Thermal Conductivity Of Samples Research Articles

Thermo-mechanical characterization of a building material based on Typha Australis

This paper is a contribution to the valorization of Typha Australis as building material. To do this, the crushed fibers of Typha have been agglomerated using clay as binder. The weight percentage of binder is found to be at least 75% to obtain stable materials. The influence of binder content on mechanical and thermal insulation properties was examined. The mechanical properties were evaluated by using a mechanical press. The measurements of thermal effusivity and thermal conductivity of samples have been performed using the transient hot-plate method. Compression and tensile strengths respectively varies from 0.279 to 0.796MPa and from 0.340 to 0.969MPa when the weight percentage of binder range from 77–85%. This values have a strong linear correlation with the weight percentage of binder. The thermal conductivity of dry materials varies from 0.117 to 0.153 W.m−1.K−1 while the thermal effusivity rises from 228.9 to 300.0 J.m−2.°C−1.s−12. The results show that the thermal conductivity and thermal effusivity increases with increasing moisture content of materials.

تاثیر دما و میزان ماده جامد محلول بر خصوصیات فیزیکی– حرارتی عصاره مالت

تعیین خواص حرارتی- فیزیکی عصاره مالت جهت تخمین سرعت انتقال حرارت، تولید، حمل ونقل و نگهداری همچنین ارزیابی و مدل سازی فرایند هایی از قبیل خشک کردن و تغلیظ ضروری است. در این پژوهش خواص حرارتی– فیزیکی عصاره مالت شامل گرمای ویژه، ضریب هدایت حرارتی، ضریب انتشار حرارتی و جرم حجمی در سه سطح ماده جامد محلول 60،70 و 80 درصد و در محدوده دمایی 25 تا 90 درجه سانتی گراد بررسی و تعیین گردید. نتایج حاصل از تحلیل داده‌ها نشان می‌دهد که هم درصد ماده جامد وهم دما بر خواص حرارتی– فیزیکی عصاره مالت تاثیر معنی داری دارند. با افزایش درصد ماده جامد گرمای ویژه،ضریب هدایت حرارتی و ضریب انتشار حرارتی کاهش یافته ولی جرم حجمی افزایش می-یابد. همچنین با افزایش دما گرمای ویژه، ضریب هدایت حرارتی و ضریب انتشار حرارتی افزایش یافته ولی دانسیته کاهش می یابد. از سوی دیگر با تجزیه و تحلیل رگرسیون چند گانه، مدل های پیش بینی کننده خواص حرارتی– فیزیکی عصاره مالت بر مبنای متغیر های کاری تعیین گردید.

Room temperature screening of thermal conductivity by means of thermal transient measurements

A proof of concept of the possibility to estimate thermal conductivity of bulk disc samples at room temperature by means of thermal decays is demonstrated. An experimental set-up was designed and fabricated, which is able to perform thermal transient measurements by using a specially designed multifunctional probe that has the ability to measure temperature at its tip. Initially, the probe is heated by a heater coil located in its interior until the tip temperature reaches a steady state. Then, the probe is contacted with a disc sample which produces a temperature decay until a new state is reached. The difference between the initial and final states temperatures shows a correlation with the thermal conductivity of the sample. Employing a calibration equation, obtained using reference materials, the thermal conductivity can be calculated. Reasonably good random and systematic errors (<13% and ~9% respectively) are obtained. Theoretical simulations performed using COMSOL show a good qualitative agreement with experimental results. This new method involves an inexpensive and simple set-up which can be especially useful for thermal conductivity screening and high-throughput measurements.

Effects of Concentrations of NaCl on the Sintering and Thermal Conductivity of Forsterite

This paper aims to study the effects of NaCl on the sintering of forsterite as well as the thermal conductivity of the sintering products. In the sintering process, NaCl played a role in the system by mainly providing a liquid phase sintering environment in order to promote grain growth and forsterite sintering. Moreover, the model of liquid phase sintering was established in this paper. In the initial sintering stage, the phenomenon of particle re-arrangement was not significant, and dissolution-precipitation was regarded as the dominant sintering mechanism. With the extension of the sintering time, the middle and later stages of the sintering process were mainly controlled by the diffusion mechanism. A test of the thermal conductivity of samples with 40% and 50% NaCl was carried out, which indicated that the thermal conductivity of the two samples ranged from 0.3 to 0.4 W·m-1·k-1. At the same temperatures, the thermal conductivity of the sample with a salt content of 50% was lower than that of the sample with a salt concentration of 40%. Also, the heat insulation effect of the sample with a salt content of 50% was better than that of the sample of 40% . Hence, it can be concluded that the higher the salt content in the sample, the lower the thermal conductivity of the sample.

