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Running Dysprosium-Free

March 1, 2012
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Dysprosium is one of the elements you’ll find in the bottom bar of forgetful Lanthanides when looking at the Periodic Table. However motor designers have come to know it quite well in the last few years as one of the most magnetic metals ever discovered. This makes this rare element the perfect tool for magnetic motors. But is there enough to go around?

The unprecedented and unforeseen rise in prices for these elements has led to significant cost increases throughout supply chains across a variety of industries. Because dysprosium—shorthand: “Dy”—is a key component of magnet motors (which is what hybrid engines are), the spread of hybrid automobiles is putting huge stress on the market for Dy and thus pricing these highily efficient motors out of the appliance business. The article explores whether alternatives to Dy can be used to meet the staggeringly high demand created by hybrid expansion, or whether it’s the appliance engineers who ought to start finding their own alternatives.

Rare Earth Metals: Now With More ‘Rare’

2011 was a challenging year, to say the least, for companies purchasing rare earth oxides, metals, or products containing rare earth elements (REEs). At their peaks, neodymium (Nd) and praseodymium (Pr) prices increased 460 percent since the beginning of the year. Even more abundant REEs, such as cerium (Ce) and lanthanum (La), have not been spared, experiencing 400-percent and 330-percent increases respectively (see Figure 1).

These rare earth price hikes have resulted in the increase of neodymium-iron-boron (“Nd-Fe-B”) magnet prices by 300 percent or more in some cases. Sintered Nd-Fe-B magnets prices not only have been impacted by both Nd and Pr prices, but also by the soaring prices for dysprosium (Dy). In July 2011, the price of Dy skyrocketed to RMB16,000/kg, representing a more than 800-percent increase from January 2011 prices. Even though the weight of Nd and Pr is eight times that of Dy in a standard sintered Nd-Fe-B magnet, the dollar value of Dy in this magnet is 50 percent greater than that of Nd and Pr at these higher price levels.

What has caused the escalating prices of Dy? A fundamental supply-demand imbalance. Dy is used in sintered Nd-Fe-B magnets, primarily, and in some laser technologies, as well. An average of 3 percent to 4 percent of Dy is commonly found in most of the sintered Nd-Fe-B magnets. The demand for these magnets reached over 100,000 metric tons (MT) in 2011, which means the industry consumed 3,000 MT to 4,000 MT of Dy last year.

The sintered Nd-Fe-B magnet market is estimated to be growing at 15 to 20 percent annually, propelled by expansion in air conditioner compressor motors, wind turbines, electric bicycles, and other applications. Moreover, the rate of Dy consumption is expected to increase as relatively Dy rich sintered Nd-Fe-B applications, such as those employed in hybrid electric vehicle main motors, continue to proliferate. As an example of this increasing rate of Dy consumption, sintered Nd-Fe-B magnets found in hybrid electric vehicle main motors use up to 8 percent of Dy, more than twice the amount used in common sintered Nd-Fe-B grades sold today.

While the demand for Dy is clearly growing rapidly, the supply of Dy is not. Dy concentrations found in typical heavy rare earth deposits range from 3 percent to 8 percent (see Figure 2). Given the current regulatory environment in China, mining output for heavy deposits is limited to approximately 13,400 MT of rare earth oxides annually, and the amount of Dy being recovered from these deposits is estimated to be merely 500 MT per year. Of course, many companies have embarked upon new mining projects outside of China. Although this will result in an increase in the global supply of REEs, the vast preponderance of these projects contains light rare earths, in which Dy is extremely limited, if it exists at all.

When considering the future potential supply and demand of Dy, it becomes apparent that the price of Dy should increase over time. The rising price of Dy will further translate into higher prices for sintered Nd-Fe-B magnets and for other applications using Dy. Given the likely rising costs of sintered Nd-Fe-B magnets, many consumers of sintered Nd-Fe-B magnets are considering utilizing alternative materials which do not rely on Dy.

