We continue to review some of the most important materials in heat treatment and metallurgy.
Polonium (chemical symbol: Po)
Polonium is a rare, silver-grey semi-metal that is highly radioactive and is typically found within uranium ores, but it is in such small quantities that it is not economical to extract. Polonium is obtained by showering bismuth-209 with neutrons, resulting in bismuth-210, which then decays into polonium. Polonium-209 is available from Oak Ridge National Laboratory for research purposes.
Prior to the introduction of nuclear reactors, the only source of polonium was uranium ore, which contains only trace amounts. One ton of uranium ore contains only about 100 micrograms (0.0001 grams) of polonium. It was extracted and used in anti-static devices, since the alpha particles it discharges neutralize electric charge. Anti-static devices are used in machinery that generates static electricity caused by processes such as the rolling of paper, wire or sheet metal. Today, other materials that emit beta particles are more commonly used.
Famed Polish chemists Marie and Pierre Curie discovered polonium in 1898 by extracting it from pitchblende, or uranium oxide. The Russian chemist Mendeleev first predicted the existence of polonium when he realized an element with an atomic weight of 212 would follow bismuth on the periodic table. The Curies had extracted the isotope polonium-209, which has a half-life of 103 years.
Polonium’s most important use is as a fuel for space probes, satellites and unmanned lighthouses located inside the Arctic Circle. It generates heat as a result of its nuclear decay. Each gram releases 140 watts of heat energy, and a small amount of the material contained in a capsule will reach temperatures above 500°C (932°F). This heat energy is used to generate electricity in a radioisotope thermoelectric generator (RTG).
An RTG (Fig. 1) uses an array of thermocouples to convert heat into electricity by the Seebeck effect, which is a phenomenon whereby a temperature difference between two dissimilar metals produces a voltage. Since the RTG has no moving parts, it is extremely reliable and robust, allowing space probes to operate without failure for many years.
Another notable property of polonium is its extreme toxicity. It is considered 250,000 times more toxic than hydrogen cyanide, and 1 microgram is lethal to humans. In 2006, polonium made headlines when it was famously used to poison Alexander Litvinenko, a former member of the Russian KGB, who fled Russia and defected to the United Kingdom.
Here are a few important facts about polonium.
- Atomic number: 84
- Atomic weight: 209
- Melting point: 527 K (254°C or 489°F)
- Boiling point: 1235 K (962°C or 1764°F)
- Density: 9.32 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: 16
- Group name: Chalcogen