Characterization of the aluminum/potassium chlorate mixtures by simultaneous TG-DTA

Article abstract of DOI:10.1007/s10973-005-7008-x

Thermogravimetry (TG) and differential thermal analysis (DTA) in the non-isothermal mode have been used to examine the thermal behaviour of the micron sized aluminum (Al) powder/potassium chlorate pyrotechnic systems in air in relation to the behaviour of the individual constituents. The effects of different parameters of Al powder such as particle size and its content in the mixtures on their thermal property were investigated. The results showed that the reactivity of Al powder in air increases as the particle size decreases. Also it was found that neat Al with 5 μm particle sizes (Al₅) has a fusion temperature of about 647°C that for 18 μm powder (Al ₁₈) is 660°C. Pure potassium chlorate has a fusion temperature around 356°C and decomposes at 472°C. DTA curves for Al ₅/KClO₃ (30:70) mixture showed a maximum peak temperature for the ignition of mixture at 485°C. Also by increasing the particle size of Al powder the ignition temperature of the mixture increased. On the other hand the oxidation temperature increased by enhancing the Al content of the mixtures. In this particular study we observed that the width of reaction peak for the mixtures corresponds to their Al contents of samples.

Full text of DOI:10.1007/s10973-005-7008-x

Journal of Thermal Analysis and Calorimetry, Vol 84 (2006) 3, 557–561
CHARACTERIZATION OF THE ALUMINUM/POTASSIUM CHLORATE
MIXTURES BY SIMULTANEOUS TG-DTA
S. M. Pourmortazavi*, S. S. Hajimirsadeghi and S. G. Hosseini
Faculty of Material and Chemical Engineering, Malek Ashtar University of Technology, P. O. Box 16765-3454, Tehran, Iran
Thermogravimetry (TG) and differential thermal analysis (DTA) in the non-isothermal mode have been used to examine the ther-
mal behaviour of the micron sized aluminum (Al) powder/potassium chlorate pyrotechnic systems in air, in relation to the behaviour
of the individual constituents. The effects of different parameters of Al powder, such as particle size and its content in the mixtures,
on their thermal property were investigated. The results showed that, the reactivity of Al powder in air increases as the particle size
decreases. Also, it was found that neat Al with 5 mm particle sizes (Al5) has a fusion temperature of about 647°C, that for 18 mm
powder (Al18) is 660°C. Pure potassium chlorate has a fusion temperature around 356°C and decomposes at 472°C. DTA curves for
Al5/KClO3 (30:70) mixture showed a maximum peak temperature for the ignition of mixture at 485°C. Also, by increasing the parti-
cle size of Al powder, the ignition temperature of the mixture increased. On the other hand, the oxidation temperature increased by
enhancing the Al content of the mixtures. In this particular study, we observed that the width of reaction peak for the mixtures corre-
sponds to their Al contents of samples.
Keywords: aluminum powder, ignition temperature, particle size effect, potassium chlorate, pyrotechnic, thermal behaviour
Introduction
of Al have considerably enhanced burning rates over
those are free of Al powder [13]. This paper presents the
results of study on the potassium chlorate/Al powder
mixtures using simultaneous thermogravimetry-differ-
ential thermal analysis (TG/DTA). The aim of this study
was investigation of the effect of Al content and its par-
ticle size on the thermal behavior of different composi-
tions of Al and potassium chlorate. Thermal behaviour
of neat micro-sized Al and its composition with explo-
sives have been reported previously [14, 15], but to the
best our knowledge nothing has been published on the
thermal behaviour of Al+potassium chlorate.
The decrease in the size of energetic materials, e.g.,
metal fuels, oxidizers and explosives, leads to an in-
crease of the reaction surface area, which considerably
enhances the combustion rate of propellant composi-
tions and detonation properties of explosives. The
crystal grid defects considerably accelerate the process
of the thermal decomposition of energetic materi-
als [1–3]. One of the most important types of defects –
dislocations – in a great extent defines the mechanical
and physical properties of crystalline materials. The
chemical reactions on dislocations proceed much faster
than on an ideal crystal. Very fine crystalline materials
are known of having a high defect concentration,
which is along with the large surface area, the reason to
expect the higher chemical activity of very fine materi-
als in comparison to the conventional compounds.
Potassium chlorate and potassium perchlorate have
been the main oxidizer in many compositions of propel-
lants and pyrotechnics [4–8]. Although they gives virtu-
ally smokeless products of combustion, their use in solid
pyrotechnics systems have been interested because of
their mild phase transition involving a volume change in
low temperature and their fast ignitability.
Ignition or initiation refers to the point during a
chemical reaction at which the rate of heat generation
exceeds the required input to sustain the reaction, that is
the point at which the reaction becomes self-sustaining.
The temperature at which this occurs depends on many
variables such as sample mass, geometry, atmosphere
and heating rate, so that in most cases it is difficult to
measure precisely [16, 17]. In this study as many of
these variables as possible were held constant in order to
facilitate comparisons that reflect on the particle size
and content of Al in the interested mixtures.
Very fine Al powder possess unique thermal be-
haviour, which renders it useful as a fuel in both propel-
lant and explosive formulations. It also has potential ap-
plications in pyrotechnic delays, flares and heat generat-
ing devices [9–12]. Propellants containing fine powders
Experimental
Materials
Potassium chlorate (mesh 300) was purchased from
Merck (Tehran-Iran). Neat Al powders 5 mm (Al5)
*
Author for correspondence: pourmortazavi@yahoo.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

