyzing existing theoretical provisions and methods for calculating air distribution systems for small production and technological premises; development of a mathematical model of the process of interaction of oncoming incompatible flows and to determine the parameters of the resulting air flow depending on the geometric characteristics of the device. The object of the study is the supply ventilation systems in industrial and technological premises of small volume with insignificant thermal loads. The subject of the study is the processes of formation of the resulting air flow created by the interaction of countermisaligned air flows.Methods. Methods of mathematical modeling of air flow motion based on the equations of aerodynamics.Results. The aerodynamic characteristics of the resulting jet are obtained – the Boussinesq coefficient and the velocity field coefficient.Conclusion. In the ambiguities, between two-meter-long-range extravehicular jets and two-meter-long-range converging streams in the inner regions, a spatial vortex zone is formed with a maximum extravehicular gradient velocity. At the boundary of the jet stream there is an intense interaction with the surrounding air, the vector of the propeller is directed by the opposite direction, and the quantity of the speed of the plane depends on the speed of the jet, which changes the nature of the swirling movements. The results show that the actual level of overshoot during a load surge is determined in the first approximation by the increment value multiplied by the difference in the values of thermal conductivity, winding-oil at the initial and final temperatures of the oils in the channels of the cooled windings, and this difference itself depends on the magnitude of the incremental damage. Optimal factor intervals for the values of the maximum value of the functions are obtained: t – from 40 to 120; ρ – from 800 to 900; c – from 1.4 to 2.3.

Assessment of thermophysical properties of the temperature profile created on peach by microwave energy.

Microwave energy is based on creating heat in the structure by creating vibrations in the moisture in the product used. In drying processes, drying kinetics, energy consumption, quality, and so on features are evaluated based on the temperature equivalent of the heat created by the heat source in the product. For this reason, the temperature value formed in the product in microwave drying processes is important. In this study, the effects of microwave drying powers (180, 540, 720, and 900W) on the surface temperature profile, drying kinetics, thermophysical properties, and color values of peach slices were investigated. For drying processes performed at 180, 540, 720, and 900W microwave powers, the surface temperatures of peach slices were 34.5-83.40, 49.60-89.60, 55.90-94.06, and 68.20-145.20°C, respectively. Effective diffusion values varied between 1.01×10-7 and 2.12×10-7, and the activation energy value was measured as 20.73kJ/mol. Specific heat values varied between 871.62 and 838.21J/kgK, density values varied between 839.41 and 697.93kg/m3, thermal diffusivity values varied between 5.69×10-7 and 2.344×10-7m2/s and thermal conductance values ranged between 0.44 and 0.08W/mK. As compared to the fresh fruits, the best color values of dried material were achieved at 720W microwave power. It is recommended to determine the microwave drying power value well and to determine the drying kinetic properties of each agricultural product specifically for the product.

The Utilisation of Coconut Shell (Cocos Nucifera) as a Partial Aggregate Replacement on the Properties of Concrete in Terms of Thermal Behaviour

Green environments or environmentally friendly buildings have increasingly captured the attention of researchers. This concept involves the reuse of waste materials to enhance or create new products. Accordingly, this study aims to investigate the utilization of fine coconut shell (FCS) as a partial substitute for sand, focusing on its applications with varying percentages from 10% to 100% on the properties of concrete in terms of thermal conductivity. The initial phase of the research concentrated on characterizing the properties of fine coconut shell and sand using methods such as sieve analysis, laser diffraction sieve technique, specific gravity tests, bulk density measurements, scanning electron microscopy (SEM), and water absorption tests. Subsequently, the mechanical properties of fine coconut shell in concrete, replacing sand partially, were evaluated through slump tests, compressive strength tests, flexural strength tests, modulus of elasticity tests, splitting tensile strength tests, water absorption tests, and water permeability tests. The second phase of the study focused on exploring the low thermal conductivity applications of fine coconut shell concrete, assessed through thermal conductivity tests (k-value) and thermal resistance (r-value) calculations. Upon collecting data, a relationship analysis was conducted to determine the optimal percentage of fine coconut shell replacement. Using this optimal percentage, wall panels were constructed to assess heat penetration into buildings, and the temperature data was validated using Autodesk Ecotect software. The findings indicated that fine coconut shell particles were finer (≤ 600 μm) compared to sand (4.25 mm - 150 μm). In terms of mechanical properties, concrete containing fine coconut shell as a partial replacement for fine aggregate demonstrated superior performance to normal concrete. Moreover, the thermal conductivity values of specimens containing coconut shell were lower than those of normal concrete. In conclusion, the study determined that replacing 50% of fine aggregate with fine coconut shell was optimal, meeting British Standard requirements and aligning with previous research findings.

