Advances in phase stabilization techniques of AN using KDN and other chemical compounds for preparing green oxidizers

Pratim Kumar

Dr.K.N.Modi Institute of Engineering and Technology,Modinagar,Ghaziabad,U.P,India

ABSTRACT Research and development of green oxidizers and green fuels as a possible replacement for ammonium perchlorate(NH4ClO4,AP)and hydrazine(N2H4)respectively has been increased considerably in the recent years.AP and hydrazine are the oxidizer and fuel entities,and used in solid and liquid rocket motors respectively.AP is highly toxic and led to adverse health effects,while hydrazine is carcinogenic in nature.AP is in use from the last several decades for rocket and space shuttle propulsion,while hydrazine is used in upper stage liquid propelled rocket motors.It's a tough task to replace AP with the currently available green oxidizers;since their ballistic properties are weaker when compared to AP and also they can't be successfully deployed in a solid rocket motor at present.Some important available solid green oxidizers are ammonium nitrate(AN),ammonium dinitramide(ADN),hydroxyl ammonium nitrate(HAN),and hydrazinium nitroformate(HNF).However,AN is one of the cheap and readily available oxidizer,and has great potential to use in solid/liquid rocket motors.Tremendous progress has been envisaged till now,and more progress will be there in the coming future over the development of AN based green energetic materials(GEM's).A concise overview has been presented over the development of phase stabilized ammonium nitrate(PSAN)and AN/KDN based green oxidizers in the present review paper.

Keywords:Green oxidizers Ammonium nitrate(AN) Phase stabilized ammonium nitrate(PSAN) Dinitramide anion(DA) Potassium dinitramide(KDN) Phase stabilizers

1. Introduction

Development of green propellants is an important step for keeping Earth's environment healthy since the growth of space industry is increasing at exaggerated rate worldwide. Space agencies of various space enthusiastic countries are putting their numerous telecommunications,meteorological,and surveillance satellites in different earth orbits every year.International space station(ISS)is also installed in the lower earth orbit for carrying out various types of zero gravity experiments,and to study universe with the help of telescopes and scientific instruments.Several space missions have also been observed each year from several space agencies around the globe with a basic aim to study and to explore the components of the outer universe.Apart from it,space tourism and space travel is also in its initial state,and will start in the coming future.Recently,space missions for cleaning scattered debris of outdated satellites which pose a threat to the working satellites is also being suggested and tested.For conducting all these activities,rockets and space shuttles are mandatory and hence their uses will keep increasing in the coming future.

Currently,AP/HTPB based solid propellants(SP)are used in the booster stages of all kinds of rocket propulsion.The reason is the generation of sufficient amount of thrust in a very small interval of time which is mandatory for creating rocket lift-off condition[1].However,the major drawbacks of AP are their chlorinated exhaust which includes hydrochloric acid(HCl),chlorine oxides(ClOx),and chlorine(Cl)gas[2].All these chemical entities are highly corrosive in nature,detrimental for the earth environment,and for the upper ozone layer.As an example,European Space Launcher(Ariane-5)was filled with about 476t of AP based composite solid propellant,which upon combustion converted into nearly 270t of concentrated HCl[3].In this way,AP was solely responsible for the detrimental effects on the flora and fauna near to the vicinity of rocket launch site,and also for the upper ozone layer.Similarly,fumes of hydrazine are also carcinogenic in nature,highly corrosive,and can't be stored in a fuel container for a long duration space flights.Hydrazine is mostly used in monopropellant and bi-propellant liquid rocket motors in upper stages of rocket[4].

In order to overcome the above mentioned disadvantages associated with AP,a suitable oxidizer needs to replace AP which must be free from chlorine and should also environmentally benign.Several such green oxidizers are available for the replacement of AP such as,ammonium nitrate(NH4NO3,AN),hydrazinium nitroformate [N2H5C(NO2)3, HNF], and ammonium dinitramide[NH4N(NO2)2,ADN][5].List of several others green oxidizers and fuels as possible candidates for both solid and liquid rockets are provided in the Ref.[6].These green oxidizers and fuels when used in a rocket for propulsion,the resulting mixture of oxidizer and fuel is known as green propellant.A green oxidizer/propellant is one of the efforts for a cleaner and greener earth and comes under the topic of green chemistry[7].

