**3.1 Synthesis by arc furnace**

The synthesis of the intermetallic compounds was carried out either by induction melting, by electric arc melting or by grinding, from a mixture of pure elements (purity 99.99%). This mixture contains the elements (R: Rare Earth, M: Transition Metal, T: Metalloids) in an amount corresponding to the desired stoichiometry, except in the case of Sm-based compounds for which an excess of Sm (4% - proportion mass) to have a stable phase.

The arc furnace uses the thermal energy of the electric arc established between the tungsten electrode (W) and the metals placed in a copper crucible to obtain a sufficient temperature to melt them (**Figure 3**). The technologies used can be the cold crucible (crucible formed of cooled copper or stainless-steel sectors), the hot

crucible (graphite or others) or the use of a susceptor or muffle to radiate on the material to be heated. As a result, cold electrical insulating materials (ceramic, glass, enamels, silicon) can be treated in our induction furnaces. Plasma solutions can be considered as a means of purification of materials. The tungsten electrode and the base copper are connected to a generator that delivers a high intensity current creating an electric arc that could reach temperatures above 3000 °C.

The principle is to create an electric arc between the tungsten tip of a welding machine (Genesis 160, Imax = 160 A) and the surface of the sample. Care should be taken not to touch the sample to prevent it from sticking to the tip. The electric arc created induces a rise in temperature and the fusion of the various constituents. The crucible is cooled by circulating cold water, under high purity argon gas. Zirconium-titanium alloy was used as an O2 getter during the melting process, which allows for a sudden quenching when the electric arc is stopped. In order to ensure good homogenization, it is necessary to move the tip to the surface of the sample but also to perform several fusions by inverting the ingot between each heating. Care should be taken to work in a slight argon overpressure and to perform several air-argon rinses in order to purify the atmosphere of the bell. Systematic weighing after reaction makes it possible to assess the loss of mass of the most volatile elements; this is related to the purity of the final products and limits to a certain extent (≤1%) the appreciation of the areas of homogeneity.

The arc furnace used could hold approximately 5 g of sample. For larger masses, the use of an induction furnace is necessary. Annealing is a homogenization technique that involves putting the sample at a high temperature for a specified time. Prolonged heating causes an increase in thermal agitation and diffusion coefficients, which allows atoms to organize themselves better, to find an optimal structure corresponding to thermodynamic equilibrium at this temperature.

The furnace used for the annealing is controlled by a Microcor Coreci regulator and the thermocouple is platinum/platinum rhodium (± 5 °C). The bulk was wrapped in a tantalum sheet. Samples were sealed in vacuum quartz tubes and annealed at 800 °C for one week in order to reach a good homogenization and improve the atomic diffusion kinetics. This temperature was chosen as a good compromise between relatively fast diffusion kinetics and absence of reaction with the quartz tube (**Figure 3**).

Intermetallic single crystals of compositions, Er6Fe17.66Al5.34C0.65 [38], GdFe 0.37Ge2, GdFe 0.27Ge2 [39], ErFe2.4Al0.6 [40] and polycrystalline SmNi2 [41], SmNi5 [42], Gd2Fe17-xCux [43], Nd2Fe17 - xCox [44], Er6Fe23-xAlx [45], SmNi2Fe [46], SmNi3-xFex (x = 0; 0,3 and 0,8) [47], GdFe12 − xCrx [48] were prepared by arc furnace. These iron-based ternary diagrams R (Sm, Gd, Er, Nd)–Fe- M (Al, Cr, Cu, Co, Ni) were achieved using arc furnace method [49–56]. The stoichiometric composition of SmNi5 is confirmed by SEM and XRD analyses (**Figure 4**).

### **3.2 Synthesis by heating in a "high frequency" induction furnace**

The induction furnace or high frequency furnace (**Figure 5**) consists of an external coil inside which a non-inductive copper crucible, divided into sectors and cooled by circulating water, supports the sample.

