NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

Dr. Manfred Bohn Fraunhofer-Institut für Chemische Technologie (ICT) Postfach 1240 D-76318 Pfinztal-Berghausen GERMANY e-mail: [email protected]

ABSTRACT Ammonium dinitramide (ADN) is one of the substances, which are seen to be able to replace ammonium perchlorate (AP) as oxidizer in rocket propellants. Some of the performance data in comparison to AP substantiate this suggestion (values for AP in brackets). It has a good oxygen balance of +25.8% (+34%), a more positive enthalpy of formation, -1208 J/g (-2518 J/g) and a higher heat of explosion as AP, 3337 J/g (1972 J/g). An additional advantage is that the reaction products of ADN are free of signature. One drawback of ADN clearly is the much lower thermal stability than that of AP, together with the relatively low melting point between 92 and 94°C. But the lower stability must not exclude its application in some types of rocket systems. The intrinsic stability of ADN is not much lower than that of unstabilized nitrocellulose. If it would be possible to find stabilizers, ADN can be considered as a substitute candidate for AP. An extensive investigation was made with eight substances to find out their stabilization effect on ADN. The ADN samples were mixed intensively with an amount of 2 mol-% each of these chemicals. The experimental methods used to determine the stabilization effect have been mass loss as function of time and temperature, 65°C, 70°C, 75°C, 80°C, and adiabatic self heating. ADN without additives decomposes autocatalytically. All tested chemicals can prevent the autocatalytic increase in decomposition activity, some of them do it well. The stabilization ability changes somewhat with temperature. With well stabilized solid phase ADN the decomposition of the rate determining step is the N-N bond cleavage in the dinitramide anion. In unstabilized ADN the protonation of hydrogen dinitramide accelerates its decomposition autocatalytically.

1.0 INTRODUCTION

Ammonium dinitramide (ADN) is the ammonium salt of the dinitramine HN(NO2)2, HDN, which can be named hydrogen dinitramine when seen as covalent compound or hydrogen dinitramide when seen as ionic compound. HDN is a very strong acid in water. In pure form it decomposes immediately explosive − like. The anion N(NO2)2 has experienced some other names: dinitraminate and dinitramidate. There is some plausibility for using these names but the same holds for naming it dinitramide. So here with the latter nomenclature will be continued. ADN has a potential as performing oxidizer and may replace ammonium perchlorate (AP). The chemical structure formula of ADN can be seen in Fig. 1. In the solid phase it is present in the ionic form. The basic performance data are listed in Table 1, together with those of AP, ammonium nitrate (AN) and hydrazinium nitroformate (HNF). ADN has a high positive oxygen 0 balance of + 25.8%, the enthalpy of formation ∆Hf is more positive than the one of AP and the intrinsic heat of explosion of ADN is higher than the one of AP. ADN is chlorine free and a reduction of signature in the exhaust of rocket motors is achievable easier than with AP. Specific impulses ISP of > 250 Ns/N seem reachable. But ADN is intrinsically instable in the range of the usual service temperatures. Its melting temperature is with about 92°C relatively low and lower melting mixtures and eutectics with

Paper presented at the RTO AVT Specialists’ Meeting on “Advances in Rocket Performance Life and Disposal”, held in Aalborg, Denmark, 23-26 September 2002, and published in RTO-MP-091.

RTO-MP-091 9 - 1

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase ammonium nitrate below 80°C have to be regarded. Chemically ADN is an aggressive and strong nitration and oxidation agent. The compatibility between ADN and other components in a formulation is therefore demanding. Therewith two questions are essential: • Can ADN be stabilized reliably ? • Are the compatibility demands reachable ?

This paper deals only with the first question.

NO2 + NH4 N NO2

Figure 1: Chemical Structure Formula of ADN (Ammonium Dinitramide). In the solid phase ADN is present in ionic form as ammonium cation and dinitramide anion.

Table 1: Performance data of the oxidizers ADN, AP, AN and HNF. The values of the heats of explosion and gas volumes have been calculated by the ICT-Thermo-dynamic-Code. The other data are from the ICT-Thermochemical Data Base [1].

substance molar O2-balance enthalpy of formation QEX QEX, density gas TM 0 mass ∆Hf wg *) wl *} vol. *]

g/mol % kJ/mol J/g J/g J/g g/cm3 cm3/g °C ADN 124.056 + 25.8 − 149.8 − 1207.5 2668 3337 1.81 592 92.9 AP 117.489 + 34.0 − 295.8 − 2517.7 1396 1972 1.95 533 130; D AN 80.043 + 20.0 − 365.6 − 4567.5 1441 2479 1.73 459 169.9 HNF 183.081 + 13.1 − 76.9 − 420.0 5012 5579 1.91 568 124; D

*) wg: QEX value with water as gas.

*} wl: QEX value with water as liquid at 20°C. *] gas volume of the substance with thermodynamically controlled combustion, at 25°C without water.

D in the column of TM means decomposition at the given temperature, which may vary with the sensitivity of the observation.

In [2] a further value of −148 ± 10 kJ/mol for the heat of formation at constant volume is reported for 0 ADN. With this the value of −162.8 kJ/mol (−1312.3 J/g) results for the enthalpy of formation ∆Hf at standard conditions. Hydrogen was used instead of oxygen to burn ADN in the bomb calorimeter.

2.0 METHODS AND MATERIALS USED The following measurement methods have been used to determine the stabilizing effect of an added substance in comparison to the ADN alone.

