Significant Decrease In Thermal Conductivity Research Articles

Rational design of high‐performance epoxy/expandable microsphere foam with outstanding mechanical, thermal, and dielectric properties

AbstractPrevious studies on epoxy foams were subjected to the specific property, and improvements in thermal insulation or dielectric property were usually accompanied by the sacrifice of mechanical strength or thermal stability. The current research reported a high‐performance epoxy foam by the pre‐curing process using E‐51/DDS and expandable microsphere (EMP) as polymer matrix and physical blowing agent, respectively. The effect of EMP loading on the structure & property of epoxy foam was reported. The mean diameter of E‐51/EMP foam was in the range of 19–21 μm. The compression property of E‐51/EMP foam was fit for the power‐law model and its integrity was well maintained during compressive test. Compared with bulky epoxy, a significant decrease of thermal conductivity of 62.93% was observed in 10 wt% EMP‐filled epoxy foam. E‐51/EMP foam showed superior thermal stabilities, with a T5% of over 300°C. The dielectric constant was decreased by 54.5% from 4.92 in bulky epoxy to 2.24 in E‐51/EMP foam (10 wt% EMP). Notably, there was a percolation phenomenon of EMP (6 wt%) regarding on the structure & property of E‐51/EMP foam. This study provides lightweight, robust, high‐temperature‐resistant, and low‐dielectric‐constant epoxy foams with the wide application prospects in buoyant, electronic packaging, and energy‐saving industries.

Cyclic thermal conductivity changes of Pd-catalyzed Ni–Mg alloy films by gasochromic hydro- and dehydrogenations

To investigate the thermal-switching properties of Pd-catalyzed Ni–Mg alloy films, we conducted in situ analyses of the films’ electrical, optical, and thermal properties through hydrogen gasochromic reactions. These reactions allow the films to reversibly switch between metallic (dehydride) and semiconductor (hydride) phases. The thermal conductivities of the metallic and semiconductor states were found to be 14 and 1.0 W m−1 K−1, respectively. By applying the Wiedemann–Franz law, we attributed the significant decrease in thermal conductivity during hydrogenation to the reduction in free electrons.

Size and kink effects on thermal conductivity in nickel nanowires

The potential applications of nanowires in thermal management and thermoelectric energy conversion have sparked extensive research on thermal transport in various nanowires. Nickel nanowires, with their unique properties and promising applications, have been extensively studied. However, the influence of size, particularly the impact of kink structures, on the thermal transport behavior in nickel nanowires remains unclear. In this paper, we employed electron-beam lithography and liftoff techniques to fabricate suspended nickel nanowires with varying sizes and kinks to experimentally investigate the size and kink effect on the thermal conductivity. The experimental results revealed that the thermal transport behavior of nickel nanowires is significantly influenced by both size and kink effects. Notably, as the nanowire size decreases, the thermal conductivity also decreases. Furthermore, we discovered that the thermal conductivity can be adjusted by altering the number and angle of kinks. Increasing the number of kinks from 18 to 36 resulted in a significant decrease in thermal conductivity. In contrast, as the kink angle decreased from 157° to 90°, the thermal conductivity also decreased. However, intriguingly, when the kink angle was further decreased from 90° to 43°, the thermal conductivity increases. This non-monotonic change in thermal conductivity with the kink angle provides an interesting insight into the intricate behavior of heat carriers in kinked nickel nanowires. Additionally, we found that varying the alloy elements can profoundly alter the thermal conductivity of nanowires with kinks. These results offer valuable insights into the behaviors of heat carriers, including electrons and phonons, during heat transfer in nickel nanowires.

