RESUMEN
The electronic band structure, especially the defect states at the conduction band tail, dominates electron transport and electrical degradation of a dielectric material under an extremely high electric field. However, the electronic band structure in a dielectric is barely well studied due to experimental challenges in detecting the electrical conduction to an extremely high electric field, i.e., prebreakdown. In this work, the electronic band structure of polymer dielectric films is probed through an in situ prebreakdown conduction measurement method in conjunction with a space-charge-limited-current spectroscopic analysis. An exponential distribution of defect states at the conduction band tail with varying trap levels is observed in accordance with the specific morphological disorder in the polymer dielectric, and the experimental defect states show also a favorable agreement with the calculated density of states from the density functional theory. The methodology demonstrated in this work bridges the molecule-structure-determined electronic band structure and the macro electrical conduction behavior with a highly improved understanding of material properties that control the electrical breakdown, and paves a way for guiding the modification of existing material and the exploration of novel materials for high electric field applications.
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The fast transient evolution of electric field assisted vertical orientation and assembly of halloysite nanotubes (HNTs) in a photo-curable matrix is investigated using a custom-built real-time birefringence measurement system. The effect of applied electric field strength and HNT loadings on the kinetics of orientation and organization of halloysite nanotubes into nanocolumns is systematically investigated. The following organization in the matrix is frozen by curing the precursor under ultraviolet (UV) light. The final structure is characterized by scanning electron microscopy (SEM) and wide angle X-ray scattering (WAXS). The nanocomposite films show vertically oriented and aligned HNTs due to the electric field. The orientation factor of HNTs decreases with the increase of particle concentration due to the higher viscosity and stronger inter-particle interaction.
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Net-shape manufacture of customizable objects through three-dimensional (3D) printing offers tremendous promise for personalization to improve the fit, performance, and comfort associated with devices and tools used in our daily lives. However, the application of 3D printing in structural objects has been limited by their poor mechanical performance that manifests from the layer-by-layer process by which the part is produced. Here, this interfacial weakness is overcome using a structured, core-shell polymer filament where a polycarbonate (PC) core solidifies quickly to define the shape, whereas an olefin ionomer shell contains functionality (crystallinity and ionic) that strengthen the interface between the printed layers. This structured filament leads to improved dimensional accuracy and impact resistance in comparison to the individual components. The impact resistance from structured filaments containing 45 vol % shell can exceed 800 J/m. The origins of this improved impact resistance are probed using X-ray microcomputed tomography. Energy is dissipated by delamination of the shell from PC near the crack tip, whereas PC remains intact to provide stability to the part after impact. This structured filament provides tremendous improvements in the critical properties for manufacture and represents a major leap forward in the impact properties obtainable for 3D-printed parts.
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A roll-to-roll continuous process was developed to manufacture large-scale multifunctional poly(dimethylsiloxane) (PDMS) films embedded with thickness direction ("Z" direction) aligned graphite nanoparticles by application of electric field. The kinetics of particle "Z" alignment and chain formation was studied by tracking the real-time change of optical light transmission through film thickness direction. Benefiting from the anisotropic structure of aligned particle chains, the electrical and thermal properties of the nanocomposites were dramatically enhanced through the thickness direction as compared to those of the nanocomposites containing the same particle loading without electrical field alignment. With 5 vol % graphite loading, 250 times higher electrical conductivity, 43 times higher dielectric permittivity, and 1.5 times higher thermal conductivity was achieved in the film thickness direction after the particles were aligned under electrical field. Moreover, the aligned nanocomposites with merely 2 vol % graphite particles exhibit even higher electric conductivity and dielectric permittivity than those of the nonaligned nanocomposites at random percolation threshold (10 vol % particles), as the "electric-field-directed" percolation threshold concentration is substantially decreased using this process. As the graphite loading increases to 20 vol %, the aligned nanocomposites exhibit thermal conductivity as high as 6.05 W/m·K, which is 35 times the thermal conductivity of pure matrix. This roll-to-roll electric field continuous process provides a simple, low-cost, and commercially viable method to manufacture multifunctional nanocomposites for applications as embedded capacitor, electromagnetic (EM) shielding, and thermal interface materials.
