Insulator-conductor Transition Research Articles

Effect of fractional spectra on conductor-insulator transitions in systems of interacting fermions

Effect of fractional spectra on conductor-insulator transitions in systems of interacting fermions

Control of proton transport and hydrogenation in double-gated graphene

The basal plane of graphene can function as a selective barrier that is permeable to protons1,2 but impermeable to all ions3,4 and gases5,6, stimulating its use in applications such as membranes1,2,7,8, catalysis9,10 and isotope separation11,12. Protons can chemically adsorb on graphene and hydrogenate it13,14, inducing a conductor–insulator transition that has been explored intensively in graphene electronic devices13–17. However, both processes face energy barriers1,12,18 and various strategies have been proposed to accelerate proton transport, for example by introducing vacancies4,7,8, incorporating catalytic metals1,19 or chemically functionalizing the lattice18,20. But these techniques can compromise other properties, such as ion selectivity21,22 or mechanical stability23. Here we show that independent control of the electric field, E, at around 1 V nm−1, and charge-carrier density, n, at around 1 × 1014 cm−2, in double-gated graphene allows the decoupling of proton transport from lattice hydrogenation and can thereby accelerate proton transport such that it approaches the limiting electrolyte current for our devices. Proton transport and hydrogenation can be driven selectively with precision and robustness, enabling proton-based logic and memory graphene devices that have on–off ratios spanning orders of magnitude. Our results show that field effects can accelerate and decouple electrochemical processes in double-gated 2D crystals and demonstrate the possibility of mapping such processes as a function of E and n, which is a new technique for the study of 2D electrode–electrolyte interfaces.

Nonlinearity effects on thermal transport properties of a mass-spring chain

Using Green’s function technique, we present a self-consistent formalism to study the phonon transport properties of an extended nonlinear mass-spring chain. We calculate the phonon transmission coefficient, thermal conductivity, and specific heat for some chains with different configurations of masses feeling the nonlinearity potential. The numerical results show that in a critical value of the nonlinearity coefficient, a sharp decrease in thermal conductivity will be observed. The same scenario happens in a critical temperature proportional to the inverse of the nonlinearity coefficient for the specific heat. Indeed, thermal conductor-insulator transition can occur in the system depending on the strength and distribution of nonlinearity. The model can aid our understanding of the effect of lattice nonlinearity on the thermal properties of one-dimensional materials to design the thermal switches.

Enhanced dielectric and electrical performance of phosphonic acid-modified tantalum (Ta)-doped potassium sodium niobate (KNaNbO3)-P(VDF-HFP) composites.

PA-KNNT-P(VDF-HFP) composite films were synthesized using facile solution casting technique. Due to their wide range of applications in dielectric and electrical systems, phosphonic acid (PA)-modified tantalum-doped potassium sodium niobate (KNNT)-polyvinylidene fluoride co-hexafluoropropylene P(VDF-HFP) composite films have piqued the interest of academic researchers. Microstructural analysis showed that PA layers incorporated onto the KNNT particles within the polymer matrix. The PA-KNNT-P(VDF-HFP) composite exhibited improved dielectric and electrical performance over a broad range of frequency, and the value of the dielectric constant of the P(VDF-HFP) composites is improved by ≈119 over the P(VDF-HFP) matrix at a filler loading 19 wt.%. Moreover, PA-KNNT-P(VDF-HFP) composite also reveals higher dielectric constant (≈ 119) and AC conductivity than P(VDF-HFP)-KNNT composites, while maintaining suppressed dielectric loss ([Formula: see text] at 102Hz). It is also observed that the PA-KNNT-P(VDF-HFP) composite exhibited an insulator-conductor transition with a percolation threshold of fKNNT = 13.4 wt.%. As a result of their exceptional dielectric and electrical characteristics, PA-KNNT-P(VDF-HFP) composites have the potential to find exciting practical applications in a variety of electronic domains.

Temperature- and pressure-controlled insulator-conductor transition performance of flexible fluoroelastomer, tin bismuth alloy and carbon nanotube composites with a sandwich structure

Recently, flexible insulator-conductor transition (ICT) composites have attracted much attention, and they are of great interest for many applications, including smart switches, sensors, and visualized circuits. In this study, flexible ICT composites based on fluorelastomers as the matrix and tin bismuth alloys (SnBi) and multiwalled carbon nanotubes (MWCNTs) as conductive fillers were prepared. The effects of the SnBi alloy and MWCNT contents and the testing temperature and pressure on the ICT properties of the flexible composites were investigated. The cyclic stability of the ICT properties for the composites was evaluated. The microstructure was characterized by scanning electron microscopy (SEM) analysis. The ICT mechanism of the composites was identified. The volume resistivity of the FKM/MWCNT/SnBi composites decreased by 11 orders of magnitude from 109 to 10−2 Ω·cm when the composites were compressed at 140 °C and then recovered to an insulation state after the composites were stretched several times with a strain of 20% at room temperature. The composites showed superior cyclic stability of the ICT performance for up to 100 cycles.

