G. Polizos

Polyethylene nanocomposite dielectrics: Implications of nanofiller orientation on high field properties and energy storage

Nanocomposite formation, through the incorporation of high aspect ratio nanoparticles, has been proven to enhance the dielectric properties of thermoplastic polymers, when the mitigation of internal charges and the nature of the interfacial regions are properly adjusted. Here, we explore polyethylene/montmorillonite nanocomposites, and we specifically investigate how to impart desirable dielectric behavior through controlled nanoscale texturing, i.e., through control of the spatial arrangement of the high aspect ratio nanofiller platelets. In particular, it is shown that filler alignment can be used to improve the high electric-field breakdown strength and the recoverable energy density. The origins of the improved high field performance were traced to improved charge-trapping by a synergy of nanofillers and polar maleic anhydride (MAH) groups—introduced via polyethylene-MAH copolymers—as templated by the inorganic nanofillers. Further, it is conclusively demonstrated that the alignment of the two-dimensional nanoparticles has a measurable positive effect on the breakdown strength of the materials and, consequently, on the maximum recoverable energy density.

High-and low-field dielectric characteristics of dielectrophoretically aligned ceramic/polymer nanocomposites

Polymer/ceramic composites with controlled spatial distribution of fillers are synthesized, and the corresponding changes in their properties are studied. Using dielectrophoretic assembly, we create anisotropic composites of aligned BaTiO3 particles in silicone elastomer and study their electrical properties as a function of ceramic volume fraction and composite morphology. These structured composites show an increase in the permittivity compared to composites with the same composition and randomly dispersed (0–3) fillers. This study emphasizes the important role of conductivity, permittivity, and, particularly, local cluster distribution in controlling high-field dielectric behavior. Designed anisotropy in dielectric properties can provide unexampled paradigms for the development of high energy density materials and gain important insights into the mechanisms that control dielectric breakdown strengths and nonlinear conduction at high fields in polymer/ceramic composites.

Epoxy-based nanocomposites for electrical energy storage. I: Effects of montmorillonite and barium titanate nanofillers

Polymer nanocomposites prepared by epoxy reinforced with high permittivity barium titanate (BT) fillers or high aspect ratio montmorillonite (MMT) fillers exhibited marked changes in their high electric field properties and their relaxation dynamics, depending on the nanoparticle type and concentration, the nanoparticle size, and the epoxy matrix conversion. We investigated epoxy resin composites based on organically modified montmorillonite (oMMT) or BT (BaTiO3) nanoparticles in order to delineate the effects of the high aspect ratio of the MMT and the high permittivity of the BT particles. We also explored the potential benefits of the synergy between the two fillers in systems consisting of epoxy and both oMMT and BT particles. It was observed that the nature of the organic–inorganic interfaces dominate the glass transition temperature and the dielectric properties of these composites. Specifically, using dielectric relaxation spectroscopy, we probed the local dynamics of the polymer at the interfaces....

State of Water in Perfluorosulfonic Ionomer (Nafion 117) Proton Exchange Membranes

The nature of water in acid-form Nafion 117 was quantified at several hydration levels by dielectric relaxation spectroscopy. Two independent experimental setups were used to collect complex dielectric permittivity spectra at low frequencies (0.01 Hz to 1 MHz at -80 to 25°C) and in the microwave region (0.40-26 GHz at 25-45°C). We directly observed the states of water, manifested through three population averages with distinctly resolved dynamical behaviors, and their changes with temperature and hydration level. The fastest process observed was identified as the cooperative picosecond relaxation of free (isotropic, bulklike) water, whereas the slowest process (microsecond relaxation times) corresponded to water molecules strongly bound to the charged sulfonic groups. A third type of water was also observed, also characterized by picosecond relaxation times, close to and about three times slower than those of bulk water, which was attributed to loosely bound water and may contain substantial dynamical heterogeneities.

Epoxy-based nanocomposites for electrical energy storage. II: Nanocomposites with nanofillers of reactive montmorillonite covalently-bonded with barium titanate

Barium titanate (BT) and montmorillonite (MMT) nanoparticles were covalently-bonded by organically modifying the particle surfaces and chemically reacting them in solution. These integrated two-material hybrid inorganic nanofillers were subsequently dispersed in epoxy resin and nanocomposites were obtained at several weight fractions. The inorganic component consisted of well dispersed BT spherical nanoparticles that are surrounded by attached layered MMT nanoplatelets, with the latter having the ability to react with the epoxy matrix. The thermodynamic properties of the glass transition process, the macroscopic mechanical properties of the nanocomposites, and the dynamics of the polymer segments at the inorganic interfaces, all indicate that this filler configuration enhances the polymer-ceramic interfaces. Polarization as a function of electric field and dielectric breakdown show improvements in the electrical properties of these composites, compared to the corresponding unfilled epoxy, despite the expected reduction in crosslinking density. The resulting nanocomposites have a property set which can be utilized in energy storage and power system applications.Barium titanate (BT) and montmorillonite (MMT) nanoparticles were covalently-bonded by organically modifying the particle surfaces and chemically reacting them in solution. These integrated two-material hybrid inorganic nanofillers were subsequently dispersed in epoxy resin and nanocomposites were obtained at several weight fractions. The inorganic component consisted of well dispersed BT spherical nanoparticles that are surrounded by attached layered MMT nanoplatelets, with the latter having the ability to react with the epoxy matrix. The thermodynamic properties of the glass transition process, the macroscopic mechanical properties of the nanocomposites, and the dynamics of the polymer segments at the inorganic interfaces, all indicate that this filler configuration enhances the polymer-ceramic interfaces. Polarization as a function of electric field and dielectric breakdown show improvements in the electrical properties of these composites, compared to the corresponding unfilled epoxy, despite the expe...

Induced anisotropy in electrically modified Polymer/Ceramic nanocomposites

Polymer/Ceramic composites with controlled spatial distribution of fillers are synthesized, and the corresponding changes in their properties are studied. Using dielectrophoretic assembly we create anisotropic composites of aligned BaTiO3 particles in silicone elastomer, and study their electrical properties as a function of ceramic volume fraction, connectivity and composite morphology. We investigate these composites for a variety of electrical properties i.e. permittivity, d.c. conduction, dielectric breakdown and energy density. The energy density of these electric-field-structured composites is found to be highly dependent on the anisotropy present in the system. This study emphasizes the important role of conductivity, permittivity, and particularly local cluster distribution in controlling high field dielectric behavior. Designed anisotropy in dielectric properties can provide new paradigms for the development of high energy density materials and gain important insights into the mechanisms that control dielectric breakdown strengths and non-linear conduction at high fields in polymer/ceramic composites.

State of Water in Nafion 117 Proton Exchange Membranes Studied by Dielectric Relaxation Spectroscopy

The dynamics and the nature of water environments in a fuel cell proton exchange membrane are studied experimentally. Specifically, the dynamics of water in Nafion 117 membranes, in the acid form, were investigated at two hydration levels and several temperatures by means of dielectric relaxation spectroscopy; two different dielectric spectroscopy experimental setups were employed for low (10 -2 -10 7 Hz, 25 to –80 o C) and microwave (0.04526 GHz, 35 o C) frequency ranges. Three states of water were clearly identified: (a) water strongly bound to the sulfonic groups (in quantitative agreement with previous investigations) defining the first hydration layer; (b) loosely bound water, surrounding the first layer, and (c) free water, having similar dynamics as in the bulk/liquid water. This is the first time that the dynamics of loosely bound water are experimentally observed.