9 - 2 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

• Mass loss measurements ADN without and with additives, three samples in parallel, each 1 to 2 grams at temperatures of 65°C, 70°C, 75°C, 80°C. The samples were aged in pyrex glass vials inserted in aluminium block ovens. The temperature constancy was very good by PID control. Also in long terms the constancy was at least ± 0.3°C. Therefore the temperature was additionally controlled by calibrated devices and sensors independent from those of the PID controller. The mass loss is proportional to the ‘global’ conversion of the sample.

• Adiabatic self heating The adiabatic self heat rate was determined with an ARCTM (Accelerating Rate Calorimeter). Details about this method can be found in [3]. The adiabatic self heat rate is proportional to the ‘global’ reaction rate of the sample. The measurement temperatures are in the range of 90°C to 200°C.

With these methods two different probings of the samples have been achieved: Method Probing Mass loss sum of split-off decomp. gases (absolute effect) Adiabatic self heating sum of heats of reaction (net effect)

The use of methods with different probing assures the results [4]. The ADN used was manufactured at ICT in 1998. Eight substances S have been applied as additives to test their stabilization effect on ADN. The amounts in the mixtures have been number-normalized that means a constant molar percentage of a substance S was added to ADN, here two mol-%. The mixtures have been mixed intensively with a three axis moving mixer for one hour. A further ADN lot made at ICT and a lot from a company are compared with non-stabilized ADN-ICT-1998.

3.0 RESULTS

3.1 Mass Loss

4 ML [%] ADN-ICT-1998 3.5 non-stabilized

3 65°C 70°C 2.5 75°C 2 80°C

1.5

1

0.5 time [d]

0 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

Figure 2: Mass Loss of Unstabilized ADN-ICT-1998 at Temperatures between 65°C and 80°C.

RTO-MP-091 9 - 3

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

Fig. 2 shows the mass loss of ADN alone at the temperatures 65°C, 70°C, 75°C and 80°C. Especially at 80°C and 75°C the autocatalytic type increase of mass loss can be seen well in the scaling of the figure. At 80°C the steady increase of the mass loss starts at about 10 to 12 days. Sometimes the ADN liquefies after such times already and the mass loss increases very strong in a catastrophic like manner, Fig. 3. Additional acceleration of the decomposition is induced by liquefaction. It is therefore necessary to stabilize ADN.

0.8 ML [%] Comparison of some ADN lots 0.7 ML measurements at 80°C

0.6 ICT 3/1998 ICT 9/2000 0.5 Company 1998

0.4

0.3

0.2

0.1 time [d] 0 0 2 4 6 8 1012141618202224

Figure 3: Comparison of Three ADN Lots. ICT 9/2000 behaves qualitatively similar to ADN of a company, measured in 1998.

For the application of ADN in formulations the primary product after production is rarely suitable. The ADN should be prilled, means transformed to sphere-like shape. By this process, done at ICT, the stability of ADN is only lowered somewhat, see Fig. 4.

8 ML [%] ADN-ICT-1998 7 non-stabilized comparison unprilled - prilled 6 80°C 5 75°C 70°C 4 80°C-Prill 75°C-Prill 3 70°C-Prill

2

1 time [d] 0 0 25 50 75 100 125 150 175 200 225 250 275 300 325 -1 Figure 4: Comparison of Unprilled and Prilled ADN-ICT-1998. The prilled ADN has only a somewhat lower stability than the start material.

9 - 4 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

In Fig. 5 to Fig. 7 the mass losses of ADN alone and of ADN with added S, numbered from S1 to S8 are shown at 80°C and 75°C. In all cases the autocatalytic type increase of the mass losses of ADN alone can be suppressed. In Fig. 5 the measurements have been extended until the stabilizers have been consumed. Then the autocatalytic increase starts also with the samples with added S#. As long as the stabilizers are active, the mass losses of the samples increase linearly, means the content of ADN decreases according to the Eq.(5) of section 4.1. From this part of the measurements the reaction rate constant for the intrinsic decomposition of ADN can be obtained, but slightly influenced by the effectiveness of the stabilizer. The smaller the slope of the mass loss the better the stabilizer. For the kinetic description of mass loss data see [4] and section 4.2.

12 ML [%] ADN non-stab. S1 10 S2 ADN-ICT-1998 S3 80°C S4 8 S5 S6 S7 6 S8

4

2

time [d] 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260

Figure 5: Mass Loss at 80°C of ADN-ICT-1998 Alone and with Added Substances. The stabilizing effect is achieved with all additives. The lower the mass loss in the region up to 60 to 70 days the better the stabilizing effect.

4 ML [%] ADN-ICT-1998 3.5 80°C ADN non-stab. 3 S1 S2 S3 2.5 S4 S5 2 S6 S7 S8 1.5

1

0.5 time [d] 0 0 102030405060708090

Figure 6: As Fig. 5. Mass Loss at 80°C of ADN-ICT-1998 Alone and with Added Substances, another Coordinate Scaling.

RTO-MP-091 9 - 5

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

4 ML [%] ADN-ICT-1998 3.5 ADN non-stab. S1 75°C 3 S2 S3 S4 2.5 S5 S6 2 S7 S8 1.5

1

0.5 time [d] 0 0 20 40 60 80 100 120 140 160 180 200 220

Figure 7: Mass Loss at 75°C of ADN-ICT-1998 Alone and with Added Substances.

3.2 Adiabatic Self Heating Fig. 8 shows the adiabatic self heating of ADN without additives in comparison with nitrocellulose (NC) and some high explosives. ADN has the lowest onset temperature of all of them. It is much less thermally stable than ε-CL20 (ε-HNIW), RDX and β-HMX. But it is comparable with the stability of NC without additives. All measurements were done with similar φ-factors [see 3]. The sample masses used were between 200mg (CL20, RDX, HMX) and 300mg (ADN, NC).

