Добірка наукової літератури з теми "Vacuum electrical insulation"

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

The different low temperature electrical characteristics of insulating materials, which plays an important role in superconducting power equipment, has attracted extensive attention. The cryogenic control system is applied to the independent development of superconducting electrical characteristics of insulating materials testing device. And it is designed on the basis of LabVIEW, for the test of the electrical characteristics of the insulation materials in special environment such as cryogenic temperature and vacuum. The system which uses DC source as a heat source and G-M refrigerator as a cold source, uses PID algorithm to control the heat source with the cold source so as to achieve closed-loop control. The system can make the temperature of sample from room temperature down to 6.0K within three hours and control the temperature of sample in the range of 6.0K-300.0K. The system enables temperature error not more than ± 0.5K for a long time and provides a reliable low-temperature environment which is used for study of the electrical characteristics of the insulation materials in different cryogenic temperature and vacuum environment.

The concept of creating electrical insulating ceramic materialsproducts from powder systems representing oxide and nonoxide chemical compounds was formed; a program document for materials science and technological logistics of physical and chemical transformation of technogenic mineral raw materials into electrical materials-products of various scientific, practical and specific technological purposes was created and implemented. The principal theoretical approach and its appliedpractical aspects of the development - research - creation of thermo- and chemically resistant structural electrical insulation materials - products for various scientific and practical purposes: automatic contact welding of tubular bimetals (for example, copper - aluminum), electron beam welding in vacuum of thickwalled large-sized structures made of high-strength aluminum alloys, high-temperature (1050 oC) hardening of drilling tools in vacuum furnaces in the medium of dissociated acetylene are considered, in electric transmissions of brake installations of quarry dump trucks (k/s) BelAZ.

AbstractA method to design the composite insulation structures in pulsed power systems is proposed in this paper. The theoretical bases for this method include the Weibull statistical distribution and the empirical insulation formula. A uniform formula to describe the reliability (R) for different insulation media such as solid, liquid, gas, vacuum, and vacuum surface is derived. The dependence curves of the normalized applied field onRare also obtained. These curves show that the normalized applied field decreases rapidly asRincreases but the declining rates corresponding to different insulation media are different. In addition, ifRis required to be higher than a given level, the normalized applied field should be smaller than a certain value. In practical design, the common range of the applied fields for different insulation media should be chosen to meet a global reliability requirement. In the end, the proposed method is demonstrated with a specific coaxial high-voltage vacuum insulator.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.09.13 – техніка сильних електричних і магнітних полів. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018 р. Дисертацію присвячено удосконаленню секцій сильнострумного лінійного індукційного прискорювача (ЛІП) електронів і ЛІП зарядово-компенсованих іонних пучків, з метою забезпечення покращених енергетичних характеристик: підвищеного темпу прискорення, середньої потужності пучка (аж до мегаватного рівня), частоти посилок прискорювальних імпульсів тощо. В роботі проведено науково-технічне обґрунтування конструкції елементів секції ЛІП з індукційною системою, секціонованою по осьовій довжині, виявлено способи забезпечення максимального темпу прискорення при найменших енергетичних втратах в індукційній системі у таких секцій. Проведено обґрунтування конструкції елементів секції ЛІП зарядово-компенсованих іонних пучків, що дозволяє забезпечити необхідні параметри прискорювача – прискорювальну напругу секції не менше 2 МВ, темп прискорення не менше 2 МВ/м. В результаті проведеного дослідження динаміки імпульсного перемагнічування феромагнетику індукторів ЛІП з метою визначення впливу геометрії феромагнітних осердь індукторів і режиму їхнього навантаження на форму імпульсу прискорювальної напруги запропоновано шляхи зменшення розкиду прискорювальної напруги на столі імпульсу. Проведено дослідження теплових навантажень на елементи секції ЛІП при роботі в частотному режимі, запропоновано способи забезпечення секцією ЛІП мегаватного рівня збільшення середньої потужності пучка при збільшенні його енергії на рівні одиниць мегаелектронвольт. Проведено чисельно-аналітичне та експериментальне дослідження впливу конструктивних елементів секції сильнострумного ЛІП електронів на забезпечення її високовольтної вакуумної електричної ізоляції, запропоновано способи їх удосконалення.
