Sci. Tech. Energetic Materials, Vol.65, No.6, 2004 215 Review Review of some newly synthesized high energetic materials S. Thangadurai*, K.P.S. Kartha**, D. R. Sharma**, and S. K. Shukla** *Department of Geology and Mining, Guindy, Chennai-600 032, INDIA e-mail: [email protected] (or) [email protected] **Central Forensic Science Laboratory, “Directorate of Forensic Sciences,” Govt. of India, Hyderabad-500 013, INDIA Received: April 12, 2004 Accepted: September 1, 2004 Abstract Towards the end of the last millennium many new high energetic materials were developed. These materials may replace the presently used explosives, sooner or later. In this review, the authors try to explore some of these materials viz., polynitropolycyclic cage explosives, cyclic nitramines, cage explosives, nitro derivatized heterocyclic compounds, nickel hydrazine nitrate (NHN) complex, nitrocubanes, hafnium explosives, heat-resistant explosives, new insensitive high explosives and some other novel high energetic materials. There is a need for the development of analytical methods for identification of these materials and their post explosion residues as these are likely to be encountered in crimes. 1. Introduction much as 80% ammonium nitrate. By the beginning of During the nineteenth century the developing science of World War II, the research to discover alternative explo- chemistry began to create molecular species with explosive sives resulted in another group of explosive molecules that properties. These molecules contain not only atoms that act could be used for the filling of ordnance 2). After two major as fuels, i.e. carbon and hydrogen, but also contain nitro World Wars in twentieth century, picric acid, TNT, tetryl, groups (NO2) similar to nitrates. There are three basic struc- RDX, NG, PETN, ammonium nitrate and potassium chlo- tural types: nitro compounds containing the C-NO2 group- rate explosives were encountered in insurgent activities. ing; nitrate ester containing C-O-NO2 and nitramines con- Experimental and theoretical studies on the pyrolysis taining N-NO2. Molecules that contain nitro groups are mechanism, sensitivity and many aspects of organic explo- good candidates for explosives. The nitro group provides sives on this subject were described in the earlier litera- the essential oxygen for the combustion, and furthermore, ture 3)-5). Polynitropolycyclic cage compounds, cyclic the nitrogen atoms are converted to dinitrogen (N2), increas- nitramines other than RDX and HMX, nitro derivatized het- ing the volume of liberated gas. The advent of nitrated mol- erocyclic compounds, nitro cubanes, hafnium explosives, ecules opened the way to explosives with far better ener- heat-resistant explosives, a new primary explosive nickel getic properties but with the ability to create detonations. hydrazine nitrate (NHN) complex, promising new insensi- However, at the turn of the nineteenth century, researchers tive high explosives and some other novel high energetic were applying the concept of detonation to explosive mole- materials were developed recently. These high energetic cules, some of which had been known for almost 100 years. materials have the potential to be used for insurgent activi- One of the first to be developed into fillings for ordnance was ties by extremists groups in the coming years. The perfor- picric acid (2, 4, 6-trinitrophenol), either as a pure material or mance and other characteristic studies of these materials, mixed with dinitrophenol to lower the melting point of the reported recently show that some of them possess better mixture to aid melt casting 1).Atthe same time the explosive potential than presently used explosives. This review paper 2, 4, 6-trinitrotoluene (TNT) was also developed and found to examines them under several headings “thermally stable” or be superior to explosives based on picric acid. The use of “heat-resistant”, “high-performance”, melt-castable, insensi- TNT was highly successful not only as a pure filling but, by tive, energetic binders, and energetic materials synthesized the end of World War I, as a mixture with ammonium nitrate using N2O5 and critically examines them from the point of to give the explosive known as amatol which could contain as view of stability, reliability, safety and specific application. 