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Enhanced dielectric permittivity in surface-modified graphene/PVDF composites prepared by an electrospinning-hot pressing method
摘要： In the present work, the surface-modified graphene (SMG)/poly(vinylidene fluoride) (PVDF) fibrous membranes obtained from the electrospinning were treated by the hot pressing in the laminating mode to form the SMG/PVDF composites. The SMG was prepared by subjecting the graphene oxide to silane modification, NaBH4 reduction, and PVDF grafting in sequence. The successful surface modification of graphene was confirmed by TEM, XPS, Raman spectroscopy, FTIR, WAXD, and TGA. Furthermore, the structures of SMG/PVDF composites fabricated by the electrospinning-hot pressing method were studied by SEM, FTIR, and WAXD, which exhibited the well dispersion of SMG in the PVDF matrix. Finally, the investigation showed that the dielectric permittivities of SMG/PVDF composites increased with the SMG content, which were significantly higher than that of pristine PVDF. The dielectric permittivity of SMG (16 wt.%)/PVDF composite (83.8) at 1000 Hz was found to be ten-fold that of the corresponding value of pristine PVDF (8.3) with a relatively low dielectric loss factor (0.34) and a relatively high thermal conductivity (0.679 W/mK).
关键词： surface-modified graphene/PVDF composite,thermal conductivity.,electrospinning-hot pressing method,dielectric performance
更新于2025-09-23 15:22:29
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Ultralow Thermal Conductivity of Turbostratically Disordered MoSe2 Ultra-Thin Films and Implications for Heterostructures
摘要： Films containing 8, 16, 24, 32 and 64 MoSe2 layers were synthesized using the modulated elemental reactants (MER) method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe2 layers thick with atomically smooth interfaces. In-plane x-ray diffraction scans contained only hk0 reflections for crystalline MoSe2 monolayers. Specular x-ray diffraction patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe2 layers are parallel to the substrate. Both x-ray diffraction and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe2 layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07 – 0.09 W m-1 K-1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe2 layers. The bonding environment of the Se atoms also becomes significantly distorted from C3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.
关键词： rotational disorder,heterostructure,TMDs,turbostratic disorder,thermal conductivity,molybdenum diselenide
更新于2025-09-23 15:22:29
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Characterization of the optical absorption, electric modulus, and thermal conductivity of poly(ethylene oxide) dispersed with alum and carbon black nanoparticles
摘要： Optical, electrical, and thermal properties of hybrid polymer thin films, made of poly(ethylene oxide) (PEO) filled with electrolyte alum salt of different concentrations, and doped with conductive carbon black (CB) nanoparticles (0.1 wt%), have been investigated. The optical properties were studied in the UV-visible wavelength range, thermal properties were studied as a function of temperature, filler content, and applied field frequency. The observed physical constants of the casted thin films like optical energy gap, energy tails, refractive index, thermal conductivity, impedance, dielectric constant, and electric modulus were determined. It was found that these measured quantities vary with the incident UV-wavelength, alum content, temperature and applied field frequency. The dependence of the electric modulus on frequency exhibit a relaxation peak occurs at 600 kHz.
关键词： Dielectric,Optical,Impedance,Thermal Conductivity,PEO,CB
更新于2025-09-23 15:22:29
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Systematic microstructural development with thermal diffusivity behaviour from nanometric to micronic grains of strontium titanate
摘要： Strontium titanate is a promising candidate for applications in thermoelectric, thermal management applications, and modern electronic devices because of its desirable thermal stability, chemical stability, and semiconducting behaviour. However, the absence of its important systematic development, having grain size from several nanometric up to micronic size with evolving thermal diffusivity behaviour, triggers the need for filling up the vacuum. Two different heat treatments have been carried out onto the samples which were with presintering and without presintering. Nanometer-sized compacted powder samples were sintered from 500 to 1400 °C using 100 °C increments. The parallel characterizations of structural, microstructural and thermal diffusivity properties were systematically carried out. Interestingly, three significant value-differentiated groups: weak, moderate, and strong thermal diffusivity were observed, resulting from the influence of different phonon-scattering mechanisms through a systematic development of microstructural properties for both heat treatments.
关键词： Strontium titanate,Thermal conductivity,Thermal diffusivity,Microstructure
更新于2025-09-23 15:21:21
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Effects of vacancy defects location on thermal conductivity of silicon nanowire: a molecular dynamics study
摘要： The improvement of thermoelectric ?gure of merit of silicon nanowire (SiNW) can be achieved by lowering its thermal conductivity. In this work, non-equilibrium molecular dynamics method was used to demonstrate that the thermal conductivity of bulk silicon crystal is drastically reduced when it is crafted as SiNW and that it can be reduced remarkably by including vacancy defects. It has been found that ‘centre vacancy defect’ contributes much more in reducing the thermal conductance than ‘surface vacancy defect’. The lowest thermal conductivity that occurs is about 52.1% of that of pristine SiNW, when 2% vacancy defect is introduced in the nanowire. The vibrational density of states analysis was performed to understand the nature of this reduction and it has been found that the various boundary scatterings of phonon signi?cantly reduce the thermal conductivity. Also, larger mass difference due to voids induces smaller thermal conductivity values. These results indicate that the inclusion of vacancy defects can enhance the thermoelectric performance of SiNWs.
