UNIVERSITY OF PARDUBICE Faculty of Chemical Technology

and

EUROPEAN OFFICE OF AEROSPACE RESEARCH AND DEVELOPMENT

NEW TRENDS IN RESEARCH OF ENERGETIC MATERIALS

PROCEEDINGS OF THE VII. SEMINAR

CD version

Pardubice, Czech Republic

April 20 - 22, 2004

UNIVERSITY OF PARDUBICE Faculty of Chemical Technology

Department of Theory & Technology of Explosives CZ-532 10 Pardubice

EUROPEAN OFFICE OF AEROSPACE RESEARCH AND DEVELOPMENT London NW1 5TH United Kingdom

PROCEEDINGS of the seventh Seminar

„NEW TRENDS IN RESEARCH OF ENERGETIC MATERIALS“

CD version

held at the University of Pardubice, Pardubice, the Czech Republic

April 20 – 22, 2004

intended as a meeting of students, postgraduate students, university teachers and young research and development workers concerned

from the whole world

636

COMBUSTION OF DINITRAMIDE SALTS

V.P. Sinditskii, A.I. Levshenkov, V.Yu. Egorshev and V.V. Serushkin

Mendeleev University of Chemical Technology, 9 Miusskaya Square, 125047, Moscow, Russia

Abstract:

Burn rate studies have been carried out on dinitramide salts of common formula of L nHN(NO2)2, (where n = 1 or 2; L = methylamine, trimethylamine, tret-butylamine, tetramethylammonuim, urea, guanidine, aminoguanidine, triaminoguanidine, ethanolamine, diethanolamine, ethylenediamine, hexamethylenediamine, aniline, 3-nitroaniline, 2-toluidine, benzylamine, phenylhydrazine, morpholine, piperazine, pyridine, 3,5-dimethylpyridine, and 5-aminotetrazole) as well as lithium and barium salts of dinitramide. Most of the salts exhibit either combustion instability (like ADN) or a transition region on their rb(p) curves. The occurrence of this region depends upon the fuel reactivity and, largely, the surface temperature which is assumed to be dissociation one and dependent on the amine basicity. To disclose combustion mechanism, flame structure of hexamethylenediamine dinitramide (GMDADN) has been investigated with tungsten-rhenium microthermocouples. Besides, burn rate study of GMDADN within a wide initial temperature interval allowed the burning rate temperature sensitivity to be evaluated.

Keywords: Combustion, dinitramide salts, burning rate, temperature profile, temperature sensitivity

1. INTRODUCTION

Ammonium salt of dinitramide (ADN) is currently considered as one of the most promising substitute for the main oxidizer of composite propellants, ammonium perchlorate (AP). Unquestionable advantages of ADN over AP manifest themselves in possibility to produce higher-energy propellant compositions with no HCl in the combustion products, what is very important from ecological standpoint. At present a lot of organic and inorganic derivatives of dinitramide have been synthesized and published

[1,2]. Study of burning

behavior of these compounds is believed to allow revealing not only combustion mechanism of dinitramide salts, their place among other energetic materials, but also elucidating combustion peculiarities of ADN-based mixtures if being considered as molecular level model compositions of oxidizer-fuel premixing.

The aim of the present paper was to investigate thoroughly the burning behavior of organic and inorganic salts of dinitramide (DN) and propose a relevant combustion mechanism. The results of earlier investigations on dinitramide salts have been partly presented at some conferences

[3,4].

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2. EXPERIMENTAL Sample Preparation.

Dinitramide salts were prepared by one of the four general pathways: Scheme I

RNH2 + NH4N3O4 → RNH2· HN3O4 + NH3↑

Mixture of ADN with an excess of amine (with or without a solvent) was heated at 60°C for 4-6 hours under vacuum. The completion of the reaction was determined by the absence of NH4

+ cation in the reaction mixture. The residuum was washed and recrystallized from a suitable organic solvent.

Scheme II RNH2· H2CO3 + NH4N3O4 → RNH2· HN3O4 + NH3↑ + CO2↑ + H2O

Equimolar amounts of the reagents in excess of iso-propanol or ethanol were brought to boiling and kept at this temperature until ammonium evolution had stopped. After filtration a major portion of the solution was evaporated, and crystals of dinitramide salt were precipitated upon cooling.