Technic sciences candidate scientific degree dissertation receiving on specialty 05.09.13 – equipment strong electric and magnetic fields. – National Technical University "Kharkiv Polytechnic Institute", Kharkiv, 2018. Dissertation work is dedicated to the improvement electrons and charge-compensated ion beams high-current LIA sections, with the providing goal improved energy LIA characteristics: high acceleration, average beam power (up to MW level), parcels accelerating pulses frequency, etc. In the work the rationale of the construction of elements the LIA section with an induction system sectioned along the axial has been carried out, methods, ensuring the maximum acceleration rate with the lowest energy losses in the induction system in such sections have been found. The construction of the elements of the charge-compensated ion beams LIA section has been substantiated, that allowed to provide the necessary accelerator parameters of – the section accelerating voltage is not less than 2 MV, and the acceleration rate is not less than 2 MV/m. The dynamics of the pulsed magnetization reversal of the LIA inductors ferromagnet has been studied, the effect of the geometry of the ferromagnetic inductor cores and the regime of their loading on the accelerating voltage pulse shape is analyzed. The ways, reducing the accelerating voltage spread on the pulse table have been found. The thermal stability of the LIA section elements (induction system, power lines, vacuum isolation) was studied, the maximum possible accelerating pulse repetition frequency was determined. The methods of providing a megawatt level of the increment of the average beam power with increment of its energy at the level of megaelectronvolts by the LIA section are revealed. The research of the vacuum electrical insulation of the LIA experimental model has been carried out; the ways of the improvement of LIA section elements electrical isolation have been found.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.09.13 – техніка сильних електричних і магнітних полів. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018 р. Дисертацію присвячено удосконаленню секцій сильнострумного лінійного індукційного прискорювача (ЛІП) електронів і ЛІП зарядово-компенсованих іонних пучків, з метою забезпечення покращених енергетичних характеристик: підвищеного темпу прискорення, середньої потужності пучка (аж до мегаватного рівня), частоти посилок прискорювальних імпульсів тощо. В роботі проведено науково-технічне обґрунтування конструкції елементів секції ЛІП з індукційною системою, секціонованою по осьовій довжині, виявлено способи забезпечення максимального темпу прискорення при найменших енергетичних втратах в індукційній системі у таких секцій. Проведено обґрунтування конструкції елементів секції ЛІП зарядово-компенсованих іонних пучків, що дозволяє забезпечити необхідні параметри прискорювача – прискорювальну напругу секції не менше 2 МВ, темп прискорення не менше 2 МВ/м. В результаті проведеного дослідження динаміки імпульсного перемагнічування феромагнетику індукторів ЛІП з метою визначення впливу геометрії феромагнітних осердь індукторів і режиму їхнього навантаження на форму імпульсу прискорювальної напруги запропоновано шляхи зменшення розкиду прискорювальної напруги на столі імпульсу. Проведено дослідження теплових навантажень на елементи секції ЛІП при роботі в частотному режимі, запропоновано способи забезпечення секцією ЛІП мегаватного рівня збільшення середньої потужності пучка при збільшенні його енергії на рівні одиниць мегаелектронвольт. Проведено чисельно-аналітичне та експериментальне дослідження впливу конструктивних елементів секції сильнострумного ЛІП електронів на забезпечення її високовольтної вакуумної електричної ізоляції, запропоновано способи їх удосконалення.
Technic sciences candidate scientific degree dissertation receiving on specialty 05.09.13 – equipment strong electric and magnetic fields. – National Technical University "Kharkiv Polytechnic Institute", Kharkiv, 2018. Dissertation work is dedicated to the improvement electrons and charge-compensated ion beams high-current LIA sections, with the providing goal improved energy LIA characteristics: high acceleration, average beam power (up to MW level), parcels accelerating pulses frequency, etc. In the work the rationale of the construction of elements the LIA section with an induction system sectioned along the axial has been carried out, methods, ensuring the maximum acceleration rate with the lowest energy losses in the induction system in such sections have been found. The construction of the elements of the charge-compensated ion beams LIA section has been substantiated, that allowed to provide the necessary accelerator parameters of – the section accelerating voltage is not less than 2 MV, and the acceleration rate is not less than 2 MV/m. The dynamics of the pulsed magnetization reversal of the LIA inductors ferromagnet has been studied, the effect of the geometry of the ferromagnetic inductor cores and the regime of their loading on the accelerating voltage pulse shape is analyzed. The ways, reducing the accelerating voltage spread on the pulse table have been found. The thermal stability of the LIA section elements (induction system, power lines, vacuum isolation) was studied, the maximum possible accelerating pulse repetition frequency was determined. The methods of providing a megawatt level of the increment of the average beam power with increment of its energy at the level of megaelectronvolts by the LIA section are revealed. The research of the vacuum electrical insulation of the LIA experimental model has been carried out; the ways of the improvement of LIA section elements electrical isolation have been found.

Thesis (M.S.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on March 18, 2008) Includes bibliographical references.

