Добірка наукової літератури з теми "CERAMICS DOPED"

MnCO3, CuO, NiO and Fe2O3 doped SrTiO3 multifunction ceramics were fabricated firstly. The microstructure and electrical properties were investigated. The results show that MnCO3 doped ceramic and CuO doped ceramic possess relative higher resistivity (ρ),nonlinear coefficient (α), varistor voltage (V1mA) and relative lower dielectric constant (ε),dielectric loss (tgδ) in contrast with NiO doped ceramic and Fe2O3 doped ceramic with the same contents under the same sintering conditions. The electrical properties among the ceramics doped are different due to the different behaviors of Mn,Cu,Ni and Fe acceptor dopants during the sintering course.

Abstract To improve the properties of CuAlO2 thin films, the preparation of CuAlO2 ceramics with excellent properties is an important prerequisite, and the optimization of ceramic properties is often achieved by doping. Zn-doped CuAlO2 ceramics were prepared by the solid phase sintering method. The structural, electrical and magnetic properties of Zn-doped CuAlO2 ceramics were systematically studied. The results show that the crystal structure of the as-sintered Zn-doped CuAlO2 was rhombohedral, space group R3m. Grain size of samples grew with the amount of Zn increases, which is related to the enhancement of CuAl2O4 phases in the sample ceramics. The density of doped ceramics changed when increasing Zn-doping concentration compared to undoped ceramics. It reached a minimum 0.96 g/cm3. The resistivity of Zn-doped CuAlO2 ceramic samples climbed with the increase of Zn content, and it can reach 59.6 Ω•cm at 10 mol%. The ferromagnetic behavior was enhanced with increasing the Zn concentration.

CeO2 and Y2O3 were co-doped to (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics and sintered by conventional solid-state reaction process to form x wt.% CeO2-y wt.% Y2O3 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (CexYy-BCZT) ceramics. The effects of different contents of CeO2-Y2O3 dopants to the (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 composition were analyzed by studying the phase, surface microstructure, piezoelectric and ferroelectric properties of BCZT ceramics. In this study, we have shown that co-doping a small amount of CeO2 and Y2O3 will not change the phase structure of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics. However, the proper introduction of CeO2 and Y2O3 can improve the piezoelectric constant and electromechanical coupling coefficient of BCZT ceramic samples. Moreover, these dopants can promote the grain growth process in (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics. C0.04Y0.02 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic has the best piezoelectric properties compared with other composition, the results are as follows: Relative density = 96.9%, Kp = 0.583, and d33 = 678 pC/N, V = 8.9 V. It means that this Ce0.04Y0.02 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic is a desired material in the application of lead-free ceramics.

Bi2(Mgl/3Nb2/3-xTix)2O7(BMNT)(x=0.1, 0.15, 0.2, 0.25) ceramics was prepared by conventional solid phase method, The influence of titanium ion doped amount on the dielectric property and microstructure of BMNT ceramics were systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and other testing and analysis how to methods. Results show that there is single monoclinic pyrochlore phase and there is not the second phase in all BMNT ceramics doped with titanium ion. The grain size of BMNT ceramics increases first and then decreases, the dielectric constant (εr) increases first and then decreases, the dielectric loss (tanδ) increases first and then decreases, the ceramic density decreases all the time while titanium ion doped amount increases. The ceramic density increases first and then decreases when sintered temperature increases. The density of BMNT ceramics doped with titanium ion is biggest when sintered temperature is 980 °C. When sintered temperature is 980 °C and titanium ion doped amount is 0.15mol, the dielectric properties of BMNT ceramics is good,which εris 135(1MHz), tanδ is 0.002(1MHz) and volume density is 7.46g/cm3.

Pure, La3+doped at A site, V5+doped at B site, and La3+and V5+co-doped ferroelectric Bi4Ti3O12(BTO), Bi3.25La0.75Ti3O12(BLT), Bi4Ti2.98V0.02O3(BTV) and Bi3.25La0.75Ti2.98V0.02O12(BLTV) were successfully prepared by conventional sintering technique. The structures of the ceramics were investigated by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. X-ray diffraction indicated that assemblages of all sintered ceramics consist of a single phase of Bi4Ti3O12, implying that the A-site La3+and B-site V5+substitutions in this case do not affect the layered structure. Among these ceramics, BLTV ceramic exhibited the best electrical properties. The leakage current density of BLTV ceramic was only 1.3×10-4Acm-2at 40 KVcm-1, two orders of magnitude lower than BTO ceramic. Besides, a saturated ferroelectric hysteresis loops with largest remnant polarization 2Pr of 30.6μC/cm2was observed for this sample. These suggested that the co-doped Bi4Ti3O12ceramic by La3+and V5+at A and B sites showed advantages in application over the pure BTO, doped BLT and BTV ceramic, respectively.