216 S. Thangadurai et al. Problems associated with their processing are identified and part of broad strategy to develop the methodology 8). suggestions are made to overcome them 6). The synthetic methodology, which has been utilised to pre- Forensic scientists have to update their knowledge about the pare a number of polynitropolycyclic cage explosives, is pre- emerging high explosives, and develop systematic analytical sented byA.P.Marchand et al. 9). The results of thermodynam- procedures by including these materials so that no explosive ic studies of several polynitropolycyclic systems are dis- residues would be left undetected. During seventies and cussed along with preliminary results by explosives evalua- eighties of last century analytical methods such as chemical tion studies which have been performed on 4, 4, 7, 7, 11, 11- 2,6 3,10 5,9 spot tests, chromatographic and spectrometric techniques hexanitropentacyclo [5.4.0.0 .0 .0 ] undecane or D3- were developed for the trace analysis of explosives. Survey of hexanitrotrishomocubane10) and on 4, 4, 8, 8, 11,11-hexani- available literature and proceedings of various conferences tropentacyclo [5.4.0.0 2,6.0 3,10.0 5,9] undecane11). Thermodyna- on detection of explosives do not indicate any significant mic parameters and explosive performance characteristics of work on detection of some of the newly emerged high ener- hexanitro pentacyclo undecanes are given in Table 1. getic materials. Keeping this aspect in mind, with a view to explore the possibility of application of modern analytical 2.1 4,4,7,7,11,11-hexanitropentacyclo methods for identification of the newly synthesized high [6.3.0.02,6.03,10.05,9 ] undecane energetic materials, the present paper highlights the impor- (or) D3-hexanitrotrishomocubane tance of these compounds to the forensic community. O2N NO2 2. Chemistry of polynitropolycyclic cage explosives The performance of an explosive is determined by the NO amount of heat and gas produced in the initial reaction. 2 Since the past decade many research groups are involved NO2 in an intensive effort to synthesize new polynitropolcyclic NO2 7) “Cage” compounds . Such polycarbocyclic compounds, by NO2 virtue of their rigid compact structures, generally pack effi- ciently in the solid state and thus possess unusually high The density of this compound is 1.84 g cm-3. The detonation crystal densities. These classes of compounds having car- velocity is 7.95 km sec-1 and the detonation pressure is 2838 7 -1 bocyclic frame work often possess significant levels of µ 10 Pa. The heat of formation (∆Hf)is-36.44 k cal mol . steric strain, which correspondingly render them thermody- Shock sensitivity measurements on D3-hexanitrotr- namically unstable relative to isomeric non-cage structure. ishomocubane were performed by using the exploding foil In recent years, attention has been focussed upon the slapper technique developed at the U.S. Army Armament potential use of polycarbocyclic cage compounds as a new Research Development and Engineering Center, by Velicky 12) class of energetic materials. Significant efforts have been et al. .D3-hexanitrotrishomocubane is less shock sensitive made in several laboratories world-wide to introduce nitro than TNT. groups as substituents into highly strained cage molecules in an effort to produce high density explosives and propel- 2.2 4,4,8,8,11,11-hexanitropentacyclo lants. Compact three-dimensional cage compounds contain- [5.4.0.02,6.03,10.05,9] undecane ing optimum numbers of nitro groups as determined in the- O2N ory by thermodynamic calculations constitute a class of NO2 explosive compounds more powerful than HMX. This is due to direct result of high crystal densities, particularly in combination with high strain energies built in certain cage system. The synthesis of polynitrocage compounds has pro- NO2 gressed on several fronts and is currently in the second stage O2N of a three-stage effort.The first stage has seen the synthesis NO2 of forerunners of more highly nitrated cage compounds as O2N Table 1 Thermodynamic parameters and explosive performance characteristics of hexanitro pentacyclo undecanes11). Detonation Detonation Compound Density ÆHf pressure velocity (g cm-3) (kcal mol-1) (Pa) (km sec-1) 4,4,8,8,11,11-hexanitro pentacyclo 1.84 -22.5 2920 ´ 107 7.87 [5.4.0.02,6.03,10.05,9] undecane 4,4,7,7,11,11-hexanitro pentacyclo 2,6 3,10 5,9 7 [5.4.0.0 .0 .0 ] undecane or D3- 1.84 -36.44 2838 ´ 10 7.95 hexanitrotrishomocubane Sci. Tech. Energetic Materials, Vol.65, No.6, 2004 217 The density of this compound is 1.84g cm-3. The detonation hexane (RDX) and 1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazacy- velocity is 7.87 km s-1 and the detonation pressure is 2920 µ clooctane (HMX) are high energy containing compounds 5 -1 10 Pa. The heat of formation (∆Hf)is-22.5 k cal mol . Shock that are used extensively as both propellant and explosive sensitivity tests were also performed on 4, 4, 8, 8, 11, 11-hexa- formulations. RDX (1.77 g cm-3), HMX (1.89 g cm-3) and nitropentacyclo [5.4.0.02,6.03,10.05,9] undecane as described PETN (1.77 g cm-3) are all capable of providing very high above in the compound D3-hexanitrotrishomocubane. The shock pressures with HMX being the best. However, explosive performance of this compound could be com- PETN is significantly more sensitive than RDX over pared with that of TNT11). PETN where safety is an issue in such uses as artillery and In comparison, the corresponding test results obtained for cannon rounds. As the performances of RDX and PETN D3-hexanitrotrishomocubane indicates that this material is are very similar, many countries have chosen to use RDX slightly less powerful.