Thermal stability of ammonium dinitramide

Article abstract of DOI:10.1016/S0040-6031(01)00776-6

The low temperature thermal stability of ammonium dinitramide (ADN) and efforts to improve the thermal stability are presented. The decomposition rates of ADN at 60, 70, 80, and 90°C were investigated. Several potential thermal stabilizers were investigated to suppress the low temperature decomposition (60-90°C) of ADN. The potential stabilizers investigated were potassium fluoride, potassium dinitramide, a 6-member ring or polymeric phosphorous compound [P(C₆H₅)]_6,polymer, and perhydro-1,3,5-triazine-2,4,6-trione, commonly known as Verkade's super base (VC). Comparison of the decomposition of pure ADN and 1-2% by mass mixtures of ADN and the potential stabilizers are presented. Finally, the low temperature stabilizers are compared to hexamethylene tetramine (commonly known as urotropine or hexamine). Verkade's super base is found to be most effective.

Full text of DOI:10.1016/S0040-6031(01)00776-6

Thermochimica Acta 384 ꢀ2002)47±56
Thermal stability of ammonium dinitramide
b
a,*
Indu B. Mishra , T.P. Russell
Department of Chemistry, Howard University, ASEE Summer Visiting Faculty, Washington, DC 20059, USA
b
Chemistry Division, Code 6110, Naval Research Laboratory, Washington, DC 20059, USA
a
Abstract
The low temperature thermal stability of ammonium dinitramide ꢀADN)and efforts to improve the thermal stability are
presented. The decomposition rates of ADN at 60, 70, 80, and 90 8C were investigated. Several potential thermal stabilizers
were investigated to suppress the low temperature decomposition ꢀ60±90 8C)of ADN. The potential stabilizers investigated
were potassium ¯uoride, potassium dinitramide, a 6-member ring or polymeric phosphorous compound [PꢀC H )]6,polymer, and
6
5
perhydro-1,3,5-triazine-2,4,6-trione, commonly known as Verkade's super base ꢀVC). Comparison of the decomposition of
pure ADN and 1±2% by mass mixtures of ADN and the potential stabilizers are presented. Finally, the low temperature
stabilizers are compared to hexamethylene tetramine ꢀcommonly known as urotropine or hexamine). Verkade's super base is
found to be most effective. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ammonium dinitramide ꢀAD); Verkade's super base ꢀVC); Hexamethylene tetramine
1
. Introduction
which has the structural formula NH NꢀNO ) . ADN,
4 2 2
like ammonium nitrate ꢀAN), undergoes residueless
combustion. However, it burns more readily and pre-
dictably than AN. Therefore, it is much more attrac-
tive to use ADN as an ingredient in propellants than
AN. Several solid propellant formulations of ADN in
various binder systems have recently been accom-
plished [11,12]. Although, it is well known that
dinitramides are energetic compounds, the thermal
decomposition of dinitramides has not been studied
extensively.
In the late 1960s, Hamel and Olson [1] synthesized
the ®rst dinitramide structures. These materials were
covalently bonded alkyl N,N-dinitramides. The result-
ing dinitramides were unstable and decomposed at
7
5 8C or less, making them unsuitable as propellant
ingredients. Recently a new class of inorganic oxidi-
zers have been synthesized [2±10]. This class of
oxidizer incorporates the newly discovered dinitrami-
À
date [NꢀNO2)2 ] anion. The dinitramidic acid and its
salts are thermally more stable than the original alkyl
N,N-dinitramides discovered in the late 1960s.
One promising oxidizer from this class of com-
pounds is ammonium dinitramide ꢀADN), N O H ,
1.1. Thermal decomposition of ADN
Typical products detected during ADN decomposi-
tion are nitric acid, ammonia, nitrous oxide, nitric
oxide, and water. Two distinct temperature regimes
have been identi®ed for ADN decomposition. Between
4
4 4
*
Corresponding author. Present address: Chemical Propulsion
55 and 100 8C, ADN decomposes slowly, forming
Information Agency, The Johns Hopkins University, 10630 Little
Patuxent Pkwy, Suite 202, Columbia, MD 210443204, USA.
E-mail address: imishra@cpia.jhu.edu ꢀI.B. Mishra).
nitrous oxide N O and ammonium nitrate NH NO ,
AN. AN is produced from the recombination of
2
4
3
0040-6031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ꢀ 0 1 ) 0 0 7 7 6 - 6