Ammonium Nitrate(AN,NH4NO3),was the first promising candidate for the green oxidizer.Ammonium nitrate(AN)is one of the readily available and cheap oxidizer as compared to HNF or ADN.From the last several decades,AN is used as a fertilizer,gas generator,as an inflator for safety air bags in automobiles,and as an explosive[8,9].However,utilization of AN in solid rocket motors is still limited because of its,a)crystallographic phase changes close to the room temperature,b)high hygroscopicity,c)lower oxygen balance percentage of 20%, d) negative heat of formation(-87 kcalmol-1),e)lower burning rate,f)ignition problem,and g)incapability to burn metallic fuels[9].It has been observed that the AN based composite propellants get cracks on storage at room temperature(32°C)due to the IV to III phase transition.During this phase transition,there is a volume change of+3.6%which led to density increase of AN crystals.This density increase results in the formation of cracks over propellant surface which lead to disastrous solid rocket motor failure upon combustion[10].Phase stabilized ammonium nitrate(PSAN)is a solution to prevent these phase changes in AN crystals[11,12].Energy content of AN crystals can be increased by addition of various catalysts[13],oxidizer[14],and energetic fuel binder[15].AN has the potential to use in both solid and liquid propelled rockets.

Hydrazinium Nitroformate[HNF,N2H5C(NO2)3],is one of the oxidizer that is being considered as a suitable replacement for AP.HNF is a yellow needle shaped crystalline solid,non-hygroscopic in nature with decomposition temperature of 120°C[16,17].Pressed HNF pellets and HNF based propellants possesses high burning rate and hence suitable for actual rocket motors[18,19].Some problems associated with HNF are its incompatibility issues with binders,friction sensitivity,and thermal stability.

Ammonium Dinitramide[ADN,NH4N(NO2)2],is an ionic liquid and an important dinitramide anion(DA)based compound[20].ADN has a great potential to replace AP in solid propellants,and hydrazine in liquid monopropellant/bipropellant rockets.ADN has high oxygen balance percentage (OB%), low molecular weight combustion products,and low negative heat of formation(ΔHf),leading to a superior specific impulse(Isp)[21,22].Out of above mentioned advantages,ADN also suffers from several disadvantages including high hygroscopicity, needle shaped crystalline structures,unstability,and incompatibility issues with the fuel binders[20,23,24].Another important dinitramide based compound is potassium dinitramide,[KDN,KN(NO2)2][25].Mixture of AN/KDN can be utilized as oxidizers for solid propellants[26-28].Numerous literatures are available over the properties,thermal decomposition,and combustion characteristics of AN,ADN,and HNF based oxidizers and propellants[16,29-31].

Apart from advances in AN,ADN,and HNF based green propellants,progresses over the development of other green oxidizers are also discussed in Ref.[6,32].Projects like replacement of hydrazine within space propulsion systems(RHEFORM)[33],green propellant infusion mission (GPIM) by NASA [34], and green advanced space propulsion(GRASP)[35]are sanctioned,funded,and have successfully completed their assigned work for the development of green propellants.Apart from it,several space agencies like National aeronautics and space agency(NASA),Indian space and research organization(ISRO),Japan space exploration agency(JAXA),Russian space agency(RSA)etc are also working for the development of green fuels.On this pace,in the coming years green space propulsion will be a reality.Several review papers are available over the current progress in the development of green propellants especially for AN and ADN based green propellants[20,31,36,37].

In the present paper,recent advances and chemistry behind the phase stabilization of AN crystals by potassium based compounds,organic compounds,and copper based catalysts are discussed.Potassium based compounds like,potassium nitrate(KNO3,KN),and potassium dinitramide[KN(NO2)2,KDN]play a dual role of phase stabilizer and energy enhancer for AN crystals [38-40]. It's important to enhance the energy content of oxidizer to maximum as oxidizer percentage loading in a solid propellant is about 80%,and rest being fuel binder and metallic fuel.Oxidizer plays a significant role on thrust generation and specific impulse(Discussed separately in Section 5).

