Cermetos à base de zircónia sintetizada por detonação de emulsão (EDS)

mestrado em Ciência e Engenharia de MateriaisA realização deste trabalho tem como objetivo a produção e caracterização de compósitos de metal em matriz cerâmica (CERMETOS). Assim sendo, e tendo como base pós cerâmicos e compósitos produzidos pela INNOVNANO, foram também produzidos pós por mecanossíntese. Os pós à base de zircónia estabilizada com ítria (YSZ), produzidos por estes dois métodos, foram caracterizados cristalográfica, química, morfológica, reológica, térmica e magneticamente. Os pós foram compactados e sinterizados em vácuo, seguindo-se a sua caracterização, principalmente estrutural, microestrutural, mecânica e magnética. Todos os resultados foram analisados com olhar crítico e tentando relacionar as propriedades físicas e químicas dos pós e compactos verdes com as propriedades finais dos sinterizados. A comparação entre cerâmicos e compósitos (quer pós, quer sinterizados) foi sempre o principal objetivo deste trabalho. Os resultados incluem a avaliação da importância das várias etapas de preparação dos pós, essencialmente no processo produtivo da INNOVNANO, a avaliação da estabilização da fase tetragonal da zircónia, a resposta magnética e a interpretação do decréscimo nas propriedades mecânicas verificado nos compósitos.This work main goal is to develop metal composites on a ceramic matrix, designated as CERMETs. Hence, and having as starting materials the ceramic and composite powders produced by INNOVNANO, mechanosynthesis powders were also prepared. The yttria stabilized zirconia (YSZ) based powders (produced by those two methods) were crystallographic, chemically, morphologically, rheological, thermal and magnetically characterized. Pressed compacts from the previous powders were prepared and sintered in vacuum conditions, followed by their characterization namely in terms of structure, microstructure, mechanical and magnetic behavior. The results were analyzed with a critical mind, trying to co-relate the physical and chemical properties of the powders and green compacts with the final sintered properties. The comparison between ceramic and composites (either powders, either sintered compacts) was always the main goal during the development of this work. The results include the evaluation of the meaning and importance of the several powders preparation steps that are conducted in INNOVNANO’s producing method, the importance of the tetragonal zirconia phase stabilization, the magnetic response and the interpretation of the decrease in mechanical resistance verified in CERMETs

Atmosphere assisted FLASH sintering of KNN

The use of FLASH alternative sintering technique allows a significant decrease in sintering time and temperature, contributing to the sustainable processing of high sintering temperature piezoelectrics. This is the case of potassium sodium niobate, K0.5Na0.5NbO3 (KNN), a relevant lead-free piezoelectric, which, due to alkali evaporation, is difficult to produce by conventional sintering, at T \u3e 1100 ºC. Please click Additional Files below to see the full abstract

Flash Sintering as a Route to Produce Lead-Free Piezoelectric KNN

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Dielectric behavior of FLASH sintered KNN

The market for lead-based piezoceramics, mainly (Pb1-x ZrxTiO3, PZT) - based materials, is higher than $100 billion per year. Due to lead-toxicity, the demand for lead-free piezoceramics is increasing. Potassium Sodium Niobate solid solutions, namely K0.5Na0.5NbO3, KNN, is currently one of the most promising materials for electromechanical applications. However, monophasic conventionally sintered KNN is hard to obtain, due to alkali evaporation during sintering (T\u3e 1100 ºC, t \u3e 2h). Within this context, there is an increasing interest in sustainable sintering techniques, as FLASH, to decrease both sintering time and temperature, avoiding alkali vaporization. However, FLASH applied to bulk ceramics, frequently produces inhomogeneous specimens. Figure 1 – Variation of length with temperature of FLASH sintered KNN, after a 2 h isothermal step. SEM micrograph showing the uniformly dense microstructure. In this work, we propose an experimental approach that allows the production of homogeneous, highly dense, KNN. In this method, the use of FLASH sintering contributed to reduce KNN sintering temperature for more than 200 ºC and the cycle time in ~3h. Uniform densification was achieved by using an isothermal step before the application of the electric field. Scanning Electron Microscopy (SEM) and Specific Surface Area (SSA) measurements were performed to characterize the pre-FLASH sintering microstructure. Please click Additional Files below to see the full abstract

Behind the High Electrical Performance of Flash Sintered Potassium Sodium Niobate Piezoelectric Ceramics

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Modeling of Joule heating in KNN FLASH sintering

