Thermal lens spectrometry (TLS) technique was used to obtain the thermal diffusivity of electrospun Titania nanofibers (TiO2), with average diameter size of 50-80 nm, in water. TiO2 nanofibers have ...been successfully prepared by sol-gel and electrospining techniques. TLS provides reliable alternative to measure the thermal diffusivities of semitransparent materials and low thermal diffusivities. The results show that the nanofluid thermal diffusivity increases with the presence of nanofibers. Complementary techniques: scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) were employed to characterize the nanofibers morphology, average fiber diameter and chemical composition, respectively.
Phase change material (PCM) aggregate concrete mitigates frost heave by increasing the minimum temperature. However, its matrix remains ordinary. In this paper, the minimum temperature was further ...increased by improving the thermal inertia of the matrix. Specifically, the study investigated the impact of incorporating 10 %-20 % rice husk ash (RHA) and 5 %-15 % fly ash (FA) on the workability, mechanical properties, thermal properties, and pore structure. The results indicated that incorporating RHA reduced the slump of concrete, while incorporating FA enhances workability. The filling and pozzolanic effects of RHA and FA increase the strength, but excessive additions have a detrimental impact. RHA uniform distributed in the cement matrix changed the heat transfer process. Time-temperature curve results indicated that the minimum temperature of all the concretes containing RHA and FA was higher than that of the reference concrete, with a maximum temperature difference of 4.3 °C. Nuclear magnetic resonance (NMR) results indicated that all pores, especially the capillary pores, increased with the increase of RHA. Conversely, the pores initially decreased and then increased with the increase of FA. The ability of computer tomography (CT) to identify pores larger than 100 μm effectively compensates for the limitations of NMR. CT results showed that pores larger than 1 mm3 may be related to workability, and these pores increased with the increase of RHA, while the results were opposite with the increase of FA. Pores smaller than 1 mm3 may be related to the strength. With the increase of RHA, these pores slightly increased. However, these pores initially decreased and then increased with the increase of FA.
•RHA-FA matrix further improved the insulation performance of PCM aggregate concrete.•The mechanical and thermal properties after incorporating RHA and FA were evaluated.•RHA-FA matrix increased the minimum temperature of PCM aggregate concrete by 4.3 ℃.•The distribution of concrete pores was evaluated through NMR and CT.•The influence of pores on strength and thermal performance was explained.
We study the thermal diffusivity DT in models of metals without quasiparticle excitations (“strange metals”). The many-body quantum chaos and transport properties of such metals can be efficiently ...described by a holographic representation in a gravitational theory in an emergent curved spacetime with an additional spatial dimension. We find that at generic infrared fixed points DT is always related to parameters characterizing many-body quantum chaos: the butterfly velocity vB and Lyapunov time τL through DT∼vB2τL. The relationship holds independently of the charge density, periodic potential strength, or magnetic field at the fixed point. The generality of this result follows from the observation that the thermal conductivity of strange metals depends only on the metric near the horizon of a black hole in the emergent spacetime and is otherwise insensitive to the profile of any matter fields.
The thermal diffusivity in the ab plane of underdoped YBCO crystals is measured by means of a local optical technique in the temperature range of 25–300 K. The phase delay between a point heat source ...and a set of detection points around it allows for high-resolution measurement of the thermal diffusivity and its in-plane anisotropy. Although the magnitude of the diffusivity may suggest that it originates from phonons, its anisotropy is comparable with reported values of the electrical resistivity anisotropy. Furthermore, the anisotropy drops sharply below the charge order transition, again similar to the electrical resistivity anisotropy. Both of these observations suggest that the thermal diffusivity has pronounced electronic as well as phononic character. At the same time, the small electrical and thermal conductivities at high temperatures imply that neither well-defined electron nor phonon quasiparticles are present in this material. We interpret our results through a strongly interacting incoherent electron–phonon “soup” picture characterized by a diffusion constant
D
~
v
B
2
τ
, where vB
is the soup velocity, and scattering of both electrons and phonons saturates a quantum thermal relaxation time τ ∼ ħ/kBT.
