Role of Na₂CO₃ Addition in Epitaxial Growth of Ruby Crystal Films on Sapphire Crystal Substrates via MoO₃ Flux Evaporation

Abstract

Ruby (Al2O3:Cr) has a wide variety of industrial applications because of its excellent properties. Herein, we report the role of sodium carbonate (Na2CO3) addition in the epitaxial growth of ruby crystal films on sapphire crystal substrates via molybdenum trioxide (MoO3) flux evaporation. It is empirically known that Na2CO3 easily reacts with MoO3 to act as an evaporation inhibitor of MoO3, but its mechanism has not been elucidated. First, the compound produced by sodium oxide (Na2O) and MoO3 was identified as Na6Mo10O33. It was found that Na6Mo10O33 acted as a chemical lid, suppressing MoO3 flux evaporation. Next, the effects of the inhibitor on the flux evaporation rate and on the growth rate of ruby crystal films were investigated. The crystal growth rate in Na2O–MoO3 flux was approximately half of that in MoO3 flux because the inhibitor slowed the flux evaporation rate. Finally, the effect of inhibitor on deviation from equilibrium in ruby crystal growth was investigated. The ruby crystal film epitaxially grew on the substrate as a seed crystal under stable conditions because crystal growth was very close to equilibrium. Control of the flux evaporation by Na2CO3 addition enables the growth of high-quality ruby crystal films with few defects.

Form Design of Bulky Ruby Crystals with Well-Developed (112̅3) Faces—Epitaxial Growth of Crystal Films on Sapphire Substrates via MoO3 Flux

“…We focused on the MoO 3 flux because it is easy to handle. Thus far, we have conducted research to understand various conditions, i.e., solubility, 14,15 holding temperature, 15 and flux evaporation control, 16 for growing ruby crystals.…”

“…The previous study demonstrated that Na 2 CO 3 easily reacted with MoO 3 and acted as a chemical lid that suppresses the evaporation of MoO 3 flux by floating on the surface of the solution. 16,20 Crystal growth under stable conditions close to equilibrium The crucible was then loosely closed with a platinum lid, placed in a refractory block, and inserted into an electric furnace. The platinum crucible containing the mixture was heated to 1100 °C at a rate of 400 °C•h −1 and held at 1100 °C for 50 h. Subsequently, the platinum crucible was cooled to 500 °C at a rate of 150 °C•h −1 using a temperature program, and then allowed to cool to room temperature (approximately 20 °C) in the furnace.…”

“…If ruby single crystals with few defects can be easily grown, they can be used as jewelry and industrial materials. Numerous studies on the artificial growth of ruby crystals were performed using various techniques, such as Verneuil, , Czochralski, hydrothermal, vapor-phase, and flux methods. − In particular, the flux methodclassified as solution growthis expected to grow high-quality ruby crystals because the generation of defects can be inhibited by the crystal growth that is close to equilibrium.…”

“…The ruby crystals were grown from lead fluoride–boron trioxide (PbF 2 -B 2 O 3 ) and cryolite (Na 3 AlF 6 ) flux. − However, the separation of flux from the ruby crystals was difficult. Evaporation of molybdenum trioxide (MoO 3 ) flux is useful for the growth of ruby crystals. − The flux was completely evaporated and easily separated from the ruby crystals. We focused on the MoO 3 flux because it is easy to handle.…”

“…We focused on the MoO 3 flux because it is easy to handle. Thus far, we have conducted research to understand various conditions, i.e., solubility, , holding temperature, and flux evaporation control, for growing ruby crystals. Further, it is necessary to understand the relative difference in the growth rate of the ruby crystal grown from MoO 3 to control the form of the crystal.…”