Decrease In Thermal Diffusivity Research Articles

Determination of thermal diffusivity and thermal conductivity of Fe-Al alloys in the concentration range 22 to 50 at.% Al

The thermal diffusivity of Fe-Al alloys in the concentration range 22 to 50 at.% Al was measured within a temperature range of 20 to 600 °C. The thermal diffusivity of the Fe-25 and 28 at.% Al alloys decreases with increasing temperature up to the Curie temperature, and then it increases up to the temperature when the D03 ↔ B2 transformation occurs. The thermal diffusivity of Fe-22 at.% Al alloys increases with rising temperature up to the temperature when D03 ↔ B2 transformation occurs, and then it decreases. A further decrease in thermal diffusivity follows up to the Curie temperature. The thermal diffusivity of Fe-34 and Fe-40 at.% Al increases monotonically with the rising temperature. The thermal diffusivity of Fe-50 at.% Al alloys decreases only up to 100 °C, and does not change any further with increasing temperature. Thermal conductivity is the highest for Fe-25 and Fe-50 at.% Al alloys at room temperature. Thermal conductivity rises for all studied alloys with increasing temperature. The smallest increase was registered for Fe-25 at.% Al alloys.

Accuracy and uncertainty of single-shot, nonresonant laser-induced thermal acoustics

We study the accuracy and uncertainty of single-shot nonresonant laser-induced thermal acoustics measurements of the speed of sound and the thermal diffusivity in unseeded atmospheric air from electrostrictive gratings as a function of the laser power settings. For low pump energies, the measured speed of sound is too low, which is due to the influence of noise on the numerical data analysis scheme. For pump energies comparable to and higher than the breakdown energy of the gas, the measured speed of sound is too high. This is an effect of leaving the acoustic limit, and instead creating finite-amplitude density perturbations. The measured thermal diffusivity is too large for high noise levels but it decreases below the predicted value for high pump energies. The pump energy where the error is minimal coincides for the speed of sound and for the thermal diffusivity measurements. The errors at this minimum are 0.03% and 1%, respectively. The uncertainties for the speed of sound and the thermal diffusivity decrease monotonically with signal intensity to 0.25% and 5%, respectively.

Effects of temperature-dependent thermal diffusivity on shear instability in a viscoelastic zone: implications for faster ductile faulting and earthquakes in the spinel stability field

The introduction of a new model of thermal diffusivity has motivated us to reinvestigate a one-dimensional viscoelastic shear zone model with realistic rheology, temperature-dependent thermal diffusivity ( κ( T)) and viscous dissipation. Although thermal diffusivity in the shear zone is spatially heterogeneous with κ( T) and viscous heating, the spatial distribution of κ( T) does not affect shear zone evolution for the mesh resolution used in the model. As temperatures increase above room temperature, thermal diffusivity decreases. The lower thermal diffusivity causes a slight spatial thinning of the shear zone and an acceleration of the onset of instability relative to cases using a room temperature value of thermal diffusivity. Increasing the nonlinearity of κ( T) enhances shear zone thinning and speed-up of instability; the amount of enhancement depends on temperature, mineralogy and the rate of shear heating. The rheology of spinel creates a more unstable situation for the shear zone than that of olivine, but the boundary separating instability and stability is sensitive to changes in material properties. A decrease in the grain size does not influence the timescale of instability, unless grain size reduction causes diffusion creep to be the dominant deformation mechanism. Viscoelastic thermal–mechanical instabilities occur on timescales ranging from a few hundred to several thousand years. In most slabs, no instability is found to occur in spinel regions at temperatures above 1200 K. Likewise, shear instability in olivine at upper mantle depths will not occur at temperatures greater than 1100 K.

Mode-mismatched thermal lens spectrometry for thermo-optical properties measurement in optical glasses: a review

