High Temperature Thermal Conductivity Measurements Research Articles

Design and heat transfer analysis of a compound multi-layer insulations for use in high temperature cylinder thermal protection systems

Thermal protection systems are very essential for high temperature thermal conductivity measurement system to reduce the heat loss to environment at the range of 600–1800 K. A compound multi-layer insulations structure which composed of inner carbon fibrous materials and outer alternately arranged alumina fibrous materials and high reflectivity foils is proposed for use in high temperature cylinder thermal protection systems. A coupled conductive and radiation governing equations is presented for heat transfer analysis of the structure. The finite volume method and the discrete ordinate method are used to solve the governing equations. The optimization structure of the compound multi-layer insulations is investigated by considering the pressure of the gas, the density of the carbon fibrous materials, the density of the alumina fibrous materials, the number of reflective foil layers and the emissivity of reflective foils. The results show that the compound structure has the best thermal insulation performance when the pressure of the gas is below 0.01 kPa, the density of carbon fibrous materials is 180 kg m−3, the density of alumina fibrous materials is 256 kg m−3 and the number of reflective foil layers is 39. In addition, the thermal insulation performance is much better when the emissivity of reflective foils is lower.

Low temperature thermoelectric properties of p-type doped single-crystalline SnSe

SnSe single crystals have been widely studied lately as a result of their record high ZT and controversial low thermal conductivity. Much research has focused on the high-temperature properties of single crystals and polycrystalline SnSe, but few studies were carried out on the low-temperature properties of doped single-crystalline SnSe. To study the mechanism of the charge carrier and phonon scattering, and to eliminate the ambiguity of the high temperature thermal conductivity measurement, we performed low temperature transport characterization of Na-doped and Ag-doped single-crystalline SnSe by a longitudinal steady-state technique. The electronic transport property measurements suggest that Na is a more efficient p-type dopant in SnSe than Ag. In the thermal conductivity data, we observe pronounced dielectric peak around 10 K with magnitude dependent on the doping level. In the p-type doped samples, we found that our room temperature lattice thermal conductivities (>1.74 W m−1 K−1) are in general higher than those previously reported. Based on these findings, our study implies that the lattice thermal conductivity values of doped and pure single-crystalline SnSe were underestimated.

Synthesis, structure and thermoelectric properties of $$\mathrm{La}_{1-x}\mathrm{Na}_{x}\mathrm{CoO}_{3 }$$ La 1 - x Na x CoO 3 perovskite oxides

Monovalent ion doped lanthanum cobaltate $$\hbox {La}_{1-x}\hbox {Na}_{x}\hbox {CoO}_{3 }$$ ( $$0 \le x \le 0.25$$ ) compositions were synthesized by the nitrate–citrate gel combustion method. All the heat treatments were limited to below 1123 K, in order to retain the Na stoichiometry. Structural parameters for all the compounds were confirmed by the Rietveld refinement method using powder X-ray diffraction (XRD) data and exhibit the rhombhohedral crystal structure with space group R-3c (No. 167). The scanning electron microscopy study reveals that the particles are spherical in shape and sizes, in the range of 0.2–0.5 $$\upmu $$ m. High temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements were performed on the high density hot pressed pellets in the temperature range of 300–800 K, which exhibit p-type conductivity of pristine and doped compositions. The X-ray photoelectron spectroscopy (XPS) studies confirm the monotonous increase in $$\hbox {Co}^{4+}$$ with doping concentration up to $$x = 0.15$$ , which is correlated with the electrical resistivity and Seebeck coefficient values of the samples. The highest power factor of $$10~\upmu \hbox {W~mK}^{-2 }$$ is achieved for 10 at% Na content at 600 K. Thermoelectric figure of merit is estimated to be $${\sim }1 \times 10^{-2}$$ at 780 K for 15 at% Na-doped samples.

High-Temperature Thermal Conductivity Measurement Apparatus Based on Guarded Hot Plate Method

An alternative calibration procedure has been applied using apparatus built in-house, created to optimize thermal conductivity measurements. The new approach compared to those of usual measurement procedures of thermal conductivity by guarded hot plate (GHP) consists of modified design of the apparatus, modified position of the temperature sensors and new conception in the calculation method, applying the temperature at the inlet section of the specimen instead of the temperature difference across the specimen. This alternative technique is suitable for eliminating the effect of thermal contact resistance arising between a rigid specimen and the heated plate, as well as accurate determination of the specimen temperature and of the heat loss at the lateral edge of the specimen. This paper presents an overview of the specific characteristics of the newly developed “high-temperature thermal conductivity measurement apparatus” based on the GHP method, as well as how the major difficulties are handled in the case of this apparatus, as compared to the common GHP method that conforms to current international standards.

