We continue to review some of the most important materials in heat treatment and metallurgy.
Selenium (chemical symbol: Se)
Selenium is a nonmetallic element found in soil (Fig. 1) and occurring naturally in certain foods such as whole grains, Brazil nuts, sunflower seeds and seafood. Selenium is found in metal-sulfide ores, where it partially replaces the sulfur. The most stable form of the element, crystalline hexagonal selenium, is a metallic gray, while crystalline monoclinic selenium is a deep red.
Selenium was discovered in 1817 by the renowned Swedish chemist Jöns J. Berzelius. Berzelius noticed a red deposit left behind after sulfur had been burned in a sulfuric-acid factory. The factory was co-owned by Berzelius and his friend, chemist Johann Gahn, the discoverer of manganese. Berzelius initially thought the deposit was the already known element tellurium due to the horseradish smell it emitted, which was similar to tellurium. When he later realized there were no other tellurium compounds in the mine where the sample originated, he concluded he had discovered a new element. This new element had properties similar to both sulfur and tellurium, which are directly above and below selenium on the periodic table. Berzelius named his discovery selenium from the Greek word Selene meaning "moon goddess."
In 1873, English electrical engineer Willoughby Smith found that selenium was more electrically conductive when struck by light. Smith had discovered the photovoltaic effect, which was later explained by Einstein as photons knocking electrons from their atomic orbits and generating electricity. Smith's stunning discovery eventually led to the modern solar energy industry (Fig. 2).
Selenium is used with bismuth in brasses and as an additive to stainless steel. When selenium is added to iron- and copper-based metals it improves their machinability. As little as 0.05-0.10% is effective in carbon and low-alloy steels, but about 0.30% is needed in high-alloy and stainless steels. The "internal lubricant" properties of selenium are retained in steels even when they are heat treated up to at least 1,300 MPa (200 ksi). Selenium is, however, a relatively expensive free-machining additive, and its use versus cheaper replacements such as sulfur or lead must be weighed against added benefits gained.
Selenium has both photovoltaic (converts light to electricity) and photoconductive (electrical resistance decreases with increased illumination) properties. It is therefore useful in photocells, solar cells, photocopiers, electric eyes and light meters for cameras. It can also convert AC electricity to DC electricity and is used in rectifiers. Selenium can also be used to reduce the transmission of sunlight in architectural glass, giving it a bronze tint. It is used to make pigments for ceramics, paint and plastics and to provide a red color to glass (Fig. 3). Selenium sulfide is the main ingredient used in dandruff shampoos because it is toxic to the scalp fungus that causes dandruff.
Here are a few scientific and engineering facts about selenium.
- Atomic number: 34
- Atomic weight: 78.96
- Density: 4.819 g/cm3
- Melting point: 221°C
- Boiling point: 685°C
- Shear modulus: 3.7 GPa
- Young modulus: 10 GPa
- Poisson ratio: 0.33
- Refractive index: 1.000895
- Speed of sound: 3,350 m/s
- Thermal conductivity: 0.52 W/(m K)