IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015
2101204
Coercivity and Thermal Stability Enhancement for Spark-Plasma-Sintered Nanocrystalline NdFeB Magnets With Dy2 O3 and Zn Additions Z. W. Liu1,2 , L. Z. Zhao1 , S. L. Hu1 , H. Y. Yu1 , X. C. Zhong1 , and X. X. Gao2 1 School
of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
2 State
NdFeB bulk magnets were synthesized by spark plasma sintering (SPS) using the precursors of melt spun nanocrystalline Nd10.15 Pr1.86 Fe80.41 Al1.67 B5.91 ribbons mixed with Zn and/or Dy2 O3 powders. The addition of 0.6 wt.% Zn or 2 wt.% Dy2 O3 is effective in improving the magnetic properties of the magnets, and their coercivities are about 11% or 15%, respectively, higher than that of the additive free magnet. Both additives can suppress the grain growth of Nd2 Fe14 B phase. Dy2 O3 is not beneficial to the densification during SPS, but Dy is considered to partly diffuse into the grains near particle boundaries and to form (Nd, Dy)2 Fe14 B phase. NdZn and NdZn5 phases were observed in the SPSed magnets with Zn additions more than 1 wt.%, which leads to coarse grains and porosities between the particles. The magnet with combined additions of 2 wt.% Dy2 O3 and 0.6 wt.% Zn showed good thermal stability with small temperature coefficients and optimal magnetic properties with high coercivity and maximum energy product. Index Terms— Magnetic properties, NdFeB magnets, spark plasma sintering (SPS), thermal stability.
I. I NTRODUCTION
N
dFeB permanent magnets have found various applications in devices. The poor thermal stability is one of the main problems with the sintered NdFeB magnets [1]. A relatively large grain size, typically >3 μm, is also not beneficial to achieve high coercivity. In addition, high rare earth (RE) content results in high magnet price, which is not favorable for the industry. Hence, current efforts are directed to improve their magnetic properties and reduce their cost by compositional modification, microstructure optimization, and new process employment [2]. Spark plasma sintering (SPS) is one of the novel sintering techniques for preparing NdFeB magnets in a laboratory scale [3]. Low sintering temperature and short holding time make it possible to sinter nanocrystalline powders into a fully dense bulk magnet [4], [5]. However, completely avoiding the grain coarsening seems still impossible, and the coercivity reduction in SPSed magnets is inevitable [6]. Thus, it is extremely important to maintain high coercivity for SPSed magnets. Fuerst and Brewer [7] indicated that the magnetic powders can be mixed with some metal powders for SPS. During sintering, the metal powders can presumably diffuse into the Nd-rich phase surrounding the Nd2 Fe14 B grains, which can enhance the magnetic anisotropy and the coercivity [7]. It was found that Cu and Zn powder addition enhanced the coercivity of hot-pressed magnets by ∼37% [8] and Zn addition increased the coercivity of die-upset magnets by ∼91% [9]. Another effective way to enhance the coercivity is to develop a core–shell microstructure, which is formed Manuscript received March 16, 2015; revised April 27, 2015; accepted May 12, 2015. Date of publication May 19, 2015; date of current version October 22, 2015. Corresponding author: Z. W. Liu (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2015.2434943
by the segregation of Dy along the grain boundary, by grain boundary diffusion process, or simply by the addition of powdered Dy compounds [10]–[13]. It was reported that 4 wt.% Dy2 O3 enhanced the coercivity from 11.6 to 16 kOe, for a sintered Dy-free NdFeB magnet [10]. In this paper, we report the effects of separated and combined Dy2 O3 and Zn additions on the magnetic properties, microstructure, and thermal stability of SPSed nanocrystalline NdFeB magnets. II. E XPERIMENTAL P ROCEDURE Commercial melt spun Nd10.15Pr1.86 Fe80.41 Al1.67 B5.91 powders with particle size up to 200 μm and grain size