A feasibility study of using CeO2 as a surrogate material during the investigation of UO2 thermal conductivity enhancement

ABSTRACTA possible substitution of UO2 for research purposes is the cerium dioxide (CeO2) owing to its chemical and physical properties. Neutronic properties are different and fission is absent in the case of CeO2; however, similarities were studied recently to have a possibility to compare the neutronic influence of secondary additives into the matrix. This paper deals with increasing the thermal conductivity of UO2 nuclear fuel on surrogate material (CeO2); the main focus of the research is given on the sintering behaviour of CeO2. The incorporation of highly thermally conductive material (SiC) is the investigated concept of thermal conductivity enhancement. Conventional sintering and spark plasma sintering (SPS) were applied to compare the behaviour of CeO2 and UO2 reported in the literature. High temperature thermal conductivity measurements did not confirm the positive influence of SiC additive inside the CeO2 matrix mainly due to grain boundary disruptions. Similar behaviour was also previously reported for UO2 pellets with SiC.

Fabrication of setup for high temperature thermal conductivity measurement.

In this work, we report the fabrication of an experimental setup for high temperature thermal conductivity (κ) measurement. It can characterize samples with various dimensions and shapes. Steady state based axial heat flow technique is used for κ measurement. Heat loss is measured using parallel thermal conductance technique. Simple design, lightweight, and small size sample holder is developed by using a thin heater and limited components. Low heat loss value is achieved by using very low thermal conductive insulator block with small cross-sectional area. Power delivered to the heater is measured accurately by using 4-wire technique and for this, the heater is developed with 4 wires. This setup is validated by using Bi0.36Sb1.45Te3, polycrystalline bismuth, gadolinium, and alumina samples. The data obtained for these samples are found to be in good agreement with the reported data. The maximum deviation of 6% in the value κ is observed. This maximum deviation is observed with the gadolinium sample. We also report the thermal conductivity of polycrystalline tellurium from 320 K to 550 K and the nonmonotonous behavior of κ with temperature is observed.

The design of high-temperature thermal conductivity measurements apparatus for thin sample size

This study presents the designing, constructing and validating processes of thermal conductivity apparatus using steady-state heat-transfer techniques with the capability of testing a material at high temperatures. This design is an improvement from ASTM D5470 standard where meter-bars with the equal cross-sectional area were used to extrapolate surface temperature and measure heat transfer across a sample. There were two meter-bars in apparatus where each was placed three thermocouples. This Apparatus using a heater with a power of 1,000 watts, and cooling water to stable condition. The pressure applied was 3.4 MPa at the cross-sectional area of 113.09 mm 2 meter-bar and thermal grease to minimized interfacial thermal contact resistance. To determine the performance, the validating process proceeded by comparing the results with thermal conductivity obtained by THB 500 made by LINSEIS. The tests showed the thermal conductivity of the stainless steel and bronze are 15.28 Wm-1K-1 and 38.01 Wm-1K-1 with a difference of test apparatus THB 500 are −2.55% and 2.49%. Furthermore, this apparatus has the capability to measure the thermal conductivity of the material to a temperature of 400°C where the results for the thermal conductivity of stainless steel is 19.21 Wm-1K-1 and the difference was 7.93%.

Thermal-Conductivity Measurement of Thermoelectric Materials Using $$3{{\upomega }}$$ 3 ω Method

In this work, a measurement system for high-temperature thermal-conductivity measurements has been designed, constructed, and characterized. The system is based on the $$3{\upomega }$$ method which is an ac technique suitable for both bulk and thin-film samples. The thermal-conductivity measurements were performed in a horizontal three-zone tube furnace whose sample space can be evacuated to vacuum or alternatively a protective argon gas environment can be applied to prevent undesired oxidation and contamination of the sample material. The system was tested with several dielectric, semiconductor, and metal bulk samples from room temperature up to 725 K. The test materials were chosen so that the thermal-conductivity values covered a wide range from $$0.37\,\hbox {W}\!\cdot \! \hbox {m}^{-1}\!\cdot \! \hbox {K}^{-1}$$ to $$150\,\hbox {W}\!\cdot \! \hbox {m}^{-1}\!\cdot \!\hbox {K}^{-1}$$ . An uncertainty analysis for the thermal-conductivity measurements was carried out. The measurement accuracy is mainly limited by the determination of the third harmonic of the voltage over the resistive metal heater strip that is used for heating the sample. A typical relative measurement uncertainty in the thermal-conductivity measurements was between 5 % and 8 % ( $$k=2$$ ). An extension of the $$3{\upomega }$$ method was also implemented in which the metal heater strip is first deposited on a transferable Kapton foil. Utilizing such a prefabricated sensor allows for faster measurements of the samples as there is no need to deposit a heater strip on each new sample.

