Results Of Thermal Conductivity Measurements Research Articles

Research of foam concrete quality by two-stage foam injection method in comparison with classical foam concrete

The article presents a method of production of foam concrete, which involves two-stage injection of foam. The proposed method involves improving the pore structure of foam concrete, due to a more uniform distribution of pores throughout the volume of the material. Laboratory tests were carried out for two types of samples, represented by the proposed method of foam concrete production by two-stage foam injection with the use of modified additive in comparison with the classical foam concrete. The density of Type 1 samples varies from 410 to 793 kg/m3 (coefficient of variation from 5.12 to 7.31%), while the density of fiberboard samples lies within the range from 539 to 655 kg/m3 (coefficient of variation from 2.66 to 3.14%). The results of greater variation of densities by height in the Type 1 sample relative to the Type 2 sample indicate the influence of the technological process of foam concrete production on the quality of the pore structure of the material. The results of strength evaluation showed a large scatter of Type 1 samples in relation to Type 2 samples. The highest values of CM strengths are logically observed in the lower part of the sample and the lowest in the upper location: 44.04 and 32.33 kg/cm2 (coefficient of variation from 5.15 to 9.54%). For Type 2 samples, the same values are 55.18 and 44.44 kg/cm2, for the lower and upper locations, respectively (coefficient of variation from 2.79 to 5.35%). The results of thermal conductivity measurements of Type 1 samples range from 0.098 to 0.203 W/m0C (coefficient of variation from 4.59 to 11.88%), while the densities of Type 2 samples lie between 128 and 162 W/m0C (coefficient of variation from 3.38 to 3.55%).

Overview of the thermal properties of rocks and sediments in Slovenia

The use of geothermal energy, which comes from both deep geothermal systems and the shallow underground, has been developing rapidly in the last few decades. The purpose of the paper is to present the results of measurements of the thermal properties of all rock samples and sediments that were available from boreholes, two tunnels and numerous surface locations in Slovenia in the period from 1982 to the end of 2022. In relation to the shallow geothermal potential, a special effort is needed to characterize the thermal properties of the rocks and sediments and to implement thermal energy transfer technology. In this sense, knowledge of the thermal conductivity of rocks and sediments is required to assess the possibility of low-enthalpy heat exchange in a given local area. The largest number of measurements was taken to determine thermal conductivity. Determinations of thermal diffusivity were carried out on a much smaller number of rock and sediment samples, as well as determinations of radiogenic heat production in rocks. The results of thermal conductivity measurements on 430 samples from 119 wells, 20 samples from two tunnels and 156 samples from surface locations are shown. The highest thermal conductivities are shown by samples of dolomite, quartz conglomerate and conglomerate, phyllonite, quartz phyllite and gneiss, while the lowest are measured in sediments such as clay, lignite with clay, peat and dry sand. The determined radioactive heat generation is the lowest for milonitized dolomite and highest for dark grey sandstone with shale clasts. Our results are comparable to those already published worldwide, and they could be the basis for the possible future Slovenian standard for the thermal properties of measured rocks and sediments.

Thermal Conductivity of Detonation Nanodiamond Hydrogels and Hydrosols by Direct Heat Flux Measurements.

The methodology and results of thermal conductivity measurements by the heat-flow technique for the detonation nanodiamond suspension gels, sols, and powders of several brands in the range of nanoparticle concentrations of 2–100% w/w are discussed. The conditions of assessing the thermal conductivity of the fluids and gels (a FOX 50 heat-flow meter) with the reproducibility (relative standard deviation) of 1% are proposed. The maximum increase of 13% was recorded for the nanodiamond gels (140 mg mL−1 or 4% v/v) of the RDDM brand, at 0.687 ± 0.005 W m−1 K−1. The thermal conductivity of the nanodiamond powders is estimated as 0.26 ± 0.03 and 0.35 ± 0.04 W m−1 K−1 for the RUDDM and RDDM brands, respectively. The thermal conductivity for the aqueous pastes containing 26% v/v RUDDM is 0.85 ± 0.04 W m−1 K−1. The dignities, shortcomings, and limitations of this approach are discussed and compared with the determining of the thermal conductivity with photothermal-lens spectrometry.

