Conductivity Percolation Research Articles

Seamless integration of bioelectronic interface in an animal model via invivo polymerization of conjugated oligomers

Leveraging the biocatalytic machinery of living organisms for fabricating functional bioelectronic interfaces, in vivo, defines a new class of micro-biohybrids enabling the seamless integration of technology with living biological systems. Previously, we have demonstrated the in vivo polymerization of conjugated oligomers forming conductors within the structures of plants.Here, we expand this concept by reporting that Hydra, an invertebrate animal, polymerizes the conjugated oligomer ETE-S both within cells that expresses peroxidase activity and within the adhesive material that is secreted to promote underwater surface adhesion. The resulting conjugated polymer forms electronically conducting and electrochemically active μm-sized domains, which are inter-connected resulting in percolative conduction pathways extending beyond 100 μm, that are fully integrated within the Hydra tissue and the secreted mucus. Furthermore, the introduction and in vivo polymerization of ETE-S can be used as a biochemical marker to follow the dynamics of Hydra budding (reproduction) and regeneration. This work paves the way for well-defined self-organized electronics in animal tissue to modulate biological functions and in vivo biofabrication of hybrid functional materials and devices.

Highly stretchable porous composite hydrogels with stable conductivity for strain sensing

Stretchable hydrogels have undergone a rapid development in recent years particularly for wearable strain sensors. However, most of hydrogel-based sensors exhibit poor mechanical properties. Here, we report a highly stretchable porous composite hydrogel that has a tensile strain of 400% and a compressive strain of 80%. The porous hydrogel is composed of polyvinyl alcohol (PVA) and copolymer of N-(3-sulfopropyl)-N-methacroyloxyethyl-N,N-dimethylammonium betaine (SBMA) and acrylamide (AM) prepared via graphene oxide (GO)-stabilized Pickering aqueous foam templates. The pore morphology of the composite hydrogels was observed by field emission scanning electron microscope (FE-SEM). The tensile and compressive curves of porous hydrogels with different GO concentrations were characterized. GO can act as Pickering stabilizer to produce aqueous foams and also construct the physical cross-linking network in porous hydrogels to improve their mechanical strength. In addition, when GO is reduced by L-ascorbic acid, it provides effective electric pathways in porous hydrogels with the conductive percolation of less than 0.40 wt%. The composite hydrogels thus exhibit excellent strain sensitivity and can be used to detect various human motions. The facile preparation process of the porous composite hydrogels with outstanding performance properties facilitates the development of flexible wearable strain sensing materials.

Polycarbonate based multifunctional self-sensing 2D and 3D printed structures for aeronautic applications

Different polycarbonate materials have been reinforced with carbon nanotubes to tune electrical conductivity and to induce piezoresistive self-sensing capabilities. Further, the composites were processed by solvent casting and fused deposition modelling (filament for 3D printing) methods. An electrical conductivity percolation threshold of ≈0.3 wt.% has been found for solvent casted films, whereas samples processed by 3D printing show higher thresholds, ≈2 wt.%, presenting all samples a similar maximum electrical conductivity (σ ≈ 1 × 10−3 S m−1), thermal and chemical properties. Overall mechanical properties are larger for the solvent cast films concerning the 3D printed ones, particularly the elongation at break. The piezoresistive sensitivity, obtained after four-point-bending and uniaxial strain experiments, shows gauge factors up to 1.7, independently of the processing method. The functionality of the materials has been demonstrated by the implementation of an airplane wing section model with self-sensing capabilities. Two implemented strategies showed the suitability of the developed materials for real-time monitoring of the wing mechanical deformation.

Ultra-Low Percolation Threshold Induced by Thermal Treatments in Co-Continuous Blend-Based PP/PS/MWCNTs Nanocomposites.

The effect of the crystallization of polypropylene (PP) forming an immiscible polymer blend with polystyrene (PS) containing conductive multi-wall carbon nanotubes (MWCNTs) on its electrical conductivity and electrical percolation threshold (PT) was investigated in this work. PP/PS/MWCNTs composites with a co-continuous morphology and a concentration of MWCNTs ranging from 0 to 2 wt.% were obtained. The PT was greatly reduced by a two-step approach. First, a 50% reduction in the PT was achieved by using the effect of double percolation in the blend system compared to PP/MWCNTs. Second, with the additional thermal treatments, referred to as slow-cooling treatment (with the cooling rate 0.5 °C/min), and isothermal treatment (at 135 °C for 15 min), ultra-low PT values were achieved for the PP/PS/MWCNTs system. A 0.06 wt.% of MWCNTs was attained upon the use of the slow-cooling treatment and 0.08 wt.% of MWCNTs upon the isothermal treatment. This reduction is attributed to PP crystals’ volume exclusion, with no alteration in the blend morphology.