Machine learning-assisted investigation of the impact of lithium-ion de-embedding on the thermal conductivity of LiFePO4

In this study, we employ a machine-learning potential approach based on first-principles calculations combined with the Boltzmann transport theory to investigate the impact of lithium-ion de-embedding on the thermal conductivity of LiFePO4, with the aim of enhancing heat dissipation in lithium-ion batteries. The findings reveal a significant decrease in thermal conductivity with increasing lithium-ion concentration due to the decrease in phonon lifetime. Moreover, removal of lithium ions from different sites at a given lithium-ion concentration leads to distinct thermal conductivities, attributed to varying anharmonicity arising from differences in bond lengths and bond strengths of the Fe-O bonds. Our work contributes to a fundamental understanding of the thermal transport properties of lithium iron phosphate batteries, emphasizing the pivotal role of lithium-ion detachment and intercalation in the thermal management of electrochemical energy storage devices.

Enhanced thermoelectric properties of bismuth telluride (Bi2Te3) and multiwall carbon nanotube (MWCNT) composites

Solid solutions based on bismuth telluride are the traditional thermoelectric materials that are commercially utilized in the industry close to room temperature applications, where energy scavenging is a big challenge due to low quality and energy density. However, their broader applications are still limited due to low efficiency. The focus of the present work is to enhance the energy conversion efficiency of Bi2Te3 by mixing one-dimensional (1D) multiwall carbon nanotubes (MWCNTs) via facile powder processing by synergistically utilizing the nanostructuring and quantum confinement effects. Thus, different vol% of MWCNTs ranging from 0.5, 1.0, 1.5, and 2 were uniformly dispersed in a fine powder of Bi2Te3 that was ball milled from course Bi2Te3 in an inert environment. The coarse Bi2Te3 and fine composite powders were consolidated in a spark plasma-sintering furnace at ∼400 °C under uniaxial pressure of ∼40 MPa. Thermoelectric properties of Bi2Te3-based MWCNTs composites were evaluated in the temperature range from ∼300 to ∼525 K. The electrical conductivity of 1.5 vol% MWCNT composite has significantly enhanced from pristine Bi2Te3. This increase indicates that electrical transport dominantly involves a percolating network of MWCNTs. The concomitant increase in the Seebeck coefficient of 1.5 vol% Bi2Te3 composite suggests the enhanced energy-dependent scattering of charge carriers. Consequently, a substantially improved power factor is observed for the composite. The thermal conductivity of all the composites decreases, and considerable reduction is achieved for 0.5 vol% due to enhanced phonon scattering through nanostructuring and uniform distribution of the nanotubes. Hence, the significant decrease in thermal conductivity with improved power factor leads to substantial improvement in the thermoelectric figure of merit of 1.5 vol% Bi2Te3 composite from pristine Bi2Te3.

Growth and integration of aligned carbon nanotube-based field emission cathode for electron gun device-level fabrication

This study elucidates on growth and integration of aligned carbon nanotube-based field emission cathode for electron gun-device fabrication. Particularly, it emphasises on field emission (FE) and thermal transport (TT) properties of carbon nanotube (CNT) arrays with controlled atomistic defects (AD) to design a cold cathode (CC) for electron gun. To assess the influence of AD on FE and TT, aligned undoped (pure CNT) and N-doped herringbone (doped CNT) were fabricated using droplet-assisted solvent-thermolysis to use them as CC. Results demonstrated that doped CNT with higher degree of AD exhibited a lower turn-on field in FE in comparison to pure CNT could be ascribed to N addition in carbon lattice. Ambiguously, doping-mediated AD in doped CNT helps to achieve the best FE but lowers TT with significant decrease in thermal conductivity (111 to 5.1 W/m.k at room temperature) in comparison to pure CNT. Based on the FE behavior, a doped CNT arrays-based cathode was integrated into an electron gun, which exhibited maximum emission current of 81.3 mA/cm2 at a grid voltage of 2 kV.

