Absolute Thermal Conductivity Research Articles

Thermoelectric Properties of Annealed Bilayer SnSe/PbTe and SnSe/PbSe Thin Films by Thermal Evaporation Method

Tin Selenide, Lead Selenide, and Lead Telluride are known best thermoelectric materials for mid and high-temperature electric generation applications. The bilayer of these materials could enhance the quality of a thermoelectric generation. The present work deals with bilayer deposition of SnSe/PbTe and SnSe/PbSe in glass substrates using physical vapor deposition followed by annealing at 323K, 423K, and 523K. The structure and morphology of the films have been investigated by XRD, SEM, and FESEM studies. The thermoelectric pursuance of both bilayer thin films was studied with the temperature as a function in the range of 300K to 623K. Both films exhibit the maximum Seebeck coefficient. The electrical Conductivity and Power factor increased gradually for SnSe/PbTe thin films and SnSe/PbSe thin films for the samples annealed up to 573K and then decreases. The electronic thermal conductivity of both films was very low compared to the total thermal conductivity. The absolute thermal conductivity at room temperature was calculated by Transient Hot Wire (THW) method. The maximum Figure of Merit (ZT) value obtained for SnSe/PbTe and SnSe/PbSe at room temperature was 0.81 and 1.3 for 573K annealed thin films respectively.

Effect of Y, La, and Yb simultaneous doping on the thermal conductivity and thermoelectric performances of CaMnO3 ceramics

Triple doping of CaMnO3 in different stoichiometric proportions has been studied in bulk sintered materials. They were prepared through the classical ceramic route using planetary milling to decrease the precursors particle sizes. It has been found that particle sizes decrease with the amount of dopant, both in the precursors and in the sintered bodies. XRD patterns showed that all samples were nearly single phase, with small amounts of CaMn2O4 phase. SEM and TEM observations revealed a homogeneous distribution of dopants in the CaMnO3 phase, while EDX showed a composition close to the nominal one. However, HREM imaging has shown some dark regions in the crystallites, with different Ca(Y,La,Yb)/Mn ratios found through STEM image-chemical mapping, which points out to the coexistence of CaMnO3 and CaMn2O4–type domains in the crystallites. Electrical resistivity, absolute Seebeck coefficient and thermal conductivity have been drastically decreased with doping. The highest PF values at 800 ºC have been achieved in 0.02(Y,La,Yb) doped samples (∼ 0.37 mW/K2m), which is among the best reported values in literature. On the other hand, lattice thermal conductivity is dramatically decreased with doping due to the phonon scattering produced by the dopants, the decrease in the grain sizes, and the strains present inside the crystallites, reaching the minimum values at 800 ºC in 0.03(Y,La,Yb) doped samples (∼ 0.8 W/K m). Consequently, ZT reaches the maximum values (∼ 0.29) in 0.03(Y,La,Yb) doped samples due to their very low thermal conductivity, being higher than the best ZT reported values in the literature.

Characterization of ZnO/nanofluid for improving heat transfer in thermal systems

Nanofluids are currently regarded a new growing area of research with numerous advantages over conventional heat transport fluids in heat transfer applications. Because, nanofluids possess a better thermo-physical characteristics. Hence, they can perform better when compared to traditional working fluids. The thermo-physical characteristics of zinc oxide nanoparticles disbanded water (ZnO/water) nanofluid was evaluated in this study by altering the volume proportions of nano-ZnO particles and temperature experimentally. The volume proportion of ZnO/water nanofluids was adjusted from 0.01 to 0.2 in four different (0.01, 0.05, 0.1 and 0.2) fractions. Using a field emission scanning electron microscope, the surface properties of ZnO nanoparticles were investigated. The results evidenced that the absolute viscosity, thermal conductivity, and density of the water was augmented by 1.43%, 37.0%, and 1.57%, respectively through the diffusion of 0.2 vol% of nano-ZnO particles in water. In contrast, the specific heat of water was lessened by 2.7% with 0.2 vol% of nano-ZnO particles in water.

