The diagnostic technique, polarization and depolarization current (PDC) is useful for insulation testing. It requires applying a DC step voltage to the test sample and measuring the current. To measure fast PDC phenomena a fast step is needed. One way of applying a fast high voltage step is to use power electronic switches. Series connection can be used to increase the voltage limit, but this result in unequal voltage sharing unless equipped with voltage balancing. In this work a high voltage vacuum reed switch is investigated as a simple and low-cost alternative to power electronic switches, handling up to 10 kV with a single device. The switch turn on and off behavior was studied. It was found that the initial turn-on is good, in the range of nanoseconds, but there is a problem with the vacuum recovering its insulating properties at low currents before the contacts fully close. The required output voltage level is therefore obtained only after a further settling time that increases with increased input voltage and is much longer than the initial breakdown, e.g. 20 µs for the case of 4.5 kV input voltage. Other limitations of the fast high voltage PDC were also studied. The output voltage was measured across the test sample without adding an intentional resistor in the circuit. There were large oscillations for 1 µs but these oscillations are damped due to inherent resistance of the connecting leads, series resistance of the capacitors and resistance of the reed switch. A comparison is made between the measured and the simulated results using MATLAB to see the effect of parasitic inductance. A damping resistor was added in the circuit and the output results were again compared. With the addition of the damping resistor, the number of oscillations were reduced and their time scale was limited to 0.1 µs . An analysis is made at the end which describes the limitation occurring in determining the high frequency component of PDC. The current during the step is many orders of magnitude higher than the polarization current even at 1 µs , so measurement of the current and protection of the apparatus is not trivial.

The typical construction of a stator coil includes the use of end corona protection (ECP)coating, which is made of semi-conductive materials like silicon carbide (SiC). Thepurpose of ECP is to smooth the electric stress distribution near the slot exit, limitingthe electric field and partial discharge (PD) activity within the insulation system. Thisthesis investigates how ECP affects the dielectric response of a stator coil in highvoltagemeasurements, both in the time-domain and frequency-domain. It also studieshow well time-domain results transformed to the frequency-domain correspond to directmeasurements. As a further point, the effect of the ECP on PD activity was demonstrated.Measurements of dielectric response and PD were made on new coil-halves producedin the usual way in a production run for a motor. The results show that applying theECP design significantly reduces the PD activity and increases the inception voltage. Inaddition, the dielectric response of the coil with ECP tape has shown to have a nonlinearvoltage dependent characteristic due to the presence of ECP. As the voltage goes higher,this causes a shift of the loss peak toward higher frequency. A distributed-element modelof the end section of a stator coil was used to model the nonlinear effect due to ECP, andits results are compared to the measured results. Lastly, FEM simulation of a stator coilend section is presented, showing the contribution of ECP tape in evenly distributing theelectric stress near the slot exit. / Den typiska konstruktionen av en statorhärva använder ändglimmskydd (ECP), somär tillverkad av halvledande material som kiselkarbid (SiC). Syftet med ECP är attjämna ut det elektriska fältet vidövergången från spåret tilländlindningen, för attundvika högpåkänning och partiell urladdningsaktivitet (PD). Arbetet som presenteras här undersöker hur ECP påverkar den dielektriska responsen hos en statorspole, vid tidsdomänoch frekvensdomänmätningar vid höga spänningar. Den studerar också hur väl tidsdomänresultat som omvandlas till frekvensdomänen motsvarar direkta mätningar. Som en annan punkt visades ECP:s inverkan på PD-aktivitet. Mätningar av dielektrisk respons och PD gjordes på nya statorhärvor som tillverkades för en stor motor. Resultaten visar att tillämpningen av ECP-konstruktionen avsevärt minskar PD-aktiviteten och ökar inceptionsspänningen. Dessutom har spolens dielektriska respons med ECP-tejp visat sig ha en ickelinjär spänningsberoende egenskap på grund av närvaron av ECP. Ö kad spänning orsakar en förskjutning av förlusttoppen mot högre frekvens. En modell för ECP användeseteendet, och dess resultat jämförs med mätningarna.utligen presenteras FEM-simulering som visar hur ECP gör fältet jämnare.

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.

