Thermal Conductivity of Magnesium Alloys in the Temperature Range from −125 °C to 400 °C

Magnesium alloys have been widely used in recent years as lightweight structural materials in the manufacturing of automobiles, airplanes, and portable computers. Magnesium alloys have extremely low density (as low as 1738 kg · m^?3) and high rigidity, which makes them suitable for such applications. In this study, the thermal conductivity of two different magnesium alloys made by twin-roll casting was investigated using the laser-flash technique and differential scanning calorimetry for thermal diffusivity and specific heat capacity measurements, respectively. The thermal diffusivity of the magnesium alloys, AZ31 and AZ61, was measured over the temperature range from ?125 °C to 400 °C. The alloys AZ31 and AZ61 are composed of magnesium, aluminum, and zinc. The thermal conductivity gradually increased with temperature. The densities of AZ31 and AZ61 were 1754 kg · m^?3 and 1777 kg · m^?3, respectively. The thermal conductivity of AZ31 was about 25 % higher than that of AZ61, and this is attributed to the amount of precipitation.     

Design and Validation of a High-Temperature Comparative Thermal-Conductivity Measurement System

A measurement system has been designed and built for the specific application of measuring the effective thermal conductivity of a composite, nuclear-fuel compact (small cylinder) over a temperature range of 100 °C to 800 °C. Because of the composite nature of the sample as well as the need to measure samples pre- and post-irradiation, measurement must be performed on the whole compact non-destructively. No existing measurement system is capable of obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guarded-comparative-longitudinal heat flow technique. The system uniquely demonstrates the use of a radiative heat sink to provide cooling which greatly simplifies the design and setup of such high-temperature systems. The design was aimed to measure thermal-conductivity values covering the expected range of effective thermal conductivity of the composite nuclear fuel from 10 W . m⁻¹ . K⁻¹ to 70 W . m⁻¹ . K⁻¹. Several materials having thermal conductivities covering this expected range have been measured for system validation, and results are presented. A comparison of the results has been made to data from existing literature. Additionally, an uncertainty analysis is presented finding an overall uncertainty in sample thermal conductivity to be 6 %, matching well with the results of the validation samples.     

Effet Hall dans les films d'argent déposés par voie chimique

Study of the Hall effect on silver films prepared by chemical reduction shows an electronic conductivity. At 25°C the Hall coefficient R_H is ?(12 ± 1) × 10^?11 m³ C^?1 and the number of conduction electrons n = 0.89 electrons per atom agrees within 10% of the result obtained from measurement of the conductivity.The mobility of the electron transport in the annealed films, μ_Hr(25°C) = 4.85 × 10^?3 m²V^?1s^?1 is six to seven time greater than that of the same unannealed films, μ_Hi(25°C) = 0.75 × 10^?3 m² V^?1 s^?1.     

Conductive,self-cleaning,and short-circuit proof multi-functional graphene aerogel composite fibers

This work reports the superior properties of flexible multi-functional composite fibers based on graphene aerogel fibers. With the addition of phase change materials, the graphene aerogel fibers were synthesized by wet spinning and supercritical drying. The phase change materials can improve the structural uniformity and thermal stability of the composite fibers. The fibers coated with polydimethylsiloxane and fluorocarbon can respond to various external stimuli (e.g., electrons, photons, and heat), as well as have excellent properties of shape compliance, self-cleaning, and insulated surfaces. After coating fluorocarbon, the maximum water contact angle of graphene aerogel fibers increases from 132.18° to 151.77°. It is worth mentioning that adding an insulation layer of polydimethylsiloxane avoids the high-temperature problem caused by the short circuit of graphene aerogel fibers. The short-circuit temperature of graphene aerogel fibers is as high as 65 °C, while that of the composite fiber is only 41.5 °C after coating with polydimethylsiloxane. The temperature of graphene aerogel fibers with polyethylene glycol can increase to 39.3 °C under simulated sunlight. In addition, graphene aerogel fibers have excellent electrical conductivity (4.85?×?10³ S m^?1) at 300 K. After coating with polyethylene glycol, its electrical conductivity is still as high as 2.95?×?10³ S m^?1. The good electrical conductivity makes the aerogel fibers have promising application in advanced wearable systems.

