Sintered Sm

8/1/2019 Sintered Sm

1/4

IEEE

Proof

WebVersion

IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 1

Sintered Sm2

Co1 7

-Based Magnets With Small Additions of Indium

Alexander M. Gabay 1 , Melania Marinescu 2 , JinFang Liu 2 , and George C. Hadjipanayis 1

Department of Physics and Astronomy, University of Delaware, Newark, DE 19716 USA

Electron Energy Corporation, Landisville, PA 17538 USA

In an attempt to develop anisotropic high-temperature permanent magnets consisting of Sm2 (

Co,Fe) 1 7

grains separated by thinlayers of a nonmagnetic phase, small amounts of Ag, C, Ga, In, and Sn were added to the Sm2

Co1 7

-based alloys and sintered magnets.Grain-boundary phase(s) with the melting temperature of 9851070 C was obtained by adding 0.51.5 at. % In to SmCo, SmCoFe,and SmCoMn alloys with the Sm content slightly enriched compared to the 2 : 17 stoichiometry. Iron, manganese and, apparently,oxygen tend to decrease the melting temperature of the grain-boundary phase(s). Due to this low-melting-temperature phase, sinteringof the milled Sm

2

Co1 7

-based alloys becomes possible at temperatures as low as 10251075 C. This phase is also believed to be respon-sible for the coercivity of 68 kOe observed in the SmCoFeMnIn alloys. The coercivity was found to be critically sensitive to thepostsintering heat treatment: it dramatically deteriorates when the magnets are annealed below the melting temperature of the grainboundary phase(s).

Index TermsCoercive force, indium, permanent magnets, samarium alloys.

I. INTRODUCTION

IN the best currently used high-temperature SmCo perma-nent magnets, the coercivity is developed in the bulk state

through a multiphase cellular nanostructure. To assure this,

some excess of Sm and 710 at. % of Cu and Zr must be added

to the alloy, which then decreases the amount of high-mag-

netization Sm Co,Fe phase. The earlier attempts to obtain

pure 2 : 17 magnets were not very successful [1], [2], although

a maximum energy product of 28 MGOe has been reported for

magnets containing Mn [3]. Coercivity of magnets fabricated

via sintering of anisotropic micron-size powders critically de-

pends on the state of their grain boundaries. In particular, the

high performance of sintered NdFeB magnets is assured by

the Nd-rich phase, which magnetically insulates the Nd Fe Bgrains and inhibits nucleation of reversed magnetic domains

[4], [5]. Due to its low melting temperature, this phase also

serves as a sintering aid. In the ternary SmCoFe system, the

Sm Co,Fe phase is in equilibrium with other ferromagnetic

phases [6]; the desired insulating phase may, therefore, be in-

duced only via doping.

In our efforts to induce insulating grain-boundary phases in

the Sm Co , Sm Co,Fe , and Sm Co,Fe,Mn alloys, we

explored small additions of Ag, C, Ga, In, Sn, and some of their

combinations. Additions of indium were found to be promising

for both the powder metallurgy and hard magnetic properties of

the Sm Co -based alloys. This paper gives a detailed account

of our experiments with In-added alloys and sintered magnets;

it also briefly summarizes the results obtained for the other ele-

ments.

II. EXPERIMENT

The Sm Co Fe Mn M alloys with M Ag,

C, Ga, In, Sn, Ag C , Ag Sn , ,

, , and were prepared by

Digital Object Identifier 10.1109/TMAG.2008.2001540

arc-melting in argon from pure elements and a CoC master

alloy. The ingots were remelted at least four times to assure their

homogeneity; appropriate excesses of Sm and Mn were addedto compensate evaporation losses of these elements during arc-

melting. Some of the ingots were additionally homogenized

at 900 C. For sintering, the ingots were crushed to less than

0.3 mm and ball-milled with a rotary mill for 15 h in toluene.

After drying, the powders were pressed in a magnetic field of

17 kOe. The obtained green compacts were degassed by step

heating in vacuum to 650 C and then sintered in argon for

1 h at 9751075 C. After sintering, the samples were pulled

out of the furnace (still under argon) for air cooling. Addi-

tional postsintering heat treatment consisted of a one-hour an-

nealing under argon at 9001050 C followed by quenching

in water. The ingots and sintered magnets were characterized

by: 1) powder x-ray diffraction (XRD) with a Philips diffrac-

tometer operating at the Cu-K radiation, 2) scanning elec-

tron microscopy (SEM) with a JEOL JSM-6335F instrument,

3) energy dispersive spectrometry (EDS) with an IXRF Sys-

tems instrument and software, and 4) differential thermal anal-

ysis (DTA) with a PerkinElmer Pyris Diamond TG/DTA instru-

ment. Magnetic properties were measured on cube-shaped spec-

imens with a vibrating sample magnetometer. Densities of the

ingots and sintered magnets were measured by a water immer-

sion technique or, in the case of too porous specimens, derived

from their outer dimensions.

