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Boron Nitride

Boron Nitride

Gist

Boron nitride (BN) is a synthetic compound made of boron and nitrogen, primarily existing in hexagonal (h-BN) and cubic (c-BN) crystalline forms. It is a refractory, high-temperature ceramic with excellent thermal and chemical resistance, with \(h-BN\) having a graphite-like structure that makes it soft and slippery, while (c-BN) is extremely hard. Applications range from high-temperature lubricants and electrical insulators to cutting tools and components in the aerospace and electronics industries.

Boron Nitride (BN) is another high-performance ceramic used in bulletproof armor. Known for its exceptional thermal conductivity and chemical stability, BN provides a unique set of properties that make it suitable for ballistic protection.

Summary

Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (zincblende aka sphalerite structure) variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite but slightly harder than the cubic form. It has been reported to be 18% stronger than diamond.

Because of excellent thermal and chemical stability, boron nitride ceramics are used in high-temperature equipment and metal casting. Boron nitride has potential use in nanotechnology.  

Properties:

Physical

The partly ionic structure of BN layers in h-BN reduces covalency and electrical conductivity, whereas the interlayer interaction increases resulting in higher hardness of h-BN relative to graphite. The reduced electron-delocalization in hexagonal-BN is also indicated by its absence of color and a large band gap. Very different bonding – strong covalent within the basal planes (planes where boron and nitrogen atoms are covalently bonded) and weak between them – causes high anisotropy of most properties of h-BN.

For example, the hardness, electrical and thermal conductivity are much higher within the planes than perpendicular to them. On the contrary, the properties of c-BN and w-BN are more homogeneous and isotropic.

Those materials are extremely hard, with the hardness of bulk c-BN being slightly smaller and w-BN even higher than that of diamond. Polycrystalline c-BN with grain sizes on the order of 10 nm is also reported to have Vickers hardness comparable or higher than diamond Because of much better stability to heat and transition metals, c-BN surpasses diamond in mechanical applications, such as machining steel. The thermal conductivity of BN is among the highest of all electric insulators.

Boron nitride can be doped p-type with beryllium and n-type with boron, sulfur, silicon or if co-doped with carbon and nitrogen. Both hexagonal and cubic BN are wide-gap semiconductors with a band-gap energy corresponding to the UV region. If voltage is applied to h-BN or c-BN, then it emits UV light in the range 215–250 nm and therefore can potentially be used as light-emitting diodes (LEDs) or lasers.

Little is known on melting behavior of boron nitride. It degrades at 2973 °C, but melts at elevated pressure.

Details

Boron nitride (chemical formula BN) is a synthetically produced crystalline compound of boron and nitrogen, an industrial ceramic material of limited but important application, principally in electrical insulators and cutting tools. It is made in two crystallographic forms, hexagonal boron nitride (H-BN) and cubic boron nitride (C-BN).

H-BN is prepared by several methods, including the heating of boric oxide (B2O3) with ammonia (NH3). It is a platy powder consisting, at the molecular level, of sheets of hexagonal rings that slide easily past one another. This structure, similar to that of the carbon mineral graphite (see the Figure), makes H-BN a soft, lubricious material; unlike graphite, though, H-BN is noted for its low electric conductivity and high thermal conductivity. H-BN is frequently molded and then hot-pressed into shapes such as electrical insulators and melting crucibles. It also can be applied with a liquid binder as a temperature-resistant coating for metallurgical, ceramic, or polymer processing machinery.

Related Topics: nitride borazon

C-BN is most often made in the form of small crystals by subjecting H-BN to extremely high pressure (six to nine gigapascals) and temperature (1,500° to 2,000° C, or 2,730° to 3,630° F). It is second only to diamond in hardness (approaching the maximum of 10 on the Mohs hardness scale) and, like synthetic diamond, is often bonded onto metallic or metallic-ceramic cutting tools for the machining of hard steels. Owing to its high oxidation temperature (above 1,900° C, or 3,450° F), it has a much higher working temperature than diamond (which oxidizes above 800° C, or 1,475° F).

