OIL CORROSION AND CONDUCTING CU 2 S DEPOSITION IN POWER TRANSFORMER WINDINGS

SUMMARY Copper corrosion and Cu 2 S deposition, has recently led to failures of transformers & shunt reactors during service. Units from several different operators and several OEMs have been affected. Analysis of actual failure cases showed that these transformers were well within established industry standard practice regarding design as well as operation, and used oil that fulfilled international material standards. Sealed units with rubber sack appear primarily to have been affected. Final failure has occurred as a turn-to-turn breakdown in the HV windings, and inspection of the failed windings showed deposits in the winding consisting of Cuprous Sulphide, Cu 2 S, an electrically conducting compound. The deposits varied within the winding, usually with more deposits in the upper part of the windings, but the complex deposition pattern could not be explained simply by the temperature distribution. We have carried out comprehensive corrosion research aiming at a thorough understanding of the phenomenon, and have reproduced the corrosion as well as the deposition of Cu 2 S in our laboratories. The influence of the gas content of the oil, and other parameters have been studied under very well controlled circumstances, and a number of different oils have been evaluated at temperatures ranging from 80 to 150 °C. The main finding is that Cu 2 S deposition occurs only for certain oils, which can cause corrosion and deposition under the typical environment of a sealed power transformer. With such oils deposition has been demonstrated over a wide temperatures range, from as low as 80 °C, and on a variety of solid materials. These oils further show a very strong oxygen dependence of the deposition, which explains much of the observed complex deposition patterns in the failed units. Real failures have all occurred with transformer oils that passed the corrosion tests in the international standards, DIN 51353 and ASTM D1275 respectively. Improved test methods are clearly needed that can take into account the observed oxygen dependence, an important factor present in real transformers. A Covered Conductor Deposition (CCD) test has demonstrated a superior selectivity for identifying the corrosive oils that have caused actual failures in transformers, and is thus a very promising type of test

Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films

Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model which captures this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device.Comment: To appear in Nature Communications (2014); Expanded version of http://arxiv.org/abs/1307.548

Hedgehog Spin-texture and Berry's Phase tuning in a Magnetic Topological Insulator

Understanding and control of spin degrees of freedom on the surfaces of topological materials are key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal (TR) symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi2Se3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies, which directly demonstrate the mechanics of TR symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunnelling between surfaces exhibits spin texture modulation at low energies but respects TR invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for the future realization of many predicted exotic magnetic phenomena of topological origin.Comment: 38 pages, 18 Figures, Includes new text, additional datasets and interpretation beyond arXiv:1206.2090, for the final published version see Nature Physics (2012

Spin-orbit coupling induced Van Hove singularity in proximity to a Lifshitz transition in Sr4Ru3O10

Funding: CAM, MN and PW gratefully acknowledge funding from the Engineering and Physical Sciences Research Council through EP/R031924/1 and EP/S005005/1, IB through the International Max Planck Research School for Chemistry and Physics of Quantum Materials and LCR from a fellowship from the Royal Commission of the Exhibition of 1851. RA, RF and AV thank the EU’s Horizon 2020 research and innovation program under Grant Agreement No. 964398 (SUPERGATE).Van Hove singularities (VHss) in the vicinity of the Fermi energy often play a dramatic role in the physics of strongly correlated electron materials. The divergence of the density of states generated by VHss can trigger the emergence of new phases such as superconductivity, ferromagnetism, metamagnetism, and density wave orders. A detailed understanding of the electronic structure of these VHss is therefore essential for an accurate description of such instabilities. Here, we study the low-energy electronic structure of the trilayer strontium ruthenate Sr4Ru3O10, identifying a rich hierarchy of VHss using angle-resolved photoemission spectroscopy and millikelvin scanning tunneling microscopy. Comparison of k-resolved electron spectroscopy and quasiparticle interference allows us to determine the structure of the VHss and demonstrate the crucial role of spin-orbit coupling in shaping them. We use this to develop a minimal model from which we identify a new mechanism for driving a field-induced Lifshitz transition in ferromagnetic metals.Peer reviewe

Spin-orbit coupled spin-polarised hole gas at the CrSe2-terminated surface of AgCrSe2

