We continue to review some of the most important materials in heat treatment and metallurgy.
Zirconium (chemical Symbol: Zr)
Zirconium is a ductile, silver-gray transition metal (Fig. 1) that readily forms compounds with other elements and is highly corrosion-resistant. Zirconium alloys have been known since biblical times and are used as gemstones and for a variety of other purposes. The most common minerals containing zirconium are zircon and badeleyita. Zirconium is always found mixed together with Hafnium (Hf), and it is very difficult to separate them.
Zirconium was discovered by Martin Heinrich Klaproth, a German chemist, while analyzing the composition of the mineral jargon (zircon) in 1789. With over 200 papers to his name, Klaproth was the leading chemist of his time in Germany. He also discovered uranium (1789) and cerium (1803). The moon crater Klaproth was named after him.
Zirconium was isolated in 1824 by Jöns Jacob Berzelius, a Swedish chemist. Berzelius, considered one of the founders of modern chemistry, conducted pioneering experiments in electrochemistry and established the law of constant proportions, which states that the elements in inorganic substances are bound together in definite proportions by weight. Berzelius produced the metal by heating a mixture of potassium and potassium zirconium fluoride in an iron tube. Since 1945, zirconium has been produced by a pyrometallurgical industrial process known as the Kroll process, developed by William Justin Kroll, in which zirconium tetrachloride is reduced by magnesium. The Kroll process is also used to produce titanium.
Zirconium has an affinity for oxygen, sulfur and nitrogen, which makes it useful in steelmaking. By forming stable compounds with these elements it allows the control of nonmetallic (sulfide and oxysulfide) inclusions and the fixation of nitrogen, primarily in boron steels. On a more limited basis, zirconium is also used to inhibit grain growth and prevent strain aging.
Zirconium dioxide (ZrO2) is used in dental implants. This form of zirconium is a ceramic material. It is covered in porcelain for color matching. Transformation-toughened zirconia is used to make ceramic cutlery. Due to the material's hardness, ceramic-edged knives stay sharp longer than steel-edged ones. The cubic phase of zirconium is commonly used as a simulated diamond, which has the same cubic structure and high index of refraction as zirconium.
Here are a few interesting facts about zirconium.
- Atomic number: 40
- Atomic weight: 91.224
- Melting point: 2128 K (1855°C or 3371°F)
- Boiling point: 4682 K (4409°C or 7968°F)
- Density: 6.52 grams per cubic centimeter
- Phase at room temperature: solid
- Element classification: metal\
1. Jefferson Lab (www.jlab.org)