In this work, the application of thermal lens spectrometry (TLS) to study thermo-optical and spectroscopic properties of optical glasses is described. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the method as compared with conventional measurements. The technique is applied to determine the thermal diffusivities, temperature coefficient of optical path length changes, ds/ dT , and the fluorescence quantum efficiencies of several glasses such as fluorides, chalcogenides, chalcohalides, soda lime and low silica calcium aluminosilicate. For some of these glasses, the effect of glass composition on thermo-optical properties was studied. For aluminosilicate glasses was observed an 8% decrease of the thermal diffusivity with the increase of Nd 2O 3 doping. Five kinds of fluoride glasses were studied and it was observed that fluorindate and fluoaluminate present better thermo-optical properties compared to fluorozirconate glasses: thermal diffusivity ∼20% higher and ds/ dT ∼50% lower (in modulus). All fluoride glasses had negative ds/ dT and all the other glasses a positive ds/ dT . For fluoride glasses, the TLS measurements were performed up to the glass transition temperature ( T g ). Near T g thermal diffusivity decreases and ds/ dT increases, both by one order of magnitude. The TLS was used to determine fluorescence quantum efficiency and concentration quenching in Nd 3+-doped glasses (aluminosilicate and fluorozirconate). These results were compared with the Judd–Ofelt calculations.

Structure and properties of water free Nd 2O 3 doped low silica calcium aluminate glasses

In this work, Nd 2O 3 doped low silica calcium aluminate glasses have been melted under vacuum conditions. Measurements of thermal diffusivity, density, optical absorption coefficient, glass transformation temperature ( T g), crystallisation temperature ( T x), dilatometric softening temperature ( T d) and Vickers microhardness have been carried out. The results provided information about the sample's structure and showed that for Nd 2O 3 concentration up to 5 wt% there was no devitrification of the samples. The decrease in hardness and thermal diffusivity was about 8%, while the density increased about 8% with the increasing concentration of Nd 2O 3. The thermomechanical properties combined with their quantum efficiency indicate that this material is a candidate for an active medium for solid-state lasers.

Improved confinement and transport studies in the reversed field experiment (RFX)

This paper presents results obtained on the Reversed Field eXperiment, RFX [G. Rostagni, Fusion Engineering and Design 25, 301 (1995)], on the study of local transport properties of the RFP configuration both in the standard regime and during improved confinement phases obtained with poloidal current drive. The plasma core is dominated by magnetic turbulence, as indicated by both energy and particle transport analysis. Diffusivity decreases toward the edge and is reduced in the core at high Lundquist number. The edge region is apparently not affected by dynamo mode turbulence and determines global confinement in standard operation. Particle and heat diffusivity in this region show a favorable dependence with increasing density. The most interesting feature of the edge region of RFX consists of a sheared radial electric field with shearing rate comparable to that observed in tokamaks. Indications of turbulence reduction in the shear layer have been obtained. Enhanced energy confinement has been produced by pulsed poloidal current drive (PPCD). A clear reduction of the dynamo activity has been observed with a decrease of core thermal diffusivity to level comparable to the edge and global confinement improvement up to a factor of 3.

Evidence of scattering effects on the thermal transport inindium-doped CdTe films

Photoacoustic (PA) heat transmission measurements were used to study thermal transport in n-type polycrystalline CdTe films doped with metallic indium. Thermal diffusivity (TD) at room temperature was determined using an open cell PA technique. The TD obtained for each sample was correlated with x-ray diffraction measurements. It can be shown that the TD of the undoped films under investigation is significantly reduced compared with corresponding doped films. This reduction is a result of the high concentration of indium atoms and defects depending on the deposition parameters. It is shown that the effects of CdTe lattice shrink produced by indium doping, induces an increase in phonon scattering reflected in the large TD decrease even with moderated indium concentration. We have calculated the relation between the TD and the lattice parameter as a polynomial function (cm2 s-1) = -35225.2+14259a-1532.7a2-27.9a3+8.4a4.

Low thermal diffusivity measurements of thin films using mirage technique

Mirage technique is proved to be powerful in measurements of thermal diffusivity. Its contactless nature makes it suitable for delicate samples such as thin films and single crystals. However, as the damping of the thermal wave profile increases progressively upon the decrease in thermal diffusivity of the medium, mirage technique becomes more difficult to be applied to low thermal diffusivity measurements. Moreover influences from substrate signals make analysis difficult when the samples are thermally thin. Recently a thermal-wave-coupling method for mirage signal analysis [P. K. Wong, P. C. W. Fung, H. L. Tam, and J. Gao, Phys. Rev. B 51, 523 (1995)] was reported for thermal diffusivity measurements of thin film down to 60 nm thick. In this article we apply the thermal-wave-coupling method to thin films of low thermal diffusivity, especially polymer films. A new lower limit of thermal diffusivity measurable by mirage technique has been reached.

