Difference In Thermal Diffusivity Research Articles

Structural and Thermal Diffusivity Analysis of an Organoferroelastic Crystal Showing Scissor-Like Two-Directional Deformation Induced by Uniaxial Compression.

A two-directional ferroelastic deformation in organic crystals is unprecedented owing to its anisotropic crystal packing, in contrast to isotropic symmetrical packing in inorganic compounds and polymers. Thereby, finding and constructing multidirectional ferroelastic deformations in organic compounds is undoubtedly complex and at once calls for deep comprehension. Herein, we demonstrate the first example of a two-directional ferroelastic deformation with a unique scissor-like movement in single crystals of trans-3-hexenedioic acid by the application of uniaxial compression stress. A detailed structural investigation of the mechanical deformation at the macroscopic and microscopic levels by three distinct force measurement techniques (including shear and three-point bending test), single crystal X-ray diffraction techniques, and polarized synchrotron-FTIR microspectroscopy highlighted that mechanical twinning promoted the deformation. The presence of two crystallographically equivalent faces and the herringbone arrangement promoted the two-directional ferroelastic deformation. In addition, anisotropic heat transfer properties in the parent and the deformed domains were investigated by thermal diffusivity measurement on all three axes using microscale temperature-wave analysis (μ-TWA). A correlation between the anisotropic structural arrangement and the difference in thermal diffusivity and mechanical behavior in the two-directional organoferroelastic deformation could be established. The structural and molecular level information from this two-directional ferroelastic deformation would lead to a more profound understanding of the structure-property relationship in multidirectional deformation in organic crystals.

Thermal diffusivity, microstructure and nanohardness of laser-welded proton-irradiated Eurofer97

Eurofer97 steel, a candidate structural material for future fusion reactors, was examined following 1.9 MeV proton-irradiation up to 0.91(5) dpa at 450 ∘C with and without a prior post-weld heat-treatment at 760 ∘C for 4 hours in a laser-welded state. A nanoindentation study found a pile-up-corrected nanohardness of 4.0(4) GPa in the as-welded fusion zone, decreasing to 2.1(3) GPa in the parent material. Irradiation temperature and post-weld heat treatment were both found to have a recovery effect on weld hardness, with the latter being entire. Proton-irradiation damage was not found to contribute to nanohardness at the temperature investigated. X-ray diffraction analysis found increased 1-dimensional dislocation density in the as-welded fusion zone, diminishing to 3(2) – 19.2(1.4)×1014 cm−2 in the parent material, dependent on irradiation and heat-treatment. Transient grating spectroscopy of Eurofer97 was attempted, finding a systematic underestimation of thermal diffusivity of average 15.6% from room-temperature to 600 ∘C when compared to laser flash analysis. Transient grating spectroscopy was, nevertheless, applied determining a room-temperature thermal diffusivity of 7.8(3) mm2 s−1 in the parent material and 6.7(4) mm2 s−1 in the as-welded fusion zone. Irradiation at 450 ∘C alleviated this difference in thermal diffusivity; recovery of weld-induced changes was observed up to 20% in the fusion zone due to irradiation conditions, distinct from temperature effects alone. Such results bode well for Eurofer97's application in fusion reactors, where welding will be essential. Thermal diffusivity has also been mapped at a fine scale across a heterogeneous structure, a technique applicable widely outside the realm of radiation materials science.

Understanding the behavior of laser surface remelting after directed energy deposition additive manufacturing through comparing the use of iron and Inconel powders

Laser directed energy deposition with powder (pDED-L) is one of the most popular techniques for additive manufacturing. However, for some applications, the resulting surface finish requires post-processing machining. Laser remelting processing has been cited as a means of decreasing machining needs in other applications. The objective of this study was to assess the performance and gain a better physical understanding of the laser remelting process as an alternative to the conventional post-processing techniques applied after pDED-L. pDED-L and remelting were performed using a 10 kW fiber laser source and iron and Inconel 625 powders as feedstock to compare the surface roughness and waviness under different processing conditions (three heat input levels). Through 2D and 3D surface measurements, as well as scanning electron microscopy (SEM), surface features were qualified and quantified. SEM and optical microscopy (OM) were also employed for metallurgical characterization of the roughness. The surface micro-notch effect was assessed through the stress concentration factor (kt). The results indicated attenuation of the average roughness (Ra) by around 30% for iron and 70% for Inconel when laser remelting was employed. In addition, kt presented a 31% reduction for iron and 29% reduction for Inconel. The performance varied according to the type of material used and was mainly related to differences in thermal diffusivity and electrical resistivity. It was concluded that laser remelting is a promising technology for coupling with pDED-L aimed at producing 3D metal components with superior quality while allowing a faster production rate in comparison to current practices.