Present paper is divided into six sections viz.Introduction(Section 1),Dinitramide anion(DA)and potassium dinitramide(KDN)(Section 2),methods to improve AN properties(Section 3),phase stabilization and energy enhancement of AN crystals(section 4),Role of Oxidizer in generation of specific Impulse(Section 5),and conclusion(Section 6).Paper will be helpful for the scientists and researchers who are working towards the development of PSAN,and PSAN based green propellants.

2. Dinitramide Anion(DA)and potassium dinitramide(KDN)

Potassium dinitramide(KDN)is one of the member of dinitramide anion[N-(NO2)2,DA]family[41].Some other DA based compounds other than ADN and KDN are,pyridinium dinitramide[42],silver dinitramide[43],dinitro azetidinium dinitramide[44],1,5-diamino-4-methyltetraolium dinitramide[45],biguanidinium dinitramide [46,47], lithium and cesium dinitramide [48],polyamino-substituted guanyl-triazole dinitramide[49],nitrogen containing ligands dinitramides[50],and melaminium dinitramide[51].Numerous application areas have been reported for various dinitranide based compounds in Ref.[52].

DA was first discovered in 1971 at the Zelinsky Institute in Russia or former USSR.However,the discovery of DA was kept secret by USSR to the rest of the world till DA was again independently rediscovered in the U.S.A.by Bottaro and Co-workers in 1989 in their lab[58].At present there are numerous number of new dinitramide salts,out of which ammonium dinitramide(ADN)and potassium dinitramide(KDN)are of considerable interest.To understand the complexities of dinitramide anion,it is worthwhile to understand its chemical,physical and structural properties.Many research papers[59,60]are completely devoted to understand this highly versatile anion,while many papers describe properties of compounds based on DA[42-51].The simplest structure of dinitramide anion is shown in Fig.1.

Fig.1.Structure of DA[48].

From Fig.1,it can be observed that,there is some bond length and bond angle between variousN-N andN-O bonds.These bond length and bond angle varies for different compounds based on the type of cations reacted with dinitramide anion i.e.cesium dinitramide,lithium dinitramide,ammonium dinitramide,potassium dinitramide etc.,each have different twist angle,bond length,and bond angle.

2.1. KDN preparation and crystal structure

There are several feasible chemical routes to synthesis dinitramide anion as discussed in Ref.[61].However,KDN crystals were reported to be prepared by thenitration of potassium sulfamatein a mixture of nitric and sulfuric acid at-40°C[27,62].The obtained KDN crystals through nitration method are yellowish in color and pure in nature.Values of some of the important parameters related to KDN crystal are summarized in Table 1[25,48,63].

2.2. KDN decomposition and combustion

KDN crystals start to melt at around 130°C[64-69].Before melting,it was observed that KDN crystals undergo one phase transformation step at around 95°C [64]. After melting, KDN decomposition starts and completes in four distinct steps.Complete decomposition of KDN takes place up to 1200°C in four steps[27].The steps are as follows:

1ststep:KN(NO2)2→KNO3+N2O+O2(in between 200 and 250°C)

2ndstep:KNO3→KNO2+O2(in between 600 and 700°C)

3rdstep:KNO2→K2O+O2+N2(in between 700 and 800°C)

4th step:K2O →K2+O2(above 1200°C)

In the first step,KDN decomposes to form potassium nitrate(KNO3,KN)along with nitrous oxide(N2O)and nitrogen(N2)gas,afterwards N2O further breaks into N2and O2.In the second step,KN breaks into potassium nitrite(KNO2)and gaseous oxygen.In the third step,KNO2breaks into potassium oxide(K2O)along with nitrogen and oxygen gas.In the last and final step,K2O breaks into potassium(K2)and oxygen gas.

The decomposition curve of KDN is shown in Fig.2[64].

From Fig.2,it is clear that the first phase transition of KDN wasobserved at 91.1°C,afterwards one melting peak at 108.4°C,and finally two decomposition peaks are observed at 231.5°C and 237.2°C.Although,all these values i.e.phase transformation temperature,melting point temperature,and decomposition temperature of KDN samples varied from samples to samples.Two major papers which are completely devoted to understand the effects of storage time,storage conditions,humidity,pressure,crystal size and structures on KDN physical properties are in Ref.[65,66],while other paper which deals with decomposition studies and other physical properties of KDN are Ref.[67-70].Thermal decomposition studies of KNO3were carried out in Ref.[71,72].