     

Temperature-Dependent Thermal Conductivity of High Strength Lightweight Raw Perlite Aggregate Concrete

Twenty-four types of high strength lightweight concrete have been designed with raw perlite aggregate (PA) from the Erzincan Mollaköy region as new low-temperature insulation material. The effects of the water/cement ratio, the amount of raw PA, and the temperature on high strength lightweight raw perlite aggregate concrete (HSLWPAC) have been investigated. Three empirical equations were derived to correlate the thermal conductivity of HSLWPAC as a function of PA percentage and temperature depending on the water/cement ratio. Experimentally observed thermal conductivities of concrete samples were predicted 92 % of the time for each set of concrete matrices within 97 % accuracy and over the range from 1.457 W · m^?1 · K^?1 to 1.777 W · m^?1 · K^?1. The experimental investigation revealed that the usage of raw PA from the Erzincan Mollaköy region in concrete production reduces the concrete unit mass, increases the concrete strength, and furthermore, the thermal conductivity of the concrete has been improved. The proposed empirical correlations of thermal conductivity were considered to be applicable within the range of temperatures 203.15 K ≤ T ≤ 303.15 K in the form of λ = a(PAP ^b ) + c(T ^d ).     

Thermal conductivity improvement of copper–carbon fiber composite by addition of an insulator: calcium hydroxide

The effects of adding calcium hydroxide (Ca(OH)₂) to a copper–CF (30 %) composite (Cu–CF(30 %)) were studied. After sintering at 700 °C, precipitates of calcium oxide (CaO) were included in the copper matrix. When less than 10 % of Ca(OH)₂ was added, the thermal conductivity was similar to or higher than the reference composite Cu–CF(30 %). A thermal conductivity of 322 W m^?1 K^?1 was measured for the Cu–Ca(OH)₂(3 %)–CF(30 %) composite. The effects of heat treatment (400, 600, and 1000 °C during 24 h) on the composite Cu–Ca(OH)₂(3 %)–CF(30 %) were studied. At the lower annealing temperature, CaO inside the matrix migrated to the interface of the copper matrix and the CF. At 1000 °C, the formation of the interphase calcium carbide (CaC₂) at the interface of the copper and CFs was highlighted by TEM observations. Carbide formation at the interface led to a decrease in both thermal conductivity (around 270 W m^?1 K^?1) and the coefficient of thermal expansion (CTE (10.1 × 10^?6 K^?1)).     

An apparatus for accurate determinations of vapour-liquid equilibrium properties and gas PVT properties

A precision apparatus for the experimental determination of vapour-liquid equilibrium properties and PVT properties of gas mixtures has been constructed and thoroughly tested. The apparatus used in the phase equilibrium determinations is of the vapour recirculation type, and that used in the PVT measurements is a Burnett apparatus.The experiments may be performed in the temperature range ?180 to +25°C, the temperature being controlled to the nearest ±0.01°C. The temperature can be measured to the nearest 0.01 K using a high precision ac bridge and a calibrated resistance thermometer. The pressure range is from 3 to 350 atm (1 atm = 101.3 kN m^?2) on the vapour recirculation apparatus and up to 700 atm on the Burnett apparatus; the pressure is determined with a dead weight piston gauge, which has an accuracy of 0.01% of the reading or 0.005 lb in^?2, whichever is the greatest. The composition of the gas and liquid phases is determined using a carefully calibrated gas chromotograph. Both the gas and the liquid samples are injected directly into the carrier gas of the gas chromatograph.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.     

Nanospheres Containing Urea: Photothermic Properties

In the present research, nanospheres of chitosan (CS), maltodextrin, and sodium tripolyphosphate (STPP), loaded with urea, were synthesized by using an ionic gelation technique. In the nanosphere synthesis was used a central composite experimental design, obtaining nanospheres with an average size of 275?±?32 nm and 27.5 mV zeta potential. The nanospheres were characterized by their hydrodynamic diameter, polydispersity index, nitrogen content, and thermal properties such as thermal diffusivity (α), effusivity (e), and conductivity (k); also melting temperature was obtained by differential scanning calorimetry. The thermal properties of nanospheres show that the sample with the smallest size has a thermal diffusivity value of (14.4?±?0.4)?×?10^?8 m²·s^?1 and a thermal conductivity value of (6.4?±?0.1)?×?10^?1 W·m^?1·K^?1, and the obtained melting temperature was 157 °C. Higher concentrations of CS increase the values of these thermal properties, probably because chitosan interacts ionically with STPP forming a reticular network due to the opposite charges of both molecules.     

Simultaneous Absorptance and Thermal-Diffusivity Determination of Optical Components with Laser Calorimetry Technique

The laser calorimetry (LCA) technique is used to determine simultaneously the absorptances and thermal diffusivities of optical components. An accurate temperature model, in which both the finite thermal conductivity and the finite sample size are taken into account, is employed to fit the experimental temperature data measured with an LCA apparatus for a precise determination of the absorptance and thermal diffusivity via a multiparameter fitting procedure. The uniqueness issue of the multiparameter fitting is discussed in detail. Experimentally, highly reflective (HR) samples prepared with electron-beam evaporation on different substrates (BK7, fused silica, and Ge) are measured with LCA. For the HR-coated sample on a fused silica substrate, the absorptance is determined to be 15.4?ppm, which is close to the value of 17.6?ppm, determined with a simplified temperature model recommended in the international standard ISO11551. The thermal diffusivity is simultaneously determined via multiparameter fitting to be approximately 6.63?×?10^?7?m² · s^?1 with a corresponding square variance of 4.8?×?10^?4. The fitted thermal diffusivity is in reasonably good agreement with the literature value (7.5?×?10^?7?m² · s ^?1). Good agreement is also obtained for samples with BK7 and Ge substrates.     