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A facile method to fabricate hierarchically structured fiber composites is described based on the electrospinning of a dope containing nickel and manganese nitrate salts, citric acid, phenolic resin, and an amphiphilic block copolymer. Carbonization of these fiber mats at 800 °C generates metallic Ni-encapsulated NiO/MnOx/carbon composite fibers with average BET surface area (150 m(2)/g) almost 3 times higher than those reported for nonporous metal oxide nanofibers. The average diameter (â¼900 nm) of these fiber composites is nearly invariant of chemical composition and can be easily tuned by the dope concentration and electrospinning conditions. The metallic Ni nanoparticle encapsulation of NiO/MnOx/C fibers leads to enhanced electrical conductivity of the fibers, while the block copolymers template an internal nanoporous morphology and the carbon in these composite fibers helps to accommodate volumetric changes during charging. These attributes can lead to lithium ion battery anodes with decent rate performance and long-term cycle stability, but performance strongly depends on the composition of the composite fibers. The composite fibers produced from a dope where the metal nitrate is 66% Ni generates the anode that exhibits the highest reversible specific capacity at high rate for any composition, even when including the mass of the nonactive carbon and Ni(0) in the calculation of the capacity. On the basis of the active oxides alone, near-theoretical capacity and excellent cycling stability are achieved for this composition. These cooperatively assembled hierarchical composites provide a platform for fundamentally assessing compositional dependencies for electrochemical performance. Moreover, this electrospinning strategy is readily scalable for the fabrication of a wide variety of nanoporous transition metal oxide fibers.
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A roll to roll continuous processing method is developed for vertical alignment ("Z" alignment) of barium titanate (BaTiO3) nanoparticle columns in polystyrene (PS)/toluene solutions. This is accomplished by applying an electric field to a two-layer solution film cast on a carrier: one is the top sacrificial layer contacting the electrode and the second is the polymer solution dispersed with BaTiO3 particles. Flexible Teflon coated mesh is utilized as the top electrode that allows the evaporation of solvent through the openings. The kinetics of particle alignment and chain buckling is studied by the custom-built instrument measuring the real time optical light transmission during electric field application and drying steps. The nanoparticles dispersed in the composite bottom layer form chains due to dipole-dipole interaction under an applied electric field. In relatively weak electric fields, the particle chain axis tilts away from electric field direction due to bending caused by the shrinkage of the film during drying. The use of strong electric fields leads to maintenance of alignment of particle chains parallel to the electric field direction overcoming the compression effect. At the end of the process, the surface features of the top porous electrodes are imprinted at the top of the top sacrificial layer. By removing this layer a smooth surface film is obtained. The nanocomposite films with "Z" direction alignment of BaTiO3 particles show substantially increased dielectric permittivity in the thickness direction for enhancing the performance of capacitors.
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In this study, the chaining and preferential alignment of barium titanate nanoparticles (100 nm) through the thickness direction of a polymer matrix in the presence of an electric field is shown. Application of an AC electric field in a well-dispersed solution leads to the formation of chains of nanoparticles in discrete rows oriented with their primary axis in the E-field direction due to dielectrophoresis. The change in the orientation of these chains was quantified through statistical analysis of SEM images and was found to be dependent on E-field, frequency and viscosity. When a DC field is applied a distinct layer consisting of dense particles was observed with micro-computed tomography. These studies show that the increase in DC voltage leads to increase in the thickness of the particle rich layer along with the packing density also increasing. Increasing the mutual interactions between particles due to the formation of particle chains in the "Z"-direction decreases the critical percolation concentration above which substantial enhancement of properties occurs. This manufacturing method therefore shows promise to lower the cost of the products for a range of applications including capacitors by either enhancing the dielectric properties for a given concentration or reduces the concentration of nanoparticles needed for a given property.
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A combination of transparency, electrical conductivity and flexibility is desired in the emerging flexible electronics industry for current and future applications. In this paper, we report the development of through thickness electrical conductivity in polystyrene films filled with nickel nanopowder by external magnetic field application. This process leads to the formation of nanocolumns of nickel spanning across the thickness direction while generating nanoparticle depleted regions in between. This leads to directionally dependent enhancement in optical light transmission particularly in the normal direction of the films. With the use of as little as 2 wt% (0.22 vol%) nickel we were able to achieve high through thickness conductivity under the influence of a magnetic field. While these films exhibit high through thickness conductivity they remain non-conductive in their planes as a result of the unique nanomorphology created which eliminates potential side branch formations. These films are anticipated to be used as electrodes for touch screens, electric dissipative materials for electronic packaging and other sensors.
