Anisotropic Conductivity Properties Research Articles

Anisotropic thermally conductive composite with wood-derived carbon scaffolds

In this study, we reported a thermally conductive composite with wood-derived 3D scaffold. The composites were easily fabricated by impregnating the polyamide-imide (PAI) into the wood scaffold and then in-situ carbonized. The well-aligned cellulose mircochannels in natural wood are maintained during the carbonization, leading to the improved anisotropic thermal conductive properties of the composite. The thermal conductivity of the composite reached 0.56 W·m−1·K−1 in cross-plane direction increasing by 250% and 0.22 W·m−1·K−1 in parallel direction, increasing by 37.5%, respectively. The unique structure of the composite also plays an important role in enhancing the mechanical performance. The Young’s modulus of the composite in vertical direction reached to 451.9 MPa, 4-times higher than that of natural wood in the same orientation. The integrated performance of the composites could be attributed to the alignment of cellulose nanofibers inherited from the natural wood. This study will provide an innovative design and fabrication of composite for thermal management.

Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba2In2O5 Thin Films

Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba2In2O5 (BIO) exhibits both oxygen ion and proton conduction, in dry and humid environment, respectively. In dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers. To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here the proton and oxygen ion conductivity along different crystallographic directions. Using epitaxial thin films with different crystallographic orientations the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained through the layered structure of the oxygen sublattice in brownmillerite BIO. The anisotropic proton conduction however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chain-like or layered structures.

Large anisotropic thermal conductivity and excellent thermoelectric properties observed in carbon foam

Using first-principles calculations in combination with the Boltzmann transport equation and empirical potential models, we investigate the thermal transport and thermoelectric properties of carbon foam. The results show that large anisotropic thermal conductivity can be observed in carbon foam. The Z direction of carbon foam has the highest thermal conductivity of 22.97 W/m K at room temperature, which is about 25 times greater than that of the Y direction. This is due to the lower phonon group velocity and stronger anharmonic interaction in the Y direction than those in the Z direction. Moreover, we find that the carbon form has excellent thermoelectric properties in the Y direction, the ZT can reach 0.5 at room temperature, and the ZT can be further improved to 0.84 at 800 K, which is two orders of magnitude higher than that in the Z direction. This results from the large power factor and ultralow thermal conductivity in the Y direction.

A Versatile Method to Fabricate Highly In‐Plane Aligned Conducting Polymer Films with Anisotropic Charge Transport and Thermoelectric Properties: The Key Role of Alkyl Side Chain Layers on the Doping Mechanism

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high‐temperature rubbing with a soft‐doping method based on spin‐coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3‐alkylthiophene)s and poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) yields highly oriented conducting polymer films that display polarized UV–visible–near‐infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV–vis–NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F4TCNQ− anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm−1 and S = 60 ± 2 µV K−1). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.

Anisotropic thermal conductivity and mechanical properties of phagraphene: a molecular dynamics study

In-plane anisotropy in the thermal conductivity of phagraphene.

Anisotropic optical and conductive properties of oriented 1D-nanoparticle thin films made by spray-assisted self-assembly.

We report on the fabrication of oriented anisotropic metal nanoparticle thin films made by Grazing Incidence Spraying (GIS) and on the anisotropic plasmonic properties of the resulting thin films. Gold nanorods of two different aspect ratios and silver nanowires were self-assembled as a uniaxially aligned monolayer with the GIS approach. In particular, we examine the influence of the nanowire/nanorod length and diameter on the degree of ordering determined by electron microscopy pictures. Furthermore, we show that the anisotropy of the optical properties (probed by polarized UV-visible-near infrared spectroscopy) strongly depend on the quality of alignment. The prepared monolayer thin films have an orientation order parameter of up to 0.83 for silver nanowires, which is reflected in an optical anisotropy of 0.57 in the UV-visible and 0.76 in the near infrared through the selective excitation of transverse and longitudinal surface plasmon resonance modes. The electronic transport in oriented silver nanowire monolayers is also shown to be highly directional, with the sheet resistance varying over almost an order of magnitude depending on the transport direction. Such anisotropic conductive plasmonic thin films may find applications in various fields like biochemical sensing, energy transport and harvesting or optoelectronic devices.

Anisotropic electrical conductivity and dielectric properties of LiTaO3 crystals in the temperature range 290–900 K

We have studied anisotropic electrical conductivity and dielectric properties of a LiTaO3 crystal in the temperature range 290–900 K. The anisotropy in its dielectric characteristics is associated with specific features of dielectric relaxation in the polar and nonpolar directions of the crystal. In the temperature range 290–450 K, the electrical conductivity in a nonpolar direction slightly exceeds that in the polar direction and there is anisotropy in electron mobility μe. At temperatures from 600 to 900 K, conductivity anisotropy shows up in both the magnitude of conductivity and the energetic and kinetic characteristics of charge transport processes.

