Transport In Nanocomposites Research Articles

Development and Characterization of Ethylene‐Vinyl Acetate‐Graphene Nanocomposites

AbstractThe prospective applications of carbon graphene nanoplatelets (GNPs) reinforced ethylene vinyl acetate (EVA) nanocomposites in flexible electronics, energy storage devices, and thermal management systems have attracted a lot of attention. These nanocomposites offer an effective solution for enhancing the electrical conductivity of interfacing materials in electronic systems and addressing issues related to thermal management. GNPs that possess excellent thermal conductivity of 2000–4000 W m−1 K−1 and an electrical conductivity of 107 S m−1 are utilized in this investigation. The homogeneous dispersion of nanofiller and the effective load transfer between components through strong filler/polymer interfacial interactions are essential for the creation of multifunctional polymer nanocomposites. Particularly, their electrical conductivity and dielectric properties are highlighted in this study, with an emphasis on the synthesis and characterization of nanocomposite materials. This work investigates the utilization of graphene nanocomposites in EVA using a melt‐mixing technique assisted by sonication to fabricate multifunctional composites. Incorporating different weight percentages of GNPs (ranging from 0.1 to 0.4 wt.%) within the polymer matrix is carried out. In the present study, the nanocomposites are characterized by scanning electron microscopy investigation and impedance spectrum analysis. The distinctive characteristics of the AC electrical transport in nanocomposites are investigated between 10 Hz and 1 MHz. Excellent interfacial compatibility has been demonstrated by the homogenous distribution of GNPs within the matrix, confirmed by SEM examination. These EVA nanocomposites demonstrate improved electrical conductivity, thereby rendering them very useful for energy storage applications, such as electronic capacitors, wherein supports with a significant dielectric constant and minimal dielectric loss are required.

GO induced grain-boundary modification in transparent TiO2-GO nanocomposite thin films: Study by DC bias dependent impedance spectroscopy

Here we present the study on GO induced grain boundary modification of solution-processed transparent TiO2-GO nanocomposite thin films by dc bias dependent impedance spectroscopy. The experimental Nyquist plot obtained from impedance spectra is fitted by an appropriate model electrical circuit comprised of two parallel R-C circuits connected with a series resistance in order to evaluate different contributions arising from core grains and grain boundaries. Variation of relaxation times (τ = RC) with external bias and its modification after GO reinforcement confirms grain boundary modification and its influence on charge transport in nanocomposites.

Invariable resistance of conductive nanocomposite over 30% strain.

The dependence of the electrical resistance on materials’ geometry determines the performance of conductive nanocomposites. Here, we report the invariable resistance of a conductive nanocomposite over 30% strain. This is enabled by the in situ–generated hierarchically structured silver nanosatellite particles, realizing a short interparticle distance (4.37 nm) in a stretchable silicone rubber matrix. Furthermore, the barrier height is tuned to be negligible by matching the electron affinity of silicone rubber to the work function of silver. The stretching results in the electron flow without additional scattering in the silicone rubber matrix. The transport is changed to quantum tunneling if the barrier height is gradually increased by using different matrix polymers with smaller electron affinities, such as ethyl vinyl acetates and thermoplastic polyurethane. The tunneling current decreases with increasing strain, which is accurately described by the Simmons approximation theory. The tunable transport in nanocomposites provides an advancement in the design of stretchable conductors.

Enhanced thermoelectric ZT in the tails of the Fermi distribution via electron filtering by nanoinclusions: Model electron transport in nanocomposites

Silicon carbide nanoparticles with diameters around 8 nm and narrow size distribution have been finely mixed with doped silicon nanopowders and sintered into bulk samples to investigate the influence of nanoinclusions on electrical and thermal transport properties. We have compared the thermoelectric properties of samples ranging from 0%--5% volume fraction of silicon carbide. The silicon carbide nanoinclusions lead to a significant improvement in the thermoelectric figure of merit ZT largely due to an enhancement of the Seebeck coefficient. A semiclassical Boltzmann transport equation is used to model the electrical transport properties of the Seebeck coefficient and electrical conductivity. The theoretical analysis confirms that the enhancements in the thermoelectric properties are consistent with the energy selective scattering of electrons induced by the offset between the silicon Fermi level and the carbide conduction band edge. This study proves that careful engineering of the energy-dependent electron scattering rate can provide a route towards relaxing long-standing constraints in the design of thermoelectric materials.

