Thermal Stability and Performance Data for Sm-Co 2 17 High-Temperature

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 5, MAY 2005 899

Thermal Stability and Performance Data forSmCo 2:17 High-Temperature Magnets

on PPM Focusing StructuresJ. F. Liu , Senior Member, IEEE, and M. H. Walmer , Member, IEEE

Abstract—This paper covers new results on the subjects of thermal stability and performance data for high-temperatureSmCo magnets in periodic permanent magnet configurations.Based on several years of experiments and observations, thesehigh-temperature magnets are metallurgically stable at tempera-tures up to 550 C. Following initial thermal stabilization for 2 h,these uncoated magnets, held at 300 C in air for more than threeyears, had losses of only 0.3%. No detectable loss was measuredfor the magnets coated with Al ion vapor deposition.

Index Terms—High-temperature magnets, magnetic properties,permanent magnets, thermal stabilities, traveling wave tubes.

I. INTRODUCTION

SAMARIUM cobalt-based rare earth permanent magnets

were discovered in the late 1960s [1]. There are two

types—RECo and RE TM —where RE represents rare

earth metals, such as Sm, Gd, Er, etc. and TM represents the

transition metal group which includes Co, Cu, Fe, and Zr. Both

types of magnets are characterized by high intrinsic coercivity,

, as a result of their high magnetocrystalline anisotropy.

The second type, RE TM magnets, were developed in themid 1970s [2] and have higher saturation magnetization as

compared to RECo magnets. Both RECo and RE TM can

be used at elevated temperatures up to 300 C and 330 C,

respectively.

The emergence of samarium cobalt magnets in the 1970s

has made possible a tremendous variety of high-performance

devices, both new and enhanced. For the standard grades of

SmCo and Sm TM , the reversible temperature coefficients

of , , are 0.04 and 0.035% C, respectively. For appli-

cations in inertial devices, constant field strength as a function

of temperature is required. In response to that need, Electron

Energy Corporation (EEC) developed a series of magnet grades

that are temperature compensated by substituting gadoliniumfor a portion of the samarium in the alloy. This series of mag-

nets, which includes 1:5TC-9, 2:17TC-15, and 2:17TC-16, are

widely accepted by the inertial device industry for applications

in accelerometers and gyroscopes. The traveling wave tube

(TWT) industry has also embraced the benefits of these “near

zero” temperature coefficient grades.

Manuscript received June 29, 2004; revised December 23, 2004. The reviewof this paper was arranged by Editor W. L. Menninger.

The authors are with the Electron Energy Corporation, Landisville, PA 17538USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/TED.2005.845868

Specifically, for the TWT industry, it is important to note

that the reversible temperature coefficient (RTC) is stated with

respect to residual induction ( ). The measurements are

made in a closed-circuit hysteresigraph (also known as a per-

meameter). The actual working point of a typical TWT periodic

permanent magnet (PPM) stack is very different than that found

in permeameter testing. As a result, the RTC of a typical PPM

system using zero RTC material (like 2:17TC-15) is not truly

zero. The system, magnets+pole pieces+Hall–Effect probe, has

an RTC of its own. Although the net RTC, the actual perfor-

mance of a system using these zero RTC grades is not zero (as

might be implied by permeameter testing data), it is an order of

magnitude better than the standard, uncompensated grades.

Until now, the TWT industry has had choices including un-

compensated, fully compensated and anything in between. The

maximum energy product of the zero RTC grades (fully com-

pensated) is 16 MGOe.

II. NEW SERIES OF RARE EARTH COBALT MAGNET GRADES

In 1999, the EEC introduced a series of patented high-tem-

perature magnets including T400, T450, T500, and T550. (U.S.

Patent 6 451 132 B1) [3]. These magnets are based on further

compositional refinement of the SmCo five-element system

Sm Co Fe Cu Zr .

These high-temperature magnets are characterized by

• High intrinsic coercivity at elevated temperatures

to resist demagnetization;

• Low-temperature coefficient of ;

• Straight demagnetization curves at maximum oper-

ating temperatures;

• Magnets can be made for any specified 1 up to

550 C with highest possible (BH) .

Table I shows the typical magnetic properties of SmCo high-

temperature magnets. Standard SmCo 2:17 grade (EEC2:17-24)

is also listed for comparison. Typical demagnetization curves

of EEC24-T400, EEC20-T500, and EEC16-T550 high-temper-

ature magnets at various temperatures are shown in Figs. 1–3,

respectively. It can be seen from Figs. 1–3, the normal (extrinsic)

demagnetization curves are straight lines up to their respective

maximum operating temperatures which provides design

flexibilities for many critical applications.

1T is the maximum use temperature at which a straight-line demagnetiza-

tion curve can exist.

0018-9383/$20.00 © 2005 IEEE



900 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 5, MAY 2005

TABLE ITYPICAL MAGNETIC PROPERTIES OF EEC HIGH-TEMPERATURE SmCo

MAGNETS AND STANDARD Sm TM MAGNETS

Fig. 1. Typical demagnetization curves at various temperatures for EEC24-T400 high-temperature magnets.

Fig. 2. Typical demagnetization curves at various temperatures for EEC20-T500 high-temperature magnets.

III. LONG-TERM THERMAL STABILITY OF

HIGH-TEMPERATURE MAGNETS

Fig. 4 shows the long-term thermal stability of SmCo 2:17

magnets and patented high-temperature SmCo magnets at

300 C [4]. All samples were uncoated except one set of T500

magnets, which was coated with aluminum ion vapor deposi-tion (IVD). These magnets have been held at 300 C in air for

Fig. 3. Typical demagnetization curves at various temperatures for EEC

16-T550 high-temperature magnets.

Fig.4. Long-term thermal stability at300 C inair for selectedSmComagnetswith maximum operating temperatures of 250 C–550 C.

more than three years. The total irreversible magnetic losses for

all samples were less than 4%.

Based on several years of experiments and observations, these

high-temperature magnets are metallurgically stable at temper-atures up to 550 C. Following initial thermal stabilization for

two hours, these uncoated high-temperature magnets, held at

300 C in air for more than three years, had losses of only about

0.3%. No detectable loss was measured for the magnets coated

with aluminum IVD.

Coating is not necessary for SmCo magnets at 300 C al-

though aluminum coating could offer further surface protec-

tion from gradual oxidation. Intermetallic compounds, such as

SmCo magnets, are intrinsically very brittle. Magnets are han-

dled extensively in many applications. Aluminum coating on the

surface could also prevent chipping, which would be an added

benefit. The durability of aluminum IVD coating has proved to

be good enough for many applications in the past although wedo not have statistical data at this point.



LIU AND WALMER: THERMAL STABILITY AND PERFORMANCE DATA 901

Fig. 5. Sketch showing normal demagnetization curve.

IV. HIGH-TEMPERATURE MAGNETS FOR TWT APPLICATIONS

Our measurements have confirmed that the RTC of the axial

field in PPM stacks using this new series of high-temperature

magnet materials is comparable to that obtained with the previ-

ously considered “best materials available” that have low to zero

(RTC of ) [5]. Accordingly, this new series of magnet mate-

rials offers additional choices to tube design engineers and could

result in significantly higher field strengths while maintainingthe important characteristics of low reversible temperature co-

ef ficient of the axial field. Our measurements are based on plot-

ting the on axis field of PPM arrangements of magnets at various

temperatures. These are dif ficult measurements to make due to

limitations related to the Hall Effect-device (the probe). Feed-

back from customers that have evaluated these materials on trav-

eling wave tubes has been favorable in a number of situations.

V. EXPLANATION FOR THE IMPROVED PERFORMANCE

Fig. 5 is a sketch showing a normal demagnetization curve.

The working point is assumed to be at and the de-

magnetization curve is assumed to be linear. ( is a magneticconstant and equal to 1 in CGS units and, in the following dis-

cussions, will not appear for the sake of simplicity.)

The flux density can be written as

(1)

Assuming the magnets in the devices are exposed to a con-

stant magnetic field , which includes self demagnetization

field, and let , the change of flux density when the

temperature changes from to can then be written as

(2)

Therefore, an empirical equation was deduced as follows:

(3)

where is the change of flux density over a temperature

range from to at a working point of ; is the re-

versible temperature coef ficient (RTC) of the residual induction

; is the temperature change; is defined as

the ratio of ; is the change of value when temper-

ature changes from to .

As a function of temperature, the change in flux density is a

result of two components— and . The first

component is related to , the reversible temperature coef ficient(RTC) of . The second component is related to the quantity

TABLE IITYPICAL VALUES OF

AND1

OF SmCo MAGNETS

Fig.6. Values of 1

of SmCo magnets from 0 0 0 C to various temperaturesup to 250 C.

, defined as the change of ratio from temperature

to . Because of the working point associated with the magnets

in a PPM device, this parameter is as important, if not more

important, than (RTC of ).The thermal behavior of various SmCo magnets has been

studied extensively. Table II lists typical values of , RTC of

, and the quantity . Fig. 6 shows the value of of SmCo

magnets as the temperature changes from C to various

temperatures up to 250 C.

As seen fromTableII andFig.6, both and are relatively

large for the standard SmCo 2:17 grade EEC2:17-24, therefore

relative large change of flux density, , would be expected

over the same temperature range. The data in Fig. 6 are cal-

culated from closed circuit measurements, which eliminate the

effect of self-demagnetization factor and variations of working

point of each individual magnet. The data are closely related to

the extrinsic demagnetization curves, which is critical for any

magnetic circuit designs.

The reversible temperature coef ficient of the axial field of

PPM stacks is related to the RTC of as well as the RTC of

. PPM stacks with standard SmCo 2:17-24 magnets would be

expected to have a relatively large reversible temperature coef-

ficient, because

1) SmCo 2:17-24 magnets have a relatively large re-

versible temperature coef ficient (RTC) of as shown

inTableII.TheRTCof isan intrinsic propertyof the

material, and is measured in closed circuit conditions

using a KJS Hysteresigraph. PPM stacks made of mag-

nets with large RTC of would have a large systemreversible temperature coef ficient of axial field.



902 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 5, MAY 2005

2) The value, (Br/Hc), increases rapidly with tem-

perature as shown in Fig. 6. Reversible temperature co-

ef ficient of is more relevant to dynamic applica-

tions. The reversible temperature coef ficient of PPM

stacks would be larger if the value is larger even

when the RTC of is the same.

For EEC2:17-TC15 magnets, the RTC of is almost zero,the contribution of the first component, , in (3) to

the change of flux density is insignificant. The change of

flux density, , over a specified temperature range is largely

due to the change of .

Although the RTC of is the same as that of the standard

SmCo grades, EEC high-temperature magnets (T400, T450,

T500, and T550) have significantly lower values of re-

sulting in improved performance for magnets used in PPM

configurations.

VI. CONCLUSION

The new series of high-temperature magnets are metallurgi-cally stable at temperatures up to 550 C. More than three years

exposure to 300 C in air resulted in only 0.3% irreversible loss

after the initial 2-h thermal stabilization. The superior magnetic

properties and thermal stability of these high-temperature mag-

nets attracted many new applications. Feedback from customers

that have evaluated these materials on traveling wave tubes has

been favorable in a number of situations. A new empirical equa-

tion was used to explain the improved performance of devices

with high-temperature magnets. And, most importantly, these

magnets are generally available at no additional cost. The prices

are comparable to those for standard magnet grades.

REFERENCES

[1] K. Strnat and W. Ostertag, Program for an in-house investigation of the yttrium-cobalt alloy system, in Technical Memorandum, May 64-4,Projects 7367and 7360, AFML, Wright-Patterson AFB, OH,Mar.1964.

[2] A. E. Ray and K. J. Strnat, “Easy directions of magnetization in ternaryR ( Co; Fe) phases,” IEEE Trans. Magn., vol. 8, p. 516, MAG–1972.

[3] M. Walmer, C. Chen, and M. Walmer, “A new class of permanent mag-netic materials,” in Proc. IVEC Conf., Monterey, CA, 2000.

[4] C. Chen, M. H. Walmer, and S. Liu, “Thermal stability and the effec-tiveness of coatings for Sm-Co 2:17 high-temperature magnets at tem-peratures up to 550 C,” IEEE Trans. Magn., no. 5, pp. 2928–2930, Oct.2004.

[5] C. H. Chen and M. H. Walmer, “A new series of rare earth cobalt magnetgrades,” in Proc. Tri-Service VED Workshop, Norfolk, VA, 2003.

J. F. Liu (SM’00) received the Ph.D. degree inphysics from the Institute of Physics, ChineseAcademy of Sciences, China, in 1990.

He is now the Director of Technology, ElectronEnergy Corporation, Landisville, PA. He has over15 years of research experience in the field of rareearth permanent magnets and magnetic systems. He

has designed various magnetic systems using finiteelement analysis, and coordinated many prototyping

projects for critical applications. He has worked with

almost all the existing and new permanent magneticmaterials. He has published more than 100 papers in scienti fic journals andinternational/national conference proceedings.

M. H. Walmer (M’00) received the B. S. degree inmaterials science and engineering from Lehigh Uni-

versity, Bethlehem, PA.He has 34 years of experience with rare earth mag-

nets. He joined Electron Energy Corporation (EEC),Landisville, PA, at its inception in 1970. He helpedto develop the SmCo 1:5 type products and 2:17 typeproducts for numerous military applications, and has

devoted hisentire careerto thedevelopmentand man-ufacture of rare earth magnet materials.

Mr. Walmer currently serves as an InternationalAdvisory Committee member for the International Workshop for High Perfor-mance Magnets and Their Applications.

Thermal Stability and Performance Data for Sm-Co 2 17 High-Temperature

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