48
I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
ammonia, NH and HNO , which is subsequently
3
inhibit the ADN decomposition, while free radical
scavengers like DPPH, 2,2-diphenyl-1-picryl hydra-
zyl, show no improvement on the decomposition
of ADN. However, compounds that are both free-
radical scavengers and bases like methyl nitro aniline
3
produced during dinitramidic acid, HN O , ꢀHDN)
3
4
decomposition. This reaction process is shown in
reactions ꢀ1)±ꢀ4).
NH4NꢀNO2† ! ꢀNH3 ‡ H3NꢀNO2† †
ꢀ1)
NH3 ‡H3NꢀNO2†2†absorbed , NH3ꢀg† ‡H3NꢀNO2†2ꢀg†
2)
2
2 absorbed
ꢀ
MNA)and 2- nitro-diphenylamine ꢀ2-NDPA ), dela-
yed the decomposition by 8 and 4 h, respectively, at
0 8C. Their pK s are 3.5±4 and 1±2, respectively.
ꢀ
7
a
ꢀ
Politzer and co-workers [17±19] and Mebel et al.
[20±22] have reported theoretical study of ADN
decomposition.
In the present work, we report ADN low tempera-
ture decomposition and the effect of thermal stabiliz-
ing additives such as a somewhat basic salts potassium
¯uoride ꢀKF)[23] and potassium dinitramide ꢀKDN),
Lewis bases such as phenylphosphorous pentamer,
hexamer and polymers, and a super base perhydro-
ꢀ
HNꢀNO † †
! N O ‡ HNO3ꢀg†
ꢀ3)
ꢀ4)
2
2 absorbed
2
ꢀg†
HNO3 ‡ NH3ꢀg† ! NH4NO3ꢀs†
ꢀg†
ADN decomposes rapidly when NO2ꢀg) is detected.
During high heating rate and high temperature decom-
position studies ꢀabove 110 8C), reaction ꢀ5) is the
favored mechanism representing the early chemistry
of the combustion of ADN.
1
,3,5-triazine-2,4,6-trione [24±26] ꢀVerkade's super
HNꢀNO † †
! NO2ꢀg† ‡ HNNO2
ꢀ5)
2
2 absorbed
base, VC)on the stabilization of ADN over this
temperature regime. We would like to emphasize
that a need exists for stabilization of ADN decomposi-
tion. The theme of our approach is to ®nd a suitable
stabilizer for ADN, which will promote ammonia evo-
lution and prevent the decomposition of HDN. This
may be accomplished by trapping the HDN [27],
dinitramidic acid and its subsequent product, HNO3
as a salt and reduce the photochemically favored reac-
tion. A base stabilizer, which is a UVabsorber, wouldbe
an ideal choice.
In the combustion regime, a complex reaction scheme
is suggested. In order to explain the overall approx-
imate stoichiometry of the product gases, a 14-step
reaction sequence has been proposed by Brill et al.
[13] ꢀThe thermodynamic parameters of many of the
steps are still unknown).
1
.2. ADN stabilization studies
The stabilization of ADN has also been a recent
topic of much interest. If the initial step in ADN
decomposition is the transfer of a proton to form the
freebase and dinitramidic acid, ADN decomposition
will be stabilized with the addition of a Lewis base.
Further ADN decomposition is autocatalyzed by
2. Experimental procedures
2.1. Materials
build up of HNO through the re-arrangement shown
3
in reaction ꢀ3). Therefore, a good base stabilizer for
ADN low temperature decomposition would be a
strong Lewis base, which would scavenge both dini-
tramidic acid and nitric acid upon formation. This
fundamental base stabilization has been established.
Low temperature decomposition of ADN has been
inhibited or decreased and a dependence of the rate
ADN and KDN were received from William
Koppes and Al Stern of Naval Surface Warfare Center,
Indian Head Division, Indian Head MD. The samples
had 99% purity and no further puri®cation was
attempted. The samples were carefully stored in the
dark and were not exposed to light. The moisture
content of the samples was not determined. However,
all samples were dried under vacuum and stored in a
dry nitrogen atmosphere before analysis. Anhydrous
KF and hexamethylene tetramine were purchased
from J.T. Baker Chemicals, and VC was purchased
from Strem Chemicals. These materials were used
without further puri®cation. Toxicity data of ADN
of ADN decomposition on pK of the base has been
b
observed. Manelis et al. [14] have reported several
base stabilizing additives. The best one was urotro-
pine ꢀhexamethylene tetramine)at 0.1±0.6 wt.%.
Russell et al. [15,16] have reported that bases like
DPA, diphenylamine and NH3 with pKb of 9 do

