Synthesis and characterization of fullerene based systems for photovoltaic applications: Evidence for percolation threshold

Acceptor polymers for photovoltaic applications were synthesized by grafting fullerene C₆₀ onto polystyrene. The quality of the reaction was verified by various analytical techniques after each of the three steps of the reaction: nuclear magnetic resonance, infrared and UV-visible spectroscopies, and thermo-gravimetric analysis. In order to determine the optimal amount of C₆₀, a series of polymers were prepared containing from 4 to 59 vol.% of fullerene. The optical (absorption, optical gap energy) and electrical (electron mobility, conductivity) properties have been measured. A percolation threshold at around 4 vol.% was identified for both conductivity and mobility measurements. This provides the lowest amount of C₆₀ required for solar cells applications.     

Percolation modeling investigation of TPB formation in a solid oxide fuel cell electrode-electrolyte interface

A Monte Carlo percolation model has been developed and utilized to characterize the factors controlling triple phase boundary (TPB) formation in an SOFC electrode. The model accounts for (1) electronic conductor, ionic conductor, and gas phase percolation, (2) competition between percolation of gas and electronically conducting phases, and (3) determination of continuous, though not necessarily fully percolating, paths from TPBs to the bulk phases. The model results show that physical processes near the TPB, such as sorbate transport, significantly affect TPB formation in a composite electrode. Active TPB formation is found to be most significantly dependent upon continuous and competing percolation of multiple phases. Simultaneously requiring continuous paths and accounting for non-continuous boundary conditions results in lower active TPB formation levels (up to 8% of possible sites) than presented in the literature (75% of possible sites). In addition, the varying ratio of active to potential TPB sites predicted by the current model (up to 80%) differs significantly from the constant reported in the literature (80%), which lacks analyses of three-phase percolation, gas phase paths, and gas/current collector boundary conditions. This dependence of active TPB formation on percolation of all three phases is important to understand as a basis for determining SOFC performance and optimization.     

Superfluid Onset and Capillary Condensation

We present experimental results on the superfluid onset of ⁴He adsorbed onto porous gold and evaporated CaF₂ films grown on the electrodes of quartz microbalances (QCM). Adsorption and desorption isotherms were measured on the porous substrates in the range of 1.2 to 2.2 K. The isotherms were used to construct phase diagrams in the μ–T plane showing hysteresis closure points and superfluid onsets. At low coverage, the superfluid onset showed both the abrupt frequency shift and dissipation peak characteristic of a conventional KT transition. Within hysteresis loops, a dissipation peak is observed, but the nearly discontinuous frequency shift is replaced by a smooth and gradual decoupling of the superfluid component as the coverage is increased. The gradual onset of superflow was analyzed using ideas from percolation theory.      

Flammability Studies of Isomeric Structures of Ethane Derivatives and Percolation Theory

In order to analyze how isomeric structures will affect the combustion hazard, the lower flammability limits (LFL) of 1,1,1-trifluoromethane (R-143a) and 1,1,2-trifluoroethane (R-143) have been measured using a modified ASTM E681 method. The modification is a spark ignition source in which the current is set at 10 mA and the voltage is adjusted (7–12 kV) so that dielectric breakdown just begins to occur rather than 30 mA at 15 kV specified by the ASTM E681-04 method. An earlier study on the LFL of difluoromethane indicated that flammability limits by the modified ASTM E681 method are affected by vessel size. In order to investigate the vessel volume effect on the measured LFL, experimental measurements of the LFL of R-143 and R-143a are made at 21.5°C using 5-, 12-, and 20-L vessels. For vessel volumes of 5-L and larger, the LFL systematically shift to higher concentrations of R-143 and R-143a with increasing vessel size which is consistent with a percolation model. Extrapolation of the measured LFL data to infinite vessel size using a percolation model yielded 3.57 and 2.96 mol · m⁻³ for R-143a and R-143, respectively.     

Large dielectric constant of the chemically purified carbon nanotube/polymer composites

The chemically purified multiwalled carbon nanotube/poly(vinylidene fluoride) (MWCNT/PVDF) composites were fabricated. Raman spectroscopy and transmission electron microscopy micrographs indicated that the catalysts metal particles and amorphous carbon had been removed from the purified MWCNTs. The percolation threshold of the composites is relatively large, about 3.8 vol.%. The most important result is that the dielectric constant of the composites is enhanced remarkably, and the dielectric constant of 3600 is obtained in the composite with 8 vol.% purified MWCNT at 1 kHz. The large dielectric constant can be attributed to the preparation procedure and the interface effect between the MWCNTs and the polymer.     