The term "topological insulator" (TI) represents a novel class of compounds which are insulating in the bulk, but simultaneously and unavoidably have a metallic surface. The reason for this is the non-trivial band topology, arising from particular band inversions and the spin-orbit interaction, of the bulk. These topologically protected metallic surface states are characterized by massless Dirac dispersion and locked helical spin polarization, leading to forbidden back-scattering with robustness against disorder. Based on the extraordinary features of the topological insulators an abundance of new phenomena and many exciting experiments have been proposed by theoreticians, but still await their experimental verification, not to mention their implementation into applications, e.g. the creation of Majorana fermions, advanced spintronics, or the realization of quantum computers. In this perspective, the 3D TIs Bi2Te3 and Bi2Se3 gained a lot of interest due to their relatively simple electronic band structure, having only a single Dirac cone at the surface. Furthermore, they exhibit an appreciable bulk band gap of up to ~ 0.3 eV, making room temperature applications feasible. Yet, the execution of these proposals remains an enormous experimental challenge. The main obstacle, which thus far hampered the electrical characterization of topological surface states via transport experiments, is the residual extrinsic conductivity arising from the presence of defects and impurities in their bulk, as well as the contamination of the surface due to exposure to air. This thesis is part of the actual effort in improving sample quality to achieve bulk-insulating Bi2Te3 films and study of their electrical properties under controlled conditions. Furthermore, appropriate capping materials preserving the electronic features under ambient atmosphere shall be identified to facilitate more sophisticated ex-situ experiments. Bi2Te3 thin films were fabricated by molecular beam epitaxy (MBE). It could be shown that, by optimizing the growth conditions, it is indeed possible to obtain consistently bulk-insulating and single-domain TI films. Hereby, the key factor is to supply the elements with a Te/Bi ratio of ~8, while achieving a full distillation of the Te, and the usage of substrates with negligible lattice mismatch. The optimal MBE conditions for Bi2Te3 were found in a two-step growth procedure at substrate temperatures of 220°C and 250°C, respectively, and a Bi flux rate of 1 Å/min. Subsequently, the structural characterization by high- and low-energy electron diffraction, photoelectron spectroscopy, and, in particular, the temperature-dependent conductivity measurements were entirely done inside the same ultra-high vacuum (UHV) system, ensuring a reliable record of the intrinsic properties of the topological surface states. Bi2Te3 films with thicknesses ranging from 10 to 50 quintuple layers (QL; 1QL~1 nm) were fabricated to examine, whether the conductivity is solely arising from the surface states. Angle resolved photoemission spectroscopy (ARPES) demonstrates that the chemical potential for all these samples is located well within the bulk band gap, and is only intersected by the topological surface states, displaying the characteristic linear dispersion. A metallic-like temperature dependency of the sheet resistance is observed from the in-situ transport experiments. Upon going from 10 to 50QL the sheet resistance displays a variation by a factor 1.3 at 14K and of 1.5 at room temperature, evidencing that the conductivity is indeed dominated by the surface. Low charge carrier concentrations in the range of 2–4*10^12 cm^−2 with high mobility values up to 4600 cm2/Vs could be achieved. Furthermore, the degradation effect of air exposure on the conductance of the Bi2Te3 films was quantified, emphasizing the necessity to protect the surface from ambient conditions. Since the films behave inert to pure oxygen, water/moisture is the most probable source of degeneration. Moreover, epitaxially grown elemental tellurium was identified as a suitable capping material preserving the properties of the intrinsically insulating Bi2Te3 films and protecting from alterations during air exposure, facilitating well-defined and reliable ex-situ experiments. These findings serve as an ideal platform for further investigations and open the way to prepare devices that can exploit the intrinsic features of the topological surface states