100 h [°C/min] adiabatic self heating

10 ADN RDX

HMX 1 NC

0.1 CL20 FOX 7

T [°C] 0.01 100 120 140 160 180 200 220 240

Figure 8: Adiabatic Self Heating of ADN-ICT in Comparison with Nitrocellulose (NC), N=12.6 Mass-%, and some Typical High Explosives.

9 - 6 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

In Fig. 9 one can see the adiabatic self heating of ADN alone and ADN with the additives S#. The curve of ADN alone shows initially an upwards convex bending. This feature is indicative for autocatalytic acceleration of the decomposition of ADN, which is in the liquid state in this type of investigation. All curves are shown up to the transition from controllable self heating to deflagration. At the higher temperature side the ADN decomposition is followed / superimposed by the AN decomposition. Important for the assessment of the stabilization effect of an additive is the initial range of the curves, from 110°C up to about 125°C. Here all the curves ADN + S# are below the one of ADN alone. Because the self heat rate is proportional to the decomposition rate the data show that the decomposition of ADN is retarded by the additives and the conclusion is its autocatalytic decomposition is suppressed.

Figure 9: Adiabatic Self Heating of ADN-ICT-1998 without and with Additives S#. All curves of ADN with additive are in the initial range below the curve of ADN without additive, indicating the effect of stabilization.

4.0 DISCUSSION

4.1 Principle of Stabilization Chemical stability means to know the state (actual conversion) and the rate of chemical decomposition reactions in a material. Stabilization means to reduce the rate of chemical decomposition reactions at least to an acceptable extent. If one assumes that the decomposition of ADN proceeds according to reaction scheme Eq.(1), the stabilization can be achieved by removing the autocatalytically effective decomposition product B by the chemical substance S which binds B chemically. C stands for gaseous products and D for solid products. k A →A B + C + D intrinsic decomposition k (1) A + B auto→ 2 B + C + D autocatalytic decomposition k S + B →S B-S stabilization reaction

RTO-MP-091 9 - 7

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

The first reaction of Eq.(1) describes the intrinsic decomposition, which cannot be stopped with usual stabilization techniques in applying only a few mass-% or mol-% of the additive. A change of the intrinsic stability behaviour could be achieved by complexation, which must be done on a 1:1 ratio. The second reaction of Eq.(1) stands for the autocatalytic decomposition and the third reaction describes the stabilization. The corresponding system of differential rate equations is shown in Eq.(2).

dA(t ,T) = − k (T) ⋅ A(t ,T) − k (T) ⋅ A(t ,T)⋅B(t ,T) dt A auto dB(t ,T) (2) = k (T) ⋅ A(t ,T) + k (T) ⋅ A(t ,T) ⋅B(t ,T) − k (T) ⋅ S(t ,T) ⋅B(t ,T) dt A auto S dS(t ,T) = − k (T) ⋅ S(t ,T) ⋅B(t ,T) dt S

Because of the stabilizer reaction, the autocatalytic decomposition in Eq.(1) is suppressed and the reaction scheme Eq.(3) results. Eq.(4) shows the differential equations of Eq.(3) and Eq.(5) to Eq.(8) the solutions of Eq.(4). Approximations are used as indicated. More details about this can be found in [5]. Eq.(8) describes the stabilizer decrease well if S has no consecutive stabilizing products or these have only small reactivities compared to the primary stabilizer. k A →A B + C + D intrinsic decomposition k (3) S + B →S B-S stabilization reaction

dA(t ,T) = − k (T) ⋅ A(t ,T) dt A dB(t ,T) (4) = k (T) ⋅ A(t ,T) − k (T) ⋅ S(t ,T) ⋅B(t ,T) dt A S dS(t ,T) = − k (T) ⋅ S(t ,T) ⋅B(t ,T) dt S

(5) A(t ,T) = A(0)⋅ exp()− k A (T) ⋅ t ≅ A(0) ⋅ (1− k A (T) ⋅ t ) pseudo-zero-order approximation

(6) B(t ,T) − S(t ,T) = A(0) + B(0) − S(0) − A(0)⋅ exp(− k A (T)⋅ t ) ≅ A(0)⋅k A (T)⋅ t with A(0) + B(0) − S(0) ≈ A(0)

dS(t ,T) (7) ≈ −k (T) ⋅ S(t ,T) ⋅ ()S(t ,T) + A(0) ⋅k (T) ⋅ t dt S A

exp()− A(0) ⋅k (T) ⋅k (T) ⋅ t 2 (8) S(t ,T) = A S 1 π k (T)  A(0) ⋅k (T) ⋅k (T)  + ⋅ S ⋅ erf  A S ⋅ t    S(0) 2 A(0) ⋅k A (T)  2 

In Eq.(9) a reaction scheme is shown by which the additional acceleration of the decomposition by liquefaction can be described.

