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Over a long period of time, micron-sized powders of aluminium have been widely used for enhancing the energy-and-mass and ballistic characteristics of high-energy condensed systems, such as thermits, explosives, gun powders, propellants. Due to their large specific surface area, nanosized electroexplosive particles of aluminium can provide certain advantages over conventional aluminium powder, in particular with respect to combustion rate [1-5].
Thermal properties (differential thermal analysis) of the electroexplosive aluminium powder (ALEX) manufactured by APT was compared to those of the commercial aluminium powder, during their heating in air, oxygen, and nitrogen [6]. For ALEX powder, the exothermic process begins at the temperature below aluminium melting point (660 ºС), while aluminium powder with particle size about 20 μm does not react with oxygen, air, or nitrogen till about 1,000 ºC. In addition, nanoaluminium ignites much faster compared to micron-sized powders. When combusted in an air shock tube, ALEX had ignition delay of only 3 microseconds, while aluminium powder with average particle diameter of about 3 μm had a delay of 600 microseconds.
ALEX powders have been tested by a number of researchers for their potential use in high-energy compositions.
The effect of aluminium nanomaterial powder on detonation velocity was experimentally demonstrated in several research organizations which studied the possibility of ALEX application in explosives. In [7], when mixed with ammonium dinitramide (AND or SR12), ALEX demonstrated the increase of detonation velocity from от 4,380 m/s to 5,070 m/s (73:24:3 ADN/ALEX/Viton fluoroelastomer). Adding conventional coarse aluminium powder did not produce any visible effect on detonation velocity. In [6], comparative tests were carried out on flaky aluminium of 40XD type and ALEX mixed with dispersed lactose and ammonium perchlorate, where detonation velocity was measured in a detonation tube in nitrogen atmosphere. The results of these experiments show the advantage of ALEX, with respect to detonation velocity value, over flaky aluminium, at four different concentrations of aluminium.
Data from [8] show the enhancement of both detonation velocity characteristics and brisance for a series of compositions based on ALEX-containing TNTs. When ALEX was substituted for the commercial aluminium powder, the increase of detonation velocity was 200-300 m/s, and in some discharges there was an increase of brisance up to 27%.
An increase in the combustion rate of a conventional propellant results in the increase of draught and the rate of gas cleanup from the rocket engine. In [9-11], double increase of fuel combustion rate was observed when ALEX was substituted for micron-sized aluminium powder in conventional types of solid propellants, such as Al/AP/HTPB (aluminium/ammonium perchlorate/isobutylene-based adhesive).
An increase in the combustion rate is due to smaller sizes of aluminium nanomaterial powder particles. Models of aluminium particle combustion in a rocket engine demonstrate that the lifetime of a burning particle is proportional to the square of the particle diameter. Experimental data show that an aluminium particle of 5 μm in size burns in the engine in about 4 milliseconds. Extrapolation based on the above models reveals that a particle 100 nm in diameter would burn in about 0.6 microseconds – a value which is lower by a factor of four than that of a micron-sized particle. High-speed photography of the surface of the combusting propellant shows that a particle of nanosized aluminium completely burns out on the
surface of a combusting propellant granule instead of being expulsed by an exhaust flow, as it happens with micron-sized aluminium, i. e. particle combustion ends inside the engine, not in the rocket exhaust.
The use of nanosized aluminium powders allows the enhancement of hybrid rocket engines as well. A typical hybrid engine contains liquid oxygen and rubber-based granules (e. g., HTPB). These granules does not contain any oxidizer or contain it in a quantity sufficient to react with granules. Rubber pyrolysis gives rise to the formation of organic molecules with low molecular weight which get into the engine and react with liquid oxygen. If aluminium is introduced into a solid propellant, such as HTPB, engine pulse increase is theoretically possible. But micron-sized aluminium does not combust efficiently in similar engines.
Data from [12] demonstrate that adding 10% of ALEX to HTPB results in pulse increase by 70%, and the combustion process becomes evener compared to the combustion of pure HTPB.
Adding aluminium to kerosene increases specific energy in the volume unit of a liquid propellant. However, micron-sized aluminium in kerosene does not burn out completely. Adding ALEX nanomaterial powder results in the complete combustion of metal [13]. Correspondingly, high temperatures which occur during aluminium combustion lead to the increase of kerosene combustion rate.
One of the issues which prevent the use of nanosized aluminium powders in high-energy applications is their high reactivity. The metal can react with other components of pyrotechnical compositions. In order to prevent the chemical reaction, a procedure was developed for aluminium powder microcapsulation. During microcapsulation, the surface of particles is covered with layers of palmitic acid which protects powder particles from contact with oxidizing media (the commercial name of this nanomaterial powder is L-ALEX). In [14, 15], the impact of moisture on L-ALEX, ALEX powders and the commercial powder Cap45a with particle size of about 17 μm was studied.
During tests, the above powders were put in thin layers in cups inside a test chamber. The quantity of active aluminium was measured on a daily basis. The temperature varied from room temperature to 60 ºС, with relative air humidity of 75%.
The results of these tests show that Cap45a loses up to 20% of active aluminium, and ALEX loses up to 70% of the original quantity of active aluminium in 8-12 days. At the same time, the quantity of active aluminium in L-ALEX hardly decreases after 40 days of testing.
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