Boron Nitride Guide: Properties, Structure & Applications

1 Introduction

In the quest for faster chips and more durable devices, thermal bottlenecks and material failure in extreme environments have become insurmountable obstacles. Industries such as aerospace, nuclear energy, and high-end manufacturing have been seeking stable materials under extreme conditions such as high temperatures, high pressure, strong corrosion, and intense radiation. While graphene and silicon carbide have been in the public eye for some time, another compound, boron nitride (BN), is quietly tackling these challenges with its unique structural properties.

It serves as a heat dissipation coating enabling supercomputing chips to operate at full speed; as a protective layer shielding rocket engines from temperatures exceeding thousands of degrees Celsius; as a tool material harder than diamond for machining quenched steel; and even as a critical material for detecting nuclear radiation. This is BN, a versatile material combining high-temperature stability, extreme insulation, ultra-high thermal conductivity, superhard wear resistance, and chemical inertness.

Underpinning these exceptional applications is the profound structure-property relationship between BN's intricate crystal structure (allotropic form) and its performance. This article will delve into how boron nitride creates miracles from atomic arrangements, uncover the performance secrets of its various forms (such as hexagonal h-BN and cubic c-BN), outline the core challenges of its preparation techniques, and explore its immense potential in addressing critical challenges in future energy, information, and manufacturing sectors.

Fig. 1 Application of BN in Rocket Engines

2 Concepts and Material Structure

Boron Nitride (BN) is a binary covalent compound composed of boron (B) and nitrogen (N) atoms in a 1:1 ratio. The B-N bond exhibits both strong covalent character and significant polarity (electronegativity difference ≈ 1.0), with bond energy exceeding that of C-C bonds, establishing the foundation for the material's high stability. The unique value of BN stems from its rich allotropic properties: differences in atomic arrangement lead to fundamental shifts in macroscopic properties.

Hexagonal boron nitride (h-BN) is the most common form, featuring a graphite-like layered structure. Boron and nitrogen atoms form hexagonal rings through sp2 hybridization, with interlayer bonding maintained by van der Waals forces. This structure confers h-BN with high anisotropy: in-plane directions exhibit excellent thermal conductivity (≈400 W/m·K), mechanical strength, and wide-bandgap insulating properties (~6 eV); while interlayer weak interactions endow it with an ultra-low friction coefficient (0.03–0.1) and high-temperature lubricity, remaining stable in air above 1000°C.

In contrast, cubic boron nitride (c-BN) and wurtzite boron nitride (w-BN) are constructed through sp3 hybridization to form three-dimensional covalent networks. c-BN exhibits a diamond-like tetrahedral structure (cubic crystal system), while w-BN has a hexagonal close-packed structure (hexagonal crystal system). Both are renowned for their extremely high hardness (c-BN has a hardness of 45–50 GPa, second only to diamond). This dense structure also confers nearly isotropic high thermal conductivity (c-BN ≈ 750 W/m·K), thermal stability above 1400°C (in an inert atmosphere), and wide-bandgap semiconductor properties (c-BN bandgap ~6.4 eV).

Fig. 2 The Structure of hBN, cBN, & wBN

All BN variants exhibit exceptional chemical inertness, resisting corrosion by acids, alkalis, and molten metals. The layered slip properties of h-BN and the ultra-hard wear resistance of c-BN/w-BN fundamentally stem from the direct implications of their sp2 layered structure and sp3 spatial network in terms of atomic bonding patterns and crystal symmetry. This structural-performance correlation forms the core logic for understanding the boron nitride material system.

Table 1 Comparison of Different Structural Types BN

3 Physical and Chemical Properties

3.1 Thermal Properties

Boron nitride demonstrates unparalleled performance in extreme thermal management applications. Hexagonal boron nitride (h-BN) exhibits ultra-high thermal conductivity along the atomic layer plane (approximately 400 W/m·K), rivaling that of graphene, while its thermal conductivity in the perpendicular direction is significantly reduced. This strong anisotropy makes it an ideal choice for directional heat dissipation materials. Cubic boron nitride (c-BN), on the other hand, exhibits isotropic high thermal conductivity (approximately 750 W/m·K), surpassing most metals. More importantly, h-BN remains stable in an oxidizing atmosphere at temperatures above 1000°C, while c-BN can withstand temperatures exceeding 1400°C in an inert environment. Both materials have extremely low thermal expansion coefficients and excellent thermal shock resistance, providing a material foundation for high-temperature device thermal barrier coatings and heat dissipation substrates.

