Percolative Composites Research Articles

Communication—Tunable Lorentz-Type Negative Permittivity of PANI/Epoxy Resin Composites in the Frequency Range from 3 kHz to 1 MHz

Negative permittivity in percolation composites garnered significant interest due to its promising implications for practical applications. This study demonstrates that the percolation threshold of the polyaniline(PANI)/epoxy resin composite falls within the range of 40 wt% to 50 wt%. Beyond this percolation threshold, the composites exhibit a corresponding negative dielectric behavior. Notably, at a high PANI content level of 90 wt%, the permittivity exhibits characteristics akin to Lorentz resonance type behavior. This research presents an effective approach to exhibit tunable low-frequency negative permittivity through Lorentz resonance.

Self-Assembled MXene@Fluorographene Hybrid for High Dielectric Constant and Low Loss Ferroelectric Polymer Composite Films.

Modern electrical applications urgently need flexible polymer films with a high dielectric constant (εr) and low loss. Recently, the MXene-filled percolative composite has emerged as a potential material choice because of the promised high εr. Nevertheless, the typically accompanied high dielectric loss hinders its applications. Herein, a facile and effective surface modification strategy of cladding Ti3C2Tx MXene (T = F or O; FMX) with fluorographene (FG) via self-assembly is proposed. The obtained FMX@FG hybrid yields high εr (up to 108 @1 kHz) and low loss (loss tangent tan δ = 1.16 @ 1 kHz) in a ferroelectric polymer composite at a low loading level (the equivalent of 1.5 wt % FMX), which is superior to its counterparts in our work (e.g., FMX: εr = 104, tan δ = 10.71) and other studies. It is found that the FG layer outside FMX plays a critical role in both the high dielectric constant and low loss from experimental characterizations and finite element simulations. For one thing, FG with a high F/C ratio would induce a favorable structure of high β-phase crystallinity, extensive microcapacitor networks, and abundant interfacial dipoles in polymer composites that account for the high εr. For another, FG, as a highly insulating layer, can inhibit the formation of conductive networks and inter-FMX electron tunneling, which is responsible for conduction loss. The results demonstrate the potential of a self-assembled FMX@FG hybrid for high εr and low loss polymer composite films and offer a new strategy for designing advanced polymer composite dielectrics.

Identification of percolation threshold of spray‐dried cellulose nanocrystals in homopolymer polypropylene composites

AbstractUnderstanding the percolation threshold is essential for determining the performance of particle‐reinforced polymer composites. Spray‐dried cellulose nanocrystals (SDCNC) of micrometer size reinforced homopolymer polypropylene (HPP) composites at 20, 30, 40, and 50 wt.% were prepared to investigate the percolation threshold of SDCNC particles in HPP. The effect of a compatibilizer (maleic anhydride polypropylene (MAPP)) at 3, 5, and 7 wt.%, on the SDCNC percolation networks and composites performance were also studied. The results indicated that SDCNC particle percolation networks in HPP were established between 30 and40 wt.%. For composites without MAPP, the impact strength significantly increased by up to 23% below the percolation threshold and declined beyond it. The peak crystallization temperature of HPP was steadily increased until 30 wt.% SDCNC particles were added due to the SDCNC saturated nucleation function at the threshold. Introducing MAPP significantly improved tensile strength (58%), tensile strain (61%), flexural strength (45%), and impact strength (91%) compared with the corresponding composites without MAPP, attributed to the enhanced interfacial adhesion between the SDCNC particles and HPP. Water absorption results indicated that adding MAPP changed the SDCNC particle distribution networks within the matrix above the percolation threshold but did not change it below the threshold.

Poly(dimethyl siloxane)-based copolymer dielectric composites with copper sulfide loaded carbon nanotubes doped with carbon quantum dots

The renewal of electromechanical transducers is tightly linked to the ongoing development of elastomers with excellent dielectric properties, but this positive progress is often plagued by the dilemma of the lower dielectric constant of the elastomers themselves. In this work, carbon nanotubes loaded with copper sulfide and doped with carbon quantum dots (abbreviated as CD-CuS@CNT) were successfully fabricated through hydrothermal reaction, followed by the incorporation of this filler into poly(dimethyl siloxane)-based copolymers (H-PDMS for short) to form percolation composites, additionally co-blended composite films of H-PDMS and carbon nanotubes loaded with copper sulfide were prepared as a control. Results exhibit that the permittivity of 2.33 vol% CD-CuS@CNT/H-PDMS composites is 53.34 at 100 Hz, which is 1.3 and 19.2 times that of 2.12 vol% CuS@CNT/H-PDMS and pure H-PDMS, while the loss is only 0.05. Moreover, the investigation revealed the role of carbon quantum dots in the whole system to enhance the dielectric properties of the composites.

