Microstructure design of Titanate-based electroceramics

Electrocerâmicos são uma classe de materiais avançados com propriedades eléctricas valiosas para aplicações. Estas propriedades são geralmente muito dependentes da microestrutura dos materiais. Portanto, o objectivo geral deste trabalho é investigar o desenho da resposta dieléctrica de filmes espessos obtidos por Deposição Electroforética (EPD) e cerâmicos monolíticos, através do controlo da evolução da microestrutura durante a sinterização de electrocerâmicos à base de titanatos. Aplicações sem fios na indústria microelectrónica e de comunicações, em rápido crescimento, tornaram-se um importante mercado para os fabricantes de semicondutores. Devido à constante necessidade de miniaturização, redução de custos e maior funcionalidade e integração, a tecnologia de filmes espessos está a tornar-se uma abordagem de processamento de materiais funcionais cada vez mais importante. Uma técnica adequada neste contexto é EPD. Os filmes espessos resultantes necessitam de um passo subsequente de sinterização que é afectada pelo substrato subjacente, tendo este um forte efeito sobre a evolução da microestrutura. Relacionado com a miniaturização e a discriminação do sinal, materiais dieléctricos usados como componentes operando a frequências das microondas em aplicações na industria microelectrónica de comunicações devem apresentar baixas perdas dieléctricas e elevadas permitividade dieléctrica e estabilidade com a temperatura. Materiais do sistema BaO-Ln2O3- TiO2 (BLnT: Ln = La ou Nd), como BaLa4Ti4O15 (BLT) e Ba4.5Nd9Ti18O54 (BNT), cumprem esses requisitos e são interessantes para aplicações, por exemplo, em estações de base para comunicações móveis ou em ressonadores para telefones móveis, onde a miniaturização dos dispositivos é muito importante. Por sua vez, o titanato de estrôncio (SrTiO3, STO) é um ferroeléctrico incipiente com constante dieléctrica elevada e baixas perdas, que encontra aplicação em, por exemplo, condensadores de camada interna, tirando partido de fronteiras de grão altamente resistivas. A dependência da permitividade dieléctrica do campo eléctrico aplicado torna este material muito interessante para aplicações em dispositivos de microondas sintonizáveis. Materiais à base de STO são também interessantes para aplicações termoeléctricas, que podem contribuir para a redução da actual dependência de combustíveis fósseis por meio da geração de energia a partir de calor desaproveitado. No entanto, as mesmas fronteiras de grão resistivas são um obstáculo relativamente à eficiência do STO para aplicações termoeléctricas. Para além do efeito do substrato durante a sinterização constrangida, outros factores, como a presença de fase líquida, a não-estequiometria ou a temperatura de sinterização, afectam significativamente não apenas a microestrutura dos materiais funcionais, mas também a sua resposta dieléctrica. Se adequadamente compreendidos, estes factores podem ser intencionalmente usados para desenhar a microestrutura dos electrocerâmicos e, desta forma, as suas propriedades dieléctricas. O efeito da não-estequiometria (razão Sr/Ti 0.995-1.02) no crescimento de grão e resposta dieléctrica de cerâmicos de STO foi investigado neste trabalho. A mobilidade das fronteiras de grão aumenta com a diminuição da razão Sr/Ti. A resistividade do interior dos grãos e das fronteiras de grão é sistematicamente diminuída em amostras não-estequiométricas de STO, em comparação com o material estequiométrico. O efeito é muito mais forte para as fronteiras de grão do que para o seu interior. Dependências sistemáticas da não-estequiometria foram também observadas relativamente à dependência da condutividade da temperatura (muito mais afectada no caso da contribuição das fronteiras de grão), à capacitância do interior e fronteiras de grão e à espessura das fronteiras de grão. Uma anomalia no crescimento de grão em cerâmicos de STO ricos em Ti foi também observada e sistematicamente analisada. Foram detectadas três descontinuidades na dependência do tipo Arrhenius do crescimento de grão relativamente à temperatura com diminuições no tamanho de grão a temperaturas em torno de 1500, 1550 e 1605 °C. Além disso, descontinuidades semelhantes foram também observadas na dependência da energia de activação relativamente à condutividade das fronteiras de grão e na espessura das fronteiras de grão, avaliadas por Espectroscopia de Impedância. Estas notáveis coincidências suportam fortemente a formação de diferentes complexos de fronteira