We continue to review some of the most important materials in heat treatment and metallurgy.
Cerium (chemical symbol: Ce)
Cerium is a malleable, pale-silver metal (Fig. 1) that becomes tarnished when exposed to air and is soft enough to be cut with a blade. It is considered a rare-earth element and is always found in combination with other rare-earth elements. Cerium is the 25th-most-abundant element, making it as prevalent as copper. In air, cerium oxidizes quickly, forming a scale that sloughs off such that a centimeter-sized sample completely corrodes in 1 year.
Cerium was discovered in 1803 by Swedish chemist Jöns Jakob Berzelius and physicist Wilhelm Hisinger. Berzelius is considered one of the founders of modern chemistry and is noted for his determination of atomic weights, as well as being the discoverer of several elements. The same year, cerium was independently discovered in Germany by chemist Martin Heinrich Klaproth. Considered the leading German chemist of his time, Klaproth also discovered uranium and zirconium. Cerium was named by Berzelius after the dwarf planet Ceres, which had been discovered two years earlier and named after the Roman goddess of agriculture, grain crops, fertility and motherly relationships.
Cerium metal is highly pyrophoric, meaning that it will combust and emits sparks when scratched or struck. For this reason it is a major component of the alloy mischmetal (Fig. 2), one application for which is the flint in cigarette and gas lighters.
Cerium has been recently combined with aluminum to form a new alloy with the promise of high-temperature resistance. The key to the alloy’s high-temperature performance is a specific aluminum-cerium compound, or intermetallic, which forms inside the alloys as they are melted and cast. This intermetallic melts only at temperatures above 1093°C (2000°F).
That heat tolerance makes aluminum–cerium alloys very attractive for use in internal-combustion engines. Tests have shown the new alloys to be stable at 300ºC (572ºF), a temperature that would cause traditional alloys to begin disintegrating. In addition, the stability of this intermetallic sometimes eliminates the need for heat treatments typically needed for aluminum alloys.
Here are a few interesting facts about cerium.
- Atomic number: 58
- Atomic weight: 140.116
- Melting point: 1071 K (798°C or 1468°F)
- Boiling point: 3697 K (3424°C or 6195°F)
- Density: 6.770 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
1. Images of the elements (www.images-of-elements.com)
2. Wikipedia (www.wikipedia.org)
3. Schmadel, Lutz (2003), Dictionary of minor planet names (5th ed.). Germany: Springer. p. 15. ISBN 978-3-540-00238-3.
4. KnowledgeDoor (www.knowledgedoor.com)
5. Jefferson Lab (www.jlab.org)
6. Oak Ridge National Laboratory (www.ornl.gov)