Realizing high thermoelectric performance for p-type SiGe in medium temperature region via TaC compositing

SiGe is recognised as an excellent thermoelectric material with superior mechanical properties and thermal stability in regions with high temperatures. This study explores a novel strategy for co-regulating thermoelectric transport parameters to achieve high thermoelectric properties of p-type SiGe in the mid-temperature region by incorporating nano-TaC into SiGe combined ball milling with spark plasma sintering. By optimizing the amount of TaC in the SiGe matrix, the power factors were significantly increased due to the modulation doping effect based on the work function matching of SiGe with TaC. Simultaneously, the ensemble effect of the nanostructure leads to a significant decrease in thermal conductivity. Thus, a high ZT of 1.06 was accomplished at 873 K, which is 64 % higher than that of typical radioisotope thermoelectric generator. Our research offers a novel strategy for expanding and enhancing the thermoelectric properties of SiGe materials in the medium temperature range.

Magneto-thermal conductivity effect and enhanced thermoelectric figure of merit in Ag2Te

In this study, we report a large magneto-thermal conductivity effect, potentially usable in heat flow switches and thermoelectric devices, in Ag2Te over a wide temperature range, including room temperature. When a magnetic field of μ0H = 9 T is applied to Ag2Te at 300 K along the direction perpendicular to the heat and electric currents, the thermal conductivity κ decreases by a remarkable 61%. This effect is mainly caused by the suppressed electronic thermal conductivity in association with a significant magnetoresistance effect, but the suppression of the thermal conductivity is larger than that of the electrical conductivity, presumably due to a field-induced decrease in the Lorenz ratio. Its very low lattice thermal conductivity, as low as 0.5 W m−1 K−1, also greatly contributes to the large relative magneto-thermal conductivity effect. The significant decrease in thermal conductivity and the 18% increase in the Seebeck coefficient S lead to a nearly 100% increase in the thermoelectric figure of merit zT = S2σTκ−1 despite the 43% decrease in electrical conductivity σ.

Low thermal conductivity and thermoelectric properties of Si80Ge20 dispersed Bi2Sr2Co2Oy ceramics

Bi2Sr2Co2Oy is a thermoelectric material with low thermal conductivity. The Bi2Sr2Co2Oy/Si80Ge20 composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si80Ge20 alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si80Ge20 in the Bi2Sr2Co2Oy matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi2Sr2Co2Oy + 0.2 wt% Si80Ge20 sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi2Sr2Co2Oy sample.

Slowdown of photoexcited spin dynamics in the non-collinear spin-ordered phases in skyrmion host GaV4S8

Formation of magnetic order alters the character of spin excitations, which then affects transport properties. We investigate the photoexcited ultrafast spin dynamics in different magnetic phases in Néel-type skyrmion host GaV4S8 with time-resolved magneto-optical Kerr effect experiments. The coherent spin precession, whose amplitude is enhanced in the skyrmion-lattice phase, shows a signature of phase coexistence across the magnetic phase transitions. The incoherent spin relaxation dynamics slows down by a factor of two in the skyrmion-lattice/cycloid phases, indicating significant decrease in thermal conductivity triggered by a small change of magnetic field. The slow heat diffusion in the skyrmion-lattice/cycloid phases is attributed to the stronger magnon scattering off the domain walls formed in abundance in the skyrmion-lattice/cycloid phase. These results highlight the impact of spatial spin structure on the ultrafast heat transport in spin systems, providing a useful insight for the step toward ultrafast photocontrol of the magnets with novel spin orders.

Thermo-physical and mechanical characterization of cement-based mortar incorporating spent tea

The use of biomass waste and plant fibers as aggregates for traditional binders has proven to be a green promising solution for reducing the overall energy consumption and carbon dioxide emissions related to the construction sector. Most research on plant-based additives reported an improvement in thermal insulation at the expense of mechanical strength. Thus, balancing the thermo-mechanical properties is a key to produce innovative insulating materials. In this work, the potential use of Spent Tea (ST) in the manufacturing of bio-sourced cement-based mortars was studied. Different weight ratios ranging of 0–10% were used to evaluate the influence of ST content on the cement performance. Thermal, mechanical, and physical properties of the produced bio-composites were characterized. The results showed that increasing ST content leads to a significant decrease in thermal conductivity (67%), thermal diffusivity (57%), density (24%), and compressive strength (97%). At last, it was found that adding up to 2.5 wt% ST meets the structural requirements of lightweight concretes (>3.5 MPa) and could be used for wall structures. However, the composite incorporating 7.5 wt% of ST should exclusively serve for insulation purposes.

