This thesis describes a study into the production of polycrystalline BaFe120 19 (BaM) specimens possessing properties appropriate to microwave device applications. Principal requirements were controlled stoichiometry, single-crystal density and a high degree of crystallographic alignment; all of these contributing to the reduction of ferromagnetic resonance (FMR) losses and linewidths to levels comparable with more expensive single-crystal alternatives. The stoichiometry of precursor BaM powder was analysed by X-ray diffraction (XRD) and found to be critically dependent on reaction conditions and batch reproducibility of constituent chemicals. The reaction process of BaM was analysed in the temperature range 500 - 1200 °C and phase content elucidated. The XRD spectrum of the single-phase BaM contained peaks not in the powder diffraction file (PDF) for this compound. Careful analysis eliminated the possibility of impurities and second phases. A comprehensive theoretical calculation from first principles allowed a complete fit between the calculated and observed XRD spectra. A revised spectrum has been proposed to the Joint Committee on Powder Diffraction Standards (JCPDS) in order to improve the PDF data reported by earlier workers. Adaptation of the theory has enabled the development of a tool for the determination of site preferences of dopant systems, such as Zn2+ -Ti4+ pairs. A novel magnetic press system was devised and constructed for the simultaneous alignment and compaction of powders in an aqueous slurry. Sintering of compacts resulted in relative densities up to 95 % coupled with improved alignment through grain growth. In an improved technique, aligned compacts were pre-sintered and hot-pressed in an oversize die, allowing viscous flow of grains. Near 100 % relative densities were obtained at temperatures significantly lower than for conventional sintering. Alignment was enhanced by subsequent annealing, but did not exceed levels achieved by normal sintering. Suggestions were made for achieving higher alignments. Microwave resonance linewidths were significantly lower than for isotropic polycrystals and suggestions for further improvements were identified.

L’objectif de cette thèse est d’explorer de nouvelles technologies permettant de faciliter l’intégration des isolateurs dans les chaînes radiofréquences de satellites. Ces composants sont utilisés pour contrôler l’adaptation des amplificateurs dans les sections d’entrée et de sortie des équipements RF bas niveaux. Nous proposons deux voies de réalisation de ces isolateurs. Une première voie basée sur l’utilisation de matériaux ferricomposites est étudiée pour concevoir des composants en bande Ku. Une deuxième voie, basée sur l’utilisation d’hexaferrites de strontium et de baryum préorientés, a été étudiée pour concevoir des composants auto-polarisés (sans aimants) en bandes Q et Ka. / The goal of this PhD thesis is to explore new technologies that make possible to improve the integration of isolators in radiofrequency chain of satellites. These components are especially used to control matching of amplifiers.We propose two ways of producing these isolators. At first, ferricomposite materials are studied to design low-cost isolators at Kuband.Then, we studied pre-oriented strontium or barium hexaferrites to design self-biased components (without magnets) for Q and Ka band frequencies.