9 - 8 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

kA s As → B + C + L + D intrinsic decomposition, solid part

kli AS + (L > LG) → Ali liquefaction ‘reaction’

kAli (9) Ali → B + C + L + D intrinsic decomposition, liquid part

kauto s As + B → 2 B + C + L + D autocatalytic decomp., solid part

kauto li Ali + B → 2 B + C + L + D autocatalytic decomp., liquid part

4.2 Description of ADN Decomposition with Mass Loss The decomposition reaction is shown again with Eq.(10).

kA (10) A → B + C + D + (-∆HR,A)

A stands for ADN, C represents all gases, D are the remaining solids and B remains in the residue. The expression for the corresponding masses is given with Eq.(11), see [4]. The symbols mi stand for the molar masses of A, B, C, D. The mean molar mass of the gases is given in Eq.(12). The mass M(0) is given by Eq.(13), whereby a non-reacting component MN can be present in the formulation or mixture. For MAr(t,T) = MA(t,T)/MA(0) the corresponding kinetic expression has to be inserted. Important to notice is the difference between the measured quantity M(t,T) and the looked for quantity MA(t,T).

M(t ,T) m A − mB − mD M A (0) (11) Mr (t ,T) = = 1 − ⋅ ⋅ ()1− M Ar (t ,T) M(0) m A M(0)

(12) mC = mA – mB – mD

(13) M(0) = MA(0) + MB(0) + MD(0) + MN

To find the kinetic expression for MAr(t,T), a reaction of first order is assumed for the initial ADN decomposition, Eq.(14). With small conversions Eq.(15) results as approximation, which describes a pseudo-zero order reaction. From Eq.(15) one obtains Eq.(16) immediately by multiplying Eq.(15) with the molar mass mA. The Eq.(17) and Eq.(18) show the definitions of the mass loss ML as percentage quantity and of the mass loss MD as quantity in parts per 1.

(14) A(t ,T) = A(0) ⋅ exp()− k A (T) ⋅ t

(15) A(t ,T) ≈ A(0) ⋅ ()1− k A (T) ⋅ t

(16) M Ar (t ,T) = 1− k A (T) ⋅ t

(17) ML(t,T) = 100% (1 − Mr(t,T)) mass loss in parts per 100

(18) MD(t,T) = (1 − Mr(t,T)) mass loss in parts per 1

RTO-MP-091 9 - 9

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

One arrives at Eq.(19) by combining Eq.(11), Eq,(16) and Eq.(18).

mA − mB − mD M A (0) (19) MD(t ,T) = ⋅ ⋅k A (T) ⋅ t mA M(0)

With pure ADN the ratio MA(0)/M(0) equals to 1. To obtain a value for the molar mass ratio in Eq.(19) the approximation of the ADN decomposition during the initial part is made according to Eq.(20). ADN decomposes in ammonium nitrate and the gas dinitrogen oxide. For mB the hydrogen mass is taken. The ratio has then the value of about 0.347. In Eq.(21) the corresponding mass loss MD(t,T) is shown. The interesting quantity MAr(t,T) is given in Eq.(22). With one percent of measurable mass loss already 2.88 percent of ADN are decomposed. The autocatalytic decomposition is described using an autocatalytic rate expression for MAr(t,T), see [4, 13].

(20) ADN → AN + N2O

(21) MD(t ,T) ≈ 0.347 ⋅k A (T) ⋅ t and ML(t ,T) ≈ 100% ⋅ 0.347 ⋅k A (T) ⋅ t

(22) M Ar (t ,T) ≈ 1− 2.88 ⋅MD(t ,T)

4.3 Decomposition Reactions of ADN About the decomposition mechanisms of ADN already several publications exists. Some are experimental [6 to 14], and in the other part of them the authors try to find out the stability and decomposition by quantum mechanical (QM) calculations [15 to 19]. In Fig. 10 some reactions are shown, which can describe the more important steps in ADN decomposition. It is assumed that especially at higher temperatures the ionic form of ADN transforms to a non-ionic complex of ammonia and hydrogen dinitramide, shown in reaction 1 of Fig.10. In the gas phase this complex is the thermodynamically stable form of ADN, as the QM-calculations have shown. The complex my dissociate and the HDN is able to exist in several conformations. One conformation has the H-atom at the middle N, another conformation is with the H atom at an O-atom of a NO2 group and a third conformation may have a hydrogen bridge type bonding of the H-atom between two NO2 groups. The conformations are shown in Fig. 10 with reaction 2. The advantage of the latter conformation can be a resonance stabilization of HDN as shown in reaction 3. In [8] the resonance form of HDN was postulated from the band shift to lower wavenumbers of the NO2 groups in the IR spectrum.

It is now widely assumed that the decomposition of ADN starts with the decomposition of any configurational form of HDN. In [11, ADN in liquid phase] it was proven by N-15 isotope marking, that the formed N2O must come out of the HDN by breaking one N-N bond. The nitrogen N2 is formed with the NH3 nitrogen atom in secondary reactions. The experimentally found activation energies are not very high, they range mostly between 130 and 170 kJ/mol. Several research groups have reported about an autocatalytic type decomposition of ADN [6,12,13]. Also by theoretical means a catalyzed decomposition mechanism is discussed [15], whereby the protonation of HDN seems to reduce the activation energy to a great extent. In [12] it was experimentally shown that the protonation of HDN reduces the activation energy of decomposition from 114 kJ/mol to about 60 to 65 kJ/mol. This was found in aqueous and acidic solutions of ADN. So the protonation of HDN seems the kernel of the autocatalytic decomposition of ADN or HDN respectively. This is shown with reaction 4 in Fig. 10. Here protonated HDN decomposes via electronic rearrangements to protonated HNO3 and N2O. Protonated HNO3 is considered as a very effective autocatalyst species for decomposing HDN and ADN. These reactions and others are compiled in Table 2. Also included are the secondary reactions with NH3. The collection of reactions may be extended, but this one already can explain qualitatively the found decomposition product spectrum. Main products

9 - 10 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

• • are NH4NO3 (AN), N2O, H2O, N2. The species HNO3, NO, NO2 and NH3 may also be recognized by • • • suitable gas phase analysis methods, as well as the intermediary radicals NH, NH2, HN NO2 and others.

