Dependent Thermal Conductivity Research Articles

The impact of sintering temperature on the thermoelectric performance of Cu2Se synthesized by solid state reaction method

In this study, copper selenide (Cu2Se) compounds were synthesized using a solid-state reaction technique and post-sintering processes (573 ​K–973 ​K). The mass density of the Cu2Se compounds decreased with increasing sintering temperatures due to changes in grain morphology and an increase in the number and size of pores in the samples. The sintering temperature also affected carrier generation mechanisms, which directly impacted the electrical transport characteristics and thermal properties. The α→β phase transition at around 400 ​K was observed in all samples. The low thermal conductivity was mainly attributed to the low lattice thermal conductivity (κlattice). As the result, the sample sintered at 773 ​K showed an outstanding ZT value of 1.44 ​at 773 ​K, with a power factor of 12.1 μWm−1K−2 and the thermal conductivity of 0.65 Wm−1K−1. The sintering temperature dependence of thermal conductivity was elucidated in terms of the growth of microstructural pores.

Thermal conductivity assessment of moist building insulation material using a Heat Flow Meter apparatus

Thermal conductivity is one of the most important parameters of building insulation materials. This parameter is dependent on the temperature, and on the moisture content for hygroscopic materials. When measured by steady-state methods (with the HFM device and the guarded hot plates), moisture will migrate from the hot side to the cold, influencing the measured heat flux and, thus, the measured thermal conductivity. This study aims to analyze the effects of moisture transfer on the thermal conductivity measurement of wet hygroscopic light building insulation material (namely wood fiber insulation). Several experiments are performed on instrumented samples to evaluate the influence of parameters like the thickness, the mean moisture content, the mean temperature, or the temperature gradient on the measured heat flux, on the temperature and relative humidity distributions and on the time to reach hygrothermal equilibrium. Particularly, the measurement errors made when shortening the experiment are discussed and the moisture dependence of true thermal conductivity is evaluated experimentally by considering the notions of apparent and diffusive thermal conductivities. Last, by using a validated heat and moisture transfer model, this dependency is compared to the one predicted by the methods proposed in the standards EN 12664 or ISO 10051.

Thermal conductivity of metallic nanoparticle using surface effect and kapitza resistance

To estimate the size dependence of thermal conductivity of metallic nanoparticles, different theoretical models and experimental techniques have been used by different groups of researchers. But it still requires an accurate model that supports the experimental results at low dimensions. A model is proposed to predict the change in thermal conductivity of metallic nanoparticles with size by extending the bond-energy model assuming core shell structure. In present work, we have taken metallic nanoparticles as samples for the verification of proposed model. It is observed that the coefficient of thermal conductivity of nanoparticles decreases by lowering the size of nanoparticles. The variation in thermal conductivity of metallic nanoparticles with their shape and size can be successfully explained by introducing the theory including the surface effect and kapitza resistance. It is found that the computed value has a deviation of maximum ±5% with experimental results which is in the permissible range of experimental measurement.

