Typical Percolation Behavior Research Articles

X-ray induced insulator-metal transition in a thin film of electron-doped VO2

We have observed a persistent x-ray induced insulator-metal transition at low temperature for an epitaxial film of electron-doped VO${}_{2}$, a material as characterized by strong electron-electron and electron-lattice interactions. The volume fraction of the photo-generated metallic patches, ranging from 0 to 100% of the whole film, can be scaled well with the total dose of x-ray irradiation, irrespective of the x-ray intensity. This indicates the monomolecular process of the insulator-metal phase conversion that corresponds to the instantaneous creation of the metallic patch extending over as many as 10${}^{5}$ V sites per one x-ray absorbed photon. The typical percolation behavior is observed in the conductivity change with the finely photo-controlled volume ratio of the metallic phase.

Mechanical and electrical properties of carbon nanofibers reinforced aluminum nitride composites prepared by plasma activated sintering

High density carbon nanofibers (CNFs) reinforced aluminum nitride (AlN) composites were successfully fabricated by plasma activated sintering (PAS) method. The effects of CNFs on the microstructure, mechanical and electrical properties of the AlN composites were investigated. The experimental results showed that the grain growth of AlN was significantly inhibited by the CNFs. With 2 wt.% CNFs added into the composites, the fracture toughness and flexural strength were increased, respectively to 5.03 MPa m 1/2 and 354 MPa, which were 20.9% and 13.4% higher than those of monolithic AlN. The main toughening mechanisms were CNFs pullout and bridging, and the main reason for the improvements in strength should be the fine-grain-size effect caused by the CNFs. The DC conductivity of the composites was effectively enhanced through the addition of CNFs, and showed a typical percolation behavior with a very low percolation threshold at the CNFs content of about 0.93 wt.% (1.51 vol.%).

Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites

Poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites have been fabricated to evaluate the potential of PTT composites as electromagnetic interference (EMI) shielding material. The room temperature electrical conductivity, complex permittivity, and shielding effectiveness (SE) of PTT/MWCNT composites were studied in the frequency range of 8.2–12.4 GHz (X-band). The dc conductivity (σ) of composites increased with increasing MWCNT loading and a typical percolation behavior was observed at 0.48 vol% MWCNT loading. The highest EMI SE of PTT/MWCNT composites was ~23 decibel (dB) at 4.76 vol% MWCNT loading which suggest that these composites can be used as light weight EMI shielding materials. The correlation among the SE, complex permittivity, and electrical conductivity was also studied. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss.

Novel capacitance-type humidity sensor based on multi-wall carbon nanotube/SiO2 composite films

A novel capacitance-type relative humidity (RH) sensor based on multi-wall carbon nanotube/SiO2 (MWCNTs/SiO2) composite film is reported. Details of the fabrication process, possible sensing mechanism and sensing characteristics, such as linearity and sensitivity, are described. The capacitance of the MWCNTs/SiO2 composite film shows typical concentration percolation behavior with increasing MWCNT loading. At loadings below the percolation threshold (1.842wt%), the sensor capacitance increases obviously with increasing MWCNTs. The water condensed in the MWCNTs/SiO2 layer can lower the percolation threshold and increase the sensor capacitance. The sensor with MWCNT concentration of 1wt% has the best properties. The sensor has a humidity sensitivity of about 673 pF/% RH and a linearity correlation of 0.98428. The response time of the sensor to RH is about 40 s and the recovery time is about 2 s.

Tuning electrical and thermal connectivity in multiwalled carbon nanotube buckypaper

We find that the electrical and thermal connectivity in multiwalled carbon nanotubebuckypaper can be tuned using a spark plasma sintering (SPS) technique. Elevated SPStemperatures promote the formation of inter-tube connections and consequentlyimpact the electrical resistivity, thermoelectric power and thermal conductivityof the buckypaper. In particular, the electrical resistivity as a function of SPStemperature exhibits a percolation-type behavior while the low temperature latticethermal conductivity shows a crossover behavior in the sample dimensionality. Theresults are discussed in terms of the quasi-one-dimensional metallic nature ofmultiwalled carbon nanotubes, the packing density and the electron–phonon coupling.

