Further, the combustion products mainly consist of CO₂ and H₂O without NO₂, which exhibit less-toxic performance as propellants and additives for rockets.

After decades of extensive exploration, introducing hydrogen into solid propellants has been considered as one of the best solutions to enhance the heat of combustion and specific impulse for solid propellants because hydrogen can improve the heat of combustion, shorten ignition delay, and improve combustion performance. Solid hydrogen storage materials, such as metal hydride, complex hydride (borohydrides, alanates), and boron−nitrogen−hydrogen (denoted as BNH) compounds, among others, were then proposed to be potential candidates of solid propellants owing to their relatively high hydrogen contents.

For the application of hydrogen storage materials in solid propellants, metal hydrides can effectively improve the burning performance of solid propellants, but generally, their hydrogen contents are relatively low and they have an inferior long stability. … Compared with metal hydrides, complex hydrides (borohydrides, alanates) usually possess higher hydrogen storage capacity … [but] Boranes and borohydrides as rocket and jet fuels have many issues that need to be addressed, like thermally unstable, unsafe, and toxic issues. Moreover, the major combustion product of boron oxides would possibly tear the jet engine apart.

BNH compounds were typical hydrogen storage materials that have gained intense attention due to their high hydrogen storage capacity and tunable hydrogen storage properties. … Recently, a new BNH hydrogen storage system-amine metal borohydride was developed, such as Mg(BH₄)₂·nEDA and Al(BH₄)₃·nEDA, which has high hydrogen contents and tunable dehydrogenation properties that are comparable with amine borane. Moreover, they are easy to synthesize and can stay relatively stable. Therefore, this system may also have the potential for propulsion applications, and it will be of interest to explore their combustion properties.

—Huang et al.

In addition to the findings on the heat of combustion and ignition delay times referenced above, the team also found that:

• The material is air-stable;

• The minimum ignition energy is very low (0.3− 0.5 mJ);

• The combustion of the material undergoes three sequential stages of dehydrogenation, combustion of hydrogen and impurities, and combustion of dehydrogenated framework; and

• No aluminum oxide or boron oxide is detected in the combustion residue, and the crystal structures of the material are totally destroyed during combustion.

Owing to excellent performance of high hydrogen content, suitable dehydrogenation temperature, short ignition delay, and substantial heat of combustion, ethylene diamine aluminum borohydrides Al(BH₄)₃·nEDA (n = 3, 2) appear to have potential for solid rocket fuels or additives.

—Huang et al.

Resources

• Xuefeng Huang, Shengji Li, Xiongfei Zheng, Shu Yang, and Yanhui Guo (2016) “Combustion Mechanism of a Novel Energetic Fuel Candidate Based on Amine Metal Borohydrides” Energy & Fuels 10.1021/acs.energyfuels.5b02430