Im Rahmen dieser Arbeit wurden verschiedene Teilchendetektoren zur Verwendung in einem gepulsten Überschallmolekularstrahlexperiment entworfen und hergestellt. In den hier durchgeführten Experimenten kollidiert ein Molekularstrahl mit einer sensitiven Detektoroberfläche, nämlich einer Polyvinylidenfluorid(PVDF)-Folie. Da PVDF sowohl pyroelektrisch wie auch piezoelektrisch ist, entstehen durch die Kollision der Molekularstrahlteilchen auf der Folie Oberflächenladungen. Vorteile von PVDF-Detektoren sind die hohe Effizient der Detektion, die Detektion von neutralen Atomen und Molekülen (eine Ionisierung der zu detektierenden Teilchen ist nicht notwendig), ein einfaches und leicht anzupassendes Detektordesign und außerdem eine schnelle Antwortzeit (im Mikrosekundenbereich). Da nur Ladungen im Bereich von wenigen Pikocoulomb generiert werden, sind verschiedene Verstärker getestet worden. Zur Analyse und Beschreibung des detektierten Signals wird der piezoelektrische Anteil durch ein Materialmodell in Verbindung mit einem schwingenden System, nämlich der erzwungenen Schwingung einer gedämpften Kreismembran, beschrieben. Der pyroelektrische Anteil wird durch einen Energieaustausch beschrieben. Durch die Ausnutzung des pyroelektrischen sowie des piezoelektrischen Effektes können zusätzliche wichtige Informationen wie zum Beispiel der Restitutionskoeffizient oder der Energieakkommodationkoeffizient experimentell erhalten werden. Zur Demonstration der Anwendungsmöglichkeiten sind Detektoren in verschiedenen Größen zur Messung von unterschiedlichen Strahleigenschaften verwendet worden. Untersucht wurde dabei die Strahlgeschwindigkeit von verschiedenen Edelgasen über einen großen Stagnationsdruck- und Stagnationstemperaturbereich. Außerdem wurden Strahlprofile zur Bestimmung der Strahldichte gemessen und mathematisch beschrieben. Zusätzlich wird eine Methode zur Bestimmung der Strahltemperatur mit Hilfe der Strahldichte und der Strahlgeschwindigkeit vorgestellt. / In this study, different particle detectors with a foil of polyvinylidene difluoride have been designed and built for use in a pulsed supersonic molecular jet experiment. Here, the molecular jet collides with the sensitive detector area and generates a charge. This generated charge is caused by the piezo- and the pyroelectric effect. Advantages of polyvinylidene difluoride detectors are a high detection efficiency, the detection of neutral atoms or molecules --- no ionization is required, a simple setup which can be easily incorporated in an existing experiment, an easy adjustment of the detector design because the shape and size can be changed easily, and a fast response-time in the sub-microsecond regime. Because the amount of charges generated is in the order of some picocoulomb, different amplifiers are used. In this analysis of the detected signal, the piezoelectric contribution is defined by the constitutive equations of piezoelectricity, which are used in combination with the concept of a driven damped circular membrane in order to obtain an analytic solution. The pyroelectric contribution is described via the exchanged energy. Because both the piezo- and the pyroelectric effects can be exploited, valuable additional information such as the coefficient of energy accommodation or the coefficient of restitution can be determined experimentally. In order to demonstrate the application possibilities of polyvinylidene difluoride detectors, detectors of different sizes are used as a local jet probe to determine different jet properties. The mean velocities of different rare gases for a wide range of source conditions are determined. Density profiles of various supersonic jets are measured and described mathematically in detail. In addition, both quantities, the velocity and the density, are used to determine the temperature of the supersonic jet.