Novel resistive-type humidity sensor based on multiwall carbon nanotube/polyimide composite films

Abstract A novel resistive-type relative humidity (RH) sensor based on plasma-treated multiwall carbon nanotube/polyimide (p-MWCNT/PI) composite films is reported. Details of the fabrication process and sensing characteristics such as linearity and sensitivity are described. The electrical resistance of the p-MWCNT/PI composite film shows typical concentration percolation behavior with increasing p-MWCNT loading. At loadings beyond the percolation threshold (0.05 wt%), this sensor exhibits very good linearity with a positive slope over the entire operational RH range. The sensor has a humidity sensitivity of about 0.0047/% RH and a linearity correlation (R2) of 0.9999. A humidity sensing mechanism for the p-MWCNT/PI composite film is proposed on the basis of charge transfer between adsorbed water molecules and the p-MWCNTs.

Electrical and dielectric properties of multiwall carbon nanotube/polyaniline composites

In this paper, electrical and dielectric properties of multiwall carbon nanotubes (MWCNTs)/insulating polyaniline (PANI) composites were studied. A mixture of MWCNTs and insulating polyaniline was dispersed in an ethanol solution by ultrasonic process, subsequently dried, and was hot-pressed at 200 °C under 30 MPa. Electrical and dielectric properties of the composites were measured. The experimental results show that the dc conductivities of the composites exhibit a typical percolation behavior with a low percolation threshold of 5.85 wt.% MWCNTs content. The dielectric constant of the composites increases remarkably with the increasing MWCNTs concentration, when the MWCNTs concentration was close to percolation threshold. This may be attributed to the critical behavior of the dielectric constant near the percolation threshold as well as to the polarization effects between the clusters inside the composites.

Modification of local order in FePd films by low energy He+ irradiation

Owing to their strong perpendicular magnetic anisotropy, FePd, CoPd, and their Co(Fe)Pt counterparts are candidate materials for ultrahigh density magnetic recording. The stability and magnetic properties of such films are largely dependent on the orientation and local distribution of the L10 FePd phase fraction. Therefore, the formation and transformation of the L10 phase in such thin films have been the subject of continued interest. Highly ordered epitaxial FePd(001) thin films (with an L10 phase fraction of 0.81) were prepared by molecular-beam epitaxy on a MgO(001) substrate. The effect of postgrown room temperature, 130 keV He+ irradiation was investigated at fluences up to 14.9×1015 ions/cm2. X-ray diffraction and conversion electron Mössbauer spectroscopy revealed that with increasing fluence, the L10 FePd phase decomposes into the face centered cubic phase with random Fe and Pd occupation of the sites. A partially ordered local environment exhibiting a large hyperfine magnetic field also develops. Upon He+ irradiation, the lattice parameter c of the FePd L10 structure increases and the long range order parameter S steeply decreases. The Fe–Fe nearest-neighbor coordination in the Fe-containing environments increases on average from Fe47Pd53 to Fe54Pd46, indicating a tendency of formation iron-rich clusters. The equilibrium parameters corresponding to the equiatomic L10 phase were found at different fluences by conversion electron Mössbauer spectroscopy and by x-ray diffraction a difference, from which a plane-perpendicular compressive stress and a corresponding in-plane tensile stress are conjectured. The steep increase in the interface roughness above 7.4×1015 ions/cm2 is interpreted as a percolation-type behavior related to the high diffusion anisotropy in the L10 phase.

Percolation-type Photoswitching Behavior in Conductance of Diarylethene–Silver Nanoparticle Networks

Abstract A silver nanoparticle network interlinked by photochromic diarylethene dithiophenols was fabricated and its photoreactive and conductive properties were investigated. The network showed reversible changes in absorption spectra and conductance by photoirradiation. The results indicate the percolation-type behavior.

