Dissertations / Theses on the topic 'Neodymium-iron-boron magnets'

Magnetization processes in a sintered Nd-Fe-B permanent magnet (NEOMAX-35) were examined on a small scale using a Hall-effect microprobe with an active area 75 μm on a side. Probes were made by evaporating bismuth through a stencil mask onto glass slides. Experiments were performed by placing a probe onto the polished pole face of a Nd-Fe-B magnet and inserting the probe-magnet assembly into an electromagnet. Barkhausen steps, indicating rapid domain wall motion, were observed (superimposed upon the blank probe signal) in the demagnetization of a fully magnetized magnet. Magnetization traces for a thermally demagnetized Nd-Fe-B magnet did not exhibit measurable Barkhausen steps until a field of approximately 1.2 T was applied. The following observations were made for two thermally demagnetized samples which were cycled through minor hysteresis loops (maximum applied field of approximately 2 T): (1) virgin magnetization traces did not contain measurable Barkhausen steps, however all other forward and reverse magnetization traces did; (2) the initial reverse magnetization trace exhibited more and larger Barkhausen steps than subsequent traces; and (3) some Barkhausen steps were repeatable, that is, occurring at approximately the same field on each subsequent forward or reverse trace. Hall voltage signals were on the order of millivolts for probe currents of 10 mA.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996.
Vita.
Includes bibliographical references (p. 383-388).
by Steven A. Attanasio.
Ph.D.

Strip casting is a rapid processing technique used in the production of alloys for both sintered and HDDR bonded NdFeB magnets. During strip casting, molten alloys are gravity fed onto a rotating copper wheel. The rapid cooling rate produces NdFeB microstructures which have many advantages over conventionally cast NdFeB book-mould alloys such as less alpha iron, near stoichiometric alloy compositions and a finer grain structure. The wheel texture of a pilot scale strip caster was altered in order to optimize the microstructure and improve the consistency of strip cast flakes for the production of NdFeB sintered magnets. This made it possible to reduce the flake thickness distribution and alpha iron content, improve the grain size consistency and increase lamellar alignment. In NdFeB alloys for HDDR (Hydrogenation Disproportionation Desorption and Recombination) powders, which are used in bonded magnets, a large grained material is preferable with little or no alpha iron. By implementing a combination of surface texture and reduced wheel speed, the grain width was increased from ~5 to ~40 μm, whilst maintaining a low alpha iron content. The magnetic performance has been shown to be comparable to conventional book-mould alloys that have been heat treated for 10 hours at 1140°C.

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Shock compaction experiments were performed on soft magnetic phases Fe₄N and Fe₁₆N₂, and hard magnetic phases Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ in order to determine their thermo-mechanical stability during shock loading and explore the possibility of fabricating a textured nanocomposite magnet. Gas gun experiments performed on powders pressed in a three capsule fixture showed phase transformations occurring in Fe₄N, Fe₁₆N₂, and Nd₂Fe₁₄B, while Sm₂Fe₁₇N₃ was observed to be relatively stable. Shock compaction of FCC Fe₄N resulted in a partial transformation to HCP Fe₃N, consistent with previous reports of the transition occurring at a static pressure of ~3 GPa. Shock compaction of Fe₁₆N₂ produced decomposition products alpha-Fe, Fe₄N, and FeN due to a combination of thermal effects associated with dynamic void collapse and plastic deformation. Decomposition of Nd-Fe-B, producing alpha-Fe and amorphous Nd-Fe-B, was observed in several shock consolidated samples and is attributed to deformation associated with shock compaction, similar to decomposition reported in ball milled Nd-Fe-B. No decomposition was observed in shock compacted samples of Sm-Fe-N, which is consistent with literature reports showing decomposition occurring only in samples compacted at a pressure above ~15 GPa. Nd-Fe-B and Sm-Fe-N were shown to accommodate deformation primarily by grain size reduction, especially in large grained materials. Hard/Soft composite magnetic materials were formed by mixing single crystal particles of Nd-Fe-B with iron nanoparticles, and the alignment-by-magnetic-field technique was able to introduce significant texture into green compacts of this mixture. While problems with decomposition of the Nd₂Fe₁₄B phase prevented fabricating bulk magnets from the aligned green compacts, retention of the nanoscale morphology of the alpha-Fe particles and the high alignment of the green compacts shows promise for future development of textured nanocomposite magnets through shock compaction.

