Azide decomposition, radiation

The relatively stable azides of the strongly electropositive metals undergo controlled (i.e. non-explosive or "slow") decomposition. For this reason, sodium and barium azides are probably the most completely studied salts in the azide group [12], Their decompositions are characterized by relatively large apparent activation energies and well-defined induction periods to onset of reaction [10], Mechanistic aspects of azide decompositions have been reviewed by Tompkins [17], The radiation stabilities of the Group lA metal azides do not correspond to the sequence of thermal stabilities [10]. The catalytic decomposition of HNj has also been investigated [12]. 

A few diazonium salts are unstable in solution, and many are in the solid state. Of these, the azides, chromates, nitrates, perchlorates (outstandingly), picrates, sulfides, triiodides and xanthates are noted as being explosive, and sensitive to friction, shock, heat and radiation. In view of their technical importance, diazonium salts are often isolated as their zinc chloride (or other) double salts, and although these are considerably more stable, some incidents involving explosive decomposition have been recorded. 

Tompkins et al. [22, 85] studied the photochemical decomposition of potassium and barium azide. Originally they found that the rate of photolysis was proportional to the square of the intensity of the radiation. 

D.J. Moore, Thermal Decomposition of Barium Azide , Nature 203, 860—61 (1964) 132) J. Roth, Initiation of Lead Azide by High Intensity Light , JChemPhys 41, 1929—36 (1964) 133) G. Odian et al, Radiation-... 

R. Bird A.J. Power. Thermal Decomposition of Tetrazene at 90°C , MRL-R-710, Australia (1978) [The authors report that Tetrazene is converted into 5-aminotetrazole in less than three days at 90°, thus losing its stab sensy property. Spectroscopic evidence indicates that the 5-aminotetrazole is derived from both the side chain (via guanyl azide) and the Tetrazole ring] 24) G.B. Franklin C.F. Parrish, Radiation Polymerized Priming Compositions , USP 4056416 (1977) CA 88, 52661 (1978) [The inventors claim that extrudable primers with good percussion sensy are prepd from Tetrazene 3.9—4.1, n-Pb Styphnate 32—42,... 

The problem of the action of radiation on azides, particularly lead and barium azide has been review cd [116]. Irradiation prior to thermal decom-Jposition often effects a reduction or elimination of the induction period, a de-ICrcasc in activation energy and an increase in the rate of decomposition. Zak-rov and co-workers [117] have found that the application of a moderate electric field can affect the rate of thermal decomposition of azides. 

Irradiation of some solids prior to, or even during, thermal decomposition may have a mariced effect on the kinetics of decomposition [68], Such effects are usually interpreted in terms of the imperfections generated by the treatment. Types of radiation used have ranged from visible light to ionizing radiation to neutrons. Solids whose stabilities have been shown to be particularly sensitive to irradiation include the azides (Chapter 11), the permanganates (Chapter 14) and some metal carboxylates (Chapter 16). Comparisons of kinetics of radiolysis with those of pyrolysis can provide useful mechanistic information. Examples of such comparisons are given in the chapters mentioned above. 

Gamer and Moon [34] reported effects of radiation other than color changes the effects produced by the emission from radium on barium azide were dependent on temperature and led to the acceleration in the thermal decomposition. On the other hand the thermal decomposition of mercury ftilmi-nate was not affected in the same environment. 

A continued evolution of gas from lead azide after the radiation source had been removed suggested that either gas trapped in the sample during irradiation or that decomposition continued after irradiation [74]. When samples was irradiated at 71 °C and cooled, no such gasing occurred. In all cases more gas was evolved than could have resulted from heat alone. 

Thermal decompositions of barium and strontium azides, preirradiated with 1 MeV gamma rays, were conducted by Prout and Moore [78,79]. With dehydrated barium azide a total gamma dose of 20 Mrad (2.24 X 10 R) eliminated the induction period and increased the acceleration of the decomposition. A somewhat greater effect was evidenced with strontium azide. Avrami et al. [80] subjected barium azide to Co gamma radiation to exposure levels up to 1 XIO R (Table XIII). Differential thermal analyses (Figure 16) showed a steady decomposition of the sample, and after 1 X 10 R exposure (W hr at room temperature), infrared analysis indicated that the residue was in the form of barium carbonate. 