Enhanced dielectric and electrical properties of polystyrene-2% divinyl benzene (PDB) embedded in SrTiO3-Poly (vinylidene fluoride) three phase composite films

A facile solution casting technique has been employed to fabricate ceramic SrTiO3; STO particle and polystyrene-2% divinyl benzene (PDB) filled three phase composite films using poly(vinylidene fluoride) (PVDF) matrix. The frequency dependence of dielectric and electrical properties of the three phase PVDF-STO-PDB composite films of various weight percentage of PDB (fPDB) were analyzed. With the small change in the AC conductivity and dielectric loss the dielectric constant of the PVDF-STO-PDB composite systems is improved noticeably. The experimental results suggested that, the introduction of 10 wt% of PDB particles in the PVDF-STO composites enhanced the dielectric constant (≈76) and suppressed dielectric loss value (< 0.5) at 100 Hz. This may be attributed to the addition of PDB particles improved the establishment of polarization. The morphological analysis depicts that the PDB particles are uniformly dispersed and well compatibility between filler particles and polymer matrix which may be ascribed to the minimization of dielectric loss of the resultant composites. Further, the strong interaction between PDB particles and PVDF matrix at the interface is the crucial factor in the improvement of the dielectric and electrical properties. The percolation theory was used to elucidate the dielectric and electrical performance of PVDF-STO-PDB composites. It is noticed that the three phase PVDF-STO-PDB composite films exhibited an insulator-conductor transition with percolation threshold of fPDB = 5 wt%. The three phase composites with high dielectric constant (≈76) at percolation threshold might be helpful to understand the influence of PDB particles on the interfaces between the ceramics and polymer matrix. This work will provide an easy and effective solution to fabricate high performance three phase composites to the development of high energy storage applications.

Heat Conductor-Insulator Transition in Electrochemically Controlled Hybrid Superlattices.

Designing materials with ultralow thermal conductivity has broad technological impact, from thermal protection to energy harvesting. Low thermal conductivity is commonly observed in anharmonic and strongly disordered materials, yet a microscopic understanding of the correlation to atomic motion is often lacking. Here we report that molecular insertion into an existing two-dimensional layered lattice structure creates a hybrid superlattice with extremely low thermal conductivity. Vibrational characterization and ab initio molecular dynamics simulations reveal strong damping of transverse acoustic waves and significant softening of longitudinal vibrations. Together with spectral correlation analysis, we demonstrate that the molecular insertion creates liquid-like atomic motion in the existing lattice framework, causing a large suppression of heat conduction. The hybrid materials can be transformed into solution-processable coatings and used for thermal protection in wearable electronics. Our work provides a generic mechanism for the design of heat insulators and may further facilitate the engineering of heat conduction based on understanding atomic correlations.

Electrical properties of melt-mixed polypropylene and as-grown carbon nanofiber composites: Analysis of their interphase via the AC conductivity modeling

The morphology, crystallinity, and electrical conductivity (σ′ and σ″) as a function of frequency of polypropylene (PP) melt-extruded with different amounts of as-grown carbon nanofibers (CNFs) from 0 to 1.4 vol. % are examined. The PP/CNF composites present CNF aggregates randomly distributed within the PP and an insulator–conductor transition at CNF contents near 0.9 vol. %. The degree of crystallinity of PP/CNF composites with loadings of 1.4 vol. % increases ∼15% with respect to the neat PP (∼34%), with σ´ ∼ 8.6 × 10−5 S m−1 (σ″ ∼ 8.3 × 10−4 S m−1) at 2 MHz. In addition, the values of the electrical conductivity σint´ ∼2.9 × 10−6 S m−1 (σint″∼3.7 × 10−4 S m−1) at 2 MHz, as a result of the interphase (ϕint ∼0.05 vol. %) of the 1.4 vol. % PP/CNF composites, are estimated by the use of a modified generalized effective medium model (GEM). The analysis gathered in here indicates that the interphase between the polymer and the conducting particle may have a quantifiable effect on the electrical properties of carbon-based polymer composites, and this fact should not be neglected in the production of conducting polymer composites (CPCs) with enhanced electrical properties.

