Electric Field-induced Alignment Research Articles

Electric Field-Induced Ordered-Structural Aerogels Enable Superinsulation and Multifunctionality.

1D flexible fibers assembled 3D porous networked ceramic fiber aerogels (CFAs) are developed to overcome the brittleness of traditional ceramic particle aerogels. However, existing CFAs with disordered and quasi-ordered structures fail to balance the relationship between flexibility, robustness, and thermal insulation. Creating novel architectural CFAs with an excellent combination of performances has proven extremely challenging. In this paper, a novel strategy is adopted to fabricate porous mullite fibrous aerogels (MFAs) with ordered structures by combining fiber sedimentation and electric field-induced fiber alignment techniques. For the first time, electric field-induced alignment of ceramic fibers is utilized to prepare bulk aerogels on a large scale. The resulting MFAs exhibit ultra-low high-temperature thermal conductivity of 0.0830 W m-1 K-1 at 1000°C, anisotropic mechanical and sound absorption performances, and multifunctionality in terms of the combination of thermal insulation, sound absorption, and hydrophobicity. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional CFAs for various applications.

Remarkable Conductive Anisotropy of Ag-Coated Glass Microbeads/UV Adhesive Composites Made by Electric Field Induced Alignment

We successfully prepared highly anisotropic conductive composites (ACCs) made of Ag-coated glass microbeads/UV adhesive via electric field-induced alignment, which was achieved using custom patterned microelectrode arrays. An optimized AC electric field (2 kV/cm, 1 kHz) with pole-plate spacing (50 μm) was utilized to effectively assemble the microbeads into chain arrays, which were precisely positioned on the microelectrode arrays to construct ordered conductive channels. In this case, tangling and cross-connection of the assembled microchains could be reduced, resulting in enhanced performance of the ACCs with high conductivity as well as excellent anisotropy. With a minor loading (3 wt %), the conductivity in the alignment direction could reach ∼24.9 S/m, which was the highest among the reported ACCs to the best of our knowledge, and it could also be 6 orders of magnitude higher than that within the plane. Besides, the samples exhibited high reliability in wire connections with low resistances. Due to these fascinating properties, the ACCs demonstrate promising applications for reliable electrical interconnects and integrated circuits.

Theoretical modeling for piezoresistive behavior of aligned carbon nanotube/polymer nanocomposites accounting for evolution of agglomerates morphology

In this study, the electromechanical response of carbon nanotube (CNT)-reinforced polymer nanocomposites subjected to uniaxial deformation was investigated theoretically. For this purpose, a comprehensive analytical model that incorporated critical factors affecting the piezoresistive properties of CNT/polymer nanocomposites was proposed. To determine the effects of field-induced CNT alignment and the corresponding prolonged agglomeration morphology, an effective resistor network was established to predict piezoresistivity defined as the normalized resistance change of CNT/polymer nanocomposites. The proposed model accurately described the piezoresistive properties of randomly aligned CNT-based composites compared with a previously developed model, and its further validation was performed using experimental data obtained from the literature for CNTs aligned in a polymer matrix. A similar approach was used for the determination of a percolation threshold, which strongly correlated with piezoresistivity. In addition, the effect of each individual critical factor was examined to establish a desirable set of parameters that would either decrease the percolation threshold or increase the piezoresistivity of composites. • The theoretical model is proposed for the piezoresistive behavior of carbon nanotube (CNT)/polymer composites. • The effects of electric field-induced alignment of CNTs and their aggregation morphology have been taken into account. • An optimal set of critical parameters is determined to increase the piezoresistivity of CNT/polymer nanocomposites.

Graphene nanoplatelets/polydimethylsiloxane electromagnetic shielding composites based on bipolar pulsed electric field-induced alignment

Conductive polymer composites require high conductive filler loadings to achieve relatively ideal electromagnetic interference (EMI) shielding performance, which brings problems such as heavy weight and poor processability and ultimately limits their application in the aerospace field. To solve the above problems and realize the preparation of excellent EMI shielding composites with low filler loadings, a bipolar pulsed electric field-induced graphene nanoplatelets (GNPs) alignment method is proposed in this paper to improve the electrical conductivity and EMI shielding performance. The orientation and alignment of GNPs in the matrix were characterized by x-ray diffraction and scanning electron microscopy, and the results showed that GNPs formed a conductive network. The electrical conductivity of the composite material in the X-band was measured, and the results showed that the bipolar pulsed electric field could significantly improve the electrical conductivity of the composite material; the improvement percentage reached 100%–300% under the five filler loadings. In addition, the EMI shielding effectiveness of the composite material in the X-band was tested. The results showed that the EMI shielding effectiveness of the composite material with a mass fraction of 14% increased from 16.48–15.86 dB to 21.37–20.99 dB with the action of the bipolar pulsed electric field, and the improvement percentage was 32%. This is the first study on improving the EMI shielding performance of composite materials by applying a bipolar pulsed electric field, and it provides an effective method to prepare EMI shielding composites with low filler loadings.

