Boron Essential for Advanced Materials
Boron advanced materials have unique characteristics which make them extremely useful in multiple applications. Boron’s many forms include Boron Nitride, a superior heat conductor and insulator; hexagonal boron nitride tubes, a structure with superior tensile strength, stability, and thermal conductivity; and cubic boron nitride, whose hardness characteristics are used for coating cutting tools and as grit for polishing and grinding wheels.
Boron is a simple atom. It has five protons, five or six neutrons, and five electrons. It constitutes just 0.001 percent of the earth’s crust, but water-soluble borates are abundant on the earth’s surface, especially in Turkey and California. Boron’s unique chemical and metallurgical characteristics make it suitable for manufacturing advanced materials.
Boron advanced materials are indispensable in a broad range of industrial applications. They are the hardest manufactured materials, resistant to ultra-high temperatures, while more robust and lighter than steel. In addition to their thermal and protective properties, boron advanced materials have unique lubricating and electrical properties.
These advanced materials are produced from borates (boron oxides). Some of the most vital boron advanced materials for industry and their applications are discussed here.
Boron Nitride, BN
Boron nitrides, BN, are approximately 46 percent boron by weight. They are isoelectronic with carbon, enabling them to form analogs of the carbon allotropes graphite and diamond. Although they share some physical properties with their carbon analogs, boron nitrides play a distinct and valuable role in industrial applications.
Lubricants. 2D sheets of BN, like graphite, can slip past each other. For this reason, BN is sometimes called “white graphite.” However, unlike graphite, BN does not require gas or liquid to be trapped between its layers to act as a lubricant. It can be used in a vacuum in many aerospace applications. But it can also provide lubricity in skincare products and cosmetics. BN is available in greases and aerosol sprays.
Electronics. Heat is the most common cause of failure in electronic devices. Hexagonal boron nitride, h-BN, is a superior conductor of heat but a good insulator. These attributes facilitate heat management in electronic devices.
Greases, ribbons, pads, and elastomeric sheets containing h-BN maintain necessary insulation while dissipating heat from computer components. h-BN also serves as a diffusion source in semiconductor manufacture and a dielectric in high-frequency electronics.
Hexagonal Boron Nitrides
At a molecular level, hexagonal boron nitrides, h-BN, consist of two-dimensional, well-ordered sheets of covalently bonded boron and nitrogen atoms analogous to the carbon sheets in graphite. h-BN is a white solid with a melting point of about 3000 °C.
The hexagonal boron nitrides include t-BN, turbostratic boron nitride, p–BN, pyrolytic boron nitride, and BNNT, boron nitride nanotubes, BNNT.
t-BN consists of randomly configured sheets of BN. p-BN presents parallel layers resulting in extraordinary tensile strength, exceptional thermal properties, and the highest electric dipole moment among readily available industrial materials. p-BN is also notably chemically non-reactive, even at high temperatures.
BNNT is roiled into sheets to make high aspect ratio tubes. These tubes display a structure with superior tensile strength, stability, and thermal conductivity, along with beneficial piezoelectric and dielectric properties. The strongest known nanomaterials, BNNT, can be manufactured as BNNT consists of BN sheets rolled into high aspect ratio tubes, a structure that gives rise to exceptional strength, stability, thermal conductivity, dielectric, and piezoelectric properties. BNNT is among the most potent known nanomaterials. BNNT can be prepared as multi-walled or single tubes of variable length.
Metallurgy. Most molten metals do not wet BN. This property makes BN a beneficial mold release agent with a high melting point. It can be used for crucibles, boats, and implements to handle molten metals. p-BN possesses an anisotropic structure that gives it exceptional thermal conductivity, thermal shock resistance, and extraordinary corrosion resistance even at high temperatures.
Cubic Boron Nitride
Cubic boron nitride has a 3D structure comparable to diamond. Approaching the hardness of Diamond, it is supplied as a powder under trade names like Amborite and Borazon for coating cutting tools and as grit for polishing and grinding wheels.
Boron Carbide, B4C
Boron carbide is approximately 78 percent boron by weight. It is among the hardest of all manufactured materials. Its melting point of 2400 °C is chemically unreactive and resistant to corrosion. B4C has a variety of industrial applications.
Armored vests and tank armor. The low density and extreme hardness of B4C make it highly protective in personal and vehicular armor plating. Ballistic armor is one of the most significant applications of B4C.
Industrial abrasive. B4C is sold in a range of particle sizes as an industrial abrasive. It is affixed to drill bits, polishing and grinding wheels, and cutting blades that operate at high temperatures.
Machine parts. In addition to being extremely hard and corrosion-resistant, boron carbide also has a relatively low density, about one-third that of steel. This combination of attributes makes it ideal for use as challenging, abrasion-resistant, lightweight parts that can be used in harsh environments. Boron carbide often appears in extreme wear parts, such as nozzles for sandblasting equipment and water jet-cutting tools.
Nuclear power. Boron carbide absorbs neutrons and is thermally stable, making it suitable for radiation shielding and reactor control rods.
B4C is also used to make padlocks.
In addition to compounds with carbon and nitrogen, metal boride with chemical formulas of MB6 to MB60 or greater is among the most complex materials known. Among the metal boride in widespread use are:
- Titanium boride–aluminum nitride crucibles used for evaporating aluminum for aluminizing.
- Titanium diboride, TiB2, makes cathodes in Hall-Heroult cells for aluminum production.
- Zirconium diboride, ZrB2, with a melting point of 3246 °C, is used as a high-temperature ceramic in aerospace applications, such as nosecones for hypersonic missiles and shielding for reentry of vehicles from space.
- Metal borides are also used for brake pads, magnetic shielding, and wear-resistant machine parts.
Emerging Applications of Boron Advanced Materials
h-BN is generally biocompatible, giving a range of potential medical applications. BNNTs are being investigated for use in drug delivery systems and for neutron capture in cancer therapies.
Nanostructured forms of BN, like BNNT, have a usable capacity to absorb and store hydrogen. This property makes boron competitive with carbon for hydrogen fuel cells and portable postproduction with hydrogen.
Boron salts are being studied for improving the performance of lithium-ion batteries,
Boron nitride nanotubes have one-sixth of steel weight and are 100 times stronger. These qualities make them ideal for constructing solid but lightweight composites. The ability of boron nitride to absorb neutrons makes it a material of interest for future space missions.