3.2 Electrical Properties

The wide bandgap properties of boron nitride define its unique position in the electronics industry. h-BN, as a wide bandgap insulator (bandgap width ~6 eV), has a breakdown field strength as high as 800 kV/cm and no dangling bonds on its surface, making it an ideal dielectric layer for two-dimensional transistors (such as graphene and molybdenum disulfide devices), effectively suppressing interface scattering. c-BN, on the other hand, combines an ultra-wide bandgap of 6.4 eV with controllable p-type doping capability. Its stable semiconductor properties at high temperatures open up possibilities for developing deep ultraviolet optoelectronic devices, detectors for harsh radiation environments, and high-frequency, high-power electronic components.

3.3 Mechanical Properties

Boron nitride exhibits extreme differentiation in its mechanical properties, combining both rigidity and flexibility. The interlayer van der Waals forces in h-BN confer an ultra-low friction coefficient (0.03–0.1), making it an ideal "solid lubricant" under high-temperature conditions. In vacuum or inert environments, its friction performance even surpasses that of graphite. Meanwhile, c-BN's three-dimensional network formed by sp3 bonds gives it a Vickers hardness of 45–50 GPa, second only to diamond, along with higher thermal stability and unique chemical inertness—it does not catalyze graphitization when processing iron-group metals. This characteristic grants c-BN tools an irreplaceable advantage in the field of hard alloy machining.

3.4 Chemical Properties

The chemical inertness of boron nitride forms the foundation of its survival in corrosive environments. Both h-BN and c-BN exhibit exceptional resistance to most acids, alkalis, and molten metals (such as aluminum, copper, and steel). h-BN can withstand molten aluminum erosion at 900°C, far surpassing traditional ceramics; c-BN remains stable in high-temperature iron-based alloy contacts, avoiding the carbon diffusion failure commonly seen in diamond tools. This "passive" property makes it a key candidate material for molten metal container linings, semiconductor manufacturing consumables, and neutron absorption components in nuclear reactors.

Fig. 3 Hexagonal Boron Nitride Nanocoating Reduces Scaling in Pipes in Actual Water Environments

3.5 Special Functional Properties

The unique properties of boron nitride are opening up new avenues in cutting-edge technology fields. h-BN single-photon sources (boron vacancy color centers) show promise in quantum communication, with their atomically flat surfaces supporting research into novel quantum states such as topological insulators. c-BN phonon polaritons enable subwavelength control of infrared light, offering new avenues for metasurface technology. Additionally, the deep ultraviolet fluorescence properties of h-BN nanosheets offer breakthroughs in bio-marking and anti-counterfeiting coding, while the ultra-high-pressure electric conductivity of w-BN points toward next-generation mechatronic transducer materials.

4 Preparation Methods

The synthesis technology system for boron nitride revolves around crystal structure control and application performance requirements. Chemical vapor deposition (CVD) serves as the core method for preparing high-performance thin films, achieving atomically controlled deposition through the reaction of gaseous precursors (such as the BCl3-NH3 system) on a heated substrate surface. Plasma-enhanced CVD enables the growth of amorphous BN insulating layers (with a dielectric constant as low as 1.16) at low temperatures of 400°C, while thermal CVD is used for epitaxial growth of large-area hexagonal boron nitride single crystals (e.g., 4×4 cm^2 single-layer h-BN on nickel substrates), achieving film thickness precision at the nanometer level and purity exceeding 95%. But industrialization is constrained by equipment costs and deposition rates.

For the large-scale production of porous BN materials, the template method dominates due to its spatial confinement effect. Among these, the hard template method uses mesoporous silicon/carbon as a scaffold, followed by impregnation with a boron source (such as boron azide), high-temperature pyrolysis (>800°C), and template etching (HF solution) to obtain mesoporous BN with uniform pore sizes (2–50 nm) and a specific surface area >1000 m^2/g, suitable for catalyst supports and gas adsorption. The soft template method, though operationally simple (relying on surfactant self-assembly), is limited in application due to low product orderliness.