The change of the spontaneous polarization due to a change of temperature is known as the pyroelectric effect and is restricted to crystalline, non-centrosymmetric and polar matter. Its main application is the utilization in infrared radiation sensors, but usage for waste heat energy harvesting or chemical catalysis is also possible. A precise quantification, i.e. the measurement of the pyroelectric coefficient p, is inevitable to assess the performance of a material. Hence, a comprehensive overview is provided in this work, which summarizes and evaluates the available techniques to characterize p. A setup allowing the fully automated measurement of p by utilizing the Sharp-Garn method and the measurement of ferroelectric hysteresis loops is described. It was used to characterize and discuss the behavior of p with respect to the temperature of the doped bulk III-V compound semiconductors gallium nitride and aluminum nitride and thin films of doped hafnium oxide, as reliable data for these materials is still missing in the literature. Here, the nitride-based semiconductors show a comparable small p and temperature dependency, which is only slightly affected by the incorporated dopant, compared to traditional ferroelectric oxides. In contrast, p of HfO2 thin films is about an order of magnitude larger and seems to be affected by the present dopant and its concentrations, as it is considered to be responsible for the formation of the polar orthorhombic phase.:1. Motivation and Introduction 2. Fundamentals 2.1. Dielectrics and their Classification 2.2. Polarization 2.3. Pyroelectricity 2.4. Ferroelectricty 2.5. Phase Transitions 2.6. Applications and Figures of Merit 3. Measurement Methods for the Pyroelectric Coefficient 3.1. General Considerations 3.1.1. Heating Concepts 3.1.2. Thermal Equilibrium 3.1.3. Electric Contact 3.1.4. Separation of Contributions 3.1.5. Thermally Stimulated Currents 3.2. Static Methods 3.2.1. Charge Compensation Method 3.2.2. Hysteresis Measurement Method 3.2.3. Direct Electrocaloric Measurement 3.2.4. Flatband Voltage Shift 3.2.5. X-ray Photoelectron Spectroscopy Method 3.2.6. X-ray Diffraction and Density Functional Theory 3.3. Dynamic Methods 3.3.1. Temperature Ramping Methods 3.3.2. Optical Methods 3.3.3. Periodic Pulse Technique 3.3.4. Laser Intensity Modulation Methods 3.3.5. Harmonic Waveform Techniques 4. Pyroelectric and Ferroelectric Characterization Setup 4.1. Pyroelectric Measurement Setup 4.1.1. Setup and Instrumentation 4.1.2. Automated Sharp-Garn Evaluation of Pyroelectric Coefficients 4.1.3. Further Examples 4.2. Hysteresis Loop Measurements 4.2.1. Instrumentation 4.2.2. Measurement and Evaluation 4.2.3. Examples 5. Investigated Material Systems 5.1. III-Nitride Bulk Semiconductors GaN and AlN 5.1.1. General Structure and Spontaneous Polarization 5.1.2. Applications 5.1.3. Crystal Growth and Doping 5.1.4. Pyroelectricity 5.2. Hafnium Oxide Thin Films 5.2.1. General Structure and Applications 5.2.2. Polar Properties in Thin Films 5.2.3. Doping Effects 5.2.4. Pyro- and Piezoelectricity 6. Results 6.1. The Pyroelectric Coefficient of Free-standing GaN and AlN 6.1.1. Sample Preparation 6.1.2. Pyroelectric Measurements 6.1.3. Lattice Influence 6.1.4. Slope Differences 6.2. Pyroelectricity of Doped Hafnium Oxide 6.2.1. Sharp-Garn Measurement on Thin Films 6.2.2. Effects of Silicon Doping 6.2.3. Dopant Comparison 7. Summary and Outlook A. Pyroelectric Current and Phase under Periodic Thermal Excitation B. Loss Current Correction for Shunt Method C. Conductivity Correction D. Comparison of Pyroelectric Figures of Merit Bibliography Publication List Acknowledgments / Die Änderung der spontanen Polarisation durch eine Änderung der Temperatur ist bekannt als der pyroelektrische Effekt, welcher auf kristalline, nicht-zentrosymmetrische und polare Materie beschränkt ist. Er findet vor allem Anwendung in Infrarot-Strahlungsdetektoren, bietet aber weitere Anwendungsfelder wie die Niedertemperatur-Abwärmenutzung oder die chemische Katalyse. Eine präzise Quantifizierung, d. h. die Messung des pyroelektrischen Koeffizienten p, ist unabdingbar, um die Leistungsfähigkeit eines Materials zu bewerten. Daher bietet diese Arbeit u.a. einen umfassenden Überblick und eine Bewertung der