Ba(Mg1/3Ta2/3)O3 Thermal Barrier Coating Deposited by Liquid Precursor Plasma Spray: Phases and Microstructure Control

This study explores a new route to deposit complex oxide thermal barrier coatings (TBCs) Ba(Mg1/3Ta2/3)O3 (BMT) from liquid precursors, using a radio frequency (RF) induction plasma spray (IPS), which differs from the most frequently used technique, direct current (DC) air plasma spray (APS). Among the known oxide materials, BMT has the highest melting point (2900-3100 °C). Accordingly, it is envisioned as one of the most promising TBC candidate materials for the aeronautic field. Due to the limits imposed by BMT feedstock decomposition mainly caused by Mg evaporation during the APS process, and to the low service lifespan of dense lamellar coatings, liquid precursors were chosen as feedstock in this work. Indeed, they can attenuate feedstock decomposition by adding an excess of Mg into the initial precursor mixture. Furthermore, liquid precursors, normally prepared by solution and/or nanoscale solid particles suspensions, facilitate the formation of columnar structures. Therefore, the process adopted in this work is named hybrid suspension plasma spray (SPS)/ solution precursor plasma spray (SPPS). X-ray photoelectron spectroscopy (XPS) was used to evaluate quantitatively the element evaporation during plasma spraying. Thermogravimetric / differential thermal analysis (TG/DTA) was applied to investigate the BMT formation. The phase and microstructure were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Plasma diagnostic was performed by optical emission spectroscopy (OES). This project first proved that complex oxide synthesis and coating deposition can occur simultaneously in the Hybrid SPS/SPPS or SPPS processes. The formation mechanism and the correlation between the main BMT phase and four other secondary phases (BaTa2O6, Ba3Ta5O15, Ba4Ta2O9, Mg4Ta2O9) were discussed clearly. The key parameters (plasma power, spraying distance, substrate pre-heating, precursor chemistry and particle size of suspended particles), were explored in order to optimize phase structure and deposition rate of as-sprayed coatings. In addition, this research demonstrated the feasibility of applying RF SPPS to deposit BMT coating with an adjustable microstructure. Lamellar, columnar and dense vertically-cracked structures of BMT can be tuned by adjusting the spraying parameters. This result represents an opportunity to apply RF plasma spray in the TBCs field, in addition to that of DC plasma spray and EB-PVD. Short spray distance and bimodal atomized droplet size distribution were identified as key prerequisites for column formation. Parameters such as substrate roughness, precursor concentration and feed rate were studied with regards to the columnar morphology. A distinctive TBC microstructure, namely vertical grains, was presented in the as-sprayed BMT coatings. The novel material and microstructure differentiate themselves from any commonly-seen TBCs. The phase structure of vertical grains was analyzed with the assistance of elemental mapping and BaO-MgO-Ta2O5 ternary phase diagrams. The formation mechanism of vertical grains was discussed in terms of temperature gradient. Finally, based on the microstructure optimized in this work and other physical property of thermal expansion coefficient, BMT presents the potential for acting as a thermal protection coating on niobium alloy in the foreseeable future.Cette étude explore une nouvelle voie pour déposer des revêtements d’oxydes complexes pour les barrières thermiques (TBC) Ba(Mg1/3Ta2/3)O3 (BMT) à partir de précurseurs liquides. Les dépôts sont faits en utilisant la projection par plasma radiofréquence (RF) à couplage inductif, qui diffère de la technique la plus fréquemment utilisée