• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 90
  • 37
  • 16
  • 7
  • 7
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 240
  • 240
  • 41
  • 24
  • 24
  • 21
  • 19
  • 17
  • 16
  • 16
  • 14
  • 13
  • 13
  • 13
  • 12
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
71

The effect of alumina coatings on the oxidation behavior of nickel-base alloys

Enin-Okut, Edu Owominekaje 05 1900 (has links)
No description available.
72

Chemical analysis of ceramic materials by means of the emission spectrum

Calvert, Jane January 1939 (has links)
M. S.
73

ELECTROPHORETIC DEPOSITION OF ORGANIC - INORGANIC NANOCOMPOSITES

Sun, Yanchao 10 1900 (has links)
<p>Electrochemical deposition methods have been developed for the fabrication of organic - inorganic nanocomposite coatings. The methods are based on electrophoretic deposition of ceramic nanoparticles and polymers.</p> <p>EPD method has been developed for the deposition of nanostructured TiO<sub>2</sub> films using new dispersing agents. The stabilization and charging of the nanoparticles in suspensions was achieved using these organic molecules, which belong to catecholate and salicylate families. Anodic deposition was achieved using caffeic acid, 2,3-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid and 5-sulfosalicylic acid. Cathodic deposition was performed using 2,4 dihydroxycinnamic acid, p-coumaric acid and trans cinnamic acid. The deposition yield has been studied as a function of the additive concentration and deposition time. The deposition mechanism has been investigated. The fundamental adsorption mechanism is based on the complexation of metal ions at the surfaces of oxide nanoparticles. The method enabled the co-deposition of TiO<sub>2</sub> and other oxides and the formation of composite films.</p> <p>Electrophoretic deposition method has been used for the deposition of TiO<sub>2</sub> nanoparticles modified with organic dyes. Alizarin red, alizarin yellow, pyrocatechol violet and Aurintricarboxylic acid dyes were used for the dispersion and charging of TiO<sub>2</sub>. The microstructures of the nanocomposite coatings were studied. The deposition yield was investigated under a variety of conditions. Obtained results could pave the way for the fabrication of dye-sensitized TiO<sub>2</sub> films.</p> <p>EPD method has also been developed for the fabrication of (Poly[3-(3-N,N-diethylaminopropoxy)thiophene]) PDAOT-TiO<sub>2</sub>, (polyethylenimine) PEI-TiO<sub>2</sub> and PEI-hydrotalcite composite films. The microstructures of the nanocomposite coatings were studied by Scanning Electron Microscopy, Thermogravimetric Analysis, which showed the co-deposition of inorganic nanoparticles and organic polymer. Electrochemical test of the composite film has been conducted. The results showed that PEI film provided corrosion protection of the stainless steel substrates.</p> / Master of Science (MSc)
74

Estudo da influência do teor de TiB2, obtido pela reação in situ de B4C e TIC, nas propriedades mecânicas de carâmicas a base de B4C / Influence study of TiB2 content in mechanical properties of B4C ceramic based, obtained by in situ reaction of B4C and TiC

Coelho, Marcelo Luis Ramos 15 June 2012 (has links)
O carbeto de boro é um material sintético com ligações químicas essencialmente covalentes, tem um alto ponto de fusão só é sinterizável em elevada temperatura. Possui excepcional dureza, baixa densidade, resistência a abrasão, elevada velocidade sônica e boas propriedades mecânicas, características ideais para as aplicações balísticas. Tem como principal característica a alta seção de choque para nêutrons térmicos para aplicações nucleares. O presente trabalho teve por objetivo avaliar as propriedades mecânicas do carbeto de boro, pela introdução de diferentes teores de diboreto de titânio, pela reação in situ com pós de carbeto de titânio, e adição do carbono durante a sinterização, em forno resistivo sem pressão e prensagem isostática a quente dos componentes cerâmicos. Em menores temperaturas valores obtidos da densidade teórica para o carbeto de boro puro, foram alcançados com o emprego do aditivo. Os resultados obtidos na sinterização mostram a eficiência da introdução do carbeto de titânio para o aumento da densificação do material. Com percentuais de 20% de carbeto de titânio, obteve-se os máximos valores para microdureza (HV) de 35 GPa e tenacidade a fratura (KlC) de 3,16 MPa.m1/2. Comprovadamente a dificuldade de sinterização em elevadas temperaturas, para maior densificação, de componentes cerâmicos de carbeto de boro pode ser minimizada com a introdução de percentuais de carbeto de titânio. / Boron Carbide is a synthetic material with essentially covalent chemical bonds with high fusion point. The main characteristics are: it is sintered at high temperature, high hardness, low density, abrasion resistant, high sonic velocity, good mechanical properties and high neutron absorption cross section (10BxC, x>4). Those features are ideal for ballistics applications. The aim of this study was to evaluate the changes in mechanical properties of Boron Carbide with different concentration of Titanium Diboride by reaction in situ with TiC powders. The addition of carbon during sintering without pressure and hot isostatic pressing of ceramic components was studied. At low temperatures, the nearly values for the theoretical density for pure Boron Carbide were obtained only with the use of additives. In sintering, the use of TiC increased Boron Carbide density. At concentrations up to 20% of TiC, the maximum values for hardness (HV) and fracture toughness (KlC) were obtained. With the introduction of different levels of additive, the difficulty of sintering at high temperatures was minimized and the density of ceramic components was maximized.
75