Effective Thermal-Conductivity Measurements on Supporting Structures of the Mercury Probe Bepi Colombo

Measurements were done on a carbon fiber reinforced composite (CFC) sample tested for the space probe Bepi Colombo by using the guarded hot-plate (GHP) method. The values of interest were the thermal transmittance through the samples, (56.3 ± 3.6) W · m^?2 · K^?1, and the effective thermal conductivity (1.06 ± 0.07) W · m^?1 · K^?1. The samples consist of a light honeycomb core attached to thicker surface plates. Due to this construction, the effective thermal conductivity parallel to the face plates is higher than the effective thermal conductivity through the sample. This leads to lateral heat gains or losses during the GHP measurement, which in return can lead to erroneous results. Furthermore, due to the high rigidity of the CFC material, there will be high contact resistances between the samples and the GHP apparatus plates. The separation of these thermal contact resistances from the total measured thermal resistance is essential in order to achieve correct results. Good results were achieved using a special measurement setup and a lateral correction method designed to reduce errors due to lateral heat flows.     

Thermal Conductivity of Thermoplastics Reinforced with Natural Fibers

With restrictions for environmental protection being strengthened, thermoplastics reinforced with natural fibers such as jute, kenaf, flax, etc., have replaced automotive interior materials such as chemical plastics. In this study, the thermal conductivity of several kinds of thermoplastic composites in the form of board composed of 48.5 mass% polypropylene (PP) and 48.5 mass% natural fiber (NF), and reinforced with 3.0 mass% maleated polypropylene (MAPP) and 0.3 mass% silane as the coupling agents, were measured at temperatures of −10, 10, and 30°C, using a heat flow meter apparatus. The results show that the thermal conductivity is in the range of 0.05–0.07 W · m⁻¹ · K⁻¹, and the thermal conductivity increased about 10–15% by adding MAPP and about 10–25% by soaking in a silane aqueous solution. The tensile strength was also measured, and the result shows similar trends as the thermal conductivity.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

The effect of microstructure on thermal expansion coefficients in powder-processed Al2Mo3O12

Orthorhombic Al₂Mo₃O₁₂ was investigated as a model anisotropic phase to understand the influence of powder preparation routes and bulk microstructure (mean grain size) on the bulk coefficient of thermal expansion (CTE) and to compare it to the intrinsic CTE of powder samples. A co-precipitation route was used for the synthesis of pure single-phase nanopowders, while a polyvinyl alcohol-assisted sol–gel method was utilized for the synthesis of micron-sized powders. Sintered samples prepared from both powders exhibited different microstructures in terms of mean crystal sizes and porosity. Bulk samples obtained from nanopowders were highly porous and contained crystals of approximately 100-nm diameter, while the bulk pieces produced from the micron-sized powders were denser, contained crystals larger than 5 μm, and showed occasional intergranular and transgranular microcracks. Such different microstructures hugely impact the bulk CTE: the nanometric sample possesses a bulk CTE (0.9 × 10^?6 °C^?1, from 200 to 700 °C) closer to the instrinsic CTE (2.4 × 10^?6 °C^?1) than for the micrometric sample, which showed a negative CTE (?2.2 × 10^?6 °C^?1) from 200 to 620 °C, and an even more negative CTE above 620 °C (?35 × 10^?6 °C^?1). A finite element analysis showed that the local maximum thermal tensile stresses could be as high as 220 MPa when simulating a temperature drop of 700 °C as an example of thermal treatment following sintering. This tensile stress is expected to exceed the tensile strength of Al₂Mo₃O₁₂, explaining the origin of microcracks in bulk samples prepared from the micron-sized powders. The thermal behavior of the microcracks leads to differences between the intrinsic and bulk thermal expansion; we show experimentally that such differences can be reduced by nanostructuring.     

PVDF hollow fiber and nanofiber membranes for fresh water reclamation using membrane distillation

Polyvinylidene fluoride hollow fiber and nanofibrous membranes are engineered and successfully fabricated using dry-jet wet spinning and electrospinning techniques, respectively. Fabricated membranes are characterized for their morphology, average pore size, pore size distribution, nanofiber diameter distribution, thickness, and water contact angle. Direct contact membrane distillation (DCMD) performances of the fabricated membranes have been investigated using a locally designed and fabricated, fully automated MD bench scale unit and DCMD module. Electrospun nanofibrous membranes showed a water flux as high as 36 L m^?2 h^?1 whereas hollow fiber membranes showed a water flux of 31.6 L m^?2 h^?1, at a feed inlet temperature of 80 °C and at a permeate inlet temperature of 20 °C.     

