This thesis presents research of two novel magnetic materials, LiHoF4 and Tb2Ti2O7. Experiments were performed at low temperatures and in an applied magnetic field to study thermal expansion and magnetostriction using a capacitive dilatometer designed during this project. This thesis presents 3 distinct topics. This manuscript begins with a thermodynamic description of thermal expansion and magnetostriction. The design of a capacitive dilatometer suitable for use at ultra-low temperatures and in high magnetic fields is presented. The thermal expansion of oxygen free high conductivity copper is used as a test of the absolute accuracy of the dilatometer. The first material studied using this dilatometer was LiHoF4. Pure LiHoF4 is a dipolar coupled Ising ferromagnet and in an applied transverse magnetic field is a good representation of the transverse field Ising model. An ongoing discrepancy between theoretical and experimental work motivates further study of this textbook material. Presented here are thermal expansion and magnetostriction measurements of LiHoF4 in an applied transverse field. We find good agreement with existing experimental work. This suggests that there is some aspect of LiHoF4 or the effect of quantum mechanical fluctuations at finite temperatures which is not well understood. The second material studied is the spin liquid Tb2Ti2O7. Despite theoretical predictions that Tb2Ti2O7 will order at finite temperature, a large body of experimental evidence demonstrates that spins within Tb2Ti2O7 remain dynamic to the lowest temperatures studied. In addition Tb2Ti2O7 also exhibits anomalous thermal expansion below 20K, giant magnetostriction, and orders in an applied magnetic field. Thermal expansion and magnetostriction measurements of Tb2Ti2O7 are presented in applied longitudinal and transverse fields. Zero-field thermal expansion measurements do not repeat the previously observed anomalous thermal expansion. A large feature is observed in thermal expansion at 100mK, in rough agreement with existing experimental work. Longitudinal and transverse magnetic fields were applied to Tb2Ti2O7. Longitudinal magnetostriction measurements show qualitatively di erent behavior than previous observations. These measurements were taken along di erent crystal axes so direct comparison cannot be made. Thermal expansion measurements in an applied transverse field show evolution with the strength of the applied field. This evolution may relate to an ordering transition, however difficulties in repeatability in a transverse field require that these results be repeated in an improved setup.

This thesis presents research of two novel magnetic materials, LiHoF4 and Tb2Ti2O7. Experiments were performed at low temperatures and in an applied magnetic field to study thermal expansion and magnetostriction using a capacitive dilatometer designed during this project. This thesis presents 3 distinct topics. This manuscript begins with a thermodynamic description of thermal expansion and magnetostriction. The design of a capacitive dilatometer suitable for use at ultra-low temperatures and in high magnetic fields is presented. The thermal expansion of oxygen free high conductivity copper is used as a test of the absolute accuracy of the dilatometer. The first material studied using this dilatometer was LiHoF4. Pure LiHoF4 is a dipolar coupled Ising ferromagnet and in an applied transverse magnetic field is a good representation of the transverse field Ising model. An ongoing discrepancy between theoretical and experimental work motivates further study of this textbook material. Presented here are thermal expansion and magnetostriction measurements of LiHoF4 in an applied transverse field. We find good agreement with existing experimental work. This suggests that there is some aspect of LiHoF4 or the effect of quantum mechanical fluctuations at finite temperatures which is not well understood. The second material studied is the spin liquid Tb2Ti2O7. Despite theoretical predictions that Tb2Ti2O7 will order at finite temperature, a large body of experimental evidence demonstrates that spins within Tb2Ti2O7 remain dynamic to the lowest temperatures studied. In addition Tb2Ti2O7 also exhibits anomalous thermal expansion below 20K, giant magnetostriction, and orders in an applied magnetic field. Thermal expansion and magnetostriction measurements of Tb2Ti2O7 are presented in applied longitudinal and transverse fields. Zero-field thermal expansion measurements do not repeat the previously observed anomalous thermal expansion. A large feature is observed in thermal expansion at 100mK, in rough agreement with existing experimental work. Longitudinal and transverse magnetic fields were applied to Tb2Ti2O7. Longitudinal magnetostriction measurements show qualitatively di erent behavior than previous observations. These measurements were taken along di erent crystal axes so direct comparison cannot be made. Thermal expansion measurements in an applied transverse field show evolution with the strength of the applied field. This evolution may relate to an ordering transition, however difficulties in repeatability in a transverse field require that these results be repeated in an improved setup.

