**3. EML in physical research**

The emergence of new metallurgical processes, such as electroslag melting, electron beam melting, arc melting, induction vacuum melting and plasma melting, revealed the limitations of the available thermodynamic and kinetic data necessary for the correct refining of liquid metal. A characteristic feature of these methods are higher temperatures in comparison with the temperatures at which traditional methods of melting steel and alloys are carried out. The melting temperature of high alloyed steels can reach 1700–1750°C, and temperatures up to 2000–2500°C develop in the reaction zone when the steel is purged with oxygen or air. When melting refractory metals in the arc and electron beam setups, local overheating of the metal is possible at 1000°C above the melting temperature. There are almost no experimental data on the behavior of liquid metals at such high temperatures, which is explained by the limited capabilities of experimental methods. Electromagnetic levitation significantly extends the temperature range of studies related to the solubility of gases in liquid metals, the processes of interaction of metal and slag melts with the participation of the gas phase, etc. It is known that such studies are impossible due to chemical reactions developing between the melt and the refractory material of the crucible.

### **3.1 Physical properties and chemical reactions studied by EML**

The study of heterogeneous systems with the help of EML encounters a number of difficulties with which it was not necessary to deal with the study of liquid

metals by traditional methods. One of the main difficulties is the strong evaporation of the metal. This process arises due to two characteristic features of the electromagnetic levitation method: high temperature and a large specific surface of a molten metal drop (**Figure 12**). Evaporation is particularly active when the liquid metal is held in a deep vacuum. For example, the temperature of the carbon iron melt can reach 2000–2100°С in a few seconds, and its five-minute exposure in vacuum is 10<sup>−</sup><sup>5</sup> –10<sup>−</sup><sup>6</sup> mm Hg and leads to a decrease in mass by 3–4 times. Such intense evaporation of metal in a vacuum makes it difficult to study liquid metals by electromagnetic levitation with prolonged exposure.

To reduce the contribution of evaporation processes during electromagnetic levitation, an inert gas or gas mixtures are used at different pressures, which can be changed from a few mm Hg to several atmospheres. Although metal evaporation proceeds more slowly in this case, metal vapors condense not only on cold surfaces, but also in less heated areas of the reaction vessel, forming condensate flakes. Their appearance makes it difficult to observe the liquid drop and distorts the temperature measurements. In addition, chemical reactions can occur between the gas phase and the metal vapor, which distort the results. Since the temperatures of the liquid metal droplet and the gas phase are different, two processes affect the rate of metal evaporation: natural gas convection and vapor condensation. Both of these processes, together or separately, increase the rate of evaporation with an increase in the temperature gradient in the system. Around the drop of molten metal, a boundary gas layer appears, the thickness of which can vary from 0.06 cm at an ambient temperature of 0 K to 0.32 cm at an ambient temperature equal to the temperature of the melt. If the surface of a molten metal, for example, iron, has a temperature of ~2000 K, then at the boundary of the gas layer, it will be 1650 K at an ambient temperature of 0 K. In this regard, when studying heterogeneous processes using electromagnetic levitation, it is necessary to take into account the possibility of reactions between metal vapors and the gas phase in the boundary gas layer at temperatures different from the parameters of the molten metal.

Intensive evaporation of the metal of a liquid droplet leads to strong dusting of the sight glasses. In those cases when the experiment is conducted in a gas atmosphere, the evaporating metal does not pollute them; however, the condensate formed in the chamber, along with the gas flows rising from the liquid droplet, continuously moves along the reaction chamber. For this reason, the temperature of a liquid droplet is usually measured from the side or from the bottom through special devices installed in the reaction vessel. For example, to measure the temperature of liquid iron under conditions of strong evaporation, you can use a

#### **Figure 12.**

*Top and side views of the same Ø 10 mm liquid iron drop in the multi-coil inductor. The top view is almost a sphere with a disfigured surface, more like a dried apple, but during the shooting on the surface of the sphere, the author observed traces of that "muscle play" under the covers that took place in the volume of the sphere. The side view also contains no evidence of the ideal surface of a metal drop during levitation [7].*

**179**

*Electromagnetic Levitation of Metal Melts DOI: http://dx.doi.org/10.5772/intechopen.92230*

another nest of the table.

