Дисертації з теми "POTASSIUM SODIUM NIOBATE ( KNN )"

Doutoramento em Ciência e Engenharia de Materiais
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.
O niobato de sódio e de potássio, K0.5Na0.5NbO3 (KNN), é o material isento de chumbo mais promissor para substituir o titanato zirconato de chumbo (PZT), que ainda é o líder de mercado utilizado para sensores e actuadores. Para tornar o KNN verdadeiramente competitivo, é necessário compreender e melhorar as suas propriedades. Esse objectivo é perseguido no presente trabalho através de diferentes abordagens, visando o estudo das propriedades intrínsecas do KNN e a subsequente identificação de estratégias apropriadas, como por exemplo a dopagem e a texturização, para desenhar melhores materiais à base de KNN para as aplicações pretendidas. Assim, no presente trabalho, fabricaram-se e caracterizaram-se cerâmicos de KNN dopado e não dopado, de KNN não estequiométrico e de KNN texturizado. Adicionalmente cresceram-se e caracterizaram-se cristais simples de KNN de elevada qualidade. Os cerâmicos de KNN (não dopado, dopado com Mn e não-estequiométrico (NST)) foram preparados pelo método convencional de mistura de óxidos, tendo-se subsequentemente medido as suas propriedades eléctricas e analisadas as respectivas estruturas e microestruturas. No caso dos cerâmicos dopados com Mn bem como no dos cerâmicos NST verificou-se existir uma estreita janela de composição monofásica associada à dopagem e não estequiometria na posição-A. Nos cerâmicos NST a variação da razão (A/B) influencia o ciclo de histerese da polarização ferroeléctrica (P-E), verificandose a obtenção de respostas dieléctricas e piezoeléctricas melhoradas para pequenos desvios da estequiometria (A/B = 0.97). No que se refere ao KNN dopado com Mn, quando comparado com o KNN não dopado cujos ciclos de histerese são não saturados, verificou-se que a dopagem no lugar B conduz a uma curva (P-E) mais saturada e a uma melhoria significativa de propriedades. Usou-se a espectroscopia de impedância para esclarecer o papel do Mn, tendo-se estabelecido uma correlação entre defeitos/transporte de carga e a resposta ferroeléctrica do K0.5Na0.5NbO3 (KNN) e do KNN dopado com Mn. À temperatura ambiente a condução eléctrica no KNN, associada ao transporte por buracos, é minimizada pela dopagem com Mn. A adição de Mn incrementa assim a resistividade do cerâmico, o que permite melhorar a saturação do ciclo (P-E). A temperaturas elevadas a condução passa a ser dominada pela movimento de lacunas de oxigénio ionizadas cuja concentração aumenta com a dopagem com Mn. Preparam-se também cristais simples de KNN recorrendo-se a um método de fluxo de alta temperatura. Usou-se um fluxo modificado com adição de B2O3 para crescer cristais a uma temperatura relativamente baixa. Caracterizou-se a qualidade química e cristalográfica dos cristais por análise de DRX, EDS e ICP. Os cristais obtidos exibiram propriedades com valores elevados, designadamente uma permitividade dieléctrica de 29,100 à temperatura de transição da fase tetragonal para fase cúbica, uma polarização remanescente 19,4 μC/cm2 e um coeficiente piezoeléctrico de 160 pC/N, valores estes superiores aos dos cerâmicos convencionais de KNN. Usou-se pela primeira vez a microscopia de força piezoeléctrica para caracterizar a estrutura de domínios dos monocristais de KNN. Foi possível observar que os cristais simples de KNN orientados segundo <001>, evidenciaram um padrão de estrutura de domínios, com domínios de 180º dispostos paralelamente e domínios de 90º dispostos em zig-zag. Com base na comparação entre cristais e cerâmicos de KNN é possível sustentar-se que a presença nos cristais simples de um tal padrão de domínios ordenados com longo alcance, ausente nos cerâmicos, é responsável pelas propriedades melhoradas dos cristais simples de KNN. Espera-se que os presentes resultados, ao estabelecerem uma relação entre o padrão de estrutura de domínios, uma espécie de impressão digital, e a resposta dielétrica e ferroelétrica dos cristais simples e ao indicarem vias para se atingirem propriedades maximizadas em materiais de KNN, venham a ter fortes implicações na expansão do uso dos materiais de KNN. Caracterizaram-se também os monocristais ferroeléctricos de KNN por espectroscopia de Raman, obtendo-se pela primeira vez evidências que permitem a atribuir a estrutura cristalina de simetria monoclínica ao KNN. As intensidades dos fonões A′ , A" e A' + A" foram calculadas teoricamente e comparadas com os dados experimentais em função da rotação de cristal, o que permitiu a determinação precisa dos coeficientes do tensor Raman para modos (non-leaking) em KNN monoclínico. No contexto da literatura este estudo confirma que a atribuição da simetria monoclínica é mais adequada do que a ortorrômbica. Este estudo constitui ainda uma base para a avaliação não-destrutiva da distribuição de domínios por espectroscopia Raman em materiais ferroelétricos isentos de chumbo, à base de KNN e com estruturas complexas. Procurando aprofundar a compreensão do comportamento eléctrico dos cristais simples e dos cerâmicos de KNN, com o objectivo de desenhar materiais com propriedades optimizadas, realizou-se um estudo comparativo ao nível de defeitos e transporte de carga, usando-se para tal a espectroscopia de impedância. Os monocristais apresentam menor condutividade do que os materiais policristalinos homólogos para temperaturas até 200 ºC ao passo que, acima desta temperatura, são os materiais policristalinos quem apresenta menor condutividade. O comportamento de baixa temperatura (T < 200 ºC) reflecte as diferentes condições de processamento dos cerâmicos e dos cristais, que são responsáveis pelo menor teor de defeitos transportadores de carga no caso dos cristais simples. À medida que a temperatura aumenta, (T > 200 ºC) os monocristais tornam-se agora mais condutores do que as amostras policristalinas nas quais as fronteiras de grão actuam como barreiras ao transporte de carga eléctrica. Para temperaturas ainda mais elevadas a diferença de condutividade entre cristais e cerâmicos é incrementada devido à contribuição da condução iónica nos cristais. Efectivamente, para a gama de temperatura elevada (T > 300 ºC),calcularam-se valores da energia de activação de 1,60 e 0,97 eV que confirmam um transporte de carga associado a condução iónica e a lacunas de oxigénio ionizadas, em cristais simples e em cerâmicos, respectivamente. Sugere-se assim que, dependendo da aplicação em em vista, os cristais, com baixo teor de defeitos e propriedades electromecânicas melhoradas serão uma escolha indicada para aplicações a temperaturas próximas da temperatura ambiente ao passo que, para altas temperaturas, os cerâmicos, menos condutores, serão a opção mais indicada. Com o objectivo de desenhar as propriedades dos materiais policristalinos de KNN na mira de um desempenho semelhante ao dos cristais simples, prepararam-se e estudaram-se as propriedades de cerâmicos de (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) com orientação (00l), usando cristais simples de KNN como partículas modelo para produzir cerâmicos texturizados por crescimento de grão modelado ( do inglês “template grain growth”). A escolha dos cristais simples como partículas modelo baseou-se no facto destas possuírem boas propriedades, aqui usadas como ferramenta indutora de melhores propriedades nos cerâmicos de KNN. A análise DRX revelou que os cerâmicos preparados com partículas modelo evidenciavam uma estrutura monoclínica à temperatura ambiente e um fator de Lotgering (f) de 40 %, o que confirma o desenvolvimento de textura cristalográfica. Estes cerâmicos texturizados apresentam um padrão de domínios ordenado com longo alcance que consiste em domínios de 90º e de 180º, semelhante ao observado nos cristais simples. Observaram-se valores elevados de constante dieléctrica (13017 na transição de fase C/T), de polarização ferroelétrica (2Pr = 42,8 μC/cm2) e de coeficiente piezoelétrico (d33 = 280 pC/N ) nos cerâmicos KNNL texturizados, quando comparados com os cerâmicos não orientados. Sugerese que esta resposta eléctrica se deve ao padrão de domínioordenados, observado nas amostras texturizadas. Os resultados obtidos, quando comparados com dados anteriormente reportados para cerâmicos de KNN texturizados confirmam a superioridade das propriedades obtidas, que se atribui à estrutura de domíneos observada. Este estudo fornece uma abordagem que pode ser de grande utilidade para a melhoria das propriedades dos cerâmicos piezoelétricos à base de KNN. Globalmente considerados, os presentes resultados configuram um importante contributo para o conjunto dos conhecimentos sobre as propriedades do niobato de sódio e de potássio: demonstrou-se que existe uma relação entre o padrão de estrutura de domínios e a resposta dieléctrica, ferroeléctrica e piezoeléctrica de cristais simples e de cerâmicos de KNN e apontou-se uma via para a melhoria das propriedades dos cerâmicos através da texturização. Prevê-se assim que este contributo tenha um impacto significativo na viabilização do uso generalizado do KNN em detrimento dos materiais à base de chumbo.