We investigated thermal properties of the epoxy-based composites with the high loading fractionup to f ≈ 45 vol %of the randomly oriented electrically conductive graphene fillers and electrically ...insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction fT > 20 vol %. The graphene loading required for achieving thermal percolation, fT , was substantially higher than the loading, fE , for electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with high filler loadings, f ≥ fT , is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.
A novel high‐entropy carbide ceramic, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a ...single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.
A spot periodic heating method is a highly accurate, non-contact method for the evaluation of anisotropy and relative thermophysical property distribution. However, accurately evaluating thermal ...diffusivity is difficult due to the influence of temperature wave reflection from the whole surface of the sample. This study proposes a method to derive thermal diffusivity using a parameter table based on heat transfer equations using the concept of optimum distance between heating-point and measurement point. This method considers finite sample size, sensitivity distribution of infrared ray detector, intensity distribution of heating laser and sample thickness. In these results, the obtained thermal diffusivity of pure copper corresponded well with previous literature values. In conclusion, this method is considered highly effective in evaluating the thermal diffusivity in the horizontal direction.
•Critical properties, heat capacities, and thermal diffusivities were measured.•Measurements were performed for liquid α-angelica lactone and triacetin.•The thermal conductivities of the compounds ...under study were calculated.•The compounds under study are involved in the production of fuels and fuel additives.
The critical temperatures, critical pressures, heat capacities, and thermal diffusivities of α-angelica lactone and triacetin have been measured. The compounds under study are involved in the production of fuels and fuel additives from lignocellulose. The critical properties of the compounds have also been calculated by the group contribution methods of Wilson and Jasperson (WJ), Nannoolal et al. (NRR), Marrero and Gani, Hukkerikar et al. in two variants (H-sw and H-s). The NRR method provides the best estimation of the critical temperature and pressure of α-angelica lactone: the differences between experimental and estimated values are about 1.6 and 3.0 %, respectively. For triacetin, all the methods used give approximately the same discrepancy between the experimental and estimated critical temperature in the range from 1.2 to 2.0 % which is less than the uncertainty of the measurement. However, the H-s and H-sw techniques slightly overestimate the critical temperature, while the rest of the methods underestimate it. All the methods overestimate the critical pressure of triacetin; the H-s method provides the minimum difference between the measured and calculated values of the critical pressure (1.3 %).
The heat capacities of liquid α-angelica lactone and triacetin have been measured at atmospheric pressure in the temperature range from 298.6 to 440.6 K (α-angelica lactone) and from 298.4 to 530.2 K (triacetin). The data obtained have been fitted with first-order polynomials. The heat capacities of the compound under study have also been estimated using the group-contribution method of Kolska et al. This method has been recommended to estimate the heat capacities of α-angelica lactone and triacetin.
The thermal diffusivities of liquid α-angelica lactone and triacetin have been measured at atmospheric pressure in the temperature range from 303.15 to 373.15 K. The thermal conductivities have been calculated from the results of the experiment. The experimental data on thermal diffusivity and thermal conductivity have been approximated with linear polynomials. The thermal conductivities of α-angelica lactone and triacetin have been calculated by the methods of Govender et al. and Sastri. In general, the method of Govender et al. gives thermal conductivity values of the compounds under study that are closer to the experimental values than those obtained using the Sastri method.
We present a study of thermal conductivity, κ, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow. In undoped SrTiO_{3}, κ ...evolves faster than cubic with temperature below its peak and in a narrow temperature window. Such behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones. The effect disappears in the presence of dopants. In SrTi_{1-x}Nb_{x}O_{3}, a significant reduction in lattice thermal conductivity starts below the temperature at which the average inter-dopant distance and the thermal wavelength of acoustic phonons become comparable. In the high-temperature regime, thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time (τ_{p}=(ℏ/k_{B}T)).
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