Magnetic Materials, Trends in Loudspeakers, and Alternatives (Update)

News Blog20112012

Jul 28

Available Magnets

When we look at the list of available magnetic materials, there are only a few that have the right combination of magnetic strength, size, and stability.

Due to the different characteristics of the materials, different magnet materials have aspect ratio requirements which are directly related to the optimization of the magnetic strength of a given formed piece.

Some important properties used to compare permanent magnets are: remanence (Br), which measures the strength of the magnetic field; coercivity (Hci), the material’s resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (Tc), the temperature at which the material loses its magnetism. Rare earth magnets have higher remanence, much higher coercivity and energy product than Ferrite or Alnico magnets.

Ferrite-based magnets increase their magnetic strength with larger surface area, therefore the larger diameter of the piece the stronger its BHmax it will be. The height of the piece, though important, is not nearly as important as the diameter. Ring magnets are the most common.

For rare-earth and Alnico magnets, the height becomes more important, and slug or cylinder shapes are more common.

Alnico magnet slugs need to become quite large to have an equivalent strength of a ferrite magnet, due to the low Br values of Alnico, but they still have a smaller diameter and lower weight advantage over ferrite. They suffer from the ease at which they demagnetize in high temperatures when compared to other magnets and ferrite magnets. Their Hci values indicate how easily they can be demagnetized. Their shelf life is also very low. With all the contrary values they possess, their use has been mostly abandoned and aren’t recommended.

Samarium-Cobalt magnet slugs have an increased magnetic strength over ferrite magnets but less than Neodymium and are very stable at high temperatures, but are extremely brittle and not good for most industrial and consumer applications. They are used in aerospace for their greater high temperature stability. The material costs are somewhat lower than Neodymium at the moment, but follow the trends of Neodymium.

Neodymium magnet slugs have a much increased magnetic strength over most magnetic materials either ferrite or rare earth (10 times more than ferrite). They require much, much less material to be comparable to ferrite pieces. They have similar stability at high temperatures as do ferrite materials.

Of all the existing magnet materials Neodymium is the most optimal rare earth magnet. Even though its availability in nature in large quantities is concentrated primarily in the northern regions of the Asian continent, and prices are increasing by demand increasing for automotive hybrids, wind turbines, laptops products, and other consumer audio products, its availability is not diminishing very quickly. It’s affordability, however, is diminishing.

(MMPA 0100-00, Standard Specifications for Permanent Magnet Materials)

Alternatives to the typical Neodymium-iron-boron compounds could include the use of Dysprosium.

Dysprosium has one of the highest magnetic strengths of the elements, especially at low temperatures (<-190ºC). So on its own it has limited use in typical magnetic motor structures. Dysprosium is one of the components of Terfenol-D[1], along with iron and terbium. Terfenol-D has the highest room-temperature magnetostriction of any known material for electric motors. It is employed in transducers, wide-band mechanical resonators, and high-precision liquid fuel injectors.

Neodymium-iron-boron magnets can have up to 6% of the neodymium substituted with Dysprosium to raise the coercivity for demanding applications such as drive motors for hybrid electric vehicles. Based on some car makers projected 2 million units per year, the use of Dysprosium in applications such as this would quickly exhaust the available supply of the metal. And, though its price was low initially, it is rising with Neodymium’s.

State of Rare Earth Magnet Costs

It is no surprise to those working in automotive to hear that the prices of Neodymium and Dysprosium, these key raw materials for rare earth magnets, continue to rise steeply due to mining restrictions and the cutting of export quotas for rare earth minerals by China, which is the main world supply source for these materials. Until now, the selling prices of rare earth magnet products were decided on a sliding scale tied to the change in the prices of rare earth elements, and a price revision was implemented every three to six months.

However, the recent prices of Neodymium and Dysprosium are increasing at an abnormal pace of 3 times in a 2-month period, and the existing price-revision period became no longer in accordance with the actual situation. The price of some rare earth magnets will increase more than 40%[2].

China produces the vast majority of the world’s rare earth minerals supply (97% of it). A question in mind should be how real is the price of Neodymium from China, and are there alternatives in Russia or South America, or the United States. China has realized the strategic importance of Neodymium and are acting in their own best interest. If they horde the neodymium for their own use, or make the price unaffordable to other nations, they will be the only country that can remain dominant in the manufacture of devices that use these minerals.

China isn’t the only country where rare earth minerals are known to exist, and there are known locations in the US. There is currently only one rare earth mine in the US, at Mountain Pass, California. That mine was shut down in 2002 because of environmental violations and low market prices for rare earth elements, but with projected shortages and price increases the mine reopened in December of 2010 and is expected to be fully functional by 2012. That particular mine does not have the capability of supplying all of the demand for Neodymium, but a Canadian mining company has applied for permits to mine rare earth minerals at the Bear Lodge area in the Black Hills of Wyoming, where deposits are known to exist. A 1983 USGS report stated that these deposits are one of the largest resources of rare earths in the United States. They weren’t exploited at the time because China had already flooded the market with rare earth materials, and the prices were to low to make it economical to mine in the US. Even in 2012 the majority of the production is likely to go to the government and US industry use. [3]

Samarium-cobalt magnets are the next alternative, and have been about 25% less expensive on average. The have issues with brittleness which needs to be overcome. They are likely to follow the Neodymium trend in price for the same reasons as Neodymium is rising.

There are no other suitable and low-cost replacements to be found at the moment, nor likely in the near future.