 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Cubic zirconia" – news ·newspapers ·books ·scholar ·JSTOR(January 2008) (Learn how and when to remove this message) |
| Cubic zirconia | |
|---|---|
 A roundbrilliant-cut cubic zirconia | |
| General | |
| Category | |
| Crystal system | Cubic |
| Identification | |
| Color | Various |
| Mohs scale hardness | 8.0–8.5 |
| Specific gravity | 5.6–6.0 g/cm3 |
| Refractive index | 2.15–2.18 |
Cubic zirconia (abbreviatedCZ) is the cubic crystalline form ofzirconium dioxide (ZrO2). The synthesized material is hard and usually colorless, but may be made in a variety of different colors. It should not be confused withzircon, which is azirconium silicate (ZrSiO4). It is sometimes erroneously calledcubic zirconium.
Because of its low cost, durability, and close visual likeness todiamond, synthetic cubic zirconia has remained the mostgemologically and economically important competitor for diamonds since commercial production began in 1976. Its main competitor as a syntheticgemstone is a more recently cultivated material, syntheticmoissanite.
Cubic zirconia iscrystallographicallyisometric, an important attribute of a would-bediamond simulant. During synthesis zirconium oxide naturally formsmonocliniccrystals, which are stable under normal atmospheric conditions. A stabilizer is required for cubic crystals (taking on thefluorite structure) to form, and remain stable at ordinary temperatures; typically this is eitheryttrium orcalcium oxide, the amount of stabilizer used depending on the many recipes of individual manufacturers. Therefore, the physical and optical properties of synthesized CZ vary, all values being ranges.
It is a dense substance, with adensity between 5.6 and 6.0 g/cm3—about 1.65 times that of diamond. Cubic zirconia is relatively hard, 8–8.5 on theMohs scale—slightly harder than most semi-precious naturalgems.[1] Itsrefractive index is high at 2.15–2.18 (compared to 2.42 for diamonds) and itsluster isAdamantine lustre. Itsdispersion is very high at 0.058–0.066, exceeding that of diamond (0.044). Cubic zirconia has nocleavage and exhibits aconchoidal fracture. Because of its high hardness, it is generally consideredbrittle.
Under shortwaveUV cubic zirconia typicallyfluoresces a yellow, greenish yellow or "beige". Under longwave UV the effect is greatly diminished, with a whitish glow sometimes being seen. Colored stones may show a strong, complexrare earthabsorption spectrum.
Discovered in 1892, the yellowish monoclinic mineralbaddeleyite is a natural form of zirconium oxide.[2]
The high melting point of zirconia (2750 °C or 4976 °F) hinders controlled growth of single crystals. However, stabilization of cubic zirconium oxide had been realized early on, with the synthetic productstabilized zirconia introduced in 1929. Although cubic, it was in the form of apolycrystallineceramic: it was used as arefractory material, highly resistant to chemical and thermal attack (up to 2540 °C or 4604 °F).[3]
In 1937, Germanmineralogists M. V. Stackelberg and K. Chudoba discovered naturally occurring cubic zirconia in the form of microscopic grains included inmetamict zircon. This was thought to be a byproduct of the metamictization process, but the two scientists did not think the mineral important enough to give it a formal name. The discovery was confirmed throughX-ray diffraction, proving the existence of a natural counterpart to the synthetic product.[4][5]
As with the majority ofgrowndiamond substitutes, the idea of producing single-crystal cubic zirconia arose in the minds of scientists seeking a new and versatile material for use inlasers and other optical applications. Its production eventually exceeded that of earlier synthetics, such as syntheticstrontium titanate, syntheticrutile,YAG (yttriumaluminiumgarnet) andGGG (gadoliniumgallium garnet).
Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France, much work being done by Y. Roulin and R. Collongues. This technique involved molten zirconia being contained within a thin shell of still-solid zirconia, with crystal growth from the melt. The process was namedcold crucible, an allusion to the system of water cooling used. Though promising, these attempts yielded only small crystals.
Later,Soviet scientists under V. V. Osiko in theLaser Equipment Laboratory at theLebedev Physical Institute in Moscow perfected the technique, which was then namedskull crucible (an allusion either to the shape of the water-cooled container or to the form of crystals sometimes grown). They named the jewelFianit after the institute's nameFIAN (Physical Institute of the Academy of Science), but the name was not used outside of the USSR.[citation needed] This was known at the time as the Institute of Physics at the Russian Academy of Science.[6] Their breakthrough was published in 1973, and commercial production began in 1976.[7] In 1977, cubic zirconia began to be mass-produced in the jewelry marketplace by the Ceres Corporation, with crystals stabilized with 94% yttria. Other major producers as of 1993 includeTaiwan Crystal Company Ltd,Swarovski and ICT inc.[8][5] By 1980, annual global production had reached 60 millioncarats (12 tonnes) and continued to increase, with production reaching around 400 tonnes per year in 1998.[8]
Because the natural form of cubic zirconia is so rare, all cubic zirconia used in jewelry has been synthesized, one method of which waspatented by Josep F. Wenckus & Co. in 1997.[9][10][11]
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 °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.[3][12]
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.[3][13]
Zirconium dioxide thoroughly mixed with a stabilizer (normally 10%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.[12][13]
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.[3] The diameter of the crystals is heavily influenced by the concentration of Y2O3 stabilizer.
