The thermal behavior of potassium dinitramide: Part 2. Mechanism of thermal decomposition

Article abstract of DOI:10.1016/s0040-6031(99)00172-0

In this paper, the thermal decomposition processes of KDN were investigated by means of thermal analysis and FT-IR with in situ cell. The results show that the decomposition of KDN in solid state is different from that of liquid state. The main condensed phase product of the decomposition in solid state is KNO₃. While the decomposition of liquid state form KNO₃ and KNO₂ simultaneously. The possible mechanism of the thermal decomposition in solid state has been proposed. A eutectic can be formed by product KNO₃ with KDN. The eutectic temperature of KNO₃/KDN system is about 109°C. Moreover another eutectic system can be formed by KNO₃ with KNO₂ to be eutectic temperature of about 315°C.

Full text of DOI:10.1016/s0040-6031(99)00172-0

Thermochimica Acta 335 (1999) 113±120
The thermal behavior of potassium dinitramide
Part 2. Mechanism of thermal decomposition
Ming Lei*, Zi-Ru Liu, Yang-Hui Kong, Cui-Mei Yin, Bo-Zhou Wang, Yuan Wang, Pei Zhang
Xi'an Modern Chemistry Research Institute, PO Box 18, Xi'an 710065, China
Received 1 February 1999; accepted 22 June 1999
Abstract
In this paper, the thermal decomposition processes of KDN were investigated by means of thermal analysis and FT-IR with
in situ cell. The results show that the decomposition of KDN in solid state is different from that of liquid state. The main
condensed phase product of the decomposition in solid state is KNO3. While the decomposition of liquid state form KNO3 and
KNO2 simultaneously. The possible mechanism of the thermal decomposition in solid state has been proposed. A eutectic can
be formed by product KNO3 with KDN. The eutectic temperature of KNO3/KDN system is about 1098C. Moreover another
eutectic system can be formed by KNO3 with KNO2 to be eutectic temperature of about 3158C. # 1999 Elsevier Science B.V.
All rights reserved.
Keywords: Mechanism of thermal decomposition; Eutectic system; Thermal analysis; In situ FT-IR
1. Introduction
follow the process of the decomposition of KDN at
heating. DSC found the eutectic systems and their
temperatures for the process of decomposition.
Since potassium dinitramide (KDN) is a new type
highly energetic oxidizer for solid propellants, the
investigations on its thermal decomposition are of
great importance. Dubovitskii et al. [1,2] investigated
KDN thermal decomposition and proposed the
mechanism of decomposition in its solid state and
liquid state. Christe et al. [3] had also investigated the
2. Experimental
2.1. Sample
geometry of NꢀNO2† anion in solid and solution.
KDN, a pale yellow crystal, was synthesized by
Xi'an Modern Chemistry Research Institute and was
puri®ed in methane alcohol±water.
2
These works are meaningful to investigation into
stability and thermal decomposition of KDN.
In this work, based on Part 1, the mechanism of
thermal decomposition of KDN was investigated by
using a differential scanning calorimeter (DSC) and
FT-IR with a heating in situ cell, which can in real time
2.2. Apparatus and experiment condition
The thermal analyses were carried out on a differ-
ential scanning calorimeter, model TA 2910, with
sample mass of 2±4 mg. The measurements were
*Corresponding author.
0040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 9 9 ) 0 0 1 7 2 - 0

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M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
Fig. 1. DSC curves of KNO3/KDN system.
performed in a dynamic atmosphere of nitrogen with a
1
¯ow rate of 40.0 ml min and at heating rate of
shape and its change of the two endothermic peaks
represent the characteristics of fusion and liquefaction
process of typical binary eutectic system on DSC
curve. It is shown that a binary eutectic system was
formed by KDN with its decomposition product.
It is believed that the binary eutectic system consists
of KDN and KNO3 due to the formation of KNO3 as
main condensed phase product in decomposition of
KDN in solid state (in following discussion). More-
over, no peak of the phase transition of KNO3 at 1328C
appears on DSC owing to the excess of KDN.
1
108C min
.
In addition, a Nicolet 60 SXR FT-IR spectrometer
was used to follow the process of thermal decomposi-
tion of KDN in real time, with scanning 18 pieces IR
spectra per min. The in situ cell, which used to follow
the process of decomposition, was heated at about
208C min 1, and sample were tabletted with KBr.
3. Results and discussion
In order to prove the existence of the above-men-
tioned binary system, the DSC tests a set of samples,
which were obtained by means of the mixing KNO3 to
KDN in various ratios, were performed and the curves
were obtained, as shown in Fig. 1. From Fig. 1, it is
shown that only enthalpy of the ®rst endothermic peak
has been increased owing to adding KNO3, but its
peak temperature has not been changed on the whole.
While the endothermic enthalpy has been attained to
maximum of 109.5 J/g in KNO3/KDN ˆ 10/90 wt%
(see Table 1) and only an endothermic peak appears
here (b curve). On adding KNO3 to 30 wt%, the
enthalpy of the peak has been decreased again and
another endothermic peak appears at 132.48C. This
3.1. The eutectic system formed by KDN and its
decomposition product KNO3
As known in the ®rst part of this paper [4] there is a
set of endothermic peaks on all the DSC curves of
KDN before 1408C. The temperature of an endother-
mic peak at 108±1108C do not changes, while its
strength (endothermic enthalpy) changes with the
quantity of decomposition product. But the tempera-
ture and endothermic enthalpy of another endothermic
peak, immediately following above mentioned peak,
reduces with the quantity of KNO3 increases. The