A flash heating method for measuring thermal conductivity at high pressure and temperature: Application to Pt

The transport properties of matter at high pressure and temperature are critical components in planetary interior models, yet are challenging to measure or predict at relevant conditions. Using a novel flash-heating method for in-situ high-temperature and high-pressure thermal conductivity measurement, we study the transport properties of platinum to 55GPa and 2300K. Experimental data reveal a simple high-pressure and high-temperature behavior of the thermal conductivity that is linearly dependent on both pressure and temperature. The corresponding electrical resistivity evaluated through the Wiedemann–Franz–Lorenz law is nearly constant along the melting curve, experimentally confirming the prediction of Stacey for an ideal metal. This study together with prior first-principles predictions of transport properties in Al and Fe at extreme conditions suggests a broad applicability of Stacey’s law to diverse metals, supporting a limit on the thermal conductivity of iron at the conditions of Earth’s outer core of 90W/mK or less.

Corrections for Thermal Expansion in Thermal Conductivity Measurement of Insulations Using the High-Temperature Guarded Hot-Plate Method

The anticipation of recently published European product standards for industrial thermal insulation has driven improvements in high-temperature thermal conductivity measurements in an attempt to obtain overall measurement uncertainties better than 5 % (k = 2). The two measurement issues that are focused on in this article are the effect of thermal expansion on in situ thickness measurement and on determining the metering area at high temperatures. When implementing in situ thickness measurements, it is vital to correct the thermal expansion of components in a high-temperature guarded hot plate (HTGHP). For example, in the NPL HTGHP this could cause 3.2 % measurement error for a 50 mm thick specimen at 800 °C. The thermal expansion data for nickel 201 measured by NPL are presented, and the effect of this on the metering area of NPL’s heater plate (nickel 201) is discussed.

Intercomparison of thermal conductivity measurements on a calcium silicate insulation material

The determination of reliable thermal conductivity values of insulation materials at high temperatures is important for target-oriented material research, further improvement of products and quality management. However, there is a lack of reference materials for high temperature thermal conductivity measurements which are needed to ensure and improve good measurement practise. In order to investigate porous calcium silicate as reference material for temperatures up to 1100 K, the German Thermophysics Working Group within GEFTA initiated an intercomparison of thermal conductivity measurements on a commercially available calcium silicate insulation material with seven participating laboratories. Stationary and instationary measurement methods were used to determine the effective total thermal conductivity of the calcium silicate material in the temperature range from 300 K to 1100 K. The derived weighted mean value of the thermal conductivity increases from 0.0846 W m −1 K −1 at 300 K to 0.173 W m −1 K −1 at 1100 K. Within the same temperature interval the relative uncertainty increases from 3.5% to 7%. The investigated product is commercially available and it could be therefore used in the daily laboratory work as reference material for thermal conductivity measurements at high temperatures.

Thermoelectric enhancement in PbTe with K or Na codoping from tuning the interaction of the light- and heavy-hole valence bands

The effect of K and K-Na substitution for Pb atoms in the rock salt lattice of PbTe was investigated to test a hypothesis for development of resonant states in the valence band that may enhance the thermoelectric power. We combined high temperature Hall-effect, electrical conductivity and thermal conductivity measurements to show that K-Na co-doping do not form resonance states but2 can control the energy difference of the maxima of the two primary valence sub-bands in PbTe. This leads to an enhanced interband interaction with rising temperature and a significant rise in the thermoelectric figure of merit of p-type PbTe. The experimental data can be explained by a combination of a single and two-band model for the valence band of PbTe depending on hole density that varies in the range of 1-15 x 10^19 cm^-3.

