Thermal Conductivity Of Samples Research Articles

Formation of Carbon with One-Dimensional Nanostructure on the Surface of Graphite Coated by Ni-Salt

In this paper, the catalyst precursors were coated on the surface of natural flake graphite with micron size by liquid coating method. Carbon with one-dimensional nanostructures formed on the surface of graphite after the heat treatment of the mixture of phenolic resin and coated graphite. The effects of the forms and amounts of catalyst precursors and the temperature on the formation of carbon with one-dimensional nanostructures were studied. And the thermal conductivities of the samples were measured. The results showed that the catalyst precursor with the form of NiO would be better. And the optimum amounts of the catalyst precursor were 1mass % ~2 mass % of resin. It was not conducive to the reaction when the temperature was too low or too high, but 1100°C was appropriate. The thermal conductivities of samples were increasing with the increase of the qualities of carbon with one-dimensional nanostructure.

Dynamic Measurements of the Thermal Conductivity of Insulators

Measurements of the thermal conductivity of insulators that are commonly used in civil engineering are as a rule performed using Ponsgen’s guarded hot-plate method under steady-state conditions. Achieving these steady-state conditions is a time consuming and relatively expensive procedure. Therefore, the application of a method that is less time consuming and less costly to common building insulating materials is of interest. The method should also have the accuracy and repeatability comparable to that of presently used methods. One such method is the transient hot-wire method (predominantly used for liquids, non-Newtonian fluids, plastics, semi-plastics, and similar materials), a dynamic method that uses a very thin pure platinum wire that functions as a thermal source in combination with a temperature sensor that detects temperature transients. This article describes the application of the transient hot-wire method to most commonly used building thermal insulating materials. The transient hot-wire measurements of the thermal conductivity were performed on many building material samples. For the sake of comparison, the thermal conductivity of samples made from the same materials was also tested using the stationary Ponsgen’s guarded hot-plate method. This article describes the comparison and evaluation of the measurement results obtained from both methods as well as the estimation of pertinent measurement uncertainties. The results are presented in graphical and numerical form in tables and diagrams for each type of thermal insulator.

Permeability and thermal conductivity of host compressed natural graphite for consolidated activated carbon adsorbent

Permeability and thermal conductivity test units were set up to study the heat and mass transfer performance of the host material, i.e. expanded natural graphite (ENG), for consolidated activated carbon (AC) adsorbent. The permeability was tested with nitrogen as the gas source, and the thermal conductivity was studied using steady-state heat source method. The results showed that the values of permeability and thermal conductivity were 10−15 to 10−12 m2 and 1.7 to 3.2 W/(m·K), respectively, while the density compressed expanded natural graphite (CENG) varied from 100 to 500 kg/m3. The permeability decreased with the increasing density of CENG, whereas the thermal conductivity increased with the increasing density of CENG. Then the thermal conductivity and permeability of granular AC were researched. It was discovered that the thermal conductivity of samples with different grain size almost kept constant at 0.36 W/(m·K) while the density was approximately 600 kg/m3. This means that the thermal conductivity was not related to the grain size of AC. The thermal conductivity of CENG was improved by 5 to 10 times compared with that of granular AC. Such a result showed that CENG was a promising host material for AC to improve the heat transfer performance, while the mass transfer performance should be considered in different conditions for utilization of adsorbent.

Effect of Nanoparticle Coating on the Thermal Conductivity of Microporous Thermal Insulations

Microporous thermal insulations were prepared from mixtures of nano-sized fumed silica, micron-sized fibers and opacifier particles. Those micron-sized particles were dry coated with nano-sized fumed silica particles by mechanical process using a compressive-shear type mill. The effect of nanoparticle coating on the thermal conductivity of the insulation media was investigated using a hot-wire method. Effect of nanoparticle coating was found to be more pronounced for the insulation composed of fumed silica and fiber than for the one composed of fumed silica, fiber and an opacifier. By adding 15% SiC or TiO2 opacifier, the thermal conductivity of the insulation samples could be lowered to 0.08 Wm(-1) K(-1) at temperature range of 805 approximately 817 degrees C. The temperature dependent thermal conductivity of the sample containing glass fiber did not exhibit any remarkable changes compared to the one containing ceramic fiber.

