Temperature Dependence Of Thermal Diffusivity Research Articles

Thermophysical properties of densified pitch based carbon/carbon materials—I. Unidirectional composites

Unidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN and pitch based carbon fibers of varying microstructure as reinforcements and different types of pitches as matrix precursors. The composites have been given final heat treatment to 2500–2700 °C. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties, i.e., thermal conductivity, coefficient of thermal expansion and specific heat have been evaluated. It is found that the type of fibers and matrix present in the composites influences the absorption (specific heat) and transmission (conductivity) of thermal energy. The temperature dependence of thermal diffusion, specific heat, thermal conductivity and coefficient of thermal expansion has been studied and correlated with microstructure of carbon/carbon composites.

Temperature dependence of the thermal diffusivity in high-collisionality regimes in the large helical device

The positive density dependence of energy confinement times, τE, as expressed in the international stellarator scaling 95 (ISS95), where ( is the line-averaged density and P is the heating power), declines in high-collisionality regimes in the large helical device experiments. In the low-collisionality regime, where parameter dependences in ISS95 agree well with the experiment, the temperature dependence of thermal diffusivity is as strong as predicted by the gyro-Bohm model and/or the neoclassical theory. As the collisionality increases to the plateau regime, the temperature dependence becomes moderate, where the thermal diffusivity is proportional to the square root of the electron temperature. Furthermore, the thermal diffusivity is inversely proportional to the magnetic field strength and the electron temperature gradient is proportional to the electron temperature in the high-collisionality regime. Compared with ISS95, the energy confinement time expected from these observations has a weaker density dependence together with mitigated power degradation: .

Thermal diffusivity of binary mixture of n-tricosane and n-tetracosane by Fourier transform temperature wave analysis

The phase transitions of binary mixtures of n-tricosane (C 23H 48)/ n-tetracosane (C 24H 50) are investigated by Fourier transform temperature wave analysis (FT-TWA) and differential scanning calorimetry (DSC). The phase diagram determined by both methods gives a good agreement. Thermal diffusivity shows an odd–even effect at the solid state, and the temperature dependence of thermal diffusivity of the mixture is classified into two types, odd-like type and even-like type, depending on the mixing ratio. The increase of thermal diffusivity in the rotator phase RI (orthorhombic rotator phase) prior to the phase transition to α-RII (rhombohedral rotator phase with hexagonal subcell) is found for odd number n-tricosane (C 23H 48). A detailed study of the rotator phase transition is shown by the measurement of thermal diffusivity.

Heat transfer in quartz, orthoclase, and sanidine at elevated temperature

The thermal diffusivity of quartz, orthoclase, and sanidine was measured in different directions by a transient method, to determine the second-rank tensor of thermal transport properties as a function of temperature up to 800 °C. Thermal diffusivity of phonons is described as D=1/3 v l, where v is the phonon velocity and l the mean free path length of phonons. In quartz the thermal diffusivity at room temperature is 3.60 mm2 s−1 in [1 0 0] and 7.00 mm2 s−1 in [0 0 1] direction. A 1/T dependency of thermal diffusivity is observed for both directions up to 550 °C with a pronounced anisotropy (A=62%; A=(D max − D min)/D average), which decreases with increasing temperature. Close to the α–β-phase transition, we observed a minimum in thermal diffusivity and a negligible anisotropy. Up to 573 °C the maximam of thermal diffusivity, average phonon velocity, and mean free path length are observed in [0 0 1] direction. However, above the temperature of phase transition the maximum values of the properties are observed in [1 0 0] direction. The observed behavior is explained by a model which interrelates velocity and mean free path length of phonons. A further contribution to the thermal diffusivity behavior is related to the higher symmetry of β-quartz, resulting in a reduced phonon–phonon-scattering probability. At ambient temperature, orthoclase and sanidine show similar values of thermal diffusivities (≈1 mm2 s−1) and a temperature-independent anisotropy behavior between 22 and 350 °C of ≈30%. For both feldspars a small 1/T temperature dependence of thermal diffusivity up to 350 °C is observed. The transparent sanidine in gemstone quality shows a strong increase in thermal diffusivity above 350 °C proportional to T 3. The contribution of the radiative heat-transfer mechanism leads to values more than two times higher in sanidine than in a milky-cloudy orthoclase at 800 °C. For the feldspars the mean free path length of phonons shows a small temperature dependence on reaching ≈0.4–0.5 nm at 800 °C and thus already in the range of the interatomic distances of ≈0.27–0.34 nm. The pronounced difference in thermal diffusivity behavior between quartz and orthoclase, which have a related tectosilicate structure, is connected to additional cations of K, Na, and Al within the feldspar structure.