Narrow linewidth tunable solid state lasers based on dye-doped sol-gel derived silica =: 窄線寬固體顔料激光的產生. / 窄線寬固體顔料激光的產生 / 窄線寬固體顔料激光的產生 / Narrow linewidth tunable solid state lasers based on dye-doped sol-gel derived silica =: Zhai xian kuan gu ti yan liao ji guang de chan sheng.

January 1995 (has links)
by Lam King Shun. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves [40-41]). / by Lam King Shun. / Acknowledgement --- p.i / Biographical Sketch --- p.ii / Abstract --- p.iii / Table of Contents --- p.iv / Chapter Chapter One --- Introduction --- p.1 / Chapter Chapter Two --- The Sol-Gel Derived Silica / Chapter 2.1 --- Introduction to the so-gel science --- p.3 / Chapter 2.2 --- The procedure of the silica formation --- p.5 / Chapter 2.3 --- During the sol-gel process --- p.7 / Chapter 2.4 --- The silica characterization --- p.12 / Chapter 2.4.1 --- Physical properties --- p.12 / Chapter 2.4.2 --- Optical properties --- p.13 / Chapter 2.5 --- Sample preparation --- p.15 / Chapter Chapter Three --- Laser Experiments / Chapter 3.1 --- Basic principle of dye laser --- p.16 / Chapter 3.2 --- Experimental procedure --- p.20 / Chapter 3.3 --- Data and discussion --- p.23 / Chapter 3.3.1 --- Spectral and temporal behaviour --- p.2 3 / Chapter 3.3.2 --- The performance of the sol-gel silica laser --- p.37 / Chapter Chapter Four --- Conclusions and Suggestions for Further Work --- p.39 / References
76

Synthesis and characterization of C-TiC bioceramics. / 碳-碳化鈦生物陶器之合成與表徵 / Synthesis and characterization of C-TiC bioceramics. / Tan-tan hua tai sheng wu tao qi zhi he cheng yu biao zheng