Universal temperature corrections to the conductivity of niobium-carbon nanocomposites

The evolution of wide-temperature range (4.2–300 K) electron transport in niobium-carbon nanocomposites was studied at niobium concentration range 0.15–0.35. It was found that electron transport in the nanocomposites has the features of universality, being expressed in the existence of two characteristic temperature intervals on the temperature dependences of conductivity. The crossover temperature between the intervals is in the range 20–30 K. Within each temperature interval, corrections to the conductivity are found to be as power-like ones. Power exponent p is characterized by the non-monotonic dependences on niobium concentration and varies in the ranges 0.5–1.4 and 0.2–1.4 in the low- and high-temperature intervals with a minimum at 0.30 and 0.27 of Nb content, respectively. The satisfactory description of electron transport in niobium-carbon nanocomposites was achieved within the model of the inelastic tunneling of the electrons between the metal grains in the framework of the effective medium approximation.

Structure and electrical transport properties of carbon nanofibres/carbon nanotubes 3D hierarchical nanocomposites: Impact of the concentration of acetylacetonate catalyst

The aim of this study was an extensive analysis of the correlations linking the structure with the electrical properties of hierarchical nanocomposites – electrospun carbon nanofibres/carbon nanotubes (eCNF/CNT). Herein, we focus primarily on the determination and separation of the impact of iron (III) acetylacetonate (Fe(Acac)3) on the structure of core-eCNFs from the overall effect it exerts on the global ordering and electrical properties of nanocomposites. The structure of materials was evaluated using highly local microscopic and diffraction techniques as well as global spectroscopic methods. The charge transport properties were determined through analysis of the temperature-dependent conductivity via Mott's variable-range hopping model. The investigation revealed that increasing concentration of Fe(Acac)3 results in higher surface density of CNTs, which affects the electrical transport in nanocomposites positively (158% increase in σ298K; notable decline in T0). However, it was proved that high catalyst concentrations simultaneously cause amorphisation of core-eCNFs and increase the activation energy of hopping conduction in them. As a consequence of the above, we estimated the concentration of Fe(Acac)3 (~3.0%), ensuring the best electrical properties. Additionally, it was demonstrated that desorption of electrically active guest molecules causes notable changes in the electronic transport in nanocomposites.

Graphene Wrinkles affect electronic transport in nanocomposites: Insight from molecular dynamics simulations

Molecular dynamics (MD) simulations were carried out to study the physical properties of graphene-oxide (GO) and polydimethylsiloxane (PDMS) interfacial systems. Simulations were performed for GO molecules dispersed into short-chain, long-chain, and long-chain and cross-linked PDMS polymers. Various structural properties, dipole moments and dielectric constants of the graphene-oxide molecules were calculated, which were correlated with the electron transport properties of the GO/PDMS system. The effects of polymer length and type of linkage on transport properties were also examined.

Electronic Transport in Alloys with Phase Separation (Composites)

A measure for the efficiency of a thermoelectric material is the figure of merit defined by ZT = S2T/ρκ, where S, ρ and κ are the electronic transport coefficients, Seebeck coefficient, electrical resistivity and thermal conductiviy, respectively. T is the absolute temperature. Large values for ZT have been realized in nanostructured materials such as superlattices, quantum dots, nanocomposites, and nanowires. In order to achieve further progress, (1) a fundamental understanding of the carrier transport in nanocomposites is necessary, and (2) effective experimental methods for designing, producing and measuring new material compositions with nanocomposite-structures are to be applied. During the last decades, a series of formulas has been derived for calculation of the electronic transport coefficients in composites and disordered alloys. Along the way, some puzzling phenomenons have been solved as why there are simple metals with positive thermopower? and what is the reason for the phenomenon of the “Giant Hall effect”? and what is the reason for the fact that amorphous composites can exist at all? In the present review article, (1), formulas will be presented for calculation of σ = (1/ρ), κ, S, and R in composites. R, the Hall coefficient, provides additional informations about the type of the dominant electronic carriers and their densities. It will be shown that these formulas can also be applied successfully for calculation of S, ρ, κ and R in nanocomposites if certain conditions are taken into account. Regarding point (2) we shall show that the combinatorial development of materials can provide unfeasible results if applied noncritically.

Influence of oxide matrix on electron transport in (FeCoZr)x(Al2O3)1-x nanocomposite films

Electron transport in (Fe0.45Co0.45Zr0.10)x(Al2O3)1-x granular nanocomposites (NCs) produced by the ion-beam sputtering of compound target was studied in the temperature range of 2–300 K. Conductivity of the films synthesized in the inert (Ar) atmosphere is determined below percolation threshold by the thermally activated electron tunneling over metallic granules at low temperatures and replaced by the Mott variable range hopping (VRH) with increasing temperature. Introduction of oxygen to the sputtering chamber suppresses VRH leading to retention of the thermally activated tunneling in the whole studied temperature range for the metallic phase atomic concentrations up to х = 0.62. The model of thermally activated tunneling over metallic granules gives an excellent agreement with experimental data when alumina matrix permittivity is considered as increasing with concentration of the metallic phase fraction in the films, which is due to increase in number of localized electronic states. The established influence of the sputtering atmosphere on the electron transport in nanocomposites is explained by reduction of concentration of the defects in matrix when oxygen is added to the sputtering atmosphere.