Photothermal Beam Deflection Spectroscopy for the Determination of Thermal Diffusivity of Soils and Soil Aggregates

Photothermal beam deflection spectroscopy (BDS) with a red He–Ne laser (632.8 nm, 35 mW) as an excitation beam source and a green He–Ne laser (543.1 nm, 2 mW) as a probe was used for estimating thermal diffusivity of several types of soil samples and individual soil aggregates with small surfaces (2 × 2 mm). It is shown that BDS can be used on demand for studies of changes in properties of soil entities of different hierarchical levels under the action of agrogenesis. It is presented that BDS clearly distinguishes between thermal diffusivities of different soil types: Sod-podzolic [Umbric Albeluvisols, Abruptic], 29 ± 3; Chernozem typical [Voronic Chernozems, Pachic], 9.9 ± 0.9; and Light Chestnut [Haplic Kastanozems, Chromic], 9.7 ± 0.9 cm2·h−1. Aggregates of chernozem soil show a significantly higher thermal diffusivity compared to the bulk soil. Thermal diffusivities of aggregates of Chernozem for virgin and bare fallow samples differ, 53 ± 4 cm2·h−1 and 45 ± 4 cm2·h−1, respectively. Micromonoliths of different Sod-podzolic soil horizons within the same profile (topsoil, depth 10–14 cm, and a parent rock with Fe illuviation, depth 180–185 cm) also show a significant difference, thermal diffusivities are 9.5 ± 0.8 cm2·h−1 and 27 ± 2 cm2·h−1, respectively. For soil micromonoliths, BDS is capable to distinguish the difference in thermal diffusivity resulting from the changes in the structure of aggregates.

Local structures in ionic liquids probed and characterized by microscopic thermal diffusion monitored with picosecond time-resolved Raman spectroscopy

Vibrational cooling rate of the first excited singlet (S(1)) state of trans-stilbene and bulk thermal diffusivity are measured for seven room temperature ionic liquids, C(2)mimTf(2)N, C(4)mimTf(2)N, C(4)mimPF(6), C(5)mimTf(2)N, C(6)mimTf(2)N, C(8)mimTf(2)N, and bmpyTf(2)N. Vibrational cooling rate measured with picosecond time-resolved Raman spectroscopy reflects solute-solvent and solvent-solvent energy transfer in a microscopic solvent environment. Thermal diffusivity measured with the transient grating method indicates macroscopic heat conduction capability. Vibrational cooling rate of S(1) trans-stilbene is known to have a good correlation with bulk thermal diffusivity in ordinary molecular liquids. In the seven ionic liquids studied, however, vibrational cooling rate shows no correlation with thermal diffusivity; the observed rates are similar (0.082 to 0.12 ps(-1) in the seven ionic liquids and 0.08 to 0.14 ps(-1) in molecular liquids) despite large differences in thermal diffusivity (5.4-7.5 × 10(-8) m(2) s(-1) in ionic liquids and 8.0-10 × 10(-8) m(2) s(-1) in molecular liquids). This finding is consistent with our working hypothesis that there are local structures characteristically formed in ionic liquids. Vibrational cooling rate is determined by energy transfer among solvent ions in a local structure, while macroscopic thermal diffusion is controlled by heat transfer over boundaries of local structures. By using "local" thermal diffusivity, we are able to simulate the vibrational cooling kinetics observed in ionic liquids with a model assuming thermal diffusion in continuous media. The lower limit of the size of local structure is estimated with vibrational cooling process observed with and without the excess energy. A quantitative discussion with a numerical simulation shows that the diameter of local structure is larger than 10 nm. If we combine this lower limit, 10 nm, with the upper limit, 100 nm, which is estimated from the transparency (no light scattering) of ionic liquids, an order of magnitude estimate of local structure is obtained as 10 nm < L < 100 nm, where L is the length or the diameter of the domain of local structure.

