Academic literature on the topic 'Diffuse Ferroelectric Transition'

La plupart des matériaux ferroélectriques actuellement utilisés pour les applications industrielles sont des céramiques à base de plomb. De tels composés sont nocifs à l’environnement en raison de la toxicité et de la volatilité de l’oxyde de plomb durant le processus de préparation de ces matériaux. Dans le cadre du respect de l'environnement, la recherche de céramiques sans plomb ou de faible teneur en plomb ayant des propriétés équivalentes est devenue l'une des principales alternatives aux céramiques à base de plomb. C’est dans cette perspective que se situe le présent travail, focalisé sur l’investigation de matériaux de formules : Ba1-xPbx(Ti1-yZry)O3 (2.5% ≤ à x ≤ 10%) et Ba1-xEr2x/3Ti1-yZryO3. La synthèse par voie solide a été utilisée pour l’élaboration des matériaux. Les techniques expérimentales utilisées sont l’analyse thermogravimétrique, la dilatomètrie, la diffraction des rayons X, la microscopie électronique à balayage couplée à un système de microanalyse EDX et la spectrométrie d’impédance. L’utilisation de ces diverses techniques nous a permis d’optimiser les conditions d’élaboration et d’obtenir des céramiques denses de densité supérieures à 90%. Par ailleurs, les paramètres structuraux ont été affinés par la méthode de Rietveld puis discutés. En outre, ces céramiques possèdent des propriétés ferroélectriques classiques et/ou relaxeurs dont certaines ont des caractéristiques diélectriques très intéressantes au voisinage de la température ambiante. De telles compositions dépourvues de plomb ou contenant très peu de plomb permettent de limiter considérablement la pollution de l’environnement. Grâce à leurs bonnes performances diélectriques, ils pourraient être des candidats potentiels pour remplacer les matériaux à base de plomb actuellement usagés dans divers appareillages électroniques<br>Most of ferroelectric materials currently used in the industrial applications are lead-based ceramics. Such compounds are harmful to the environment due to the toxicity and volatility of lead oxide during the preparation process of these materials. Therefore, environmental friendly lead-free ceramics or low-lead content materials with equivalent properties has become one of the main topics as an alternative to lead-based ceramics. In this view, the present work focuses on investigating materials with formulas :Ba1-xPbx(Ti1-yZry)O3 (2.5% ≤ x ≤ 10%) and Ba1-xEr2x/3(Ti1-yZry)O3.Solid state route was used for materials elaboration. The experimental techniques used are thermogravimetric analysis, dilatometry, X-ray diffraction, scanning electron microscopy coupled to an EDX microanalysis system and impedance spectrometry. The use of these various processes allowed us to optimize the synthesis conditions and to obtain dense ceramics with density superior to 90%. In addition, the structural parameters were refined by the Rietveld method and then discussed. Furthermore, these ceramics exhibited normal and/or relaxor ferroelectric properties then some of them have very interesting dielectric characteristics in the vicinity of room temperature. Such lead-free ceramics or low-lead content materials reduce significantly the environmental pollution. Due to their good dielectric performances, they could be potential candidates for replacing lead-based materials currently used in various electronic equipments