Negative permittivity of polyimide-based percolative composites with zero/one dimensional functional fillers

The construction of conductor/insulator composites based on percolation theory is a hot method for achieving negative permittivity in the radio frequency range. The morphology of functional fillers strongly affects the electron transport behavior and the performance of negative permittivity. However, research on the influence between the morphology of the functional fillers and negative permittivity seems to have been overlooked in the past. In this work, different contents of acetylene black/polyimide (ACET/PI) and multiwalled carbon nanotubes/polyimide (MWCNTs/PI) composites were successfully prepared to reveal the relationship between the morphology of the functional fillers, performance of negative permittivity, and the electron transport behavior. The difference in morphology of fillers directly affects the formation of conductive networks and the numerical value of percolation threshold, ultimately affecting the characteristics of negative permittivity. Moreover, the Drude model is used to analyze the negative permittivity behavior of composites with different conductive fillers. Research on reactance indicates that materials with negative permittivity exhibit inductive characteristics. This work provides a basis for the selection of functional fillers and performance control of materials with negative permittivity.

Understanding the negative permittivity behavior of polyvinyl alcohol based metamaterials under radio frequency

Functional metamaterials that exhibit negative dielectric permittivity through percolative network formation have attracted immense attention due to their significant potential for a wide range of applications in electronic devices. Negative permittivity has taken off as a research topic recent years in percolative composites. The polymer nanocomposites (PVA/H-Pa-G) were prepared using the α-Fe2O3 modified GNP and HCl doped polyaniline (Pani) based nanofillers (H-Pa-G). The as prepared samples were characterized in order to analyze their dielectric response for understanding the negative permittivity behavior. Below the percolation threshold for the polymer nanocomposite with 0.10%, 0.25% H-Pa-G nanofiller weight fraction, an increase in σac conductivity is observed with increase in frequency. The dielectric study confirmed the negative permittivity behavior for the polymer nanocomposite (0.50%, 1.00% and 2.00%) above the percolation threshold. This can be mainly attributed to the presence of the conductive network of H-Pa-G nanofiller in the PVA matrix. On the contrary, the positive permittivity is due to the insulative nature of PVA matrix. The empirical data of real permittivity was fitted using the Drude model that corresponded well with the experimental data with the fp = 1.31 GHz which is further certified by the equivalent circuit model fitting. This proposed work strongly suggest the implication of the as prepared polymer nanocomposite for the negative permittivity materials based applications.

Progress in percolative composites with negative permittivity for applications in electromagnetic interference shielding and capacitors

Progress in percolative composites with negative permittivity for applications in electromagnetic interference shielding and capacitors

MOF-derived ZrO2/C-polyvinylidene fluoride composite towards negative permittivity regulation mechanism

Composites featuring negative permittivity have garnered significant attention for their potential in novel capacitance designs, coil-less electrical inductors, and electromagnetic shielding applications. In this study, we prepared polyvinylidene fluoride (PVDF) matrix composites filled with ZrO2/C nanoparticles derived from metal-organic frameworks (MOFs) via a hot-pressing method. With an increase in the ZrO2/C content to 30 wt.%, electrical percolation was observed, accompanied by a transition mechanism from hopping conduction to metal-like conduction. This enabled the realization of ZrO2/C/PVDF composites with tailorable negative permittivity properties, attributed to the plasmonic oscillation of free electrons in the composites beyond the percolation threshold (30 wt.%). Furthermore, the permittivity transition along to a shift in the electrical behavior of the percolative composites from capacitive to inductive. We explored the regulatory mechanism behind the negative permittivity in this random composite system, and our findings highlight the potential of these tunable negative permittivity media as promising candidates for diverse electromagnetic applications.

