Effect of heating rates on TG-DTA results of aluminum nanopowders prepared by laser heating evaporation

Article abstract of DOI:10.1007/s10973-008-9374-7

Aluminum (Al) nanopowders with mean diameter of about 50 nm and passivated by alumina (Al₂O₃) coatings were prepared by an evaporation route: laser heating evaporation. Thermal properties of the nanopowders were investigated by simul

Full text of DOI:10.1007/s10973-008-9374-7

Journal of Thermal Analysis and Calorimetry, Vol. 96 (2009) 1, 141–145
EFFECT OF HEATING RATES ON TG-DTA RESULTS OF ALUMINUM
NANOPOWDERS PREPARED BY LASER HEATING EVAPORATION
1
L. Chen , W. L. Song , J. Lv , L. Wang and C. S. Xie
1,2,*
1
2
1
1
State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology,
Wuhan 430074, P.R. China
2
Analytical and Testing Center, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
Aluminum (Al) nanopowders with mean diameter of about 50 nm and passivated by alumina (Al
an evaporation route: laser heating evaporation. Thermal properties of the nanopowders were investigated by simultaneous
thermogravimetric-differential thermal analysis (TG-DTA) in dry oxygen environment, using a series of heating rates (5, 10, 20,
2 3
O ) coatings were prepared by
–
0, 50 and 90°C min ) from room temperature to 1200°C. With the heating rates rise, the onset and peak temperatures of the oxi-
1
3
dation rise, and the conversion degree of Al to Al O varies. However, the specific heat release keeps relatively invariant and has
2 3
–
1
an average value of 18.1 kJ g . So the specific heat release is the intrinsic characteristic of Al nanopowders, which can represent
the ability of energy release.
Keywords: aluminum, energetic materials, nanopowders, reactivity, TG-DTA
Introduction
understand the relationship between thermal behavior
and heating rate applied on TG-DTA.
Aluminum (Al) powders are commonly added to
propellants, explosives and pyrotechnics compositions
to add energy to the burning reaction in propellants
and to enhance the blast effect of explosives, as well
as their underwater performance. Conventional use of
Al powders are typically micron-sized, however, the
usual problems of using them are agglomeration,
uncompleted degree of oxidation and two-phase losses
Al nanopowders can be prepared using a variety
of techniques, including electro-exploded wire [12, 13],
plasma synthesized process [14], sol–gel [15] and heating
evaporation [16]. The heating evaporation methods
based on vaporization of metals followed by condensation
and particle formation have the ability to produce Al
nanopowders below 100 nm or even below 50 nm.
These evaporation methods distinguish themselves by
the heating apparatuses, such as electric cooker,
high-frequency induction and laser. A high-frequency
[1]. Recently, Al nanopowders have attracted
widespread interest to solve the problems their micro-
counterparts meet due to their unusual energetic
properties [2, 3]. These are associated with higher
specific surface area, higher reactivity, potential
ability to store energy in surface defects, and lower
melting points [4–6].
power and high-power CO laser were combined as
2
heating source in the laser heating evaporation method
to prepare Al nanopowders with mean particle diameter
of 50 nm. Then a series of heating rates were applied
in the simultaneous TG-DTA to investigate the thermal
behavior of Al nanopowders, and then thermal
parameters for the estimation of the reactivity of Al
nanopowders were discussed.
Characterization of Al nanopowders including
the particle diameters and morphology, oxide layer
thicknesses, thermal behavior is important in predicting
performance for its applications. Among them, the
thermal behavior which can be applied in the evaluation
of the reactivity of Al nanopowders is the main
characteristic for the application in explosives and
propellants. As for the instruments for thermal
analysis, simultaneous thermogravimetric and differential
thermal analysis (TG-DTA) is a convenient and useful
one [7, 8]. However, TG-DTA results vary under
different heating rates for a certain type of Al
powders [9–11]. Therefore, more work is needed to
Experimental
Materials preparation
Al nanopowders were prepared using an evaporation
route: laser heating evaporation. As illustrated in Fig. 1,
the bulk pure Al (99.6%) was put in a crucible. The
chamber was evacuated below 10 Pa and maintained
at 2 kPa argon pressure. Then the high-frequency
induction power of 5 kW was initiated, and 5 min later,
*
Author for correspondence: wulins@126.com
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2009 Akadémiai Kiadó, Budapest