M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
115
Table 1
Influence of adding KNO3 on thermal behavior of KDN
Sample
KNO3/
KDN wt%
KNO3/KDN system
KNO3 phase
transition Tc
KNO3/KNO system
Decomposition in
liquid state
Te
ÁHe
Tl
Te0
ÁHe ‡ ÁHl0
Tl0
Tm1
Tm2
a
b
c
d
0/100
10/90
30/70
50/50
108.8
110.1
108.3
108.9
42.8
109.5
105.8
61.9
125.3
43.7
40.9
42.3
62.5
338.2
327.2
321.9
231.5
230.8
229.5
237.2
236.1
234.8
234.9
132.4
132.4
315.1
315.2
0
Te, Te0, T1, T1 (in 8C) are eutectic and liquefaction temperatures of excess components for KNO3/KDN and KNO3/KNO2 binary systems,
respectively.
1
ÁHe and ÁHe (in J g ) are the fusion enthalpy of eutectic of the two systems, respectively. ÁHl0 (in J g 1) is the liquefaction enthalpy of
0
KNO3/KNO2 system.
Tm1 and Tm2 (in 8C) are the temperatures of two exothermic peaks of decomposition of KDN in liquid state, respectively.
peak would be large when more KNO3 was added, but
the peak temperature did not change, indicating that it
is the phase transition peak of KNO3 and that KNO3 is
already excess. For the fusion peak of KNO3 is at
3358C far away, the liquefaction peak of KNO3 cannot
be seen. The results prove further that product KNO3
of the decomposition of KDN can form binary eutectic
system with KDN and its eutectic temperature is 108±
1108C and the value has also been reported in the
literature [2].
in Fig. 1). Therefore, it is considered that the last
endothermic peak on the DSC of KDN is attributed to
the liquefaction of KNO2 of KNO3/KNO2 binary
system.
It is deserved attention that the different tempera-
tures of liquefaction peak of KNO2 have been found
from all DSC curves of various KDN samples in Part 1
of this paper. When solid KDN decomposed more
deeply or fusion peak of KNO3/KNO2 was stronger,
the liquefaction peak temperature was also lower. So,
KNO3 is considered to be the main solid product of
decomposition of KDN in solid state.
3.2. The eutectic system formed by decomposition
products KNO3 and KNO2
3.3. Mechanism of thermal decomposition of KDN
There is a set of endothermic peaks at last part of
DSC curve of KDN. Based on temperature these peaks
are neither attributed to fusion of KNO3 (375.58C), nor
that of KNO2 (428.18C). IR analyses (see following)
show that the ®nal solid products of decomposition of
KDN are KNO3 and KNO2. The DSC curves of KNO3/
KNO2 system (see Fig. 2f in Part 1 of this paper) are
those of a typical binary system with eutectic tem-
perature at 315.28C.
No eutectic peak appears distinctly in all DSC
curves of KDN, the reason being a few of the decom-
position products formed, in comparison to sample
itself, and being that the content of KNO3 in the
eutectic binary system should be more than 70±
80 wt%. So when KNO3 was added up to 30 wt%,
the eutectic peak of KNO3/KNO2 could be distin-
guished from this set of endothermic peak (curve c in
Fig. 1). Only a fusion peak of eutectic could be seen at
312.58C when KNO3 was added up to 50 wt% (curve d
The partial IR spectra obtained from following the
decomposition process of KDN by a heating in situ
cell are shown in Fig. 2. It is obviously found from
Fig. 2 that the processes of disappearance of the
absorption bands of N±nitro group (N±NO2) (1530,
1431, 1344 and 1178 cm 1) and N±N (1023 cm
1
)
and those of the increase in strength of the bands of ±
NO3 (1384 cm 1) ±NO2 (1249 cm 1) could occur as
increase in temperature. The IR spectra in Fig. 2 can
be divided into four groups. In ®rst group, from room
temperature to 157.28C (spectrum 1±4), N±NO2
absorption band changes greatly and do not disappear,
while ±NO3 absorption band at 1384 cm 1 is strength-
ened somewhat. In this temperature range the thermal
behavior of KDN on DSC is corresponding to the
processes of decomposition in solid state and of
liquefaction. In second group from 157.28C to
190.68C (spectrum 4±7), changes in spectra are rela-

