Thermal stability of habit modified ammonium nitrate: Insights from isoconversional kinetic analysis

Article abstract of DOI:10.1016/j.tca.2011.07.009

Compared to the conventional ammonium perchlorate based solid propellants, burning of ammonium nitrate (AN) based propellants produce environmentally innocuous combustion gases. However, the near room temperature phase transition limits the application of AN as a propellant ingredient. A method to produce phase modified ammonium nitrate (PMAN) was reported by means of habit modification route using potassium ferrocyanide. In the present study, the thermal stability and moisture absorption characteristics of PMAN were evaluated to understand the viability of its application in the environmentally benign propellant systems. The energy of activation of decomposition reaction of AN, PMAN and an admixture of AN-potassium ferrocyanide trihydrate were computed by Friedman's differential and Vyazovkin's non-linear integral isoconversional methods and used to ascertain their thermal stability. The moisture absorption characteristics were determined at 53 and 62% RH and the cyclic DSC analysis, combination of non-isothermal and isothermal, was used to trace the phase behaviour of the PMAN. The study indicates that PMAN and AN has comparable properties suggesting similar processing and handling conditions.

Full text of DOI:10.1016/j.tca.2011.07.009

Thermochimica Acta 524 (2011) 165–169
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Thermal stability of habit modified ammonium nitrate: Insights from
isoconversional kinetic analysis
Anuj A. Vargeesea,b,∗, Krishnamurthi Muralidharana,b, V.N. Krishnamurthyb
a
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad 500 046, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Compared to the conventional ammonium perchlorate based solid propellants, burning of ammonium
nitrate (AN) based propellants produce environmentally innocuous combustion gases. However, the near
room temperature phase transition limits the application of AN as a propellant ingredient. A method to
produce phase modified ammonium nitrate (PMAN) was reported by means of habit modification route
using potassium ferrocyanide. In the present study, the thermal stability and moisture absorption char-
acteristics of PMAN were evaluated to understand the viability of its application in the environmentally
benign propellant systems. The energy of activation of decomposition reaction of AN, PMAN and an admix-
ture of AN-potassium ferrocyanide trihydrate were computed by Friedman’s differential and Vyazovkin’s
non-linear integral isoconversional methods and used to ascertain their thermal stability. The moisture
absorption characteristics were determined at 53 and 62% RH and the cyclic DSC analysis, combination of
non-isothermal and isothermal, was used to trace the phase behaviour of the PMAN. The study indicates
that PMAN and AN has comparable properties suggesting similar processing and handling conditions.
Received 29 March 2011
Received in revised form 6 July 2011
Accepted 8 July 2011
Available online 19 July 2011
Keywords:
Ammonium nitrate
Phase and habit modification
Thermal stability
Decomposition, Isoconversional kinetics
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
phase transitions, low performance and low burning rate, AN based
propellant systems are receiving an invigorate interest in the pro-
pellant field due to eco-friendly plumes, smokelessness and low
sensitivity [4].
Ammonium nitrate (AN) finds extensive use as a fertilizer and
as a major ingredient in commercial explosives, as well as in gas
generator propellant formulations. AN has a positive oxygen bal-
ance and as a commercial explosive or propellant oxidizer, the
nitrate ion acts as the source of oxygen. Combustion gases of
conventional solid propellants, which typically incorporate (70%
by weight) ammonium perchlorate (AP) as oxidizer, contain nox-
ious materials like HCl, reactive chlorine compounds and heavy
toxic metal oxides [1]. Investigations indicate that the exhaust
plume predominantly converts the exhaust product HCl into Cl
or Cl2 which consequently destroys the ozone in the stratosphere
through the chlorine catalyzed mechanism [2]. Also, relatively
uncreative perchlorate ion, originating from AP, remains in water
for decades and its high solubility and mobility in water eventu-
ally cause health hazards. Hence, there is a quest for a greener
alternative oxidizer, such as AN or ammonium dinitramide [3].
