Magnetics Business & Technology - Spring 2016 - (Page 6)

FEATURE ARTICLE Permanent Magnets in a Changing World Market By Steve Constantinides, Director of Technology | Arnold Magnetic Technologies There are several commercially available permanent magnets. In sequence of invention they are: alnico, ferrite, SmCo (samarium cobalt), NdFeB (neodymium iron boron, Neo), and SmFeN (samarium iron nitride). The last three types are considered "rare earth" magnets because they contain one or more rare earth elements. In terms of total quantity produced each year the permanent magnet market is dominated by the ferrite and rare earth types. Approximately 70 weight percent of ferrite and rare earth permanent magnets are utilized in motors. Permanent magnet motors have several advantages over induction (non-magnet) types, the most notable being efficiency. This coupled with the pervasive use of motors means that permanent magnets are a key enabling material in modern economies. Magnet Alternatives The 20th century saw remarkable developments in the area of permanent magnet materials. Approximately every 12 years a new and significantly improved magnet material was invented. The first major invention was for the alnico family of magnetic materials commencing in late 1931 with continued development through 1975. Then in 1952 a magnetic ceramic was marketed by Philips[1]. The material is today referred to as a permanent magnet ferrite, a hard ferrite or, simply, ferrite magnet. The composition is represented by MO*(Fe2O3)6 where the metal M can be barium, lead or strontium. Almost all ferrite magnets are today made using strontium because of environmental and safety issues associated with both barium and lead. The development of ferrite magnets overlapped the end of the alnico development period. (Note there are soft magnetic ferrites based on manganese, nickel and/or zinc and these should xnot be confused with the permanent magnet ferrites). Starting in the 1950s researchers worked with rare earth elements, notably yttrium, in combination with the naturally ferromagnetic elements iron, nickel and cobalt. Yttrium cobalt (YCo5) was found to have interesting magnetic properties. Continuing research on the family of REE+(Co,Fe) resulted in the discovery in 1965 of SmCo5 by Karl Strnat at Wright Patterson AFB in Ohio, USA. Dr. Strnat relocated to the nearby University of Dayton Research Institute and along with Herb Mildrum and Al Ray continued development of SmCo[2]. As with much research, numerous laboratories were exploring similar compositions and assignment of the discovery is often based on who received the patent or first published the data. A part of the metallurgical research was a hunt for stable alloys of samarium and cobalt with reduced rare earth content. The result was Sm2Co17. This alloy, while promising, had insufficient coercivity (resistance to demagnetization) resulting in inferior maximum energy product. After numerous trials, the team of Strnat/Ray/Mildrum arrived at additions of copper and a refractory metal, preferably zirconium, to optimize residual magnetic induction (Br), maximum energy product and coercivity[2]. This enhanced Sm2(Co,Fe,Cu,Zr)17 alloy, also referred to as SmCo 2:17, was commercialized by 1975 and immediately became the material of choice for demanding applications. By 1978, SmCo 2:17 was widely utilized in high performance motors and in sensors in hostile ambient conditions such as automotive under-the-hood applications. Civil unrest in Zaire (Belgium Congo) in 1978 disrupted the supply of cobalt and the price of co- 6 Magnetics Business & Technology * Spring 2016 balt increased 6.5x over the base. As a result the search for a cobaltfree high performance magnet began in earnest. Many laboratories were experimenting with rare earth elements and iron but these alloys resulted in poor permanent magnet properties though some had promising soft magnetic properties. N.C. Koon, along with B.N. Das and others at the Naval Research Laboratory, was attempting to develop an ultra-high performance soft magnetic material. As part of an attempt to prevent crystallization during melt-spinning the misch metal-iron alloy, the glass-forming element boron was added to the composition[3,5,6]. The surprising result was a moderately high performance permanent magnet material for which the Navy filed for and received composition, process and product patents[4]. In the fall of 1980, Koon reported on this discovery and shortly thereafter both Musato Sagawa (Sumitomo) and John Croat (GM) optimized the composition and processes for Neo (NdFeB) magnets[7]. Competitively, NdFeB was reputed to have been identified by Kus'ma and coworkers in the USSR in the late 1970s. The first commercial product was reportedly sold by Crucible Magnetics (Elizabethtown, KY, USA) in November 1984. The commercialization of NdFeB did not stop research for other high performance magnetic materials. In Europe the Concerted European Action on Magnets (CEAM) ended up influencing the discovery of nitride magnets, specifically SmFeN, by J.M.D. Coey at Trinity College. With a maximum energy product of ~40 MGOe, this alloy had the potential of a major invention. While important, the limitation is that the material is formed as a powder and decomposes above ~450°C preventing consolidation to full density and limiting its use to bonded magnet applications. Due to the dilution effect of the binder, maximum energy products achieved in commercial bonded magnet products are below 20 MGOe. Thus there are currently two major commercial materials (ferrite and NdFeB) and three lesser used materials (alnico, SmCo and SmFeN). Key Parameters There is a long list of magnet characteristics to consider for use in an application. Among the more important is the temperature of use. As one might imagine, the magnetic field is not constant: it changes with temperature, getting either stronger or weaker. Figure 1 shows the usable temperature range for each of the materials. Conveniently, most applications utilize magnets between -40°C and 150°C and all listed materials are usable. Figure 1. Acceptable Magnet Use Temperatures www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2016

Editor's Choice
Permanent Magnets in a Changing World Market
Design Considerations Using Permanent Magnets
Insulated Iron Powders, SMC, Current State and Future Possibilities
Research & Development
The Design of a Permanent Magnet Motor for a Direct Drive Fan Application
Tailoring of Pole Shapes of Multipolar Injection Molded Magnets
Electromagnetics
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: Seventeen Mistakes People Make with Permanent Magnets

Magnetics Business & Technology - Spring 2016