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Propriedades electromecânicas de materiais à base de niobato de potássio e sódio
Full text linkDoutoramento em Ciência e Engenharia de MateriaisK0.5Na0.5NbO3 (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 - 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 , 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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