Pb-free piezoelectric materials have grown in importance through increased environmental concern related to the presence of Pb and the subsequent legislation that has arisen including directives such as Waste Electrical and Electronic Equipment (WEEE) and the Restriction of Hazardous Substances Directive (RoHS). While much progress has been made on producing Pb-free bulk materials, the need to integrate these next generation Pb-free piezoelectric materials with substrates to form functional micro devices has received less attention and raises a number of challenges. With respect to the high temperature mixed oxide synthesis method, a simple, cost effective and robust low temperature molten hydroxide synthesis (MHS) method derived from the molten salt synthesis (MSS) method, has been developed to produce K0.5Na0.5NbO3 (KNN) small grain powders and is a method that lends itself easily to industrial scale up. A powder/sol gel composite ink film forming technique has been used to produce KNN thick films on silicon substrates. Characterisation of the produced films has shown the films to exhibit piezoelectric coefficients for un-doped material in the region of 30pC/N. The work will report on the Na ion favouring mechanism of the MSS and the related mechanism of the MHS. The work will also report on the dielectric and piezoelectric characteristics of initial KNN thick films produced and an investigation into use of dopants and process modification to improve the KNN thick film’s characteristics.

Mestrado em Engenharia de Materiais
This work is about lead-free ceramic materials intended for electromechanical applications and candidates to replace lead-based electroceramics. One of the most widely used piezoelectric ceramics is lead zirconate titanate (PZT). However, it contains more than 60% of lead and it is toxic for humans and environment. In 2003, a directive from European Union has prohibited the use of potentially hazardous elements as lead. Due to the lack of competitive materials for PZT replacement an exception was created until a competitive alternative be found. Potassium and sodium niobate due to its high Curie temperature and moderate piezoelectric properties is currently one of the most promising lead-free materials for PZT substitution. However, its effective industrial adoption requires, among others, optimization of its properties. In this context, in this work we initially studied the effect of dopants, texturing and sintering temperature of KNN ceramics. For this purpose KNN ceramics doped with i) 1.5 mol% CuO + 2.0 mol% Li2O, ii) 1.5 mol% CuO + 4.0 mol% Li2O and iii) 1.5 mol% CuO + 0.5 mol% MnO using different sintering temperatures (1050, 1065 and 1080 °C) were prepared. In addition in order to maximize the preferential crystallographic orientation of the ceramic KNN (texturing), in this case in the direction (h00), KNN single crystals were produced. These crystals were used as seeds in the texturing process KNN ceramics. It was found that the composition doped with copper and manganese was the only single phase one of the studied compositions. Dense (> 95%) ceramics, textured and non-textured, and with a high Lotgering factor among the studied compositions (≈ 20%) were prepared. The dependence of the dielectric properties of the Lotgering factor was demonstrated. In the attempt to optimize the Lotgering factor to top up the piezoelectric properties, the effect of the quantity of added crystals, heating and cooling rate and duration of sintering cycle were studied for the composition doped with copper and manganese. To this end, KNN textured ceramics and doped with 1.5 mol% of CuO and 0.5 mol% MnO, using 2.5, 5.0 and 10.0 wt% of single crystals were processed. For the same composition the heating rate of 2 °C/min and 20 °C/min and sintering level between 4 and 24 h was varied. Dense single phase KNN ceramics with an increase in the Lotgering factor from ≈20% to ≈38% for KNN ceramics doped with 1.5 mol% of copper and 0.5 mol% of manganese, textured with 5 wt% crystals and sintered at 1065 °C for 24 h with a heating rate/cooling of 10 °C/min have been achieved. These ceramics exhibit a relative permittivity at room temperature ≈ 300 for a Curie temperature value which remained high (TC ≈ 400 °C). The piezoelectric coefficient increased (d33 = 65 pC/N) with increased texturing. Despite the value of the piezoelectric coefficient achieved is still modest, the obtained piezoelectric voltage constant revealed values (g33 = 23.9 * 10-3 Vm/N) comparable to the values reported for certain compositions of commercial PZT, showing clearly competitive opportunities in applications (such as piezoelectric sensors) for KNN ceramics. The results of this study definitely contribute to the knowledge in the field of lead-free piezoelectric materials.
Este trabalho é acerca de materiais cerâmicos isentos de chumbo destinados a aplicações electromecânicas e candidatos à substituição de electrocerâmicos à base de chumbo. O titanato zirconato de chumbo (PZT) é o cerâmico piezoeléctrico mais utilizado em todo o mundo. No entanto, contém mais de 60 wt% de chumbo que é um elemento tóxico para os seres humanos e para o ambiente. Em 2003, a União Europeia aprovou uma directiva proibindo e restringindo o uso de elementos potencialmente perigosos como o chumbo. Devido à inexistência de materiais aptos para a substituição do PZT, foi feita uma exceção até ser encontrado um material alternativo competitivo. O niobato de potássio e sódio (K0.5Na0.5NbO3, KNN), devido à sua elevada temperatura Curie e propriedades piezoeléctricas moderadas, é um dos materiais isentos de chumbo mais promissores para substituição do PZT. No entanto, a sua efetiva adopção industrial requer, entre outros aspectos, a optimização das suas propriedades. Neste contexto, estudou-se neste trabalho o efeito de dopantes, da temperatura de sinterização e da texturização em cerâmicos de KNN. Foram fabricados cerâmicos de KNN dopados com i) 1,5 mol% CuO + 2,0 mol% Li2O, ii) 1,5 mol%CuO + 4,0 mol% Li2O e iii) 1,5 mol% CuO + 0,5 mol% MnO e sinterizados a diferentes temperaturas (1050, 1065 e 1080 ºC). Complementarmente com o objectivo de maximizar a orientação cristalográfica preferencial dos cerâmicos de KNN (texturização), neste caso segundo a direcção (h00), foram produzidos monocristais de KNN. Estes cristais foram usados como sementes no processo de texturização de cerâmicos de KNN. Verificou-se que a composição dopada com cobre e manganês foi a única das composições estudadas que se apresentou monofásica. Foram conseguidos cerâmicos, texturizados e não texturizados, densos (> 95 %) e com factor de Lotgering mais elevado dentre as composições estudadas (≈ 20 %). Foi possível demonstrar a dependência das propriedades dieléctricas do factor de Lotgering. Na tentativa de optimizar o factor de Lotgering para majorar as propriedades piezoeléctricas, foi estudado, para a composição dopada com cobre e manganês, o efeito da quantidade de monocristais adicionada, da taxa de aquecimento e arrefecimento e da duração do patamar de sinterização. Para tal, foram processados cerâmicos de KNN texturizados e dopados com 1,5 mol% de CuO e 0,5 mol% MnO, usando 2,5 wt%, 5,0 wt% e 10,0 wt% de monocristais. Para a mesma composição foi variada a taxa de aquecimento entre 2 ºC/min e 20 ºC/min e o patamar de sinterização entre 4 e 24 h. Foram conseguidos cerâmicos densos e monofásicos e um incremento no factor de Lotgering de ≈20 % para ≈38 %, para cerâmicos de KNN dopados com 1.5 mol % de cobre e 0.5 mol % de manganês, texturizados com 5 wt% de monocristais e sinterizados a 1065 ºC por 24 h com uma taxa de aquecimento / arrefecimento de 10 ºC/min. Estes cerâmicos exibem uma permitividade relativa de ≈ 300 à temperatura ambiente, para um valor da temperatura de Curie que se manteve elevado (TC ≈ 400 ºC). O coeficiente piezoeléctrico aumentou (d33 = 65 pC/N) com o aumento de texturização. Apesar do valor do coeficiente piezoelétrico conseguido ser ainda modesto, a constante de voltagem piezoeléctrica destes cerâmicos revelou valores (g33 = 23.9*10-3 Vm/N) comparáveis com os valores apresentados por certas composições de PZT comercial, mostrando claramente oportunidades competitivas em aplicações (nomeadamente como sensores piezoeléctricos) de cerâmicos de KNN. Os resultados obtidos neste trabalho contribuem para o conhecimento na área dos materiais piezoeléctricos isentos de chumbo.

