Effect of carbon on the crystal structure, microstructure and magnetic properties of neodymium-iron-(boron,carbon) nanocomposite magnets

The effect of carbon substitution for boron has been investigated on the crystallization behavior, structural, microstructural and magnetic properties of nano-composite Nd₁₀Fe₈₂B_8−xC _x, (x = 0, 2, 4, 5, 6, 7) ribbons. Four different structural states were observed in as-spun ribbons (i) 2:14:1 and α-Fe, (ii) 2:14:1, 2:17:C_x, and α-Fe, (iii) 2:17:C _x and α-Fe, and (iv) amorphous), depending on the wheel speed and the amount of carbon. After annealing all samples were composed of a mixture of the magnetically hard 2:14:1 and the soft α-Fe phases. Crystallization studies on amorphous samples revealed one exothermic peak for x ≤ 4 and two for x > 4. In the former case the 2:14:1 phase crystallizes along with the α-Fe directly from the amorphous phase, whereas in the latter, they crystallize from the metastable 2:17:C_x phase. The activation energy for the formation of the 2:14:1 phase increases with the amount of carbon (290 kJ/mol for x = 2 to 480 kJ/mol for x = 6), whereas, it decreases (367.5 kJ/mol for x = 5 to 302.9 kJ/mol for x = 6) for the formation of the 2:17:C_x phase.; The best hard magnetic properties were obtained for x = 2 with H_C = 8.7 kOe and (BH)_max = 15.2 MGOe. The exchange interactions between the fine grains are also slightly enhanced in this composition, as shown by δM plots. However, these parameters are reduced drastically for x > 4, due to smaller amount of the magnetically hard 2:14:1 phase and the coarsening of the microstructure. Micro-structural studies revealed finer and more uniform grain sizes (20–50 nm) for the as-spun Nd₁₀Fe₈₂B₆C₂ ribbons. For compositions with larger carbon content, the microstructure becomes coarser and inhomogeneous, and dendritic regions of α-Fe appear.; The effect of small substitutions (Nb, Zr, Ga and Cr) on the magnetic properties of Nd₁₀Fe₈₂B₆C₂ and Nd₁₀Fe₈₂B₂C₆ was examined. DSC and x-ray studies showed that Nb and Zr additions stabilize the amorphous phase in the as-spun ribbons. Also, the amount and size of α-Fe phase in both as-spun and annealed Nd₁₀Fe₈₂B₂C ₆ ribbons was significantly reduced for Nb and Zr substitutions, but increased for all the other substitutions. From the magnetic properties point of view Nb was proven to be more beneficial in Nd₁₀Fe₈₂B ₆C₂ ribbons, and Zr in Nd₁₀Fe₈₂B ₂C₆.; The field and temperature dependence of coercivity were examined and fitted to current magnetic hardening models. The former type of measurement showed a behavior characteristic of a mixture of domain wall pinning and nucleation of reversed domains for x = 0 and 2 and for the Nb/Zr substituted compositions. For x > 2, a behavior closer to pinning mechanism was observed. In contrast, the H_C versus T data fitted very well the nucleation model. (Abstract shortened by UMI.)...