de grão com transições entre os regimes de crescimento de grão observados, que podem ser correlacionados com diferentes mobilidades de fronteira de grão e propriedades dieléctricas. Um modelo é sugerido, que se baseia na diminuição da fase líquida localizada nas fronteiras de grão, como o aumento da temperatura de sinterização, um cenário compatível com um fenómeno de solubilidade retrógrada, observado anteriormente em metais e semicondutores, mas não em cerâmicos. A EPD de filmes espessos de STO em substratos de folha de Pt e a sinterização constrangida dos filmes fabricados foram também preliminarmente tratadas. Filmes espessos de STO foram depositados com êxito por EPD sobre substratos de Pt e, depois de sinterizados, atingiram densidades elevadas. Um aumento da densificação e do tamanho de grão assim como o alargamento da distribuição de tamanho do grão foram observados com a diminuição da razão Sr/Ti, tal como anteriormente observado em amostras cerâmicas. Grãos equiaxiados foram observados para todas as composições, mas um certo grau de anisotropia na orientação dos poros foi detectado: os poros revelaram uma orientação vertical preferencial. Este trabalho focou-se também na sinterização constrangida do sistema BLnT (Ln = La ou Nd), nomeadamente de filmes espessos de BLT e BNT sobre substratos de folha de platina, e na relação do desenvolvimento de anisotropia microestrutural com as propriedades dieléctricas. As observações durante a sinterização constrangida foram comparadas com cerâmicos monolíticos equivalentes sinterizados livremente. Filmes espessos de BLnT (Ln = La ou Nd) com elevada densidade foram obtidos por EPD e subsequente sinterização constrangida. A anisometria cristalográfica do material em conjunto com um passo de sinterização constrangida resultou em grãos alongados e microestruturas anisotrópicas. O efeito do stress do substrato durante a sinterização constrangida originou graus mais elevados de anisotropia (grãos e poros alongados e orientação preferencial, bem como textura cristalográfica) nos filmes sinterizados relativamente aos cerâmicos equivalentes sinterizados livremente, não obstante o estado equivalente das amostras em verde. A densificação dos filmes de BLnT (Ln = La ou Nd) é retardada em comparação com os cerâmicos, mas depois de longos tempos de sinterização densidades semelhantes são obtidas. No entanto, em oposição a observações na sinterização constrangida de outros sistemas, o crescimento do grão em filmes de BLnT (Ln = La ou Nd) é favorecido pelo constrangimento causado pelo substrato. Além disso, grãos e poros alongados orientados paralelamente ao substrato foram desenvolvidos durante a sinterização constrangida de filmes espessos. Verificou-se uma forte correlação entre a evolução de grãos e poros, que começou assim que o crescimento do grão se iniciou. Um efeito da tensão do substrato no aumento do crescimento de grão, bem como um forte “Zener pinning”, origina microestruturas altamente texturizadas, o que também é observado a nível cristalográfico. Efeitos marcantes da anisotropia microestrutural foram também detectados nas propriedades dieléctricas dos filmes de BLnT (Ln = La ou Nd). Juntamente com o aumento da razão de aspecto dos grãos, do factor de orientação e do grau de textura cristalográfica, a permitividade relativa é ligeiramente diminuída e o coeficiente de temperatura da permitividade evolui de negativo para positivo com o aumento do tempo isotérmico de sinterização. Este trabalho mostra que a não-estequiometria pode ser usada para controlar a mobilidade das fronteiras de grão e, portanto, desenhar a microestrutura e as propriedades dieléctricas de electrocerâmicos à base de STO, com ênfase nas propriedades das fronteiras de grão. O papel da não-estequiometria no STO e dos complexos de fronteira de grão no desenvolvimento microestrutural é discutido e novas oportunidades para desenhar as propriedades de materiais funcionais são abertas. As observações relativamente à sinterização constrangida apontam para o efeito de tensões mecânicas desenvolvidas devido ao substrato subjacente no desenvolvimento da microestrutura de materiais funcionais. É assim esperado que a escolha adequada de substrato permitia desenhar a microestrutura de filmes espessos funcionais com desempenho optimizado. “Stress Assisted Grain Growth” (SAGG) é então proposto como uma técnica potencial para desenhar a microestrutura de materiais funcionais, originando microestruturas anisotrópicas texturizadas com propriedades desejadas.

