Cette thèse porte sur l’analyse microstructurale d’intermétalliques formés par réaction à l’état solide entre une couche mince de métal Ni(Co) et un substrat d’InGaAs et s’inscrit dans le cadre du développement de reprises de contact pour les dispositifs MOSFET sub-10 nm ou les applications photoniques. Ce travail comporte une partie relative au développement d’une méthodologie de diffraction des rayons x adaptée à ces composés très texturés et deux parties distinctes où nous décrivons et discutons les résultats expérimentaux.L’étude microstructurale (phase, texture…) des intermétalliques obtenus par réaction à l’état solide est rendue complexe par la formation transitoire de phases métastables, non stœchiométriques parfois, contraintes mécaniquement, et présentant en général des orientations cristallines (texture cristallographique) très marquées. Du fait de cette complexité microstructurale, ces intermétalliques n’ont souvent pas été caractérisés de façon complète et peu de connaissances se trouvent sur leur structure et leur formation. Aussi, et pour caractériser de façon complète et sans omettre des phases ou des orientations dans le système Ni-In-Ga-As, nous avons contribué au développement d’une méthode de mesure globale et rapide par diffraction des rayons X permettant de reconstruire une cartographie large 3D de l’espace réciproque. Les données recueillies par cette méthode sont reconstruites afin d’extraire soit des diagrammes de diffraction dits « détexturés », soit des figures de pôle…permettant une analyse semi quantitative de la microstructure des échantillons.Dans la première partie des résultats expérimentaux, nous nous intéressons à la caractérisation des intermétalliques formés à partir d’empilements Ni/InGaAs/InP recuits ex-situ à différentes températures. Nous décrivons la formation des intermétalliques, leurs textures, et leurs paramètres structuraux. Nous relevons certains aspects de la microstructure qui évoluent en fonction de la température de recuit comme l’anisotropie de texture, la stœchiométrie des intermétalliques et le domaine d’existence thermique et nous proposons des hypothèses qui peuvent expliquer l’évolution de ces phénomènes. Cette première étude faite sur des substrats InP a été confrontée aux résultats obtenus pour des intermétalliques similaires réalisés sur substrats InGaAs/GaAs/Si. En effet, les substrats Si sont les substrats industriellement ciblés pour la réalisation de composés logiques à canal III-V à grande échelle (sur des plaquettes de 300mm de diamètre). Ensuite, nous avons comparé la métallisation de la couche d’InGaAs dans le cas de Ni pur avec la métallisation d’InGaAs lorsqu’un élément d’alliage (Co) est ajouté à la couche de Ni. Ainsi, l’analyse microstructurale révèle des différences notamment sur les textures qui ont été interprétées sur la base de considérations thermodynamiques, mais aussi structurales comme l’alignement des deux couches entre elles, liées à des aspects plus cinétiques.Dans la deuxième partie de ce travail, nous présentons les résultats des analyses in situ effectuées par cartographies de l’espace réciproque en 3D au synchrotron ESRF à Grenoble. Il s’agit de suivre en temps réel par diffraction des rayons X, les réactions à l’état solide des échantillons du type Ni (7 nm et 20 nm)/InGaAs/InP et Ni0.9Co0.1 (20 nm)/InGaAs/InP lors de recuits par rampes... Ensuite, nous avons effectué des recuits isothermes sur les échantillons de type Ni (20 nm)/InGaAs/InP. Ces différentes mesures, couplées avec des hypothèses sur la croissance nous ont permis d’extraire les paramètres cinétiques relatifs à la formation de la première phase d’intermétallique. Les textures observées et leur évolution lors des recuits thermiques in situ sont différents des recuits ex situ. Ceci peut notamment être expliqué par un mode de recuit différent dans le cas in situ où la cinétique du système est plus lente, favorisant ainsi les structures et textures les plus stables. / This thesis focuses on the microstructural analysis of intermetallics formed by solid-state reaction between a thin layer of Ni (Co) metal and an InGaAs substrate and was carried out in the framework of contact development for sub-10 nm MOSFET but have also photonic applications. This work includes a part related to the development of an X-ray diffraction methodology adapted to highly textured compounds and two distinct parts in which we describe and discuss the experimental results.The microstructural study (phase, texture ...) of intermetallics obtained by solid-state reaction is complicated due to the formation of transient metastable, often non-stoichiometric and