Coercivity of nd fe b hot deformed magnets produced by the spark plasma sintering method

Coercivity of Nd Fe B hot deformed magnets produced by the spark plasma sintering method Coercivity of Nd Fe B hot deformed magnets produced by the spark plasma sintering method Tetsuji Saito, Shunji[.]

Coercivity of Nd-Fe-B hot-deformed magnets produced by the spark plasma sintering method

Tetsuji Saito, Shunji Nozaki, Yusuke Sajima, and Daisuke Nishio-Hamane

Citation: AIP Advances 7, 056205 (2017); doi: 10.1063/1.4973438

View online: http://dx.doi.org/10.1063/1.4973438

View Table of Contents: http://aip.scitation.org/toc/adv/7/5

Published by the American Institute of Physics

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Coercivity of Nd-Fe-B hot-deformed magnets produced

by the spark plasma sintering method

Tetsuji Saito,1, aShunji Nozaki,1Yusuke Sajima,1

and Daisuke Nishio-Hamane2

1Department of Mechanical Science and Engineering, Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan

2Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan

(Presented 1 November 2016; received 23 September 2016; accepted 17 October 2016;

published online 27 December 2016)

The effects of Nd-Cu alloy powder addition on the microstructures and magnetic properties of Nd-Fe-B hot-deformed magnets produced by the spark plasma sin-tering (SPS) method were investigated The addition of a small amount of

Nd-Cu alloy powder, up to 2%, significantly increased the coercivity of the Nd-Fe-B hot-deformed magnets without deteriorating the crystallographic alignment of the

Nd2Fe14B phase The Nd-Fe-B hot-deformed magnet with 2% Nd-Cu alloy powder had the same remanence value as the Fe-B hot-deformed magnet without

Nd-Cu alloy powder addition, but the magnet with 2% Nd-Nd-Cu alloy powder exhibited higher coercivity and a higher maximum energy product than the magnet without

Nd-Cu alloy powder addition © 2016 Author(s) All article content, except where

otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4973438]

I INTRODUCTION

Rare-earth-based magnets serve as the backbone of many products, from mobile phones and computers to hybrid and electric vehicles When such magnets are applied to the motors of electric vehicles and wind-turbine generators, their temperature rises due to the evolution of eddy currents.1,2 The addition of DyN to Nd-Fe-B magnets and the grain refinement of Nd-Fe-B magnets are under consideration as measures to reduce these eddy currents.3,4In practice, however, Nd-Fe-B magnets cannot be used for these motors, and Nd-Dy-Fe-B magnets, with Dy added to increase the coercivity, are applied instead With the extremely short supply of Dy for Nd-Dy-Fe-B permanent magnets in the international market, demand has arisen for the development of a new permanent magnet that does not contain Dy.5,6Various new compounds such as NdFe12and Sm5Fe17have been studied for this purpose.7 10These compounds are highly promising, but their practical application can be expected

to take a long time

Another approach is to increase the coercivity of Nd-Fe-B magnets without using Dy A number

of studies have been conducted with the aim of increasing the coercivity of Nd-Fe-B magnets.11 – 14In particular, the addition of Nd-Cu alloys such as Nd70Cu30and Nd80Cu20has been found to be highly effective in increasing the coercivity of Nd-Fe-B magnets Nd-Fe-B magnets can be categorized into two types: one produced by sintering, and the other by hot deformation Recently, hot-deformed magnets have been successfully produced by the spark plasma sintering (SPS) method.15–19However, the magnetic properties of these Nd-Fe-B hot-deformed magnets produced by the SPS method are not yet comparable to those of Nd-Fe-B sintered magnets In order to improve the magnetic properties of Nd-Fe-B hot-deformed magnets, it is necessary to improve their coercivity by the addition of Nd-Cu alloy In this study, we produced Nd-Fe-B hot-deformed magnets by the SPS method and examined the effects of Nd-Cu alloy powder addition on the microstructures and magnetic properties of the magnets

a

E-mail address: tetsuji.saito@it-chiba.ac.jp

2158-3226/2017/7(5)/056205/5 7, 056205-1 © Author(s) 2016

056205-2 Saito et al. AIP Advances 7, 056205 (2017)

II EXPERIMENT

A commercially available Nd-Fe-B alloy powder (MQ powder) was used as the starting material

