We continue to review some of the most important materials in heat treatment and metallurgy.
Tellurium (chemical symbol: Te)
Tellurium is a brittle silver-white material referred to as a metalloid, an element whose properties are in between those of metals and nonmetals with some properties of both. It is rarely found alone, but most commonly in minerals such as calaverite, sylvanite and tellurite. Perhaps the most important element you’ve never heard of, tellurium is used as a metallurgical alloying agent; in the processing of rubber, glass and ceramics; and in the oil-refining and the semiconductor industries. With its content in the Earth's crust comparable to that of platinum, tellurium is one of the rarest stable solid elements.
Tellurium was discovered in a compound form with gold by Franz-Joseph Muller von Reichenstein in 1782 in Transylvania, Romania. Muller was an Austrian mineralogist and mining engineer. Muller was unable to identify the new material he had found. In 1798, he sent it to Martin Heinrich Klaproth, an esteemed German chemist and the discoverer of uranium (1789), zirconium (1789) and cerium (1803). He took Reichenstein’s sample, a combination of gold and tellurium (later named calaverite), and isolated a new element from it. Klaproth named it tellurium after the Latin word “tellus,” for Earth.
Like its neighbor above it on the periodic table, tellurium is a semiconductor and demonstrates an increased electrical conductivity in certain directions or when exposed to light. In metallurgical applications, tellurium can be used to improve the machinability of copper and stainless steel.
Adding tellurium to lead in batteries makes it more resistant to sulfuric acid and increases its hardness and strength. Rubber vulcanized with tellurium instead of the more common sulfur or other materials improves its heat resistance. Solar panels made with cadmium telluride (CdTe) are among the most efficient available, converting a greater percentage of light into electricity than most photovoltaics. This application has increased the demand for tellurium in recent years.
Cadmium is used in X-ray detectors, providing an alternative to single-use film badges, which are wearable radiation dosimeters used to warn workers of radiation overexposure. Bismuth telluride is used to make thermoelectric heat pumps via the Peltier effect. When a DC voltage is applied, one side gets cold while the other side gets hot. Reversing the polarity reverses the direction of heat flow, making the hot side cold and vice versa.
Here are some interesting facts about tellurium.
- Density (g/cc): 6.24
- Melting point (K): 722.7
- Boiling point (K): 1263
- Appearance: silvery-white, brittle semimetal
- Atomic radius (pm): 160
- Atomic volume (cc/mole): 20.5
- Covalent radius (pm): 136
- Ionic radius: 56 (+6e) 211 (-2e)
- Specific heat (@20°C J/g mole): 0.201