The spark plug’s thermal state simulation

BACKGROUND: Spark plugs are among the most important components of the ignition system of an internal combustion engine (ICE). The spark-over produced by the ignition system should have sufficient energy for combustion initiation at any engine operation mode in all the service conditions. Start properties, reliability, power, fuel efficiency and exhaust toxicity of an ICE essentially depend on excellence of design and manufacturing quality of a spark plug. On the other hand, functional properties of spark plugs depend on concordance to ICE with main dimensions, design, heat properties and value of a spark gap. AIMS: Use of the methods of the spark plugs temperature field simulation in order to reduce the amount of experimental studies and to reduce the number of specimen options subjected to laboratory and on-road engine tests.
 METHODS: The simulation model of the temperature field of a spark plug of an internal combustion engine (ICE) was developed in this study.
 RESULTS: The representation of temperature distribution in the insulators thermal cone and other ceramic elements of a spark plug was obtained. The dependence of thermal conductivity and specific thermal capacity of corundum ceramics on temperature was derived. The dependencies of thermal conductivity coefficient of the plugs ceramic insulator are presented. The analysis of thermophysical properties of the substances made of various materials was carried out. The equations of the temperature field of all parts of a spark plug (a contact head, glass sealant, an insulator, a central electrode, a spark plugs body, a heat-sinking washer, a sealing ring, a cylinder head) were developed. The schematic representation of the spark plug geometry, used in the temperature field calculation, was obtained. The requirements for boundary conditions for calculation of the temperature field of a spark plug were determined. The calculation scheme of the spark plug is presented. The condition of heat exchange at the boundary between the selected part of a cylinder head and the cooling system of an internal combustion engine was considered. The study of heat transfer between the structural elements of a spark plug and the air inside the ICE compartment was carried out. The dependencies describing the heat exchange between the structural elements of a spark plug being in thermal contact with each other were determined and the internal boundary conditions were established.
 CONCLUSIONS: The presented methods and algorithms of spark plugs thermal state simulation helped to perform the calculation of dependencies of thermal conductivity and specific thermal capacity of the insulators ceramics as well as the thermal conductivity coefficient of the plugs ceramic elements on temperature.

Molecular dynamics study on the dependence of thermal conductivity on size and strain in GaN nanofilms

The thermal conductivity of GaN nanofilm is simulated by using the molecular dynamics (MD) method to explore the influence of the nanofilm thickness and the pre-strain field under different temperatures. It is demonstrated that the thermal conductivity of GaN nanofilm increases with the increase of nanofilm thickness, while decreases with the increase of temperature. Meanwhile, the thermal conductivity of strained GaN nanofilms is weakened with increasing the tensile strain. The film thickness and environment temperature can affect the strain effect on the thermal conductivity of GaN nanofilms. In addition, the analysis of phonon properties of GaN nanofilm shows that the phonon dispersion and density of states of GaN nanofilms can be significantly modified by the film thickness and strain. The results in this work can provide the theoretical supports for regulating the thermal properties of GaN nanofilm through tailoring the geometric size and strain engineering.

Low-temperature specific heat and heat transport of Tb2Ti2−xZrxO7 single crystals

We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample.

Response of vibrational properties and thermal conductivity of perovskites to pressure

We study the response of SrTiO3 and KTaO3's vibrational properties and thermal conductivity Λ to pressurization. Our goal is to improve the understanding of the relationship between crystal structure, vibrational dynamics, and thermal conductivity in perovskites. We measure the thermal conductivity of SrTiO3 and KTaO3 up to 28 GPa by time-domain thermoreflectance. We also perform Raman scattering and stimulated Brillouin scattering measurements of SrTiO3 and KTaO3 to characterize changes in vibrational dynamics with pressure. The thermal conductivity of SrTiO3 increases under pressure with a slope comparable to that of other perovskites whose thermal conductivity has been measured versus pressure. Alternatively, the thermal conductivity of KTaO3 has a stronger pressure dependence than that of other materials with similar crystal structure. We correlate pressure-induced changes in Raman and Brillouin spectra with pressure-induced changes in thermal conductivity. We show that pressure-induced changes in phonon lifetimes dominate the pressure dependence of thermal conductivity. This study provides benchmark knowledge of why Λ depends on pressure and improves understanding of structure/thermal-property relationships.

Analysis of thermoelectric effect of wet spun graphene fiber composites

Abstract In this manuscript, we have theoretically reanalyzed data prepared and already published by Foroughi et al. (RSC Adv 2016;6:46427–32). These data concern the thermal energy dependence of the electrical conductivity of the samples: graphene fiber, graphene/poly(3,4-ethylenedioxythiophene) (graphene/PEDOT), graphene/carbon nanotube (graphene/CNT) and graphene/carbon nonotube/poly(3,4-ethylenedioxythiophene) (graphene/CNT/PEDOT) composite fibers. We investigate the behavior of the thermal conductivity for these four samples. These samples are considered as thermoelectric materials or green energy conversion materials using the Seebeck effect to transform heat into electrical energy and vice versa. In this context, the optimization of the merit factor ZT remains a challenge for the scientific community. The objective of this investigation is to evaluate and characterize the thermoelectric efficiency of the above-mentioned samples. We confront the experimental data with the existing theoretical models. The thermoelectric efficiency is generally characterized by the merit factor ZT. The maximum ZT of about 1,2 is obtained in the graphene fiber and between 0,1 and 0,2 in the others samples. The performance of the material depends on ZT; indeed a high ZT ($\mathrm{ZT}\ge 1$) corresponds to a better optimization of the composite.

