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The Important Role of Dysprosium in Modern Permanent Magnets

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    The Important Role of Dysprosium in Modern Permanent Magnets
    Issue Time:2018-12-26
    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%.