Field-induced orientational switching produces vertically aligned Ti3C2Tx MXene nanosheets

Controlling the orientation of two-dimensional materials is essential to optimize or tune their functional properties. In particular, aligning MXene, a two-dimensional carbide and/or nitride material, has recently received much attention due to its high conductivity and high-density surface functional group properties that can easily vary based on its arranged directions. However, erecting 2D materials vertically can be challenging, given their thinness of few nanometres. Here, vertical alignment of Ti3C2Tx MXene sheets is achieved by applying an in-plane electric field, which is directly observed using polarised optical microscopy and scanning electron microscopy. The electric field-induced vertical alignment parallel to the applied alternating-current field is demonstrated to be reversible in the absence of a field, back to a random orientation distribution. Interdigitated electrodes with uniaxially aligned MXene nanosheets are demonstrated. These can be further modulated to achieve various patterns using diversified electrode substrates. Anisotropic electrical conductivity is also observed in the uniaxially aligned MXene nanosheet film, which is quite different from the randomly oriented ones. The proposed orientation-controlling technique demonstrates potential for many applications including sensors, membranes, polarisers, and general energy applications.

Enhanced Dielectric Relaxation in Self-Organized Layers of Polypeptides Coupled to Platinum Nanoparticles: Temperature Dependence and Effect of Bias Voltage

Using alternative current impedance spectroscopy, we investigate the dynamical conductivity of hybrid nanomaterials composed of helical polypeptide layers containing platinum nanoparticles (PtNP). The electrical characteristics of the self-organized poly(γ-benzyl-l-glutamate) (PBGL) in bidimensional lamellar assembly in the presence of Pt nanoparticles are well modeled and described by a single equivalent circuit of parallel resistance and capacitance. The latter are determined using a comparison between the measured and calculated Nyquist plots, which allows extracting the characteristic relaxation time and frequency of the dipolar relaxation process. We found that the relaxation frequency in the PBLG–PtNP hybrid materials is enhanced by 4 orders of magnitudes compared to pure PBLG, which indicates a much faster dielectric relaxation in PBLG–PtNP due to dipole orientation and dipole–dipole interactions. The temperature dependence of the relaxation time is analyzed using Arrhenius plots, from which the activation energy of the relaxation process is found to be around 0.1 eV. Such a value close to the peptide vibration energy of the PBLG indicates a vibration-assisted relaxation process and a polaronic charge transport mechanism. An advantage of the PBLG–PtNP nanocomposite material is that the activation energy can be finely tuned by the PBLG degree of polymerization. Finally, an important outcome of this work is the investigation of the dielectric relaxation process in PBLG–PtNP under applied DC bias. We found that the activation energy decreases with increasing bias voltage for all degrees of polymerization of the PBLG molecule. This effect is interpreted in terms of electric field-induced alignment of the dipoles and of increased mobility of the polaronic charge carriers. The presence of piezoelectricity in the hybrid material gives the possibility to use the DC bias as a simple mean of monitoring the dynamical conductivity involving polaronic states.

Effect of electric field-induced alignment of conductive fillers in polyurethane

Effect of an electric field (Joule heating) on the electrical conductivity of four different systems of polyurethane filled with multiwall carbon nanotube, graphite, Cu particulates, and Cu flakes ranging from 10 to 80 wt%. The effect of filler shape on percolation threshold and maximum conductivity was also discussed. In the Joule effect test, the amount of adhesive resistance and surface temperature of the samples were instantaneously measured. It was observed that all four systems showed a negative temperature coefficient (NTC) behavior upon applying an electric field. Characterization was performed via scanning/transmission electron microscopy, Raman/infrared spectroscopy, and four-probe electrical conductivity measurements. By mathematical modeling, the possibility of rotation of fillers was investigated, based on which mechanisms are proposed for the observed NTC behavior.

Electric Field-Induced Assembly and Alignment of Silver-Coated Cellulose for Polymer Composite Films with Enhanced Dielectric Permittivity and Anisotropic Light Transmission.