CuO and MnCO3 doped SrTiO3-based multifunction ceramics were prepared respectively with solid state reaction method under vacuum condition. The electrical properties’ change with the oxidation temperature and microstructure were investigated. The results show that acceptor dopants behaves differently with the same sintering conditions. CuO-doped ceramic possess relative higher dielectric loss, dielectric constant and relative lower nonlinear coefficient, varistor voltage in contrast with MnCO3 doped ceramic with the same contents under the same sintering conditions. The grain size of CuO-doped sample is significantly larger than the sample doped with MnCO3. The grain distributions of CuO-doped sample is more homogeneous. The electrical properties of the ceramics are different due to the different behaviors of CuO and MnCO3 accepter dopants during the sintering process.

Using a pulse electric current sintering (PECS) method, β–SiC and α–SiC powders doped with a few weight percent of Al2O3–Y2O3oxide or Al4C3–B4C–C nonoxide additives were rapidly densified to high densities (95.2–99.7%) within less than 30 min of total processing time. When Al2O3–Y2O3additive was used, both ceramics resulting from β–SiC and α–SiC had fine, equiaxed microstructures. In contrast, when Al4C3–B4C–C additive was used, the ceramic resulting from α–SiC had a coarse, equiaxed microstructure, whereas the ceramic resulting from β–SiC was composed of large elongated grains whose formation was accompanied by the β →?α phase transformation of SiC. Compared with the Al2O3–Y2O3-doped SiC ceramics, the Al4C3–B4C–C-doped SiC ceramics had higher densities, lower fracture toughness, and higher hardness. The fracture mode of the oxide-doped SiC was mainly intergranular, whereas the nonoxide-doped SiC exhibited almost complete intragranular fracture that was attributed to the higher interfacial bonding strength.

Broadband near-infrared (IR) luminescence in transparent alkali gallium silicate glass-ceramics containing Ni2+-doped β-Ga2O3 nanocrystals was observed. This broadband emission could be attributed to the 3T2g (3F) → 3A2g (3F) transition of octahedral Ni2+ ions in glass-ceramics. The full width at half-maximum (FWHM) of the near-IR luminescence and fluorescent lifetime of the glass-ceramic doped with 0.10 mol% NiO were 260 nm and ∼1220 μs, respectively. It is expected that transparent Ni2+-doped β-Ga2O3 glass-ceramics with this broad near-IR emission and long fluorescent lifetime have potential applications as super-broadband optical amplification media.

La0.67LixTi1-xAlxO3 ceramics (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) were synthesized with the use of Al2O3 and an aqueous solution of Al(NO3)3 as an aluminium source. In both cases, the ceramics preserve a high dielectric constant ε ~ 105. It was found that the single-phase La0.67LixTi1-xAlxO3 perovskite structure is formed at temperatures above 1200 C. It was shown that the use of Al(NO3)3 allows simplifying the synthesis: reduction in the sintering temperature by 20 C, Li loss and, as a result, an increase in the density of ceramics. Ceramics La0.67Li0.15Ti0.85Al0.15O3 with a maximum density higher than 85 % were sintered at about 1280 and 1300 C of with the use of Al(NO3)3 and Al2O3 respectively. Frequency spectra of ima­ginary parts of impedance and electrical modulus demonstrate two dispersion regions that refer to processes in the ceramic grains’ boundaries and ceramic grains. Ceramic samples synthesized using Al(NO3)3 solutions tend to exhibit higher dielectric constants than those synthesized using Al2O3. At a frequency of 100 Hz, the dielectric constant for ceramics synthesized using Al(NO3)3 aqueous solution is 70600, whereas that for ceramics synthesized using Al2O3 is 44300. Obtained materials are useful for microelectronics, energy storage and harvesting devices.

Ba (Zr0.2Ti0.8) O3-50% (Ba0.7Ca0.3) TiO3 (BZT-0.5BCT) ceramics with different doping contents of Pr3+ were prepared by the conventional solid-state reaction. The phase structure and crystallinity of the fabricated ceramics were investigated by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Photoluminescence (PL) emission spectra were measured to analyze the PL characteristics. The strong intensities of a green band at 489 nm and a red band at 610 nm were observed. The maximum emission intensity of the PL spectrum was achieved in the BZT-0.5BCT ceramic with 0.2% mol of Pr3+ ions. Furthermore, the PL spectra of BZT-0.5BCT ceramics were found to be sensitive to polarization of the ferroelectric ceramics. Compared with the unpoled ceramics, the green emission increased about 42% and a new emission peak at 430 nm appeared for the poled ceramics. With excellent intrinsic ferroelectricity and an enhanced PL property, such material has potential to realize multifunctionality in a wide application range.

