The developments and improvements in nanotechnology in last decades have provided the use of nanoparticles in various areas for many applications such as biomedical, electronics, pharmaceutical, cosmetics, aerospace and so on. Inorganic nanoparticles have been widely used in various areas such as biomedicine, electronics, photonics, and thermotics due to their unique chemical and physical properties. With the help of rapid development of science technology, inorganic nanoparticles and their applications have been drawn great attention, and hence research and experiments on inorganic nanomaterials became more and more important. Among inorganic nanomaterials, boron nitride nanoparticles have attracted high interest due to their unique chemical, physical, thermal and electrical properties.
Boron nitride is an inorganic compound with the chemical formula BN. Boron nitride exists in non-crystalline and in several crystalline forms which are isoelectronic to a similarly structured carbon lattice.
Structure of Boron Nitride
Boron nitride exists in multiple forms (non-crystalline and crystalline) and these forms differ in the arrangement of boron and nitrogen atoms, leading to vary the bulk properties of the compound. Boron nitride has three crystalline and an amorphous structure.
- Amorphous BN (a-BN)
- Hexagonal BN (h-BN)
- Cubic BN (c-BN)
- Wurtzite form BN (w-BN)
Figure 1: Different crystalline forms of boron nitride.
The hexagonal boron nitride (analogous to graphite) is the most stable and the softest among BN polymorphs. The cubic boron nitride (analogous to diamond) is the hardest among them, it is the second hardest material in the world. It is softer than diamond, however, its thermal and chemical stability is superior. The wurtzite structure is a rare form of BN (analogous to lonsdaleite). However, it is slightly softer than the cubic BN. In addition, amorphous form of boron nitride is a non-crystalline material which lacks any long-distance regularity in the arrangement of their atoms. It is analogous to amorphous carbon.
Boron Nitride Nanoparticles
When compared to three-dimensional bulk (3D) substances, nano-size structures are envisaged to show new properties due to surface and interfacial effects. Therefore, nanoparticles have been drawn great attention because of the fact that these new properties would promote novel applications in engineering. Boron nitride (BN) nanoparticles are considered as the most promising inorganic nanomaterials so far. These boron nitride nanoparticles can have different forms such as nanotubes, nanosheets, nanofibers, nanorods and nanoribbons. Boron nitride nanotubes (BNNTs) were experimentally discovered in 1995 after four years of the discovery of the carbon nanotubes (CNTs). Subsequently, other one-dimensional boron nitride nanoparticles including nanosheets, nanorods, nanofibers were discovered and synthesized.
Properties of Boron Nitride Nanoparticles
Bulk h-BN has been a topic of significant interest since their discovery due to its low density, excellent inertness, high thermal conductivity, high thermal shock resistance, electrical insulation, low friction coefficient, and superb oxidation resistance. BNNTs and BNNSs (nanosheets) also exhibit these properties as they are inheritors of bulk h-BN.
BNNTs possess significant thermal conductivity and high specific heat. According to theoretical calculations, high specific heat of BNNTs and their thermal conductivity is higher than that of carbon nanotubes. Also, they have high thermal shock resistance. Another research on thermal stability of BNNTs stated that oxidation resistance of BNNTs can reach up to 830℃ in air. Moreover, this study shows that the onset temperature for the oxidation of BNNTs (800℃) is much higher than the onset temperature for the oxidation of CNTs (400℃) under the same conditions.
Superior mechanical properties of BN nanoparticles (BNNPs) make them interesting to utilize them in various applications. BNNPs have very high elastic modulus and tensile strength. The elastic modulus and tensile strength values may vary for different types of nanostructures of BN, and theoretical studies have revealed the elastic modulus of BNNTs to vary between 1.22 and 1.48 TPa, which is similar to elastic modulus of CNTs. According to another experiments, tensile strength of BNNTs were estimated as 33 GPa. This result clearly shows that BNNTs have much higher tensile strength than the most used industrial material in the world which is steel. On the other hand, in the case of bamboo-like BNNTs, the elastic modulus and tensile strength were found as 225 GPa and 8 GPa, respectively. These superior properties of BN structure draw attention as nanofillers in composite materials. To exemplify, BNNTs can be used as reinforcements in polymer matrices and they can improve thermal conductivity of polymers while transparency of the polymers is still preserved. In addition, the density of BN is approximately 2.1 g/cm3, it only weighs nearly 1/6 of steel. Therefore, BN can be considered as a light material. The low density of BN can be advantageous in many areas such as aerospace.
Electronic and Optical Properties
BN nanoparticles are generally known as insulators because the have wide bandgaps of 5.0-6.0 eV. These wide bandgap property, meaning electrical insulating behavior, can encourage the utilization of BN nanoparticles on applications as protective shields encapsulating nanomaterials. Moreover, BNNPs exhibit distinctive violet or ultraviolet luminescence emissions, which promote their usage in surgeries, photocatalysis, and optical storage. Also, BNNPs do not have colorization problem as CNPs have. This advantageous property can be quite useful in different application areas.