Thermal Conductivity of Silicon Thin Films Deposited On Porous Alumina by PECVD

In this paper we have studied the thermal properties of silicon thin films deposited by plasma enhancement chemical vapor deposition (PECVD) on porous aluminum using the phothermal deflection technique (PTD). The aim of this work is to investigate the influence of anodisation current (between 200 to 400 mV) on the thermal conductivity of samples. We present a computational model to determine the thermal conductivity K. The coincidences between the experimental and theoretical curves permit to deduce the values of thermal conductivity with a good accuracy. In fact, it was found that K decrease with the anodisation current.

Thermal and Flammability Properties of Polypropylene/Silica Aerogel Composites

Polypropylene (PP)/silica aerogel (SA) composites were prepared and their thermal and flammability properties were studied. The PP/SA composites with different weight percent were prepared via melt compounding method using an internal mixer. Their morphology, thermal conductivity, thermal stability and combustion behavior were characterized. The SEM images confirmed the homogenous mixing of the components. The measurement of the thermal conductivities of samples indicated that PP would be a better thermal barrier in the presence of SA. The thermal gravimetric analysis results showed that combining the silica aerogel particles into polypropylene increased the decomposition temperature. The resultant composites displayed improved flame retardancy with a significant reduction in the peak heat release rate and increase of limited oxygen index value. It can be concluded that the flame retardant mechanism of PP/SA composites is associated with two decisive factors: a coat-like char effect and a physical crosslinking effect.

Thermoelectric properties of In-filled and Te-doped CoSb3 prepared by high-pressure and high-temperature

A novel chemical alloying method of high-pressure and high-temperature (HPHT) has been used for the synthesis of bulk-skutterudite [Formula: see text]. Through HPHT method, the synthesis time has been shortened from a few days to 30[Formula: see text]min. The samples of [Formula: see text] skutterudites were synthesized at 1.8–3.3[Formula: see text]GPa. We have studied the phase, the microstructure, and the temperature-dependent thermoelectric properties. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the temperature range of 295–673[Formula: see text]K. As we expected, the thermal conductivity of sample [Formula: see text] decreased with the increase of the synthetic pressure. A maximal ZT of 0.64 was achieved for the [Formula: see text] synthesized at 1.8[Formula: see text]GPa at 673[Formula: see text]K. These results revealed that HPHT method may be helpful for optimizing electrical conductivity and thermal conductivity in a comparatively independent way.

Intrinsic thermal conductivity in monolayer graphene is ultimately upper limited: A direct estimation by atomistic simulations

In the sample size domain so far explored, the thermal conductivity of graphene shows an intriguing dependence on the sample length ${L}_{z}$ along the heat flux direction. An extrapolated infinite value for such a thermal conductivity is sometimes suggested for infinite samples, while other investigations predict anyway an upper limit for it. We address this issue by avoiding any guess or approximation on the underlying microscopic heat transport mechanisms; we rather perform direct atomistic simulations aimed at estimating thermal conductivity in samples with increasing size up to the unprecedented value of ${L}_{z}=0.1$ mm. Our results provide evidence that thermal conductivity in graphene is definitely upper limited in samples long enough to allow a diffusive transport regime for both single and collective phonon excitations.

Simultaneous measurements of high-temperature total hemispherical emissivity and thermal conductivity using a steady-state calorimetric technique

A method was developed to simultaneously measure the total hemispherical emissivity and the thermal conductivity of samples at high temperatures. The inverse problem to determine the emissivity and thermal conductivity from steady-state high-temperature calorimetric experiments was established based on models for these two quantities. The accuracy of the inverse solution was numerically analyzed for various noise levels for samples with various thermophysical properties. The simulation results illustrate that the calculation accuracies for the emissivity and thermal conductivity strongly depend on the proportions of the radiation and conduction heat fluxes in the strip sample arising from the temperature distributions in the sample. Steady-state high-temperature experiments with nickel samples were used to experimentally verify the method. The inverse solution results for the emissivity and thermal conductivity calculated from the measured data agree well with reported data in the literature. This research provides a useful reference for measuring the total hemispherical emissivity and thermal conductivity of conductive samples at high temperatures.

The Preparation of Modified Fiber Reinforced Desulfurization Gypsum Insulation Board

Hydrogen peroxide was used as the foaming agent to reduce the density and thermal conductivity of composite insulation materials, and glass fibers were adopted to reinforce the performance of foamed composite insulation materials. Meanwhile, the mechanical properties of foamed composite insulation materials is studied, and the internal micro-morphology of samples were observed by SEM. The strengthening mechanism of fiber was analyzed. The results indicated that mixing foaming agent significantly decrease the density and thermal conductivity of samples, when the content of foaming agent was 4%, the minimum rate of thermal conductivity was 0.051W/(m•K). Wrapped VAE emulsion on the surface of glass fiber, glass fiber after surface modification could improve the strength of samples. The flexural and compressive strength of samples that adding glass fiber after surface modification were increased by 18.75% and 6.98% compared with samples that adding unmodified glass fiber.

