Temperature Dependence Of Thermal Diffusivity Research Articles

Microstructure-thermal transport property correlation after cold/cryo rolling deformation of alloy D9

ABSTRACT Plastic deformation alters the microstructure of a material due to the formation of twins, high density of dislocations and strain-induced phases and grain refinement. Such microstructural modifications can affect thermal transport properties of a material. In this work, variation in thermal diffusivity (TD) was investigated in cold worked (CW) and cryo rolled (CR) alloy D9 (ASTM designation A771/UNS S38660), using laser flash analyser after 20–80% thickness reduction. Deformed samples were characterised using XRD and microscopy techniques for structure and microstructure modifications besides correlating with the change in hardness. Temperature dependence of thermal diffusivity was measured from room temperature up to 1273 K. Microstructural features confirmed the formation of deformation-induced martensite in CR specimen. This was accompanied by up to 172% increase in hardness in 80% CR compared to annealed specimen. The dislocation density estimated from XRD structural profile showed an increase with the percentage of deformation. For all specimens, thermal diffusivity showed an increasing trend with temperature but decreased with deformation. The reduction in thermal diffusivity was non-negligible beyond 40% thickness reduction for all deformed specimen and was substantial specifically for CR alloy D9.

Тепловые, магнитные и магнитотранспортные свойства быстрозакаленнго ленточного образца Ni-=SUB=-50-=/SUB=-Mn-=SUB=-35-=/SUB=-Al-=SUB=-2-=/SUB=-Sn-=SUB=-13-=/SUB=-

The heat capacity, thermal diffusivity, thermal conductivity, magnetization, and electrical resistance in a tape sample of Ni50Mn35Al2Sn13 alloy were studied. The temperature dependence of thermal diffusion shows pronounced anomalies in the form of minima associated with magnetic and magnetostructural transformations. The behavior of thermal conductivity and thermal diffusion indicates that the mean free path of heat carriers is limited by the structural imperfections of the tape. The transition of the sample to the martensitic phase is accompanied by a sharp increase in the electrical resistance, suggesting an increase in the electron relaxation rate due to structural distortions inherent in martensite. The magnitude of the magnetoresistive effect ∆ρ/ρ_0 in the field of 1.8T near the martensitic transition is reached 40%. The temperature hysteresis was revealed in the dependence ρ(T) near TC, which indicates the structural heterogeneity of the austenitic phase.

Appropriate metallic coating for thermal diffusivity measurement of nonopaque materials with laser flash method and its effect

Laser flash method is used as a standard method for the thermal diffusivity measurements, which required the absorber of the laser pulse and the emitter of the infrared ray. However, due to the transmission, the nonopaque materials are difficult to be measured with the laser flash method directly. Quartz glass disks were evaluated with the laser flash method to ensure the appropriate measuring condition. To verify the appropriate metallic conditions, different thick platinum and gold coatings were sputtered on the both surfaces of the glass disks by controlling the sputtering time. Sample thickness dependence and temperature dependence of thermal diffusivity for quartz glass were also evaluated with the metallic coatings. In addition, thermal diffusivity of Al2O3-TiC ceramics disks, which is a certified reference material supplied by NMIJ, were also evaluated with and without the metallic coating to ensure the effect of metallic coating by the laser flash method. A 32.7 nm-thick gold coating was found to be the appropriate measurement condition in consideration of stability, operability and economic efficiency for the laser flash method.

Magnetocaloric properties in the Pr0.7Sr0.3-xCaxMnO3: Direct and indirect estimations from thermal diffusivity data

Magnetization, specific heat, thermal diffusivity, and magnetocaloric effect were studied in Pr0.7Sr0.3-xCaxMnO3 system (x = 0; 0.1; 0.2; 0.3) in the temperature range of 80–320 K and in a magnetic field of 18 kOe. A correlation was found between the coefficients under study and microscopic characteristics (〈rA〉 – average ionic radius and σ2 – cationic disorder). The decrease in σ2 and 〈rA〉, caused by the substitution of Sr atoms by Ca atoms, causes a weakening of the exchange interactions and a decrease in ТС. The observed minima in the temperature dependence of thermal diffusivity η(T) near ТС are associated with the appearance of an additional channel of phonon relaxation in fluctuations of the magnetic order parameter. A relationship was found between the MCE value and the change in thermal diffusivity in a magnetic field near ТС. This relationship can be applied as another MCE evaluating indirect method. It is shown that MCE value estimation from thermal diffusivity data is consistent with the results of caloric measurements.