Poly(phenylene sulfide) magnetic composites. I. Relations of percolation between rheology, electrical, and magnetic properties

AbstractPoly(phenylene sulfide)/ferrosoferric oxide composites (PPS/Fe3O4) with various loading levels were prepared by melt compounding. The microstructure of composites was investigated using SEM and XRD. The rheological, electrical and magnetic properties were characterized respectively by the parallel plate rheometer, high resistance meter, and magnetometer. The results reveal that the Fe3O4 particles are well dispersed in the PPS matrix due to their nice affinity, which results in a weak strain overshoot at large amplitude oscillatory level. Both the rheological and the electrical responses of the composites show a typical percolation behavior. But the rheological percolation presents lower threshold (< 40 wt %) than that of electrical percolation (∼ 50 wt %), which is attributed to the difference structure of the percolation network. The magnetic response, however, shows good linear relation with Fe3O4 loadings, indicating that the physical percolation has little influence on the magnetic properties. This is mainly due to the yielded long‐range magnetic interactions among Fe3O4 particles in the applied field, which are far stronger than those nonmagnetic physical interactions accounting for percolation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 233–243, 2008

Effect of Multiwall Carbon Nanotubes on Electrical and Dielectric Properties of Yttria‐Stabilized Zirconia Ceramic

MWCNT/3Y‐TZP (3 mol% yttria‐stabilized tetragonal polycrystalline zirconia) composites with different multiwall carbon nanotube (MWCNT) contents were prepared by the spark plasma sintering technique. The effect of MWCNT addition on the electrical and dielectric properties of the composites at room temperature was studied. The experimental results showed that the DC conductivity of the composites demonstrated a typical percolation behavior with a very low percolation threshold between 1.0 and 2.0 wt% MWCNT content, and the dielectric constant was greatly increased when the MWCNT concentration was close to the percolation threshold, which was attributed to dielectric relaxation, the space charge polarization effect, and the percolation effect.

Complexity in charge transport for multiwalled carbon nanotube and poly(methyl methacrylate) composites

We report on studies of the charge transport mechanism of the composites of multiwalled carbon nanotubes MWCNTs and poly methyl methacrylatePMMA. The MWCNTs were synthesized by a chemical vapor deposition method using Fe as a catalyst. The dc conductivity dc and its temperature dependence dcT were measured in a temperature range of 0.5– 300 K to study the charge transport mechanism of the composites. The dc of composites at room temperature increased as the MWCNT concentration increased, which shows typical percolation behavior with percolation threshold of pc at 0.4 wt % of the MWCNTs. The dcT of the MWCNT-PMMA composites were fitted to the combination of Sheng’s fluctuation induced tunneling FIT model and the one-dimensional variable range hopping 1D VRH model. The tunneling mechanism and the 1D VRH transport were attributed to the charge tunneling between MWCNT clusters and the charge hopping through 1D MWCNTs, respectively. Magnetoresistance MR of MWCNT-PMMA composites was measured up to 9 T of magnetic field. The results of MR and dcT showed that the FIT model was dominant in the low temperature region T 10 K, while the 1D VRH process became dominant as the temperature increased 10 KT 300 K. We observed unusually large negative MR in the composites at the lower temperatures T 4K due to FIT conduction. We related the parameters specifying charge transport to the percolation structures of MWCNT-PMMA composites based on the results of temperature dependence of dc and of the MR.