January 2006 (has links)
by Tang Wing Chi = 碳-碳化鈦生物陶器之合成與表徵 / 鄧詠芝. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Tang Wing Chi = Tan-tan hua tai sheng wu tao qi zhi he cheng yu biao zheng / Deng Yongzhi. / Acknowledgement --- p.i / Abstract --- p.ii / 摘要 --- p.iv / Table of contents --- p.vi / List of figures --- p.xi / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Biomorphic materials --- p.1-1 / Chapter 1.2 --- Synthesis of biomorphic ceramics --- p.1-1 / Chapter 1.2.1 --- Pyrolysis --- p.1-2 / Chapter 1.2.1.1 --- Pyrolysis conditions --- p.1-3 / Chapter 1.2.2 --- Infiltration --- p.1-4 / Chapter 1.2.2.1 --- Vapor or gas infiltration --- p.1-4 / Chapter 1.2.2.2 --- Liquid infiltration --- p.1-4 / Chapter 1.2.2.3 --- Chemical vapor infiltration (CVI) --- p.1-5 / Chapter 1.2.2.4 --- Sol-gel processing --- p.1-5 / Chapter 1.2.3 --- Sintering --- p.1-6 / Chapter 1.3 --- Potential applications of biomorphic ceramics --- p.1-7 / Chapter 1.3.1 --- Homogenous porosity structures --- p.1-7 / Chapter 1.3.2 --- Heterogeneous porosity structures --- p.1-7 / Chapter 1.4 --- Methods and raw materials --- p.1-8 / Chapter 1.4.1 --- Infiltration --- p.1-8 / Chapter 1.4.2 --- Starting materials --- p.1-8 / Chapter 1.4.2.1 --- Ti source --- p.1-8 / Chapter 1.4.2.2 --- Biotemplates --- p.1-9 / Chapter 1.5 --- Objectives and approaches --- p.1-9 / Chapter 1.6 --- Thesis layout --- p.1-10 / References --- p.1-11 / Figures --- p.1-13 / Chapter Chapter 2 --- Methodology and Instrumentation / Chapter 2.1 --- Introduction --- p.2-1 / Chapter 2.2 --- Synthesis --- p.2-1 / Chapter 2.2.1 --- Biomorphic C-TiC ceramics from dragon tree --- p.2-1 / Chapter 2.2.2 --- Biomorphic C-TiC ceramics from wool sponge --- p.2-2 / Chapter 2.3 --- Characterization methods --- p.2-3 / Chapter 2.3.1 --- Differential thermal analyzer (DTA) --- p.2-3 / Chapter 2.3.2 --- Scanning electron microscopy (SEM) --- p.2-4 / Chapter 2.3.3 --- X-Ray powder diffractometry (XRD) --- p.2-4 / Chapter 2.3.4 --- Transmission electron microscopy (TEM) --- p.2-4 / Chapter 2.3.5 --- X-ray photoelectron spectroscopy (XPS) --- p.2-6 / Chapter 2.3.6 --- Compression tests --- p.2-6 / References --- p.2-9 / Figures --- p.2-8 / Chapter Chapter 3 --- The starting materials / Chapter 3.1 --- Introduction --- p.3-1 / Chapter 3.2 --- Fresh dragon tree --- p.3-1 / Chapter 3.2.1 --- SEM results --- p.3-1 / Chapter 3.2.2 --- Discussions --- p.3-2 / Chapter 3.3 --- Fresh sea wool sponge --- p.3-2 / Chapter 3.3.1 --- SEM results --- p.3-3 / Chapter 3.3.2 --- Discussions --- p.3-3 / Chapter 3.4 --- Tyzor-LA solution --- p.3-3 / Chapter 3.4.1 --- Physical properties --- p.3-4 / Chapter 3.4.2 --- DTA