UNSTEADY COOLING OF A SOLID SPHERE IN A RADIATIVELY ACTIVE MEDIUM

The transient thermal characteristics of a hot spherical body were investigated when it was suddenly introduced to an initially cool radiatively active medium. The governing equations modeled were solved numerically. The radiative transfer equation was solved by the discrete ordinates method. The results were discussed of changing various physical parameters. The heat transfer mode inside the spherical body was conduction only. But both conduction and radiation were assumed to occur in the medium. Faster cooling of the sphere was accompanied by a decrease in optical thickness, an increase in medium thermal conductivity, an increase in sphere surface emissivity, or a decrease in thermal diffusivity of the sphere. However, faster cooling of the sphere body did not always induce faster heating of the medium. This was found to be dependent on the physical properties of the medium.

Collinear mirage effect measurement of the thermal diffusivity in Ferronematics

Thermal diffusivity results obtained for a lyotropic nematic liquid crystal sample doped with different amounts of ferrofluid (ferronematics) are reported. The collinear mirage technique was used for measurements along axes parallel and perpendicular to the director of this anisotropic material. The results show that as the ferrofluid concentration increases the thermal diffusivity decreases, leading to values even lower than those already reported for plain lyotropic nematic liquid crystals. The ratio of the two diffusivities increases as the ferrofluid concentration increases, reaching the exact value theoretically expected, though not achieved experimentally, for plain lyotropic nematic liquid crystals.

Measurement of thermal diffusivities of thin metallic films using the ac calorimetric method

The thermal diffusivities of thin films of evaporated metal less than 1×10−6 m thick have been measured using the light-irradiation ac calorimetric method. The thermal diffusivities of metallic thin films become smaller than those of the bulk as the thickness of the metallic thin films decreases. It is determined, using x-ray diffraction measurement, that the decrease of thermal diffusivity of thin films of evaporated metal is caused by a decrease in crystalline size.

Energy Transport in Tokamak Plasmas with Central Current Density Control Using Fast Waves.

Fast wave current drive has been used to substantially modify the central current density profile in a tokamak plasma. Counter-fast wave current drive (FWCD) applied to discharges with negative central magnetic shear enhances the shear reversal and leads to a distinct transition to a mode of improved core confinement. In this state, the electron thermal diffusivity decreases by $(50\ifmmode\pm\else\textpm\fi{}20)%$ and the ion diffusivity by $(80\ifmmode\pm\else\textpm\fi{}20)%$, compared to just before the transition. The FWCD and electron heating elucidates the role of the current profile on confinement and stability.

Reduced thermal diffusivity of nanostructured silver

Abstract The thermal diffusivity of nanostructured Ag, synthesized by inert-gas condensation and in situ compaction, was measured at room temperature by photoacoustic technique. The results indicated that the thermal diffusivity of nanostructured Ag decreased catastrophically as compared to that of conventional polycrystalline Ag and ranged from about 27 to 74% that of polycrystalline Ag for all the nanostructured Ag specimens investigated. Annealing experiments showed that although the thermal diffusivity of nano-structured Ag is related to its grain size, it is dependent strongly on annealing temperature. In addition, it was found that the variation of thermal diffusivity became weak as its relative density reached 93%, below which it decreased steeply with decrease of specimen density. Preliminary analysis showed that the decrease of thermal diffusivity originates mainly from its thermal conductivity, which can be understood in terms of intense electron scattering within interfacial component.

An experimental study on thermal diffusivity of nanocrystalline Ag

Thermal diffusivity (TD) and its relation to the microstructure of nanocrystalline Ag (n-Ag), synthesized by inert-gas condensation and in-situ compaction, was investigated by photoacoustic technique. The results indicated that a dramatic decrease of TD in n-Ag (which is about 27% that of p-Ag for the n-Ag with grain size 20 nm) is related to its small grain size and large volume fraction of interfacial component, and originates mainly from reduction of its thermal conductivity. Annealing experiments showed that TD is not only affected by its grain size, but also strongly dependent on the microstructure of the interfaces. The increase of TD with increasing grain size can be explained by attributing it mainly to the rise of effective electron density; while increasing TD by annealing where no substantial grain growth occurred, reflects an increase of electron transmission coefficient during annealing, which could also lead to a rise in effective electron density. In addition, the prompt decrease of TD with decreasing specimen density as it is lower than 93% could be reasonably attributed to the increasing number of extended boundaries in n-Ag.

