Este estudo teve como objetivo principal desenvolver uma técnica para síntese de um nanocompósito de Y-TZP/MWCNT-COOH (Zircônia estabilizada por 3 mol% de ítria reforçada por nanotubos de carbono funcionalizado em -COOH) com propriedades mecânicas e ópticas que permitam a sua futura utilização como infraestrutura de próteses fixas dentárias e pilares protéticos para implantes. Assim, foram avaliados a microestrutura, resistência à flexão, tenacidade à fratura, limite de fadiga e propriedades ópticas do nanocompósito e comparada àquelas medidas para Y-TZP convencional (controle). O material Y-TZP/MWCNT-COOH foi desenvolvido pelo processo de co-precipitação de hidróxidos mistos associado ao tratamento hidrotérmico/solvotérmico e prensagem uniaxial em formato de blocos para sistemas CAD/CAM. O pó de MWCNT-COOH foi caracterizado por meio de MEV-FEG, TEM, TGA, DRX e FRX previamente a sua utilização para desenvolvimento do nanocompósito. Espécimes foram obtidos a partir do material Y-TZP/MWCNT-COOH para caracterização por meio de DRX, MEV-FEG e TEM, e comparação de suas propriedades estruturais (densidade e contração), ópticas, resistência à flexão, tenacidade à fratura e limite de fadiga com a Y-TZP convencional. O MWCNT-COOH apresentou-se em feixes de nanotubos de carbono recobertos por sílica tendo comprimento médio de 5,10 ± 1,34 ?m, com 90% dos comprimentos medidos (D90) estando abaixo de 6,9 ?m. Foi verificado a não possibilidade da utilização de líquidos orgânicos em nenhum passo da fabricação dos compósito Y-TZP/MWCNT-COOH por levar ao escurecimento do compósito, inviabilizando sua futura aplicação clínica. O tratamento hidrotérmico sem uso de líquidos orgânicos mostrou-se eficaz em proporcionar o revestimento do nanotubo de carbono por partículas de óxido de zircônio e ítrio. Entretanto, ocasionou a formação de aglomerados e partículas de Y-TZP com tamanho maiores que 5 ?m. Uma densidade relativa de 97,4% foi alcançada para o compósito experimental de Y-TZP contendo MWCNT-COOH, tendo uma razão de contraste de 0.9929 ± 0.0012 e um valor de diferença de cor da Y-TZP convencional de 6,1 ± 3,1 ( ?E). As propriedades mecânicas da Y-TZP/MWCNT-COOH, dureza Vickers (10,14 ± 1,27 GPa; p=0,25) e tenacidade à fratura (4,98 ± 0,30 MPa.m1/2; p=0,39), não apresentaram diferença significativa da Y-TZP convencional (dureza: 8,87 ± 0,89; tenacidade à fratura: 4,98 ± 0,30 MPa.m1/2). Entretanto, para a resistência à flexão (p=0,003) e limite de fadiga cíclica (LFC) foram obtidos valores inferiores para o material experimental Y-TZP/MWCNT-COOH (resistência à flexão: 299,4 ± 30,5 MPa; LFC: 179,4 ± 22,5 MPa) quando comparado à Y-TZP controle (resistência à flexão: 623,7 ± 108,8 MPa; LFC: 439,0 ± 56,4 MPa). Com base nos resultados apresentados, é possível concluir que a síntese de um nanocompósito de Y-TZP/MWCNT-COOH com propriedades ópticas adequadas para aplicação na odontologia restauradora foi possível por meio dos métodos descritos, entretanto algumas adequações nos métodos de síntese e processamento para criação do nanocompósito devem ser realizadas para se evitar a acentuada diminuição de importantes propriedades mecânicas do material. / This study aim was to develop a technique for synthetize nanocomposite of Y-TZP/MWCNT-COOH (3 mol% Yttria-Stabilized Tetragonal Zirconia reinforced with COOH functionalized carbon nanotubes) with mechanical and optical properties that allow their future use as fixed dental prosthesis infrastructure and implant abutments. Thus, the following properties of the nanocomposite were investigated and compared to those measured for conventional Y-TZP (control): microstructure, flexural strength, fracture toughness, fatigue limit and optical properties. Y-TZP/MWCNT-COOH material was developed by the co-precipitation of mixed hydroxides associated with the