Percolation effects in functionally graded SOFC electrodes

A solid oxide fuel cell (SOFC) composite electrode exhibits a superior performance compared to a single phase electrode since the electrochemically active zone is spread into its volume. A functionally graded composite electrode consisting of monosized spherical electrocatalyst and electrolyte particles is sintered numerically by the discrete element method (DEM). The electrochemical performance is evaluated by a resistance network approach using Kirchhoff's current law. In the network discretization each contact between two particles is substituted by a bond resistance defined by contact size and the type of materials in contact.The graded electrode is optimized by varying its composition at the electrolyte/electrode interface and the degree to which the composition decreases linearly towards the current collector/electrode interface. Regarding its electrochemical activity, the graded electrode does not perform significantly better than an optimized uniformly randomly mixed composite electrode but percolation of the graded electrode is improved. In order to demonstrate the importance of percolation effects, a novel better performing electrode is developed which contains electronically conducting particle chains arranged within a random packing of ionically conducting particles.     

Wettability and capillary behavior of fibrous gas diffusion media for polymer electrolyte membrane fuel cells

The relationship of capillary pressure to liquid saturation for the water-air fluid pair in two different types of gas diffusion media (GDM) used in polymer electrolyte membrane fuel cell (PEMFC) electrodes is elucidated. It is experimentally demonstrated that GDM samples with and without treatment with poly(tetrafluoroethylene) (PTFE) ubiquitously display permanent capillary pressure hysteresis. Water does not imbibe spontaneously into a dry GDM, neither is it ejected spontaneously from a water-saturated GDM. Rather, positive displacement pressure is required to force both water and air into GDMs, whereas the main effect of adding PTFE is to increase the amount of work required for forcing water into the GDM, and to decrease the work required for water removal. Irrespective of PTFE content, the GDM samples tested are generally shown to behave as materials of intermediate (neutral) wettability. The US Bureau of Mines (USBM) wettability index nevertheless shows that water is the preferentially non-wetting phase in PTFE-treated GDMs and the preferentially wetting phase in untreated GDMs. Water-air capillary pressure curves are found to depend on sample thickness, clearly demonstrating that finite size effects are important. Finally, compression of the GDM is found to increase the capillary pressures for water injection and decrease the capillary pressures required for water withdrawal. These results should aid the design of GDMs with improved water management properties and the modeling of PEMFC electrodes in general.     

Percolation analysis of large-scale wireless balloon networks

Recent advancements in wireless technology have tested Wireless Balloon Networks (WBNs) as an ideal solution for the provision of internet facilities in deprived and challenging areas. A few high profile companies, such as Google, Space Data Inc., etc., have already made news by initiating projects based on high-altitude WBNs in order to provide internet facilities in remote areas. Unfortunately, the technical details have mainly been kept confidential so far. In this paper, we attempt to analyze the percolation properties of large-scale WBNs, considering both homogenous and heterogenous wireless nodes. In order to do so, we modeled a WBN as a large-scale random network where the path-loss models of homogenous and heterogenous WBNs were reduced to GDM (Gilbert's Disk Model) and RGDM (Random Gilbert's Disk Model), respectively. The bounds of the critical density regime were derived for both percolation models. Additionally, this paper implemented an experimental test bed for the WBN percolation model. Consequently, the findings of this research may prove crucial in estimating critical network properties.     

Characterizing Colloidal Structures of Pseudoternary Phase Diagrams Formed by Oil/Water/Amphiphile Systems

Two pseudoternary phase diagrams were constructed using ethyl oleate, water, and a surfactant blend containing poly (oxyethylene 20) sorbitan monooleate and sorbitan monolaurate with or without the cosurfactant 1-butanol. Two colloidal regions were identified in the cosurfactant-free phase diagram; a microemulsion (ME) and a region containing lamellar liquid crystals (LC). The addition of 1-butanol increased the area in which systems formed microemulsions and eliminated the formation of any liquid crystalline phases. Samples that form the colloidal regions of both systems were investigated by freeze-fracture transmission electron microscopy and by viscosity and conductivity measurements. The three techniques were compared and evaluated as characterisation tools for such colloidal systems and also to identify transitions between the colloidal systems formed. A droplet ME was present at a low water volume fraction (?_w) in both systems (?_w <0.15) as revealed by electron microscopy. At higher ?_w values, LC structures were observed in micrographs of samples taken from the cosurfactant-free system while the structure of samples from the cosurfactant-containing system was that of a bicontinuous ME. The viscosity of both systems increased with increasing ?_w to 0.15 and flow was Newtonian. However, formation of LC in the cosurfactant-free system resulted in a dramatic increase in viscosity that was dependent on ?_w and a change to pseudoplastic flow. In contrast, the viscosity of the bicontinuous ME was independent of ?_w. Three different methods were used to estimate the percolation threshold from the conductivity data for the cosurfactant-containing system. The use of nonlinear curve fitting was found to be most useful yielding a value close to 0.15 for the ?_w.