Effect of Reactive-Al<sub>2</sub>O<sub>3</sub> Addition on the Pore Size Distribution and Thermal Conductivity of Carbon Blocks for Blast Furnace

The effect of reactive-Al2O3 addition on the pore size distribution and thermal conductivity of carbon blocks for blast furnace was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray, mercury porosimetry, and a laser thermal conductivity meter. The results showed that the pore size distribution and the thermal conductivity of carbon block were mainly influenced by SiC whiskers and reactive-Al2O3. With increasing addition of reactive-Al2O3, the mean pore diameter reduced and < 1μm pore volume increased of open pores owing to the more efficient filling of pores by SiC and reactive-Al2O3, and the thermal conductivities of samples increased due to the facts that the higher thermal conductivity of reactive-Al2O3 than that of brown corundum and the more formation amount of high thermal conductivity of SiC.

Relationship between changes in the crystal lattice strain and thermal conductivity of high burnup UO2 pellets

Abstract Two kinds of disk-shaped UO2 samples (4 mm in diameter and 1 mm in thickness) were irradiated in a test reactor up to about 60 and 130 GWd/t, respectively. The microstructures of the samples were investigated by means of optical microscopy, scanning electron microscopy/ electron probe micro-analysis (SEM/EPMA) and micro-X-ray diffractometry. The measured lattice parameters tended to be considerably smaller than the reported values, and the typical cauliflower structure which is often observed in high burnup fuel pellet is hardly seen in these samples. Thermal diffusivities of the samples were also measured by using a laser flash method, and their thermal conductivities were evaluated by multiplying the heat capacity of unirradiated UO2 and sample densities. While the thermal conductivities of sample 2 showed recovery after being annealed at 1500 K, those of sample 4 were not clearly observed even after being annealed at 1500 K. These trends suggest that the amount of accumulated irradiation-induced defects depends on the irradiation condition of each sample. From the comparison of the changes in the lattice parameter and strain energy density before and after the thermal diffusivity measurements, it is likely that the thermal conductivity recovery in the temperature region from 1200 to 1500 K is related to the migration of dislocation.

Influence of moisture content on measurement accuracy of porous media thermal conductivity

AbstractThe thermal conductivity measurement accuracy of sand was experimentally studied with a hot disk thermal constant analyzer and water morphologies, distribution, and evolution at the pore scale were observed with a charge coupled device (CCD) combined with a microscope. It was found that thermal conductivities of samples with low moisture content (<25%) could not be accurately measured. For samples with low moisture content, the analysis showed that the water in the region adjacent to the analyzer sensor mainly existed as isolated liquid bridges between/among sand particles and would evaporate and diffuse to relatively far regions because of being heated by the sensor during measurement. Water evaporation and diffusion caused the sample constitution in the region adjacent to the sensor to vary throughout the whole measurement process, and accordingly induced low accuracy of the obtained thermal conductivities. Due to high water connectivity in pores, the rate of water evaporation and diffusion in porous media of high moisture content was relatively slow when compared with that of low moisture content. Meanwhile, water in the relatively far regions flowed back to the region adjacent to the sensor by capillary force. Therefore, samples consisting of the region adjacent to the sensor maintained the constant and thermal conductivities of porous media with relatively high moisture content and could be measured with high accuracy. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20272

Thermo-electrical characterization of Sn–Zn alloys

The thermo-electrical properties of Sn–Zn alloys have been investigated for four different compositions. The SEM micrographs and EDX analysis of the samples have been obtained. The electrical resistivity measurements were performed by using four-point probe technique in the temperature range 300 K–575 K. The resistivity of samples increases linearly with temperature and the electrical conductivity is inversely proportional to temperature. The electrical current preferentially flows through the pure Sn phase having lower resistivity in the matrix. Also, thermal conductivity of samples has been measured by using the radial heat flow method. It has been found that the thermal conductivity decreases with the increasing temperature and composition of Sn. The results were consistent with available experimental results of other studies. Finally, the temperature coefficient of electrical resistivity and the temperature coefficient of thermal conductivity have been determined, which were independent from the composition of Sn and Zn.