Os magnetos de Neodímio Ferro Boro (Nd2Fe14B) apresentam excelentes propriedades magnéticas e compatibilidade biológica, embora sejam frágeis e possuam baixa resistência à corrosão. As forças de atração magnética de magnetos de terras raras têm sido empregadas em sistemas de retenção, principalmente associadas aos implantes osseointegráveis, em próteses bucais e próteses faciais, bem como em aparelhos ortodônticos. Este estudo avalia o comportamento das forças de repulsão magnética, em função do volume e comprimento do eixo magnético, quando submetidos a afastamentos, de modo que esta propriedade física seja empregada em odontologia. Vinte e quatro corpos de prova, com um ou dois pares de magnetos de Nd2Fe14B, com diferentes formas e volumes variados, foram submetidos a ensaio mecânico para mensuração das forças de repulsão magnética, em condições de afastamentos que variaram entre 0,0 e 20,0 mm. Os dados observados mostram que, considerando-se uma mesma área superficial de polo, a força de repulsão magnética varia em função do comprimento do eixo magnético. Os dados relativos ao afastamento foram submetidos a análise de variância (p0,05), tendo-se observado que o aumento da distância entre os magnetos diminui significantemente a força de repulsão magnética na maioria dos corpos de prova. No afastamento 0,0 mm as forças magnéticas de repulsão dos subgrupos mantêm uma sequência crescente a partir do subgrupo de menor volume de magnetos ao subgrupo de maior volume de magnetos, os quais permanecem nessa sequência nos afastamentos iniciais. A força de repulsão magnética gerada por magnetos pequenos varia menos em função do aumento do afastamento. Conclui-se que a variação da força de repulsão magnética, dada a mesma área superficial de polo, apresenta uma relação direta com o volume e com o eixo magnético onde, quanto maior o volume ou o comprimento do eixo magnético, maior a força de repulsão magnética. A relação direta entre volume e força de repulsão magnética não se altera frente aos afastamentos inicais, tendendo a se tornar imensurável com o aumento da distância. A força de repulsão magnética mantém uma relação inversa com a distância entre os pares de magnetos. Este ensaio mecânico indica que a força de repulsão magnética apresenta, assim como a força de atração magnética, uma curva hiperbólica de redução frente ao aumento do afastamento.
Neodymium Iron Boron (Nd2Fe14B) magnets exhibit excellent magnetic properties and demonstrate biological compatibility, despite of their fragility and low resistance to corrosion. The magnetic attraction forces of rare earth magnets have been used in retention systems mainly associated with osseointegrated implants, in oral and facial prostheses, as well as orthodontic appliances. This study evaluates the repulsion forces of magnets, varying volume and length of magnetic axis when subjected to vertical distance, in order to assess the usefulness of such physical property in clinical situations. Twenty-four specimens, with one or two pairs of Nd2Fe14B magnets, presenting different shapes and various volumes, were subjected to mechanical testing so as to measure magnetic repulsion forces, under conditions of vertical pole face separation ranging from 0.0 to 20.0 mm. Repulsive magnetic force varies depending on the length of the magnetic axis, when considering the same surface area of a pole. Analysis of Variance - (p0.05) indicated that by increasing the distance between the pairs of magnets, magnetic repulsion force significantly reduces in most of the specimens. Starting at a 0.0 mm distance, the magnetic repulsion forces keep an enhancing sequence from lower to higher magnetic volume, holding these characteristics during initials distances. Repulsion magnetic forces generated by small magnets varies less as the distance increases. The results indicate the repulsion magnetic force variation, fixing the same pole surface area, presenting a direct relationship between volume and magnetic axis, since the increase of the volume or of the magnetic axis length leads to a higher magnetic repulsive force. The relationship between volume and magnetic repulsion force does not vary at initial distance. Nevertheless, when such distance increases, the force between the magnets significantly decreases. The magnetic repulsion force holds an inverse relationship with respect to the distance between magnets pairs. This mechanical test indicates that the magnetic repulsion force, as well the magnetic attraction force, present a hyperbolic curve of force reduction due to the increasing vertical distance.