Suppression of thermal conductivity in LaCoO3 ceramic by lattice disorder and mass fluctuation scattering for thermoelectric application

Lanthanum Cobaltate (LaCoO 3 ) ceramic has high seebeck coefficient ( S ) at room temperature which makes it a promising material for thermoelectric application, but suffer from low figure of merit ( ZT ) due to its large thermal conductivity ( κ ). Herein, a strategy of isovalent substitution in Co site was effectively attempted to induce phonon scattering in order to decrease κ . In this study, the perovskites were prepared by solid state reaction using corresponding metal oxide precursors at 1373 K. XRD analysis of the prepared perovskites revealed excellent phase purity and the homogeneity of the prepared samples were confirmed by HR-SEM mapping. The low κ value of 1.38 W/mK was achieved at 753 K for Al- substituted LaCoO 3 . The significant decrease of thermal conductivity was due to the lattice disorder and effect of mass upon Al substitution. The strategy of isovalent substitution to achieve low thermal conductivity potentially opens up a new way for enhancing the thermoelectric performance.

Thermal Transport in Graphene Nanomesh: Unraveling the Role of Brillouin Zone Folding, Phonon Localization and Phonon Confinement

Coherent thermal transport has recently become a subject of considerable interest, but whether some intriguing phenomena of phonon coherence (including Brillouin zone folding, phonon localization, and phonon confinement) occur in certain nanostructures remains unclear. We have investigated the thermal transport in graphene nanomesh using molecular dynamics simulations together with first-principles calculations. It is found that the thermal conductivity of nanomesh is significantly reduced compared with graphene, mainly due to the bandgaps, flattened phonon dispersions and reduced phonon group velocities induced by Brillouin zone folding. Meanwhile, the significant decrease in thermal conductivity is induced by phonon localization. Phonon confinement caused by reflecting the optical phonon modes back into the C-H part further leads to the reduction of thermal conductivity. We highlight the coherent mechanism for lowering thermal conductivity, which is useful for manipulating heat conduction for thermoelectrics and thermal insulation applications.

Impact of biochar addition on soil thermal properties: Modelling approach

Little information is available on the effect of biochar on both measured and model-predicted soil thermal properties although they play a crucial role in the accumulation and transfer of heat and water through the soil. This study aimed at evaluation of the impact of biochar addition on soil thermal properties in field experiments conducted in China and Poland. The experimental data included biochar doses, soil texture, quartz and other minerals, bulk density, particle density, water content, air-filled porosity, and organic matter content. The data were then used to validate three models (analytic of de Vries, analytic of Zhao et al., and statistical-physical) and improve the predictability of the soil thermal properties. The biochar doses in the field experiments were from 4.5 to 40 Mg ha−1 and those in the modelling study ranged from 52 to 267 Mg ha−1. Pure biochar was used as well. The biochar additions caused a significant decrease in thermal conductivity and thermal diffusivity and exerted a less pronounced effect on the heat capacity. As shown by classic statistics and Bland-Altman plots, the agreement between the predicted and measured heat capacity of the Chinese soil was good and similar to the Zhao et al. and de Vries models. The best agreement between the statistical-physical model predicted and measured the thermal conductivity and diffusivity was obtained using the geometric mean of the native soil organic matter and biochar and after reduction of the quartz content in the sand fraction (2–0.05 mm) from 100 to 72%. The thermal diffusivity decreased with increasing biochar dose at comparable water contents and its characteristic maximum shifted towards higher water contents with increasing biochar dose. As thermal diffusivity enhances the smoothing of the temperature wave, these results imply that application of different biochar rates may help in regulation of soil temperature, which is an essential determinant of plant growth and soil processes and functions.