temperature measurement.

movable tube mounted on bellows and brought almost to the melting inductor on the side. When measuring temperature at the bottom of a melt drop, the pyrometer is sighted through an opening in the turntable. After performing several operations to measure the temperature, a mold is placed under the drop, which is installed in

The microdistribution of temperature over the sample was studied, and the measurement was carried out in the central and peripheral zones of a solid sample heated to premelting temperatures. The difference in temperature measurements of these sections did not exceed 10–12°. Apparently, in the case of molten metal, the temperature difference becomes completely insignificant due to intensive mixing and continuous updating of the surface of the liquid droplet. This eliminates the accumulation of impurities on the surface of the droplet, which could affect the

The temperature control of liquid metal during levitation can be carried out by optical, radiation and color pyrometers. The use of the first two is allowed in the absence of noticeable smoke or contamination of the sight glasses. The surface of the metal when measuring temperature should be free of oxide or any other film. Color pyrometers are not very sensitive to the appearance of films on glasses; therefore, they are used especially often, although they are difficult to operate. In the case of using a brightness pyrometer, it is necessary to first measure the emissivity of metals in a special setup, since a strong dependence of the emissivity on temperature has been revealed. To determine the temperature dependence of the emissivity of some refractory metals in the solid state, it is desirable to use a model of an absolutely black body. The shape of such samples should be chosen so that the samples had the greatest stability in the inductor and the direction of the hole in an absolutely black body looked vertically upward. The calibration of the pyrometers was carried out on the reference points of pure metals, molten during levitation or in a small ceramic crucible. A drop of metal was slowly melted and then crystallized. To do this, you can take advantage of a change in the gas flow rate or a copper crystallizer cooled by liquid nitrogen, supplied to the drop from above. When it comes into contact with metal, it is possible to observe the crystallization front and make multiple measurements of the melting point. When using pyrometers with automatic recording of the melting and crystallization temperatures, they are fixed with the corresponding areas or bursts (**Figure 13**) [7]. The use of conventional thermocouples is difficult, since it is difficult to eliminate the influence of a highfrequency electromagnetic field on the junction and, secondly, the volume of the metal is usually not large enough to neglect the errors introduced in the temperature

readings when a solid junction is introduced into a liquid metal drop.

**in zero gravity in parabolic flights and ISS**

**3.2 Measurement of surface tension and melt viscosity during levitation** 

als science. The main advantage of these materials is their superior mechanical properties compared to conventional crystalline materials. Amorphous alloys are solid metal materials with a disordered glass-like structure of an atomic scale. They are formed when their cooling from a liquid state occurs much faster than the critical cooling rate. During supercooling of the melt, the increasing thermodynamic driving force of crystallization and amorphous atomic kinetics compete with each other. A strong increase in viscosity during cooling and a high probability of amorphization of the melt also establish boundary conditions for the correct choice of parameters of the process. To justify superplasticity, it is important to know the temperature-dependent viscosity of the alloy. Thus, in order to create technology

Volumetric metal glasses or amorphous alloys are a new phenomenon in materi-

#### *Electromagnetic Levitation of Metal Melts DOI: http://dx.doi.org/10.5772/intechopen.92230*

*Magnetic Materials and Magnetic Levitation*

–10<sup>−</sup><sup>6</sup>

by electromagnetic levitation with prolonged exposure.

in vacuum is 10<sup>−</sup><sup>5</sup>

metals by traditional methods. One of the main difficulties is the strong evaporation of the metal. This process arises due to two characteristic features of the electromagnetic levitation method: high temperature and a large specific surface of a molten metal drop (**Figure 12**). Evaporation is particularly active when the liquid metal is held in a deep vacuum. For example, the temperature of the carbon iron melt can reach 2000–2100°С in a few seconds, and its five-minute exposure

intense evaporation of metal in a vacuum makes it difficult to study liquid metals