The synthesis of industrial-grade micron-sized BN powder primarily relies on high-temperature pyrolysis methods. The borax-ammonium chloride method involves sintering raw materials at 1200°C in an ammonia atmosphere, offering continuous production advantages but resulting in high impurity residues (including carbon); the borax-urea method involves nitriding at 900–1100°C followed by acid washing for purification, achieving h-BN micropowder with purity >95%, becoming the mainstream process for thermal conductive fillers and lubricants; while the organic precursor method (such as boron azide decomposition) produces high-purity porous BN (>97% purity), it is limited to high-end ceramic applications due to the high cost of raw materials.

The preparation of cubic boron nitride (c-BN) requires high-pressure high-temperature (HPHT) technology to drive phase transformation. The catalyst-free method requires extreme conditions (11–12 GPa, 1700°C). Industrially, alkali metal nitrides (Li3N, etc.) are commonly used as catalysts to reduce pressure to 5 GPa and temperature to 1400°C, synthesizing c-BN grains (hardness 45–50 GPa) that meet the requirements for superhard abrasives and tools. Emerging plasma synthesis methods activate N2-BH3 gas at 400–600°C to deposit c-BN thin films, avoiding substrate thermal damage, and are suitable for optical coatings.

Frontier breakthroughs focus on precise structural control, such as oblique epitaxial growth using symmetry-broken substrates (Ni(520) oblique step surfaces) to sequentially lock ABC stacking, successfully preparing 4×4 cm^2 rhombohedral BN (rBN) single-crystal films. Their ferroelectricity (Curie temperature >600°C) opens new pathways for electronic devices.

Method Selection and Industrialization Logic

Application adaptability: Borate-urea method (low-cost h-BN micropowder) is preferred for thermal conductivity/lubrication applications; CVD films are relied upon for semiconductor insulating layers; HPHT-synthesized c-BN is essential for superhard tools; and rBN single crystals grown via edge-tilted epitaxy are explored for quantum devices.

Technological Evolution: Current research focuses on low-temperature processes (plasma-assisted), green processes (low-energy templates), and improved epitaxial precision, driving the adoption of BN in advanced electronic and energy systems.

Fig. 4 Schematic Diagram of The Device for Synthesizing Hexagonal Boron Nitride Nanosheets

5 Real-World Applications and Recent Breakthroughs

5.1 Industrial Applications

The layered structure of hexagonal boron nitride (h-BN) endows it with unique in-plane strong bonding/interlayer weak interaction dual properties. In high-temperature gears and aerospace engines, h-BN powder achieves an ultra-low friction coefficient (0.03–0.1) through interlayer slip. Its sp2 bond network remains stable in an 800°C oxidizing environment, addressing the pain point of traditional lubricants failing at high temperatures. Cubic boron nitride (c-BN), with its diamond-like sp3 three-dimensional covalent network, achieves hardness second only to diamond (45–50 GPa) and does not undergo iron-catalyzed graphitization like diamond when machining quenched steel, making it an indispensable tool material for high-hardness alloy processing. In the field of 5G chip thermal management, h-BN flakes, with their ultra-high in-plane thermal conductivity (≈400 W/m·K), are embedded in a polymer matrix to form anisotropic thermal pathways, reducing local hotspot temperatures by over 30%. Their wide bandgap insulating properties (~6 eV) also prevent current leakage.

5.2 Electronic Device Raw Materials

The atomically flat surface and absence of dangling bonds in h-BN, a raw material for electronic devices, make it an ideal dielectric substrate for two-dimensional electronic devices. When single-layer graphene is placed on h-BN, the shielding effect of its layered structure enhances carrier mobility to 140,000 cm^2/(V·s), a tenfold increase over traditional SiO2 substrates, due to its surface charge trap density being below 10^10 cm^-2. c-BN, on the other hand, leverages its 6.4 eV ultra-wide bandgap and indirect bandgap characteristics, enabling room-temperature lasing in deep ultraviolet lasers (wavelength <200 nm). The boron vacancy defects in its three-dimensional lattice can also capture high-energy particles and convert them into electrical signal pulses, enabling the construction of radiation-resistant detectors with a lifespan 100 times longer than silicon-based devices in nuclear power plant monitoring.