α-Li2TiO3: a new ultrastable anode material for lithium-ion batteries.

Herein, sol-gel-synthesized α-Li2TiO3 was evaluated as a new promising anode material for lithium-ion batteries. The results show ultrastable release of discharge capacity within the range of 290-350 mA h g-1 in 400 cycles. Decent rate performances were also observed. A capacity of ca. 113 mA h g-1 was retained at a current density of 3 C. A 2 × 2 × 1 supercell of the lowest energy ordering structure was used in density functional theory simulations. The calculations show that in the intercalation process, Li+ preferentially enters the tetrahedral voids, leading to the activation of lithium-ion diffusion on the a-b plane with a minimal energy barrier of 0.06 eV (compared with 0.82 eV for the fully charged state). The activation of cation mobility at Li+ intercalation and insulator-conductor transition both contribute significantly to the ultrastability of the material. However, Li+ propagation along the c-axis is highly limited during the whole intercalation process. The enumeration of all the ordering structures on the tetrahedral sites shows two intermediate phases, α-Li2.25TiO3 and α-Li3.0TiO3, as observed from the formation energy convex hull.

Using static linear response theory to describe field emission field enhancement and a field-induced insulator-conductor transition

Experiments on field electron emission (FE) of single carbon nanotubes (CNTs) indicate that they exhibit a nearly linear Fowler–Nordheim plot, and the field enhancement factor (FEF) near the CNT apex is independent of the applied (macroscopic) field (FM) for small radii field emitters. Recent results, based on density functional theory calculations considering CNTs with small radii, retrieved the constancy of the FEF defined in terms of the corresponding induced electron density. As a consequence, it has been reported that the constancy of the FEF with FM could be connected with the linear response of the CNT. In this paper, we reinforce this connection, considering the problem of a floating (6,6) hybrid single-walled nanotube, whose cylindrical body is an insulating one and composed of alternating boron and nitrogen atoms end-capped with carbon atoms. Our results show that the constancy of the FEF is achieved when a linear dependence between the longitudinal component of the induced system dipole moment (μi,z) and FM is observed. Two regimes of constant polarizabilities have been found at sufficiently low and high FM-values. In the intermediate range 0.3V/nm≲FM≲5 V/nm, a crossover from insulating-to-conducting behavior, exhibiting a nonlinear dependence of μi,z on FM, is found accompanied by an increase of the FEF with FM. This result reveals circumstances that could lead to dependence of the FEF on FM, being timely for interpretation of FE characteristics in the context of vacuum nanoelectronic devices.

High fluoride-ion conductivity and fluoride-ion conductor−insulator transition in fluorinated hexagonal boron nitride

Several two-dimensional (2D) materials exhibit a rich variety of transport properties from insulating to metallic conduction, and further to superconductivity. However, almost all charge carriers for ionic conduction have been so far limited to the cations such as sodium ions and protons. Here, we report a superhigh in-plane fluoride-ion conductivity of 200 mS cm−1 at room temperature for 2D fluorinated boron nitride (F-BN). F-BN also exhibits unique fluoride-ion transport at higher temperatures. An ionic conductor–insulator transition (iCIT) is responsible for the temperature variation of fluoride-ion conductivity at higher temperatures. The most drastic iCIT shows a change of approximately six orders of magnitude in conductivity. Neutron diffraction and X-ray photoemission spectroscopy measurements reveal that the mobile fluoride ions in the 2D void space are trapped by the formation of covalent N–F bonds, which is the primary factor influencing the iCIT. Tuning the bonding state yields exciting guidelines for designing fluoride-ion conductors, with broad implications for fluoride-ion batteries, chemical sensors, and heat-responsive on/off-switching electric devices.

Dielectric spectroscopy of melt-extruded polypropylene and as-grown carbon nanofiber composites.

In this work, different weight contents of as-grown carbon nanofibers (CNFs), produced by chemical vapor deposition, were melt-extruded with polypropylene (PP) and their morphologic, structure and dielectric properties examined. The morphologic analysis reveals that the CNFs are randomly distributed in the form of agglomerates within the PP matrix, whereas the structural results depicted by Raman analysis suggest that the degree of disorder of the as-received CNFs was not affected in the PP/CNF composites. The AC conductivity of PP/CNF composites at room temperature evidenced an insulator-conductor transition in the vicinity of 2 wt.%, corresponding to a remarkable rise of the dielectric permittivity up to [Formula: see text] 12 at 400 Hz, with respect to the neat PP ([Formula: see text] 2.5). Accordingly, the AC conductivity and dielectric permittivity of PP/CNF 2 wt.% composites were evaluated by using power laws and discussed in the framework of the intercluster polarization model. Finally, the complex impedance and Nyquist plots of the PP/CNF composites are analyzed by using equivalent circuit models, consisting of a constant phase element (CPE). The analysis gathered in here aims at contributing to the better understanding of the enhanced dielectric properties of low-conducting polymer composites filled with carbon nanofibers.