Tetragonal polycrystalline zirconia, commonly stabilised with 3 mol% yttria (3Y-TZP), became one of the most interesting ceramics for biomedical applications due to its biocompatibility and high mechanical properties. Among them, its high fracture toughness should be highlighted, which is due to the stress-induced tetragonal-to-monoclinic (t-m) phase transformation near a crack tip. However, the tetragonal grains on the Surface can also spontaneously transform to monoclinic phase in a humid environment, phenomenon known as low-temperature degradation (LTD), which is an important issue for applications in which water is present. Several methods have been proposed to increase the LTD resistance in zirconia, which range from improving the fabrication process in terms of grain size, density or residual stresses, to doping zirconia with other oxides, like magnesia or ceria. Particularly, ceria-stabilised zirconia (Ce-TZP) possesses higher LTD resistance and fracture toughness than 3Y-TZP, but lower fracture strength and hardness, partly because of its larger grain size. The approach proposed in this work consists in the improvement of mechanical properties of Ce-TZP by reducing grain size, without reducing LTD resistance. With this objective, starting from two compositions of Ce-TZP (10 and 12 mol% CeO2), different amounts of CaO and Y2O3 have been added to reduce the grain growth during sintering, and thus increasing the critical t-m transformation stress, and consequently flexural resistance and hardness. On the other hand, as flexural resistance is determined by fracture toughness from small superficial cracks, a novel reproducible methodology to produce superficial micro-notches by means of ultra-short pulsed laser ablation has been developed to measure this property. It has been highlighted that with the developed methodology very sharp cracks are produced, with the size in the order of natural cracks. Results show that small-crack fracture toughness is very different from values measured from methods using large cracks or from the indentation method. This thesis is submitted for the degree of Doctor of Philosophy in the program "Materials Science and Engineering" at the Universitat Politècnica de Catalunya as a compendium of published articles. The research described in this work was carried out by the author during the period from December 2012 to October 2017 under the supervision of Prof. M. Anglada in the Department of Materials Science and Metallurgical Engineering at the Universitat Politècnica de Catalunya, and during 3 research stays during 2015, 2016 and 2017 (406 days in total) in the Department of Materials Science (MTM) at KU Leuven (Belgium) under the supervision of Prof. J. Vleugels. The work described in this dissertation is original, unless otherwise detailed references are provided.
L'òxid de zirconi estabilitzat amb un 3 mol% d'itria és una ceràmica policristal·lina amb estructura tetragonal (denominada freqüentment zircònia, o bé 3Y-TZP) presenta propietats molt interessants per a aplicacions biomèdiques degut a la seva biocompatibilidad i altes propietats mecàniques. Entre elles, destaca la seva alta tenacitat de fractura, la qual és deguda a la transformació de fase tetragonal a monoclínica (t-m) induïda sota tensió al voltant de la punta d'una esquerda. No obstant, els grans tetragonals superficials també poden transformar-se de forma espontània a fase monoclínica en ambients humits, fenomen conegut com a degradació a baixa temperatura (LTD, per les seves sigles en anglès), la qual és un problema important en aplicacions on l'aigua hi és present. Diversos mètodes s'han proposat per tal d'augmentar la resistència a la LTD de la zircònia, des de la millora dels processos de fabricació en termes de tamany de gra, densitat o tensions residuals, fins al dopatge de la zircònia amb altres òxids, com magnèsia o cèria. Particularment, la zircònia estabilitzada amb cèria (Ce-TZP) posseeix una alta resistència a la LTD i major tenacitat de fractura que la 3Y-TZP, però menor resistència a fractura i duresa, en part degut a la seva major grandària de gra. L'estratègia seguida en aquest treball consisteix en la millora de les propietats mecàniques de la Ce-TZP mitjançant la reducció de la grandària de gra, sense disminuir la resistència a la LTD. Amb aquest objectiu, partint de dues composicions de Ce-TZP (10 i 12 mol% de CeO2), s'han afegit diferents quantitats de CeO i de Y2O3 per a reduir el creixement de gra durant la sinterització i així augmentar la tensió crítica per a la transformació t-m, i per tant la resistència a flexió i duresa. Per altra banda, donat que la resistència a flexió ve determinada per la tenacitat de fractura d'esquerdes petites superficials, s'ha desenvolupat una metodologia reproduïble per a produir micro-entalles superficials mitjançant l'ablació làser de puls ultra-curt per així mesurar aquesta propietat. S'ha posat en relleu que amb la metodologia desenvolupada es produeixen esquerdes molt afilades i de dimensions de l'ordre de les esquerdes naturals presents. Els resultats posen de manifest que la tenacitat de fractura per esquerdes petites són molt diferents de les obtingudes per mètodes amb esquerdes grans o a partir del mètode d'indentació. Aquesta tesis és dipositada pel grau de doctor en el programa "Ciència i Enginyeria dels Materials" de la Universitat Politècnica de Catalunya com a compendi de publicacions. La investigació descrita en aquest treball es va desenvolupar per l'autor durant el període de desembre de 2012 a octubre de 2017 sota la supervisió del professor M. Anglada al Departament de Ciència dels Materials i Enginyeria Metal·lúrgica de la Universitat Politècnica de Catalunya, i durant tres estades d'investigació durant 2015, 2016 i 2017 (406 dies en total) al Departament de Ciència de Materials (MTM) de KU Leuven (Bèlgica), sota la supervisió del professor J. Vleugels. El treball descrit en aquesta dissertació és original, llevat quan es proporcionen referències detallades.