5.3 Emerging Applications

In nuclear reactors, the boron-10 isotope of h-BN has a neutron absorption cross-section as high as 3,840 target ev, and its layered structure can be processed into porous ceramic bodies that can effectively capture thermal neutrons at high temperatures of 800°C while maintaining chemical inertness to resist coolant corrosion. In the field of quantum technology, the boron vacancy color centers (VB-) in the h-BN lattice emit stable single photons with a quantum efficiency of 85%. The interlayer isolation environment extends the decoherence time to the millisecond level, making it a candidate material for room-temperature quantum storage devices. At rocket engine nozzles, h-BN coatings achieve dual protection through a gradient densification structure: the sp² ring on the surface resists 3,000°C oxidizing flame streams, while the inner sp³ bond network blocks heat diffusion from the base alloy, extending nozzle lifespan to three times that of traditional silicon carbide coatings.

Fig. 5 Boron Nitride Nuclear Reactor Control Rod

6 New Discoveries and Future Focus Areas

6.1 Major Technical Challenges and Solutions

1. Difficulties in Growing Large-Area Single Crystals of c-BN

Cubic boron nitride (c-BN), as an ultra-hard material (with a hardness of 45–50 GPa), can replace diamond in the field of cutting tools (especially when processing iron-group metals, as it does not cause graphitization without a catalyst). However, the preparation of its single crystals faces core challenges:

Interfacial stress and phase purity issues: Traditional PVD/CVD methods require high-energy ion bombardment to induce phase transformation, resulting in mixed phases (hexagonal h-BN and cubic c-BN coexisting) and residual stress within the film. Additionally, the interface often contains amorphous boron nitride (a-BN) and disordered layer structures (t-BN) transition layers, which degrade crystal quality.

Size limitations: High-pressure high-temperature (HPHT) methods require extreme conditions (5–12 GPa, 1400–1700°C), which can produce high-purity c-BN grains but struggle to achieve wafer-scale single-crystal growth.

Breakthrough directions:

Epitaxial growth technology: Recent studies have shown that columnar epitaxial c-BN single-crystal films can be grown on diamond substrates, avoiding intermediate layer defects.

Plasma-assisted CVD: Low-temperature plasma-enhanced CVD (e.g., 350°C PECVD) controls crystallinity by regulating plasma irradiation time, offering potential for large-area growth.

2. Optimization of Interlayer Thermal Conductivity Mechanism in h-BN

Hexagonal boron nitride (h-BN) exhibits an in-plane thermal conductivity as high as 400 W/m·K, but its interlayer thermal conductivity is insufficient, limiting its application in vertical heat dissipation. The primary issues include:

Anisotropic Constraints: The layered structure of h-BN results in strong covalent bonds within the plane and weak van der Waals forces between layers, making it difficult for heat to transfer across layers.

Topography-Dependent Thermal Conductivity Behavior: Flake-shaped h-BN optimizes horizontal heat dissipation, but spherical particles are needed to improve filling efficiency in the vertical direction; however, the preparation process for spherical h-BN is complex and costly.

Therefore, optimization strategies mainly focus on the following aspects:

• Micro/nano-structure design:
  • Plate-like h-BN: Ultra-thin nanosheets (thickness < 10 nm) are prepared via ionic liquid exfoliation, enhancing interlayer phonon transport efficiency and improving thermal paste performance by 30%.
  • Spherical h-BN: High-frequency plasma vapor deposition synthesizes spherical particles, enabling high-filling composite materials suitable for vertical thermal management applications such as battery cooling.
• Interface engineering: Oriented arrangement of h-BN nanosheets in a polymer matrix to construct anisotropic thermal conduction pathways, such as 5G chip heat dissipation films that can reduce local hotspot temperatures by over 30%.

3. Low-Cost Large-Scale Production

Currently, the mass production cost of BN materials is high, especially for high-performance forms (such as nanotubes and single-crystal thin films):

Cost reduction pathways:

Ionic Liquid Stripping Technology: A method based on inexpensive ionic liquids enables the large-scale production of h-BN nanosheets (yield of 25%), with costs reduced to one-third of traditional methods.

