CUBIC ZIRCONIA
DESCRIPTION
Cubic zirconia (or CZ) is based on zirconium oxide (ZrO2), a mineral that is extremely rare in nature but is widely synthesized. Generally, Cubic Zirconia is the name given to the family of monocrystals, created on the base of high temperature oxides. The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors.
Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has been the most gemologically and economically important diamond simulant. It can be produced either transparent or opaque in nearly any color and can be faceted in many cuts. Its main competition as a synthetic gemstone is the more recently cultivated material moissanite.
Cubic zirconia is, as its name would imply, crystallographically isometric. Synthesized material contains a certain mole percentage (10-15%) of metal oxide stabilizer, typically Yttrium oxide. In colored CZ other oxides are added to produce the colors. The stabilizer is required for cubic structure formation, otherwise zirconium oxide would form monoclinic crystals during synthesis. The amount and the type of stabilizer used depends on the recipes of individual manufacturers. Therefore the physical and optical properties of synthesized CZ vary, all values being ranges.
PROPERTIES
Cubic Zirconia is a dense substance with the following properties:
· Hardness (Mohs's scale): 8.5
· Density: 5,65 – 5,95
· Refractive index: 2.088 – 2.176
· Dispersion: 0.058 – 0.066
· Cleavage: none
· Fracture: conchoidal
Cubic zirconia is considered brittle. Under shortwave UV cubic zirconia typically luminesces a yellow, greenish yellow or "beige." Under longwave UV the effect is greatly diminished, with sometimes a whitish glow being seen. Colored stones may show a strong, complex rare earth absorption spectrum. CZs are excellent thermal insulators.
HISTORY
The extremely high melting point of zirconia (2750°C) posed a hurdle to controlled single-crystal growth, as no existing crucible could hold it in its molten state. However, stabilization of zirconium oxide had been realized early on, with the synthetic product stabilized zirconia introduced in 1930. Although cubic, it was in the form of a polycrystalline ceramic and was used as a highly resistant refractory material (up to 2540°C).
Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s in
France
, where much work had been done by Y. Roulin and R. Collongues. The technique developed was used for growing refractory oxide crystals from the melt. One of their most important results was the melting of dielectrics in a cold container. Though promising, these pursuits yielded only small crystals.
Later, Russian scientists under V. V. Osiko at the Lebedev Physical Institute in
Moscow
perfected the technique and used it for commercial production of cubic zirconia crystals. The technique of growing CZ crystals is called skull melting (an allusion to the thin shell-layer of the powdered material containing the melt). The grown crystals were named Fianit, but the name was not much used outside of the
USSR
. Their breakthrough was published in the early 70s, and commercial production began in 1976. By 1980 annual global production had reached 50 million carats (10,000 kg). In the late 70s the main production concentrated in the
USA
, then later – in
Taiwan
and
China
.
Our factory producing CZ is base in
Zelenograd
,
Russia
. We are making all simple and complex colors of Cubic Zirconia.
SYNTHESIS
The method used to synthesize CZ crystals is called "scull melting" and is used for gems that have exceptionally high melting points or very reactive. The melting point for CZ is ~2750°C.
The most important element of the system is a cold crucible, a water-cooled container in which crystals are grown composed of an array of water-filled copper pipes. Radio frequency induction coils are positioned perpendicular to the copper pipes and function in a manner similar to a microwave: the technique employs direct radio frequency heating of the material in the container.
The zirconia powder and yttrium oxide stabilizer are located within the container. To begin the process, a solid piece of zirconium metal is required to act as a catalyst for the reaction. When the metal is heated, it melts, which transfers the heat to the surrounding powder that also starts melting. The molten areas become, in turn, sources of heat. On the other hand, the water-filled copper pipes have an opposing cooling effect in the outer area. This results in a thin layer of solid outer surface with molten content. Powder can be added into the crucible during the melting process. When enough melt is produced, the crystallization process begins.
As a rule, crystallization is produced by lowering the crucible containing the melt out of the induction coil. This method of crystal growth is called spontaneous directional solidification in a cold container. It yields ingots consisting of many single crystals and there is no practical way to limit their quantity. However, as crystallization proceeds, the number of crystals is reduced due to selection of growth rates. As a result, the crystals in the ingot are elongated and parallel to each other, forming a so-called column structure.
The addition of certain metal oxide dopants into the feed powder results in a variety of vibrant colors. For example:
- Cerium: yellow, orange, red
- Chromium: green
- Neodymium: purple
- Erbium: pink
- Titanium: brown
USES
Due to its excellent properties, CZ is widely used both in industry and in jewellery. The main uses include the following:
· gemstone (diamond simulant)
· optical components: lenses, prisms
· substrates in microelectronics
· insulators
· medical instruments (scalpels)
· ceramic components for different applications
COLOURS AVAILABLE
