Michael Bell

Dielectric breakdown strength of epoxy bimodal-polymer-brush-grafted core functionalized silica nanocomposites

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

Investigation of dielectric breakdown in silica-epoxy nanocomposites using designed interfaces

Adding nano-sized fillers to epoxy has proven to be an effective method for improving dielectric breakdown strength (DBS). Evidence suggests that dispersion state, as well as chemistry at the filler-matrix interface can play a crucial role in property enhancement. Herein we investigate the contribution of both filler dispersion and surface chemistry on the AC dielectric breakdown strength of silica-epoxy nanocomposites. Ligand engineering was used to synthesize bimodal ligands onto 15nm silica nanoparticles consisting of long epoxy compatible, poly(glycidyl methacrylate) (PGMA) chains, and short, π-conjugated, electroactive surface ligands. Surface initiated RAFT polymerization was used to synthesize multiple graft densities of PGMA chains, ultimately controlling the dispersion of the filler. Thiophene, anthracene, and terthiophene were employed as π-conjugated surface ligands that act as electron traps to mitigate avalanche breakdown. Investigation of the synthesized multifunctional nanoparticles was effective in defining the maximum particle spacing or free space length (Lf) that still leads to property enhancement, as well as giving insight into the effects of varying the electronic nature of the molecules at the interface on breakdown strength. Optimization of the investigated variables was shown to increase the AC dielectric breakdown strength of epoxy composites as much as 34% with only 2wt% silica loading.

The effects of nanoparticles and organic additives with controlled dispersion on dielectric properties of polymers: Charge trapping and impact excitation

This work presents a comprehensive investigation into the effects of nanoparticles and organic additives on the dielectric properties of insulating polymers using reinforced silicone rubber as a model system. TiO2 and ZrO2 nanoparticles (d = 5 nm) were well dispersed into the polymer via a bimodal surface modification approach. Organic molecules with the potential of voltage stabilization were further grafted to the nanoparticle to ensure their dispersion. These extrinsic species were found to provide deep traps for charge carriers and exhibited effective charge trapping properties at a rather small concentration (∼1017 cm−3). The charge trapping is found to have the most significant effect on breakdown strength when the electrical stressing time is long enough that most charges are trapped in the deep states. To establish a quantitative correlation between the trap depth and the molecular properties, the electron affinity and ionization energy of each species were calculated by an ab initio method and were compared with the experimentally measured values. The correlation however remains elusive and is possibly complicated by the field effect and the electronic interactions between different species that are not considered in this computation. At high field, a super-linear increase of current density was observed for TiO2 filled composites and is likely caused by impact excitation due to the low excitation energy of TiO2 compared to ZrO2. It is reasoned that the hot charge carriers with energies greater than the excitation energy of TiO2 may excite an electron-hole pair upon collision with the NP, which later will be dissociated and contribute to free charge carriers. This mechanism can enhance the energy dissipation and may account for the retarded electrical degradation and breakdown of TiO2 composites.

Enhanced charge trapping in bimodal brush functionalized silica-epoxy nanocomposite dielectrics

This manuscript details the processing, and investigates the dielectric properties, of surface ligand engineered epoxy nanocomposites. They display significant improvements in dielectric breakdown strength (DBS). Thermally stimulated depolarization current (TSDC) measurements and pulsed electroacoustic analysis (PEA) results are used to investigate space charge evolution and trapping. These techniques reveal the potential underlying phenomena behind the DBS enhancement.

Prediction of interface dielectric relaxations in bimodal brush functionalized epoxy nanodielectrics by finite element analysis method

Finite element 2-D analysis was implemented to simulate the dielectric spectra of nanodielectrics. As a test case, silica modified with a high graft density of short molecules and a low graft density of epoxy compatible chains were incorporated into epoxy. TEM images of the composites filler distribution were used to construct the model geometry with the interfacial area specifically included. The interfacial area was found to have dielectric relaxation behavior different from that of the matrix, as described by additional fitting parameters. This modeling method has the potential to improve our understanding of the impact of interface properties on the dielectric properties of composites.

Free volume in nanodielectrics

It is now well established that the incorporation of nanoparticulates into a polymer matrix to form a nanodielectric can bring about useful improvements in electric strength provided that the processing results in acceptable particle dispersion. One of the theories of electrical breakdown in polymers seeks to associate breakdown with the polymer free volume, and the well-documented substantial changes in electric strength occurring at glass transition provide some credence to that theory. While it is clear that interactions at the large internal interfacial area in a nanodielectric facilitate the augmentation of dielectric strength, the exact mechanism has not been clarified. Very early work using a compression methodology to estimate free volume in an epoxy nanodielectric indicated that the particles did little to affect free volume. However, this contribution seeks to use positron annihilation lifetime spectroscopy to estimate free volume in an epoxy and several of its SiO2-based nanocomposites to estimate the changes in free volume brought about both through the incorporation of nanoparticulates and also through their functionalization. This study broadly confirms the earlier work and provides good evidence that the free volume is little affected, and thus is not an explanation for the observed change in dielectric strength.