Electron cyclotron heating combined with lower hybrid current drive on WT-3

Lower hybrid current drive (LHCD) is applied to improve the confinement in the WT-3 tokamak. With increasing the lower hybrid (LH) power, a sharp decrease in electron thermal diffusivity χ e is observed at a certain LF power. The χ e is directly measured by applying the simple time-to-peak method to propagation of a heat pulse induced by a short electron cyclotron heating (ECH) pulse. A high electron temperature is obtained by applying ECH to the LHCD plasma with improved χ e. The efficiency of ECH in the LHCD plasma with improved χ e becomes three times that in ohmic heating plasmas.

Thermal diffusivity of neutron-irradiated carbon-carbon composites

Abstract Thermal diffusivity of two kinds of two-directional carbon-carbon composites were studied after neutron irradiation at temperatures 640–1070°C to a maximum neutron fluence 1.98 × 10 25 n/m 2 ( E > 29 fJ , 0.18 MeV) in the JMTR (Japan Materials Testing Reactor). The decrease of thermal diffusivity was observed after irradiation, and its significant decrease occurred at low neutron fluence, followed by gradual decrease with neutron fluence. The changes in thermal diffusivity, which were normalized to unirradiated values, were large at low measurement temperatures and decreased with temperature and also increased with neutron fluence. The changes in thermal conductivity related to thermal diffusivity are also discussed.

Thermal diffusivity and electrical conductivity of electropolymerized polypyrrole films

AbstractThis article presents measurement of thermal diffusivity and electrical conductivity of polypyrrole films prepared by electropolymerization. Thermal diffusivity was measured by laser radiometry (former flash radiometry). Electrical conductivity was determined by a conventional four‐probe method. Increase of thermal diffusivity is observed when increasing the supporting electrolyte concentration, which is also shared with the increase of electrical conductivity. Both thermal diffusivity and electrical conductivity significantly depended on the types of counter anion incorporating into polymer bulk. Thermal diffusivity of polypyrrole film is larger than that for common nonelectrical conductive polymers. Temperature profile of thermal diffusivity for as‐grown polypyrrole films shows that thermal diffusivity increases with increasing temperature (first running profile), whereas remeasured temperature profile of thermal diffusivity (second or third running profiles) shows the decrease of thermal diffusivity with increasing temperature. Electrical conductivity monotonically increases until the significant decrease of it occurs at the temperature above 130°C. Investigation of these temperature profiles of thermal diffusivity and electrical conductivity has been made by corresponding to thermal analysis data. © 1994 John Wiley & Sons, Inc.

Effect of Thermal History on the Thermal Diffusivity and Thermal Expansion of an Alumina–Aluminum Titanate Composite

A study was conducted of the temperature dependence of the thermal diffusivity and thermal expansion of an alumina–aluminum titanate composite heated to a range of maximum temperatures followed by cooling to room temperature. Heating to temperatures above about 600°C resulted in a hysteresis behavior in which the data on cooling differed from the data obtained during heating. For both the thermal diffusivity and thermal expansion, the degree of hysteresis increased with increasing maximum temperature. On return to room temperature, following heating to a temperature of about 1200°C, the thermal diffusivity exhibited a significant decrease, with a corresponding increase in specimen size. This effect was attributed to an increase in microcrack density over the corresponding value prior to heating. On subsequent cycles of heating and cooling for a maximum temperature of 1200°C this decrease in thermal diffusivity was partially recovered, indicative of the structural integrity of the alumina–aluminum titanate composite of this study in practical applications involving temperatures of at least 1200°C.

Thermal properties of AlAs/GaAs superlattices

Thermal properties of semiconductor superlattices have been investigated for the first time. The thermal properties of AlAs/GaAs superlattices were measured with the ac calorimetric method. It is found that the thermal diffusivity and thermal conductivity of the AlAs/GaAs superlattices are larger than those of the AlGaAs alloy due to the suppression of alloy scattering in the superlattice. However, the thermal diffusivity and thermal conductivity decrease with a decrease in the superlattice period and seem to approach those of the Al0.5Ga0.5As alloy in the limit of short-period superlattices.

A transient thermal failure mode in metal film resistors

A failure mode has been observed in metal film resistors when they are subjected to single shot high voltage pulses. The conductor fails at a high field point which may be inherent in the design or may be due to manufacturing defects. The localised nature of the failure leads to the conclusion that there is a closed loop mechanism: the decrease in thermal diffusivity of the structure with rising temperature leads to an increase in the rate of temperature rise.