Papers by Keyword: Percolation

Abstract: Heavy metal contamination in waste water is a problem of paramount concern. Instant measurement of the degree of contamination is the long term aim of this work. This project proposed the fabrication of mask printed graphite paste electrodes based on natural graphite and micronized graphite powder which has potential for sensing heavy metal in water. The graphite paste were prepared by mixing paraffin oil and graphite powder at certain ratios and they were coated via a mask on a paper substrate using squeegee method. A two-probe station was used to characterize the I-V curve of the mask printed electrodes, in which the result was used for determining the resistivity of the graphite paste electrodes. A field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) was used to investigate the surface structure of the graphite paste electrode and determining the purity of the carbon in the electrode. The result shows that natural and micronized graphite paste electrode has a mean resistivity of 1.69 x 10^-3 Ωm and 1.25 x 10^-3 Ωm, respectively. The slight difference found in the conductivity of both electrodes is associated with the particle gap size, density and dimension of graphite electrodes which are associated with the percolation theory.

Abstract: Electrical properties and morphology of carbon black (CB) filled ethylene-butyl-acrylate copolymer (EBA) composites were studied in compact and chemically foamed versions. Conductivity percolation threshold for foamed composites was lowered by five weight percent of carbon black concentration. It means that the same conductivity values were achieved at reduced concentration of filler than for non-foamed version of composite. As a result semiconducting material with reduced density (weight) and content of carbon black is obtained. The sensitivity of foamed composite conductivity to mechanical stimulus induced by uniaxial compression was observed.

Abstract: A new kind of two phase ammonium dihydrogen phosphate (NH₄H₂PO₄ or ADP)/polyvinyl alcohol (PVA) composite films have been prepared by solution cast technique. Significant enhancement of dielectric permittivity (~442) of ADP/PVA composites are observed at low ADP concentration (f_ADP f_C ~5 wt %) which is the percolation threshold value estimated from the concentration dependent transport and dielectric data. Nearly 85 times increase in dielectric permittivity compared to that of the bare PVA film in the composite film around f_C with low loss (~0.14) at 1 kHz and room temperature is observed. The present ADP based PVA composite material might be processed for application in devices.

Abstract: Among the broad class of electro-active polymers, dielectric elastomer actuators represent a rapidly growing technology for electromechanical transduction. In order to further develop this applied science, the high driving voltages currently needed must be reduced. For this purpose, one of the most promising and adopted approach is to increase the dielectric constant while maintaining both low dielectric losses and high mechanical compliance. In this work, a dielectric elastomer was prepared by dispersing functionalised carbon nanotubes into a polyurethane matrix and the effects of filler dispersion into the matrix were studied in terms of dielectric, mechanical and electro-mechanical performance. An interesting increment of the dielectric constant was observed throughout the collected spectrum while the loss factor remained almost unchanged with respect to the simple matrix, indicating that conductive percolation paths did not arise in such a system. Consequences of the chemical functionalisation of carbon nanotubes with respect to the use of unmodified filler were also studied and discussed along with rising benefits and drawbacks for the whole composite material.

Abstract: The composites composed of micro-sized calcium copper titanate (CCTO) and nano-sized metallic nickel (Ni) fillers in the polyvinylidene fluoride (PVDF) matrix (Ni/CCTO/PVDF) were prepared, in which the filler content (volume fraction) of Ni and CCTO was set at 60 %. The impedance spectra and a serial equivalent circuit model consisting three RC units were used to investigate the behaviors of the Ni/CCTO/PVDF three-phase composite system near the percolation threshold. The real (Z′) and imaginary (Z″) parts of the impedance dramatically decreased as the Ni content was increased from 22% to 24% indicating a transition from an insulating to a conducting state. This transition process has been realized by the changes in the capacitance derived from the model, and the investigation has been carried out to clarify the release mechanism of the entrapped electrons at the interfaces.