Modification of computer-aided modelling input data based on medium-scale fire tests of wooden beams

The aim of the paper was to optimize the settings of the material properties of a computer model describing heat transfer in a wooden beam exposed to thermal loading from a porcelain radiation panel. The methodology was based on performing medium-scale fire tests as a basis for a creation of finite element model with 6 different setups of material characteristics based on the outputs of tests. When adjusting the settings, the T-history method was used to determine a beginning and end of a phase change of the water content in the wood, a thermal conductivity was adjusted based on a density and a moisture content, and enthalpy was used instead of a specific heat. The results of the simulations were compared with the real medium-scale fire tests, which showed the importance of adjusting the input data. Based on the T-history method, the setting with a thermal conductivity value of 0.35 W·m-1·K-1 at a temperature of 114.8 °C was shown to be the best, with a coefficient of determination 98.7%.The results of the simulations showed that there could be a correlation between the moisture content of the wood and the maximum value of the thermal conductivity of the wood in the phase change of water.

Measurement of Thermal Conductivity of ZnO and SiO2 Water Based Nanofluids

The preparation and definition of thermal characteristics of novel-type nanofluids are critical for understanding the fluidity mechanism of nanofluids and selecting appropriate nanofluids for application. This study seeks to give an alternative fluid for various applications and to fill a new type of nanofluid requirement in the literature that has been identified by many research groups. In this study, a water-based ZnO-SiO2 nanofluid is synthesized in two steps, and the thermal conductivity values are experimentally determined. It is found that thermal conductivity increases with the increase in the concentration of particles. The maximum deviation ofthermal conductivity of SiO2 nanofluid is found to be 1.30% whereas in the case of ZnO nanofluid it is found to be 2.65%.

Addressing the Nusselt number divergence across varied fluid–solid combinations in Darcy–Bénard convection

Natural convection in a fluid-saturated, horizontal, porous medium, warmed from bottom and cooled from top, is essential to understand the physics behind many systems such as geothermal reservoirs and solar thermal storage facilities. Despite extensive research over the years, the reason for divergence in porous-media Nusselt number results across various fluid-solid combinations still remains unclear and, consequently, a universally applicable correlation is yet to be developed. The present study aims to address this gap through three-dimensional numerical simulations employing distinct fluid and solid phases that constitute the porous medium having a Darcy number of 2.91×10−4 and an aspect ratio of unity. This study confirms that different fluid–solid combinations yield distinct porous-media Nusselt number (Nupm) values for the same stagnant thermal conductivity of the porous medium (kpm) and Rayleigh–Darcy number (RaK). Nupm values are found to be independent of the fluid Prandtl number (Prf) and exhibit consistent and overlapping trends with RaK when Prf is greater than 0.1. Below this threshold value of Prf, Nupm decreases as Prf decreases. Based on these insights, two distinct correlations are developed for the porous-media Nusselt number: (i) Nupm=0.037RaK0.4(kf/kpm)0.5(Fs)−0.5 for Prf>0.1 and (ii) Nupm=0.165RaK0.4Prf0.5(kf/kpm)0.5(Fs)−0.5 for Prf≤0.1.