Thermal properties of pure tungsten and its alloys for fusion applications

Tungsten is a promising candidate for plasma-facing materials in a fusion reactor, and several research studies have been conducted to improve the mechanical properties of pure W. However, the thermal properties are also important characteristics of plasma-facing materials. In this study, the thermal properties of pure W and its alloys were measured, and the effect of alloying on the thermal properties of pure W was investigated. Potassium-bubble (K-bubble) dispersion, which is one of the major methods utilized to improve the mechanical properties of W, did not affect the thermal diffusivity and conductivity of pure W. On the other hand, the presence of rhenium, which is major alloying element of W, affected the absolute values and the temperature dependence of thermal diffusivity and conductivity. The effect of alloying on specific heat and the anisotropy in thermal diffusivity of pure W and its alloys were also investigated. Measurements of the specific heat showed that K-bubble dispersion and Re addition had insignificant effects on the values obtained for pure W. Anisotropy in thermal diffusivity was not observed, and the effect of anisotropic grain structure and alloying was insignificant.

PbGa6Te10 crystals for IR laser operated piezoelectricity

A ternary compound PbGa6Te10 grown by Bridgman method was studied with respect to IR induced piezoelectricity. We have found an opportunity to induce piezoelectric features using the external infrared laser (IR) beams for PbGa6Te10 crystals. The three principal wavelengths corresponding to fundamental 10.6 μm, doubled frequency 5.3 μm and third harmonic 3.5 μm laser beams were explored as photoinduced beams. Maximally achieved IR induced piezoelectricity was observed at laser induced wavelength 5.3 μm. The optimal photoinduced power density was varied within 150–160 MW/cm2 . The observed space gratings are very crucial for the observed photo-induced piezoelectricity, which is indicated, that we deal with interference of the photoinduced coherent beams. The role of the surface states was discussed based on the XPS spectra. Additionally, the temperature dependence of thermal diffusivity was measured. The determined thermal conductivity indicates prevailingly the phonon contribution and possesses very low values in the measured temperature ranges.

Temperature dependence of thermal diffusivity and conductivity for sandstone and carbonate rocks

Thermal diffusivity and conductivity of sandstone and carbonate rocks are essential parameters for modeling surface heat flow and thermal regimes in sedimentary basins. The temperature dependence of the thermal diffusivity (D) of dolomite, limestone, Carrara marble, and four types of sandstone was studied using the laser flash technique, which is accurate (±2%) and isolates the lattice component from the direct radiative transfer, at temperatures up to 973 K. At room temperature, D was 2.62, 1.34, 1.22, and 1.30–2.22 mm2 s−1 for the dolomite, limestone, Carrara marble, and sandstone samples, respectively. The sandstone D correlated strongly with the quartz content of the samples. D of all the samples decreased monotonically to 0.30–0.62 mm2 s−1 at 973 K. The results were compared with laser flash analysis data from previous studies. Combining these results with the heat capacity derived from modal mineralogy and the measured density, we obtained the thermal conductivity (k). The measured k was near or lower than that calculated from the rock’s mineral components using structural models. This suggests that thermal conduction is more complicated than that represented in the models, and that in situ measurement is essential to accurately obtain the thermal conductivity of rocks. Finally, the calculated temperature-dependent k value was used in the heat flow calculation for the sampled rock.

Temperature Dependence of Thermal Diffusivity and Conductivity of FeO Scale Produced on Iron by Thermal Oxidation

The thermal diffusivity/conductivity of FeO scales produced on iron substrates by thermal oxidation have been determined as functions of temperature. Iron plates (99.99%) were oxidised at 973 K in air to obtain oxide scales of FeO, Fe3O4, and Fe2O3, and then were reduced at 1273 K in nitrogen to obtain FeO-only. The densities of these scales were 5.85–6.05 g cm−3. The laser flash method was used to measure the apparent thermal diffusivity of the whole sample from room temperature to 1164 K during the heating/cooling cycles. This was converted to the thermal diffusivity of the scale only, which in turn was converted to the thermal conductivity. However, these values depended on the scale thickness, which suggests an interfacial heat resistance occurs between the scale layer and iron substrate. In addition, scanning electron microscopy (SEM) observations revealed that the scales contained Fe and Fe3O4 phases after heating. The scale thermal diffusivity/conductivity were corrected considering the interfacial heat resistance and dispersed phases to derive the corresponding values for FeO only. The interfacial heat resistance derived from the thickness dependence of the scale thermal conductivity was 8.3 × 10−6 m2 K W−1. Using this value, the thermal diffusivity of FeO was derived as 3.7 × 10−7–5.8 × 10−7 m2 s−1 and the thermal conductivity as 1.8–2.5 W m−1 K−1 between room temperature and 1164 K. The temperature coefficients of the thermal conductivity were mostly negative, which would be dominated by the phonon mean free path.

