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Combustion Characteristics of Composite Solid propellants
Containing Different Coated Aluminum nanopowders
YAO Er Gang a *, ZHAO Feng Qi b, XU Si Yu c, HU Rong Zu d, XU Hui Xiang e, and HAO Hai Xia f
Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an 710065, P. R. China
aemail [email protected],
Keywords: Aluminum nanopowders, Combustion Characteristic, Solid propellant, Coating
Abstract. Aluminum nanopowders coated with oleic acid (nmAl+OA), perfluorotetradecanoic acid
(nmAl+PA) and nickel acetylacetonate (nmAl+NA) were prepared. The combustion characteristics of
hydroxyl terminated polybutadiene (HTPB) composite solid propellants containing different coated
aluminum nanpowders were investigated. The result shows that the burning rate of the propellant
sample containing nmAl+NA is the highest at different pressure, the maximum burning rate is up to
26.13 mm·s-1
at 15 MPa. The burning rates of propellant samples containing nmAl+OA and
nmAl+PA are almost the same at different pressures, and higher than the propellant samples
containing untreated aluminum nanopowders only at the pressure range of 10 ~ 15 MPa. The flame
brightness of different propellants under different pressure is not the same. The flame brightness is
increased with the pressure increasing. The flame center zone brightness of the propellant containing
nmAl+PA and nmAl+NA is brighter under 4 MPa, and the brightness of nmAl+NA is the brightest.
The surface coating of aluminum nanopowder has little effect on the combustion flame temperature of
solid propellant. The burning surface temperature increases with the pressure increasing.
Introduction
Aluminum nanopowders have attracted an intense interest over the years as reactive additives in
formulations of explosives[1-3]
and propellants [4-6]
, as well as components of nanocomposite thermite
materials[7-9]
, mostly because of their higher and faster energy release as compared to micrometerscale
aluminum powders. Hydroxyl terminated polybutadiene (HTTB)-based composite propellants are
widely used because this type of propellant not only has excellent burning characteristics but also
good processability and storability. The replacement of micrometer-sized by aluminum nanopowders
in HTTB composite solid propellants is expected to improve burning rate, reduce pressure exponent,
increase specific impulse, reduce nozzle two-phase flow losses and decrease ignition delay time[10]
.
Baschung et al. [11]
showed that the burning rate of HTPB-based solid fuels can be increased by 70%
with a mass addition of 20% Alex. It has been demonstrated by Ivanov et al.[12]
that propellants
containing Alex exhibit burning rates much greater than those of the same propellant formulations
containing regular aluminum powders. In some cases the burning rate of the propellant with Alex was
as much as 5 to 20 times greater. In addition, ammonium perchlorate (AP)-based propellants
containing Alex have been shown to burn cleanly with essentially no residuals, as opposed to a
significant amount of condensed-phase particulate and surface residues for AP-based propellants with
regular aluminum.
In spite of many benefits associated with the usage of aluminum nanopowders described above,
there are significant challenges when moving to the nanoscale. The aluminum nanopowders also
suffer from serious drawbacks. Aluminum particles exposed to air undergo rapid aging to produce a
2-6 nm thick oxide shell[13]
. For micrometerscale aluminum particles, this oxide shell may account for
< 0.5 % of the particle mass, but for aluminum nanopowders with diameters < 20 nm the oxide layer
can account for > 70 % of the particle mass. For the aluminum nanopowders this oxide layer results in
Advanced Materials Research Vol. 924 (2014) pp 200-211Online available since 2014/Apr/17 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.924.200
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a much lower energy density. In addition, thicker oxide layers result in particles with elevated
temperatures for ignition[14-16]
.
Another obstacle associated with the use of aluminum nanopowders is its processability. Typically
metallized energetic formulations are composed of between 10 % and 40 % Al by mass with binder,
high explosive, oxidizer, and plasticizers making up the bulk of the other components. Processing
difficulties are experienced with aluminum nanopowders because of its extremely high surface area.
The binder is simply, in some cases, incapable of wetting all of the solids. The result can be a crumbly,
fragile material unsuitable for use[17]
. The aluminum nanopowders also react more readily with
energetic components, such as RDX[18]
, and produce gases. This minor incompatibility can cause
problems in mixes kept in long-term storage.