Tuning the electronic, mechanical, thermal, and optical properties of tetrahexcarbon via hydrogenation

Recently, Tetrahexcarbon (TH-carbon), a new two-dimensional(2D) carbon allotrope, has been identified with an intrinsic direct bandgap, which makes it promising for practical applications in optoelectronic devices. Using first-principles calculations, we examined the possibility of manipulating the physical and chemical properties of TH-carbon sheet by controlled hydrogenation. We systematically studied pristine TH-carbon and its hydrogenated derivatives with various configurations such as single- and double-sided hydrogenation. Our study revealed their stability in energetic, dynamic, thermal, and mechanical aspects. Depending on the hydrogen coverage and configurations, we observed the tunability of the phononic and electronic bandgap, and the direct-indirect-direct bandgap transitions. These results suggest the plausibility of modulating its electronic properties by hydrogenation. The heat transport in TH-carbon is anisotropic. A significant decrease in thermal conductivity was observed in the fully hydrogenated TH-carbon. The thermal conductivity in TH-carbon can be controlled by the sp3 C–H low conduction domains. A notable increase in specific heat capacity was observed in hydrogenated derivatives of TH-carbon, which would make them useful in nanoscale engineering of thermal transport. The hydrogenation was found to reduce the in-plane stiffness and Young’s modulus, but increase the ultimate strength. These findings would provide important guidelines for practical applications of TH-carbon.

Enhanced Thermoelectric Performance of Bi2Te2.7Se0.3/Bi2S3 Synthesized by Anion Exchange Method

Decoupling the electric conductivity, Seebeck coefficient, and thermal conductivity is a core issue to improve the properties of thermoelectric materials by tuning the electron and phonon transport properties. Herein, Bi2Te2.7Se0.3/Bi2S3 heterostructure is successfully synthesized by a simple anion exchange method with Bi2Te2.7Se0.3 as the starting material. The Bi2Te2.7Se0.3/Bi2S3 heterointerfaces can achieve an effective phonon scattering and significant decrease in thermal conductivity, while maintaining a high power factor, resulting in an enhanced figure of merit (ZT). By controlling the concentration of Bi2Te2.7Se0.3/Bi2S3 heterointerfaces, the carrier‐transporting and phonon‐scattering behaviors can be simultaneously optimized. A maximum ZT of 0.7 is obtained at 473 K, which is 23% higher than that of pure Bi2Te2.7Se0.3. In addition, the average ZT at 300–548 K increases from 0.49 to 0.61, which is 25% higher than that of pure Bi2Te2.7Se0.3.

Experimental study of the fibres content effect on the heat insulation capacity of new vegetable composite plaster-pea pod fibres

The purpose of this work is to study the thermal conductivity and density of a new vegetable composite composed of a mixture of plaster fibres and pea pods. The plaster used in this research is chosen for its wide use as a building material. The natural reinforcing fibres are obtained after grinding the previously dried pea pods. Its mass fraction in the vegetable composite varies from 0 to 25% in various test samples prepared in cylindrical form with a diameter of 25 mm and a height of 30 mm. The experimental tests were carried out with a thermal conduction unit under a heat flow of 15 W and a cooling water flow rate of 2 l/h. Experimental results showed that increasing the mass fraction of fibres in the vegetable composites induced a significant decrease in thermal conductivity and density.