Evaluation of the rheological and electrical percolation of high‐density polyethylene/carbon black composites using mathematical models

AbstractIn this work, conductive polymer composites (CPCs) of bio‐based polyethylene (BioPe) containing different concentrations of carbon black (CB) were developed. By using oscillatory rheology analysis, a Newtonian plateau was observed in BioPe, and all BioPe/CB composites had a behavior of a pseudo‐solid and that composites with volume fractions ranging from 0.24 to 0.56 presented higher viscosity, storage, and loss modulus. This suggests the formation of a percolated network and by using the power‐law models, it was observed that the electrical percolation threshold was higher than the rheological percolation threshold. The electrical conductivity was measured using the four‐point probe method and a sigmoid model was used to predict the CPCs' electrical conductivity percolation threshold. The results indicated that the four‐point probe method presented satisfactory results according to the calculated standard deviations and voltage–current characteristics for each round of measurements considering the same ranging as used in rheology analysis. The analytical model used showed a coefficient of determination (R2) higher than 95%, allowing the prediction of the electrical conductivity of the CPC and the percolation threshold as a function of the volumetric fraction of the CB.

Effect of the selective localization of carbon nanotubes and phase domain in immiscible blends on tunable microwave dielectric properties

In recent years, the immiscible polymer blend system has attracted much attention as the matrix of nanocomposites. Herein, from the perspective of dynamics, the control of the carbon nanotubes (CNTs) migration aided with the interface of polystyrene (PS) and poly (methyl methacrylate) (PMMA) blends was achieved through a facile melt mixing method. Thus, we revealed a comprehensive relationship between several typical CNTs migrating scenarios and the microwave dielectric properties of their nanocomposites. Based on the unique morphologies and phase domain structures of the immiscible matrix, we further investigated the multiple microwave dielectric relaxation processes and shed new light on the relation between relaxation peak position and the phase domain size distribution. Moreover, by integrating the CNTs interface localization control with the matrix co-continuous structure construction, we found that the interface promotes double percolation effect to achieve conductive percolation at low CNTs loading (~1.06 vol%). Overall, the present study provides a unique nanocomposite material design symphonizing both functional fillers dispersion and location as well as the matrix architecture optimization for microwave applications.

Graphene Nanoribbon Grids of Sub-10 nm Widths with High Electrical Connectivity.

Quasi-one-dimensional (1D) graphene nanoribbons (GNRs) have finite band gaps and active edge states and therefore can be useful for advanced chemical and electronic devices. Here, we present the formation of GNR grids via seed-assisted chemical vapor deposition on Ge(100) substrates. Nucleation seeds, provided by unzipped C60, initiated growth of the GNRs. The GNRs grew toward two orthogonal directions in an anisotropic manner, templated by the single crystalline substrate, thereby forming grids that had lateral stitching over centimeter scales. The spatially uniform grid can be transferred and patterned for batch fabrication of devices. The GNR grids showed percolative conduction with a high electrical sheet conductance of ∼2 μS·sq and field-effect mobility of ∼5 cm2/(V·s) in the macroscopic channels, which confirm excellent lateral stitching between domains. From transconductance measurements, the intrinsic band gap of GNRs with sub-10 nm widths was estimated as ∼80 meV, similar to theoretical expectation. Our method presents a scalable way to fabricate atomically thin elements with 1D characteristics for integration with various nanodevices.

Dielectric properties of PMMA/KCTO(H) composites for electronics components

AbstractPolymethyl methacrylate/hollandite‐like copper doped potassium titanate high‐k ceramics composites with different filler content (20–60 vol.%) are prepared by simple method of mixing of the polymer solution and ceramics dispersion followed evaporation of the solvent and hot pressing. The particle morphology and size are estimated by SEM. The composites obtained are studied by XRD, FTIR, and TGA methods. The influence of the additive and the concentration of hollandite‐like copper doped potassium titanate high‐k ceramics on dielectric properties (permittivity, conductivity, dielectric loss tangent) are investigated at frequency range from 10−1 to 106 Hz. It is shown that the increasing of ceramics concentration cause the increasing of main dielectric characteristics, namely permittivity up to 40 and dielectric loss up to 0.13 at 60 vol.% of filler and 0.1 Hz. Conductivity percolation threshold for PMMA/KCTO(H) composites was estimated (vc = 0.0773 vol. [20.9 wt. %]). Experimental data on permittivity are compared with different theoretical models (Lichtenecker's model and the Effective Medium Theory model).