hydrothermal/solvothermal treatment and uniaxial pressing to form blocks for CAD/CAM systems. The MWCNT-COOH powder was characterized by SEM-FEG, TEM, TGA, XRD and XRF prior to its use for the development of nanocomposite. Specimens were obtained from the Y-TZP/MWCNT-COOH material and characterized by XRD, SEM-FEG and TEM. After characterization, the material had their structural properties (density and contraction), optical, flexural strength, fracture toughness and fatigue limit compared to a conventional Y-TZP. The MWCNT-COOH material was observed to be a bundle formation of carbon nanotube covered with silica with an average length of 5.10 ± 1.34 ?m, with 90% of the measured lengths (D90) being below 6.9 ?m. It has been found to be not possible to use organic liquids on any step of the Y-TZP/MWCNT-COOH manufacturing process due to darkening of the composite, making it unfeasible to future clinical application. The hydrothermal treatment without the use of organic liquids was effective in providing the carbon nanotube coating by zirconium and yttrium oxide particles. However, this treatment led to the formation of agglomerates and particles of Y-TZP with larger than 5 ?m. A relative density of 97.4% was achieved for the Y-TZP/MWCNT-COOH composite, having a contrast ratio of 0.9929 ± 0.0012, and a color difference value from the conventional Y-TZP of 6.1 ± 3.1 (?E). The mechanical properties of Y-TZP/MWCNT-COOH, Vickers hardness (10.14 ± 1.27 GPa; p = 0.25) and fracture toughness (4.98 ± 0.30 MPa.m1/2; p = 0.39), showed no significant difference from the conventional Y-TZP (hardness: 8.87 ± 0.89; fracture toughness: 4.98 ± 0.30 MPa.m1/2). However, flexural strength (p = 0.003) and cyclic fatigue limit (CFL) showed lower values for the experimental material Y-TZP/MWCNT-COOH (flexural strength: 299.4 ± 30.5 MPa; CFL: 179.4 ± 22.5 MPa) compared to Y-TZP control (flexural strength: 623.7 ± 108.8 MPa; CFL: 439.0 ± 56.4 MPa). Based on the results presented, it could be conclude that the synthesis of a nanocomposite of Y-TZP/MWCNT-COOH with optical properties suitable for application in restorative dentistry was made possible by the described methods, however some adjustments in synthesis and processing methods for the nanocomposite creation should be taken; to avoid the sharp decrease of important mechanical properties of the material.

Este estudo teve como objetivo principal desenvolver uma técnica para síntese de um nanocompósito de Y-TZP/MWCNT-COOH (Zircônia estabilizada por 3 mol% de ítria reforçada por nanotubos de carbono funcionalizado em -COOH) com propriedades mecânicas e ópticas que permitam a sua futura utilização como infraestrutura de próteses fixas dentárias e pilares protéticos para implantes. Assim, foram avaliados a microestrutura, resistência à flexão, tenacidade à fratura, limite de fadiga e propriedades ópticas do nanocompósito e comparada àquelas medidas para Y-TZP convencional (controle). O material Y-TZP/MWCNT-COOH foi desenvolvido pelo processo de co-precipitação de hidróxidos mistos associado ao tratamento hidrotérmico/solvotérmico e prensagem uniaxial em formato de blocos para sistemas CAD/CAM. O pó de MWCNT-COOH foi caracterizado por meio de MEV-FEG, TEM, TGA, DRX e FRX previamente a sua utilização para desenvolvimento do nanocompósito. Espécimes foram obtidos a partir do material Y-TZP/MWCNT-COOH para caracterização por meio de DRX, MEV-FEG e TEM, e comparação de suas propriedades estruturais (densidade e contração), ópticas, resistência à flexão, tenacidade à fratura e limite de fadiga com a Y-TZP convencional. O MWCNT-COOH apresentou-se em feixes de nanotubos de carbono recobertos por sílica tendo comprimento médio de 5,10 ± 1,34 ?m, com 90% dos comprimentos medidos (D90) estando abaixo de 6,9 ?m. Foi verificado a não possibilidade da utilização de