Microstructure, melting behavior and thermal conductivity of the Sn–Zn alloys

Four hypereutectic Sn-Zn alloys with 69.5, 48.3, 28.1 and 14.1 at% of Zn were produced by mixing and melting of pure metals Sn and Zn followed by cooling in air to room temperature. Microstructure and chemical compositions of prepared samples were analyzed by using scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS). It was found that morphology of primary (Zn) phase changes from rounded and equiaxed dendritic structure to plate-like grains and finally to needle-like shape of grains with decreasing Zn content. Formation of characteristic broken-lamellar type of eutectic microstructure was observed in all investigated alloys. Melting behavior of the alloys was studied using differential scanning calorimetry (DSC). Phase transition temperatures and corresponding heat effects were experimentally determined and compared with the results of thermodynamic and phase equilibria calculations using the CALPHAD (CALculation of PHAse Diagrams) method and optimized thermodynamic parameters from literature. Thermal diffusivity of the studied alloys was measured using xenon flash method in the temperature range from 25 to 150 °C. Based on the measured values of thermal diffusivity and calculated specific heat capacity data, thermal conductivities of the solid alloys were obtained. It was found that thermal conductivity decreases monotonously with increasing temperature and Sn content. The results of thermal conductivity measurements were compared with literature data and with the data obtained from the Wiedemann-Franz law and measured electrical conductivities.

Planar and vertical nanostructures for thermoelectric generation

The reduction of phonon propagation at the nanoscale offers the opportunity to exploit, for thermoelectric applications, materials which are abundant on Earth and technologically feasible, such as silicon, but which have a high bulk thermal conductivity. We propose two strategies for the fabrication of thermoelectric devices based on large collections of silicon nanostructures, and we report the results of thermal conductivity measurements in both cases. At first, we describe the fabrication process of a silicon nanowire thermoelectric generator based on nanowire forests. We describe also a possible solution to reduce the parasitic electrical resistance of the substrate. Then, we illustrate a strategy based on planar silicon nanofabrication techniques. Devices based on suspended nanostructures are shown, and the effect of localized phonon scattering centers on the reduction of the thermal conductivity is reported..

Study on the role of crumb rubber on the thermal and mechanical properties of natural rubber nanocomposites

ABSTRACT Crumb rubber represents a series hazardous waste that causes environmental pollution that needs to be treated. Such a problem consumes a high budget in controlling its consequences. The main objective of this study is to get maximum benefit from this waste and use it as a filler in green processing to obtain useful materials such as heat insulators. Thus, the black filler was removed from the natural rubber mixes and crumb rubber from waste in different ratios was used instead to form rubber composite that contains a small amount of organo modified nanoclay to maintain the mechanical properties of the vulcanized rubber composites. A controlling mix containing black carbon filler was used to compare the obtained results from crumb/NR nanocomposites. These mixes were examined by Scanning Electron Microscopy (SEM) and the graphs revealed that at higher ratios of crumb rubber in the mix, crumb forms a dispersible network within the rubber blend matrix which enhances the miscibility between the rubber and all other ingredients. TGA data indicated high thermal stability of all crumb/NR nanocomposites. All results showed that the addition of crumb to natural rubber nanocomposites enhanced the levels of the properties especially for the mix of the ratio 4:6 for crumb:natural rubber. Results of thermal conductivity measurements assured that such mix acts as an insulating material and may be used in constructural applications for shielding purposes.