The objectives of this research were to investigate the possibility of controlling the '/' phase ratio and morphology in Sialon ceramics. These objectives have been sought by the control of the starting composition, and by post sintering heat treatment. The main emphasis has been on the production of a series of ' and (+') Sialon ceramics with a minimum amount of the glass phase by the pressureless sintering technique and using ytterbium (Yb) as an ' stabilising element. The Yb additions were made via the oxide or the alumino-silicate presynthesised glass; the latter was found to improve the density. The XRD analysis of the as sintered materials revealed ' to be the dominant phase with minor contributions from ' sialon and/or or 12H AIN polytype. Additions of SiO2 or -Si3N4 were made to various materials to assess potential mechanisms for obtaining control over the microstructural development of '/' sialon materials. The addition of silica (SiO2) to sialon with high ' content in Yb system significantly improves the densification and increased the amount of ' phase. The incorporation of -Si3N4 as a seeding agent had a very small effect on the '/' phase ratio and the phase morphology. Further experiments were aimed at optimizing sinterability and sialon microstructure through the introduction of two ' stabilizing cations. Compositions were prepared that contained a combination of light and heavy rare-earth (Yb-Nd and Gd-Nd), and then pressureless sintered and compared with the single cation materials. Materials in the as sintered state were composed of a high ' sialon content with a minor amount of ' sialon and 12H AIN polytype indicating that the heavy rare-earth (which is the principal ' stabilizer) has a dominant effect although EDAX analysis confirmed the presence of both cations (light and heavy) within the ' structure. The research also compared, and developed an understanding of, the thermal stability of '-sialon using single Yb or mixed cations. The Yb single cation '/' materials exhibited excellent stability over a range of temperature (1200 - 1600°C) and for different periods of time up to 168 hrs. The heat treatments result in the crystallisation of the residual phase as a Yb garnet phase which formed at ˜1300°C. The mixed cation '/' materials showed some '-' transformation. The transformation was accompanied by dissolution of RA1O3 (normally crystallized with R=Nd,Gd) and crystallization of melilite.

The influence of the Na/Bi ratio on the electrical properties of nonstoichiometric NBT has been investigated in the Na0.5-xBi0.5+xTiO3+x series. The solid solution limit is very small -0.02 < x < 0.02, but the bulk conductivity has a dramatic change of ~ 4 orders of magnitude at 600 ℃, where it switches from an insulator to an ionic conductor for Na/Bi ≥ 1. Within the solid solution limit, the maximum bulk conductivity is achieved by replacing 1 at% Bi with Na atoms on the A-site (acceptor doping), which generates oxygen vacancies. Furthermore, the limit of oxygen vacancies is identified to be ~ 0.33 at%, which is illustrated by the geometric-independent parameter fmax. In general, ceramics prepared by hand grinding have better control of the nominal starting composition. The bulk conductivity of NBT can also be enhanced by Ga-acceptor doping on the B-site. By decreasing the Na/Bi ratio (donor doping) which compensates for oxygen vacancies, it switches from being an ionic conductor to an insulator. In contrast, by increasing the Na/Bi ratio (acceptor doping) which generates oxygen vacancies, the bulk conductivity remains the same, which is independent of the nominal oxygen content. In fact, the actual composition changes with increasing nominal oxygen vacancies, in order to maintain the actual oxygen content at a certain level. This self-recovery behaviour restricts the enhancement of ionic conductivity in NBT-based ceramics. The influence of B-site acceptor dopants and highly polarised Bi ions on the electrical properties has been investigated in the solid solution of NBT with Bi-based perovskite end members, such as BiGaO3 (BG), BiFeO3 (BF) and Bi(Ni0.5Ti0.5)O3 (BNiT). In general, the ionic conductivity is suppressed with increasing doping level due to the mobile oxygen ions being trapped by the accepter dopants on the B-site. The electronic conductivity is enhanced by transition metal ions, such as Fe3+ and Ni2+. For the BG-NBT series, the solid solution limit is ~ 6%, where the ionic conductivity is eliminated. The actual composition may be influenced by the presence of secondary phases, leading to a slight mismatch of the charge configuration between the A and B-site results in a dramatic change in conductivity. The rhombohedral phase is stabilised by BF-doping, where Tm associated with the permittivity increases from 320 to 445 ℃ when the BF content increases from 0 to 50%. The ionic conductivity is slowly supressed by the BF-doping, where p-type electronic conductivity becomes dominant for samples prepared in air when the BF content is 60%; however, ionic conduction may be dominant when annealed under N2 at 600 ℃. Furthermore, 1 at% Nb-doping for Ti can efficiently suppress the ionic conductivity. For the BNiT-NBT series, the solid solution limit is ~ 60%. Neutron diffraction studies confirm the high temperature tetragonal phase is stabilised at room temperature when the BNiT content is ≥ 40%. The coexistence of rhombohedral and tetragonal phases leads to permittivity peak broadening and a decrease of Tm to a lowest value of 186 ℃ during cooling when the BNiT content is 60%. p-type electronic conductivity becomes dominant when the BNiT content is ≥ 30% for samples processed and measured in air.

