# Zirconia

In brilliant cut polished zirconia (cubic zirconia)

Zirconia (also zirconia and fianite ) is a name for artificially produced single crystals from zirconium (IV) oxide (formula: ZrO 2 , zirconium dioxide), which were stabilized in the cubic high-temperature phase. Zirconia is used as an imitation diamond for jewelry and for the manufacture of optical components.

## Manufacturing

The abbreviation KSZ denotes cubic stabilized zirconium oxide ( English Cubic Zirconia , CZ ). The two German mineralogists Mark Freiherr von Stackelberg and Karl Chudoba discovered it for the first time around 1937 as small inclusions in natural zircon without examining it further.

At the beginning of the 1970s, the Lebedev Institute of the Academy of Sciences of the USSR ( Cyrillic ФИАН FIAN , hence the name Fianit) recognized the value of artificial cubic zirconia and this was first synthesized a little later in a new process developed at the institute. The so-called skull crucible ( English skull crucible ) allowed a high-temperature melt (melting point ZrO 2 : 2680 ° C), the usual crucible materials would destroy. The zirconium oxide powder is heated in a water-cooled crucible by induction heating and partially melted so that a layer of the powder sintered together at the edge and thus represents a heat protection layer that also prevents contamination with the crucible material. The shape of the sintered layer is similar to that of the skull bone, which protects the brain, hence the name of the crucible concept. Since the cold powder is not electrically conductive, you have to start with a metallic zirconium piece in the oxide charge, which liquefies and then melts further amounts of oxide. An additive is required so that the cubic lattice does not revert to the monoclinic phase when it cools down. Usually about 10% yttrium oxide is added to the melt; however, other stabilizers are also possible. By reducing the induction power, the melt slowly cools down, so that at the end of the process a ZrO 2 block of several kilograms results, in which a sintered protective cover has been created on the outside and the crystals have grown inside.

## application

Due to its hardness of 8–8.5 on the Mohs scale and an advanced production process, it has developed into a high quality, inexpensive gem stone and is in demand in the jewelry industry today. A one- carat zirconia costs less than a thousandth of what you have to pay for a good quality diamond of the same size (only around 1 euro compared to around 8,000 euros - as of the end of 2017).

As gemstones, these can either be stabilized with yttrium oxide (Y-KZP) or with calcium oxide (CSZ). Calcium-stabilized zirconia can be burned when it comes into contact with boric acid , which is used, for example, to protect diamonds and other precious stones during fire work during jewelry production. Since it is not possible to determine which oxide was used to stabilize the stone without extensive examinations, there is a risk of damage to the stone, especially when repairs to alleged diamond jewelery are actually KSZ.

KSZ is made in all sizes and shapes, and even with artificial inclusions. Due to its isomorphism, zirconia can be doped with various elements to change the color of the crystal. The table below gives a selection of doping elements and the resulting color.

element symbol colour
cerium Ce yellow-orange-red
chrome Cr green
Cobalt Co lilac-violet-blue
Erbium He pink
Europium Eu pink
iron Fe yellow
holmium Ho champagne
manganese Mn brown-purple
copper Cu yellow-aquamarine
Neodymium Nd purple
nickel Ni yellow-brown
Praseodymium Pr amber yellow
Thulium Tm yellow-brown
titanium Ti gold-brown
color range Doping with
yellow-orange-red ${\ displaystyle {\ ce {CeO2}}}$, ${\ displaystyle {\ ce {Ce2O3}}}$
yellow-amber-brown ${\ displaystyle {\ ce {CuO, Fe2O3, NiO, Pr2O3, TiO2}}}$
pink ${\ displaystyle {\ ce {Er2O3, Eu2O3, Ho2O3}}}$
green-olive ${\ displaystyle {\ ce {Cr2O3, Tm2O3, V2O3}}}$
lilac-purple ${\ displaystyle {\ ce {Co2O3, MnO2, Nd2O3}}}$

In addition to the jewelry industry, yttrium-stabilized zirconia crystals are also used in technical optics for windows, lenses, prisms, filters and laser components due to their optical properties . In the chemical industry they are used as a viewing window for corrosive liquids.

## Demarcation

Even experts cannot distinguish good zirconia stones from diamonds by looking at them, but only by measuring the thermal conductivity : While diamonds are particularly good at conducting heat, zirconia are particularly poor at conducting heat (see zirconium dioxide, usage ). Other relatively simple differences to diamonds that can be determined by non-destructive measuring methods are the different refraction of light ( refractive index zirconia 2.18; diamond 2.42) and density (zirconia 5.8 g · cm −3 , diamond 3.5 g · cm −3 ).

The mineral zircon (ZrSiO 4 ), a combination of zirconium , silicon and oxygen , is also often used as a gemstone in its transparent form, which sometimes leads to confusion. Despite the similar name, a distinction must be made between zirconia (zirconium dioxide) and zirconium ( zirconium silicate ).

## literature

• Mark von Stackelberg, Karl Chudoba : Density and structure of the zircon. In: Crystallography. Volume 97, 1937, pp. 252–262.
• Yu S. Kuzminov: Cubic Zirconia And Skull Melting. Viva Books, 2010, ISBN 978-81-309-1234-9 .

## Individual evidence

1. ^ Florian Neukirchen: Gemstones: Brilliant witnesses for the exploration of the earth . Springer DE, 2012, ISBN 978-3-8274-2922-3 , pp. 234 ( limited preview in Google Book search).
2. ^ SD Scott, DE Hull, CC Herrick: Skull Melting of Synthetic Minerals. In: LA-7080-MS Informal Report. los alamos scientific laboratory, December 1977, accessed January 17, 2019 .
3. Rayner W. Hesse: Jewelrymaking Through History: An Encyclopedia . Greenwood Publishing Group, 2007, ISBN 978-0-313-33507-5 , pp. 72 ( limited preview in Google Book search).
4. mg-quartz.de: Zirconia price list , accessed on November 29, 2017.
5. diamanten-infos.com: Diamond price: calculate the value of a diamond, calculated there for color I (slightly tinted white) and quality SI1 (small inclusions), accessed on November 29, 2017.
6. Marijan Tadin: X-ray and neutron diffraction studies on CaO, Y2O3, CeO2 and TiO2 ... Herbert Utz Verlag, 1996, ISBN 3-931327-50-7 , p. 2 ( limited preview in Google Book search).
7. Article by United Precious Metal: CASTING CUBIC ZIRCONIA IN PLACE WITH UNITED DEOXIDIZED STERLING SILVER ALLOYS , accessed on May 17, 2014.
8. Qiang Shen, Changlian Chen, Fei Chen, Junguo Li, Lianmeng Zhang: Change of phase compositions in calcia stabilized zirconia ceramics using a boric acid additive. In: Journal of the Ceramic Society of Japan. 117, 2009, pp. 449-451, doi: 10.2109 / jcersj2.117.449 .
9. eurogem.biz: Cubic Zirconia Gemstone Lexicon - Information & Properties , accessed on May 17, 2014.
10. a b E. E. Lomonova, VV Osiko: Growth of Zirconia Crystals by skull melting Technique . J. Wiley, Chichester, West Sussex 2004, pp. 461-484 .
11. ^ A b Kurt Nassau: Cubic zirconia: An Update . In: Gems & Gemology . tape 1 , 1981, p. 9-19 .
12. a b cubic- Zirkonia.de: Cubic Zirkonia ( Memento from August 30, 2014 in the Internet Archive ), accessed on May 17, 2014.