Prolonged annealing at high temperature under secondary vacuum between 1200 and 1750 °C should be carried out for two reasons:


#### **Figure 4.**

*SmNi5 characterization: (a) SEM image and (b) XRD pattern and the corresponding Rietveld refinement.*

#### **Figure 5.**

*(a) Diagram of the induction furnace (b) photograph of the induction furnace.*

The coil is traversed by a high frequency current (from 10 to 100 kHz), which generates a variation of the magnetic field. Therefore, the metal sample, located in the center of the coil, is subjected to an induced current or eddy current, which causes the metal to heat up by Joule. The sample preparation is identical to that for the arc furnace preparation. The pure metals are placed in the copper crucible.

As the temperature rises, each sector of the latter being cooled with water, any contamination metal through the crucible is avoided. The crucible is protected from the outside environment by a glass tube in which a secondary vacuum is created before melting. This makes it possible to get rid of all gaseous species adsorbed on the internal walls of the tube and on the surface of the sample. The power of the high frequency generator is then gradually increased until all the metals melted to combine with each other. The evaporation of metals is controlled with the manometer indicating the vacuum state. Because of a significant increase in pressure is

observed, argon is introduced into the tube. Five fuses in total are performed with successive turning of the alloy between each of them to ensure the homogeneity of the sample.

The induction furnace could synthesize up to 12 g of massive compounds. Unlike the arc furnace, the melting is less violent because the temperature is controllable. However, the preparation time in the induction furnace is longer because to return the sample between each fusion, it is necessary to disassemble the glass tube and therefore make the secondary vacuum again later to proceed with the next fusion.

The polycrystalline PrTiFe11-xCox (x ≤ 3) [57] and YFe11-xCoxTiC alloys (x = 0; 0.5; 1; 1.5, 2) [58]. Ingots were prepared by induction melting under argon atmosphere.

### **3.3 Synthesis by high energy grinding**

To carry out the fragmentation of the particles, it is necessary to set them in motion in suitable equipment. Several studies [59, 60] have been carried out on the grinding of polymers using vibrating fragmentation systems. Even if this technique gives interesting results, this type of device is difficult to extrapolate industrially because of the technological difficulties linked to the vibrating system. Thus, the two crushers used were ball mill and agitated ball mill. These two mills have the same operating principles but the energy transmitted to the powder to effect the fragmentation is different. We would see later that some processes require several tens of hours in the ball mill while they only require a few tens of minutes in the agitated ball mill.

For preparation by mechanosynthesis, the starting materials should be in powder form. This poses a problem due to their sensitivity to oxidation, especially in the case of rare earths, which are very reactive to air. By means of the mechanical composition, it is possible to manufacture amorphous alloys, supersaturated solid solutions of immiscible elements in thermodynamic equilibrium, semi-crystalline compounds, as well as unregulated metal alloys.

The "Pulverisette 4" planetary vario-mill [61] is a mill consisting of an animated plate, with a rotational movement on which two jars are placed; rotating around their axis in both directions relative to that of the movement without rotation of

#### **Figure 6.**

*Planetary mill Fritsch (a) planetary mill "Pulverisette 4"; (b) jar and grinding balls; (c) movement of the balls inside the jars.*

#### *Polycrystalline Powder Synthesis Methods DOI: http://dx.doi.org/10.5772/intechopen.97006*

the plate. The ratio between the speed of the jars ω and the plate speed Ω is called the multiplicity factor, if Ω/ω > 1 the grinding is in shock mode, and if Ω/ω < 1 we say that the grinding is in friction. As the directions of rotation of the disc and the jars are opposite, the centrifugal forces resulting from these movements act on the contents of the jars by creating high-energy effects of shocks, friction of the balls on the walls of the jars in all directions and cause the powder to fission. Grinding is carried out discontinuously; it is interrupted every 30 minutes for 15 minutes, to limit the temperature rise inside the jars, as well as to avoid the problems of powder clogging on the walls of the jars, which prevents further crushing. Previous experimental studies have identified the necessary parameters; to avoid increasing the temperature inside the jars, as well as to obtain a satisfactory nanometric powder. The polycrystalline LaFe13-xSix (x = 1.4, 1.6, 1.8, 2.0) compounds were synthesized by high energy ball milling using LaSi as a precursor to prevent oxidation of lanthanum (**Figure 6**) [62].