Multifarious wearable electronics with flexible touch screens have been invented for extensive outdoor activities. One challenge associated with these wearable electronics is the development of materials with both high dielectric permittivity and anisotropic light transmission, which is responsible for high touch sensitivity and screen peep-proof protection, respectively. Herein, we demonstrated a scalable approach for assembling and aligning anisotropic cellulose in a polymer matrix through the thickness direction via the assistance of an electric field to address this challenge. The alignment of silver-coated fibrillated celluloses in the polymer matrix not only significantly increases dielectric permittivity but also effectively enhances optical anisotropy. The impact of alignment degree and filler content on the dielectric and optical properties of polymer composite films has been systematically studied. The kinetics and aligning mechanisms of silver-coated fibrillated celluloses are revealed by in situ optical microscope images while an electric field is applied. We believe that this study provides a facile strategy to enhance both dielectric permittivity and optical anisotropy of polymer composite films by the alignment of embedding nanoparticles via an AC electric field, which is essential for future flexible electronics and display technology.

Direct Patterning and Spontaneous Self-Assembly of Graphene Oxide via Electrohydrodynamic Jet Printing for Energy Storage and Sensing.

The macroscopic assembly of two-dimensional materials into a laminar structure has received considerable attention because it improves both the mechanical and chemical properties of the original materials. However, conventional manufacturing methods have certain limitations in that they require a high temperature process, use toxic solvents, and are considerably time consuming. Here, we present a new system for the self-assembly of layer-by-layer (LBL) graphene oxide (GO) via an electrohydrodynamic (EHD) jet printing technique. During printing, the orientation of GO flakes can be controlled by the velocity distribution of liquid jet and electric field-induced alignment spontaneously. Closely-packed GO patterns with an ordered laminar structure can be rapidly realized using an interfacial assembly process on the substrates. The surface roughness and electrical conductivity of the LBL structure were significantly improved compared with conventional dispensing methods. We further applied this technique to fabricate a reduced graphene oxide (r-GO)-based supercapacitor and a three-dimensional (3D) metallic grid hybrid ammonia sensor. We present the EHD-assisted assembly of laminar r-GO structures as a new platform for preparing high-performance energy storage devices and sensors.

Multifunctional Polymer Nanocomposites Reinforced by Aligned Carbon Nanomaterials.

Carbon nanomaterials such as carbon black (CB), carbon nanotubes (CNTs), and graphene have demonstrated significant potential as fillers to improve the electrical, thermal, and mechanical properties of polymers and their fiber-reinforced polymer composites. The level of improvement has been found to depend significantly on the degree of alignment of carbon nanomaterials. Due to the very small scale and complex interactions of carbon nanomaterials with polymers and structural fibers, alignment in a given direction has been a major challenge. Over the past decade, considerable effort has been devoted to developing effective strategies to align carbon nanomaterials in polymer matrices. However, significant technological challenges remain, and there is still a lack of understanding of the alignment mechanisms and their effects on the properties of polymers and composites. This paper reviews in situ alignment techniques including shear deformation, mechanical stretching, electrospinning, and application of an external magnetic or electric field, and ex situ techniques including using vertically grown CNTs or graphene. This review particularly focuses on physical mechanisms underpinning the magnetic or electric field-induced alignment and theoretical analyses that describe the different motions occurring and the major parameters controlling alignment. Moreover, this review highlights the recent research findings of the effects of alignment on the properties of polymer nanocomposites. The outlook towards the challenges and opportunities in this field are also discussed in this review.

Electric field-induced alignment of MWCNTs during the processing of PP/MWCNT composites: effects on electrical, dielectric, and rheological properties

Abstract High-frequency electric field (HEF) was applied to prepare aligned carbon nanotube (CNT)-reinforced polypropylene (PP) matrix composites during the compression molding process in this article. The effects of the alignment of multiwalled CNTs (MWCNTs) in the PP matrix under HEF on the electrical, dielectric, and rheological properties of the resulting composites were reported. The results showed that the composites prepared in the presence of the electric field had better conductivity than those of the untreated composites. The dielectric property measurement indicates that MWCNTs aligning along the direction of the imposed electric field greatly improved the dielectric properties of composites. Rheological analysis showed that the storage modulus of the aligning direction samples is higher than the value of the untreated composites and the microstructure of the composite has been changed due to the effect of HEF.

