We report on the temperature dependence of in-plane E2g and out-of-plane A1g Raman modes in high-quality few-layer MoS2 (FLMS) prepared using a high-temperature vapor-phase method. The materials ...obtained were investigated using transmission electron microscopy. The frequencies of these two phonon modes were found to vary linearly with temperature. The first-order temperature coefficients for E1 2g and A1g modes were found to be (1.32 and 1.23) × 10–2 cm–1/K, respectively. The thermal conductivity of the suspended FLMS at room temperature was estimated to be ∼52 W/mK.
Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively powered electronic ...systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device’s electrical conductance to its thermal conductance. In this paper, a recently fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal not only that phosphorene possesses a spatially anisotropic electrical conductance, but that its lattice thermal conductance exhibits a pronounced spatial-anisotropy as well. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT may reach the criterion for commercial deployment along the armchair direction of phosphorene at T = 500 K and is close to 1 even at room temperature given moderate doping (∼2 × 1016 m–2 or 2 × 1012 cm−2). Ultimately, phosphorene hopefully stands out as an environmentally sound thermoelectric material with unprecedented qualities. Intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (∼300 K), one whose performance does not require any sophisticated engineering techniques.
This paper presents a thorough analysis on the temperature dependence of the thermo-optic coefficient, dn/dT, of four bulk annealed pure-silica glass samples (type I—natural quartz: Infrasil 301; ...type II—quartz crystal powder: Heraeus Homosil; type III—synthetic vitreous silica: Corning 7980 and Suprasil 3001) from room temperature down to 0 K. The three/four term temperature dependent Sellmeier equations and respective coefficients were considered, which results from fitting to the raw data obtained by Leviton et al. The thermo-optic coefficient was extrapolated down to zero Kelvin. We have obtained dn/dT values ranging from 8.16 × 10sup.−6 up to 8.53 × 10sup.−6 for the four samples at 293 K and for a wavelength of 1.55 μm. For the Corning 7980 SiOsub.2 glass, the thermo-optic coefficient decreases monotonically, from 8.74 × 10sup.−6 down to 8.16 × 10sup.−6, from the visible range up to the third telecommunication window, being almost constant above 1.3 μm. The Ghosh’s model was revisited, and it was concluded that the thermal expansion coefficient only accounts for about 2% of the thermo-optic coefficient, and we have obtained an expression for the temperature behavior of the silica excitonic bandgap. Wemple’s model was also analyzed where we have also considered the material dispersion in order to determine the coefficients and respective temperature dependences. The limitations of this model were also discussed.
We study the photoresponse of single-layer MoS2 field-effect transistors by scanning photocurrent microscopy. We find that, unlike in many other semiconductors, the photocurrent generation in ...single-layer MoS2 is dominated by the photothermoelectric effect and not by the separation of photoexcited electron–hole pairs across the Schottky barriers at the MoS2/electrode interfaces. We observe a large value for the Seebeck coefficient for single-layer MoS2 that by an external electric field can be tuned between −4 × 102 and −1 × 105 μV K–1. This large and tunable Seebeck coefficient of the single-layer MoS2 paves the way to new applications of this material such as on-chip thermopower generation and waste thermal energy harvesting.
This study focuses on examining the influence of bast fibers on the flammability and thermal properties of the polylactide matrix (PLA). For this purpose, Urtica dioica and Vitis vinifera fibers were ...subjected to two types of modifications: mercerization in NaOH solution (M1 route) and encapsulation in an organic PLA solution (M2 route). In a further step, PLA composites containing 5, 10, and 15 wt% of unmodified and chemically treated fibers were obtained. The results of the tests show that only biocomposites containing mercerized fibers had a nearly 20% reduced flammability compared to that of PLA. Moreover, the biofiller obtained in this way belongs to the group of flame retardants that generate char residue during combustion, which was also confirmed by TGA tests. The M2 modification route allowed to achieve higher mass viscosity than the addition of unmodified and M1-modified fibers. The reason is that fibers additionally encapsulated in a polymer layer impede the mobility of the chain segments. The inferior homogenization of the M2-modified fibers in the PLA matrix translated into a longer combustion time and only a 15% reduction in flammability.
If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure ...capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.
A controlled alignment of graphite nanoplatelets in a composite matrix will allow developing materials with tailored thermal properties. Achieving a high degree of alignment in a reproducible way, ...however, remains challenging. Here we demonstrate the alignment of graphite nanoplatelets in copper composites produced via high-energy ball milling and spark plasma sintering. The orientation of the nanoplatelets in the copper matrix is verified by polarized Raman scattering and electron microscopy showing an increasing order with increasing platelet size. The thermal conductivity k along the alignment direction is up to five times higher than perpendicular to it. The composite with the highest degree of alignment has a thermal diffusivity (100 mm2s–1) comparable to copper (105 mm2s–1) but is 20% lighter. By modeling the thermal properties of the composites within the effective medium approximation we show that (i) the Kapitza resistance is not a limiting factor for improving the thermal conductivity of a copper-graphite system and (ii) copper-graphite-nanoplatelet composites may be expected to achieve a higher thermal conductivity than copper upon further refinement.
Nanofluids are considered to offer important advantages over conventional heat transfer fluids. Over a decade ago, researchers focused on measuring and modeling the effective thermal conductivity and ...viscosity of nanofluids. Recently important theoretical and experimental research works on convective heat transfer appeared in the open literatures on the enhancement of heat transfer using suspensions of nanometer-sized solid particle materials, metallic or nonmetallic in base heat transfer fluids. The purpose of this review article is to summarize the important published articles on the enhancement of the forced convection heat transfer with nanofluids.
The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance ...values of 11–12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m–1 K–1, approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m–1 K–1 at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.
•Elastic properties of titanium borides are calculated by first principles calculation.•Thermodynamical stability of titanium borides is analyzed.•Heat capacity and thermal expansion coefficient for ...titanium borides are calculated and compared.•Grüneisen parameters of titanium borides are calculated.
The anisotropic elastic and thermal expansions of the titanium borides (TiB2, Ti3B4, TiB_Pnma and TiB_Fm3¯m) are calculated from first-principles using density functional theory. All borides show different anisotropic elastic properties; the bulk, shear and Young’s moduli are consistent with those determined experimentally. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at low temperature. When the temperature is higher than 500K, the volumetric coefficient is increased linearly by increasing temperature. Meanwhile, the heat capacities of titanium borides are obtained based on the knowledge of thermal expansion coefficient and the elasticity, the calculations are in good agreement with the experiments.
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