results --- p.3-4 / Chapter 3.4.3 --- XRD results --- p.3-5 / Chapter 3.4.4 --- Discussions --- p.3-5 / Chapter 3.5 --- Conclusions --- p.3-6 / References --- p.3-7 / Figures --- p.3-8 / Chapter Chapter 4 --- Results and discussions of biomorphic products fabricated from dragon tree / Chapter 4.1 --- Introduction --- p.4-1 / Chapter 4.2 --- Fabrication of biomorphic C-TiC ceramics --- p.4-1 / Chapter 4.2.1 --- Biotemplates from dragon tree --- p.4-1 / Chapter 4.2.1.1 --- XRD results --- p.4-1 / Chapter 4.2.1.2 --- SEM results --- p.4-2 / Chapter 4.2.1.3 --- Discussions --- p.4-2 / Chapter 4.2.2 --- Biomorphic C-TiC ceramics --- p.4-3 / Chapter 4.2.2.1 --- XRD results --- p.4-3 / Chapter 4.2.2.1.1 --- Effects of sintering temperature --- p.4-3 / Chapter 4.2.2.1.2 --- Effects of time duration --- p.4-3 / Chapter 4.2.2.2 --- SEM results --- p.4-4 / Chapter 4.2.2.3 --- TEM results --- p.4-4 / Chapter 4.2.2.3.1 --- Sample sintered at 1400。C for 6 hours --- p.4-4 / Chapter 4.2.2.3.2 --- Sample sintered at 1200°C for 6 hours --- p.4-5 / Chapter 4.2.2.3.3 --- Sample sintered at 1100。C for 6 hours --- p.4-5 / Chapter 4.2.2.3.4 --- Sample sintered at 900°C for 32 hours --- p.4-6 / Chapter 4.2.2.4 --- XPS results --- p.4-6 / Chapter 4.2.2.5 --- Results of compression tests --- p.4-7 / Chapter 4.2.2.6 --- Discussions --- p.4-7 / Chapter 4.3 --- Biomorphic C-TiC ceramics by repeated infiltration --- p.4-10 / Chapter 4.3.1 --- XRD results --- p.4-10 / Chapter 4.3.2 --- Discussions --- p.4-10 / Chapter 4.4 --- Conclusions --- p.4-11 / References --- p.4-12 / Figures --- p.4-13 / Chapter Chapter 5 --- Results and discussions of biomorphic products fabricated from sea wool sponges / Chapter 5.1 --- Introduction --- p.5-1 / Chapter 5.2 --- Fabrication of C-TiC biomorphic ceramics with sea wool sponges --- p.5-1 / Chapter 5.2.1 --- XRD results --- p.5-1 / Chapter 5.2.2 --- SEM results --- p.5-2 / Chapter 5.2.3 --- TEM results --- p.5-3 / Chapter 5.2.4 --- Discussions --- p.5-3 / Chapter 5.3 --- Dilution of Tyzor-LA solution --- p.5-5 / Chapter 5.3.1 --- XRD results --- p.5-5 / Chapter 5.3.2 --- SEM results --- p.4-5 / Chapter 5.3.3 --- Discussions --- p.5-6 / Chapter 5.4 --- Further annealing of biomorphic C-TiC ceramics in air --- p.5-6 / Chapter 5.4.1 --- XRD results --- p.5-7 / Chapter 5.4.2 --- SEM results --- p.5-7 / Chapter 5.4.3 --- TEM results --- p.5-8 / Chapter 5.4.4 --- Discussions --- p.5-9 / Chapter 5.5 --- Conclusions --- p.5-11 / References --- p.5-12 / Figures --- p.5-13 / Chapter Chapter 6 --- Conclusions and future works / Chapter 6.1 --- Conclusions --- p.6-1 / Chapter 6.2 --- Future works --- p.6-2 / References --- p.6-4
77

