Dielectric Breakdown in Chemical Vapor Deposited Hexagonal Boron Nitride

Insulating films are essential in multiple electronic devices because they can provide essential functionalities, such as capacitance effects and electrical fields. Two-dimensional (2D) layered materials have superb electronic, physical, chemical, thermal, and optical properties, and they can be effectively used to provide additional performances, such as flexibility and transparency. 2D layered insulators are called to be essential in future electronic devices, but their reliability, degradation kinetics, and dielectric breakdown (BD) process are still not understood. In this work, the dielectric breakdown process of multilayer hexagonal boron nitride (h-BN) is analyzed on the nanoscale and on the device level, and the experimental results are studied via theoretical models. It is found that under electrical stress, local charge accumulation and charge trapping/detrapping are the onset mechanisms for dielectric BD formation. By means of conductive atomic force microscopy, the BD event was triggered at several locations on the surface of different dielectrics (SiO2, HfO2, Al2O3, multilayer h-BN, and monolayer h-BN); BD-induced hillocks rapidly appeared on the surface of all of them when the BD was reached, except in monolayer h-BN. The high thermal conductivity of h-BN combined with the one-atom-thick nature are genuine factors contributing to heat dissipation at the BD spot, which avoids self-accelerated and thermally driven catastrophic BD. These results point to monolayer h-BN as a sublime dielectric in terms of reliability, which may have important implications in future digital electronic devices.Fil: Jiang, Lanlan. Soochow University; ChinaFil: Shi, Yuanyuan. Soochow University; China. University of Stanford; Estados UnidosFil: Hui, Fei. Soochow University; China. Massachusetts Institute of Technology; Estados UnidosFil: Tang, Kechao. University of Stanford; Estados UnidosFil: Wu, Qian. Soochow University; ChinaFil: Pan, Chengbin. Soochow University; ChinaFil: Jing, Xu. Soochow University; China. University of Texas at Austin; Estados UnidosFil: Uppal, Hasan. University of Manchester; Reino UnidoFil: Palumbo, Félix Roberto Mario. Comisión Nacional de Energía Atómica; Argentina. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lu, Guangyuan. Chinese Academy of Sciences; República de ChinaFil: Wu, Tianru. Chinese Academy of Sciences; República de ChinaFil: Wang, Haomin. Chinese Academy of Sciences; República de ChinaFil: Villena, Marco A.. Soochow University; ChinaFil: Xie, Xiaoming. Chinese Academy of Sciences; República de China. ShanghaiTech University; ChinaFil: McIntyre, Paul C.. University of Stanford; Estados UnidosFil: Lanza, Mario. Soochow University; Chin

An investigation into solid dielectrics

Direct measurement techniques for the investigation of electrical processes in solid dielectrics are reviewed and their respective strengths and weaknesses are discussed, particularly the complementary nature of thermally stimulated current measurements. The successful design and construction of a new Thermally Stimulated Discharge Current (TSDC) Spectrometer at the University of Southampton is presented and its correct function validated with experimental measurements of the well known and often characterized synthetic polymers low density polyethylene (LDPE) and polyethylene terephtalate (PET). Results were found to correspond well to published data. First TSDC observations of filled and oil impregnated papers are presented.The second aspect of this work is the investigation of natural polymer insulation materials,specifically paper for oil-paper insulation systems. For the first time, electrical insulation papers with filler contents up to 50% were investigated. Bentonite and talcum were compared as filler materials and found to have negative and positive effects respectively.The superior electrical strength of a talcum filled kraft paper was verified, and a series of constructive modifications was undertaken to further maximise its electrical strength at comparable or improved dielectric performance. An increase in electrical breakdown strength of 20% to 30% has been observed, but the substitution of such great amounts of fiber with fillers also lead to a reduction in mechanical strength of the paper. Further trials with chemical additives were conducted to counteract this effect and polyvinyl alcohol and starch were found to enhance the paper strength. Additional trials also comprised sizing agents, guar gum and wet strength agents. Uncharged or slightly charged chemical additives provided best results with regard to dielectric performance. The significance of the trialled paper modifications is judged in light of statistical analysis