I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
49
[
3
28] and discussion of sensitivity of nitramines [29±
2] have been reported.
The phenylphosphorous pentamer, hexamer and
2.2. Data collection and analysis
All samples were loaded into an infrared gas cell,
evacuated and then ®lled with dry nitrogen to atmo-
spheric pressure, and ®nally sealed. Typical sample
masses were 100 mg of material. Typical stabilization
studies were accomplished with 1±2% by a mass
of additive and 99±98% by mass ADN. After loading
the cell, the sample was wrapped with heating tape
and was placed inside the sample compartment of a
Nicolet 800 FTIR spectrometer. A typical spectrum
polymer were synthesized in our laboratory. The
synthesis of cyclic phenylphosphine polymer was
carried out following the procedure of Henderson
and Epstein [33]. The reaction was done in a three-
necked ¯ask. Nitrogen was slowly introduced through
one port, a re¯ux condenser connected in the middle
and dropping a funnel connected at the third port.
Phenylphosphine ꢀ6.1 g, 0.065 mol)in 25 ml cyclo-
hexane was stirred at room temperature while 10.0 g
À1
was collected with 100 scans at 1 cm resolution
ꢀ
0.065 mol)of phenyl dichlorophosphine in 15 ml of
cyclohexane was added slowly during the course of
h. The mixing was continued for an additional hour
using MCT-A or MCT-B detector. Spectra were taken
at 1±2 h time intervals up to 7 days. Fig. 1 shows the
typical experimental setup. The IR beam was focused
2±3 mm above the sample for product analysis. The
cell chamber was then heated to a pre-determined
temperature and kept heated for the desired length of
time. An Omega temperature controller employing a
thyristor controller and a silicon-controlled recti®er
ꢀSCR)controlled the power input to the heater. Auto-
matic control of the temperature was accomplished
with proportional, integral, and derivative features.
This system permits routine heating of the sample in
the infrared gas cell. The temperature was measured by
achromel-alumelthermocouplewithitsbeadimbedded
1
and then the temperature was raised slowly to ꢁ60 8C
while continuing stirring. A precipitate was formed
and there was gas evolution. As soon as the precipitate
appeared, the heating was discontinued. The stirring
was continued for an additional hour. The solution was
then slowly cooled while under re¯ux. The precipitate
formed was then ®ltered and the solid product was
washed with hexane. The product was dried under
vacuum. Finally, crystalline material was obtained in
ꢁ
95% yield. The dried crystalline product was used as
made.
Fig. 1. Experimental setup showing FTIR and an enlarged gas IR cell

Fig. 2. Ammonium nitrate formation with time from ADN heated at 90 8C.

Fig. 3. Nitrous oxide formation with time from ADN heated at 90 8C

52
I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
into the glass below the sample. The sample tempera-
ture can be set and automatically maintained to Æ1 8C
with an estimated accuracy of Æ1 8C. The sample tem-
perature was calibrated using a ®xed-point tempera-
ture calibration method on the melting point of pure
materials.
decomposition and stabilization of ADN in the low
temperature decomposition regime.
3. Results and discussion
The spectra of the gaseous decomposition products
N O and NH and the condensed ammonium nitrate
3.1. Thermal decomposition of ADN
2
3
on the cold part of the cell window were monitored
every 1, 2, or 4 h. The data collected was stored in the
computer and later analyzed using the 680-DSP spec-
tral workstation for N O, NH and NH NO evolution.
Figs. 4 and 5 show the detected amount of AN and
N O produced during the decomposition of ADN at
2
60, 70, 80, and 90 8C. The evolution of the products as
a function of time was described in the experimental
section. AN is determined by the measured infrared
2
3
4
3
A typical sequence for N O and AN is shown in Figs. 2
2
and 3. Each spectra was collected in situ during iso-
thermal heating at a constant temperature of 90 8C.
Over the duration of the experiment the only products
spectrum of gaseous N O. These two products provide
2
an in situ analysis of early reaction proposed for the
decomposition of ADN. Nitrogen dioxide is a direct
measurement of HDN decomposition ꢀreaction ꢀ3))
while AN is a measurement of the recombination of
detected were N O and AN. As can be seen in Figs. 2
2
and 3 an increase in the concentration of N O and
2
AN is observed during the experiments. Determining
the relative concentration of N O and AN produced
HNO
ꢀreaction ꢀ4)). ADN decomposition is observed to be
®rst order for the production of N O and AN over the
temperature regime 60±90 8C. As the temperature is
increased, the rate of formation of N O and AN is also
increased. At a temperature of 80 8C and above, a
steady increase in vapor phase ammonium nitrate is
detected. Below 80 8C, AN formation increases stea-
dily until it reaches an asymptotic limit ꢀ®rst order in
3 3
and NH produced during this reaction process
2
during decomposition was accomplished by integ-
rating over the infrared absorption of the ammonium
2
À1
ion of AN ꢀ3350±2850 cm )while the concentration
of N O was accomplished by integrating over the
2
2
À1
wavelength region 2260±2180 cm . The changes
in concentration of these two products in time at a
constant temperature were employed to evaluate the
Fig. 4. Ammonium nitrate concentration with time for ADN heated at 60, 70, 80 and 90 8C.