Shock Structures Using the OBurnett Equations in Combination with the Holian Conjecture

In the present work, we study the normal shock wave flow problem using a combination of the OBurnett equations and the Holian conjecture. The numerical results of the OBurnett equations for normal shocks established several fundamental aspects of the equations such as the thermodynamic consistency of the equations, and the existence of the heteroclinic trajectory and smooth shock structures at all Mach numbers. The shock profiles for the hydrodynamic field variables were found to be in quantitative agreement with the direct simulation Monte Carlo (DSMC) results in the upstream region, whereas further improvement was desirable in the downstream region of the shock. For the discrepancy in the downstream region, we conjecture that the viscosity–temperature relation (μ∝Tφ) needs to be modified in order to achieve increased dissipation and thereby achieve better agreement with the benchmark results in the downstream region. In this respect, we examine the Holian conjecture (HC), wherein transport coefficients (absolute viscosity and thermal conductivity) are evaluated using the temperature in the direction of shock propagation rather than the average temperature. The results of the modified theory (OBurnett + HC) are compared against the benchmark results and we find that the modified theory improves upon the OBurnett results, especially in the case of the heat flux shock profile. We find that the accuracy gain is marginal at lower Mach numbers, while the shock profiles are described better using the modified theory for the case of strong shocks.

A novel method for spatially-resolved thermal conductivity measurement by super-resolution photo-activated infrared imaging

Thermal conductivity measurements play a crucial role in most areas of the applied physical sciences, thereby constantly demanding for new non-invasive methods capable of providing high-resolution spatial mapping of absolute thermal conductivities on heterogeneous samples ranging from solid-state bulk materials to low-dimensionality structures. In this work, we lay the theoretical foundations and provide the experimental demonstration of a novel method for quantitative thermal conductivity mapping at tunable ∼10-μm resolution, by non-contact infrared photo-activated thermography. Starting from Fourier’s heat transfer law, we surmise a universal dependence of the thermal response of a laser-irradiated sample on its thermal conductivity, irrespectively of density and specific heat capacity. We demonstrate such a dependence over the three conductivity decades 0.1–100 W/mK by finite-element numerical simulations, and exploit it for proof-of-principle single-point thermal conductivity measurements on both thermally thick and thermally thin reference solid samples. We exemplify the feasibility of space-resolved measurements on eighteenth-century tin organ pipe fragments, where the product of the thermal conductivity times the sample thickness, imaged at 40-μm sub-diffraction resolution, is pointed out as a relevant parameter for the non-destructive characterization of the sample conservation state. By coupling temperature maps with the extraction of thermal properties at high spatial resolution, our approach significantly expands the capability of state-of-the-art infrared imaging technology in fully capturing the compositional heterogeneity and/or functional state of the imaged materials.

RETRACTED: A new fractal model on fluid flow/heat/mass transport in complex porous structures

Abstract A new fractal theoretical model with periodic pore morphology, which idealizes the pore channels of the porous media as gourd-shaped structure, is established to model the transport in complex porous media. Based on the fractal model, the flow/thermal/mass transport properties of single-phase fluid and two-phase fluid flowing through porous structures are predicted, and analytical solutions to the permeability, thermal conductivity, diffusion coefficient of single-phase fluid in porous media, and the permeability, and the thermal conductivity of two-phase fluid in unsaturated porous media are derived. The analytical results of the proposed model for predicting the single-phase gaseous flow, heat and mass transfer in porous media are compared with the previous model data based on the idealized tubule-shaped structure of the real pore channels in porous media, which shows that the previous tubule-shaped structure is a limiting case of the present gourd-shaped structure. Analytical expressions of absolute permeability, relative permeability, and effective thermal conductivity of two-phase flow and heat transport in unsaturated porous media are derived for the present model and the previous tubule model. A parametric analysis is performed for presenting the effects of the structural parameters and fluid properties on the effective thermal conductivity, permeability and effective diffusion coefficient predicted. Meanwhile, the results show that the predicted data of the theoretical model are in good agreement with experimental data. The present study is significant for enhancing the flow, heat and mass transfer in accurately predicting transport properties of real porous media applications, and can provide an important parameter basis and guidance for the design of the transport system incorporating porous media.

Unified Effect of Dispersed Xe on the Thermal Conductivity of UO2 Predicted by Three Interatomic Potentials

Thermal conductivity is a critical fuel performance property of uranium dioxide (UO2)-based nuclear fuels. Numerous studies have shown that xenon (Xe) fission gas plays a major role in fuel thermal conductivity degradation. It has also been shown that dispersed Xe atoms can cause a stronger phonon-scattering effect than their clustered form. In this work, molecular dynamics simulations are conducted to study the dispersed Xe-induced thermal conductivity reduction using three different interatomic potentials. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, the normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the realistic thermal conductivities of UO2 at different dispersed Xe concentrations and temperatures. Using this new model, the critical Xe concentration that offsets the grain boundary Kapitza resistance effect in a high burnup structure is revisited.

Functional Properties of a Pitch-Based Carbon Fiber-Mortar Composite as a Thin Overlay for Concrete Pavement.