mechanically stressed phases. These phases have generally very marked crystalline orientations (crystallographic texture). Because of this microstructural complexity, these intermetallic have often been not fully characterized and there is little knowledge about their structure and formation. Moreover, and to fully characterize the Ni-In-Ga-As system without omitting phases or textures, we have contributed to the development of a comprehensive method of rapid measurement by X-ray diffraction to reconstruct large 3D maps of the reciprocal space. The collected data through this method are reconstructed to extract either diffraction diagrams called "detextured" diagrams or pole figures ... which allows a semi-quantitative analysis of the intermetallic microstructure.In the first part of the experimental results, we focus on the characterization of intermetallic formed through Ni / InGaAs / InP stacks and annealed ex situ at different temperatures. We describe the formation of the intermetallics, textures, and structural parameters. We note some aspects which vary depending on the annealing temperature such as the texture anisotropy, the stoichiometry of intermetallic and range of thermal existence and propose hypotheses that can explain the evolution of these phenomena. The studies on InP substrates were compared to results obtained for similar intermetallic made on GaAs / Si substrates. Indeed, the Si substrates are targeted for industrials to achieve logic compounds III-V channel large-scale (on 300 mm wafers). Then, we compared the metallization of the InGaAs layer in the case of pure Ni metallization with the results when an alloying element (cobalt) was added to the Ni layer. The microstructural analysis revealed several differences especially texture differences. These differences were interpreted on the basis of thermodynamic considerations, but also on the basis of structural alignment of the two layers together which are also linked to more kinetic aspects.In the second part of this work, we present the analysis results of studies performed by in situ 3D Reciprocal Space Mapping on the ESRF synchrotron in Grenoble. We followed the formation and stability of the intermetallics by real-time X-ray diffraction measurements, for different kind of samples: Ni (7 nm and 20 nm) / InGaAs / InP and Ni0.9Co0.1 (20nm) / InGaAs / InP, using ramp annealing... Then, we performed isothermal annealings for Ni(20 nm) / InGaAs / InP samples. These measurements, coupled with assumptions on the intermetallic growth, allowed us to extract the kinetic parameters for the formation of the first phase of the intermetallic. The observed textures and their evolution during in situ thermal annealings are different than ex situ annealing. This can be explained by a different mode of annealing in the case of in situ where the kinetics of the system is slower, thus favoring the most stable structures and textures.

The A15-based Nb3Sn and V3Ga superconducting compounds are an integral part of synchrotrons and magnetic fusion reactor technology, especially where a magnetic field higher than 10 T is required, which lies beyond the limit of conventional Nb-Ti superconductors (~8 T). These brittle intermetallic compounds are difficult to manufacture in the form of wires, required for the application purpose, using the traditional wire-drawing process. Hence, bronze technique is adopted to fabricate such filamentary wires. This is based on the solid-state diffusion where A3B compound (A=Nb or V, B=Sn or Ga) forms during the interaction of Cu(B) and A. The operation of pure superconducting wires gets restricted to the field of 12 T, however, the ever-increasing demands for an improved efficiency have promoted the development of these A15 wires with the addition of alloying elements such as Ti and Zr. Many important physical and mechanical properties of such wires depend on the growth behaviour of these compounds. Therefore, understanding the growth of such compounds necessitates an in-depth analysis on diffusion behaviour of various elements in both bronze-based solid solutions as well as A15-intermetallics. Estimation of diffusion parameters makes use of the most commonly used diffusion couple technique. There are mainly three methods available for the estimation of the interdiffusion coefficients, proposed by Matano-Boltzmann (MB), Den Broeder (dB), same as Sauer-Freise (SF) and Wagner. Among these three, MB treatment is known to be the least accurate method, especially when there is a deviation of molar volume in a system from the ideality. At the same time molar volume might affect the estimation process differently for