ChemInform Abstract: Structure and High‐Temperature Thermoelectric Properties of SrAl2Si2.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Isothermal and cycling properties of zirconia-based thermal barrier coatings deposited by PECVD

Columnar 100–200-μm-thick Y 2O 3 partially stabilised ZrO 2 thermal barrier coatings have been deposited by PECVD at temperatures between 700 and 900°C on alumina and NiPtAl-coated superalloy substrates, at rates which could exceed 250 μm h −1. The high temperature behaviour of the coatings has been investigated with respect to isothermal (at 1100–1400°C) and cycling (at 1100°C) ageing. The tetragonal phase t′ destabilisation scheme leads to the formation of a low yttria tetragonal phase t and a second high yttria tetragonal phase, labelled t′ 1. The good thermal cycling behaviour of coatings deposited at 900°C (>400 1-h cycles), as well as room and high temperature thermal conductivity measurements, confirm the similarity with EBPVD TBCs. The thermally grown oxide formation, which was not optimised here, limits the lifetime in cyclic conditions. The more porous 700°C coatings, that are less resistant to sintering at high temperatures, also exhibit a good resistance to thermal cycling. These results and the high growth rates position the PECVD technique as a valuable alternative process for TBC manufacturing suitable for industrial requirements.

Preparation and thermoelectric properties of IrxCo1 − xSb2 alloys

The preparation and characterization of the binary arsenopyrite compounds CoSb2 and IrSb2 and IrxCo1 − xSb2 alloys is reported. Single crystals of CoSb2 were grown by the vertical gradient-freeze technique from solutions rich in antimony. Polycrystalline samples of IrSb2 and IrxCo1 − xSb2 alloys were prepared by hot-pressing of prereacted elemental powders. Samples were investigated by X-ray diffractometry, microprobe analysis and density measurements. It was found that a range of solid solutions exists in the system IrxCo1 − xSb2. Samples were also characterized by high temperature electrical resistivity, Seebeck coefficient and thermal conductivity measurements. All materials have p-type conductivity and are semiconductors. A band gap of about 0.98 eV was calculated for IrSb2. Preliminary measurements of the thermoelectric properties of these materials showed that their potential for thermoelectric applications is limited.

Effects of Cation Substitution on Electrical and Thermal Transport Properties of YCrO3 and LaCrO3

High‐temperature electrical conductivity, Seebeck coefficient, and thermal conductivity measurements were used to study the effects of different cation substituents on electrical and thermal transport in YCrO3 and LaCrO3. The substitution of divalent Ca and Sr for Y and La, respectively, resulted in the formation of small polarons as charge carriers. The additional substitution of Mn for Cr resulted in the formation of a second charge carrier associated with the Mn. The electrical conductivity results were consistent with thermally activated transport by hopping of a temperature‐independent carrier concentration. The activation energies were 0.20 and 0.12 eV for (Y,Ca)CrO3 and (La,Sr)CrO3, respectively, and increased to about 0.50 eV with the substitution of Mn for Cr. The Seebeck coefficient increased linearly with temperature and decreased with substituent concentration for both (Y,Ca)CrO3 and (La,Sr)CrO3. The substitution of Mn for Cr resulted in a Seebeck coefficient with a more complex dependence on temperature and substituent concentration. The thermal conductivity did not change significantly with either cation substitution or temperature.

Method of Simultaneous Measurement of Radiative and Lattice Thermal Conductivity

A new technique of high-temperature thermal-conductivity measurement is described. A CO2 gas laser is used to generate a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. A mathematical expression is derived which enables phase data at several frequencies to be used for the simultaneous determination of thermal diffusivity and mean extinction coefficient. Lattice and radiative thermal conductivities are then calculated. Test results for sintered aluminum oxide at temperatures from 530 to 1924 °K are within the range of error of previously existing data.

Thermal Conductivity of Silicon from 300 to 1400°K

The thermal diffusivity of pure silicon has been measured from 300 to 1400\ifmmode^\circ\else\textdegree\fi{}K. The specific heat of the same material over the same temperature range has been measured by Dennison. The thermal conductivity was obtained from the product of the thermal diffusivity, specific heat, and density. At 1400\ifmmode^\circ\else\textdegree\fi{}K about two-thirds of the thermal conductivity was caused by lattice vibrations and one-third by bipolar diffusion. Wiedemann-Franz type diffusion accounted for less than one percent of the total thermal conductivity at 1400\ifmmode^\circ\else\textdegree\fi{}K. Thermal transport by direct transmission of radiation appeared to be negligible up to 1400\ifmmode^\circ\else\textdegree\fi{}K. The Gr\uneisen constant for silicon from these high-temperature thermal conductivity measurements was 1.96, if the Debye temperature is taken as 636\ifmmode^\circ\else\textdegree\fi{}K.