**4. Manufacturing process**

The manufacturing process is very crucial for production of good permanent magnet to achieve excellent magnetic properties. Even in an opposing magnetic field, a permanent magnet will exhibit magnetic characteristics. The favorable magnetic characteristics such as Curie temperature, coercivity and anisotropy are achieved from the mixtures of the constitute matrix designed for the magnets through structural and thermodynamic modifications. Few known manufacturing processes are used for the fabrication of permanent magnets.

Bonding is the common process used for manufacturing og Ne-Fe-B and Sm-Co magnets. The main processing routes such as calendaring, injection molding, extrusion, and compression bonding are used for manufacturing of most bonded magnets [36]. Other emerging magnet manufacturing processes are extrusion, additive manufacturing, spark plasma sintering, shock compaction, and thermomagnetic processing [37]. Furthermore, rare earth magnets fabricated by rapidly solidified process shows better magnetic properties than conventual processed route. The kinetics and thermodynamics of different processing routes such as melt spinning, atomization and melt extraction also determines the quality and characteristics of the magnets.

**Figure 10** shows the standard process steps for manufacturing of the Sm-Co magnets. The steps include alloy preparation, powder production, particle alignment, pressing, sintering, heat treatment, machining, and finally magnetizing. SmCo *Current Advances in Nanocrystalline Rare Earth Based Modern Permanent Magnet DOI: http://dx.doi.org/10.5772/intechopen.114227*

**Figure 10.**

*Manufacturing process steps for Sm-Co based magnets [37, 38].*

magnets indicates high temperature stability, which can operate in conditions up to 500°C without suffering any magnetization loss.

The magnets alloy composition and microstructure are the critically important for the processing of NdFeB magnets. A well-defined sintering treatment at a suitable temperature result in high density final product magnets. The basic process step for the manufacture of NeFeB magnet is shown in **Figure 11**.

Mostly sintering, polymer bonding and hot deformation are used as fabrication routes for manufacture of NdFeB-based bulk magnets. In recent days melt quenching nanocrystalline material are used as bonded and hot deformed components and sintering microcrystalline powder.

The SmFeN compounds are thermodynamically metastable and begin to decompose above 600°C, making traditional sintering or hot-pressing methods unsuitable

#### **Figure 11.**

*The manufacturing process steps for the Nd-Fe-B-based magnets [37, 39].*

for manufacturing magnets. However, bonded magnets can be produced from SmFeN alloy powders. The alloy powders can be prepared using traditional powder metallurgy methods or by using melt spinning or mechanical alloying techniques to improve powder properties. The preparation of Sm2Fe17Nx compound magnetic powder is divided into two steps: the first is to prepare the single-phase Sm2Fe17 compound, and the second is to nitride the Sm2Fe17 compound to produce Sm2Fe17Nx. The methods of preparing Sm2Fe17 compound include rapid quenching, reductive diffusion, powder metallurgy and hydrogenation disproportionation. The reduction diffusion method is used for synthesis of Sm-Fe-N powders. The synthesized powders are used in injection molded or compression bonded magnets. Further, the Low temperature milling process are successfully implemented for fabricating the highperformance Sm–Fe–N bonded magnets [40].

In general, a sintered magnet process is used for manufacturing of anisotropic high remanence regular shaped magnet of large volume. However, hot pressed or dieupset magnet are suitable for net-shape formability and powder stability.

#### **4.1 Improvement in properties**

The magnetic properties of permanent magnet alloys are always sensitive to the microstructure, and the addition of rare earth elements mainly increases the magnetic crystal anisotropy of the material. The crystal anisotropy is an intrinsic factor for the strong coercivity of rare earth-transition group alloys which enhance the properties of magnets. The Curie temperature, coercivity and magnetization of rare-earth permanent magnets are the primary properties of interest for application. Making intermetallic with suitable doping with metals and non-metals atom for Fe or inserting interstitials atoms that can change lattice site and can significantly increase in Curie temperature. Grain boundary diffusion process is an emerging technology implemented to improve the coercivity of Nd-Fe-B magnets with low rare earth consumption. **Figure 12** shows the coercivity enhancement mechanism by grain boundary optimization. In this process, the grain boundary phase is modified to increase the decoupling of exchange interactions for coercivity enhancement.

It is also reported that the properties of rare earth magnets can be improved through microstructure modification [42]. **Figure 13** shows the mechanism of microstructural modification for the enhancement of coercivity. The coercivity depends on the intrinsic magnetic properties and the microstructure of the magnets. Generally,

**Figure 12.**

*The schematic diagram of the coercivity enhancement mechanism by GB optimization [41].*

*Current Advances in Nanocrystalline Rare Earth Based Modern Permanent Magnet DOI: http://dx.doi.org/10.5772/intechopen.114227*

**Figure 13.** *Coercivity enhancement by microstructural modification [41, 42].*

a demagnetizing field is easily forms around the hard magnetic grains in a magnet which reduces the coercivity of the magnet.

Further, the grain surface and the grain boundary are the weak regions in the magnet, where the magnetization reversal starts [43]. Microstructure modification will be performed to retard the magnetic reversal and increase the coercivity. This can be done by the enhancement of the anisotropy field by compositional modification. The defect on the surface of the grain will be reduced through smoothing of grain boundary or optimizing the grain boundary phase distribution. This process will improve the coercivity of the magnet. Grain boundary diffusion process is under test and research for improving the performance rare earth permanent magnets.

Further, introduction of other rare earth elements can modify the intrinsic magnetic properties of magnets. For example, Dy in Nd–Fe–B permanent magnet provide enough coercivity at elevated temperatures by controlling the nanostructures of the materials [44]. The size of the particle alters the magnetic properties of the magnets. The domain structure and chemistry of respective phases of the heterogenous nanostructure can change the coercivity of the magnetic materials. Now a day machinelearning techniques are also used for designing magnetic materials with reduced rare earth components by integrating different physical model [45]. The advancement and improvement in properties of rare-earth will be further done with the addition of nanocrystalline materials.