líquidos orgânicos em nenhum passo da fabricação dos compósito Y-TZP/MWCNT-COOH por levar ao escurecimento do compósito, inviabilizando sua futura aplicação clínica. O tratamento hidrotérmico sem uso de líquidos orgânicos mostrou-se eficaz em proporcionar o revestimento do nanotubo de carbono por partículas de óxido de zircônio e ítrio. Entretanto, ocasionou a formação de aglomerados e partículas de Y-TZP com tamanho maiores que 5 ?m. Uma densidade relativa de 97,4% foi alcançada para o compósito experimental de Y-TZP contendo MWCNT-COOH, tendo uma razão de contraste de 0.9929 ± 0.0012 e um valor de diferença de cor da Y-TZP convencional de 6,1 ± 3,1 ( ?E). As propriedades mecânicas da Y-TZP/MWCNT-COOH, dureza Vickers (10,14 ± 1,27 GPa; p=0,25) e tenacidade à fratura (4,98 ± 0,30 MPa.m1/2; p=0,39), não apresentaram diferença significativa da Y-TZP convencional (dureza: 8,87 ± 0,89; tenacidade à fratura: 4,98 ± 0,30 MPa.m1/2). Entretanto, para a resistência à flexão (p=0,003) e limite de fadiga cíclica (LFC) foram obtidos valores inferiores para o material experimental Y-TZP/MWCNT-COOH (resistência à flexão: 299,4 ± 30,5 MPa; LFC: 179,4 ± 22,5 MPa) quando comparado à Y-TZP controle (resistência à flexão: 623,7 ± 108,8 MPa; LFC: 439,0 ± 56,4 MPa). Com base nos resultados apresentados, é possível concluir que a síntese de um nanocompósito de Y-TZP/MWCNT-COOH com propriedades ópticas adequadas para aplicação na odontologia restauradora foi possível por meio dos métodos descritos, entretanto algumas adequações nos métodos de síntese e processamento para criação do nanocompósito devem ser realizadas para se evitar a acentuada diminuição de importantes propriedades mecânicas do material. / This study aim was to develop a technique for synthetize nanocomposite of Y-TZP/MWCNT-COOH (3 mol% Yttria-Stabilized Tetragonal Zirconia reinforced with COOH functionalized carbon nanotubes) with mechanical and optical properties that allow their future use as fixed dental prosthesis infrastructure and implant abutments. Thus, the following properties of the nanocomposite were investigated and compared to those measured for conventional Y-TZP (control): microstructure, flexural strength, fracture toughness, fatigue limit and optical properties. Y-TZP/MWCNT-COOH material was developed by the co-precipitation of mixed hydroxides associated with the hydrothermal/solvothermal treatment and uniaxial pressing to form blocks for CAD/CAM systems. The MWCNT-COOH powder was characterized by SEM-FEG, TEM, TGA, XRD and XRF prior to its use for the development of nanocomposite. Specimens were obtained from the Y-TZP/MWCNT-COOH material and characterized by XRD, SEM-FEG and TEM. After characterization, the material had their structural properties (density and contraction), optical, flexural strength, fracture toughness and fatigue limit compared to a conventional Y-TZP. The MWCNT-COOH material was observed to be a bundle formation of carbon nanotube covered with silica with an average length of 5.10 ± 1.34 ?m, with 90% of the measured lengths (D90) being below 6.9 ?m. It has been found to be not possible to use organic liquids on any step of the Y-TZP/MWCNT-COOH manufacturing process due to darkening of the composite, making it unfeasible to future clinical application. The hydrothermal treatment without the use of organic liquids was effective in providing the carbon nanotube coating by zirconium and yttrium oxide particles. However, this treatment led to the formation of agglomerates and particles of Y-TZP with larger than 5 ?m. A relative density of 97.4% was achieved for the Y-TZP/MWCNT-COOH composite, having a contrast ratio of 0.9929 ± 0.0012, and a color difference value from the conventional Y-TZP of 6.1 ± 3.1 (?E). The mechanical properties of Y-TZP/MWCNT-COOH, Vickers hardness (10.14 ± 1.27 GPa; p = 