Plateau in the temperature dependence of the thermal conductivity of solid hydrogen containing heavy isotopic neon impurities at ultimately low concentrations

The thermal conductivity of solid hydrogen with 0.0001–0.0002 at. % Ne in the form of equilibrium samples grown at a low rate after remelting is investigated in the temperature range 1.5–10.0 K. It is demonstrated that the temperature dependence of the thermal conductivity for the samples containing neon at concentrations considerably lower than the limiting solubility of the heavy impurity exhibits a symmetric plateau. This behavior of the temperature dependence of the thermal conductivity differs qualitatively from the previously observed resonance minimum in the temperature dependence of the thermal conductivity for desublimated samples. A relaxation model is proposed for explaining the observed effect. According to this model, the plateau is explained by the formation of linear impurity structures that are located along dislocation lines and considerably enhance phonon scattering by dislocation cores. The density of linear impurity structures is estimated. The influence of these structures on the thermal conductivity is compared with the corresponding effect of uniformly distributed individual neon atoms in solid hydrogen.

Thermal conductivity of composite catalysts containing metallic copper as a reinforcing component

The dependences of the electrical and thermal conductivities of porous composite materials containing a metallic component (copper) on the volume copper content are investigated experimentally. The measured thermal conductivities of samples prepared according to the proposed technique indicate that the thermal conductivity of monolithic catalysts with a copper content of no less than 15 vol % exceeds 1 W m−1 K−1. This corresponds to the formation of a connected cluster consisting of conducting spheres in a random packing of conducting and insulating spheres. A comparative analysis of the thermal and electrical conductivities of the composites demonstrates that, at a copper content of higher than 20 vol %, the thermal conduction through a percolation cluster formed by copper particles makes the dominant contribution. In employing composite materials containing a catalytically active component in exothermic catalytic processes (Fischer-Tropsch synthesis, steam conversion of carbon monoxide CO, etc.), their high thermal conductivity is an important advantage that makes it possible to decrease the temperature gradient across the porous composite catalyst bed. A semiempirical method for calculating the thermal conductivity of composites is developed. The results of the calculations performed using the proposed method are in good agreement with experimental data.

Thermal conductivity of CaO–SiO2–Al2O3 glassy slags: Its dependence on molar ratios of Al2O3/CaO and SiO2/Al2O3

Thermal conductivities of CaO–SiO2–Al2O3 glassy slags have been determined using the nonstationary hot wire method as functions of (mol.-%Al2O3)/(mol.-%CaO) and (mol.-%SiO2)/(mol-%Al2O3) ratios over a temperature range 300–1270 K. The thermal conductivities of the samples investigated were in the range between 1·0 and 1·6 W m−1 K−1. The thermal conductivities of samples having a constant concentration of SiO2 increased with increasing the (mol.-%Al2O3)/(mol.-%CaO) ratio as long as the constant pressure heat capacity per unit volume was kept constant. This increase in thermal conductivities would be due to increases in the speed of sound and/or the phonon mean free path by re-polymerisation of the network structure of silicate glasses by Al2O3. On the other hand, the thermal conductivities of samples having a constant value of NBO/T (the ratio of the number of non-bridging oxygen ions per tetrahedrally coordinated cation) decreased with increasing the (mol.-%SiO2)/(mol.-%Al2O3) ratio. This decrease in thermal conductivities would be dominated by the constant pressure heat capacities per unit volume of the samples.

Prediction of thermal conductivity of kernels and a packed bed of brown rice

The effective thermal conductivity of brown rice was measured by the transient heat flow method using twin probes at selected moisture contents, temperatures and bulk densities. Air and corn oil were used as fluids to fill the gap in the sample-packed bed. The effects of temperature, bulk density, and moisture content on the effective thermal conductivity of samples were investigated. The thermal conductivity of brown rice kernels was estimated using a compound model of series and parallel structural heat transfer models, and represented as a function of moisture content. In addition, the partial area of the fluid layer arranged in parallel toward the heat flow of the compound model was related to temperature and porosity, and a new model to predict the effective thermal conductivity of brown rice was proposed.

