Significant Decrease In Thermal Conductivity Research Articles

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.

Nonthermal plasma synthesis of silicon nanoparticles and their thermal transport properties

Silicon-based nanostructured materials have been studied to utilize and observe the novel properties of silicon nanoparticles (SiNPs). Nonthermal plasma was employed to synthesize SiNPs, producing particles below 10 nm with narrow size distribution. The synthesis process of nonthermal plasma is discussed including the parameters that determine the particle size, yield and crystallinity of SiNPs. In this study, nanostructured material of SiNP/polystyrene (PS) nanocomposite has been observed to have a significant decrease in thermal conductivity. In this paper, post-processing of thermal annealing was applied, and the thermal conductivity measurement result shows the increase of thermal conductivity values. Although there is no significant enhancement of SiNP dispersion in the PS matrix, the thermal annealing induced molecular structure change, resulting in thermal transport enhancement at the interfacial region. The preservation of unique properties of nanoscale materials and low-cost fabrication by a silicon ink process at room temperature provide the promising potential of the SiNP-based nanostructured material. In addition, the combination of interfacial region engineering and post-processing provides controllable thermal transport properties for application in heat transfer management.

Effect of the initial composition and irradiation on phase composition, transfer coefficients and solid state swelling of uranium-plutonium nitride

Nitride nuclear fuel is a perspective type of fuel for fast reactors. This type of fuel has a number of advantages, including high thermal conductivity, high density of uranium and low fission gas (FG) release together with moderate swelling. Modeling the change of fuel properties with burnup is of interest from the point of justifying the safe operation of fuel rods. Changes in the phase composition of the fuel during irradiation lead to a complex changes in its properties. This paper examine the thermal conductivity, diffusion coefficient of FG and solid-state swelling change in (U, Pu)N fuel with 20% plutonium content at various stoichiometry and oxygen and carbon impurities. The thermodynamic calculation of the phase composition was carried out with the help of the multi-purpose program complex ASTRA-4 in the burnup range 0-15% fima and temperature range 900-1800 K. The dependences of concentrations of phase inclusions from content of nitrogen and oxygen in the initial fuel are received. Their fraction can attain ∼ 50% by volume at significant burnups and deviations from the stoichiometric composition. It is shown, that the main phase inclusion is U2N3, the concentration of which increases with the burnup and stoichiometric coefficient growth of the initial fuel. The increase in stoichiometry leads to a significant decrease in thermal conductivity, an increase in the diffusion coefficient of FG in fuel, and with increasing burnup this effect is enhanced. The calculated solid-state swelling of the fuel is in range 0,25-0,5% per burnup %, which is in satisfactory agreement with the literature data.

Comparative study of thermal conductivity in crystalline and amorphous nanocomposite

Silicon nanocrystals (SiNCs)/polystyrene (PS) nanocomposite has been observed to have a significant decrease in thermal conductivity in terms of the SiNC fraction with unspecified factors remained unclear. In this paper, amorphous silicon nanoparticles (a-SiNPs) with a mean diameter of 6 nm and PS nanocomposites were synthesized, and their thermal conductivity, including the density and specific heat, was compared with our previous work which investigated well-crystalized SiNPs (6 nm) and PS nanocomposite. The difference between amorphous and crystalline structure is insignificant, but phonon scattering at SiNPs and PS boundary is the key influencing factor of thermal conductivity reduction. The effective thermal conductivity models for nanocomposite revealed that the thermal boundary resistance, explained by Kapitza principle, is estimated to be 4 × 10−7 m2K/W, showing the significant effect of nanostructured heterogenic surface resistance on overall heat transfer behavior. Preservation of unique properties nanoscale materials and low-cost fabrication by silicon inks process at room temperature give the promising potential of SiNPs based heat transfer management.

Thickness Effects for Thermoelectric Property of Antimony Telluride Nanoplatelets via Solvothermal Method.

Nanostructures have the potential to exhibit good thermoelectric properties by tuning and controlling their size and thickness, and the competing electrical and thermal properties can be decoupled by engineering the interface and grain boundary. In the present study, Sb2Te3 nanoplatelets with different sizes were fabricated using a practical solvothermal method. The thickness of the platelets were regulated between sizes of 10 nm and 100 nm, and the opposite edge length was varied between 1 and 10 μm by altering chemical conditions. Consequently, manipulating the grain size made it suitable to benefit the carrier transport and also block phonons for the thin platelets, resulting in a significant decrease in thermal conductivity and simultaneous increase in electrical conductivity. The results showed that the optimized figure of merit ZT, increased from 0.2 to 1.0 for thin samples, providing a comprehensive understanding of size-dependent thermoelectric performance.