Radiation-induced changes in a-lead azide caused by X-rays were noted by Todd and Party [88, 89]. The hardness was changed by exposure to soft X-rays. With an X-ray dose rate of 1.4 X 10 R/min in air, the decomposing crystal expanded prefentially along the b axis of its crystal structure. More than 97% de-stmction of the azide was achieved with a total dose of 3.4 X 10 R. It was also shown that the stable endproduct of X-ray decomposition in air was basic lead carbonate of the formula 2PbCO3 Pb(OH)3. After a dose of 6.7 X lO R, there was evidence of residual lead azide together with an unidentified phase. Higher doses produced a further unidentified phase before stable basic lead carbonate was finally formed. 

Color centers can be produced in the alkali metal azide by ultraviolet light and ionizing radiation at low temperatures. The phenomenon has been of interest for some time since the defects produced are involved in the process of photochemical decomposition (cf. Chapter 7). In earlier studies [54a, b, c] purely speculative identifications of optical absorption bands with F, V, and aggregate F centers were made by analogy with the alkali halides. The most prominent visible absorption band in each case was attributed to the F center—a defect involving an electron trapped at an azide (N3) vacancy. In the case of NaNa, spin resonance [55] and recent point ion calculations [56] clearly point to the existence of a F center. However, in the case of KN3, spin-resonance studies [54a] point to the existence of molecular centers of type N2 (on low-temperature irradiation) and NJ (on room-temperature irradiation). Infrared absorptions [57] and Raman scattering [58] have been observed in the irradiated alkali azides, which can be correlated with modes associated with these defects. 

The most detailed consideration of the phenomenon is given by Torkar and Spath [121], who attribute the comparatively low intensity of radiation accompanying the decomposition of the alkaline-earth azides to the formation of nitride species. The reaction scheme involves bimolecular decomposition of N radicals at the reaction interface ... 

Radiation-induced decomposition of insulating solids has been the subject of extensive research for many years. Because of their structural simplicity, the alkali and silver halides have perhaps received the widest attention. Studies of radiation-induced decomposition in azides could represent the next logical step in structural complexity. The azides in many respects are similar to the halides. Like the alkali halides, the alkali azides are primarily ionically bonded with band gaps of the order of 8 eV. Like the halides, there are azides with smaller band gaps (less than 4 eV). Important differences between the halides and azides are the presence of the triatomic azide anion and the lattice symmetry differences, which are perhaps a result of the nonspherical charge distribution on the azide ion. The salient questions which arise for the purpose of this chapter when one compares the azides to the hahdes are How does the the presence of the molecular anion influence radiation-induced decomposition are new and/or different kinds of defects produced how does the azide molecular anion influence the defect production process ... 

There are, of course, other reasons why radiation-induced decomposition of azides is of interest. Some of the azides have technological applications related to their chemical or explosive instability. This instability, as discussed in other chapters, is clearly related to the electronic and lattice properties of the materials. It has been pointed out that there is a possible relationship between stability and electronic structure [1], and others have indicated the possible role of... 

Often in research unforeseen applications emerge which were not apparent beforehand. Studies of the decomposition processes in the azides have led to such bonuses. For example, radiation-induced decomposition of thallous azide produces colloidal disorder efficiently at low temperatures. Thus, this material could be used in a low-temperature photographic process [7-9]. 

The purpose of this chapter is to review and assess the various approaches that have been used to study the radiation-induced decomposition processes in the azides with the hope of giving a clear picture of the state of the art. While the effects of both electromagnetic and particle irradiations have been investigated, most of the work reported in this chapter is concerned with the former, i.e., ultraviolet light and to a lesser extent visible light. X-rays, and 7-rays. This chapter is limited to those azides in which radiation-induced decomposition has been studied. 

What follows is a description of the paramagnetic defects in azides which have been studied by ESR. Although the chapter deals primarily with solid azides, there have been some studies by ESR of decomposition of hydrogen azide, which is a liquid, and these are included for completeness. Similarly, defects created by doping rather than radiation are included, since they are of value in characterizing the host material. Table I summarizes the types of defects, the azides in which they have been identified, and the conditions under which they were produced. 

As noted in the introduction, the small band gap azides of lead, silver, and thallium exhibit many similar properties which differentiate them from the large band gap azides. Barium azide may be an intermediate case since with irradiation it shows properties similar to both groups of materials. The small band gap azides in question detonate while barium azide deflagrates but will not sustain detonation. When the small band gap azides, barium azide, and silver and lead halides are exposed to radiation, decomposition appears to take place in both the metal and anion sublattices. Apparently, colloidal metal is formed from the metal sublattice [7,8,81-84] and, in addition, nitrogen [85,86] or halogen gas [87,88] is liberated from the anion sublattice. The relationship... 

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