NO2 NO2 + N NH4 NH3 . H-N NO2 NO2

Reaction 1: Dissociation of ionic ADN to a complex between NH3 and HN(NO2)2

O δ+ H δ− δ− NO2 N O OH O H-N N δ+ δ+ N N NO2 NO2 N O O δ−

Reaction 2: Isomeric forms of HN(NO2)2

δ+ δ+ H H δ− δ− δ− δ− O O O O

δ+ δ+ δ+ δ+ N N N N N N O O O O δ− δ−

Reaction 3: Resonance stabilization in HN(NO2)2

H δ− H H δ− O O O O H + O δ+ δ+ N δ+ δ+ δ+ O δ− N N δ− N N N δ+ O O O N O O δ− O +H δ− N H + N

+ Reaction 4: Protonation of HN(NO2)2 and decomposition of H2N(NO2)2 to N2O and protonated HNO3

Figure 10: Reactions of ADN with the Corresponding Structural Changes.

RTO-MP-091 9 - 11

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

Table 2: Chemical Reactions in ADN Decomposition. TS means Transition State.

No. Reaction Description

1 NH4 N(NO2)2 → NH3 + HN(NO2)2 ‘dissociation’ of ADN

+ 2− 2 HN(NO2)2 + HN(NO2)2 → H2N(NO2)2 + N(NO2) self proton. of HN(NO2)2 + + + 3 H2N(NO2)2 → H2NO3 + N2O decomposition of H2N(NO2)2 , via TS

• • 4 HN(NO2)2 → H NNO2 + NO2 homolytical decomposition of HDN • 5 H NNO2 • decomposition type 1 of nitramine radical → N H + NO2

6 HN(NO2)2 → O2N-NN(O)OH formation of HDN isomer, 1,3-H re-arrang.

7 O2N-NN(O)OH → N2O + HNO3 decomposition of HDN isomer via TS

• • 8 O2N-NN(O)OH → NO2 + NN(O)OH decomp. of HDN isomer, NO2 form. No. 9 to 17: secondary reactions

• • • • 9 NH2 + NO2 → H2NO + NO part of secondary reaction series • • 10 NH2 + NO → N2 + H2O part of secondary reaction series • • 11 NH3 + NO2 → NH2 + HONO part of secondary reaction series

12 NH3 + HONO → NH2NO + H2O part of secondary reaction series

13 NH3 + HONO → N2 + 2 H2O formation of nitrogen

• • 14 H NNO2 → NN(O)OH 1,3-H rearrangement • • 15 NN(O)OH → N2O + OH decomposition type 2 of nitramine radical • • 16 OH + NH3 → H2O + NH2 14, 15 + 16 + 17 together with 4 sum up to • • 17 NH2 + NO2 → N2O + H2O ADN --> 2 H2O + 2 N2O

No. 18 to 21: autocatalytic attack of HNO3 on ADN and HN(NO2)2

18 HNO3 + [NH3] [HN(NO2)2] → NH4NO3 + HN(NO2)2 re-salting by HNO3

+ − 19 HNO3 + HN(NO2)2 → H2N(NO2)2 + NO3 protonation of HN(NO2)2 + + 20 H2NO3 + HN(NO2)2 → H2N(NO2)2 + HNO3 protonation of HN(NO2)2 + + 21 H2NO3 + [NH3] [HN(NO2)2] → H2N(NO2)2 + NH4NO3 protonation of HN(NO2)2

At lot of QM work was done to clarify the reaction mechanisms of HDN decomposition, see Table 3. The precision of the calculations increase from the older work of Politzer [15] to the more recent work of Mebel a.o. [18] and Chakraborty a.o. [19]. Surprisingly the activation energies found for the gas phase decompositions are in the range of some experimentally found activation energies for the liquid phase decomposition of ADN at higher temperatures, Table 4.

9 - 12 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

Table 3: Calculated Bond Energies in Gas Phase Molecules. As indicated, in two cases the given energy is a transition state (TS) activation energy (Ea).

Bond Resulting products by Bond Reactant energy Reference bond scission type [kJ/mol]

− • − • 2ON-N(NO2) → NNO2 + NO2 N - N 190 Politzer [15] + − H-N(NO2)2 → H + N(NO2)2 H - N 313 Politzer [15] • • H-N(NO2)2 → H + N(NO2)2 H - N 401 Politzer [15] • • H2N-NH2 → H + N(H)NH2 H - N 364 Politzer [15] • • HN(NO2)2 → H NNO2 + NO2 N - N 184 Politzer [15] + + (NO2)N-NO(OH) + H → HONO2 + H + N2O TS (Ea ≈ 0) Politzer [15]

• • HN(NO2)2 → H NNO2 + NO2 N - N 201±34 Michels [16]

(NO2)N-NO(OH) → NNO2 + HONO N - N 247±34 Michels [16]

• • HN(NO2)2 → H NNO2 + NO2 N - N 161 Mebel [18] • • (NO2)-NNO(OH) → NNO(OH) + NO2 N - N 168 Mebel [18]

(NO2)N-NO(OH) → HONO2 + N2O TS (Ea=177) Mebel [18] • • HN(NO2)2 → H NNO2 + NO2 N - N 159 Chakraborty [19]

• • H NNO2 → N H + NO2 N - N 159.4 Chakraborty [19]

