Ultralow Electrical Percolation Threshold Research Articles

Unified modeling and experimental realization of electrical and thermal percolation in polymer composites

Correlations between electrical and thermal conduction in polymer composites are blurred due to the complex contribution of charge and heat carriers at the nanoscale junctions of filler particles. Conflicting reports on the lack or existence of thermal percolation in polymer composites have made it the subject of great controversy for decades. Here, we develop a generalized percolation framework that describes both electrical and thermal conductivity within a remarkably wide range of filler-to-matrix conductivity ratios (Yf/Ym), covering 20 orders of magnitude. Our unified theory provides a genuine classification of electrical conductivity with typical Yf/Ym≥1010 as insulator–conductor percolation with the standard power-law behavior and of thermal conductivity with 102≤Yf/Ym≤104 as poor–good conductor percolation characterized by two universal critical exponents. Experimental verification of the universal and unified features of our theoretical framework is conducted by constructing a 3D segregated and well-extended network of multiwalled carbon nanotubes in polypropylene as a model polymer matrix under a carefully designed fabrication method. We study the evolution of the electrical and thermal conductivity in our fabricated composites at different loading levels up to 5 vol. %. Significantly, we find an ultralow electrical percolation threshold at 0.02 vol. % and a record-low thermal percolation threshold at 1.5 vol. %. We also apply our theoretical model to a number of 23 independent experimental and numerical datasets reported in the literature, including more than 350 data points, for systems with different microscopic details, and show that all collapse onto our proposed universal scaling function, which depends only on dimensionality.

An Effective Strategy to Achieve Ultralow Electrical Percolation Threshold with CNTs Anchoring at the Interface of PVDF/PS Bi‐Continuous Structures to Form an Interfacial Conductive Layer

AbstractConductive composites based on polymers and conductive nanofillers are widely studied as a promising material. The rational design of 3D conductive networks in composites is crucial to improve their electrical conductivity and reduce the dosage of nanofillers. Herein, poly(vinylidene fluoride) (PVDF) and polystyrene (PS) bi‐continuous structures with modified carbon nanotubes (CNTs) tailored to anchor at the interface are designed to achieve an ultralow electrical percolation threshold because of the formation of a thin interfacial conductive layer. In this work, the modification of CNTs with poly(methyl methacrylate) (PMMA), which contributes to the improvement of the compatibility between PVDF and CNTs, is effective to control the distribution of CNTs in composites. It promotes the migration of CNTs from the PS phase to the interface of PVDF and PS. Consequently, the interfacial conductive layer is formed at a low CNT content, and the electrical percolation threshold of PVDF/PS/CNTs‐PMMA nanocomposites is only 0.07 vol%, having a great decrease of about 50% compared with that of PVDF/PS/CNTs nanocomposites. Thus, it is demonstrated that the distribution of CNTs can be tailored to anchor at the interface by proper chemical modification to form an interfacial conductive layer and a decrease of percolation threshold can also be achieved.

Fabrication of pristine graphene-based conductive polystyrene composites towards high performance and light-weight

Although highly electrically conductive polymer composites filled with two-dimensional graphene-based nanofillers have been considerably explored, it still remains a challenge for constructing these nanocomposites with high filler efficiency, mild fabricating condition, as well as more effective conductive network at lower nanofiller contents. To meet above objectives, a facile preparation strategy was designed, wherein the pristine graphene (pGR) was utilized as nanofiller in polystyrene (PS) particle matrix via latex technology. Owing to the absence of reduction process, the hot-processing parameters were optimized to be more gentle ones. The structural, morphological, electrical, thermal and mechanical properties of resultant PS/pGR nanocomposites with various pGR contents were systematically investigated. SEM and TEM images showed the excellent interfacial adhesion between these two materials and segregated conductive structure was well-established. The as-fabricated PS/pGR composites achieved a quite high electrical conductivity of 20.5 S/m at a relatively low pGR loading of 0.957 vol% and an ultralow electrical percolation threshold of 0.0475 vol%. The thermal conductivity (TC) increased by 2.1 times at 0.957 vol% pGR loading than neat PS and the mechanical properties also exhibited applaudable results. This novel strategy to fabricate high performance PS conductive composites based on pristine graphene as nanofillers was thus proved to be effective.

Impact of carbon nanotube prelocalization on the ultra-low electrical percolation threshold and on the mechanical behavior of sintered UHMWPE-based nanocomposites

Nascent powder particles of Ultra High Molecular Weight Polyethylene have been covered by multi-walled carbon nanotubes and then processed by means of sintering. The prelocalization of carbon nanotubes in the interfacial region between powder particles of UHMWPE allowed reaching an electrical percolation threshold as low as 0.04 wt%. It is also a possible way to reinforce specifically the interfacial regions with a very small amount of fillers. Indeed, the nucleation effect of the nanotubes is very strong on the strain-induced crystallization in the melting state of the composite. The tensile experiments above the melting point allowed by the extremely broad rubbery plateau of the UHMWPE show a significant increase in strain-hardening due to the addition of CNTs from 0.02 wt%. In the solid state the elastic properties, such as yield stress, and the crystallinity can also be increased for higher CNT concentrations despite the presence of aggregates which reduce the elongation at break.

Graphene-Elastomer Composites with Segregated Nanostructured Network for Liquid and Strain Sensing Application.