dB and Wagner’s approach. MB method is still being used neglecting the actual molar volume variation. On the other hand, the implementation of dB or Wagner’s approach for the estimation remains to be random. For the first time, we have critically examined the role of molar volume on estimated diffusion parameters and indicated the more accurate approach. Similar analysis for the estimation of the intrinsic diffusion coefficient is conducted considering Heumann and van Loo’s methods. Furthermore, the discussion is extended to the estimations of various diffusion parameters considering the measured composition profile in the V-Ga system. A detailed diffusion study has been conducted on Cu(Ga) and Cu(Sn) solid solutions to examine the role of the vacancy wind effect on interdiffusion. The interdiffusion, intrinsic and impurity diffusion coefficients are determined to facilitate the discussion. It is found that Ga and Sn are the faster diffusing species in the respective systems. The trend of the interdiffusion coefficients is explained with the help of the driving force. Following that, the tracer diffusion coefficients of the species are calculated with and without consideration of the vacancy wind effect. We found that the role of the vacancy wind is negligible on the minor element in a dilute solid solution, which is the faster diffusing species in this system and controls the interdiffusion process. However, consideration of this effect is important to understand the diffusion rate of the major element, which is the slower diffusing species in this system. Major drawback of studying diffusion in multi-component systems is the lack of suitable techniques to estimate the diffusion parameters. In this study, a generalized treatment to determine the intrinsic diffusion coefficients in multi-component systems is developed utilizing the concept of pseudo-binary approach. This is explained with the help of experimentally developed diffusion profile in the Cu(Sn, Ga) solid solution. Based on an interdiffusion study using an incremental diffusion couple in the V-Ga binary system, we have shown that V diffuses via lattice, whereas Ga does so via grain boundaries for the growth of the V3Ga phase. We could estimate the contributions from two different mechanisms, which are, usually, difficult to delineate in an interdiffusion study. Available tracer diffusion studies and the atomic arrangement in the crystal structure have been considered for a discussion on the diffusion mechanisms. Diffusion–controlled growth rate of V3Ga at the Cu(Ga)/V changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase in Ga leads to more than double the product phase layer thickness and a significant decrease in activation energy. Kirkendall marker experiment indicates that V3Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V3Ga are discussed. The growth of Nb3Sn by bronze technique on two different single crystals and deformed Nb is studied. The grain boundary diffusion-controlled growth rate is found to be different for each of these three specimens. The difference is explained on the basis of the grain size of Nb3Sn. Elemental additions such as Ti and Zr to either bronze or metal are found to improve the superconducting properties. We have examined their effects on the growth rates of A15-phase formed in Cu(B,x)/A and Cu(B)/(A,x), where x is Ti or Zr. In either cases Ti and Zr-additions result in an improved growth rate of the product phase and reduces activation energy with increase in alloying addition; however few precipitates are formed in the interdiffusion zone for Cu(B,x)/A. Wavelength dispersive spectrometry (WDS)-mapping reveals these to be x-rich. Scanning transmission electron microscopy (STEM)-analysis suggests having composition gradient inside a single precipitate. TEM-diffraction demonstrates these to be Ti(A) solid solution crystallizing as BCC-structure for Cu(B,Ti)/A. These are located on grain boundaries of A15-phase. Electron back-scattered diffraction (EBSD)-analysis demonstrates grain morphology of product phase and found the average grain size to exhibit a decreasing trend with increasing x content. Columnar grains, on Ti and Zr addition tend to form as equiaxed ones. Based on the morphology and grain size pattern, the role of grain boundary diffusion is speculated to have a dominant effect with increase in elemental additions. The texture evolution of the product phase is also investigated and found the product phase to grow as a strongly textured one with the elemental additions. A peculiar pattern is observed for the texture of the product phase and its adjacent A or A(x) grains.