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Roll-to-roll (R2R) processing enables the rapid fabrication of large-area sheets of cooperatively assembled materials for production of mesoporous materials. Evaporation induced self-assembly of a nonionic surfactant (Pluronic F127) with sol-gel precursors and phenolic resin oligomers (resol) produce highly ordered mesostructures for a variety of chemistries including silica, titania, and tin oxide. The cast thick (>200 µm) film can be easily delaminated from the carrier substrate (polyethylene terephthalate, PET) after cross-linking the resol to produce meter-long self-assembled sheets. The surface areas of these mesoporous materials range from 240 m(2)/g to >1650 m(2)/g with these areas for each material comparing favorably with prior reports in the literature. These R2R methods provide a facile route to the scalable production of kilograms of a wide variety of ordered mesoporous materials that have shown potential for a wide variety of applications with small-batch syntheses.
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Large-scale roll-to-roll (R2R) fabrication of vertically oriented nanostructures via directed self-assembly of cylindrical block copolymer (c-BCP) thin films is reported. Nearly 100% vertical orientation of cylinders in sub-100 nm c-BCP films under optimized processing via a dynamic sharp temperature gradient field termed Cold Zone Annealing-Sharp or 'CZA-S' is achieved, with successful scale-up on a prototype custom-built 70 ft × 1 ft R2R platform moving at 25 µm/s, with 9 consecutive CZA units. Static thermal annealing of identical films in a conventional vacuum oven fails to produce comparable results. As a potential for applications, we fabricate high-density silicon oxide nanodot arrays from the CZA-S annealed BCP thin film template.
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Combinations of rigid and flexible aromatic diamines were used to tailor the properties of octa(aminophenyl)-silsesquioxane (OAPS) cross-linked polyimide aerogels. 2,2'-Dimethylbenzidine (DMBZ) or p-phenylenediamine (PPDA) was used in combination with the more-flexible diamine, 4,4'-oxydianiline (ODA). The amount of rigid diamine was varied from 0% to 100% of the total diamines in the backbone. The resulting aerogels vary in density, shrinkage, porosity, surface area, mechanical and thermal properties (depending on the type of diamine and the proportions of rigid diamine to flexible diamine used). Replacing ODA with PPDA increases shrinkage that occurs during gelation and processing, while increasing the DMBZ fraction decreases shrinkage. Replacing ODA with 50 mol% of DMBZ maintains the flexibility of thin films, while the moisture resistance of the aerogels is greatly improved.
RESUMEN
Polyimide gels are produced by cross-linking anhydride capped polyamic acid oligomers with aromatic triamine in solution and chemically imidizing. The gels are then supercritically dried to form nanoporous polyimide aerogels with densities as low as 0.14 g/cm(3) and surface areas as high as 512 m(2)/g. To understand the effect of the polyimide backbone on properties, aerogels from several combinations of diamine and dianhydride, and formulated oligomer chain length are examined. Formulations made from 2,2'-dimethylbenzidine as the diamine shrink the least but have among the highest compressive modulus. Formulations made using 4,4'-oxydianiline or 2,2'dimethylbenzidine can be fabricated into continuous thin films using a roll to roll casting process. The films are flexible enough to be rolled or folded back on themselves and recover completely without cracking or flaking, and have tensile strengths of 4-9 MPa. Finally, the highest onset of decomposition (above 600 °C) of the polyimide aerogels was obtained using p-phenylene diamine as the backbone diamine with either dianhydride studied. All of the aerogels are suitable candidates for high-temperature insulation with glass transition temperatures ranging from 270-340 °C and onsets of decomposition from 460-610 °C.