Der Begriff "Topologischer Isolator" (TI) beschreibt eine neuartige Klasse von Verbindungen deren Inneres (engl. Bulk) isolierend ist, dieses Innere aber gleichzeitig und zwangsläufig eine metallisch leitende Oberfläche aufweist. Dies ist begründet in der nicht-trivialen Topologie dieser Materialien, welche durch eine spezielle Invertierung einzelner Bänder in der Bandstruktur und der Spin-Bahn-Kopplung im Materialinneren hervorgerufen ist. Diese topologisch geschützten, metallischen Oberflächenzustände sind gekennzeichnet durch eine masselose Dirac Dispersionsrelation und gekoppelte Helizität der Spinpolarisation, welche die Rückstreuung der Ladungsträger verbietet und somit zur Stabilisierung der Zustände gegenüber Störungen beiträgt. Auf Grundlage dieser außergewöhnlichen Merkmale haben Theoretiker eine Fülle neuer Phänomene und spannender Experimente vorhergesagt. Deren experimentelle Überprüfung steht jedoch noch aus, geschweige denn deren Umsetzung in Anwendungen, wie zum Beispiel die Erzeugung von Majorana Teilchen, fortgeschrittene Spintronik, oder die Realisierung von Quantencomputern. Aufgrund ihrer relativ einfachen Bandstruktur, welche nur einen Dirac-Kegel an der Oberfläche aufweist, haben die 3D TI Bi2Te3 und Bi2Se3 in den letzten Jahren großes Interesse erlangt. Weiterhin besitzen diese Materialien eine merkliche Bandlücke von bis zu ~0,3 eV, welche sogar Anwendungen bei Raumtemperatur ermöglichen könnten. Dennoch ist deren experimentelle Umsetzung nachwievor eine enorme Herausforderung. Das Haupthindernis, welches bis jetzt insbesondere die elektrische Charakterisierung the topologischen Oberflächenzustände behindert hat, ist die zusätzliche Leitfähigkeit des Materialinneren, welche durch Kristalldefekte und Beimischungen, sowie die Verunreinigung der Probenoberfläche durch Luftexposition bedingt wird. Die vorliegende Arbeit liefert einen Beitrag zu aktuellen den Anstrengungen in der Verbesserung der Probenqualität der TI um die Leitfähigkeit des Materialinneren zu unterdrücken, sowie die anschließende Untersuchung der elektrischen Eigenschaften unter kontrollierten Bedingungen durchzuführen. Weiterhin sollen geeignete Deckschichten identifiziert werden, welche die besonderen elektronischen Merkmale der TI nicht beeinflussen sowie diese gegen äußere Einflüsse schützen, und somit die Durchführung anspruchsvoller ex situ Experimente ermöglichen können. Die untersuchten Bi2Te3 Schichten wurden mittels Molekularstrahlepitaxie (MBE) hergestellt. Es konnte gezeigt werden, dass es allein durch Optimierung der Wachstumsbedingungen möglich ist Proben herzustellen, die gleichbleibend isolierende Eigenschaften des TI Inneren aufweisen und Eindomänen-Ausrichtung besitzen. Die zentralen Faktoren sind hierbei die Aufrechterhaltung eines Flussratenverhältnisses von Te/Bi ~8 der einzelnen Elemente, sowie die Wahl einer ausreichend hohen Substrattemperatur, um ein vollständiges Abdampfen (Destillation) des überschüssigen Tellur zu erreichen. Weiterhin müssen Substrate mit gut angepassten Gitterparametern verwendet werden, welches bei BaF2 (111) gegeben ist. Optimales MBE Wachstum konnte durch ein Zwei-Stufen Prozess bei Substrattemperaturen von 220°C und 250°C und einer Bi-Verdampfungsrate von 1 Å/min erreicht werden. Die nachfolgende Charakterisierung der strukturellen Eigenschaften, Photoelektronenspektroskopie, sowie temperaturabhängige Leitfähigkeitsmessungen wurden alle in einem zusammenhängenden Ultrahochvakuum-System durchgeführt. Auf diese Weise wird eine zuverlässige Erfassung der intrinsischen Eigenschaften der TI sichergestellt. Zur Überprüfung, ob die Leitfähigkeit der Proben tatsächlich nur durch die Oberflächenzustände hervorgerufen wird, wurden Filme mit Schichtdicken im Bereich von 10 bis 50 Quintupel-Lagen (QL; 1QL~ 1 nm) hergestellt und charakterisiert. Winkelaufgelöste Photoelektronenspektroskopie (ARPES) belegt, dass das chemische Potential (Fermi-Niveau) in allen Proben innerhalb der Bandlücke der Bandstruktur des Materialinneren liegt und nur von den topologisch geschützten Oberflächenzuständen gekreuzt wird, welche die charakteristische lineare Dirac Dispersionsrelation aufweisen. Die temperaturabhängigen Widerstandsmessungen zeigen ein metallisches Verhalten aller Proben. Bei der Variation der Schichtdicke von 10 zu 50QL wird eine Streuung des Flächenwiderstandes vom Faktor 1,3 bei 14K und 1,5 bei Raumtemperatur beobachtet. Dies beweist, dass die gemessene Leitfähigkeit vorrangig durch die topologisch geschützten Oberflächenzustände hervorgerufen wird. Eine geringe Oberflächenladungsträgerkonzentration im Bereich von 2–4*10^12 cm^−2 und hohe Mobilitätswerte von bis zu 4600 cm2/Vs wurden erreicht. Weiterhin wurden die negativen Auswirkungen auf die Eigenschaften der TI durch Luftexposition quantifiziert, welches die Notwendigkeit belegt, die Oberfläche der TI vor Umgebungseinflüssen zu schützen. Die Proben verhalten sich inert gegenüber reinem Sauerstoff, daher ist Wasser aus der Luftfeuchte höchstwahrscheinlich der Hauptgrund für die beobachtbare Verschlechterung. Darüber hinaus konnte epitaktisch gewachsenes Tellur als geeignete Deckschicht ausfindig gemacht werden, welches die Eigenschaften der Bi2Te3 Filme nicht beeinflusst, sowie gegen Veränderungen durch Luftexposition schützt. Die gewonnenen Erkenntnisse stellen eine ideale Grundlage für weiterführende Untersuchungen dar und ebnen den Weg zur Entwicklung von Bauelementen welche die spezifischen Besonderheiten der topologischen Oberflächenzustände

