(616a) Mechanoactivation, Initiation and Combustion of Aluminum and Copper Oxide Mixtures

mechanoactivation, initiation and combustion of aluminum
and copper oxide mixtures

A. Yu. Dolgoborodov^1,2,3, A. N. Streletskii^2,3, B. D.
Yankovskii¹, V. G. Kirilenko²,

S. Yu. Anan'ev^1,3 , I.V. Kolbanev², G. A.Vjrob'eva²

¹Join
Institute for High Temperature RAS, Izhorskaya St. 13 Bd.2, Moscow, Russia

¹Semenov
Institute of Chemical Physics RAS, Kosygin St. 4, Moscow, Russia

³ Moscow
Institute of Physics and Technology, Dolgoprudny, Russia

e-mail: aldol@ihed.ras.ru

Thermite mixtures based on
metals and solid oxidants allow to obtain a significant exothermic effect
during combustion (up to 8 kJ/g), while the burning rate of mixtures of micron
powders usually does not exceed several tens of mm/s, which limits the field of
their application. This is due to the fact that the rate of propagation of
chemical reactions in solid mixtures is determined primarily by the effective
contact surface of the reactants. To increase this surface, various methods are
used: ultrasonic mixing of nanosized powders, electrochemical deposition of
submicron metal-oxidizer layers, etc. [1]. One of the relatively new methods
for obtaining fast-burning thermite compounds is the preliminary
mechanochemical activation of mixtures of micron-sized particles in
energy-intensive ball mills. At the same time, the components are shredded and
mixed at the submicron and nano levels, and new defects in the crystal
structure are formed, which makes it possible to increase the rate of chemical
reaction on the surface of the reagents and, accordingly, we get a way to
control the processes of energy release during combustion and shock wave
action. In Russia, the method of mechanoactivation of solid oxidant-metal fuel
mixtures has been actively used since the beginning of the 2000s [2-5], and the
resulting materials are called Mechanically Activated Energetic Composites
(MAEC).

Among the variety of thermite mixtures, special attention is drawn
to the composition of Al + CuO, which makes it possible to obtain one of the
highest exothermic effects per unit volume (more than 20 kJ/cc). In this paper
new research results on the initiation and burning of MAEC
Al/CuO based on micron and nanosized powders using the mechanoactivation are
presented.

As the initial components, micron and nanosized powders were used:
industrial pyrotechnic powder Al PP-2L (flake 50 ÷ 100 μm ´ 2 ÷ 5 μm), CuO 20÷50 μm, nanosized nAl (Alex
100 ÷ 200 nm) and nCuO (50 ÷ 80 nm). Al weight content was from
18 to 25%. Mixing and activation was carried out in the vibration mill of the
Aronov design or in the planetary mill "Activator-2sl» with steel drums and
balls. A rough estimate of the energy intensity of the two types of mills
based on the growth of the specific surface area of the test material (MoO₃)
is for "Ativator-2sl" at a total power J = 9.7 W/g, and for the
Aronov mill J = 3.7 W/g. Weight load of powders was 10-25 g, the mass of balls was
200-300 g. Hexane was added to reduce frictional heating. The starting powders
and MAEC were analyzed by X-ray diffraction, electron microscopy and thermo-gravimetric
analysis. Image analysis shows that as a result of activation the obtained material
were a polydisperse mixture of fairly large conglomerates of flat fragments of
Al particles (~ 1 - 10 μm) with submicron CuO particles (see Fig.1a). The products
of high-speed burning of MAEC Al/CuO are shown in Fig. 1b. The bulk of Al₂O₃
is spherical particles of 10 μm, which consist of stuck submicron
particles. Cu is in form of large smeared drops on the surface of Al₂O₃
or in the form of nanosized particles condensed from the gas phase.

A number of the dependences of the combustion parameters on the
activation dose D_a were
determined in experiments: ignition temperature by hot surface, brightness
temperature of burning products, burning rate in cylindrical channels and
electrical resistivity in cloud of products. The dynamics of the expansion of
products in free space during electric spark and shock wave initiation was
analyzed.

The burning rate was measured in plastic and glass tubes (diameter
4-10 mm) by recording of products emission by light fiber or high speed
photography. The porosity of charges was 60-70%. The initiation was carried out
by heating the NiCr wire or by an electric spark. The spark energy was
regulated by changing the current amplitude in the range of 1.5-50.0 mJ at
duration of the current pulse 1.2 μs. Experimental setup with electric
spark initiation and individual frames at different sparks energies are shown
in Fig. 2. Brightness temperature of products was measured by four - channel
pyrometer.

Depending on D_a
and the initiation method, the measured burning rate varies from 10 m/s to 700
m/s, and the product temperature is from 2000 K to 3700 K (see Fig.3). In case
of low-energy electric spark (<20.0 mJ), combustion has a pronounced heterogeneous
character. Photo registration shows that zones of bright glow of hot products
alternate with dark zones. As the spark energy increases, the uniformity of the
glow increases. The highest reactivity, burning rate and temperature of
products for MAEC Al/CuO were obtained at D_a
= 1.8÷2 kJ/g. With increasing D_a
there is a partial reaction of the reagents in the activation process.

In the case of shock-wave initiation of compositions in a
semi-enclosed volume, the main process of energy release proceeds in the flow
of products dispersed in the unloading wave. The initial flow rate of the
products is more than 800 m/s. The maximum brightness temperature in the cloud
of partially ionized products reaches 3700 K, the resistivity is about 107 Ohm
* mm²/m.

In general, the results of the work have shown the promise of
preliminary mechanochemical activation for the production of fast-burning
thermite compositions of Al/CuO. The use of the original nanoscale components
is inadvisable, since it does not give appreciable advantages. The reactivity
of thermite mixtures based on CuO can be controlled by the addition of metals
having catalytic action.

1.      
Energetic Nanomaterials: Synthesis,
Characterization, and Application / Ed. by
V.E. Zarko and A. Gromov. 2016. Elsevier Inc.

2.      
A.Yu. Dolgoborodov, M.F.
Gogulya, M.N. Makhov, et al. Detonation-like phenomena in Al/S mixture”, Proc. Twenty-Ninth Intern. Pyrotechnics
Seminar, 2002, p. 557-563.

3.      
A.Yu. Dolgoborodov, M.N. Makhov,
I.V. Kolbanev, A.N. Streletskii. Mechanically activated pyrotechnic
composition, RF Patent No. 2235085,
2004.

4.      
A.Yu.
Dolgoborodov. Mechanically activated oxidizer–fuel energetic composites. Combustion, Explosion, and Shock Waves.
2015, V. 51(1), p. 86–99.

5.      
A.N. Streletskii,
M.V. Sivak, A.Yu Dolgoborodov. Nature of high reactivity of
metal/solid oxidizer nanocomposites prepared by mechanoactivation: a review. J. Mater. Sci. 2017. V. 52(20), p.
11810–11825.