Mechanical, electrical, and thermal properties of the directionally solidified Bi-Zn-Al ternary eutectic alloy

A Bi-2.0Zn-0.2Al (wt%) ternary eutectic alloy was prepared using a vacuum melting furnace and a casting furnace. The samples were directionally solidified upwards at a constant growth rate (V = 18.4 μm/s) under different temperature gradients (G = 1.15–3.44 K/mm) and at a constant temperature gradient (G = 2.66 K/mm) under different growth rates (V = 8.3–500 μm/s) in a Bridgman-type directional solidification furnace. The dependence of microstructure parameter (λ) on the solidification parameters (G and V) and that of the microhardness (Hv) on the microstructure and solidification parameters were investigated. The resistivity (ρ) measurements of the studied alloy were performed using the standard four-point-probe method, and the temperature coefficient of resistivity (α) was calculated from the ρ-T curve. The enthalpy (ΔH) and the specific heat (C p ) values were determined by differential scanning calorimetry analysis. In addition, the thermal conductivities of samples, obtained using the Wiedemann-Franz and Smith-Palmer equations, were compared with the experimental results. The results revealed that, the thermal conductivity values obtained using the Wiedemann-Franz and Smith-Palmer equations for the Bi-2.0Zn-0.2Al (wt%) alloy are in the range of 5.2–6.5 W/Km and 15.2–16.4 W/Km, respectively.

General bidirectional thermal characterization via the 3ω technique.

The 3ω technique has become a popular method for determining the thermophysical properties of microscale and bulk materials. The prerequisite fabrication of a highly linear metal line a few hundred nanometers thick on the sample can be a failing point in specific material systems. This difficulty can be overcome by utilizing a bidirectional experimental geometry that employs a contact resistance between the sample and heating wire, which also allows for data collection under varying axial pressure loads. In this work, such a system is demonstrated with an emphasis on developing a thermal mount that will optimize sensitivity to the thermophysical parameters of interest: the sample's thermal conductivity, volumetric heat capacity, and the contact resistance between the sample and mount. A general thermal model is presented that can be simplified to analyze nearly any similar system. This model is then employed to analyze a sample in the mounting scheme described with varying applied pressures to demonstrate the general feasibility of the system.

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al2O3 hollow sphere and Al2O3 powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al2O3 hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.

Prediction of effective thermal conductivity of consolidated porous materials under ambient conditions

Various models (theoretical as well as empirical) have been employed for the prediction of thermal conductivity of consolidated porous limestones using available data. Predicted thermal conductivity of sample is compared with experimental results and the corresponding variations are discussed. Sugawara and Yoshizawa’s model is applied and discussed with its limitations and applicability for limestones, in particular.

Thermal Conductivity of Exfoliated p-Type Bismuth Antimony Telluride

A bulk p-type thermoelectric compound with nominal composition Bi0.5Sb1.5 Te3 has been exfoliated using dimethyl sulfoxide as a solvent. Samples have been prepared from the exfoliated platelets by pressing followed by sintering or hot pressing. The exfoliated nanoplatelets have been characterized for size distribution and composition using scanning electron microscopy and energy-dispersive spectrometry. The smallest size platelet was 40 nm, and the maximum in the size distribution was near 80 nm. The exfoliated platelets and sintered sample showed significant deficiency in Sb and Te. The nonstoichiometry in the composition of the exfoliated platelets indicates that the mechanism of exfoliation may not be between quintuplets only, with other layers also being active. The composition of the hot-pressed sample remained closer to that of the bulk. Results of x-ray diffraction indicated the presence of Bi2Te3 and Bi0.5Sb1.5Te3 phases and pure Te and Sb. Residual porosity was observed in the hot-pressed and sintered samples. The thermal conductivity of the samples was measured by transient thermoreflectance. The results showed that the thermal conductivity of the hot-pressed sample was reduced by a factor of two compared with that of the bulk as a result of the presence of a high density of interfaces and residual porosity. The thermal conductivity of the sintered sample showed an increase above that of the bulk sample, which is explained by the change in composition due to loss of Sb and Te.

Influence of Ni nanoparticle addition and spark plasma sintering on the TiNiSn–Ni system: Structure, microstructure, and thermoelectric properties

The electronic and thermal properties of thermoelectric materials are highly dependent on their microstructure and therefore on the preparation conditions, including the initial synthesis and, if applicable, densification of the obtained powders. Introduction of secondary phases on the nano- and/or microscale is widely used to improve the thermoelectric figure of merit by reduction of the thermal conductivity. In order to understand the effect of the preparation technique on structure and properties, we have studied the thermoelectric properties of the well-known half-Heusler TiNiSn with addition of a small amount of nickel nanoparticles. The different parameters are the initial synthesis (levitation melting and microwave heating), the amount of nickel nanoparticles added and the exact pressing profile using spark plasma sintering. The resulting materials have been characterized by synchrotron X-ray diffraction and microprobe measurements and their thermoelectric properties are investigated. We found the lowest (lattice) thermal conductivity in samples with full-Heusler TiNi2Sn and Ni3Sn4 as secondary phases.