Abstract: Bulk properties of polycrystalline structural and functional materials are controlled by the grain boundary microstructure defined by the grain boundary character distribution (GBCD) and grain boundary connectivity, because of percolation-dependent grain boundary phenomena. It has been found that there is a close relationship between microscale texture and grain boundary microstructure. Since percolation-controlled grain boundary phenomena are involved and playing key roles in the generation of various kinds of bulk properties, the relationship between texture and grain boundary microstructure can be effectively used as a powerful tool in development of high performance structural and functional materials by Grain Boundary Engineering (GBE).

Abstract: A kind of graphitized carbon black 40B2 was incorporated in natural rubber matrix and vulcanizates were prepared. The content of carbon black varied from 4 to 100phr for 100phr of natural rubber. Thermal conductivity of the CB/rubber composites was studied as a function of CB loadings and temperature. TEM and SEM were employed to explore the morphology of the carbon black and the composites. It was found that percolation phenomenon existed in thermal conductivity of CB/rubber composites at different filler loadings, which was much similar with percolation behavior in electrical conductivity properties of many composites, and the percolation threshold was about 13.63%. The influence of temperature on thermal conductivity was more obviously within the region from the percolation threshold to the volume fraction of 26.2%.However, no great effect could be found on the percolation behavior of the composites in thermal conductivity. The result of TEM indicated that the mean size of CB particles was about 25nm, the maximum size of CB aggregates was about 125nm, and the CB aggregates were multichain in shape. The results of SEM showed that the distribution of CB differed at different filler loadings. Before the percolation threshold, the CB aggregates existed isolatedly, nevertheless at the percolation threshold, some long chains made up of CB aggregates ran through the rubber matrix, which resulted in the rapid increase in thermal conductivity value, and then with the addition of CB loadings, the number of chains increased. When the volume fraction was up to 26.2%, some three dimensional networks formed.

Abstract: Research of alkali-activated materials has been a traditional domain of chemists. This paper exploits contribution of micromechanics to the subject. A new model for volumetric evolution of chemical phases is formulated. The first homogenization level identifies elasticity on the scale of N-A-S-H gel. Nanoindentation sensing technique yielded the intrinsic Young's modulus of N-A-S-H gel as ~18 GPa, which was further downscaled to the solid gel particles. Percolation theory had to be introduced to match an early-age elasticity. The second homogenization level takes into account an unreacted fly ash. Homogenization models match well the experimental elasticity and demonstrate stiffening of N-A-S-H gel, induced by increasing packing density of the solid gel particles. The percolation model explains a long setting time of alkali-activated materials.

Abstract: Molecular dynamics (MD) simulations based on a plastic crystal model (PCM) were performed for a Pd0.4Ni0.4P0.2 alloy in Metal-Metalloid (M-MLD) type of bulk metallic glass (BMG). Two kinds of clusters of cubeoctahedron capped with four half-octahedra and trigonal prism were used as initial atomic arrangements of the Pd0.4Ni0.4P0.2 alloy. Random rotations of clusters around their centers of gravity and subsequent structural relaxation vitrified the alloy. The high glass-forming ability of the Pd0.4Ni0.4P0.2 alloy is due to the critically-percolated, cluster-packed structure that is a universal feature for both M-MLD and M-M types of BMGs.

Abstract: Many nanocomposite materials are obtained by dispersing a charge in a matrix. Due to the conditions of mixing, the arrangement of the charge usually presents some heterogeneity at different scales. In order to predict the effective properties of such composites (like the dielectric permittivity or the elastic moduli), it is necessary to know the properties of the two components (charge and matrix), and their spatial distribution. To fulfil this project, we developed a general methodology in several steps: the morphology is summarized by multi-scale random models accounting for the heterogeneous distribution of aggregates. The identification of models is made from image analysis. It is then used for the prediction of effective properties by estimation, or by numerical simulations. Our approach is illustrated by various examples of multi-scale models: Boolean random sets based on Cox point processes and various random grains (spheres, cylinders), showing a very low percolation threshold and therefore a high conductivity or elastic moduli for a low charge content; multi-scale iterations of random media.