Combustion Synthesis Method: Using boric acid-urea as raw materials, h-BN micropowder is directly synthesized at 900–1100°C, eliminating reliance on high-purity gases, and is suitable for industrial lubricants and thermal conductive fillers.

6.2 Cutting-Edge Research Breakthroughs and Directions

1. Van der Waals Heterojunction (h-BN/Graphene/Transition Metal Dichalcogenide)

h-BN plays a central role as an insulating layer in two-dimensional heterojunctions:

Photodetector innovation: Inserting an h-BN barrier layer into a graphene/MoS₂ heterojunction suppresses dark current to the picoampere level (0.07 pA), improves response speed by 100 times (0.3 s vs. 20 s), and enhances photogenerated carrier transport using the FN tunneling effect.

Quantum effect regulation: Aligning five layers of graphene with h-BN forms a Moire superlattice, achieving the "fractional quantum anomalous Hall effect" (FQAHE) in graphene for the first time, providing a platform for zero-magnetic-field topological quantum computing.

Advantages:

h-BN's atomically flat surface reduces interface scattering, increasing graphene carrier mobility to 140,000 cm^2/(V·s)10.

The wide bandgap characteristic (~6 eV) blocks current leakage, meeting the requirements of high-frequency devices.

2. Boron Nitride Nanotubes (BNNT)

BNNTs replace the C-C bonds in carbon nanotubes (CNTs) with B-N bonds, combining high strength with insulation properties:

Mechanical properties surpass CNTs: Theoretical calculations indicate higher yield strength, stronger defect tolerance, and the highest strength among known insulating fibers.

Extreme environmental stability: They maintain structural stability in a 1000°C oxidizing environment, outperforming CNTs' oxidation threshold (~400°C).

Application scenarios:

Reinforcing phase in composite materials: Filled into polymer matrices (e.g., epoxy resin) to enhance high-temperature stability and thermal conductivity, used in spacecraft thermal management components.

Neutron shielding material: Boron-10 isotope neutron absorption cross-section reaches 3,840 target epsilons, suitable for nuclear reactor protection.

Fig. 6 Boron Nitride Nanotube

3. Boron-Nitrogen-Based Quantum Materials 

The dynamic reversibility of B-N bonds provides a new dimension for quantum material design:

Quantum light sources: Boron vacancies (VB-) in h-BN emit stable single photons with a quantum efficiency of 85% and a decoherence time reaching the millisecond level, laying the foundation for room-temperature quantum memory.

Topological flat-band control: Rhombic BN (rBN) single crystals achieve ferroelectricity (Curie temperature >600°C) via oblique epitaxial growth, supporting high-order flat bands, with potential for generating non-Abelian anyons.

B-N covalent polymers: City University of Hong Kong has synthesized single-crystal polymers (e.g., CityU-15) using B-N bonds, which, after iodine doping, achieve ultra-low-energy devices (3.3 fJ/cycle) for artificial retinal synapse simulation.

7 Conclusion

Boron nitride (BN) is a binary compound composed of boron and nitrogen atoms. It primarily exists in allotropic forms such as hexagonal (h-BN) and cubic (c-BN). The layered structure of h-BN confers it with high in-plane thermal conductivity (approximately 400 W/m·K) and high-temperature lubricity; the cubic structure of c-BN provides superhard properties (hardness of 45–50 GPa) and wide-bandgap semiconductor behavior (bandgap of 6.4 eV). Current challenges include the difficulty of growing large-area single crystals of c-BN, low interlayer thermal conductivity in h-BN, and high costs associated with large-scale production. Cutting-edge research focuses on van der Waals heterostructures (e.g., h-BN/graphene), mechanical/neutron shielding properties of hexagonal boron nitride nanotubes (BNNTs), and boron-nitrogen-based quantum materials (e.g., boron vacancy color center single-photon sources). Future efforts should optimize fabrication processes (e.g., plasma synthesis, ionic liquid exfoliation) and deepen quantum control research to advance its applications in electronics, nuclear energy, and quantum technology.

As a leading supplier in advanced materials, Stanford Advanced Materials is dedicated to providing high-quality boron nitride products and expert support to facilitate both research and industrial progress.

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