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M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
Fig. 2. The partial IR spectra for a heating in situ cell for following the decomposition process of KDN.
tively slight. This process is corresponding to rela-
tively stable stage of liquid KDN on DSC. In third
group from 190.68C to 224.38C (spectrum 7±13), all
absorption bands of N±NO2 and N±N weaken gradu-
ally to disappear, but the absorption bands of ±NO3
and ±NO2 strengthen gradually, which are to the
decomposition stage of KDN in liquid state. In the
fourth group from 224.38C to 380.98C (spectrum 13±
17), the spectra change relatively slightly, only the
relative quantity of ±NO3 to ±NO2 change (see next).

M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
117
These processes are corresponding to the later stage
and the end of decomposition of liquid KDN on DSC.
From the above analyses it is shown that KNO3 is
the main solid product of decomposition stage of KDN
in solid state, also con®rmed by the following facts.
On DSC curve of KDN, the strength of eutectic peak
of KNO3/KDN and the change in temperature of
liquefaction peak of KNO2 of KNO3/KNO2 are in
close relationship with the depth of solid decomposi-
tion. The strength of eutectic peak of KNO3/KDN is
increased with solid decomposition deepened. If the
mechanism of solid decomposition was the same as
liquid decomposition, namely the ratio of KNO3 and
KNO2 formed in solid decomposition would be same
as liquid, the liquefaction temperature of KNO2 in
KNO3/KNO2 system should not be changed in spite of
the depth of the solid decomposition. The opposite
results proved that the process of decomposition in
solid state is different from that of liquid state and the
liquefaction temperature of KNO2 decreases as the
depth of solid decomposition decreases.
The data of high pressure DSC show [5] that the
strength of the exothermic peak of decomposition in
solid state and that of eutectic peak of KNO3/KDN
would be decreased by increasing pressure, indicating
that the condition at high pressure is not favorable to
the solid decomposition reaction which released gas.
Therefore, it also supports the above-mentioned
mechanism of releasing N2O gas during decomposi-
tion process.
The DSC curves of KDN show that the two exother-
mic peaks can be found in the decomposition of liquid
KDN and their temperature are 228±2308C and 236±
2388C, respectively. Between 2308C and 2368C, there
is an obvious peak valley. It possibly indicates that
there are macroscopically at least two main stages in
decomposition of KDN in liquid state.
The relationship curves of relative strength of char-
acteristic absorption with temperature were obtained
from integrating corresponding characteristic peak,
which were obtained from the spectra of FT-IR with
the heating in situ cell. The relationships of the
characteristic peak strengths of N±NO2, ±NO3,
±NO2 and N2O in the decomposition process of
KDN with temperature (time) are shown as Fig. 3.
It is known from these results that a sudden change in
the strengths of all characteristic peaks occurs from
2248C to 2348C. It is indicated that N±NO2 disap-
peared and ±NO3 got to maximum gradually. Then the
latter basically kept constant strength with the tem-
perature increased continuously. While ±NO2 and
N2O all got to maximum and then tended to decrease
with temperature increased continuously. The IR
absorption band of N2O and its relative strength
change with temperature is shown in Fig. 4 and
Table 2.
On the basis of the analyses of the molecular
structure difference between solid and liquid KDN
in literature [2], it is believed the charge of NꢀNO2†
2
anion in liquid phase is uniformly distributed and the
anion has the plane structure with utmost stability. But
in crystal state, the two nitro groups of the anion are
spread and are not conjugate. The charge is prece-
dently gathered on one nitro group, and the two N±N
bonds are not identical. So the anion in crystal is easily
decomposed to form N2O and NO3 . From geometry of
molecular crystal and data of IR and Raman spectra,
Christe et al. [3] believed that the two nitro groups of
KDN crystal are not in the same plane, their N±NO2
bonds make an angle of 22.68 with each other and the
bond lengths are not the same, to lead to that the ±NO2
groups are asymmetrical. On the basis of the above
analyses, the following mechanism of decomposition
of KDN in solid state is proposed:
According to the above-mentioned analyses, the
possible mechanism of decomposition of KDN in
liquid state is proposed as follows:
First stage (exothermic peaks on DSC at 228±
2308C):
KNꢀNO2† ! KNO3 ‡ N2O
2
Table 2
Strength of characteristic absorption peak of product N2O at various temperatures
Temperature (8C)
135.1 157.2 168.2 179.4 190.6 212.9 224.3 269.0 313.8 358.6 380.9
0.0 0.0 0.15 0.49 1.33 3.97 4.77 3.65 2.68 1.93 1.22
Relative strength of N2O absorption peak