III. RESULTS

A. Small Additions of Silver, Carbon, Gallium and Tin

In the Sm Co and Sm Co,Fe alloys with small addi-

tions of Ag, C, Ga, and Sn, the grain-boundary phases were

observed for Ag, C, and Sn, whereas Ga was found to be

dissolved by the magnetic SmCo phases. The Sm Ag

phase in the SmCoAg alloys, though nonmagnetic and with

the melting temperature of 935 C, was found not to wet the

Sm Co grains. The grain-boundary phases in SmCoSn and

SmCoAgSn alloys were found to have high melting tem-

peratures. Grain boundaries in the alloys with carbon featured

eutectic mixture of the carbide phase and the ferromagnetic

0018-9464/$25.00 2008 IEEE
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


2/4

IEEE

Proof

WebVersion

2 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008

Fig.1. Heating DTA curvesof ingotsandsinteredmagnets: 1Sm Co ,2Sm Co In , 3Sm ( Co Fe Mn ) In ,4Sm ( Co Fe Mn ) . T and T indicate the Curietemperature and solidus temperature, respectively.

SmCo phase, which do not make these boundaries magnetic

insulators.

B. Effect of Indium Addition on Microstructure

SEM and EDS examination of the Sm Co In alloys

with and revealed a phase en-

riched with samarium and indium. The approximate composi-

tion of this phase was found to be Sm Co In . Inclusions

of this indium-rich phase, as well as those of the minor SmCo

phase, which contained only traces of indium, stretched along

the boundaries of the indium-free Sm Co grains. The DTAscan shown in Fig. 1 (curve 2) indicates the appearance of a

liquid phase at 1070 C in the alloy with indium, in contrast with

the single-phase Sm Co alloy (curve 1). The Fe substitution

for Co (not shown in Fig. 1) and the combined (Fe, Mn) substi-

tution (curve 3) decrease the solidus temperature; the same ef-

fect is also caused by sintering (curve 3 ). The most likely way

the milling/sintering could influence the phase equilibrium is

by introducing oxides. The presence of indium, however, is still

necessary for the low-melting-temperature phase(s): the solidus

temperature of the indium-free sintered magnet exceeds 1150

C (Fig. 1, curve 4).

The XRD data shown in Fig. 2 confirm that millingand sintering had modified the phase structure of the

Sm Co,Fe,Mn In alloy: the minor SmCo phase of

the as-cast alloy is now replaced by Sm O . The XRD analysis,

however, does not reveal the indium-rich phase: the scans

for sintered magnets with and without indium are virtually

identical.

The In-rich and oxide phases can be clearly seen in the SEM

image of a fractured surface shown in Fig. 3(a); in the backscat-

tered electrons (BSE) mode, these two phases appear bright.

Comparing the SEM image with the EDS elemental maps for

indium and oxygen presented in Fig. 3(b) and (c), one can see

that the indium-rich grains and the oxide grains of similar size

0.51.3 m are located next to each other along the boundariesof the larger Sm Fe,Co,Mn grains. It seems quite probable

Fig. 2. Powder XRD scans of (a) as-cast Sm ( Co Fe Mn ) Inalloy, (b) sintered Sm ( Co Fe Mn ) In magnet, and (c) sinteredSm ( Co Fe Mn ) magnet. 2 : 17 and 1: 5 denote reflections ofTh Zh -type and CaCu -type structures, respectively.

Fig. 3. (a) BSE SEM image of a fractured surface of Sm ( Co Fe Mn ) In magnet sintered at 1075 C and (b)indium and (c) oxygen elemental maps for the selected area.

that the indium-rich and oxide phases crystallized from a liquid

grain-boundary phase via a eutectic reaction.

C. Effect of Indium Addition on Densification

As it might be expected, the liquid grain-boundary phase dra-

matically facilitates sintering. Fig. 4(a) presents densities of the

indium-added sintered magnets. The magnet with only 0.5 at.%

In exhibits a density of 0.98 times that of the ingot after sin-

tering at 1075 C, which is more than 100 lower than the

usual sintering temperature of the 2 : 17 magnets. Moreover, the

density data for the different sintering temperatures shown inthe inset suggest that the critical temperature for densification
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


3/4

IEEE

Proof

WebVersion

GABAY et al.: SINTERED Sm Co -BASED MAGNETS WITH SMALL ADDITIONS OF INDIUM 3

Fig. 4. Effect of indium content on relative density and intrinsiccoercivity of Sm ( Co Fe Mn ) In (a) andSm ( Co Fe Mn ) In (b) magnets sintered at1075 C and annealed at 10001050 C. Inset shows density ofSm ( Co Fe Mn ) In magnets sintered at different tempera-tures.

is around 1000 C, just above the solidus temperature revealed

in this magnet by DTA (see curve 3 in Fig. 1).