The work presented in this thesis detail the measurement of the thermal expansion coefficient of three magnetic spin materials. Thermal expansion coefficient values were measured by capacitive dilatometry in several key low (T < 250 K) temperature regions specific to each material. This thesis is separated into several key parts. The first part establishes the theory behind observing phase transitions through the thermal expansion coefficient. Beginning with the classical definitions of the specific heat, compressibility and thermal expansion coefficient, the three properties are related using a property known as the Grüneisen parameter. To first order, the parameter allows phase transitions to be observed by the thermal expansion coefficient. The second part introduces capacitive dilatometry; a technique used to measure the thermal expansion coefficient. Three capacitive dilatometer devices are presented in this section. The silver compact dilatometer, the fused quartz dilatometer and the copper dilatometer. Each device discusses merits and weaknesses to their designs. Particular focus is made on the fused quartz dilatometer which was built during the duration of this thesis. The third part presents research on three magnetic spin materials; LiHoF4, Tb2Ti2O7 and Ba3NbFe3Si2O14. These materials are studied individually focusing on specific aspects. LiHoF4, a candidate material for the transverse field Ising model, provides insight to quantum phase transitions. Thermal expansion coefficient and magnetostriction along the c-axis for T ≈ 1.3-1.8 K and transverse field Ht ≈ 0-4 T were measured extracting critical points for a Ht-T phase diagram. Existing thermal expansion coefficient measurements had evidence of possible re-entrant behaviour. With a high density of low transverse field critical points it was established that LiHoF4 showed no evidence of re-entrant behaviour. The highly debated material Tb2Ti2O7 has a rich, controversial low temperature behaviour. Originally believed to be a spin liquid, specific heat results propose a scenario involving a sample composition dependent ordered state. Still under considerably attention, thermal expansion coefficient measurements were performed for T < 1 K. The results are interpreted to either fit into the proposed scenario or provide evidence for an alternate scenario. The material Ba3NbFe3Si2O14 exhibits a magnetoelectric multiferroic phase below TN ≈ 27 K; a phase where magnetic and electric order simultaneously exist. The formation of this phase is believed to have a similar structural shift observed in hexagonal perovskite multiferroic materials. The ferroelectric ordering in those materials are brought about through a centrosymmetric to non-centrosymmetric structural shift. The thermal expansion and thermal expansion coefficient coefficient along the a and c axis are measured for T > TN searching for a displacive structural phase transition.