Dans le cadre d'un projet de transport automobile propre pour la mise en place de véhicules électriques décarbonés grand public, les aimants représentent un enjeu important au niveau des moteurs électriques. Il est alors nécessaire de se passer des aimants à base de terres rares, peu écologiques et dont un seul pays détient le monopole. Deux voies sont explorées ici : la nanostructuration des hexaferrites de type M par frittage flash de type SPS (Spark Plasma Sintering), et la synthèse d'alliages Mn-Al-C par la méthode de la trempe rapide sur roue. Concernant le premier matériau, l'optimisation du protocole de synthèse a permis d'augmenter le champ coercitif d'un facteur 2. Deux outils de caractérisation des aimants ont été mis en place : le premier évalue la texture du matériau (modèle de Stoner et Wohlfarth), le second étudie les processus d'aimantation (Preisach). Pour les alliages Mn-Al-C, nous avons pu démontrer la présence d'un couplage antiferromagnétique entre les atomes de Mn des sites 1a et 1d. La diffraction de neutrons à également pu montrer que le moment magnétique du Mn du site 1d est par ailleurs très supérieur à celui du site 1a. Un broyage cryogénique suivi d'un frittage SPS à haute pression (400 MPa) a permis d'augmenter la valeur du champ coercitif de 40 %. Les résultats obtenus sont très prometteurs quant à la faisabilité d'alternatives aux aimants à base de terres rares. / In the context of green car development for mass market, development of low cost rare-earth-free magnets is clearly a milestone, because the rare-earth are expensive and their production is not ecological and a monopoly of one country. Two routes are explored herein : nanostructuration of M-type hexaferrites by flash sintering (Spark Plasma Sintering), and the synthesis of Mn-Al-C alloys by melt spinning. Concerning the first material, the optimization of the synthesis protocol allowed to increase the coercive field by a factor of 2. Two characterization tools of the magnets were set-up : the first evaluates the texture of the material (Stoner and Wohlfarth model), the second, based on Preisach model, is used to infer the magnetization processes. For Mn-Al-C alloys, we have evidenced from Weiss plots the antiferromagnetic coupling between 1a and 1d-sites Mn atoms. Neutron diffraction confirmed this result and shown that the magnetic moment of the 1d-site Mn is also much larger than that of 1a-site. Cryogenic ball milling followed by high-pressure SPS (400 MPa) increased the coercive field by 40 %. The results obtained are very promising as to the feasibility of alternatives to rare-earth magnets.

Structure-property relationships of two crystal structures were investigated using computational methodologies in two different length scales:electronic and atomistic length scales. Electronic structure calculations were performed using density functional theory (DFT) with generalized gradient approximation (GGA), GGA+U (U is “on-site” electronelectron repulsion) and hybrid functional forms. Atomistic calculations were performed utilizing the semi-empirical interatomic formulation, Modified Embedded Atom Method (MEAM). Classical molecular dynamics simulations were performed on the atomistic length scale in order to investigate thermal properties. In the first study, structural, elastic and thermal properties of cementite (Fe3C) were investigated using a Modified Embedded Atom Method (MEAM) potential for iron-carbon (Fe-C) alloys. Previously developed Fe and C single element potentials were used to develop a Fe-C alloy MEAM potential, using a statistically-based optimization scheme to reproduce structural and elastic properties of cementite, the interstitial energies of C in bcc Fe as well as heat of formation of Fe-C alloys in L12 and B1 structures. The stability of cementite at high temperatures was investigated by molecular dynamics simulations. The nine single crystal elastic constants for cementite were obtained by computing total energies for strained cells. Polycrystalline elastic moduli for cementite were calculated from the single crystal elastic constants of cementite. The formation energies of (001), (010), and (100) surfaces of cementite were also calculated. The melting temperature and the variation of both the specific heat and volume with respect to temperature were investigated by performing a two-phase (solid/liquid) molecular dynamics simulation of cementite. The predictions of the potential are in good agreement with first-principles calculations and experiments. In the second study the site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite (SrFe12-x(Zn0.5Sn0.5)xO19 with x = 1) were investigated using firstprinciples total-energy calculations. We find that in the ground-state configuration Zn-Sn ions preferentially occupy 4f1 and 4f2 sites unlike the model previously suggested by Ghasemi et al. where Zn-Sn ions occupy 2b and 4f2 sites. Our model predicts a rapid increase in saturation magnetic moment (Ms) as well as decrease in magnetic anisotropy compared to the pure M-type Sr-hexaferrite, which is consistent with experimental observations.