Evaluation of the Effective Sample Temperature in Thermal Diffusivity Measurements Using the Laser Flash Method

In the measurement of thermal diffusivity by the laser flash method, a temperature rise occurs in the sample as a pulsed laser hits on the sample surface. Due to the temperature dependence of thermal diffusivity of the sample, the thermal diffusivity corresponds to a temperature that is larger by T eff than the temperature before laser irradiation is applied. This effective temperature rise, T eff, has been investigated by using a numerical simulation. The results indicate that the effective temperature rise is almost equal to a maximum temperature rise, T M, of the back surface of the sample in cases where both linear and nonlinear temperature variations of thermal diffusivity are considered.

Photopyroelectric Characterization of Industrial Polymers: Temperature Dependent Thermal Diffusivity of Packaging Foils Manufactured from Low Density Polyethylene (LDPE) and Polyvinyl Chloride (PVC)

The temperature dependence of thermal diffusivity of the low density polyethylene (LDPE) and polyvinyl chloride (PVC) packaging foils was determined using the photopyroelectric (PPE) approach in the standard configuration. Glassy phase transitions observed in foils within the -30°C to 70°C range cause a 7 to 20% increase in values of thermal diffusivity for these test materials. The results obtained by the PPE approach were confirmed by differential scanning calorimetry.

Thermal Properties of Porous Ni/YSZ Particulate Composites at High Temperatures

The porosity, composition, and temperature dependence of thermal properties in porous Ni/yttria-stabilized zirconia (YSZ) cermets has been investigated at high temperatures by the laser flash diffusivity method. At a temperature range of 1073 to 1273 K, there is no temperature dependence of thermal diffusivity of the cermets. Observable, however, is a slight increase in both of specific heat and thermal conductivity with an increase in temperature. Although the specific heat is governed by the law of mixing in composition, it is independent of the porosity. Pores in the cermet act as heat barriers to prevent heat transfer, in which thermal conductivity is regarded as zero. The relationship between thermal conductivity and porosity can be expressed by the resultant conductivity calculated on the basis of the modified effective medium theory. In this calculation, a porous Ni/YSZ cermet is regarded as a simple cubic resistance network composed of two kinds of unit resistors corresponding to the solid and pore phases. Moreover, the composition dependence of the thermal conductivity of a dense Ni/YSZ cermet could be expressed using the resultant conductivity of the resistance network of unit resisters corresponding to Ni and YSZ. On the basis of these results, the following equation expressing the thermal conductivity of porous Ni/YSZ cermets λC/Wm−1 K−1, has been derived as a function of Ni-volume fraction for total solids VNi, and porosity P, for 1273 K(Remark: Graphics omitted.)where KA⁄Wm−1K−1=0.16−55.62VNi+195.7VNi2, KB⁄(Wm−1K−1)2=95.23+1432.3VNi+1257VNi2. The validity is confirmed.

Estimation of a state-dependent model parameter

A numerical algorithm was developed which estimates a state-dependent model parameter on the basis of transient state observation data. The algorithm was presented for the problem of estimating the temperature dependence of thermal diffusivity in a one-dimensional heat equation. The estimation problem was converted into a finite dimensional optimization problem by the least-squares formulation and B-splines representation of the parameter. Numerical experiments were performed using simulated observation data as well as the actual observation data obtained in a heat conduction experiment on rubber compound layers. The performance of the algorithm was discussed in relation to the effect of the parameter representation scheme, the quality and quantity of the data.