Levenberg–Marquardt Training Technique Analysis of Thermally Radiative and Chemically Reactive Stagnation Point Flow of Non-Newtonian Fluid with Temperature Dependent Thermal Conductivity

We have examined the magnetized stagnation point flow of non-Newtonian fluid towards an inclined cylindrical surface. The mixed convection, thermal radiation, viscous dissipation, heat generation, first-order chemical reaction, and temperature-dependent thermal conductivity are the physical effects being carried for better novelty. Mathematical equations are constructed for four different flow regimes. The shooting method is used to evaluate the heat transfer coefficient at the cylindrical surface with and without heat generation/thermal radiation effects. For better examination, we have constructed artificial neural networking models with the aid of the Levenberg–Marquardt training technique and Purelin and Tan-Sig transfer functions. The Nusselt number strength is greater for fluctuations in the Casson fluid parameter, Prandtl number, heat generation, curvature, and Eckert number when thermal radiations are present.

Powell-Eyring Fluid Flow Over a Stretching Surface with Variable Properties

The objective of this study is to investigate the effect of temperature dependent viscosity and thermal conductivity on the stretching surface and heat transfer of a Powell-Eyring fluid with the effect of nonlinear thermal radiation. The Prandtl number is also considered to be varying within the boundary layer. The governing model which consists of a set of coupled non-linear partial differential equations is converted into a system of ordinary differential equations via suitable similarity transformations and then solved by a recent and reliable numerical method called the spectral quasi-linearization method is used in the computational analysis. The result shows that there is large variation in the value of the Nusselt number and skin friction co-efficient.

Growth and characterization of optical and thermal properties of LiGdF4 single crystal

The opportunity of growing of several centimeters in length LiGdF4 single crystals by Bridgman–Stockbarger technique was demonstrated and the optically perfect LiGdF4 single crystals with 8 mm and 55 mm in length were grown. The phase composition, thermal conductivity, basic optical and spectral-kinetic properties of LiGdF4 single crystals were studied for the first time. For LiGdF4 crystal, the resulting linear thermal expansion coefficients α is 11% larger, than for both LiYF4 and LiYbF4 crystals. The obtained thermal conductivity dependence k(T) is more pronounced compared to LiLuF4 and LiYF4. The absorption spectrum of the LiGdF4 crystal shows absorption bands corresponding to the 8S7/2 – (6Dj, 6Ij, and 6Pj) transitions of Gd3+ ions. The measured room temperature lifetime of 6Pj multiplet of Gd3+ ions was equal to 4.25 ± 0.05 ms. The values of the temperature gradient of the LiGdF4 refractive index do not strongly depend on the wavelength.

Mutual impact of thermal radiations and temperature dependent thermal conductivity on non-Newtonian multiple flow regimes

The transfer of thermal energy is a specialized discipline of thermal engineering and hence own practical applications in an industrial thermal systems like refineries, air-conditioning plants, automotive engines, power plants, jet engines, refrigeration, food processing, and chemical processing plants to mention just a few. Owning such importance we have reported the collective impact of thermal radiations and temperature-dependent thermal conductivity for heat transfer in stagnation point magnetized Casson fluid flow field. The thermal flow regime of Casson fluid is manifested with a viscous dissipation effect as well. The flow equations are tackled numerically by the use of the shooting method. The outcomes are debated comparatively by considering various flow regimes namely (I) Thermal flow regime with thermal radiations (II) Thermal flow regime without thermal radiations (III) Heat flow with heat generation and (IV) Heat flow in the absence of heat generation. We believe that the present thermal comparative analysis will be helpful for the preceding studies which own thermal engineering standpoints for the transfer of thermal energy by means of fluid flow.