Dielectric evolution of permittivity transition from positive to negative induced by percolation in ceramic composites

Ceramic composites made up of well-selected components have enriched the varieties of the metamaterials with negative electromagnetic parameters. In this work, TiN/BaTiO3 ceramic composites were prepared with a simple dry-pressing and pressureless sintering process. Percolation was eventually determined to be the key inducements which caused the permittivity of the ceramic composites changing from positive to negative. Essentially, negative permittivity was closely related to the plasma oscillations of free electrons in percolation networks. Meanwhile, conduction mechanism of the composites varied owing to the changes in microstructure, where the percolation networks brought out pathways for electron migration and achieved high conductivity. In addition, ac impedance behavior with changes of the sign for reactance were also discussed, thus the transition from electrical capacitive to inductive for the percolative ceramic composites has been demonstrated. This work clarified the influence of percolation on the conductivity, impedance, and permittivity of ceramic composites altogether. The obtained negative permittivity composites have enormous potential in various electronic devices and electromagnetic apparatus.

Dielectric properties of a linear polymer-matrix composite filled with aligned Ag@BaTiO3 flakes

Composites with linear dielectric properties possess advantages in energy storage fields for the high charge-discharge efficiency with low energy loss. However, energy density of them is limited due to low permittivity. Here, a linear dielectric polydimethylsiloxane (PDMS) composite filled with parallel aligned core-shell Ag@BaTiO3 (BT) 2D flakes is prepared by directional shearing, which possesses high permittivity and excellent energy storage properties. Compared with traditional polymer/core-shell hybrid filler composites, this work presents modified filler composition, different filler dimension and enhanced energy storage performances. Conformably-coated Ag@BT flakes are prepared through simultaneous hydrolysis on polyvinylpyrrolidone-functionalized Ag flakes, forming a smooth ∼100 nm thick coating layer. The aligned PDMS/40 vol% Ag@BT flake composite exhibits a very high permittivity of 347 and a low dielectric loss< 0.03 at 100 Hz from 20 to 150 °C. The comp°site exhibits an enhanced breakdown strength of 26 kV cm−1 owing to BT coverage-shell blocking and parallel filler arrangement, compared with that of percolative composites filled with metal particles without shell-coverage. The composite exhibits linear dielectric characteristics with high discharged energy density of 6.18 × 10−3 J cm−3 and superhigh charge-discharge efficiency> 95% at 20 kV cm−1. This study demonstrates Ag@BT flake based linear-polymer composites for energy related applications.

Core-Shell Engineering of Conductive Fillers toward Enhanced Dielectric Properties: A Universal Polarization Mechanism in Polymer Conductor Composites.

Flexible dielectric and electronic materials with high dielectric constant (k) and low loss are constantly pursued. Encapsulation of conductive fillers with insulating shells represents a promising approach, and has attracted substantial research efforts. However, progress is greatly impeded due to the lack of a fundamental understanding of the polarization mechanism. In this work, a series of core-shell polymer composites is studied, and the correlation between macroscopic dielectric properties (across entire composites) and microscopic polarization (around single fillers) is investigated. It is revealed that the polarization in polymer conductor composites is determined by electron transport across multiple neighboring conductive fillers-a domain-type polarization. The formation of a core-shell filler structure affects the dielectric properties of tpolymer composites by essentially modifying the filler-cluster size. Based on this understanding, a novel percolative composite is prepared with higher-than-normal filler concentration and optimized shell's electrical resistivity. The developed composite shows both high-k due to enlarged cluster size and low loss due to restrained charge transport simultaneously, which cannot be achieved in traditional percolative composites or via simple core-shell filler design. The revealed polarization mechanism and the optimization strategy for core-shell fillers provide critical guidance and a new paradigm, for developing advanced polymer dielectrics with promising property sets.