Review of recent research towards power cable life cycle management

Power cables are integral to modern urban power transmission and distribution systems. For power cable asset managers worldwide, a major challenge is how to manage effectively the expensive and vast network of cables, many of which are approaching, or have past, their design life. This study provides an in-depth review of recent research and development in cable failure analysis, condition monitoring and diagnosis, life assessment methods, fault location, and optimisation of maintenance and replacement strategies. These topics are essential to cable life cycle management (LCM), which aims to maximise the operational value of cable assets and is now being implemented in many power utility companies. The review expands on material presented at the 2015 JiCable conference and incorporates other recent publications. The review concludes that the full potential of cable condition monitoring, condition and life assessment has not fully realised. It is proposed that a combination of physics-based life modelling and statistical approaches, giving consideration to practical condition monitoring results and insulation response to in-service stress factors and short term stresses, such as water ingress, mechanical damage and imperfections left from manufacturing and installation processes, will be key to success in improved LCM of the vast amount of cable assets around the world

Impulse ageing of polymeric materials

Impulse over-voltage is a common phenomenon in electric power systems. A switching impulse is created by a switching surge or local fault while a lightning impulse is due to direct lightning strike to high voltage plant such as an overhead line. Both impulse events create travelling waves in the system, damaging insulation components and equipment. This work is concerned with the hypothesis that lightning impulses can lead to accelerated ageing of extruded polymeric cables. The results show that there may well be a reduction in electric field strength of the insulation of a power cable that experiences a lot of impressed lightning impulse over-voltages. Pre-designed shaped polyethylene material sample discs have been manufactured using a mould tool. The samples then have been electrically aged using an impulse generator. A real-time software based monitoring tool has been designed to control the impulse wave-shape and process the measurement data. Sets of identical lightning impulses were applied to samples and this was then followed by ramped AC breakdown tests. The obtained results were analyzed using the Weibull distribution to identify any differences in lifetime between aged and un-aged samples. This thesis also provides insight into the dominant ageing processes through the analysis of dielectric spectroscopy and space charge measurement data. In order to quantify the effects of dielectric ageing due to impressed lighting impulse over voltages, experiments have also been undertaken using samples that have been aged under UV light and thermally. Analysis of obtained results reveals that mechanisms of these two ageing processes are significantly different from the mechanisms due to lightning impulse agein

Process development and reliability of thin gate oxides

The Semiconductor Industry Association\u27s (SIA) current National Technological Roadmap calls for the development of a suitable dielectric material for use in gate oxide for the 0.18|micrometers generation of chips and beyond. Some of the key challenges identified are resistance to oxide trapped charge generation from higher levels of tunneling currents and/or plasma processing, and formation of an effective barrier to dopant penetration during the gate processing. One promising material to meet these challenges is nitrided thermal oxide. Development of a growth process that yields high quality, lOnm thick, thermally grown Si02 films at RJT for use as a gate dielectric is described. Thin oxides (8nm - 20nm) were grown by thermal oxidation followed by inert anneals in Ar and N2. Nitrided oxides were created by implanting N2 (dose range: 5el3 - lei 5 /cm2) into the substrate prior to gate oxidation. Test equipment was setup to study Fowler Nordheim (FN) tunneling and dielectric breakdown. Test structures consisted of conventional and novel MOS capacitor structures with aluminum and poly-silicon gate electrodes. Scaling RJT\u27s existing, 20nm oxidation process to lOnm resulted in degradation of dielectric strength from \u3e lOMV/cm to ~6-7MV/cm for Al-gate MOS capacitors. Replacing the Al gate material with poly-silicon restored the dielectric strength to lOMV/cm. Performing an N2 implant through a screening oxide, prior to gate oxidation, was investigated as a means of obtaining a nitrided thermal oxide. For certain doses (5el3 - 5el4 /cm2), Al-gate MOS capacitors exhibited an improved dielectric strength as the mean value increased from 6- 7MV/cm to ~9MV/cm. Poly-Si gate MOS capacitors showed a similar improvement for the nitrided oxides, exhibiting mean dielectric strength values in the 10-12MV/cm range. Fowler- Nordheim (FN) tunnel current measurements showed that the nitrided films exhibit lower leakage levels and less charge trapping than their thermal Si02 counterparts. Results indicate that a 12nm nitrided oxide, for a certain dose (5el4/cm2), exhibited equivalent electrical performance to a 20nm thermally grown Si02 oxide. In conclusion, a process was developed for yielding reliable thin gate oxides (~10nm) in a university fab