POURMORTAZAVI et al.
Table 1 Summary of tested sample’s compositions and TG/ DTA results
Transition temperature/°C
decomposition
Sample No.
Component
Composition
fusion
T*
1
2
3
4
5
6
7
Al5
100
100
647
660
356
361
367
366
372
–
484–603 (increase)
490–610 (increase)
472–500 (decrease)
485–550 (decrease)
493–580 (decrease)
535–575 (decrease)
550–600 (decrease)
Al18
–
KClO3
100
472
485
493
535
550
Al5/KClO3
Al5/KClO3
Al18/KClO3
Al18/KClO3
30/70
50/50
30/70
50/50
T* is the temperature when there is a variation in the sample’s mass
and 18 mm (Al18) particles sizes were prepared from
metals laboratory of Malek Ashtar University of
Technology. Four Al/KClO3 mixtures, as well as pure
potassium chlorate, Al5 and Al18 were examined. Ta-
ble 1 summarizes the tested samples compositions.
Before testing, all samples were stored in a vacuum
oven at 65°C for at least three hours. The mixtures
were prepared by well mixing of fine Al powder and
potassium chlorate powder.
Potassium chlorate
The DTA and TG curves for pure potassium chlorate
are shown in Fig. 2. No thermal event observed prior
to the melting point near 356ºC. At this point (356ºC),
potassium chlorate undergoes a sharp endothermic
phenomenon containing melting, without any change
in the mass of sample. As shown in the DTA curve in
Fig. 2, there is a long temperature interval of 115ºC
between the melting (356ºC) and the first decomposi-
tion step (472ºC) of potassium chlorate. On the other
hand, TG curve for potassium chlorate showed con-
siderable decreasing in the sample mass (approxi-
mately 40%) in temperature range between
472–500ºC. Therefore, this behavior would suggest
that, pure potassium chlorate is kinetically stable at its
melting point [19]. After the complete decomposition
of the sample, oxygen and potassium chloride pro-
duced [7, 20]. This statement agrees with TG curve of
this sample in Fig. 2.
Procedure
A thermobalance (Stanton, model TR-01, sensitivity
0.1 mg) with a differential thermal analysis attach-
ment (STA 1500) was used for TG/DTA studies of
pyrotechnic mixtures. Approximately, 4.0 mg of sam-
ple and reference (Pt foil) were placed in alumina
pans and heated 10ºC min–1 from 30 to 800ºC. In this
study, the flow rate of purge gas (air) was 10ºC min–1
at 1 bar. TG mass, DTA baseline and temperature cali-
brations were performed prior to the experiments [18].
Binary mixtures
Al5/potassium chlorate mixtures
Results and discussion
The TG and DTA curves for Al5/KClO3 mixture
(30:70) are shown in Fig. 3a. The DTA curve showed
an endothermic peak maximum at about 361ºC,
which corresponded well with the temperature of fu-
sion of the potassium chloride eutectic. On the other
hand, a sharp exothermic occurs near 530ºC and a
mild reduction in the mass of sample that it corre-
sponds to the reaction of Al powder with potassium
chlorate according following equation:
Thermal behaviors of the individual components
Aluminum powders
Al5 and Al18 were examined in air using simultaneous
TG/DTA to assess its oxidation characteristics. The
TG/DTA results for the two different Al powders in
air are shown in Fig. 1. The powders exhibit an exo-
thermic peak, which is below the melting point of Al.
The first mass gain for Al18, Dm1=12.5%, was ob-
served at the temperature interval 490–610ºC. How-
ever, this range (Dm1=15.4%) for Al5 was 484–603ºC.
It means that Al5 has a higher reactivity rather than
Al18 powder. Also, it has a lower melting point
(647ºC) rather than Al18 (~660ºC).
2Al+KClO3®Al2O3+KCl
Figure 3b shows the TG/DTA results for
Al5/KClO3 with mass ratio of 50:50. An endotherm
