We continue to review some of the most important materials in heat treatment and metallurgy.
Scandium (chemical symbol: Sc)
Scandium is a bright, silvery-white metal (Fig. 1) that oxidizes easily in air, burns readily and reacts strongly with water. In nature, it is found in over 800 minerals as scandium oxide (Sc2O3) and is often referred to as scandia.
Aluminum-scandium alloys are used in the aerospace industry and other applications such as bicycle frames, fishing rods, golf iron shafts and baseball bats, to name a few (Fig. 2). When used as an alloying element, scandium increases the strength and durability of aluminum alloys by approximately 50%.
The presence of scandium aligns the grains of the alloy and makes the material less susceptible to failure. With a density similar to aluminum but with a higher melting point, it has significant metallurgical potential. Due to the high cost associated with scandium, it has had limited commercial use despite its many favorable properties. Scandium oxide costs several thousand dollars per kilogram, and pure scandium costs a few hundred thousand dollars per kilogram.
In 1869, famed Russian chemist Dmitri Ivanovich Mendeleev (Fig. 3) observed that there was a gap in atomic weights between calcium (atomic weight of 40) and titanium (atomic weight of 48). He predicted an undiscovered element whose oxide would have the form X2O3. His prediction was confirmed in 1879 when Swedish chemist Lars Frederik Nilson discovered scandium oxide (Sc2O3). Nilson successfully extracted several oxides from euxenite, a complex mineral containing eight metal oxides. From this mineral, he extracted erbium oxide to which he had obtained ytterbium oxide and then another oxide of an unknown, lighter element. Remarkably, this was the metal that Mendeleev had predicted, and its oxide was Sc2O3.
Extracting pure metallic scandium from scandium oxide is a tedious and expensive process. It was not until 1937 that metallic scandium was isolated, through the electrolysis of molten scandium chloride, by German chemists Werner Fischer, Karl Brünger and Hans Grienseisen. It took another 20 years before a large sample – weighing 1 pound – was produced, and it wasn’t until 1960 that a 99%-pure sample of scandium was produced. Scandium’s atomic weight is 45, between 40 and 48 as predicted by Mendeleev.
Here are a few important facts about scandium.
- Atomic number (number of protons in the nucleus): 21
- Atomic weight (average mass of the atom): 44.9559
- Density: 1.734 ounces per cubic inch (3.0 grams per cubic cm)
- Phase at room temperature: solid
- Melting point: 2804ºF (1540ºC)
- Boiling point: 5126ºF (2830ºC)
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