Study of oil/pressboard progressive creeping discharge under highly divergent electric field

As one of the most dangerous failure modes of transformers, the oil/pressboard progressive creeping discharge, as well as the corresponding insulation failure mechanisms, have thus far not been fully understood. The present thesis aims to gain a sound fundamental knowledge of the progressive creeping discharge, their influential factors, and the relevant failure mechanisms, as well as to contribute to an effective monitoring and diagnostic methodology. This study explored the progressive creeping discharges under highly divergent AC and DC voltages, respectively. The tests conditions were configured to investigate the impacts of critical influential factors, including ageing, voltage level, temperature, stress waveform, and oil flowing, on creeping discharges. The discharge processes were systematically evaluated using tools such as partial discharge, gas analysis, temperature monitoring, and finite-element simulation. Comparative analyses and theoretical examinations were made to the insulation faults as well as their correlated physicochemical phenomena. A conventional techniques-based monitoring framework was proposed for the dangerous creeping discharges, and the applicability of a new detection technique based on synchronous PD and dielectric measurements was tested as well. The characteristics of damage-free AC progressive creeping discharge, as well as the influences of pressboard ageing and field intensity, were investigated. It was found the progressive creeping discharge is a compound discharge that comprises mainly oil corona discharge and surface discharge. Pressboard ageing affected the discharge inception but had no influence on the interface breakdown strength. The damage-free discharge exhibited a constant PRPD pattern but had a diminishing PD intensity, where the pressboard ageing and voltage level jointly determine whether the discharges can be sustained or not. The damage-free discharge generated hydrogen principally and was located between D1/ D2 fault zones in the classical Duval’s Triangle. The insulation failures due to dangerous creeping discharges were comparatively studied. It was found the AC progressive creeping discharge could generate two faults to the pressboard, i.e., internal treeing (internal treeing) and surface tracking (surface tracking). The faults exhibited distinct electrical as well as physicochemical characteristics, and their occurrences were subject to pressboard ageing, voltage level, and pressboard density. A four-staged model was proposed to illustrate the development of internal treeing. The model highlighted the central role of the filament-like carbonization tracks in the pressboard interior, of which the onset and growth were theoretically expounded as well. A detection methodology was proposed for internal treeing, which was mainly based on continuous PD measurement, online temperature monitoring, and gas analysis. However, it was found still hard to detect a severe surface tracking fault due to the PD disappearance. The influences of temperature and voltage waveforms on AC progressive creeping discharge were studied. Temperature affected the short-time electric strength of either interface or oil gap and could form a synergistic effect with a strong field in exacerbating the dissipation factor of pressboard. Moreover, it was found temperature can determine the fault types and discharge severity under either constant or step-wise stress, whereas the stress waveform had an obvious effect on the fault type only at high temperatures. Depending on the test temperature and stress waveforms, the test specimens failed due to tracking-free interface flashover, surface tracking, and internal treeing, respectively. Their PD quantities were comparatively evaluated. Moreover, it was found the synchronous PD and dielectric measurement is highly indicative of the internal treeing