An Experimental Study of Thermophysical Properties for Quinary High-Entropy NiFeCoCrCu/Al Alloys**Supported by the National Natural Science Foundation of China under Grant Nos 51571163, 51371150, 51271150 and 51327901

Two quinary high-entropy alloys (HEAs) with equiatomic concentrations formed by doping either Cu or Al elements into the quaternary NiFeCoCr alloy are produced by arc melting and spray casting techniques. Their entropy of fusion, thermal expansion coefficient and thermal diffusivity are experimentally investigated with differential scanning calorimetry, dilatometry and laser flash methods. The NiFeCoCrCu HEAs contain a face-centered cubic high-entropy phase plus a minor interdendritic (Cu) phase and display a lower entropy of fusion and the Vickers hardness. The NiFeCoCrAl HEAs consist of two body-centered cubic high-entropy phases with coarse dendritic structures and show higher entropy of fusion and the Vickers hardness. Both the thermal expansion coefficient and the thermal diffusivity of the former Cu-doped alloy are significantly larger than those of the latter Al-doped alloy. Although the temperature dependence of thermal diffusivity is similar for both HEAs, it is peculiar that the thermal expansion curve of the NiFeCoCrAl alloy exhibits an inflexion at temperatures of 860–912 K.

Determination of the enthalpy of fusion and thermal diffusivity for ternary Cu60−xSnxSb40 alloys

The liquidus and solidus temperatures, enthalpy of fusion, and the temperature dependence of thermal diffusivity for ternary Cu60−xSnxSb40 alloys were systematically measured by DSC and laser flash methods. It is found that both the liquidus temperature and the enthalpy of fusion decrease with the rise of Sn content, and their relationships with alloy composition were established by polynomial functions. The thermal diffusivity usually drops from the solid toward the semi-solid state. The undercoolability of those liquid Cu60−xSnxSb40 alloys with primary Cu2Sb solid phase is stronger than the others with primary β(SnSb) intermetallic compound, and the increase of cooling rate facilitates further undercooling. Microstructural observation indicates that both of the primary Cu2Sb and β(SnSb) intermetallic compounds in ternary Cu60−xSnxSb40 alloys grow in faceted mode, and develop into coarse flakes and polygonal blocks.

Anisotropic Thermal Diffusivity Measurements in High-Thermal-Conductive Carbon-Fiber-Reinforced Plastic Composites

This paper presents the temperature dependence of in-plane thermal diffusivity and anisotropy distribution for pitch-based carbon-fiber-reinforced polymers (CFRPs). The measurement was performed using the laser-spot periodic heating method. The samples were unidirectional (UD) and crossply (CP) CFRPs. All carbon fibers of the UD samples ran in one direction, while those of the CP samples ran in two directions. In both UD and CP CFRPs, from -80°C to +80°C, temperature dependence of thermal diffusivity values increased as temperature decreased. In this temperature range, the anisotropic ratio between the fiber direction and its perpendicular direction of the UD CFRP was 106 - 124. During the anisotropy distribution measurement, it was found that thermal anisotropy can be visualized by scanning the laser in a circle on the sample. The thermal diffusivity of the UD CFRP in the fiber direction was 17 times larger than that in the 15° direction, and the thermal diffusivity in the other directions was lower than that in the 15° direction. The anisotropy distribution for the CP CFRP reflected its inhomogeneous structure.

Thermophysical properties of ceramic materials of molds and cores for casting the heat-resistant alloy-based blades of gas-turbine engines

The thermal diffusivity, thermal conductivity, temperature coefficient of linear expansion (TCLE), and specific heat capacity of ceramic materials of shell molds and cores for casting parts of heatresistant nickel-based alloys for gas-turbine engines (GTEs) are measured in a temperature range from 20 to 1500°C by the use of methods of laser flash, relative dilatometer with a pusher, and differential scanning calorimetry (DSC). The effect of the sample thickness, various process coatings, and environment is investigated. The composition and temperature dependences of thermal diffusivity and heat expansion are considered.