Funding: We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Engineering and Physical Sciences Research Council (Grant No. EP/T02108X/1), and the Leverhulme Trust (Grant No. RL-2016-006). S.-J.K., E.A.M., A.Z., and I.M. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.In half-metallic systems, electronic conduction is mediated by a single spin species, offering enormous potential for spintronic devices. Here, using microscopic-area angle-resolved photoemission, we show that a spin-polarised two-dimensional hole gas is naturally realised in the polar magnetic semiconductor AgCrSe2 by an intrinsic self-doping at its CrSe2-terminated surface. Through comparison with first-principles calculations, we unveil a striking role of spin-orbit coupling for the surface hole gas, unlocked by both bulk and surface inversion symmetry breaking, suggesting routes for stabilising complex magnetic textures in the surface layer of AgCrSe2.Publisher PDFPeer reviewe

Hearing screening by the public switched telephone network : Development and validation of test application

En stor del av befolkningen har en hörselnedsättning men långt ifrån alla som skulle kunna dra nytta av hörselhjälpmedel söker vård i tid. Det finns ett behov av att kunna göra ett snabbt och enkelt screening-test hemifrån och i flera andra europeiska länder har det för det ändamålet tagits fram telefon- och Internetbaserade hörseltest av tresiffertyp.  Dessa tester spelar upp tre inlästa siffror samtidigt som ett störande brus-ljud och en adaptiv metod används för att automatiskt uppskatta testpersonens genomsnittliga, uppfattbara signal-brus-förhållande. Studier har visat att förmågan att urskilja tal i brus försämras för personer med hörselnedsättning och att den absoluta uppspelningsvolymen inte spelar väsentlig roll så länge ljudet är klart hörbart.  Ett svenskt talmaterial för ett sådant test har tagits fram inom EU-projektet HearCom. Syftet med examensarbetet var att implementera och validera en funktionell och prisvärd lösning i samarbete med en aktör inom telekommunikationsbranschen.  Efter en upphandlingsprocess valdes ett Linux-baserat system med den öppna telefonserver-programvaran Asterisk. En normalstudie genomfördes där resultaten pekar på att det utvecklade testet är jämförbart med liknande test i andra länder. Telefontestet jämfördes även med en befintlig Internetversion och båda testerna utvärderades med avseende på användbarhet. Vissa förbättringsåtgärder har föreslagits för testerna.  Rapporten tar också upp förutsättningar för en relevant gränssättning av testerna samt de återstående stegen till en färdig talsvarstjänst som allmänheten kan ha nytta av

Photoelectron Spectroscopy of Cuprates and Manganites

Abstract In this thesis, the high temperature superconductors (HTSC) La2CuO4+x and Bi2Sr2CaCu2O8+d and the colossal magnetoresistive (CMR) La1-xSrxMnO3 have been studied by photoelectron spectroscopy. Techniques such as angular resolved photoemission (ARPES), resonant photoemission spectroscopy (RESPES) and x-ray absorption spectroscopy have been applied, using synchrotron radiation. The aim of the study has been to investigate the electronic structure of these materials. By using resonance photoemission spectroscopy on La2CuO4+x single crystals, we have found the presence of La states in a large region in the valence band and strong evidence for the existence of surface derived La-O electronic bands in the c^surface orientation. We have also investigated the influence of surface termination effects by using different methods of cleaving the samples. This reveals that the topmost layer can be either La-O or Cu-O, giving rise to remarked different photoelectron spectra. The generally accepted picture of the Fermi-surface topology of B2Sr2CaCu2O8, is large hole-like crossings centered around the X(Y) symmetric point. We have performed angular resolved measurements showing that by choosing a proper photon energy and experimental geometry, an electron-like Fermi surface can clearly be seen centered at the G-point. We have also found that the ARPES holelike Fermi surface is photon energy dependent. Magnetoresistant materials such as the La1-xSrxMnO3 exhibit a complicated phase diagram as function of doping and temperature. Many questions remain to be answered regarding their electronic and magnetic structure. By core level spectroscopy on single crystal in a wide doping region and for different temperatures temperature. Doping and temperature dependent shifts were seen in the core levels, reflecting the changes in screening and band filling connected to the different regime in the phase diagram. The properties of the states in the valence band and close to the Fermi level are of fundamental importance of describing the electronic properties of the complex metal oxides. Up to now, few systematic studies of the valence band with respect to doping and temperature have been performed. By angular resolved and angular integrated valence band spectroscopy on single crystals we have observed binding energy shifts of the spectral features in the valence band for increasing Sr concentration, as well as an increase of the density of states close to the Fermi level. A feature, which has not previously been observed, was found in the valence band. Resonant photoemission close to the Mn 2p threshold shows that this feature is related to Mn states. The nature of this feature is discussed in terms of band structure and Mn 3d orbital orientation