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POLARIZATION SWITCHING KINETICS IN FERROELECTRIC CERAMICS.

Khachaturyan, Ruben (2019):
POLARIZATION SWITCHING KINETICS IN FERROELECTRIC CERAMICS.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8572],
[Ph.D. Thesis]

Abstract

The classical Kolmogorov-Avrami-Ishibashi (KAI) model successfully describes polarization switching kinetics of single crystals. Later on, Tagantsev et al. introduced a statistical distribution of switching time (SwT) in the KAI model; the improved statistical model is known as the nucleation limited switching (NLS) model and became suitable for thin films. Based on a robust dependence of SwT on an electric field, Lupascu et al. have proposed that the nature of the SwT statistical distribution can be attributed to a corresponding distribution of local electric fields inside a material. Using this assumption, the inhomogeneous field mechanism (IFM) model was developed. Although the NLS and IFM models are able to describe experimental measurements with high accuracy they neglect several crucial physical aspects of the poling problem. A disordered granular structure in a polarized state is unavoidably accompanied by charge formation on grain boundaries (GB), therefore giving rise to additional electric fields. Charges on grain boundaries should vary depending on polarization, therefore depolarization fields should be also time dependent. The latter fact, however, is omitted in the NLS as well as in the IFM models. Another questionable assumption, included in all statistical models, is the statistical independence of switching regions, which is doubtful since the huge depolarization fields have to produce grain correlations. Statistical independence of switching events disable accounting for non-180 switching events because these events are sequential processes rather than independent. Hence an independent switching mechanism is not suitable for this process. These all make the aforementioned statistical models and the reliability of extracted parameters questionable. This work aims to shed some light on the reliability of statistical models by investigation of charge formation on grain boundaries during a poling process as well as produced depolarization fields and their evolution. Correlations of polarization and electric field components are analyzed. All studies are carried out for tetragonal, rhombohedral and orthorhombic symmetries and are in good agreement with previous theoretical results and experimental measurements. A new statistical model which involves non-180 switching events is presented and successfully applied to the recent polarization-time and strain-time measurements. A statistical distribution of electric fields is additionally studied for the case of porous ceramics as they are materials where the distribution can be controlled by modifications to the structure.

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Khachaturyan, Ruben
Title: POLARIZATION SWITCHING KINETICS IN FERROELECTRIC CERAMICS.
Language: English
Abstract:

The classical Kolmogorov-Avrami-Ishibashi (KAI) model successfully describes polarization switching kinetics of single crystals. Later on, Tagantsev et al. introduced a statistical distribution of switching time (SwT) in the KAI model; the improved statistical model is known as the nucleation limited switching (NLS) model and became suitable for thin films. Based on a robust dependence of SwT on an electric field, Lupascu et al. have proposed that the nature of the SwT statistical distribution can be attributed to a corresponding distribution of local electric fields inside a material. Using this assumption, the inhomogeneous field mechanism (IFM) model was developed. Although the NLS and IFM models are able to describe experimental measurements with high accuracy they neglect several crucial physical aspects of the poling problem. A disordered granular structure in a polarized state is unavoidably accompanied by charge formation on grain boundaries (GB), therefore giving rise to additional electric fields. Charges on grain boundaries should vary depending on polarization, therefore depolarization fields should be also time dependent. The latter fact, however, is omitted in the NLS as well as in the IFM models. Another questionable assumption, included in all statistical models, is the statistical independence of switching regions, which is doubtful since the huge depolarization fields have to produce grain correlations. Statistical independence of switching events disable accounting for non-180 switching events because these events are sequential processes rather than independent. Hence an independent switching mechanism is not suitable for this process. These all make the aforementioned statistical models and the reliability of extracted parameters questionable. This work aims to shed some light on the reliability of statistical models by investigation of charge formation on grain boundaries during a poling process as well as produced depolarization fields and their evolution. Correlations of polarization and electric field components are analyzed. All studies are carried out for tetragonal, rhombohedral and orthorhombic symmetries and are in good agreement with previous theoretical results and experimental measurements. A new statistical model which involves non-180 switching events is presented and successfully applied to the recent polarization-time and strain-time measurements. A statistical distribution of electric fields is additionally studied for the case of porous ceramics as they are materials where the distribution can be controlled by modifications to the structure.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Materials Modelling
Date Deposited: 07 Apr 2019 19:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8572
URN: urn:nbn:de:tuda-tuprints-85725
Referees: Genenko, Apl. Prof. Yuri and Hongbin, Prof. Zhang
Refereed / Verteidigung / mdl. Prüfung: 17 December 2018
Alternative Abstract:
Alternative abstract Language
Das klassische Kolmogorov-Avrami-Ishibashi (KAI) Modell beschreibt erfolgreich die Polarisationsumschaltkinetik in Einkristallen. Später führte Tagantsev et al. eine statistische Verteilung der Schaltzeit (SwT) im KAI-Modell ein; das verbesserte statistische Modell ist als durch Nukleation begrenztes Umschalten (NLS) Modell bekannt und wurde erfolgreich für dünne Schichten umgesetzt. Auf der Grundlage einer robusten Abhängigkeit von SwT von einem elektrischen Feld hat Lupascu et al. vorgeschlagen, dass die Art der statistischen Verteilung des SwT auf eine entsprechende Verteilung der lokalen elektrischen Felder innerhalb eines Materials zurückzuführen ist. Anhand dieser Annahme wurde das inhomogene Feld- mechanismus (IFM)-Modell entwickelt. Obwohl die NLS und IFM Modelle in der Lage sind, experimentelle Messungen mit hoher Genauigkeit zu beschreiben, vernachlässigen sie mehrere entscheidende physikalische Aspekte des Polarisationsproblems. Eine ungeordnete körnige Struktur wird von Ladungsbildung an Korngrenzen (GB) begleitet, wodurch zusätzliche elektrische Felder entstehen. Die Ladungen an Korngrenzen sollten je nach Polarisation variieren, daher sollten Depolarisationsfelder auch zeitabhängig sein. Letzteres wird aber sowohl in der NLS, als auch in den IFM-Modellen ausgelassen. Eine weitere fragwürdige Annahme, die in allen statistischen Modellen enthalten ist, ist die statistische Unabhängigkeit von umschaltenden Regionen, was zweifelhaft ist, da die riesigen Depolarisierungsfelder Korrelationen von Körner erzeugen müssten. Die angenommene statistische Unabhängigkeit von Schaltereignissen macht es unmöglich die nicht-180 Schaltereignissen zu beschreiben, da diese keine unabhängige sondern sequenzielle Prozesse sind. All dies macht die obengenannten statistischen Modelle und Verlässlichkeit der daraus extrahierten Parameter fragwürdig. Ziel dieser Arbeit ist es, die Zuverlässigkeit statistischer Modelle durch die Untersuchung der Ladungsbildung an den Korngrenzen während eines Polarisationsprozesses zu prüfen, sowie die produzierten Depolarisierungsfelder und deren Entwicklung zu beleuchten. Es werden Korrelationen von Polarisierung und elektrischen Feldkomponenten analysiert. Alle Studien werden für tetragonale, rhomboedrische und orthorhombische Symmetrien durchgeführt und sind in guter Übereinstimmung mit bekannten theoretischen Ergebnissen und experimentellen Messungen. Ein neues statistisches Modell, das nicht-180 Schaltereignisse beinhaltet, wird vorgestellt und erfolgreich auf die jüngsten zeitabhängigen Polarisations- und Dehnungsmessungen angewendet. Unter anderem wird für den Fall der porösen Keramik eine statistische Verteilung der elektrischen Felder untersucht, da es sich um Materialien handelt, bei denen diese Verteilung durch Änderungen der Struktur gesteuert werden kann.German
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