CTViz: A tool for the visualization of transport in nanocomposites

A visualization tool (CTViz) for charge transport processes in 3-D hybrid materials (nanocomposites) was developed, inspired by the need for a graphical application to assist in code debugging and data presentation of an existing in-house code. As the simulation code grew, troubleshooting problems grew increasingly difficult without an effective way to visualize 3-D samples and charge transport in those samples. CTViz is able to produce publication and presentation quality visuals of the simulation box, as well as static and animated visuals of the paths of individual carriers through the sample. CTViz was designed to provide a high degree of flexibility in the visualization of the data. A feature that characterizes this tool is the use of shade and transparency levels to highlight important details in the morphology or in the transport paths by hiding or dimming elements of little relevance to the current view. This is fundamental for the visualization of 3-D systems with complex structures. The code presented here provides these required capabilities, but has gone beyond the original design and could be used as is or easily adapted for the visualization of other particulate transport where transport occurs on discrete paths.

Nanocomposites for thermoelectrics and thermal engineering

Abstract

Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications

ABSTRACTSilicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocomposites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures, while for nanowires we use the Monte Carlo method to solve the phonon Boltzmann equation. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite and at the boundaries of nanowires, we employ a momentum-dependent specularity parameter. Our results show thermal transport in SiGe nanocomposites and nanowires is reduced significantly below their bulk alloy counterparts.

Lattice Thermal Transport in Si-based Nanocomposites for Thermoelectric Applications

Silicon-germanium (SiGe) superlattices (SLs) have been studied for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so Si-based nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Because the lattice thermal conductivity of SiGe SLs is reduced by scattering from rough boundaries between layers, it is expected that grain boundary properties, for example roughness, orientation, and composition, will also substantially effect thermal transport in nanocomposites, resulting in many ways of adjusting their thermal conductivity by manipulation of grain size, shape, and crystal angle distributions. A model of phonon transport in nanocomposites was developed on the basis of the phonon Boltzmann transport equation. When nanocomposite structures were modeled by using a Voronoi tessellation to mimic the grains and their distribution, agreement with experimentally observed structures was excellent. To accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we used a momentum-dependent specularity variable. Our results revealed thermal transport in Si-based nanocomposites is highly anisotropic and suggest further utilization of grain morphology to minimize thermal conductivity.

Polymer-mediated tunneling transport between carbon nanotubes in nanocomposites.

Electron transport in nanocomposites has attracted a good deal of attention for some time now; furthermore, the ability to control its characteristics is a necessary step in the design of multifunctional materials. When conductive nanostructures (for example carbon nanotubes) are inserted in a non-conductive matrix, electron transport below the percolation threshold is dominated by tunneling and thus the conductive characteristics of the composite depends heavily on the characteristics of the tunneling currents between nanoinserts. A parameter-free approach to study tunneling transport between carbon nanotubes across a polymer matrix is presented. The calculation is done with a combination of Density Functional Theory and Green functions (an approach heavily used in molecular electronics) which is shown here to be effective in this non-resonant transport condition. The results show that the method can effectively capture the effect of a dielectric layer in tunneling transport. The current is found to exponentially decrease with the size of the gap for both vacuum and polymer, and that the polymer layer lowers the tunneling barrier enhancing tunneling conduction. For a polyacrylonitrile matrix, a four-fold decrease in the tunneling constant, compared to tunneling in vacuum, is observed, a result that is consistent with available information. The method is very versatile as any DFT functional (or any other quantum mechanics method) can be used and thus the most accurate method for each particular system can be chosen. Furthermore as more methods become available, the calculations can be revised and improved. This approach can be used to design functional materials for fine-tunning the tunneling transport, for instance, the effect of modifying the nanoinsert-matrix interface (for example, by adding functional groups to carbon nanotubes) can be captured and the comparative performance of each interface predicted by simulation.

Molecular dynamics studies of thermal boundary resistance at carbon–metal interfaces

Molecular dynamics is used to study the interfacial thermal conductance between graphitic structures and metals. It is shown that with different metals, the conductance can vary by ∼4-fold, allowing the control of thermal transport in nanocomposites and nanoelectronic devices. The experimental values of conductance are higher by 10–20 MW m−2 K−1 compared to simulations. We suggest that in addition to lattice vibrations, an electromagnetic coupling between Johnson–Nyquist electric currents in the metal and graphite may contribute to the interfacial thermal conductance.