Mestrado em Engenharia de Materiais
Este trabalho é sobre materiais cerâmicos isentos de chumbo destinados a aplicações electromecânicas e candidatos à substituição de electrocerâmicos à base de chumbo. O titanato zirconato de chumbo (PZT) é o cerâmico piezoeléctrico mais utilizado em todo o mundo. No entanto, contém mais de 60 % de chumbo que é um elemento tóxico para os seres humanos e para o ambiente. Em 2003, a União Europeia aprovou uma directiva que proíbe e restringe o uso de elementos potencialmente perigosos, tais como o chumbo. Devido à inexistência de materiais aptos para a substituição do PZT, foi feita uma exceção até ser encontrado um material alternativo competitivo. O niobato de potássio e sódio (KNN), devido à sua elevada temperature de Curie e propriedades piezoeléctricas moderadas, é um dos materiais isentos de chumbo mais promissores para substituição do PZT. No entanto, a sua efetiva adopção industrial requer, entre outros aspectos, a optimização das suas propriedades. A maioria da literatura está focada em materiais cerâmicos densos com base em KNN. Recentemente, os filmes de KNN receberam bastante atenção, pois é uma das alternativas mais promissoras para várias aplicações, como por exemplo, sensores, atuadores, sistemas de colheita de energia e sistemas microelectromecânicos (MEMS). Essa atenção deve-se às altas propriedades piezoelétricas nas suas contrapartidas cerâmicas. No entanto, duas questões principais ainda impedem a fabricação de filmes de KNN de alta qualidade: tensão exercida entre o filme de KNN e o substrato e a perda de óxidos alcalinos durante a sua preparação. Neste contexto, este trabalho tem como objectivo o estudo da influência de tensões existentes nos filmes de KNN nas propriedades elétricas. Para este fim, filmes de KNN com i) 20% de excesso de potássio e sódio e uma concentração molar de 0,4; ii) 20% de excesso de potássio e sódio e concentração molar de 0,2; iii) 5% de potássio com concentração molar de 0,4 e iv) 5% de potássio e concentração molar de 0,2 foram depositados em substratos de Si/SiO2, Al2O3 policristalino, Si/SiO2/TiO2/Pt, Al2O3/Pt e SrTiO3/Pt. Verificou-se que os filmes finos de KNN têm uma estrutura perovskita sem fases secundárias. Os filmes finos de KNN com 20% de excesso de potássio e sódio depositado nos substratos de Al2O3/Pt e SrTiO3/Pt mostram uma orientação preferencial ao longo do pico (100), tendo um fator de Lottering maior que 38% (f100> 38%) Os filmes finos de KNN depositados nos substratos de Si/SiO2/TiO2/Pt encontram-se sob uma tensão de tracção, enquanto que os filmes finos de KNN depositados nos substratos de SrTiO3/Pt e Al2O3/Pt estão sob uma tensão compressiva. Entre os filmes finos de KNN com 20% de excesso de potássio e sódio e concentração de 0,2 M, o filme que apresenta a permitividade mais elevada (ε´ = 585 (10 kHz) with tanδ = 0.182) é filme depositado no subtrato de SrTiO3/Pt e o filme depositado em Si/SiO2/TiO2/Pt é o que apresenta as perdas mais baixas (ε' = 382 (10 kHz) com tanδ = 0,093). O ultimo filme, apresenta valores de polarização remanescente mais elevados (Pr = 9,57 μC/cm2 (a 50 Hz) com Ec = 36 kV/cm). Os filmes finos de KNN com 5% de excesso de potássio com uma concentração molar de 0,2 têm o Pr mais elevado nos filmes depositados nos substratos de SrTiO3/Pt (Pr = 4,55 μC/cm2 (a 50 Hz) com Ec = 34 kV/cm). Os filmes depositados em Al2O3/Pt têm a menor permitividade e polarização moderada, mas são os que mais sustentam altos campos elétricos, mostrando “loops” de histerese quadrados. As imagens de PFM mostram que os filmes finos de KNN com uma concentração molar de 0,4 depositados nos substratos de Al2O3/Pt e SrTiO3/Pt têm domínios bem definidos, com um tamanho médio que varia entre os 75 e os 100 nm, sendo separados por paredes com um domínio de 180o. Para os filmes com uma concentração molar de 0,2, são observados domínios com escala micrométrica e obtidas curvas de histerese piezoeléctricas locais. Os resultados deste estudo contribuem definitivamente para o conhecimento no campo dos materiais piezoelétricos sem chumbo.
This work is about lead-free piezoelectric materials intended for electromechanical and energy harvesting applications. One of the most widely used piezoelectric ceramics is lead zirconate titanate (PZT). However, it contains more than 60% of lead that is toxic for humans and environment. In 2003, a directive from European Union has prohibited the use of potentially hazardous elements as lead. Due to the lack of competitive materials for PZT replacement an exception was created until a competitive alternative be found. Potassium and sodium niobate (KNN) due to its high Curie temperature and moderate piezoelectric properties is currently one of the most promising lead-free materials for PZT substitution. However, its effective industrial adoption requires, among others, optimization of its properties. Most literature is focused on KNN-based bulk materials. Recently, KNN based films have received more attention as one of the promising alternatives in various applications, such as sensors, actuators, energy harvesting systems and microelectromechanical systems (MEMS). This attention is due to the high piezoelectric properties in their bulk counterparts. However, two main issues still inhibit the fabrication of high-quality KNN-based films: stress/strain exerted between the KNN film and the substrate and the loss of alkali oxides during its preparation. In this context, in this work the influence of stress/strain applied to KNN films on the electrical properties is studied. For this purpose, KNN films with i) 20% excess of potassium and sodium and 0.4 M concentration, ii) 20% excess of potassium and sodium and 0.2 M concentration, iii) 5% of potassium and 0.4 M concentration and iv) 5% of potassium and 0.2 M concentration were deposited on: Si/SiO2, polycrystalline Al2O3, Si/SiO2/TiO2/Pt, Al2O3/Pt and SrTiO3/Pt substrates. It was found that KNN thin films have a perovskite structure without secondary phases. KNN thin films with 20% excess of potassium and sodium deposited on Al2O3/Pt and SrTiO3/Pt substrates show a preferential orientation along (100) direction and have Lottering factor higher than 38% (f100 > 38%). KNN thin films deposited on Si/SiO2/TiO2/Pt substrates are found to be under a tensile strain, while the KNN films deposited on SrTiO3/Pt and Al2O3/Pt substrates are under a compressive strain. Among the KNN thin films with 20% excess of potassium and sodium and 0.2 M concentration, the film that show the highest permittivity (ε´ = 585 (10 kHz) with tanδ = 0.182) is that on SrTiO3/Pt, while the one deposited on Si/SiO2/TiO2/Pt substrate possesses the lowest losses (ε´ = 382 (10 kHz) with tanδ = 0.093). The later film shows as well the highest values of remnant polarization (Pr = 9.57 μC/cm2 (at 50 Hz) with Ec = 36 kV/cm). However, the KNN thin films with 5% excess of potassium and 0.2 M concentration that has the highest Pr is the film deposited on SrTiO3/Pt substrates (Pr = 4.55 μC/cm2 (at 50 Hz) with Ec= 34 kV/cm). The films deposited on Al2O3/Pt have the lowest permittivity and moderate polarization, but they are the most sustainable to high electric field, showing square-like hysteresis loops. The PFM images shows that the KNN thin films with 0.4 M concentration deposited on SrTiO3/Pt and Al2O3/Pt substrates have well defined domains with average size between 75 and 100 nm, separated by 180o domain walls. For the films with 0.2 M concentration micrometre scale domains are observed and local piezoelectric loops are obtained. The results of this study definitely contribute to the knowledge in the field of lead-free piezoelectric materials.