ENHANCING THERMAL PROPERTIES OF FIBER-REINFORCED POLYMER COMPOSITES TO BE USED IN BATTERY CASINGS: A REVIEW

Fiber-reinforced polymer composites, particularly those reinforced with carbon fiber, hold significant promise for use in battery casings. Since the performance of the batteries of electric and hybrid vehicles is highly dependent on the operating temperature, the thermal management system of the battery pack is essential. For this thermal management to be efficient, the battery enclosure must have sufficient thermal conductivity and thermal diffusivity values. However, most of the fiber-reinforced polymer composite materials have poor thermal properties. Thus, it is necessary to increase the thermal conductivity and thermal diffusivity values of such materials. In this paper, studies that have succeeded in increasing the thermal properties of fiber-reinforced polymer composites by applying different methods have been examined. The methods and obtained results are reviewed and discussed. Also, some suggestions are given to the potential applicators. Therefore, the most applicable methods, to enhance the thermal properties of the composite battery cases, can be determined and compared with one another.

Synthesis of ferrofluid using magnetite (Fe3O4), citric acid and deionized water and its characterizations for the application of a non-uniform magnetic thermosyphon

The performance improvement of a thermosyphon for disseminating heat is significant for the thermal management of a system. Furthermore, the water as the working liquid in a thermosyphon has an operating limit. Therefore, adding particles in the nanofluids as a working liquid could improve the thermal properties of thermosyphon and boiling temperature. Additionally, the effects of a non-uniform magnetic field on the ferrofluid thermosyphon at three locations: evaporator, adiabatic, and condenser were analysed. In this investigation, a two-step method was utilized, the ferrofluid was synthesized using magnetite (Fe3O4), citric acid, and deionized water. First, the Fe3O4 nanoparticles and associated ferrofluid were fully characterized from X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), magnetic properties analysis, zeta potential, and thermal properties, respectively. Then, the performance of thermosyphon heat pipes filled using high filling ratios of 70, 80, 90, and 100 % of stable synthesized ferrofluid at 0.5 wt% were examined. The results were compared to water using similar filling ratios that have been experimentally evaluated. The thermosyphon heat pipe (copper container) has a diameter of 12.7 mm and a length of 200 mm. The thermal performance test of the thermosyphon filled using the ferrofluid suggested a thermal resistance decrease from 1.95 to 1.20 °C/W with filling ratios of 70–90 % of evaporator volume. The effective thermal conductivity of the thermosyphon is improved from 685 to 1160 W/m°C at similar filling ratios. On the other hand, higher thermal resistance and lower effective thermal conductivity of thermosyphon filled using water were obtained at similar filling ratios of 70–90 %. The values are 2.2–1.3 °C/W and 600–1000 W/m °C, respectively. Moreover, the thermosyphon filled using the ferrofluid with a filling ratio of 90 % and a non-uniform magnetic field (two sides) on the evaporator section has a thermal resistance and an effective thermal conductivity of 1.9 °C/W and 690 W/m °C. The result has a lower thermal conductivity value compared to the thermosyphon filled with ferrofluid without a non-uniform magnetic field at a similar filling ratio of 90 % of 1160 W/m °C. This occurs due to less particle deposition on the inner surface of the thermosyphon as an effect of the magnetic field. A non-uniform magnetic field effect to the several sections of the thermosyphon (e.g., evaporator, adiabatic, and condenser) as the novelty of the research required to be modified to optimize the results.