he short-range order in triglycine sulphate (TGS) was investigated using x-ray and neutron di raction techniques. Complete deuteration of TGS was required for the neutron di raction experiments and a new method was developed to grow single crystals of fully deuterated TGS by vapour di usion crystallisation. The long-range structure of fully deuterated TGS was re ned at several temperatures from single crystal neutron di raction data and found to be consistent with the published structure of hydrogenous TGS. The phase transition temperature was found to increase from about 322 K to about 334 K with complete deuteration. The evolution of the long-range structure with temperature was investigated using x-ray and neutron powder di raction. All of the lattice parameters had a single cusp at the phase transition, except for the b lattice parameter, which also had a second cusp about 34 K below TC. In contrast to the lattice parameter behaviour, the unit cell volume was found to increase monotonically with temperature. The length of the hydrogen bonds between the disordered N atom on glycine 1 (G1) and the surrounding molecules was found to increase with temperature, whereas the length of the short hydrogen bond between G2 and G3 decreased slightly with temperature. This supports the suggestion that weakening of the hydrogen bonds decouples G1 from G2 and G3, allowing the system to become disordered. Except around the ferroelectric to paraelectric phase transition temperature, no abnormalities in the behaviour of any of the re ned parameters were observed, suggesting that TGS only has a single phase transition. The short-range order in TGS was investigated by collecting single crystal x-ray and neutron di use scattering at several temperatures from well below to well above TC. Well below TC, the di use scattering was purely thermal di use scattering due to correlations of the atomic displacements. Close to the phase vii viii transition, di use streaks perpendicular to b were also present in the di use scattering patterns, which were due to short-range order of the G1 orientations parallel to the ferroelectric b axis. The onset of signi cant short-range order appears to occur about 40 K below TC. The correlations are strongest at the phase transition and then decrease with temperature above TC. The short-range order was modelled using a combination of a displacive disorder and an orientational disorder Monte Carlo simulation. The intermolecular interactions that give rise to correlated atomic displacements were modelled by treating them like Hooke's law springs. The force constants for the interactions were parameterised in a number of ways, the most successful of which was an empirical formula developed by Chan et al. The short-range order of the G1 orientations was modelled using an Ising-type model. The G1 interactions that lead to short-range order along the ferroelectric b axis appear to be mediated by the short hydrogen bond between G2 and G3. This suggests that the hydrogen bonding, rather than the dipole-dipole interactions, plays the dominant role in the ferroelectric ordering of TGS. While the hydrogen bonding gives rise to strongly correlated chains of G1 molecules along b, it is likely that there are also weaker correlations between the chains due to dipole-dipole interactions. This provides a mechanism for TGS to go from short-range ordered in 1-dimension, to long-range ordered in 3-dimensions as it is cooled through TC.

博士<br>國立成功大學<br>電機工程學系碩博士班<br>95<br>In this thesis, the low-field dielectric behaviors of the Pb(Fe2/3Ti1/3)O3-PbTiO3 (PFW-PT) based ceramic system are investigated by using the diffuse phase transition (DPT) laws, the spin-glass model, the Curie-Weiss law and the modified-Landau theory. The effects of the PbTiO3 composition, the sintering temperature, the MnO dopant, and the synthesized method are discussed on the dielectric properties, the grain structure and the lattice structure. In the first phase, we analyze the diffuse phase transition models in the (1-x)Pb(Fe2/3W1/3)-xPbTiO3 relaxor system by using the statistical regression theory. We find that the laws of both Burfoot and Eiras show the same adaptability and provide smaller estimation bias on the samples than the models of Smolensky and Isupov despite the complete or incomplete DPT characteristics. Regarding the physical and mathematical meanings of parameters in their laws, the law of Eiras is better to explain the behaviour of both the more complete relaxor system and the incomplete relaxor system. The effects of the sintering temperature are investigated on the 0.7PbFe2/3W1/3O3-0.3PbTiO3 relaxor ferroelectrics sintered. We find that the lattice structure is changed from cubic to tetragonal, the diffuse phase transition characteristics and the spin glass behavior are deviated as the sintering temperature increases. In conclusion, the normal ferroelectric characteristics are more obvious as the sintering temperature increases in this system. The low-field dielectric responses of pure and 0.15wt% MnO additive (1-x)Pb(Fe2/3W1/3)O3-xPbTiO3 were investigated by using the changes of the lattice structure and the space charge polarization, the diffuse phase transition model, the Curie-Weiss law, the modified-Landau theory and the spin-glass model. Furthermore, we suggest that the lead vacancy is substituted or the lattice interstice is intervened by manganese ions, and the ordering degree of B-site cations is enhanced by PbTiO3 compositions are possible mechanism. The 0.75Pb(Fe2/3W1/3)O3-0.25PbTiO3-0.15wt%MnO ceramics are synthesized by two different methods. The low field dielectric properties are changed with the lattice structure, the grain structure, the space charge polarization, the diffuse phase transition characteristics, the frequency dispersion, the dielectric loss, the Curie-Weiss behavior and the local order parameters. Furthermore, we suggest that the ordering degree of B-site cations is enhanced by using the indirect synthesized (IS) method.