0.25) and fracture toughness (4.98 ± 0.30 MPa.m1/2; p = 0.39), showed no significant difference from the conventional Y-TZP (hardness: 8.87 ± 0.89; fracture toughness: 4.98 ± 0.30 MPa.m1/2). However, flexural strength (p = 0.003) and cyclic fatigue limit (CFL) showed lower values for the experimental material Y-TZP/MWCNT-COOH (flexural strength: 299.4 ± 30.5 MPa; CFL: 179.4 ± 22.5 MPa) compared to Y-TZP control (flexural strength: 623.7 ± 108.8 MPa; CFL: 439.0 ± 56.4 MPa). Based on the results presented, it could be conclude that the synthesis of a nanocomposite of Y-TZP/MWCNT-COOH with optical properties suitable for application in restorative dentistry was made possible by the described methods, however some adjustments in synthesis and processing methods for the nanocomposite creation should be taken; to avoid the sharp decrease of important mechanical properties of the material.

There has been an increase in the interest in fluoride materials over the last decade. This interest has focused on multiferroic materials and kagome lattices, to name but a few areas. This thesis focuses on the synthesis and crystallographic characterisation of selected transition metal fluorides and oxyfluorides. Work is presented on the tetragonal tungsten bronze solid solutions of KₓFeF₃, where x = 0.58 and x ≈ 0.5, and the copper analogue, K₃Cu₃Fe₂F₁₅; the kagome structure of Cs₂ZrCu₃F₁₂; and hydrothermal reactions using vanadium, manganese, or molybdenum as the transition metals in the formation of new fluorides and oxyfluorides. The tetragonal tungsten bronze compounds KₓFeF₃ (x = 0.58 and x ≈ 0.5) are both tetragonal at 500 K. In the variant with the lower K-content, there is a clear phase separation into two tetragonal phases even at this temperature. The K₀.₅₈FeF₃ sample separates into two distinct phases below 340 K to possess one tetragonal and one orthorhombic phase. Then at roughly 300 K, both samples undergo a phase transition where the tetragonal phase in the P4/mbm space group in K₀.₅₈FeF₃ changes to an orthorhombic phase with a larger unit cell; and the tetragonal phase in P4₂bc for the K₀.₅FeF₃ sample changes to the same orthorhombic model, whilst the P4/mbm model remains unchanged. The evolution of the lattice parameters and phase fractions is studied in detail using synchrotron powder X-ray diffraction (sPXRD). The kagome structure investigated, Cs₂ZrCu₃F₁₂, possesses the “ideal” kagome lattice at room temperature, but previous work has suggested that there is a phase transition at 225 K. The two structures are determined by single crystal X-ray diffraction at 300 K and 125 K. Variable temperature sPXRD studies are performed between these two temperature ranges to determine the phase evolution as a function of temperature. The structure changes from a rhombohedral to a monoclinic phase at low temperature. This is the result of the buckling of the kagome layers at the phase transition. The Zr⁴⁺ ion changes from 6 to 7 coordinate and this is seen as the main driving force for the distortion of the kagome layer from its “ideal” planar arrangement. ii The phase transition is first-order as seen from the electrical impedance measurements. The hydrothermal reactions presented reveal seven new materials and their crystal structures. Sr₂V₂F₁₀·H₂O is new and found to be isostructural to Sr₂Fe₂F₁₀·H₂O. BaVO₂F₃ is a cubic material that is potentially piezoelectric. Two hybrid organic inorganic manganese compounds are reported. The ladder structure (C₃N₂H₅)[Mn₂F₆(H₂O)₂] crystallises in a polar space group and shows promise as a candidate for multiferroic studies. The second hybrid material, (C₇NH₁₆)₂[MnF₅(H₂O)]·2H₂O, crystallises in a centrosymmetric space group. The Mo hybrid materials are all centrosymmetric and possess isolated molybdenum-centred monomeric or dimeric octahedral units.

