Teses / dissertações sobre o tema "Antiferroelectric materials"

This thesis examined two-dimensional ferroelectric materials and their applications using density functional theory calculations. The research has revealed several novel applications for 2D ferroelectric materials. It illustrated that ferroelectric materials can be used to modify electronic, photocatalytic and magnetic properties of two-dimensional materials. In addition to exploring applications, new two-dimensional ferroelectric material which exhibits metallic properties was discovered through high through output search. Two-dimensional ferroelectric metals are extremely rare and only handful of materials were ever discovered.

Mesures entre 7 et 300K, dans le domaine 15-400 CM**(-)1 et réalisation des spectres du cristal d'ADP, dans la phase anti-ferroélectrique, en lumière polarisée. Détermination des paramètres des modes de vibration par ajustement au moyen d'un modèle de la fonction diélectrique. Mise en évidence de l'importance des modes de basse fréquence, liés aux mouvements collectifs des protons, dans le mécanisme de la transition de phase

Cette thèse examine les films minces d’oxyde de hafnium (HfO2) pour leur potentiel dans lesnanocondensateurs, répondant aux besoins en miniaturisation et haute performance del’électronique moderne. Le HfO2 est compatible avec la Déposition par Couches Atomiques(ALD), ce qui permet des dépôts minces précis et homogènes, essentiels pour garantir la fiabilitédes dispositifs électroniques. Les films minces sont soumis à différentes techniques de fabricationet de caractérisation pour analyser leur morphologie et leurs propriétés électriques, notamment laconstante diélectrique, la tension de claquage et la capacité de stockage d’énergie. Cette approchepermet de déterminer comment optimiser ces matériaux, à la fois en configurations amorphes eten structures cristallines, pour des performances maximales.Pour les diélectriques amorphes linéaires, HfO2 est combiné avec d’autres oxydes, tels quel’alumine et la silice, dans des structures de nanolaminés et de solutions solides. Ces combinaisonssont conçues pour stabiliser la constante diélectrique et offrir une résistance au claquage,améliorant l'efficacité du stockage d’énergie. La linéarité et la stabilité de ces matériaux amorphesles rendent particulièrement adaptés aux applications nécessitant une capacitance stable.L’étude approfondit aussi les propriétés des diélectriques cristallins non linéaires, dopés avec dusilicium ou de la zircone. Différentes températures de déposition et de recuit révèlent descomportements ferroélectriques et antiferroélectriques, augmentant la densité d’énergie et lastabilité. Cependant, les matériaux ferroélectriques, bien que prometteurs pour des applications àhaute densité, sont sensibles aux variations de tension, ce qui limite leur usage dans les applicationsnécessitant une capacitance constante. Les matériaux antiferroélectriques, en revanche, présententune stabilité accrue face aux variations de tension, mais ils font encore face à des défis d’efficacitéénergétique et de gestion thermique. La recherche souligne la variabilité de la constantediélectrique comme un défi majeur pour l'utilisation de ces matériaux dans des applicationsnécessitant une capacitance stable, comme le filtrage de signaux. Les matériaux nanolaminés etles solutions solides sont privilégiés pour obtenir une capacitance linéaire, mais leur efficacité restelimitée en termes de permittivité. L’exploration des phases non linéaires, cependant, ouvre la voieà des performances accrues dans certaines applications avancées.En conclusion, cette étude apporte un éclairage précieux sur les films minces d’oxyde de hafniumet leur rôle dans les nanocondensateurs, en explorant des solutions d’optimisation pour améliorerles performances diélectriques, notamment par les techniques de fabrication et les compositionsde matériaux. Les matériaux linéaires et non linéaires présentent chacun des avantages distincts,mais des recherches supplémentaires sont nécessaires pour surmonter les défis liés à la durabilité,l’efficacité électrique et la gestion thermique, afin de développer des condensateurs plusperformants pour les technologies électroniques modernes<br>This research investigates the use of hafnium oxide (HfO2)-based thin films in nanocapacitors, focusing on both their linear and non-linear electrical properties to meet the growing demands of high-performance and miniaturized electronic devices. Starting with the fundamental physics of energy storage capacitors, the investigation highlights the essential characteristics of effective dielectric materials, such as a high dielectric constant and a substantial band gap. Hafnium-based materials are particularly promising due to their compatibility with Atomic Layer Deposition (ALD), which allows for precise and uniform thin-film deposition—crucial for ensuring reliable performance in electronic devices.To understand the potential of these materials, various fabrication and characterization techniques were employed. This includes specific deposition processes to create the thin films and morphological tests to study the physical structure of the capacitors. Electrical testing plays a key role in evaluating critical parameters like dielectric constant, breakdown voltage, and overall energy storage capacity. By analyzing these factors, a comprehensive view of how both linear and non-linear hafnium-based dielectrics perform is provided.When exploring linear, amorphous hafnium-based dielectrics, HfO2 is combined with aluminum oxide and silicon dioxide to enhance dielectric properties. Different configurations, such as nanolaminates and solid solutions, are tested to find the optimal balance. The goal is to achieve materials that maintain a high dielectric constant and resist voltage breakdown, thereby improving their ability to store energy efficiently. On the other hand, a detailed look into non-linear, crystalline dielectrics examines the effects of doping hafnium oxide with elements like zirconia and silicon. Different deposition and annealing temperatures are assessed for their impact on crystalline structure and polarization behavior, revealing complex ferroelectric and antiferroelectric behaviors that could offer high energy density and stability.The findings suggest that while ferroelectric materials might not be suitable for applications requiring linear capacitance due to their sensitivity to voltage variations, antiferroelectric materials show promise. However, they still face challenges related to electrical efficiency and thermal management. Finding materials that can effectively stabilize voltage variations is crucial, as capacitors are increasingly used to manage these fluctuations in modern electronics.A significant challenge identified is the variability in the dielectric constant, which can limit the use of these materials in applications demanding stable capacitance, such as signal filtering. To address this issue, solid solutions and laminated materials, which provide consistent linear capacitance, are prioritized. Although these materials are effective up to a certain permittivity threshold, exploring non-linear phases opens the door to potentially higher performance under specific conditions.In summary, understanding of HfO2-based thin films and their role in nanocapacitors is advanced by this research. By examining both linear and non-linear dielectric materials, insights into how to optimize fabrication techniques and material compositions to improve dielectric properties are provided. Ongoing research into issues like material endurance, electrical efficiency, and thermal management is essential for developing reliable and high-performing capacitors that meet the evolving demands of modern electronic technologies

