Boron nanoparticles/nanopowder have numerous applications as fuel additives and high-energy density fuels, hydrogen generation, cancer therapy. The elemental boron is particularly the subject of a lot of research in various field of science due to its interesting properties. Boron has the melting and boiling points of 2394˚C and 4200˚C, respectively and is highly resistant against corrosion. Boron behaves like carbon when interacting with other compounds covalently forming stable molecular networks. Boron has a common oxidation state (III) forming oxides like sulfides, halides and nitrides. Boron trihalides oxides are chemically Lewis acids that engage in reactions with electron pair donor Lewis bases. As an example of this class of compounds, tetrafluoroborate is a catalytic anion with applications in petrochemical industry. Boron occurs naturally as various boron (III) oxides on Earth crust.
Even the amorphous boron with irregular crystalline structure contains regular boron randomly bonded to other boron elements. The crystalline boron is particularly hard black metalloid with four different crystalline structures. Chemically, boron is more similar to silicon compared to aluminum and is resistant against acids like hydrochloric and hydrofluoric acids. However, boron is slightly attacked by hydrogen peroxide, chromic acids, nitic acid and sulfuric acid. The crystalline behavior of boron, its particle size temperature and purity determine its oxidization rate making it unreactive when exposed to air at room temperature. It should be noted that boron is oxidized to boron trioxide at higher temperatures. The highest volumetric energy and the third gravimetric energy density are the properties that make boron a potential agent to be applied in fuels and propulsion. Elemental boron is used in devices that function at higher temperatures, thermoelectric devices, nuclear reactors control unit and refractory electric materials. The materials that are rich in boron adopt unique electronic properties to be qualified is semiconductors 1.
Boron Nanoparticles/Nanopowder Properties
Due to the dramatic increase in surface area, nanoparticles turn out to have the intensified behavior and properties of their bulky forms. So far, various nanostructures of boron namely boron nanoribbons, boron nanowires, single-crystalline boron nanobelts and nanocones have been synthesized which proves the feasibility of boron to be shaped into different structures in nanoscale 1. Considering this, boron nanoparticles have found increasing applications and interest as fuel additives and propulsion agents. Boron nanoparticles can be oxidized completely prior to any diffusing all over their surface oxidization layer is limited. It has to be proved that inorganic nanoparticles reactivity increases as the particles size falls to the nanoscale 2.
Synthesis and Preparation of Boron Nanoparticles/Nanopowder
Various synthesis methods have been suggested to achieve boron nanostructures and nanoparticles. Based on a study, amorphous boron nanoparticles are synthesized through heating boron trioxide with sodium chloride at 800˚C under argon atmosphere. The sodium chloride is serves to lover the maximum combustion temperature generated during the reaction as the inert material. The boron nanoparticles synthesized this way range from 30 to 300nm. By increasing the concentration of sodium chloride five times in this method, the specific surface area increases as well 3. In a low-cost and simple preparation method, boron nanoparticles are synthesized using the pulse ire discharge method. To do so, a large pulsed current is applied by micro-sized boron powder to obtain crystalline boron nanoparticles under Ar gas sized less than 100nm 1. Room temperature method, which is scientifically and practically interesting, has also been introduced to synthesize highly pure amorphous boron nanoparticles. Accordingly, the boron particles are prepared only in a single step and a gas-phase process through pyrolyzing mixtures of SF6 and B2H6 by carbon dioxide lase. The resulting boron nanoparticles size range almost homogenously between 10 to 15nm with specific surface area over 250m2 and shape spherically 2. Another method takes the advantage of thermal plasma. In so doing, boron nanoparticles are prepared by injecting boron into thermal plasma with a consecutive nucleation to generate the nanoparticles 4. Water dispersed boron nanoparticles are synthesized though mechanical milling along with 10-undecenoic acid surface ligands with a subsequent hydrolysis of tetra ethoxy silane to obtain boron nanoparticles of 10 nanometers. The boron nanoparticles in this method appear to be spherically shaped and hydrophobic and monodispersed in water 5. In addition to the above-mentioned methods, other preparation approaches such as metallic reduction of boron compounds, ball milling, gas phase reduction/decomposition if gas phase boron agents and reduction of the BBr3 solution using sodium naphthalenide have been suggested as well.
Boron Nanoparticles/Nanopowder Applications
Primarily, boron nanoparticles are used extensively because of their thermal properties. They have a broad range of applications like energy generation purpose specifically as high-energy agents in solid propellants and fuels. This is due to the fact that boron’s gravimetric heat of combustion is much more than the conventionally used liquid hydrocarbon fuels. Moreover, boron nanoparticles are used as protective coatings, anti-freezing agents, semiconductors, braking system fluids, neutron capturer in cancer therapy, hydraulic systems fluids, hydrogen generation from water, corrosion inhibitor. Actually, anywhere iron and metallic parts are employed, boron and is nanostructures can serve so efficiently. Boron and its compounds are widely used as abrasive material and surface and high-speed cutting. Boron nanopowder has applications in electrolytic condensation, tanning, photography and fuel cells. In particular, boron nanopowder is a black or brown powder which is basically used for its thermal stability and properties. There are reports of using boron nanoparticles as pesticides to fight insects and pests too. In addition to what mentioned so far, it must be pointed out that elemental boron and its nanoscale compounds are specifically used for their excellent thermal properties, ignition aid as well as high-energy generation applications.
Boron is a versatile element to be obtained in nanoscales such as boron nanoparticles, nanopowder, nanowires, nanoribbons and nano belts with interesting applications that are mostly due to the excellent thermal properties, higher energy density compared to liquid hydrocarbon fluids and the highest volumetric energy density. The excellent fuel and energy properties of boron intensify even more when it is achieved as nanoparticles and nanopowder with applications in science and industry.