Made available in DSpace on 2014-06-11T19:24:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2002-03-25Bitstream added on 2014-06-13T18:51:42Z : No. of bitstreams: 1 conti_ecg_me_arafo.pdf: 2274247 bytes, checksum: bd8fd8849fb076ead7d056ec179f83a2 (MD5) / O objetivo deste trabalho foi caracterizar química e metalograficamente o Ti.c.p. antes e após a fundição por Plasma-Skull, e estudar as interfaces entre esse metal e as porcelanas Vitatitankeramik, Triceram e Noritake TI22, empregando-se M.E.V., EDS, Mapeamento Elementar e Dilatometria. Trinta e cinco placas nas dimensões de 25mm x 3mm x 1mm foram fundidas pela máquina nacional Discovery - Plasma, EDG Equipamentos, sendo 5 amostras empregadas para a análise química e metalográfica e as 30 restantes, utilizadas para aplicação das porcelanas, sendo divididas em 3 grupos de 10 placas cada. O tratamento da superfície do metal, bem como a aplicação e as cocções das porcelanas foram realizadas segundo as recomendações de cada fabricante. Posteriormente, as amostras tiveram uma de suas faces laterais lixadas, até a exposição da interface e preparadas para análise por M.E.V. Para os ensaios de Dilatometria, 2 amostras de Ti c.p. e 2 para cada sistema de porcelanas foram confeccionadas na forma de cilindro com 12 mm de comprimento x 2 mm de diâmetro, e em cada amostra foi soldado um termopar de cromel-alumel, para medida de temperatura dos ensaios. As curvas de expansão térmica do Ti c.p. e das três porcelanas, foram levantadas para estudar a compatibilidade entre os conjuntos metalocerâmicos. Os resultados da análise química e das imagens obtidas por Microscopia Óptica, demonstram que o Ti c.p. após a fundição apresenta condições físico-químicas adequadas para receber a aplicação da porcelana. Dos resultados de M.E.V., observa-se que todos os sistemas apresentaram integridade nas interfaces Ti/Bonder, livre de fendas ou poros, com um bom contato entre a porcelana e o metal, confirmado pelas imagens do Mapeamento Elementar, sugerindo uma adequada interação entre esses materiais. Da análise das imagens do EDS... . / The aim of this work was to perform a chemical na metalographical characterization of the commercially pure titanium before and after melting by the Plasma-Skull method, and to study the interfaces between this metal and the porcelains Vitatitankeramik, Triceram e Noritake TI22, using SEM, EDS, elementary mapping and dilatometry. Thirty five samples with dimensions 25mm x 3mm x 1mm were melted in a national Discovery-Plasma, EDG Equipaments, five of which were used for chemical analysis and metalography and the remaining thirty, divided in three groups, were used for applications of the porcelains. The metal surface treatment, as well as the applications and firing of the porcelains, were performed according to the recommendations of each manufacturer. Subsequently, one of the lateral surfaces of the samples was polished until the interfaces was exposed and then was prepared for the SEM analysis. Two samples of commercially pure titanium and other two for each system of porcelains were cylindrically shaped with lengths of 12mm and 2mm of diameter, and in each sample Chromel-Alumel thermocouple was welded to measure the temperature of the tests. The thermal expansion curves of the commercially pure titanium and porcelains were measured to study the compatibility of the metal-ceramic systems. The chemical analysis and optical microscopy results show that the commercially pure titanium after the melting exhibits appropriate physical and chemical conditions for the application of the porcelain. The SEM results indicate that all the systems exhibits integrity of the Ti/bonder interface, free of gap and porous, with good contact between metal and porcelain, which was confirmed by the elementary mapping images, suggesting and adequate interaction between these materials. From the analysis of the EDS images... (Complete abstract, click electronic address below).