Table 4 lists the experimentally found activation energies of ADN decomposition at different conditions. Of main interest is the decomposition in the solid phase at temperatures experienced at service conditions, say in the temperature range 20°C to 80°C. Not much data are available at these conditions. The two groups [6, 13, this work] which have published some data have found with about 136 to 138 kJ/mol nearly equal activation energies for the decomposition of solid ADN in the lower temperature range. All other results have been obtained in the liquid phase or with a solution of ADN in water. Some results are surprising: the very low activation energy of 75 kJ/mol found by Oxley [11] in the temperature range 120°C to 160°C and the very high activation energy of 237 kJ/mol found by Trubert [14] with a DSC-method. The latter work reports an exceptional high value for the frequency factor also. Maybe some methodological influences caused these special values. The two respectively three values of the activation energy given for solid ADN with the assumption of a first order decomposition are equal within the experimental error range. Further these both groups [6,13, this work] have found an autocatalytic type acceleration of the decomposition in the solid phase too. With one ADN lot (different to the one used here to investigate the effect of additives) both sets of Arrhenius parameters are given.

RTO-MP-091 9 - 13

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

Table 4: Experimental Arrhenius Parameters of Thermal Decomposition of ADN at Different Conditions. All evaluations assume a first order decomposition, the two exceptions are indicated, there autocatalysis is included.

Act. Energy Pre-exponential Corr. Temp. Range [°C] Reference [kj/mol] factor Coeff. conditions decomposition of ADN without additives 40 to 80°C, solid phase, gas 138.2 2.379 E+13 cm3/g/s 0.9999 Manelis [6] formation 97 to 115°C and 150 to 170°C, 148.5 2.51 E+14 1/s ? Manelis [6] liquid phase, gas formation 65 to 80°C, solid phase, mass loss 136 8.72 E+12 %/s 0.9943 this work up to 0.5% 65 to 80°C, solid phase, mass loss 138 6.31 E+11 1/s 0.99 Bohn [13] up to 3% non-autocatalytic part 132 1.59 E+13 1/s 0.98 autocatalytic part 100 to 300°C, fast heating up, 133.9 3.5 E+15 1/s ? Korob. [9] product analysis with TOF MS 160°C, 180°C, 200°C, liquid, 166.9 8.81 E+16 1/s 0.9994 Oxley [11] dinitramide depletion 120°C, 140°C, 160°C, liquid, 75.4 7.7 E+5 1/s 0.9991 Oxley [11] dinitramide depletion 120 to 200°C, 20 mass-% ADN in 157.4 1.007 E+16 1/s 0.9999 Oxley [11] water, DN depletion liquid, DSC 237.1 1.2 E+26 1/s ? Trubert [14] (Kissinger method ?) 113 1.8 E+11 1/s ? liquid, DSC-ASTM-method Oxley [11] dilute aqueous solution of ADN, 170.4 1.7 E+17 1/s ? − Kazakov [12] decomp. of N(NO2)2 anion dilute aqueous solution of ADN, 113.9 3.3 E+14 1/s ? Kazakov [12] non-dissociated HN(NO2)2 aqueous + H SO solution of 65.7 1 E+12 1/s ? 2 4 Kazakov [12] ADN, protonated HN(NO2)2 aqueous + HNO solution of ADN, 60.7 6.3 E+2 1/s ? 3 Kazakov [12] protonated HN(NO2)2 144.8 7.7 E+14 ? liquid ADN Kazakov [12] non-autocatalytic part 93.1 5.6 E+12 ? autocatalytic part decomposition of solid ADN with additives 235 (average) 65 to 80°C, solid phase with this work additives, mass loss up to 2%

9 - 14 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

4.4 Stabilization Effect From the above discussion one can conclude that a stabilizing additive for ADN should suppress the protonation of HDN, which then results in the difference shown with Eq.(1), (2) and Eq.(3), (4) in section 4.1. Table 4 contains also the averaged activation energy of the decomposition of ADN with the additives S1 to S8 given above with exception of additive S2. This additive caused a decomposition with nearly the same activation energy as for ADN without additive. Therefore it is not considered as a possible stabilizer. The activation energies of the mixtures range between 210 and 280 kJ/mol. This results in some temperature dependent stabilization ability of the investigated additives. At 70°C the following series of the stabilization ability was found: S8 > S7 > S3 > S4 ≈ S6 > S5 ≈ S1 > S2.

From the measurements of adiabatic self heating in the range 110°C to 125°C a somewhat different series of stabilizer effectiveness was found: S7 > S8 > S1 > S6 > S3 > S5 > S2. The additive S4 was not investigated. Compared with the series above the positions of S1 and S3 have been changed to a greater extent. The distribution of the additive in liquid ADN may change compared to the one in solid ADN. Further influence on the effectiveness may come from a changing ability to act as proton scavenger as function of temperature because of the temperature dependence of chemical equilibriums.