Compressional stress effect on thermal conductivity of powdered materials: Measurements and their implication to lunar regolith

Thermal conductivity of powdered materials under vacuum conditions is a valuable physical parameter in the context of planetary sciences. We report results of thermal conductivity measurements of 90–106μm and 710–1000μm glass beads, and lunar regolith simulant using two different experimental setups for varying the compressional stress and the temperature, respectively. We found the thermal conductivity increase with the compressional stress, for example, from 0.003 to 0.008Wm−1K−1 for the glass beads of 90–106μm in diameter at the compressional stress less than 20kPa. This increase of the thermal conductivity is attributed the areal enlargement of the contacts between particles due to their elastic deformation. The thermal conductivity increased also with temperature, which primarily represented enhancement of the radiative heat conduction between particles. Reduction of the estimated radiative conductivity from the effective thermal conductivity obtained in the first experiment yields the relation between the solid conductivity (conductive contribution through inter-particle contacts) and the compressional stress. We found that the solid conductivity is proportional to approximately 1/3 power of the compressional stress for the glass beads samples, while the regolith simulant showed a weaker exponent than that of the glass beads. We developed a semi-empirical expression of the thermal conductivity of the lunar regolith using our data on the lunar regolith simulant. This model enabled us to estimate a vertical distribution of the lunar subsurface thermal conductivity. Our model provides an examination for the density and compressional stress relationships to thermal conductivity observed in the in-situ measurements in Apollo 15 and 17 Heat Flow Experiments.

Thermal conductivity of high performance carbon nanotube yarn-like fibers

In the present paper, we present results of thermal conductivity measurements in free standing carbon nanotube (CNT) yarn-like fibers. The measurements are made using a T-type experimental configuration utilizing a Wollaston-wire hot probe inside a scanning electron microscope. In this technique, a suspended platinum wire is used both as a heater and a thermal sensor. A low frequency alternating current source is used to heat the probe wire while the third harmonic voltage across the wire is measured by a lock-in amplifier. The conductivity is deduced from an analytical model that relates the drop in the spatially averaged temperature of the wire to that of the sample. The average thermal conductivity of the neat CNT fibers and the CNT –polymer composite fibers is found to be 448 W/m-K and 225 W/m-K, respectively. These values for conductivity are amongst the highest measured for CNT yarn-like fibers fabricated using a dry spinning process from vertically aligned CNT arrays. The enhancement in thermal conductivity is understood to be due to an increase in the CNT fiber elastic stiffness during the draw and twist operations, lower CNT thermal contact resistance due to increase in CNT contact area, and better alignment of the CNT fibrils along the length of the fiber.

Thermal conductivity of individual carbon nanofibers

In the present paper, we present results of thermal conductivity measurements in commercially-available, chemical vapor deposition grown, heat-treated and non-heat-treated individual carbon nanofibers (CNFs). The thermal conductivity measurements are made using the T-type probe experimental configuration using a Wollaston wire probe inside a high resolution scanning electron microscope. The results show a significant increase in the thermal conductivity of CNFs that are annealed at 2800°C for 20h when compared with the non-heat-treated CNF samples. When adjusted for thermal contact resistance, the highest measured thermal conductivity is 449±39W/m-K. The average thermal conductivity of the heat-treated samples is 163W/m-K, while the average thermal conductivity of the non-heat-treated samples is 4.6W/m-K. The results demonstrate the importance of the quality of the CNFs, in particular their heat treatment (high temperature annealing), in controlling their thermal conductivity for thermal management applications.

Thermal Properties of ReBCO Copper Stabilized Superconducting Tapes

In recent years, high temperature superconductors (HTSs) have been developed intensively for different applications such as high field magnets, fault current limiters, and current leads. Two-dimensional HTS-based conductors, the coated conductors, used for these applications are multilayered structures on substrate tapes, which can be laminated in addition by a metal, e.g., by copper or stainless steel. Depending on the constituents and the conductor architecture, the thermal conductivity of the whole conductor can vary over several orders of magnitude. Thermal conductivity data are required for calculation of the temperature distribution in a superconducting tape or cable, and for prediction of quench propagation in the conductor itself, and finally in a superconducting device (e.g., coil arrangement). Thermal conductivity data should be measured first of all for the HTS tape, which is used for production of the superconducting appliance. In this paper, we present results of thermal conductivity measurements of the yttrium-barium-copper-oxide copper stabilized superconducting tape in the direction along the tape. Moreover, measurements of the thermal conductivity of the tape in the normal direction are presented.

