Percolation Threshold Region Research Articles

Electrical properties of stretchable and skin–mountable PDMS/MWCNT hybrid composite films for flexible strain sensors

Flexible strain sensors based on carbon nanofillers have great potential in the application of skin-adhesive sensors, wearable sensors, and tactile sensors, due to their superior electrical properties. Herein, the electrical properties of highly sensitive PDMS/MWCNT strain sensors made by vacuum filtration method were investigated. In order to obtain the electrical percolation curve of the flexible conductive films, first different samples were made with the same surface area but with different wt. % of CNTs. Then, depending on CNT content, the obtained conductive films exhibited initial electrical resistance in the range of 12.5 KΩ to 22.8 MΩ. The piezoresistive films with the CNT concentration of 1.4 to 2.9 [Formula: see text] had shown superior resistance drop, so this interval was determined as the percolation threshold region. According to the SEM images, the nanocomposite layer thickness of the flexible strain sensors in this region was 790 nm to 1210 nm. Afterward, the percolation curve was obtained using curve fitting to the experimental data and the exact value of the percolation threshold was defined as [Formula: see text]. Finally, in order to determine the minimum gauge factor ([Formula: see text]) of the sensors in percolation region, a flexible strain sensor in the upper limit of this region was selected and the piezoresistive properties of the selected sample were investigated.

Electrical and thermal phenomena in low‐density polyethylene/carbon black composites near the percolation threshold

ABSTRACTLow‐density polyethylene (LDPE)/carbon black (CB) composites were fabricated via melt‐compounding technique. The percolation threshold was found to be around 20 wt % CB, and an electrical network formed by conductive CB was proven by scanning electron microscopy investigation. Dielectric responses depicted an interfacial relaxation peak at 20 wt % CB content. LDPE/CB composites showed an electric field‐dependent conductivity as and a hysteresis behavior around the percolation threshold region. The CB particles with high thermal conductivity increased the heat conductance of the LDPE/CB20 up to 56%. The dynamic mechanical analysis of the LDPE/CB composites exhibited a noticeable contribution of CB throughout the composites, increasing the storage and loss modulus. The physical interactions between CB particles in the filler network enhanced the thermal degradation of the LDPE/CB25 composite for more than 76°C. The maximum breakdown strength of the LDPE/CB composites appeared with an approximately 10% improvement for LDPE/CB5 than pure LDPE. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47043.

Study on a novel dosimeter based on polyethylene–carbon nanotube composite

In this research work, the electric current of polyethylene-carbon nanotube composite, in the electrical percolation threshold region, over the absorbed dose under the applied bias voltage was investigated via the finite element method. The investigated geometry was formed by a two-dimensional cross-sectional view of randomly orientated nanotubes as ellipses. The variable range hopping model developed by Mott and thermally activated hopping model was used to calculate the electrical conductivity of carbon nanotubes and polymer, respectively. Regarding the calorimetric approach, we considered the absorbed dose equal to heat capacity of polyethylene-carbon nanotube composite multiplied by temperature rise. Results showed that this kind of composite can be used for low dose rate applications for monitoring and radiation protection utilizations.

Electrical and Mechanical Properties of Short-Carbon-Fiber Reinforced Polymerized Cyclic Butylene Terephthalate Composites

Polymerized cyclic butylene terephthalate (pCBT) resin casts filled with short carbon fibers were prepared by the melt-mixing approach. The electrical conductivity of short-carbon-fiber (SCF) reinforced thermoplastic pCBT resin casts were investigated with a special attention paid to the properties in the percolation threshold region and the mechanical properties of the composites were also studied. The percolation threshold value of the novel material system was determined which was also verified by SEM images and the thermoelectric behavior of the specimens. Even though the electrical properties of SCF/pCBT composites enhanced significantly, the material becomes more brittle than neat pCBT and all the specimens appear brittle fracture during the mechanical test. Moreover, fiber pull-out is the main damage form in three-point-bending test.

Effect of compounding sequence on localization of carbon nanotubes and electrical properties of ternary nanocomposites

Multiwalled carbon nanotubes (MWNTs) were incorporated into polyamide 12 (PA12) and PA12/polyethylene-octene elastomer grafted with maleic anhydride (POE-g-MA) components by melt compounding. The addition of MWNTs improves electrical, dynamic mechanical and thermal properties of PA12. In the presence of 20wt.% POE-g-MA, PA12/POE-g-MA/MWNT ternary nanocomposites exhibit substantially improved electrical conductivities as compared to PA12/MWNT binary nanocomposites with the same loading of MWNTs. The electrical conductivity shows 5–6 orders of magnitude increase in the percolation threshold region due to the volume exclusion effect of POE-g-MA. Variation of the compo unding sequence of the three components results in large difference in electrical conductivity of the ternary nanocomposites at the low loading of 2wt.% MWNTs, while the difference becomes slight at high MWNT loading of 4wt.%.