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-07T21:13:51Z No. of bitstreams: 1 FlavioMouraSilva.pdf: 12265404 bytes, checksum: 15367785bb1dabbe1c0f6398c05ce1c2 (MD5) / Made available in DSpace on 2017-06-07T21:13:51Z (GMT). No. of bitstreams: 1 FlavioMouraSilva.pdf: 12265404 bytes, checksum: 15367785bb1dabbe1c0f6398c05ce1c2 (MD5) Previous issue date: 2016-08-31 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this study we investigated the hexaferrites type Y Ba2(Mg1-xCox)2Fe12O22 (x = 0; 0.2; 0.4; 0.6; 0.8 and 1), Ba2Zn2Fe12O22 (Zn2Y) and type M BaFe12O19 (BaM). The experimental techniques used were the X-ray diffraction, XANES (X-ray Absorption Near Edge Structure), XPS (X-ray Photoelectron Spectroscopy), Specular Reflectance IR Spectroscopy, Raman Spectroscopy, Magnetometry and Impedance Spectroscopy. The investigated samples were synthesized by ceramic conventional method. The results of X – ray diffraction confirmed to obtain the desired phase with a small amount of subphases, except for BaM did not show traces of impurities or subphases. XANES and XPS measurements indicated a predominance of 3+ valence for Fe ion and the presence of the 2+ and 3+ oxidation states to Co ions in the samples containing cobalt. As for measures of Raman spectroscopy and reflectance speculate at room temperature it was possible to classify the main bands, qualitatively, making a comparison of the spectra measured with ferrites spectra reported in the literature. From this analysis it was possible associate the high-frequency bands (560-700 cm -1) and low frequency bands (<130 cm-1) with vibration tetrahedra (Mg, Co and Zn)O4 and Ba-O, respectively. As for the system Ba2(Mg1-xCox)2Fe12O22, the specular reflectance spectra and X-ray diffraction patterns showed no appreciable change with magnesium substitution by cobalt, showing no change of space group in the samples due to this substitution. Analysis of the Raman spectra of this system showed that the bands shift to low frequency regions as it grows cobalt content in the structure and new bands at 623 cm-1 and 459 cm-1 appear, which are due to vibration Co-O bonds. Anomalies were observed in the phonon parameters (frequency, width and intensity) in Raman spectroscopy measurements at high and low temperatures in Ba2Mg2Fe12O22 (Mg2Y) samples, Ba2Zn2Fe12O22 (Zn2Y) and BaFe12O19 (BaM) at temperatures where they exhibit magnetic phase transitions. With the aid of additional magnetic measurements, we analyze these results in terms of the spin-phonon coupling. Furthermore, the observation of spin-phonon coupling in the low temperature on Mg2Y suggested magnetostriction as magnetoelectricity mechanism for this material. The electrical measurements on Mg2Y showed that the relaxation process is due to conductive mechanisms being thermally activated and showed grain and grain boundary contributions to the impedance of the sample. / Neste trabalho foram investigadas as hexaferritas tipo Y Ba2(Mg1-xCox)2Fe12O22 (x=0; 0,2; 0,4; 0,6; 0,8 e 1), Ba2Zn2Fe12O22 (Zn2Y) e tipo M BaFe12O19 (BaM). As técnicas experimentais utilizadas foram a Difração de raios X, XANES (X-ray Absortion Near Edge Structure), XPS (X-ray Photoelectron Spectroscopy), Espectroscopia no Infravermelho por Refletância Especular, Espectroscopia Raman, Magnetometria e Espectroscopia de Impedância. As amostras investigadas foram sintetizadas por método convencional cerâmico. Os resultados de difração de raios X confirmaram a obtenção das fases pretendidas com uma pequena quantidade de subfases, exceto para BaM que não apresentou traços de impurezas ou subfases. Medidas XANES e XPS indicaram uma redominância a valência 3+ para o Íon Fe e a presença dos estados de oxidação 2+ e 3+ para os Íons Co nas amostras que contém cobalto. Quanto ¾s medidas de Espectroscopia Raman e Refletância Especular ¾ temperatura ambiente foi possível classificar a principais bandas, de forma qualitativa, fazendo um comparativo dos espectros medidos com os espectros de ferritas reportados na literatura. Dessa análise foi possível associar as bandas de alta frequência (560-700 cm-1) e as bandas de baixa frequência (<130 cm-1) ¾s vibrações de tetraedros (Mg, Co e Zn)O4 e de Ba-O, respectivamente. Quanto ao sistema Ba2(Mg1-xCox)2Fe12O22, os espectros de refletância especular e padrões de difração de raios X não apresentaram mudanças apreciáveis com a substituição de magnésio por cobalto, mostrando que não há mudança de grupo espacial nas amostras devido a essa substituição. A análise dos espectros Raman desse sistema, mostrou que as bandas se deslocam para regiões de baixa frequência ¾ medida que cresce o teor de cobalto na estrutura e que novas bandas em 623 cm-1 e 459 cm-1 surgem, sendo estas devido ¾ vibração de ligações Co-O. Foram observadas anomalias nos parâmetros de fônons (frequência largura e intensidade) nas medidas de espectroscopia Raman em altas e baixas temperaturas nas amostras Ba2Mg2Fe12O22 (Mg2Y), Ba2Zn2Fe12O22 (Zn2Y) e BaFe12O19 (BaM) em temperaturas nas quais estas exibem transições de fase magnética. Com o auxílio de medidas magnéticas complementares, analisamos tais resultados em termos do acoplamento spin-fônon. Além disso, a observação do acoplamento spin-fônons em baixa temperatura na Mg2Y sugeriu a magnetostrição como mecanismo de magnetoeletricidade para este material. As medidas elétricas na Mg2Y mostraram que o processo de relaxação se deve a mecanismos condutivos termicamente ativados e revelaram contribuições de grão e contorno de grão para a impedância da amostra.