Elaboration and characterization of clay-sand composite based on Juncus acutus fibers

The study reported in this paper was undertaken to investigate the feasibility of lightweight construction materials, based on vegetable fibres. This innovative material consisted of mixture of natural clay (60%) and natural sand (40%) reinforced with different levels of fibers extracted from Jancus acutus “Smar”. The fibers were used as partial replacement of sand in mixture by volume at: 0% (Control Specimen), 5%, 10%, and 20%.The objective of this work is to evaluate the physico-mechanical properties, through the examination of materials lightning, mechanical strengths (compressive and flexural). Due to the high hygroscopic nature of the vegetable fibers, the thermal conductivity of the composite materials was measured at both wet and dry state at different volumes of Juncus fibers replacement. Test-results have shown that the addition of 20% fibers decreased the composite bulk density from 1900 kg/m3 to 1100 kg/m3, which results in a high reduction of mechanical performances in terms of compressive and flexural strengths. The experimental investigation of thermal behaviour of this composite has shown that the increase of fibers volume leads to a significant decrease in thermal conductivity. For a composite containing 20% of fibers replacement, the dry thermal conductivity decreased from 0.902 W/m.K for control specimen (without fibers) to a value of 0.327 W/m.K. However, at wet state, the corresponding thermal conductivity decreased from 1.543 W/m.K to 0.361 W/m.K. Despite the decrease in mechanical strengths, the resulting composite can thus be considered as a promising candidate for use in thermal insulation material, because a conductivity of 0.350 W/m.K is generally considered as the worst acceptable value for insulating building material.

Evaluation and Optimization of Composite Thermal Insulators from Waste Materials

The present work includes a study on the effect of loading rubber waste into cement mortar on the thermal and mechanical properties of a thermal insulator.The experimental work of the study included the preparation of ten models of 35 mm diameter and 5 mm thickness. Portland cement and natural sand were used as a matrix and rubber waste (extracted from the consumed tires) as a filler was added in weight percentages ( 5% ,10% ,15% ,20% ,25% ,30% ,35% ,40%,45% and 50%). Water was also used as a binder.Also, the experimental work included conducting a thermal conductivity test using Lee’s Disk method, and a hardness test using the Shore scale. The theoretical side included extraction of empirical equations, depending on the experimental results. The thermal conductivity equation was for two variables, temperature and mass fraction. While the hardness equation was for one variable, mass fraction. Theoretically determined heat capacity was extracted using the equations of the composites. Based on the empirical equations of thermal conductivity and hardness and using the technique of multi-objectives genetic algorithm, the optimum values of temperature and mass fraction were extracted, which achieve the best thermal insulation of the mortar.The results showed a significant decrease in thermal conductivity. The reduction in thermal conductivity was (90.3%) at 5% and reduced to (95.73%) at 50%. The specific heat capacity was increasing as the percentage of rubber waste increase. The results also indicated a decrease in hardness. The optimal value of thermal insulation was (0.02658 W/m2.ºC ) as a thermal conductivity and (58.07 N/m2) as a hardness, at temperature (50°C) and mass fraction (27.764%) of rubber waste.Index Terms— rubber wastes ,empirical data , genetic algorithm.

Thermal insulation improvement in construction materials by adding spent coffee grounds: An experimental and simulation study

Improving thermal insulation of buildings envelopes can reduce their energy consumption and related CO2 emissions considerably. Searching for new and innovative insulation materials is therefore a key challenge. This paper presents an experimental study to show the effect of adding spent coffee grounds (SCGs) on thermal properties of plaster composite. The obtained results using the box method show a significant decrease of thermal conductivity along with an increase of the coffee ground content, indicating the possible usage of the resulting material as efficient insulation in buildings. It is found that the thermal conductivity of the resulting building material can be decreased from 0.5 to 0.31 W/m.K by adding 6 wt% of CGs to the initial material. To illustrate energetically and environmentally the benefit of using the proposed material, annual simulations were carried out for a typical Moroccan house located in Marrakech. The results indicated that up to 20% of the cooling and heating loads of the building can be reduced if the proposed material substitutes the conventional one. From an environmental point of view, the utilization of the proposed material can reduce up to 1500 kg CO2, annually for the studied case.