Thermal conductivity of geomaterial foams based on silica fume

Porous materials have been prepared from a solution containing sodium silicate and sodium hydroxide with the addition of silica fume. Kaolin and diatomite were also tested as additives to the initial formulation. This method yields consolidated geomaterial foams without requiring thermal treatment above 50 ^°C50\,{}^{\circ}\hbox{C}. The influence of chemical composition on the thermal conductivity was studied. The choice of raw materials was found to play an important role. The accuracy of thermal conductivity measurements was evaluated by comparing the steady state heat flow method with the laser flash technique for five different reference materials giving values within 6%. Using the steady state heat flow method, a value of 0.12 ±0.01 W m^-1 K^-10.12 \pm 0.01\,{\rm W}\,{\rm m}^{-1}\,{\rm K}^{-1} was then obtained for consolidated foams, made with kaolin as the precursor, containing approximately 70% of porosity.     

Highly Thermoconductive,Thermostable, and Super-Flexible Film by Engineering 1D Rigid Rod-Like Aramid Nanofiber/2D Boron Nitride Nanosheets

Polymer-based thermal management materials have many irreplaceable advantages not found in metals or ceramics, such as easy processing, low density, and excellent flexibility. However, their limited thermal conductivity and unsatisfactory resistance to elevated temperatures (<200 °C) still prevent effective heat dissipation during applications with high-temperature conditions or powerful operation. Therefore, herein highly thermoconductive and thermostable polymer nanocomposite films prepared by engineering 1D aramid nanofiber (ANF) with worm-like microscopic morphologies into rigid rod-like structures with 2D boron nitride nanosheets (BNNS) are reported. With no coils or entanglements, the rigid polymer chain enables a well-packed crystalline structure resulting in a 20-fold (or greater) increase in axial thermal conductivity. Additionally, strong interfacial interactions between the weaved ANF rod and the stacked BNNS facilitate efficient heat flux through the 1D/2D configuration. Hence, unprecedented in-plane thermal conductivities as high as 46.7 W m⁻¹ K⁻¹ can be achieved at only 30 wt% BNNS loading, a value of 137% greater than that of a worm-like ANF/BNNS counterpart. Moreover, the thermally stable nanocomposite films with light weight (28.9 W m⁻¹ K⁻¹/10³ (kg m⁻³)) and high strength (>100 MPa, 450 °C) enable effective thermal management for microelectrodes operating at temperatures beyond 200 °C.     

Etude de la conduction ionique de verres a base de ZrF4

The electrical and dielectric properties of new ZrF₄ based glasses have been studied in the frequency and temperature ranges 5Hz-500kHz and 130–280°C. Transport number measurements following Tubant's method showed that the glasses investigated are F^? conductors. The conductivities and activation energies for conduction are about 10^?6(Ωcm)^?1 at 200°C and 18 kcal.mole^?1 respectively. Preliminary interpretation of the change of conductivity with composition is given.     

Fast ionic conductivity in lithium nitride

Analysis of the frequency dependence of ac measurements with ionically-blocking electrodes, as well as transmission electron microscopic observations have enabled the transcrystalline and intercrystalline resistances of polycrystalline Li₃N to be separately evaluated. At 25°C the transcrystalline ionic conductivity is 6.6×10^?4 (ohm cm)^?1 and the activation enthalpy is 24.1 kJ/mole. The intercrystalline conductivity has an activation enthalpy of 68.5 kJ/mole, and its magnitude varies with thermal history. By optimized thermal treatment, the microstructure can be controlled so the bulk conductivity becomes approximately equal to that for transport in the fast direction in this very anisotropic structure.     

A Steady-State Apparatus to Measure the Thermal Conductivity of Solids

This paper presents a steady-state apparatus to measure the thermal conductivity of solids based on the use of heat flux sensors. The uncertainty assessment of the experimental apparatus is performed in accordance with the ISO “Guide to the Expression of Uncertainty in Measurement” (GUM): the combined standard uncertainty (k = 1) with reference to the properties of Pyrex Glass CRM 039 at 20°C is ±1.9%. The standard uncertainty has also been assessed in the measurement range from 0.1 to 10W · m⁻¹ · K⁻¹. The experimental apparatus has been used to measure the certified reference material Pyrex Glass CRM 039 in the temperature range from 3.1 to 60.3°C: the standard deviation (k = 1) between the experimental measurements and the certified values is ±1.1% with a maximum deviation of 2.4%. Reasonable agreement is found between the uncertainty assessment and the experimental validation.