Rutile, TiO2, has drawn significant attention due to its unique properties. In this project, the structural and electrical properties of undoped and Cr/Al/Ga/Zn-doped rutile have been investigated. Undoped rutile is increasingly oxygen-deficient on heating in air above ~700 °C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The lattice parameters of quenched samples show an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increase with a small oxygen loss, δ; chemical expansion of the lattice occurs and is attributed to increase in average Ti-O bond lengths. At higher δ, lattice parameters start to decrease, giving a chemical contraction effect attributed to partial collapse of columns of edge-sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear, CS, plane formation. Oxygen-deficient rutile samples quenched from above 700 °C are n-type, whereas samples annealed and measured at 450-500 °C are p-type and believed to be slightly oxygen-rich. Holes are located on oxide ions at or near the sample surface and arise from redox electron transfer between under-bonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600 °C show cross-over between n- and p-type behavior. Cr-doped rutile, Ti1-xCrxO2-x/2-δ, samples show a volume expansion at low Cr content, which is associated with the larger size of Cr3+ and the increase in average M-O bond length. At relatively higher Cr content, samples show an unusual volume contraction effect, which is also believed to be attributed to the formation of shear planes. The electrical properties of Cr-doped rutile are very dependent on composition x. Impedance results show that at low dopant concentration (0.001 ≤ x ≤ 0.01), samples are two-phase mixtures. The phase identified by R2C2 is more resistive than R1C1 and has larger activation energy. R2C2 represents a phase with very small Cr3+ dopant concentration, i.e. x = 0.001 ± 0.0005, and R1C1 represents a phase with relatively higher dopant content, i.e. x = 0.008 ± 0.002. In addition, since Cr content of both phases are different, R2C2 may have random point defects, such as oxygen vacancies. With relatively higher dopant content, oxygen vacancies collapse to form shear planes, i.e. defects present in the phase R1C1 may be random shear planes. At low x, Cr-doped rutile samples show mixed conduction, with p-type electronic conduction and oxide ion conduction. The location of holes is thought to be related with O2- ions as well. At higher x, a p-n transition is observed; samples are semiconducting and have small activation energy (~0.2 eV). Such semiconductivity is attributed to intrinsic hopping of charge carriers associated with Cr-Cr or Cr-Ti. Since orbital overlap may occur, charge carriers could hop between Cr3+ and Ti4+ and thus, Cr4+ and Ti3+ are generated and samples become semiconducting. Moreover, if oxygen vacancies collapse to form shear planes, the distance between Cr-Cr or Cr-Ti in face-sharing octahedra in the shear planes are smaller, and hopping of charge carriers becomes easier and the conductivity increases. Moreover, polaron hopping is an alternative description of the amount of semiconductivity. Electrical properties of Al-doped TiO2 are dependent on cooling condition. Samples show a p-n transition with increasing quench temperature. The p-type conductivity is driven by surface absorption of oxygen and associated with O- at the sample surface. Ga-doped TiO2 shows an n-p transition with increasing dopant concentration, whereas such electronic conduction is quite weak. Maybe with Ga3+ doping in TiO2, ionic compensation is the main compensation mechanism. With Zn2+ doping, it is clear that the homogeneity of rutile ceramics improves in some degree. However, Zn-doped samples still show evidence of a constriction resistance or a dipole orientation-related impedance. Zn-doped rutile samples show n-type conduction.