Electromechanical properties of engineered lead free potassium sodium niobate based materials

K0.5Na0.5NbO3 (KNN), is the most promising lead free material for substituting lead zirconate titanate (PZT) which is still the market leader used for sensors and actuators. To make KNN a real competitor, it is necessary to understand and to improve its properties. This goal is pursued in the present work via different approaches aiming to study KNN intrinsic properties and then to identify appropriate strategies like doping and texturing for designing better KNN materials for an intended application. Hence, polycrystalline KNN ceramics (undoped, non-stoichiometric; NST and doped), high-quality KNN single crystals and textured KNN based ceramics were successfully synthesized and characterized in this work. Polycrystalline undoped, non-stoichiometric (NST) and Mn doped KNN ceramics were prepared by conventional ceramic processing. Structure, microstructure and electrical properties were measured. It was observed that the window for mono-phasic compositions was very narrow for both NST ceramics and Mn doped ceramics. For NST ceramics the variation of A/B ratio influenced the polarization (P-E) hysteresis loop and better piezoelectric and dielectric responses could be found for small stoichiometry deviations (A/B = 0.97). Regarding Mn doping, as compared to undoped KNN which showed leaky polarization (P-E) hysteresis loops, B-site Mn doped ceramics showed a well saturated, less-leaky hysteresis loop and a significant properties improvement. Impedance spectroscopy was used to assess the role of Mn and a relation between charge transport – defects and ferroelectric response in K0.5Na0.5NbO3 (KNN) and Mn doped KNN ceramics could be established. At room temperature the conduction in KNN which is associated with holes transport is suppressed by Mn doping. Hence Mn addition increases the resistivity of the ceramic, which proved to be very helpful for improving the saturation of the P-E loop. At high temperatures the conduction is dominated by the motion of ionized oxygen vacancies whose concentration increases with Mn doping. Single crystals of potassium sodium niobate (KNN) were grown by a modified high temperature flux method. A boron-modified flux was used to obtain the crystals at a relatively low temperature. XRD, EDS and ICP analysis proved the chemical and crystallographic quality of the crystals. The grown KNN crystals exhibit higher dielectric permittivity (29,100) at the tetragonal-to-cubic phase transition temperature, higher remnant polarization (19.4 μC/cm2) and piezoelectric coefficient (160 pC/N) when compared with the standard KNN ceramics. KNN single crystals domain structure was characterized for the first time by piezoforce response microscopy. It could be observed that <001> - oriented potassium sodium niobate (KNN) single crystals reveal a long range ordered domain pattern of parallel 180° domains with zig-zag 90° domains. From the comparison of KNN Single crystals to ceramics, It is argued that the presence in KNN single crystal (and absence in KNN ceramics) of such a long range order specific domain pattern that is its fingerprint accounts for the improved properties of single crystals. These results have broad implications for the expanded use of KNN materials, by establishing a relation between the domain patterns and the dielectric and ferroelectric response of single crystals and ceramics and by indicating ways of achieving maximised properties in KNN materials. Polarized Raman analysis of ferroelectric potassium sodium niobate (K0.5Na0.5)NbO3 (KNN) single crystals was performed. For the first time, an evidence is provided that supports the assignment of KNN single crystals structure to the monoclinic symmetry at room temperature. Intensities of A′, A″ and mixed A′+A″ phonons have been theoretically calculated and compared with the experimental data in dependence of crystal rotation, which allowed the precise determination of the Raman tensor coefficients for (non-leaking) modes in monoclinic KNN. In relation to the previous literature, this study clarifies that assigning monoclinic phase is more suitable than the orthorhombic one. In addition, this study is the basis for non-destructive assessments of domain distribution by Raman spectroscopy in KNN-based lead-free ferroelectrics with complex structures. Searching a deeper understanding of the electrical behaviour of both KNN single crystal and polycrystalline materials for the sake of designing optimized KNN materials, a comparative study at the level of charge transport and point defects was carried out by impedance spectroscopy. KNN single crystals showed lower conductivity than polycrystals from room temperature up to 200 ºC, but above this temperature polycrystalline KNN displays lower conductivity. The low temperature (T < 200 ºC) behaviour reflects the different processing conditions of both ceramics and single crystals, which account for less defects prone to charge transport in the case of single crystals. As temperature increases (T > 200 ºC) single crystals become more conductive than polycrystalline samples, in which grain boundaries act as barriers to charge transport. For even higher temperatures the conductivity difference between both is increased due