NKN doped samples, (100-x)NKN-xSBN (0 ≤ x ≤ 10) were produced using the conventional mixed oxide route with 0.45 wt% Fe2O3 sintering aid (xSBNF). After 20-24 hours mixing, samples were calcined at 850°C and sintered at 1100–1140°C (± 180°C/hour) for 4 hours. By XRD 4 mol% SBN was found to be the solubility limit for single phase structure. By SEM, second phases were visible when 2 ≤ x ≤ 4; their structure was subsequently shown to be tungsten bronze type (TBT). 2-4 SBNF samples were high density, over 96% theoretical. For x = 0, TC = 457°C, TO-T = 234°C, Pr = 22 μC/cm2 and EC = 16.5 kV/cm. TC was found to decrease by 14.7°C and TO-T by 9.0°C per 1 mol% addition SBN. 2SBNF was the optimal formulation in terms of microstructure and electrical properties, with average grain size 3 μm, Pr = 25 μC/cm2 and EC = 8.8 kV/cm, ρ = 4.7 kΩm and Q = 1.16 eV. This material comprised approximately 90% orthorhombic and 10% tetragonal phases coexisting. Pseudo-cubic lattice parameters are a’ = c’ = 3.947180 Å, and b’ = 3.999996 Å for orthorhombic phase; the tetragonal has a’ = c’ = 3.989798 Å, and b’ = 3.975777 Å.Synchrotron XRD studies were undertaken as a function of temperature on 99.5NKN-0.5CuO + 0.6 wt% Nb2O5 solid and powder samples. The data were Rietveld refined. The solid sample underwent two polymorphic phase transitions at 300°C and 515°C; the latter was between two tetragonal phases: lattice parameters for the tetragonal phase (300-520°C) were a’ = c’ = 4.99557 Å, and b’ = 4.0363 Å; high temperature tetragonal (>500°C) exhibited a’ = c’ = 4.9519 Å, and b’ = 4.4941 Å. The powder sample of the same formulation exhibited more, smaller transformations. It was only orthorhombic at temperatures <140°C with a’ = c’ = 4.10680 Å, and b’ = 4.02620 Å. Above 140°C both orthorhombic and tetragonal phases were present. Another significant transformation occurred at 360°C where the structural unit cell parameters changed significantly. Parameter lengths are provided. P-E data was characterised by Pr = 19.9 μC/cm2 and EC = 13.5 kV/cm. Synchrotron X-ray diffraction analysis of 94 NKN-6LiTaO3 showed that tetragonal phase was present at 20-390°C, although an orthorhombic phase was identified at 20-200°C and again at 340-390°C just before the cubic transition temperature at 390°C. This is a new observation for NKN. A new and simple method for tape casting was developed to reduce powder wastage, enabling thick films of 50 μm to be cast. The reactive templated grain growth (RTGG) method was employed to orient 95NKN-5LiNbO3 and 94NKN-6LiNbO3 samples; CuO was utilised as a sintering aid. Pre-cursor BNN and NN template particles were produced using the molten salt synthesis (MSS) method, using a salt to oxide ratio of 1:1. Resulting NN particles were 15 μm wide and 0.5 μm thick. Eight layered 6LN + 0.4 wt% tapes produced using 10 wt% template particles resulted in 210 μm thick tapes with 67% orientation when sintered at 1150°C. Resulting properties include TC = 440ºC and TO-T = 70ºC, 25 kΩ resistance and capacitance 21.6 pF.