verfügbaren Messmethoden zur Charakterisierung von p. Weiterhin wird ein Messaufbau beschrieben, welcher die voll automatisierte Messung von p mit Hilfe der Sharp-Garn Methode und auch die Charakterisierung der ferroelektrischen Hystereseschleife ermöglicht. Aufgrund fehlerender Literaturdaten wurde dieser Aufbau anschließend genutzt, um den temperaturabhängigen pyroelektrischen Koeffizienten der dotierten III-V-Verbindungshalbleiter Gallium- und Aluminiumnitrid sowie dünner Schichten bestehend aus dotiertem Hafniumoxid zu messen und zu diskutieren. Im Vergleich zu klassichen ferroelektrischen Oxiden zeigen dabei die nitridbasierten Halbleiter einen geringen pyroelektrischen Koeffizienten und eine kleine Temperaturabhängigkeit, welche auch nur leicht durch den vorhandenen Dotanden beeinflusst werden kann. Dagegen zeigen dünne Hafniumoxidschichten einen um eine Größenordnung größeren pyroelektrischen Koeffizienten, welcher durch den anwesenden Dotanden und seine Konzentration beeinflusst wird, da dieser verantwortlich für die Ausbildung der polaren, orthorhombischen Phase gemacht wird.:1. Motivation and Introduction 2. Fundamentals 2.1. Dielectrics and their Classification 2.2. Polarization 2.3. Pyroelectricity 2.4. Ferroelectricty 2.5. Phase Transitions 2.6. Applications and Figures of Merit 3. Measurement Methods for the Pyroelectric Coefficient 3.1. General Considerations 3.1.1. Heating Concepts 3.1.2. Thermal Equilibrium 3.1.3. Electric Contact 3.1.4. Separation of Contributions 3.1.5. Thermally Stimulated Currents 3.2. Static Methods 3.2.1. Charge Compensation Method 3.2.2. Hysteresis Measurement Method 3.2.3. Direct Electrocaloric Measurement 3.2.4. Flatband Voltage Shift 3.2.5. X-ray Photoelectron Spectroscopy Method 3.2.6. X-ray Diffraction and Density Functional Theory 3.3. Dynamic Methods 3.3.1. Temperature Ramping Methods 3.3.2. Optical Methods 3.3.3. Periodic Pulse Technique 3.3.4. Laser Intensity Modulation Methods 3.3.5. Harmonic Waveform Techniques 4. Pyroelectric and Ferroelectric Characterization Setup 4.1. Pyroelectric Measurement Setup 4.1.1. Setup and Instrumentation 4.1.2. Automated Sharp-Garn Evaluation of Pyroelectric Coefficients 4.1.3. Further Examples 4.2. Hysteresis Loop Measurements 4.2.1. Instrumentation 4.2.2. Measurement and Evaluation 4.2.3. Examples 5. Investigated Material Systems 5.1. III-Nitride Bulk Semiconductors GaN and AlN 5.1.1. General Structure and Spontaneous Polarization 5.1.2. Applications 5.1.3. Crystal Growth and Doping 5.1.4. Pyroelectricity 5.2. Hafnium Oxide Thin Films 5.2.1. General Structure and Applications 5.2.2. Polar Properties in Thin Films 5.2.3. Doping Effects 5.2.4. Pyro- and Piezoelectricity 6. Results 6.1. The Pyroelectric Coefficient of Free-standing GaN and AlN 6.1.1. Sample Preparation 6.1.2. Pyroelectric Measurements 6.1.3. Lattice Influence 6.1.4. Slope Differences 6.2. Pyroelectricity of Doped Hafnium Oxide 6.2.1. Sharp-Garn Measurement on Thin Films 6.2.2. Effects of Silicon Doping 6.2.3. Dopant Comparison 7. Summary and Outlook A. Pyroelectric Current and Phase under Periodic Thermal Excitation B. Loss Current Correction for Shunt Method C. Conductivity Correction D. Comparison of Pyroelectric Figures of Merit Bibliography Publication List Acknowledgments

The structural origin of the temperature-dependent ferroelectricity in Si-doped HfO₂ thin films is systematically examined. From temperature-dependent polarization-electric field measurements, it is shown that remanent polarization increases with decreasing temperature. Concurrently, grazing incidence X-ray diffraction shows an increase in the orthorhombic phase fraction with decreasing temperature. The temperature-dependent evolution of structural and ferroelectric properties is believed to be highly promising for the electrocaloric cooling application. Magnetization measurements do not provide any indication for a change of magnetization within the temperature range for the strong crystalline phase transition, suggesting that magnetic and structural properties are comparatively decoupled. The results are believed to provide the first direct proof of the strongly coupled evolution of structural and electrical properties with varying temperature in fluorite oxide ferroelectrics.

