New Grade of Temperature Compensated Samarium Cobalt Permanent Magnets and Design Considerations

K. Takasumi2, P. Dent1, J. Liu1, M., Marinescu1, M. Walmer1 1Electron Energy Corporation, 924 Links Ave Landisville, PA 17538 USA

2Electron Energy Corporation 633 Corte Loren, San Marcos, CA 92069 USA

Abstract: Electron Energy has successfully developed a new grade of zero Reversible Temperture Coeffeicient (RTC) magnets with maximum energy product, (BH)max , of 18.0 MGOe and Hk > 12 kOe. The new grade of zero RTC 18 augments the existing product line and surpasses the prior highest energy zero RTC grade with a (BH)max of 16 MGOe. The RTC of the newly developed magnets is suitable for TWT applications where little change in magnetic induction, Br, over a wide temperature range of up to -50° to +150° C is desired. Excellent magnetic properties were obtained with reversible temperature c o e f f i c i e n t s o n t h e o r d e r o f - 0 . 0 0 1 % / o C .

Keywords: reversible temperature coefficient; RTC; samarium cobalt; magnets; maximum energy product; magnetic induction.

Introduction Improved magnetic properties have been obtained by composition and process optimization studies for magnets synthesized from precursor powders Sm-based, Gd-based, and Er-based. The figure below shows demagnetization curves at different temperatures for magnet specimens with optimized composition and process parameters.

Figure 1. Demagnetization curves at different temperatures for the new Gd-Er-Sm based

Zero RTC magnet with a (BH)max of 18 MGOe

Background Information Br changes with temperature and it is one of the important character i s t ics o f magnet per for mance. So me applications, such as inertial gyroscopes and traveling wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature c o e f f i c i e n t ( R T C ) o f B r i s d e f i n e d a s :

! = ( Br/Br) x (1/ T) x 100%

To address these requirements, temperature compensated magnets were developed in the late 1970s. For convent iona l Sm-Co magnets , Br decreases as temperature increases. Conversely, for Gd-Co magnets, Br increases as temperature increases within certain temperature ranges. By substituting samarium with gadolinium in the alloy, the reversible temperature coefficient of Br can be reduced to nearly zero.

Figure 2. Br versus temperature for different

permanent magnet materials

Table 1.Permanet magnets with smaller reversible temperature coefficient (RTC) .

GRADES (BH)max RTC of Br Comment SmCo 1:5 �– 18 18 MGOe -0.04 %/oC No Compensation SmCo 1:5 TC - 15 15 MGOe -0.03 %/oC Some Compensation SmCo 1:5 TC �– 13 13 MGOe -0.02 %/oC Some Compensation SmCo 1:5 TC �– 9 9 MGOe -0.001 %/oC Full Compensation SmCo 2:17 �– 24 24 MGOe -0.035 %/oC No Compensation SmCo 2:17 TC - 18 18 MGOe -0.02 %/oC Some Compensation SmCo 2:17 TC - 16 16 MGOe -0.001 %/oC Full Compensation SmCo 2:17 0TC �– 18 18MGOe -0.001%/oC Full Compensation

Note: RTC of Br is calculated in the temperature range -50 to 150oC.

TWT Design Considerations Higher energy product magnets are often needed as the frequency and power of the TWTs increase. Open circuit measurements will not guarantee a specific field performance and it is highly recommended that the profile of the stack be measured as the stack is being put together. Some degree of flexibility is desirable when specifying magnets especially if only using open circuit values and requiring tight tolerances on the nominal open circuit value.

Using 100% zero RTC materials in a stack would be ideal but it is not possible when the axial profile has been defined and the outer diameter is fixed. Typically a mix of magnets made from different material grades will be required to achieve a specific axial profile on a stack.

Position (inch)

Figure 1. Axial Magnetic Field Profile of a Generic TWT Magnet Stack Based on Finite Element Analysis

Future Material Development A technology whose feasibility was demonstrated in NSF SBIR Phase I grant - �“Fe-nanoparticle Coating of Anisotropic Magnet Powder for Nanocomposite Permanent Magnets with Enhanced (BH)max�” will be further researched in a Phase II grant. The innovation consists in manipulating the regular Fe nanoparticles onto anisotropic hard magnetic powder that acts as a seed substrate. The proposed approach allows for a control at nanometer scale of the soft magnetic Fe particles unlike all the previously employed electroless and electrolytic techniques. If successfully reduced to practice, the Fe-nanoparticle coating process will lead to superior nanocomposite powders for the production of a new class of high performance nanocomposite permanent magnets with potentially much higher (BH)max.

Magnetic Flu

x (T)