BNNT films can achieve superhydrophobic state with contact angle with water above 150 degrees, while h-BN films are partially wetted by water with a contact angle of only ~ 50 degrees. From the difference of these two materials, it can be said that a significant change from hydrophilicity to superhydrophobicity. Because of the extreme chemical inertness of BN, pH value of water does not affect the wetting characteristics of BNNT and BNNS films. Thus, BN nanostructure films are very likely to be used in industrial applications such as water-repelling, self-cleaning, anti-corrosion systems.
It is important to state that BN absorbs neutrons very well, and it can be a good and suitable material in radiation protection applications. Also, the neutron absorption concept can be used in cancer treatments which will be discussed on the following pages.
Synthesis of Boron Nitride Nanoparticles
Most of the techniques known for the synthesis of CNTs have been modified to synthesize BNNPs as well. These techniques can be divided into two groups.
1) High Temperature Synthesis (>2000℃)
2) Medium/ Low Temperature Synthesis (<2000℃)
High temperature synthesis techniques include some methods such as arc discharge and laser ablation. On the other side, carbothermal methods, chemical vapor deposition (CVD), and ball milling are the examples of medium/ low temperature synthesis methods.
Arc discharge is a continuous process using the ac-jet method at high temperatures (5000-20000K) for the production large-scale of single and multi-walled BNNTs. In this method, a mixture of h-BN and catalyst metal powders are used as precursor and a combination of argon (Ar) and N2 gas is used as the plasma gas during production.
Laser ablation is a laser assisted technique to produce single-, double-, and triple-walled BNNTs by using a mixture of Ni, Co, and BN powders in an inert atmosphere. A rotating catalyst-free BN target in a continuous laser ablation reactor under N2 gas is used to obtain bulk quantities of BNNTs. Therefore, high yields of single-, and multi-walled BNNTs are obtained based on the vaporization of h-BN particle targets via a continuous CO2 laser under N2 flow at approximately at 3500K. However, the final product can contain unwanted morphologies besides tubular structures.
Figure 2: Schematic of the substitutional reactions in BNNTs, developed with temperature (T) and time (t).
Carbothermal methods are alternative ways of synthesizing BNNTs. The idea is based on turning CNTs to BNNTs through the substitution of carbon atoms with B and N atoms. These methods can lead to the generation of BNNTs with smaller diameters than those of starting CNTs.
Chemical Vapor Deposition
Chemical vapor deposition (CVD) is a route to synthesize significant amounts of BNNTs through heating a mixture of B, MgO, and FeO powders up to 1300℃. The growth vapors are transported by an argon gas flow to the reaction chamber, where a substrate is placed in it and heated up to 1550℃ under NH3 flow. As a result, BNNTs (both cylindrical and bamboo-like) with 20-100 nm can be produced by this method. This technique is successfully applied to obtain high yields of thin BNNTs with an average diameter of 10 nm by using a mixture of Li2O and B powders as precursors.
By ball milling of h-BN powders, high yields of BNNSs can be obtained under an N2 atmosphere. Ball milling conditions are accommodated to generate gentle shear forces which results in production of BNNSs. It is considered that this method could be applicable to produce nanosheets from any layered materials.
Application Areas of Boron Nitride Nanoparticles
Boron nitride nanoparticles are used in various areas in many applications, and the application areas can be listed as given below.
- Radiation protection
- Cancer treatment and cancer diagnosability methods
As it can be seen clearly, boron nitride nanoparticles are used in various application areas due to their exceptional and superior chemical, mechanical, electrical, and thermal properties. The list below indicates the most common application areas of BN nanoparticles deriving from their thermal, mechanical, chemical, electrical, thermal, and optical properties.
- In subsurface investigation, oil drilling and high-speed cutting tools
- As a metal forming and metal drawing lubricant release agent
- In transistors and as plastic resin sealing desiccant polymer additives
- Pressed into various shapes and can be used as high temperature, high voltage, insulation, cooling components
- As a special high-temperature electrolysis state, resistor material
- In the aerospace industry, for use in heat shielding material
- As a catalyst for high temperature, high pressure treatment
- Mold release for die casting and injection molding
- Thermally conductive filler for polymers
- Composite ceramics
- Starting material for hot-pressed BN parts
- For high-voltage high frequency electricity, plasma arc’s insulators, high-frequency induction furnace materials, semi-conducting solid phase admixtures, atom reactor’s structure materials, automatic welding high-temperature coating, preventing neutron radiation’s packing materials, radar antenna’s mediums, radar’s pass box, and rocket engine’s components
- As high-temperature lubricants
- Mold pressing boron nitride can be used in making high-temperature crucible and other products.
An overview of research on boron nitride nanoparticles has been presented in terms of structure, properties, synthesis, and application areas. Although BNNPs have interesting properties and promising outlook, the research progress on physical aspects and potential applications are limited so far. Therefore, more research and experiments are needed to find and explore new sides of these nanomaterials.