Why and which opacifier for perlite based vacuum insulation panels (VIPs) in the average temperature range of 10–70 °C

Little is known on the effect of opacifiers in perlite core vacuum insulation panels at lower temperatures in the range of 10–70 °C, which are important for applications like refrigerators, transport boxes, buildings, and domestic hot water storage tanks. This paper shows that the effect of opacifier on perlite core Vacuum Insulation Panels (VIPs) is radically different than that on fumed silica VIPs. Identifying an optimum proportion of opacifiers for perlite core VIPs is not a trivial task because of a concomitant rise in solid conductivity as the radiative conductivity decreases. Further, the effect of opacifiers on temperature dependent thermal conductivity of VIPs has been resolved. Opacifiers in perlite are shown to yield clear benefits by reducing the overall thermal conductivity for insulation exposed to ≥70 °C. This was not the case for ≤20 °C. Several VIPs were manufactured using a low-density expanded perlite and 3 different proportions of 4 opacifiers i.e., carbon black, graphite and two variants of silicon carbide (SiC I, SiC II). These VIPs were tested at various combinations of sealing pressure and four different average temperatures i.e., 10 °C, 20 °C, 40 °C and 70 °C. A novel methodology has been developed to derive the evacuated thermal conductivity of perlite core VIPs by curve fitting technique, in order to eliminate the effect of gaseous conduction. The results were also compared with fumed silica core vacuum insulation panels with different proportions of carbon black as opacifier. Among the opacifiers tested (carbon black, graphite, SiC I and SiC II) in this research, Silicon carbide I with a mean particle size of 20 μm was found out to be the best opacifier, with one of its composite resulting in a 50% lower thermal conductivity at 70 °C in comparison to pristine perlite core VIPs.

Процессы молекулярного переноса в жидкостных питательных средах дрожжевой и пивной отраслях промышленности. Часть 3. Исследование теплофизических свойств водных растворов мелассы и концентрата пивного сусла

Thermophysical properties of aqueous solutions of molasses and beer wort concentrate with different dry matter content in a wide range of temperature changes were investigated. Seven samples of aqueous solution of beet molasses with an initial dry matter concentration of 77.7 wt.% and seven samples of beer wort concentrate with an initial dry matter concentration of 81 wt.% were studied. The temperature variation range was 10–70℃ with a discreteness of 10℃. Thermal conductivity, specific heat capacity, and thermal diffusivity of the initial solutions were determined by unsteady plane source and differential scanning calorimetry methods. Numerical values of the above-mentioned thermal constants for aqueous solutions of beet molasses in the range of dry matter mass concentrations n of from 16 to 70% at the temperatures of from 293 to 353 K and of beer wort concentrate in the range of dry matter mass concentrations of from 10.2 to 72.4% at temperatures from 284 to 342 K have been experimentally established. Graphical dependences of heat capacity, thermal conductivity, and thermal diffusivity of these parameters on temperature are given. Design equations of the required parameters are obtained, which are of interest both from the point of view of fundamental science on the development of the theory for liquid state of matter, and from the applied point of view of engineering calculations and design of biotechnological equipment.

On the description of the temperature-baric dependence of the effective thermal conductivity of granites

On the description of the temperature-baric dependence of the effective thermal conductivity of granites

Intrinsic and strain dependent ultralow thermal conductivity in novel AuX (X = Cu, Ag) monolayers for outstanding thermoelectric applications.