CHEN et al.
connected to a computer, using the Muse version 5.4U
thermal analysis system. The used crucibles were of
0 mL alumina. The samples were heated at different
7
–
1
rates of 5, 10, 20, 30, 50 and 90°C min from room
temperature to 1200°C and all the experiments were
carried out under dry oxygen (99.995%) atmosphere,
–
with a fixed gas flow of about 20 mL min .
1
Results and discussion
The general morphologies and structures of Al
nanopowders are shown in Figs 2a and b. The low
magnification of TEM image Fig. 2a reveals that the
produced nanopowders are well dispersed particles
with the shape of uniformed sphere and a mean
diameter of 50 nm. The high resolution TEM image
Fig. 2b shows that the particle has a structure of
Fig. 1 Schematic diagram of the equipment applied in
laser-induction complex heating evaporation
the bulk pure Al was molten. Sequentially, continuous
core-shell: the shell part alumina (Al
O ) coats the
2 3
wave CO
2
laser irradiation (with a maximum output
core Al tightly. The shell part Al O with a thickness
2 3
of 2 kW) with the power of 1.8 kW was incorporated
into the system. After the focused laser beam (4 mm
in diameter) radiated on the Al liquid for about one
minute, a plasma arc was ignited and rapidly propagated
in the reverse direction to the laser beam and in the
transverse direction. The plasma expands cools and
condenses into clusters, which continue to migrate
away from target Al surface until they are collected
on the water-cooled wall of the chamber.
of about 3 nm was formed in the air passivation
process to protect the core Al from intensive oxidation
or even spontaneous combustion.
The XRD patterns of air-passivated Al nanopowders
are shown in Fig. 3a. The strong diffraction peaks can
be indexed to metallic Al and the weak ones correspond
to Al
2
O
3
, which means the sample mainly contains
, which is in
metallic Al and a small quantity of Al
O
3
2
good agreement with the HRTEM result of the
core-shell structure. No diffraction peaks from other
impurities were detected in the sample, which means
the high purity of the Al nanopowders.
The reaction lasted for about 15 min, and then
the powers were turned off, simultaneously gas mixture
of argon and air (volume ratio about 20:1) was added
into the chamber maintaining a pressure of about
Thermal properties of the Al nanopowders
investigated by simultaneous thermogravimetric-differential
thermal analysis (TG-DTA) under a series of heating
rates are revealed in Figs 4a and b. The mass of all the
samples added into the alumina crucible is of fixed
quantification of 1.2±0.1 mg. Seen from the figures,
the shape of all the curves can be classified into two
3
20 Pa to passivate Al nanoparticles. Passivating of
about 10 h later, the resulting dark gray, air passivated
Al nanopowders (about 3.0 g) were collected for
further testing and analysis.
Microstructures and thermal analysis
–
types: one is the low heating rates (5, 10, 20°C min ,
1
The phases and crystal structures of the powders were
analyzed with XRD (PANalytical B.V. X’Pert PRO)
using CuKa radiation (l Ka 1=0.15406 nm), generating
voltage 40 kV and current 40 mA. Spectra were
collected at a step size of 0.0167° and were scanned
from 10 to 90°. The morphology and internal structure
of Al nanopowders were investigated using TEM on a
Tecnai G2 20 (accelerating voltage 180 kV) and
HRTEM on a JEM-2100FEL.
Fig. 4a), the other is the high heating rates (30, 50,
–
90°C min , Fig. 4b). There is a mass loss (2–5%) in
1
each TG curves from room temperature to about
The thermal properties of the as-prepared
powders were tested by TG-DTA with a Perkin Elmer
TG/DTA 6300 thermal analysis system, which has a
–1
maximum heating rate of 100°C min . The equipment
consists of a pair of horizontal electrobalances, with a
.1 mg of resolution, a furnace and sensors of temperature,
Fig. 2 a – A TEM image of the Al nanopowders; b – An
0
HRTEM image of a single particle
142
J. Therm. Anal. Cal., 96, 2009

