Thin Carbon Nanofibers Research Articles

Development of Kovacs model for electrical conductivity of carbon nanofiber–polymer systems

This study develops a model for electrical conductivity of polymer carbon nanofiber (CNF) nanocomposites (PCNFs), which includes two steps. In the first step, Kovacs model is developed to consider the CNF, interphase and tunneling regions as dissimilar zones in the system. In the second step, simple equations are expressed to estimate the resistances of interphase and tunnels, the volume fraction of CNF and percolation onset. Although some earlier models were proposed to predict the electrical conductivity of PCNFs, developing of Kovacs model causes a better understanding of the effects of main factors on the nanocomposite conductivity. The developed model is supported by logical influences of all factors on the conductivity and by experimented conductivity of several samples. The calculations show good accordance to the experimented data and all factors rationally manage the conductivity of PCNFs. The highest conductivity of PCNF is gained as 0.019 S/m at the lowest ranges of polymer tunnel resistivity (ρ = 500 Ω m) and tunneling distance (d = 2 nm), whereas the highest levels of these factors (ρ > 3000 Ω m and d > 6 nm) cannot cause a conductive sample. Also, high CNF volume fraction, poor waviness, long and thin CNF, low “k”, thick interphase, high CNF conduction, high percentage of percolated CNFs, low percolation onset and high interphase conductivity cause an outstanding conductivity in PCNF.

Structure-rate performance relationship in Si nanoparticles-carbon nanofiber composite as flexible anode for lithium-ion batteries

A flexible silicon-carbon nanofibre composite is reported as an anode material for lithium-ion batteries. Self-standing, binder-free and flexible anodes composed of Si nanoparticles embedded inside carbon nanofibers of different fibre diameter are fabricated via electrospinning. The silicon nanoparticles are effectively protected from direct exposure to the electrolyte by the carbon fibre encapsulation, leading to vastly improved capacity retention during galvanostatic half-cell cycling. Cycling results also showed that an electrode with 230 nm fibre diameter has enhanced cyclability and rate capability when compared to one with 620 nm diameter. SEM (scanning electron microscopy) and EIS (electrochemical impedance spectroscopy) post-cycling investigations of the electrodes reveals an appropriate structural stability and lower impedance during cycling for the electrode with thinner carbon fibres. This behaviour is associated with the low linear density of the Si nanoparticles along the thin carbon nanofibers, which prevents the fracture of the carbon fibres at the sites of Si clusters.

Feasibility of Carbon Nanofiber Catalyst Support for the Heterogeneous Fenton Process

AbstractThe heterogeneous Fenton oxidation reaction was investigated at two different catalytic supports including the blank alumina plates and alumina plates with thin carbon nanofiber (CNF) layer. Iron (III) oxide was used as a catalyst and propionic acid (PA) was studied as a model organic compound. A carbon nanofiber layer and a thin catalytic layer of iron (III) oxide were prepared using the chemical vapor deposition method with acetylene and ferrocene precursors. The reaction rate of heterogeneous Fenton oxidation was compared with the rate of homogeneous reaction. Obtained results indicate that the reaction rate is significantly higher at homogeneous Fenton oxidation at high temperature (70°C), however, the reaction rate for homogeneous and heterogeneous oxidation is comparable at room temperature (25°C) if carbon nanofiber support is used for the iron (III) oxide catalyst.

Method for Measuring the Distribution of Adhesion Forces on Continuous Nanoscale Protrusions Using Carbon Nanofiber Tip on a Scanning Probe Microscope Cantilever

The adhesion force on surfaces has received attention in numerous scientific and technological fields, including catalysis, thin-film growth, and tribology. Many applications require knowledge of the strength of these forces as a function of position in three dimensions, but until now such information has only been theoretically proposed. Here, we demonstrate an approach based on scanning probe microscopy that can obtain such data and be used to image the three-dimensional surface force field of continuous nanoscale protrusions. We present adhesion force maps with nanometer and nanonewton resolution that allow detailed characterization of the interaction between a surface and a thin carbon nanofiber (CNF) rod synthesized by plasma-enhanced chemical vapor deposition (PECVD) at the end of a tip on a scanning probe microscope cantilever in three dimensions. In these maps, the positions of all continuous nanoscale protrusions are identified and the differences in the adhesive forces among limited areas at inequivalent sites are quantified.

PAN Based Carbon Nanofibers as an Active ORR Catalyst

Polyacrylonitrile (PAN) based carbon nanofibers were prepared by electrospinning and their activity for oxygen reduction reaction (ORR) in acidic media was investigated. Field emission electron microscope (FE-SEM), transition electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) were carried out to investigate the surface morphology, composition, and catalytic activity. Thin carbon nanofibers of a 150 nm diameter were successfully produced by electrospinning using 8 wt% PAN in dimethylformamide, 15 cm pin to plate distance, and applying voltage of 18 kV at different carbonization temperatures of 700, 900, 1000, 1100, and 1200 °C. The ORR activity of the prepared carbon nanofibers was evaluated. The PAN based carbon nanofibers showed a considerable ORR activity and this activity was increased by increasing the carbonization temperature. The high ORR onset potentials over 700 mV vs. RHE (milli-volt versus reversible hydrogen electrode) were obtained at temperatures over 1000 °C. The activity of PAN based carbon nanofibers increased with increasing carbonization temperature from 700 to 1100 °C, this would be related to the increasing in the electrical conductivity at low carbonization temperatures, and the high Pyridine N content at the high carbonization temperatures.

The Bimetallic Effects of Catalysts on the Syntheses of Thin Carbon Nanofibers

Thin carbon nanofibers with less than 100 nm diameter were variously prepared over MgO-supported nickel and bimetallic nickel catalysts with iron, copper, and molybdenum. The second metal effects on the carbon yield and structure were investigated by using electron microscopic and gas chromatographic methods. Further, the active catalysts were examined by powdered X-ray diffraction to elucidate correlation between the catalyst and nanofiber growth. The surface composition of catalysts appeared to govern the catalyst activity and the resulting CNF structure. The second metal effects on the catalyst activity and resulting nanofiber structure is discussed in terms of the formation mechanism of carbon nanofibers, on the basis of the results.

Selective synthesis of thin carbon nanofibers: I. Over nickel–iron alloys supported on carbon black

Selective and efficient synthesis of thin carbon nanofibers (CNF) of fish bone-like texture with diameter below 40 nm was shown at low temperatures (673–873 K) from an ethylene and hydrogen (4:1) mixture, using Ni–Fe alloys supported on carbon blacks of relatively small surface area. A particular alloy of a Ni–Fe (8:2 w/w) was highly dispersed to produce thinner fibers at 753 K than Ni alone or Ni–Fe catalysts of other compositions on carbon black, although the fiber yield was similar when the Ni content was more than 60%. It must be noted that unsupported alloy showed activity when the Ni content was higher than 75% at the same temperature. Catalyst particles were observed by using TEM and XRD to confirm the roles of carbon black and alloy composition on dispersion of the catalyst, even if the catalyst particle was degraded into a finer one at the growth of CNF. Carbon black of smaller surface area carries the catalyst particles on its surface to be active for CNF synthesis.