Добірка наукової літератури з теми "POTASSIUM SODIUM NIOBATE ( KNN )"

Sodium-potassium Niobate (K0.4Na0.6NbO3, KNN) nanopowders were prepared by hydrothermal synthesis at the temperature range of 140-180°C for 12-48h using Nb2O5, NaOH and KOH as source materials. By means of XRD and SEM techniques, the effects of composition and hydrothermal treatment process, such as the rate of [R]/[Nb], the concentration of the alkali, the hydrothermal treatment temperature and the hydrothermal treatment time, on the microstructures and the crystallinity of alkali metals niobate were investigated in details. Results show that K0.4Na0.6NbO3 powders could be achieved by hydrothermal synthesis at the temperature range of 140-180°C with the alkalinity concentration of 2-8M. With the increase of hydrothermal reaction temperature and time, the crystallinity of KNN particles was improved. The obtained K0.4Na0.6NbO3 polycrystalline particles have rhombic structure.

A functional potassium sodium niobate/polyimide (KNN/PI) composite films were prepared in this paper. KNN fillers are well dispersed in the PI matrix without any accumulation through in situ polymerization process. The optical band baps of the hybrid films become smaller with the increase of KNN loading. The optical band baps of the films with 0-20 wt% KNN filler are estimated to be 2.61 eV, 2.57 eV, 2.52 eV, 4.29 eV, 2.35 eV respectively.

In the fields of sensors, actuators and ultrasonic gadgets, piezoelectric ceramics based on lead free KNN(1-8) had been brought due to the development in overall performance. The KNN based substances have the gain of low dielectric constants, excessive coupling coefficient and excessive mechanical electricity than lead containing ceramics that cause them to appropriate for excessive frequency transducers. The traits of KNN may be changed through appropriate dopants. (11).Here we use Bi3+ to dop with KNN to change the properties. Structural determinations of all studied solid solutions are completed through XRD and photoluminescence research had been achieved through the usage of spectrophotometer. Optical spectroscopy evaluation has been accomplished for every solid solution. Doping KNN with Bi3+ at concentrations of 0.5 wt%, 1 wt % and 1.5wt%, respectively, improves the optical properties

The motivation of this study was design, fabrication and characterization of bolt-clamped Langevin type transducers (BLT) from lead-free K0.5Na0.5NbO3 (KNN) based piezoceramics for high-power ultrasonic cutting applications. Hard and lead-free KNN piezoceramics was obtained by adding K4CuNb8O23 (KCN) together with ZnO and SnO2. Densification and high-power characteristics of KNN-KCN piezoceramics were enhanced in the presence of ZnO and SnO2. BLTs made from hard PZT4 (commercial Pb(Zr,Ti)O3) or Zn,Sn co-doped KNN-KCN piezoceramic rings (KNN-KCN-ZnSn) were modelled through ATILA finite element analysis software package. Simulated and experimentally measured impedance spectra, resonance modes and harmonic analysis results of BLTs were compared with each other. Longitudinal vibration displacement at the tip of the horns of BLTs at approximately 30 kHz was measured via photonic sensor device to compare their performances. At the end, based on the simulation and experimental results, a prototype ultrasonic cutting device was fabricated from lead-free KNN-KCN-ZnSn piezoceramic rings. Its cutting action on both plastic and ceramic materials was demonstrated for the first time. In summary, it was found that a hard KNN-KCN based lead-free piezoceramics were good potential replacements for their lead-based counterparts for commercial high-power BLT applications.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030°C for 8 h and 1100°C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100°C, the larger grains (up to 30 μm across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.

In the present study, the effect of varying the concentration of carbon nanotubes (CNTs) on the piezoelectric performance of a poly(vinylidene fluoride) (PVDF)/potassium sodium niobate (KNN)-based electrospun nanocomposite has been revealed.

Lead-free potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) piezoelectric ceramics have been densified at temperatures lower than 300 °C using atmosphere-water assisted FLASH sintering.

The effects of Nb₂O₅ and K₂Nb₄O₁₁ precursors on the morphology, composition, and piezoelectric properties of potassium sodium niobate (KNN) nanorods were investigated.

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.

Perovskites are an interesting class of materials that have tremendous applications as actuators, sensors and in photovoltaics. Lead based perovskites exhibit piezoelectricity and other interesting properties and thus have conquered the ceramic industry for a long time. Lead free piezoelectric perovskites are the need of the hour because lead based piezoceramics are toxic and a danger to the environment. There are various contenders of lead free alternatives of lead zirocnate-titnate (PZT) based ceramics including potassium-sodium niobate, barium titanate, bismuth based perovskites that exhibit similar piezoelectric and ferroelectric properties in comparison to PZT ceramics. These lead free piezoceramics and their important properties and respective applications such as sensors, transducers and actuators, is briefly explored in this chapter.

Perovskites are an interesting class of materials that have tremendous applications as actuators, sensors and in photovoltaics. Lead based perovskites exhibit piezoelectricity and other interesting properties and thus have conquered the ceramic industry for a long time. Lead free piezoelectric perovskites are the need of the hour because lead based piezoceramics are toxic and a danger to the environment. There are various contenders of lead free alternatives of lead zirocnate-titnate (PZT) based ceramics including potassium-sodium niobate, barium titanate, bismuth based perovskites that exhibit similar piezoelectric and ferroelectric properties in comparison to PZT ceramics. These lead free piezoceramics and their important properties and respective applications such as sensors, transducers and actuators, is briefly explored in this chapter.

During recent year, the photorefractive materials are attractive in many applications such as optical image processing and optical computing because they have more lower threshold power and easily realize phase conjugation Potassium sodium strontium barium niobate crystal is one of the best candidates. It has open structure and large structure tolerance for dopants with different valence There are two different ways to improve the photorefractive properties of the materials. 1) by adjusting the composition of materials and 2) doping cation ions. This paper is to describe the doping effects on the ferroelectric and photorefractive properties. The composition of potassium sodium strontium barium niobate was chosen as 92% of A-sites occupied by K, Na, Sr and Ba ions. The dopants such as Ce, Pr, Fe, Co, Ni, Cu, Ti and Mn with 0.01-0.1wt% were adopted for modifying the properties. Potassium sodium strontium barium niobate crystals were grown from platinum crucible by Czochralski method with induction heating in air atmosphere. The growth conditions are pulling direction: [001], the rate of pulling/rotation: 0.2-0.4. The size of as-grown crystal is up to ϕ 35 × 35 mm From the shape of hysteresis loops, the doping effect on ferroelectric properties of potassium sodium strontium barium niobate crystals were revealed to have three categories: expansive, stable and shrink for Fe, Cu and others, respectively After the temperature recycle from 100 to 500K, the d33 value for Cu-doped crystal does not change, which suggest that the Cu ion is a good polarization stabilizer. The self-pumped phase conjugation refractivities for Ce, Co and Cu-doped crystals are much higher than the others. They are 70% at 514.5nm for Cu-doped, 73% at 514.5nm for Co-doped and 84% at 632.8nm for Ce-doped respectively.