dans le domaine, à savoir la projection plasma à air (APS) en courant continu (DC). Parmi les matériaux oxydes connus, le BMT a le point de fusion le plus élevé (2900-3100 °C). En conséquence, il est considéré comme l'un des matériaux candidats TBC les plus prometteurs pour le domaine aéronautique. En raison des limites imposées par la décomposition des matières premières du BMT, principalement causée par l'évaporation du Mg pendant le processus APS, et à la faible durée de vie des revêtements lamellaires denses, des précurseurs liquides ont été choisis comme matière première dans ce travail. En effet, ils peuvent atténuer la décomposition des matières premières en ajoutant un excès de Mg dans le mélange initial de précurseurs. En outre, les précurseurs liquides, normalement préparés par des solutions et/ou des suspensions de particules solides à l’échelle nanométrique, facilitent la formation d'une structure colonnaire. Par conséquent, le procédé adopté dans ce travail est nommé projection plasma hybride de suspension (SPS) / précurseur liquide (SPPS). La spectroscopie photoélectronique X (XPS) a été utilisée pour évaluer quantitativement l'évaporation des éléments pendant la projection par plasma. L’analyse thermique thermogravimétrique / différentielle (TG / DTA) a été réalisée pour étudier la formation de BMT. La phase et la microstructure ont été analysées par diffraction des rayons X (XRD) et par microscopie électronique à balayage (SEM), respectivement. Le diagnostic du plasma a été effectué par spectroscopie d'émission optique (OES). Ce projet a d'abord prouvé que la synthèse d'oxydes complexes et le dépôt de revêtement peuvent se produire simultanément dans les processus hybrides SPS/SPPS ou SPPS. Le mécanisme de formation et la corrélation entre la phase principale du BMT et quatre autres phases secondaires (BaTa2O6, Ba3Ta5O15, Ba4Ta2O9, Mg4Ta2O9) ont été clairement discutés. Les paramètres clés (puissance du plasma, distance de projection, préchauffage du substrat, chimie des précurseurs et taille des particules en suspension) ont été étudiés afin d'optimiser la structure des phases et le taux de dépôt des revêtements projetés. En outre, cette recherche a démontré la faisabilité de l’application de RF SPPS pour déposer un revêtement de BMT ayant une microstructure contrôlable. Les structures lamellaires, colonnaires et à fissures verticales denses du BMT peuvent toutes les trois être obtenues en ajustant les paramètres de projection. Ce résultat représente une opportunité d'appliquer la projection plasma RF dans le domaine des TBC, en plus de celle de la projection plasma DC et du dépôt physique en phase vapeur par faisceau d'électrons (EB-PVD). Une courte distance de projection et une distribution bimodale de la taille des gouttelettes atomisées ont été identifiées comme des conditions préalables clés à la formation de colonnes. Des paramètres tels que la rugosité du substrat, la concentration en précurseur et le taux d'alimentation ont été étudiés pour connaître leur influence sur la morphologie en colonne. Une microstructure TBC distinctive, à savoir avec des grains verticaux, a été trouvée dans les revêtements BMT projetés. Le nouveau matériau et la microstructure se distinguent de tous les autres TBC courants. La structure de phase des grains verticaux a été analysée à l'aide d'une cartographie élémentaire et de diagrammes de phases ternaires BaO-MgO-Ta2O5. Le mécanisme de formation des grains verticaux a été discuté en termes de gradient de température. Enfin, sur la base de la microstructure optimisée dans ce travail et d'autres propriétés physiques du coefficient de dilatation thermique, le BMT présente le potentiel d'agir comme un revêtement de protection thermique sur un alliage de niobium dans un avenir prévisible

