**3. Permanent magnets**

Rare earth metals are key elements of permanent magnets, which are used as components in clean energy applications such as wind turbines or motors in electric vehicles. Permanent magnets convert electrical energy into mechanical energy (motors) or inversely (generators) by generating a magnetic field [11, 12]. In recent years, the development of wind energy technology has been observed, which is related to the desire to increase the percentage share of renewable energy sources in energy production in the world and, thus reduce carbon dioxide emissions. **Figure 3** shows the periodic table of elements with highlighted rare earth metals used for permanent magnets. As can be seen from the figure, among rare earth metals, lanthanides are used to produce permanent magnets. These elements are characterized by good magnetic properties. The properties of lanthanides are greatly influenced by the *f* orbital, which is also a factor that distinguishes them from transition metals. This orbital is located in the atomic core and therefore does not participate in chemical bonding. Since there are 7 orbitals of the 4*f* type, the number of unpaired electrons can be as high as 7. This results in the formation of large magnetic moments in lanthanide compounds [14]. Lanthanides are paramagnetic except for La3+ (4f0 5d0 6s0 ), Ce4+ (4f0 5d0 6s0 ), Lu3+ (4f145d0 6s0 ) and Yb2+ (4f145d0 6s0 ), they are diamagnetic – they have no unpaired electrons [15]. Examples of electron configurations of paramagnetic lanthanides are presented in **Figure 4**.

Magnets containing rare earth metals were discovered in the 1960s in the United States. They were based on samarium and the transition metal cobalt (SmCo). The first type of this kind of magnet was SmCo5 which had properties suitable for permanent magnets including large uniaxial magnetocrystalline anisotropy, comparatively high saturation magnetization, high Curie temperature, and maximum energy

**Figure 3.**

*Location of rare earth metals on the periodic table, elements used in permanent magnets are marked in red [13].*

*Rare Earth Elements in New Advanced Engineering Applications DOI: http://dx.doi.org/10.5772/intechopen.109248*

#### **Figure 4.**

*Electron configuration on orbital diagrams for rare earth elements neodymium and samarium [16].*

product [(BH)max] with the value of 160 kJ/m3 [17]. Next, to obtain higher magnetizations, a compound with a stoichiometry of Sm2Co17 was designed. Nevertheless, despite obtaining a higher saturation magnetization and Curie temperature compared to SmCo5, the field anisotropy in the case of Sm5Co17 was smaller [17]. The major disadvantage of these magnets was the relatively high cost of Sm and Co, which resulted that in the 1980s, researchers in Japan and the United States designed a new type of magnet containing the rare earth metal neodymium as well as iron and boron (Nd2Fe14B) [2, 17]. It was cheaper and at the same time stronger compared to the SmCo magnet. The development of permanent magnets in the world is shown in **Figure 5**.

Rare earth magnets are distinguished by excellent magnetic properties determined by high induction and coercive force. They are much more powerful per unit mass and volume than other types of magnets [19]. This is related primarily to the high magnetocrystalline anisotropy. The resistance of the crystal lattice to a change in the direction of magnetization gives these materials a very high magnetic coercivity (resistance to demagnetization), which may be attributed to a strong demagnetizing field in the finished magnet than does not reduce the magnetization of the material [2].

The development of permanent magnet technology has been achieved through the use of alloys containing rare earth elements such as Nd, Sm, Gd, and Pr. The introduction of additives in the form of these elements made it possible to produce magnets characterized by a lower mass, smaller dimensions, and high strength at the same time [20]. As a consequence, it made it possible to significantly reduce the size of various types of electronic devices and their components. Compared to other permanent magnets, rare earth magnets can significantly reduce the size and weight of generators used for the production of clean energy in wind turbines. Such magnets can increase efficiency above 20%, which is also very important from an economic point of view [8].

**Figure 5.** *Development of permanent magnets [18].*

Rare earth magnets are employed in applications that require a high magnetic field in difficult operating conditions such as high temperature and high demagnetization forces. In SmCo magnets, samarium is the dominant rare earth metal and cobalt is the primary transition metal, often used with iron, zircon, and copper. The quantity of rare earth elements in such magnets ranges from 25 to 35% by weight. Samarium cobalt magnets are used at high temperatures [2]. However, some disadvantages, include their brittleness which limited the size of the magnet and thus seriously restricts the possibility of their use in certain applications, e.g. in car motors [8]. Another rare earth element, gadolinium, is used to achieve a near-zero change in the residual induction over a wide temperature range. The difference between the SmCo and GdCo magnets is that with increasing temperature, the residual induction decreases in the first case and increases in the second, although from a significantly lower initial value. The combination of SmCo and GdCo allows to obtain magnets with high stability. Magnets based on samarium and cobalt have been used, among others e.g. in generators, actuators, or medical devices [2].

NdFeB magnets (also known as Neo or NIB magnets) [21] are now largely replacing SmCo magnets [22]. NdFeB magnets are more powerful than those of SmCo, and offer the strongest magnetic flux density, therefore are widely used in clean energy technologies such as wind turbines and electric vehicle motors [23]. The neodymium content in these materials is about one-third. The typical chemical composition of NIB magnets is 26.7% Nd, 72.3% Fe and 1.0% B. Although the content of Nd and Fe elements may differ somewhat in commercially used magnets [21]. Frequently other rare earth element praseodymium (Pr) replaced neodymium in these compounds [2, 24]. This is related, among others to the reduction of their production costs.

Previously mentioned use of such magnets are wind turbine generators. Wind turbines require the use of generators with greater power, therefore magnets such as NdFeB are widely used in this type of application. Thus it is possible to substitute the mechanical gears in wind turbines with permanent magnet generators. The

#### *Rare Earth Elements in New Advanced Engineering Applications DOI: http://dx.doi.org/10.5772/intechopen.109248*

advantages of using permanent magnets in wind turbine generators are a reduction in the total weight of the turbine as well as a reduction in the number of moving parts, which results in an increase in the efficiency and reliability of these turbines. The benefits of using permanent magnet generators are especially important for offshore installations, where reliability is a priority. Which is associated with the high cost of repair and maintenance of such turbines [8]. The magnet in a large wind turbine contains as much as 260 kg (or more) of neodymium [25]. Due to their properties, these magnets have also found application in other technologies used to produce renewable energy under the ocean floor and by waves [8].

In addition to wind turbines, where magnets play a key role, they also dominate the market for the production of motors for electric cars. In this case, the size and weight of these components are also important, and they must be adapted to the previously designed engine parameters [8, 22]. As a consequence of the high magnetic strength of NdFeB magnets, they can produce a lot of energy in relation to their weight and size. This makes them ideal for applications that need a high energy-toweight ratio, such as electric vehicle motors.

It is possible to introduce small amounts of additional ingredients in the form of heavy rare earth metals or other metals, which may improve the properties of the Neo magnets. A very important addition is dysprosium, which increases intrinsic coercivity and resistance to demagnetization of these magnets and in consequence enables the use of them at higher temperatures [22, 26]. The addition of dysprosium is usually from 2 to 5% [2, 8]. As mentioned before NdFeB magnets are requested for applications of small or large motors and generators. Small motors are used e.g. in power disc drivers in computers, while large in electric vehicles. In electric car motors, up to 200 g of neodymium and 30 g of dysprosium are used whereas wind turbine generators can contain 1 ton of neodymium per megawatt of electrical power generated [23, 27].

Another common application of this type of magnets is electronics as well as in lasers and telecommunications, there are 12 times stronger than standard iron magnets [28].