Optimum plasmon hybridization at percolation threshold of silver films near metallic surfaces

We experimentally demonstrate the strong interaction and plasmonic hybridization from a nanosystem having both localized and delocalized surface plasmon modes simultaneously in the presence of a nearby thin continuous metallic film. In situ dc resistance measurement of silver films and percolation theory were used to accurately determine the systems where the percolation threshold exists by distinguishing the nucleation and growth regions of silver films. We found an optimum plasmon hybridization existing in this percolation threshold region which can be verified from the absorption spectra. We interpret our data in terms of a fitting of the absorption spectra to the Fano-type line shape model.

An investigation of melt rheology and thermal stability of poly(lactic acid)/ poly(butylene succinate) nanocomposites

AbstractA systematic investigation of the rheological and thermal properties of nanocomposites prepared with poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and organically modified layered silicate was carried out. PLA/PBS/Cloisite 30BX (organically modified MMT) clay nanocomposites were prepared by using simple melt extrusion process. Composition of PLA and PBS polymers were fixed at a ratio of 80 to 20 by wt % for all the nanocomposites. Rheological investigations showed that high clay (> 3 wt %) contents strongly improved the viscoelastic behavior of the nanocomposites. Percolation threshold region was attained between 3 and 5 wt % of clay loadings. With the addition of clay content for these nanocomposites, liquid‐like behavior of PLA/PBS blend gradually changed to solid‐like behavior as shown by dynamic rheology. Steady shear showed that shear viscosity for the nanocomposites decreased with increasing shear rates, exhibiting shear‐thinning non‐Newtonian behavior. At higher clay concentrations, pseudo‐plastic behavior was dominant, whereas pure blend showed almost Newtonian behavior. Thermogravimetric analysis revealed that both initial degradation temperature (at a 2% weight loss) and activation energy of thermal decomposition nanocomposite containing 3 wt % of C30BX were superior to those of other nanocomposites as well as to those of PLA/PBS blend. Nanocomposite having 1 wt % of C30BX did not achieve expected level of thermal stability due to the thermal instability of the surfactant present in the organoclay. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Abnormal Magnetic Properties of Granular Co-SiO2/GaAs Nanostructures at the Percolation Threshold Region

Abnormal Magnetic Properties of Granular Co-SiO2/GaAs Nanostructures at the Percolation Threshold Region

The effect of pressure deformation on dielectric and conducting properties of siliconerubber/polypyrrole composites in the percolation threshold region

The changes in the frequency dependences of the AC conductivity and complexpermittivity during the pressure deformation of silicone rubber/polypyrrole compositeshave been examined. The experiments revealed that the pressure loading of the compositewith particle concentration in the vicinity of the upper part of the percolation region causesdisconnection of particle contacts and the transition of the semiconducting particle chainstructure to a non-conducting composite structure corresponding to mutually insulatedparticles below the percolation threshold. A close relation between the frequencydependences of the conductivity and permittivity of the composites at various pressureloadings has been confirmed.

Microwave properties of granular amorphous carbon films with cobalt nanoparticles

We have studied losses of microwave electromagnetic radiation in granular amorphous hydrogenated carbon films a-C:H(Co) containing cobalt nanoparticles at cobalt concentrations below the percolation threshold and in the percolation threshold region. The difference between the experimental values of the losses in granular structures sputtered on corundum-based substrates and those on aramide tissues is observed. It is found that this is determined by the difference in localization sizes of cluster electron states in a-C:H(Co) films. We have found that losses grow with increasing concentration of cobalt nanoparticles and with increasing frequency. The frequency dependence of losses is determined by the fast spin relaxation of nanoparticles. The anisotropy of losses in a-C:H(Co) covers on aramide tissues is observed.

Magnetotransport, magnetic, and structural properties of TM–SiO 2 (TM=Fe, Co, Ni) granular alloys

TM x (SiO 2) 1− x (transition metals, or TM=Fe, Co, Ni) thin films were prepared in a wide concentration range with the purpose of studying the giant Hall effect (GHE). The structure was studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and small angle X-ray scattering (SAXS). Magnetic, transport, and magnetotransport properties were investigated by means of magnetization, resistivity, and Hall effect measurements. TEM images show nanometer-sized spherical structures embedded in an SiO 2 amorphous matrix, with typical sizes ranging from 2 to 7 nm when TM volume concentration x is increased. SAXS measurements show a broadened peak. XRD measurements show that the system consists of amorphous SiO 2 and a crystalline peak, corresponding to small TM crystallites. The observed magnetic properties are strongly dependent on x, and clearly display an evolution resulting from the progressive increase in the mean particle size. Above the percolation threshold region all samples display GHE. Relationships between structure and magnetotransport properties are discussed.

Electrical conductivity of carbon fibres/polyester resin composites in the percolation threshold region

The electrical conductivity of composites of a polyester resin filled with short carbon fibres has been investigated with a special attention to the properties in the percolation threshold region. A very low percolation threshold (0.7–0.8 vol% of the filler) was confirmed. In contrast to S-shaped curves calculated according to the percolation theory of composites of globular particles, the experimental conductivity vs. fibre content dependence, after a steep increase in the percolation region, increased almost linearly. This atypical behaviour was explained by a different mechanism of formation of fibrous and globular conducting structures above the percolation threshold. An increase in scatter of conductivity values observed at percolation threshold as a consequence of great fluctuation of fibre arrangement manifested itself also in the conductivity–temperature dependences.