Structure and thermoelectric performance of β-Cu2Se doped with Fe, Ni, Mn, In, Zn or Sm

Cu1.99A0.01Se (A = Fe, Ni, Mn, In, Zn or Sm) alloys with high thermoelectric performance were prepared through a conventional melting, ball milling and quenching route, followed by a spark plasma sintering technique. Elemental doping did not change the structure type of Cu2Se. All the samples showed p-type conduction. All the doping elements except Indium reduced the electric resistivity and modified the carrier concentration, leading to a significant increase in the power factor. The lattice distortion and point defects due to the substitution of Cu became new phonon scattering centers, leading to a significant decrease in thermal conductivity. All the samples except the In-doped sample obtained better thermoelectric properties compared with the undoped Cu2Se sample. The values of the figure of merit ZT of the samples doped with Zn, Mn, Ni, Fe and Sm were 1.25, 1.28, 1.51, 1.07 and 1.07 at 823 K, respectively. In Cu2−xNixSe system, High ZT value of 1.51 is obtained for the sample of x = 0.0075 and 0.010 at 823 K.

Morphology, thermal and mechanical performances of SR composites containing sepiolite and HGMs as binary fillers

Abstract Binary fillers-filled silicone rubber (SR) matrix composites are prepared to improve the thermal and mechanical properties of SR as a lightweight thermal insulating material. Sepiolite and hollow glass microspheres (HGMs) can be evenly dispersed in SR matrix to obtain SR/sepiolite/HGM composites with superior performances at amounts of around 7.5 parts per hundreds of rubber (phr) and 20 phr, respectively, as observed from scanning electron microscopy (SEM). The density of the composites increases slightly with the addition of sepiolite in a linear trend, while the addition of HGMs results in the abrupt reduction in density lower than 0.7 g/cm3. Sepiolite exhibits a strengthening effect in the SR matrix; however, the addition of HGMs in this matrix slightly enhances the stress of the composite; its maximum elongation at break can increase up to 300%. The addition of sepiolite and HGMs improves the initial decomposition temperatures of SR composites a little; the excellent thermal insulating effect provided by HGMs is further verified by their significant decrease in thermal conductivity at 200°C, providing the facts that these SR/sepiolite/HGM composites can be used as lightweight materials for thermal insulating purposes.

Properties of pumice aggregate concretes at elevated temperatures and comparison with ANN models

SummaryThe mechanical properties and thermal conductivity of concretes including pumice aggregate (PA) exposed to elevated temperature were analyzed by thermal conductivity, compressive strength, flexure strength, dynamic elasticity modulus (DEM) and dry unit weight tests. PA concrete specimens were cast by replacing a varying part of the normal aggregate (0–2 mm) with the PA. All concrete samples were prepared and cured at 23 ± 10C lime saturated water for 28 days. Compressive strength of concretes including PA decreased that reductions were 14, 19, 25 and 34% for 25, 50, 75 and 100% PA, respectively. The maximum thermal conductivity of 1.9382 W/mK was observed with the control samples containing normal aggregate. The tests were carried out by subjecting the samples to a temperature of 0, 100, 200, 300, 400 500, 600 and 700 °C for 3 h, then cooling by air cooling or in water method. The results indicated that all concretes exposed to a temperature of 500 and 700 °C occurred a significant decrease in thermal conductivity, compressive strength, flexure strength and DEM. An artificial neural network (ANN) approach was used to model the thermal and mechanical properties of PA concretes. The predicted values of the ANN were in accordance with the experimental data. The results indicate that the model can predict the concrete properties after elevated temperatures with adequate accuracy. Copyright © 2016 John Wiley & Sons, Ltd.

Significant decrease in thermal conductivity of multi-walled carbon nanotube induced by inter-wall van der Waals interactions

The thermal transport properties of multi-walled carbon nanotubes (MWCNTs) were investigated by using non-equilibrium molecular dynamics simulation. The results show that the thermal conductivity of MWCNTs decreases significantly comparing to that of single-walled carbon nanotubes (SWCNTs) due to the inter-wall van der Waals interactions. The more interesting is a fact that the thermal conductance of MWCNTs is significantly greater than the thermal conductance summation of each SWCNTs. This is because the thermal conductance of a carbon nanotube protected by an outer tube is much larger than that of one that is not protected. Moreover, we also studied the thermal flux distribution of MWCNTs, and found that the outer tube plays a dominant role in heat energy transfer.

Synthesis and Properties of Highly Efficient Thermoelectric Materials Based on Lead Telluride with Antimony and Silver Impurity

The paper presents the results of research of X-ray diffraction and thermoelectric parameters (thermoelectric coefficient α, electrical conductivity σ and thermal conductivity coefficient k) of materials based on Lead Telluride: PbTe, PbTe:Sb, PbTe-Sb2Te3, Pb18Ag1Sb1Te20, Pb18Ag2Te20 and PbTe-Ag2Te. Established that the highest values of thermoelectric figure of merit have samples of PbTe:Sb (0,3 at.%) and system Pb18Ag1Sb1Te20, Pb18Ag2Te20. For PbTe:Sb is due to a significant increase of the electrical conductivity. For the other two materials is due to a increase the thermoelectric coefficient and a significant decrease of thermal conductivity compared to pure PbTe.

