Non-universal behavior well above the percolation threshold and thermal properties of core-shell-magnetite-polymer fibers
12 Dec 2011-Journal of Applied Physics (American Institute of Physics)-Vol. 110, Iss: 11, pp 113718
TL;DR: In this article, the authors investigated the electrical properties of aligned nanocomposite electrospun fibers for electrical (dc, ac, and dielectric) and thermal properties.
Abstract: Aligned nanocomposite electrospun fibers are investigated for electrical (dc, ac, and dielectric) and thermal properties (10−40 wt.%). This nanocomposite consists of poly(methylmethacrylate) (PMMA) grafted core-shell magnetite (CSM) nanoparticles and PMMA. Electrical properties as a function of tunnel gap (in between the CSM nanoparticles) are studied as the polymer shell stays intact and the “cores” do not touch each other well above the percolation threshold. The depleted improvement in dc conductivity (σdcwt%) with increasing wt.% (the improvement percentage: σdc10 to σdc20 ≈ 100%; σdc20 to σdc30 ≈ 40% and σdc30 to σdc40 ≈ 1.2%) affirms the non-touching “cores” of CSM. Interestingly, the observed ac conductive behavior (0.1−13 MHz) in the high end of the frequency range is in clear contrast to that of a typical percolating system, in fact it does not explicitly follow the universal power law. Some of the obtained critical exponents are not accommodated by the universal theory and significantly differen...
TL;DR: The factors that influence the efficiency of H2 production are addressed and effective ways of engineering catalyst combinations to overcome the current performance barriers are suggested.
Abstract: Hydrogen is an attractive alternative to fossil fuels in terms of environmental and other advantages. Of the various production methods for H2, photocatalysis requires further development so that it can be applied economically on an industrial scale. One- and two-dimensional nanostructures in both pristine and modified forms have shown great potential as catalysts in the generation of H2. We review here recent developments in these nanostructure catalysts and their efficiency in the generation of H2 under UV/visible/simulated solar light. Despite much research effort, many photocatalysts do not yet meet the practical requirements for the generation of H2, such as visible light activity. H2 production is dependent on a variety of parameters and factors. To meet future energy demands, several challenges in H2 production still need to be solved. We address here the factors that influence the efficiency of H2 production and suggest alternatives. The nanostructures are classified based on their morphology and their efficiency is considered with respect to the influencing parameters. We suggest effective ways of engineering catalyst combinations to overcome the current performance barriers.
TL;DR: In this article, the synthesis and photocatalytic activity (PCA) of PAN/ZnO nanofibrous mat decorated with nanoneedles of zinc oxide was investigated.
Abstract: We report on the synthesis and photocatalytic activity (PCA) of electrospun poly(acrylonitrile) (PAN) nanofibrous mat decorated with nanoneedles of zinc oxide (ZnO). Apart from a detailed morphological and structural characterization, the PCA has been carefully monitored and the results are discussed elaborately when juxtaposed with the photoluminescence. The present hierarchal homoassembled nanostructures are a combination of two types of ZnO with diverse optical qualities, i.e. (a) controlled deposition of ZnO coating on nanofibers with dominant oxygen vacancies and significant grain boundaries by atomic layer deposition (ALD), and (b) growth of single crystalline ZnO nanoneedles with high optical quality on the ALD seeds via hydrothermal process. The needle structure (∼25 nm in diameter with an aspect ratio of ∼24) also supports the vectorial transport of photo-charge carriers, which is crucial for high catalytic activity. Furthermore, it is shown that enhanced PCA is because of the catalytic activity at surface defects (on ALD seed), valence band, and conduction band (of ZnO nanoneedles). PCA and durability of the PAN/ZnO nanofibrous mat have also been tested with aqueous solution of methylene blue and the results showed almost no decay in the catalytic activity of this material when reused.
TL;DR: In this paper, the universal percolation behavior of the inter-cluster polarization model for the case of 3D systems is investigated. But the critical exponents s, s′ and t which characterize the divergence of eeff and σeff in the vicinity of fc show non-universal values.