Depuis plus de dix ans les capacités MIM (Métal Isolant Métal) sont des composants passifs largement intégrés au niveau des interconnections des puces de microélectronique. A cause de la miniaturisation et de la réduction de la surface des puces, la densité de capacité des capacités MIM doit être constamment augmentée. Une solution est l'utilisation d'un isolant avec une constante diélectrique élevée dit « high-k ». Pour les prochaines générations de condensateurs, des densités de capacité supérieur à 30 fF/µm² sont visées. L'oxyde de zirconium (ou zircone) a été sélectionné pour remplacer de l'oxyde de tantale actuellement utilisé. Il possède une constante diélectrique qui dépend de sa structure cristalline. Elle est respectivement de 17, 47 et 37 dans les phases monoclinique, quadratique et cubique. Il est donc nécessaire de déposer la zircone dans la phase quadratique. Cependant, les couches minces de zircone ne sont pas entièrement cristallisées dans la phase quadratique. De plus, elles ne répondent pas aux critères de fiabilité requis par la microélectronique. L'objectif de cette thèse est la stabilisation de la zircone dans la phase quadratique par le dopage. Le tantale et le germanium sont les deux dopants choisis grâce à une étude de sélection de matériaux. Des couches minces d'environ 8 nm de zircone dopée à différentes concentrations ont été réalisées par PEALD (Plasma Enhanced Atomic Layer Deposition). Après les dépôts, des recuits à 400°C pendant 30 min ont été effectués afin de reproduire les traitements thermiques subis par les couches lorsqu'elles sont intégrées dans des puces de microélectronique. Plusieurs types de caractérisations ont été effectuées afin d'étudier l'influence des dopants sur la structure cristalline de la zircone mais aussi sur ses propriétés physico-chimiques. Des tests électriques sur des capacités MIM intégrées ont permis de mesurer les propriétés électriques des couches et la fiabilité de la zircone dopée a été évaluée. Ce travail a pour but la fabrication d'une capacité MIM planaire à base d'oxyde de zirconium dopée dont la densité de capacité sera supérieure à 30 fF/µm² pour des applications de découplage. / For more than ten years Metal-Insulator-Metal capacitors (MIM) have been integrated at the level of copper interconnections. All new technology nodes have led to a decrease of the surface of chips; capacitance density must be thus enhanced. The best solution is to use a material with a high dielectric constant commonly named “high-k”. For the next MIM capacitor generation, capacitance density has to be higher than 30 fF/µm². Tantalum oxide, currently used, has reached its limits and it must be replaced. Zirconium dioxide has a high dielectric constant of 47 in the tetragonal phase with a sufficient band gap for MIM applications. When deposited in thin films, zirconia is not fully crystalized in the tetragonal phase. Moreover, this pure zirconium oxide does not fulfill the reliability criteria. The aim of this work is to stabilize zirconia in its tetragonal phase by alloying it with other elements. Tantalum and Germanium are the two dopants selected thanks to a bibliographic study. Thin layers of zirconia of 8 nm alloyed with Tantalum and Germanium have been deposited by Plasma Enhanced Atomic Layer Deposition (PEALD). Samples were annealed at 400°C during 30 minutes after deposition to reproduce the thermal conditions that microelectronic chips are submitted to. Different characterization technics have been used to study the effect of dopants on zirconia's crystalline structure and its physic-chemical properties. Tests have been made on integrated MIM capacitors with Titanium Nitride electrodes to determine the electrical properties of the layers. Reliability of zirconia doped layers was also evaluated. The purpose of this work is the production of zirconia based planar MIM capacitor with a capacitance density of 30 fF/µm².

