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Zirconia, also known as zirconium dioxide, is a white solid used in ceramic glazes and refractory coatings, as a synthetic substitute for diamonds and as crowns in dentistry.
Zirconium metal sits in the same group of the Periodic Table as titanium, hafnium, and rutherfordium, its products are often found in dinnerware in ceramic colors and pigments, in electrical fixtures and other electronic applications and in refractory products.
The most naturally occurring form of zirconium dioxide (ZrO2) is the mineral baddeleyite, which has a monoclinic crystalline structure. This means it has vectors of unequal length: it forms a rectangular prism with a parallelogram-shaped base, so two vectors are perpendicular with the third vector meeting the other two at an angle other than 90°. Baddeleyite can be transparent or translucent, exhibits high refection indices and can range in color from clear, to yellow, green and dark brown.
Zirconia can be produced in several ways:
- Thermal dissociation or calcination (heating to a high temperature in air/oxygen)
- Decomposition by fusion with:
- sodium hydroxide
- sodium carbonate
- calcium oxide and magnesium oxide
- potassium fluorosilicate
- calcium carbonate
- Chlorination (reaction of zirconium dioxide, carbon, and chlorine)
- Carbiding (carbo-thermal reduction of zirconia by graphite).
Zirconia products are characterized by good mechanical properties and stability at high temperatures – it can withstand temperatures of 2750°C. It exhibits strong thermal and corrosion resistance, chemical inertness, and consistent quality.
Super Ceramics
Zirconia is one of the most studied ceramic materials – its crystal structure is monoclinic at room temperature but changes to tetragonal and cubic at higher temperatures. Pure zirconia is incredibly tough, but as it is heated, phase changes alter the physical characteristics and reveal a weakness – internal stresses and cracks can appear. These can be stabilized with yttrium oxide, and the stabilized product has fewer stresses and cracks when the phases change.
Zirconia’s main employment is in hard ceramics but it also finds use as protective coatings, as refractory materials, in insulation, abrasives or abrasion resistance parts, in general, industrial machinery, and in enamels. The material is biocompatible and has no adverse effects on bone or muscle, making it ideal for dentistry applications, but also in hip head replacements.
Shine Bright Like a Diamond
Zirconia is widely used in jewelry as a substitute for diamond. Cubic zirconia is zirconia in its cubic crystalline form, which does not occur naturally so, therefore, must be synthesized. The resulting stone is hard, measuring 8.5 on the Mons scale (a scale of mineral hardness), optically flawless and colorless. It can, however, be colored, making it popular as a gemstone, as well as a diamond surrogate.
Cubic zirconia has near-identical properties to diamond in terms of brilliance, refraction, and durability, and even the most experienced jeweler can have difficulty telling them apart without the help of a thermal conductivity test (cubic zirconia is a poor conductor while diamond is a very good thermal conductor). It can be cut like diamond and achieve the same level of brilliance as its expensive counterpart.
The stone can set in jewelry rather than glued thanks to its ability to withstand high temperatures, but unlike precious diamonds, it is ideal for everyday use thanks to its durability.
Crowning Glory
Zirconia is often used in place of porcelain as crowns to cap a damaged tooth or implant, or in fixed partial dentures. The material’s durability makes it ideal for crowns at the back of the mouth where a high level of strength is required for chewing and grinding of food.
The crown is created from a block of zirconia and milled to produce a tooth-shape using a computerized cutting machine before being cemented in the patient’s mouth. This process can be carried out in a single appointment, making zirconia crowns a more popular choice than the traditional porcelain crown that can take several weeks just to make.
Zirconia might be best known it’s in cubic form as a replacement for diamond in jewelry, but the material has many other uses, not least in dentistry where it represents a more aesthetically pleasing and cheaper alternative to porcelain crowns. Not forgetting its electronic applications and uses as a color and pigment for ceramics.
Sources and Further Reading
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