Reconstruction of apparent orthotropic conductivity tensor image using magnetic resonance electrical impedance tomography

Magnetic resonance electrical impedance tomography visualizes current density and/or conductivity distributions inside an electrically conductive object. Injecting currents into the imaging object along at least two different directions, induced magnetic flux density data can be measured using a magnetic resonance imaging scanner. Without rotating the object inside the scanner, we can measure only one component of the magnetic flux density denoted as Bz. Since the biological tissues such as skeletal muscle and brain white matter show strong anisotropic properties, the reconstruction of anisotropic conductivity tensor is indispensable for the accurate observations in the biological systems. In this paper, we propose a direct method to reconstruct an axial apparent orthotropic conductivity tensor by using multiple Bz data subject to multiple injection currents. To investigate the anisotropic conductivity properties, we first recover the internal current density from the measured Bz data. From the recovered internal current density and the curl-free condition of the electric field, we derive an over-determined matrix system for determining the internal absolute orthotropic conductivity tensor. The over-determined matrix system is designed to use a combination of two loops around each pixel. Numerical simulations and phantom experimental results demonstrate that the proposed algorithm stably determines the orthotropic conductivity tensor.

Anisotropic thermal conductive properties of hot-pressed polystyrene/graphene composites in the through-plane and in-plane directions

Abstract The interactions between polystyrene (PS) and graphene as well as the factors leading to anisotropic thermal conductive properties in the in-plane and through-plane directions of the hot-pressed samples were studied. In this procedure, the PS/graphene composites (PG) were prepared via solution mixing followed by hot-pressing method, the graphene oxide (GO) was simultaneously modified and reduced to graphene nanosheets by p-phenylene diamine. And the polydispersity index of PS matrix was controlled at as narrow as 1.14 by reversible addition–fragmentation chain transfer (RAFT) polymerization. The characterization showed that the thermal conductivity of PG composites with 10 wt% graphene loading was enhanced by 66% to 0.244 W m−1 K−1 compared with 0.147 W m−1 K−1 of pure PS. The thermal diffusivity (α) of hot-pressed PG samples showed anisotropic behavior according to the heat transfer directions. The α of in-plane direction was almost ten times higher than that of in through-plane direction. And both of them changed slowly in function of graphene loadings. This anisotropic property may be caused by the ordered arrangement of graphene sheets and PS chains after PG composites were hot-pressed.

Parallel carbon nanotube stripes in polymer thin film with tunable microstructures and anisotropic conductive properties

It is known that shear-flow can induce units to assemble into vorticity-aligned stripe-structures in confined geometries. This study shows that the microstructure and the property of the stripe in polymer thin film can be well tuned by adjusting the viscosity ratio between dispersed phase and continuous phase. Polypropylene (PP)/poly(styrene–ethylene/butadiene–styrene) (SEBS)/octadecylamine functionalized multiwalled carbon nanotubes (ODA-MWCNTs) composites with different viscosity ratios were prepared by either pre-compounding ODA-MWCNT into PP or SEBS in a microcompounder. Under the induction of shear-flow, ODA-MWCNT and SEBS spontaneously assembled into vorticity-aligned stripes in PP thin films for all the composites with different viscosity ratios, resulting in the property of conductive anisotropy for the film. Interestingly, it was found that both the microstructures and the electrical properties of MWCNT stripes in PP thin films prepared from the composites with different viscosity ratios were significantly different.

Anisotropic highly-conductive films of poly(3-methylthiophene) from epitaxial electropolymerization on oriented poly(vinylidene fluoride)

Electrochemical polymerization of 3-methylthiophene on highly oriented poly(vinylidene fluoride) (PVDF) film was achieved by cyclic voltammetry to yield well-ordered poly(3-methylthiophene) (P3MT) thin films with anisotropic structural and conductivity properties. The conductivity of P3MT along the direction perpendicular to the chain direction of PVDF, after electrochemical dedoping, is 59 ± 3 S cm−1, while that along the PVDF chain direction is 1.2 ± 0.4 S cm−1. The high conductivity of the P3MT is attributed to the well-ordered structure with its flat-on single crystals as confirmed by electron diffraction and Reflection Absorption Infra-red Spectroscopy (RAIRS). The data are consistent with P3MT chains aligned with π–π stacking perpendicular to the chain direction of the PVDF substrate. Epitaxial electropolymerization is an unusual method of preparing highly ordered thin films of organic semiconductors.