Low Temperature Synthesis of the Microwave Dielectric Material, Barium Magnesium Tantalate (BMT)

Wireless communication systems utilize microwave dielectrics for coupling, selecting and filtering microwaves. Over the past several years there has been an increased demand for smaller, lighter and temperature stable devices. An important material that has been studied extensively for these applications is barium magnesium tantalate (BMT). Although BMT has very good dielectric properties: relatively high dielectric constant (25), temperature stability and low dielectric loss in the microwave region (Qd*fo » 150,000 GHz at 4.9 GHz), it can be expensive to produce because of the high sintering temperatures (>1600oC) required to obtain the desired properties. The objective of this study was to dope BMT with ZnGa2O4, Ga2O3, and ZnO to try and lower the sintering temperature without sever degradation of the microwave dielectric properties. The study showed that the BMT doped with both Ga2O3 and ZnO gave the best properties at the lowest sintering temperatures. BMT doped with 4mol%Ga2O3 and ZnO has been successfully sintered at 1400oC for 2 hours had an average density of 95% with a dielectric constant of 24 and a Qd*fo of 130,000 at 4.9 GHz. BMT doped with 8mol% Ga2O3 and ZnO and sintered at 1450oC for 2 hours had an average density of 94%, a dielectric constant of 24 and a Qd*fo of 135,000 at 4.9GHz. The BMT materials doped with ZnGa2O4 and Ga2O3 both had average densities of over 95% and dielectric constants of approximately 24 but high dielectric loss. The BMT doped with Ga2O3 had a Qd*fo of only 84,000 at 4.9 GHz and the BMT doped with ZnGa2O4 had a Qd*fo of 93,000 at 4.9 GHz. Phase evolution and densification behavior of these materials are described

Estruturas 2D funcionais de tantalatos alcalinos para microelectrónica e aplicações relacionadas