Size-Dependent Lattice Thermal Conductivity of Nanostructured Bulk Semiconductors

Recently, nanostructuring of bulk semiconductors has emerged as an effective approach to develop high-efficiency thermoelectric materials for large-scale device applications, where the thermal conductivity reduction predominates in the enhanced figure of merit of these materials. In this work, a quantitative nanothermodynamic model was established to calculate the lattice thermal conductivity of semiconductor nanocomposites considering the interface scattering effects. It is found that the lattice thermal conductivity can be significantly reduced in nanostructured bulk semiconductors compared with their bulk counterparts. The findings in this work may provide new insights into the fundamental understanding of phonon transport in nanocomposites and also the development of high-performance thermoelectric materials.

Semiclassical model for thermoelectric transport in nanocomposites

Nanocomposites (NCs) has recently been proposed and experimentally demonstrated to be potentially high-efficiency thermoelectric materials by reducing the thermal conductivity through phonon-interface scattering and possibly by increasing the Seebeck coefficient through energy-selective carrier scattering (low-energy filtering) or quantum-size effect of electrons. In this paper, we develop a Boltzmann transport equation based semiclassical electron transport model to describe the thermoelectric transport processes in semiconductor NCs. This model considers multiband transport of electrons and holes with both the intrinsic carrier scatterings and the carrier-interface scattering. A relaxation-time model is developed for carrier-interface scattering. After fitting the model with bulk thermoelectric alloys that gives reasonable material input parameters for bulk alloys, which are close to handbook values, the model is further validated by comparing the modeled thermoelectric properties with recently reported measurement values of thermoelectric properties in high efficiency NCs. The model is then applied to predict thermoelectric properties of both the particle-host-type and the particle-particle-type semiconductor NCs such as the $p$-type $({\text{Bi}}_{\text{y}}{\text{Sb}}_{2\text{-y}}{\text{Te}}_{3})\text{\ensuremath{-}}({\text{Bi}}_{0.5}{\text{Sb}}_{1.5}{\text{Te}}_{3})$ NCs and the $n$-type $({\text{Mg}}_{2}{\text{Si}}_{\text{y}}{\text{Ge}}_{1\text{-y}})\text{\ensuremath{-}}({\text{Mg}}_{2}{\text{Si}}_{0.6}{\text{Ge}}_{0.4})$ NCs. The dependence of thermoelectric transport coefficients on the size of nanoconstituent, doping concentration and temperature are studied. Our study could shed some light to optimally design high-efficiency thermoelectric NCs which could contribute to solar-thermal utilization or waste heat recovery.

Ion transport in nanocomposites

Successive analysis of interface defect formation and its implications for the ionic conductivity was carried out for ionically conducting nanocomposites that hold much significance for technological applications. It was shown that the most efficient way to prepare such composites is through the use of nanosized particles. A review was made of the current state of research on inorganic composites and hybrid membrane materials that are already in use in alternative power engineering.

Modeling study of thermoelectric SiGe nanocomposites

Nanocomposite thermoelectric materials have attracted much attention recently due to experimental demonstrations of improved thermoelectric properties over those of the corresponding bulk material. In order to better understand the reported data and to gain insight into transport in nanocomposites, we use the Boltzmann transport equation under the relaxation-time approximation to calculate the thermoelectric properties of $n$-type and $p$-type SiGe nanocomposites. We account for the strong grain-boundary scattering mechanism in nanocomposites using phonon and electron grain-boundary scattering models. The results from this analysis are in excellent agreement with recently reported measurements for the $n$-type nanocomposite but the experimental Seebeck coefficient for the $p$-type nanocomposite is approximately 25% higher than the model's prediction. The reason for this discrepancy is not clear at the present time and warrants further investigation. Using new mobility measurements and the model, we find that dopant precipitation is an important process in both $n$-type and $p$-type nanocomposites, in contrast to bulk SiGe, where dopant precipitation is most significant only in $n$-type materials. The model also shows that the potential barrier at the grain boundary required to explain the data is several times larger than the value estimated using the Poisson equation, indicating the presence of crystal defects in the material. This suggests that an improvement in mobility is possible by reducing the number of defects or reducing the number of trapping states at the grain boundaries.

Thermal conductivity modeling of compacted type nanocomposites

Due to different interface densities and arrangements, the compacted type nanocomposites may yield even lower thermal conductivity than embedded type nanocomposites. In this paper, the phonon transport and thermal conductivity in compacted type nanocomposites (nanowires and nanoparticles) are investigated using a deterministic phonon Boltzmann transport equation solver. The effects of interface density and characteristic size on the phonon energy transport in nanocomposites are studied. It is found that the silicon-germanium compacted nanoparticle composites can have lower value of thermal conductivity than that of compacted nanowire composites under the same characteristic size (21.6% lower when the characteristic size is 3 nm).