Study of change in thermal diffusivity of amorphous polymers during glass transition

The thermal diffusivity of amorphous polymers was investigated by the newly developed a.c. joule heating technique in the practical temperature range including the melt state, glass transition and glassy state. The temperature dependence of thermal diffusivity was measured both in the glassy state and the melt state and its discontinuous change at the glass transition was clearly observed. Δα, defined as the change of thermal diffusivity during the glass transition, was closely related to the glass transition temperatures and to the change in heat capacity, ΔC p. It is confirmed that the thermal diffusivity is one of the basic thermophysical properties which can connect the chemical structure and molecular weight of polymer to the glass transition.

Thermal diffusivity, specific heat, and thermal conductivity of (Ca1−x,Mgx)Zr4(PO4)6 ceramic

The thermal diffusivity (a), specific heat (Cp), and thermal conductivity (K) of single phase (Ca1−x,MgxZr4 (PO4)6 (CMZP) ceramic with x=0.4 were investigated in terms of porosity and temperature. The dense CMZP ceramic exhibited values of thermal conductivity lower by a factor of approximately two up to 600°C. The conductivity can be further reduced, to as low as 0.4 W M−1 K−1, by 30% porosity. The temperature dependence of thermal diffusivity became less pronounced with increasing porosity, and a near-constant coefficient of thermal conductivity about 1.1 × 10−4 conduction unit per degree Celsius at temperatures of 30–600 °C, was observed for different porosities. These findings indicate that the CMZP ceramic may be an attractive candidate for thermal insulating applications.

Photoacoustic Investigation of Glass Transition in AsxTe1−x Glasses

Photoacoustic (PA) technique is used to study glass transition and temperature dependence of thermal diffusivity in AsxTe1-x glasses with 0.25 0.60. PA amplitude goes through a minimum and the phase shows a maximum at glass transition temperature Tg. The variation of thermal diffusivity with temperature shows sharp decrease near Tg. The variation of thermal diffusivity with composition shows maximum at = 0.40 for all temperatures T Tg.

Temperature dependence of thermal diffusivity measured by photothermal radiometry

Thermal diffusivity is generally measured by impulse or modulated flux methods; the temperature distribution inside the sample is measured by thermocouples. The nonintrusive photothermal techniques do not induce geometrical or thermal changes inside the sample; an indirect procedure gives the temperature variations on the sample surface. Photothermal radiometry, based on the measurement of the radiative flux emitted by the sample, is all the more accurate as the temperature is elevated. We have used this method to measure thermal diffusivities of thin and opaque solid samples at temperatures above 400 K. The temperature field is calculated by using a standard model accounting for the emitted radiative flux. The experimental apparatus is briefly described and experimental results for selected materials (nickel, stainless steel) and cast-iron samples are presented. The influence of the material structure on the thermal diffusivity is discussed.

Measurement of thermal diffusivity for polymer film by flash radiometry

A new method to determine the thermal diffusivity of polymer film by flash radiometry is presented. The transient infrared radiation from the sample surface was measured after the sample film was heated by a short-duration pulsed light. The thermal diffusivity was determined by fitting the transient infrared signal to the theoretical curve of a one-dimensional heat diffusion model. The transient infrared signal of polymethylmethacrylate (PMMA) film could be well fitted by the theoretical curve using appropriate thermal diffusivity in the wide temperature range between 100 K and near the softening point of PMMA. The temperature dependence of thermal diffusivity of PMMA film showed that the thermal diffusivity sensitively reflects the molecular motion of polymer chain.