Dans cette thèse nous avons évalué l'effet de la substitution partielle du cobalt par des éléments non-magnétiques de type p (M) sur les propriétés structurales et magnétiques des composés RCo5. Les échantillons ont été synthétisées par fusion dans un four à induction, puis caractérisés en utilisant des nombreuses techniques expérimentales: diffraction de rayons X et de neutrons, microscopie électronique à balayage, magnétométrie, susceptométrie en courant alternatif et spectroscopie photoélectronique par rayons X.Tous les composés de type RCo5-xMx (R= Pr, Sm, Tb, Er et Tm; M= Si, Ge, Al, Ga; x=0,5 et x=1) étudiés gardent la structure cristalline de type CaCu5 des composés de départ RCo5, mais les paramètres de la maille cristalline sont modifiés par la présence des éléments M. Le domaine de stabilité thermique des phases RCo5-xMx est notablement modifié par la présence d'élément métalloïde en substitution sur les sites du cobalt. Il peut être étendu pour Al et Ga et réduit pour Si et Ge. Les atomes de l'élément M sont localisés préférentiellement sur le site cristallographique Co 3g. La solubilité des éléments Ge et Si dans la structure RCo5 est trouvée être inférieure à celle des métalloïdes ayant un électron de moins tels que Al et Ga.La substitution M/Co a une influence importante sur les propriétés magnétiques des composés RCo5. La température d'ordre et l'aimantation spontanée diminuent significativement après le remplacement partiel du cobalt par l'élément métalloïde. Ces modifications sont induites par l'évolution des interactions d'échange en particulier Co-Co mais aussi par la réduction de l'aimantation du Co liée à sa sensibilité au voisinage atomique et magnétique local. La direction de facile aimantation à la température ambiante est préservée le long de l'axe cristallographique c pour tous les échantillons étudiés. Dans quelques composés RCo5-xMx (R = Sm, Er et Tm) la substitution de M au cobalt change le mécanisme de coercitivité par rapport aux composés de départ RCo5. Une coercitivité élevée a été détectée à basse température, surtout pour les composés SmCo4Al et SmCo4Ga, qui présentent aussi des champs d'anisotropie énormes, bien supérieurs à ceux de phase SmCo5.Les expériences XPS sur les composés de type RCo5-xMx ont détecté la réduction de la densité d'états électroniques au niveau de Fermi par rapport aux composés de type RCo5. La bande 3d du cobalt est remplie lors de la substitution M/Co et conduit à un moment magnétique du cobalt réduit. / In this work we have evaluated the effect of the partial substitution of cobalt with non-magnetic p-type elements (M) on the structural and magnetic properties of RCo5 compounds. The samples were prepared by alloying in an induction furnace and were characterized using various experimental techniques: X-ray and neutron diffraction, scanning electron microscopy, magnetometry, AC susceptometry and X-ray photoelectron spectroscopy.All of the studied RCo5-xMx compounds (R=Pr, Sm, Tb, Er and Tm; M=Si, Ge, Al and Ga; x=0.5 and x=1) maintain the CaCu5 crystal structure of the RCo5 compounds, however the lattice parameters are modified due to the presence of M elements. The thermal stability range of the RCo5-xMx phases is modified significantly due to the Co site substitutions. The thermal stability increases for Al and Ga substitutions and decreases when M is Si or Ge. The M atoms were found to preferentially occupy the Co 3g site. The solubility of Ge and Si in the RCo5 structure is inferior to that of metalloid elements with one less electron, such as Al and Ga. The M/Co substitution has an important influence on the magnetic properties of RCo5 compounds. The ordering temperature and the spontaneous magnetization are significantly reduced after the partial substitution of cobalt by the metalloid elements. These changes are induced in particular by the evolutions of the Co-Co exchange interactions and also by the reduction of the Co magnetization due to the sensitivity of Co to the local atomic and magnetic vicinity. The easy magnetization direction at room temperature is preserved along the c-axis for all of the studied samples. In several RCo5-xMx (R = Sm, Er and Tm) compounds the substitution of Co with M atoms changes the coercivity mechanism compared to RCo5. A higher coercivity was found at low temperatures, especially for SmCo4Al and SmCo4Ga compounds, which also show huge values of the anisotropy field, well above the ones found in SmCo5.The XPS measurements on RCo5-xMx compounds show a reduction of the density of states at the Fermi level compared to the RCo5 compounds. There is a filling of the Co 3d band following the M/Co substitution, leading to a lower Co moment.