Thermal Properties of Transparent Yb-Doped YAG Ceramics at Elevated Temperatures

This work presents the thermal properties of ytterbium-doped yttrium aluminium garnet (Yb:YAG) transparent ceramics at elevated temperatures in dependence on the dopant concentration and on temperature. Transparent polycrystalline Yb:YAG ceramics were prepared by solid-state reaction of oxide powders sintered under high vacuum. The dopant amount varied from 0 to 20 at.% of Yb. Thermal diffusivity of the sintered samples was measured by the laser and xenon flash methods at temperatures ranging from room temperature to 900°C. Both the thermal diffusivity and thermal conductivity values decreased with increasing dopant content, and until 500°C a decrease was observed also with increasing temperature. When available, the measured values were compared to data published in literature, and were found to be in good agreement. Based on the measured values, empirical relations in the form of shifted power laws are proposed for the temperature dependence of thermal diffusivity.

Temperature dependence of thermal diffusivity, specific heat capacity, and thermal conductivity for several types of rocks

The thermo-physical properties for four rock types (granite, granodiorite, gabbro, and garnet amphibolite) from room temperature to 1,173 K were investigated. Thermal diffusivity and specific heat capacity were measured using the laser-flash technique and heat flux differential scanning calorimetry, respectively. Combined with the density data, rock thermal conductivities were calculated. Rock thermal diffusivity and conductivity decrease as the temperature increases and approach a constant value at high temperatures. At room temperature, the measured thermal conductivity is consistently near or lower than the calculated conductivity using the mineral series model, which suggests that real thermal conduction is more complicated than is depicted in the model. Therefore, in situ measurement remains the best method for accurately obtaining thermal conductivity for rocks.

The influence of temperature-dependent thermal diffusivity on the conductive cooling rates of plutons and temperature-time paths in contact aureoles

We explore the conductive cooling rates of plutons and temperature-time paths of their wall rocks using numerical methods that explicitly account for the temperature dependence of thermal diffusivity (α) and heat capacity (CP). We focus on α because it has the strongest influence on the temperature-dependence of thermal conductivity (k=ρ·CP·α) at high temperatures. Latent heats of crystallization are incorporated into the models as apparent CP's. Two sets of models are presented, one for a 50m thick basalt sill emplaced into rocks with diffusivity of the average crust, and one for a 5km wide and 2km thick granite pluton emplaced into dolostones. The sill's liquidus is 1230°C, the solidus 980°C, and the sill is emplaced into wall rocks that are at 150°C. The pluton's liquidus is 900°C, the solidus 680°C, and the pluton is emplaced into wall rocks with initial geothermal gradient of 30°C/km. The top of the pluton is at 3km depth. Incorporating appropriate α=f(T) into calculations can more than double the solidification times of intrusions in comparison with incorporating constant α of 1mm2·s−1 that is used in most petrologic heat transport models. The instantaneously emplaced basalt sill takes ~51a to completely solidify, whereas the granite pluton takes ~52ka to completely solidify. The solidification time is longer because of low thermal diffusivity of magma and because wall rocks become more insulating as temperature rises.In contact aureoles, maximum temperatures reached with α=f(T) are somewhat lower in comparison with α=1mm2·s−1; however, temperatures in inner aureoles stay elevated significantly longer with α=f(T), and consequently promote approach to mineralogical and textural equilibrium. The elevated temperature regime in inner aureoles enhances temperature gradients that may promote greater fluid fluxes if fluid pore pressures are controlled by temperatures. The results demonstrate the need to incorporate temperature-dependent transport properties of magmas and wall rocks into models of metamorphism and fluid flow in contact aureoles.