Simulation of the percolation of water into rigid polyurethane foams at applied hydraulic pressures

The hydraulic resistance of polyurethane foams is studied by means of simulations of water penetration into model foams. The model foams of cubical shape are constructed by generating the centers of the cells randomly. The strength of the window separating two cells is assumed to be a function of the distance between the centers of the cells in one set of computations. In another set of computations the strengths of the windows are assigned randomly from a specified distribution. The foam is exposed to an elevated pressure at its boundaries and water penetrates into the foam by rupturing the windows with strengths lesser than the applied pressure. The variation of equilibrium volume fraction of the foam filled with water for increasing hydraulic pressures shows typical percolation behavior: there is a sharp increase in the volume filled beyond a threshold pressure. Simulations show that beyond a certain sample size there is no change in the percolation curve with sample size, and indicate that it is mainly the weaker windows that control the hydraulic resistance of the foam. The simulation results are compared with experimental data. POLYM. ENG. SCI. 46:970–983, 2006. © 2006 Society of Plastics Engineers

Electrical conductivity and electromagnetic interference shielding of polyaniline/polyacrylate composite coatings

AbstractLightweight and flexible composite coatings of p‐toluene sulfonic acid doped polyaniline (PANI–TSA) with various mass fractions and polyacrylate were prepared for electromagnetic interference (EMI) shielding. Both the volume and surface conductivities of the composite coatings increased with increasing PANI–TSA; furthermore, the volume conductivity showed a typical percolation behavior with a percolation threshold at about 0.21. The EMI shielding effectiveness (SE) of the PANI–TSA/polyacrylate coatings over the range of 14 kHz to 15 GHz increased with increasing PANI–TSA as the direct‐current conductivity did. EMI SE of the coatings at the low frequencies (14 kHz to 1 GHz) was around 30–80 dB, higher than that at the high frequencies (1–15 GHz); this indicated possible commercial application of the coatings for far‐field EMI shielding. The highest EMI SE value was 79 dB at 200 MHz with a coating thickness of 70 ± 5 μm. The moderate SE, light weight, and easy preparation of the coating are advantages for future applications for EMI shielding. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2149–2156, 2005

Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotube composites containing Fe catalyst

Thin and flexible composite films of raw or purified multiwalled carbon nanotube (MWCNT) with various mass fractions and poly(methylmethacrylate) (PMMA) were synthesized for electromagnetic interference (EMI) shielding material. From scanning electron microscopy and high-resolution transmission electron microscopy photographs, we observed the formation of a conducting network through MWCNTs in an insulating PMMA matrix and the existence of an Fe catalyst in MWCNTs. The dc conductivity (σdc) of the systems increased with increasing MWCNT mass fraction, showing typical percolation behavior. The measured EMI shielding efficiency (SE) of MWCNT–PMMA composites by using the extended ASTM D4935-99 method (50 MHz–13.5 GHz) increased with increasing MWCNT mass fraction as σdc. The highest EMI SE for raw MWCNT–PMMA composites was ∼27 dB, indicating commercial use for far-field EMI shielding. The contribution of absorption to total EMI SE of the systems is larger than that of reflection. Based on magnetic permeability, we suggest raw MWCNTs and their composites can be used for near-field EMI shielding.

Electrical Conductivity and Electromagnetic Interference Shielding of Multi-walled Carbon Nanotube Filled Polymer Composites

ABSTRACTMulti-walled carbon nanotube (MWNT) filled polystyrene (PS) composites were synthesized for electromagnetic interference (EMI) shielding applications. SEM images of composites showed the formation of the conducting networks through MWNTs within the PS matrix. The measured DC conductivity of composites increased with increasing MWNT loading, showing a typical percolation behavior. EMI shielding characteristics of MWNT-PS composites were investigated in the frequency range of 8.2–12.4 GHz (X-band). It was observed that the shielding effectiveness (SE) of such composite increased with the increase of MWNT loading. The SE of the composite containing 7 wt% MWNTs could reach more than 26 dB in the measured frequency region.