Additional Information:

Overview

Boron nitride is a non-toxic thermal and chemical refractory compound with high electrical resistance and low density, commonly found in colorless crystals or white powder. As an advanced ceramic material, boron nitride has a unique structure that gives it properties similar to both graphite and diamond, earning it nicknames like "white graphene" or "inorganic graphite." With its diverse applications and remarkable physical properties, boron nitride is widely studied and used in industries ranging from electronics to cosmetics. In this article, we will explore its properties, density, structure, production methods, and uses.

Properties of Boron Nitride

The structure of boron nitride consists of equal numbers of boron and nitrogen atoms, forming a robust lattice that gives rise to its unique physical and chemical properties. Depending on how the atoms are arranged, boron nitride exists in three main crystalline forms:

* Hexagonal Boron Nitride (h-BN): A layered, graphite-like structure known for its lubricating and insulating properties.
* Cubic Boron Nitride (c-BN): A diamond-like structure with exceptional hardness and oxidation resistance.
* Wurtzite Boron Nitride (w-BN): A rarer form, considered even harder than cubic boron nitride under certain conditions.

Other key properties of boron nitride include:

* High Thermal Conductivity: Essential for heat dissipation in electronics and high-temperature environments.
* Chemical Inertness: Makes it resistant to corrosion by acids, alkalis, and molten metals.
* Low Density: h-BN has a density of ~2.1 g/cm³, while c-BN is denser at ~3.48 g/cm³.
* Electrical Insulation: Ensures reliable performance as a dielectric material.
* High Melting Point: Withstands temperatures up to 2,973°C, making it suitable for extreme conditions.

Production of Boron Nitride

Boron nitride is typically synthesized through chemical reactions between boric acid or boron oxide and nitrogen under controlled conditions. The production methods include:

Hexagonal Boron Nitride (h-BN):
* Produced by reacting boric acid with ammonia in a nitrogen atmosphere.
* Dense shapes are formed through hot pressing due to its poor sinterability.

Cubic Boron Nitride (c-BN):

Created by subjecting hexagonal boron nitride to high pressure and temperature, mimicking the process used to produce synthetic diamonds.

Wurtzite Boron Nitride (w-BN):

Formed under slightly different conditions compared to c-BN, specifically at lower temperatures (~1,700°C).

Boron nitride can be manufactured in various forms, including powders, bars, rods, and plates. The material’s density and grade (e.g., A, AX, 05, HP, M) vary depending on its intended application, ensuring adaptability across industries.

Uses Of Boron Nitride

Boron nitride's unique structure and density enable it to serve a wide range of applications across multiple industries. Its versatility stems from its various crystalline forms, including hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), and wurtzite boron nitride (w-BN). These forms collectively contribute to its exceptional performance in challenging environments. Below are the key applications of boron nitride.

Industrial and Manufacturing:

Boron nitride is widely used in cutting and grinding tools for hard materials such as hardened steel and wear-resistant cast iron, thanks to its high hardness and chemical stability.

Its thermal conductivity and resistance to molten metals make it a preferred material in high-temperature furnaces, vacuum systems, and thermal spraying applications.

Electronics and Optics:

The material's low dielectric constant, excellent thermal stability, and electrical insulation properties make it suitable for use in semiconductor heat sinks and as a substrate material for graphene-based devices.

In the optics industry, boron nitride's ability to resist oxidation and its high thermal conductivity enable its application in advanced optical coatings and electronics.

Automotive and Aerospace:

Hexagonal boron nitride is commonly used for creating seals and insulating components in the automotive industry, such as oxygen sensors and thermal shields.

Its lightweight density and structure contribute to its use in aerospace materials where weight reduction and thermal resistance are critical.

Cosmetics and Medical:

Boron nitride’s lubricious nature and non-toxicity make it ideal for cosmetics, including eye shadows, foundations, and lipsticks, where it improves smoothness and spreadability.

Emerging research suggests potential applications in the biomedical field, such as implants and biocompatible coatings.

Other Applications:

Boron nitride is frequently used in the production of coatings for tools and molds to enhance their wear resistance.

It also finds applications in ceramics, paints, resins, and high-performance alloys.

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