This study investigates relaxation polarization in some dielectric components of tester UNISPOT S40 developed by company UNITES Systems a.s. Relaxation polarization in the PCB substrates and in tester’s cable insulation causes undesirable deceleration of some test steps. Relaxation polarization in selected PCB substrates and various cables are examined by method of dielectric relaxation spectroscopy in the frequency domain and by measurement of polarization or depolarization current.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Samples of Bi 2Fe4O9 mullite and ReFe 0.5M0.5O3 (Re = Nd, Sm, Eu, Gd, Dy; M = Mn, Al) complex perovskites were successfully synthesized by using the combustion synthesis method. While the mullite was obtained after thermal treatment at temperatures below 1000ºC, complex perovskites are produced after thermal treatments between 1250ºC and 1500ºC for at most 24 h. The X-ray diffraction data analysis suggests the formation of single phase orthorhombic structure, after suitable heat treatment. Scanning electron microscopy (SEM) revealed the formation of micrometric grain size, giving to the sample the relative density. EDS spectra confirmed the homogeneity and purity of complex perovskites. Magnetization measurements as a function of field and temperature showed the diversity of magnetic behavior of the samples. Among the behaviors we can highlight the reorientation of spin for ortoferritas ReFe0.5Mn0.5O3 (Re = Dy, Gd, Eu, Sm, Eu) and ReFe0.5Al0.5O3 (Re = Dy, Nd). Another interesting finding was the effect of magnetic reversal of the samples ReFe0.5Al0.5O3 (Re = Nd, Gd) and EuFe0.5Mn0.5O3. Raman spectra show anomalous bands of second order in the perovskite DyFe0.5Al0.5O3 with characteristics of resonant effects. The evolution of the band associated with the symmetric vibrational mode of the octahedron shows hardening at the temperature range of ordered magnetic phase, thus suggesting a possible spin-phonon coupling. Depolarization current measurements highlights a relaxation process due to charge carriers in the samples ReFe0.5Mn0.5O3 (Re = Dy, Gd). The application of magnetic field profoundly influences the depolarization current behavior of DyFe0.5Mn0.5O3. Dielectric permittivity measurements showed no anomalies between 10 K and 300 K, that could can be associated to a ferroelectric phase. / Amostras da mulita Bi2Fe4O9 e das perovskitas complexas ReFe0.5M0.5O3 (Re= Nd, Sm, Eu, Gd, Dy; M = Mn, Al) foram sintetizadas com sucesso utilizando o método de síntese por reação de combustão. Enquanto a mulita foi obtida após tratamento térmico com temperaturas abaixo de 1000ºC, as perovskitas complexas foram produzidas após tratamentos térmicos entre 1250ºC e 1500ºC por, no máximo, 24 h. A análise de dados de difração de raios X sugerem a formação de estruturas ortorrômbicas de fase única, após os tratamento térmico apropriado. Medidas de microscopia evidenciaram formação de grãos micrométricos conferindo às amostras relativa densidade. Os espectros EDS confirmaram a homogeneidade e pureza das perovskitas complexas. As medidas de magnetização em função do campo e da temperatura evidenciaram a diversidade de comportamentos magnéticos das amostras estudadas. Entre os comportamentos destacamos reorientação de spin para as ortoferritas ReFe0.5Mn0.5O3 (Re = Dy, Gd, Eu, Sm, Eu) e ReFe0.5Al0.5O3 (Re = Dy,Nd). Outro interessante achado foi o efeito de inversão magnética das amostras ReFe0.5Al0.5O3 (Re = Nd, Gd) e EuFe0.5Mn0.5O3. Espectros Raman demonstram bandas anômalas de segunda ordem na perovskita DyFe0.5Al0.5O3 com características de efeitos ressonantes. A evolução da banda associada ao modo vibracional simétrico do octaedro apresenta endurecimento em temperaturas na faixa da fase magnética ordenada, sugerindo assim, um possível acoplamento spin-f onon. Medidas de corrente de despolarização destaca um processo de relaxação devido aos portadores de cargas nas amostras ReFe0.5Mn0.5O3 (Re=Dy,Gd). A aplicação de campo magnético influencia profundamente o comportamento da corrente de despolarização do DyFe0.5Mn0.5O3. Medidas de permissividade dielétrica não exibiram anomalias entre 10 K e 300 K que possam a ser associadas a uma fase ferroelétrica.