Metal Profile: Boron
A look at the semi-metal boron
Boron is an extremely hard and heat-resistant semi-metal that can be found in a variety of forms and is widely used in compounds to make everything from bleaches and glass to semiconductors and agricultural fertilizers.
The properties of boron are:
- Atomic Symbol: B
- Atomic Number: 5
- Element Category: Metalloid
- Density: 2.08g/cm3
- Melting Point: 3769 F (2076 C)
- Boiling Point: 7101 F (3927 C)
- Moh’s Hardness: ~9.5
Characteristics of Boron
Elemental boron is an allotropic semi-metal, meaning that the element itself can exist in different forms, each with its own physical and chemical properties. Also, like other semi-metals (or metalloids), some of boron's properties are metallic in nature while others are more similar to non-metals.
High purity boron exists either as an amorphous dark brown to black powder or a dark, lustrous, and brittle crystalline metal.
Extremely hard and resistant to heat, boron is a poor conductor of electricity at low temperatures, but this changes as temperatures rise. While crystalline boron is very stable and not reactive with acids, the amorphous version slowly oxidizes in air and can react violently in acid.
In crystalline form, boron is the second hardest of all elements (behind only carbon in its diamond form) and has one of the highest melt temperatures. Similar to carbon, for which early researchers often mistook the element, boron forms stable covalent bonds that make it difficult to isolate.
Element number five also has the ability to absorb a large number of neutrons, making it an ideal material for nuclear control rods.
Recent research has shown that when super-cooled, boron forms yet an altogether different atomic structure that allows it to act as a superconductor.
History of Boron
While the discovery of boron is attributed to both French and English chemists researching borate minerals in the early 19th century, it is believed that a pure sample of the element was not produced until 1909.
Boron minerals (often referred to as borates), however, had already been used by humans for centuries. The first recorded use of borax (naturally occurring sodium borate) was by Arabian goldsmiths who applied the compound as a flux to purify gold and silver in the 8th century A.D.
Glazes on Chinese ceramics dating from between the 3rd and 10th centuries A.D. have also been shown to make use of the naturally occurring compound.
Modern Uses of Boron
The invention of thermally stable borosilicate glass in the late 1800s provided a new source of demand for borate minerals. Making use of this technology, Corning Glass Works introduced Pyrex glass cookware in 1915.
In the postwar years, applications for boron grew to include an ever-widening range of industries. Boron nitride began to be used in Japanese cosmetics, and in 1951, a production method for boron fibers was developed. The first nuclear reactors, which came on-line during this period, also made use of boron in their control rods.
In the immediate aftermath of the Chernobyl nuclear disaster in 1986, 40 tons of boron compounds were dumped on the reactor in order to help control radionuclide release.
In the early 1980s, the development of high-strength permanent rare earth magnets further created a large new market for the element. Over 70 metric tonnes of neodymium-iron-boron (NdFeB) magnets are now produced every year for use in everything from electric cars to headphones.
In the late 1990s, boron steel began being used in automobiles to strengthen structural components, such as safety bars.
Production of Boron
Although over 200 different types of borate minerals exist in the earth's crust, just four account for over 90 percent of commercial extraction of boron and boron compounds: tincal, kernite, colemanite, and ulexite.
To produce a relatively pure form of boron powder, boron oxide that is present in the mineral is heated with magnesium or aluminum flux. The reduction produces elemental boron powder that is roughly 92 percent pure.
Pure boron can be produced by further reducing boron halides with hydrogen at temperatures over 1500 C (2732 F).
High-purity boron, required for use in semiconductors, can be made by decomposing diborane at high temperatures and growing single crystals via zone melting or the Czolchralski method.
Applications for Boron
While over six million metric tons of boron-containing minerals are mined each year, the vast majority of this is consumed as borate salts, such as boric acid and boron oxide, with very little being converted to elemental boron. In fact, only about 15 metric tonnes of elemental boron are consumed each year.
The breadth of use of boron and boron compounds is extremely wide. Some estimate that there are over 300 different end-uses of the element in its various forms.
The five major uses are:
- Glass (e.g., thermally stable borosilicate glass)
- Ceramics (e.g., tile glazes)
- Agriculture (e.g., boric acid in liquid fertilizers).
- Detergents (e.g., sodium perborate in laundry detergent)
- Bleaches (e.g., household and industrial stain removers)
Boron Metallurgical Applications
Although metallic boron has very few uses, the element is highly valued in a number of metallurgical applications. By removing carbon and other impurities as it bonds to iron, a tiny amount of boron—only a few parts per million—added to steel can make it four times stronger than the average high-strength steel.
The element's ability to dissolve and remove metal oxide film also makes it ideal for welding fluxes. Boron trichloride removes nitrides, carbides, and oxide from molten metal. As a result, boron trichloride is used in making aluminum, magnesium, zinc and copper alloys.
In powder metallurgy, the presence of metal borides increases conductivity and mechanical strength. In ferrous products, their existence increases corrosion resistance and hardness, while in titanium alloys used in jet frames and turbine parts borides increase mechanical strength.
Boron fibers, which are made by depositing the hydride element on tungsten wire, are strong, light structural material suited for use in aerospace applications, as well as golf clubs and high-tensile tape.
The inclusion of boron in NdFeB magnet is critical to the function of high-strength permanent magnets that are used in wind turbines, electric motors, and a wide range of electronics.
Boron's proclivity toward neutron absorbing allows it to be used in nuclear control rods, radiation shields, and neutron detectors.
Finally, boron carbide, the third-hardest known substance, is used in the manufacture of various armors and bulletproof vests as well as abrasives and wear parts.
USGS. Minerals Information. Boron