Materials Science Forum Vol. 802

Abstract: The coercivity in soft and hard magnetic materials has different origin. The high coercivity of barium ferrite, SmCo₅, Sm₂Co₁₇ or Nd₂Fe₁₄B is due to high magnetocrystalline anisotropy, and the processing aims very small grain size (nanocrystalline). In the case of soft magnetic materials, the coercivity has origin in defects that are able to stop domain wall movement, as for example grain boundaries, inclusions or dislocations. Soft magnetic materials in general present large domain wall thickness (thousands of Angstroms for pure iron), whereas domain wall thickness is ~ 50 Angstroms for SmCo₅ and Nd₂Fe₁₄B. The differences between hard and soft magnetic behavior are commented and discussed. The domain wall energy and thickness can be used as parameters for classifying soft and hard magnetic behavior. Other examples of soft magnetic materials are the amorphous alloys and the nanocrystalline soft magnetic materials with grain size very below the single domain particle size. The soft behaviour in amorphous and soft nanocrystalline materials is also discussed.

Abstract: The Bitter method is an adequate method for domains observation, and depends on careful metallographic preparation and also on a proper magnetic colloid solution. In this study, Bitter patterns were obtained for a Nd-Fe-B as-cast alloy. The Nd₂Fe₁₄B grains show strong anisotropy during growing, and are larger for the direction perpendicular to the easy magnetization axis. The equilibrium distance between domain walls allows an estimate of intrinsic parameters of ferromagnetic phases as the domain wall energy and the single domain particle size.

Abstract: Powder injection molding (PIM) is a well-known process for the production of geometrically complex shaped parts at a high production rate. The possibility of recycling the raw material (feedstock) is economically necessary for industrial applications, especially when processing very expensive materials, such as rare-earth alloys. In this study, Nd-Fe-B magnets were fabricated by powder injection molding, in which the injection residues (gates, sprue and surplus green samples) were reprocessed, up to 6 times. Subsequently, samples underwent binder extraction by chemical and thermal procedures, being finally sintered under high vacuum. Physical characterizations were carried out and the magnetic properties were measured via B-H trace analysis in each batch to study the influence of the recycled feedstock on the magnetic properties of produced samples. It was concluded that the properties are significantly affected after the second reprocessing lot, mainly due to the contamination of the feedstock by organic elements by-products of the binder decomposition.

Abstract: The aim of this work is the development of an electronic device for the rapid characterization of soft ferromagnetic materials for electrical purposes named single sheet tester (sst). The basis of this study consists in determining the magnetic induction, B and the magnetic field, H, by using a simplified circuit. Results are compared with classical epstein test and the obtained losses present an error less than 10%.

Abstract: The shortage of dysprosium as an alloying element has directed the research on the grain size refining of NdFeB, since higher coercivities can be obtained by decreasing the grain size, without Dy addition. The Spark Plasma Sintering (SPS) is a consolidation process which allows densification at lower temperatures and shorter dwell times of sintering, thus avoiding the grain growth. Therefore, the typical temperature of sintering of NdFeB magnets can be decreased from 1050°C to around 800°C, as it was evidenced by means of SPS shrinkage curves and the high densified microestructure obtained in this work.

Abstract: NdFeB bonded magnets are produced by compression molding and curing. Typical raw materials are mixtures of epoxy resin and Nd₂Fe₁₄B flakes, produced mostly by melt spinning. The curing temperature should be adjusted for obtaining the best in terms of mechanical properties without overheating the pressed component. High curing temperatures can strongly oxidize Nd, generating more heat, and burning may occurs. The curing reaction is exothermic, thus the actual curing temperature will depend on the cured mass. This paper investigates this heating generation during curing, using DSC experimental apparatus.

Abstract: Hysteresis loops were calculated according the Stoner-Wohlfarth model. Using as values for constants of magnetocrystalline anisotropy K₁ =4.5 and K₂=0.66 (J m⁻³), and 1.61 T for magnetization of saturation of Nd₂Fe₁₄B, the maximum coercivity for isotropic Nd₂Fe₁₄B was predicted as mi0 H = 2.95 T (29.5kOe). For a very well aligned magnet, with M_r/M_s=0.96, following the f (alpha)=cosⁿ(alpha) distribution, the theoretical coercivity limit was estimated as mi0 H = 3.6 T (36 kOe). These estimates are valid for the ternary Nd₂Fe₁₄B alloy. It is predicted the upper limit for the coercive field as function of grain size for NdFeB and PrFeB magnets. Addition of Praseodymium is an effective method for increasing coercivity of NdFeB magnets.

Abstract: Hysteresis curves were calculated according the Stoner-Wohlfarth (SW) model using values of K₂/K₁ from zero up to 0.25 (where K₂ is the 2^nd order anisotropy constant and where K₁ is the 1^st order anisotropy constant). The SW calculations assume Nd₂Fe₁₄B nanocrystalline magnets with single domain particle size (grain diameter less than 150 nm). The effect of K₂ on several index of merit of magnets as BH_max and squareness are discussed. As a general result, increasing K₂, the intrinsic coercivity increases. However, the increase of K₂ almost does not improve the BH_max of isotropic magnets. It is found that the effect of the K₂/K₁ ratio is coupled with the effect of texture, i.e., the effect of K₂ is more significant for pronounced texture.

Abstract: A model for simulating the reduction of metallic neodymium from molten salts is presented. The model was formulated with basis on the Navier-Stokes equations coupled with the Maxwell's relations. The model is useful for predicting optimum parameters of processing, as for example cell geometry and current density.

Abstract: Micromagnetics and its particular case, the Stoner-Wohlfarth model, are reviewed. The possibility of occurrence of curling is discussed. The use of the Felix Bloch approximation for exchange energy description should be avoided. A tensor 3x3 has to be used for appropriate evaluation of the exchange energy in phases with less symmetry, as Nd₂Fe₁₄B, Sm₂Co₁₇ and SmCo₅. The assumptions used in classic Brown micromagnetics make it less suitable for phases with high magnetocrystalline anisotropy. The role of the exchange term in the Stoner-Wohlfarth Callen-Liu-Cullen model is clarified.