As seen on thephase diagram, the cubic phase will crystallize first as the solution is cooled down no matter theconcentration of Y2O3. If the concentration of Y2O3 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 Y2O3 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.[12]
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.
| Dopant[12][13] | 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 |
| Color Range[12][13] | Dopant Used |
|---|---|
| yellow-orange-red | , |
| yellow-amber-brown | |
| pink | |
| green-olive | |
| lilac-violet |
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.[12] However some samples contain defects with the most characteristic and common ones listed below.
Due to its optical properties yttrium cubic zirconia (YCZ) has been used for windows, lenses, prisms, filters and laser elements. Particularly in the chemical industry it is used as window material for the monitoring of corrosive liquids due to its chemical stability and mechanical toughness. YCZ has also been used as a substrate for semiconductor and superconductor films in similar industries.[12]
Mechanical properties of partially stabilized zirconia (high hardness and shock resistance, low friction coefficient, high chemical and thermal resistance as well as high wear and tear resistance) allow it to be used as a very particular building material, especially in the bio-engineering industry: It has been used to make reliable super-sharp medical scalpels for doctors that are compatible with bio-tissues and contain an edge much smoother than one made of steel.[12]
In recent years[when?] manufacturers have sought ways of distinguishing their product by supposedly "improving" cubic zirconia. Coating finished cubic zirconia with a film ofdiamond-like carbon (DLC) is one such innovation, a process usingchemical vapor deposition. The resulting material is purportedly harder, more lustrous and more like diamond overall. The coating is thought to quench the excessfire of cubic zirconia, while improving its refractive index, thus making it appear more like diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature inRaman spectra.
Another technique first applied toquartz andtopaz has also been adapted to cubic zirconia: Aniridescent effect created by vacuum-sputtering onto finished stones an extremely thin layer of a precious metal (typicallygold), or certain metal oxides, metal nitrides, or other coatings.[14] This material is marketed as "mystic" by many dealers. Unlike diamond-like carbon and other hard synthetic ceramic coatings, theiridescent effect made with precious metal coatings is not durable, due to their extremely low hardness and poor abrasion wear properties, compared to the remarkably durable cubic zirconia substrate.
Key features of cubic zirconia distinguish it from diamond:
Cubic zirconia, as adiamond simulant and jewel competitor, can potentially reduce demand forconflict diamonds, and impact the controversy surrounding the rarity and value of diamonds.[15][16]
Regarding value, the paradigm that diamonds are costly due to their rarity and visual beauty has been replaced by an artificial rarity[15][16] attributed to price-fixing practices ofDe Beers Company which held a monopoly on the market from the 1870s to early 2000s.[15][17] The company pleaded guilty to these charges in an Ohio court in 13 July 2004.[17] However, while De Beers has less market power, the price of diamonds continues to increase due to the demand in emerging markets such as India and China.[15] The emergence of artificial stones such as cubic zirconia with optic properties similar to diamonds, could be an alternative for jewelry buyers given their lower price and noncontroversial history.
An issue closely related to monopoly is the emergence of conflict diamonds. TheKimberley Process (KP) was established to deter the illicit trade of diamonds that fund civil wars inAngola andSierra Leone.[18] However, the KP is not as effective in decreasing the number of conflict diamonds reaching the European and American markets. Its definition does not include forced labor conditions or human right violations.[18][19] A 2015 study from theEnough Project, showed that groups in theCentral African Republic have reaped between US$3 million and US$6 million annually from conflict diamonds.[20] UN reports show that more than US$24 million in conflict diamonds have been smuggled since the establishment of the KP.[21] Diamond simulants have become an alternative to boycott the funding of unethical practices.[20] Terms such as “Eco-friendly Jewelry” define them as conflict free origin and environmentally sustainable.[22] However, concerns from mining countries such as theDemocratic Republic of Congo are that a boycott in purchases of diamonds would only worsen their economy. According to the Ministry of Mines in Congo, 10% of its population relies on the income from diamonds.[18] Therefore, cubic zirconia are a short term alternative to reduce conflict but a long term solution would be to establish a more rigorous system of identifying the origin of these stones.