Doutoramento em Ciência e Engenharia de MateriaisAlkali tantalates and niobates, including K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, are a very promising ferroic family of lead-free compounds with perovskite-like structures. Their versatile properties make them potentially interesting for current and future application in microelectronics, photocatalysis, energy and biomedics. Among them potassium tantalate, KTaO3 (KTO), has been raising interest as an alternative for the well-known strontium titanate, SrTiO3 (STO). KTO is a perovskite oxide with a quantum paraelectric behaviour when electrically stimulated and a highly polarizable lattice, giving opportunity to tailor its properties via external or internal stimuli. However problems related with the fabrication of either bulk or 2D nanostructures makes KTO not yet a viable alternative to STO. Within this context and to contribute scientifically to the leverage tantalate based compounds applications, the main goals of this thesis are: i) to produce and characterise thin films of alkali tantalates by chemical solution deposition on rigid Si based substrates, at reduced temperatures to be compatible with Si technology, ii) to fulfil scientific knowledge gaps in these relevant functional materials related to their energetics and ii) to exploit alternative applications for alkali tantalates, as photocatalysis. In what concerns the synthesis attention was given to the understanding of the phase formation in potassium tantalate synthesized via distinct routes, to control the crystallization of desired perovskite structure and to avoid low temperature pyrochlore or K-deficient phases. The phase formation process in alkali tantalates is far from being deeply analysed, as in the case of Pb-containing perovskites, therefore the work was initially focused on the process-phase relationship to identify the driving forces responsible to regulate the synthesis. Comparison of phase formation paths in conventional solid-state reaction and sol-gel method was conducted. The structural analyses revealed that intermediate pyrochlore K2Ta2O6 structure is not formed at any stage of the reaction using conventional solid-state reaction. On the other hand in the solution based processes, as alkoxide-based route, the crystallization of the perovskite occurs through the intermediate pyrochlore phase; at low temperatures pyrochlore is dominant and it is transformed to perovskite at >800 °C. The kinetic analysis carried out by using Johnson-MehlAvrami-Kolmogorow model and quantitative X-ray diffraction (XRD) demonstrated that in sol-gel derived powders the crystallization occurs in two stages: i) at early stage of the reaction dominated by primary nucleation, the mechanism is phase-boundary controlled, and ii) at the second stage the low value of Avrami exponent, n ~ 0.3, does not follow any reported category, thus not permitting an easy identification of the mechanism. Then, in collaboration with Prof. Alexandra Navrotsky group from the University of California at Davis (USA), thermodynamic studies were conducted, using high temperature oxide melt solution calorimetry. The enthalpies of formation of three structures: pyrochlore, perovskite and tetragonal tungsten bronze K6Ta10.8O30 (TTB) were calculated. The enthalpies of formation from corresponding oxides, ∆Hfox, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -203.63 ± 2.84 kJ/mol, - 358.02 ± 3.74 kJ/mol, and -1252.34 ± 10.10 kJ/mol, respectively, whereas from elements, ∆Hfel, for KTaO3, KTa2.2O6 and K6Ta10.8O30 are -1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol, and -13393.04 ± 31.15 kJ/mol, respectively. The possible decomposition reactions of K-deficient KTa2.2O6 pyrochlore to KTaO3 perovskite and Ta2O5 (reaction 1) or to TTB K6Ta10.8O30 and Ta2O5 (reaction 2) were proposed, and the enthalpies were calculated to be 308.79 ± 4.41 kJ/mol and 895.79 ± 8.64 kJ/mol for reaction 1 and reaction 2, respectively. The reactions are strongly endothermic, indicating that these decompositions are energetically unfavourable, since it is unlikely that any entropy term could override such a large positive enthalpy. The energetic studies prove that pyrochlore is energetically more stable phase than perovskite at low temperature. Thus, the local order of the amorphous precipitates drives the crystallization into the most favourable structure that is the pyrochlore one with similar local organization; the