Mestrado em Ciência e Engenharia de Materiais
Cerâmicas ferroeléctricas do tipo relaxador têm despertado um renovado interesse científico, devido às suas muitas aplicações em tecnologia: memórias dinâmicas de acesso aleatório, condensadores “bypass”, dispositivos com aplicações na gama das microondas, sensores piroeléctricos de baixa temperatura, etc. Estas aplicações estão associadas a problemas conceptuais, relacionados com a distribuição de agregados polares e suas interacções, e também com o efeito mediador das matrizes em que os agregados estão inseridos. Este trabalho tem como objectivo principal o estudo das propriedades polares de cerâmicas em matrizes altamente polarizáveis. Entre os materiais fortemente polarizáveis, os paraeléctricos quânticos são os que mais se destacam. São muito sensíveis a defeitos, impurezas, tensões e campos eléctricos. Um dos mais interessantes é o paraeléctrico quântico Titanato de Estrôncio (SrTiO3), que apresenta uma transição de fase paraantiferrodistorsiva a ~ 110 K e que para concentrações adequadas de dopantes podem ser induzidos estados ferroeléctricos ou do tipo relaxador, geralmente localizados a baixas temperaturas. Estes estados podem corresponder à activação de novas bandas Raman de primeira ordem, a valores elevados de constante dieléctrica e a diferentes processos de relaxação polares. Assim, este trabalho está focado no estudo comparativo da rede, das propriedades dieléctricas e de relaxação polar em cerâmicas de SrTiO3 dopadas com Ítrio, Lantânio e Manganês, designados pelos sistemas Sr1- 1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.13), Sr1-1.5xYxTiO3 (x=0.005 and 0.01), Sr1- xMnxTiO3 (x=0.01, 0.02 and 0.05) and SrTi1-xMnyO3 (y=0.01 and 0.05). Estes sistemas foram sintetizados pelo método convencional. A sua microestrutura e a estrutura cristalográfica foram analisadas através de difracção de raios X, microscopia electrónica de varrimento e transmissão, assim como por espectroscopia de energia dispersiva. Estudos detalhados de dinâmica de rede e de processos polares foram realizados utilizando as técnicas de espectroscopia Raman e de Correntes Termicamente Estimuladas (CTE), num amplo intervalo de temperaturas variando entre 10 K e a temperatura ambiente. Os resultados experimentais foram analisados através de modelos teóricos e discutidos com base na estequiometria química, características dos iões e a sua localização na rede. Pretende-se, a partir do conjunto de resultados obtidos, compreender melhor as propriedades do Titanato de Estrôncio. Embora, a dinâmica da rede do Sr1- 1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.0833) ter sido já estudada, o efeito da dopagem de Ítrio é ainda desconhecido. No presente estudo, propomos analisar o espectro Raman obtido em ambos os sistemas e diferentes concentrações, com o objectivo de clarificar o comportamento da rede do SrTiO3 aquando a dopagem por iões aliovalentes. Os resultados experimentais indicam que a substituição do ião de Estrôncio por iões de Lantânio e Ítrio, dá origem a novas bandas nos espectros Raman, devido à perda do centro de inversão originado pelo efeito de dopagem do Titanato de Estrôncio. Este facto permite o acesso ao comportamento das bandas activas em infravermelho, sendo assim possível obter informações relativas à existência de estados polares a baixas temperaturas. O amortecimento parcial do modo polar TO1 obtido em todos os sistemas estudados aponta para a ausência de uma fase ferroelétrica a baixas temperaturas, confirmando resultados publicados anteriormente. O desvio da temperatura de transição para-antiferrodistorsiva para temperaturas mais elevadas nos sistemas dopados com Ítrio e Lantânio mostra que Ta é fortemente dependente do factor de tolerância da estrutura. Apesar das mudanças observadas em Ta nos sistemas dopado de SrTiO3, o comportamento em temperatura do modo mole associado à transição de fase antiferrodistorsiva A1g, pode ser descrito em termos de uma lei de potência semelhante à lei utilizada em trabalhos previamente realizados no sistema de SrTiO3 não dopado. Observam-se ainda alguns modos de baixa frequência, que por não poderem ser identificados com nenhum modo da rede, podem efectivamente estar associados a efeitos de desordem originados pela presença dos iões dopantes nestes sistemas. Vários trabalhos focaram-se já no estudo do SrTiO3 dopado, contudo os processos de relaxação polar observados em alguns destes sistemas não foram ainda completamente analisados. Assim, os processos de relaxação polar observados nas cerâmicas de SrTiO3 dopadas com Lantânio,Ítrio e Manganês foram estudados a partir de medidas de Currentes Termicamente Estimuladas (CTE) na gama de temperatura 10-300 K. Os resultados experimentais obtidos foram analisados através de modelos de relaxação dipolar e cargas espaciais, com o objectivo de determinar a natureza dos processos de relaxação. Estes resultados revelam a existência de diferentes processos de relaxação localizados na gama de temperaturas estudada. Com base em resultados previamente publicados, os mecanismos de relaxação detectados por CTE nos sistemas estudados e que ocorrem a baixas temperaturas foram associados a mecanismos de origem dipolar, enquanto que aqueles que se observam a altas temperaturas foram atribuídos à deslocação de cargas espaciais. Os resultados obtidos nos sistemas em que o ião dopante substitui o catião no lugar A ou B da rede apontam para a existência de mecanismos de relaxação polares muito distintos. Considerando apenas os processos de tipo dipolar observados a baixas temperaturas, foi possível identificar os correspondentes mecanismos de relaxação e concluir que estes dependem principalmente do raio iónico do ião dopante e do nodo da rede onde a substituição ocorre. Esta técnica (CTE) não convencional tem revelado ser de grande interesse como complemento de resultados previamente obtidos por espectroscopia dieléctrica. Esta dissertação é constituída por quatro principais capítulos. O capítulo 1 inclui uma revisão da literatura sobre materiais relacionados com SrTiO3 e concluí com os objectivos deste trabalho. No capítulo 2 são apresentados resultados referentes à dinâmica da rede dos sistemas Sr1-1.5xLaxTiO3 e Sr1- 1.5xYxTiO3 obtidos por espectroscopia Raman. Ainda neste capítulo, é introduzido o fenómeno da difusão Raman e apresentada um estudo comparativo da transição de fase para-antiferodistorsiva e a natureza paraléctrica quântica destes dois sistemas. Uma técnica distinta foi também utilizada na identificação dos mecanismos de relaxação em cerâmicas de SrTiO3 dopadas: Correntes Termicamente Estimuladas (CTE). No capítulo 3, este método eficaz é descrito e aplicado no estudo das cerâmicas de SrTiO3 dopadas com Lantânio, Ítrio e Manganês. Neste capítulo é desenvolvida uma discussão geral que relaciona estes resultados com os resultados apresentados na literatura. A análise teórica dos resultados experimentais permitirá uma melhor compreensão dos processos de relaxação observados a partir da determinação dos