to the contribution of ionic conduction in single crystals. Indeed the values of activation energy calculated to the high temperature range (T > 300 ºC) were 1.60 and 0.97 eV, confirming the charge transport due to ionic conduction and ionized oxygen vacancies in single crystals and polycrystalline KNN, respectively. It is suggested that single crystals with low defects content and improved electromechanical properties could be a better choice for room temperature applications, though at high temperatures less conductive ceramics may be the choice, depending on the targeted use. Aiming at engineering the properties of KNN polycrystals towards the performance of single crystals, the preparation and properties study of (001) – oriented (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) ceramics obtained by templated grain growth (TGG) using KNN single crystals as templates was undertaken. The choice of KNN single crystals templates is related with their better properties and to their unique domain structure which were envisaged as a tool for templating better properties in KNN ceramics too. X-ray diffraction analysis revealed for the templated ceramics a monoclinic structure at room temperature and a Lotgering factor (f) of 40% which confirmed texture development. These textured ceramics exhibit a long range ordered domain pattern consisting of 90º and 180º domains, similar to the one observed in the single crystals. Enhanced dielectric (13017 at TC), ferroelectric (2Pr = 42.8 μC/cm2) and piezoelectric (d33 = 280 pC/N) properties are observed for textured KNNL ceramics as compared to the randomly oriented ones. This behaviour is suggested to be due to the long range ordered domain patterns observed in the textured ceramics. The obtained results as compared with the data previously reported on texture KNN based ceramics confirm that superior properties were found due to ordered repeated domain pattern. This study provides an useful approach towards properties improvement of KNN-based piezoelectric ceramics. Overall, the present results bring a significant contribution to the pool of knowledge on the properties of sodium potassium niobate materials: a relation between the domain patterns and di-, ferro-, and piezo-electric response of single crystals and ceramics was demonstrated and ways of engineering maximised properties in KNN materials, for example by texturing were established. This contribution is envisaged to have broad implications for the expanded use of KNN over the alternative lead-based materials.

Electrolytes for ceramic oxide fuel cells

The main objective of this dissertation is the development and processing of novel ionic conducting ceramic materials for use as electrolytes in proton or oxide-ion conducting solid oxide fuel cells. The research aims to develop new processing routes and/or materials offering superior electrochemical behavior, based on nanometric ceramic oxide powders prepared by mechanochemical processes. Protonic ceramic fuel cells (PCFCs) require electrolyte materials with high proton conductivity at intermediate temperatures, 500-700ºC, such as reported for perovskite zirconate oxides containing alkaline earth metal cations. In the current work, BaZrO3 containing 15 mol% of Y (BZY) was chosen as the base material for further study. Despite offering high bulk proton conductivity the widespread application of this material is limited by its poor sinterability and grain growth. Thus, minor additions of oxides of zinc, phosphorous and boron were studied as possible sintering additives. The introduction of ZnO can produce substantially enhanced densification, compared to the un-doped material, lowering the sintering temperature from 1600ºC to 1300ºC. Thus, the current work discusses the best solid solution mechanism to accommodate this sintering additive. Maximum proton conductivity was shown to be obtained in materials where the Zn additive is intentionally adopted into the base perovskite composition. P2O5 additions were shown to be less effective as a sintering additive. The presence of P2O5 was shown to impair grain growth, despite improving densification of BZY for intermediate concentrations in the range 4 – 8 mol%. Interreaction of BZY with P was also shown to have a highly detrimental effect on its electrical transport properties, decreasing both bulk and grain boundary conductivities. The densification behavior of H3BO3 added BaZrO3 (BZO) shows boron to be a very effective sintering aid. Nonetheless, in the yttrium containing analogue, BaZr0.85Y0.15O3- (BZY) the densification behavior with boron additives was shown to be less successful, yielding impaired levels of densification compared to the plain BZY. This phenomenon was shown to be related to the undesirable formation of barium borate compositions of high melting temperatures. In the last section of the work, the emerging oxide-ion conducting materials, (Ba,Sr)GeO3 doped with K, were studied. Work assessed if these materials could be formed by mechanochemical process and the role of the ionic radius of the alkaline earth metal cation on the crystallographic structure, compositional homogeneity and ionic transport. An abrupt jump in oxide-ion conductivity was shown on increasing operation temperature in both the Sr and Ba analogues.