118
M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
Fig. 3. Relationship curves of relative strength of IR characteristic peak with time and temperature.
_
KNꢀNO2† ! KNꢀNO2† ‡ NO2
KNO2 would occurred and KNO3 would be produced.
2
This moment, a stronger exothermic peak would over-
lap another exothermic peak on DSC. Thus the later
exothermic peak (236±2388C) rose abruptly and
KNO2 reduced, while KNO3 increased continuously
(Fig. 3) and N2O also increased to maximum at 2248C
(Fig. 4 and Table 2).
_
KNꢀNO2† ‡ NO2 ! KNOꢀNO† ‡ NO
2KNꢀNO2† ! 2KNO2 ‡ NꢀNO2†
KNO2 ‡ NO2 ! KNO3 ‡ NO
Second stage (exothermic peaks on DSC at 236±
_
_
:
_
Dubovitskii et al. [2] also discovered that the yield
of N2O is more at 2208C during the decomposition of
KDN at constant temperature.
It can be seen from Fig. 1 that adding KNO3 would
cause the second exothermic peak of liquid decom-
position to change, i.e. the temperature of the peak
would be advanced and the strength of the peak would
be strengthened. In addition, adding KNO2 had similar
results. It is possible for the reason that KNO3
or KNO2 has the ability to capture the double free
2388C):
_
NꢀNO2† ! ‰OŠ ‡ N2O
:
_
KNO2 ‡ ‰OŠ ! KNO3
Dubovitskii et al. [1] studied the thermal decom-
position of KDN in liquid state by labeled atom
technology. He proposed that KDN ®rstly splits off
a nitro group, forming NO2, and the stoichiometric
reaction equation is shown as follows:
_
radical NꢀNO2† and hence the concentration of this
:
KNꢀNO2† ! NO ‡ 0:5 N2O ‡ 0:5 KNO2
2
intermediate product is increased. So the decomposi-
tion temperature can be decreased and the exothermic
peak increased.
‡0:5 KNO3:
He also believed that the intermediate product,
_
double free radical NNO2, has relative good stability
In addition, it is shown from the data of high-
pressure DSC [5] that for the ®rst exothermic
peak of decomposition of KDN in liquid state its
peak temperature is risen and strength is strengthened
with pressure increase. The corresponding activation
energy of the ®rst exothermic peak is decreased.
:
and it may not be decomposed until relative high
_
temperature. When NNO2 is decomposed at rela-
:
tively high temperature to produce free [O], which
has much stronger oxidizing property than that of
NO2, the stronger oxidizing reaction of [O] with

M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
119
Fig. 4. Characteristic absorption peak of product N2O at various temperatures.
But the second exothermic peak and its correspond-
ing activation energy are not changed. Obviously,
increasing pressure is favorable to increase the
concentration of NO2 with relative stronger oxidiz-
ing property in liquid state and to the second and
the forth step process in the ®rst stage of above
decomposition mechanism. Therefore the ®rst
exothermic peak is advanced and strengthened. To
second stage, the reaction system may be in solid
_
state already and the free radical NꢀNO2† may be
:
captured on KNO3 and KNO2. So, the pressure in¯u-
ences the reaction process of the second stage very
slightly.
Thus, it is clear that these results all support the
above-mentioned decomposition mechanism of KDN
in liquid state.

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M. Lei et al. / Thermochimica Acta 335 (1999) 113±120
4. Conclusion
decomposition in liquid is divided into two main
stages.
1. There are two binary eutectic systems in the
decomposition process of KDN, One system
consists of KDN and decomposition product
KNO3, with eutectic temperature at about 1098C
and the other consists of the two products KNO3
and KNO2, with eutectic temperature at about
312.58C.
2. The main products of decomposition of KDN in
solid state are KNO3 and N2O. While the decom-
position in liquid state, besides KNO3 and N2O,
KNO2 and NO (or NO2) are the main products. The
References
[1] F.I. Dubovitskii et al., Dokl. Akad. Nauk. 347 (1996) 763±765.
[2] F.I. Dubovitskii et al., Dokl. Akad. Nauk. 348 (1996) 205±206.
[3] K.O. Christe, W.W. Wilson et al., Inorg. Chem. 35 (1996)
5068±5071.
[4] M. Lei et al., The thermal behavior of potassium dinitramide.
Part 1. Thermal stability..
[5] C.-M. Yin, Z.-R. Liu et al., The thermal behavior of potassium
dinitramide at high pressure, private communication.