Thermophysical and structural investigation OP Eu 2O 3 - Mo system

The paper presents the results of temperature dependence of thermal diffusivity of 3 composition Eu 2O 3 - Mo mixtures in temperature range (900–1300) K as well as the results of rtg. diffraction analysis. An anomalous temperature dependence of thermal diffusivity was found as a function of Mo concentration. In the case of Eu 2O 3 there exists the structure transformation from cubic to monoclinic phase at temperature 1323 K. It was found, that this change is not reversible. The monoclinic phase was observed also after cooling to room temperature. The content of Mo lowers substantialy the temperature of phase transformation of Eu 2O 3. The anomaly of temperature dependences of thermal diffusivity can be explained by different values of thermal parameters of cubic and monoclinic phase of Eu 2O 3, respectively.

Moisture and temperature dependence of thermal diffusivity of cod minces

Moisture and temperature dependence of thermal diffusivity of cod minces

Electrical resistance and thermophysical properties of solid solutions of germanium in iron

Temperature dependence of thermal diffusivity, thermal conductivity, and electrical resistance is studied in solid solutions of germanium in iron at impurity concentrations of 0.49–18.0 at.% Ge over the temperature range 300–1300‡K. The results obtained are evaluated and compared to similar dependences for the system Fe-Si.

Potential Parameters for Xenon on Morse Potential Model

Two sets of the potential parameters for Xenon on Morse potential model (exp:exp) are evaluated by means of temperature dependence of thermal diffusion and temperature dependence of viscosity. In order to test the suitability of the parameters, experimental thermal conductivity data are reproduced for different temperatures in the range 300 K–500 K. A good agreement between the experimental and theoretical thermal conductivities shows that the Morse potential is a satisfactory choice for explaining the interaction forces in Xenon. It is also found that the parameters obtained by using thermal diffusion properly are superior to those obtained by using viscosity property.

Temperature dependence of thermal diffusion in CO isotopic gas mixtures

The temperature dependence of thermal diffusion in several different isotopic gas mixtures of CO is investigated theoretically and the results are compared with experiment. The temperature range includes the inversion temperatures for all mixtures. Thermal conductivity and diffusion coefficients are also calculated. The basis of the study is a previously presented theory which provides expressions for classical collision integrals for molecules with rotational structure in terms of spherical Ω* integrals.

A Measuring Method of Thermal Diffusivity by Continuous Heating

A new transient measuring method of thermal diffusivity by continuous heating of a slab-like specimen to obtain the temperature dependence of thermal diffusivity covering a wide temperature range in a short time, is investigated. The principle is based on the solution of equation of linear heat conduction and the thermal diffusivity is determined from the temperature response at the center face of the specimen heated continuously from both surfaces. The measuring conditions are determined by theoretical analysis. By comparing the linear solutions of the measuring principle with the non-linear solutions of the measurement obtained analytically or numerically, the heating conditions of the specimens under which the linear principle is satisfied within required accuracies are determined. Results of testing polyethylene up to the softening temperature with an apparatus based on the above conditions are shown.

Intermolecular Potentials for Unlike Interaction on Exp-Six Model

The potential parameters for the unlike pairs of molecules A–He, A–Xe and Ne–Xe have been determined for the ``exp-six'' model from the temperature dependence of thermal diffusion of these gas mixtures following the method developed earlier by Srivastava and Madan. Unlike the graphical method of Srivastava and Srivastava a purely computational method has been employed here for determining the parameters α12 and ε12. First coupled values of α12 and ε12 were determined by neglecting the temperature variation of the ``g'' factor and using Kihara's first approximation formula. Then the absolute value of the ``g'' factor was calculated at a fixed temperature for various values of α12 and that value of α12 selected which gave the experimental value of the thermal diffusion factor αT. Precise value of ε12 was determined by taking into account the temperature variation of ``g.'' The values of the potential parameters α12, ε12, and (rm)12 thus obtained were found to reproduce satisfactorily the experimental data on thermal diffusion except for very low temperatures where T* is less than 1.5 and the ``exp-six'' law probably does not hold.