The term "topological insulator" (TI) represents a novel class of compounds which are insulating in the bulk, but simultaneously and unavoidably have a metallic surface. The reason for this is the non-trivial band topology, arising from particular band inversions and the spin-orbit interaction, of the bulk. These topologically protected metallic surface states are characterized by massless Dirac dispersion and locked helical spin polarization, leading to forbidden back-scattering with robustness against disorder. Based on the extraordinary features of the topological insulators an abundance of new phenomena and many exciting experiments have been proposed by theoreticians, but still await their experimental verification, not to mention their implementation into applications, e.g. the creation of Majorana fermions, advanced spintronics, or the realization of quantum computers. In this perspective, the 3D TIs Bi2Te3 and Bi2Se3 gained a lot of interest due to their relatively simple electronic band structure, having only a single Dirac cone at the surface. Furthermore, they exhibit an appreciable bulk band gap of up to ~ 0.3 eV, making room temperature applications feasible. Yet, the execution of these proposals remains an enormous experimental challenge. The main obstacle, which thus far hampered the electrical characterization of topological surface states via transport experiments, is the residual extrinsic conductivity arising from the presence of defects and impurities in their bulk, as well as the contamination of the surface due to exposure to air. This thesis is part of the actual effort in improving sample quality to achieve bulk-insulating Bi2Te3 films and study of their electrical properties under controlled conditions. Furthermore, appropriate capping materials preserving the electronic features under ambient atmosphere shall be identified to facilitate more sophisticated ex-situ experiments. Bi2Te3 thin films were fabricated by molecular beam epitaxy (MBE). It could be shown that, by optimizing the growth conditions, it is indeed possible to obtain consistently bulk-insulating and single-domain TI films. Hereby, the key factor is to supply the elements with a Te/Bi ratio of ~8, while achieving a full distillation of the Te, and the usage of substrates with negligible lattice mismatch. The optimal MBE conditions for Bi2Te3 were found in a two-step growth procedure at substrate temperatures of 220°C and 250°C, respectively, and a Bi flux rate of 1 Å/min. Subsequently, the structural characterization by high- and low-energy electron diffraction, photoelectron spectroscopy, and, in particular, the temperature-dependent conductivity measurements were entirely done inside the same ultra-high vacuum (UHV) system, ensuring a reliable record of the intrinsic properties of the topological surface states. Bi2Te3 films with thicknesses ranging from 10 to 50 quintuple layers (QL; 1QL~1 nm) were fabricated to examine, whether the conductivity is solely arising from the surface states. Angle resolved photoemission spectroscopy (ARPES) demonstrates that the chemical potential for all these samples is located well within the bulk band gap, and is only intersected by the topological surface states, displaying the characteristic linear dispersion. A metallic-like temperature dependency of the sheet resistance is observed from the in-situ transport experiments. Upon going from 10 to 50QL the sheet resistance displays a variation by a factor 1.3 at 14K and of 1.5 at room temperature, evidencing that the conductivity is indeed dominated by the surface. Low charge carrier concentrations in the range of 2–4*10^12 cm^−2 with high mobility values up to 4600 cm2/Vs could be achieved. Furthermore, the degradation effect of air exposure on the conductance of the Bi2Te3 films was quantified, emphasizing the necessity to protect the surface from ambient conditions. Since the films behave inert to pure oxygen, water/moisture is the most probable source of degeneration. Moreover, epitaxially grown elemental tellurium was identified as a suitable capping material preserving the properties of the intrinsically insulating Bi2Te3 films and protecting from alterations during air exposure, facilitating well-defined and reliable ex-situ experiments. These findings serve as an ideal platform for further investigations and open the way to prepare devices that can exploit the intrinsic features of the topological surface states.:Abstract Kurzfassung Acronyms List of Symbols Introduction 1 Topological insulators 1.1 Basic theory of topological insulators 1.2 3D topological insulator materials: bismuth