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A indústria automobilística, ao visar o aumento da garantia à corrosão, emprega na construção das carrocerias aços IF (intersticial free) galvanizados, já que estes atendem aos critérios de qualidade superficial, conformabilidade, soldabilidade, entre outras características requeridas. Dentro deste contexto, a resistência à corrosão de um aço livre de intersticiais (IF) com revestimento galvanizado comum (GI) e diferentes gramaturas (85 g/m2 (Z85), 100 g/m2 (Z100), 120 g/m2 (Z120), 144 g/m2 (Z144) e 180 g/m2 (Z180), fosfatizadas e com cataforese, foram avaliadas neste estudo por intermédio do ensaio de corrosão cíclica acelerado. O resultado deste ensaio mostrou que mesmo com a variação da gramatura do revestimento (GI) a resistência à corrosão foi praticamente a mesma, levando-se a hipótese que a camada intermetálica que está presente em todas as amostras independente da gramatura, pode possui uma grande influência na resistência à corrosão. Assim ensaios suplementares foram feitos para compreender o efeito da camada de zinco e a camada intermetálica na resistência à corrosão. A caracterização das camadas formadas durante o processo de galvanização GI foi realizado na amostra com gramatura de 100 g/m2 (Z100). Tal amostra foi escolhida por ser a mais empregada pela indústria automobilística e a mesma não sofreu nenhum pré tratamento já que o objetivo foi analisar apenas as camadas do galvanizado comum GI. Os ensaios realizados foram de microestrutura (XRD, MEV e EDS) e ensaio eletroquímico (dissolução eletroquímica e polarização potenciodinâmica). Concluiu-se que a camada intermetálica é formada pelas fases Fe2Al5 e FeAl3, com predominância da fase Fe2Al5. O ensaio de dissolução eletroquímica demonstrou que a resistência o corrosão da camada intermetálica é no mínimo 7 vezes maior que a do zinco, além deste resultado o ensaio de polarização potenciodinâmica apresentou que a camada intermetálica passiva, retardando a velocidade de oxidação, ou seja, aumenta a resistência à corrosão do galvanizado comum GI. / The automobile industry, when seeking to increase warranty against corrosions, employs galvanized IF (intersticial free) steels to the body shell, since these meet the superficial, compliance, weldability and other quality criteria. In this context, the corrosion resistance of an IF steel with galvanic coating (GI) and different weights (85 g/m2 (Z85), 100 g/m2 (Z100), 120 g/m2 (Z120), 144 g/m2 (Z144) and 180 g/m2 (Z180), phosphated and with cataphoresis, were evaluated through an accelerated cyclical corrosion experiment. The result of this experiment showed that even with the variation of the galvanic coating (GI) the result of the corrosion resistance was the same, leading to the hypothesis that the intermetallic layer which is present in all samples, regardless of the weight, must influence corrosion resistance. Thus, supplementary experiments were done to comprehend the effect of the zinc layer and the intermetallic layer in corrosion resistance. The characterization of the layers formed in the GI galvanizing process was done in the Z100 (100g/m²) sample. This sample was chosen because it is the most used in the automobile industry and it did not suffer any previous treatment since the objective was to analyze only the layers of galvanized GI. The experiments done were in the microstructure (XRD, MEV and EDS) and electrochemical experiment (potenciodinamic polarization). We concluded that the intermetallic layer is formed by phases Fe2Al5 and FeAl3, with predominance of phase Fe2Al5. It was verified through the electrochemical dissolution experiment that the intermetallic corrosion resistance is at minimum 7 times greater than of the zinc, further on this result, the potentiodynamic polarization experiment shows that the passive intermetallic layer slows the oxidation velocity, which means, the galvanic coating (GI) corrosion resistance is increased.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O efeito magnetocalórico, base da refrigeração magnética, é caracterizado por duas quantidades: a variação isotérmica da entropia (ΔST) e a variação adiabática da temperatura (ΔTad) as quais podem ser obtidas sob variações na intensidade de um campo magnético aplicado. Em sistemas que apresentam anisotropia magnética, pode‐se definir o efeito magnetocalórico anisotrópico, o qual, por definição, é calculado através da variação na direção de aplicação de um campo magnético cuja intensidade se mantém fixa. Nos materiais de nosso interesse, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta a rede magnética (que pode ser composta por diversas sub-redes magnéticas acopladas), rede cristalina e a dinâmica dos elétrons de condução. No hamiltoniano magnético são consideradas as interações de troca, Zeeman e campo cristalino (esta ultima responsável pela anisotropia magnética). Recentemente, estudamos o efeito magnetocalórico convencional e o efeito magnetocalórico anisotrópico nos compostos mononitretos com terras-raras, a saber: Ho(y)Er(1-y)N para as concentrações y= 0,1,0.5 e 0.75. Comparações entre nossos resultados teóricos e os dados experimentais para o EMC foram bastante satisfatórias [3,9]. Além disso, diversas predições teóricas como a existência de uma fase ferrimagnética no sistema Ho(y)Er(1-y)N (para a concentração y=0.5) e reorientações de spin nas sub-redes do Ho e Er foram feitas [25]. / The magnetocaloric effect, magnetic refrigeration base, is characterized by two quantities: the isothermal entropy change (ΔST) and the adiabatic temperature change (ΔTad) which can be obtained through variations in the intensity of a magnetic field applied. In systems which present magnetic anisotropy, one can define anisotropic magnetocaloric effect, which, by definition, is calculated through the variation the direction of application of a magnetic field whose intensity remains fixed. In the materials of our interest, the magnetocaloric effect is studied theoretically starting from a model Hamiltonian which takes into account the magnetic lattice (that can be composed of several magnetic sublattices coupled), crystalline lattice and the dynamics of the conduction electrons. In the magnetic hamiltonian are considered the exchange interactions, Zeeman and crystalline electrical field (this latter responsible for the magnetic anisotropy). Recently, we studied the conventional magnetocaloric effect and anisotropic magnetocaloric effect in mononitrides compounds with rare earths, namely: o(Y)Er(1-Y)N for concentrations y= 0,1,0.5 e 0.75 . Comparisons between our theoretical results and experimental data for EMC were quite satisfactory [26].Furthermore, several theoretical predictions how to the existence of a phase ferrimagnetic in the system Ho(y)Er(1-y)N (for concentration ) and spin reorientations in the sublattices of Ho and Er were made [25].

