Neodymium (Nd) is one of the chemical elements employed for the high-strength, permanent magnets in high-output/low-weight loudspeakers. Over the past several years, the small, tight-knit pro audio industry has seen volatile neodymium price changes on the global commodities market. And with Nd becoming an important component of modern, high-performance loudspeakers (particularly in flying systems), this price volatility has created uncertainty in end-user pricing of loudspeaker systems.
Today, although new sources of neodymium are opening up, Nd supply and pricing fluctuations continue, and manufacturers can’t rule out a scenario where they would need to return to older, heavier magnetic materials some day.
Aside from its light weight, how much do we really know about this mystery element? Let’s discuss Neodymium’s history, processing and integration into magnets. We’ll also look at the material in context of loudspeaker designs and consider the future of the global neodymium supply.
History
Neodymium was discovered in 1885 by one of the most prolific scientific geniuses you’ve never heard of — an Austrian named Carl Auer von Welsbach (1858-1229). Investigating Nd and related elements led von Welsbach to develop the gas lantern “mantle” in 1891. Lantern mantles glow a brilliant white color when heated and this lighting technology made von Welsbach wealthy.
Beyond gas lighting, von Welsbach developed early metal filament light bulbs in the late 1890s, improving on Thomas Edison’s carbon filament designs. In 1903, von Welsbach developed ferrocerium, which remains the “flint” striker material of lighters and survival kits to this day. Anyone who uses a gas camping lantern or flicks a lighter benefits from von Welsbach’s inventions.
Processing
From his research, von Welsbach became known as the father of “rare earth” metallurgy. Rare earths are primarily elements 57 to 71 on the periodic table (neodymium is element 60). However, neodymium and most other rare earths are not particularly scarce and are readily available in low concentrations from specific ores.
In the course of manufacturing his gas mantles, von Welsbach needed to efficiently extract another rare earth, cerium, from mineral ore. During the course of this extraction, he formed an alloy of rare earths called mischmetal (“mixed metal”). Alloys are mixtures of metals in solution together, like mixing different alcohols for a cocktail. One of the elements in mischmetal is Nd, and purification of neodymium from mischmetal remains the primary source of Nd today.
Extracting metals from ore is often messy business. The production of rare earth mischmetal from ore is no exception. The rare earths are extracted from two different mineral ores called monazite and bastnaesite. The process involves powerful acids, large amounts of water and extensive leftover mineral remains. For example, mischmetal production from monazite commonly begins with dissolving the raw ore in hot sulfuric acid.
The earth’s crust contains similar amounts of neodymium and copper. Unlike copper, though, neodymium and the other rare earths are not found in rich pockets of high concentration and must be patiently concentrated from large amounts of starting material. The various rare earths are then separated from the alloy via complicated solvent extraction. After solvent extraction of Nd, one is left with a chemical compound known as a neodymium “salt.” That salt goes through one further chemical reaction, called reduction, to produce pure Nd metal.
Neodymium metal is far removed from the ore. Much processing is required to reach the finished product, and much Nd extraction occurs in China, a country with historically lax environmental standards. From 1965 to 1995, most of the global rare earth metal supply came from the single Mountain Pass Mine in California. Today, nearly all of the world’s rare earth metals are both mined and processed in China.
Neodymium and Magnets
Producing neodymium is only part of the equation for the pro audio industry. That metal must then be alloyed and formed into magnets that are used in loudspeaker products. Neodymium magnets are not pure Nd but, rather, a chemical compound, Nd2Fe14B, commonly called neodymium-iron-boron, NiB or “neo.”
In 1982, methods of making Nd2Fe14B were developed by General Motors and Japan’s Sumitomo Special Metals. Ironically, the development of neo was driven by market volatility in sourcing cobalt. Cobalt is mined in the historically unstable African country of Zaire. It is used to make samarium cobalt (SmCo) permanent magnets. Neo magnets are stronger than SmCo magnets, and Nd-based magnetic materials quickly grew to dominate over SmCo in many segments of the permanent magnet market.
There are two major classes of neo magnets: “bonded” and “sintered.” Bonded magnets are created by a process known as “melt spinning.” In melt spinning, a small stream of liquid alloy containing iron, Nd and boron is poured onto a chilled, rotating drum. The chilled drum rapidly cools the liquid, creating very fine alloy grains. This fine-grained alloy is made into a powder and mixed with a binding agent, usually a type of plastic. Bonded magnets have lower magnetic strength than sintered magnets and are popular for applications requiring intricate shapes.
Of interest to the pro loudspeaker industry, sintered magnets begin by melting iron, boron and Nd in the absence of oxygen. The resulting liquid is cast into alloy ingots. The ingots are milled into powder with small particle sizes. This loose powder is then placed in an aligning magnetic field and pressure is applied to form a “powder compact” of the desired shape and size. Because of the aligning magnetic field, the magnetic domains of the powder particles are oriented in a specific direction that facilitates the later creation of the permanent magnet.