Finally, their electrostatic discharge sensitivity (ESD), and that of mixtures containing the
powders, can be much higher than the ESD of micron-size powders[19]
.
One way to solve those problems is to coat the nanoparticles with a protective layer of material[20]
.
Cliff et al.[21]
demonstrated that a coating of palmitic acid on Alex nanopowders seriously decreased
the sensitivity of the aluminum powders to humid conditions and slowed down the ageing of the
powders. Kwon et al.[22]
showed that electro-exploded aluminum nanopowders coated with
liquids-nitrocellulose (NC), oleic acid (C17H33COOH), stearic acid (C17H35COOH) and
fluoropolymer can increase stability to oxidation in air during the storage period and higher reactivity
by heating. Jouet et al.[23]
also studied the aluminum nanopowders coated with perfluoroalkyl
carboxylic acids. It showed that the coating appears to prevent the oxidation of the particles in air.
Foley et al.[24]
founded that aluminum nanoparticles capped by transition metal contain less aluminum
oxide than an aluminum sample that was not treated with a transition metal when exposure to air at
ambient conditions, and the nickel treated sample contained as much or more metallic aluminum as
the untreated aluminum sample.
At present, there are few on the combustion characteristics of composite solid propellants
containing different coated aluminum nanopowders. The objectives of this research are therefore to
investigate the effect of the coated aluminum nanopowders to the combustion characteristics of
composite solid propellant. In view of the many advantages of the oleic acid, perfluorotetradecanoic
acid and transition metal nickel to protect the aluminum particles against oxidation, the oleic acid,
perfluorotetradecanoic acid and nickel acetylacetonate were used to coating materials. The
combustion characteristics of composite solid propellants containing the coated aluminum
nanopowders were studied.
1. Experimental section
1.1. Preparation of composite solid propellant samples
Hydroxyl terminated polybutadiene (HTPB, industrial grade), ammonium perchlorate (AP,
industrial grade), dioctyl sebacate (DOS, analytical grade), toluene diisocyanate (TDI, analytical
grade), 5 µm aluminum (µmAl, > 98.0 %), 50 nm aluminum nanopowders (nmAl, >98.0 %) and
aluminum nanopowders coated with oleic acid (nmAl+OA), perfluorotetradecanoic acid (nmAl+PA)
and nickel(II) acetylacetonate (nmAl+NA) were used as components of HTTP composite solid
propellant. The nmAl+OA, nmAl+PA and nmAl+NA were prepared in our laboratory and the mass
ratio of nmAl to coating materials was 9:1. All chemical reagents were used as received without any
further purification. The propellant compositions tested in this study are shown in Table 1.
A solid propellant formulation containing 67 % oxidizer (ammonium perchlorate), 15 % metallic
fuel (aluminum) and 14 % binder (HTPB: hydroxyl terminated polybutadiene 11 % and DOA:
di-octyl adipate 4 %) is prepared by vacuum casting. Cross-linking agent, process aids, and burn rate
modifiers are added in small percentage (less than 0.5 %) to meet the ballistic requirements. The cast
propellant composition is cured using toluene diisocynate (TDI) at 50 °C for 5 d.
Advanced Materials Research Vol. 924 201
Table 1 Compositions of HTTB composite solid propellant
No. Coating materials HTPB(%) AP(%) 5 µmAl(%) nmAl(%) Others*(%)
11HT-0 — 10 70 15 0 5
11HT-1 — 10 70 5 10 5
11HT-2 OA 10 70 5 10 5
11HT-3 PA 10 70 5 10 5
11HT-4 NA 10 70 5 10 5 *others are DOS and TDI.
The HTPB composite propellant samples were prepared by mixing thoroughly of the ingredients
(list in Table 1) in a 2 L vertical kneader. The air bubbles trapped in the propellant slurry while mixing
was removed by degassing in a vacuum-casting chamber. The slurry was casted into aluminum plates
having dimensions about 3 cm × 4 cm × 15 cm, and then cured in an oven maintained at 70 °C for 7
days.
1.2. Characterizations
The morphology of the sample was characterized by a FEI Quanta 600 field-emission environment
scanning electron microscope (SEM). The composition of the sample was roughly examined by using
an OXFORD INCA Penta FET×3 energy-dispersive X-ray spectroscopy (EDS) attached with SEM.