Scheme III RNH2·HX + Ba(N3O4)2 → RNH2·HN3O4 + BaX2↓

HX = 1/2 H2SO4, HNO3

Solid barium dinitramide was dissolved in ethanol and treated with a solution of amine sulfate in aqueous ethanol. After separation of barium sulfate the resulting solution was evaporated until dry followed by recrystallization of the residue.

The metal and tetramethylammonium salts of dinitramidic acid were prepared as following:

Scheme IV

MOH + NH4N3O4 → MN3O4 + NH3↑ + H2O

Equimolar amounts of ADN and MOH (M = Li, Ba, Me4N) in water were heated to boiling and kept for one hour, filtered, and evaporated until dry.

ADN was twice-recrystallized from ethanol and used as the starting material in all the preparation reactions. The only stated impurity was about 0.4 per cent by weight of ammonium nitrate.

Physico-chemical properties of dinitramide salts studied are presented in Table 1.

Burn Rate Measurements. Burn rate measurements were carried out in a window constant pressure bomb of 1.5 liter volume. The pressure range studied was 0.1-36 MPa. A slit camera was used to determine a character of the combustion process as well as burning rate values.

The combustion strands were prepared by compacting the thoroughly comminuted substances in transparent acrylic tubes of 4 or 7 mm i.d. at 150-200 MPa.

Thermocouple Study. Temperature profiles were measured using thin tungsten-rhenium thermocouples embedded in the pressed samples. The thermocouples were welded from 25 µm diameter tungsten—5% rhenium and tungsten—20% rhenium wires and rolled in bands to obtain 5-7 µm bead size. A Tektronix TDS-210 digital oscilloscope was used to record the thermocouple signal. Calculations of adiabatic combustion temperatures were performed using the computer code REAL [5].

638

Table 1. Preparation methods and physical properties of dinitramide salts

Compound

Prepa-ration

method

Density of

pressed strand,

g/cc

Melting point °C

Ignition point °C

Enthalpy of

formation (calc.), kJ/mol

Ref.

Aminoguanidine dinitramide II 1.63 87-88 205 -48 [1]

5-Aminotetrazole dinitramide III 1.87 83-86 170 201 [3]

Aniline dinitramide III 1.52 95-97 182 39 [1] Benzylamine dinitramide I 1.44 59-61 - -19 [3] Tret-Butylamine dinitramide I 1.32 87-89 - -267 This

work Diethanolamine dinitramide I 1.55 75-78 198 -598 [3]

3,5-Dimethylpyridine dinitramide III 1.41 70-73 - 19 [3]

Ethanolamine dinitramide I 1.53 37-39 175 -335 [3]

Ethylenediamine bisdinitramide I 1.78 129-131 174 -209 [1]

Guanidine dinitramide II 1.53 133-136 - -168 [1] Hexamethylenediamine bisdinitramide I 1.40 89-90 192 -154 [3]

Methylamine dinitramide I 1.53 43-45 177 -121 [1]

Morpholine dinitramide I 1.49 82-84 180 -226 [3] m-Nitroaniline dinitramide III 1.61 101-103 195 -9 [3]

Phenylhydrazine dinitramide III 1.45 93-95 206 +151 This

work Piperazine dinitramide I 1.47 200-205 - -124 This

work Piperazine bisdinitramide I 1.50 212-214 215 -236 [3]

Pyridinium dinitramide III 1.53 89-91 - +35 This work

Tetramethylammonuim dinitramide IV 1.22 234-238 - -191 [1]

o-Toluidine dinitramide III 1.43 70-71 178 +6 [3] Triaminoguanidine dinitramide III 1.56 88-90 210 +168 [1]

Trimethylamine dinitramide I 1.28 128-130 - -90 [1]

Urea dinitramide III 1.70 96-98 - -346 [1] Barium bisdinitramide IV 2.70 - - -525 [2] Lithium dinitramide IV 1.96 164-169 - -238 [2]

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3. RESULTS AND DISCUSSION

One of peculiarities of dinitramide-anion is known to be the unique reaction of its transformation to form nitrate-anion and N2O.[6,7,8] Because of this exothermic reaction takes place, the studied inorganic dinitramide salts, barium and lithium, proved to be capable of self-sustained burning, although they did not contain any combustible elements. Combustion of these salts (Fig 1) is accompanied by formation of large amounts of condensed products as smoke and solid residue.

