The Important Role of Dysprosium in Modern Permanent Magnets
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- Dec 26,2018
Introduction
Dysprosium is one of a group of elements called the Rare Earths. Rare earth elements consist of the
Lanthanide series of 15 elements plus yttrium and scandium. Yttrium and scandium are included
because of similar chemical behavior. The rare earths are divided into light and heavy based on atomic
weight and the unique chemical and magnetic properties of each of these categories. Dysprosium
(Figure 1) is considered a heavy rare earth element (HREE).
One of the more important uses for dysprosium is in neodymium‐iron‐
boron (Neo) permanent magnets to improve the magnets’ resistance to
demagnetization, and by extension, its high temperature performance.
Neo magnets have become essential for a wide range of consumer,
transportation, power generation, defense, aerospace, medical,
Figure 1: Dysprosium Metal
industrial and other products. Along with terbium (Tb), Dysprosium (Dy)
is also used in magnetostrictive devices, but by far the greater usage is in permanent magnets.
The demand for Dy has been outstripping its supply. An effect of this continuing shortage is likely to be a
slowing of the commercial rollout or a redesigning of a number of Clean Energy applications, including
electric traction drives for vehicles and permanent magnet generators for wind turbines. The shortage
and associated high prices are also upsetting the market for commercial and industrial motors and
products made using them.
Background
Among the many figures of merit for permanent magnets two are of great importance regarding use of
Dy. One key characteristic of a permanent magnet is its resistance to demagnetization, which is
quantified by the value of Intrinsic Coercivity (HcJ or Hci). Substituting Dy for a portion of the neodymium
(Nd) in Neo magnets increases the room temperature value of HcJ and also reduces the rate at which it
falls with increasing temperature. Thus Dy‐containing Neo magnets have greater resistance to
demagnetization over a wider temperature range. The downside of adding Dy is a drop in Residual
Induction (Br). The second key characteristic, Energy Product, is proportional to the square of Br.
Therefore, even a small drop in Br results in significantly lower magnet strength.
For applications such as motors and generators, resistance to demagnetization is a critical performance
objective and the amount of Dy is dictated, not only by demagnetizing stress, but also by the expected
maximum temperature of the application. Grades of Neo are most often denoted by a suffix indicating
the minimum HcJ ( at 20 °C) and a corresponding recommended maximum operating temperature.
These notations are shown in Figure 2. At the top of the chart are some typical applications. Letters in
ovals along the HcJ curve are suffixes denoting minimum HcJ of Neo magnets. For example, “SH”
magnets have a minimum HcJ of 20,000 Oe (1590 kA/m). The Dy percentages shown in Figure 2 are
typical for materials where all the Dy is added to the starting alloy. Despite recent improvements in
alloying and processing, considerable dysprosium is still required for high temperature applications.
Increased demand for higher Dy grades of Neo magnets is one reason why Dy has been in short supply
and why prices have stayed relatively higher even
after the pricing bubble collapsed. (Efforts to reduce dysprosium will be discussed later)
A list of applications and most likely Dy content
(traditional alloying) is shown in Table 1. Two green
energy applications are Wind Power, in which designs
have used 4 to 5% Dy to resist demagnetization at the
operating temperature seen in the generators (up to
150 °C), and hybrid or full electric vehicle traction
drives (EVs), in which demagnetization stress can be
severe, especially along the leading and trailing edges
of the magnets thus requiring Dy as high as 11%.
Recent introduction of dysprosium diffusion ‐ more
accurately HRE diffusion since terbium can also be
used ‐ is permitting reduction of HRE content from
4+% Dy for SH grades to ~2% while 8‐11% grades are
reduced to 4‐5%.