Vapor-Mediated Stretchable and Reversible Conductors from Microporous Liquid Metal Polymers.

Flexible electronic devices have penetrated into a variety of industry sectors (i.e., consumer electronics, automotive, and medical) in human life, and this calls for better properties of stretchable conductive composites as the crucial elements. Traditionally, conductive inorganic fillers are incorporated in flexible polymers to prepare conductive composites, which falls short of the required properties in more demanding devices nowadays due to limited deformation, low conductivity, and poor processability. Herein, liquid metals were successfully incorporated in microporous polymer matrixes using a simple codissolving and film casting/solvent evaporation approach. The microporous liquid metal-embedded polymer (LMEP) was insulative as fabricated due to discontinuous liquid metals (LMs), while it became conductive upon stretching. Interestingly, the LMEP films showed a reversible insulator-conductor transition due to the regenerated pores in polymer matrix under organic vapor. Negligible changes in the resistance value were seen even after 50 solvent exposure-tensile strain cycles, demonstrating the excellent stability of the electrical properties of these films. Furthermore, most of the commercially available soluble polymers including rigid plastics and soft elastomers are suitable for the fabrication of LMEP. With the ideal characteristics, they have been successfully exploited in model alarm systems to prevent temperature overloads and solvent leakage, showcasing the great potential in next generation sensors used in industry settings.

Temperature dependence of the conductivity of filled diblock copolymers.

We demonstrate that an insulating diblock copolymer system (DBC) filled with conductive fillers can be used as an electrically responsive soft composite that changes its conductivity in response to temperature-induced changes in its morphology. By combining the phase field model describing the morphology of the DBC system with the Monte Carlo simulations and the resistor network model describing electrical properties of the filler network, we calculate the conductivity of this composite. The calculated conductivity is found to essentially depend, in particular, on the temperature of the composite. Changing the temperature is shown to result in morphological changes in the DBC system causing the structural changes in the filler network. In particular, the order-disorder transition in the host DBC system is found to be accompanied by the conductor-insulator transition in the filler network. The effect of the difference between the affinities of the fillers for dissimilar copolymer blocks on the composite conductivity, as well as the effect of the repulsive and attractive interaction between fillers, is considered in detail.

Stretchable, Phase‐Transformable Ionogels with Reversible Ionic Conductor–Insulator Transition

AbstractStretchable iontronics with reversible conductor–insulator transitions are attracting great interest due to their promising applications in various fields of flexible electronics. However, effective conductivity switching of the ion conductors is still challenging. Here, a phase‐transformable ionogel (PTIG) that shows a reversible conductor–insulator transition with a very sharp change in bulk ionic conductivity (≈107 times) at a mild transition temperature of −15 °C is introduced. Moreover, this unique material shows good stretchability (840%), high transparency (97% transmittance in the visible range), and good stability. The ionogel material is used to prepare a long‐storage capacitor, which demonstrates good performance. Hence, the new PTIG has the potential for application in the development of flexible iontronics.

Conductivity of Insulating Diblock Copolymer System Filled with Conductive Particles Having Different Affinities for Dissimilar Copolymer Blocks.

We investigate the electrical response of the insulating diblock copolymer system (DBC) filled with conductive spherical fillers depending on the affinities of these fillers for copolymer blocks and the interaction between fillers. We demonstrate that the contrast (difference) between the affinities of the fillers for dissimilar copolymer blocks is a decisive factor that determines the distribution of these fillers in the DBC system. The distribution of filler particles, in turn, is found to be directly related to the electrical response of the DBC-particle composite. In particular, increasing the affinity contrast above a certain threshold value results in the insulator-conductor transition. This transition is found to be caused by the preferential localization of the fillers in the microphases of the DBC system having larger affinity for these fillers. The effect of the interaction between fillers is found to be secondary to the described effect of the affinity contrast that dominates in determining the distribution of fillers in the composite. This effect of the inter-particle interactions is shown to be significant only when the affinity contrast and filler volume fraction are sufficiently large.