A large power factor and ultralow lattice thermal conductivity in 2D-monolayers of AuX (X = Cu and Ag) are achieved via first principles calculations. Low phonon frequency, small Debye temperature and high Gruneisen parameter limit the intrinsic thermal conductivity of both the studied materials. An ultra-low lattice thermal conductivity of 0.13 (0.30) W m-1 K-1 and 0.66 (1.59) W m-1 K-1 is obtained for unstrained AuCu and AuAg monolayers, respectively, at 700 (300) K, which further reduces to 0.04 (0.09) and 0.26 (0.63) W m-1 K-1 at 6% biaxial tensile strain. Such values of thermal conductivity are lower than the critical thermal conductivity for the state-of-art thermoelectric materials (kl < 2 W m-1 K-1). The peak values of ZT for unstrained monolayers are 2.20 and 1.40, which enhances to 3.61 and 2.91 at 6% strain for AuCu and AuAg monolayers, respectively. Interestingly pudding-mold band textures are found to be responsible for this unusual thermoelectric behaviour. The stability concerns (chemical/dynamic/mechanical) of these monolayers are ensured to stimulate experimental determinations for novel synthesis and possible applications.

Mathematical modeling of the temperature field of the arc of a conductive channel taking into account the temperature dependence of thermal conductivity

The article is devoted to mathematical modelling of the plasma temperature field taking into account the temperature dependence of thermal conductivity. The channel model was taken as a basis. Neglecting a very small fraction of the energy received by the ions during their acceleration in the longitudinal field, it can be assumed that all the energy taken by the arc discharge from an external source in the arc column passes directly to the plasma electrons. To find the temperature field by the integration method, the equation of thermal conductivity was solved. At the same time, the arc column is considered as a cylindrical continuous conductive rod in which all the supplied electrical energy is diverted due to thermal conductivity to the cooled walls of the discharge tube. The arc itself is represented by two regions: conductive and non-conductive. Based on mathematical modelling, the law of temperature field change in the arc cross-section was obtained. A formula was also derived to determine the effective radius of an electrically conductive channel.

Ablation threshold and temperature dependent thermal conductivity of high entropy carbide thin films

High entropy carbides (HECs) are a promising new class of ultra-high temperature ceramics that could provide novel material solutions for leading edges of hypersonic vehicles, which can reach temperatures &gt; 3,500 �C and experience extreme thermal gradients. Although the mechanical and thermal properties of HECs have been studied extensively at room temperature, few works have examined HEC properties at high temperatures or considered these materials� responses to thermal shock. In this work, we measure the thermal conductivity of a five-cation HEC up to 1200 �C. We find that thermal conductivity increases with temperature, consistent with trends demonstrated in single-metal carbides. We also measure thermal conductivity of an HEC deposited with varying CH4 flow rate, and find that although thermal conductivity is reduced when carbon content surpasses stoichiometric concentrations, the films all exhibited the same temperature dependent trends regardless of carbon content. To compare the thermal shock resistance of HECs with a refractory carbide, we conduct pulsed laser ablation measurements to determine the fluence threshold the HECs can withstand before damaging. We find that this metric for the average bond strength trends with the theoretical hardness of the HECs as expected.

Thermoelectric transport in holographic quantum matter under shear strain

We study thermoelectric transport under shear strain in two spatial dimensional quantum matter using the holographic duality. General analytic formulae for the DC thermoelectric conductivities subjected to finite shear strain are obtained in terms of black hole horizon data. Off-diagonal terms in the conductivity matrix also appear at zero magnetic field, resembling an emergent electronic nematicity, which cannot nevertheless be identified with the presence of an anomalous Hall effect. For an explicit model study, we numerically construct a family of strained black holes and obtain the corresponding nonlinear stress–strain curves. We then compute all electric, thermoelectric, and thermal conductivities and discuss the effects of strain. While the shear elastic deformation does not affect the temperature dependence of thermoelectric and thermal conductivities quantitatively, it can strongly change the behavior of the electric conductivity. For both shear hardening and softening cases, we find a clear metal-insulator transition driven by the shear deformation. Moreover, the violation of the previously conjectured thermal conductivity bound is observed for large shear deformation.

Concentration and Temperature Dependences of Thermal and Electrical Conductivity of Polymer Hybrid Composites Graphite Nanoplatelets/Fe/Epoxy

Concentration and Temperature Dependences of Thermal and Electrical Conductivity of Polymer Hybrid Composites Graphite Nanoplatelets/Fe/Epoxy