Depressed thermal conductivity of mechanically alloyed nanocrystalline 10 mol% yttria-stabilized zirconia

We present the results of thermal conductivity measurements on bulk nanocrystalline 10 mol% yttria-stabilized zirconia (10 mol%) in the temperature range between 173 and 773 K. The samples studied in this work, with grain sizes from d = 17 nm to d ≈ 1 µm, were obtained by cold compaction and sintering of mechanically alloyed powders. A strong grain-size dependent reduction of the thermal conductivity is observed. We will discuss our results in terms of two different models: (i) the Kapitza thermal resistance model, which considers the low thermal conductivity as an effect of the thermal contact resistance at grain boundaries and (ii) the phonon hopping model, based on localization of short wavelength phonons due to enhanced scattering at grain boundaries.

Steady-state thermal conductivity measurements of super-hard materials

An improved method for the direct measurement of the bulk thermal conductivity of super-hard materials in a stationary regime is described. The method is based on a technique known as the ‘squeezing’ of the heat flux at the flat specimen–cone-shaped indentor interface. Results of thermal conductivity measurements of natural and synthetic diamonds, CVD diamond films, and AlN ceramics are presented. The uncertainty of the measurement method is discussed.

Thermal conductivities of some novel nonlinear optical materials

Results of thermal conductivity measurements are reported for several of the more recently developed nonlinear optical crystals. New or substantially revised values of thermal conductivity were obtained in six materials. Notable thermal conductivities measured were those for AgGaS(2) [0.014 W/(cm K) and 0.015 W/(cm K)], AgGaSe(2) [0.010 W/(cm K) and 0.011 W/(cm K)], beta barium borate [0.016 W/(cm K) and 0.012 W/(cm K)], and ZnGeP(2) [0.36 W/(cm K) and 0.35 W/(cm K)], with values quoted for directions respectively parallel and perpendicular to the optic axis for each material. These new data provide necessary input for the design of high-power optical frequency converters.

Thermal conductivity of REIn 3 (RETb, Dy, Tm, Lu) single crystals

Some results of thermal conductivity measurements on REIn 3 (RETb, Dy, Tm, Lu) single crystals over the temperature range 4–80 K are reported. On the basis of experimental data, an extraction of the magnetic contribution to the thermal conductivity was performed. In the antiferromagnetic state the thermal conductivities of TbIn 3 and DyIn 3 are dominated by magnons. In the paramagnetic state the thermal conductivities of both compounds increase nearly linearly with increasing temperature. Contrary to other analogous compounds the temperature dependence of the thermal conductivity of TmIn 3 is more complex. To separate the contribution resulting from the 4f electrons scattering from the total thermal conductivity in the studied compounds, the thermal conductivity of isomorphous and non-magnetic LuIn 3 was measured.

Hysteresis of thermal conductivity and electrical resistivity of niobium hydrides

We studied thermal conductivity, κ, and electrical resistivity, ϱ, in niobium hydrides between 5 and 420 K, during both cooling and heating. The temperature dependences of κ and ϱ show hysteresis, revealed for the first time in the system Nb-H. The results of thermal conductivity measurements suggest occurrence of a hysteretic ordering change in the β phase between 215.8 and 280 K for NbH 0.87 and between 208.4 and 350 K for NbH 0.90. The beginning of hysteresis takes place at the structural phase transition at ∼200 K. At this transition a jump of thermal conductivity was found - for NbH 0.87 (215.8 K) during heating and for NbH 0.90 (208.4 K) during both cooling and heating. The jump of electrical resistivity was observed only for NbH 0.90. The appearance of hysteresis of κ was found also in other ranges of temperature.