chalcogenides 2 Experimental techniques 2.1 General layout of the UHV-system 2.2 Molecular beam epitaxy 2.3 Structural and spectroscopic characterization 2.3.1 RHEED and LEED 2.3.2 Photoelectron spectroscopy 2.3.3 Ex situ x-ray diffraction 2.4 In situ electrical resistance measurements 2.4.1 In situ transport setup 2.4.2 Measurement equipment and operation modes 2.5 Substrates and sample holders 3 MBE growth and structural characterization of Bi2Te3 thin films 3.1 Bi2Te3 growth optimization and in situ structural characterization 3.1.1 1-step growth on Al2O3 (0001) 3.1.2 2-step growth on Al2O3 (0001) 3.1.3 2-step growth on BaF2 (111) 3.2 Ex situ structural characterization 4 In situ spectroscopy and transport properties of Bi2Te3 thin films 4.1 In situ spectroscopy of Bi2Te3 thin films 4.1.1 XPS 4.1.2 ARPES 4.2 Combined ARPES and in situ electrical resistance measurements of bulk-insulating Bi2Te3 thin films 4.2.1 Quality of the in situ electrical sample contacts 4.2.2 Verification of the intrinsic conduction through topological surface states of bulk-insulating Bi2Te3 thin films 5 Effect of surface contaminants on the TI properties 5.1 Effect of air exposure on the electrical conductivity of Bi2Te3 surfaces 5.2 Determination of the contaminants causing degradation of the TI properties 5.3 Long-time resistance behavior of a Bi2Te3 film exposed to minimal traces of contaminants 6 Protective capping of bulk-insulating Bi2Te3 thin films 6.1 Capping with BaF2 6.1.1 MBE growth and structure of BaF2 on Bi2Te3 thin films 6.1.2 Electron spectroscopy and electrical transport properties of BaF2 capped Bi2Te3 6.2 Capping with tellurium 6.2.1 MBE growth and structure of Te on Bi2Te3 thin films 6.2.2 Photoelectron spectroscopy and electrical transport properties of Te capped Bi2Te3 6.2.3 De-capping of Te 6.2.4 Efficiency of Te capping against air exposure 7 Conclusion and outlook Bibliography Versicherung Curriculum vitae Veröffentlichungen
Der Begriff "Topologischer Isolator" (TI) beschreibt eine neuartige Klasse von Verbindungen deren Inneres (engl. Bulk) isolierend ist, dieses Innere aber gleichzeitig und zwangsläufig eine metallisch leitende Oberfläche aufweist. Dies ist begründet in der nicht-trivialen Topologie dieser Materialien, welche durch eine spezielle Invertierung einzelner Bänder in der Bandstruktur und der Spin-Bahn-Kopplung im Materialinneren hervorgerufen ist. Diese topologisch geschützten, metallischen Oberflächenzustände sind gekennzeichnet durch eine masselose Dirac Dispersionsrelation und gekoppelte Helizität der Spinpolarisation, welche die Rückstreuung der Ladungsträger verbietet und somit zur Stabilisierung der Zustände gegenüber Störungen beiträgt. Auf Grundlage dieser außergewöhnlichen Merkmale haben Theoretiker eine Fülle neuer Phänomene und spannender Experimente vorhergesagt. Deren experimentelle Überprüfung steht jedoch noch aus, geschweige denn deren Umsetzung in Anwendungen, wie zum Beispiel die Erzeugung von Majorana Teilchen, fortgeschrittene Spintronik, oder die Realisierung von Quantencomputern. Aufgrund ihrer relativ einfachen Bandstruktur, welche nur einen Dirac-Kegel an der Oberfläche aufweist, haben die 3D TI Bi2Te3 und Bi2Se3 in den letzten Jahren großes Interesse erlangt. Weiterhin besitzen diese Materialien eine merkliche Bandlücke von bis zu ~0,3 eV, welche sogar Anwendungen bei Raumtemperatur ermöglichen könnten. Dennoch ist deren experimentelle Umsetzung nachwievor eine enorme Herausforderung. Das Haupthindernis, welches bis jetzt insbesondere die elektrische Charakterisierung the topologischen Oberflächenzustände behindert hat, ist die zusätzliche Leitfähigkeit des Materialinneren, welche durch Kristalldefekte und Beimischungen, sowie die Verunreinigung der Probenoberfläche durch Luftexposition bedingt wird. Die vorliegende Arbeit liefert einen Beitrag zu aktuellen den Anstrengungen in der Verbesserung der Probenqualität der TI um die Leitfähigkeit des Materialinneren zu unterdrücken, sowie die anschließende Untersuchung der elektrischen Eigenschaften unter kontrollierten Bedingungen durchzuführen. Weiterhin sollen geeignete Deckschichten identifiziert werden, welche die besonderen elektronischen Merkmale der TI nicht beeinflussen sowie diese gegen äußere Einflüsse schützen, und somit die Durchführung anspruchsvoller ex situ Experimente ermöglichen können. Die untersuchten Bi2Te3 Schichten wurden mittels Molekularstrahlepitaxie (MBE) hergestellt. Es konnte gezeigt werden, dass es allein durch Optimierung der Wachstumsbedingungen möglich ist Proben herzustellen, die gleichbleibend isolierende Eigenschaften des TI Inneren aufweisen und Eindomänen-Ausrichtung besitzen. Die zentralen Faktoren sind hierbei die Aufrechterhaltung eines Flussratenverhältnisses von Te/Bi ~8 der einzelnen Elemente, sowie die Wahl einer ausreichend hohen Substrattemperatur, um ein vollständiges