process, and it exhibited both high detection sensitivity and discharge identification capability. The DC progressive creeping discharge was evaluated against temperatures and forced oil flows. Temperature affected the DC conductivity of both oil and pressboard, the resistivity ratio of pressboard to oil, and the oil/pressboard interface charge relaxation time. Accordingly, the interface field distributions were evaluated against temperatures using the finite-element method. Moreover, both oil flow speed and temperatures affected the short-time DC interface electric strength as well as the progressive creeping discharge. Two types of discharges were found under static oil condition. They exhibited distinct PD characteristics and showed different responses to temperature changes. They were presumably associated with discharges in oil and pressboard, respectively. Furthermore, it was found that forced oil flows can alter the behaviors of DC creeping discharges drastically and generated a peculiar PD type (Pulse Train). The occurrence of the pulse train was subject to applied voltage level, temperature, and oil flowing velocity. A theoretical model was proposed to elucidate the pulse train, in which the interactions between space charges from different sources were discussed against voltage levels, oil flows, and temperatures. The present thesis extends the knowledge of the oil/pressboard progressive creeping discharges. The research results are expected to serve as a base for future scientific studies and a piece of reference information for practical applications pertinent to progressive creeping discharges.Eine der gefährlichsten Fehlerarten in Transformatoren, die fortschreitende Kriechentladung in Öl/Feststoff Anordnungen sowie der dazugehörige Mechanismus des Isolationsfehlers sind noch nicht völlig geklärt. Die vorliegende Arbeit zielt auf die Erreichung einer soliden fundamentalen Kenntnis der fortschreitenden Kriechentladung, ihrer Einflussfaktoren und der entsprechenden Fehlermechanismen ab. Weiterhin soll sie einen Beitrag zur effektiven Überwachung und diagnostischen Methoden liefern. Die Untersuchungen beinhalten die fortschreitenden Kriechentladungen bei stark inhomogenen elektrischen Feldern unter Wechsel- und Gleichspannung. Die Versuchsbedingungen wurden so konfiguriert, dass die Wirkung der kritischen Einflussfaktoren auf die Kriechentladungen wie Alterung, Höhe der Spannung, Temperatur, Beanspruchungsform und Ölfluss, untersucht werden konnte. Die Entladungsvorgänge wurden systematisch ausgewertet unter Verwendung von Methoden wie Teilentladung, Gasanalyse, Temperaturerfassung und Simulation mit Finiten Elementen. Vergleichende Analysen und theoretische Prüfungen wurden im Hinblick auf Isolierungsfehler und der dazugehörigen physio-chemischen Erscheinungen durchgeführt. Ein auf konventioneller Technik basierendes Monitoring Konzept wurde für die gefährlichen Kriechentladungen vorgeschlagen und die Anwendung neuer Detektionstechniken basierend auf synchroner Teilentladungserfassung und dielektrischen Messungen geprüft. Die Eigenschaften von beschädigungsfreien fortschreitenden Kriechentladungen bei Wechsel-spannung und der Einfluss der Alterung des Pressspan sowie der Feldstärke wurden untersucht. Es wurde herausgefunden, dass die fortschreitende Kriechentladung eine zusammengesetzte Entladung ist, bestehend aus Korona-Entladungen in Öl und Oberflächenentladungen. Die Alterung von Pressspan beeinflusst den Einsatz der Entladung aber hat keinen Einfluss auf die Durchschlagsfestigkeit der Grenzfläche. Die beschädigungsfreie Entladung zeigt ein konstantes PRPD Muster aber mit abnehmender Teilentladungs-Aktivität, wobei die Alterung des Pressspans und die Spannungshöhe gemeinsam bestimmen, ob die die Entladungen unterbrochen werden oder nicht. Die beschädigungsfreie Entladung erzeugt grundsätzlich Wasserstoff und ist den Fehlerzonen D1/D2 des klassischen Duval Dreiecks zugeordnet. Die Fehler der Isolierung