1.3. Equipment methods and Test conditions
1.3.1. Burning rate test method
The steady burning rates (u) of the propellant strands were measured at the initial temperature of
20 °C and pressure range of 4 MPa ~ 15 MPa. The strands were held vertically and ignited electrically
with the help of a nichrome wire at the top. The propellants were cut into samples of 5 mm × 5 mm ×
150 mm. A thin layer of inhibitor (polyvinyl alcohol) was coated on the four side-surfaces of the
samples to ensure that the burning surface area of the sample remained the same during regression.
1.3.2. Burning flame profiles test method
In order to investigate the flame structure of the different HTPB propellants, the flame profiles of
the different propellants under different pressures were recorded by the single frame amplification
photography method. Propellant samples without any side coating were cut into 1.5 mm × 4.0 mm ×
25.0 mm, then placed vertically on the combustion rack vertically and sealed in a chamber which was
filled with four windows. The combustion chamber was filled with nitrogen gas to reach the different
pressures. In order to ensure photo quality, the gas should remove immediately by form a bottom-up
flowing nitrogen atmosphere. The ignition source was 20 V DC power. The Φ 0.15 mm
nickel-chromium alloy wire was used to ignition by the process controller. The samples were ignited
from top, and then start the camera to take the flame profiles images at the appropriate time.
1.3.3. Combustion wave test method
The Π type double tungsten-rhenium thermocouple was used to test the combustion wave
distribution of the solid propellant. The thermocouple (Φ = 25 µm) was embedded in the propellant
sample (diam. = 7 mm, length = 120 mm) whose profile was coated by polyvinyl alcohol solvent as a
flame-retardant and then exposure to air for drying[25,26]
.
The nickel-chromium alloy wire (Φ = 0.15 mm) with direct-current voltage of 200 V was adopted
for ignition. The automatic trigger acquisition system began to record the data output by the
thermocouple after the ignition of the propellant. With the sample burning out, the thermocouple
approached the burning surface gradually and finally got into the flame zone. In this way, the whole
202 Frontiers in Micro-Nano Science and Technology
burning process of the propellant was recorded and the combustion wave structure from the
condensed phase to gas phase was obtained ultimately.
2. Results and discussion
2.1. SEM-EDS analysis
Figure 1 shows SEM images of the nmAl, nmAl+OA, nmAl+PA and nmAl+NA. It could be
clearly seen from Fig. 1a that the morphologies of the nmAl particles were mainly spherical shape
with the average diameter of 50 nm. Due to the bonding effect of the surface coating oleic acid, as
shown in Fig. 1b, the nmAl+OA sticked together, the average diameters of particles increased to 200
nm. Fig. 1c shows that the nmAl+PA particles had better dispersion and the evenest distribution of
particle size. This may be because the PA is long carbon chain. The terminal carboxyl group of PA
can react with the aluminum atoms and form a strong chemical bond[22]
.The steric effect of the other
side can play a role of dispersant, which makes the aluminum nanoparticles separate from each other,
so the dispersion of aluminum nanopowders increase. Fig. 1d shows that the surface of nmAl+NA is
smooth, uniform distribution of particle size and the dispersity improved, but there are still certain
degrees of agglomeration.
(a) nmAl (b) nmAl+OA
(c) nmAl+PA (d) nmAl+NA
Fig. 1 SEM images of different aluminum nanopowders
In order to confirm the the existence of coating materials at aluminum nanopowders surface, EDS
analysis was performed. Fig. 2 shows EDS spectra of different aluminum nanopowders. The
appearance of C element from Fig.2b revealed that the OA had successfully coated on the aluminum
nanopowder surface. Moreover, the content of C, O and Al are close to 38.30 %, 7.97 % and 53.72 %,
respectively. The C element content is relatively high, which was caused by the oleic acid molecules
containing 18 carbon atoms. The EDS results (Fig. 2c) also demonstrate only the elements of C, O, F
and Al are contained in the sample and the content of C, O, F and Al are close to 6.63 %, 6.65 %,
18.20 % and 68.52 %, respectively. The ratio of F to C is close to the stoichiometric content ratio of
PA (C13F27COOH). This shows that the PA had coated on the aluminum nanopowders surface. It can
Advanced Materials Research Vol. 924 203
be seen form Fig. 2d that the sample is mainly composed of C, O, Ni and Al. The content of C, O, Ni
and Al are 17.21 %, 14.46 %, 13.47 % and 54.86 %. As show in Fig.2, the coating effect of OA and
PA is better from the Al element content compared with the NA. The reason is that the OA and PA are
carboxylic acid which can react with the aluminum and form chemical bonds, but the NA only
adsorbed on the aluminum nanopowders surface by physical adsorption.