Electric field-induced alignment of nanofibrillated cellulose in thermoplastic polyurethane matrix

Nanofibrillated fibers (NFC) has been largely used as reinforcing filler to improve the properties of polymer matrix because of their unique properties such as high aspect ratio, good aqueous stability, high specific strength and stiffness. However, the full potential of NFC as reinforcing agent has not been realized because of its poor dispersion and random distribution in polymer matrix. In this work, we demonstrate the importance of alignment of NFC to determine the mechanical properties of polymer via applying alternative current (AC) electric field and using thermoplastic polyurethane (TPU) as the matrix. The TPU/NFC nanocomposites were prepared via solution casting method and the effect of parameters of the applied electric field (amplitude, frequency as well as the duration of application) on the orientation degree of NFC and the final properties of TPU/NFC nanocomposites are systematically investigated. It was found that NFC could be easily oriented under effect of AC due to electric induced polarization. The prepared anisotropic TPU/NFC composites exhibit 2.07 and 1.82 times increase of tensile strength and elongation at break in the parallel direction than that in the vertical direction for the anisotropic samples, respectively. It was also interesting, to find that the samples with aligned NFC exhibit an increased dielectric constant with lower dielectric loss, which could provide an idea to fabricate high performance dielectric materials.

Remarkable Conductive Anisotropy of Metallic Microcoil/PDMS Composites Made by Electric Field Induced Alignment

We successfully fabricated a highly anisotropic electrical conductive microcoil/polydimethylsiloxane (PDMS) composite based on helical Spirulina-templated metallic particles using an electric field-induced alignment method. The optimized AC electric field (2 kV/cm, 1 kHz) could efficiently assemble the lightweight conductive microcoils into continuous long chains and form unique end-to-end physical contacts between adjacent particles in the alignment direction, leading to highly conductive channels. Furthermore, the electrical conductivity in the alignment direction reached up to ∼10 S/m for 1 wt % loading and exhibited almost 7-8 orders of magnitude higher than that in perpendicular directions, which is by far the most remarkable conductive anisotropy for anisotropic conductive composites (ACCs). In addition, the anisotropic composites exhibit excellent current-carrying capability in a functional light emitting diode (LED) circuit. Therefore, due to the superior conductive anisotropy and high conductivity, the composites have promising potential in high reliability electrical interconnections and subminiature integrated circuits.

A study of electric field-induced conductive aligned network formation in high structure carbon black/silicone oil fluids

An electric field-induced alignment characteristics of high structure carbon black (HSCB) nanoparticles in silicone oil base fluids with different viscosities were investigated in this work. Resistivity change versus time of HSCB/silicone oil fluids with different concentrations of HSCB filler and different silicone oil viscosities was determined at different electric field strengths. Measurement results were analyzed using percolation theory by calculating critical concentration at which rapid increase of conductivity appears. An analytic model based on tunneling percolation theory was developed and successfully applied to explain the experimental results. The obtained results potentially can be used in the development of polymer composites with aligned nanoparticle structure.

Reorientation mechanisms of block copolymer/CdSe quantum dot composites under application of an electric field.

Time- and temperature-resolved in situ birefringence measurements were applied to analyze the effect of nanoparticles on the electric field-induced alignment of a microphase separated solution of poly(styrene)-block-poly(isoprene) in toluene. Through the incorporation of isoprene-confined CdSe quantum dots the reorientation behavior is altered. Particle loading lowers the order-disorder transition temperature, and increases the defect density, favoring nucleation and growth as an alignment mechanism over rotation of grains. The temperature dependent alteration in the reorientation mechanism is analyzed via a combination of birefringence and synchrotron SAXS. The detailed understanding of the effect of nanoparticles on the reorientation mechanism is an important prerequisite for optimization of electric-field-induced alignment of block copolymer/nanoparticle composites where the block copolymer guides the nanoparticle self-assembly into anisotropic structures.

B-Stage Graphene-Epoxy Composite Films: Effects of film viscosity changes on the electric field-induced alignment of graphene flakes

Graphene, which is a layer of carbon atoms in a hexagonal lattice, has been reported to have excellent mechanical, electrical, thermal, and gas barrier properties. One of the most widely used applications of graphene is graphene?polymer composites, which are one of the most promising solutions, since highly functional materials can be produced with a small amount graphene flakes incorporated in a lightweight polymer matrix. Graphene?polymer composites have been investigated for various types of applications such as thermal interface materials, electromagnetic interference shielding materials, gas sensor materials, and gas barrier materials.

