1Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University
2Department of Materials Science, Graduate School of Engineering, Tohoku University
Abstract:
The phase components at the grain boundaries in the Nd-Fe-B permanent magnet materials have the important influence on the coercivity. Some elements such as oxygen and copper in the boundary phase can greatly improve the properties of the Nd-Fe-B magnet. It is found that a disordered NdOx-fcc phase formed at the interface between Nd-rich and Nd2Fe14B phases[1] which believed to play an important role in enhancing the coercivity. Existing of small amount of copper in the boundary phase also increases the coercivity. In order to understand the microscopic mechanism of the effect of there elements on the properties of the Nd-Fe-B magnet, first-principles calculations based on density functional theory coupled with cluster expansion method are performed to investigate the ground state stability of disordered NdOx-fcc phase[2]. Formation energies of single O-vacancy, single Cu in various structures in Nd-O are also calculated.
Figure 1(a) is the comparisons of stabilities of O-doped structures for fcc-based ZnS vs. hcp-based hP5 series, it shows that the ZnS based tends to adopt oxygen deficiency so as to produces a stable NdOx-fcc solid solution in a wide oxygen concentration. In contrary, the hP5 structure is most stable phase but it is hard to accommodate O-vacancies at various oxygen concentration, which explain the formation mechanism of this important Nd oxides and also coincides to experimental observation. Figure 1(b) is the case ZnS vs. B1 structures. Both are fcc-based structure but B1 is slightly more stable than ZnS at NdO composition. B1-based shows similar behavior to the hP5 structure that it is hard to accommodate O-vacancies at various oxygen concentration.
Table 1 lists the calculated formation energies for several Cu-doped Nd-O structures. Although, the formation energies of Cu in Nd-dhcp structure are positive but still relatively small which indicates that there is a possibility of Cu solubility in Nd-dhcp phase. The formation energy of Cu in NdO-ZnS structure is the only case in which the Cu-doped has lower formation energy than un-doped structures. The Cu concentration of Cu soluble is possible in NdO-ZnS structure has been also estimated.
Figure 1(a) is the comparisons of stabilities of O-doped structures for fcc-based ZnS vs. hcp-based hP5 series, it shows that the ZnS based tends to adopt oxygen deficiency so as to produces a stable NdOx-fcc solid solution in a wide oxygen concentration. In contrary, the hP5 structure is most stable phase but it is hard to accommodate O-vacancies at various oxygen concentration, which explain the formation mechanism of this important Nd oxides and also coincides to experimental observation. Figure 1(b) is the case ZnS vs. B1 structures. Both are fcc-based structure but B1 is slightly more stable than ZnS at NdO composition. B1-based shows similar behavior to the hP5 structure that it is hard to accommodate O-vacancies at various oxygen concentration.
Table 1 lists the calculated formation energies for several Cu-doped Nd-O structures. Although, the formation energies of Cu in Nd-dhcp structure are positive but still relatively small which indicates that there is a possibility of Cu solubility in Nd-dhcp phase. The formation energy of Cu in NdO-ZnS structure is the only case in which the Cu-doped has lower formation energy than un-doped structures. The Cu concentration of Cu soluble is possible in NdO-ZnS structure has been also estimated.
References
[1] T. Fukagawa, S. Hirosawa, T. Ohkubo, K. Hono, J. Appl. Phys. 105, 07A724 (2009).
[2] Y. Chen, A. Saengdeejing, M. Matsuura, S. Sugimoto, JOM 66, 1133 (2014).
[1] T. Fukagawa, S. Hirosawa, T. Ohkubo, K. Hono, J. Appl. Phys. 105, 07A724 (2009).
[2] Y. Chen, A. Saengdeejing, M. Matsuura, S. Sugimoto, JOM 66, 1133 (2014).
Figure 1: Formation energies of NdOx (a) ZnS vs. hP5 structures, (b) ZnS vs.B1 structures
Table 1: Formation energies of Cu-doped