(a) nmAl (b) nmAl+OA
(c) nmAl+PA (d) nmAl+NA
Fig. 2 EDS spectra of different aluminum nanopowders
2.2. Burning rate and its pressure exponent
Figure 3 is the burning rate (u)—pressure (P) curves of different composite solid propellant
containing different aluminum powders. The strand burn rate experiments were conducted in the
pressure range of 4 ~ 15 MPa. The base formulation (11HT-0) exhibited a burn rate in the order of
8.14 ~ 13.55 mm·s-1
(Fig.3). The addition of aluminum nanopowders treated and untreaded with other
coating materials leads to increase in the burn rate of the propellant as compared with the 11HT-0.
The burning rate of 11HT-4 sample which contains nmAl+NA is the highest at different pressure, and
the maximum burning rate is up to 26.13 mm·s-1
at 15 MPa. The burning rates of 11HT-2 and 11HT-3
samples are almost the same at different pressures, and higher than those of the 11HT-1 only at the
pressure range of 10 ~ 15 MPa. The burn rate trend at the pressure of 15 MPa was as follows:
11HT-4>11HT-2>11HT-3>11HT-1>11HT-0.
0 2 4 6 8 10 12 14 165
10
15
20
25
30
u /
mm
·s-1
P / MPa
11HT-0
11HT-1
11HT-2
11HT-3
11HT-4
Fig. 3 Burning rates of the propellants containing different aluminum powder at different pressures
204 Frontiers in Micro-Nano Science and Technology
The burning rate increase (∆) of propellants containing different aluminum powders at different
pressures were showed in Table 2. According to Table 2, the ∆ of 11HT-4 is the highest at the pressure
of 13 MPa, up to 94.4 %. This was attributed to the coating effect of nickel acetylacetonate which can
protect the aluminum particles against oxidation and make the aluminum content of aluminum
nanopowders be higher. Meanwhile, the combustion catalysis of nickel acetylacetonate at high
pressure was also important for another reason. The ∆ of 11HT-2 and 11HT-3 were both higher than
those of 11HT-1 at different pressure except 4MPa. This was owing to the OA and PA were easily to
decompose at low pressure. The decomposition was an endothermic process which decreased the
burning surface temperature and burning rates. As a whole the burning rate of propellant containing
nmAl+OA and nmAl+PA were higher than that of the propellant containing untreated aluminum
nanopowder.
Table 2 ∆∆∆∆ of different propellants containing different aluminum powders at pressure of 4 ~ 15 MPa
Samples ∆(%)
4 MPa 7 MPa 10 MPa 13 MPa 15 MPa
11HT-0 — — — — —
11HT-1 56.1 62.1 67.8 58.9 57.9
11HT-2 45.6 63.6 70.5 77.3 78.5
11HT-3 48.3 63.6 68.7 78.8 76.3
11HT-4 60.8 78.7 88.9 94.4 92.8
The burning rate pressure exponents of different propellants at different pressure range were
shown in Table 3. The burning rate pressure exponent of 11HT-1 was the lowest at the pressure range
of 10 ~ 13 MPa. The pressure exponents all increased when adding the different aluminum
nanopowders, and the pressure exponents of propellants containing coated aluminum nanopowders
were higher than those of the untreated aluminum nanopowders at the pressure range of 4 ~ 7 MPa.
