Due to the characteristics of high energy density and a high calorific value, boron has become a high-energy fuel and shows great potential to be a high-performance candidate for propellants. However, the wide applications of boron are still limited by the characteristics of easy oxidization, ignition difficulty, a long combustion duration, and combustion products that readily adhere to the surface and inhibit full combustion. The boron-based energetic materials can be prepared by surface coating, mechanical milling, and ultrasonic mixing methods. The boron-based composites with different additives had different combustion characteristics. The combustion of boron-based energetic materials can be optimized by removing surface oxide layers, providing extra heat, inhibiting the formation of or the rapid removal of the combustion intermediates, and increasing the diffusion rate of oxygen.
Until now, the reported preparation methods of boron-based composites mainly include surface coating [11][12], mechanical milling [13][14][15], ultrasonic mixing [16], ultrasonic dispersion [17], high-energy mechanical ball milling [18], cold spraying [19], self-propagating high-temperature synthesis (SHS) [20][21], mechanical mixing [22], and cryomilling [23]. The cryomilling method achieves a relatively high specific surface area compared with the conventional methods. This method also has several advantages compared with room-temperature milling such as the production of small particles without agglomeration and a reduction in the oxidation of powder because the milling process is performed in a nitrogen or argon atmosphere. How to improve the ignition and combustion performance are the major objects for designing the preparation methodology of boron-based composites.
Metal oxides can affect the ignition and combustion performance of boron-based thermite. The thermites could be prepared by a facile method. For example, B/nano-NiO thermite composites can be obtained by a simple procedure. First, the nano-Ni(OH)2 was prepared through the reaction of NiCl2·6H2O and NaOH. Then, the prepared nano-Ni(OH)2 was coated on the surface of boron powder by precipitation [32]. After the pyrolysis of B/nano-Ni(OH)2, the B/nano-NiO composite was obtained. XRD tests confirmed that two crystals co-existed in the composite, which indicated that the boron powder was in the amorphous state and the nickel oxide was in a crystalline state. NiO has a high specific surface area and relatively low surface free energy, making it easier for boron to release energy and undergo full combustion. Several synthetic methods of B–metal oxide-based composites had been presented [33].
Other methodologies have been developed to prepare B–metal oxide-based composites that contain CuO, Bi2O3, and Fe2O3 [16][34]. Amorphous B powder with a purity of 99.5% and the metal oxide were first ultrasonically treated for 30 min in ethane. Then, the ethane was evaporated, and the precipitate was passed through a 45 micron sieve to obtain the designed composites. The composite containing MoO3 and Co3O4 was prepared by similar methodology.
Metal carbide-based materials have the advantage of high activity, high combustion heat, a high combustion rate, and high combustion velocity. Under the normal condition, the metal carbide can maintain high stability and decrease the mechanical sensitivity of energetic materials. The metal carbides are usually adopted in the composite of ramjet fuels. The B–nano carbide composites are usually synthesized by mechanical mixing under an argon atmosphere [17]. The high-purity amorphous boron were first mixed with n-TiC, n-ZrC, and n-SiC, separately. Then, the mixtures were ground to ensure the uniformity of the composites. In addition to the metal carbide, other materials such as boron carbide were also adopted to prepare the composites.
Metal oxides have been investigated and adopted in practical application as an important component of metal thermites. The metal oxide can react with more active metals and other reducing agents with the release of a large amount of heat. Boron-based thermites form an important branch of thermites. For example, the high combustion performance of NiO-coated B-based thermite has been confirmed. Compared with Mg-PTFE-coated B, the propellant of the B–NiO/Mg-PTFE composite shows enhanced combustion efficiency, a faster combustion rate, and a higher combustion temperature. This is because B–NiO can absorb the heat generated by the meteorological heat flow and condensed phase composition. At the same time, the composite of B–NiO has a higher specific surface and relatively higher surface free energy; therefore, B–NiO can easily react and release a large amount of heat in the combustion procedure.
Metal oxides, such as MgO, Al2O3, Bi2O3, CeO2, Fe2O3, CuO, and SnO2, can also dissolve into liquid B2O3 [43]. With the increase in the surface temperature of B particles, the metal oxide that dissolved into the liquid oxide layer produced mechanical stress at the interface, which broke through the oxide layer and played a positive role in the removal of boron oxide and promoted the combustion of B particles.
Some metal oxides have a relatively high thermal conductivity coefficient, which can enhance the diffusion of oxygen from metal oxides to boron particles. At the same time, the volume density of oxygen in metal oxides is much higher than that of gaseous oxygen. The oxygen can directly contact and diffuse into the core of boron particles, which eventually promotes the combustion of B particles. Bi2O3 has higher oxygen ion conductivity [44]; therefore, the boron-Bi2O3 composite can be ignited by the minimum ignition input energy with the minimum ignition delay time. CuO, MoO3, and Co3O4 can also improve the combustion efficiency of boron particles. C
The combustion of boron-containing propellant can generate carbon dioxide, water vapor, and ammonia, which can break the oxide film on the surface of boron particles [45]. It has been proved that metal carbides can be used as effective components of solid propellants [46]. The combustion of metal carbide can generate solid components and carbon dioxide gas. The solid components, i.e., the metal oxide, act as catalysts for accelerating the oxidation and combustion of boron particles, while carbon dioxide destroys the external oxide layer and expands the contact area between boron and oxygen. Therefore, metal carbides can be adopted as additives to promote boron combustion.
This entry is adapted from the peer-reviewed paper 10.3390/catal13020378