The role of water on dehydrofluorination in PVDF/K2CO3 percolative composites

Percolative composites of polyvinylidene fluoride/potassium carbonate (PKC) represent a new way of incorporating activated carbon in the polymer matrix with high dielectric constant and electrical conductivity, making it promising for flexible energy storage applications. In this paper, a series of characterization techniques were applied to investigate the role of water in the PKC mixture on PVDF dehydrofluorination and the structure of the subsequently developed activated carbon. The mechanism of the initiation of PVDF dehydrofluorination was clarified from the structural variation of the activated carbon generated under different conditions. The results revealed that the presence of a suitable amount of hydrated water or sesquicarbonate of potassium in the initial mixture could initiate the dehydrofluorination reaction at low temperatures upon thermal treatment and effectively modify the structure of activated carbon to enable performance optimization of the percolative composite.

Low-frequency plasmonic state and tunable negative permittivity in percolative graphite / barium titanate composites

Percolative composites with negative permittivity have attracted widespread attentions due to their great potential in electromagnetic shielding and microwave devices. Targeting at achieving epsilon-negative properties, the percolative graphite/barium titanate (GR/BaTiO 3 ) composite is herein designed and prepared using hot-pressing sintered process. It's found that the plasma oscillations of delocalized electrons result in the epsilon negative permittivity behaviors when the GR contents exceeds the 2 wt% (percolation threshold), and frequency dependence of the negative permittivity which is in well agreement with the Drude model. Meanwhile, it's demonstrated that the ac conductivity represents a typical metal-like behavior as the conductive networks formed within the composites by the increasing GR loadings. Moreover, the equivalent circuit analysis reveals the relationships between capacitive-inductive transition and the conversion of permittivity changing from positive to negative. This work provides effective possibility for developing excellent dielectric properties of percolative GR/BaTiO 3 composite for capacitors and coil-less electrical inductors applications.

Communication—Tunable Negative Permittivity of Cobalt and Epoxy Composites at 3 kHz ∼ 1 MHz Frequency Regions

Negative permittivity in percolation composites has drawn much attention owing to their potential application. This study set out to explore whether tunable negative permittivity can be realized in metal composites at 3 kHz ∼ 1 MHz frequency regions. When the content of cobalt is 80 wt%, the permittivity turns to negative in the test band. These findings show that the Co conductive network has formed and provides large numbers of free electrons. Meanwhile, the percolation composites with negative permittivity show inductive characteristic. This research supports the idea that tunable low-frequency negative permittivity can be realized in metal composites.

Enhanced dielectric properties of acrylic resin elastomer (AE)-based percolative composite with modified MXene

Excellent dielectric properties are essential for electromechanical applications of dielectric elastomer. This work confirms that acrylic resin elastomer (AE)-based percolative composites using MXene (Ti3C2Tx) nanosheets modified with PANI (referred to as PANI-Ti3C2Tx) as conductive fillers exhibit improved dielectric performances. Ti3C2Tx/AE and PANI-Ti3C2Tx/AE composites with different filler contents were prepared, and their percolation threshold are 6.01 vol% and 6.43 vol%, respectively. The dielectric constant of 6.39 vol% PANI-Ti3C2Tx/AE is 298 at 100 Hz, which 1.67 times that of 5.98 vol% Ti3C2Tx/AE composites (178), and is 85 times that of the pure AE matrix (3.5). Moreover, the dielectric loss of 6.39 vol% PANI-Ti3C2Tx/AE composite still maintains at a low level (0.29 at 100 Hz), which is lower than that of 5.98 vol% Ti3C2Tx/AE composite (0.55 at 100 Hz). We suppose that the improved dielectric constant of PANI-Ti3C2Tx/AE composites is attributed to the enhanced micro capacitors effect and Maxwell-Wagner-Sillars effect.

Communication—A Weakly Negative Permittivity of Carbonyl Iron and La-Doped Barium Ferrite Composites at Radio Frequency

Percolative composites with negative permittivity have attracted much attention. For carbonyl iron and La-doped barium ferrite composites, the conduction mechanism changed from hopping conduction to metal-like conduction when carbonyl iron content increased. When carbonyl iron content was 80 wt%, which exceeded but approached the percolation threshold, a weak value of negative permittivity between −200 and 0 was achieved. In addition, the analysis of reactance revealed that inductance characteristic resulted in the negative permittivity. This work provides a way to design the metal composites with a weakly negative permittivity to apply in different fields.