But it is different at high pressure. The pressure exponent of 11HT-3 can decrease to 0.36 lower than
the 11HT-1 (0.42). The pressure exponent of 11HT-4 can also decrease to 0.40 for the combustion
catalysis of nickel acetylacetonate and the exothermic reaction of the Al and F at high pressure[27-29]
. Table 3 Burning rate pressure exponent at different pressure range
Samples Pressure exponent (n) at different pressure range
4 ~ 7 MPa 7 ~ 10 MPa 10 ~ 13 MPa 13 ~ 15 MPa
11HT-0 0.35 0.42 0.36 0.46
11HT-1 0.42 0.53 0.14 0.42
11HT-2 0.56 0.55 0.50 0.51
11HT-3 0.52 0.52 0.57 0.36
11HT-4 0.54 0.59 0.46 0.40
2.3. Burning flame profiles
Flame profile is an important indicator of the combustion mechanism of the propellant. The length
and intensity of the flame, the dark zone and its thickness, and the morphology of the burning surface
can be observed from the flame picture. Fig. 4 shows the flame photographs of four typical composite
solid propellants containing different coated aluminum nanopowders at pressures of 1 MPa and 4
MPa. The splash of aluminum particles is not observed, and the propellant combusts stably. The
flame is continuous and not some large granular lumps in the flame, indicating that the 6propellant
combust fully. It also can be seen from the Fig. 4 that the dark zone of the samples is eliminated
almost completely and the luminous flame approaches the burning surface.
Advanced Materials Research Vol. 924 205
11HT-1 (1 MPa) 11HT-2 (1 MPa) 11HT-3 (1 MPa) 11HT-4 (1 MPa)
11HT-1 (4 MPa) 11HT-2 (4 MPa) 11HT-3 (4 MPa) 11HT-4 (4 MPa)
Fig.4 Flame photographs of four typical composite solid propellant containing different coated
aluminum nanopowders at pressures of 1 MPa and 4 MPa
As shown in Fig. 4, the flame shape exists large difference to different propellants. The flame of
11HT-1 and 11HT-2 samples showed radialized at the pressure of 1 MPa. On the one hand, the
gas-phase zone is far from the burning surface at low pressure, the heat feedback is decreased. The
heat of the burning surface is uneven. It makes the burning rate not be uniform at different surface
districts. On the other hand, the diameter of aluminum powder used in propellant is nanoscale. It is
easily to agglomerate for high specific surface area and difficult to uniformly dispersed in the
propellant. So the burning rate is not uniform. The burning flame of 11HT-3 and 11HT-4 samples is
relatively concentrated, approximately cylindrical cone, and flame length is relatively longer, flame
brightness is higher, and the flame structure is denser. This showed that the combustion efficiency of
aluminum particles was increased and propellant combustion is more fully. The flame profile of the
propellant at the pressures of 4 MPa is significantly different at the 1 MPa. The flame is brighter and
the flame zone closer to the propellant burning surface at the pressure of 4 MPa. The flame length is
longest and burning surface uniformity.
The brightness of the flame center region can directly reflect the effect of the propellant heat
release and heat release rate. The flame brightness of different propellants under different pressures is
not the same. Overall, the flame brightness is increased with the pressure increasing. This mainly
because that the gas-phase zone is more closer to the burning surface, and the heat flux transferred
back from the gas phase and the heat of reaction at the burning surface are increased. Those promote
the solid phase thermal decomposition, so the combustion zone brightness is higher. Compared with
the propellant sample containing untreated aluminum nanopowders (11HT-1), the flame center zone
brightness of propellant containing nmAl+PA (11HT-3) and nmAl+NA (11HT-4) is brighter under 4
MPa, and the brightness of 11HT-4 is the brightest. This maybe because the nickel acetylacetonate
can be used as combustion catalyst and accelerates the solid phase decomposition of the composite
solid propellant. The perfluorotetradecanoic acid can take strong exothermic reaction with aluminum.
Those make the flame temperatures and brightness of 11HT-3 and 11HT-4 increase.