An Analytical Method for Parameter Extraction in Oxide Semiconductor Field-Effect Transistors

This article presents an analytical method for parameter extraction in oxide semiconductor field-effect transistors (OS FETs), including threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ), effective channel length (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ), source-drain series resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SD</sub> ), and intrinsic effective mobility ( μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">int</sub> ). An analytical expression of effective mobility ( μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) is implemented with the consideration of percolation conduction and R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SD</sub> effects. This method relies on the concept of Y-function method and the channel length dependent measurement. The extraction results are compared to those from the combination of transfer length method (TLM). It has been shown that a better accuracy of the extracted parameters is achieved with the proposed method. Furthermore, transfer characteristics are resimulated based on the extracted parameters, in which a good match between measurement and simulation is obtained.

3D Printing Temperature Tailors Electrical and Electrochemical Properties through Changing Inner Distribution of Graphite/Polymer.

The rise of 3D printing technology, with fused deposition modeling as one of the simplest and most widely used techniques, has empowered an increasing interest for composite filaments, providing additional functionality to 3D-printed components. For future applications, like electrochemical energy storage, energy conversion, and sensing, the tuning of the electrochemical properties of the filament and its characterization is of eminent importance to improve the performance of 3D-printed devices. In this work, customized conductive graphite/poly(lactic acid) filament with a percentage of graphite filler close to the conductivity percolation limit is fabricated and 3D-printed into electrochemical devices. Detailed scanning electrochemical microscopy investigations demonstrate that 3D-printing temperature has a dramatic effect on the conductivity and electrochemical performance due to a changed conducive filler/polymer distribution. This may allow, e.g., 3D printing of active/inactive parts of the same structure from the same filament when changing the 3D printing nozzle temperature. These tailored properties can have profound influence on the application of these 3D-printed composites, which can lead to a dramatically different functionality of the final electrical, electrochemical, and energy storage device.

Dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) are soft, electrically powered actuators that have no discrete moving parts, yet can exhibit large strains (10%–50%) and moderate stress (∼100 kPa). This Tutorial describes the physical basis underlying the operation of DEA's, starting with a simple linear analysis, followed by nonlinear Newtonian and energy approaches necessary to describe large strain characteristics of actuators. These lead to theoretical limits on actuation strains and useful non-dimensional parameters, such as the normalized electric breakdown field. The analyses guide the selection of elastomer materials and compliant electrodes for DEAs. As DEAs operate at high electric fields, this Tutorial describes some of the factors affecting the Weibull distribution of dielectric breakdown, geometrical effects, distinguishing between permanent and “soft” breakdown, as well as “self-clearing” and its relation to proof testing to increase device reliability. New evidence for molecular alignment under an electric field is also presented. In the discussion of compliant electrodes, the rationale for carbon nanotube (CNT) electrodes is presented based on their compliance and ability to maintain their percolative conductivity even when stretched. A procedure for making complaint CNT electrodes is included for those who wish to fabricate their own. Percolative electrodes inevitably give rise to only partial surface coverage and the consequences on actuator performance are introduced. Developments in actuator geometry, including recent 3D printing, are described. The physical basis of versatile and reconfigurable shape-changing actuators, together with their analysis, is presented and illustrated with examples. Finally, prospects for achieving even higher performance DEAs will be discussed.

Effect of spherical copper particle size on the negative permittivity behavior of copper/polypropane composite

In this work, the conductive composites were manufactured by hot pressing with polypropylene (PP) as matrix resin and copper powder as conductive filler. The effects of copper particle radius and content on the dielectric properties of the composites were studied. The results show that dielectric constant of the composite was greatly improved than that of pure PP material. With the increase of copper content, the conductive percolating network was established within the composites. Along with the percolation phenomenon of microstructure, the conductive mechanism simultaneously changed. The absolute value of the negative permittivity also increases to 2.2 × 106. Moreover, the negative permittivity was observed by tuning their compositions and tailoring their microstructures. Further investigation revealed that there was a constitutive relationship between permittivity and reactance.

A Three-Dimensional Printable Liquid Metal-Like Ag Nanoparticle Ink for Making a Super-Stretchable and Highly Cyclic Durable Strain Sensor.

