Amira Barhoumi Meddeb

Extreme enhancement and reduction of the dielectric response of polymer nanoparticulate composites via interphasial charges

An analytical solution is constructed for the homogenized (i.e., macroscopic) dielectric response of particulate composites comprising a random distribution of particles bonded to a matrix material through interphases of finite size that contain space charges. By accounting for interphasial charges, the solution is able to describe and explain both the extreme enhancement and the reduction of the dielectric response typically exhibited by emerging polymer nanoparticulate composites. More generally, the solution reveals that judicious manipulation of interphasial charges provides a promising path forward for the design of materials with exceptional dielectric properties.

Nano-enhanced polymer composites for energy storage applications

Polymer nanocomposites containing high dielectric permittivity ceramic particles embedded into a dielectric polymer represent promising candidates to overcome the limitations of monolithic materials in both energy storage and energy conversion. Indeed, monolithic materials are hitting a plateau in terms of high energy storage capabilities due to the trade-off between the dielectric constant, the dielectric loss and the dielectric breakdown. Since ceramics have high dielectric constant but low dielectric breakdown, while polymers have high dielectric breakdown and low loss but low dielectric constant, the strategy of simply filling a polymer with ceramic particles will only yield incremental and limited success. In this study, we investigate the effect of adding commercial metal oxide nanoparticles, TiO2, to a ferroelectric polymer on the dielectric constant, breakdown, ferroelectric behavior and energy density of the system; specifically, we focus on impact of the particles size, aspect ratio, and interaction with the polymer dipole. We find that at a very low TiO2 content, namely 4.6vol%, the energy density increased by more than 400% as compared to the pristine polymer, with an enhancement in both the dielectric constant and the dielectric breakdown while the dielectric loss remained in the same range as that of the pure polymer. We also investigate the mechanism for this large improvement and demonstrate that the high aspect ratio particles have a planar distribution in the nanocomposite film, resulting in a low local field, and therefore a high dielectric breakdown.

Plasma surface modification of P(VDF-TrFE): Influence of surface chemistry and structure on electronic charge injection

Reactive ion plasma treatments have been used to alter the high field electrical properties of organic dielectrics via a grafting process of chemical species within the plasma to the surface of the dielectric. This study determines the effect of a CF4/O2 plasma based processing procedure on polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] on low and high field electrical performance. Plasma treatment in conjunction with a thermal annealing procedure is analyzed in the following ways: X-ray Photoelectron Spectroscopy to determine the changes in surface chemistry of films post plasma treatment, optical profilometry to measure evolution in surface topology, water contact angle to track surface polarity as a function of plasma treatment time, and current-voltage measurements at low and high-fields to capture the electrical behavior of the films. The results indicate that plasma treatment causes the chemical modification of P(VDF-TrFE) surface through the addition of carbonyl (C=O) groups, as well as oxygen and fluorine based moieties (CF-O, C-O) which are dependent on processing condition. Contact angle with water shows an increase as a function of plasma treatment time from ∼84° to 111° in plasma treated films, indicating decreased surface polarity after plasma treatment. Finally, plasma treatment decreases film resistivity by one order of magnitude, from 8.0 × 1011 Ω m in untreated control samples to 0.8 × 1011 Ω m, as well as resulted in enhanced Schottky emission caused by decreased Schottky barrier height. Modeling I(V) data using both a surface limited (Schottky) and bulk limited (Poole-Frenkel) approaches suggest that conduction in P(VDF-TrFE) thin films results from Schottky emission and is dependent on the chemical environment of the metal/dielectric contact. This study ultimately demonstrates the ability to alter the electrical properties by plasma surface treatment and also the importance of surface chemistry in organic dielectrics to control conduction through the material for high energy and power applications.Reactive ion plasma treatments have been used to alter the high field electrical properties of organic dielectrics via a grafting process of chemical species within the plasma to the surface of the dielectric. This study determines the effect of a CF4/O2 plasma based processing procedure on polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] on low and high field electrical performance. Plasma treatment in conjunction with a thermal annealing procedure is analyzed in the following ways: X-ray Photoelectron Spectroscopy to determine the changes in surface chemistry of films post plasma treatment, optical profilometry to measure evolution in surface topology, water contact angle to track surface polarity as a function of plasma treatment time, and current-voltage measurements at low and high-fields to capture the electrical behavior of the films. The results indicate that plasma treatment causes the chemical modification of P(VDF-TrFE) surface through the addition of carbonyl (C=O) groups, as well as ox...