One of the critical issues for the fabrication of desirable sensing materials has focused on the construction of an effective continuous network with a low percolation threshold. Herein, graphene-based elastomer composites with a segregated nanostructured graphene network were prepared by a novel and effective ice-templating strategy. The segregated graphene network bestowed on the natural rubber (NR) composites an ultralow electrical percolation threshold (0.4 vol %), 8-fold lower than that of the NR/graphene composites with homogeneous dispersion morphology (3.6 vol %). The resulting composites containing 0.63 vol % graphene exhibited high liquid sensing responsivity (6700), low response time (114 s), and good reproducibility. The unique segregated structure also provides this graphene-based elastomer (containing 0.42 vol % graphene) with exceptionally high stretchability, sensitivity (gauge factor ≈ 139), and good reproducibility (∼400 cycles) of up to 60% strain under cyclic tests. The fascinating performances highlight the potential applications of graphene-elastomer composites with an effective segregated network as multifunctional sensing materials.

Gelatin-Graphene Nanocomposites with Ultralow Electrical Percolation Threshold.

Gelatin-graphene conductive biopolymer nanocomposites (CPCs) with ultralow percolation threshold are designed by reducing in situ graphene oxide nanosheets with ascorbic acid and suppressing the aggregation of the graphene nanosheets. The resulting conductive nanocomposites show a record-low electrical percolation threshold of 3.3 × 10(-2) vol%, which arises from the homogeneous dispersion of the graphene nanosheets within the gelatin matrix.

Fractal dendrite-based electrically conductive composites for laser-scribed flexible circuits.

Fractal metallic dendrites have been drawing more attentions recently, yet they have rarely been explored in electronic printing or packaging applications because of the great challenges in large-scale synthesis and limited understanding in such applications. Here we demonstrate a controllable synthesis of fractal Ag micro-dendrites at the hundred-gram scale. When used as the fillers for isotropically electrically conductive composites (ECCs), the unique three-dimensional fractal geometrical configuration and low-temperature sintering characteristic render the Ag micro dendrites with an ultra-low electrical percolation threshold of 0.97 vol% (8 wt%). The ultra-low percolation threshold and self-limited fusing ability may address some critical challenges in current interconnect technology for microelectronics. For example, only half of the laser-scribe energy is needed to pattern fine circuit lines printed using the present ECCs, showing great potential for wiring ultrathin circuits for high performance flexible electronics.

Ultralow percolation threshold in polyamide 6.6/MWCNT composites

When incorporating multiwalled carbon nanotubes (MWCNTs) synthesised by the aerosol-CVD method using different solvents into polyamide 6.6 (PA66) by melt mixing an ultralow electrical percolation threshold of 0.04 wt.% was found. This very low threshold was assigned to the specific characteristic of the MWCNTs for which a very high aspect ratio, a good dispersability in aqueous surfactant dispersions, and relatively low oxygen content near the surface were measured. The investigation of the composites by transmission electron microscopy on ultrathin cuts as well as by scanning electron microscopy combined with charge contrast imaging on compression moulded plates illustrated a good MWCNT dispersion within the PA66 matrix and that the very high aspect ratio of the nanotubes remained even after melt processing. Additionally, the thermal behaviour of the PA66 composites was investigated using differential scanning calorimetry (DSC) showing that the addition of only 0.05 wt.% MWCNT leads to an increase of the onset crystallization temperature of 11 K.

Self-assembly of miscible homopolymer/quasi-block copolymer blends/MWNT composites: a strategy to obtain ultralow electrical percolation threshold and mechanism

We utilized self-assembly of miscible polymer blends/CNTs composites to obtain ultralow electrical percolation threshold, with different results of both miscibility and glass transition temperature variations from antecedent works.

Glass Fibers with Carbon Nanotube Networks as Multifunctional Sensors

AbstractA simple approach to deposit multiwalled carbon nanotube (MWNT) networks onto glass fiber surfaces achieving semiconductive MWNT–glass fibers is reported, along with application of fiber/polymer interphases as in‐situ multifunctional sensors. This approach demonstrates for the first time that the techniques of conducting electrical resistance measurements could be applicable to glass fibers for in situ sensing of strain and damage; the techniques were previously limited to conductive and semiconductive materials. The electrical properties of the single MWNT–glass fiber and the “unidirectional” fiber/epoxy composite show linear or nonlinear stress/strain, temperature, and relative humidity dependencies, which are capable of detecting piezoresistive effects as well as the local glass transition temperature. The unidirectional composites containing MWNT–glass fibers exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold. Based on this approach, the glass fiber—the most widely used reinforcement in composites globally—along with the surface electrical conductivity of MWNTs will stimulate and realize a broad range of multifunctional applications.

Conducting probe atomic force microscopy applied to organic conducting blends

Atomic force microscopy (AFM) is used in contact mode with a conducting tip to probe the conducting network of the conductive polymer polyaniline blended in an insulating polymer matrix. The high resistance contrast and sharp boundaries between conductive and insulating phases is observed down to scales in the 10 nm range. The very low scale electric dispersion corresponds to the morphologic phase segregation known from conventional AFM or transmission electron microscopy measurements, which is responsible for the ultralow electrical percolation threshold previously demonstrated in this system.

Thermomechanical Studies of Poly(aniline)/Poly(methyl methacrylate) Blends: Relationship with Conducting Properties

We report on the first dynamic mechanical studies of blends of conducting polymer poly- (aniline) (PANI) in an insulating matrix that exhibit ultralow electrical percolation threshold. Whereas the matrix undergoes an irreversible flow slightly above its glass-rubber transition temperature, blends with PANI mass fraction as low as 1% show a well-defined rubber plateau with a stability range of ca. 100 K. These results corroborate the image of a ramified network of conducting phase, inferred from electrical results. In situ resistance measurements during tensile testing also probe the conductive network. The increase of conductivity in the elastic regime could be interpreted as a consequence of a better alignment of PANI bundles.