关键词： molecular dynamics,silicon nanowire,thermoelectric performance,thermal conductivity,vacancy defects
更新于2025-09-23 15:21:21
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Thermal conductivity and diffusivity of triple-cation perovskite halide materials for solar cells
摘要： We report on the measurement of thermal conductivity and thermal diffusivity by a modulated thermoreflectance microscopy technique on a mixed-cation perovskite material [Cs0.05(formamidinium0.83methylammonium0.17)0.95Pb(I0.83Br0.17)3] widely applied for solution-processed perovskite solar cells. Such materials are supposed to present improved thermal stability compared to methylammonium-based single cation perovskites. Our measurements are performed on perovskite/TiO2/SnO2:F/SiO2 structures, with perovskite thicknesses ranging between 250 nm and 1000 nm. This configuration is the one of a real solar cell, with the same substrate and intermediate layers as of an operating device. We measured a thermal conductivity kper of 0.26 ± 0.03 W m?1 K?1 and a thermal diffusivity Dper of 3.5 × 10?7 ± 0.5 m2 s?1. The value for thermal conductivity is comparable to the one measured on single cation perovskites, which is generally in the 0.2–0.6 range. The value for thermal diffusivity has not been reported previously.
关键词： perovskite solar cells,thermal conductivity,modulated thermoreflectance microscopy,thermal diffusivity
更新于2025-09-23 15:21:01
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A field study on the application of distributed temperature sensing technology in thermal response tests for borehole heat exchangers
摘要： Although the enhanced thermal response test (ETRT) method has been used to determine the distribution of ground temperatures and effective thermal conductivities, there are a number of obstacles which limit the wide application of this technology in the discipline of geoengineering. In this literature, four aspects of ETRT technology were investigated: (a) acquisition of ground temperature, (b) installation of the heat exchange tubes, (c) optimization of the monitoring positions, and (d) the difference in thermal conductivity obtained by the ETRT and numerical simulation. To investigate these issues, a field trial was carried out in Heze, Shandong Province, China, and the corresponding numerical models were built. The results demonstrate that: (i) the conventional methods that infer undisturbed ground temperature using water in tubes have large errors, whereas the distributed temperature sensing (DTS) technique enables the measurement of precise temperature profiles; (ii) the thermal conductivity measured using double U-tubes reflects the soil thermal property more accurately than that for a single U-tube; (iii) it is more reasonable to install optical fibers outside the U-tube sidewall than inside the observation tube; and (iv) it is essential to quantitatively consider various interface thermal impedance when estimating ground thermal conductivities using numerical simulation.
关键词： Fiber optic sensor,Distributed temperature sensing (DTS),Ground-coupled heat pump (GCHP),Thermal conductivity,Thermal response test (TRT)
更新于2025-09-23 15:21:01
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Effects of Strains on Thermal Conductivity of Si/Ge Superlattices
摘要： The effect of strains on the thermal conductivity of Si/Ge superlattices was investigated by nonequilibrium molecular dynamics (NEMD) simulation. The thermal conductivities experienced a near linear drop with increasing tensile and compressive strains. It was explained by the fact that the decrease of the phonons velocities and a mass of structural defects generated under strains. Meanwhile, a theoretical calculation based on Modified-Callaway model was performed，and it was found that the theoretical results were in good agreement with the molecular dynamics results.
关键词： nonequilibrium molecular dynamics,thermal conductivity,Si/Ge superlattics,strains
更新于2025-09-23 15:21:01
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Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide
摘要： Hybrid organic–inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron–phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron–phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.
关键词： solar cells,inelastic neutron spectroscopy,lattice dynamics,hybrid perovskite,thermal conductivity
更新于2025-09-23 15:21:01
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Probing of Thermal Transport in 50-nm Thick PbTe Nanocrystal Film by Time-Domain Thermoreflectance
摘要： Bottom-up fabrication of thermoelectric (TE) materials from colloidal nanocrystal (NC) building blocks can substantially increase their TE efficiency, e.g., by reducing lattice thermal conductivity. In this work, 10-nm spherical phase-pure oleate-capped PbTe NCs with narrow size distribution were synthesized and employed to fabricate 50-nm thick films on insulating SiO2/Si substrates. The spin-coating, with subsequent ligand exchange procedure, was applied to enhance coupling interactions between the NCs. Using dark conductivity measurements, we confirmed the semiconducting behavior and the Schottky-type electrical field-dependent conductivity mechanism in the resultant thin films. The thermal transport in the thin-film was probed by means of a time-domain thermoreflectance (TDTR) method. For this purpose, we used a customized state-of-the-art system based on a picosecond thermoreflectance instrument, which enables area-selective analysis with spatial resolution down to 5 μm. The results show that as-fabricated PbTe NC films exhibit ultralow thermal conductivity of 0.9 W m–1 K–1 at 300 K. The transport property findings suggest potential in the proposed quick and cost-effective spin-coating strategy for bottom-up fabrication of nanostructured TE films from high-quality colloidal NC building blocks.
关键词： colloidal nanocrystals,spin-coating,thermoelectric materials,thermal conductivity,time-domain thermoreflectance,PbTe
更新于2025-09-23 15:21:01