Orientador: Flávio César Guimarães Gandra / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T22:36:59Z (GMT). No. of bitstreams: 1 Reis_RicardoDonizethdos_M.pdf: 3698782 bytes, checksum: 685ad61061d7b02d4c3347f86a4822eb (MD5) Previous issue date: 2011 / Resumo: O efeito magnetocalórico (EMC) é a base da refrigeração magnética. O potencial magnetocalórico é caracterizado por duas quantidades termodinâmicas: a variação isotérmica da entropia (?S) e a variação adiabática da temperatura (?T), as quais são calculadas sob uma variação na intensidade do campo magnético aplicado ao sistema. Em sistemas que apresentam anisotropia magnética é observada uma mudança no efeito magnetocalórico porque este potencial torna-se fortemente dependente da direção de aplicação do campo magnético. A anisotropia em sistemas magnéticos pode levar à definição de um efeito magnetocalórico anisotrópico, o qual, por definição, é obtido para um campo cuja intensidade é mantida constante e cuja orientação variamos de uma direção difícil de magnetização para a direção fácil de magnetização. Neste trabalho apresentaremos os resultados obtidos para o efeito magnetocalórico anisotrópico nos compostos monocristalinos de DyAl2, RBi(R=Dy,Ho) e RGa2 (R=Er,Ho). Para o composto DyAl2 , utilizando o hamiltoniano de campo cristalino (CC) e a aproximação de campo médio, foi possível simular as curvas de magnetização e calor específico obtendo boa concordância com os resultados experimentais. Neste composto a variação isotérmica da entropia ?Sanisotrópico obtida pela variação da direção do campo H (EMC anisotrópico) é maior do que ?Siso convencional que, entretanto, ocorre na temperatura de reorientação de spin (T=42K). A forte anisotropia do ErGa2 e do HoGa2 contribui para uma expressiva diferença no ?Smag (~12 e 23J/kgK@5T, respectivamente, para T~10K) quando o campo é aplicado paralela ou perpendicularmente ao eixo fácil. Em ambos os casos a variação anisotrópica de entropia com a temperatura é semelhante ao ?S convencional com o campo magnético aplicado paralelamente ao eixo fácil de magnetização (eixo c para o ErGa2 e plano ab para o HoGa2). Observamos ainda que o EMC do ErGa2 é fortemente afetado pelo campo cristalino. Medidas de calor específico mostraram um acentuado pico tipo Schottky centrado em 40K e, conseqüentemente, somente parte da entropia magnética total se apresenta na temperatura de ordenamento antiferromagnética. Nos compostos de DyBi e HoBi o valor obtido para o EMC anisotrópico foi maior do que o EMC convencional ( cerca de 15% para o DyBi e 45% para o HoBi). Para os dois compostos foi obtido o EMC anisotrópico para os campos magnéticos de 5T, 6T e 7T. Para o HoBi obtivemos um resultado bastante interessante, no qual o EMC anisotrópico encontrado para µ0H= 5T, 24.7J/KgK, é aproximadamente o dobro do obtido para µ0H =7T / Abstract: The magnetic refrigeration is based on the magnetocaloric effect. The magnetocaloric potential is characterized by two thermodynamic quantities: the isothermal entropy change (?S) and the adiabatic temperature change (?Tad), which are calculated upon under a change in the intensity of the applied magnetic field. In anisotropic magnetic systems it is observed a change in the magnetocaloric effect, since this potential becomes strongly dependent on the direction in which the external magnetic field is applied. The anisotropy in such magnetic systems can lead to an inverse magnetocaloric effect, as well as to the definition of an anisotropic magnetocaloric effect, that by definition is calculated upon a magnetic field which intensity is kept fixed and which orientation is changed from a hard direction of magnetization to the easy direction of magnetization. For DyAl2 compound, using crystal field and mean field approximations, it was possible to simulate the magnetization curves and specific heat obtaining a good agreement with experimental results. In this compound the isothermal entropy change ?Sanisotrópico obtained by varying the direction of the field H (anisotropic EMC) is higher than