ALUMINUM NANOPOWDERS PREPARED BY LASER HEATING EVAPORATION
Fig. 4 TG-DTA curves of the Al nanopowders under different
–1
heating rates (dry oxygen 20 mL min ): a – 5, 10,
–1
–1
20°C min ; b – 30, 50, 90°C min
mass gain stage begins at 450°C and finishes at 600°C.
The second one ranges from 750 to 900°C. This
two-stage mass gain phenomenon can be explained by
the mechanism of low temperature oxidation of
aluminum nanopowders. The low temperature oxidation
of Al particles occurs at least in two steps [17], as
illustrated in Fig. 5. The first step, dominated by chemical
Fig. 3 Results of X-ray diffraction for the powders: a – the
as-prepared Al nanopowders; b – collected powders
after TG-DTA analysis
4
00°C because of the outgassing of the absorbed air
(
carbon dioxide) and water vapor after preparation of
the nanopowders. A general tendency that with the
lower heating rates is the more mass loss can also be
observed, which means that lower heating rates are
propitious to outgas the absorbed substrates in the
surface of the nanopowders. Therefore, the mass gain
should be calculated from about 400°C after the finish
of outgassing. After the mass loss, there is a mass gain
in each TG curves especially when the temperature
reaches 500–550°C. An intensive mass gain is
observed from 500°C (550°C, high heating rates) to
kinetics, builds a layer (g-Al
1
2
O
3
and q-Al
0 nm thickness composed of crystallites of the same
O ) of 6 to
2 3
size independent on the initial particle size. The
second step combines diffusion and chemical reaction
and proceeds therefore slowly. When the temperature
reaches and exceeds 660°C, Al particles melt and
expand because the density of the core aluminum
–
3
–3
decrease from 2.7 kg m at solid-state to 2.3 kg m at
the current liquid state, when Al melts it would
expand by 12%. The outer Al shells are in tension,
O
3
2
6
00°C because of the reaction between Al and O :
2
4
Al(s)+3O =2Al O
2 2
(1)
3
For the reaction between Al and O , the mass of
2
the samples would increase 1.125 times of the initial mass.
The exothermic reaction would occur simultaneously,
which can be evidenced by the intensive exothermal
peaks in DTA curves.
In the low heating rates, there are two intensive
mass gain stages in each TG curve. The first intensive
Fig. 5 A schematic representation of the two-stage oxidation
mechanism
J. Therm. Anal. Cal., 96, 2009
143