4.5 Discussion of the Decomposition in Stabilized ADN The average activation energy for ADN with additives is with about 235 kJ/mol much higher than the one for the decomposition of ADN without additives with about 138 kJ/mol. For this result the following argumentation is given. The activation energy of 136 to 138 kJ/mol is the one of the transition state in forming a complex between NH3 and HDN out of the ionic educts. For ADN alone the dissociation is assumed as rate controlling step. The following decomposition of HDN is catalysed by proton transfer to HDN. It was shown theoretically and proven experimentally that the acid catalysed decomposition has a much lower activation energy or even nearly no activation energy, see Tables 3 and 4. In the case of added stabilizers this proton transfer to HDN is blocked and one measures the intrinsic non-catalysed − decomposition of HDN or even of anion N(NO2)2 in the solid state and this would then be the rate controlling step. This decomposition is controlled by the scission of a N-N bond in the dinitramide. It is not easy to get plausible values for the N-N bond energy in HDN. Some theoretical works postulated values between 159 to 201 kJ/mol, whereby the lower value seems the more accurate one because of the use of refined quantum mechanical methods. Nearly all the theoretical calculations deal with ADN or HDN in the gaseous state or more precisely with isolated single molecules. So all N-N bond scissions in these investigations were attributed to bonds with a bond length of about 145 to 148 pm, Fig. 11. This is the length of the N-N bond in hydrazine and there the bond energy is about 160 kJ/mol. But in solid − ADN the bond length was found by X-ray scattering to be 136 pm for the anion N(NO2)2 , Fig. 12, + − much shorter than in hydrazine. A theoretical calculation [18] on gaseous ionic ADN, [NH4 ][N(NO2)2 ] has found a N-N bond length of 138.1 pm, also shown in Fig. 12. The average length of the N-N double bond is 125 pm with an average bond energy of 418 kJ/mol. If one just scale the bond lengths with the bond energies one gets for a N-N bond length of 137 pm the bond energy of 277 kJ/mol (=418-(418-160)/(125-147)*(125-137)). This is in the range of the found activation energy of ADN decomposition with additives. Taking into account the crude estimate and the possibility of a residual catalyzed decomposition of HDN the found activation energies between 210 and 280 kJ/mol are − explainable as based upon the scission of the N-N bond in the anion N(NO2)2 in the solid ADN phase.

RTO-MP-091 9 - 15

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

O O O O 122.5 122.4 120.9 146.3 120.9 N N N N 129.3 122.5 121.7 139.3 N 148.1 O 121.7 N O 102.6 O O H 98.5

H Figure 11: Calculated Bond Lengths in pm [18] n Hydrogen Dinitramine (HDN) in the Gas Phase for two Isomeric Forms of HDN: left one with the H-atom at the middle N atom, right one with the H-atom at a NO2 group.

O O 123.50 123.55 124 123

N 138.1 138.1 N 136 135 127.0 127.0 N 125 125 O O

Figure 12: Bond Lengths in pm in the Dinitramide Anion in Ionic ADN. + - Normal letters are calculated values [18] for gas phase [NH4 ][N(NO2)2 ]. Italic letters are from experimental X-ray data of solid ADN [20].

There is a publication on the stabilization of ADN with additives [21], but investigated with molten ADN in the temperature range 100°C to 120°C. The reduction of the initial gas formation rate and the times of an induction period were used to assess the effectiveness. The stabilization effect of additives is explained + on their ability to protect NH4 from oxidation by HDN or its anion. The authors concluded to use easily + oxidateable additives as stabilizers, because the oxidation of NH4 would form an autocatalytic active species. In spite of the fact that for potassium jodide a stabilization ability is claimed, it seems a not conclusive explanation.

5.0 SUMMARY AND CONCLUSION It was found that ADN alone decomposes autocatalytically. From what is known now one can conclude that the ‘dissociation’ of ionic solid ADN to a complex of ammonia and hydrogen dinitramide is the first step to decomposition. HDN itself may then decompose. But the protonation of HDN increases the rate of HDN decomposition very strongly. The protonation of HDN is part of an autocatalytical reaction cycle including several reactions. To stabilize ADN the autocatalytic decomposition reaction must be suppressed. This was achieved with additives. Seven of eight additives have shown an acceptable to good

9 - 16 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase stabilization ability. Some stabilizers kept the mass loss below 1.5% up to 60 days at 80°C. This is considered as well stabilized. Two methods with different probing of the samples were used. One was mass loss as function of time and temperature (the probing is the split-off of decomposition gases) in the temperature range 65°C to 80°C, means solid ADN samples. The other method was the adiabatic self heating measured with an ARCTM (the probing is the net effect of the heats of reaction), in the temperature range 110°C to 125°C, means liquid ADN samples. The stabilization effect varies somewhat with temperature. From the obtained activation energies with the mass loss measurements on ADN samples − with additives it is concluded that the intrinsic decomposition of the anion N(NO2)2 by N-N bond splitting is the rate controlling step with well stabilized ADN samples.

6.0 ACKNOWLEDGMENT The work was financed by the German Bundesministerium für Verteidigung (BMVg).

7.0 REFERENCES [1] H. Bathelt, F. Volk, M. Weindel. The ICT Thermochemical Data Base. Proceed. 30th International Annual Conference of ICT, pages 56-1 to 56-12, June 29 – July 2, 1999, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany. [2] H. Östmark, U. Bemm, A. Langlet, R. Sanden, N. Wingborg. The Properties of Ammonium Dinitramide (ADN): Part 1, Basic Properties and Spectroscopic Data. J. Energetic Materials 18, 123-138 (2000). [3] M.A. Bohn. Determination of Kinetic Data of the Thermal Decomposition of Energetic Plasticizers and Binders by Adiabatic Self Heating. Thermochim. Acta 337, 121-139 (1999). [4] M.A. Bohn. Kinetic Description of Mass Loss Data for the Assessment of Stability, Compatibility and Aging of Energetic Components and Formulations Exemplified with є-CL20. Propellants Explosives Pyrotechnics 27, 125-135 (2002). [5] M.A. Bohn, N. Eisenreich. Kinetic Modelling of the Stabilizer Consumption and the Consecutive Products of the Stabilizer in a Gun Propellant. Propellants Explosives Pyrotechnics 22, 125-136, (1997). [6] G.B. Manelis. Thermal Decomposition of Dinitramide Ammonium Salt. Paper 15 in Proceed. of the 26th Internat. Annual Conference of ICT 1995, pages 15-1 to 15-17, July 4-July 7, 1995, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany. [7] T.P. Russel, A.G. Stern, W.M. Koppes, C.D. Bedford. Thermal Decomposition and Stabilization of Ammonium Dinitramide. CIPA Publication 593, 29th JANNAF Combustion Subcommittee Meeting, Hampton, VA, USA, October 1992; Vol II, pages 339 -345. [8] V.A. Shlyapochnikov, N.O. Cherskaya, O.A. Luk’yanov, V.P. Gorelik, V.A. Tartakovsky. Dinitramide and Iits Salts. 4. Molecular Structure of Dinitramide. Russian Chem. Bulletin 43, 1522-1525 (1994). [9] O. Korobeinichev, A. Shmakov, A. Paletsky. Thermal Decomposition of Ammonium Dinitramide and Ammonium Nitrate. Paper 41 in Proceed. of the 28th Internat. Annual Conference of ICT 1997, pages 41-1 to 41-11, June 24-27, 1997, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany.