The diploma thesis deals with influence of fast pressure-less sintering on the microstructure of advanced ceramic materials, namely -Al2O3 and tetragonal ZrO2 (doped by 3 mol% Y2O3) with particle sizes ranging from 60 nm to 270 nm. Fast and controlled heating rate was enabled by utilization of the special superkanthal furnace with moving sample holder. Defect-free bulk and dense samples were prepared using heating rates in order of 100-200 °C/min. Higher densities reached the samples pressed by higher pressures; the specimens with densities higher than 99 % t.d. were prepared within tens of minutes for alumina as well as for zirconia with very low thermal conductivity. Different behavior was observed only for material TZ-3Y, which exhibited core-shell structure with dense surface and porous centre after sintering at heating rates higher than 10 °C/min. It was shown in this work that such behavior was not primarily caused by the high thermal gradient resulting from high heating rates. Its creation was probably caused by chlorine impurities. The mechanism of desintering of these samples was described and eliminated by calcination of the samples at 1000 °C for 10 hours prior to fast sintering at 1500 °C, so even this material could be fast sintered up to 99.9 % theoretical density.

The effect of dental technical tools on the phase composition and roughness of 3/4/5 yttria-stabilized tetragonal zirconia polycrystalline (3y-/4y-/5y-TZP) for application in prosthetic dentistry was investigated. Additionally, the X-ray diffraction methods of Garvie-Nicholson and Rietveld were compared in a dental restoration context. Seven plates from two manufacturers, each fabricated from commercially available zirconia (3/4/5 mol%) for application as dental restorative material, were stressed by different dental technical tools used for grinding and polishing, as well as by chewing simulation and thermocycling. All specimens were examined via laser microscopy (surface roughness) and X-ray diffraction (DIN EN ISO 13356 and the Rietveld method). As a result, the monoclinic phase fraction was halved by grinding for the 3y-TZP and transformed entirely into one of the tetragonal phases by polishing/chewing for all specimens. The tetragonal phase t is preferred for an yttria content of 3 mol% and phase t″ for 5 mol%. Mechanical stress, such as polishing or grinding, does not trigger low-temperature degradation (LTD), but it fosters a phase transformation from monoclinic to tetragonal under certain conditions. This may increase the translucency and deteriorate the mechanical properties to some extent.

Theoretische Berechnungen sagen für tetragonal gedehntes Fe-Co eine hohe magnetokristalline Anisotropie voraus, wie sie für seltenerdfreie Dauermagnetwerkstoffe vorteilhaft wäre. In dieser experimentellen Arbeit werden epitaktische Fe-Co-Schichten strukturell und magnetisch charakterisiert. Zur Untersuchung der Dehnung in diesen Schichten eignen sich AuxCu100-x-Pufferschichten besonders, da über die Stöchiometrie (x) deren lateraler Gitterparameter eingestellt werden kann. Wird Fe-Co auf einer solchen Pufferschicht abgeschieden, erfolgt aufgrund dessen hoher elastischer Energie schon in den ersten Monolagen eine vollständige Relaxation der pufferinduzierten Dehnung. In ternären Fe-Co-X-Schichten, in denen kleine X-Atome (X = B, C oder N) Oktaederlücken besetzen, wird jedoch eine spontane tetragonale Dehnung c/a bis zu 1,05 beobachtet. Entlang der gedehnten c-Achse tritt eine uniaxiale magnetokristalline Anisotropie auf, die für B- oder C-Zulegierungen von 2 at% eine maximale Anisotropiekonstante von 0,4 MJ/m³ zeigt. Wird der X-Gehalt weiter erhöht, nehmen die Kristallinität der Schichten und die magnetische Anisotropie ab. Neben der magnetokristallinen Anisotropie des Schichtvolumens wird an den Fe-Co(-X)-Schichten eine hohe Grenzflächenanisotropie beobachtet. Der Beitrag der freien Oberfläche übersteigt den der Au-Cu-Grenzfläche dabei deutlich. / Theoretical calculations predict a high magnetocrystalline anisotropy for tetragonally strained Fe-Co, which would be beneficial for rare-earth free permanent magnet materials. In this experimental work, epitaxial Fe-Co films are investigated structurally and magnetically. AuxCu100-x buffer layers are very suitable to study the strain in these films since their in-plane lattice parameter can be tailored via the applied stoichiometry (x). However, when Fe-Co is deposited on such a buffer layer, the induced strain of the Fe-Co lattice relaxes completely within the first monolayers, due to its high elastic energy. In ternary Fe-Co-X films, where small atoms X like B, C or N occupy octahedral interstitial sites, a spontaneous strain c/a up to 1.05 is observed. A uniaxial magnetocrystalline anisotropy along the strained c axis appears. Their maximum anisotropy constant is 0.4 MJ/m³ for B or C contents of 2 at%. If the X content is further increased, the crystallinity and thus, the magnetic anisotropy of the films degrade. Together with the magnetocrystalline anisotropy of the films’ volumes, a high interface anisotropy is observed for the Fe-Co(-X) films. The contribution of the free surface clearly exceeds the contribution of the Au-Cu interface.