Investigation on giant dielectric constant material CaCu₃Ti₄O₁₂. / 高介電常數材料CaCu₃Ti₄O₁₂χχχ / CUHK electronic theses & dissertations collection / Investigation on giant dielectric constant material CaCu₃Ti₄O₁₂. / Gao jie dian chang shu cai liao CaCu₃Ti₄O₁₂ de yan jiu

January 2010 (has links)
CaCu3Ti4O12 (CCTO), an unusual perovskite-like material, is known for extraordinarily high (often reaching 104) and relatively frequency independent dielectric constant. Recently, it has drawn a lot of attention, because of its potential applications in microelectronics and microwave devices. / In this investigation, CCTO powders were synthesized by two routes, a conventional solid-state reaction and a wet-chemistry method. Three kinds of materials, polyvinyl alcohol (PVA, an organic binder), boric oxide (B 2O3, a well-known glass former), hafnium oxide (HfO 2, a material with high dielectric constant &sim;25), were added to the pre-reacted CCTO powder and sintered into ceramics. The effects of these additives on the microstructures, or electric and dielectric properties of CCTO ceramics were investigated. In addition, CCTO thin films were also successfully prepared. The AC conductivity, impedance, and complex dielectric permittivity were used to analyze the data. These observations were well explained in terms of an internal barrier layer capacitor (IBLC) model with Maxwell-Wagner (MW) dielectric relaxation. / Yuan, Wenxiang = 高介電常數材料CaCu₃Ti₄O₁₂χχχ / 苑文香. / Adviser: S.K. Hark. / Source: Dissertation Abstracts International, Volume: 73-01, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Yuan, Wenxiang = Gao jie dian chang shu cai liao CaCu₃Ti₄O₁₂ de yan jiu / Yuan Wenxiang.
78

Formação da eflorescência em cerâmica vermelha : fatores de influência no transporte dos íons SO4 e Ca