Un creuset froid inductif a ete mis en place sous champ magnetique intense. Ce dispositif permet d'appliquer au cours de la solidification de materiaux tres reactifs un champ magnetique homogene ou inhomogene dans l'espace. Il permet egalement de realiser la levitation stabilisee par courants de foucault d'alliages metalliques liquides paramagnetiques. De nombreuses applications sont ouvertes dans le domaine de l'elaboration de materiaux ultra-purs. Nd#2fe#1#4b est le compose le plus utilise a la fabrication d'aimants permanents performants. Les conditions de texturation de ce compose par solidification sous champ magnetique sont etudiees. Malgre la decroissance thermique de l'energie d'anisotropie, le champ oriente des cristallites de nd#2fe#1#4b dans l'etat paramagnetique en suspension dans un liquide eutectique ndcu. L'anisotropie de la susceptibilite magnetique de nd#2fe#1#4b existe jusqu'au point de fusion et elle est toujours dirigee suivant la direction cristallographique (001). Cependant, l'energie magnetocristalline n'est pas suffisante pour induire une orientation des germes de cristallisation de nd#2fe#1#4b. Par consequent, la solidification sous un champ de 8t d'alliages a base de nd#2fe#1#4b a partir du liquide faiblement surchauffe ne conduit pas a un materiau texture, au contraire de dy#2fe#1#4b qui s'oriente partiellement suivant une direction de facile aimantation. Des mesures de susceptibilite en fonction de la temperature et du temps ont permis d'observer les effets de la surchauffe sur la solidification d'un alliage a base de nd#2fe#1#4b. Une forte surchauffe a pour consequence de detruire les germes de nd#2fe#1#4b presents dans le liquide lorsque celui-ci est maintenu a une temperature proche de la fusion. La metastabilite des germes de cristallisation est mis en evidence par la formation d'une phase solide metastable apres forte surchauffe du liquide

La spectrométrie Mössbauer du fer 57 a été utilisée en relation avec d'autres techniques (diffractions de rayons X, microscopie électronique à balayage, microsonde électronique) pour étudier la corrosion d'aimants permanents industriels Nd-Fe-B entre 90°C (avec humidité) et 400°C, et d'un acier faiblement allié (42CD4) en eau de mer naturelle en fonction de la pression hydrostatique. Dans les aimants Nd-Fe-B, l'évolution de la phase Nd2Fe1#4B a été plus particulièrement suivie. Cette phase semble disparaître sous l'effet de l'oxydation du Nd, en formant principalement du fer alpha. Un net ralentissement de ce processus a été mis en évidence dans les aimants avec Co et V préparés par décrépitation par l'hydrogène. La composition de la couche de rouille formée sur des plaques d'acier 42CD4 immergées en eau de mer naturelle pendant 15 jours ne varie pas pour des pressions supérieures à 3MPa. L'akaganéite n'est observée qu'aux pressions inferieures a 3MPa

Les alliages amorphes métal de transition-terre rare peuvent présenter de l'anisotropie magnétique aléatoire due au désordre structural et au couplage spin-orbite de la terre rare. Les propriétés structurales et magnétiques de rubans amorphes hypertrempés Fe80-xTrxB20 (Tr = Nd, Ho, Dy ; x 16 % at. ) Ont été étudiées par spectrométrie Mössbauer, spectroscopie d'absorption X, analyse thermique différentielle et diffraction X aux grands angles. On montre par Mössbauer que les propriétés structurales des alliages ne dépendent pas de la nature de la terre rare. Une discontinuité des variations des paramètres hyperfins a lieu dans le domaine de concentration X#T#R#C 8-9% at. . Elle a été attribuée à une modification structurale autour du fer. Au-dessous de X#T#R#C , les sites de fer apparaissent similaires à ceux de Fe#8#0B#2#0. Un autre type d'environnement, plus magnétique, est suggère au-delà de X#T#R#C. La discontinuité et son origine structurale sont confirmées par spectroscopie d'absorption X. La modification intervient déjà dans la première sphère de coordination du fer. Les caractéristiques des paires Fe-Fe, identiques pour X#T#R < X#T#R#C, sont analogues à celles de Fe#8#0B#2#0. Au voisinage de X#T#R#C, un atome de fer supplémentaire apparait dans la première sphère de coordination et la distance moyenne Fe-Fe augmente. Au-delà de X#T#R#C, la première sphère de coordination du fer se contracte progressivement. D'après l'étude de cristallisation de deux alliages situés de part et d'autre de X#T#R#C, les premiers composés formés sont en accord avec des environnements locaux différents dans les deux domaines. L'étude Mössbauer sous champ des alliages les plus concentrés en terre rare indique un rapport anisotropie/échange plus faible dans l'alliage à base de Ho que dans ceux à base de Nd et Dy. Un modèle décrivant l'évolution sous champ de la structure magnétique du fer en termes de déformation angulaire a été développé. Il explique quantitativement les résultats dans le cas de Ho et Dy.