distance between nearest neighbours in the amorphous or short-range ordered phase is very close to that in pyrochlore. Taking into account the stoichiometric deviation in KTO system, the selection of the most appropriate fabrication / deposition technique in thin films technology is a key issue, especially concerning complex ferroelectric oxides. Chemical solution deposition has been widely reported as a processing method to growth KTO thin films, but classical alkoxide route allows to crystallize perovskite phase at temperatures >800 °C, while the temperature endurance of platinized Si wafers is ~700 °C. Therefore, alternative diol-based routes, with distinct potassium carboxylate precursors, was developed aiming to stabilize the precursor solution, to avoid using toxic solvents and to decrease the crystallization temperature of the perovskite phase. Studies on powders revealed that in the case of KTOac (solution based on potassium acetate), a mixture of perovskite and pyrochlore phases is detected at temperature as low as 450 °C, and gradual transformation into monophasic perovskite structure occurs as temperature increases up to 750 °C, however the desired monophasic KTaO3 perovskite phase is not achieved. In the case of KTOacac (solution with potassium acetylacetonate), a broad peak is detected at temperatures 700 °C. Infrared analysis indicated that the differences are due to a strong deformation of the carbonate-based structures upon heating. A series of thin films of alkali tantalates were spin-coated onto Si-based substrates using diol-based routes. Interestingly, monophasic perovskite KTaO3 films deposited using KTOacac solution were obtained at temperature as low as 650 °C; films were annealed in rapid thermal furnace in oxygen atmosphere for 5 min with heating rate 30 °C/sec. Other compositions of the tantalum based system as LiTaO3 (LTO) and NaTaO3 (NTO), were successfully derived as well, onto Si substrates at 650 °C as well. The ferroelectric character of LTO at room temperature was proved. Some of dielectric properties of KTO could not be measured in parallel capacitor configuration due to either substrate-film or filmelectrode interfaces. Thus, further studies have to be conducted to overcome this issue. Application-oriented studies have also been conducted; two case studies: i) photocatalytic activity of alkali tantalates and niobates for decomposition of pollutant, and ii) bioactivity of alkali tantalate ferroelectric films as functional coatings for bone regeneration. Much attention has been recently paid to develop new type of photocatalytic materials, and tantalum and niobium oxide based compositions have demonstrated to be active photocatalysts for water splitting due to high potential of the conduction bands. Thus, various powders of alkali tantalates and niobates families were tested as catalysts for methylene blue degradation. Results showed promising activities for some of the tested compounds, and KNbO3 is the most active among them, reaching over 50 % degradation of the dye after 7 h under UVA exposure. However further modifications of powders can improve the performance. In the context of bone regeneration, it is important to have platforms that with appropriate stimuli can support the attachment and direct the growth, proliferation and differentiation of the cells. In lieu of this here we exploited an alternative strategy for bone implants or repairs, based on charged mediating signals for bone regeneration. This strategy includes coating metallic 316L-type stainless steel (316L-SST) substrates with charged, functionalized via electrical charging or UV-light irradiation, ferroelectric LiTaO3 layers. It was demonstrated that the formation of surface calcium phosphates and protein adsorption is considerably enhanced for 316L-SST functionalized ferroelectric coatings. Our approach can be viewed as a set of guidelines for the development of platforms electrically functionalized that can stimulate tissue regeneration promoting direct integration of the implant in the host tissue by bone ingrowth and, hence contributing ultimately to reduce implant failure.Tantalatos e niobatos alcalinos, como K(Ta / Nb)O3, Li(Ta / Nb)O3 and Na(Ta / Nb)O3, são uma família atrativa de compostos ferroeléctricos livres de chumbo com estrutura perosvquítica. As suas propriedades versáteis fazem destes potencialmente interessantes para aplicações em microelectrónica, foto catálise, energia