2 4 6 8 2 4 6 8 2 40.1 1.0 10.0Pressure, MPa

4

6

8

2

4

6

8

10

100

Bur

ning

Rat

e, m

m/s

BaDN

LiDN

KDN

Fig 1. Effect of pressure on the burning rate of inorganic salts of dinitramide

At low pressures, the combustion occurs without luminous flame, which however appears as pressure grows. Both barium and lithium salts demonstrate rather high burning rates in the low-pressure region, decreasing with pressure. The lithium salts has probably a region of unstable burning characterized by a large scatter of data points. The combustion products of barium dinitramide were found to contain mainly barium nitrate and partially barium nitrite and oxide. At high pressures, a contribution of the gas-phase exothermic decomposition of N2O to the heat balance probably increases, making for stable burning again. Fig 1 presents also burn rate data for potassium dinitramide obtained earlier by A.P.Glazkova at Chemical Physic Institute (Moscow).

Combustion of the organic salts under investigation at low pressures (less than 1-2 MPa) is generally also accompanied by copious fumes and solid residua with no visible flame observed. As the pressure increases, a luminous flame appears several millimeters above the surface, becoming more pronounced at pressures above 10-15 MPa. The condensed combustion products collected after burning at low pressure were analyzed to contain nitrate anion in the composition. In particular, the condensed combustion products of guanidine dinitramide, which unlike other salts were free from resinous impurities, were identified with IR spectroscopy as guanidine nitrate.

640

2 4 6 8 2 4 6 8 2 40.1 1.0 10.0Pressure, MPa

68

2

4

68

2

4

68

2

4

1

10

100B

urni

ng R

ate,

mm

/s

1

2

3

Fig 2. Effect of pressure on the burning rate of dinitramide salts of ethylenediamine (1), ethanolamine (2), and m-nitroaniline (3)

All the salts studied can be divided into 3 main groups according to their burning characteristics (Fig 2): (1) dinitramide salts which show a transition region characterized by the reduced pressure exponent on the rb(p) curve in the medium pressure range. This is the most commonly encountered group which includes dinitramide salts of amines of both high and low basicity; (2) salts which burning rate-pressure dependence includes a region of combustion instability, characterized by a significant scatter of burning rates in parallel tests. These are salts of amines of high basicity, such as benzylamine and ethanolamine. Methylamine dinitramide can be probably placed into this group as well, although its transition region is difficult to distinguish, because the salt is incapable of burning at all in the low-pressure region; (3) salts which demonstrate stable combustion, showing invariant pressure exponent through the whole pressure range. These includes salts of amines of the least basicity: 5-aminotetrazol and m-nitroaniline.

Fig 2 demonstrates burn rate characteristics for representatives of each group: ethylenediamine dinitramide (1st group), ethanolamine dinitramide (2nd group) and m-nitroaniline dinitramide (3rd group).

To disclose the combustion mechanism, the flame structure of hexamethylenediamine bisdinitramide (GMDADN) has been investigated with tungsten-rhenium microthermocouples. As in the case of ADN, GMDADN shows a local maximum on the burning rate vs. pressure dependence, situated at pressure lower than that of ADN burning rate maximum (Fig 3).

641

2 4 6 8 2 4 6 8 2 4 6 8 2 40.01 0.10 1.00 10.00

Pressure, MPa

68

2

4

68

2

4

68

1

10

100

Bur

ning

Rat

e, m

m/s

1890 K

2050 K

Fig 3. Effect of pressure on the burning rate of dinitramide salts of hexamethylenediamine (crosses) and ammonium (points). Figures are calculated adiabatic combustion temperature

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0Distance, mm

0

400

800

1200

1600

2000

Tem

pera

ture

, C

10

2.1

0.05TsT1

T1

Tf

Fig 4. Temperature profiles of GMDADN at 0.05, 2.1 and 10 MPa

Temperature profiles have been measured with thin thermocouples at pressures of 0.05,

0.1, 0.25, 2.1, and 10 MPa, that is before the local burning rate maximum, at the maximum, and after it. The typical profiles are presented in Fig 4.