What If We Change Materials?

Could sintered Nd-Fe-B magnets simply stop using Dy? In most cases, it is difficult or impractical to implement sintered Nd-Fe-B without Dy. A simple motor case study illustrates that using sintered Nd-Fe-B without Dy is not so feasible. Without Dy, sintered Nd-Fe-B magnets are much more inclined to have a knee in the second quadrant engineering curve at elevated temperature as indicated by points A and B in Figure 3. The knee is largely due to the high remanence values of sintered Nd-Fe-B and can result in irreversible losses in a motor. These irreversible losses occur when the magnet’s load line intersects the engineering curve below the knee point as shown in Figure 3.

In contrast, isotropic bonded Nd-Fe-B magnets such as those made from MQP powder grades have very good linearity in the second quadrant engineering curve up to temperatures as high as 180oC. Hence, even without Dy, bonded Nd-Fe-B magnets offer equivalent performance to sintered Nd-Fe-B magnets for applications operating up to 180oC without a significant increase in motor size, as shown in Figure 4 for a motor discussed in this design study. Since all of these powder grades are Dy-free, solutions based on Nd-Fe-B will be less expensive in many applications.

Motor Design Case Study:

To illustrate how a knee in the demagnetization curve influences a motor design, the following design study has been undertaken. A 100 W rated motor with 250 W peak was designed with both a N35SH sintered magnet and a high performance MQP grade bonded magnet. A solution for near-zero Dy sintered Nd-Fe-B (N35) was also included in the study to complete the comparison. Figure 5 shows that the performance of the motors are comparable in terms of speed and current for a given load torque, and Table 1 presents detail on the major dimensions and amount of raw materials used for the motors.

Figure 6 shows the magnet operating point at no-load and at stall conditions. For the motor designed with N35SH, the higher Dy grade, the operating point is above the knee at 120°C (point A in Fig. 6). It can be observed from Figure 6 that reduced Dy in the sintered magnets results in the knee points’ shifting to the right, toward the origin, on the coercivity, H, axis. If the motor is designed with sintered Nd-Fe-B magnets that contain no Dy or reduced Dy, then the magnet’s operating point is below the knee at 120°C (points B & C in Fig. 6).

When designing this motor with Dy free N35 material the magnet’s load line needs to be increased to avoid irreversible losses under the most extreme operating condition, stall at elevated temperature. At a given air gap between the magnet and armature teeth, increasing the magnet’s load line will require a thicker magnet. This effect can be seen clearly in Table 1 with the magnet weight increasing as the material is switched from N35SH to N35. Furthermore, the increased thickness of the magnet will result in higher flux. To avoid saturation with this higher flux the thickness of the soft iron components of the motor, such as the back iron and armature teeth needs to be increased. In this particular design study, the motor with the N35 grade Dy-free sintered magnet requires a stator back iron thickness of 7 mm to avoid saturation. A typical stator back iron thickness is about 2mm, and 7mm is rather unusual.

Alternatively, the magnet designer could use a bonded Nd-Fe-B magnet for a Dy-free motor. Figure 7 illustrates the magnet operating points at no-load and at stall condition for the bonded Nd-Fe-B magnet based motor. It is clear that even under stall conditions and at maximum operating temperature the operating point is on the linear part of the curve, and the knee point is safely in the third quadrant. Bonded Nd-Fe-B magnets provide a Dy-free option to the motor designer, and compared to designs using reduced Dy or Dy-free sintered Nd-Fe-B magnets, the bonded Nd-Fe-B solution requires less magnet material and soft magnetic components.

Companies and designers working today on next generation motors must take into consideration that prices for sintered Nd-Fe-B magnets will likely increase throughout a typical product life cycle. Bonded Nd-Fe-B magnets, however, will not be impacted by the impending supply-demand constraints and resulting price increase because they do not rely on Dy. This will make bonded Nd-Fe-B an ideal solution for many applications.

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