Field-induced ferroelectricity in (doped) hafnia and zirconia has attracted increasing interest in energy-related applications, including energy harvesting and solid-state cooling. It shows a larger isothermal entropy change in a much wider temperature range compared with those of other promising candidates. The field-induced phase transition occurs in an extremely wide temperature range, which contributes to the giant electrocaloric effect. This article examines the possible origins of a large isothermal entropy change, which can be related to the extremely broad phase transitions in fluorite-structured ferroelectrics. While the materials possess a high entropy change associated with the polar–nonpolar phase transition, which can contribute to the high energy performance, the higher breakdown field compared with perovskites practically determines the available temperature range.

Icing on materials surface causes operational failures as well as technical and safety issues. Furthermore, it reduces the energy efficiency of the power supply and passenger/freight transportation systems. Conventional active deicing methods are widely used to remove ice, but are often associated with uneconomically high energy consumption and high maintenance costs, often not being aware of their environmental impact. Instead, passive anti-icing methods are being sought to prevent or delay ice formation by means of physico-chemical surface treatment. Pyroelectric materials can be used as possible anti-icing surfaces after their ability to inhibit ice nucleation has been experimentally demonstrated. This makes use of the effect of the pyroelectrically induced surface charge, which changes with the ambient temperature and thus, hypothetically, exerts an influence on the dipole orientation of the water molecules at the surface. This is expected to affect the hydrogen bonding network of the interfacial water in the supercooled liquid phase, depending on the sign of surface charge. However, the Classical Nucleation Theory predicts an increased nucleation rate with increasing electric field strength of the pyroelectric surface charge irrespective of its polarity, as confirmed by many experiments. This raises the question of what exactly influences the ice nucleation. The main purpose of this thesis is to find a relationship between the pyroelectricity and the ice nucleation rate. Various theoretical and experimental investigation methods have been used to examine which of the possible influencing factors related to the pyroelectric material surface plays a major role in promoting or inhibiting ice nucleation.:Contents Abstract i List of figures xi List of tables xv 1 Introduction 1 1.1 Motivation 1 1.2 Objective and Tasks 4 1.3 Structure of the thesis 6 2 Basics 7 2.1 Pyroelectric materials 7 2.1.1 Fundamental properties 7 2.1.2 Lithium niobate, LiNbO3 14 2.2 Ice nucleation and water freezing 21 2.2.1 Thermodynamics of ice nucleation 21 2.2.2 Factors influencing ice nucleation 26 3 Materials and Methods 29 3.1 Sample materials used for the investigation 29 3.2 Theoretical methods 31 3.2.1 Theoretical background of computational quantum mechanical modeling 31 3.2.2 LiNbO3 model system 38 3.2.3 DFT implementation in CP2K 41 3.3 Experimental methods 42 3.3.1 Optical and vibrational spectroscopy 43 3.3.2 X-ray spectroscopy 47 3.3.3 Atomic force microscopy 48 3.3.4 Environmental scanning electron spectroscopy 51 3.3.5 Pyroelectric measurement 52 3.3.6 Contact angle measurement 53 3.3.7 Icing temperature measurement 54 3.4 Tabular overview of the different methods 57 ix4 Results and Discussion 59 4.1 Results 59 4.1.1 Several results of DFT calculations 59 4.1.2 MD simulations of interfacial water 75 4.1.3 Results of optical and vibrational spectroscopy 80 4.1.4 X-ray spectroscopy on LiNbO3 surfaces 96 4.1.5 Extended treatment of the Classical Nucleation Theory 100 4.1.6 