Materials Science Forum Vols. 727-728

Abstract: The search for energetic alternatives and the concern about water preservation are novel ideas that challenge the humanity. This project was inspired in these challenges, being based in the utilization of solar energy for desalination of brackish water. A parabolic solar concentrator captures the solar radiation heating thermal fluid, which is closed-circuit circulated into a evaporator, which has three trays. The first has the functions of storing, pre-heating and condensing. The second works as an evaporator and the third stores the evaporated water. The system was tested with and without pump. The thermal fluid temperature reached 105°C and the amount of evaporated water reached 15kg/day. The equipment meets the social demand for sustainable inventions.

Abstract: The calculation of hysteresis curves with the Stoner-Wohlfarth model is somewhat laborious. For the Nd₂Fe₁₄B phase, the second order magnetocrystalline anisotropy (K₂) constant is relevant, and this case only has been discussed in some studies. In this article, a simple algorithm for the calculation of the Stoner-Wohlfarth model for isotropic NdFeB magnets is described. This algorithm makes easier the modeling of the hysteresis curves of bonded melt-spun NdFeB magnets.

Abstract: In this work, Metal Injection Molding (MIM) process was applied to manufacture Nd-Fe-B magnets, where carbon residues were quantified. In a separated test, controlled additions of carbon were added prior to sintering in the conventional processing of Nd-Fe-B magnets, aiming to simulate the binder residues with more accuracy. The carbon contents in the sintered magnets were related to final magnetic properties such as remanence and coercivity. It was found that the rare-earth content in the alloy influence the threshold where further additions of carbon will degrade coercivity. This study gives directions on developing binder systems and debinding processes, focusing on reaching adequate carbon levels to maximize final magnetic properties.

Abstract: A model, based on hyperbolic tangent curve, was found to be able to reproduce several different types of magnetization curves of soft magnetic materials. The model is especially suitable for curves with sigmoid shape. The model is compared with experimental data for bonded soft iron powders, and also other ferromagnetic materials (1045 steel). Three adjusting parameters were used in the original hyperbolic tangent equation. Each parameter has different physical meaning.

Abstract: The crystallographical texture is relevant information for NdFeB magnets, since the maximum energy product is directly related to orientation of the crystals. EBSD (Electron Backscattered Diffraction) is a very suitable tool for preferred orientation measurement of NdFeB magnets. The advantages of EBSD against X-ray Diffraction (XRD) pole figures for texture determination are discussed. EBSD identifies misaligned grains, and this is not feasible with XRD pole figures. EBSD is also helpful on the identification of oxides.

Abstract: The high uniaxial magnetic anisotropy makes phases like Nd₂Fe₁₄B, Sm₂Co₁₇, SmCo₅ or BaFe₁₂O₁₉ are very suitable as hard permanent magnetic materials. In the case of ferromagnetic materials, the global magnetostatic energy changes, according to the domain wall configuration. The simplest case to be studied is that of spherical grain with only one domain wall, in phases with uniaxial symmetry. The magnetostic energy in this system is calculated by means of Legendre polynomials. Studying the maximums and minimums of energy in this system, a simple hysteresis model can be developed. The model provides insight about the dependence of the coercive force on the grain size.

Abstract: In Nd₂Fe₁₄B sintered magnets there is increase of coercivity after Dy diffusion in the region near the surface of the grains. A kinetical model evaluating the Dy surface diffusion is presented. This model is based on a shell hypothesis: the most relevant place for the nucleation is a tiny shell near the grain boundaries. The kinetical analysis shows that a heat treatment during 1 hour at 850°C is able to produce a Dy-rich layer on the surface of the Nd₂Fe₁₄B grains.

Abstract: The nucleus size was determined for the following phases: Nd₂Fe₁₄B, Sm₂Co₁₇, SmCo₅ and BaFe₁₂O₁₉. These phases are the basis for the most relevant sintered permanent magnets. For the determination, it was assumed a spherical grain, and that the reversal of magnetization occurs in a tiny shell near the surface. The Magnetostatic energy for each domain configuration was found by means of Legendre polynomials. The nucleus is a spherical cap, near the equator. After reaching the nucleus critical size, the reversal of magnetization continues spontaneously, reducing the global energy of the system. It was found that, for each grain size, there is a different nucleus size.

Abstract: The effect of crystalline imperfections (as for instance, grain size, inclusions and dislocations) on the coercivity of soft magnetic materials is additive. This only can be explained by an Energy Balance Model. By another hand, the angular dependence of the coercivity only can be explained with a Force Balance Model. Thus both models, Energy Balance and Force Balance have to be invoked for the construction of a general model. The effect of dislocations on the coercivity can be treated as short range magnetostrictive effect. The effect of inclusions needs the consideration of the global magnetostatic energy of the system. The dependence of the coercivity with the grain size can also be explained analyzing the magnetostatic energy. The idea of pinning of domain walls by crystalline defects is reformulated, and it is shown that the coercivity increase due to crystalline defects is principally caused by variations of the magnetostatic energy in the system.

Abstract: In the case of the modeling of sintering and heat treatments, the diffusion coefficients are an essential input. However, experimental data in the literature about diffusion coefficients for rare-earth transition metal intermetallics is scarce. In this study, the available data concerning diffusion coefficients relevant for rare-earth transition metal magnets are reviewed and commented. Some empirical rules are discussed, for example the activation energy is affected by the size of the diffusing impurity atom. Diffusion coefficients for Dy, Nd and Fe into Nd₂Fe₁₄B are given according an Arrhenius equation D=D₀ exp (-Q/RT). For Dy diffusion into Nd₂Fe₁₄B, Q 315 kJ/mol and D₀ 8 . 10^-4 m²/s.