In this work, we propose the use of FLASH sintering as an alternative technique to densify Potassium Sodium Niobate, K0.5Na0.5NbO3, KNN, a piezoceramic with relevant promising applications and a possible viable substitute of lead zirconate titanate based compositions (Pb1-x ZrxTiO3, PZT). We aim to increase this material performance by densifying KNN ceramics without secondary phase segregation. Furthermore, FLASH will contribute to a more sustainable processing of piezoelectrics as lead-free ceramics at reduced sintering temperature and time. Please click Additional Files below to see the full abstract

Particle characteristics’ influence on FLASH sintering of potassium sodium niobate: A relationship with conduction mechanisms

Funding Information: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate, co-financed by FEDER under the PT2020 Partnership Agreement. This work was also financed by Portugal 2020 through European Regional Development Fund (ERDF), in the frame of Operational Competitiveness and Internationalization Programme (POCI), in the scope of the project "FLASH sintering of lead-free functional oxides towards sustainable processing of materials for energy and related applications-FLASH", POCI-01-0247-FEDER-029078. Ricardo Serrazina acknowledges FCT for financial support (SFRH/PD/BD/128411/2017).The considerable decrease in temperature and time makes FLASH sintering a more sustainable alternative for materials processing. FLASH also becomes relevant if volatile elements are part of the material to be processed, as in alkali‐based piezoelectrics like the promising lead‐free K0.5Na0.5NbO3 (KNN). Due to the volatile nature of K and Na, KNN is difficult to process by conventional sintering. Although some studies have been undertaken, much remains to be understood to properly engineer the FLASH sintering process of KNN. In this work, the effect of FLASH temperature, TF, is studied as a function of the particle size and impurity content of KNN powders. Differences are demonstrated: while the particle size and impurity degree markedly influence TF, they do not significantly affect the densification and grain growth processes. The conductivity of KNN FLASH‐sintered ceramics and KNN single crystals (SCs) is compared to elucidate the role of particles’ surface conduction. When particles’ surfaces are not present, as in the case of SCs, the FLASH process requires higher temperatures and conductivity values. These results have implications in understanding FLASH sintering towards a more sustainable processing of lead‐free piezoelectrics.publishersversionpublishe

High Frequency Characterization and Nonlinear Analysis of BST Thick Films Produced by Electrophoretic Deposition

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Exploitation of industrial application of FLASH to sinter ceramics

FLASH is an electric field-assisted sintering technique recently proposed to densify materials in a more sustainable, energy reductive and cost-effective way than conventional sintering (CS). FLASH sintering promotes the densification of materials by using a combination of temperature and electric field. The use of electric field allows a decrease in the sintering temperature, and as important as well, in the sintering cycle duration. The advantages of FLASH, when compared with other field-assisted techniques, are: low investment, no need for specific atmosphere and dies, and specimen shape versatility. Please click Additional Files below to see the full abstract

Sinterização flash de óxidos de perovesquites sem chumbo para o processamento sustentável de materiais para aplicações em energia