Thermal conductivity in a sample simulating the winding structure of Bi-2223/Ag epoxy impregnated coils

We measured the effective thermal conductivity in samples simulating the structure of Bi-2223/Ag impregnated coils at liquid nitrogen temperature. We also studied the effect of the sample orientation with respect to the coolant surface: samples with the cooled surface in the horizontal and vertical positions were measured. The effect of heat drains located between the layers of the tape on the effective thermal conductivity was studied. Its increase is important for coils conductively cooled by refrigerators. Our experimental results are compared with those obtained by other authors using model calculations.

The role of heat-conducting walls in the measurement of heat and mass transport in transient grating experiments

We present a two-dimensional model to account for the role of heat-conducting walls in the measurement of heat transport and Soret-effect--driven mass transport in transient holographic grating experiments. Heat diffusion into the walls leads to non-exponential decay of the temperature grating. Under certain experimental conditions it can be approximated by an exponential function and assigned an apparent thermal diffusivity D(th, app)<D(th, s), where D(th,s) is the true thermal diffusivity of the sample. The ratio D(th, app)/D(th, s) depends on only three dimensionless parameters, d/l(s), kappa(s)/kappa(w), and D(th, s)/D(th, w). d is the grating period, l(s) the sample thickness, kappa(s) and kappa(w) the thermal conductivities of sample and wall, respectively, and D(th,w) the thermal diffusivity of the wall. If at least two measurements are performed at different d /l(s), both D(th,s) and kappa(s) can be determined. Instead of costly solving PDEs, D(th,s) can be obtained by finding the zero of an analytic function. For thin samples and large grating periods, heat conduction into the walls plays a predominant role and the concentration grating in binary mixtures is no longer one-dimensional. Nevertheless, the normalized heterodyne diffraction efficiency of the concentration grating remains unaffected and the true thermal and collective diffusion coefficient and the correct Soret coefficient are still obtained from a simple one-dimensional model.

Utilization of boon and chaff for manufacturing lightweight walling materials

Abstract Lightweight composites for walls and thermal insulation, containing anhydrite (An) or aluminate cement (Al), vinyl acetate (VA) or cellulose (Cl) polymeric binders and cellulose fiber fillers (boon, chaff) were produced. The best results were obtained for transportation and construction of items containing: An ≥ 30–45 %, Al ≥ 30–50 %, VA ≥ 1–5 %, Cl ≥ 0,5–5,0 %, boon or chaff ≥ 40–47 %. Polymeric binder VA for both kinds of cement is better than Cl. An is better for boon than for chaff. Aluminate cement is a good binder for both types of fiber fillers. Density r of a composite containing cements 50–60 % is less than 400 kg/m3. According to density such composite materials are light‐weight heat‐insulating materials. Density (p ≤ kg/m3) depends on the amount of cement content. Bending strength for samples with p ≤ 400 kg/m3, containing CMC is 0,6–1,3 MPa. Coefficient of thermal conductivity for samples, density with 400 kg/m3 is 0,06 W/m‐K.

UTILIZATION OF BOON AND CHAFF FOR MANUFACTURING LIGHTWEIGHT WALLING MATERIALS

Lightweight composites for walls and thermal insulation, containing anhydrite (An) or aluminate cement (Al), vinyl acetate (VA) or cellulose (Cl) polymeric binders and cellulose fiber fillers (boon, chaff) were produced. The best results were obtained for transportation and construction of items containing: An ≥ 30–45 %, Al ≥ 30–50 %, VA ≥ 1–5 %, Cl ≥ 0,5–5,0 %, boon or chaff ≥ 40–47 %. Polymeric binder VA for both kinds of cement is better than Cl. An is better for boon than for chaff. Aluminate cement is a good binder for both types of fiber fillers. Density r of a composite containing cements 50–60 % is less than 400 kg/m3. According to density such composite materials are light‐weight heat‐insulating materials. Density (p ≤ kg/m3) depends on the amount of cement content. Bending strength for samples with p ≤ 400 kg/m3, containing CMC is 0,6–1,3 MPa. Coefficient of thermal conductivity for samples, density with 400 kg/m3 is 0,06 W/m‐K.