A high-precision method to measure thermal conductivity of solids using reversible heat flux

In the thermal conductivity measurement of solids using the traditional steady-state methods, it is observed that the test results differ from their standard values when only one directional heat flux is introduced. Analysis of the results shows that the additive errors are considerable and cannot be ignored. To minimize the additive errors, a high-precision method by alternating the direction of heat flux in steady-state thermal conductivity measurement of solids is proposed. In this method, the additive errors are diminished by using the data corresponding to both forward and reverse heat flux. Meanwhile, a modified thermal instrument is designed and established to measure the thermal conductivity of solids. Simultaneous calibration of the thermal instrument has been processed to improve the measurement accuracy of temperature readings. The thermal conductivities of samples 99.999% standard pure copper and Elkonite copper–tungsten alloy 30W3 are measured and compared with their reference values to verify the accuracy of the method. The results indicate that the processing data well match the reference values.

Laboratory Study of Conductive Properties of Contaminated Riverbed Sands in Ado-Odo Ota Local Government Area of Ogun State, Nigeria

As industrial activities increase, the risk of contamination of rivers in Ado-Odo Ota Local Government Area of Ogun State in Nigeria is becoming higher. In this study, riverbed sands were collected from five major rivers in Ado-Odo Ota Local Government Area, and conductivity properties were determined after the samples have been treated with varying concentration of petrol, engine oil, diesel, caustic soda and H2SO4. HANNAN Electrical Conductivity Meter, KD2 Thermal Conductivity Meter and Constant Head Method were used to determine the electrical, thermal and hydraulic conductivities respectively. After treatment of the samples with different concentration of the contaminants, it was found that thermal, electrical and hydraulic conductivities of the samples were largely proportional to contaminants concentration in all the samples in general. However, with increase in concentration of caustic soda and H2SO4, increase in electrical and thermal conductivities of samples were observed, while increase in the concentration of other contaminants decreased the hydraulic conductivity of the samples.

A hot-wire method based thermal conductivity measurement apparatus for teaching purposes

The implementation of an automated system based on the hot-wire technique is described for the measurement of the thermal conductivity of liquids using equipment easily available in modern physics laboratories at high schools and universities (basically a precision current source and a voltage meter, a data acquisition card, a personal computer and a high purity platinum wire). The wire, which is immersed in the investigated sample, is heated by passing a constant electrical current through it, and its temperature evolution, ΔT, is measured as a function of time, t, for several values of the current. A straightforward methodology is then used for data processing in order to obtain the liquid thermal conductivity. The start point is the well known linear relationship between ΔT and ln(t) predicted for long heating times by a model based on a solution of the heat conduction equation for an infinite lineal heat source embedded in an infinite medium into which heat is conducted without convective and radiative heat losses. A criterion is used to verify that the selected linear region is the one that matches the conditions imposed by the theoretical model. As a consequence the method involves least-squares fits in linear, semi-logarithmic (semi-log) and log–log graphs, so that it becomes attractive not only to teach about heat transfer and thermal properties measurement techniques, but also as a good exercise for students of undergraduate courses of physics and engineering learning about these kinds of mathematical functional relationships between variables. The functionality of the experiment was demonstrated by measuring the thermal conductivity in samples of liquids with well known thermal properties.

Reducing Thermal Conductivity of Polymer Derived SiC Ceramics via Microwave Sintering Processing

Generally, high thermal conductivity of SiC ceramics is required. However, in some circumstances, low thermal conductivity SiC is also needed, for example, for good thermoelectricity or heat insulation. In this work, the product of pyrolysis of polycarbosilane (PCS) is chosen as raw materials. The microwave sintering process was applied to obtain low thermal conductivity at a relatively low sintering temperature. The microwave sintering processes were performed at 1200 °C under Ar atmosphere with dwelling time of 10, 20 and 30 min. The morphology and phases were analyzed by electric microscopy and X-ray diffraction (XRD), respectively. The thermal diffusivities, specific heat and thermal conductivities were measured by laser-flashed method using a thermal analyzer. The thermal conductivity of samples is within the range from 0.81 to 1.37 W/m•K. The sample with dwelling time of 30 min shows potential for multiple applications, due to its good crystallization in XRD spectra and relatively low thermal conductivity in the considered temperature range. The results also suggest a phase transformation at a dwelling time near 20 min that may have occurred.