Ferreira, Cristiano Correa January 2009 (has links)
Neste trabalho, investigou-se a migração dos íons SO4 -2 e Ca+2 em uma cerâmica vermelha aditivada com CaSO4.2H2O simulando o processo de eflorescência. Para tanto, uma massa cerâmica com tendência não patológica, à base de uma argila da região de Gravataí-RS, foi formulada com 0, 4, 8, 12 e 16% em peso de CaSO4.2H2O. No processamento cerâmico, utilizou-se a conformação por extrusão a vácuo e queima em forno elétrico tipo mufla a diferentes temperaturas (800, 850, 900 e 950ºC) com patamar de doze horas. Os corpos cerâmicos obtidos em dimensões de 70x27x9mm3 foram caracterizados quanto às propriedades físicas (absorção de água e porosidade aparente). Para avaliar a mobilidade dos íons SO4 -2 e Ca+2, foram realizados ensaios de eflorescência segundo a norma ASTMC67/2003, análise da distribuição de tamanho de poros nos corpos cerâmicos investigados, ensaios de solubilização durante os períodos de uma hora com os corpos cerâmicos imersos em água a 90°C e imersão dos corpos cerâmicos por sete, quatorze e vinte e oito dias consecutivos em temperatura ambiente. Também foram realizados testes de condutividade elétrica nos solubilizados dos corpos cerâmicos durante os períodos de uma hora e de sete dias de ensaio e avaliou-se a potencialidade desses íons na formação do fenômeno em função das variáveis temperatura e pH. Para a quantificação da eflorescência, foi desenvolvida uma nova metodologia por análise de imagem utilizando o software gráfico Image Tools versão 3.0. Os resultados obtidos permitiram identificar que a nova metodologia baseada em análise de imagens mostrou-se bastante eficiente para quantificar a eflorescência em corpos cerâmicos, diminuindo a subjetividade da avaliação visual. Constatou-se que o íon cálcio possui maior mobilidade do que o íon sulfato após sete dias de repouso. Para maiores tempos de repouso, quatorze e vinte e oito dias, a diferença entre as concentrações por solubilização dos íons Ca+2 e SO4 -2 diminui significativamente. Também foi possível identificar uma tendência (com poucas exceções): os valores da concentração do íon Ca+2 são maiores do que os da concentração de SO4 -2 para um mesmo volume total e diâmetro médio de poros e mesma quantidade de sulfato de cálcio na formulação. Da mesma forma, quando a concentração por solubilização dos íons Ca+2 e SO4 -2 é relacionada à absorção de água dos corpos cerâmicos investigados, e comparando-os com os resultados dos ensaios de eflorescência por imagem, constata-se uma similaridade entre os resultados da eflorescência e da concentração do íon Ca+2 após sete dias de repouso. Já para o íon SO4 -2, os resultados de eflorescência assemelham-se aos de quatorze dias de repouso. A formação da eflorescência está intimamente ligada à porosidade e aos índices de concentração de sal e absorção de água dos corpos cerâmicos. Ao mesmo tempo, os ensaios de solubilização investigados mostraram-se ótimos instrumentos para análise do fenômeno, permitindo, de forma bastante confiável, que se disponha de dados sobre a formação da eflorescência em corpos cerâmicos. / In this work, the displacement of SO4 -2 and Ca+2 ions in red ceramic was investigated simulating the efflorescence process. Ceramic bodies were molded (70x27x9mm3) by vacuum extrusion formulated with different contents of CaSO4.2H2O (0, 2, 4, 8, 16% in weight) and burnt at different temperatures (800, 850, 900, 950°C) for 12 hours. Microstructure was evaluated for water absorption, apparent porosity, and pore size distribution. Efflorescence was evaluated according to the ASTMC67/2003 norm. Tests of SO4 -2 and Ca+2 ion solubilizations after periods of 1 hour with the ceramic bodies immerged in hot water and periods of 7, 14 and 28 consecutive days with the ceramic bodies in cold water were carried out. Tests were also made of electrical conductivity in solubilized from ceramic bodies during one hour and during seven days and evaluated the potential of these ions to formation of efflorescence regarding the variables temperature and pH. In the efflorescence quantification, an image analysis methodology was developed using the graphic software Image Tools 3.0. Results allowed establishing a relationship between the efflorescence of the investigated ions, physical properties (water absorption and apparent porosity), pore size distribution, and solubilization. The new methodology based on the analysis of images is sufficiently efficient to quantify efflorescence in ceramic bodies, reducing the subjectivity of the visual evaluation. Calcium ion has larger mobility than sulfate ion after 7 days of rest. For longer periods of rest (14 and 28 days), the difference between the concentrations by solubilization of Ca+2 and SO4 -2 ions significantly reduces. It was also possible to identify the tendency (with few exceptions) that the concentration values of the Ca+2 ion are bigger than the concentration values of SO4 -2 for the same total volume and average diameter of pores as well as the same amount of calcium sulfate in the formularization. Likewise, when the concentration for the solubilization of the Ca+2 and SO4 -2 ions is related to water absorption of the ceramic bodies investigated as well as when we compare them with the results of the assays of efflorescence for image, a similarity between the results of the efflorescence and the concentration of the Ca+2 ion after 7 days of rest is observed. For the SO4 -2 ion, the efflorescence results are similar to 14 days of rest. The formation of efflorescence is closely linked to the porosity and to the indices of concentration of salt and water absorption contained in the products at the same time. The investigated assays of solubilization showed to be excellent instruments for analysis of the phenomenon allowing, sufficiently trustworthy, the obtaining of data on the formation of efflorescence in ceramic bodies.
79