To reduce the contribution of evaporation processes during electromagnetic levitation, an inert gas or gas mixtures are used at different pressures, which can be changed from a few mm Hg to several atmospheres. Although metal evaporation proceeds more slowly in this case, metal vapors condense not only on cold surfaces, but also in less heated areas of the reaction vessel, forming condensate flakes. Their appearance makes it difficult to observe the liquid drop and distorts the temperature measurements. In addition, chemical reactions can occur between the gas phase and the metal vapor, which distort the results. Since the temperatures of the liquid metal droplet and the gas phase are different, two processes affect the rate of metal evaporation: natural gas convection and vapor condensation. Both of these processes, together or separately, increase the rate of evaporation with an increase in the temperature gradient in the system. Around the drop of molten metal, a boundary gas layer appears, the thickness of which can vary from 0.06 cm at an ambient temperature of 0 K to 0.32 cm at an ambient temperature equal to the temperature of the melt. If the surface of a molten metal, for example, iron, has a temperature of ~2000 K, then at the boundary of the gas layer, it will be 1650 K at an ambient temperature of 0 K. In this regard, when studying heterogeneous processes using electromagnetic levitation, it is necessary to take into account the possibility of reactions between metal vapors and the gas phase in the boundary gas

layer at temperatures different from the parameters of the molten metal.

Intensive evaporation of the metal of a liquid droplet leads to strong dusting of the sight glasses. In those cases when the experiment is conducted in a gas atmosphere, the evaporating metal does not pollute them; however, the condensate formed in the chamber, along with the gas flows rising from the liquid droplet, continuously moves along the reaction chamber. For this reason, the temperature of a liquid droplet is usually measured from the side or from the bottom through special devices installed in the reaction vessel. For example, to measure the temperature of liquid iron under conditions of strong evaporation, you can use a

*Top and side views of the same Ø 10 mm liquid iron drop in the multi-coil inductor. The top view is almost a sphere with a disfigured surface, more like a dried apple, but during the shooting on the surface of the sphere, the author observed traces of that "muscle play" under the covers that took place in the volume of the sphere. The side view also contains no evidence of the ideal surface of a metal drop during levitation [7].*

mm Hg and leads to a decrease in mass by 3–4 times. Such

**178**

**Figure 12.**

movable tube mounted on bellows and brought almost to the melting inductor on the side. When measuring temperature at the bottom of a melt drop, the pyrometer is sighted through an opening in the turntable. After performing several operations to measure the temperature, a mold is placed under the drop, which is installed in another nest of the table.

The microdistribution of temperature over the sample was studied, and the measurement was carried out in the central and peripheral zones of a solid sample heated to premelting temperatures. The difference in temperature measurements of these sections did not exceed 10–12°. Apparently, in the case of molten metal, the temperature difference becomes completely insignificant due to intensive mixing and continuous updating of the surface of the liquid droplet. This eliminates the accumulation of impurities on the surface of the droplet, which could affect the temperature measurement.

The temperature control of liquid metal during levitation can be carried out by optical, radiation and color pyrometers. The use of the first two is allowed in the absence of noticeable smoke or contamination of the sight glasses. The surface of the metal when measuring temperature should be free of oxide or any other film. Color pyrometers are not very sensitive to the appearance of films on glasses; therefore, they are used especially often, although they are difficult to operate. In the case of using a brightness pyrometer, it is necessary to first measure the emissivity of metals in a special setup, since a strong dependence of the emissivity on temperature has been revealed. To determine the temperature dependence of the emissivity of some refractory metals in the solid state, it is desirable to use a model of an absolutely black body. The shape of such samples should be chosen so that the samples had the greatest stability in the inductor and the direction of the hole in an absolutely black body looked vertically upward. The calibration of the pyrometers was carried out on the reference points of pure metals, molten during levitation or in a small ceramic crucible. A drop of metal was slowly melted and then crystallized. To do this, you can take advantage of a change in the gas flow rate or a copper crystallizer cooled by liquid nitrogen, supplied to the drop from above. When it comes into contact with metal, it is possible to observe the crystallization front and make multiple measurements of the melting point. When using pyrometers with automatic recording of the melting and crystallization temperatures, they are fixed with the corresponding areas or bursts (**Figure 13**) [7]. The use of conventional thermocouples is difficult, since it is difficult to eliminate the influence of a highfrequency electromagnetic field on the junction and, secondly, the volume of the metal is usually not large enough to neglect the errors introduced in the temperature readings when a solid junction is introduced into a liquid metal drop.