Abdampfen (Destillation) des überschüssigen Tellur zu erreichen. Weiterhin müssen Substrate mit gut angepassten Gitterparametern verwendet werden, welches bei BaF2 (111) gegeben ist. Optimales MBE Wachstum konnte durch ein Zwei-Stufen Prozess bei Substrattemperaturen von 220°C und 250°C und einer Bi-Verdampfungsrate von 1 Å/min erreicht werden. Die nachfolgende Charakterisierung der strukturellen Eigenschaften, Photoelektronenspektroskopie, sowie temperaturabhängige Leitfähigkeitsmessungen wurden alle in einem zusammenhängenden Ultrahochvakuum-System durchgeführt. Auf diese Weise wird eine zuverlässige Erfassung der intrinsischen Eigenschaften der TI sichergestellt. Zur Überprüfung, ob die Leitfähigkeit der Proben tatsächlich nur durch die Oberflächenzustände hervorgerufen wird, wurden Filme mit Schichtdicken im Bereich von 10 bis 50 Quintupel-Lagen (QL; 1QL~ 1 nm) hergestellt und charakterisiert. Winkelaufgelöste Photoelektronenspektroskopie (ARPES) belegt, dass das chemische Potential (Fermi-Niveau) in allen Proben innerhalb der Bandlücke der Bandstruktur des Materialinneren liegt und nur von den topologisch geschützten Oberflächenzuständen gekreuzt wird, welche die charakteristische lineare Dirac Dispersionsrelation aufweisen. Die temperaturabhängigen Widerstandsmessungen zeigen ein metallisches Verhalten aller Proben. Bei der Variation der Schichtdicke von 10 zu 50QL wird eine Streuung des Flächenwiderstandes vom Faktor 1,3 bei 14K und 1,5 bei Raumtemperatur beobachtet. Dies beweist, dass die gemessene Leitfähigkeit vorrangig durch die topologisch geschützten Oberflächenzustände hervorgerufen wird. Eine geringe Oberflächenladungsträgerkonzentration im Bereich von 2–4*10^12 cm^−2 und hohe Mobilitätswerte von bis zu 4600 cm2/Vs wurden erreicht. Weiterhin wurden die negativen Auswirkungen auf die Eigenschaften der TI durch Luftexposition quantifiziert, welches die Notwendigkeit belegt, die Oberfläche der TI vor Umgebungseinflüssen zu schützen. Die Proben verhalten sich inert gegenüber reinem Sauerstoff, daher ist Wasser aus der Luftfeuchte höchstwahrscheinlich der Hauptgrund für die beobachtbare Verschlechterung. Darüber hinaus konnte epitaktisch gewachsenes Tellur als geeignete Deckschicht ausfindig gemacht werden, welches die Eigenschaften der Bi2Te3 Filme nicht beeinflusst, sowie gegen Veränderungen durch Luftexposition schützt. Die gewonnenen Erkenntnisse stellen eine ideale Grundlage für weiterführende Untersuchungen dar und ebnen den Weg zur Entwicklung von Bauelementen welche die spezifischen Besonderheiten der topologischen Oberflächenzustände.:Abstract Kurzfassung Acronyms List of Symbols Introduction 1 Topological insulators 1.1 Basic theory of topological insulators 1.2 3D topological insulator materials: bismuth chalcogenides 2 Experimental techniques 2.1 General layout of the UHV-system 2.2 Molecular beam epitaxy 2.3 Structural and spectroscopic characterization 2.3.1 RHEED and LEED 2.3.2 Photoelectron spectroscopy 2.3.3 Ex situ x-ray diffraction 2.4 In situ electrical resistance measurements 2.4.1 In situ transport setup 2.4.2 Measurement equipment and operation modes 2.5 Substrates and sample holders 3 MBE growth and structural characterization of Bi2Te3 thin films 3.1 Bi2Te3 growth optimization and in situ structural characterization 3.1.1 1-step growth on Al2O3 (0001) 3.1.2 2-step growth on Al2O3 (0001) 3.1.3 2-step growth on BaF2 (111) 3.2 Ex situ structural characterization 4 In situ spectroscopy and transport properties of Bi2Te3 thin films 4.1 In situ spectroscopy of Bi2Te3 thin films 4.1.1 XPS 4.1.2 ARPES 4.2 Combined ARPES and in situ electrical resistance measurements of bulk-insulating Bi2Te3 thin films 4.2.1 Quality of the in situ electrical sample contacts 4.2.2 Verification of the intrinsic conduction through topological surface states of bulk-insulating Bi2Te3 thin films 5 Effect of surface contaminants on the TI properties 5.1 Effect of air exposure on the electrical conductivity of Bi2Te3 surfaces 5.2 Determination of the contaminants causing degradation of the TI properties 5.3 Long-time resistance behavior of a Bi2Te3 film exposed to minimal traces of contaminants 6 Protective capping of bulk-insulating Bi2Te3 thin films 6.1 Capping with BaF2 6.1.1 MBE growth and structure of BaF2 on Bi2Te3 thin films 6.1.2 Electron spectroscopy and electrical transport properties of BaF2 capped Bi2Te3 6.2 Capping with tellurium 6.2.1 MBE growth and structure of Te on Bi2Te3 thin films 6.2.2 Photoelectron spectroscopy and electrical transport properties of Te capped Bi2Te3 6.2.3 De-capping of Te 6.2.4 Efficiency of Te capping against air exposure 7 Conclusion and outlook Bibliography Versicherung Curriculum vitae Veröffentlichungen