Electrical properties of single-wall carbon nanotube and epoxy composites

The homogeneous composites of single-wall carbon nanotube (SWNT) and Stycast 1266 epoxy were made with the nanotube concentrations of 0.01–0.21 wt % by using a high frequency sonication method. Direct current conductivities of various mass fractions show typical percolation behavior and the percolation threshold pc is found to be 0.074 wt % and the conductivity exponent t is estimated as 1.3±0.2. The anomalously small pc is attributed to the large aspect ratio of the nanotubes. Dielectric measurement was performed at temperatures 220–300 K with the frequency range of 20 Hz–1 MHz. There appeared different dielectric aspects according to the mass fractions p≪pc, p∼pc, or p>pc. The dielectric relaxation of epoxy dominates at low concentrations (p≪pc), while at high concentrations (p>pc) we found a scaling property in the frequency dependence of loss tangent. The scaled loss tangent spectra show a Debye type relaxation. Near the percolation threshold (p∼pc), the composites show peaks in the frequency dependence of the imaginary part of their electric moduli. Log–log plots of ac conductivity versus frequency for the midrange (p∼pc) composites exhibit the scaling and universal behavior regardless of temperature and mass fraction of SWNT.

Polymer composites prepared by compression molding of a mixture of carbon black and nylon 6 powder

AbstractThe results of an experimental study on the effect of processing variables and filler concentration on the electrical resistivity of conductive composites based on nylon 6 filled with carbon black are reported. A typical percolation behavior in the effect of electroconductive filler content on the resistivity was found. The electrical resistivity of the composites is > 1012 ohm°Cm unless the carbon black content reaches the percolation threshold at ∼9 wt%, beyond which the resistivity decreases markedly by as much as twelve orders of magnitude. Two parameters of molding process—temperature and time—were shown to have a notable effect on the resistivity of composites, whereas pressure has no influence on this property in the pressure range considered. There is no sharp variation in the density due to the onset of percolation, and the hardness of samples is not influenced by the presence of the filler.

Problems in the determination of pore connectivity by digital image processing

In the practical use of porous materials, pore blocking processes can occur leading to a decrease of the accessibility of the pore space. This percolation-type behavior is mainly influenced by the connectivity of the pore network. In order to characterize this property of porous media, the usefulness of a method is discussed which determines a value for pore connectivity with the help of digital image processing. Starting with a microphotograph of a prepared section of the porous sample, the proposed algorithm, in a first step, covers the pore space of the digitized image with convex areas. In the second step, it determines the number of convex areas connected to a given one. The mean number of the neighbouring areas is said to be the measure of the connectivity of a pore network. To check the usefulness of the method, the connectivities of different pore textures are measured and the results are evaluated critically.

The impact of fragmentation on char conversion during pulverized coal combustion

Synthetic char particles with controlled macroporosity are used to study char fragmentation behavior during pulverized coal combustion. The chars are burned in an atmospheric laminar flow reactor that permits the control of gas temperature and composition. Char particles are extracted from the reactor at selected residence times and characterized for extent of mass loss and particle size distribution. Chars of 23% and 36% porosity are subjected to environments containing 12 mol % O2 at nominally 1500 K. Number distributions show a large increase in the numbers of small particles during devolatilization, the higher porosity char exhibiting the larger increase. Results show that char burn off increases with porosity at any given residence time, demonstrating an impact of fragmentation on char burn off. The data indicate that both particle diameter and apparent density decrease during burn off and support power-law relations between char particle mass, apparent density, and diameter. A particle population balance model is developed and used to characterize the type of fragmentation that occurs during char oxidation and to quantify the rates of fragmentation events. The model allows for attrition-type behavior (in which only fines are produced), breakage-type behavior (in which particles break into two or three smaller particles), and percolation-type behavior (in which particles fragment into a distribution of smaller-size particles). Calculations using the model indicate that fragmentation during burn off is percolative in nature and char burning rate parameters determined from mass loss, size, and temperature measurements are too high if account is not made for the effects of particle fragmentation. Calculations also suggest that fragmentation during develatilization is percolative in nature and the extent of fragmentation increases with coal volatile yield. Fragmentation rates during devolatilization are estimated to be as high as five times the fragmentation rates during char oxidation.