e biomédica. Entre os compostos acima citados, os compostos de tantalato de potássio, KTaO3 (KTO), tem atraído bastante atenção como substitutos para o amplamente conhecido titanato de estrôncio, SrTiO3 (STO). KTO é um óxido perovsquítico com comportamento paraelétrico quântico, quando eletricamente estimulado, e elevada polaribilidade tornando viável engenhar as suas propriedades através de estímulos internos e externos. No entanto os problemas na sua produção, quer em macroescala quer em nanoestruturas 2D, tornam estes compostos numa alternativa pouco viável para a substituir o STO. Consequentemente, e de forma a contribuir cientificamente para aumentar o conhecimento sobre as aplicações dos tantalatos, os principais objectivos desta tese são: i) produzir e caracterizar filmes finos de tantalatos alcalinos através de deposição de solução química em substratos rígidos, à base de silício, e a baixas temperaturas de forma a serem compatíveis com a tecnologia de silício; ii) complementar o conhecimento científico sobre estes materiais funcionais relativamente às suas características termodinâmicas; iii) explorar aplicações alternativas para os tantalatos alcalinos, como a foto catálise. No que diz respeito à síntese, foi focalizada no entendimento da formação de fase no tantalato de potássio sintetizado por diferentes métodos, de modo a controlar a cristalização da estrutura perovsquítica desejada e evitar a formação da fase pirocloro a baixas temperaturas e fases deficientes em potássio. Em tantalatos alcalinos o processo de formação da fase desejada está longe de estar plenamente analisado, como é o caso das perovsquites que contêm chumbo, consequentemente o trabalho foi inicialmente focado na compreensão da relação processo-fase para identificar as forças motrizes responsáveis por regular o processo de síntese. Foi realizada um estudo comparativo da formação de fase via método convencional de reação do estado sólido e via método de sol-gel. A análise estrutural revelou que a estrutura piroclórica intermédia K2Ta2O6 não foi formada em nenhuma etapa da reação via método do estado sólido. Por outro lado em processos baseados em solução, como os baseados em alcóxidos, a cristalização perovsquítica ocorre através da indesejada fase pirocloro intermédia; a baixas temperaturas a fase pirocloro é dominante e sofre a transformação para perovsquite a >800 °C. A análise cinética efectuada usando o modelo Johnson-Mehl-Avrami-Kolmogorow e a difração de raio-X quantitativa (DRX), demonstraram que nos pós obtidos pelo método sol-gel, a cristalização ocorre em duas etapas: i) no estágio inicial a reação é denominada por nucleação primária, o mecanismo é controlado por fronteira de fase, e ii) no segundo estágio, o baixo valor do expoente de Avrami, n ~ 0.3, não segue nenhuma categoria reportada impossibilitando assim uma clara identificação do mecanismo. Posteriormente, e em colaboração com o grupo da Professora Alexandra Navrostky da Universidade da Califórnia, Davis, foram realizados estudos de termodinâmica, usando calorimetria de solução de óxidos fundidos a alta temperatura. Foram calculadas as entalpias de formação das três estruturas: pirocloro, perovsquite e tetragonal tungsténio bronze K6Ta10.8O30 (TTB). As entalpias de formação relativas aos óxidos correspondentes, ∆Hfox, para KTaO3, KTa2.2O6 e K6Ta10.8O30, são -203.63 ± 2.84 kJ/mol, 358.02 ± 3.74 kJ/mol e -1252.34 ± 10.10 kJ/mol, respectivamente; enquanto que as relativas aos elementos, ∆Hfel, para KTaO3, KTa2.2O6 e K6Ta10.8O30 são 1408.96 ± 3.73 kJ/mol, -2790.82 ± 6.06 kJ/mol e -13393.04 ± 31.15 kJ/mol, respectivamente. As possíveis reações de decomposição, de KTa2.2O6 para KTaO3 e Ta2O5 (reação 1) ou para K6Ta10.8O30 e Ta2O5 (reação 2), foram propostas e o cálculo das entalpias resultou em 308.79 ± 4.41 kJ/mol e 895.79 ± 8.64 kJ/mol, respectivamente. As reações são fortemente endotérmicas, indicando que estas decomposições são energeticamente desfavoráveis, uma vez que é improvável que qualquer termo de entropia possa sobrepor-se a uma entalpia tão positiva. Os estudos termodinâmicos provaram que o pirocloro é energeticamente mais estável que a perovsquite para temperaturas baixas. Assim, a organização local dos precipitados amorfos canaliza a cristalização para a estrutura mais favorável, que é a pirocloro com uma organização local similar; a distância entre os vizinhos mais próximos na fase amorfa, ou na fase ordenada a baixo alcance, é similar