Nd70Cu30melt-spun ribbons were prepared in an Ar atmosphere and then mechanically comminuted into powders The MQ powder was blended with Nd-Cu alloy powder (0–3 wt%) in an Ar-filled glove box The blended powders were placed into a carbon die and hot-pressed in a vacuum at

723 K for 300 s under a pressure of 100 MPa by the SPS method, forming a columnar shape of

10 mm in diameter and 10 mm in height The hot-pressed magnets were then deformed to an 80% reduction in height in a vacuum at 923 K under a pressure of ∼200 MPa by the SPS method For property measurements, specimens were cut from these magnets using a low-speed diamond saw The phases in the specimens were examined by X-ray diffraction (XRD) using Cu Kαradiation The microstructures of the specimens were examined using a transmission electron microscope (TEM) after ion-beam thinning The magnetic properties of the specimens were measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 25 kOe The samples were magnetized

in a pulsed field of 50 kOe prior to the VSM measurements

III RESULTS AND DISCUSSION

The Nd-Fe-B hot pressed magnets produced at temperatures lower than 723 K had low densities and was not hot-deformed by the SPS method Thus, the Nd-Fe-B hot pressed magnets were pro-duced at 723 K by the SPS method Since the hot-pressing temperature was lower than the melting temperature of the Nd70Cu30alloy powder (793 K), the small addition of Nd-Cu alloy powder did not affect the magnetic properties of the Nd-Fe-B hot-pressed magnets Figure1shows the dependence

of the coercivity and remanence of the Nd-Fe-B hot-pressed magnets on the content of Nd-Cu alloy powder The coercivity of the Nd-Fe-B hot-pressed magnets showed little change, while the rema-nence gradually decreased as the content of Nd-Cu alloy powder increased This confirms that the small addition of the Nd-Cu alloy powder did not deteriorate the magnetic properties of the Nd-Fe-B hot-pressed magnets

Figure 2shows the XRD patterns of the Nd-Fe-B hot-pressed magnet with 1% Nd-Cu alloy powder and the Nd-Fe-B hot-deformed magnet produced from the Nd-Fe-B hot-pressed mag-net by the SPS method The XRD patterns were obtained from the surfaces of these magmag-nets

to examine the crystallographic alignment of their Nd2Fe14B phase Due to the small amounts

FIG 1 Dependence of the coercivity and remanence on the content of Nd-Cu alloy powder in the Nd-Fe-B hot-pressed magnets produced by the SPS method.

FIG 2 XRD patterns of (a) the Nd-Fe-B hot-pressed magnet with 1% Nd-Cu alloy powder and (b) the Nd-Fe-B hot-deformed magnet produced from the Nd-Fe-B hot-pressed magnet by the SPS method.

of the Nd-Cu addition, no clear diffraction peaks of the Nd and Nd-Cu phases were seen in the XRD patterns Although the Nd-Fe-B hot-pressed magnet shows the typical XRD patterns of the

Nd2Fe14B phase, prominent (006) and (008) peaks of the Nd2Fe14B phase are noted in the XRD pattern of the Nd-Fe-B hot deformed magnet This indicates that the Nd-Fe-B hot-deformed mag-net possesses the crystallographic alignment of the Nd2Fe14B phase In other words, the Nd-Fe-B hot-deformed magnet is crystallographically anisotropic

The microstructures and magnetic properties of the Nd-Fe-B hot-deformed magnets were exam-ined by TEM and VSM Figure3 shows the dependence of the coercivity and remanence of the Nd-Fe-B hot-deformed magnets on the content of Nd-Cu alloy powder The coercivity of the Nd-Fe-B hot-deformed magnets were sharply increased with increasing content of Nd-Cu alloy powder, while the remanence remained unchanged up to the addition of 2% Nd-Cu alloy powder and then decreased with further increasing Nd-Cu alloy powder content With regard to the coercivity of the Nd-Fe-B hot-deformed magnets, the addition of Nd-Cu alloy powder is considered to increase the

FIG 3 Dependence of the coercivity and remanence on the content of Nd-Cu alloy powder in the Nd-Fe-B hot-deformed magnets produced by the SPS method.