parâmetros característicos, tais como a energia de activação e tempo de relaxação a temperatura infinita. Esta informação proporcionará a identificação dos mecanismos subjacentes ao comportamento de relaxador anteriormente observado. Finalmente, os resultados obtidos serão resumidos no capítulo 4, bem como apresentadas algumas sugestões de trabalho futuro. ABSTRACT: Ferroelectric ceramics and relaxors have awaken a renewed scientific interest, due to their many applications in technology: dynamic random access memories (DRAMs), bypass capacitors, tuneable microwave devices, low temperature pyroelectric sensors, etc. These applications are associated with conceptual problems, which deal with the distribution of polar aggregates and their interactions, and also with the mediator effects of the matrices in which the aggregates are inserted. This work is centred in the study of the properties of polar ceramics in highly polarizable matrices. Among the highly polarizable materials the quantum paraelectrics stand out. Their electrical properties are very sensitive to defects, impurities, stresses, and applied electric fields. One of the most interesting quantum paraelectrics is Strontium Titanate (SrTiO3), which presents a paraantiferrodistortive phase transition at ~110 K and atomic lattice substitutions can induce ferroelectric or relaxor states, commonly located at low temperatures. These may yield the activation of Raman first order bands, high dielectric constant values and distinct polar relaxation processes. Consequently this work is focused on the comparative study of the lattice, dielectric and polar relaxation properties of SrTiO3 ceramics doped with Yttrium, Lanthanum, and Manganese ions, namely Sr1-1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.13), Sr1-1.5xYxTiO3 (x=0.005 and 0.01), Sr1-xMnxTiO3 (x=0.01, 0.02 and 0.05) and SrTi1-xMnyO3 (y=0.01 and 0.05) systems. These systems were synthesised by the conventional mixed oxide method. Their crystallographic and micro structures were analysed through X-ray diffraction, scanning and transmission electron microscopy, together with energy dispersive spectroscopy methods. Detailed studies of both lattice dynamics and polar relaxations processes have been carried out through Raman and Thermally Stimulated Currents techniques, in a wide interval of temperatures ranging between 10 K and room temperature. The experimental results have been analysed through comprehensive theoretical models, and discussed on the basis of charge and chemical stoichiometry, ion characteristics, and site occupancy. With the ensemble of these results it is expected to provide a better understanding of the physical properties of Strontium Titanate. Although, lattice dynamics of Sr1-1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.0833) have already been reported, the Yttrium doping effect is still unknown. In the present work we propose to analyze Raman spectra for both systems and different concentrations in order to clarify the aliovalent doping behaviour on SrTiO3 lattice. The obtained results show that Lanthanum and Yttrium substitution gives rise to new features in the Raman spectra, due to the loss of the center of inversion originated by doping SrTiO3 ceramics. Those features enable us to have also access to the behavior of infrared active bands, which provides additional information regarding the existence of polar states at low temperatures. The absence of the softening of the TO1 polar mode in all the systems studied supports the non-existence of a ferroelectric phase at low temperatures, which has been reported previously. The shift of the para-antiferrodistortive transition temperature (Ta) towards higher temperatures on both Lanthanum, and Yttrium doped systems clearly evidences that Ta is strongly dependent on the structural tolerance factor. Despite the observed changes in Ta in doped SrTiO3, the temperature behaviour of the antiferrodistortive soft mode A1g can be described in terms of a power law as it has been previously reported for pure SrTiO3. Moreover some low frequency modes, which could not be assigned to other lattice modes, may be apparently associated with disorder effects, stemming from the presence of dopants ions. Although there are several works addressed to the study of doped SrTiO3 systems, the polar relaxation processes observed in some of these systems have not been fully investigated. Therefore, polar relaxation processes in La-, Y- and Mn- doped SrTiO3 ceramics were studied by undertaking Thermally Stimulated Currents (TSC) measurements from room temperature to 10 K. The experimental results were analyzed by using dipolar and space-charge relaxation models in order to determine the nature of the relaxation processes involved. The results reveal the existence of different relaxation processes within the studied range of temperatures. Whereas at low temperatures (18-80 K), relaxation mechanisms of dipolar type were disclosed, space-charge relaxation processes could be identified at higher temperatures (150-300 K) confirming the previous dielectric results. In addition differences in the relaxation processes are observed for different substitute site of the lattice. Regarding dipolar relaxation processes observed at low temperatures, we were able to identify the corresponding mechanisms, which mainly depend on the ionic size, charge and site occupancy. This non conventional technique (TSC technique) has revealed to be a powerful technique to complement the results previously obtained by dielectric spectroscopy. This thesis will comprise four main chapters. Chapter one includes a survey of literature addressing the background of SrTiO3-related materials and concludes with the aims and objectives of this work. Chapter 2 disclose the lattice dynamics of Sr1-1.5xLaxTiO3 and Sr1-1.5xYxTiO3 systems revealed by Raman spectroscopy. Here, Raman diffusion theory is briefly introduced as well as the experimental set-up used in this work. This chapter concludes with a comparison between both systems regarding the para-antiferrodistortive phase transition and quantum paraelectricity features. A different technique was used to identify relaxation mechanisms in doped SrTiO3: Thermally Stimulated Depolarization Currents (TSDC) technique. In Chapter 3, this powerful method is reviewed and applied to La-, Y- and Mn- doped SrTiO3 ceramics. This chapter outlines the general discussion and links the results to the literature survey from the Chapter 1. The theoretical analysis of the experimental data should enable us to gain a better understanding of the relaxation processes through the knowledge of their characteristic parameters such as the activation energy, and relaxation time at infinite temperature. This information will allow determining which mechanisms are underlying the relaxation behaviour previously reported. Finally, a summary of the results obtained for the studied systems is presented in Chapter 4, which ends with a few concluding remarks and directions for further work.