was observed at an onset temperature of 367ºC which
is due to the fusion of potassium chlorate. Also, an
exotherm with a limited decreasing in the mass was
occurred at maximum peak temperature 543ºC, which
558
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CHARACTERIZATION OF THE ALUMINUM/POTASSIUM CHLORATE MIXTURES
Fig. 2 TG and DTA curves for potassium chlorate (sample
mass 4.0 mg; heating rate 10°C min–1; air atmosphere
and potassium chlorate with mesh 300)
been observed. Consequently, it can be related to the
reaction between Al powder and potassium chlorate.
Figure 4b shows the TG and DTA curves for
Al18/KClO3 (50:50) mixture. As seen in this figure, the
potassium chlorate has fusion temperature of 372ºC.
There is also a complicated exotherm at about 580ºC
with a fall in the mass of sample corresponding to the
Al/potassium chlorate reaction. These results showed
that the ignition temperature of these mixtures in-
creased when higher content of Al powder were used.
Comparison of mixtures
As can be seen from the Table 1 and Figs 3 and 4, the
particle size of Al powder and the content of Al in the
mixture could affect the fusion temperature of potas-
sium chlorate. The larger particle size of Al and the
more content of this powder in the mixture increase the
fusion temperature of potassium chlorate. Also, in-
creased Al content of the mixtures, will enhance the ig-
nition temperature of the mixture. On the other hand, the
width of ignition peak for the combustion reaction of
Al+potassium chlorate varies over a wide range depend-
ing on the Al content of the mixture. By increasing the
amount of Al in the mixture (from 30 to 50%), the wide
of ignition reaction peak will increase. This phenome-
non was observed, because increasing the amount of Al
powder in the mixture will increase the amount of its
product (Al2O3) in the reaction. Due to the higher heat
capacity of Al2O3 in comparison with other components
presented in the reaction, the heat in the cell area will be
absorbed by Al2O3. This process could produce wide
peak in comparison with other mixtures that have a
lower amount of Al2O3 in their products.
Fig. 1 TG and DTA curves for neat Al powder a – Al5 and
b – Al18 (sample mass 4.0 mg; heating rate 10°C min–1;
air atmosphere)
corresponds with the reaction between Al and potas-
sium chlorate. As the results showed that the decom-
position temperature of these mixture varied as a
function of the amount of Al contained in the sample.
As shown in Table 1 and Fig. 3, by using a higher
content of the Al powder in the mixture, the decom-
position temperature of the mixture increases.
Al18/potassium chlorate mixture
TG/ DTA data for Al18/KClO3 mixtures are presented
in Fig. 4. As can be seen in Fig. 4a, there are two ther-
mal transitions, one is endothermic and another is exo-
thermic. The endotherm with maximum 366ºC is in
good agreement with the fusion point of pure KClO3.
Another is exothermic peak on the DTA curve with its
maximum at 565ºC, where the stage of mass losses has
J. Therm. Anal. Cal., 84, 2006
559

POURMORTAZAVI et al.
Fig. 3 TG and DTA curves for mixture of Al5 and potassium
Fig. 4 TG and DTA curves for mixture of Al18 and potassium
chlorate with mass ratio a – 30:70 and b – 50:50 (sam-
ple mass 4.0 mg; heating rate 10°C min–1; air atmo-
sphere and potassium chlorate with mesh 300)
chlorate with mass ratio a – 30:70 and b – 50:50 (sam-
ple mass 4.0 mg; heating rate 10°C min–1; air atmo-
sphere and potassium chlorate with mesh 300)
Conclusions
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