This experimental study investigated the utility of a pitch-based carbon fiber–mortar composite, which could replace polyacrylonitrile carbon fiber, as a thin overlay for concrete pavement. The objective was to explore the utility of the low-cost carbon fiber, which was produced via a melt-blown method, i.e., blowing at high pressure after melting the pitch residue following crude oil purification. The mechanical properties, durability, and thermal properties of the pitch-based carbon fiber were explored to maximize strength, durability, functionality, and economy by using micro-sized fibers that are closer in size to the constituents of cementitious materials. Melt-blown pitch-based carbon fiber has low individual fiber strength but generally excellent thermal conductivity. Thermal conductivity tests were conducted on mortar panels (560 mm × 560 mm; thickness = 25, 40 or 60 mm) containing 0, 0.4, 0.5 or 0.6 wt % pitch-based carbon fiber. The absolute thermal conductivity tended to improve with higher wt % of pitch-based carbon fiber, in the range of 9~11 W/°C. However, thermal conductivity tended to be lower under the 0.6 wt % condition, possibly due to the effect of dispersion. Compressive strength degradation was tested over 350 cycles of freezing and thawing: the strength of the 0.4, 0.5 or 0.6 wt % samples was 91, 89, and 82%, respectively, relative to the control specimen (0 wt %). Thus, all specimens had a compressive strength of 80% or more after 350 cycles compared to the control specimen. To test the adhesion performance for new thin overlays and old concrete surfaces, concrete cylinders (100 × 200 mm; thickness = 10 mm) were cut at an angle of 46 degrees, and the pitch-based carbon fiber-mortar composite was used to bond the various sections. The bond strength of the test specimens was more than twice that of the reference specimen.

Hygrothermal and Acoustical Performance of Starch-Beet Pulp Composites for Building Thermal Insulation.

This article deals with the elaboration and the characterization of an innovative 100% plant-based green composite made solely of beet pulp (BP) and potato starch (S). Using this type of material in insulation applications seems a good solution to reduce the CO2 gas emissions in building. The influence of the starch amount on composite characteristics was studied. Four mixtures were considered with different S/BP mass ratios (0.1, 0.2, 0.3 and 0.4). The physical properties of these materials were studied in terms of porosity, apparent and absolute densities, thermal conductivity, and hygric properties. The influence of humidity content on acoustical properties was studied as a function of frequency. Test results show a real impact of both starch and humidity contents on the hygrothermal and acoustical properties of the studied material due to the porosity. The composite with the lowest amount of starch (S/BP = 0.1) seems to be the optimal composition in terms of the hygrothermal and acoustical behaviors.

Hygrothermal performance of various Typha–clay composite

This article deals with the influence of both morphology and amount of Typha on hygrothermal behavior of a Typha–clay composite for building application. An agromaterial containing the fiber mix of Typha Australis and clay was made in three samples: three fiber mixtures were prepared with different amounts Typha and cut type (transversal or longitudinal). The physical properties of these materials were studied in terms of porosity, apparent and absolute density, thermal conductivity, and hygric properties. Results show a real impact of the Typha fraction type and its volume content on hygrothermal properties of the studied material due to the porosity. The transversal fraction of Typha (80% in volume weight) seems to be the optimal composition for a better hygrothermal behavior.

Effect of simultaneous K, and Yb substitution for Ca on the microstructural and thermoelectric characteristics of CaMnO3 ceramics

CaMnO3-based materials are very attractive among n-type thermoelectric oxides for high-temperature applications when they are appropriately doped. The main drawback of these materials is the cost associated to the necessary rare earth cations. This work aims decreasing the amount of these materials through a partial substitution of Ca2+ by an equimolar mixture of K+ and Yb3+, Ca1-x(K0.5Yb0.5)xMnO3, with x = 0.05, 0.10, 0.15, and 0.20. XRD studies have confirmed that the thermoelectric phase is the major one in all samples. Microstructure has shown the formation of large crystals, and an increasing porosity when the substitution is raised. This evolution has been confirmed through density measurements. Electrical resistivity has been drastically decreased for the 0.10 substituted samples, compared with the 0.05 ones, slightly increasing for higher substitution. On the other hand, absolute Seebeck coefficient and thermal conductivity are lower when the substitution is raised. The best ZT values have been achieved for the 0.10 substituted samples, which are around the typical reported in the literature for higher doping level. These results clearly point out to a decrease of the necessary rare earth dopant content to achieve similar performances in CaMnO3 ceramics, which is of the main economic significance for their industrial production.