Phase change material with graphite foam for applications in high-temperature latent heat storage systems of concentrated solar power plants

A high-temperature latent heat thermal energy storage (LHTES) system was analyzed for applications to concentrated solar power (CSP) plants (utilizing steam at ∼610 °C) for large-scale electricity generation. Magnesium chloride was selected as the phase change material (PCM) for the latent heat storage because of its high melting point (714 °C). Because the thermal conductivities of most salt materials are very low, usually less than 1 W/m K, graphite foam was applied as an additive to considerably enhance the overall thermal conductivity of the resulting graphite foam–PCM combination in the LHTES system. The heat transfer performance and the exergy efficiency in the graphite foam–MgCl2 LHTES system were considered for the design and optimization of the storage system. Three-dimensional (3-D) heat transfer simulations were conducted for the storage system using commercial software COMSOL. Three groups of analyses were performed for an LHTES system: using PCM alone without graphite foam, using average material properties for graphite foam–PCM combination, and using anisotropic thermal conductivity and temperature-dependent material properties for graphite foam–PCM. Results presented show that the graphite foam can help to significantly improve the heat transfer performance as well as the exergy efficiency in the LHTES system. They also show the effects of the anisotropic thermal conductivity and indicate capital cost savings for a CSP electric power plant by reducing the number of heat transfer fluid (HTF) pipes in the LHTES tank by a factor of eight.

Hybrid hydrogels containing vertically aligned carbon nanotubes with anisotropic electrical conductivity for muscle myofiber fabrication

Biological scaffolds with tunable electrical and mechanical properties are of great interest in many different fields, such as regenerative medicine, biorobotics, and biosensing. In this study, dielectrophoresis (DEP) was used to vertically align carbon nanotubes (CNTs) within methacrylated gelatin (GelMA) hydrogels in a robust, simple, and rapid manner. GelMA-aligned CNT hydrogels showed anisotropic electrical conductivity and superior mechanical properties compared with pristine GelMA hydrogels and GelMA hydrogels containing randomly distributed CNTs. Skeletal muscle cells grown on vertically aligned CNTs in GelMA hydrogels yielded a higher number of functional myofibers than cells that were cultured on hydrogels with randomly distributed CNTs and horizontally aligned CNTs, as confirmed by the expression of myogenic genes and proteins. In addition, the myogenic gene and protein expression increased more profoundly after applying electrical stimulation along the direction of the aligned CNTs due to the anisotropic conductivity of the hybrid GelMA-vertically aligned CNT hydrogels. We believe that platform could attract great attention in other biomedical applications, such as biosensing, bioelectronics, and creating functional biomedical devices.

Transparency Enhancement for Photoinitiated Polymerization (UV Curing) through Magnetic Field Alignment in a Piezoresistive Metal/Polymer Composite

We use a magnetic field to align nickel particles into stringlike assemblies in urethane oligomer mixtures and create a semitransparent UV-curable nickel particle/polymer composite with anisotropic electrical conductivity and piezoresistive properties. When the particles are uniformly distributed in the oligourethane matrix, the mixture is moderately conductive at higher particle fractions but becomes insulating once the fraction is below about 5 vol %. With the particle fraction below this threshold and using an external magnetic field, the particles are aligned into continuous pathways through the oligomer mixtures following the magnetic flux lines. The matrix is subsequently cured by UV light. This results in conductivity and piezoresistivity along the alignment direction, while the material is not conducting perpendicular to the alignment direction. The lower particle fraction results in a lower number of absorbers for UV light: the decrease from 5 to 1 vol % increases optical transmission from 10% to 50% in the UV/vis region. This leads to a shorter photocuring time, typically from tens of seconds to seconds for 300-μm-thick films at a wavelength of 365 nm. We propose that this concept could be applied in areas such as pressure sensors.

A three-dimensional multi-physics model for a Li-ion battery

A multi-geometry and multi-physics model is developed for a Li-ion battery module which includes three cells connected in series by electrical busbars. The model can be used to predict the 3D profiles of the electrical potentials and temperature in the battery. The physics-based porous electrode (P2D) model is used to predict the electrochemical behavior of the cells, and the coupling between the P2D model and the electrical/thermal equations is simplified through a linear approximation method. This approximation is useful at rates of at least 5C and reduces the computation time significantly. The anisotropic conductive properties for the regions where conductors are separately arrayed are discussed in detail, and the model predictions are presented and discussed.