Foram coletadas amostras de uma chapa grossa temperada, de aço microligado ao boro com 8 mm de espessura. O aço foi produzido em aciaria do tipo LD, laminado na forma de chapa grossa em laminador de cadeira única reversível e finalmente recebeu tratamento de têmpera em forno de tubos radiantes e máquina de têmpera com fluxo de água. Todas as etapas de produção tiveram os parâmetros de processo controlados para obtenção das melhores propriedades possíveis. As amostras foram retiradas da região situada a um quarto da largura da chapa e submetidas a diversos ciclos térmicos em aparelho de dilatometria. Com auxílio de técnicas de microscopia ótica, microscopia eletrônica de varredura e medidas de dureza, o aço pôde ser caracterizado e foi possível levantar as curvas de resfriamento contínuo (TRC). As microestruturas revelaram a presença de múltiplos microconstituintes, principalmente em resfriamentos a partir de resfriamentos 25°C/s, quando surgem elementos aciculares como ferrita acicular e bainita misturados com ferrita poligonal e perlita. Acima desta taxa de resfriamento o campo perlítico é suprimido e a partir de 45°C/s surge a martensita. A 100°C/s a microestrutura é dominada predominantemente por martensita. As amostras resfriadas nas 3 taxas mais severas também revelaram a presença de austenita retida e constituinte MA, detectados por meio de difração de raios-x e ataque com reagente de Klemm. / Samples were taken from a quenched plate with 8mm thickness and chemical composition consisting of carbon, manganese, silicon, aluminum, niobium, titanium and boron. The steel was produced in LD converter, rolled in a reversible single chair rolling mill and quenched in a radiating tube furnace and quenching machine with water flow. All production stages had the process parameters controlled to obtain the best possible properties. The samples were extracted from a position situated at one quarter of the plate´s width and submitted to various thermal cycles in dilatometry device. With the aid of optical microscopy, scanning electron microscopy and hardness measurements, the steel could be characterized and the continuous cooling transformation curves could (CCT) be plotted. The microstructures revealed the presence of multiple components, especially when cooling rates reach 25°C/s, where acicular elements, like acicular ferrite and bainite, are found along with polygonal ferrite and pearlite. Above this cooling rate the pearlite field is suppressed and starting at 45°C/s martensite was found. At 100°C/s the microstructure is mainly martensitic. The samples cooled at the 3 most severe rates also revealed the presence of retained austenite and MA constituent, which were detected by x-ray diffraction and attack using Klemm reagent.

A proposta de desenvolvimento deste trabalho de dissertação veio de encontro com o objetivo de adaptar um laboratório para pesquisas em transformações de fases, utilizando o método dilatométrico, onde foram estudadas as reações no estado sólido que sofrem os materiais submetidos a ciclos térmicos. Para atingir este objetivo, instalou-se recentemente um dilatômetro de resfriamento rápido, modelo DT 1000 da Adamel Lhomargy. Completada a etapa de instalação do equipamento, o passo seguinte consistiu em explorar, através de ciclos térmicos variados, todos os recursos manipuláveis do dilatômetro. Aplicando a técnica dilatometrica em um aço de características termomecânicas conhecidas - SAE 1070 - possibilitou-se, a partir deste trabalho, que pesquisas mais complexas pudessem ser executadas com tal equipamento ou que o mesmo servisse como um instrumento complementar para estudos de transformações de fases. Nessa fase inicial de investigações estudou-se as curvas dilatométricas a fim de verificar os tempos de início e fim de transformação isotérmica em várias temperaturas de decomposição da austenita. Traçou-se as curvas de transformação isotérmica - curvas TTT - para o aço ensaiado (tratado termicamente), onde analisou-se a influência desse tratamento térmico prévio (recozimento) sobre a transformação \'alfa\' para \'gama\'. Finalizando a parte prática deste trabalho, comprovou-se as transformações de fases isotérmicas nas várias temperaturas ensaiadas, através de análise metalográfica e de medidas de microdureza dos produtos obtidos. A fim de solucionar eventuais problemas relacionados ao manuseio e manutenção do equipamento, com base na experiência adquirida ao longo do desenvolvimento deste trabalho, são dadas sugestões de possíveis aplicações do dilatômetro de têmpera rápida DT 1000 em pesquisas de transformações de fases. / The purpose of this work comes together with the objective to adapt a phase transformation research laboratory using dilatometric method, where it will be possible to study the solid state reactions of materials submited to thermal cycles. To fulfil this purpose, it was installed a high speed quenching dilatometer, model DT 1000 manufactured by Adamel Lhomargy. After this stage, our next step was to explorer all equipment resources through variable thermal cycles. Appling dilatometric method to a well known thermomechanical steel - SAE 1070 - it becomes possible the accomplishment of more complex experiments with this equipment. This equipment, itself, may be a powerfull auxilliary instrument to phase transformations studies. In this first investigation, it was studied the dilatometric curves to find the beginning and the end times of isothermal phase transformation for several austenite decomposition temperatures - TTT diagram - for the tested steel (thermal treated). The effect of this previous heat treatment (annealing) on the \'alfa\' for \'gama\' transformation were observed. Finally, the isothermal phase transformation at the different tested temperatures could be confirmed through metallographic and microhardness analysis. In order to solve possible problems associated to the equipment use or maintenance, some suggestions of applications of the high speed quenching dilatometer are indicated for further works.

