Interfacial effects on dielectric properties of polymer-particle nanocomposites

Dielectric materials that are capable of efficiently storing large amounts of electrical energy are desirable for many electronic and power devices. Since the electrical energy density in a dielectric material is limited to εVb²/2, where ε is the dielectric permittivity of the material and Vb is the breakdown strength, increased permittivity and breakdown strength are required for large energy storage density. Interfacial effects can influence the dielectric properties, especially dielectric breakdown resistance in polymer-particle nanocomposites. Several functional organophosphates were used to modify the surface of titania and barium titanate nanofiller particles in order to achieve covalent interface when interacted with polymer and to study the influence the electronic nature of filler surfaces on dielectric properties, in particular the breakdown resistance. Surface modified powders were analyzed by thermogravimetric analysis (TGA) and by X-ray photoelectron spectroscopy (XPS). The dielectric composite films obtained by incorporating surface modified powders in epoxy thermosetting polymer were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), impedance spectroscopy, and dielectric breakdown strength measurements. At 30 vol-% filler concentration, a calculated energy density of ~8 J/cm³ was observed for titania based composites and ~8.3 J/cm³ for barium titanate based composites involving electron scavenging interface with minimal dielectric losses compared to pure polymer. Covalent interface composites yielded energy density of ~7.5 J/cm³ for barium titanate based composites at 30 vol.-%. The data indicate that improved dispersion, breakdown strengths and energy densities resulted when electron-poor functional groups were located at the particle surfaces even compared to covalent interface --Abstract, page iv

Chapter 10 -- Dielectric Spectroscopy and Stimulated Current Analyses of Polymer-Ceramic Nanocomposites

Spectroscopy is an absorption or emission response to a stimulus where a device utilizes the excitation to perform a function. Spectroscopy is, therefore, a primary tool to assess materials properties for application in a device. Choice of the characterization method and frequency must account for interfacial properties to obtain an appropriate response signal. In this review, dielectric spectroscopy is discussed as a means to characterize the dielectrical and mechanical properties of powders, powder compacts, and composites. Dielectric and impedance spectroscopy are commonly utilized tools for examining the polarization response of dielectric materials, powder compacts, and composites, as a function of temperature and frequency. The specific advantage of these techniques is their ability to measure relaxation phenomena over a very wide frequency range, from ~10-4 to 109Hz. Stimulated current analyses, on the other hand, release stored a charge to characterize matrix relaxation events, the energy stored and/or the kind of charge carrier to elucidate structure, quantify loss, or diagnose dielectric breakdown mechanisms

Chapter 10 -- Dielectric Spectroscopy and Stimulated Current Analyses of Polymer-Ceramic Nanocomposites

Spectroscopy is an absorption or emission response to a stimulus where a device utilizes the excitation to perform a function. Spectroscopy is, therefore, a primary tool to assess materials properties for application in a device. Choice of the characterization method and frequency must account for interfacial properties to obtain an appropriate response signal. In this review, dielectric spectroscopy is discussed as a means to characterize the dielectrical and mechanical properties of powders, powder compacts, and composites. Dielectric and impedance spectroscopy are commonly utilized tools for examining the polarization response of dielectric materials, powder compacts, and composites, as a function of temperature and frequency. The specific advantage of these techniques is their ability to measure relaxation phenomena over a very wide frequency range, from ~10-4 to 109Hz. Stimulated current analyses, on the other hand, release stored a charge to characterize matrix relaxation events, the energy stored and/or the kind of charge carrier to elucidate structure, quantify loss, or diagnose dielectric breakdown mechanisms

Improved Polymer Nanocomposite Dielectric Breakdown Performance through Barium Titanate to Epoxy Interface Control

A composite approach to dielectric design has the potential to provide improved permittivity as well as high breakdown strength and thus afford greater electrical energy storage density. Interfacial coupling is an effective approach to improve the polymer-particle composite dielectric film resistance to charge flow and dielectric breakdown. A bi-functional interfacial coupling agent added to the inorganic oxide particles\u27 surface assists dispersion into the thermosetting epoxy polymer matrix and upon composite cure reacts covalently with the polymer matrix. The composite then retains the glass transition temperature of pure polymer, provides a reduced Maxwell-Wagner relaxation of the polymer-particle composite, and attains a reduced sensitivity to dielectric breakdown compared to particle epoxy composites that lack interfacial coupling between the composite filler and polymer matrix. Besides an improved permittivity, the breakdown strength and thus energy density of a covalent interface nanoparticle barium titanate in epoxy composite dielectric film, at a 5 vol.% particle concentration, was significantly improved compared to a pure polymer dielectric film. The interfacially bonded, dielectric composite film had a permittivity ∼6.3 and at a 30 μm thickness achieved a calculated energy density of 4.6 J/cm3