AN burning produces less toxic or innocuous gases. AN released
to the environment in moderate quantities, dissolves in the mois-
ture present in soil and the nitrogen in the nitrate is immediately
available for the plant uptake. Irrespective of its drawbacks, such as
Nevertheless, both producers and users of ammonium nitrate
are troubled by serious product quality problems, for instance,
caking and physical breakdown of particles, due to its inherent
hygroscopicity and density decrease caused by the near room tem-
perature polymorphic transition. Numerous methods have been
reported to modify the crystallographic phases of AN to pre-
vent subsequent density decrease [5–8] but, by incorporating high
+
molecular weight compounds such as K salts. The specific impulse
of propellants is inversely related to the molecular weight of com-
bustion products and hence potassium is highly undesirable. In
view of the situation, a method to produce phase and habit modified
ammonium nitrate (PMAN) using trace amount of potassium fer-
rocyanide trihydrate (KFCT) as crystal habit modifier was reported
[9]. Since the particle size, shape and phase behaviour were modi-
fied, the thermal stability and hygroscopicity of PMAN needs to be
evaluated, which could significantly affect the handling, processing
as well as the performance of propellant formulations.
Even though, the moisture absorption characteristics of AN are
well studied and the understanding is near perfect, when two com-
ponents with different critical relative humidity (CRH) are mixed,
the CRH (humidity of air in equilibrium with saturated solution)
of the mixture may vary substantially from the CRH of individual
components. As the capacity to hold moisture and caking tendency
∗ Corresponding author at: School of Chemistry, University of Hyderabad, Hyder-
abad 500 046, India. Tel.: +91 40 66794919; fax: +91 40 23012460.
E-mail address: dr anuj@ymail.com (A.A. Vargeese).
0
040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.07.009

1
66
A.A. Vargeese et al. / Thermochimica Acta 524 (2011) 165–169
of AN vary with the size, shape, and porosity of particles, and in
turn, these properties vary with the process used for the prepara-
tion. In the present study, the moisture absorption characteristics
special saturated salt solutions [15]. Two different relative humidi-
◦
ties (RH) 53% and 62% (at 25 ± 2 C) were used to find out the
moisture absorption characteristics. Moisture absorption studies
were done by keeping accurately weighed quantity of PMAN sam-
ple in the constant humidity chamber and recording the increase
in weight at different intervals. The experiments were repeated to
ensure the reproducibility of the results.
◦
and CRH of AN at 53 and 62% RH (at 25 ± 2 C) were determined and
the results are discussed.
Transition metal ions, such as Cr3+, Fe3+ and Cu2+ have a con-
siderable catalytic influence on AN thermal decomposition [10].
Considering the safety and processing point of view, the stabil-
ity of AN has great importance. If some impurities/additives have
some catalytic effect on decomposition, it will be clearly indicated
as a change in the activation energy required for the decomposition
reaction. As per the ICTAC kinetics committee recommendations,
obtaining the E␣ vs. ˛ dependence by an isoconversional method
from quality kinetic data at not less than three different temper-
ature programs itself is sufficient for making kinetic predictions
2.3. Thermal analysis
◦
The PMAN sample was thermally cycled between 20 and 100 C
◦
(at 5 C/min) on a Mettler-Toledo DSC 1 instrument.
Thermogravimetric analysis (TGA) measurements were carried
out on a Mettler-Toledo TGA/DSC 1 instrument. In all experiments
∼2 mg of samples were loaded in open 40 ␮L alumina pans and
heated. Nitrogen, at a flow rate of 30 mL/min, was used as the purge
gas and protective gas.
[
11,12]. Considering the ICTAC kinetic committee recommenda-
tions, the isoconversional principle based model-free methodology
was employed in the present study to evaluate the E␣ vs. ˛
dependence. Two different isoconversional methods namely, Fried-
man’s differential method [13] and Vyazovkin’s non-linear integral
method [14] have been employed to evaluate the activation energy
of AN, PMAN and admixture of AN and KFCT.
2.4. Decomposition kinetic analysis
Thermogravimetric analysis (TGA) measurements were used to
determine the decomposition kinetics of the sample. Nonisother-
mal TGA runs were conducted at heating rates 2.5, 5, 10 and
◦
2
0 C/min and the extend of conversion (˛) has been computed
2
. Materials and methods
from the weight loss data using the reported standard method [16].
˛ of 0.01 and ꢀ˛ of 0.025 were respectively used to compute the
ꢀ
The chemicals used for the experiments were of commercial
activation energy from the differential and the integral methods.