Electric-Field Alignment of Chitin Nanorod–Siloxane Oligomer Reactive Suspensions

Uniaxially anisotropic chitin-silica nanocomposite solids have been obtained thanks to the electric field-induced macroscopic alignment of liquid-crystalline reactive cosuspensions. We demonstrate how chitin nanorods (260 nm long, 23 nm thick) can be aligned upon the application of an alternating current (ac) electric field, and within water-ethanol suspensions containing reactive siloxane oligomers (D(h) ∼ 3 nm). The alignment at the millimeter length scale is monitored by in situ small-angle X-ray scattering (SAXS) and polarized light optical microscopy. The composition and state (isotropic, chiral nematic) of the cosuspensions are proven to be determining factors. For nematic phases, the alignment is preserved when the electric field is switched off. Further solvent evaporation induces sol-gel transition, and uniaxially anisotropic chitin-silica nanocomposites are formed after complete drying of the aligned nematic suspensions. Here, the collective response of colloidal mesophases to external electric fields and the subsequent formation of ordered nanocomposite solids would represent a new opportunity for materials design.

Fabrication of a Large-Scale Conductive Composite Film Containing Electrically Aligned Carbon Nanotubes

Application of high electric field is effective for the alignment of carbon nanotubes (CNTs) in a nanocomposite film. The conventionally used one pair of parallel plate electrode is not applicable to large-sized nanocomposite fabrication due to limited output voltage of high voltage source. We have proposed a new method using parallel array electrode system to prepare large-scale conductive composite film containing aligned CNTs. In the electrode system, the electric field generated by an array of interdigitated parallel wire electrodes was applied to the CNTs by using an electric field averaging scheme. The array electrode system made a large-scale (15 × 15 cm2) composite film containing uniformly aligned CNTs. In this study, we investigated the relationship between weight fraction of the CNTs and electrical conductivity of the composite film. Measurement of the surface electric potential of the composite film after corona discharge exposure revealed that the film with an electrical conductivity increased by electric field-induced CNTs alignment could serve as an antistatic film.

AC Electric Field-Induced Alignment and Long-Range Assembly of Multi-Wall Carbon Nanotubes Inside Aqueous Media

The present work examines the behavior of multiwall carbon nanotubes (MWCNT) inside AC electric fields created by three-dimensional electrodes. The response of carbon nanotubes stably suspended in water with the aid of a nonionic surfactant is monitored by combining microscopic observations with on-line measurements of the suspension resistivity. It is found that polarization effects induced by the externally applied AC electric field on MWCNTs can cause their unidirectional orientation and end-to-end contact that result in formations of spatially distributed, long-range, three-dimensional and electrically conducting structures that span the entire gap between the electrodes. The length of the formed structures, which in the present case was approximately 30 times larger than that of an individual carbon nanotube, can be controlled by adjusting the spacing between the electrodes. The influence of main experimental parameters, namely, MWCNT concentration, applied voltage, AC field frequency, and electrode surface topography on the suspension behavior is experimentally examined. Results are demonstrated for applied voltage values, AC field frequencies, and carbon nanotube concentrations in the range 4-40 Vptp, 10 Hz-5 MHz, and 0.001-2.0 wt%, respectively. While higher electric field strengths accelerate the formation of aligned structures, higher frequency values were found to result in suspensions that exhibit smaller electrical resistivity. Carbon nanotube dispersions exposed to an AC electric field exhibit a 100-fold or more decrease in their electrical resistivity, even when carbon nanotube concentrations as low as 0.005 wt% are used.

Preparation and properties of alumina composites modified by electric field-induced alignment of carbon nanotubes

Catalytic chemical vapor deposition (CVD) grown multi-walled carbon nanotubes (MWNTs) are treated with HF and deionized water and are then placed into alumina ceramics for improvement of both electrical conductivity and mechanical properties. In particular, an alternating current (ac) electric field is applied during the coagulation of the alumina slurries to induce the formation of aligned MWNT networks in the alumina matrix. The coagulated alumina matrix composite bases filled with 2 wt. % ac electric field-induced aligned MWNTs, are then sintered by hot pressing. The electrical conductivities of the prepared composites in directions both parallel and perpendicular to the MWNTs alignment, reach values of 6.2×10-2 S m-1 and 6.8×10-9 S m-1, respectively, compared with that of 4.5×10-15 S m-1 for pristine alumina ceramics. The fracture toughness and flexing strengths of the prepared composites in the two directions are 4.66±0.66 MPa m0.5, 390±70 MPa, and 3.65±0.46 MPa m0.5, 191±5 MPa, respectively, compared with 3.78±0.66 MPa m0.5 and 302±50 MPa for pristine alumina, 4.09±0.15 MPa m0.5 and 334±60 MPa for alumina filled with 2 wt. % MWNTs prepared without the effect of an electric field, respectively. The results indicate that the electric field leads to anisotropic behaviour. The properties of the composites along the direction of the MWNTs alignment are much improved with the addition of a small amount of CVD grown MWNTs.