à do pirocloro. Tendo em conta a derivação estequiométrica no sistema KTO, selecionar a técnica de fabricação / deposição de filmes finos mais apropriada é uma questão-chave, especialmente no que concerne aos óxidos ferroeléctricos complexos. A deposição por solução química tem sido o método de processamento mais reportado, para crescimento de filmes finos de KTO, mas o método clássico de alcóxidos permite cristalizar a fase perovsquite a temperaturas >800 °C enquanto que a temperatura máxima de estabilidade para os substratos de silício platinizado é ~700 °C. Portanto, foi usado um processo alternativo baseado em dióis, com precursores carboxilados de potássio, com o objectivo de estabilizar os precursores em solução, evitando assim o uso de solventes tóxicos e diminuindo a temperatura de cristalização da fase perovsquite. A análise dos pós revelou que no caso de KTOac (solução baseada em acetato de potássio), uma mistura de fase perovsquite e pirocloro foi detectada a uma temperatura de apenas 450 °C, e a transformação gradual em estrutura perovsquítica monofásica ocorre quando as temperaturas sobem acima de 750 °C, no entanto a fase KTaO3 monofásica não é obtida. No caso do KTOacac (solução com acetil-acetona de potássio, cadeia alquílica longa carboxilato de metal), um amplo pico é detectado a temperaturas 700 °C. A análise de infravermelhos mostrou que estas diferenças acontecem devido à deformação da estrutura base dos carbonatos sob aquecimento. Uma série de filmes finos de tantalatos alcalinos foram depositados por spincoating em substratos de silício, usando a metodologia baseada em dióis. Filmes monofásicos de perovsquite KTaO3 depositados usando solução de KTOacac foram obtidos a uma temperatura de apenas 550 °C; os filmes foram recristralizados em fornos de aquecimento rápido em atmosfera de oxigénio durante 5 minutos com taxa de aquecimento de 30 °C/seg. Outras composições, LiTaO3 (LTO) e NaTaO3 (NTO), foram depositados com sucesso em substratos de silício a 650 °C. O carácter ferroeléctrico do LTO à temperatura ambiente foi provado. Infelizmente, não foi possível medir as propriedades eléctricas do KTO no condensador paralelo devido às interfaces filme-substrato ou filme-eléctrodo. Assim sendo, estudos futuros são necessários para compreender esta questão. Foram também conduzidos estudos com vista às possíveis aplicações; dois casos de estudo: i) estudo da atividade fotocatalítica de tantalatos e niobatos alcalinos para decomposição de poluentes, e ii) estudo de bioatividade de filmes ferroelétricos de tantalatos alcalinos como revestimento funcional para regeneração óssea. Recentemente, tem sido dedicada muita atenção ao desenvolvimento de novos materiais fotocatalíticos, e as composições à base de óxido de tântalo e nióbio tem demonstrado capacidade de fotocatálise na reação de separação da água devido ao elevado potencial das bandas de condução. Assim, várias composições das famílias dos tantalatos e niobatos alcalinos foram testadas como catalisadores para degradação do azul de metileno. Os resultados mostram valores de atividade promissores para alguns dos compostos, sendo o KNbO3 o mais ativo de entre os testados, alcançando valores acima de 50 % na degradação do pigmento após 7 h sob exposição a UVA. No entanto algumas modificações nas composições dos pós podem melhorar a sua performance. No que concerne à regeneração óssea, é importante obter plataformas que através de estímulos apropriados consigam assegurar a adesão e direcionar o crescimento, a proliferação e a diferenciação celular. Neste contexto, foi aqui explorada uma estratégia alternativa para revestimento de implantes ósseos, baseada na regeneração óssea mediada por sinais elétricos. Esta estratégia implica revestir substratos metálicos de aço inoxidável tipo 316L (316L-SST), com camadas de LiTaO3 ferroeléctrico, funcionalizadas através de polarização elétrica ou de irradiação com luz UV. Foi demonstrado que a formação de fosfato de cálcio na superfície e a adsorção de proteínas é consideravelmente melhorada quando o 316L-SST é revestido com filmes ferroelétricos funcionalizados. Esta estratégia pode ser encarada como um conjunto de orientações para o desenvolvimento de plataformas eletricamente funcionalizadas, capazes de estimular a regeneração de tecidos, promovendo a associação direta do implante com os tecidos hospedeiros, contribuindo assim para a redução de falhas na reabilitação com implantes ósseos

