Ruby
(Al2O3:Cr) is used not only for jewelry
but also in various industrial materials because of its excellent
mechanical, optical, and chemical properties. Ruby crystals have been
grown using the molybdenum trioxide (MoO3) flux evaporation
method. Herein, we report the effect of holding temperature on the
growth of ruby crystal films via MoO3 flux evaporation.
All temperature effects were investigated considering the epitaxial
growth of ruby crystal film on sapphire crystal substrates. First,
the solubility curve of aluminum oxide (Al2O3) in MoO3 flux was examined in the temperature range 1050–1200
°C. The difference in solubilities between 1050 and 1200 °C
was ∼0.4 mol %. However, higher temperatures increased the
crystal film growth rate due to increased flux evaporation rate. The
difference in the crystal growth rate produced a difference in the
surface pattern of the ruby crystal films. The surface of the ruby
crystal films exhibited ellipsoidal patterns with a wide step interval
at 1100 °C and circular patterns with a narrow step interval
at 1200 °C. Finally, the material balance between the dissolved
mass supplied by the dissolution of the substrates and the mass of
grown ruby crystals was investigated. All dissolved solutes were found
to be crystallized in ruby crystals, either in film or in particle
form.
Ruby
(Al2O3:Cr) crystal films were, for the
first time, epitaxially grown on sapphire (Al2O3) crystal substrates via the evaporation of molybdenum trioxide (MoO3) flux. The crystal films were grown by heating the flux placed
on the sapphire crystal substrates at 1100 °C for 1200 min. The
films thus obtained exhibited a red color and were approximately 200
μm in thickness. Based on the X-ray diffraction and electron
backscatter diffraction analyses data, the crystallographic orientation
of the crystal films was found to be the same as that of the substrate
crystals. A ruby crystal film with a (0001) face developed on the
(0001) substrate face, and a film with a (112̅0) face developed
on the (112̅0) substrate face. In addition, the solubility of
Al2O3 in the flux at 1100 °C was estimated
to be approximately 9.6 mol % by measuring the mass loss of the substrate
and the amount of evaporated flux when the crystallization of the
ruby crystals occurred.
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.
Ruby thin film was grown on sapphire substrate by evaporation method of MoO3 flux. Small amount of Na2CO3 was added in order to decrease the evaporation rate of flux. The contact angles of liquids and crystal surfaces were measured for water and formamide droplets, and the specific surface free energy of each face of the crystal was calculated using Fowkes approximation and Wu's mean equations. The growth hillocks on the synthesized thin film were observed. The addition of Na2CO3 incrased the density of hillocks and the specific surface free energy was larger for the ruby thin film, which has larger number of hillocks.
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