Results of atomic force microscopy 108 4.1.7 ESEM images of ice crystals grown on LiNbO3 116 4.1.8 Results of pyroelectric measurements 122 4.1.9 Results of contact angle measurements 124 4.1.10 Results of icing temperature measurements 126 4.2 Discussion 135 4.2.1 Surface charge 135 4.2.2 Surface structure 144 4.2.3 Surface reactivity 149 4.3 Conclusion of the findings and remarks 151 5 Summary and Outlook 157 5.1 Conclusion of the thesis 157 5.2 Recommendations for further investigations 161 5.3 Outlook 164 Appendix 167 A.1 Additional information to the DFT calculations 167 A.2 Background spectrum for ATR spectroscopy 175 A.3 Additional information to SFG/SHG spectroscopy 176 A.4 Additional information to the XPS results 181 A.5 Additional information to the AFM measurement 182 A.6 ESEM images of ice accretion in the sample system 187 A.7 FEM simulation of local temperature and flow velocity distribution 190 A.8 Additional information to the icing temperature measurement 203 A.9 Temperature-dependent pH variation of water at LiNbO3 surface 207 List of abbreviations and symbols 213 References 217 Publications 276 Acknowledgements 277 Erklärung 281 / Vereisung auf Werkstoffoberflächen führt einerseits zu Betriebsausfällen und andererseits zur Reduzierung der Energieeffizienz von Energieversorgungs- sowie Personen- und Gütertransportsystemen. Sie stellt nicht selten ein sicherheitstechnisches und gesundheitliches Risiko dar. Da die konventionellen aktiven Enteisungsmethoden mit hohem Energieaufwand und hohen Wartungskosten verbunden sind, wird nach passiven Anti-icing-Methoden als vorbeugende Maßnahmen zur Vermeidung/Verzögerung von Eisbildung auf physikalisch-chemisch behandelten Oberflächen gesucht. Der Einsatz dieser Werkstoffoberflächen senkt nicht nur den Energieverbrauch, sondern soll auch die Umwelt schonen. Pyroelektrische Materialien kommen als passive Anti-icing-Oberflächen in Frage, nachdem ihre eiskeimbildungshemmende Fähigkeit experimentell nachgewiesen wurde. Dabei wird der Effekt der pyroelektrisch induzierten Oberflächenladung ausgenutzt, die sich mit der Umgebungstemperatur ändert und somit, hypothetisch gesehen, einen Einfluss auf die Dipolorientierung der Wassermoleküle an der Oberfläche ausübt. Das hat je nach Vorzeichen der Oberflächenladung Auswirkungen auf das Wassermolekülbindungsnetzwerk des Grenzflächenwassers in der unterkühlten flüssigen Phase. Da die klassische Keimbildungstheorie jedoch eine erhöhte Keimbildungswahrscheinlichkeit mit zunehmender Stärke des elektrischen Feldes der pyroelektrischen Oberflächenladung unabhängig von ihrem Vorzeichen voraussagt, wie es ebenfalls in vielen Experimenten nachgewiesen wurde, stellt sich die Frage, was genau die Eiskeimbildung beeinflusst. Das Hauptanliegen dieser Arbeit ist, einen Zusammenhang zwischen der Pyroelektrizität der Oberfläche und der Eiskeimbildungsrate zu finden. Mithilfe einer Vielzahl von verschiedenen theoretischen und experimentellen Methoden wird untersucht, welcher der möglichen Einflussfaktoren im Zusammenhang mit der pyroelektrischen Materialoberfläche eine große Rolle bei der Eiskeimbildung spielt.:Contents Abstract i List of figures xi List of tables xv 1 Introduction 1 1.1 Motivation 1 1.2 Objective and Tasks 4 1.3 Structure of the thesis 6 2 Basics 7 2.1 Pyroelectric materials 7 2.1.1 Fundamental properties 7 2.1.2 Lithium niobate, LiNbO3 14 2.2 Ice nucleation and water freezing 21 2.2.1 Thermodynamics of ice nucleation 21 2.2.2 Factors influencing ice nucleation 26 3 Materials and Methods 29 3.1 Sample materials used for the investigation 29 3.2 Theoretical methods 31 3.2.1 Theoretical background of computational quantum mechanical modeling 31 3.2.2 LiNbO3 model system 38 3.2.3 DFT implementation in CP2K 41 3.3 Experimental methods 42 3.3.1 Optical and vibrational spectroscopy 43 3.3.2 X-ray spectroscopy 47 3.3.3 Atomic force microscopy 48 