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Cubic zirconia - Technical Aspects, History

crystals oxide stabilizer diamond

Cubic zirconia is a synthetic diamond. It is crystallographically isometric like a diamond. Synthesized material is filled with ten to fifteen percent of a metal oxide stabilizer. This is to keep the zirconium oxide from forming crystals during the synthesis. These crystals are monoclinic and are considered the natural, stable form under regular atmospheric conditions. The stabilizer is needed in order for the crystals to form cubically. The kind of stabilizer used is usually calcium oxide with the amount of stabilizer being utilized depends on the number of recipes used by separate manufacturers. Because of this, the physical and optical properties of synthesized cubic zirconia can vary. All values are ranges. Cubic zirconia is a very dense substance and has a specific gravity from 5.5-6.0. Although it is made of a harder substance than most gems, it is not as hard as a diamond, which is considered one of the hardest known to man. Cubic zirconia is about an 8 on the Mohs scale. The refractive index of cubic zirconia is higher at over 2, with a subadamtine luster. Cubic Zirconia’s dispersion is higher than that of a diamond, at 0.057-0.066. Cubic zirconia has no cleavage and is considered brittle because it exhibits a conchoidal fracture. Cubic zirconia is viewed as green, beige or yellowish. This effect is highly diminished when viewed under longwave UV light and appears white.

History

Since being discovered in 1892, the yellow, monoclinic mineral known as baddeleyite is the only known form of zirconium oxide, occuring naturally. Cubic zirconia has a melting point of 2750°C which was a problem to controlling the growth of single crystals because no crucible made could hold it in its melted state. Because the stabilization of zirconium oxide was realized, the synthetic product was able to stabilize zirconia. Cubic zirconia was introduced in the 1930s. Cubic zirconia is very resistant to chemical and thermal attack, which led to it being used as a refractory material.

Stackelberg and Chudoba, German mineralogists, discovered Cubic zirconia occuring naturally in tiny grains of metamict zircon. They believed these grains were the result of the process of metamictization. The scientists dismissed their discovery, not even bothering to name the substance because they believed it to be unimportant.

Early research into controlling the growth of single crystals took place in the 1960s in France. Much of the work was done by Roulin and Collongues. Their technique included the containment of cubic zirconia in its melted state by enclosing it in a thin shell of solid cubic zirconia. The process became known as cold crucible which alluded to the water cooling system used to form crystals from the melt. Although advanced, these melts only produced small crystals.

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