I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
53
Fig. 5. Nitrous oxide concentration with time for ADN heated at 60, 70, 80 and 90 8C.
reactant; it indeed tapers off asymptotically). N O
2
KDN actually enhanced the formation of AN during
decomposition at 90 8C. VC suppressed the produc-
tion of AN for 65 h. However, an initial NH ꢀg)
3
production ꢀFig. 5)shows a similar temperature
dependence as AN. However, an asymptotic limit is
also achieved at 80 8C. A steady production of N O is
evolution is observed in ADN decomposition when
VC is added as a stabilizer to ADN. However, no more
ammonia evolution was detected. This indicated that
VC is a good base stabilizer and probably reacts with
2
only observed at 90 8C. Again, this represents a ®rst
order decomposition process over the temperature
regime studied. It is interesting to note that a rate
change in the decomposition of ADN is observed
the HNO thereby consuming it. We do observe some
3
between 60 and 70 8C. The early N O production at
N O evolution and do not observe any NH NO ,
3
2
2
4
6
0 8C is higher than what is observed at 70 8C. How-
which leads us to conclude that perhaps it reduces
the production of HNO during decomposition. The
ever, after ꢂ100 h, the N O production rate at 60 8C
2
3
falls below the production rate at 70 8C. This appears
to be reproducible within the limited number of
experiments performed rather than an experimental
error. This interesting rate change is still under inves-
tigation. Therefore, no explanation for this observa-
tion will be discussed at this time.
rate of N O production in Verkade's super base
2
ꢀlabeled VC2 in Fig. 7)is similar to that observed
in pure ADN, although it is markedly lower. All other
materials studied enhance the rate of N O production
2
during decomposition at 90 8C. In a second experi-
ment, ADN was mixed with Verkade's super base
ꢀlabeled VC in Fig. 7)and the decomposition was
3
.2. Thermal stabilization of ADN
performed with exposure to room light in an effort.
Because VC absorbs in the ultra violet, VC was used to
determine whether small mass percent additions of an
ultra violet absorber could be a protection against
ADN photochemistry. At a 1% by mass addition of
VC, no stabilization of ADN is observed in the pre-
sence of room light ꢀlabeled VC in Fig. 7 ). Therefore,
if a stabilizer for photolytic effects is to be discovered
a higher mass percent addition will most likely be
necessary.
Figs. 6 and 7 show the detected amount of AN and
N O produced during decomposition of ADN at a
constant temperature of 90 8C, when potential stabi-
lizers are added. The stabilizers studied were KDN,
2
KF, VC, and PP. The amount of N O and AN produced
2
during stabilization is compared to the decomposition
of pure ADN. Only VC and KF reduced the amount of
AN produced during decomposition ꢀFig. 6). PP and

54
I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
Fig. 6. Ammonium nitrate concentration with time for ADN, ADN-KDN, ADN-KF, ADN-PP, and ADN-VC at 90 8C
Fig. 7. Nitrous oxide concentration with time for ADN, ADN-KDN, ADN-KF, ADN-HEX, and ADN-VC at 90 8C.
4
. Conclusions
The thermal decomposition of ADN in the solid
during decomposition. The reaction is observed to be
rst order in reactant over the temperature regime
investigated. In order to slow the decomposition of
ADN in this temperature regime, several additives
were examined. While monitoring AN production,
®
state has been examined at 60, 70, 80, and 90 8C by
monitoring the evolution of N O and AN produced
2

I.B. Mishra, T.P. Russell / Thermochimica Acta 384 ꢀ2002) 47±56
55
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KF and VC show reductions in AN formation. How-
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While monitoring N Oꢀg)production, all additives
2
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