infolge der gefährlichen Kriechentladungen wurden vergleichend untersucht. Es wurde herausgefunden, dass die fortschreitende Kriechentladung bei Wechselspannung zwei Fehlerarten im Pressspan erzeugen kann, die schnelle Kriechwegbildung (internes Treeing) und die langsame Kriechwegbildung (Oberflächen Kriechweg). Die Fehler weisen deutlich elektrische als auch physio-chemische Eigenschaften auf und ihr Erscheinen war abhängig vom Alterungszustand des Pressspans, Spannungshöhe und Dichte des Pressspans. Ein vierstufiges Modell wurde vorgeschlagen um die Entwicklung der schnellen Kriechwegbildung darzustellen. Das Modell betont die zentrale Rolle der faserähnlichen karbonisierten Kriechwege im Inneren des Pressspans, wobei Einsatz und Wachstum theoretisch erklärt werden. Eine Erkennungsmethode für die Schnelle Kriechwegbildung wurde vorgeschlagen, die im Wesentlichen auf Teilentladungsmessungen, online Temperaturerfassung und Gas Analyse basiert. Es war dennoch schwierig eine langsame Kriechwegbildung zu erfassen, da Teilentladungen verschwanden. Die Einflüsse von Temperatur und Spannungsform auf fortschreitende Kriechwegentladungen wurden untersucht. Die Temperatur beeinflusste die kurzzeitige elektrische Feldstärke der Grenzschicht und der Ölstrecke und kann eine synergetische Auswirkung mit einem starken Feld auslösen, indem sich der Verlustfaktor des Pressspans erhöht. Es wurde weiterhin herausgefunden, dass die Temperatur die Fehlertypen und Entladungsstärke unter konstanter oder stufenweise Beanspruchung beeinflussen kann, wobei die Art der Beanspruchung einen klar erkennbaren Einfluss auf die Fehlerart nur bei hohen Temperaturen hat. Abhängig von der Prüftemperatur und der Beanspruchungsforme versagten die Prüflinge durch Grenzschichtüberschlag ohne Kriechwegspuren, langsame Kriechwegbildung und schnelle Kriechwegbildung. Ihre Teilentladungsgrößen wurden vergleichend ausgewertet. Darüber hinaus wurde herausgefunden, dass synchrone Teilentladungen und dielektrische Messungen eine gute Kennzeichnung für schnelle Kriechwegbildungsprozesse sind und es zeigt die Fähigkeit einer hohen Erkennungsempfindlichkeit und Entladungserkennung. Die fortschreitenden Kriechentladungen bei Gleichspannung wurden in Abhängigkeit der Temperatur und der Ölflussstärke ermittelt. Die Temperatur beeinflusst die Gleichspannungsleitfähigkeit von beiden, Öl und Pressspan, das Widerstandsverhältnis von Pressspan und Öl, und die Relaxationszeit der Grenzschichtladungen Öl/Pressspan. Dementsprechend wurde die Grenzschichtfeldverteilung in Abhängigkeit der Temperatur mit Hilfe der Finite-Elemente Methode ausgewertet. Außerdem beeinflussten die Geschwindigkeit des Ölflusses und die Temperaturen die kurzzeitige Grenzflächenfeldstärke bei Gleichspannung und fortschreitenden Kriechwegentladung. Zwei Arten von Entladungen wurde bei ruhendem Öl gefunden. Sie zeigen deutliche Teilentladungsmerkmale und unterschiedliches Verhalten bei Temperaturänderungen. Sie waren vermutlich verbunden mit Entladungen im Öl und Pressspan. Es wurde weiterhin herausgefunden, dass forcierte Ölströmung das Verhalten der Kriechentladungen bei Gleichspannung drastisch verändert und einen besonderen Typ von Teilentladungen erzeugt (Pulsfolge). Die Erscheinung dieser Pulsfolge war abhängig von der Höhe der angelegten Spannung, der Temperatur und der Geschwindigkeit des Ölflusses. Es wurde ein theoretisches Modell zur Verdeutlichung der Pulsfolge vorgeschlagen, bei dem die Wechselwirkungen zwischen Raumladungen von verschiedenen Quellen behandelt wurden in Abhängigkeit der Spannungshöhe, des Ölflusses und der Temperatur. Die vorliegende Arbeit erweitert die Kenntnis der fortschreitenden Kriechentladungen in der Anordnung Öl/Pressspan. Es wird erwartet, dass die Untersuchungsergebnisse als Basis für zukünftige wissenschaftliche Studien dienen und einen Beitrag mit hinweisenden Informationen für die praktische Anwendung zur Thematik fortschreitender Kriechentladungen liefern