After alignment and pressing, the powder compacts are heated, again in the absence of oxygen, to chemically bond the individual particles together. The process of bonding small particles together via heating, but not melting, is known as “sintering.” During normal sintering, solid particles fuse together without any liquid forming, as atoms from adjacent particles diffuse into other neighboring particles. The sintering process is also used in the formation of ceramic materials like porcelain.
Neo magnets undergo “liquid phase” sintering, where a thin layer of liquid forms on the surface between adjacent particles, bonding them together. After the small neo particles coalesce by sintering, the magnet is carefully cooled, and the surface is coated with other metals (e.g., nickel) to prevent corrosion. The finished, unmagnetized neo magnet is then sent to a loudspeaker manufacturer, who places it in the loudspeaker driver’s motor structure. The motor structure is then placed in a strong electromagnet to induce permanent magnetization of the neo.
Loudspeaker Magnet Considerations
Common sintered magnet geometries for professional loudspeakers include disc (Fig. 1), ring (Fig. 2) and button-shapes. The neo magnet is sandwiched top and bottom by the low-carbon steel of the loudspeaker motor structure. This steel directs the magnetic field towards the gap where the voice coil is located.
There are several important parameters of magnetic behavior that affect the performance of loudspeaker drivers. The first is “remnant induction” — a measure of how strong the permanent magnet remains after an external field magnetizes it. Loudspeaker designers generally desire the strongest magnet possible and therefore the largest remnant induction. A second parameter, the “coercive force,” is a measure of the magnet’s resistance to being demagnetized. Clearly, the demagnetization of a loudspeaker’s magnet is undesirable, so a high coercive force is preferred.
Another important parameter used to describe magnet behavior is the “Curie temperature.” Above the Curie temperature, the magnet permanently loses its intrinsic magnetic field. Even below the Curie temperature, heat can reduce the neo’s magnetic performance. As heat is a fact of life for loudspeakers, improving the temperature-dependent performance of neo is of great interest to loudspeaker driver manufacturers. Neo’s temperature stability is improved by adding small quantities of other rare earth elements, in a process called “doping.” Neo magnets doped with terbium or dysprosium are utilized for pro audio driver applications.
There are multiple performance benefits of neo magnets for loudspeaker transducers. The most obvious is the reduced weight relative to conventional ferrite-based magnets. Other benefits for the transducer designer include stronger loudspeaker motors and physically smaller magnets, which allow for increased flexibility in placement. Different magnet locations open up new possibilities for transducer architecture. For instance, neo magnets are powerful enough that a transducer designer can now place a sufficiently strong neo magnet inside the voice coil of a loudspeaker (Figures 1, 2). As an example, JBL’s Differential Drive technology sandwiches a disc of neodymium material inside the voice coil, centered between the two voice coil windings. Multiple other manufacturers also place magnets inside the voice coil, or in other unique geometries afforded by neo. Conventional, ferrite magnets are too large for these placements (Fig. 3).
The Performance Dilemma
Manufacturers must go through the calculus of where to utilize neodymium-based drivers. In the simplest case, the supply of magnet material is constrained, and the decision is based on supply considerations.
On the demand side of the fence, manufacturers must determine where the price/performance curve stops supporting the use of neo drivers. Neo’s weight benefits are evident to most users, but some of the design advantages are not widely known. If neo transducers are chosen primarily for weight, then the decision amounts to picking a price point where neo’s expense prevents competitiveness in the product category.
Driver selection for a product is substantially more complicated when neo is enabling other technologies, like JBL’s Differential Drive. Here, the decision is about more than weight, and a complete transition and/or re-design of the transducers would be on the table in the absence of Nd. As manufacturers weigh the cost and supply of neo magnets, difficult decisions about the driver complement of mid-market products must be discussed.
Future Trends
We are on the cusp of substantial production of rare earths outside of China, which will hopefully increase availability worldwide. Molycorp (molycorp.com) is re-opening the Mountain Pass rare earth mine in California, while Lynas Corporation (lynascorp.com) has nearly opened the Mount Weld mine in Australia, with ore processing in Malaysia. Despite these new sources, the near-term future of rare earth supply remains unclear. The Chinese may continue to place export restrictions on their rare earth resources, or they may try to flood the market with cheap product to drive out these new suppliers. Moving forward, pricing stability is hardly assured.
Transducer manufacturers need to monitor the pulse of the neodymium market. That may mean stockpiling material, purchasing futures, or increasing relationships with Nd suppliers. Regardless of approach, commodity sourcing would seem to be a new, critical facet of loudspeaker manufacturing.
Phil Graham is the senior engineering consultant of PASSBAND, llc. His formal education includes graduate research on the reaction chemistry of high-temperature ceramic materials. Email him at: [email protected].
