|Monthly Tech-Tip |
Alternate Names: Zirconia, ZrO2, zirconium oxide
Zirconium Dioxide or zirconia (ZrO2) is a metallic oxide either processed from the mineral Baddeleyite (zirconium oxide) or extracted from zirconium silicate sand. While there is an abundance of raw material (mostly from Australia and South Africa), processes to extract the ZrO2 are varied and expensive (e.g. fusing, leaching, plasma arc, dissolution and precipitation). Purities range from 75 to >99%. Each process produces zirconias that have their own unique properties. Considerable tonnages of zirconia are used each year (10s of 1000s of tons), far more than hitech materials used for similar purposes.
The form in which zirconia crystals exist changes with temperature (monoclinic to 1170C, tetragonal to 2370C, cubic to melting at 2880C).
In ceramics zirconia is used for a number of things:
-It is employed in stain formulations to stabilize and assist certain colors.
-It is added to non-oxide ceramics as a sintering aid (to help glue the particles together).
-Added to body and glaze formulations to promote hardness.
-Used in crucibles, nozzles, valves and even refractory bricks to resist the attack of molten metals.
-Used as an opacifier in glazes and frits (makes transparents white). Opacifying power is similar to zirconium silicate (6-9% for semi-opacity, 10-15% for full opacity).
-Used as a whitener in porcelains.
Zirconia has other interesting uses also:
-It has chemical and corrosion resistance at temperatures well above the melting point of alumina. It's hardness and resistance to heat make it suitable for use in abrasives, cutting tools and engine parts.
-It is useful as a medical implant material.
-Its ionic conductivity makes it valuable in sensors and fuel cells.
In its pure form, mechanical strength is limited, but very high strength and hardness can be produced by adding a stabilizer. Partially Stabilized Zirconias (PSZ) add small amounts of lime, yttrium or magnesia to create a super tough multi-phase matrix.
Opacifiers are powders that turn transparent glazes opaque by various chemical and physical mechanisms (and combinations of mechanisms).