Interplay between Particle Momentum and Heat Transport

The interlinkage of particle, momentum, and heat transport in plasmas appears as a nondiffusive term of each transport equation. The physical mechanisms determining the diffusive and nondiffusive terms of particle, momentum, and heat transports are described. The nondiffusive term in the particle transport and impurity transport, which causes an inward pinch or outward flux, is driven by the temperature gradient and the magnetic field curvature. One significant piece of evidence of the nondiffusive term of particle transport is observed in the impurity transport as an impurity hole, where the impurity profiles become extremely hollow and the inward flow due to the density gradient is balanced with the outward flow driven by the ion temperature gradients. The outward convection of impurity observed contradicts the neoclassical prediction but is expected to contribute to the purity of plasma in the ion root even if the radial electric field is negative. The nondiffusive term in the momentum transport, which drives spontaneous toroidal rotation, is also observed in the plasmas in the Large Helical Device (LHD). The spontaneous rotations are driven by the electric field near the plasma edge and the ion temperature gradient at the midradius in the plasma. In the heat transport, no clear nondiffusive term is observed, and it is considered to be diffusive. The temperature and temperature gradient dependences of the diffusive terms are studied with the perturbation transport study and the slow transition between two transport branches that have a weak and a strong temperature dependence of thermal diffusivity.

Influence of filler content, particle size and temperature on thermal diffusivity of polypropylene‐iron silicon composites

AbstractThe filler fraction, particle size and temperature dependence of thermal diffusivity in polypropylene‐iron silicon composites were examined by laser flash method. Results show that raising the filler content raises the thermal diffusivity of the composite material only slightly. Nevertheless, the higher the filler content the higher the influence of the filler material expressed by a altered course of the thermal diffusivity versus temperature. Measurement values are compared with selected existing mathematical models whereas a model, originally developed for magnetic permeability, by Hashin and Shtrikman shows the best congruence. Further measurements show that the particle size of the filler does not have an influence on the composite's thermal diffusivity at the examined filler content. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Thermal diffusivity estimation of mashed potatoes and olive oil at high pressure

The effect of high pressure on thermal diffusivity of mashed potatoes and olive oil was studied at a temperature range about 25–100 °C. The preheated sample filled into a cylindrical Cu cup was loaded by nominal pressures of 400 and 500 MPa in a high-pressure chamber. The thermal diffusivity was estimated from the time–temperature decay during pressure holding time. Temperatures were measured at three different positions in the sample (wall, centre and at given radial distance). The recorded time–temperature profiles were compared with the prediction of a finite-element model assuming the linear temperature dependence of thermal diffusivity. Wall temperature data were used as a boundary condition. The difference between the measured and the calculated temperatures at the sample centre was minimised by the Marquardt Levenberg algorithm, giving optimal values of the model parameters A and B. Thermal diffusivity for both samples increased with temperature. The thermal diffusivity of mashed potatoes increased with pressure. The thermal diffusivity of olive oil decreased with pressure.

Temperature dependence of the relationship of thermal diffusivity versus thermal conductivity for crystalline rocks

Thermal diffusivity governs the transient heat transport equation. Thus, a realistic characterisation of this parameter and its temperature dependence is crucial for geothermal modelling. Due to sparse information from boreholes, lack of samples, and elaborate measurement procedures, there is often insufficient data on thermal diffusivity at elevated temperatures. We make use of existing data on crystalline (metamorphic and magmatic) rock samples from the Kola Peninsula and the Eastern Alps and develop a general relationship for the temperature dependence of thermal diffusivity up to 300°C. The temperature dependence of thermal conductivity is parameterised itself, using an empirical relationship which is set up for both data sets as well. Hence, only thermal conductivity at ambient temperatures is required for determining the temperature dependence of thermal diffusivity. We obtain different coefficients for both data sets which can be explained by different geological settings, and therefore different mineral compositions and internal structures. Comparisons with other expressions for these rock physical parameters show a good agreement at ambient conditions. General relations for thermal diffusivity at elevated temperatures are rare. A comparison of our results with data from two crystalline samples from the KTB and data from the Southern Indian Granulite Province highlights the need for further data, which will help to quantify uncertainties.

Thermophysical properties of densified pitch based carbon/carbon materials—II. Bidirectional composites

Bidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN based carbon fabric as reinforcement and coal tar and synthetic pitches as matrix precursors. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties i.e. thermal conductivity and specific heat have been evaluated both at room temperature and between 40 and 300 °C. The temperature dependence of thermal diffusion, specific heat and thermal conductivity has been studied and correlated with microstructure of carbon/carbon composites. It is found that the specific heat of carbon/carbon composites shows increase with temperature with an inverse slope in the temperature range of 150–200 °C. Accordingly, though the thermal conductivity decrease with temperature due to increased phonon interactions, it shows reversible action between 150 and 200 °C.