For comparison and plotting, constant ˛ values were shown at an
interval of 0.025.
(
MerckIndia Ltd.) analyticalreagentgrade and used withoutfurther
purification.
Two isoconversional methods namely, Friedman’s, and Vya-
zovkin’s were employed for the kinetic analysis. The third degree
approximation proposed by Senum and Yang [17] has been used
in the present study to evaluate the integral in Vyazovkin’s
method. The theoretical background of kinetic analysis is given as
supplementary data.
2
.1. Synthesis of phase and habit modified ammonium nitrate
Phase modified ammonium nitrate was synthesized using the
reported procedure [9]. In brief, saturated solution of AN was
prepared by dissolving 12.5 g AN in 5 mL of distilled water at
8
◦
0 C. The solution was kept in a water bath and gently agitated
(
(
<50 rpm) for 30 min. After complete dissolution of the salt, 0.1 wt.%
0.0125 g) of KFCT, was added to the solution. The agitation was
3. Results and discussion
further continued for 15 min and then the solution was cooled to
room temperature and kept in ambient condition in a sealed con-
tainer for 240 h. After 240 h, the sample was collected by filtration
through a Whatmann 40 filter paper and washed several times with
methanol. The sample was dried in desiccators under vacuum and
used for further analysis.
AN crystals formed from its saturated solutions, in spite of hav-
ing the crystal habit modifier in the solution, were of needle-like
morphology. However, by keeping these AN crystals in solutions
containing KFCT modifier, for 240 h, chemical milling of AN occurs
in the solution changing the needle morphology of AN particles
to smaller particles with aspect ratio nearer to one. Presence of
hydroxylammonium nitrate (HAN) clusters were observed in the
powder XRD pattern and these clusters modify the IV ↔ III transi-
tion of AN. The particle size obtained in the study is approximately
3.1. Analysis of PMAN phase transition behaviour
◦
The PMAN was thermally cycled from 20 to 100 C and the
◦
◦
◦
cycling sequence was 20–100 C → 100–20 C → 20–100 C → 100–
◦
◦
20 C → 20–100 C. The samples were held for 2 min at isothermal
condition at the final temperatures before reversing the tempera-
ture. The DSC thermograms of the PMAN thermal cycling is shown
in Fig. 1. During the thermal cycling, the PMAN sample showed
◦
the first transition at 55 C and on cooling the reverse transition
◦
occurred at 45 C. This transition behaviour was similar in the next
heating cycle. However, in the cooling curve a small hump appeared
◦
around 28 C indicating the possibility of lower temperature tran-
sition during the next heating cycle. Even if, the modified transition
◦
re-appeared around 40 C, the intensity was very low and a higher
◦
1
00 ␮m, compared to 1000 ␮m particles generally produced by the
intensity peak was still exhibited at 55 C. These observations indi-
recrystallization method [9].
cate that PMAN sample looses the stabilized nature on increasing
the number of heating cycles and started exhibiting the inherent
transition behaviour of AN. The observed behaviour is similar to
that explained in the earlier work [9]. However, in the present
study the cooling curve just before the re-appearance of transi-
In another experiment, an admixture of AN and KFCT was pre-
pared by evaporative crystallization. For the admixture sample
preparation, above procedure was repeated and instead of cool-
ing the solution to room temperature, it was evaporated to dryness
◦
◦
over a paraffin oil bath controlled at 100 C. Commercially available
tion at 40 C is enough to predict the phase behaviour of PMAN.
AN obtained from the manufacturer was used for the thermogravi-
metric and kinetic analyses.
The HAN clusters getting located in ideal sites or forming a weak
networked structure with AN presumably influence the phase tran-
sition of PMAN. Even though the decomposition temperature of
◦
HAN is below 100 C, because of the location of the incorporated
2.2. Moisture absorption characteristics
HAN clusters, it may require more heating cycles to get decomposed
◦
Moisture absorption characteristics of the AN samples were
off. Therefore, PMAN exhibits only one transition in 20–100 C for
determined by using constant humidity chambers, created with
at least two heating cycles.

A.A. Vargeese et al. / Thermochimica Acta 524 (2011) 165–169
167
Compared to AN, no appreciable change in the moisture absorp-
tion characteristics of PMAN was observed. Our results indicate
that gravimetric measurements of moisture uptake as a function
of increasing relative humidity can provide reasonable estimates
of moisture uptake by compounds under investigation.