Table 1Crystal structure data of KDN at 298 K.

Fig.2.DSC-TG of KDN[64].

The major reason for the variation in phase transition temperature and melting temperature of KDN is that upon storage KDN slowly decomposes to form potassium nitrate,KNO3(KN).KN forms a eutectic mixture with KDN and a eutectic melting point(m.p.)was observed at and above 105°C.The value of eutectic m.p.varies from sample to sample as it depends upon the percentage of KN present in the KDN to form a eutectic mixture. Phase transformation occurs at or above 95°C because at this temperature,KDN crystals began to crack,and it partially decomposes to form KNO3with a minor weight loss.The KNO3which formed is adsorbed by the KDN crystal,and due to which KDN crystal changed from transparent to opaque[27,64].

As previously discussed, there are several papers over the thermal decomposition properties of KDN crystals under various conditions.However,research papers over the detailed combustion characteristics or on combustion wave structure of KDN combustion are not available.Hitherto,original research and review papers over the combustion of ADN,and ADN based solid propellants are in abundance[20-23,29,30,73-75].ADN combustion is controlled by the condensed phase(c-phase)combustion mechanism.Sinditskii et al.[29,30,76,77]studied and presented research paper over the combustion behavior of various dinitramide salts[77].Hence from the previous literatures,it can be stated that the combustion mechanism of KDN crystals and propellant based is also controlled by condensed phase combustion mechanism as observed in Ref.[27].Similarly,AN and AN based solid propellants also have condensed phase combustion mechanism[76,78].Several review papers are available over the properties,decomposition,and combustion mechanism of AN crystals[13,36,39,76].

Importance of green propellants based on AN and KDN was first reported by Borman et al.[26]in 1994.Borman was the first to report the application of KDN in improving the phase transition and energetic properties of AN crystals.Authors also patented the prepared propellant on AN and KDN by successfully deploying it in a solid propulsion system[26].After this important step towards the development of green propellants based on AN/KDN,no other literatures are available on the advancement of this technique.Recently,Sinditskii et al.[79]have conducted combustion study of potassium dinitramide and its binary mixtures with nitroester binders.In another paper by Kumar et al.[27],burning rate and flame structure studies were performed over catalyzed AN/KDN and hydroxyl terminated polybutadiene(HTPB)based composite solid propellants.Kumar et al.also performed kinetic studies of AN,KDN,and AN/KDN based oxidizer and propellant samples using Ozawa-Flynn-Wall(OFW)method[28].Although,detailed combustion modeling or combustion wave structure of KDN and AN/KDN based solid propellants are still missing.

Before the advent of KDN,potassium nitrate(KNO3,KN)was used to phase stabilize and to enhance the combustion properties of AN crystals.However,there is a limit to use the amount of KN in AN crystals i.e.upto 15%of KN and 85%of AN.This is because of the generation of potassiun oxide(K2O)and molten potassium(K)upon decomposition of KN.Both potassium oxide and molten potassium are solid in nature and have high molecular weight.Generation of high molecular weight gases and solid particles upon combustion led to two phase flow inside rocket nozzle,which drastically reduces the overall performance by inhibiting the optimal expansion of gases while flowing through the nozzle.In this way,excess of K2O and molten potassium will generate less thrust when used in rocket and missiles.

For AN/KDN propellant system a maximum of 30%KDN can be added with 70%of AN.The reason lies in the decomposition mechanism of KDN as we observed previously.From Fig.2,it was observed that after the first step decomposition of KDN(consider it 100%initially),there was a mass loss of around 40%i.e.after first step KDN decomposition,60%of KN was left in the system.In this way,25%-30%of KDN can be used with 75%-70%AN in an/KDN system,so that final KN percentage in the system will be about 15%-18%.Kumar et al.[27],conducted burning rate studies on three ratios of AN/KDN i.e.90/10,75/25,and 50/50,and observed that 75/25 and 50/50 AN/KDN system show sufficiently high burn rate.