D. Effect of Indium Addition on Coercivity

The addition of indium also led to a significant increase in

the intrinsic coercivity . Fig. 4(b) presents the highestvalues for as-sintered and heat-treated samples (typically, the

coercivity of the magnets sintered at 1075 C slightly improved

after the postsintering annealing at 10001025 C). Fig. 5 shows

the coercivity as a function of Sm and Mn concentrations. Al-

thoughthe results for the Smeffect ofon are not veryconclu-

sive, they may suggest that there exists an optimum samarium

concentration. Such optimum concentrations of both indium and

samarium may have a simple explanation: at a given indium

content, the grain-boundary phase can accommodate only cer-

tain amount of samarium, and vise versa. As for the Mn, it had

been added to the Sm Co,Fe matrix phase in the very be-

ginning of this study in order to enhance its magnetocrystalline

anisotropy [3], [7]. As one can see in Fig. 5(b), the magnets

with the Sm Co,Fe matrix do not develop any significant

coercivity even though they also had reached the density of

97.5%98%.

E. Effect of Postsintering Annealing

Postsintering annealing below certain temperature dramat-

ically decreases of the sintered SmCoFeMnIn mag-

nets. Fig. 6 shows that the temperature critical for coin-

cides with the solidus temperature of the sintered magnet. It is

reasonable to assume that the more chemically homogeneous

grain-boundary phase achieved by freezing the liquid phase is

a better magnetic insulator than multiple phases produced by aeutectic crystallization. Indeed, the grain-boundary regions of

Fig. 5. Effects of Sm and Mn contents on intrinsic coer-civity of (a) Sm ( Co Fe Mn ) In and (b)Sm ( Co Fe Mn ) In magnets sintered at 1075 C andannealed at 10001050 C.

Fig. 6. Heating DTA curve of Sm ( Co Fe Mn ) In magnetsintered at 1075 C and coercivity of this magnet versus temperature of postsin-tering annealing. T indicates the Curie temperature.

the high-coercivity sample quenched from 1000 C, which is

shown in Fig. 7(a), appear to be more uniform than those of the

low-coercivity sample quenched from 950 C [Fig. 7(b)]. The

black spots in Fig. 7 correspond to a CoFeMn phase. This

soft magnetic phase might appear as a result of inhomogeneous

oxidation during milling and/or sintering and it is not neces-

sarily involved in the above grain-boundary transformation. In

any case, this phase must be harmful for the coercivity.

IV. DISCUSSION AND CONCLUSION

Permanent magnets with high-temperature capabilities and

room-temperature energy product greater than 50 MGOe

promised by the intrinsic properties of the Sm Co,Fe

compound [2] still remain an attractive target. During the last

decade, the pure 2 : 17 magnets were mostly sought via

nonequilibrium techniques such as mechanical alloying [8], [9]

and melt-spinning [10][12].

This work shows that even the old-fashioned powder

sintering, which has the undeniable advantage of delivering

anisotropic massive magnets, may not yet run out of options.

We are still far away from the ultimate goal: the coercivity of the

magnets we report is hardly sufficient even for room-tempera-ture applications, and their remanence[AU: Remanence
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


4/4

IEEE

Proof

WebVersion

4 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008

Fig.7. BSE SEM imagesof Sm ( Co Fe Mn ) In magnetsin-tered at 1075 C and annealed at (a) 1000 C and (b) 950 C.

correct word?], which typically does not exceed 9 kG,is disappointingly low. Nevertheless, the idea that motivated

this study appears to be working: magnetic insulation can be

induced in the single-magnetic-phase Sm Co,Fe magnets

(we assume that the indium-rich grain boundary phase is

nonmagnetic) and it leads to the magnetic hardness.