Hallmarks of mechanochemistry: From nanoparticles to technology

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).Slovak Grant Agency VEGA 2/0009/11, 2/0043/11Slovak Agency for Science and Development APVV VV-0189-10, VV-0528-11Russian Foundation for Basic Research 10-03-00942a, 12-03-00651aMinistry of Science and Higher education in Poland CUT/c-1/DS/KWC/2008-2012, PB1T09B02330, NN209145136, NN20914893

Study, selection, and preparation of solid cationic conductors

Crystal chemical principles and transport theory have been used to predict structures and specific compounds which might find application as solid electrolytes in rechargeable high energy and high power density batteries operating at temperatures less than 200 C. Structures with 1-, 2-, and 3-dimensional channels were synthesized and screened by nuclear magnetic resonance, dielectric loss, and conductivity. There is significant conductivity at room temperature in some of the materials but none attain a level that is comparable to beta-alumina. Microwave and fast pulse methods were developed to measure conductivity in powders and in small crystals

Modern microwave methods in solid state inorganic materials chemistry: from fundamentals to manufacturing

No abstract available

Physical and electrical properties of Li1+xTi2-xAlx(PO4)3 and Li1+2xTa1-xAlx+1(PO4)3 electrolytes

Sodium superionic conducting materials (NASICON) are promising solid electrolytes for Li-ion rechargeable batteries. In this study, two compositions; lithium titanium aluminium phosphate (LTAP), Li1+xTi2-xAlx(PO4)3 (x = 0.0, 0.2, 0.6, 1.0) and lithium tantalum aluminium phosphate (LTaAP), Li1+2xTa1-xAlx+1(PO4)3 (0 ~ 0.5) solid electrolyte were synthesized via solid state reaction techniques at various sintering temperature ranging from 700 to 1000 °C for 8 and 12 h respectively. Lithium carbonate (Li2CO3), titanium dioxide (TiO2), aluminium dioxide (Al2O3), tantalum oxide (Ta2O5) and ammonium dihydrogen phosphate (NH4H2PO4) of high purity grade were used as the starting material. Physical properties of LTAP and LTaAP electrolyte show bulk density of 2.83 and 3.63 g/cm3 at 900 and 800 °C sintering temperature. XRD revealed major phase of LiTi2(PO4)3 NASICON structure and secondary phases (Ti4(PO5)3, TiO2 and AlPO4) co-exist in LTAP and LTaAP samples. FTIR shows presence of NASICON phosphate peaks which were dominated with vibration of PO4 ion in all prepared LTAP and LTaAP electrolytes. This also confirms the presence of LiTi2(PO4)3 in all of the samples. The ionic conductivity of solid electrolytes was analyzed with IS at room temperature. The highest conductivity was 1.06 x 10-4 and 9.854 x 10-6 S/cm for Li1.2Ti1.8Al0.2(PO4)3 and Li1.2Ta0.9Al1.1(PO4)3 electrolytes at room temperature. Conductivity behavior is enhanced when the sample was doped with aluminium, x = 0.2 for LTAP and x = 0.1 for LTaAP composition. The high ionic conductivity of LTAP-0.2 was supported by density data and lower impurity peaks, as reported in XRD. LTAP had better conductivity behavior compared to LTaAP composition which could be due to hard nature of tantalum in the stoichiometry ratio of LTaAP compound resulted into wide grain boundary and lower its conductivity. However, from ac conductivity analysis, the conductivity values for LTAP-0.2 and LTaAP-0.1 are within the range of 0 < s < 1

Structure Processing Properties Relationships in Stoichiometric and Nonstoichiometric Oxides

The interrelation among composition, microstructure, and properties of stoichiometric and nonstoichiometric compounds is a major field of research for both scientific and technological reasons. As such, this book focuses on metal oxides, which present a large diversity of electrical, magnetic, optical, optoelectronic, thermal, electrochemical, and catalytic properties, making them suitable for a wide range of applications. By bringing together scientific contributions with special emphasis on the interrelations between materials chemistry, processing, microstructures, and properties of stoichiometric and nonstoichiometric metal oxides, this book highlights the importance of tightly integrating high-throughput experiments (including both synthesis and characterization) and efficient and robust theory for the design of advanced materials

Are lead-free relaxor ferroelectric materials the most promising candidates for energy storage capacitors?

Dielectric capacitors offer high-power density and ultrafast discharging times as compared to electrochemical capacitors and batteries, making them potential candidates for pulsed power technologies (PPT). However, low energy density in different dielectric materials such as linear dielectrics (LDs), ferroelectrics (FEs), and anti-ferroelectric (AFEs) owing to their low polarization, large hysteresis loss and low breakdown strength, respectively, limits their real time applications. Thus, achieving a material with high dielectric constant, large dielectric breakdown strength and slim hysteresis is imperative to obtain superior energy performance. In this context, relaxor ferroelectrics (RFEs) emerged as the most promising solution for energy storage capacitors. This review starts with a brief introduction of different energy storage devices and current advances of dielectric capacitors in PPT. The latest developments on lead-free RFEs including bismuth alkali titanate based, barium titanate based, alkaline niobite based perovskites both in ceramics and thin films are comprehensively discussed. Further, we highlight the different strategies used to enhance their energy storage performance to meet the requirements of the energy storage world. We also provide future guidelines in this field and therefore, this article opens a window for the current advancement in the energy storage properties of RFEs in a systematic way.This study has been partially supported by (i) DST-SERB, Govt. of India through Grant ECR/2017/000068 (KCS), (ii) UGC through grant nos. F.4-5(59-FRP)/ 2014(BSR) and (iii) Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/FIS/04650/2020 (JPBS). The author A. R. Jayakrishnan acknowledges the Central University of Tamil Nadu, India for his Ph. D fellowship. The authors acknowledge the CERIC-ERIC Consortium for access to experimental facilities and financial support under proposal 20192055