Editing Cubic zirconia (section)

==Synthesis==
[[Image:1000kwCZLpKelley.jpg|thumb|Worker monitoring melting zirconium oxide and yttrium oxide in an induction-heated "cold crucible" to create cubic zirconia]]

The skull-melting method refined by Josep F. Wenckus and coworkers in 1997 remains the industry standard. This is largely due to the process allowing for temperatures of over 3000 &nbsp;°C to be achieved, lack of contact between crucible and material as well as the freedom to choose any gas atmosphere. Primary downsides to this method include the inability to predict the size of the crystals produced and it is impossible to control the crystallization process through temperature changes.<ref name="growth" /><ref name=":1" />

The apparatus used in this process consists of a cup-shaped crucible surrounded by radio frequency-activated (RF-activated) copper coils and a water-cooling system.<ref name="growth" /><ref name=":2" />

Zirconium dioxide thoroughly mixed with a stabilizer (normally 10% [[Yttrium(III) oxide|yttrium oxide]]) is fed into a cold crucible. Metallic chips of either zirconium or the stabilizer are introduced into the powder mix in a compact pile manner. The RF generator is switched on and the metallic chips quickly start heating up and readily oxidize into more zirconia. Consequently, the surrounding powder heats up by thermal conduction, begins melting and, in turn, becomes electroconductive, and thus it begins to heat up via the RF generator as well. This continues until the entire product is molten. Due to the cooling system surrounding the crucible, a thin shell of sintered solid material is formed. This causes the molten zirconia to remain contained within its own powder which prevents it from being contaminated from the crucible and reduces heat loss. The melt is left at high temperatures for some hours to ensure homogeneity and ensure that all impurities have evaporated. Finally, the entire crucible is slowly removed from the RF coils to reduce the heating and let it slowly cool down (from bottom to top). The rate at which the crucible is removed from the RF coils is chosen as a function of the stability of crystallization dictated by the phase transition diagram. This provokes the crystallization process to begin and useful crystals begin to form. Once the crucible has been completely cooled to room temperature, the resulting crystals are multiple elongated-crystalline blocks.<ref name=":1" /><ref name=":2" />

This shape is dictated by a concept known as crystal degeneration according to Tiller. The size and diameter of the obtained crystals is a function of the cross-sectional area of the crucible, volume of the melt and composition of the melt.<ref name="growth" /> The diameter of the crystals is heavily influenced by the concentration of Y<sub>2</sub>O<sub>3</sub> stabilizer.

=== Phase relations in zirconia solids solutions  ===
As seen on the [[phase diagram]], the cubic phase will crystallize first as the solution is cooled down no matter the [[concentration]] of Y<sub>2</sub>O<sub>3</sub>. If the concentration of Y<sub>2</sub>O<sub>3</sub> is not high enough the cubic structure will start to break down into the tetragonal state which will then break down into a monoclinic phase. If the concentration of Y<sub>2</sub>O<sub>3</sub> is between 2.5-5% the resulting product will be PSZ (partially stabilized zirconia) while monophasic cubic crystals will form from around 8-40%. Below 14% at low growth rates tend to be opaque indicating partial phase separation in the solid solution (likely due to diffusion in the crystals remaining in the high temperature region for a longer time). Above this threshold crystals tend to remain clear at reasonable growth rates and maintains good annealing conditions.<ref name=":1" />

=== Doping ===
Because of cubic zirconia's isomorphic capacity, it can be doped with several elements to change the color of the crystal. A list of specific dopants and colors produced by their addition can be seen below.

{| class="wikitable" border="1"
|-
!Dopant<ref name=":1">{{cite book |doi=10.1002/0470871687.ch21 |chapter=Growth of Zirconia Crystals by Skull-Melting Technique |title=Crystal Growth Technology |date=2003 |last1=Lomonova |first1=E. E. |last2=Osiko |first2=V. V. |pages=461–485 |isbn=978-0-471-49059-3 }}</ref><ref name=":2">{{cite journal |last1=Nassau |first1=Kurt |title=Cubic Zirconia: An Update |journal=Gems & Gemology |date=Spring 1981 |volume=17 |issue=1 |pages=9–19 |doi=10.5741/GEMS.17.1.9 }}</ref>
!Symbol
!Color(s)
|-
|[[Cerium]]
|Ce
| yellow-orange-red
|-
|[[Chromium]]
|Cr
|green
|-
|[[Cobalt]]
|Co
|lilac-violet-blue
|-
|[[Copper]]
|Cu
|yellow-aqua
|-
|[[Erbium]]
|Er
|pink
|-
|[[Europium]]
|Eu
|pink
|-
|[[Iron]]
|Fe
|yellow
|-
|[[Holmium]]
|Ho
|Champagne
|-
|[[Manganese]]
|Mn
|brown-violet
|-
|[[Neodymium]]
|Nd
| purple
|-
|[[Nickel]]
|Ni
|yellow-brown
|-
|[[Praseodymium]]
|Pr
|amber
|-
|[[Thulium]]
|Tm
|yellow-brown
|-
|[[Titanium]]
|Ti
|golden brown
|-
|[[Vanadium]]
|V
|green
|}
{| class="wikitable"
|+
!Color Range<ref name=":1" /><ref name=":2" />
!Dopant Used
|-
|yellow-orange-red
|<chem>CeO2 
</chem>, <chem>Ce2O3</chem>
|-
|yellow-amber-brown
|'''<chem>CuO, Fe2O3, NiO, Pr2O3, TiO2</chem>'''
|-
|pink
|<chem>Er2O3, Eu2O3, Ho2O3</chem>
|-
|green-olive
|<chem>Cr2O3, Tm2O3, V2O3</chem>
|-
|lilac-violet
|<chem>Co2O3, MnO2, Nd2O3</chem>
|}
<gallery>
Image:Baguette Double Side Checkerboard Cut CZ.JPG|Purple cubic zirconia with checkerboard cut
Image:Multicolor Cubic zirconia.JPG|Multi-color cubic zirconia
Image:Multi Colour CubicZirconia.JPG|Three-tone cubic zirconia gems
Image:Yellow cubic zirconia.JPG|Yellow cubic zirconia
</gallery>

=== Primary growth defects ===
The vast majority of YCZ (yttrium bearing cubic zirconia) crystals are clear with high optical perfection and with gradients of the refractive index lower than <math>5\times 10^{-5}</math>.<ref name=":1" /> However some samples contain defects with the most characteristic and common ones listed below.

* Growth striations: These are located perpendicular to the growth direction of the crystal and are caused mainly by either fluctuations in the crystal growth rate or the non-congruent nature of liquid-solid transition, thus leading to non-uniform distribution of Y<sub>2</sub>O<sub>3</sub>.
* Light-scattering phase inclusions: Caused by contaminants in the crystal (primarily precipitates of silicates or aluminates of yttrium), typically of magnitude 0.03-10 μm.
* Mechanical stresses: Typically caused by the high temperature gradients of the growth and cooling processes, causing the crystal to form with internal mechanical stresses acting on it. This causes refractive index values of up to <math display="inline">8\times 10^{-4}</math>, although the effect of this can be reduced by annealing at 2100&nbsp;°C followed by a slow enough cooling process.
* Dislocations: Similar to mechanical stresses, dislocations can be greatly reduced by annealing.