Coupling of magnetic, strain and electric polarization fields in the structure of multiferroic material

The comprehensive study on the coupling of magnetism, electrical polarization and the crystalline lattice with the off-stoichiometric effects in self-doped multiferroic hexagonal h-LuMnxO3±δ (0.92≤x≤1.12) ceramic oxides was carried out for the PhD work. There is a complex coupling of the three ferroic degrees. The cancelation of the magnetic moments of ions in the antiferromagnetic order, electric polarization with specific vortex/antivortex topology and lattice properties have pushed researchers to find out ways to disclose the underlying physics and chemistry of magneto-electric and magneto-elastic couplings of h-RMnO3 multiferroic materials. In this research work, self-doping of Lu-sites or Mn-sites of h-LuMnxO3±δ ceramics prepared via solid state route was done to pave a way for deeper understanding of the antiferromagnetic transition, the weak ferromagnetism often reported in the same crystalline lattices and the ferroelectric properties coupled to the imposed lattice changes. Accordingly to the aim of the PhD thesis, the objectives set for the sintering study in the first chapter on experimental results were two. First, study of sintering off-stoichiometric samples within conditions reported in the bibliography and also extracted from the phase diagrams of the LuMnxO3±δ, with a multiple firings ending with a last high temperature step at 1300ºC for 24 hours. Second, explore longer annealing times of up to 240 hours at the fixed temperature of 1300 ºC in a search for improving the properties of the solid solution under study. All series of LuMnxO3±δ ceramics for each annealing time were characterized to tentatively build a framework enabling comparison of measured properties with results of others available in literature. XRD and Rietveld refinement of data give the evolution the lattice parameters as a function to x. Shrinkage of the lattice parameters with increasing x values was observed, the stability limit of the solid solution being determined by analysis of lattice parameters. The evolution of grain size and presence of secondary phases have been investigated by means of TEM, SEM, EDS and EBSD techniques. The dependencies of grain growth and regression of secondary phases on composition x and time were further characterized. Magnetic susceptibility of samples and magnetic irreversibility were extensively examined in the present work. The dependency of magnetic susceptibility, Neel ordering transition and important magnetic parameters are determined and compared to observation in other multiferroics in the following chapter of the thesis. As a tool of high sensitivity to detect minor traces of the secondary phase hausmannite, magnetic measurements are suggested for cross-checking of phase diagrams. Difficulty of previous studies on interpreting the magnetic anomaly below 43 K in h-RMnO3 oxides was discussed and assigned to the Mn3O4 phase, with supported of the electron microscopy. Magneto-electric coupling where AFM ordering is coupled to dielectric polarization is investigated as a function of x and of sintering condition via frequency and temperature dependent complex dielectric constant measurements in the final chapter of the thesis. Within the limits of solid solubility, the crystalline lattice of off-stoichiometric ceramics was shown to preserve the magneto-electric coupling at TN. It represents the first research work on magneto-electric coupling modified by vacancy doping to author’s knowledge. Studied lattices would reveal distortions at the atomic scale imposed by local changes of x dependent on sintering conditions which were widely inspected by using TEM/STEM methods, complemented with EDS and EELS spectroscopy all together to provide comprehensive information on cross coupling of distortions, inhomogeneity and electronic structure assembled and discussed in a specific chapter. Internal interfaces inside crystalline grains were examined. Qualitative explanations of the measured magnetic and ferroelectric properties were established in relation to observed nanoscale features of h-LuMnxO3±δ ceramics. Ferroelectric domains and topological defects are displayed both in TEM and AFM/PFM images, the later technique being used to look at size, distribution and switching of ferroelectric domains influenced by vacancy doping at the micron scale bridging to complementary TEM studies on the atomic structure of ferroelectric domains. In support to experimental study, DFT simulations using Wien2K code have been carried out in order to interpret the results of EELS spectra of O K-edge and to obtain information on the cation hybridization to oxygen ions. The L3,2 edges of Mn is used to access the oxidation state of the Mn ions inside crystalline grains. In addition, rehybridization driven ferroelectricity is also evaluated by comparing the partial density of states of the orbitals of all ions of the samples, also the polarization was calculated and correlated to the off-stoichiometric effect.