The Effect of Structural Defects on the Thermal Conductivity of ZnS, ZnSe, and CdTe Polycrystals

ZnS, ZnSe, and CdTe polycrystals are experimentally investigated. Mechanisms are treated which restrict the thermal conductivity in samples prepared by recrystallization pressing and by deposition from the vapor phase and subjected to additional strain. Anomalies are observed on the temperature dependences of the thermal resistance of investigated samples, which are due to the special features of their phonon spectra and to the variation of the contribution made by longitudinal and transverse phonon branches to the heat transfer in the Debye temperature region and higher.

Measurement of the thermal conductivity of thin layers using a scanning thermal microscope

A scanning thermal microscope (SThM) was used to measure the thermal conductivity of thin sputter-deposited films in the thickness range of 10 nm–10 μm. The SThM method is based on a heated tip that is scanned across the surface of a sample. The heat flowing into the sample is correlated to the local thermal conductivity of the sample. Issues like the contact force, the surface roughness of the sample, and tip degradation, which determine to a great extent the contact area between tip and surface, and thus the heat flow to the sample, are addressed in the paper. A calibration curve was measured from known reference materials to quantify the sample heat flow. This calibration was used to determine the effective thermal conductivity of samples. Further, the heat diffusion through a layered sample due to a surface heat source was analyzed with an analytical and numerical model. Measurements were performed with films of aluminum, ZnS–SiO2, and GeSbTe phase change material of variable thickness and sputter-deposited on substrates of glass, silicon, or polycarbonate. It is shown in the paper that the SThM is a suitable tool to visualize relative differences in thermal structure of nanometer resolution. Determination of the thermal conductivity of thin layers is possible for layers in the micrometer range. It is concluded that the SThM is not sensitive enough to measure accurately the thermal conductivity of thin films in the nanometer range. Suggestions for improvement of the SThM method are given.

Thermal conductivity of coal ash and slags and models used

A one-dimensional heat transfer method was used to determine the thermal conductivity for a range of coal ash and synthetic ash samples at elevated temperatures. The effect of parameters such as temperature, porosity, and sintering time were investigated. The thermal conductivity of the samples was generally observed to increase with increasing temperature. During heating of the samples, softening of minerals and sintering reactions resulted in changes in the physical structure of the ash, which then altered the observed thermal conductivity. The thermal conductivity of sintered ash samples was found to be higher than that of unsintered samples. The sintering temperature and sintering time were found to increase the observed thermal conductivity irreversibly. A decrease in sample porosity was also observed to increase the thermal conductivity. Chemical composition was found to have little effect on the thermal conductivity, apart from influencing the extent of sintering. Predictions of the thermal conductivity of ash samples based on Rayleigh's model are also presented. The thermal conductivity of slag and particulate structures was modelled by considering spherical pores distributed in a continuous slag phase. A particulate layer structure was modelled by considering solid particles dispersed in a continuous gas phase. The Brailsford and Major model of random distribution for mixed phases gives results within 20% of the measured values for a partially sintered sample.

Thermal properties of vesicular rhyolite

Abstract Thermal diffusivity of rhyolite melt and rhyolite foam (70–80% porosity) has been measured using the radial heat transfer method. Cylindrical samples (length ∼ 50–55 mm, diameter ∼ 22 mm) of rhyolite melt and foam have been derived by heating samples of Little Glass Mountain obsidian. Using available data on heat capacity and density of rhyolite melt, the thermal conductivity of samples has been determined. The difference in thermal conductivity between rhyolite melt and foam at igneous temperatures (∼ 1000°C) is about one order of magnitude. The effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection.