Permanent magnet blocks of NdFeB have a relatively high maximum energy product. Because of its relatively low Curie temperature, however, NdFeB has a large temperature coefficient for its residual induction. The temperature coefficients of the relative magnetic fields ({Delta}B/B)/{Delta}T in the air gap of NdFeB dipole magnets were reduced from {minus}1.1 {times} 10{sup {minus}3}/c to less than 2 {times} 10{sup {minus}5}/{degree}C under operating temperatures of {+-} 6 C. This was achieved passively by using 1.25-mm-thick strips of 30%-Ni-Fe alloy as flux shunts for the NdFeB blocks. The magnets with soft-steel poles and flux-return yokes were assembled and measured in a temperature-controlled environment

Doose, C.

Kim, S. H.

UNT Digital Library

c TEMPERATURE COMPENSATION OF NDFEB PERMANENT MAGNETS S .  H. Kim and C. Doose Advanced Photon Source, Argonne National Laboratoq. 9700 South Cass Avenue, Argonne, Illinois 60439 Abstract Permanent magnet blocks of NdFeB have a relatively high maximum energy product. Because of its relatively low Curie temperature, however, NdFeB has a large temperature coefficient for its residual induction. The temperature coefficients of the relative magnetic fields (ABnS)/AT in the air gap of NdFeB dipole magnets were reduced from -1.1 x 10-3/0C to less than 2 x IO-'/OC under operating temperatures of 2 6 T .  This was achieved passively by using 1.25-mm-thick strips of 30%-Ni-Fe alloy as flux shunts for the NdFeB blocks. The magnets with soft-steel poles and flux-return yokes were assembled and measured in a temperature-controlled environment. 1 INTRODUCTION Stable permanent magnets could be used for charged- beam storage rings and for other new magnetic devices which require system reliability and lower construction and operation costs. One drawback of permanent magnets for these types of applications. however, is maintaining the required magnetic field when the residual induction and coersive forces change due to the ambient temperature variation. The bulk magnetization of a permanent magnet is the result of a collective alignment of atomic spins within magnetic domains. As the temperature increases, thermal fluctuations induce random variations in individual atomic spin orientations. At Curie temperature Tc. all the spins are randomly oriented and the bulk magnetization disappears. Therefore, at ambient temperature, a permanent magnet w + higher T, has a lower temperature coefficient for the variation of the magnetization. Permanent magnet blocks of neodymium-iron-boron (NdFeB) have relatively high maximum energy-product in the demagnetizing quadrant coiilpared to other permanent magnets such as Alnico, ferrite. and rare-earth cobalt [I]. Because of its relatively low Curie temperature of approximately 300'T,  NdFeB has a high temperature coefficient for its residual induction. The purpose of this work is to reduce the temperature coefficient of the magnetic fields of NdFeB permanent magnets better than lO-'/OC withir: the range of an operation temperature using parallel flux-shcnts. At lower temperatures the shunt  permeability increases, enabling the shunt to carry or divert more flux from the A i r  y3.p of the magnet. At higher temperatures, the reverse cc,:diticin exists: the shunt permeability decreases and less air-gap f iux is diverted. In order to have higher temperature sensitivity, the shunt material should have a Curie point close to ambient temperature. Shunt materials made of Curie alloys (30-32% Ni-Fe) and Monte1 alloys (67% Ni- Cu-Fe) have relatively low Curie temperatures of less than 100°C. Temperature compensations for ferrite permanent magnets have been extensively studied at Fermilab for use at relatively low magnetic field [2]. fBZ7ZZj Steel I M I  Compensation strips NbFeB Magnet Figure 1: (a) Symmetrical cross section of a quadrant of a dipole magnet with a soft-steel pole and flux-return yoke. The NdFeB block is magnetized 111 the vertical direction. (b) Top view for the arrangement of the permanent magnet blocks and the temperature cornpersation strips inserted between th? blocks. 2 TEMPERATURE COMPENSATION T h  symmetrical cross section of a quadrant of a typical dipole :Tagnet used for this study is shown in Fig. 1. The major components of the magnets are NdFeB perinanent magnet blocks, magnetic flux shunts for temperature compensation. flux-return yokes, and soft-steel poles for field-shaping. The NdFeB blocks have a cross section of 35.4 x 25.3 and a thickness in  the magnetized direction of The submitted manuscript has h e n  created b! the Universjry of Chicago as Oprator of Argonne National Laborarory ("Argonne") under Contract No. W-31-109-ENG-35 with the U.S. Department of Ensrev. The U S. Government retains for itself, and others actinp. on its behalf. a paid-up, nonexclusive. inevh-able worldwide license in said artlcle to reproduce. prepm dem&e works. msmbute copits to the public. and perform publicly and d,plaY publicly. by or on behalf of the Government OF 34% DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or wfulness of any information, appa- ratus, product, or process disdosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessan7y constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not uecessar- ily state or reflect those of the United States Government or any agency thereof. DISCLAIMER 12.7 mm. The residual induction and coercive force of the blocks were approximately 1.1 T and 10.5 kOe, respectively. For the temperature compensation flux shunts, 1.27-mm-thick and 25-mm-wide strips of 30%-Ni- Fe alloy were used. The alloy has a Curie temperature of 60°C and saturation induction of 0.22-0.1 T at 20-45OC. First a dipole magnet without temperature compensators was put in a temperature-controlled incubator and its temperature and the magnetic field in the air gap were measured. The magnet was assembled using six NdFeB blocks in each of the top and bottom poles. Figure 2 shows that the temperature coefficient for the relative magnetic field (AB/B)/AT in the air gap was -1.1 x 10-3/0C in the temperature range of 2545°C. The Hall probes used for the magnetic field measurements were calibrated to +IO-' of readingPC with a resolution of 0.2 G. The temperature measurement has an absolute accuracy of 0.5"C with a resolution of 0.1"C. -250 ' ' ' I . . . , J  20 25 30 35 40 45 50 Tempe- [C] Figure 2: Magnetic field data under a slow thermal cycle for a NdFeB permanent dipole magnet. Temperature coefficient for the relative magnetic field in the air gap was (AB/B)/AT = -1.1 x 10'3/cC in the te ,perature range of 25-35"C. The data shown in Fig. 3 are measurements of a magnet with an optimum number of temperature compensation strips. The volume ratio of the smpMdFeB for the magnet was 20%. In this magnet the compensation strips were inserted between the gaps of the NdFeB blocks as shown the top view in Fig l(b). The strips cannot be distributed uniformly, but the magnetic field distribution in the air gap can be adjusted by the geometry of the magnet poles. The measured data show that within the relative field variation of 2 x the temperature coefficient was less than 2 x 10-5/"C in the temperature range of 31 k 6°C. (For better control of the temperature inside the incubator, the temperature at the compensation field was set higher than the room temperature.) In the whole range of the thermal cycle the coefficient was less than 1 x lO-'/OC. ' During measurement the temperature was controlled aE a rate of approximately 0.8"C/hr to ensure the uniformity of the magnet temperature. Ideally the magnetic field in the air gap can be calculated from the magnetomotive force of the NdFeB blocks as well as the reluctances of the air gap and the compensation strips. Because of the nonlinear magnetic properties of the strips, however, the temperature range for a fully compensated field is limited. The relative magnetic permeability of the strips in the magnet was calculated to be less than 5. Figures 4(a) and (b) show the effects of consecutive thermal cycles on the compensated field and temperature. The temperature of the magnet was controlled rather slowly at a rate of only 0.15"C/h. The compensated relative magnetic field increased by approximately 1 x (0.5 G) for the first to the third cycle. On the other hand, the temperature at the compensated field decreased approximately 1°C. After an additional seven thermal cycles, the compensated fields and temperatures remained within 0.1 G and 0.5"C, respectively. 20 25 30 35 40 45 50 Temperature [C] : ............. ............ .................... ... ..... . . . . . . . . . . . . . . . . . .  ...... . .  . . .  a . . .  . .  ....... ......... - 1 5 ~ ' " " " "  " '  " " " '  " '  ' '  I "  " 20 (b) 0 10 20 30 40 50 60 70 Time [hours] Figure 3: Temperature compensation of a dipole magnet with a volume ratio of the Compensation stripMdFeB of 20%, the temperature coefficient wi?s 1.7 x in the temperature range of 31 -+ 6°C. (a) ABfB vs. temperature and (b) ABfB and temperature vs. time. 0 1 T g  -2 3 -3 $ 4  -5 -6 - 7 : '  " " ' I . .  1 25 30 35 40 4s M Tempenwe [C] .......................... ....................... ...................... 50 100 150 200 254 300 Time [hours] Figure 4: Three thermal cycles of a compensated dipole magnet at a thermal cycle rate of 0.15"Ch (a) B I B  vs. temperature and (b) AB/B and temperature vs. time. 3 SUMMARY The temperature coefficients of NdFeB permanent magnets were reduced from -1 .1  x lO"PC to less than 2 x 10-5/0C under operating temperatures of k 6°C by passively using Ni-Fe alloy as flux shunts for the NdFeB blocks. After the initial increase of the compensated field by 1 x in the first three thermal cycles, the field remained within 2 x after an additional seven thermal cycles. The effects of thermal cycles, aging, and radiation have to be investigated further. 4 ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract NO. W-3 1-109-ENG-38. 5 REFERENCES [ I ]  Rollin J. Parker, Advances in P e m e n t  Magnetism, John Wiley & Sons, New York, 1990. [2] K. Bertsche, J.-F. Ostiguy. and W. B. Foster, "Temperature Considerations in the Design of a Permanent Magnet Storage Ring," Proc. of the 1995 Particle Accelerator Conference, IEEE 992835843. pp. 1381-1383 (1996). 

Temperature compensation of NdFeB permanent magnets

Permanent magnet blocks of NdFeB have a relatively high maximum energy product. Because of its relatively low Curie temperature, however, NdFeB has a large temperature coefficient for its residual induction. The temperature coefficients of the relative magnetic fields ( D B/B)/D T in the air gap of NdFeB dipole magnets were reduced from-1.1 ·  10-3/ ° C to less than 2 ·  10-5/ ° C under operating temperatures of ± 6 ° C. This was achieved passively by using 1.25-mm-thick strips of 30%-Ni-Fe alloy as flux shunts for the NdFeB blocks. The magnets with soft-steel poles and flux-return yokes were assembled and measured in a temperature-controlled environment. 

Temperature compensation of NdFeB permanent magnets

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