Molten zone

The length of the zone and the diameter of the tod are chosen in such a way that surface tension and interactions between circulating electric currents in the molten zone and the radio-frequency (r-f) field from the surrounding induction coil keep the molten zone in place. As of this writing (ca 1996), the maximum sihcon rod diameter that can be purified in this manner is ca 125 mm. Initially, additional purification can be obtained by making mote sweeps of the zone. Eventually, however, more sweeps do not remove any additional impurities. The limiting profile is given by equation 4 ... 

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

Molten zones are also formed by radiant heating (71). The light source may be focused carbon arcs, xenon lamps, sunlight, or lasers. Very high temperatures have been achieved with all of these. For example, sapphire has been float-zoned in this manner, at over 2000°C. 

Electric heaters have also been directly immersed in the molten zone. Zone refining has been accompHshed with a single hehcal heater rotating in an annular sample space (71). 

Some materials are so reactive that they cannot be zone-melted to a high degree of purity in a container. Floating-zone techniques in which the molten zone is held in place by its own surface tension have been developed by Keck et al. [Phys. Rev., 89, 1297 (1953)]. 

Figure 4.4(f) shows what moving the molten zone along the bar has done to it we have removed impurity from the left-hand end of the bar and dumped it at the right-hand end that is, we have zone refined the left-hand part of the bar. 

A multilayer-type structure probably due to cords in the molten zone between single arc sprayed (0.25 MPa) Ni droplets and steel substrate were found in AES point depth profiles [2.158]. That particular arc spraying condition turned out to yield the best adhesion. Plasma-sprayed AI2O3 layers separated from pre-oxidized Ni Substrate had a micrometer-thick NiO layer on the substrate-sided face and micrometer-deep oxide interdiffusion [2.159]. In this work also, AES point depth profiling substantiated technological assumptions about adhesion mechanisms. 

FIGURE 14.34 In the technique of zone refining, a molten zone is passed repeatedly from one end of the solid sample to the other. The impurities collect in the zone and move along the solid with the heater, leaving a pure substance behind. 

To exploit this observation, a cylindrical bar of material is passed slowly through an induction heater and a narrow molten zone is moved along its length. This causes the impurities to segregate at one end of the bar and super-pure material at the other. In general, the impurities move in the same direction as the molten zone moves if the impurities lower the melting point of the material (see p. 212). 

Equation (9.3.22) can be used when volume to mass-fraction conversion is needed. In the right-hand side braces, the first term corresponds to the amount of element i entering the molten zone, the last two terms to the amount left behind at z. Assuming... 

Figure 9.12 The zone-refining model described by simplified equation (9.4.27) for a completely molten zone. Concentration in the solid left behind the zone for different values of the bulk solid-liquid D,. Steady-state is achieved over distances much shorter for compatible than for incompatible elements.

This relationship has been displayed in Figure 9.13. For small values of d> and compatible elements are such that /ciRssfclL. This means that , L and compatible elements such as Ni, Cr, or Mg are virtually unaffected by zone-refining. Incompatible elements are such that ktR/ktLat

efficient scavenging by ascending molten zones. Again, residual porosity is a critical factor for incompatible-element distributions. 

---