Fabrication of metal nanoparticle (NP)-based strain sensors with both a broad working range and linearity range is still a significant challenge. Typically, homogeneous conductive percolation networks are indispensable for linear sensing performance, whereas inhomogeneous microstructures may inevitably arise under large strain due to the formation of defects in rigid NPs. In this study, a sandwich-structured strain sensor with an extraordinarily large stretchability (800%) yet self-healing property is fabricated by three-dimensional printing using a liquid metal-like Ag NP ink. The strain sensor shows an initial conductivity of 248 S cm-1, a good linearity in two strain ranges, and a long-term stability after undergoing 5000 cycles under a strain level of 100%. Such highly comprehensive sensing performance is attributed to the unique structure of the Ag NP ink, in which Ag NPs coalesce together after room-temperature sintering triggered by chlorides, and then, the sintered Ag aggregates tend to form continuous conductive networks through hydrogen bonds between polyacrylic acid and carboxymethylcellulose. Further, the free flow of Ag aggregates is the root cause that leads to the change of relative resistance as demonstrated by finite element simulation. This Ag NP-based strain sensor shows high potential for application in monitoring human knuckle motion.

Thermal and electrical characterization of poly(vinyl)alcohol)/poly(vinylidene fluoride) blends reinforced with nano-graphene platelets

Nano-graphene/polymer composites can function as pressure induced electro-switches, at concentrations around their conductivity percolation threshold. Close to the critical point, the pressure dependence of the electron tunneling through the polymer barrier separating nano-graphene's results from the competition among shortening of the tunneling length and the increase of the polymer's polarizability. Such switching behavior was recently observed in polyvinyl alcohol (PVA) loaded with nano-graphene platelets (NGPs). In this work, PVA is blended with α-poly(vinylidene fluoride) (PVdF) and NGPs. Coaxial mechanical stress and electric field render the nano-composite piezoelectric. We investigate the influence of heterogeneity, thermal properties, phase transitions and kinetic processes occurring in the polymer matrix on the macroscopic electrical conductivity and interfacial polarization in casted specimens. Furthermore, the effect of electro-activity of PVdF grains on the electric and thermal properties are comparatively studied. Broadband Dielectric spectroscopy is employed to resolve and inspect electron transport and trapping with respect to thermal transitions and kinetic processes traced via Differential Scanning Calorimetry. The harmonic electric field applied during a BDS sweep induces volume modifications of the electro-active PVdF grains, while, electro-activity of PVdF grains can disturb the internal electric field that free (or bound) electric. The dc conductivity and dielectric relaxation was found to exhibit weak dependencies.

Printable anisotropic magnetoresistance sensors for highly compliant electronics

Printed electronics are attractive due to their low-cost and large-area processing features, which have been successfully extended to magnetoresistive sensors and devices. Here, we introduce and characterize a new kind of magnetoresistive paste based on the anisotropic magnetoresistive (AMR) effect. The paste is a composite of 100-nm-thick permalloy/tantalum flakes embedded in an elastomer matrix, which promotes the formation of appropriately conductive percolation networks. Sensors printed with this paste showed stable magnetoresistive properties upon mechanical bending. The AMR value of this sensor is 0.34% in the field of 400 mT. Still, the response is stable and allows to resolve sub-mT field steps. When printed on ultra-thin 2.5-upmu hbox {m}-thick Mylar foil, the sensor can be completely folded without losing magnetoresistive performance and mechanically withstand 20, upmu {hbox {m}} bending radius. The developed compliant printed AMR sensor would be attractive to implement on curved and/or dynamic bendable surfaces for on-skin applications and interactive printed electronics.

The Entangled Conductive Structure of CB/PA6/PP MFCs and Their Electromechanical Properties.

In this work, carbon black (CB)/polyamide 6 (PA6)/polypropylene (PP) microfibrillar composites (MFCs) were fabricated through an extrusion (hot stretching) heat treatment process. The CB-coated conductive PA6 microfibrils with high aspect ratio were in situ generated as a result of the selective accumulation of CB at the interface. At the proper temperature, a 3D entangled conductive structure was constructed in the PP matrix, due to topological entanglement between these conductive microfibrils. This unique conductive structure provided the PP composites with a low electrical conductivity percolation threshold. Moreover, the electromechanical properties of conductive MFCs were investigated for the first time. A great stability, a high sensitivity and a nice reproducibility were achieved simultaneously for CB/PA6/PP MFCs. This work provides a universal and low-cost method for the conductive polymer composites’ (CPCs) fabrication as sensing materials.