206 Frontiers in Micro-Nano Science and Technology
2.4. Combustion wave structure and flame temperature distribution
A schematic representation of the combustion wave structure of a typical solid propellant is shown
in Fig. 5. It is shown from the Fig.5 that the combustion wave structure consists of three main zones:
condensed-phase zone (zone I), condensed-phase reaction zone (zone II), gas-phase zone (zone III)
which can be divided into gas-phase diffusion reaction zone and luminous flame zone. In zone I, no
chemical reactions occur and the temperature increases from the initial temperature (T0) to the
decomposition temperature (Tu), through conductive heat feedback from the burning surface. In zone
II, in which there is a phase change from solid to liquid and/or to gas and reactive gaseous species are
formed in endothermic or exothermic reactions, the temperature increases from Tu to the burning
surface temperature (Ts). Since the condensed phase reaction zone is very thin (<0.1mm), Ts is
approximately equal to Tu. In gas-phase diffusion reaction zone, in which exothermic gas-phase
reactions occur, and the temperature increases rapidly from Ts to the combustion flame temperature
(Tf). In luminous flame zone, the exothermic reaction terminates and the final combustion products
are formed, and the temperature reaches the maximum.
Fig.5 Schematic representation of the combustion wave structure of a typical solid propellant
Temperature profiles in combustion wave of four different propellants at the pressures of 4 MPa
and 7 MPa were shown in Fig. 6. According to Fig. 6, the temperature in the solid phase increases
rapidly from T0 to the Ts. The AP and HTTP directly change from solid phase to gas phase by the
thermal decomposition or sublimation. In the burning surface, the AP and HTPB do not pre-mixed,
but escape from the burning surface as a kind of “airbag”. Then the decomposition products of AP
take exothermic reaction in the gas phase near the burning surface, formed the AP premixed flame
and oxygen-enriched air flow. In the gas phase far away from the burning surface, the gas
decomposition products of AP and HTTB took place diffusion combustion with aluminum, generated
the final combustion products and released a lot of heat.
14400 14500 14600 14700 14800 14900 15000 15100 15200
0
500
1000
1500
2000
2500
3000
Tf
Ts
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm
4 MPa
21500 21600 21700 21800 21900 22000 22100
0
500
1000
1500
2000
2500
3000
7 MPa
Ts
Tf
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm 11HT-1 (4 MPa) 11HT-1 (7 MPa)
Advanced Materials Research Vol. 924 207
18800 18900 19000 19100 19200 19300 19400 19500 19600
0
500
1000
1500
2000
2500
3000
Ts
Tf4 MPa
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm 12000 12100 12200 12300 12400 12500 12600 12700 12800
0
500
1000
1500
2000
2500
3000
7 MPa
Ts
Tf
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm 11HT-2 (4 MPa) 11HT-2 (7 MPa)
14500 14520 14540 14560 14580 14600 14620 14640 14660 14680
0
500
1000
1500
2000
2500
Tf
Ts
4 MPa
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm
12020 12040 12060 12080 12100 12120 12140 12160 12180 12200
0
500
1000
1500
2000
2500
7 MPaTf
Ts
T /
℃℃℃℃
λ / µλ / µλ / µλ / µm 11HT-3 (4 MPa) 11HT-3 (7 MPa)
11650 11700 11750 11800 11850 11900 11950 12000 12050 12100
0
500
1000
1500
2000
2500
Ts
Tf4 MPa
T / ℃℃℃℃
λ / µλ / µλ / µλ / µm
13400 13450 13500 13550 13600 13650 13700 13750
0
500
1000
1500
2000
2500
3000
Tf
Ts
7 MPa
T / ℃℃℃℃
λ / µλ / µλ / µλ / µm 11HT-4 (4 MPa) 11HT-4 (7 MPa)
Fig. 6 Temperature profiles of 11HT-4 at the pressures of 4 MPa and 7 MPa Table 4 is the combustion wave structure parameters and burning rate of four different propellants
at the pressure of 4 MPa and 7 MPa. Where δ is the thickness of the gas-phase diffusion reaction zone. Table 4 Combustion wave structure parameters and burning rate of four different propellants at the
pressure of 4 MPa and 7 MPa
Samples δ(µm) Ts(°C) Tf(°C)
4 MPa 7 MPa 4 MPa 7 MPa 4 MPa 7 MPa
11HT-1 185.62 109.61 370.3 463.5 2335.3 2389.2
11HT-2 170.90 179.60 392.3 490.9 2389.2 2407.2
11HT-3 27.72 41.94 460.3 485.0 2137.7 2173.6
11HT-4 117.58 84.12 718.6 697.1 2389.2 2385.0
It can be seen from Table 4, the combustion flame temperature (Tf) of four different propellants at
the pressure of 4 MPa and 7 MPa are almost same. This indicates that the surface coating of aluminum
nanopowders has little effect on the combustion flame temperature of solid propellant. The burning
surface temperature (Ts) increases with the pressure increasing as a result of the flame zones more
closer to the burning surface of the solid propellant under high pressure. The heat feedback from the
flame zone to the burning surface increase, so that the burning surface temperature increases.
