Resistive Electrical Field Grading of Insulation Oil-Solid Interfaces

There is always a need for more compact designs of power transformers free of partial discharges, in order to save cost on the construction and required material resources. The physical geometric constrictions inside the transformer tank would demand field-grading techniques to homogenise the field strength distribution on oil-solid interfaces, when required. Standard filler materials such as carbon black or silicon carbide (SiC) have a too high electrical conductivity yielding an appropriate grading field strength values for air-related applications. Because insulation oil has a higher electrical breakdown strength, the electrical conductivity must be engineered to lower values in order to reach a higher effective grading field strength. This paper presents the investigation of a new material system based on a phenolic resin Lerg FL-500 and the electrically functionalized ceramic filler particles Merck Iriotec®7550 that enable a resistive electrical field grading in insulation oil. In order to verify the principle functionality of the proposed field grading system, a layer is applied on a substrate surface representing possible oil-solid-interface inside oil-filled power transformers. First, the manuscript describes the methods of specimen preparation and the measurement of the nonlinear current-time behaviour under AC voltage stress for different filler contents. Second, a concurring optical and electrical determination of the partial discharge inception and extinction voltage of a modified Toepler arrangement allows the indirect determination of the electrical field strength distribution along the functionalized layer without the need of direct measurement. To do so, the radius of the circular functional layer is varied and with it the specific grading length. In analogy to state of the art SiC-filled systems, a linear dependency between the effective grading length and the PD inception voltage is observed. The quotient of voltage drop over a varied radius yields the effective graded electric field strength

Synthese und Charakterisierung von schichtartigen Doppelhydroxiden (LDHs) unter besonderer Berücksichtigung von Hydrocalumiten und Hybridmaterialien mit organischen Farbstoffen

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Substrate effects on the surface topography of evaporated gold films: a scanning tunnelling microscopy investigation

Vancea J, Reiss G, Schneider F, Bauer K, Hoffmann H. Substrate effects on the surface topography of evaporated gold films: a scanning tunnelling microscopy investigation. Surface science. 1989;218(1):108-126.Direct observation of surface roughness on metal films is a longstanding problem in thin film characterization. In this work the high quality of scanning tunnelling microscopy (STM) was used for investigation of evaporated gold films. A scanning tunnelling microscope able to scan areas up to 0.8 × 0.8 micro m with high reproducibility is presented. The topography of 80 nm thick gold films grown under identical evaporation conditions was investigated as a function of the selected substrate material (Corning glass, silicon, NaCl, mica and highly oriented pyrolitic graphite (HOPG)). The incipient growth mechanism on the substrate is the primary reason for the surface roughness. The real space images of the surface topography correlate very well with knowledge achieved from former growth experiments given in the literature. Moreover, very flat gold surfaces on HOPG allowed the observation of atomic corrugations in air environments

Mechanical behavior and size effects of polymer/amorphous NiB composites with 3D micro‐ architectures

Micro-architectured materials are a new class of hierarchical cellular material with outstanding properties. By designing advantageous cellular geometries and combining the material size effects at the nanometer scale, lightweight hybrid micro-architectured materials with hierarchical cellular structures and tailored structural properties are achieved. Previous papers have reported the mechanical properties of ceramic/polymer composites but few studies have examined the properties of similar structures with metal coatings instead of ceramic. To estimate the mechanical performance of polymer cellular structure reinforced with a metal coating, we combined 3D laser lithography and electroless deposition of an amorphous layer of NiB to produce metal/polymer hybrid structures. In this poster, the fabrication of 3D hybrid structures by electroless deposition aiming at achieving high and yet low density material will be presented. We also studied the mechanical response of micro-architectured structures as a function of the architecture design and the thickness of the amorphous NiB layer on their deformation mechanisms. In situ SEM microcompression experiments revealed a change in the deformation behavior with the NiB layer thickness, suggesting that the deformation mechanism and the buckling behavior are controlled by the size induced brittle-to-ductile transition in the NiB layer. In addition, the energy absorption properties demonstrate the possibility of tuning the energy absorption efficiency with adequate designs. Please click Additional Files below to see the full abstract

Does voter polarisation induce party extremism? The moderating role of abstention

This article contributes to the literature on representation by examining how the ideological polarisation of the electorate affects parties’ programmatic positions in multiparty systems. The main argument is that parties face incentives to adopt more extreme positions when the electorate becomes more ideologically polarised and the share of non-moderate voters is higher. The reason is that by adopting moderate positions parties will prompt their non-moderate core constituents to sit out the election. This risk is conditioned by voters’ propensity to abstain. A higher (lower) propensity to abstain means that parties alienate a larger (smaller) share of their core constituents when adopting a moderate position. Parties therefore respond to greater voter polarisation by adopting more extreme positions, but the effect declines as voters’ propensity to abstain decreases. An empirical analysis of parties’ programmatic positions in 11 Western European countries between 1977 and 2016 strongly supports this expectation