056205-4 Saito et al. AIP Advances 7, 056205 (2017)

FIG 4 Microstructures of the Nd-Fe-B hot-deformed magnets produced by the SPS method (a) without Nd-Cu addition and with (b) 1%, (c) 2%, and (d) 3% Nd-Cu alloy powder.

amount of the liquid grain boundary phase because the melting point of the Nd-Cu alloy powder is

793 K, far below the hot-deformation temperature Both the excess of the grain boundary phase and the addition of Cu in the grain boundary phase increase the coercivity of the Nd-Fe-B hot-deformed magnets The above results indicate that the coercivity of the Nd-Fe-B hot-deformed magnets can be increased without the loss of remanence by the addition of up to 2% Nd-Cu alloy powder

Figure 4 shows TEM micrographs of the Nd-Fe-B hot-deformed magnets with Nd-Cu alloy powder addition produced by the SPS method The micrographs of the magnets with 1% and 2% Nd-Cu alloy powder show oriented plate-like grains The mechanism of formation of plate-like

Nd2Fe14B grains during hot deformation has been studied by several researchers.20–22 The grain boundary phase of the Nd2Fe14B grains is melted at the hot-deformation temperature and grain growth of the Nd2Fe14B grains occurs during hot deformation at the expense of the liquid grain boundary phase Such grain growth is not isotropic but anisotropic when the compression pressure is applied by hot deformation In the Nd2Fe14B grains, the grain growth takes place much more rapidly normal to the c-axis of the Nd2Fe14B phase than parallel to it, so plate-like Nd2Fe14B grains are obtained by the hot deformation Although plate-like grains were still observed in the microstructure

of the Nd-Fe-B hot-deformed magnet with 3% Nd-Cu alloy powder, the microstructure of that magnet was different from that of the other magnets Some equiaxed grains were also seen in the magnet with 3% Nd-Cu alloy powder This indicates that the crystallographic alignment of the Nd2Fe14B grains

in the Nd-Fe-B hot-deformed magnets was retained up to a 2% content of Nd-Cu alloy

Figure5 shows hysteresis loops of the Nd-Fe-B hot-deformed magnets without Nd-Cu alloy powder addition and with 1-3% Nd-Cu alloy powder produced by the SPS method Since these magnets were magnetically anisotropic, the hysteresis loops were measured parallel to the deforma-tion direcdeforma-tion The Nd-Fe-B hot-deformed magnet with 2% Nd-Cu alloy powder has the same high remanence value as the Nd-Fe-B hot-deformed magnet without Nd-Cu alloy powder addition On the other hand, the coercivity of the magnet with 2% Nd-Cu alloy powder is much higher than that

of the magnet without Nd-Cu alloy powder addition This confirms that the coercivity of Nd-Fe-B hot-deformed magnets can be increased without loss of the remanence up to 2% Nd-Cu alloy powder

FIG 5 Hysteresis loops of the Nd-Fe-B hot-deformed magnets produced by the SPS method without Nd-Cu alloy powder addition (MQ) and with 1-3% Nd-Cu alloy powder.

addition The maximum energy product increased from 30 MGOe for the Nd-Fe-B hot-deformed magnet without Nd-Cu alloy powder addition to 42.4 MGOe for the Nd-Fe-B hot-deformed magnet with 2% Nd-Cu alloy powder This confirms that the addition of Nd-Cu alloy powder can improve not only the coercivity but also the maximum energy product of Nd-Fe-B hot-deformed magnets

IV CONCLUSION

Nd-Fe-B hot-deformed magnets with Nd-Cu alloy powder were produced from Nd-Fe-B hot-pressed magnets by the SPS method The coercivity of the Nd-Fe-B hot-deformed magnets significantly increased with increasing Nd-Cu alloy powder content, while their remanence remained almost unchanged up to the addition of 2% Nd-Cu alloy powder addition and then decreased as the content of Nd-Cu alloy powder further increased TEM and VSM studies revealed that the Nd-Cu alloy powder addition did not deteriorate the crystallographic alignment of the Nd2Fe14B phase, so the magnets exhibited a high remanence value up to 2% addition Although the remanence of the Nd-Fe-B hot-deformed magnet with 2% Nd-Cu alloy powder was same as that of the Nd-Fe-B hot-deformed magnet without Nd-Cu alloy powder addition, the magnet with 2% Nd-Cu alloy powder exhibited a high coercivity of 10.3 kOe with a high maximum energy product of 42.4 MGOe

ACKNOWLEDGMENTS

The use of the facilities of the Materials Design and Characterization Laboratory at the Institute for Solid State Physics, The University of Tokyo, is gratefully acknowledged

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