Piezoelectrics as K0.5Na0.5NbO3 (KNN) have currently an emerging importance due to their lead-free nature and high transition temperature, which permits a wide range of high-tech applications as sensors, actuators, energy harvesters, biosensors, etc. However, monophasic dense KNN products are yet difficult to obtain, due to the high temperature and long time of conventional sintering processes. This PhD proposes a new method to densify materials abruptly above a threshold condition using FLASH sintering, where the densification occurs by a combination of furnace environment (temperature and/or atmosphere) and electrical field directly applied to the specimen. There are several proposed mechanisms for FLASH. Joule heating is the most reported one, but also defectrelated theories have been proposed. The phenomena are not yet completely understood, but most probably, FLASH sintering is a combination of both effects, with particle surfaces energy and conductivity performing a significant role. The present work aims to exploit FLASH for sintering of KNN ceramics but also to depict the fundamental phenomena behind FLASH sintering, and specifically, FLASH sintering of KNN. The ultimate goal is to develop sintering of ceramics towards room temperature, contributing to the energy economy low thermal budget of ceramic industry. The use of Finite Element Modelling (FEM) tools allowed to study the particle orientation effect on the Joule heating during FLASH, while the simulated temperature gradients were used to explain the presence of FLASH sinteringinduced stresses in dense ceramics. The production of different size and purity KNN powders permitted to establish the link between FLASH temperature (TF) and particle size/purity. Following, the establishment of an engineered thermal cycle before the application of the electric field for the FLASH was responsible for increasing the final densification of KNN ceramics to 95%. The link between FLASH parameters, as current density and holding time, was determined and the relationship with final density and grain size of ceramics studied. TEM and FEM studies allowed to propose a FLASH sintering mechanism for KNN, in which the current flow through particles’ surfaces promotes the partial melting of contacts and the particle sliding towards pore removal and compact densification. To allow a significant decrease on the TF of KNN, atmosphere-assisted FLASH sintering (AAFS) was presented, and the temperatures were decreased to TF ≈ 265 ºC, however, the final densification was limited to 79%. The ferroelectric and dielectric performance of FLASH sintered KNN was studied and compared with that of conventionally sintered ceramics. Similar performance was attained after a heat treatment for electrode cure; however, a detailed analysis revealed FLASH sintering-fingerprints in both as-sintered and heattreated ceramics. This work presents a clear contribution for the development of FLASH sintering in ceramics, namely in piezoelectric KNN.Piezoelétricos como o K0.5Na0.5NbO3 (KNN) têm uma importância emergente devido à sua natureza livre de chumbo e variada aplicabilidade em componentes como sensores, atuadores, dispositivos de recolha de energia, biossensores, etc. No entanto, o KNN monofásico continua a ser difícil de produzir devido à elevada temperatura e tempo associados ao processo de sinterização convencional. Este doutoramento propõe a utilização de um método alternativo de densificação, a sinterização FLASH, que acima de uma condição limite promove a densificação repentina de cerâmicos por uma combinação de ambiente do forno (atmosfera e/ou temperatura) com a aplicação de campo elétrico diretamente no material. Existem vários mecanismos reportados para explicar a sinterização FLASH. O aquecimento por efeito de Joule é um dos mais reportados e aceites, mas também têm sido sugeridos mecanismos envolvendo a criação e movimento de defeitos por efeito do campo elétrico. Uma compreensão clara do fenómeno continua por ser apresentada, mas muito provavelmente a sinterização por FLASH resulta duma combinação destes dois efeitos, sendo que a energia e condutividade das superfícies das partículas desempenham um papel fundamental. Este trabalho pretende explorar a sinterização por FLASH de cerâmicos, mas também estudar os seus fenómenos fundamentais, mais especificamente, na sinterização FLASH de KNN. O objetivo último deste trabalho é o desenvolvimento de processos de sinterização de cerâmicos que operem à temperatura ambiente, contribuindo para a economia energética e sustentabilidade da indústria cerâmica. A utilização de ferramentas de Modelação por Elementos Finitos (MEF, ou FEM) permitiu estudar o efeito da orientação das partículas na geração de calor por efeito de Joule durante o FLASH, enquanto a modelação da distribuição temperatura local e respetivos gradientes térmicos foram usados para explicar tensões induzidas em cerâmicos densos. A produção de pós de KNN com diferentes tamanhos e pureza permitiu estabelecer a sua relação com a temperatura de FLASH (TF). Em consequência, o estabelecimento de um ciclo térmico apropriado, antes da aplicação do campo elétrico, permitiu obter cerâmicos de KNN com densidade relativa de 95%. A ligação entre os parâmetros de FLASH, como densidade de corrente e tempo, foi determinada, e a relação com a densidade final e tamanho de grão dos cerâmicos foi estudada. Estudos em TEM e FEM permitiram propor um mecanismo para a sinterização por FLASH de KNN, em que o fluxo de corrente pelas superfícies das partículas promove uma fusão parcial nos seus contactos e o rearranjo para a remoção de poros e densificação do compacto. De forma a permitir um decréscimo acentuado na TF do KNN, a sinterização FLASH assistida por atmosfera foi apresentada, e a temperatura foi diminuída para TF ≈ 265 ºC. No entanto, a densificação final foi limitada aos 79%. As propriedades ferroelétricas e dielétricas do KNN sinterizado por FLASH foram estudadas e comparadas com as de cerâmicos sinterizados convencionalmente. Um desempenho semelhante entre ambos foi obtido após um tratamento térmico para cura de elétrodos. No entanto, uma análise detalhada mostrou que as propriedades são afetadas pelo processo de FLASH em cerâmicos tratados ou não termicamente. Este trabalho apresenta uma contribuição clara no desenvolvimento da sinterização FLASH de cerâmicos, especificamente, no piezoelétrico KNN.Programa Doutoral em Materiais e Processamento Avançado