3.3.4 Environmental scanning electron spectroscopy 51 3.3.5 Pyroelectric measurement 52 3.3.6 Contact angle measurement 53 3.3.7 Icing temperature measurement 54 3.4 Tabular overview of the different methods 57 ix4 Results and Discussion 59 4.1 Results 59 4.1.1 Several results of DFT calculations 59 4.1.2 MD simulations of interfacial water 75 4.1.3 Results of optical and vibrational spectroscopy 80 4.1.4 X-ray spectroscopy on LiNbO3 surfaces 96 4.1.5 Extended treatment of the Classical Nucleation Theory 100 4.1.6 Results of atomic force microscopy 108 4.1.7 ESEM images of ice crystals grown on LiNbO3 116 4.1.8 Results of pyroelectric measurements 122 4.1.9 Results of contact angle measurements 124 4.1.10 Results of icing temperature measurements 126 4.2 Discussion 135 4.2.1 Surface charge 135 4.2.2 Surface structure 144 4.2.3 Surface reactivity 149 4.3 Conclusion of the findings and remarks 151 5 Summary and Outlook 157 5.1 Conclusion of the thesis 157 5.2 Recommendations for further investigations 161 5.3 Outlook 164 Appendix 167 A.1 Additional information to the DFT calculations 167 A.2 Background spectrum for ATR spectroscopy 175 A.3 Additional information to SFG/SHG spectroscopy 176 A.4 Additional information to the XPS results 181 A.5 Additional information to the AFM measurement 182 A.6 ESEM images of ice accretion in the sample system 187 A.7 FEM simulation of local temperature and flow velocity distribution 190 A.8 Additional information to the icing temperature measurement 203 A.9 Temperature-dependent pH variation of water at LiNbO3 surface 207 List of abbreviations and symbols 213 References 217 Publications 276 Acknowledgements 277 Erklärung 281

Ferroelectricity in hafnium oxide thin films is known to be induced by various doping elements and in solid-solution with zirconia. While a wealth of studies is focused on their basic ferroelectric properties and memory applications, thorough studies of the related pyroelectric properties and their application potential are only rarely found. This work investigates the impact of Si doping on the phase composition and ferro- as well as pyroelectric properties of thin film capacitors. Dynamic hysteresis measurements and the field-free Sharp-Garn method were used to correlate the reported orthorhombic phase fractions with the remanent polarization and pyroelectric coefficient. Maximum values of 8.21 µC cm−2 and −46.2 µC K−1 m−2 for remanent polarization and pyroelectric coefficient were found for a Si content of 2.0 at%, respectively. Moreover, temperature-dependent measurements reveal nearly constant values for the pyroelectric coefficient and remanent polarization over the temperature range of 0 °C to 170 °C, which make the material a promising candidate for IR sensor and energy conversion applications beyond the commonly discussed use in memory applications.

Zur Messung pyrelektrischer Koeffzienten wurde ein Messplatz nach einem erweiterten SHARP-GARN-Verfahren entwickelt und zur Untersuchung von Phasenumwandlungen in Pyroelektrika eingesetzt. Einerseits konnten pyroelektrische Messungen im elektrischen Feld die Pyroelektrizität einer neuen durch elektrisch angetriebene Defektmigration erzeugten Phase in Strontiumtitanat nachweisen. Andererseits gelang es, Ferroelektrizität in der Hochtemperaturphase von Poly(Vinylidenfluorid-Trifluorethylen), mittels phasenreiner Präparation der Hochtemperaturphase unterhalb der CURIEtemperatur und anschließender Polarisierung, nachzuweisen. Ferner ließen sich mittels thermisch angeregter Pyroelektrika Redoxprozesse antreiben, was durch Desinfektion von Escherichia coli Bakterien in wässriger Lösung mittels Lithiumniobat und -tantalat gezeigt wurde. Die Hypothese der Desinfektion durch reaktive Sauerstoffspezies konnte durch spektroskopisch nachgewiesene OH-Radikale - erzeugt mittels thermisch angeregter Bariumtitanatnanopartikel - belegt werden.