Wolfgang Bauhofer

A review and analysis of electrical percolation in carbon nanotube polymer composites

We review experimental and theoretical work on electrical percolation of carbon nanotubes (CNT) in polymer composites. We give a comprehensive survey of published data together with an attempt of systematization. Parameters like CNT type, synthesis method, treatment and dimensionality as well as polymer type and dispersion method are evaluated with respect to their impact on percolation threshold, scaling law exponent and maximum conductivity of the composite. Validity as well as limitations of commonly used statistical percolation theories are discussed, in particular with respect to the recently reported existence of a lower kinetic (allowing for re-aggregation) and a higher statistical percolation threshold.

Electric anisotropy of carbon nanofibre/epoxy resin composites due to electric field induced alignment

Abstract We report the application of alternating electric fields for the alignment and network formation of carbon nanofibres in an epoxy resin dispersion during curing. In situ optical microscopy verified the electrostatic stabilisation of the fibres in the dispersion and the orientation and agglomeration caused by the electric field. An explanation for the interaction forces between particles and the external electric field is given taking into account electric dipoles induced on the fibres. For the cured composites, the structural anisotropy of the fibre network was evidenced by electrical measurements. A maximal anisotropy for the dc resistivity of about 10 and for the dielectric constant at 1 kHz of about 20 was found for composites with a fibre loading ⩾1 wt.%.

Alternating electric field induced agglomeration of carbon black filled resins

Abstract This letter reports on our observation that an alternating electric field is able to induce the formation of an electrically conducting network in carbon black (CB) filled resins well below the zero-field percolation threshold. Compared with the recently presented dc method, the ac agglomeration is more efficient in two respects: it proceeds significantly faster under equivalent conditions and is still effective at higher ionic concentration. In contrast to the ramified form of dc-induced CB networks, ac agglomeration favors the formation of parallel CB chains. The experimental results can be explained taking into account ionic conductivities of the matrices as well as charges and field induced dipoles on the CB particles.

In situ observation of electric field induced agglomeration of carbon black in epoxy resin

This letter reports on the influence of a static electric field applied by metal electrodes on the agglomeration process of carbon black CB in epoxy resin. The growth of dendrites from the anode into the material is observed in situ by optical transmission microscopy. A percolating network is seen to form, combined with a drastic reduction in the sample resistivity. This behavior can be explained by taking into account the electrostatic interaction of the charged CB particles. The final resistance for composites with a given CB content can be controlled within a range of several decades by varying the applied voltage and the curing temperature of the mixture.

Analyzing the quality of carbon nanotube dispersions in polymers using scanning electron microscopy

The ability to examine conducting filler particles in an insulating polymer matrix by scanning electron microscopy (SEM) was investigated. The detection of selected secondary electrons is necessary to resolve sub-micron scale filler particles, but not every SEM detector seems to be able to monitor the small changes introduced by the conducting filler particles. The influence of SEM parameters and the challenge of image interpretation in view of the apparent lack of appropriate information in literature are discussed. In accordance with other experiments on light element samples, all monitored electrons seem to be emitted within approximately 50 nm of the sample depth and no information is accessible from deeper regions even by increasing the acceleration voltage.

Spin-cast composite gate insulation for low driving voltages and memory effect in organic field-effect transistors

The authors report on a solution-processed composite film based on poly(vinylidene fluoride/trifluoroethylene) copolymer and barium titanate (BT) nanopowder, to be used as ferroelectric high-κ gate insulation in organic field-effect transistors (OFETs). Flexible films of up to 50vol% BT powder content are produced by preparing homogeneous dispersion of the powder in the polymer solutions. The films exhibited high specific volume resistivities combined with dielectric constants of up to 51.5 at 1kHz. Low-voltage OFETs with ferroelectric hysteresis and good memory retention properties were demonstrated by using the composite films.

A simulation study on the combined effects of nanotube shape and shear flow on the electrical percolation thresholds of carbon nanotube/polymer composites

Here we investigate the combined effects of carbon nanotube (CNT) properties such as aspect ratio, curvature, and tunneling length and shear rate on the microstructure and electrical conductivities of CNT/polymer composites using fiber-level simulations. Electrical conductivities are calculated using a resistor network algorithm. Results for percolation thresholds in static systems agree with predictions and experimental measurements. We show that imposed shear flow can decrease the electrical percolation threshold by facilitating the formation of conductive aggregates. In agreement with previous research, we find that lower percolation thresholds are obtained for nanotubes with high aspect ratio. Our results also show that an increase in the curvature of nanotubes can make more agglomeration and reduce the percolation threshold in sheared suspensions.

A comparative investigation of electrical resistance and acoustic emission during cyclic loading of CFRP laminates

This paper reports on simultaneous monitoring of electrical resistance and acoustic emission (AE) during cyclic tensile loading of cross-ply carbon-fibre-reinforced plastics (CFRP). The detection of the AE allows further insight into the damage mechanisms causing the electrical response. During loading and unloading the samples the electrical measurements show a hysteresis in the corresponding resistance vs strain plot. When the previous load maximum is exceeded in the subsequent load cycle a characteristic increase in the measured slope of the resistance vs strain curve appears to combined with a sudden rise in the AE (Kaiser effect). After unloading the resistance relaxes to a new equilibrium value. This behaviour is consistent with the formation, opening and closing of cracks in the CFRP samples. The observed time dependence in the resistance hysteresis and relaxation can be attributed to the influence of the stressed polymeric matrix. Thus, the reported in situ observation of the electrical resistance allows virgin and damaged CFRP samples to be identified and the previous load maximum to be determined by the characteristic change in the resistance-strain slope.

On the influence of nanotube properties, processing conditions and shear forces on the electrical conductivity of carbon nanotube epoxy composites.

We analyse statistical and kinetic percolation thresholds and maximum electrical conductivities of carbon nanotube epoxy composites as a function of shear forces, processing conditions, nanotube type and dimensions. Entangled and non-entangled nanotubes of different lengths (15-100 microm) and thicknesses (12-80 nm) have been obtained with three different synthesis methods based on catalytic or plasma enhanced chemical vapour deposition. The dispersions were processed either solely with a dissolver disk or additionally with a three roll calender. Care was taken to prevent unintentional shearing (e.g. through convection) in all samples that were not subject to deliberate shearing. It was found that shear forces have a similar influence on kinetic percolation thresholds and composite conductivities independent of nanotube types and dimensions.

Electronic properties of GaAs(100) surface passivated in alcoholic sulfide solutions

Using Raman spectroscopy and the Kelvin probe method the surface band bending and electron work function of GaAs(100) passivated in aqueous and alcoholic solutions of ammonium sulfide have been studied. It is shown that the solvent strongly affects the position of energy levels on the sulfide-treated surface. With the decrease of the solution dielectric constant the surface band bending decreases and the electron work function increases both in n-GaAs and p-GaAs. The ionisation energy of the semiconductor increases after sulfur treatment and in n-GaAs it increases with the decrease of the dielectric constant of the solution.