Medidas de transporte sob pressão em materiais cerâmicos / Transport measurement under pressure in ceramic materials

Solange de Andrade 06 October 2008 (has links)
Uma célula de pressão do tipo pistão-cilindro fabricada com BeCu foi utilizada na realização de medidas de transporte eletrônico (resistividade elétrica ½) sob pressão hidrostática P e como função da temperatura T. O procedimento de montagem do arranjo experimental compreendeu: (i) elaboração de novos acessórios para a célula de pressão, (ii) calibração do sensor de temperatura e (iii) calibração do sensor de pressão. A montagem e calibração da célula de pressão foram realizadas com sucesso e o resultado desse trabalho de instrumentação é que a referida célula pode operar no intervalo de temperatura compreendido entre 77 e 350 K e sob pressões tão altas quanto » 9.0 kbar. Uma característica dos procedimentos utilizados é que a determinação dos parâmetros termodinâmicos T e P é realizada de maneira precisa, de tal forma que P pode ser inferida exatamente em qualquer valor de temperatura. Para a realização de um estudo sistemático das propriedades de transporte eletrônico utilizando a referida célula de pressão, uma família de óxidos de níquel foi escolhida. Os compostos cerâmicos Nd1¡xEuxNiO3; x = 0, 0.10, 0.15, 0.25, 0.30 e 0.35; apresentam uma transição de fase metal-isolante MI em um amplo intervalo de temperatura 200 T 350 K. Além disso, esses compostos exibem uma histerese térmica entre as medidas de aquecimento e resfriamento, que caracteriza a transição de fase MI como sendo de primeira ordem. Por outro lado, a amostra com x =0.35 não apresenta histerese térmica e, por isso, a transição de fase MI é caracterizada como sendo de segunda ordem. Em especial, esses compostos quando submetidos a variações de pressão hidrostática apresentam diferentes magnitudes da temperatura de transição de fase metal-isolante TMI, exibindo forte dependência dos parâmetros termodinâmicos P e T. Um estudo sistemático acerca da característica da transição de fase MI sob variação dos parâmetros termodinâmicos P e T foi realizado via medidas de resistividade elétrica sob pressão e como função da temperatura ½(P; T). Os resultados obtidos são utilizados no mapeamento da dependência de TMI como função de P para cada material. O diagrama de fases TMI versus P evidencia duas regiões: uma isolante e outra metálica, para todos os compostos estudados. Ainda acerca do diagrama de fases, a relação dTMI dP foi estimada e valores de » - 4.4 K/kbar foram obtidos para x < 0.15, ou seja, para os compostos com x = 0, 0.10 e 0.15, o valor de dTMI dP foi observado ser praticamente constante. Esses compostos encontram-se no limite de diluição (x 0.165), onde as propriedades dos materiais são alteradas de maneira suave e, portanto, similares às da matriz NdNiO3. Contudo, os compostos com x > 0.15 possuem taxas de variação dTMIdP crescentes como função do aumento da substituição de Nd por Eu e preservam a ocorrência de uma histerese térmica, mas com intervalos ¢T muito menos pronunciados (da ordem de alguns K). No caso da amostra com x = 0.35 a histerese térmica não é observada à pressão atmosférica, mas ocorre de forma sistemática com a aplicação de P. Logo, a aplicação de P e/ou substituição de Nd por Eu podem alterar o caráter da transição de fase de segunda para primeira ordem, ou vice-versa, nessa família de óxidos. / A piston-cylinder self-clamping pressure cell made of BeCu was used to perform electronic transport measurements (electrical resistivity ½) as a function of temperature T and under hydrostatic pressure P. The procedure for assembly of the experimental arrangement included: (i) development of new accessories for the cell pressure, (ii) temperature sensor calibration and (iii) pressure sensor calibration. The assembly and calibration of the pressure cell were performed successfully, and the result of this experimental work is that of this cell can operate in a range of temperature between 77 and 350 K and under pressures as high as » 9.0 kbar. A feature of the procedures developed is that determination of thermodynamics parameters T and P is performed accurately, so that P can be determined exactly at any temperature. To perform a systematic study of the electronic transport properties using the self-clamped cell, a family of nickel oxides was chosen. The ceramic compounds Nd1¡xEuxNiO3; x = 0, 0.10, 0.15, 0.25, 0.30 e 0.35; exhibit a metal-insulator MI phase transition in a broad range of temperature 200 T 350 K. Besides, these compounds display a thermal hysteresis between measures of heating and cooling, which characterizes the MI transition as a first order phase transition. Except to x = 0.35 that does not show the thermal hysteresis and, therefore, the MI phase transition is characterized as of the second order. In particular, this compound when submitted to variations of hydrostatic pressure have different magnitude of the metal-insulator phase transition temperature TMI showing strong dependence of thermodynamics parameters P and T. The systematic study regarding the properties of the MI phase transition subject to changes in thermodynamic parameters P and T was done by measurements of electrical resistivity under pressure and as a function of temperature ½(P; T). The results are used to map the dependence of TMI as a function of P for each material. The phase diagram of TMI versus P shows a broadenring line that exhibits two regions: one insulating and other metallic, to all compounds studied. Also on the phase diagram, the ratio of dTMI dP was estimated values of » -4.4 K/kbar were obtained for x < 0.15, or for compounds with x = 0, 0.10 and 0.15, the value of dTMIdP was observed to be almost constant. These compounds are at the limit of dilution (x 0.165), where properties of the materials are changed softly and therefore similar to the matrix NdNiO3. On the other hand, the compounds with x > 0.15 have rates of change dTMIdP growing as a function of increase the replacement of Nd by Eu and preserve the occurrence of a thermal hysteresis, but with intervals T much less pronounced (the order of some K). In the case of sample with x = 0.35 the thermal hysteresis is not observed at atmospheric pressure, but occurs systematically under aplied P. Hence, the application of P and/or replacement of Nd by Eu can change the character of the phase transition from second to first order, or vice versa, in this family of oxides.
80