Electrical and thermal conductivity of polymers filled with metal powders

The electrical and thermal conductivity of systems based on epoxy resin (ER) and poly(vinyl chloride) (PVC) filled with metal powders have been studied. Copper and nickel powders having different particle shapes were used as fillers. The composite preparation conditions allow the formation of a random distribution of metallic particles in the polymer matrix volume for the systems ER–Cu, ER–Ni, PVC–Cu and to create ordered shell structure in the PVC–Ni system. A model is proposed to describe the shell structure electric conductivity. The percolation theory equation σ∼( ϕ− ϕ c) t with t=2.4–3.2 (exceeding the universal t=1.7 value) holds true for the systems with dispersed filler random distribution, but not for the PVC–Ni system. The percolation threshold ϕ c depends on both particle shape and type of spatial distribution (random or ordered). In contrast to the electrical conductivity, the concentration dependence of thermal conductivity shows no jump in the percolation threshold region. For the description of the concentration dependence of the electrical and thermal conductivity, the key parameter is the packing factor F. F takes into account the influence of conductive phase topology and particle shape on the electrical and thermal conductivity.

Dielectric Properties of Polymers Filled with Dispersed Metals

The authors have studied the dielectric properties of composite materials based on both thermoplastic and thermoset resins filled with nickel or copper, with various particle sizes and shapes. In addition, two types of particle distribution, random and segregated, were produced for composites filled with nickel. The main objective was to study the effect of the above factors on the dielectric properties of the composites. The concentration dependence of the dielectric parameters (i.e. the real, ∊′, and the imaginary, ∊″, parts of the complex dielectric permittivity and the dielectric loss tangent, tanδ), calculated for all the systems studied, demonstrates a critical behaviour in the percolation threshold region, with maximum values reached at a volume fraction ϕ = ϕc. The dependence of the dielectric parameters on concentration follows power-law behaviour in the ϕ < ϕc region. The critical exponent value for ∊′ is q = 0.75, in agreement with the theoretical one. The dielectric characteristics of the filled composites are more sensitive to the spatial filler distribution. For the segregated PVC-Ni system with an ordered filler distribution, the value of ϕc is much lower than for ER-Ni composites with a random filler distribution. Besides, for the segregated PVC-Ni system, the value of q is not constant, as it depends on the filler concentration. A model for the structure, which explains this behaviour, is proposed.

Fabrication process and electrical behavior of novel pressure-sensitive composites

Abstract A novel route was developed to fabricate a new pressure-sensitive composite by dispersing homogeneously conductive carbon particles in an insulating silicone rubber matrix. The composites showed a gradual change in electrical resistivity with applied pressure within percolation threshold region at a constant temperature. This type of gradual fall of resistivity with applied pressure is very important to fabricate pressure sensors. Various amounts of carbon particles were dispersed in a rubber matrix to understand the effect of volume fraction of conductive filler with applying external pressure on resistivity. A quantitative general effective media (GEM) theory was used to understand the resistivity of carbon–rubber composites system over a large range of volume fraction of carbon with applied pressure. The use of two different sizes of silicon rubber particles showed a significant effect in gradual fall of resistivity with applied pressure in the narrow range of percolation threshold. However, a large variation in resistivity from 1st measuring to 10th measuring was observed. A significant improvement in successive measuring of resistivity variation from 1st measuring to 10th measuring was observed when composites were fabricated in hexane solvent media. Finally, nano-sized Al2O3 was dispersed to control the resistivity variation upon successive measurement and to improve the mechanical properties of the composites. The material was suggested to use as unique materials as pressure sensors in practical applications mainly for robots.

Compositonal dependence of sound velocity and elastic moduli of amorphous Se1−xGex systems: comparative study with percolation theory

Abstract A comparative study of thermal and elastic properties has been made on amorphous Se 1− x Ge x alloy by employing a dynamical treatment. It has been observed that all these properties show an increasing linear trend with the increase of germanium content in Se 1− x Ge x amorphous system and an excellent agreement with the experimental results. Further, the linear variation shows no anomaly in the elastic moduli–coordination number relation in the studied rigidity percolation threshold region ( m =2.4–2.6).

Electromagnetic-field-induced oscillations of the lipid domain structures in the mixed membranes

The effect of external electromagnetic field (EMF) on a percolation structure formed during phase separation in the mixed phospholipid membranes was studied by computer simulation. Decay of the percolation structure under electromagnetic radiation was detected. It was shown that oscillation regime can be realized in this system: periodic alternation of formation and decay of the percolation cluster was observed under 10 kHz EMF. The decay of the lipid domain structure in the EMF results from anomalous increase of the permittivity of the continuous fluid lipid phase in the percolation threshold region. It is proposed that detected EMF effect can influence the signal and transport processes associated with percolation properties of biomembranes.