Hence,it can be stated that the KDN is primarily used with AN crystals as a phase stabilizer and to increase its energy capacity.With the addition of only 5%KDN,AN can be phase stabilized,and an increase in percentage addition of KDN in AN crystals boost the energy capacity of AN can be increased significantly as shown in Ref.[27,40].From the next section,various methods to improve the decomposition and combustion properties of AN with the help of potassium based compounds,catalysts,and organic compounds have been discussed sequentially for enhanced understanding.

3. Phase transition of AN and methods to resolve it

As discussed in the introduction section,some of the problems associated with AN are its,a)crystallographic phase transitions,b)high hygroscopicity,and c)low energy content.Ammonium nitrate(AN)undergoes 5 phase transitions in the temperature range of-16°C to 170°C.The various phase transitions are shown in Table 2.

Table 2Phase transitions of AN[27].

The occurrence of phase transitions in AN crystals at and above ambient room temperature is because of the free rotation of the ammonium cation()in the crystal lattice of NH4NO3.Thisshows the property of a single-atom cation with an effective radius of 1.48 ?.Such free rotation can be suppressed,if AN is doped with a cation of nearly same ionic radii.This cation forms a solid solution and results in suppression of phase transitions.Potassium cation(K+)has ionic radii of 1.33 ?,and thus addition of K+results in reduction in the IV→III transition,and raises the III→II temperature range[80].Similarly,rubidium(1.48 ?),cesium(1.65 ?),and thallium(1.50 ?)form solid solutions;but sodium(0.98 ?)does not.The effect of substitution of these cations in the crystal lattice of AN is on the polymorphic transition temperatures.This polymorphic transition temperature of the resulting solid solution depends on the ionic radii of the cation introduced.

The problem of the phase transitions of AN has been solved by developing phase stabilized ammonium nitrate(PSAN).Now attempts were made by the researchers to achieve uniform and stable combustion of PSAN based composite propellants.PSAN can be achieved by adding suitable phase stabilizers in the AN crystals.Various phase stabilizers such as metal halides[13,81],potassium based compounds[82,83],copper based catalyst[84],and organic compounds[85,86]has been reported to produce PSAN.Hygroscopicity problem can be removed by using prilling and recrystallization techniques[87].Low energy content of AN crystals can be enhanced by using various catalyst such as Fe2O3, MnO2etc.[88,89],nitramines such as HMX and RDX[14],potassium dinitramide[KN(NO2)2,KDN][26,27],transition metal salts[90],energetic binders such as GAP and AGAT[91,92].

4. Phase stabilization and energy enhancement of AN crystals

4.1. Phase stabilization of AN by potassium based compounds

Some of the additives used for obtaining phase stabilized ammonium nitrate(PSAN)are metal halides,potassium based compounds,copper based catalyst,and organic compounds as discussed previously.Out of above mentioned entities,potassium based compounds are the most preferred,most efficient,and the most studied phase stabilizer for AN,since potassium based compounds not only helps in phase stabilization of AN,but they are good energy enhancer for AN crystals.Although,other additives are also efficient in phase stabilization and the basic chemistry behind the phase stabilization is the same.

Under potassium based compounds,potassium nitrate(KN)[82],potassium ferrocyanide[83],potassium fluoride(KF)[81,93],potassium iodide(KI)[39],potassium chloride(KCl)[39],potassium sulfate(K2SO4)[39],and potassium dinitramide(KDN)[27]are generally used for phase stabilization of AN.1%by weight of potassium fluoride(KF)is sufficient to stabilize AN crystals[93],as shown in Fig.3a.A patent has been filed in 1992 by Mehrotra et al.[81]for stabilizing AN with 3.5%of KF.This result indicates that KF is good phase stabilizer in small amount but,KF is not an energy enhancer for AN crystals.

Potassium nitrate(KN)is the most widely used ingredient for firework and pyrotechnic compositions [72]. Like, potassium perchlorate(KClO4,KP),potassium nitrate(KN)was also used for low range missiles and in small rockets applications in early days of rocketry.With the advent of AP as a better option for oxidizer,the uses KP and KN were completely removed.The main reason was the generation of low thrust from KP and KN based propellants because of the generation of high molecular weight combustion products as discussed previously.