Percolation conductivity model in A site-deficient La0.8−xoxNa0.2−xoxMnO3 manganites and universal model of magnetic field-dependent resistivity and magnetoresistance behavior

The electrical and magnetoresistance behaviors of La0.8−xoxNa0.2−xoxMnO3 [with x = 0 (N0) and 0.1 (N0.1)] polycristalline samples were investigated. Resistance measurements versus temperature under different magnetic applied fields were performed to investigate the electronic transport as well as the magnetoresistance properties. The metal/insulator transition temperature decreases from 280 K for x = 0 to 275 K for x = 0.1. The electrical conduction in these compounds has been well described in the context of the percolation model based on the phase separation of ferromagnetic metallic regions with paramagnetic insulating ones. The temperature-dependent resistance shows the existence of a minimum at low temperature. This behavior is due to the existence of a competition between inelastic and elastic interactions expressed by Kondo-like spin-dependent scattering. An empirical model was used to describe the variation of the magetoresistance and normalized resistance as a function of the magnetic applied field. The model was successfully widely tested on several magnetic specimens and the evolution of the fitting parameters was discussed in detail.

Vitrimer chemistry enables epoxy nanocomposites with mechanical robustness and integrated conductive segregated structure for high performance electromagnetic interference shielding

Constructing segregated pathways in the conductive polymer composites (CPCs) is efficient to achieve satisfactory electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) with low content of nanofillers. Conventionally, most of segregated CPCs were based on thermoplastics and precisely controlled processing conditions were essential to achieve structural integrity and mechanical robustness. Herein, by taking the advantage of the associative dynamic bonding rearrangement in the epoxy vitrimer, we fabricated segregated MWCNTs/epoxy composites in a wide range of compression temperature and pressure. The segregated CPCs show a low conductive percolation threshold at 0.066 wt% and an EMI SE of 22 dB when only 2 wt% of MWCNTs were loaded. More promisingly, the mechanical robustness was achieved by the volatile ethylene glycol (EG) involved decomposition and repolymerization of β-hydroxyl ester at the interface even when numerous MWCNTs were in between. The segregated structure and macroscopic properties can be reserved after reprocessing and the nanofillers can be recovered with excessive EG at elevated temperature.

Effects of Expandable Graphite at Moderate and Heavy Loadings on the Thermal and Electrical Conductivity of Amorphous Polystyrene and Semicrystalline High-Density Polyethylene

In this work, we prepared and investigated two series of polymer composites, wherein the matrix was either an amorphous polystyrene (PS) or a semicrystalline high-density polyethylene (HDPE) filled with expandable graphite (EGr) at relatively high loadings within the range 5–55 wt %. For the investigation we employed a thermogravimetric analysis and differential scanning calorimetry to assess the thermal transitions and evaluate the various polymer fractions (crystalline (CF), mobile (MAF) and rigid amorphous (RAF)) in addition to broadband dielectric spectroscopy and a laser flash analysis to evaluate the EGr effects on electrical conductivity, σ, and thermal conductivity, λ, respectively. In PS, EGr was found to impose an increase of the glass transition temperature and a systematic decrease of the corresponding heat capacity change. The latter was rationalized in terms of the formation of an interfacial RAF. No glass transition was recorded for HDPE whereas the fillers increased the CF moderately. As expected, σ increased with the filler loading for both matrices, up to 10−3–10−2 S/cm, resulting in a conductive percolation threshold for electrons at &gt; 8 wt % EGr. Simultaneously, the λ of PS and HDPE were strongly increased, from 0.13 and 0.38 W·K–1·m–1 up to 0.55 and ~2 W·K–1·m–1, respectively. λ demonstrated an almost linear EGr loading dependence whereas the semicrystalline composites exhibited a systematically higher λ.

Analytical Surface Potential-Based Compact Model for Independent Dual Gate a-IGZO TFT

A surface potential-based compact model for independent dual gate (IDG) amorphous In-Ga-Zn-O thin-film transistors (IDG a-IGZO TFTs) is proposed here. The transport theories of percolation conduction, trap-limited conduction (TLC), and variable range hopping (VRH) in extended and localized states are first considered simultaneously via Schroder method, obtaining a physical description of the transport mechanism under different conditions of temperature and gate voltage. Moreover, a single formulation of front and back surface potentials which is valid and extremely accurate in all operation regimes is developed. Based on the transport theories and surface potentials, the complete compact model is developed and verified using both numerical simulation and experiment with an excellent agreement, and the threshold compensation effect is also included. Finally, the compact model is coded in Verilog-A, and implemented in a vendor CAD environment, which suggested that the proposed model can be successfully applied to circuit design.