Dielectric spectroscopy and AC conductivity measurements have been employed to investigate the electrical properties of disordered materials in a wide range of temperature. In this work, conducting polypyrrole–titanium dioxide (PPy–TiO2) hybrid nanocomposites have been synthesized by oxidizing pyrrole in presence of different concentration (wt%) of TiO2 nanoparticles. Thermal stability of the nanocomposites increases with the increase in TiO2 wt%. The dielectric and electric modulus studies of as prepared samples were carried out over a frequency range of 42 Hz–5 MHz. The dielectric constant and dielectric loss for all the nanocomposite samples were observed to decrease with the increase of frequency. Electric modulus analysis has been carried out to understand the electrical relaxation processes. The dielectric relaxation time of nanocomposites decreases with the increase of TiO2 nanoparticles concentration. AC conductivity measurements show that correlated barrier hopping conduction mechanism can be employed for pure PPy and PPy‐5 wt% TiO2 nanocomposites, whereas large polaron tunnelling mechanism is applicable for higher concentration of TiO2 (10–25 wt%) in PPy–TiO2 nanocomposites. 相似文献
This paper reports that the nanostructured β-FeSi2 bulk materials are prepared by a new synthesis process by combining melt spinning(MS) and subsequent spark plasma sintering(SPS).It investigates the influence of linear speed of the rolling copper wheel,injection pressure and SPS regime on microstructure and phase composition of the rapidly solidified ribbons after MS and bulk production respectively,and discusses the effects of the microstructure on thermal transport properties.There are two crystalline phases(α-Fe2Si5 and ε-FeSi) in the rapidly solidified ribbons;the crystal grains become smaller when the cooling rate increases(the 20 nm minimum crystal of ε-FeSi is obtained).Having been sintered for 1 min above 1123 K and annealed for 5 min at 923 K,the single-phase nanostructured βFeSi2 bulk materials with 200-500 nm grain size and 98% relative density are obtained.The microstructure of β-FeSi2 has great effect on thermal transport properties.With decreasing sintering temperature,the grain size decreases,the thermal conductivity of β-FeSi2 is reduced remarkably.The thermal conductivity of β-FeSi2 decreases notably(reduced 72% at room temperature) in comparison with the β-FeSi2 prepared by traditional casting method. 相似文献
The thermal conductivity (TC) of compression-moulded polypropylene (PP) and PP filled with 5–15% zinc oxide (ZnO) or calcium carbonate (CaCO3) nanoparticles, prepared by extrusion, was studied using a thermal conductivity analyzer (TCA). The effect of nanoparticle content and crystallinity on the thermal conductivity was investigated using conventional methods, including SEM, XRD, and DSC. The incorporation of nanoparticles improved the crystallinity and thermal conductivity simultaneously. The experimental TC values of the PP nanocomposites with different level of nanoparticles concentration showed a linear increase with an increase in crystallinity. The TC improvement in PP/ZnO nanocomposite was greater than that of PP/calcium carbonate nanocomposites. This fact can be attributed to the intrinsic, better thermal conductivity of the ZnO nanoparticles. Several models were used for prediction of the TC in the nanocomposites. In the PP/ZnO nanocomposites the TC values correlated well with the values predicted by the Series, Maxwell, Lewis and Nielson, Bruggeman, and De Loor models up to 10 wt%. 相似文献
Manipulating thermal conductivities are fundamentally important for controlling the conduction of heat at will. Thermal cloaks and concentrators, which have been extensively studied recently, are actually graded materials designed according to coordinate transformation approaches, and their effective thermal conductivity is equal to that of the host medium outside the cloak or concentrator. Here we attempt to investigate a more general problem: what is the effective thermal conductivity of graded materials? In particular, we perform a first-principles approach to the analytic exact results of effective thermal conductivities of materials possessing either power-law or linear gradation profiles. On the other hand, by solving Laplace's equation, we derive a differential equation for calculating the effective thermal conductivity of a material whose thermal conductivity varies along the radius with arbitrary gradation profiles. The two methods agree with each other for both external and internal heat sources, as confirmed by simulation and experiment. This work provides different methods for designing new thermal metamaterials (including thermal cloaks and concentrators), in order to control or manipulate the transfer of heat. 相似文献
Copper‐decorated carbon nanotubes (CNTs) were synthesized and used as conductive filler to improve the heat transport capabilities of copper matrix. Thermal properties, i.e., thermal diffusivity and thermal conductivity, of copper composite were measured and compared with those containing pristine and functionalized CNTs. Experimental results revealed that composites enriched with nanohybrids where Cu nanoparticles were covalently bonded to CNTs had thermal conductivity four times higher than those containing the same content of pristine CNTs.
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