Convective effects and traveling waves in transparent oxide materials processed with the floating zone technique

Zone melting (or zone refining or floating zone process, FZ) is a group of similar methods, specifically conceived for the purification of crystals, in which thermally-driven flows of both gravitational and surface-tension natures are typically produced when the considered material is processed. Since the melt never comes into contact with anything but vacuum (or inert gases), there are no contaminants that the melt may incorporate. Even though compounds with higher purity and improved quality can be obtained with this technique, a typical drawback is represented by the defects potentially induced in the crystalline structure by the unavoidable convection emerging in the fluid phase. In the present chapter, special attention is paid to a specific category of materials known as transparent oxides. A range of conditions is explored, differing in the dominant effect (buoyancy or Marangoni flow), the thermal conditions (heating being provided along the radial or axial direction) and the relative direction of gravity and applied temperature gradient. The hallmark of the entire chapter is our commitment to identify situations in which “waves” are produced and provide a systematic classification of such convective instabilities together with a description of related features based on advanced numerical simulations.