Dimensionless Scaling Parameters During Thermal Flooding Process in Porous Media

The scaling concept is important, effective, and consistent in any application of science and engineering. Scaled physical models have inimitable advantages of finding all physical phenomena occurring in a specific process by transforming parameters into dimensionless numbers. This concept is applicable to thermal enhanced oil recovery (EOR) processes where continuous alteration (i.e., memory) of reservoir properties can be characterized by various dimensionless numbers. Memory is defined as the continuous time function or history dependency which leads to the nonlinearity and multiple solutions during modeling of the process. This study critically analyzed sets of dimensionless numbers proposed by Hossain and Abu-Khamsin in addition to Nusselt and Prandtl numbers. The numbers are also derived using inspectional and dimensional analysis (DA), while memory concept is used to develop some groups. In addition, this article presents relationships between different dimensionless numbers. Results show that proposed numbers are measures of thermal diffusivity and hydraulic diffusivity of a fluid in a porous media. This research confirms that the influence of total absolute thermal conductivities of the fluid and rock on the effective thermal conductivity of the fluid-saturated porous medium diminishes after a certain local Nusselt number of the system. Finally, the result confirms that the convective ability of the fluid-saturated porous medium is apparently more pronounced than its conductive ability. This study will help to better understand the modeling of the EOR process thus improving process design and performance prediction.

Time Resolved Temperature Measurement of Single Gold Structures via Luminescence Thermometry

ABSTRACTHere we report the use of luminescence thermometry to measure the temperature decay from single gold structure into the substrate of AlGaN:Er3+ film. We looked at Er3+ ion photoluminescence upon illumination by modulated 532 nm laser and recorded time-resolved luminescence of 2H11/2 → 4I15/2 and the 4S3/2 → 4I15/2 energy transitions. We calculated the heat generated from gold microdisk and observed the rate of heat dissipation to the environment. We directly calculated the absolute thermal conductivity of 1.7 W/mK for AlGaN: Er3+ film which was in agreement with the literature.

Further elucidation of nanofluid thermal conductivity measurement using a transient hot-wire method apparatus

The Transient Hot-Wire Method (THWM) was developed to measure the absolute thermal conductivity of gases, liquids, melts, and solids with low uncertainty. The majority of nanofluid researchers used THWM to measure the thermal conductivity of test fluids. Several reasons have been suggested for the discrepancies in these types of measurements, including nanofluid generation, nanofluid stability, and measurement challenges. The details of the transient hot-wire method such as the test cell size, the temperature coefficient of resistance (TCR) and the sampling number are further investigated to improve the accuracy and consistency of the measurements of different researchers. It was observed that smaller test apparatuses were better because they can delay the onset of natural convection. TCR values of a coated platinum wire were measured and statistically analyzed to reduce the uncertainty in thermal conductivity measurements. For validation, ethylene glycol (EG) and water thermal conductivity were measured and analyzed in the temperature range between 280 and 310 K. Furthermore, a detailed statistical analysis was conducted for such measurements, and the results confirmed the minimum number of samples required to achieve the desired resolution and precision of the measurements. It is further proposed that researchers fully report the information related to their measurements to validate the measurements and to avoid future inconsistent nanofluid data.

Thermal conductivity model for powdered materials under vacuum based on experimental studies

The thermal conductivity of powdered media is characteristically very low in vacuum, and is effectively dependent on many parameters of their constituent particles and packing structure. Understanding of the heat transfer mechanism within powder layers in vacuum and theoretical modeling of their thermal conductivity are of great importance for several scientific and engineering problems. In this paper, we report the results of systematic thermal conductivity measurements of powdered media of varied particle size, porosity, and temperature under vacuum using glass beads as a model material. Based on the obtained experimental data, we investigated the heat transfer mechanism in powdered media in detail, and constructed a new theoretical thermal conductivity model for the vacuum condition. This model enables an absolute thermal conductivity to be calculated for a powder with the input of a set of powder parameters including particle size, porosity, temperature, and compressional stress or gravity, and vice versa. Our model is expected to be a competent tool for several scientific and engineering fields of study related to powders, such as the thermal infrared observation of air-less planetary bodies, thermal evolution of planetesimals, and performance of thermal insulators and heat storage powders.

Continuous Carbon Nanotube-Based Fibers and Films for Applications Requiring Enhanced Heat Dissipation.