Melting of Phase Change Materials With Volume Change in Metal Foams

Melting of phase change materials (PCMs) embedded in metal foams is investigated. The two-temperature model developed accounts for volume change in the PCM upon melting. Volume-averaged mass and momentum equations are solved, with the Brinkman–Forchheimer extension to Darcy’s law employed to model the porous-medium resistance. Local thermal equilibrium does not hold due to the large difference in thermal diffusivity between the metal foam and the PCM. Therefore, a two-temperature approach is adopted, with the heat transfer between the metal foam and the PCM being coupled by means of an interstitial Nusselt number. The enthalpy method is applied to account for phase change. The governing equations are solved using a finite-volume approach. Effects of volume shrinkage/expansion are considered for different interstitial heat transfer rates between the foam and PCM. The detailed behavior of the melting region as a function of buoyancy-driven convection and interstitial Nusselt number is analyzed. For strong interstitial heat transfer, the melting region is significantly reduced in extent and the melting process is greatly enhanced as is heat transfer from the wall; the converse applies for weak interstitial heat transfer. The melting process at a low interstitial Nusselt number is significantly influenced by melt convection, while the behavior is dominated by conduction at high interstitial Nusselt numbers. Volume shrinkage/expansion due to phase change induces an added flow, which affects the PCM melting rate.

Vibrational Cooling Process of S1 trans-Stilbene in Ionic Liquids Observed with Picosecond Time-resolved Raman Spectroscopy

Abstract Vibrational cooling rates in room temperature ionic liquids were measured with picosecond time-resolved Raman spectroscopy. The 1570-cm−1 Raman band of the first excited singlet (S1) state of trans-stilbene was used as a “Raman thermometer.” The recorded vibrational cooling rates in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N) were close to those in ordinary molecular solvents despite a large difference in thermal diffusivity. The result is consistent with the assumption that local structures are formed in the ionic liquids.

Non-contact bruise detection in apples by thermal imaging

Thermal imaging is a non-destructive and non-contact infrared sensing technique. Such imaging creates a bit-map called a thermogram by detecting infrared radiation emitted from an object. Up to 100% of apple bruises were detected using thermal imaging during warming of the fruits by discriminating surface temperature between bruised and sound tissues. Apples were bruised by dropping them from 0.46 m onto a smooth concrete floor and then were held at 26 °C and 50% RH for 48 h. They were then thermally imaged using a ThermaCam™ PM390 (FLIR Systems, Inc., Portland, OR) during heating and cooling treatments. Thermal images of bruised tissue showed at least 1–2 °C difference from sound tissue within 30–180 s. The temperature differences between bruised and sound tissues were possibly due to the differences in thermal diffusivity. Under steady-state temperature, thermal imaging did not detect bruises, indicating that the temperature differences were not due to emissivity differences. The technique could provide a basis for automatic bruise sorting, and possibly a better understanding of bruised tissue.

The thermal diffusivity of simulated DUPIC fuel for irradiation test

The thermal diffusivity of simulated DUPIC fuel with an equivalent burnup of 35,000 MWd/tU (3.73 at.%), which was to be tested for irradiation at the HANARO research reactor, was measured using the laser flash method in a temperature range from room temperature to 1623 K. The density of the simulated DUPIC fuel used in the measurement of thermal diffusivity was 10.16 g/cm3 (94.2% of theoretical density) at room temperature, and the diameter and thickness were 10 mm and 1 mm, respectively. The thermal diffusivity decreased from 0.0205 cm2/s at room temperature to 0.0060 cm2/s at 1623 K. The thermal diffusivity of simulated DUPIC fuel was lower than that of UO2 by 36.8% at 300K and by 12.7% at 1623 K. The difference in thermal diffusivity between the simulated DUPIC fuel and UO2 was large at room temperature, and decreased as the temperature increased.

Thermal and Solutal Influences of Continuous Fibers on Matrix Dendrite Growth in Composite Materials

Numerical simulation was applied to evaluate the thermal and solutal influences of fibers on the dendrite growth of matrix alloy reinforced with continuous fibers. The results of temperature and solute distribution, and dendrite tip undercooling and tip radius were compared with experimental data obtained from directional solidification studies for polyvinylidene fluoride, Pyrex glass, and copper fiber reinforced succinonitrile-acetone alloy composites. In the polyvinylidene flouride fiber/pure succinonitrile composites, dendrite in the composite region grew behind that in the bulk region, and in the direction of the heat flow. However, dendrite between copper fibers grew faster than that in the bulk region. A computer simulation revealed that a difference in thermal diffusivity influences the thermal distribution of specimens and the apparent dendrite tip location. The gap in dendrite tips between the composite and bulk regions increased as the composition of acetone increased or the fiber interstices became smaller. Numerical analysis revealed that tip composition and undercooling increased as the fiber interstices became smaller than the primary dendrite arm spacing. The constraint of growth on the secondary arm and the change of dendrite morphology were also shown in the analysis.