KDN is a compound of great interest for phase stabilizer and energy enhancer for AN.3%KDN is also sufficient to stabilize AN with slight increase in energy content too[40],as shown in Fig.3b.As it was discussed previously,KN is a good phase stabilizer and energy enhancer but,less efficient as compared to KF in phase stabilization.Other potassium based compound are moderately efficient for phase stabilization of AN.

Fig.3.DSC curves.

Comparing above two curves(Fig.3a and Fig.b)with the DSC curve of pure AN[Ref.27],it can be observed that the phase transition of AN at 57.64°C was completely removed in the presence of 1%KF and in 3%of KDN.Apart from these,a carefully examination of Fig.3b also reveals that,the area of endothermic phase transition and melting peak is decreasing with increasing weight percentage of KDN.This is because,KDN undergoes exothermic decomposition in the temperature range of 200-220°C.

4.2. Phase stabilization of AN by other compounds

Several catalysts and organic compounds are also used as a phase stabilizer for AN.Copper oxide[94],titanium dioxide[94],lithium fluoride[94],copper diamine nitrate[95],and nickel oxide[96]act both as a phase stabilizers along with decompositional catalyst for AN.The solid state reaction between AN and CuO proceeds as:

Copper diammine complex[Cu(NH3)2(NO3)2]has the property to stabilize phase transition of AN to some extent.In the presence of cupric oxide(CuO),the first phase transition was observed at 87.6°C[94].Thus,CuO is a good phase stabilizer for AN,as the first phase transition temperature increased considerably. X-ray diffraction(XRD)and IR spectrum studies revealed the existence of solid solution formation.Temperature resolved X-ray diffraction pattern showed that the decomposition product of copper phasestabilized ammonium nitrate(Cu-PSAN)was CuO.

In Ref. [94], kinetic parameters were evaluated for AN,AN+CuO,AN+TiO2and AN+LiF using Friedman,Vyazovkin and Coats-Redfern kinetic method. Various chemical kinetics approaches were used to calculate the activation energy values and have several rule and regulations[97,98].The lowest activation energy(Ea)was of 97 kJ`mol-1for AN+CuO,while highest value was 141 kJ`mol-1for AN+LiF.The study confirms the efficacy of CuO as a decompositional catalyst.Kumar et al.[99],have worked with thermal decomposition and combustion analysis of AN added with copper-cobalt based metal oxide(Cu-Co*)catalyst.In the presence of Cu-Co*, theEavalue of AN comes down to 77.87 kJ`mol-1,and also AN+HTPB propellant in the presence of Cu-Co*can sustain its combustion upto 0.8 kPa. Similarly in Ref.[100],kinetic studies were performed for PSAN in presence of HMX and RDX in ratio of 1:1.The primary aim of this study is to enhance the energetic of AN crystals. In presence of HMX,exothermic decomposition of PSAN is more pronounced as well as value ofEais also less as compared to PSAN+RDX.

After potassium and copper based compounds,another group is based of organic compounds which are used for phase stabilization of AN[8,85,101-107].Application of organic phase stabilizers is to develop new generation of smokeless environmentally safe gasgenerating formulations for safety air bags.Organic compounds consisted of C,H,N and O only.There are several organic compounds used for phase stabilization,some of them are:phthalimide[8], uracil [8], nitrouracil [8], activated carbon [107], polyvinylpyrrolidone (PVP) [103], diammonium salt of 1,3,5-trihydroxyisocyanuric acid [104], trioxypurine [104], various organic acids[106],guanidium nitrate(GN)[106],polynitro compounds[106],and nitrocellulose(NC)[85,107].

In Ref.[85,107],activated carbon(AC)and carbon black(CB)were used for enhancing the decomposition and combustion properties of AN.In presence of AC,the first phase transition was observed at 90°C,while in presence of CB,the first phase transition was near about 50°C.Some of the important points observed in[8]over the role and mechanism of organic compounds in phase stabilization of AN are:

a)Organic compounds which are close to AN in crystal-chemical parameters has phase stabilizing effect.

b)Organic compounds with functional groups that are involved in intermolecular interactions of orbital and electrostatic types and form new hydrogen bonds with AN.

c)Most effective organic groups are carbonyl,amino,purine,pyrimidine,and imino groups.