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Table 2. Surface temperature (Ts), first flame temperature (T1), and maximum flame temperature (Tf) for combustion of GMDADN

Pressure, MPa Ts, K T1, K Tf, K 0.05 710±10 860±15 - 0.1 730±15 907±15 - 0.25 765±15 940±15 - 2.1 - 1020±45 1600±35 10.0 - - 1700±20

0.0009 0.0012 0.00151/T, K-1

2

3

5

2

3

5

2

3

5

1

10

100

Pres

sure

, atm

T1

Ts

Fig 5. The plot of pressure against reciprocal characteristic temperatures of GMDADN

Temperature profiles of GMDADN reveal characteristic features at low pressures: just above the surface, the temperature is kept constant and equal to the surface temperature for some time and then increases by ~ 200 K in a stepwise manner. The gas flame appears at pressures above 2 MPa. Characteristic temperatures in the combustion wave of GMDADN: the surface temperature (Ts), the temperature above the surface (T1), and temperature of the final flame (Tf), are presented in Table 2 and Fig 5.

In the previous ADN combustion studies [9,10] it was proposed that the temperature of the combustion surface corresponded to the temperature of dissociation of ammonium nitrate formed in the condensed phase, and the temperatureT1 was controlled dissociation reaction of ammonium dinitramide. Using the same approach for GMDADN, the heats of dissociation of nitrate and dinitramide salts have been calculated from the slopes of the curves constructed in the corresponding coordinates lgP-1/Тs and lgP-1/Т1 to give values of 80 and 111 kcal/mole, respectively. These figures are in a good agreement with heats calculated as a difference

643

between heats of formation of solid GMDA nitrate and dinitramide and gaseous products of their dissociation, 89 and 104 kcal/mole, respectively.

0 100 200 300 400Temperature, K

0

2

4

6

8

Bur

ning

Rat

e, m

m/s

Fig 6. Effect of initial temperature on the burning rate of GMDADN at 0.1 MPa

0 200 400 600Temperature, K

0.000

0.004

0.008

0.012

0.016

0.020

Tem

pera

ture

Sen

sitiv

ity, K

-1

Ts

Fig 7. Comparison of the experimental temperature sensitivity of GMDADN

(crosses) at 0.1 MPa with calculated one (dashed line)

644

It is interesting to note that the surface temperatures of GMDADN and ADN measured at the pressures corresponding to the burn rate local maximums are close together (~725 K). Since heat effects of the GMDADN and ADN decomposition in the condensed phase are close also, the similarity between surface temperatures supports the early conclusion,[11] that the reason for combustion instability is the same: a deficit in the reaction heat for heating-up the solid to the surface temperature and subsequent dissociation.

GMDADN is capable of self-sustained burning at atmospheric pressure even at temperature of liquid nitrogen (77 K), allowing burning rate-initial temperature dependence to be measured in a wide temperature interval (77-383 K) (Fig 6). The burning rate of the salt measured at low temperatures demonstrates rather significant data scatter, which is characteristic for combustion with the leading role of the condensed phase reactions.

Burning rate temperature sensitivity of GMDADN (σ) at atmospheric pressure has been calculated from the experimental data. It increases from 4·10-3 to 8·10-3 K-1as the initial temperature grows from 77 to 353 K. These values are several times more than the theoretical ones evaluated from the combustion model based on the dominant role of condensed phase chemistry (Fig 7):

pmS cLTT +−=

0

1σ

This may be indicative of the thermal instability of combustion as a reason for the occurrence of the local maximum on the burning rate vs. pressure curve, as was the case of ADN.

These observations allow considering all the dinitramide salts including ADN as having similar combustion mechanism. At low pressures, the reaction of dinitramide salt decomposition to form N2O and corresponding nitrate plays a dominant role in combustion. As pressure increases, there comes a point when the heat of the reaction is no longer sufficient for heating-up the solid to the surface temperature and subsequent dissociation, while exothermic redox reactions in the gas have yet to be faster and closer to the surface to contribute noticeably to the condensed phase heat balance. This causes a large share of dinitramide salts to exhibit either a transition region or combustion instability area at these pressures. The occurrence of this region depends upon the fuel reactivity and, largely, the surface temperature, which is assumed to be dissociation one and dependent on the amine basicity. Salts of low-basicity amines generally have lower dissociation temperatures, with the result that the balance between heat required and evolved in the condensed phase is unaffected by the increasing pressure. At pressures above 10 MPa a contribution of gas-phase redox reactions gets dominant, bringing about the combustion temperature to the first place.