3.3. Decomposition kinetics and thermal stability
The thermolysis of AN depends upon various factors like pres-
sure, temperature, and experimental conditions like sample size,
rate of heating, sample purity and presence of foreign substances
[
7]. Although, the thermal decomposition of ammonium nitrate is
influenced by many intrinsic and extrinsic parameters, presence
of impurities as well as low pH conditions are most significant in
enhancing the decomposition reaction [19]. Such enhancements
may lead to an uncontrolled decomposition, or even to an explo-
sion. Concentration of acid (HNO ) in AN, either produced due to AN
3
dissociation or added externally, significantly increases the ther-
mal decomposition rate of AN through an autocatalytic reaction
Fig. 1. DSC analysis of PMAN phase behaviour.
[
7]. Considering this effect, the pH of the AN solution has been
experimentally checked, after KFCT addition (after attaining room
temperature), 24 h after KFCT addition, 48 h after KFCT addition
and 240 h after KFCT addition and found that the pH remains in the
range of 4.3 ± 0.1. This rules out the presence of acid affecting the
thermal stability of PMAN.
3
.2. Moisture absorption characteristics
Hygroscopic compounds such as AN, absorb moisture from the
atmosphere by forming saturated solutions on exposed surfaces.
Small amount of water vapour may be adsorbed by the compounds
due to van der Waal forces of attraction and also, absorbed by cap-
illary condensation. The moisture uptake of PMAN determined at
To get more insights and better comparison of the decomposi-
tion behaviour of AN, PMAN and admixture AN, their kinetic curves,
˛
vs. T, were plotted and shown in Fig. 3. Clearly indicating the ther-
5
3% and 62% RH, plotted against time is shown in Fig. 2. As per the
mal stability differences, the admixture AN decomposition starts
at significantly lower temperatures compared to AN. It can be seen
from the figure that the decomposition of admixture AN starts well
in the melt phase of AN, and hence the thermal gasification could
be a dominant process.
◦
literature, the CRH of AN is 62.7% RH at 25 C [18]. After 72 h at 53%
RH, PMAN showed a negligible increase in weight, indicating neg-
ligible amount of moisture absorption. While at 62% RH, the PMAN
absorbed significant amount moisture within 24 h.
Absorption of water will occur only when the partial pressure of
water vapour in the atmosphere is greater than the partial pressure
of water of a saturated solution of the compound and desorption is
anticipated when the latter is greater than the former. The presence
of highly soluble components in the mixture that form a eutec-
tic can appreciably alter the CRH, when slight variations occur in
moisture content or temperature. Presumably, the decrease in par-
ticle size and subsequent increase in exposed surface area as well
as the presence of HAN might have caused a negligible amount of
moisture absorption at 53% RH. Since 62% RH is above the CRH of
AN, PMAN also showed a significant moisture uptake at this RH.
Reported activation energy for ammonium nitrate decompo-
−
1
sition varies from 86.2 to 206.9 kJ mol [20]. However, all the
previous researchers in this area agree that the overall decom-
position reaction of ammonium nitrate is described by the first
order reaction kinetics. A recent work reports that the thermal
gasification of AN has a similar kinetics in the solid and liquid
−
1
phase and requires an activation energy of 90 kJ mol [21]. Even
though, the literature reports a variety of decomposition reac-
tion pathways that AN can undergo, it is widely accepted that
Fig. 2. Moisture absorption characteristics at 53 and 62% RH.
Fig. 3. ˛ − T kinetic curve for AN, PMAN and admixture AN.

1
68
A.A. Vargeese et al. / Thermochimica Acta 524 (2011) 165–169
other words the stability varies in the order AN > PMAN > admixture
AN. But, the activation energy difference between AN and PMAN is
negligible compared to that of admixture AN. It can also be seen
that, the variation in values of apparent activation energy of the
decomposition process is linear for 0.2–0.8 region of ˛. The inde-
pendence of activation energy on extent of conversion indicates
the possibility of single reaction mechanism (or the unification
of multiple-reaction mechanisms). The linearity of the isoconver-
sional Arrhenius plots for the thermal decomposition of AN, PMAN
and admixture AN obtained for Friedman method is characterized
by correlation coefficients (|r|) of 0.97614, 0.95332 and 0.98245,
respectively.
