Papers by Keyword: SmCo₅

Abstract: The SmCo₅ has three sintering stages using spheres (0.6 – 1.0 mm diameter) at temperatures between 1030 and 1200 °C. During the first stage the neck radius increases as X² ~ t for less than 1200 °C temperature, the exponent is 3 at 1200 °C. Interdiffusion is sintering mechanism for exponent of 2. Sm diffuses from inside to the surface, where it is oxidized and oxides fills the neck between the spheres. Co diffuses through the oxides. At 1200 °C the sintering mechanism is evaporation-condensation of Sm. The activation enthalpy of the first stage is 582 kJmol^-1 for temperatures above 1130 °C and 210 kJmol^-1 bellow 1130 °C, respectively. The second stage is characterized by a plateau where the neck growth is arrested. The small pores in the neck and in the sphere surface layer (formed during the first stage) shrink. When these pores disappear a continuous α-Co layer forms and the third stage starts. It is essential growth of the neck formed by dense Co layer. The law of sintering is X⁴ ~ t. The activation enthalpy (276 kJmol^-1 at temperatures above 1130 °C) closes the activation enthalpy of Co self-diffusion. This (together with the exponent of 4) suggests that the Co layer plays a role similar to that of a liquid film. Making slight changes in the chemical composition of the alloys and substituting an argon atmosphere to vacuum have no influence or stages and sintering mechanisms.

Abstract: Nanocrystalline magnets (SmCo5)94(Cr3C2)6 were prepared using melt-spinning and their magnetic behaviors were investigated by studying their structures and magnetic properties. The alloys prepared using rapid solidification consisted of SmCo5 matrix phase, MgCu2-type SmCo2 secondary phase, and small traces of Sm2Co7 phase. The solidification with higher wheel speed were found to be preferable for the formation of single SmCo2 secondary phase. Relatively high coercive values of 28–36 kOe and high reduced remanence of 0.78–0.79 were observed for the SmCo5/SmCo2 nanocrystalline magnets. The shape of the corresponding magnetization curves revealed that two magnetization processes, nucleation and pinning of domain walls, took place in these magnets. The Henkel plots indicated strong inter-grain exchange coupling effect in these ribbons, consequently resulted in the phenomena of domains interacting with each other and enhancement of the remanence in the ribbons.

Abstract: The coercivity of sintered magnets like barium ferrite (BaFe12O19), samarium-cobalt (SmCo5) or neodymium-iron-boron (Nd2Fe14B) is largely affected by the grain size. A method to evaluate coercivity behavior as function of the crystalline orientation, including also the effects of grain size and lattice defects, is presented. Expressions were deduced to estimate the critical size of nucleus for spontaneous reversion of magnetization. The model indicates that the nucleation in grains of materials with high magnetocrystalline anisotropy only can begin by domain rotation. The model also predicts that the surface condition of grains is very important for the coercivity. A qualitative explanation is offered for the fact that materials with higher coercivity (or with smaller grain size) tend to follow an angular dependence of the coercivity similar to that given by the Stoner-Wohlfarth model, while materials with lower coercivity (or with larger grain size) tend to follow an angular dependence of the coercivity similar to 1 / cos theta.

Abstract: The usual process for producing the high energy magnets based on rare-earth-transition metals as for example NdFeB, SmCo5 or Sm(CoFeCuZr)z involves powder metallurgy. In many cases, it is necessary the determination of anisotropy constants (K1 – first order and K2 second order) from polycrystalline samples. This is not the ideal situation because for more accurate determinations a single crystal is necessary. Nevertheless, in many cases it is very difficult, or not possible, obtaining a single crystal. Then, for these situations, the anisotropy constants can be evaluated from polycrystalline samples with uniaxial texture. In this study, the methodology for making such determination is described. It includes the measurement of Schulz Pole figure by X-Ray diffraction in a surface perpendicular to the c-axis, the axis of easy magnetization. The measured Pole figure can be adjusted with a Gaussian distribution f(q)=exp(-q2/2s2) or with a distribution of type f(q) = cosn q. A model to evaluate the remanence from quantitative metallography is also described. From these distributions, and using the microstructural model, it is possible to estimate the initial magnetization curves for polycrystalline samples, including the effect of the 2nd order anisotropy constant (K2) which produces a curvature in initial magnetization curve. With all these data it is finally possible to estimate the initial magnetization curves for single crystal samples (theoretical), and the anisotropy constants K1 and K2. The inadequacy of Sucksmith-Thompson plots for determination of anisotropy constants from polycrystalline samples is also commented. The described method can be used either for rare-earth transition magnets or for Barium or Strontium ferrites.