The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / Unrestricted

The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected. The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel. The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of. Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.

No presente trabalho, foi realizado um estudo sobre a influência da adição do boro em um aço C-Mn, nas transformações de fases por resfriamento contínuo, através da técnica dilatométrica. Para este estudo, utilizou-se dois aços de composição química semelhante: o aço 10B22 com 30 ppm de boro e o aço sem boro é o E1522, equivalente. Foram traçadas as curvas de CCT dos dois aços, onde analisou-se o efeito de temperabilidade do boro através das curvas de resfriamento. Os materiais foram ensaiados em um dilatômetro de resfriamento rápido, utilizando corpos de prova cilíndricos, resfriados por injeção de ar e hélio. Os resultados gráficos dos processos de resfriamento foram comparados com análises metalográficas ótica estruturais e quantitativas e também por medidas de dureza. Detalhes microestruturais, tais como, contagem de fases formadas em função das taxas de resfriamento, tamanho de grão, distribuição de inclusões e precipitados, foram extraídos, através de um sistema de análise de imagens. Complementando o trabalho, foi realizado uma completa documentação fotográfica das microestruturas, com uma indicação final do tratamento térmico mais adequado aos aços C-Mn microligados ao boro. / This work presents a study about the influence of boron on phase transformation by continuous cooling in a C-Mn steel using the dilatometric technique. For this study two close chemical composition steels were applied: a 10B2 steel with 30 ppm of boron and a E1522 steel without boron. For both steels the CCT curves were obtained, where the effect of boron on hardenability was analysed throughout continuous cooling curves. Both materials were testedcin a high speed quenching dilatometer, using cilindricals samples quenched by helium blow on their surfaces. The graphics results of cooling processes were compared with microstructural and quantitative optical metallographic analysis and hardness measurements. Microstructural features as such phases fraction as a function of cooling rates, grain size, and distributions of inclusions and precipitates were obtained through an image analysis system. In addition, complete microstructural photography documentation was carried out where it was possible to indicate the best heat treatment to boron microalloyed C-Mn steels.

This diploma thesis deals with the preparation of doped piezoceramic materials based on BaTiO3 using electrophoretic deposition. Five rare earth oxides, i.e. Er2O3, Dy2O3, Eu2O3, Tb407 and CeO2, were used as dopants in amounts of 1, 3, and 5 wt. %. The prepared deposits were evaluated in terms of preparation methodology, high temperature dilatometry, X-ray diffraction analysis, relative density, mean grain size, hardness and fractographic analysis. The study of dilatometric curves described the sintering behavior and its changes at different material compositions. X-ray diffraction analysis revealed a tetragonal phase in all samples; the tetragonality of the BaTiO3 crystalline cell decreased with dopant content. By a suitable choice of dopant, it was possible to significantly increase the relative density of sintered samples, their hardness and at the same time prevent the samples from coarsening of the microstructure during heat treatment. A relative density up to 98 %, a mean grain size below 1 m and a hardness of over 10 GPa were achieved. Analysis of the fracture surfaces revealed that the fracture mode was transcrystalline for the most of studied materials; only the samples doped with cerium dioxide had fracture surfaces with both transcrystalline and intercrystalline fracture modes. Based on the obtained results, a suitable composition of the material for the intended use in a layered piezoceramic harvester was identified, which, in addition to the BaTiO3 layers, consists of functionally-protective Al2O3 and ZrO2 layers.