Measurement of the Thermal Diffusivity of Metallic Foils and Films by the Photoacoustic Method1

The thermal diffusivities of four kinds of metallic foils from 20 to 200μμm in thickness were measured by a photoacoustic method on the basis of the Rosencwaig and Gersho theory. The measured data for continuous foils of uniform microscopic structure almost agreed with the literature values. Measurements were also carried out on two kinds of metallic thin films with of 10μμm thickness produced by sputtering. The difference in thermal diffusivity between the foils and the sputtered films depended on the uniformity of the microscopic structure.

Out-of-pile Tests of Simulated Rock-like Oxide (ROX) Fuels

A new type of fuel, that is, ROX (rock-like oxide) fuel originally being composed of PuO2-SZR (stabilized zirconia)-MgAl2 O4 is under development by JAERI. In order to increase data base, the simulated ROX fuel replaced PuO2 by UO2 was fabricated and out-of-pile tests were performed. The following results were obtained: (1) The densities of simulated ROX fuels ranged from 4.9 to 5.4g/cc, which were of order of about 47–52% of that in UO2. (2) The melting points (MPs) of simulated ROX fuel ranged from 2,206 to 2,139 K, which was lower of order of about 30% to that of UO2. (3) There was no marked difference in linear thermal expansion coefficient (LTEC) between simulated ROX fuels and UO2 ones. Linear thermal expansion coefficient was increased with increasing SZR. (4) The creep propensity compared by the stress index (the ratio of deformation rate (h−1) to log (stress)) of simulated ROX fuel to UO2 was similar. (5) The magnitude of hardness (Hv) was changed by MgAl2O4. The Hv of simulated ROX fuel is significantly large than that of UO2. (6) The difference in thermal diffusivity between simulated ROX fuels and UO2 ones was insignificant. Similarly, the difference in thermal conductivity between the two was also little.

The temperature rise beneath a light-cured cement lining during light curing

Isothermal differential thermal analysis has been used to measure the temperature rise at the lower surface of linings of a light-cured cement. It increases linearly with exposure time, and decreases linearly with lining thickness. Separating the end of the light guide from the surface of the lining results in a small reduction in temperature. The temperature rise accompanying setting (from the material alone) is not significant clinically, but the heat emitted from the light-curing unit presents a greater hazard. Whereas the heat produced by the source used in this study is acceptable, that generated by other sources can be greater and unacceptable. The difference in thermal diffusivity between the chemically cured and light-cured varieties of the cement tested (both when set) is not sufficient to produce a measurable difference for the temperature rise when an overlay of composite is light cured. The temperature rise depends more upon the depth of composite with the consequent adjustment to the curing time than upon the choice of composite product.

Photoacoustic Signal from Subsurface Defects in Ceramics

The laser scanning photoacoustic (PA) technique was used to detect subsurface slots in Si3N4, SiC and AlN ceramics. The PA signal amplitude peak width across the slot decreased with increasing chopping frequency, up to 480 Hz. At low frequencies, the peak width for SiC and AlN was considerably larger than that for Si3N4. This is ascribed to the difference in thermal diffusivity among these ceramics. The amplitude peak height vs subsurface slot depth relationships for Si3N4 are in good agreement with the calculation based on the Rosencwaig-Gersho theory. However, the relation-ships for SiC and AlN disagree with the calculation. This suggests that at low frequencies the PA signal on the slot is affected by the thermal diffusion outside the slot.

ChemInform Abstract: EFFECT OF CRYSTALLIZATION OF THE GRAIN‐BOUNDARY PHASE ON THE THERMAL DIFFUSIVITY OF A SIALON CERAMIC

Crystallization of the glassy grain-boundary phase in a series of sialon ceramics fabricated using a range of hot-pressing schedules increased the thermal diffusivity at room temperature by an average of =lo%. For samples made by a given hotpressing schedule, the relative difference in thermal diffusivity between composites containing a glassy grain-boundary phase and those in which this phase had crystallized decreased with increasing temperature. This behavior is attributed to enhanced phonon scattering in the crystalline grain-boundary phase.

Effect of Crystallization of the Grain‐Boundary Phase on the Thermal Diffusivity of a Sialon Ceramic

Crystallization of the glassy grain‐boundary phase in a series of sialon ceramics fabricated using a range of hot‐pressing schedules increased the thermal diffusivity at room temperature by an average of ∼10%. For samples made by a given hot‐pressing schedule, the relative difference in thermal diffusivity between composites containing a glassy grain‐boundary phase and those in which this phase had crystallized decreased with increasing temperature. This behavior is attributed to enhanced phonon scattering in the crystalline grain‐boundary phase.