Abstract: The universal percolation behavior i.e. σ eff ( ω , f con ≈ f c ) ∝ ω x and e eff ( ω , f con ≈ f c ) ∝ ω - y is satisfied; with x + y = 1, where σeff is the effective ac conductivity of the composite, ω is the frequency of applied ac signal and fc is percolation threshold. The exponents (x = 0.72 and y = 0.28) obtained under the inter-cluster polarization model for the case of 3D systems are consistent with the experimental values. The critical exponents s, s′ and t which characterize the divergence of eeff and σeff in the vicinity of fc show non-universal values. The non-universality of s, s′ and t is correlated with the extent of spatial connectivity of filler particles, which is reflected from the experimental values of loss tangent.
TL;DR: In this article, a comprehensive review is presented on the researches and developments related to electrospun polymer nanofibers including processing, structure and property characterization, applications, and modeling and simulations.
Abstract: Electrospinning has been recognized as an efficient technique for the fabrication of polymer nanofibers. Various polymers have been successfully electrospun into ultrafine fibers in recent years mostly in solvent solution and some in melt form. Potential applications based on such fibers specifically their use as reinforcement in nanocomposite development have been realized. In this paper, a comprehensive review is presented on the researches and developments related to electrospun polymer nanofibers including processing, structure and property characterization, applications, and modeling and simulations. Information of those polymers together with their processing conditions for electrospinning of ultrafine fibers has been summarized in the paper. Other issues regarding the technology limitations, research challenges, and future trends are also discussed.
TL;DR: In this paper, it is argued that ac universality reflects an underlying percolation determining dc as well as ac conductivity in the extreme disorder limit, i.e., when the local randomly varying mobilities cover many orders of magnitude.
Abstract: The striking similarity of ac conduction in quite different disordered solids is discussed in terms of experimental results, modeling, and computer simulations. After giving an overview of experiment, a macroscopic and a microscopic model are reviewed. For both models the normalized ac conductivity as a function of a suitably scaled frequency becomes independent of details of the disorder in the extreme disorder limit, i.e., when the local randomly varying mobilities cover many orders of magnitude. The two universal ac conductivities are similar, but not identical; both are examples of unusual non-power-law universalities. It is argued that ac universality reflects an underlying percolation determining dc as well as ac conductivity in the extreme disorder limit. Three analytical approximations to the universal ac conductivities are presented and compared to computer simulations. Finally, model predictions are briefly compared to experiment.
TL;DR: Three-dimensional percolation theory reveals that Poisson's ratio for the composite is a key parameter in determining how the conductivity changes upon stretching, and highly conductive, printable and stretchable hybrid composites composed of micrometre-sized silver flakes and multiwalled carbon nanotubes decorated with self-assembled silver nanoparticles are presented.
Abstract: highly conductive, printable and stretchable hybrid composites composed of micrometre-sized silver flakes and multiwalled carbon nanotubes decorated with self-assembled silver nanoparticles. The nanotubes were used as one-dimensional, flexible and conductive scaffolds to construct effective electrical networks among the silver flakes. The nanocomposites, which included polyvinylidenefluoride copolymer, were created with a hot-rolling technique, and the maximum conductivities of the hybrid silver–nanotube composites were 5,710 S cm 21 at 0% strain and 20 S cm 21 at 140% strain, at which point the film ruptured. Three-dimensional percolation theory reveals that Poisson’s ratio for the composite is a key parameter in determining how the conductivity changes upon stretching. Useful combinations of conductivity and stretchability have been observed in vertically aligned multiwalled carbon nanotube (MWNT) forest/polyurethane films (� 0.5–1 S cm 21 at 0% strain and electrical resistance increased upon stretching 12 ) and in textiles
TL;DR: The electrospun nanofibers developed highly oriented structure in CL-unit sequences during the electrospinning process and the biocompatibility of the nanofiber scaffold has been investigated by culturing cells on the nan ofibers.
Abstract: Poly( l -lactide- co - e -caprolactone) [P(LLA-CL)] with l -lactide to e -caprolactone ratio of 75 to 25 has been electrospun into nanofibers. The relationship between electrospinning parameters and fiber diameter has been investigated. The fiber diameter decreased with decreasing polymer concentration and with increasing electrospinning voltage. The X-ray diffractometer and differential scanning colorimeter results suggested that the electrospun nanofibers developed highly oriented structure in CL-unit sequences during the electrospinning process. The biocompatibility of the nanofiber scaffold has been investigated by culturing cells on the nanofiber scaffold. Both smooth muscle cell and endothelial cell adhered and proliferated well on the P(LLA-CL) nanofiber scaffolds.