Activation energy may be correlated to bond energy data [22]
and is generally considered as a measurement of the energy barrier
to a controlling (rate limiting) bond rupture or bond redistribution
step [23]. Though, activation energy alone is not enough to under-
stand the thermal decomposition process, it can provide reasonable
information about critical energy needed to start the decompo-
sition reaction of the compound. In turn the values will help to
understand the thermal stability of the compound under investi-
gation.
Scheme 1. Dissociative decomposition of ammonium nitrate.
Scheme 2. Decomposition of ammonium nitrate dissociation products.
the thermal decomposition is initiated by the vapourization of
melted AN accompanied by an endothermic proton transfer reac-
tion (Scheme 1).
Once the dissociation occurs the products will be subsequently
reduced by the dissociation products of HNO , as shown in
3
Scheme 2.
The activation energy values, computed using Friedman and
Vyazovkin method, are plotted as function of extends of conversion
. Fig. 4 shows activation energies computed by Friedman method
and Fig. 5 shows values obtained by Vyazovkin method. The activa-
tion energy obtained by both methods clearly indicate that AN and
PMAN have nearly similar stability compared to admixture AN, in
˛
Even though the trends were similar in the relation of apparent
activation energy with extent of conversion, the numerical values of
activation energy obtained using Friedman method and non-linear
(Vyazovkin) isoconversional methods showed small difference.
However, the confidence interval determined for the computed E␣
values indicated that the differences are statistically insignificant.
The small differences may be due to the approximation of the tem-
perature integral that were used in the derivations of the relations
of non-linear isoconversional methods [24]. While, the differen-
tial isoconversional method of Friedman employs instantaneous
rate values, it is very sensitive to experimental noise, and tends to
be numerically unstable, especially when the rate is estimated by
numerical differentiation of experimental data. Indeed, investiga-
tors have compared these two methods and put in evidence of such
differences [25].
PMAN showed similar stability as AN, with a slight difference
in the activation energy for decomposition. As stated earlier, the
pH measurements showed no significant variation and the pos-
sibility of acid influencing the thermal stability of PMAN is ruled
out. Small amount of HAN present in the PMAN system, which
eventually decomposed off at higher temperatures, would have
played a role in slightly changing the energy of activation of PMAN
decomposition reaction. There is a wide belief that the initiation
of decomposition in energetic materials is associated with hot
spots. These mechanisms involve crystal defects or deformations
in the hot spots, leading to chemical chain reactions of decom-
positions [26,27]. The activation energy required for the thermal
Fig. 4. Variation of activation energy with the extent of conversion (based on Fried-
man’s method).
−
1
decomposition of AN is 109 kJ mol
as per the integral method
−
1
(
or 119 kJ mol
as per differential method) and that of PMAN
−
1
−1
is 104 kJ mol (or 108 kJ mol ), while that of admixture AN is
−1 −1
7
6 kJ mol (77 kJ mol ). This clearly points out that, there is no
substantial change in the stability of PMAN which could pose a
handling or processing risk compared to AN. Whereas allowing the
complexes to crystallize along with AN pose a handling or process-
ing risk as indicated by a large difference in the activation energy
of AN and admixture AN.
The least stable behaviour of admixture AN may be explained
due to the presence of foreign cations. Addition of KFCT to the hot-
acidic solution of AN can result in the dissociation of the KFCT.
Ammonium ion is a weak acid and cyanide ion a weak base, during
evaporation, in the hot concentrated ammonium nitrate solution
iron (II) may be oxidized to iron (III) [9]. At higher decomposition
◦
temperatures (<200 C) these iron (II) and iron (III) species will also
decompose and affect the thermal stability of AN. Other than iron,
the admixture AN contains potassium (from KFCT) and this will
Fig. 5. Variation of activation energy with the extent of conversion (based on Vya-
zovkin’s method).

A.A. Vargeese et al. / Thermochimica Acta 524 (2011) 165–169
169
also influence the thermal stability. Hence, it is quite reasonable to
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in significantly bringing the activation energy barrier down and in
turn making the compound less stable.
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−
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