Microwave materials have been widely used in a variety of applications ranging from communication devices to military satellite services, and the study of materials properties at microwave frequencies and the development of functional microwave materials have always been among the most active areas in solid-state physics, materials science, and electrical and electronic engineering. In recent years, the increasing requirements for the development of high speed, high frequency circuits and systems require complete understanding of the properties of materials function at microwave frequencies.

Ferroelectric materials usually have high dielectric constants, and their dielectric properties are temperature and electric field dependent. The change in permittivity as a function of electric field is the key to a wide range of applications. Ferroelectric materials can be used in fabrication capacitors for electronic industry because of their high dielectric constants, and this is important in the trend toward miniaturization and high functionality of electronic products. The simple tunable passive component based on ferroelectric films is a varactor which can be made as a planar structure, and electrically tunable microwave integrated circuits using ferroelectric thin films can be developed. Therefore, it is very important to characterize the dielectric constant and tunability of ferroelectric thin films.

This thesis shows experimental results for growth, crystalline properties and microwave characterization of Na0.5K0.5NbO3 (NKN), AgTa0.5Nb0.5O3 (ATN), Ba0.5Sr0.5TiO3 (BST) as well as AgTaO3 (ATO), AgNbO3 (ANO) thin films. The films were grown by Pulsed Laser Deposition (PLD) and rf-magnetron sputtering of a stoichiometric, high density, ceramic NKN, ATN, BST target onto single crystal LaAlO3(LAO), Al2O3 (sapphire), and Nd:YAlO3, and amorphous glass substrates. By x-ray diffractometry, NKN, ATN, BST films on LAO substrates were found to grow epitaxially, whereas films on r-cut sapphire substrates were found to be preferentially (00l) oriented.

Coplanar waveguide interdigital capacitor (CPWIDC) structures were fabricated by standard photolithography processing and metal lift-off technique. Microwave properties of the NKN/Sapphire and ATN/Sapphire with CPW structures were characterized using on-wafer microwave measurement technique. Measurement setup is composed of network analyzer, probe station, and microwave G-S-G probes. External electric field through the connection between network analyzer and power supply was applied to measure voltage tunability. Measured S-parameter were used for the calculation of capacitance, loss tanδ, tunability and K-factor.

The NKN films interdigital capacitors with 2 μm finger gap on Nd:YAlO3 showed superior performance compared to ATN in the microwave range from 1 to 40 GHz. Within this range, the voltage tunability (40V, 200 kV/cm) was about 29%, loss tangent ∼ 0.13, K-factor = tunability/tanδ from 152% @ 10GHz to 46% @ 40GHz.

The microwave performance of ATN film CPWIDC with 2 μm finger gap on sapphire substrate in the microwave range from 1 to 40 GHz showed that frequency dispersion is about 4.3%, voltage tunability was 4.7% @ 20GHz and 200 kV/cm, loss tangent ∼ 0.068 @ 20GHz, K-factor = tunability/tanδ is ranged from 124% @ 10GHz to 35% @ 40GHz.

The BST films CPWIDC with 2μmfinger gap on Al2O3 substrate showed frequency dispersion of capacitance in the microwave range from 1 to 40 GHz about 17%, voltage tunability = 1 - C(40V)/C(0) ∼ 22.2%, loss tangent ∼ 0.137 @ 20GHz, and K-factor = tunability/tanδ from 281% @ 10GHz to 95% @ 40GHz.

Thesis: S.M., Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 40-41).
There have been recent concerns regarding the use, recycling, and disposal of the predominantly used leaded piezoelectric ceramic- Lead Zirconate Titanate (PZT). The European Union has initiated restricting the use of lead in commercial products, with countries such as China and Japan. These regulations provide further motivation for the development of an alternative to leaded piezoelectric materials. The aim of this thesis is to characterize the more recently researched lead-free piezoelectric alternative, Sodium Potassium Niobate (KNN). Thin films of KNN ribbons with gold interconnects are microfabricated on various conformal substrates such as Kapton, Ecoflex, Polydimethylsiloxane (PDMS), and Silbione/fabric and characterized electrically using the Keithley Semiconductor Parameter Analyzer. In this initial experimental evaluation, it was found that at the frequency of 100 KHz, the dielectric constant of the KNN on PDMS is the highest at 427 followed by the Kapton at 410. The Ecoflex and Silbione/fabric both have a dielectric of about 387. In the literature, the dielectric constant of KNN is reported to fall between 185 and 598 based on the substrate it is on, and our values are well within this reported range. The results from the other electrical characterization tests indicate that the KNN behaves similarly on the different tested substrates as the capacitance, polarization curve, and leakage current of all the devices are in the same range and are close as the ribbons are swept from -40 V to 40 V.
by Atieh Sadraei.
S.M.
S.M. Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences

Gegenstand dieser Arbeit ist die experimentelle Untersuchung der Verspannungs-Temperatur-Phasenbeziehungen in epitaktischen KxNa1-xNbO3 Dünnschichten, sowie deren Zusammenhang mit ferro- und piezoelektrischen Eigenschaften. Die präsentierten Ergebnisse ermöglichen es KxNa1-xNbO3 Dünnschichten für neuartige technologische Anwendung zu optimieren. Zunächst wird eine detaillierte strukturelle Untersuchung der ferroelektrischen Domänenstruktur in epitaktischen K0.7Na0.3NbO3 Schichten auf (110) TbScO3 vorgestellt. Eine Analyse der ferroelektrischen Domänenstruktur mittels lateral aufgelöster Piezoresponse-Kraftmikroskopie (PFM) zeigt vier Arten von Superdomänen. Durch die ergänzende Untersuchung der zweidimensionalen und dreidimensionalen Abbildung des reziproken Raumes mittels hochauflösender Röntgenbeugung (HR-XRD) wird nachgewiesen, dass dieses Domänenmuster mittels monokliner Einheitszellen in einem MC Domänenmodell beschrieben werden kann. Im Anschluss an die strukturelle Untersuchung wurden die elektromechanischen Eigenschaften der KxNa1-xNbO3 Schichten auf (110) TbScO3untersucht. Mittels Doppelstrahl-Laserinterferometrie (DBLI) wurde ein makroskopischer effektiver piezoelektrischer Koeffizient von bis zu d33,f = 23 pm/V nachgewiesen. Zudem wurden Oberflächenwellen-Experimente (SAW) durchgeführt. Diese zeigten außergewöhnlich hohe Signalstärken. Um die Temperatur der ferroelektrischen Phasenübergänge gezielt einstellen zu können, wurde der Zusammenhang zwischen epitaktischer Verspannung und der Phasenübergangstemperatur untersucht. Dazu wurden KxNa1-xNbO3 Schichten mit unterschiedlicher Verspannung gewachsen. Die Änderung der Domänenstruktur und der piezoelektrischen Eigenschaften aufgrund von Temperaturänderung wurde in-situ durch temperaturabhängige PFM, HR-XRD und DBLI Messungen untersucht. Die Untersuchung zeigte, dass die Übergangstemperatur des Übergangs von der MC- in die c-Phase mit zunehmender kompressiver Verspannung kontinuierlich um mehr als 400 °C abnahm.
The subject of this thesis is the experimental investigation of the strain-temperature-phase relations in epitaxial KxNa1-xNbO3 thin films and their connection to ferro- and piezoelectric properties. This will enable the optimization of KxNa1-xNbO3 layers for novel technological devices. First, a detailed structural investigation of the ferroelectric domain structure in epitaxial K0.7Na0.3NbO3 films on (110) TbScO3 is presented. An analysis of the ferroelectric domain structure with laterally resolved piezoresponse force microscopy (PFM) reveals four types of superdomains. By complementary two-dimensional and three-dimensional high resolution X-ray reciprocal space mapping this domain pattern is proven to be describable by an MC domain structure with monoclinic unit cells. Subsequently to the structural investigation, the electromechanical properties of KxNa1-xNbO3 layers on (110) TbScO3 were investigated. Double beam laser interferometry (DBLI) revealed a macroscopic effective piezoelectric coefficient of up to d33,f = 23 pm/V. Furthermore, surface acoustic wave (SAW) experiments were performed. They exhibited extraordinary signal intensities. In order to be able to selectively tune such phase transition temperatures, the correlation between epitaxial strain and the phase transition temperature was investigated. For this purpose, KxNa1-xNbO3 films with different compressive strain conditions were grown. The change of domain structure and piezoelectric properties upon temperature variation was investigated in-situ by temperature-dependent PFM, HR-XRD and DBLI measurements. The transition temperature between the MC- and c-phase was shown to continuously decrease by more than 400 °C with increasing compressive strain.

The topic of this thesis is synthesis and preparation of lead-free pizoceramic with perovskite structure, in particular potassium-sodium niobate (KNN). In theoretical part are described possibilities of KNN synthesis and of it’s shaping and sintering. Experimental part deals with three types of synthesis described in the theretical part – solid state reaction, hydrothermal synthesis, sol-gel synthesis, and subconsequentional shaping of prepared powders by uniaxial pressing, cold isostatic pressure method, sintering (classical in oven, in some cases SPS – spark plasma sintering) of such green body and their following properties like relative density, grain size or charge constant. Preparation of KNN powder by each described method took place, followed by shaping and sintering. Best properties were shown by SPS sintered samples, which reached the highest density and lowest grain growth.

The content of this thesis is about preparation and processing of lead-free piezoceramic materials with perovskite structure. Potassium sodium niobate (KNN) powder was prepared by solid state reaction (SSR) and liquid phase reaction (sol-gel reaction). The powders were formed by uniaxial and isostatic pressing and further sintered. The density, grain size and morphology were determined on the sintered samples. The powder, synthesised by SSR and sintered in a conventional furnace, was chosen as a standard. The maximum density achieved on samples after optimization of sintering cycle was 93 %TD. The sintering optimization involved a homogenization step at 950 °C, which promotes the correct development of the phase composition and microstructure, followed by sintering at 1120 °C. The same approach and sintering cycle were used for sintering the samples, prepared by sol-gel synthesis. The maximum density of the samples prepared by sol-gel reaction and sintered in a conventional way, was 92 %TD. For further comparison, both of the synthesised powders were sintered using SPS (spark plasma sintering), which increased their final density up to 97 %TD. The approximate value of the piezoelectric coefficient d33 (pC/N) has been measured on selected SSR samples with pure phase composition ((K0,5Na0,5)NbO3). The best measured value of d33 was around 100 pC/N.

Due to the toxicity of lead, there is an urgent need to develop lead-free alternatives to replace the currently dominant lead-based piezoelectrics such as lead zirconate titanate (PZT). (Na0.5K0.5)NbO3 (NKN)-based piezoelectrics are promising because of their relatively high Curie temperatures and piezoelectric coefficients among the non-lead piezoelectrics.How ever, it is difficult to sinter. Sodium potassium niobate [(Na0.5K0.5)NbO3 or NKN], the main material system on which the research of this thesis work is centered. The challenges for lead-free materials to replace lead-based families and the approaches to improve the piezoelectric performance of lead-free systems by adding dopants to the system.

The present project dealt with the preparation of a multiferroic composite (Na0.5K0.5)NbO3-(Ni0.6Zn0.4)Fe2O4 in which the constituent materials (Na0.5K0.5)NbO3 [NKN] and (Ni0.6Zn0.4)Fe2O4 [NZFO] were prepared by coating method and combustion method respectively. These were mixed in required proportion and densified by conventional sintering process to form composite. Four compositions were prepared to form composite (1-x)(Na0.5K0.5)NbO3-x(Ni0.6Zn0.4)Fe2O4 [NKN-NZFO] ceramics, where x=0.01, 0.10, 0.15 and 0.20. XRD of the sintered and ground sample showed no reaction between individual phases and no secondary phase formation in the final product. SEM image shows that addition of NZFO significantly modifies the microstructure of the sintered sample. Grain size found to be 10-15 µm for 1% NZFO addition. The grain size reduced to 2-5 µm for higher NZFO addition. The dielectric constant and dissipation factor were studied as a function of frequency for ceramic composites

碩士
逢甲大學
材料科學與工程學系
106
This work aims at the following aspects including (a) preparation of KxNa1-xNbO3 (KNN) nanofibers via both sol-gel and electrospinning (ESP); (b) production of rGO/KNN composite nanofibers by ESP and liquid doping method; (c) four-point probe conductivity test of rGO/KNN composite nanofiber on silicon substrate; and (d) piezoelectric force microscope test of rGO/KNN composite nanofibers deposited on silicon substrate. The results show that KNN nanofibers of uniform size and density by optimzing the process parameters to form an highly oriented and aligned nanofiber structure. The rGO/KNN composite nanofibers with uniform distribution of implanted graphene were achieved by ESP and liquid doping routes for enhancing the graphene particle doping efficiency and morphological uniformity of KNN fibers. The results show that samples via liquid doping of graphene to increasing electrical conductivity. The electrospun rGO/KNN composite nanofibers examined by a piezoelectric force microscopy (PFM) indicated that the rGO/KNN composite nanofiber with graphene doping will enhance its piezoelectric perfromnace by increasing d33 coefficient from 6.36 pm/V to 18.4 pm/V. We believe that our rGO/KNN composite nanofibers are definitely of a great potential in developing novel applications of flexible electronic devices.