Polymer laminates for high energy density and low loss

In this study, we investigate the effect of added interfaces on the dielectric and breakdown response of polyvinylidene fluoride polymer (PVDF). Multilayer laminates (1 through 4 layers) of PVDF are fabricated using hot pressing. A series of electrical characterization techniques including high voltage breakdown, dielectric spectroscopy and impedance spectroscopy, show the effect of interfacial elements on dielectric breakdown strength (displaying an 18% increase from a 1-layer to 3-layer structure) and dielectric permittivity (increasing 10% from 1- to 4-layers). Equivalent circuit modeling of impedance data is used to characterize the effect of laminated interfaces at low frequencies (10 mHz) and elevated temperatures (70 °C). Results suggest capacitive circuit elements in addition to a Debye like bulk model are necessary to describe space charge polarizations observed in multilayered structures. Better understanding the effect of added interfaces on charge transport in all organic laminates will provide insight into conduction through organic dielectric media, and offer insight into improving energy density and mitigating loss in high energy storage applications.

Spectroscopic neutron detection using composite scintillators

Shielded special nuclear material (SNM), especially highly enriched uranium, is exceptionally difficult to detect without the use of active interrogation (AI). We are investigating the potential use of low-dose active interrogation to realize simultaneous high-contrast imaging and photofission of SNM using energetic gamma-rays produced by low-energy nuclear reactions, such as 11B(d,nγ)12C and 12C(p,p′)12C. Neutrons produced via fission are one reliable signature of the presence of SNM and are usually identified by their unique timing characteristics, such as the delayed neutron die-away. Fast neutron spectroscopy may provide additional useful discriminating characteristics for SNM detection. Spectroscopic measurements can be conducted by recoil-based or thermalization and capture-gated detectors; the latter may offer unique advantages since they facilitate low-statistics and event-by-event neutron energy measurements without spectrum unfolding. We describe the results of the development and characterization of a new type of capture-gated spectroscopic neutron detector based on a composite of scintillating polyvinyltoluene and lithium-doped scintillating glass in the form of millimeter-thick rods. The detector achieves >108 neutron–gamma discrimination resulting from its geometric properties and material selection. The design facilitates simultaneous pulse shape and pulse height discrimination, despite the fact that no materials intrinsically capable of pulse shape discrimination have been used to construct the detector. Accurate single-event measurements of neutron energy may be possible even when the energy is relatively low, such as with delayed fission neutrons. Simulation and preliminary measurements using the new composite detector are described, including those conducted using radioisotope sources and the low-dose active interrogation system based on low-energy nuclear reactions.

Processing and characterisation of two– and three–phase polymer–based nanocomposites for energy storage applications

The aim of this work is to process and characterise nanocomposites which combine the advantages of a flexible lightweight polymer matrix (polyvinylidene fluoride, PVDF), the high conductivity of single wall carbon nanotubes (SWNTs) and the attractive dielectric properties of titanium dioxide (TiO2) nanospheres, to simultaneously achieve improved dielectric constant and low dielectric loss. The effect of nanoparticles and the effect of the heat treatment on the evolution of the phase, crystallinity and dielectric behaviour of the polymer host are investigated. In particular, a transition from γ to α phase due to heat treatment is identified. In addition, the crystallinity of three–phase composites is found to decrease compared to the corresponding two–phase composites indicating a possible interaction between SWNTs and TiO2 nanofillers. Furthermore, an improvement of the permittivity is achieved in all nanocomposites at low frequencies and attributed to interfacial and orientation polarisations and to the heat treatment.

Polymer Nanocomposites for Energy Storage Applications

High dielectric polymer nanocomposites are promising candidates for energy storage applications. The main criteria of focus are high dielectric breakdown strength, high dielectric constant and low dielectric loss. In this study, we investigate the effect of the addition of TiO2 particles to PVDF matrix on the dielectric constant, breakdown and energy density of the system. The dispersion of the particles is qualified by scanning electron microscopy (SEM). The morphology of the composites is characterized by polarized light microscopy, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The dielectric properties are measured by a Novocontrol system with an Alpha analyzer. Finally, the breakdown measurements are carried out by a QuadTech hipot tester.Copyright