Taming a temperamental rocket fuel

Despite much investigation as a rocket fuel, the full potential of boron has never been realised. It could get another chance to shine, thanks to a better understanding of the role impurities play in its unpredictable combustion behaviour.

Boron is the fifth lightest element, has a high intrinsic energy density, and is cheap, abundant and stable — making it of interest as a solid rocket fuel and as an additive to jet fuels to increase energy output. However, while boron has been investigated for such applications since the 1940s, this propellant has been difficult to tame due to its variable and complex combustion behaviour.

Alla Pivkina, Nikita Muravyev and colleagues from the Semenov Federal Research Center for Chemical Physics in Moscow have now conducted a series of studies to understand why this is the case.

“Boron particles are not ignited easily,” says Pivkina, “because the solid boron core is coated with an oxide shell that acts as a protective layer, preventing or delaying ignition and combustion.”

In the first series of studies, the researchers found that various boron powders had different shell compositions due to the presence of different impurities like aluminium and magnesium and their oxides that are introduced during the production process.

“We showed that boron particles with different impurities in the oxide shell have significantly different thermal behaviour,” says Pivkina.

Now, reporting in the journal Combustion, Explosion, and Shock Waves, the researchers have shown through thermal analysis of the boron oxide powder which impurities hamper or improve combustion performance¹.

“We found that aluminium and magnesium oxide impurities reduced evaporation of the boron oxide and thus decreased the activity of boron ignition, while magnesium fluoride had the opposite effect,” says Pivkina.

During heating, the oxide layer undergoes a process of dehydration, melting and evaporation. Each of these steps occurs at a different temperature depending on the impurities present. The researchers found that aluminium and magnesium undergo a metallic thermite reaction with boron oxide that can interfere with the combustion process and leaves unwanted oxide residues. Magnesium fluoride, on the other hand, evaporates efficiently at 1000 °C to form boron fluoride gas, which promotes evaporation of the boron oxide melt at around 1300 °C and helps activate the boron core, which can result in a reduction in the troublesome ignition delay.

“Currently, we are investigating a number of binders for optimal organization of the combustion process of boron-based fuels,” says Pivkina. “The results will provide pathways to facilitate the ignition and combustion of boron particles in energetic formulations.”