Agglomeration in Composite Propellants Containing Different Nano-Aluminum Powders

In order to improve nano-aluminized solid propellant properties and performance, different materials are being investigated for coating nano-aluminum particles. Two of the powders which are considered are V-ALEX, a fluoropolymer (Viton) coated nano-aluminum, and L-ALEX, a stearic acid coated nano-aluminum. It is believed that a thin layer of coating should protect the aluminum core from premature oxidation, as well as provide an exothermic source which may accelerate particle ignition, resulting in reduction of agglomeration. This paper investigates the effect on the agglomeration phenomenon of replacing 33 % of the micron aluminum powder with ALEX, V-ALEX and L-ALEX powders, in solid propellant samples containing 20 % HTPB, 65 % AP and 15 % aluminum. Propellant strands were burned inside a windowed pressure chamber, allowing measurements of the agglomerates size and number using a high-speed camera. The experimental data showed that propellants in which 33 % of the micron aluminum was replaced with nano-powders exhibited reduced agglomeration, in terms of number of agglomerates and their total volume. The reason is the lower ignition time and temperature of the nano-powders that ignite before aggregating to agglomerates. The propellants containing ALEX nano-powder exhibited a reduction of up to 36 % in agglomerates volume compared to the propellants containing only micron powder, while the propellants containing coated nano-powders, L-ALEX and V-ALEX, showed a higher reduction of up to 60 % in agglomerates volume. V-ALEX showed slightly better performance than L-ALEX. It can be concluded that by replacing some of the micron-aluminum with coated nano-aluminum in solid propellants, it is possible to significantly reduce the propulsive losses due to agglomeration.