RTO-MP-091 9 - 17

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

[10] S. Löbbecke; H.H. Krause, A. Pfeil. Thermal Analysis of Ammonium Dinitramide Decomposition. Propellants Explosives Pyrotechnics 22, 184-188 (1997). [11] J.C. Oxley, J.L. Smith, W. Zeng, E. Rogers, M.D. Coburn. Thermal Decomposition Studies on Ammonium Dinitramide (ADN) and 15N and 2H Isotopomers. J. Phys. Chem. A 101, 5646-5652 (1997). [12] A. I. Kazakov, Yu. I. Rubtsov, G.B. Manelis. Kinetics and Mechanism of Thermal Decomposition of Dinitramide. Propellants Explosives Pyrotechnics 24, 37-42 (1999). [13] M.A. Bohn. Modelling of the Stability, Ageing and Thermal Decomposition of Energetic Components and Formulations using Mass Loss and Heat Generation. Proceed. 27th International Pyrotechnics Seminar, pages 751 to 770, July 16-21, 2000, Grand Junction, Colorado USA. [14] J. Hommel, J.F. Trubert. Study of the Condensed Phase Degradation and Combustion of Two New Energetic Charges for Low Polluting and Smokeless Propellants: HNIW and ADN. Paper 10 in Proceed. of the 33rd Internat. Annual Conference of ICT 2002, pages 10-1 to 10-17, June 25-June 28, 2002, Karlsruhe, Germany. Fraunhofer-Institut für Chemische Technologie (ICT), Postfach 1240, D-76318 Pfinztal-Berghausen, Germany.

[15] P. Politzer, J.M. Seminario. Computational Study of the Structure of Dinitramide Acid, HN(NO2)2, and the Energetics of Some Possible Decomposition Steps. Chem. Phys. Letters 216, 348-352 (1993). [16] H.H. Michels, J.A. Montgomery, Jr. On the Structure and Thermochemistry of Hydrogen Dinitramide. J. Phys. Chem 97, 6602-6606 (1993). [17] Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics. Editors: V. Yang, T.B. Brill, W.-Z. Ren, Vol. 185 of the series: Progress in Astronautics and Aeronautics, publ. in 2000. Series- Editor: P. Zarchan. American Institute of Aeronautics and Astronautics, Inc. Reston, Virginia, USA. [18] Mebel, A.M.; Lin, M.C.; Morokuma, K.; Melius, C.F., Theoretical Study of the Gas-Phase Structure, Thermochemistry, and Decomposition Mechanisms of NH4NO2 and NH4N(NO2)2. J. Phys. Chem. 99, 6842-6848 (1995). [19] D. Chakraborty, M.C. Lin. Gas-Phase Chemical Kinetics of [C, H, N, O] Systems Relevant to Combustion of Nitramines. in [17] , Chapter 1.2, 33-71. [20] B.V. Gidaspov, I.V. Tselinskii, V.V. Mel’nikov, N.V. Margolis, N.V. Grigor’eva, Crystal and Molecular Structure of Dinitramide Salts and Acid-Base Properties of Dinitramide. Russian Journal of General Chemistry 65, 906-913 (1995). [21] A.B. Andreev, O.V. Anikin, A.P. Ivanov, V.K. Krylov, Z.P. Pak. Stabilization of Ammonium Dinitramide in the Liquid Phase. Russian Chemical Bulletin 49, 1974-1976 (2000).

9 - 18 RTO-MP-091

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

SYMPOSIA DISCUSSION – PAPER NO: 9

Discusser’s Name: Erik Unneberg Question: If I understand you correctly, the main issue to stabilize AND is to prevent further proton transfer from the ammonium ion to the dinitramide ion, thus preventing further decomposition. In that respect, could you specify what kind of stabilizers (S1 to S8) you used? Would it be right to assume that they were not necessarily of aromatic character, that they would have been if they were going to consume products homo-cyclically released in the composition? Should the stabilizers have acid-base properties?

Author’s Name: M. Bohn Author’s Response: As to the nature of the substances used in this study, we cannot give any information at this time.

RTO-MP-091 9 - 19

NATO UNCLASSIFIED NATO UNCLASSIFIED

Stabilization of the New Oxidizer Ammonium Dinitramide (ADN) in Solid Phase

This page has been deliberately left blank

Page intentionnellement blanche

9 - 20 RTO-MP-091

NATO UNCLASSIFIED