Ce travail porte sur la modélisation électromagnétique des circulateurs à jonction-Y polarisé non-uniformément en vue de concevoir et de réaliser des structures ultraminiatures. L'originalité de cette étude repose sur l'approche théorique permettant de mieux décrire le comportement électromagnétique de la structure. Cette approche tient compte de la non-uniformité des différents champs apparaissant dans la structure (champ statique de polarisation, champs démagnétisants et champ interne) ainsi que de l'état d'aimantation du matériau ferrite. La réalisation de circulateurs microruban en bande X a permis de confronter des résultats expérimentaux aux réponses théoriques obtenues à partir de notre approche théorique et d'observer la dégradation des performances du dispositif due à la non-uniformité des champs statiques. A l'aide de modifications apportées à l'analyse électromagnétique, les réponses de dispositifs à base d'hexaferrites autopolarisés fonctionnant à la rémanence sont obtenues.

In this dissertation were studied powders and pellets of the CayFe12-yO19-aFe2O3(0 ≤ y ≤ 1,0) obtained by the proteic sol-gel process having as precursor coconut water. Samples were analyzed by techniques of X-ray diffraction, magnetometry and impedance. The impedance analysis showed that resistivity increased with the calcium concentration to 0.0 <y ≤ 0.2, but decreased for y> 0.2 and reached saturation at 7.5×106 Ω.cm to y = 0.9. The magnetization curve as a function of the magnetic field presents high values of saturation magnetization (Ms = 40 emu / g) with low remanent field (Mr = 6.7 emu / g) and coercive field (Hc = 320 Oe). By means of X- ray diffraction we have observed that agglomerates of the type hexaferrite CaM were formed after the doping with hematite, i.e., there was the existence of the two phases occurring antiferromagnetic coupling confirmed by exchange bias field of 9.6 Oe. / Resumo : Nesta dissertação foram estudados os pós e pastilhas de CayFe12-yO19 - (0 ≤ y ≤ 1,0) obtidos por meio do processo sol-gel proteico que tem como precursor água de coco. As amostras foram analisadas através de técnicas de difratometria de raios X, magnetometria e impedanciometria. A análise de impedância mostrou que a resistividade aumentou com a concentração de cálcio para 0,0 < y 0,2, mas diminuiu para y> 0,2 e atingiu a saturação em 7,5×106 cm para y = 0,9. A curva de magnetização como uma função do campo magnético teve valores elevados de magnetização de saturação (Ms = 40 emu/g) com baixo campo remanente (Mr = 6,7 emu/g) e campo coercivo (Hc = 320 Oe). Por meio da difratometria de raios X observou-se que foram formados aglomerados da hexaferrita tipo CaM a partir da dopagem da hematita, ou seja, houve a existência das duas fases ocorrendo um acoplamento ferro-antiferro, confirmado pelo campo de exchange bias na curva de M x H de 9,6 Oe.