Les travaux présents ici portent sur l’étude et la prospection de matériaux ferroiques. Cette étude consiste en une approche revêtant plusieurs aspects que sont la chimie du solide, la physique du solide et la science des matériaux. Deux parties sont développées, avec deux approches différentes. La première se concentre sur les liens entre composition/structure/propriétés dans des matériaux de la famille des TTB dérives de Ba2NdFeNb4O15 à travers l’étude de solutions solides à base de Li et l’étude de l’impact des différents paramètres de synthèse sur la nature de l’anomalie diélectrique mesurées sur pastilles densifiées. La mise en évidence de modulation structurale dans cette famille de matériaux semble être en lien direct avec l’observation des variations de propriétés diélectriques. L’utilisation de diffraction électronique notamment permet la mise en évidence de ces modulations structurales et leur évolution en température. Dans une seconde partie, l’approche consiste à utiliser le Spark Plasma Sintering (SPS) comme technique de densification pour des matériaux dits “fragiles” mais aussi d’explorer des propriétés diélectriques jusqu’alors inaccessibles, sur matériaux massifs. Le développement de la technique SPS à basse température permet ainsi non seulement de densifier à basse température des matériaux fragiles, mais aussi d’obtenir des phases inaccessibles dans des conditions de températures similaires par traitement thermique conventionnel. De même, l’obtention de céramique moléculaire de très haute densité a pu être réalisée, malgré des températures de décomposition extrêmement faible (100 ˚C). / The present work focuses on the prospection and understanding study of ferroic materials. It consists in a multiple aspect approach, including materials chemistry, materials physics and materials processing. Two parts compose this work, with two different approaches. The first one focuses on the links between composition, structure and properties in materials belonging to the TTB family, more specifically derived from Ba2NdFeNb4O15, through the exploration of Li containing solid solutions, and the impact of synthesis parameters on measured dielectric anomalies on dense samples. The observation of structural modulation in these materials seems to be closely related to the observation of dielectric anomalies variations. Using electron diffraction techniques allowed the evidence of such anomalies and the following of their thermal evolution. In a second part, the approach consists in using Spark Plasma Sintering (SPS) as a densification technique for so called “fragile” materials but also explore dielectric properties impossible to experimentally measure thus far. Developing low temperature SPS technique not only allows to densify ceramics at low temperatures fragile materials, but also to obtain inaccessible phases in similar temperature conditions using conventional thermal treatments. Moreover, sintering of molecular ceramic at very high density was possible, even if its decomposition temperature is extremely low (100 ˚C).