CHEN et al.
which is beneficial to the diffusion of the oxygen. The
heating rates, the onset and peak temperature of the
first extensive oxidation increase. In the low heating
rates, the maximum oxidation rates were calculated
from the TG curves. With the rise of heating rates, vox
increases, however, as the mass gain occurred transiently
in the high heating rates, the vox has a high or even in-
finite value (also can be seen from the tangent of TG
following oxidation step involves the diffusion of
oxygen to the metal core and the metal to the outer
surface, and the oxygen diffusion is the dominating
factor. Then the profile of diffusing oxygen from the
surface into the reaction zone front with the metallic
fuel is given by (solution of the radial diffusion equation):
curves). It is insignificant to calculate the v in high
ox
1
- R/r
cO =cO,R +(c - c
)
(2)
heating rates. The enthalpy change (DH) is read by the
Muse thermal analysis system from the peak area of
each DTA curve. DH is a parameter that reveals the
O,S
O,R
1- R/RS
c concentration, r – radius of Al core, R – radius of
heat release of the first oxidation stage. Dm or Dm
the mass percentum increase at the corresponding
temperature range in TG curves. The conversion of
the Al to Al ).
(a ) is 1.125 times of Dm (or Dm
1
is
reaction zone, O – oxygen, S – surface.
The increase of the c and tension of Al
contributes to the second intense mass gain from 750
to 800°C because of the following reaction:
O shell
2 3
O
O
2
3
1
From Fig. 3b, we can see that there is unreacted me-
tallic Al in the remained ash after TG-DTA, which
means the conversion of the Al is not complete. As
4
Al(l)+3O =2Al O
2 2 3
(3)
In the high heating rates, there is an obvious first
mass gain stage in each TG curve, however, the
second mass gain is not observed especially in 50 and
the heating rates increase, a
variant in some degree. From Table 1 and Fig. 4b, we
1
increase and a keeps in-
–
can also see that at the heating rate of 50 °C min the
1
–
1
9
0°C min . The absence of the second mass gain is
due to the more complete oxidation of Al powders in
the first oxidation stage or instantaneous combustion
because of the rupture of Al shell under the rapid
values of DH, Dm (or a ) and Dm (or a ) are lower
1
1
–
even than the values at the heating rate of 30°C min .
1
The reason to this phenomenon is that the initial mass
–
O
3
2
1
(
m
i
) added into the TG-DTA instrument at 50°C min
increase of high temperature. The relative high
temperature is beneficial for the chemical reaction
between Al and oxygen in the first oxidation stage.
Traditionally, the reactivity of Al powders depends
on the content of metallic aluminum in powders.
However, with the decrease in particle size, the metal
content of Al powders usually decreases but the oxidation
rate can increase, especially for the nanopowders.
Ilyin and Kwon [18, 19] established four parameters
obtained from TG-DTA (DSC) to estimate the reactivity
of Al powders: the onset temperature of oxidation
is 1.295 mg which is higher than 1.169 mg at
–
1
0°C min . More samples make the oxidation of Al
3
nanopowders more incomplete. But this phenomenon
can not disturb the tendency summarized above. The
last datum listed in the table (S/Dm*) is a parameter of
specific heat release that can determine the ability of
energy release. Specific heat release (S/Dm*) means
the release of heat per g of Al because of oxidation.
Though many parameters of TG-DTA are different
with each other under different heating rates, the spe-
cific heat release keeps relatively invariant and has an
–
(Ton, °C), the maximum oxidation rate (vox, mg s ),
1
–
1
average value of 18.1 kJ g . Therefore it is necessary
to establish a reasonable heating rate to obtain the
parameters to estimate the reactivity of Al
nanopowders; however, the specific heat release is a
the degree of conversion of aluminum in a specified
temperature range (a, %), the specific heat release (S/Dm).
The TG-DTA results for the as-prepared Al nanopowders
under different heating rates are summarized in Ta-
ble 1. From Table 1, we can see that with the rise of
Table 1 Comparison of TG-DTA results for Al nanopowders under different heating rates
–1
a
b
–1
–1
c
d
–1e
Rates/°C min
Ton/°C
Tpeak/°C
v
ox/mg s
DH/kJ g
Dm/%
Dm
1
/%
S/Dm*/kJ g
5
0
0
0
0
0
493
529
556
566
560
564
541
556
578
571
620
622
2.27
3.714
3.548
4.219
6.738
6.338
7.467
37.4
40.1
38.9
40.4
38.0
44.0
17.5
16.8
22.0
34.9
30.4
36.4
18.86
18.77
17.05
17.16
18.53
18.23
1
2
3
5
9
2.44
14.27
/
/
/
a
Onset temperature at which a deflection from the established baseline of DTA is observed.
b
Peak temperature corresponding to a peak value of DTA.
c
Total mass percentum gain from lowest mass to the end point of the TG curves.
d
Mass gain percentum for first intensive mass gain obtained between about 400 and 600°C.
e
S/Dm*=DHmi/1.125Dm
1
i 1 i
m =DH/1.125m , m : initial mass of the tested samples.
144
J. Therm. Anal. Cal., 96, 2009

ALUMINUM NANOPOWDERS PREPARED BY LASER HEATING EVAPORATION
characteristic parameter for a certain type of Al
powders.
Heating rate is an important factor in the thermal
Testing Center, Huazhong University of Science and
Technology for the measurements and analysis.
calorimetry analysis and there is always a proper
heating rate for a specific analyte in a certain calorimetry.
Generally, high heating rate benefits the improvement
of the sensitivity, while low heating rate helps to
improve the resolution of the measurement. As for Al
powders which have a special application in energetic
materials, such as propellants, explosives or pyrotechnics,
have a characteristic of instantaneous combustion in
several seconds. So when we investigate or simulate
the actual situation of combustion in propellants, the
higher heating rates could be applied, however, as we
investigate the thermal behavior of Al powders, the
lower ones would be proper. From the results above
we can also find that a fixed heating rate regardless of
high or low should be applied to compare the thermal
properties of different types of Al powders.
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8
9
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a uniformed spherical shape, core-shell structure and a
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applied on TG-DTA, the shape of all the curves can
be classified into the low heating rates (5, 10,
9
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1
9
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heat release keep invariant. The specific heat release
that means the ability of energy release when oxidized
is a characteristic parameter for a certain type of Al
powders. When estimate the reactivity of Al
nanopowders, it is necessary to establish a proper
heating rate and make it fixed.
Received: June 20, 2008
Accepted: December 16, 2008
Acknowledgements
DOI: 10.1007/s10973-008-9374-7
This work was funded by the National Natural Science
Foundation of China. We would like to thank Analytical and
J. Therm. Anal. Cal., 96, 2009
145