The production of continuous carbon nanotube (CNT) fibers and films has paved the way to leverage the superior properties of individual carbon nanotubes for novel macroscale applications such as electronic cables and multifunctional composites. In this manuscript, we synthesize fibers and films from CNT aerogels that are continuously grown by floating catalyst chemical vapor deposition (FCCVD) and measure thermal conductivity and natural convective heat transfer coefficient from the fiber and film. To probe the mechanisms of heat transfer, we develop a new, robust, steady-state thermal characterization technique that enables measurement of the intrinsic fiber thermal conductivity and the convective heat transfer coefficient from the fiber to the surrounding air. The thermal conductivity of the as-prepared fiber ranges from 4.7 ± 0.3 to 28.0 ± 2.4 W m(-1) K(-1) and depends on fiber volume fraction and diameter. A simple nitric acid treatment increases the thermal conductivity by as much as a factor of ∼3 for the fibers and ∼6.7 for the thin films. These acid-treated CNT materials demonstrate specific thermal conductivities significantly higher than common metals with the same absolute thermal conductivity, which means they are comparatively lightweight, thermally conductive fibers and films. Beyond thermal conductivity, the acid treatment enhances electrical conductivity by a factor of ∼2.3. Further, the measured convective heat transfer coefficients range from 25 to 200 W m(-2) K(-1) for all fibers, which is higher than expected for macroscale materials and demonstrates the impact of the nanoscale CNT features on convective heat losses from the fibers. The measured thermal and electrical performance demonstrates the promise for using these fibers and films in macroscale applications requiring effective heat dissipation.

Thermal conductivity reduction in analogous 2D nanomaterials with isotope substitution: Graphene and silicene

We employ molecular dynamics simulations to understand how the presence of isotopes influences thermal transport across silicene, and compare the findings with that in structurally analogous graphene. The simulated structures are about 140nm long and around 4nm wide. The phonon spectra along with the variation of thermal conductivity reveal that out-of-plane modes are delocalized relative to the in-plane counterparts. The absolute thermal conductivity reductions are more pronounced in graphene than in silicene. Our computational findings agree with results of an analytical model based on mean-field approximation with appropriate corrections for the lattice anharmonicity.

Scanning Thermal Microscopy of Thermoelectric Nanostructures

We present the development and results of a new simple method for thermal conductivity characterization of thin films and thermoelectric structures using a scanning thermal microscope in pulsed current mode. The presented method does not allow measurement of absolute thermal conductivity of the studied system, but only relative to the Si substrate. We present the results of the method on the Si substrate/layer step boundary. The nano-layers of different thickness and different materials were prepared for the experiments by the pulsed laser deposition from hot-pressed targets.

Determination of the thermal conductivity of periodic APM foam models

Advanced pore morphology (APM) foam elements have a spherical outer skin and a porous inner structure. In this study, the method of Lattice Monte Carlo is applied to determining the thermal characterisation of periodic structures formed by spherical APM foam elements. Two diameters, i.e. 5mm and 10mm spheres, are considered. To this end, micro-computed tomography data of real samples is converted into numerical calculation models. This procedure allows the accurate geometric representation of the complex internal foam geometry. Lattice Monte Carlo is then used to obtain the effective thermal conductivity of partial and syntactic structures made up of APM foam elements. Samples are analysed for variation in absolute and directional (anisotropy) thermal conductivity.

Effects of Al/Sb Double Doping on the Thermoelectric Properties of Mg2Si0.75Sn0.25

Al/Sb double-doped Mg2Si0.75Sn0.25 materials were prepared by liquid–solid reaction synthesis and the hot-pressing technique. The effects of Al/Sb double doping on the thermoelectric properties were investigated at temperatures between room temperature and 900 K, and the resistivity and Hall coefficient were investigated at 80 K to 900 K. Al/Sb double-doped samples were found to be n-type semiconductors in the investigated temperature range. The absolute Seebeck coefficient (α), resistivity (ρ), and thermal conductivity (κ) for Al/Sb double-doped samples at room temperature were in the ranges of 152.5 μV K−1 to 109.2 μV K−1, 2.92 × 10−5 Ω m to 1.29 × 10−5 Ω m, and 2.50 W K−1 m−1 to 2.86 W K−1 m−1, respectively. The absolute values of α increased with increasing temperature up to a maximum, and decreased thereafter. This could be attributed to mixed carrier conduction in the intrinsic region. κ decreased linearly with increasing temperature to a minimum near the intrinsic region, then increased rapidly because of bipolar components. The highest ZT value measured was 0.94 at 850 K for Mg1.9975Al0.0025Si0.75Sn0.2425Sb0.0075. Sb doping was effective for enhancement of ZT, because of a remarkable increase in the carrier concentration. However, Al doping was almost ineffective for enhancing ZT.