Micro and Nano Machining by Electro-Physical and Chemical Processes

Understanding electrical properties of polymer nanocomposites is of great significance in order to diagnose and analyze physical and chemical behaviors of the composites as well as their own dielectric properties and can provide valuable data for improving the characteristics of the nanocomposites. In this work, the solution compounding method was used to prepare the polyethylene (PE)/aluminum (Al) nanocomposites. The dielectric permittivity and loss tangent of the PE/Al nanocomposites with different nanofiller loadings were investigated in the broad frequency range from 0.1 Hz to 10 MHz and the direct current (dc) conductivity and the dielectric breakdown strength of the PE/Al nanocomposites were also studied. The research results have shown that the dielectric permittivity shows different behaviors in the low and high frequency range. The permittivity values of the nanocomposites with the nanofiller loading levels of 0−48 wt % are found to show no percolative characteristics in the low frequency range b...

Electrical properties of polyethylene/aluminum nanocomposites

The dielectric permittivity (e′) of a polymeric material can be significantly increased when blended with conductive fillers at concentrations approaching the percolation threshold. However, reproducible synthesis of such composites is after decades of research still a major challenge and a bottleneck for their application. Difficulties arise in controlling the size and shape of the filler as well as in its homogenous distribution within the composite. These parameters strongly affect the dielectric as well as mechanical properties of the composite. While a substantial amount of literature deals with the influence of conductive fillers on the dielectric properties of composites, little is known about their mechanical properties. It is therefore still an important goal to synthesize materials with simultaneously high e′ and good mechanical properties. Here, we report the synthesis of dielectric elastomers that combine key properties such as high flexibility and stretchability, high thermal stability, increased e′, low dielectric loss and conductivity. Such materials were prepared by solution processing using quasi-spherical silver nanoparticles (AgNPs) of a defined size in a polydimethylsiloxane matrix (Mw = 692 kDa). To prevent percolation, the AgNPs were coated with a thin silica shell (<4 nm). To increase their compatibility with the silicone matrix, these core–shell nanoparticles were passivated with a silane reagent. The insulating silica shell around the particles precisely defines the minimum approach distance between the cores as twice the shell thickness. The dielectric properties of the passivated particles (filler) were measured in pellets and found to have an almost frequency independent value of e′ = 90 and a very low loss factor tan δ = 0.023 at high frequencies. When such particles were used as fillers in a polydimethylsiloxane matrix, composites with low dielectric losses were obtained. A composite containing a 31 vol% filler with e′ = 21 and tan δ = 0.03 at ∼1 kHz was achieved. At a AgNP volume fraction of 20%, the composite has e′ = 5.9 at ∼1 kHz, a dielectric strength of 13.4 V μm−1, an elastic modulus as low as 350 kPa at 100% strain, and a strain at break of 800%. Due to the high specific energy density per volume at low electric fields, these composites are attractive materials in applications involving low electric fields.

/pdf/highly-stretchable-dielectric-elastomer-composites-2ibju6sn1g.pdf

Highly stretchable dielectric elastomer composites containing high volume fractions of silver nanoparticles

Multiple-walled carbon nanotube (CNT)-and carbon black (CB)-polymer composites have been fabricated by mechanical mixing with different loadings, the polymeric matrix being identical between the two series of samples. The main focus of this work is to report measurements of physical properties of these mixtures in ambient conditions and to discuss the origin of similarities and differences among them according the kind of carbonaceous filler. The uniform dispersion of the carbonaceous phase in the dielectric matrix was probed by high-resolution transmission electron microscopy. The good dispersibility of the filler particle is also reflected in the much lower conduction threshold observed for CNT-containing samples than in the CB composites. This is likely due to the high aspect ratio of the CNTs. Mechanical properties show that the storage modulus of the two kinds of samples is close to the modulus value of the neat styrene-butadiene rubber (SBR), independent of filler content over a wide range of compositions (ϕ<0.1) encompassing the percolation threshold. Microwave measurements show that the real part of the effective permittivity exhibits a flat frequency response, with the exception of the sample containing 30 vol % CB for which an inverse-power law is observed revealing a behavior that has been seen for many random heterogeneous soft materials. No resonant dielectric absorption is evidenced within the frequency range explored and for the filler concentrations investigated. The results were also compared with analytical effective (mean-field) models. The symmetric Bruggeman model is in very good agreement with the microwave effective permittivity once account is taken of the depolarization factor which is close to the value found for a three-dimensional (3D) random dispersion of monodisperse spherical conductive inclusions within a dielectric matrix. By combining microwave frequency-domain spectroscopy with uniaxial tension, we obtain the effective permittivity as a function of the elongation ratio. Our results indicate that the effective permittivity spectrum of the CNT-polymer samples and their CB-based counterparts is not very sensitive to the applied stress in the range of elongation ratios explored. For the sample containing 30 vol % CB, the relative variation in the effective permittivity as a function of the elongation ratio is well described by the Gaussian molecular network model. The experimentally determined mechanical and microwave properties of these nanocomposites is related to the change in the mesostructure, formed by the heterogeneous 3D interconnected network of polymer and of aggregates (or agglomerates) of filler particles, as the composite is stretched. The results of this study provide another insight and opportunities to the comprehension of multifunctional materials, including novel nanoelectronic components, and carbon-based systems.

A comparison between physical properties of carbon black-polymer and carbon nanotubes-polymer composites

Silica-coated multiwalled carbon nanotubes (MWCNTs) have been prepared by the sol–gel polymerization of tetraethoxysilane (TEOS) in the presence of the acid-oxidized MWCNTs at room temperature, followed by oxidizing the MWCNTs templates at high temperature in air to produce hollow silica nanotubes. The thickness and architectures of silica shell were well controlled by rationally adjusting the concentration of TEOS, and by adding cationic surfactant as a structure-directing agent. These results also give a clear answer to prove the fact that the structures of spherical silica particles can be fully “copied” to the coating shell and the wall of silica nanotubes when prepared by the same method as the synthesis of silica particles in the presence of templates.

A facile method to fabricate silica-coated carbon nanotubes and silica nanotubes from carbon nanotubes templates

We study the dielectric properties of nanocomposites in the 0–15-GHz frequency regime These composites consist of electrically insulated metallic nanoparticles and carbon nanotubes, the elongated nanostructures in this study, uniformly dispersed in a dielectric matrix We find that the properties of these composites depend crucially on the shape and monodispersity of the nanoparticles While at high concentrations the spherical nanoparticles cause only slight modification of the dielectric function of the composite with respect to the matrix, the elongated nanoparticles produce composites with an enhanced real part of the dielectric function and low imaginary part Our theory, which is in excellent agreement with these results, suggests specific ways to further improve the dielectric properties of the composites

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/p168.pdf?origin=publication_detail

Dielectric media based on isolated metallic nanostructures

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Dielectric media based on isolated metallic nanostructures