碩士
逢甲大學
材料與製造工程所
100
The goal of this study is to investigate the fabrication potassium sodium niobate (KxNa1-xNbO3,KNN) nanowires and nanofibers by electrospinning process (ESP) and the characterization of KNN nanowires by piezoelectric force microscopy (PFM). There are a number of key parameters involved in electrospinning process such as viscosity and surface tension of precursor solution as well as the Coulombic force during the electrospinning. The effect of molecular weight and concentration of polyvinyl alcohol (PVA) on viscosity and electrospinning performance was analyzed. The size distribution of KNN nanowires and entanglement of KNN nanofibers were adjusted by the flow rate of PVA solution, needle specifications, and applied electrical field. Furthermore, the increased concentration of KNN precursor solution achieved an even uniform size distribution of KNN nanowires, of which the precipitation of niobate salts was avoided under various post-heat treatment conditions. The precursor solution, a mixture of KNN chemical precursors and PVA polymeric solution, was transformed into the form of KNN nanowires via ESP process. The electrospun samples were tested under thermal gravimetric analyzer (TGA) to identify its on-set temperature for thermal decomposition. Both crystalline phase and microstructure of acquired KNN nanowires and nanofibers were analyzed by SEM/EDS and XRD. The piezoelectric properties of KNN products, which were manufactured by optimized process conditions, were examined by a PFM. The electrospun KNN nanowires and nanofibers were successfully obtained with much outstanding piezoleectric quality, as compared to equivalent materials made y other methods and are qualified to be a candidate material for the development of flexible electronic devices in the near future.

碩士
國立成功大學
電機工程學系碩博士班
101
In this study, the development of lead-free (1-x)(Na0.535K0.48)NbO3－xLiNbO3 (NKLN) ceramics were investigated and the phase transition behavior of material, sintering temperature and poling condition were discussed. In NKLN ceramics, it was observed that the morphotropic phase boundary (MPB) not only contented the orthorhombic and tetragonal phases, but also had the formation of monoclinic phase. The best piezoelectric properties of NKLN ceramics with kp = 42%、kt = 52% were obtained at x = 0.05. In 0.95NKN-0.05LN ceramics, the sintering temperature was reduced from 1050oC to 900oC and the excellent piezoelectric properties were obtained under sintering at 950oC. Moreover, the 0.95NKN-0.05LN ceramics sintered at 950oC for different soak times was also investigated. The maximum values of kp (48%) and kt (52 %) were obtained at the optimum soak time of 4 h. In the present study, the electric properties of ceramics were significantly by the poling conditions, including poling temperature and poling electric field. The optimum poling conditions obtained were under the poling temperature of 90oC and poling electric field of 3 kV/mm. 　　Based on the properties of ceramics above, the ceramics with high kp and kt values were chose for fabrication of single-element ultrasound transducers. The acoustic impedances of the ceramics and backing layer were calculated. The pulse/echo response of the ultrasound transducers fabricated using the (Na0.5K0.5)NbO3 and 0.95(Na0.535K0.48)NbO3- 0.05LiNbO3 ceramics were examined and the performances of these two ultrasound transducers were compared. Effects of piezoelectric properties of ceramics on the performances of ultrasound transducer were also investigated.

