What is Hexagonal Boron Nitride?
Hexagonal boron nitride (H-BN) ceramics are important microwave communication materials in the aerospace field. However, H-BN is a covalently bonded compound with a low self-diffusion coefficient at high temperatures and is difficult to sinter. It is usually prepared by a hot-press sintering process. Without proper additives and high hot-pressure sintering temperature and pressure, the hot-pressure sintering process is difficult to produce ceramic products with complex shapes. Reactive sintering and high-pressure gas-solid combustion are also currently used, but it is difficult to obtain sintered products with satisfactory shapes and dimensions. The use of mechanochemical activation with hexagonal boron nitride powder followed by pressureless sintering of H-BN ceramics to obtain AlN ceramics with a relative density of 70% was used.
Characteristics of hexagonal boron nitride
Hexagonal boron nitride has attracted increasing attention worldwide as a solid material with incredible potential for applications in the optical, biological, and health sciences. Professors Bernard Gill (CNRS) and Guillaume Casabois (University of Montpellier) have made groundbreaking contributions to the physics of this interesting material and to the development of its ability to interact with and control electromagnetic radiation. They are collaborating with Prof. James H. Edgar of Kansas State University (USA) on the application of hexagonal boron nitride in emerging quantum information technologies.
Applications of Hexagonal Boron Nitride
Hexagonal boron nitride (hBN) is a versatile solid material that plays a central role in many traditional applications, from lubrication to cosmetic powder formulation, thermal control, and neutron detection. hBN was first synthesized as a fragile powder in 1842 and exhibits a layered crystal structure that differs from graphite: tightly bound B and N atoms are arranged in a network plane of weak interactions superimposed on each other. In a similar way that graphene can be obtained from graphite, monolayers of hBN can also be obtained. Indeed, hBN lies at the intersection of two worlds and is widely used in short-wave solid-state light sources, as well as in layered semiconductors such as graphene and transition metal halides. However, hBN shows some properties different from these two types of materials, making it a unique and potentially widespread material candidate.
HBN crystal growth
With the development of new techniques for the growth of large (~110.2 mm3) hBN single crystals, a new phase of hBN research and applications has been underway since 2004. Professor Edgar and his team at Kansas State University have played a key role in this area. They have studied in detail the factors that determine and control the growth process, the final crystal size and quality, and the effects of doping impurities and changing the ratio of boron isotopes in the sample. hBN crystals are grown from solutions of molten metals, such as chromium and nickel or iron and chromium, and have the ability to dissolve boron and nitrogen. Professor Edgar and his co-workers showed that crystals obtained from pure boron were of better quality than those obtained from hBN powder. They also investigated the effects of gas composition, metal-solvent selection and crucible type on the growth process.
The group also developed a unique technique to grow isotopically pure hBN crystals. Natural boron is a mixture of two isotopes, boron-10 (20%) and boron-11 (80%), which have different nuclear masses but have the same chemical properties and yield undifferentiated hBN crystal structures. However, the ratio of isotopes in the LATTICE of hBN has a profound effect on its vibrational modes (also called phonons). Crystals containing only boron-10 (h10BN) or boron-11 (h11BN) have longer phonon lifetimes. The random distribution of boron isotopes in the crystal structure leads to more frequent dispersion of phonon modes and reduces their lifetimes. When hBN contains only one boron isotope, phonon scattering is reduced and phonon lifetime is extended. This improves the thermal conductivity of hBN, making it more effective in dissipating heat. Its optical properties are also of great interest, especially its application in the field of nanophotonics, where light compression to sizes below the wavelength of free space is studied. In this case, in the case of h10BN, the wavelength of light is reduced by a factor of 150.
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