Thermal and hygrometric performances, and mold growth resistance, of novel bio-stitched insulating fiberboards made from date and Washingtonia palm surface fibers

This paper evaluates the performance of a novel bio-stitched, lightweight insulating fiberboards made from Phoenix dactylifera L. and Washingtonia filifera palm fiber waste. These materials are binder-free, rendering them fully biodegradable. In this study, measurements are made of water sorption and its impact on density and thermal conductivity. Both materials exhibited insulating properties, with thermal conductivity values ranging from 0.043 to 0.063 W.m−1.K−1 depending on density. The bio-stitched fiberboards achieved their lowest thermal conductivity at an optimum density of 85–105 kg.m−3. The results indicate also that the thermal conductivity and density of bio-stitched fiberboards increase significantly with volumetric water content. However, even at saturation, they retain competitive thermal insulation properties. The residual fibers used were subjected to different natural weathering durations. Three types of fiber were therefore evaluated: one fresh, one moderately weathered and the other extremely weathered, in an attempt to compare the hygrothermal properties of the elaborated fiberboards as a function of fiber conditioning. It was found that Washingtonia palm surface fiberboards have higher water absorption compared to weathered date palm surface fiberboards. In addition, mold growth was evaluated by exposing the fiberboards to severe relative humidity conditions, i.e., 74 % RH, 77 % RH, 86 % RH, and 94 % RH at 29ºC. It was observed that the proliferation of mold was slow, particularly on weathered fiberboards, while it remained moderate on the surface of fresh Washingtonia palm surface fiberboards.

First principles investigation of double perovskites Li2CuAsZ6 (Z = Cl, Br, I) as a suitable alternatives for energy conversion technologies

The present study employs density functional theory to examine the physical properties of halide double perovskites Li2CuAsZ6 (Z = Cl, Br, I). The stability of the phase has been confirmed by analyzing the cubic layout and the values of the octahedral and tolerance factors. The formation energies of all perovskites have been computed to guarantee thermodynamic stability. The detailed description of the electronic characteristics of Li2CuAsZ6 reveals its behavior as a semiconductor. Energy band gaps of 0.55 eV, 0.33 eV, and 0.088 eV for Li2CuAsCl6, Li2CuAsBr6, and Li2CuAsI6, respectively, are determined by our calculations. Additionally, we have investigated the optical properties of these materials with 0–4 eV energy span in detail, which are reported as strong candidates for use in optoelectronic systems. The BoltzTraP code is employed to evaluate the thermoelectric characteristics. The lower lattice thermal conductivity values for Li2CuAsCl6, Li2CuAsBr6, and Li2CuAsI6 suggest less contribution to thermoelectric functionality. Finally, this discussion indicates the suitability of studied perovskites for energy conversion systems; however, experimental confirmation is necessary before these perovskites may be used in thermoelectric and optoelectronic systems.

Synthesis, characterization, and thermal conductivity properties of graphene oxide doped chromium-titanium oxide structures

This study investigates synthesis and characterization of graphene oxide (GO) undoped and doped chromium-titanium oxide structures, along with their thermal conductivity properties in detail. The importance of low or high thermal conductivity varies depending on the type of material intended, and it is known that thermal conductivity, which determines the ability of materials to conduct heat, plays a crucial role in energy, electronics, and thermoelectric applications. Materials with low thermal conductivity are widely used in various industrial fields due to their ability to isolate heat effectively. For experimental studies, GO-undoped(T1), 1%-GO-doped(T2), and 3%-GO-doped(T3) samples were produced via sol-gel method, followed by calcination, pelletization, and sintering processes. Characterization of these pellets was performed using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Fourier Transform-Infrared Spectroscopy (FTIR). Thermal conductivity was measured using Physical Properties Measurement System (PPMS). No structural or peak changes were detected in the XRD and FTIR results, but differences in peak intensities were observed. SEM images revealed that reductions in structural dimensions occurred with GO doping, and this was explained by changes in thermal conductivity data. The thermal conductivity values of T1, T2, and T3 samples were measured as 6.49, 3.45, and 1.50 W/K·m, respectively. These findings indicate that GO-doping reduces thermal conductivity and differences in the material structure are effective. Additionally, it was observed that the reduction of the structure to the nanoscale also led to a decrease in thermal conductivity data. These materials could play an important role in developing next-generation material designs.