We continue to review some of the most important materials in heat treatment and metallurgy.
Technetium (chemical symbol: Tc)
Technetium is one of a group of man-made elements that remain relatively unknown to most people. A radioactive silvery-gray metal that slowly discolors in humid air, it is one of the 33 elements that are artificially produced by man (aka synthetic elements). Because it releases gamma rays, technetium is toxic, but very small amounts of short-lived isotopes are used in medical procedures.
The existence of technetium was first predicted by Russian chemist Dmitri Mendeleev, creator of the periodic table of elements. In 1871, Mendeleev predicted this missing element would occupy an empty place in the periodic table below manganese. Remarkably, although it would not be discovered for another 66 years, Mendeleev accurately predicted technetium’s properties and declared they would be similar to manganese. He gave the missing element the provisional name “eka-manganese” (from eka-, the Sanskrit word for one).
Technetium was produced for the first time in 1937 by Italian mineralogist Carlo Perrier and Italian physicist Emilio Segre in Italy. Segre was also known for his discovery of the antiproton – a subatomic antiparticle – for which he was awarded the Nobel Prize in physics. Perrier and Segre found eka-manganese in a sample of molybdenum that had been bombarded by deuterons in a cyclotron, a type of atom smasher. In 1962, the isotope technetium-99 was discovered in African uranium ore in extremely minute quantities. Uranium naturally decays into technetium-99. It was named technetium after the Greek word “technètos,” for "artificial," since it was the first element to be artificially produced.
Technetium shows remarkable anti-corrosion properties with steel. When steel is submerged in water containing a 55-ppm concentration of potassium pertechnetate (a compound of technetium), it protects the steel from corrosion even if the temperature is raised to 250°C (482°F) and even after 20 years (in one example). For this reason, pertechnetate has been used as an anodic corrosion inhibitor for steel, although the element’s radioactivity makes widespread use impractical, and its use is limited to self-contained systems.
Technetium-99m is used in radioactive imaging and in functional studies of the brain, heart, lungs, gall bladder, thyroid (Fig. 2), liver, kidneys, skeleton and other organs. After being introduced into the body, medical imaging equipment detects the technetium as it passes through the organ being studied, revealing the blood flow-through and function of the tissue.
Technetium is an ideal radioactive tracer because it emits easily detectable 140 keV gamma rays, and its half-life is 6.01 hours, which means that only 6% of it remains after 24 hours. The chemistry of technetium permits it to be bound to a wide range of biochemicals, each of which determines how it is metabolized and deposited in the body. Over 50 radiopharmaceuticals are based on technetium-99m.
Here are some interesting facts about technetium.
- Density (g/cc): 11.5
- Melting point (K): 2445
- Boiling point (K): 5150
- Atomic radius (pm): 136
- Covalent radius (pm): 127
- Ionic radius: 56 (+7e)
- Atomic volume (cc/mole): 8.5
- Specific heat (@20°C J/g mole): 0.243
- Fusion heat (kJ/mole): 23.8
- Evaporation heat (kJ/mole): 585