Enhanced power factor and reduced thermal conductivity of a half-Heusler derivative Ti9Ni7Sn8: A bulk nanocomposite thermoelectric material

We report a half-Heusler (HH) derivative Ti9Ni7Sn8 with VEC = 17.25 to investigate the structural changes for the optimization of high thermoelectric performance. The structural analysis reveals that the resulting material is a nanocomposite of HH and full-Heusler with traces of Ti6Sn5 type-phase. Interestingly, present nanocomposite exhibits a significant decrease in thermal conductivity due to phonon scattering and improvement in the power factor due to combined effect of nanoinclusion-induced electron injection and electron scattering at interfaces, leading to a boost in the ZT value to 0.32 at 773 K, which is 60% higher than its bulk counterpart HH TiNiSn.

Thermal Conductivity of Molten B2O3, B2O3–SiO2, Na2O–B2O3, and Na2O–SiO2 Systems

With the aid of the transient hot‐wire method, the thermal conductivity of molten B2O3, B2O3–SiO2, Na2O–SiO2, and Na2O–B2O3 systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B2O3 increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS‐NMR, 3Q‐MAS, and Raman spectroscopy, the structure of B2O3 and SiO2 in the B2O3–SiO2 system was confirmed. Findings show that an addition of B2O3 into the pure SiO2 system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na2O–SiO2 system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na2O–B2O3 system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na2O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.

Influence of Core-Shell Architecture Parameters on Thermal Conductivity of Si-Ge Nanowires

ABSTRACTIn this work, we investigate the influence of the core-shell architecture on nanowire (1D) thermal conductivity targeting to evaluate its validity as a strategy to achieve a better thermoelectric performance. To obtain the thermal conductivity values, equilibrium molecular dynamic simulations is applied to Si and Ge systems that are chosen to form core-shell nanostructures. To explore the parameter space, we have calculated thermal conductivity values of the Si-core/Ge-shell and Ge-core/Si-shell nanowires at different temperatures for different cross-sectional sizes and different core contents. Our results indicate that (1) increasing the cross-sectional area of pristine Si and pristine Ge nanowire increases the thermal conductivity (2) increasing the Ge core size in the Si-core/Ge-shell structure results in a decrease in the thermal conductivity values at 300 K (3) thermal conductivity of the Si-core/Ge-shell nanowires demonstrates a minima at specific core size (4) no significant variation in the thermal conductivity observed in nanowires for temperature values larger than 300 K (5) the predicted thermal conductivity around 10 W m−1K−1 for the Si and Ge core-shell architecture is still high to get desired ZT values for thermoelectric applications. On the other hand, significant decrease in thermal conductivity with respect to bulk thermal conductivity of materials and pristine nanowires proves that employing core–shell architectures for other possible thermoelectric material candidates would serve valuable opportunities to achieve a better thermoelectric performance.

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu3Si was found in the present work, which is evidence for the hypothesis that the formation of Cu3Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6W/(mK). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70W/(mK). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80W/(mK). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.

Thermal transport in nanoporous Si: Anisotropy and junction effects

Si-based nanoporous semiconductors have attracted great attentions recently due to their prominent properties and great potentials in thermoelectric applications. We have systematically investigated the transversal thermal transport in crystalline Si with nanocylindrical pore arrays using nonequilibrium and equilibrium molecular dynamics simulations together with lattice dynamics calculations. It is found that nanoporous Si with anisotropic pore pitches shows a remarkable tunable anisotropy in phonon transport, mainly due to the different reductions in phonon relaxation times by the channel confinement. Meanwhile, the significant decrease in thermal conductivity is caused by both the zone folding and the suppression of relaxation times. The analysis on the temperature distributions and profiles shows that the major thermal resistance is from the channel region while ballistic phonon transport is important for the other regions. Additional scattering caused by the junctions leads to an effective thermal conductivity considerably lower than that predicted by only considering diffusive boundary scattering and also enhances the anisotropy. This anomaly is attributed to the phonon dispersion mismatch and can be quantitatively modeled by a scalar model of the elastic waves, implying the importance of phonon wave behavior at nanoscale. By considering all the scattering mechanisms, a structure-based two-part model has been developed to predict the thermal conductivity in nanoporous structures, which shows good agreements with molecular dynamics results and can be readily extrapolated to meso/macro systems. The anisotropy and junction effects in nanoporous structures provide a new strategy in tailoring the thermal properties of materials for targeted applications.

Abnormal thermal conductivity in tetragonal tungsten bronze Ba6−xSrxNb10O30

Ba6−xSrxNb10O30 solid solution with 0 ≤ x ≤ 6 crystallizes in centrosymmetric tetragonal “tungsten bronze” structure (space group P4/mbm). We report on the x dependence of thermal conductivity of polycrystalline samples measured in the 2–400 K temperature interval. Substitution of Sr for Ba brings about a significant decrease in thermal conductivity at x ≥ 3 accompanied by development of a low-temperature (T ≈ 10–30 K) “plateau” region reminiscent of a glass-like compounds. We explain this behaviour based on a size-driven site occupancy and atomic displacement parameters associated with an alkaline earth atomic positions in the title compounds.

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.