4. CONCLUSION A specific region on the burning rate vs. pressure dependence, characterized by either

reduced pressure exponent or combustion instability has been found to be a peculiar feature of the combustion of most of the dinitramide salts. All the salts including ADN have been concluded to have a common combustion mechanism which involves the condensed-phase reaction to form N2O and the corresponding nitrate. This reaction determines the burning rate at low pressures.

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REFERENCES

[1] О.А. LUKJANOV, А.R. AGEVNIN, А.А. LEJCHENKO, N.М. SEREGINA and V.A. TARTAKOVSKY: Dinitramide and Its Salts. 6. Dinitramide Salts with Amine Bases, Izvestiya RAN, Ser. Khim., No 1, pp.113-117, 1995

[2] О.А. LUKJANOV, Y.V.KONNOVA, T.A.KLIMOVA, А.А. LEJCHENKO and V.A.TARTAKOVSKY: Dinitramide and Its Salts. 3. Dinitramide Salts with Metals”, Izvestiya RAN, Ser. Khim., No 9, pp.1546-1549, 1994

[3] V.P. SINDITSKII, A.E. FOGELZANG, A.I. LEVSHENKOV, V.Y. EGORSHEV and V.V. SERUSHKIN: Combustion Behavior of Dinitramide Salts, AIAA Paper 98-0808, pp.1-6, 1998

[4] V.P. SINDITSKII, A.I. LEVSHENKOV, V.Y. EGORSHEV, V.V. SERUSHKIN: Combustion of Dinitramide Salts, Proc. Int. Workshop on Unsteady Combustion and Interior Ballistics, Saint-Petersburg, June 26-30, 2000, pp.75-78

[5] G.V. BELOV: Thermodynamic Analysis of Combustion Products at High Temperature and Pressure, Propellants, Explosives, Pyrotechnics, Vol. 23, pp. 86-89, 1998

[6] S. VYAZOVKIN and C.A. WIGHT: Ammonium Dinitramide: Kinetics and Mechanism of Thermal Decomposition, J. Phys. Chem. A, Vol.101, pp.5653-5658, 1997

[7] T.P. RUSSELL, G.J. PIERMARINI, S. BLOCK and P.J. MILLER: Pressure, Temperature Reaction Phase Diagram for Ammonium Dinitramide, J. Phys. Chem. A, Vol.100, pp.3248-3251, 1996

[8] A.N. PAVLOV and G.M.NAZIN: Decomposition Mechanism of Dinitramide Onium Salts, Russ. Chem.Bull., Vol. 46, No. 11, pp.1848-1850, 1997

[9] A.E. FOGELZANG, V.P. SINDITSKII, V.Y. EGORSHEV, A.I.LEVSHENKOV, V.V.SERUSHKIN, and V.I. KOLESOV: Combustion Behavior and Flame Structure of Ammonium Dinitramide, Proc. 28th Inter. Annual Conference of ICT, Karlsruhe, FRG, 24-27 June 1997, paper 90, pp.1-14

[10] V.P. SINDITSKII, A.E. FOGELZANG, V.Y. EGORSHEV, A.I.LEVSHENKOV, V.V. SERUSHKIN and V.I. KOLESOV: Combustion Peculiarities of ADN and ADN-based Mixtures (Combustion of Energetic Materials, edited by K. K.Kuo, L.T. DeLuca), Begell House Inc., New York, 2002, pp.502-512

[11] V.P.SINDITSKII, V.Yu.EGORSHEV, V.V.SERUSHKIN, A.I.LEVSHENKOV: Chemical Peculiarities of Combustion of Solid Propellant Oxidizers, Proc 8th Inter. Workshop on ROCKET PROPULSION: PRESENT AND FUTURE, Pozzuoli, Italy, 16-20 June 2002, pp. 34-1- 34-20