Effect of Anisotropic Conductive Properties on Heat Transfer and Temperature Distribution of Coatings and Substrates

Thermal conductivity was the main thermal parameter that greatly influenced temperature distribution of materials especially in coatings. Some materials such as plasma-sprayed coatings that had a lamellar microstructure displayed the anisotropy of thermal conductivity. Thermal conductivities of isotropy and anisotropy metallic ceramic coatings were calculated and analyzed using Markworth method and parallel plate model, respectively. The influence of anisotropy on heat transfer and temperature distribution of coatings using finite element analysis and Gaussian heat source modal applied locally was investigated. Results showed that in anisotropic coatings the ratio of transverse and vertical thermal conductivity had a maximum value when metal volume fraction was 50%. The temperatures on the edge of heat sources (Th) had a maximum value, and the highest temperatures of substrate (Ts) had a minimum value with the same fraction. The value decreased by 79°C compared to that in the isotropy coatings. The highest temperatures of coatings (Tc) decreased expectedly with metal volume fraction rised. The maximum width of heat affected zone (HAZ) of coating and substrate was 4.52 mm at the metal content of 40%. The minimum HAZ depth was 0.27mm which was very small at the content of 50%, compared with 1.25 mm in isotropic coatings. It revealed that anisotropic conductive properties caused more heat transfer to outer regions and provided better protection for substrate.

Plasma-sprayed ceramic coatings: anisotropic elastic and conductive properties in relation to the microstructure; cross-property correlations

Effective anisotropic elastic stiffnesses and thermal conductivities of the plasma sprayed ceramic coating are calculated in terms of the relevant microstructural parameters. The dominant features of the porous space are identified as strongly oblate (crack-like) pores that tend to be either parallel or normal to the substrate. ‘Irregularities’ of the microstructure — the scatter in pore orientations — are shown to have a pronounced effect on the effective properties. The explicit elastic-conductive cross-property correlations are derived.

Finite element model determination of correction factors used for measurement of aortic diameter via conductance.

Traditional methods for estimating the slope alpha and offset volume Vp for determining real-time chamber volume by the conductance catheter technique are not suited to measurements made in the aorta due to the relatively low resistivity of the aortic wall. We developed three distinct three-dimensional finite element models of the conductance catheter and surrounding tissues in order to predict alpha and Vp and to examine the nature of the electric field near the aortic wall. A heterogeneous isotropic model of the catheter, aorta and surrounding tissues accurately predicted the values of alpha and Vp. A homogeneous anisotropic model was developed to examine the effects of anisotropy of blood and the layers of the aortic wall on measured values of resistance, alpha and Vp. This model demonstrated that anisotropy of blood and aortic wall tissue can increase the values of both alpha and Vp. Finally, a three-dimensional homogeneous isotropic rectangular model allowed examination of the effects of catheter position. This model showed small effects of catheter position on measured resistance (9.7% increase) and larger effects on alpha (21.2% decrease) and Vp (41.9% increase). We conclude the following: the FEA models may lead to accurate estimate values of alpha and Vp in vivo. The unique anisotropic conductive properties of the layers of the aortic wall contribute to the high observed values of alpha and Vp in the aorta. Finally, catheter position has a proportionately greater effect on alpha and Vp than on measured resistance. The results of this study should assist in the determination of aortic mechanical properties using conductance catheter measurements of vessel dimension.

Characterization of boron nitride thin films synthesized by plasma-assisted chemical vapor deposition technique

Boron nitride (BN) is a well-known non-oxide ceramic that has interesting and useful properties for potential industrial applications. The attractive properties of BN include its high-temperature shock stability, high electrical resistivity, anisotropic thermal conductivity and desirable mechanical properties. The potential uses of BN films include oxidation-resistant and anti-corrosive coatings, sensors, optical devices, and high temperature electronics. Thin films of BN have been obtained by a variety of growth techniques including sputtering, ion plating, evaporation, and chemical vapor deposition and associated techniques. To optimize the growth parameters and the performance of BN films, advanced electron microscopy techniques have been employed to study the structural evolution of BN films synthesized by plasma assisted chemical vapor deposition technique (PACVD).The BN films were grown in a capacitively coupled rf plasma reactor with a feedstock of diborane (B2H6), ammonia (NH3), and hydrogen (H2). The growth parameters were the same as previously reported. Chemical analyses of the grown BN films showed that they had significantly more boron (44 at.%) than nitrogen (33 at.%) and contained a large amount of hydrogen (23 at.%).

Magnetocardiographic localisation and modelling

In our magnetocardiographic (MCG) localisation studies, two modelling approaches have been applied: (a) modelling the sources with dipole and quadrupole moments in a general multipole expansion and using a homogeneous, semi-infinite volume conductor, and (b) using a single current dipole source in a homogeneous, realistically shaped torso. Both approaches have been successfully applied in localising the premature ventricular excitation site in patients suffering from the Wolff-Parkinson-White syndrome. In addition, we have participated in developing a model of propagation of electrical activation in the ventricles. Anisotropic conductivity properties and spiral arrangement of myocardial fibres are included in the model.