The present Thesis explores a few novel anode materials for both lithium-ion and sodium-ion rechargeable batteries. A series of layered metal titanium niobates have been synthesised and their electrochemical energy storage properties, ion transport, and reaction mechanisms are studied in detail. Alkali-titanium niobates such as Li-Ti-niobate (and it’s sodium counterpart) store lithium (sodium) via the conventional intercalation mechanism. Detailed experimental and theoretical investigations reveal interesting and non trivial ion transport, which are found to be strongly correlated to the electrochemical properties. Apart from intercalation, where amount of energy storage is limited by the crystal structure, energy storage via an alloying reaction is an important alternative strategy to boost specific capacities and energy densities of various battery systems. However, drastic volume changes during alloying/dealloying is detrimental for stable electrochemical function of the cell. The volume expansion problem associated with alloying anodes materials e.g. Sn for alkali-ion batteries have been tackled here via two different strategies. While one uses a flexible layered structure resulting in simultaneous intercalation and alloying process, the other approach uses a porous electrospun carbon fiber encapsulation for alloying compounds. The electrochemical properties as a function of Sn-content in a binary SnX (X: Sb) compound anode have been explicitly probed. This study provided invaluable information on alloying reaction mechanisms as well as identified the most optimum Sn-content for the long term stable battery operations. Usage of graphite as an anode in high energy density Li-ion cell has already been shown to be associated with severe safety issues. The thesis demonstrates a novel and very simple strategy to develop a stable non-carbonaceous anode for operation in the Li-ion (full) cell configuration. The thesis comprises of six chapters and a brief discussion of the content and highlights of the individual chapters are discussed below: Chapter 1 briefly reviews the different materials (mainly anodes) and storage mechanisms in the context of lithium-ion and sodium-ion rechargeable batteries. Energy storage via different mechanisms in metal-ion batteries has it’s own advantages and disadvantages. Thus, design of alternative novel materials is absolutely essential to nullify the detrimental factors associated with various storage methods leading to highly efficient and safe alkali metal-ion rechargeable battery systems. Development of materials for efficient alkali metal-ion batteries are very pertinent even today as the next generation high energy density rechargeable batteries based on metal-S/metal-O2 are still in the stages of infancy. They are far away from widespread commercialization and thus, do not pose any threat to the rechargeable alkali metal-ion batteries. This chapter discusses the importance of diffusion of ions inside the electrode materials, which essentially determines the rate capability of half/full cells. Chapter ends with discussion on galvanostatic intermittent titration technique (GITT) which has been used extensively for calculating the diffusion coefficients of the electrodes. Chapter 2 comprises of synthesis, characterization and investigation of electrochemical properties of novel Ti-based anode materials, namely Li-Ti-niobate and Na-Ti-niobate. These compounds are synthesized using a simple ion-exchange reaction from aqueous medium using KTiNbO5 (potassium titanium niobate) as the parent compound. Li-Ti-niobate and Na-Ti-niobate are tested in Li and Na-battery respectively as an anode material. The effects of Ti3+/Ti2+ redox couple in the electrochemical performances are also investigated in the case of Li-Ti-niobate by altering the working potential window of the battery. The electrochemical performances of Li-Ti-niobate are further improved by downsizing the particle size followed by carbon coating through hydrothermal carbonization method. Scheme 1: Layered structure of metal-titanium niobate. Electrochemical performance of Li-Ti-niobate in the voltage ranges (1-3) V and (0.2-2.75) V. The specific capacity of Li-Ti-niobate has been increased by downsizing the particles followed by carbon coating (cd-Li-Ti-niobate) in the voltage range (0.2-2.75) V. In Chapter 2 we investigated the electrochemical properties of Li-Ti-niobate as an anode material for Li-ion battery. In Chapter 3 we probed the ion diffusion inside the material, an important physical property that determines the possibility of battery operation at higher current densities. Layered Li-Ti-niobate shows pesudo-1-D Li+ ion diffusion, with ion transport taking place mainly along the crystallographic b-direction. Presence of line defects along crystallographic b-direction assists the diffusion to be pesudo-1-D in nature. Removal of line defects via sintering followed by studies on electrochemical properties suggests that presence of high density dislocation defects is crucial for superior rate performance of Li-Ti-niobate. Scheme 2: Preferential direction of ion diffusion in Li-Ti-niobate In the previous Chapters, the lithium ion intercalation behavior and its diffusion properties into titanium niobate layers have been investigated in detail. In Chapter 4, the same layered geometry has been explored to tackle the drastic volume expansion problem typically associated with anodes storing energy via the alloying method. Unique flexible non-carbonaceous layered host viz. M-Ti-niobate (Ti: Titanium; M: Al3+, Pb2+, Sb3+, Ba2+, Mg2+) has been designed which can synergistically store both lithium-ions and sodium-ions via simultaneous intercalation and alloying mechanisms. M-Ti-niobate is formed by ion-exchange of the K-ions, which are specifically located in the galleries between the layers formed by edge and corner sharing TiO6 and NbO6 octahedral units in the sol-gel synthesized potassium titanium niobate (KTiNbO5). The detrimental issues such as drastic volume changes (approximately 300-400%) typically associated with alloying mechanism of storage are completely tackled chemically viz. by the unique chemical composition and structure of the M-Ti-niobates. The free space between the adjustable Ti/Nb octahedral layers easily accommodates the drastic volume changes. Due to the presence of an optimum amount of multivalent alloying metal ions (50-75% of total K-ions) in the M-Ti-niobate, efficient alloying reaction takes place directly with the ions and completely eliminates any form of mechanical degradation of the electroactive particles. The M-Ti-niobate can be cycled over a wide voltage range (as low as 0.01 V) and displays remarkably stable Li+ and Na+ ion cyclability (> 2 Li+/Na+ per formula unit) for widely varying current densities over few hundreds to thousands of successive cycles. The simultaneous intercalation and alloying storage mechanisms demonstrated by the experiments is studied within the framework of density functional theory (DFT). DFT expectedly shows a very small variation in the volume of Al-titanium niobate following lithium alloying. Moreover, the theoretical investigations also conclusively endorse the occurrence of the alloying process of Li-ions with the Al-ions along with the intercalation process during discharge. The M-Ti-niobates studied here demonstrates a paradigm shift in chemical design of electrodes and will pave the way for development of multitude of improved electrodes for different battery chemistries Scheme 3: Scheme depicts the synergistic approach of charge storage in M-Ti-niobate anodes for alkali-ion rechargeable batteries. Colour changes in the layers indicate that the layers are electrochemically active. Chapter 5 mainly focuses on a fully Li-alloy based anode such as SnSb for prospective application in rechargeable Li-ion batteries. The Sn-content variation in SnSb nanoparticles confined inside electrically conducting carbon nanofiber is observed to significantly influence the electrochemical performance. It is a major challenge to minimize the detrimental effects arising as a result of drastic volume changes (≈ few hundred times) occurring during repeated alloying-dealloying of lithium with Group IV elements e.g. tin (Sn). An important design strategy is to have Sn as a component in a binary compound. SnSb, is an important example where the antimony (Sb) itself is redox active at a potential higher than that of Sn. The ability of Sb to alloy with Li reduces the Li uptake amount of Sn in SnSb compared to bare Sn. Thus, the volume changes of Sn in SnSb will expectedly be much lower compared to bare Sn leading to greater mechanical stability and cyclability. As revealed recently, complete reformation of SnSb (for molar ratio Sn:Sb= 1:1) during charging is not achieved due to loss of some fraction of Sn. Thus, molar concentration of Sn and Sb in SnSb is also absolutely important for the optimization of battery performance. We discuss here SnSb with varying compositions of Sn encapsulated inside an electrospun carbon-nanofiber (abbreviated as CF). The carbon-nanofiber matrix not only provides electron transport pathways for the redox process but also provides ample space to accommodate the drastic volume changes occurring during successive charge and discharge cycles. The systematic changes in the chemical composition of SnSb minimize the instabilities in the SnSb structure as well as replenish any loss in Sn during repeated cycling. The composition plays a very crucial role as magnitude of specific capacities and cyclability of SnSb is observed to depend on the variable percentage of Sn. SnSb-75-25-CF, which contains excess Sn, exhibits the highest specific capacity of 550 mAh g-1 after 100 cycles in a comparison with pure SnSb (1:1) anode material at current density (0.2 A/g) and shows excellent rate capability over widely varying current densities (0.2-5 A g-1). Scheme 4: Schematic depiction of lithiation and delithiation mechanism in SnSb. Bar diagram of specific capacity versus percentage of Sn present in SnSb-series of compounds. Percentage of Sn present is 0 %, 25%, 50%, 75% and 100% in Sb-CF, SnSb-25-75-CF, SnSb-50-50-CF, SnSb-75-25-CF and Sn-CF respectively. In Chapter 6 we discuss a binary mixture of two non-carbon coated electroactive compounds viz. anatase-titanium dioxide (TiO2) and vanadium pentoxide (V2O5) as a potential electrode for Li-based batteries. The binary mixture, whose components are synthesized using sol-gel methods and not carbon coated, can be reversibly cycled in the potential range (1.0-3.5) V against Li-metal. The physical mixture of the as-synthesized TiO2 and V2O5 (w/w = 1:1) provides a high specific capacity (≈ 190 mAh g-1 after 100 cycles at 100 mA g-1) and higher compared to the bare anatase-TiO2 and V2O5. Thus, this simple strategy enhances the operational potential of anatase-TiO2 by 0.5 V to 3.5 V against lithium and also nullifies greatly the complexities of carbon electronic wiring of electroactive particles. A Li-ion cell, comprising of the non-carbon coated binary mixture as anode and lithium manganese oxide (LiMn2O4) as the cathode, cycled in the potential range (0.2-3.5) V delivers a high specific capacity of nearly 80 mAh g-1 at 100 mA g-1 and is higher compared to the full cell capacities using the individual components as anodes. No signatures of SEI formation is observed from the cyclic voltammetry results. The presence of a second electroactive material may strongly suppress the SEI formation typically observed for Ti-oxide based materials when cycled to such a low potential (≈ 0.2 V). This may also account for the high percentage of reversibility and specific capacity of the full cell in this wide potential range. This simple approach enables the possibility of using Ti-oxide based anodes against the commercial intercalation cathodes without any compromise in the cell performance and also reduces the need for design of novel high voltage cathode materials. Scheme 5: Scheme shows a design strategy for improvement in specific capacity as a result of presence of an additional redox active species in the Li-ion configuration.