conventional ?Siso, however, occurs in spin reorientation temperature (T = 42K). The strong anisotropy of ErGa2 and HoGa2 contribute to a expressive difference in the ?Smag (~12 and 23J/kgK@50kOe, respectively at T=10K) when the magnetic field is applied parallel or perpendicular to the easy axes. In both cases the anisotropic variation of entropy with temperature is similar to conventional Ds with the applied magnetic field parallel to the easy axis of magnetization (c axis for ErGa2 and plane ab for HoGa2). We also observed that the EMC ErGa2 is strongly affected by crystal field. Specific heat measurements show a sharp peak Schottky type centered at 40K and, therefore, only part of the total magnetic entropy is presented in the antiferromagnetic ordering temperature. In the compounds of DyBi and HoBi the value obtained for the anisotropic EMC was higher than the conventional EMC (~ 15% to DyBi and 45% for HoBi). For the two compounds was obtained the EMC anisotropic for magnetic fields of 5T, 6T and 7T. HoBi obtained for a very interesting result, in which the anisotropic found for EMC µ0H = 5T, 24.7J/KgK is approximately double that obtained for µ0H = 7T / Mestrado / Física da Matéria Condensada / Mestre em Física

Orientador: Ana Maria Martinez Nazar / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-22T16:31:39Z (GMT). No. of bitstreams: 1 Nunes_CarlosAngelo_D.pdf: 12292935 bytes, checksum: ea1a755d6645281ed644bfe2879a287c (MD5) Previous issue date: 1997 / Resumo: Em trabalhos recentes têm-se proposto que materiais formados por um metal refratário em equilíbrio com uma fase intermetálica constituem a melhor opção para aplicações estruturais em temperaturas acima de 1400°C. Baseado em informações disponíveis na literatura foi identificada a existência de um campo bifásico envolvendo as fases Mo (solução sólida) e o intermetálico 'Mo IND. 5¿ ¿Si¿ ¿B IND. 2¿ ('T IND. 2¿) A 1600 º C no sistema Mo-B-Si. Tendo em vista o limitado número de informações relacionadas a este sistema objetivamos neste trabalho uma avaliação sistemática sobre estabilidade de fases na região rica em Mo. O trabalho consistiu em: determinar a relação de fases a 1600°C; desenvolver métodos de análise via microssonda eletrônica (WDS) para determinação da composição das fases; determinação da projeção liquidus; determinação da seção vertical Mo-'T IND. 2¿.A determinação da relação de fases a 1600°C confmnou a existência do campo bifásico Mo+ 'T IND. 2¿ naquela temperatura.A fase ternária 'T IND. 2¿ é rodeada pelos campos trifásicos ... Observação: O resumo, na íntegra, poderá ser visualizado no texto completo da tese digital / Abstract: In recent investigations it has been proposed that equilibrium microstructures constituted by a refractory metal and an intermetallicphase are the best choice for high temperature structural applications at temperatures higher than 1400°C. Based on the information available in the literature it has been identified the existence of a Mo (solid solution) + intermetallic 'Mo IND. 5¿ ¿Si¿ ¿B IND. 2¿ two-phase field in the Mo-B-Si system. Considering the limited information related to this system it was the objective of this study to systematically evaluate the phase stability in the Mo-rich region. Specific objectives were: determination of the phase relations at 1600°C; development of microprobe methods of analysis to determine phase compositions; determination of the liquidus projection; determination of the Mo-'T IND. 2¿ vertical section. The Mo+ 'T IND. 2¿ two-phase field was confirmed to exist at 1600°C. The ternary phase ('T IND. 2¿) is surrounded by the following three-phase field ... Note: The complete abstract is available with the full electronic digital thesis or dissertations / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Made available in DSpace on 2014-10-09T12:43:31Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:05Z (GMT). No. of bitstreams: 1 06635.pdf: 4560564 bytes, checksum: 677cab6b14a66c0989ef5859a83679e3 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP