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Developments in the processing and properties of NdFeb-type permanent magnets
Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
Topical review
Developments in the processing and properties of NdFeb-type permanent magnets David Brown*, Bao-Min Ma, Zhongmin Chen Magnequench Technology Center, 9000 Development Drive, Research Triangle Park, NC 27709-4827, USA Received 25 April 2002
Abstract The composition, microstructure and processing of NdFeB-type permanent magnets are all critical factors for the successful production of high performance magnet components. Three common fabrication routes can be used to categorize these NdFeB-based bulk magnets: sintering, polymer bonding and hot deformation. Generally, the former type of magnet has a high-energy product (30–50 MGOe), full density and a relatively simple shape. Bonded magnets have intermediate energy products (10–18 MGOe), lower density and can be formed into intricate net-shapes. Hot deformed magnets possess full density, intermediate to high-energy products (15–46 MGOe), isotropic or anisotropic properties and have the potential to be formed into net shapes. This article discusses the critical issues of improved magnetic performance, environmental stability, net-shape formability and magnetization behavior for the main categories of NdFeB magnets. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Isotropic magnets; Anisotropic magnets; Magnet fabrication
1. Introduction The production and application of NdFeB magnets has seen incredible growth in recent years, despite fluctuations in the world economy (e.g. financial downturn of South East Asia in 1998/9). It has been predicted that the total NdFeB permanent magnet market will grow from a current value of 2 to $4.8 billion over the next 5 years [1]. This spectacular growth of NdFeB production has been predominantly due to the rapid growth in the PC market over the last 10 *Corresponding author. Tel.: +919-993-5524; fax: +919993-5501. E-mail address: [email protected] (D. Brown).
years, with the voice-coil-motor (VCM) being the major application. However, the market is expected to grow into new fields like electric motor applications in the near future [1]. Table 1 gives a summary of the wide range of applications for NdFeB magnets, with many of these being predicted for substantial growth in the future [2]. One promising area is the use of permanent magnets in automotive applications, particularly in control systems. Servo and linear drives also constitute strong growth areas, especially in industrial robot-type applications where high performance, high torque motors are required. Another strong area of growth has been the huge expansion in the use of mobile phones where
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D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440 Table 1 Examples of applications of permanent magnets [2] Computer and office automation Disk drive spindle motors and voice coil motors CD-ROM spindle motors and pick-up motors Printer and fax stepper motors Printer hammer Copy machine rollers Automotive Starter motors Electric steering Sensors Electric fuel pumps Instrumentation gauges Brushless DC motors Actuators Alternators Consumer electronics VCRs and camcorders Cameras Speakers, headsets Microphones Pagers DVD players Watches Cell phones Appliance Portable power tools Household appliance motors Scales Air conditioners Water pumps Security systems Factory automation Magnetic couplings Pumps Motors Servo motors Generators Bearings Medical MRI Surgical tools Implants ‘‘Therapeutic’’
NdFeB magnets are employed in the isolators of microwave stations. Growing concern about global warming has scientific and technological implications and many
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of these impinge on the use of NdFeB magnets. Future uses could include their more widespread use in ‘‘white goods’’ such as washing machines, refrigerators, etc., in order to improve energy efficiency and energy conservation. The greatest potential market is with automotive applications, where weight reduction, safe operation and comfort improvements are required. The NdFeB magnets required for the developing markets, such as automotive devices, need to operate up to B1801C. Temperature stability is dependent upon a number of factors for the various types of NdFeB magnet. For example, fully dense, sintered magnets require significant amounts of dysprosium substitution for the neodymium [3], which results in a substantial increase in the cost of the magnet, as Dy is much less plentiful than Nd. In the case of polymerbonded magnets, the particle–particle interaction, amount of binder, magnet density and binder– particle interaction are also controlling factors, which need to be optimized for successive operation in potentially aggressive environments. These issues represent just a few of the many scientific/ technological challenges associated with future NdFeB magnet development. Predominant NdFeB magnet manufacturing routes include melt quenching nanocrystalline material for use in bonded and hot deformed components, and sintering microcrystalline powder into high energy, fully dense components. Each of these techniques has certain strengths and weaknesses in meeting the demands of the applications mentioned in Table 1. This article discusses these and alternative NdFeB magnet fabrication techniques in relation to magnetic properties, environmental stability, net-shape fabrication and magnetization behavior.
2. Types of NdFeB magnet fabrication NdFeB magnets can be categorized by their microstructure: nanocrystalline and microcrystalline. Nanocrystalline NdFeB magnets tend to be produced by melt spinning ribbon with a composition close to Nd2Fe14B [4]. This ribbon is subsequently crushed into a powder and
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either polymer bonded into a magnet or hot deformed into a fully dense magnet. Other than melt spinning, nanograin material can be produced via atomization [5], hydrogen–disproportionation–desorption–recombination (HDDR) [6] and mechanically alloying [7]. Atomization offers the advantages of high production rate and uniform spherical particle morphology, but is limited by the compositional changes required by the lower quench rate involved. HDDR has the ability to produce magnetically anisotropic powder, but is limited by the thermal stability of the powder. Mechanical alloying can be employed to produce NdFeB nanocrystalline and nanocomposite powder, however, the drawbacks with this technique have been low yields (B70%) and the reactivity of the fine powder. Microcrystalline magnets tend to be sintered to a fully dense shape using a traditional powder metallurgy route. The process for producing sintered magnets tends to start with an ingot being cast with a relatively high rare-earth component (Nd, DyB15 at%). This additional rare-earth limits a-Fe precipitation, and facilitates the subsequent liquid phase grain boundary sintering operation. Developments in the casting technology (e.g. strip casting [8]) have led to faster cooling rates, more homogeneity and finer scale microstructures which require less rare-earth content (Nd+DyB14 at%) and no post-cast annealing. Following the casting of NdFeB material for sintered magnets, it is pulverized with a course grind and then jet-milled. The resulting powder has a mean particle size around 5 mm, and can then be aligned in a magnetic field (B10–20 kOe) and pressed into a partially dense compact. Sintering then takes place at elevated temperatures for several hours (1000–11001C), and is followed by a post-sinter heat treatment (B6001C) to refine the grain boundary texture and relieve internal stresses. Cylindrical, arc and rectangular bars are typical shapes for sintered components. These shapes require machining to the desired dimensions and coating with a protective layer of nickel, zinc, aluminum or epoxy [9].
3. Common themes of improvement for NdFeB magnets There are a number of common themes for future improvements to both nanocrystalline and microcrystalline NdFeB permanent magnets. These include: * *
* *
Higher maximum energy product. Improved environmental stability and corrosion resistance at elevated operating temperatures. Near-net-shape manufacture. Improved magnetization behavior.
3.1. Higher maximum energy product If any type of permanent magnetic material has sufficient coercivity and a uniform microstructure then there are three factors that affect the remanence, and hence the (BH)max of the magnets. These factors and how they are influenced are detailed below. Factors that affect remanence have a larger effect on (BH)max, as (BH)max is proportional to the square of the remanence. (1) The saturation magnetization: This is an intrinsic property of the magnetic phase and can only be affected by compositional changes. (2) The proportion of magnetic phase: The remanence of a magnet is proportional to the volume fraction of magnetic phase and can only be improved by increasing the density of the magnet and/or decreasing the proportion of non-magnetic secondary phases or binder. (3) The degree of crystal alignment for anisotropic magnets: The maximum remanence of an anisotropic magnet is proportional to the cosine of the angle of misalignment averaged over each grain. Therefore, maximum alignment would require the c-axis of each grain to be orientated in the direction of magnetization as shown in Fig. 1.
3.1.1. Bonded magnets In terms of bonded magnets the melt-spun powder is produced close to the Nd2Fe14B or
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Microcrystalline Powder Metallurgy Route
Melt Quench Nanocrystalline Route
Alternative Nanocrystalline Routes
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Casting of basic composition from Nd, Fe, Fe2B
HDDR or Mechanical Alloying
Melt spin ribbon or Atomize powder
Ingot, strip or atomization casting of micro-structure
Coarse mill/HD to~100-200µm particles
Coarse grind ribbon to ~ 100-200µm particles
Jet Mill to ~ 1 to 10µm particles Annealing to full crytallinity~600°C, few minutes Blend to correct composition and add molding agent Blend with binder and lubricant
Align and compact into preform Compression mold magnet
Injection mold magnet
Extrude or calender form magnet Transverse Press
Axial Press
Rubber Isostatic Press
Magnetize Sinter ~ 1100°C, few hours Isotropic:Br~5-7.1kG, Hci~4.2-16.5kOe(BH)max~5-10MGOe Heat treat ~ 600°C, few hours
Anisotropic:Br~10kG,Hic~13kOe(BH)max~18MGOe
Compact preform Slice and machine to shape
Die Upset or Back Extrude into ring
Hot Press
Apply protective coating to magnet
Magnetize
Anisotropic:
Isotropic: Br ~ 8.3kG, Hci ~ 18kOe BH)max ~ 15MGOe
Anisotropic:
Br~12-13.1kG, Hci~12.5-20kOe (BH)max~32-42 MGOe
Br ~ 12-15 kG, Hci ~ 13-27kOe(BH)max ~ 30-50MGOe
Fig. 1. Basic schematic for common magnet production routes.
Nd2Fe14B/a-Fe compositions with small additions of elements like cobalt to maximize the saturation magnetization. One would expect the remanence and energy product of a bonded magnet to be directly linked to the amount of binder used, typically 2 wt% in compression molded magnets and 8–15 wt% in injection molded magnets. However, pore volume and internal magnetic shear loss also lead to lower than the expected Br values [1,10]. Pores reduce the magnet density and hence the Br : They can be limited by particle morphology, type of binder and consolidation technique. Internal shear loss is the effect caused by isolated magnetic particles magnetically shearing with one another within the polymer matrix. The effect increases with higher levels of polymer and powders with low rare-earth content or high Br =Hci values. A typical example of the
change in properties from powder to compression molded magnet is: MQPTM-B+ isotropic meltspun powder has a Br B9:0 kG and a (BH)max B16.3 MGOe, and the related magnet MQ1TM10–10 has a Br B7:1 kG and an energy product of 10 MGOe. Anisotropic-bonded magnets tend to suffer from a similar reduction in remanence from the powder to the magnet (e.g. powder MQATM-T has Br B12 kG and the related magnet MQ1TM 20–13 has Br B9:5 kG). The alignment of anisotropic nanograin material is a challenging operation as large aligning fields (B24 kOe) are required for full alignment [11]. 3.1.2. Sintered magnets For fully dense magnets the most successful method of improving ðBHÞmax has been to increase
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the proportion of magnetic Nd2Fe14B phase. Currently, sintered NdFeB-type magnets are based on the composition Nd14Fe78B8 (at%), which contains only B89.4 vol% Nd2Fe14B, giving a theoretical maximum ðBHÞmax of B52 MGOe, as compared with B64 MGOe for 100% Nd2Fe14B material [12]. However, moving towards higher proportions of Nd2Fe14B causes problems such as: excessive precipitation of a-Fe during equilibriumstate casting, the need for greater control over the liquid phase sintering operation and a heightened sensitivity to oxidation with less rare-earth phase at grain boundaries. Developments to tackle points 1 and 2 above have included the use of strip cast and atomized alloy to increase the cooling rate from the melt and limit the amount of a-Fe precipitation [8,13–15]. These casting techniques have allowed material with lower rare-earth contents and more homogenized microstructures to be produced, such that post-cast heat treatment and extensive coarse grinding are no longer required. By limiting the amount of a-Fe, strip cast alloy also has improved crushability over conventionally cast alloy, and hence greater milling throughputs are possible [13]. Further refinements have focused on the alignment and pressing of milled powder (issue 3, above). Conventionally, green compacts for sintered magnets have been aligned and pressed in a magnetic field, which is either parallel or perpendicular to the axis of pressing. The parallel pressing technique has the advantage of being able to produce parts with more near-net shape (86–88% alignment), while the perpendicular route produces parts that need more machining to shape. However, the perpendicular version yields magnets with higher magnetic properties, due to a greater degree of Nd2Fe14B grain alignment (90–93%) [16]. Rubber isostatic pressing (RIP) [17] or cold isostatic pressing (CIP) [13] techniques can produce even higher degrees of alignment (B98%). CIP has recently been used in conjunction with strip casting to produce a NdFeB magnet with the highest recorded energy product (55.8 MGOe) [13]. Isostatic-type pressing appears to be in a developmental stage and mass production magnets tend to have slightly lower maximum magnetic properties (B50 MGOe).
One outstanding challenge for sintered magnet production is limiting the oxidation of powder particles. With the fine milled particles size (2– 5 mm) and high rare-earth content (B14 at%) required for sintering, powder handling and oxygen pick-up are serious drawbacks to the process. A recent article has observed improvements in remanences (14.1–14.7 kG), density (7.47–7.52 g/cm3), intrinsic coercivity (up 36%) and energy product (up 17%) of a magnet prepared with 0.1 wt% as opposed to one with 0.3 wt% [14]. 3.1.3. Hot-pressed melt-spun magnets Many of the factors mentioned above for sintered magnets, such as composition and Nd2Fe14B density apply to hot-pressed and dieupset nanocrystalline magnets. However, the alignment mechanism for anisotropic die-upset magnets is fundamentally different and relies upon a solution–precipitation–creep mechanism driven by the axial compression of a material with liquidphase grain boundaries [18,19]. The degree to which the grains elongate and align with one another is strongly effected by the rare-earth (typically 13–14 at%), additives to improve hot workability (like Ga), die lubrication and design, initial grain size and temperature (700–7501C). Die-upset shapes and back-extruded tube magnets with energy products over 46 MGOe (Br B13 kG, Hci B16 kOe) are commercially available. 3.2. Environmental stability The environmental stability of a magnet can be measured by the change in its magnetic properties over time at a particular temperature. Aging behavior of a magnet can be determined by the flux loss over time for a given temperature. The total flux loss is composed of reversible loss, recoverable irreversible loss and structural loss. Flux loss is incurred by a magnetization reversal mechanism occurring with increased operating temperature, as illustrated in Fig. 2. On cooling part of this loss is recoverable (known as reversible loss, R in Fig. 2), and part is not recovered and is known as irreversible loss (I in Fig. 2). Irreversible loss is partly recoverable through remagnetization
D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
and partly permanent due to oxidation. Structural losses are related to the presence of the rare-earth grain boundary phase and tend to increase proportionally with the amount of this phase. Recoverable losses (R and I) are inversely proportional to the intrinsic coercivity and tend to increase with lower Hci values. The irreversible loss can be predicted by temperature coefficients of Br (commonly known as a/%1C1) and Hci (known as b/%1C1), as defined by the following equations: a ¼ ½ðBrðTÞ BrðRTÞ Þ=ðBrðRTÞ fT RTgÞ 100%; b ¼ ½ðHciðTÞ HciðRTÞ Þ=ðHciðRTÞ fT RTgÞ 100%; where, BrðTÞ and HciðTÞ denote the Br and Hci values at temperature T measured in degrees Celsius, and
I = Recoverable Irreversible Loss plus Structural Loss
Magnetic Moment (arb.units)
R = Reversible Loss 120-
I 80-
R
40-
00
50
100
150
200
Aging Temperature / °C Fig. 2. Illustration of the flux losses experienced by a NdFeB magnet at elevated temperatures [23].
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BrðRTÞ and HciðRTÞ are the Br and Hci values at room temperature, respectively. Typical values for bonded and fully dense magnets are presented in Table 2. 3.2.1. Sintered magnets Except for the least demanding of applications, sintered NdFeB-type permanent magnets have been disadvantaged in terms of environment stability, as they contain significant amounts of rare-earth-rich grain boundary phase. Improvements can be made by increasing the Curie temperature (Tc ) of the magnet (cobalt additions), modifying the magnetic domain configuration (dysprosium additions) and improving the corrosion stability of the grain boundary phase (Co, Cu and/or Ga) [14]. Powder oxidation during the magnet fabrication stage can be a considerable danger without careful control. Recent process improvements have achieved greater chemistry control and improved environmental stability for sintered magnet compositions, via powder blending techniques [14,20]. One study has shown that the hydrogen decrepitated, jetmilled powder can be very effectively blended with metal hydrides or metal powders (e.g. Cu, Co, etc.) to ‘‘fine tune’’ the composition. This is particularly interesting in the case of DyH2, where it has been shown that Dy can be concentrated around the grain boundaries of Nd2Fe14B grains, thus maximizing the effect of the Dy additions on the coercivity whilst minimizing the negative effect Dy has on the remanence and magnet cost. Protective coatings can improve the corrosion resistance of magnets. Many different materials
Table 2 Comparison of the fundamental magnetic properties of four commonly used permanent magnets [24] Material property
Bonded MQ1-B
Hot pressed MQ2-E
Die-upset MQ3-F
Sintered VCM
Br (kG) ðBHÞmax (MGOe) Hci (kOe) a (%/1C) b (%/1C) Tc (1C) Electrical conductivity Grinding required?
6.9 10 9 0.105 0.41 360 Fair No
8.25 15 17.5 0.10 0.50 335 Good No
13.1 42 16 0.09 0.60 370 Good No
13.1 42 14 0.12 0.60 310 Good Yes
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D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
can be used as a barrier coating for NdFeBsintered magnets, but the most widely used are based on Ni for higher value magnetic parts and zinc for lower cost applications. Using these compositional and coatings modifications some grades of sintered magnet (Trade names 32EH, NMX-27VH) can operate up to 2201C without irreversible flux loss [16]. 3.2.2. Hot-pressed melt-spun magnets Hot-pressed and die-upset magnets have been shown to exhibit significantly lower corrosion rates than sintered magnets. The reduction in corrosion rate is probably due to smaller grain size and less amounts of Nd-rich material [21]. 3.2.3. Bonded magnets Bonded magnets have the advantage of each NdFeB particle being covered in a protective polymer layer during the magnet compaction process, and ideally preventing the powder from coming into contact with humid atmospheres during operation. This feature has been enhanced by the development of new binder application techniques. One successful example is the liquid coating (LC) technique, which encapsulates the surface of each magnetic particle with an epoxybased polymer and allows higher powder fractions to be used in the magnet fabrication process [22]. A further advantage of melt-spun powder is its stability during magnet fabrication, as it can be handled in air without risk of oxidation. A simple experiment has been run to compare the aging loss of a melt-spun material (MQPTMB+) with a HDDR (MQATM-T) material. Both types of powder were blended with 2 wt% epoxy binder and compression molded to form cylindrical magnets with a length to diameter ratio of approximately 1. All magnets were pulse magnetized at 40 kOe. Flux aging loss was determined for samples held at 1001C, 1251C, 1501C and 1751C for 1000 h in air. The results are illustrated in Fig. 2, and show HDDR-prepared material to have significantly higher flux losses than a standard melt-spun material. This demonstrates the reason why HDDR processing has not received widespread commercial acceptance todate. It is also worth noting that MQP-B+materi-
al in epoxy is not an ideal candidate for magnets that operate above 1251C, and melt-spun powders with modified chemistries (Nb addition), such as MQP-O offered more stable aging characteristics up to 1751C [1]. 3.3. Net-shape manufacture Fully dense NdFeB-type materials are extremely brittle at room temperature and hence machining such fully dense magnets can be an expensive and laborious operation. As a result one important goal for magnet manufacturers is to develop netshape forming processes to avoid or minimize machining costs. Bonded magnets possess a high degree of netshape formability as such materials can be processed at relatively low temperatures (170– 3001C) depending on the binder and molding technique being used. Hot-pressed nanocrystalline magnets also achieve a high level of net shape, with the advantage of full density (MQ2) and anisotropy (MQ3, die upset or backward-extruded parts). The drawbacks tend to be the additional cost of further process stages and the elevated temperatures (700– 8001C) required. However, the hot deformation technique is the most straightforward method for producing fully dense, high-energy product netshaped magnets. Sintering is fundamentally limited in its ability to produce net-shape components as the green compacts can lose up to 25% in volume through shrinkage during the sintering treatment. Netshape capability could be achieved by applying a pressure during the sintering stage, similar to the MQ3 process, only with longer times and higher temperatures. Isostatic powder consolidation techniques can go some way to improving green compact density and hence reducing shrinkage during sintering, although a degree of surface machining is still required (Fig. 3). 3.4. Improved magnetization behavior The assembly of high BHmax and Hci permanent magnet devices is a complex procedure, particularly with the drive for miniaturization of many
D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
devices. Post-assembly magnetization is one preferred technique for applications, although a less effective magnetization field can be applied when the magnets are in situ. Hence, the magnetization behavior of magnets is an important factor for many applications, and the lower the applied field that is required for saturation is an advantage. Fig. 4 is a schematic comparison of the magnetization behavior of two types of sintered NdFeB-type magnet and three types of bonded NdFeB-type magnet [1,25]. For comparison, hot-pressed and die-upset magnets tend to require magnetizing
Fig. 3. Comparison of the aging behavior of melt-spun and HDDR processed bonded magnet.
100
Sintered NdFeCoCuB magnet
80 % Saturation of Flux
fields of 45 and 35 kOe for full saturation, respectively. Isotropic bonded magnets appear to need higher magnetizing fields than the sintered magnets shown in Fig. 4, due to differences in the magnetic mechanisms. The high coercivity (Hci B15 kOe) MQ1TM-A magnet requires an applied field of 20 kOe to achieve 65% of the Br value, while the lower coercivity (10 kOe) MQ1TM-B magnet achieves 85% saturation with a similar applied field. The development of Nd2Fe14B/a-Fe and Nd2Fe14B/Fe3B nanocomposite materials with lower coercivities has allowed higher saturation of bonded magnets with lower magnetizing fields. For example, MQ1TM-Q (Hci B4.2 kOe) bonded magnets can achieve 75% of the Br value with a 12.5 kOe applied field. Improving the magnetizing behavior of bonded magnets remains one of the challenges for future permanent magnet research and development. Sintered magnets exhibit a greater ability for magnetization, as illustrated in Fig. 4. Research has shown that by increasing the Dy content of the rare-earth component in the composition, even lower fields can be used to saturate sintered magnets. Fig. 4 shows how a 9 wt% Dy addition to a Nd–Fe–Co–Cu–B material can bring the required magnetizing field for full Br saturation down from 20 to 10 kOe. Dysprosium and terbium have the additional benefit of improving the
Sintered (Nd,Dy)FeCoCuB magnet
90 70 60
MQ1-B
50 MQ1-A
40
MQ1-Q
30 20
Sintered no Dy Sintered 9wt.% Dy MQ1-A MQ1-B MQ1-Q
10 0 0
2
4
6
439
8 10 12 14 Magnetizing field / kOe
16
18
Fig. 4. Comparison of magnetization field required for saturation [1,25].
20
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D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
temperature stability for magnets. However, Dy and Tb are expensive additives and have the affect of lowering the remanence. Future research in this area may focus on reducing the amount of Dy required or developing an alternative. 4. Summary The principle types of NdFeB magnets have been discussed in terms of their fabrication techniques and operating characteristics. Important concerns of magnet producers and users, like magnetic performance, environmental stability, net-shape manufacture and the magnetization behavior have been addressed. Bonded and fully dense magnets have their own advantages and drawbacks. If isotropic properties, net-shape formability and low cost are required, then a bonded magnet will be selected. On the other hand, if a high remanence, anisotropic, regular-shaped magnet is required in large volume then a sintered magnet route should be selected. If net-shape formability, powder stability, together with high remanence is required, then a hotpressed (isotropic MQ2TM) or die-upset (anisotropic MQ3TM) magnet would be suitable. References [1] V. Panchanathan, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 431. [2] P. Campbell, Private communication 2001. [3] M. Velicescu, W. Fernengel, W. Rodewald, P. Schrey, B. Wall, J. Magn. Magn. Mater. 158 (1996) 47. [4] J.J. Croat, J.F. Herbst, R.W. Lee, E.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078. [5] C.H. Sellers, D.J. Branagan, T.A. Hyde, L.H. Lewis, V. Panchanathan, Proceedings of the 14th International Workshop on Rare-Earth Magnets and Their Applications, San Paulo, Brazil, 1996, p. 28. [6] O. Gutfleisch, I.R. Harris, J. Phys. 29 (1996) 2255.
[7] L. Schultz, J. Wecker, E. Hellstern, J. Appl. Phys. 61 (1987) 3583. [8] Y. Hirose, H. Hasegawa, S. Saski, M. Sagawa, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 77. [9] P.B. Gwan, S.J. Collocott, J.B. Dunlop, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 325. [10] J.M.D. Coey, K. O’Donnell, J. Appl. Phys. 81 (1997) 4810. [11] R. Nakayama, T. Takeshita, M. Itakura, N. Kuwano, K. Oki, J. Appl. Phys. 76 (1994) 412. [12] M. Sagawa, S. Fujimura, M. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [13] Y. Kaneko, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 83. [14] S. Sasaki, J. Fidler, M. Sagawa, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 109. [15] W. Rodewald, R. Blank, B. Ball, G.W. Reppel, H.D. Zilg, Proceedings of the 16th International Workshop on RareEarth Magnets and Their Applications, Sendai, Japan, 2000, p. 119. [16] S. Hirosawa Y. Kaneko, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 43. [17] M. Sagawa. Proceedings of the 13th International Workshop on Rare-Earth Magnets and Their Applications, Birmingham, UK, 1996 [18] W. Grunberger, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications. Dresden, Germany, 1998, p. 333. [19] O. Gutfleisch, J. Appl. Phys. 33 (2000) R157–172. [20] R.S. Mottram, A. Kianvash, I.R. Harris, J. Alloys Compounds 283 (1999) 282. [21] B.M. Ma, D. Lee, B. Smith, S. Gaiffi, B. Owens, H. Bie, G.W. Warren, 8th Joint MMM-Intermag Conference, San Antonio, TX, January 2001, Paper BG-06. [22] D. Li, S. Gaiffi, D. Kirk, K. Young, J. Herchenroeder, J. Appl. Phys. 85 (1999) 4871. [23] B.M. Ma, D.W. Scott, Y.L. Liang, L.W. Lui, C.O. Bounds. Proceedings of the 10th Conference on Magnetism and Magnetic Technology, 1995, p. 83. [24] B.M. Ma, S. Gaiffi. 8th International Conference on Ferrites, 2000, Paper No: 19ApI-2. [25] A. Kim, F.E. Camp, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 55.
Topical review
Developments in the processing and properties of NdFeb-type permanent magnets David Brown*, Bao-Min Ma, Zhongmin Chen Magnequench Technology Center, 9000 Development Drive, Research Triangle Park, NC 27709-4827, USA Received 25 April 2002
Abstract The composition, microstructure and processing of NdFeB-type permanent magnets are all critical factors for the successful production of high performance magnet components. Three common fabrication routes can be used to categorize these NdFeB-based bulk magnets: sintering, polymer bonding and hot deformation. Generally, the former type of magnet has a high-energy product (30–50 MGOe), full density and a relatively simple shape. Bonded magnets have intermediate energy products (10–18 MGOe), lower density and can be formed into intricate net-shapes. Hot deformed magnets possess full density, intermediate to high-energy products (15–46 MGOe), isotropic or anisotropic properties and have the potential to be formed into net shapes. This article discusses the critical issues of improved magnetic performance, environmental stability, net-shape formability and magnetization behavior for the main categories of NdFeB magnets. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Isotropic magnets; Anisotropic magnets; Magnet fabrication
1. Introduction The production and application of NdFeB magnets has seen incredible growth in recent years, despite fluctuations in the world economy (e.g. financial downturn of South East Asia in 1998/9). It has been predicted that the total NdFeB permanent magnet market will grow from a current value of 2 to $4.8 billion over the next 5 years [1]. This spectacular growth of NdFeB production has been predominantly due to the rapid growth in the PC market over the last 10 *Corresponding author. Tel.: +919-993-5524; fax: +919993-5501. E-mail address: [email protected] (D. Brown).
years, with the voice-coil-motor (VCM) being the major application. However, the market is expected to grow into new fields like electric motor applications in the near future [1]. Table 1 gives a summary of the wide range of applications for NdFeB magnets, with many of these being predicted for substantial growth in the future [2]. One promising area is the use of permanent magnets in automotive applications, particularly in control systems. Servo and linear drives also constitute strong growth areas, especially in industrial robot-type applications where high performance, high torque motors are required. Another strong area of growth has been the huge expansion in the use of mobile phones where
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D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440 Table 1 Examples of applications of permanent magnets [2] Computer and office automation Disk drive spindle motors and voice coil motors CD-ROM spindle motors and pick-up motors Printer and fax stepper motors Printer hammer Copy machine rollers Automotive Starter motors Electric steering Sensors Electric fuel pumps Instrumentation gauges Brushless DC motors Actuators Alternators Consumer electronics VCRs and camcorders Cameras Speakers, headsets Microphones Pagers DVD players Watches Cell phones Appliance Portable power tools Household appliance motors Scales Air conditioners Water pumps Security systems Factory automation Magnetic couplings Pumps Motors Servo motors Generators Bearings Medical MRI Surgical tools Implants ‘‘Therapeutic’’
NdFeB magnets are employed in the isolators of microwave stations. Growing concern about global warming has scientific and technological implications and many
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of these impinge on the use of NdFeB magnets. Future uses could include their more widespread use in ‘‘white goods’’ such as washing machines, refrigerators, etc., in order to improve energy efficiency and energy conservation. The greatest potential market is with automotive applications, where weight reduction, safe operation and comfort improvements are required. The NdFeB magnets required for the developing markets, such as automotive devices, need to operate up to B1801C. Temperature stability is dependent upon a number of factors for the various types of NdFeB magnet. For example, fully dense, sintered magnets require significant amounts of dysprosium substitution for the neodymium [3], which results in a substantial increase in the cost of the magnet, as Dy is much less plentiful than Nd. In the case of polymerbonded magnets, the particle–particle interaction, amount of binder, magnet density and binder– particle interaction are also controlling factors, which need to be optimized for successive operation in potentially aggressive environments. These issues represent just a few of the many scientific/ technological challenges associated with future NdFeB magnet development. Predominant NdFeB magnet manufacturing routes include melt quenching nanocrystalline material for use in bonded and hot deformed components, and sintering microcrystalline powder into high energy, fully dense components. Each of these techniques has certain strengths and weaknesses in meeting the demands of the applications mentioned in Table 1. This article discusses these and alternative NdFeB magnet fabrication techniques in relation to magnetic properties, environmental stability, net-shape fabrication and magnetization behavior.
2. Types of NdFeB magnet fabrication NdFeB magnets can be categorized by their microstructure: nanocrystalline and microcrystalline. Nanocrystalline NdFeB magnets tend to be produced by melt spinning ribbon with a composition close to Nd2Fe14B [4]. This ribbon is subsequently crushed into a powder and
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either polymer bonded into a magnet or hot deformed into a fully dense magnet. Other than melt spinning, nanograin material can be produced via atomization [5], hydrogen–disproportionation–desorption–recombination (HDDR) [6] and mechanically alloying [7]. Atomization offers the advantages of high production rate and uniform spherical particle morphology, but is limited by the compositional changes required by the lower quench rate involved. HDDR has the ability to produce magnetically anisotropic powder, but is limited by the thermal stability of the powder. Mechanical alloying can be employed to produce NdFeB nanocrystalline and nanocomposite powder, however, the drawbacks with this technique have been low yields (B70%) and the reactivity of the fine powder. Microcrystalline magnets tend to be sintered to a fully dense shape using a traditional powder metallurgy route. The process for producing sintered magnets tends to start with an ingot being cast with a relatively high rare-earth component (Nd, DyB15 at%). This additional rare-earth limits a-Fe precipitation, and facilitates the subsequent liquid phase grain boundary sintering operation. Developments in the casting technology (e.g. strip casting [8]) have led to faster cooling rates, more homogeneity and finer scale microstructures which require less rare-earth content (Nd+DyB14 at%) and no post-cast annealing. Following the casting of NdFeB material for sintered magnets, it is pulverized with a course grind and then jet-milled. The resulting powder has a mean particle size around 5 mm, and can then be aligned in a magnetic field (B10–20 kOe) and pressed into a partially dense compact. Sintering then takes place at elevated temperatures for several hours (1000–11001C), and is followed by a post-sinter heat treatment (B6001C) to refine the grain boundary texture and relieve internal stresses. Cylindrical, arc and rectangular bars are typical shapes for sintered components. These shapes require machining to the desired dimensions and coating with a protective layer of nickel, zinc, aluminum or epoxy [9].
3. Common themes of improvement for NdFeB magnets There are a number of common themes for future improvements to both nanocrystalline and microcrystalline NdFeB permanent magnets. These include: * *
* *
Higher maximum energy product. Improved environmental stability and corrosion resistance at elevated operating temperatures. Near-net-shape manufacture. Improved magnetization behavior.
3.1. Higher maximum energy product If any type of permanent magnetic material has sufficient coercivity and a uniform microstructure then there are three factors that affect the remanence, and hence the (BH)max of the magnets. These factors and how they are influenced are detailed below. Factors that affect remanence have a larger effect on (BH)max, as (BH)max is proportional to the square of the remanence. (1) The saturation magnetization: This is an intrinsic property of the magnetic phase and can only be affected by compositional changes. (2) The proportion of magnetic phase: The remanence of a magnet is proportional to the volume fraction of magnetic phase and can only be improved by increasing the density of the magnet and/or decreasing the proportion of non-magnetic secondary phases or binder. (3) The degree of crystal alignment for anisotropic magnets: The maximum remanence of an anisotropic magnet is proportional to the cosine of the angle of misalignment averaged over each grain. Therefore, maximum alignment would require the c-axis of each grain to be orientated in the direction of magnetization as shown in Fig. 1.
3.1.1. Bonded magnets In terms of bonded magnets the melt-spun powder is produced close to the Nd2Fe14B or
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Microcrystalline Powder Metallurgy Route
Melt Quench Nanocrystalline Route
Alternative Nanocrystalline Routes
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Casting of basic composition from Nd, Fe, Fe2B
HDDR or Mechanical Alloying
Melt spin ribbon or Atomize powder
Ingot, strip or atomization casting of micro-structure
Coarse mill/HD to~100-200µm particles
Coarse grind ribbon to ~ 100-200µm particles
Jet Mill to ~ 1 to 10µm particles Annealing to full crytallinity~600°C, few minutes Blend to correct composition and add molding agent Blend with binder and lubricant
Align and compact into preform Compression mold magnet
Injection mold magnet
Extrude or calender form magnet Transverse Press
Axial Press
Rubber Isostatic Press
Magnetize Sinter ~ 1100°C, few hours Isotropic:Br~5-7.1kG, Hci~4.2-16.5kOe(BH)max~5-10MGOe Heat treat ~ 600°C, few hours
Anisotropic:Br~10kG,Hic~13kOe(BH)max~18MGOe
Compact preform Slice and machine to shape
Die Upset or Back Extrude into ring
Hot Press
Apply protective coating to magnet
Magnetize
Anisotropic:
Isotropic: Br ~ 8.3kG, Hci ~ 18kOe BH)max ~ 15MGOe
Anisotropic:
Br~12-13.1kG, Hci~12.5-20kOe (BH)max~32-42 MGOe
Br ~ 12-15 kG, Hci ~ 13-27kOe(BH)max ~ 30-50MGOe
Fig. 1. Basic schematic for common magnet production routes.
Nd2Fe14B/a-Fe compositions with small additions of elements like cobalt to maximize the saturation magnetization. One would expect the remanence and energy product of a bonded magnet to be directly linked to the amount of binder used, typically 2 wt% in compression molded magnets and 8–15 wt% in injection molded magnets. However, pore volume and internal magnetic shear loss also lead to lower than the expected Br values [1,10]. Pores reduce the magnet density and hence the Br : They can be limited by particle morphology, type of binder and consolidation technique. Internal shear loss is the effect caused by isolated magnetic particles magnetically shearing with one another within the polymer matrix. The effect increases with higher levels of polymer and powders with low rare-earth content or high Br =Hci values. A typical example of the
change in properties from powder to compression molded magnet is: MQPTM-B+ isotropic meltspun powder has a Br B9:0 kG and a (BH)max B16.3 MGOe, and the related magnet MQ1TM10–10 has a Br B7:1 kG and an energy product of 10 MGOe. Anisotropic-bonded magnets tend to suffer from a similar reduction in remanence from the powder to the magnet (e.g. powder MQATM-T has Br B12 kG and the related magnet MQ1TM 20–13 has Br B9:5 kG). The alignment of anisotropic nanograin material is a challenging operation as large aligning fields (B24 kOe) are required for full alignment [11]. 3.1.2. Sintered magnets For fully dense magnets the most successful method of improving ðBHÞmax has been to increase
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the proportion of magnetic Nd2Fe14B phase. Currently, sintered NdFeB-type magnets are based on the composition Nd14Fe78B8 (at%), which contains only B89.4 vol% Nd2Fe14B, giving a theoretical maximum ðBHÞmax of B52 MGOe, as compared with B64 MGOe for 100% Nd2Fe14B material [12]. However, moving towards higher proportions of Nd2Fe14B causes problems such as: excessive precipitation of a-Fe during equilibriumstate casting, the need for greater control over the liquid phase sintering operation and a heightened sensitivity to oxidation with less rare-earth phase at grain boundaries. Developments to tackle points 1 and 2 above have included the use of strip cast and atomized alloy to increase the cooling rate from the melt and limit the amount of a-Fe precipitation [8,13–15]. These casting techniques have allowed material with lower rare-earth contents and more homogenized microstructures to be produced, such that post-cast heat treatment and extensive coarse grinding are no longer required. By limiting the amount of a-Fe, strip cast alloy also has improved crushability over conventionally cast alloy, and hence greater milling throughputs are possible [13]. Further refinements have focused on the alignment and pressing of milled powder (issue 3, above). Conventionally, green compacts for sintered magnets have been aligned and pressed in a magnetic field, which is either parallel or perpendicular to the axis of pressing. The parallel pressing technique has the advantage of being able to produce parts with more near-net shape (86–88% alignment), while the perpendicular route produces parts that need more machining to shape. However, the perpendicular version yields magnets with higher magnetic properties, due to a greater degree of Nd2Fe14B grain alignment (90–93%) [16]. Rubber isostatic pressing (RIP) [17] or cold isostatic pressing (CIP) [13] techniques can produce even higher degrees of alignment (B98%). CIP has recently been used in conjunction with strip casting to produce a NdFeB magnet with the highest recorded energy product (55.8 MGOe) [13]. Isostatic-type pressing appears to be in a developmental stage and mass production magnets tend to have slightly lower maximum magnetic properties (B50 MGOe).
One outstanding challenge for sintered magnet production is limiting the oxidation of powder particles. With the fine milled particles size (2– 5 mm) and high rare-earth content (B14 at%) required for sintering, powder handling and oxygen pick-up are serious drawbacks to the process. A recent article has observed improvements in remanences (14.1–14.7 kG), density (7.47–7.52 g/cm3), intrinsic coercivity (up 36%) and energy product (up 17%) of a magnet prepared with 0.1 wt% as opposed to one with 0.3 wt% [14]. 3.1.3. Hot-pressed melt-spun magnets Many of the factors mentioned above for sintered magnets, such as composition and Nd2Fe14B density apply to hot-pressed and dieupset nanocrystalline magnets. However, the alignment mechanism for anisotropic die-upset magnets is fundamentally different and relies upon a solution–precipitation–creep mechanism driven by the axial compression of a material with liquidphase grain boundaries [18,19]. The degree to which the grains elongate and align with one another is strongly effected by the rare-earth (typically 13–14 at%), additives to improve hot workability (like Ga), die lubrication and design, initial grain size and temperature (700–7501C). Die-upset shapes and back-extruded tube magnets with energy products over 46 MGOe (Br B13 kG, Hci B16 kOe) are commercially available. 3.2. Environmental stability The environmental stability of a magnet can be measured by the change in its magnetic properties over time at a particular temperature. Aging behavior of a magnet can be determined by the flux loss over time for a given temperature. The total flux loss is composed of reversible loss, recoverable irreversible loss and structural loss. Flux loss is incurred by a magnetization reversal mechanism occurring with increased operating temperature, as illustrated in Fig. 2. On cooling part of this loss is recoverable (known as reversible loss, R in Fig. 2), and part is not recovered and is known as irreversible loss (I in Fig. 2). Irreversible loss is partly recoverable through remagnetization
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and partly permanent due to oxidation. Structural losses are related to the presence of the rare-earth grain boundary phase and tend to increase proportionally with the amount of this phase. Recoverable losses (R and I) are inversely proportional to the intrinsic coercivity and tend to increase with lower Hci values. The irreversible loss can be predicted by temperature coefficients of Br (commonly known as a/%1C1) and Hci (known as b/%1C1), as defined by the following equations: a ¼ ½ðBrðTÞ BrðRTÞ Þ=ðBrðRTÞ fT RTgÞ 100%; b ¼ ½ðHciðTÞ HciðRTÞ Þ=ðHciðRTÞ fT RTgÞ 100%; where, BrðTÞ and HciðTÞ denote the Br and Hci values at temperature T measured in degrees Celsius, and
I = Recoverable Irreversible Loss plus Structural Loss
Magnetic Moment (arb.units)
R = Reversible Loss 120-
I 80-
R
40-
00
50
100
150
200
Aging Temperature / °C Fig. 2. Illustration of the flux losses experienced by a NdFeB magnet at elevated temperatures [23].
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BrðRTÞ and HciðRTÞ are the Br and Hci values at room temperature, respectively. Typical values for bonded and fully dense magnets are presented in Table 2. 3.2.1. Sintered magnets Except for the least demanding of applications, sintered NdFeB-type permanent magnets have been disadvantaged in terms of environment stability, as they contain significant amounts of rare-earth-rich grain boundary phase. Improvements can be made by increasing the Curie temperature (Tc ) of the magnet (cobalt additions), modifying the magnetic domain configuration (dysprosium additions) and improving the corrosion stability of the grain boundary phase (Co, Cu and/or Ga) [14]. Powder oxidation during the magnet fabrication stage can be a considerable danger without careful control. Recent process improvements have achieved greater chemistry control and improved environmental stability for sintered magnet compositions, via powder blending techniques [14,20]. One study has shown that the hydrogen decrepitated, jetmilled powder can be very effectively blended with metal hydrides or metal powders (e.g. Cu, Co, etc.) to ‘‘fine tune’’ the composition. This is particularly interesting in the case of DyH2, where it has been shown that Dy can be concentrated around the grain boundaries of Nd2Fe14B grains, thus maximizing the effect of the Dy additions on the coercivity whilst minimizing the negative effect Dy has on the remanence and magnet cost. Protective coatings can improve the corrosion resistance of magnets. Many different materials
Table 2 Comparison of the fundamental magnetic properties of four commonly used permanent magnets [24] Material property
Bonded MQ1-B
Hot pressed MQ2-E
Die-upset MQ3-F
Sintered VCM
Br (kG) ðBHÞmax (MGOe) Hci (kOe) a (%/1C) b (%/1C) Tc (1C) Electrical conductivity Grinding required?
6.9 10 9 0.105 0.41 360 Fair No
8.25 15 17.5 0.10 0.50 335 Good No
13.1 42 16 0.09 0.60 370 Good No
13.1 42 14 0.12 0.60 310 Good Yes
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can be used as a barrier coating for NdFeBsintered magnets, but the most widely used are based on Ni for higher value magnetic parts and zinc for lower cost applications. Using these compositional and coatings modifications some grades of sintered magnet (Trade names 32EH, NMX-27VH) can operate up to 2201C without irreversible flux loss [16]. 3.2.2. Hot-pressed melt-spun magnets Hot-pressed and die-upset magnets have been shown to exhibit significantly lower corrosion rates than sintered magnets. The reduction in corrosion rate is probably due to smaller grain size and less amounts of Nd-rich material [21]. 3.2.3. Bonded magnets Bonded magnets have the advantage of each NdFeB particle being covered in a protective polymer layer during the magnet compaction process, and ideally preventing the powder from coming into contact with humid atmospheres during operation. This feature has been enhanced by the development of new binder application techniques. One successful example is the liquid coating (LC) technique, which encapsulates the surface of each magnetic particle with an epoxybased polymer and allows higher powder fractions to be used in the magnet fabrication process [22]. A further advantage of melt-spun powder is its stability during magnet fabrication, as it can be handled in air without risk of oxidation. A simple experiment has been run to compare the aging loss of a melt-spun material (MQPTMB+) with a HDDR (MQATM-T) material. Both types of powder were blended with 2 wt% epoxy binder and compression molded to form cylindrical magnets with a length to diameter ratio of approximately 1. All magnets were pulse magnetized at 40 kOe. Flux aging loss was determined for samples held at 1001C, 1251C, 1501C and 1751C for 1000 h in air. The results are illustrated in Fig. 2, and show HDDR-prepared material to have significantly higher flux losses than a standard melt-spun material. This demonstrates the reason why HDDR processing has not received widespread commercial acceptance todate. It is also worth noting that MQP-B+materi-
al in epoxy is not an ideal candidate for magnets that operate above 1251C, and melt-spun powders with modified chemistries (Nb addition), such as MQP-O offered more stable aging characteristics up to 1751C [1]. 3.3. Net-shape manufacture Fully dense NdFeB-type materials are extremely brittle at room temperature and hence machining such fully dense magnets can be an expensive and laborious operation. As a result one important goal for magnet manufacturers is to develop netshape forming processes to avoid or minimize machining costs. Bonded magnets possess a high degree of netshape formability as such materials can be processed at relatively low temperatures (170– 3001C) depending on the binder and molding technique being used. Hot-pressed nanocrystalline magnets also achieve a high level of net shape, with the advantage of full density (MQ2) and anisotropy (MQ3, die upset or backward-extruded parts). The drawbacks tend to be the additional cost of further process stages and the elevated temperatures (700– 8001C) required. However, the hot deformation technique is the most straightforward method for producing fully dense, high-energy product netshaped magnets. Sintering is fundamentally limited in its ability to produce net-shape components as the green compacts can lose up to 25% in volume through shrinkage during the sintering treatment. Netshape capability could be achieved by applying a pressure during the sintering stage, similar to the MQ3 process, only with longer times and higher temperatures. Isostatic powder consolidation techniques can go some way to improving green compact density and hence reducing shrinkage during sintering, although a degree of surface machining is still required (Fig. 3). 3.4. Improved magnetization behavior The assembly of high BHmax and Hci permanent magnet devices is a complex procedure, particularly with the drive for miniaturization of many
D. Brown et al. / Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
devices. Post-assembly magnetization is one preferred technique for applications, although a less effective magnetization field can be applied when the magnets are in situ. Hence, the magnetization behavior of magnets is an important factor for many applications, and the lower the applied field that is required for saturation is an advantage. Fig. 4 is a schematic comparison of the magnetization behavior of two types of sintered NdFeB-type magnet and three types of bonded NdFeB-type magnet [1,25]. For comparison, hot-pressed and die-upset magnets tend to require magnetizing
Fig. 3. Comparison of the aging behavior of melt-spun and HDDR processed bonded magnet.
100
Sintered NdFeCoCuB magnet
80 % Saturation of Flux
fields of 45 and 35 kOe for full saturation, respectively. Isotropic bonded magnets appear to need higher magnetizing fields than the sintered magnets shown in Fig. 4, due to differences in the magnetic mechanisms. The high coercivity (Hci B15 kOe) MQ1TM-A magnet requires an applied field of 20 kOe to achieve 65% of the Br value, while the lower coercivity (10 kOe) MQ1TM-B magnet achieves 85% saturation with a similar applied field. The development of Nd2Fe14B/a-Fe and Nd2Fe14B/Fe3B nanocomposite materials with lower coercivities has allowed higher saturation of bonded magnets with lower magnetizing fields. For example, MQ1TM-Q (Hci B4.2 kOe) bonded magnets can achieve 75% of the Br value with a 12.5 kOe applied field. Improving the magnetizing behavior of bonded magnets remains one of the challenges for future permanent magnet research and development. Sintered magnets exhibit a greater ability for magnetization, as illustrated in Fig. 4. Research has shown that by increasing the Dy content of the rare-earth component in the composition, even lower fields can be used to saturate sintered magnets. Fig. 4 shows how a 9 wt% Dy addition to a Nd–Fe–Co–Cu–B material can bring the required magnetizing field for full Br saturation down from 20 to 10 kOe. Dysprosium and terbium have the additional benefit of improving the
Sintered (Nd,Dy)FeCoCuB magnet
90 70 60
MQ1-B
50 MQ1-A
40
MQ1-Q
30 20
Sintered no Dy Sintered 9wt.% Dy MQ1-A MQ1-B MQ1-Q
10 0 0
2
4
6
439
8 10 12 14 Magnetizing field / kOe
16
18
Fig. 4. Comparison of magnetization field required for saturation [1,25].
20
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temperature stability for magnets. However, Dy and Tb are expensive additives and have the affect of lowering the remanence. Future research in this area may focus on reducing the amount of Dy required or developing an alternative. 4. Summary The principle types of NdFeB magnets have been discussed in terms of their fabrication techniques and operating characteristics. Important concerns of magnet producers and users, like magnetic performance, environmental stability, net-shape manufacture and the magnetization behavior have been addressed. Bonded and fully dense magnets have their own advantages and drawbacks. If isotropic properties, net-shape formability and low cost are required, then a bonded magnet will be selected. On the other hand, if a high remanence, anisotropic, regular-shaped magnet is required in large volume then a sintered magnet route should be selected. If net-shape formability, powder stability, together with high remanence is required, then a hotpressed (isotropic MQ2TM) or die-upset (anisotropic MQ3TM) magnet would be suitable. References [1] V. Panchanathan, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 431. [2] P. Campbell, Private communication 2001. [3] M. Velicescu, W. Fernengel, W. Rodewald, P. Schrey, B. Wall, J. Magn. Magn. Mater. 158 (1996) 47. [4] J.J. Croat, J.F. Herbst, R.W. Lee, E.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078. [5] C.H. Sellers, D.J. Branagan, T.A. Hyde, L.H. Lewis, V. Panchanathan, Proceedings of the 14th International Workshop on Rare-Earth Magnets and Their Applications, San Paulo, Brazil, 1996, p. 28. [6] O. Gutfleisch, I.R. Harris, J. Phys. 29 (1996) 2255.
[7] L. Schultz, J. Wecker, E. Hellstern, J. Appl. Phys. 61 (1987) 3583. [8] Y. Hirose, H. Hasegawa, S. Saski, M. Sagawa, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 77. [9] P.B. Gwan, S.J. Collocott, J.B. Dunlop, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 325. [10] J.M.D. Coey, K. O’Donnell, J. Appl. Phys. 81 (1997) 4810. [11] R. Nakayama, T. Takeshita, M. Itakura, N. Kuwano, K. Oki, J. Appl. Phys. 76 (1994) 412. [12] M. Sagawa, S. Fujimura, M. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [13] Y. Kaneko, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 83. [14] S. Sasaki, J. Fidler, M. Sagawa, Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, Japan, 2000, p. 109. [15] W. Rodewald, R. Blank, B. Ball, G.W. Reppel, H.D. Zilg, Proceedings of the 16th International Workshop on RareEarth Magnets and Their Applications, Sendai, Japan, 2000, p. 119. [16] S. Hirosawa Y. Kaneko, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 43. [17] M. Sagawa. Proceedings of the 13th International Workshop on Rare-Earth Magnets and Their Applications, Birmingham, UK, 1996 [18] W. Grunberger, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications. Dresden, Germany, 1998, p. 333. [19] O. Gutfleisch, J. Appl. Phys. 33 (2000) R157–172. [20] R.S. Mottram, A. Kianvash, I.R. Harris, J. Alloys Compounds 283 (1999) 282. [21] B.M. Ma, D. Lee, B. Smith, S. Gaiffi, B. Owens, H. Bie, G.W. Warren, 8th Joint MMM-Intermag Conference, San Antonio, TX, January 2001, Paper BG-06. [22] D. Li, S. Gaiffi, D. Kirk, K. Young, J. Herchenroeder, J. Appl. Phys. 85 (1999) 4871. [23] B.M. Ma, D.W. Scott, Y.L. Liang, L.W. Lui, C.O. Bounds. Proceedings of the 10th Conference on Magnetism and Magnetic Technology, 1995, p. 83. [24] B.M. Ma, S. Gaiffi. 8th International Conference on Ferrites, 2000, Paper No: 19ApI-2. [25] A. Kim, F.E. Camp, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Dresden, Germany, 1998, p. 55.