Decoupling of inter-particle polarization and intra-particle polarization in core-shell structured nanocomposites towards improved dielectric performance

Conductive-filler filled polymer composites have exhibited great potential due to the high dielectric constant that can be reached near the percolation threshold, yet the associated high dielectric loss prohibits wide use in practice. Recently, extensive efforts have been devoted to encapsulating conductive fillers with an insulating shell, with the aim to constrain the dielectric loss; but such efforts also significantly reduce dielectric constant. This dilemma raises the question of whether core-shell structured fillers are ultimately beneficial for the dielectric performance of composites, which is less explored. In this study, we investigate polymer nanocomposites containing a series of Al@Al2O3 nanofillers with different shell thicknesses. It shows that the high dielectric constant of percolative composites is contributed by a fast intra-particle polarization and a slow inter-particle polarization. Formation of an insulating shell enables the independent control of the two polarizations, which are otherwise coupled (increasing or decreasing together) in regular percolative composites. By promoting intra-particle polarization and suppressing inter-particle polarization, the core-shell structured nanocomposites can obtain a high dielectric constant and concurrently low dielectric loss, far surpassing the unmodified nanofiller composites. Moreover, the thermal conductivity and high field resistivity are also improved, resulting in a stable and low temperature during operation. This work offers a new paradigm for the design of percolative polymer composite with high dielectric constant and low dielectric loss as well as improved thermomechanical properties.

Tunable piezoresistivity of low percolation threshold micro-nickel wires/PDMS conductive composite regulated by magnetic field

High-performance flexible pressure sensors with percolative composites using aligned μNi wires as conductive fillers for clinical diagnosis.

Epsilon-negative media from the viewpoint of materials science

A comprehensive review of the fundamentals and applications of epsilon-negative materials is presented in this paper. Percolative composites, as well as homogeneous ceramics or polymers, have been investigated to obtain the tailorable epsilon-negative properties. It's confirmed the anomalous epsilon-negative property can be realized in conventional materials. Meanwhile, from the perspective of materials science, the relationship between the negative permittivity and the composition and microstructure of materials has been clarified. It's demonstrated that the epsilon-negative performance is attributed to the plasmonic response of delocalized electrons within the materials and can be modulated by it. Moreover, the potential applications of epsilon-negative materials in electromagnetic interference shielding, laminated composites for multilayered capacitance, coil-less electric inductors, and epsilon-near-zero metamaterials are reviewed. The development of epsilon-negative materials has enriched the connotation of metamaterials and advanced functional materials, and has accelerated the integration of metamaterials and natural materials.

Scaling behavior and variable-range-hopping conduction of localized polarons in percolative BaTiO3-Ni0.5Zn0.5Fe2O4 ceramic composite with colossal apparent permittivity

The percolative BaTiO3-Ni0.5Zn0.5Fe2O4 (BTO-NZFO) ceramic composite represents a family of multifunctional materials exhibiting multiferroic properties and colossal apparent permittivity. It is of fundamental interest to investigate the conduction mechanism in such percolative composites from both macroscopic and microscopic perspectives. Herein, three representative systems with the NZFO content locating below the percolation threshold fc, near fc, and above fc, respectively, were investigated, using pure NZFO ceramic as a comparison. The conductivity of the composite as a function of NZFO content agrees well with the McLachlan model, which takes percolation into consideration and essentially equivalent to the Kirkpatrick model. The electrical conductivity of the composite conforms to Mott's variable-range-hopping (VRH) model in the temperature range of 303–573 K, suggesting that VRH conduction of localized polarons dominates the electrical behavior microscopically. Parameters including the most probable hopping range (R); the density of localized states at the Fermi level [N(EF)]; and the activation energies of VRH (W), dc conductivity (Edc), hopping (Eon), and relaxation (Er) were obtained and analyzed. Scaling behaviors of the conductivity and the imaginary part of complex impedance of the composite have been observed, implying that the distribution of relaxation times is temperature independent. The impedance data measured at different temperatures exhibit typical semiconducting behavior, which can be well fitted by an equivalent circuit model considering both grain and grain boundary responses. The correlation between conductivity and colossal apparent permittivity has also been revealed. The discoveries deepen the understanding of the conduction mechanism in such multifunctional composites composed of an insulating phase and a semiconducting phase.