This work presents a new proposal for supporting the sustainability of a single-family house in very cold climates by installing many vacuum-tube solar collectors and a small water tank in order to fulfill the whole dweller demands of heat: space heating, sanitary hot water, and warming both, a greenhouse (spring and autumn) and a swimming pool (summer). This way is obtained a sustained demand that maximizes the utilization of heat from solar collectors throughout the year. This system is designed intending to use the smallest tank that fulfills the winter heating demand, supported by vacuum-tube solar collectors and a little help from electrical heaters working just on the valley tariff. This innovative design gets the most sustainable (but affordable) solution. This goal can be achieved by using a small well-insulated overheated aboveground water tank, instead of the huge underground reservoir of heat used by most projects tested up today. These large communal projects use huge reservoirs to provide seasonal thermal storage (STES) capacity, but their costs are huge too. Besides, it was observed that all these huge STES suffer large heat losses (about 40%), due to constraints for thermally insulating such very heavy systems. On the contrary, our small aboveground water tank can be thermally insulated very well and gets affordable costs. In this work is developed dynamical solar-thermal modeling for studying this novel approach and are discussed its major differences with traditional design. This modeling is used to study the whole demands of heat for one family living in the same conditions of the Okotoks’ project. The Okotoks’ project is based on many flat solar collectors (2,290 m2) and a huge (2,800 m3) rocky-underground STES system in order to almost fulfill (97%) the space heating demand of 52 houses (15,795 kWh/y ea.) in Alberta (Canada), having an overall cost of 9 MU$ (173,000 U$ ea.). We have already shown in previous work that this new proposal could reach noticeably lower costs (€30,500) than the Okotoks’ project in order to provide the same heating demand, by taking advantage of using 18 vacuum-tube collectors (solar area 37 m2) and a small (72 m3) well-insulated (heat losses 18%) water tank heated up to 85°C, which is the same temperature used in Okotoks and other traditional projects. Now, this proposal is enhanced by using a holistic approach to include other low-temperature demands (sanitary hot water and warming a greenhouse and swimming pool) that enhance the sustainability of dweller living. This way, the full production of heat from solar collectors is utilized (about six times larger than the single space heating demand, but using only 20 vacuum-tube solar collectors (21 m2 solar area) and a very small (10m3) water tank, reaching about a lower overall cost (€20,000), and so, the economic performance is enhanced as well. Besides, it is shown that using a small fraction of electrical heaters as a backup system (2%) and slightly overheating the water (up to 120°C@2 bar), which is feasible by using commercial stainless steel water tanks designed for such purposes, its economic performance could be again noticeably enhanced (reducing the overall cost to €20,000, and getting payback period less than two years). This way here is demonstrated the overall solar-STES system can be reduced by about half size meanwhile the energy output can be increased up to seven times. Hence, the thermal analysis performed suggested us strongly critic the traditional approach of using flat solar collectors instead of vacuum-tube collectors. This analysis shows that this choice has strongly driven the selection of a huge STES, which in turn increases noticeably the overall costs of the system since for such huge STES is mandatory to use underground reservoirs. However, this analysis also shows that without including those secondary demands, this proposal achieves a modest economic performance (payback period about 11 years) regarding its lower energy saved and compared against the “most smart” standard solution (one water tank with electrical heaters, costing about 5,000 U$ and exploiting the valley tariff of nocturnal electricity costing 0.1 €/kWh). On the contrary, when these secondary demands are included, the payback period is reduced by two years. Beyond the particular case studied here, this analysis suggests that the right design of any solar + STES system should be led by the solar production. On the contrary, the traditional design intends to fulfill one demand (space heating) concentrated during winter, and so, its performance is noticeably penalized, and the solution is definitely not to put a larger tank. Unfortunately, up today the poor performance of these projects has shown that this solar technology is (by far) unaffordable. Maybe its best days have gone, considering the enormous improvements achieved by another solar technology (using photovoltaic panels + heat pump + small daily-storage water tank), as it was discussed here.