The fact that the considerable coercivity was achieved only

for the Mn-substituted magnets might suggest that there exists a

threshold anisotropy field for obtaining a magnetic hysteresis in

fine 2 : 17 grains. However, the 2 : 17 magnets with the cellular

structure demonstrate that the cell-interior Sm Co,Fe phase

containing 20and moreat. % Fe can exhibit veryhigh values

if the cell size is sufficiently small [13], [14]. Therefore, the fur-

ther development of the proposed 2 : 17 magnets with the in-

dium-rich grain-boundary phase may be sought in improvementof the comminuting technique to obtain finer particles with more

uniform size distribution (fortunately, the low sintering temper-

atures made possible by the indium additions are favorable for

the grain size control). Eventually, these efforts may allow us to

increase the remanence by avoiding the Mn substitution and/or

increasing that of Fe. Alignment of the milled powders with a

pulsed field will almost certainly provide the better texture and

the higher remanence, and, of course, the effect of oxygen that

is strongly suggested by some of our results requires a more fo-

cused study to figure out how it might be optimized.

In conclusion, the small indium additions greatly facilitate

sintering of the pure 2 : 17 magnets, but more effort is neededto obtain a high magnetic performance.

ACKNOWLEDGMENT

This work was supported by Air Force under STTR contract

FA9550-07-C-0029.

REFERENCES

[1] R. C. Carriker and A. S. Rashidi, Experiments in the liquid phasesintering of Sm Co , in AIP Conf. Proc., 1973, no. 10, pp. 608612.

[2] R. S. Perkins, S. Gaiffi, and A. Menth, Permanent magnet propertiesofSm ( C o ; F e ) ,IEEE Trans. Magn., vol. 11,no. 5, pp. 14311433,Sep. 1975.

[3] H. Nagel,Magnetic properties of sintered Sm TM magnets, inAIPConf. Proc., 1976, no. 29, pp. 603604.

[4] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, and Y. Matsuura,New material for permanentmagnetson a base of Nd andFe,J. Appl.Phys., vol. 55, pp. 20832087, Mar. 1984.

[5] F. Vial, F. Joly, E. Nevalainen, M. Sagawa, K. Hiraga, and K. T. Park,Improvement of coercivity of sintered NdFeB permanent magnets byheat treatment, J. Magn. Magn. Mater., vol. 242245, pp. 13291334,Apr. 2002.

[6] G. Schneider, E.-T. Henig, H. L. Lukas, and G. Petzow, Phaserelations in the samarium-poor Sm-Co-Fe system, J. Less-Common

Metals, vol. 110, pp. 159170, Aug. 1985.[7] R. S. Perkins, S. Strssler, andA. Menth,Upon influencing themagne-

tocrystalline anisotropy of RE TM compounds, in AIP Conf. Proc.,1976, no. 29, pp. 610612.

[8] P. A. I. Smith, J. Ding, R. Street, and P. G. McCormick, Mechani-cally alloyed Sm-(Co-Fe) permanent magnets, Scr. Mater., vol. 34,pp. 6166, Jan. 1996.

[9] Z. M. Chen, X. Meng-Burany, H. Okumura, and G. C. Hadjipanayis,Magnetic properties and microstructure of mechanically milledSm ( Co,M ) -based powders with M = Zr, Hf, Nb, V, Ti, Cr, Cu andFe, J. Appl. Phys., vol. 87, pp. 34093414, Apr. 2000.

[10] A. Yan, A. Bollero, K. H. Mller, and O. Gutfleisch, Influence of Fe,Zr, and Cu on the microstructure and crystallographic texture of melt-spun2:17 Sm-Co ribbons,J. Appl. Phys., vol. 91, pp.88258827, May2002.

[11] S. S. Makridis, G. Litsardakis, K. G. Efthimiadis, E. Pavlidou, I.Panagiotopoulos, G. C. Hadjipanayis, and D. Niarchos, Structural

and magnetic properties of rhombohedral Sm(

Co,Fe,Cr)

B andSm ( Co,Fe,Mn) B compounds, IEEE Trans. Magn., vol. 39, no.5, pp. 28722874, Sep. 2003.

[12] C. H. Chen, S. Kodat, M. H. Walmer, S.-F. Cheng, M. A. Willard, andV. G. Harris, Effects of grain size and morphology on the coercivityof Sm ( Co Fe ) based powders and spin cast ribbons, J. Appl.Phys., vol. 93, pp. 79667968, May 2003.

[13] B. Zhang, J. R. Blachere, W. A. Soffa, and A. E. Ray, AEM studiesof Sm:Co 2:17 permanent magnet alloys, J. Appl. Phys., vol. 64, pp.57295731, Nov. 1988.

[14] H. Kronmuller and D. Goll, Micromagnetic analysis of pin-ning-hardened nanostructured, nanocrystalline Sm Co basedalloys, Scr. Mater., vol. 47, pp. 545550, Oct. 2002.

Manuscript received March 3, 2008. Current version published (e-mail:[email protected]).
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-

Documents

Sintered Sm