Improved Dielectric Breakdown Strength of Covalently-bonded Interface Polymer-particle Nanocomposites

Interfacial covalent bonding is an effective approach to increase the electrical resistance of a polymer-particle composite to charge flow and dielectric breakdown. A bifunctional tether reagent bonded to an inorganic oxide particle surface assists with particle dispersion within a thermosetting epoxy polymer matrix but then also reacts covalently with the polymer matrix. Bonding the particle surface to the polymer matrix resulted in a composite that maintained the pure polymer glass transition temperature, compared to modified or unmodified particle dispersions that lacked covalent bonding to the polymer matrix, which depressed the polymer glass transition to lower temperatures. The added interfacial control, directly bonding the particle to the polymer matrix, appears to prevent conductive percolation across particle surfaces that results in a reduced Maxwell-Wagner relaxation of the polymer-particle composite and a reduced sensitivity to a dielectric breakdown event. The inclusion of 5 vol% particles of higher permittivity produces a composite of enhanced dielectric constant and, with surface modification to permit surface cross-linking into the polymer, a polymer-particle composite with a Weibull E 0 dielectric breakdown strength of 25% greater than that of the pure polymer resulted. The estimated energy density for the cross-linked interface composite was improved 260% compared to the polymer alone, 560% better than a polymer-particle composite synthesized using bare particles, and 80% better than a polymer-particle composite utilizing bare particles with a dispersant. © Koninklijke Brill NV, Leiden, 2010

Impedance Analysis of Dielectric Nanoparticles Enabled via a Self-assembled Monolayer

Impedance spectroscopy has been shown to be a powerful tool to investigate the dielectric characteristics of powders suspended in suitable liquids. the electrical and dielectrical contributions of different components of the slurry can be extracted from the impedance spectra through measurement of frequency-dependent relaxations. However, for ferroelectric powders that possess innate surface conductivity, such as BaTiO3, nanoparticles have sufficient conductivity to exclude low-frequency fields that preclude impedance characterization of the particle core. in this work, the slurry technique is shown to be effective for dielectric characterization of not only micrometer-sized particles through equivalent circuit modeling but also applicable to nanometer size dielectric particles upon remediating the conductive surface defect. Application of a self-assembled monolayer (SAM) onto the nanoparticle as a surface passivation layer reduces the surface conductivity, stabilizes the nanoparticles to dissolution, and allows a reproducible measurement and modeling of the nanoparticle dielectric characteristics including nanoparticle permittivity. the dielectric permittivity of surface passivated, ∼40 nm diameter barium titanate particles was measured to be εr ∼ 135. © 2013 the American Ceramic Society

Dielectric Properties of Polymer–Particle Nanocomposites Influenced by Electronic Nature of Filler Surfaces

The interface between the polymer and the particle has a critical role in altering the properties of a composite dielectric. Polymer-ceramic nanocomposites are promising dielectric materials for many electronic and power devices, combining the high dielectric constant of ceramic particles with the high dielectric breakdown strength of a polymer. Self-assembled monolayers of electron rich or electron poor organophosphate coupling groups were applied to affect the filler–polymer interface and investigate the role of this interface on composite behavior. The interface has potential to influence dielectric properties, in particular the leakage and breakdown resistance. The composite films synthesized from the modified filler particles dispersed into an epoxy polymer matrix were analyzed by dielectric spectroscopy, breakdown strength, and leakage current measurements. The data indicate that significant reduction in leakage currents and dielectric losses and improvement in dielectric breakdown strengths resulted when electropositive phenyl, electron-withdrawing functional groups were located at the polymer–particle interface. At a 30 vol % particle concentration, dielectric composite films yielded a maximum energy density of ∼8 J·cm^–3 for TiO₂-epoxy nanocomposites and ∼9.5 J·cm^–3 for BaTiO₃-epoxy nanocomposites