In Ref.[103,105],polyvinylpyrrolidone(PVP)was used to stabilize and to enhance the decomposition properties of AN.PVP is a polymer with medicinal,pharmaceutical and cosmetic applications.Pyrrolidone groups of PVP molecule shows large dipole moment,which makes the PVP matrix capable of holding the ammonium and nitrate ions separate.Hence,PVP provides an effective solution to the problem of phase and thermal stabilization of AN.Also,the combination of AN+PVP can be used in propellant formulation since PVP is a fuel and AN is an oxidizer.In Ref.[103],80/20(PVP/AN)sample was prepared and this sample was used to study the decomposition and kinetic properties using DSC/TG techniques.Virgin PVP undergoes major mass loss in the temperature range of 380-550°C,withEavalue of 250-300 kJ`mol-1.In PVP-AN material,the PVP matrix remained unaltered after the release of AN with effectiveEavalue of 150-200 kJ`mol-1.Similar observations were also observed in Ref.[105]with 7 AN/PVP samples in the ratio of 5/95,10/95,20/80,25/75,30/70,40/60 and 50/50.

Five phase stabilizers namely 1,3,5-trihydroxyisocyanuric acid dihydrate,diammonium salt of,3,5-trihydroxyisocyanuric acid,4-aminouracil,4-aminouracil hydrate,and trioxypurine were used as phase stabilizer of AN in[104].These compounds were selected because they are effective acceptors of electrons(due to>C=O group),and they are also good donors of electrons(due to presence ofN-H group).So the crystalline structure of these compounds can help in phase stabilization of AN.All these compounds are added in AN crystals at the melting point of AN.X-ray diffraction(XRD)and Quantum-chemical calculations methods were used to analyze the structure modifications in AN due to presence of additives.Authors comes out with the conclusion that,organic compounds having crystallographic structure close to AN crystalline structure are able to modify the phase transitions of AN as observed in Ref.[8].

In Ref.[106],organic acids and other nitrogen containing compounds were used for phase stabilization for AN.Most organic compounds increases the rate of oxidation of AN.Some of the factors which are responsible for increased reactivity are:

a)Availability of easily oxidable groups in organic compound.

b)Change of AN dissociation equilibrium temperature under the influence of component.

c)Rate of formation of active molecules like nitrogen dioxide.

Metals like aluminum(Al)is an important constituent of AP based solid propellant.It helps in increasing the flame temperature by increasing the enthalpy release upon combustion[108].However,Al metal do not burn efficiently with AN as the temperature of the order of 2000°C is required to initiate the combustion of Al metal[8].It was observed that the addition of catalysts like cryolite into AN based propellants enhances the oxidation of aluminum[109]and enhances the burning rate of AN based solid propellants.

Recently,hydroxyl ammonium nitrate(HAN)[110,111]is also getting attention as a green fuel for liquid rocket motors.Hydroxylammonium nitrate is unstable because it contains both a reducing agent(hydroxylammonium cation)and an oxidizer(nitrate)[112].HAN is usually stored as an aqueous solution,although the solution is corrosive and toxic,and may be carcinogenic.

5. Role of Oxidizer in generation of specific impulse(Isp)

An oxidizer energy contribution is determined by the heat and gas volume produced by oxidation of fuel during combustion.In general,the enthalpy release(ΔH)increases as oxygen content increases,but decreases as negative heat of formation(Hf)increases.For example,ammonium nitrate(AN)has high oxygen content,but its enthalpy release is relatively low as a result of its comparatively high heat of formation(-1090 cal`g-1).Hydrazinium nitrate(HN)has a gas equivalent to AN but with a lower oxygen content.Yet,HN is much more energetic because its heat of formation(-531 cal`g-1)is less negative than that of AN[108].Values of heat of formation,oxygen content,major combustion products,and density of AP,AN,ADN,HNF,and KDN is provided in Table 3.