Com base em resultados de DSC, análise estrutural por difração de raios X e observações de textura, analisamos os efeitos da mistura do solvente apolar hexadecano com cristais líquidos formados pelas moléculas banana 1,3-fenilenobis[4-(4-tetradecilfeniliminometil)benzoato] (MB14) e 4-cloro-1,3-fenilenobis[4-(4-tetradecilfeniliminometil) benzoato] (MB14Cl). Propusemos um modelo estrutural para explicar as modificações causadas no arranjo molecular da fase B2 pelo acréscimo gradual do solvente. Observamos a diminuição da temperatura de transição entre esta fase e a fase isotrópica, porém a transição entre a fase B2 e a fase a temperaturas mais baixas não sofre alteração significativa. Para concentrações em massa de hexadecano de 45 % no MB14 e de 55 % no MB14Cl, a fase B2 já não é mais observada. Para o MB14Cl, resultados de difração de raios X revelaram que as moléculas de hexadecano penetram entre as camadas esméticas, aumentando a distância intercamada em torno de 3 Å. Acima de 5 % de concentração do solvente, o aumento da distância intercamada satura e ocorre segregação de fases em escala nanométrica. O comportamento da fase B2 sob a ação de um campo elétrico variável também foi analisado para o MB14 puro. Apresentamos um modelo para a linha de base do sinal de corrente de polarização, que leva em conta a não linearidade da condutividade para valores altos de campo aplicado, devido à movimentação iônica na amostra. Para o cálculo da viscosidade, consideramos a não linearidade da constante dielétrica com o campo aplicado, e adaptamos outro modelo, inicialmente utilizado para cristais líquidos ferroelétricos sob a ação de um campo quadrado, para o caso de um cristal líquido antiferroelétrico sob um campo triangular. Quanto aos dois tipos de arranjo molecular da fase B2, o arranjo homoquiral se mostrou bem mais estável que o racêmico, mesmo sob aplicação de campo triangular, quando este último é inicialmente favorecido. O arranjo racêmico se apresentou mais viscoso que o arranjo homoquiral, contrariando nossas previsões.<br>Based on DSC results, structural analysis by X-ray diffraction and texture observations, we observed the effects of mixing the nonpolar solvent hexadecane with the banana molecules liquid crystals ,3-phenilenebis[4-(4-tetradecilpheniliminometil)benzoate] (MB14) and 4-chloro-1,3-phenilenebis[4-(4-tetradecilpheniliminometil) benzoate] (MB14Cl). We propose a structural model to explain the changes in the molecular ordering of the B2 phase caused by the gradual increase of the solvent. We observed a decreasing of the transition temperature between B2 and isotropic phases, however the transition between B2 and lower temperature phases did not change significantly. For hexadecane concentrations above 45 wt% in MB14 and 55 wt% in MB14Cl, the B2 phase is no longer present. In MB14Cl, X-ray diffraction results showed that the hexadecane molecules penetrate between the smectic layers, increasing the interlayer spacing by about 3 Å. Above 5 wt% of solvent concentration, the increasing of the interlayer spacing saturates, and a phase segregation in nanometric scale occurs. The behavior of the B2 phase under variable electric field was also analysed for the pure MB14. We present a model for the baseline of the polarization current signal, which considers the non-linearity of the conductivity for high values of applied field, due to the presence of ions in the sample. In order to calculate the viscosity, we considered the non-linearity of the dielectric constant with the applied field, and adapted another model, initially used in ferroelectric liquid crystals under rectangular field, for the case of an antiferroelectric liquid crystal under triangular field. Concerning the two kind of molecular ordering in the B2 phase, the homoquiral ordering proved to be far more stable than the racemic, even under triangular field, when the latest is favored. Our measurements resulted in a racemic ordering more viscous than the homoquiral, going against our predictions.