Medidas de transporte sob pressão em materiais cerâmicos / Transport measurement under pressure in ceramic materials

Andrade, Solange de 06 October 2008 (has links)
Uma célula de pressão do tipo pistão-cilindro fabricada com BeCu foi utilizada na realização de medidas de transporte eletrônico (resistividade elétrica ½) sob pressão hidrostática P e como função da temperatura T. O procedimento de montagem do arranjo experimental compreendeu: (i) elaboração de novos acessórios para a célula de pressão, (ii) calibração do sensor de temperatura e (iii) calibração do sensor de pressão. A montagem e calibração da célula de pressão foram realizadas com sucesso e o resultado desse trabalho de instrumentação é que a referida célula pode operar no intervalo de temperatura compreendido entre 77 e 350 K e sob pressões tão altas quanto » 9.0 kbar. Uma característica dos procedimentos utilizados é que a determinação dos parâmetros termodinâmicos T e P é realizada de maneira precisa, de tal forma que P pode ser inferida exatamente em qualquer valor de temperatura. Para a realização de um estudo sistemático das propriedades de transporte eletrônico utilizando a referida célula de pressão, uma família de óxidos de níquel foi escolhida. Os compostos cerâmicos Nd1¡xEuxNiO3; x = 0, 0.10, 0.15, 0.25, 0.30 e 0.35; apresentam uma transição de fase metal-isolante MI em um amplo intervalo de temperatura 200 T 350 K. Além disso, esses compostos exibem uma histerese térmica entre as medidas de aquecimento e resfriamento, que caracteriza a transição de fase MI como sendo de primeira ordem. Por outro lado, a amostra com x =0.35 não apresenta histerese térmica e, por isso, a transição de fase MI é caracterizada como sendo de segunda ordem. Em especial, esses compostos quando submetidos a variações de pressão hidrostática apresentam diferentes magnitudes da temperatura de transição de fase metal-isolante TMI, exibindo forte dependência dos parâmetros termodinâmicos P e T. Um estudo sistemático acerca da característica da transição de fase MI sob variação dos parâmetros termodinâmicos P e T foi realizado via medidas de resistividade elétrica sob pressão e como função da temperatura ½(P; T). Os resultados obtidos são utilizados no mapeamento da dependência de TMI como função de P para cada material. O diagrama de fases TMI versus P evidencia duas regiões: uma isolante e outra metálica, para todos os compostos estudados. Ainda acerca do diagrama de fases, a relação dTMI dP foi estimada e valores de » - 4.4 K/kbar foram obtidos para x < 0.15, ou seja, para os compostos com x = 0, 0.10 e 0.15, o valor de dTMI dP foi observado ser praticamente constante. Esses compostos encontram-se no limite de diluição (x 0.165), onde as propriedades dos materiais são alteradas de maneira suave e, portanto, similares às da matriz NdNiO3. Contudo, os compostos com x > 0.15 possuem taxas de variação dTMIdP crescentes como função do aumento da substituição de Nd por Eu e preservam a ocorrência de uma histerese térmica, mas com intervalos ¢T muito menos pronunciados (da ordem de alguns K). No caso da amostra com x = 0.35 a histerese térmica não é observada à pressão atmosférica, mas ocorre de forma sistemática com a aplicação de P. Logo, a aplicação de P e/ou substituição de Nd por Eu podem alterar o caráter da transição de fase de segunda para primeira ordem, ou vice-versa, nessa família de óxidos. / A piston-cylinder self-clamping pressure cell made of BeCu was used to perform electronic transport measurements (electrical resistivity ½) as a function of temperature T and under hydrostatic pressure P. The procedure for assembly of the experimental arrangement included: (i) development of new accessories for the cell pressure, (ii) temperature sensor calibration and (iii) pressure sensor calibration. The assembly and calibration of the pressure cell were performed successfully, and the result of this experimental work is that of this cell can operate in a range of temperature between 77 and 350 K and under pressures as high as » 9.0 kbar. A feature of the procedures developed is that determination of thermodynamics parameters T and P is performed accurately, so that P can be determined exactly at any temperature. To perform a systematic study of the electronic transport properties using the self-clamped cell, a family of nickel oxides was chosen. The ceramic compounds Nd1¡xEuxNiO3; x = 0, 0.10, 0.15, 0.25, 0.30 e 0.35; exhibit a metal-insulator MI phase transition in a broad range of temperature 200 T 350 K. Besides, these compounds display a thermal hysteresis between measures of heating and cooling, which characterizes the MI transition as a first order phase transition. Except to x = 0.35 that does not show the thermal hysteresis and, therefore, the MI phase transition is characterized as of the second order. In particular, this compound when submitted to variations of hydrostatic pressure have different magnitude of the metal-insulator phase transition temperature TMI showing strong dependence of thermodynamics parameters P and T. The systematic study regarding the properties of the MI phase transition subject to changes in thermodynamic parameters P and T was done by measurements of electrical resistivity under pressure and as a function of temperature ½(P; T). The results are used to map the dependence of TMI as a function of P for each material. The phase diagram of TMI versus P shows a broadenring line that exhibits two regions: one insulating and other metallic, to all compounds studied. Also on the phase diagram, the ratio of dTMI dP was estimated values of » -4.4 K/kbar were obtained for x < 0.15, or for compounds with x = 0, 0.10 and 0.15, the value of dTMIdP was observed to be almost constant. These compounds are at the limit of dilution (x 0.165), where properties of the materials are changed softly and therefore similar to the matrix NdNiO3. On the other hand, the compounds with x > 0.15 have rates of change dTMIdP growing as a function of increase the replacement of Nd by Eu and preserve the occurrence of a thermal hysteresis, but with intervals T much less pronounced (the order of some K). In the case of sample with x = 0.35 the thermal hysteresis is not observed at atmospheric pressure, but occurs systematically under aplied P. Hence, the application of P and/or replacement of Nd by Eu can change the character of the phase transition from second to first order, or vice versa, in this family of oxides.

Page generated in 0.1014 seconds