Asunto(s)
Aminas/química , Resinas Sintéticas/química , Derivados del Benceno/química , Geles/química , Nanotecnología , Polímeros/química , Porosidad , TemperaturaRESUMEN
We report the first synthesis of polyimide aerogels cross-linked through a polyhedral oligomeric silsesquioxane, octa(aminophenyl)silsesquioxane (OAPS). Gels formed from polyamic acid solutions of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), bisaniline-p-xylidene (BAX) and OAPS were chemically imidized and dried using supercritical CO(2) extraction to give aerogels having density around 0.1 g/cm(3). The aerogels are greater than 90 % porous, have high surface areas (230 to 280 m(2)/g) and low thermal conductivity (14 mW/m-K at room temperature). Notably, the polyimide aerogels cross-linked with OAPS have higher modulus than polymer reinforced silica aerogels of similar density and can be fabricated as both monoliths and thin films. Thin films of the aerogel are flexible and foldable making them an ideal insulation for space suits, and inflatable structures for habitats or decelerators for planetary re-entry, as well as more down to earth applications.
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We have developed a replaceable bioartificial pancreas to treat diabetes utilizing a unique cocontinous amphiphilic conetwork membrane created for macroencapsulation and immunoisolation of porcine islet cells (PICs). The membrane is assembled from hydrophilic poly(N,N-dimethyl acrylamide) and hydrophobic/oxyphilic polydimethylsiloxane chains cross-linked with hydrophobic/oxyphilic polymethylhydrosiloxane chains. Our hypothesis is that this membrane allows the survival of xenotransplanted PICs in the absence of prevascularization or immunosuppression because of its extraordinarily high-oxygen permeability and small hydrophilic channel dimensions (3-4 nm). The key components are a 5-10 microm thick semipermeable amphiphilic conetwork membrane reinforced with an electrospun nanomat of polydimethylsiloxane-containing polyurethane, and a laser-perforated nitinol scaffold to provide geometric stability. Devices were loaded with PICs and tested for their ability to maintain islet viability without prevascularization, prevent rejection, and reverse hyperglycemia in three pancreatectomized dogs without immunosuppression. Tissue tolerance was good and there was no systemic toxicity. The bioartificial pancreas protected PICs from toxic environments in vitro and in vivo. Islets remained viable for up to 3 weeks without signs of rejection. Neovascularization was observed. Hyperglycemia was not reversed, most likely because of insufficient islet mass. Further studies to determine long-term islet viability and correction of hyperglycemia are warranted.
Asunto(s)
Hiperglucemia/terapia , Islotes Pancreáticos/citología , Páncreas/cirugía , Aleaciones/química , Animales , Órganos Artificiales , Dimetilpolisiloxanos/química , Perros , Inmunosupresores/uso terapéutico , Trasplante de Islotes Pancreáticos/métodos , Rayos Láser , Páncreas/inmunología , Proyectos Piloto , Poliuretanos/química , Porcinos , Trasplante HeterólogoRESUMEN
The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofibers without concern for thermal contact resistance, which is commonly a barrier to accurate thermal measurement of these materials. Within a scanning electron microscope, a micromanipulator supplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response is detected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity. Glass fibers of diameter 15 microm had a measured diffusivity of 1.21x10(-7) m(2)/s; polyimide fibers of diameters 570 and 271 nm exhibited diffusivities of 5.97x10(-8) and 6.28x10(-8) m(2)/s, respectively, which compare favorably with bulk values.
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Silica aerogels are sol-gel-derived materials consisting of interconnected nanoparticle building blocks that form an open and highly porous three-dimensional silica network. Flexible aerogel films could have wide applications in various thermal insulation systems. However, aerogel thin films produced with a pure sol-gel process have inherent disadvantages, such as high fragility and moisture sensitivity, that hinder wider applications of these materials. We have developed synthesis and manufacturing methods to incorporate electrospun polyurethane nanofibers into the cast sol film prior to gelation of the silica-based gel in order to reinforce the structure and overcome disadvantages such as high fragility and poor mechanical strength. In this method, a two-stage sol-gel process was employed: (1) acid-catalyzed tetraethyl orthosilicate hydrolysis and (2) base-catalyzed gelation. By precisely controlling the sol gelation kinetics with the amount of base present in the formulation, nanofibers were electrospun into the sol before the onset of the gelation process and uniformly embedded in the silica network. Nanofiber reinforcement did not alter the thermal conductivity and rendered the final composite film bendable and flexible.