A study of alumina (AI203 ) and magnesium aluminate spinel (MgAb04) was undertaken with the aim of investigating the changes in properties and microstructural characteristics upon doping with specific rare earth elements. Microscopic imaging and analysis of RE doped polycrystalline oxide ceramics has shown convincing evidence for monolayer segregation of RE cations to grain boundaries. State of the art aberration corrected scanning transmission electron microscopy (SuperSTEM I Daresbury Laboratories) has shown monolayer segregation to grain boundaries, and atomic resolution parallel electron energy loss spectroscopy has confirmed the presence of the RE cation at the grain boundary position. The region affected by segregation has been shown to extend no further than one monolayer from the centre of the grain boundary with RE cations occupying matrix cation boundary sites. The effect of RE dopants on the powder processing and sintering of high purity commercial grade precursor powders was investigated. Differences were found between doped alumina and spinel in the sintering whereby the alumina grain growth was restricted by grain boundary mobility such that the grain size was reduced for a given sintering temperature. The grain size of spinel was unaffected by sintering temperature. Differences in the fracture behaviour between doped alumina and spinel was found. The alumina samples manifested a change from trans-granular fracture to inter-granular fracture due to the addition of RE dopants. Spinel did not show such an effect. Alumina was shown to posess an approximate Hall-Petch relationship between hardness and grain size for both doped and undoped samples, such that sub-micron grain size samples posessed high hardness. Optical characterisation has shown the potential for the use of fine grained RE doped alumina and spinel samples for hard window applications. A reduction in the grain size of alumina to below 1 μm leads to a change in the scattering mechanism, thus reducing low angle scatter and birefringence due to the refractive index mismatch. The benefits to optical properties are in addition to the benefits in mechanical properties of a submicron grain structure.