Specific impulse(Isp)is directly proportional to the square root of the flame temperature(Tf)and inversely proportional to the molecular weight(MW)of the combustion products.Flame temperature further depends on the heat of formation value of oxidizer.Thus,an oxidizer with high adiabatic flame temperature and low molecular weight is highly desirable for rocket operation.The dependency ofIspon the flame temperature and on the molecular weight is shown by equation as shown below[108]:

Table 3Thermo chemical properties of AP,AN,ADN,KDN,and HNF[108].

Where,

Isp=Specific impulse.

g=acceleration due to gravity.

R=Gas constant.

MW=molecular weight of the exhaust gases.

Tf=Flame temperature.

Pexit=Nozzle exit pressure.

Pc=Combustion chamber pressure.

?=ratio of specific heats.

In this way,mixture of AN/KDN will show high specific impulse as compared to AN alone.Specific impulse of AN/KDN can further increase by the addition of catalysts,and energetic fuel binders.Other probable measures for increasing the energy content of AN crystals are provided separately in conclusion section.

From the above discussion it can be stated that,AN as a green oxidizer has a great future ahead.Lots of researches still have to be done to achieve AN based green solid propellants having comparable burning properties as compared to AP based solid propellants.Similarly,HAN also provide better specific impulse as compared to hydrazine,although it also required more research.

6. Conclusion

AN is one the earliest discovered energetic materials dating back to 1659 by J R.Glauber.Since from its discovery,AN is also one of the most fascinating chemical compound which witnessed lots of researches which are still going on.AN is surely a compound of interest and have a great potential in various application areas.Phase stabilization technique of AN crystals is now a matured science and much knowledge has been gained in this field.However,enhancing the energetic properties of AN crystals to be in the same order of AP or to other energetic materials is still in progress.

Dinitramide anion(DA)is also a great finding in the field of energetic materials,and research over synthesis and application areas of various DA compounds have great scope in the future.Ammonium dinitramide(ADN)and KDN are two important DA based compounds.Lots of research is already going over ammonium dinitramide(ADN)to produce it commercially and to use it in solid and liquid rocket motors.Discovery,synthesis,and physical/chemical properties of new DA based compounds are reported nearly every year.Although,practical applications of these new DA based compounds in propulsion systems are not reported,which may be due to various security reasons.

KDN is also a compound of interest in the field of green energetic materials with various application areas.However,a detailed combustion analysis and combustion modeling of KDN and AN/KDN based oxidizers are still lacking.Thermal decomposition and other physical/chemical properties of KDN are well documented now.Feasible application of AN/KDN as an oxidizer ingredient in solid and liquid propulsion systems is still in progress.Recently,some articles were published which discussed the burning rate and combustion characteristics of KDN based solid propellants.AN mixed KDN show promising high burn rate as compared to AN mixed HMX/RDX based solid propellants.However,burning rate of AN/KDN based solid propellant is not in order to that of the burning rate of AP and ADN based solid propellants.

Currently,major approaches aimed at increasing the burning rate of AN based solid propellants.Addition of catalysts(copper chromites,cupric oxide,titanium dioxide,cryolite etc.),energetic fuel binders(GAP,AGAT,BAMO,NIMMO etc.),and energetic oxidizers(HMX,RDX,KDN etc.)are some of the methods to enhance the burning rate of AN.Other approach to increase the burning rate is by reducing the oxidizer particle sizes,although the effect is negligible since the combustion of AN is controlled by condensed phase mechanism.Metals like aluminum(Al)don't combust with AN since the flame temperature of AN propellant is low.For achieving higher flame temperature,metal hydrides(aluminum hydride)can be use with AN propellants along with catalysts.Phase stabilizations of AN crystals can be achieved by organic compounds,potassium based compounds,or by using copper based catalysts.

Some other approaches which can be used in future for enhancing the burning rate of AN crystals are,a)use of nano carbon,metal oxides,and catalysts b)use of energetic plasticizers,c)use of different kinds of metal hydrides,d)advancement in prilling and coating techniques for reducing hygroscopicity and for obtaining fine AN crystals,e)use of other types of energetic oxidizers which may include different DA based compounds.All these future researches will help in the development of powerful AN based solid propellants which can be used in various space and defense programs by deploying it successfully in rockets and missiles.Development and feasible production of other green energetic materials apart from AN,HNF,and ADN will also be envisage in the coming future.