The Oxygen-Related Defect in Aluminum Nitride Ceramics: a Thermal Conduction Limiting Defect in a Technologically Important Material

AbstractAluminum nitride (AlN) sintered ceramics are a critical new material for electronic packaging applications, principally because of AlN's high thermal conductivity and close thermal expansion match to silicon. AlN is expected to play a key role in the next generation of high powder electronic packages, in applications ranging from high power discrete component substrates to co-fired, multilayer packages for integrated circuits. In the following paper, a detailed picture of the oxygen-related defect in AlN ceramics is presented. This impurity is of critical technological importance, because vacancies associated with oxygen severely limit thermal conduction when present in high concentrations. The results of thermal conductivity measurements, luminescence studies, optical absorption experiments, photo-induced absorption studies and electrical measurements on both ceramic and single crystal samples will be presented in order to understand the detailed nature of this defect and to model its control over a number of important technological properties.

Physical properties in the temperature range from –180 to 400°C of nickel alloy steels for low‐temperature applications

Nickel, which is the second neighbour to iron in the periodic table of elements, has a significant effect on most of the physical properties of the steels studied here, with the nickel content of the steels varying between 3 and 9%. Density and coercive force increase as the nickel content in the steel increases. By contrast, thermal expansion, modulus of elasticity and shear modulus decrease with increasing nickel content.Because of large scatter in the data, it is not possible to notice any significant influence of nickel content on Poisson's ratio and bulk modulus. Specific heat is also found not to vary with the nickel content of the steel. Electrical resistivity and thermal resistivity values increase, as expected, with increasing nickel content. The results of thermal conductivity measurements on 12 Ni 19 and X 8 Ni 9 have led to the development of modified Smith‐Palmer relationships, which allow the calculation of the thermal conductivity values for the present set of steels and other steels with similar compositions.

Thermal conductivity of Inconel 718 and 304 stainless steel

The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C. For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity. In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy. Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references. These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known.

Thermal conductivity of various glass-reinforced plastics at temperatures below 80 K

The results of thermal conductivity measurements of glass-fabric base laminates CKTφ-5KT, CT0φ-1, CT0φ-HT and uniaxial glass-fibre base laminates 27–63C and AΓ-4C within the 4–80 K temperature range are presented. The thermal conductivities of CKTφ-5KT constituents have also been measured and the thermal conductivity in specific directions has been calculated. The technique used for calculation made it possible to find the thermal conductivities of the matrices of the glass-reinforced plastics under investigation. The effect of cooling heating cycles under vacuum on the thermal conductivity has also been studied.

The evaluation of temperature jump distances and thermal accommodation coefficients from measurements of the thermal conductivity of UO 2 packed sphere beds

The results of thermal conductivity measurements on UO 2 80 μm diameter microsphere beds in helium, argon and krypton over a range of temperatures and pressures are reported. These and previous results on UO 2 microsphere beds in helium, krypton and nitrogen obtained at Oak Ridge National Laboratory (ORNL) were analysed in terms of a previously developed model which enables thermal accommodation coefficients (TAC) and temperature jump distances to be evaluated. Values derived from ORNL results tend to be in reasonable agreement with the corresponding, rather sparse data reported previously. In particular, in the case of helium, TAC's are in reasonable agreement with the direct measurements of Ullman et al. and exhibit a similar decrease with increasing temperature. However the TAC and temperature jump distance values derived from the Harwell measurements are respectively larger and smaller than the comparable ORNL figures. Also, in the case of helium, the TAC increases with pressure in contrast with values obtained from the ORNL data which are pressure independent. These differences are attributed to the differing surface finishes that are achieved during the manufacturing processes employed in the two laboratories.