Steering Capacitor Film Development with Methods for Correct and Adequate Dielectric Performance Assessment

The transition of electric power systems towards renewable generation has created an increasing market for power electronics using film capacitors as one of their key components. Size, weight, and cost reduction can be achieved with better capacitors – an objective achievable with advanced dielectric films. The current state-of-the-art biaxially oriented polypropylene (BOPP) films are already operated close to their fundamental limits, causing a growing demand for next-generation technologies. To perform well when used in a capacitor, a film needs to have a wide range of fundamental and applied properties, all of which should be evaluated during film development to ensure there are no unwanted trade-offs. Power capacitors are used in applications with high downtime costs, e.g. HVDC, thus especially the reliability aspects must be given scrutiny. This thesis work was inspired by the lack of knowledge of the long-term performance of next generation dielectrics, e.g. polymer nanocomposites. Equally important was to fill the gaps in published knowledge of measurement methods to evaluate long-term properties, voltage endurance, and surprisingly, also the dielectric permittivity of thin (≈10 μm) low-loss films. In this thesis, a suitable measurement for each three is presented along with examples of their capability and an approach to applying them to steer film development. The large-area multi-breakdown method developed in our research group is extended to measurements at realistic operating temperatures, and industrial BOPP films are shown to exhibit an 11–20 % decrease in the DC breakdown strength between room temperature and 100 ◦C. The results align with literature, which supports the validity of the approach. BOPP films made of base materials varying in terms of molecular weight are measured: these films exhibit similar short-term breakdown performance at room temperature, yet at 100 ◦C differences emerge. The difference did not correlate with the reduction of breakdown performance after DC electro-thermal aging, demonstrating the necessity of long-term tests. Electron beam evaporation in high vacuum (P<10−6 mbar) is established as a repeatable and suitable method to metallize electrodes on ultra low-loss BOPP films, solving earlier issues of abnormally high dielectric losses or unrealistically low real permittivity. Metallization process is identified as the crucial factor: no pre- or post-treatments are required, and valid results are obtainable with various electrode metals. The method was demonstrated by measuring true “literature value” dielectric permittivity of commercial BOPP films: E≈2.25 and tanδ≈10−4. The importance of successful metallization process for measuring the intrinsic losses is demonstrated: samples with sputter deposited electrodes exhibited abnormally high dielectric losses, as also did samples metallized using another e-beam evaporator. The multi-breakdown approach is also extended to times-to-breakdown tests, and accelerated ageing tests are conducted on an industrial BOPP film. High-field degradation and drastically reduced insulation life are observed. Analysis of the Weibull failure rate supports the notion that at current design stresses, BOPP is already operated close to the fundamental material limits, and also that the life in operating conditions cannot be determined by simple inverse power law extrapolation of accelerated rapid ageing data. Again, long-term ageing testing is advocated. Space charge measurements on “classic” BOPP films reveal charge accumulation at high fields, as expected. Interestingly, no space charge accumulation is detected in a novel nanostructured material under similar conditions, demonstrating the potentiality of nanofilled DC insulation. A DC electro-thermal ageing test method is presented to investigate long-term phenomena in realistic operating conditions. Two 1000 h DC electro-thermal ageing tests associate ageing with the formation of electrically weak points. Large-area breakdown behavior, being sensitive to local changes, is established as a recommended ageing indicator. Material characterization does not reveal ageing-induced changes in bulk properties, supporting the literature-backed conclusion that early ageing progresses by localized degradation. A trial with eight pilot-scale materials demonstrate that weak point formation may be inhibited in nanostructured materials, but also that material-specific optimization of film processing is required to reach optimal dielectric performance. Ultimately, the methods developed are fused into one resource-efficient approach to capacitor film development, in which the short-, mid-, and long-term properties are evaluated in three overlapping phases. Reliance on individual performance metrics to steer film development is discouraged: all properties need to be at an appropriate level for a film to perform in application, and there are trade-offs to be managed

Insulation Design of Low Voltage Electrical Motors Fed by PWM Inverters

This paper proposes a model to determine the partial discharge inception voltage of magnet wires, including the effect of elevated temperatures, and shows its applicability to the complete range of wire geometries considered in IEC Standard 60317-13