Mattewos Tefferi

Poly(dimethyltin glutarate) as a Prospective Material for High Dielectric Applications

Poly(dimethyltin glutarate) is presented as the first organometallic polymer, a high dielectric constant, and low dielectric loss material. Theoretical results correspond well in terms of the dielectric constant. More importantly, the dielectric constant can be tuned depending on the solvent a film of the polymer is cast from. The breakdown strength is increased through blending with a second organometallic polymer.

Rational design and synthesis of polythioureas as capacitor dielectrics

Rational strategies combining computational and experimental procedures accelerate the process of designing and predicting properties of new materials for a specific application. Here, a systematic study is presented on polythioureas for high energy density capacitor applications combining a newly developed modelling strategy with synthesis and processing. Synthesis was guided by implementation of a high throughput hierarchical modelling with combinatorial exploration and successive screening, followed by an evolutionary structure search based on density functional theory (DFT). Crystalline structures of polymer films were found to be in agreement with DFT predicted results. Dielectric constants of ∼4.5 and energy densities of ∼10 J cm−3 were achieved in accordance with Weibull characteristic breakdown fields of ∼700 MV m−1. The variation of polymer backbone using aromatic, aliphatic and oligoether segments allowed for tuning dielectric properties through introduction of additional permanent dipoles, conjugation, and better control of morphology.

Optimization of Organotin Polymers for Dielectric Applications

Recently, there has been a growing interest in developing wide band gap dielectric materials as the next generation insulators for capacitors, photovoltaic devices, and transistors. Organotin polyesters have shown promise as high dielectric constant, low loss, and high band gap materials. Guided by first-principles calculations from density functional theory (DFT), in line with the emerging codesign concept, the polymer poly(dimethyltin 3,3-dimethylglutarate), p(DMTDMG), was identified as a promising candidate for dielectric applications. Blends and copolymers of poly(dimethyltin suberate), p(DMTSub), and p(DMTDMG) were compared using increasing amounts of p(DMTSub) from 10% to 50% to find a balance between electronic properties and film morphology. DFT calculations were used to gain further insight into the structural and electronic differences between p(DMTSub) and p(DMTDMG). Both blend and copolymer systems showed improved results over the homopolymers with the films having dielectric constants of 6.8 and 6.7 at 10 kHz with losses of 1% and 2% for the blend and copolymer systems, respectively. The energy density of the film measured as a D-E hysteresis loop was 6 J/cc for the copolymer, showing an improvement compared to 4 J/cc for the blend. This improvement is hypothesized to come from a more uniform distribution of diacid repeat units in the copolymer compared to the blend, leading toward improved film quality and subsequently higher energy density.

A thermo-electrodynamic electric field dependent molecular ionization model to realize positive streamer propagation in a wet-mate DC connector

Complete subsea factory concept, an equivalent of the full topsides processing facility to be operated on the seabed, is envisaged to power longer, deeper and colder subsea oil and gas fields in the future. This concept has been envisioned through a modular stacked subsea DC transmission and distribution system whose subsea umbilical cables and electrical power component on the seabed can be interfaced with each other by wet-mate (WM) DC connectors. Laboratory and theoretical investigations have been carried out to assess various electrical insulation systems and electrode geometries for a WM DC connector which should operate in the steady state as well as switching transients in a corrosive environment for high reliability and minimum maintenance in its lifetime. In this paper, the electrical insulation performance of a needle-sphere electrode geometry defined by IEC 60897 under a positive step voltage is studied. To approach the complicated solid-liquid insulation system envisaged in a WM DC connector after mating, the electrodes are covered by a dielectric solid and oil is enclosed by the dielectric solid as well. A full thermo-electrodynamic electric field dependent molecular ionization Multiphysics model was developed for the simulation of streamer initiation and growth in the oil while dielectric solid is modeled as a perfect insulator. It is shown that stabilization methods, mesh strategies and time step have a great influence on simulation results and guidelines to choose them properly are presented. Based on simulation results, it was found that the higher relative permittivity of the solid insulation the slower streamer propagation in the oil and the less electrical stress on the solid insulation.

Modeling a liquid-solid insulation system used in a DC wet-mate connector

Subsea DC transmission and distribution system is a promising technology for powering subsea oil and gas fields with high power, long distance and ultra-deepwater depth. DC connectors as the interface between cables and electrical power component and loads in such a system play a key role. An electrodynamic model is developed in this paper to study charge transport phenomena in a DC wet-mate subsea connector. The oil-solid insulation system of WM chamber contains synthetic transformer oil enclosed by dielectric solid and the electrodes covered by dielectric solid. This complicated hybrid insulation system placed in a cylindrical electrode geometry is simulated in this paper. It is shown that the oil can be susceptible to the streamer initiation and development. Moreover, it will be discussed that the free space charge carriers traveled to the interface between the oil and dielectric solid and converted to the surface charges may increase the electric field magnitude across the dielectric solid.

The “materials space” of DC polymeric dielectrics

The operation of dielectrics in DC power apparatus depends on nonlinear and temperature-dependent conductivity of the dielectric. These two parameters are (anti-)correlated. We propose a relatively simple formalism for modeling the conductivity of polymeric dielectrics which includes this correlation based on relatively few physical parameters and which fits well the available, but admittedly limited, published data.

Electrical-insulation behavior of cellular polymer foams in comparison to their piezoelectret properties

Polymer foams are widely used in daily life for thermal insulation, shock and sound absorption, packaging, etc. A more recent development is the emergence of so-called ferroelectrets (or piezoelectrets). They are completely non-polar polymer foams that exhibit piezoelectricity after internal charging of the cavities by means of a series of dielectric barrier discharges (DBDs). The internally charged cavities are considered as macroscopic dipoles, which — in combination with the heterogenous mechanical properties of the material — induce the piezoelectricity. The piezoelectric properties of ferroelectrets and their dependence on material structures have been extensively studied in recent years. Polymer materials are often used for electrical insulation e.g. in electronic devices or in AC and DC power transmission and distribution systems. Due to their unique microstructure, polymer foams possess specific electrical and functional properties and are promising for a wide range of electrical applications. Here, we comparatively study piezoelectric and electrical insulation properties of polymer foams and their dependence on material structures.

Characterization of solid-liquid interface for wet-mate subsea HVDC connectors

Wet-mate connectors are essential components in a submarine HVDC system. Solid-liquid insulation systems play an important role in the electrical insulation performance of a WM chamber. In this paper, the electric field distribution in synthetic ester oil of a solid-oil insulation system is studied by using Kerr electro-optic technique where the measurements were carried out with the presence of moisture and ionic contaminants. Moreover, effect of voltage polarity on the electric field distribution in the oil is investigated. For two solid liquid interface geometries studied, the electric field norm decreases gradually from the ground electrode to the HV electrode under positive DC while it remains almost constant for an applied negative DC voltage. The deviation of magnitudes of electric fields particularly near the electrodes suggests the presence of positive ions in the oil under positive and negative testing conditions.