208 Frontiers in Micro-Nano Science and Technology
The thickness of the diffusion reaction zone (δ) has no significant change at different pressures.
But the δ of samples contained different coated aluminum nanopowders are less than the samples
contained untreated aluminum powders. To the 11HT-3 sample, the δ under different pressures is
minimum with the decomposition of perfluorotetradecanoic acid on the surface of aluminum
nanopowders at high temperature. The decomposition products are fluorinated compounds, which
may react with the aluminum. Because the oxidative ability of fluorine is stronger, the react of
aluminum and fluorinated compounds is easier, the reaction time is shorter. Those make the diffusion
reaction zone thickness increasing. It is interesting to note that the pressure has an obvious influence
on the δ of 11HT-4 sample which contained aluminum nanopowders coated by nickel acetylacetonate.
The possible reason was that the nickel acetylacetonate has catalytic action to the HTTB propellant.
The nickel acetylacetonate was more closer to the burning surface at high pressure. This may increase
the catalytic activity and accelerate the decomposition of propellant. The reaction rate was faster so
that the δ increases.
It is essential that maximal measured temperature of burning surface at all studied pressures. It can
be seen from the Table 4 and Fig. 3 that there is a general trend, but a little discrepancies. 11HT-4 has
the highest burning surface temperature, but its burning rate is same with 11HT-3 at the pressure of 7
MPa. It appears that burning rates are dependent on many phenomena, maybe because the reaction of
aluminum and fluorine was far away from the burning surface, but the aluminum content of
nmAl+PA was higher than the nmAl+NA, and finally cause the exothermic value of nmAl+PA was
higher.
3. Conclusions
In this study, combustion characteristics of composite solid propellants containing different coated
aluminum nanopowders have been investigated. Main conclusions, which can be marked from this
work, are as follows:
(1) Due to the OA and PA are carboxylic acid which can react with the aluminum and form
chemical bonds, but the NA only adsorbed on the aluminum nanopowder surface by physical
adsorption which caused the coating was incomplete. The coating effect of OA and PA were better
than the NA.
(2) The burning rates of propellant sample containing nmAl+NA are the highest at different
pressure, the maximum burning rate is up to 26.13 mm·s-1
at 15 MPa. The burning rates of propellant
samples containing OA and PA are almost the same at different pressures, and higher than the
propellant samples containing untreated aluminum nanopowders only at the pressure range of 10 ~ 15
MPa.
(3) The flame brightness of different propellants under different pressure is not the same. The
flame brightness is increased with the pressure increasing. The flame center zone brightness of
propellant containing nmAl+PA (11HT-3) and nmAl+NA (11HT-4) is brighter under 4 MPa, and the
brightness of 11HT-4 is the brightest.
(4) The aluminum nanopowders coated with different materials have little effect on the
combustion flame temperatures of solid propellants. The burning surface temperature (Ts) increases
with the pressure increasing.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21173163)
and Science and Technology Foundation of Combustion and Explosion Laboratory in China (No.
9140C3503141006). The authors wish to express their gratitude to Ms. Wang Ying and Ms. Chen
Xue-Li for their help in combustion experiments.
Advanced Materials Research Vol. 924 209
References
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Nanopowders 10.4028/www.scientific.net/AMR.924.200
DOI References
[1] P. Brousseau, C.J. Anderson, Nanometric Aluminum in Explosives, Propellants, Explos., Pyrotech.
27(2002)300-306.
http://dx.doi.org/10.1002/1521-4087(200211)27:5<300::AID-PREP300>3.0.CO;2-# [7] M.L. Pantoya, J.J. Granier, Combustion behavior of highly energetic thermites: nano versus micron
composites, Propellants, Explos., Pyrotech. 30(2005) 53-62.
http://dx.doi.org/10.1002/prep.200400085