Les multiferroïques sont des matériaux dans lesquels plusieurs propriétés ferroïques peuvent coexister, e. g. ferromagnétisme et ferroélectricité. La recherche de tels matériaux fait l'objet d'une activité croissante en raison de l’enjeu majeur qu’ils représentent dans de nombreux domaines (mémoires, spintronique…). Les matériaux qui possèdent les propriétés nécessaires pour des applications futures sont cependant peu nombreux. Des niobates de formule Ba2LnFeNb4O15 (Ln = lanthanide), de structure bronze quadratique (TTB) susceptibles de présenter un ordre ferroélectrique et un ordre magnétique ont été synthétisés. Les propriétés magnétiques des céramiques proviennent d'une phase secondaire, faisant d’eux des composites multiferroïques. Leur souplesse cristallochimique permet de contrôler les propriétés composites par substitutions cationiques dans la matrice TTB. Afin de compléter l'étude cristallochimique, la croissance de monocristaux de TTB a été entreprise avec succès. / Multiferroics are materials which possess several ferroic properties, e.g. ferroelectricity, ferromagnetism. The search for multiferroics arises a growing activity, due to their potential applications in memories, spintronic… Yet the materials displaying the adequate properties for future application are very few. Niobates with the formula Ba2LnFeNb4O15, potentially ferroelectric and ferromagnetic, have been synthesized. The magnetic properties of the ceramics are related to a secondary phase, thus making them composite multiferroics. Their crystal-chemical flexibility allows for the composites properties tuning by cationic substitutions into the TTB framework. To complete the crystal-chemical study, the growth of TTB single-crystals was successfully engaged.

Les travaux présentés dans ce manuscrit concernent des niobates de formulation Ba2LnNb4O15 (Ln = La ou Nd) et de structure "Tetragonal Tungsten Bronze" (TTB). Ces travaux se situent à la charnière de la chimie et de la physique des matériaux diélectriques et visent à mettre en relation structure cristalline et propriétés diélectriques. L'étude a été menée sur des TTB élaborés sous forme céramique et monocristalline. Après détermination de conditions optimales de croissance, par fusion de zone en four à image, des sections monocristallines ont été obtenues et caractérisées. Les résultats obtenus suggèrent des liens étroits entre composition, modulations structurales apériodiques et comportement diélectrique. Dans le même temps, deux nouvelles solutions solides céramiques ont été explorées, avec des schémas de substitution différents menant notamment à une réflexion très large sur les liens cristallochimie-ferroélectricité dans cette famille de TTB. Ces solutions solides présentent un crossover relaxeur-ferroélectrique, un comportement original et déjà observé dans d'autres solutions solides issues de cette famille de TTB. Des caractérisations avancées (mesures pyro- et piézoélectriques, cycles de polarisation) et des études structurales résolues en composition et en température ont permis d'établir des diagrammes de phases diélectriques montrant l'existence d'un état ferroélectrique métastable. Finalement, la présence d’une modulation structurale bidimensionnelle a pu être confirmée, dans les monocristaux et dans les céramiques, et l'ensemble des résultats obtenus pointent son implication dans les comportements cristallochimiques originaux rencontrés dans ces TTB. / The present work deals with Ba2LnNb4O15 (Ln = La ou Nd) niobates crystallizing with the "Tetragonal Tungsten Bronze" (TTB) structure. These researches, at the interface of chemistry and physics of dieletrics, aim at establishing structure / dielectric properties relationships. They were performed on TTB materials elaborated in ceramic and single crystal forms. Following optimization of growth parameters with an image furnace, single crystals were obtained and characterized. The results obtained suggest that composition, aperiodically modulated structure and dielectric behavior are closely tied in TTBs. Meanwhile, two new ceramic solid solutions with different substitutions schemes were investigated, deepening the insight on crystal-chemistry and ferroelectricity of TTBs. These solid solutions exhibit a relaxor-ferroelectric crossover, an original behaviour previously observed in solid solutions derived from the same family of TTB niobates. Advanced physical characterization (ferro-, pyro- and piezoelectric measurements, polarization loops), and composition/temperature resolved structural studies, allowed for the determination of a dielectric phase diagrams showing the presence of a metastable ferroelectric state. Finally, the existence of a two-dimensional structural modulation in single crystals and ceramics has been confirmed, and the whole set of experimental results points towards its implication in the original dielectric behavior observed in these TTBs.