Insulator-conductor transition in carbon nanotube and graphene nanoplatelates reinforced plasma sprayed alumina single splat: Experimental evidence by conductive atomic force microscopy

Splats are the most fundamental and paramount unit of any plasma sprayed coating and splats are having the potential to manipulate the overall property of the same. Current study shows the insulator-conductor transition of alumina (Al2O3) splat by the hybrid addition of carbonaceous nanofillers, namely carbon nanotubes (CNTs) and Graphene nanoplatelets (GNPs) into it. Reinforcement of CNTs and GNPs changed the splat shape from fragmented to nearly disk shaped as well as from porous splat to densified one upon the reinforcement. Further, Current mapping over the splats were carried out using conductive atomic force microscopy (cAFM). Current map for Al2O3 splat showed fully black region, evincing its insulating nature; whereas hybrid addition of CNTs and GNPs into the splat dramatically improved the electrical conductivity of Al2O3 splat, coining its non-conductive map to conductive one. This transition was attributed to electron tunneling mechanism, which is a function of electron hopping distance of these conductive nanofillers. The electron hopping distance of the matrix got minimized when CNTs and GNPs were simultaneously added to Al2O3 splat, due to their higher aspect ratio. This led to the formation of a 3-dimensional conductive channels for the electron transport in the matrix, converting the whole splat as conductive.

Hybrid Composites Based on Thermoplastic Polyurethane With a Mixture of Carbon Nanotubes and Carbon Black Modified With Polypyrrole for Electromagnetic Shielding

Hybrid conducting composites comprising thermoplastic polyurethane (TPU) and mixtures of carbon black modified with polypyrrole (CB-PPy) and carbon nanotubes (CNT) were prepared by melt mixing process. The electrical conductivity, rheological properties and electromagnetic shielding effectiveness (EMI SE) of TPU/CB-PPy and TPU/CNT composites were also carried out and compared to those results observed for TPU/CB-PPy/CNT hybrid composites. TPU/CNT composites show a very sharp insulator-conductor transition and the electrical percolation threshold was about 1 wt% of CNT, which was lower than that found for TPU/CB-PPy (7 wt%). Moreover, EMI SE values of TPU/CNT composites were higher than those for TPU/CB-PPy due to the denser CNT conductive pathway into TPU matrix. In order to achieve the highest electrical conductivity and EMI SE values, considerable amount of CB-PPy was replaced by CNTs. In fact, the electrical conductivity values increased by combining CB-PPy and CNT, resulting in hybrid composites of TPU/CB-PPy/CNT with higher EMI SE values when compared to TPU/CB-PPy composites. The present study demonstrates the potential use of hybrid polymer composites containing 5 wt% or 8 wt% of CB-PPy/CNT at specific CB-PPy/CNT ratios that reach an EMI SE value as high as -20 dB, indicating the potential use of these materials for electromagnetic shielding application in the X-band frequency region.

Graphene/h-BN heterostructures under pressure: From van der Waals to covalent

Scanning probe microscopy and ab initio calculations reveal modifications on the electronic and structural properties of graphene/h-BN heterostructures induced by compression. Using AFM and EFM techniques, with charge injection being made in the heterostructures at different pressures, the charge injection efficiency monotonically decreases with increasing pressure for monolayer-graphene (MLG)+BN heterostructures, indicative of a conductor-insulator electronic transition. Bilayer-graphene (BLG)+BN and trilayer-graphene (TLG)+BN heterostructures show a non-monotonic behavior of charge injection versus pressure, indicative of competing electronic structure modifications. First-principle calculations of these systems indicate a pressure-induced van der Waals-to-covalent interlayer transition, where such interlayer covalent binding, in the presence of water molecules, results in a disordered insulating structure for the MLG + BN case, while it leads to an ordered conducting structure for both BLG + BN and TLG + BN heterostructures. These opposing effects may have a strong influence on graphene/h-BN-based electronic devices and their physics under pressurized environments.

Multiscale study of influence of interfacial decohesion on piezoresistivity of graphene/polymer nanocomposites

A multiscale strategy is proposed to study the role of interfacial decohesion on the piezoresistive properties of a graphene/polymer nanocomposite. The piezoresistive effect is a change in the electrical resistivity when mechanical strain is applied. First, a cohesive zone model is identified by atomistic simulations. This cohesive zone model enriches imperfect interfaces, which model graphene sheet, at mesoscale in our continuum mechanical model. This nonlinear mechanical model is used to generate deformed representative volume element to study the influence of strain and interfacial decohesion on the conductivity of graphene/polymer nanocomposites. The effective conductivity is studied with an electric continuum model at mesoscale that incorporates the tunneling effect. A conductor-insulator transition is observed for elongations above 2% for graphene volume fraction just above the percolation threshold. The transition appears for an elongation of 8% instead of 2%, when the interfacial decohesion is neglected.