Thermal Decomposition of Lead and Silver Azides with Copper Additives
Abstract Generated abstract
Thermal decomposition of lead and silver azides containing small amounts of divalent copper was studied by gas evolution measurements, mass spectrometry, and electron paramagnetic resonance. Copper additives sharply increased the initial decomposition rate in both azides, altered the appearance and timing of later rate maxima, and changed the gaseous products detected during lead azide thermolysis. EPR signals in the doped azides, their disappearance after slight decomposition, and mass spectral evidence support the view that highly dispersed copper azide species or copper-containing regions decompose early, affecting subsequent decomposition of the host azide. The authors interpret these effects in terms of solid solution formation, impurity “burnout,” surface or lattice water, and catalytic or inhibitory roles of copper-derived products, while noting that further single-crystal EPR and miscibility studies are needed.
Full Text
THERMAL DECOMPOSITION OF LEAD AND SILVER AZIDES WITH COPPER ADDITIVES
G. T. SHCHUKOV, G. G. SAVEL'EV, Yu. A. ZAKHAROV
(Presented by the scientific seminar of the Department of Radiation Chemistry)
Earlier we carried out studies of the electrophysical properties [1] and thermostability [2] of solid solutions based on lead and silver azides. The present work presents the results of further experiments on the thermolysis of the above-mentioned preparations, which are compared with data from mass-spectrometric analysis and EPR spectra.
Experimental Part
The synthesis of solid solutions was carried out by the method [3] developed for solid solutions based on silver azide. In the work, initial reagents of chemically pure grade were used. The amount of coprecipitated copper is given in this work according to the analysis performed earlier [5]. Thermal decomposition was studied by gas evolution on a differentiating apparatus [4]. Determination of the composition of the thermolysis products at 230°C in the initial period of decomposition of pure lead azide and lead azide containing 0.05 mol% Cu\(^{++}\) was carried out on an MX-1302 mass spectrometer. The pressure in the ionization chamber was \(2.7 \times 10^{-7}\) torr, and the accelerating voltage was 1.7 kV.
EPR spectra of lead and silver azides containing about 0.1 mol% Cu\(^{++}\) were recorded at room temperature and at the temperature of liquid nitrogen on an RE-1301 apparatus. Owing to the absence of sufficiently large single crystals, spectra were recorded only for powdered preparations.
Results of the Experiments
Divalent copper introduced into lead azide causes a sharp increase in the first maximum of the rate as the impurity concentration increases (Fig. 1). At the same time, an increase in the induction period is observed in the doped preparations as compared with the pure ones (the content of impurity ions of heavy metals is \(< 0.001\%\)). Thus, for the composition PbN\(_6\) + 0.05 mol% Cu\(^{++}\), the second maximum of the rate appears after 850 min, while for lead azide containing 1.1 mol% Cu\(^{++}\), it is absent even after 1200 min of decomposition.
In silver azide, ions of divalent copper likewise cause a sharp increase in the initial rate of the reaction. However, in this case the appearance of a second maximum of the rate is observed, which is absent in pure silver azide (Fig. 2). Studies of electron spectra—
Fig. 1. Thermolysis of lead azide containing Cu\(^{++}\) at 247°C.
1 — pure PbN\(_6\); 2 — PbN\(_6\) + 0.05 mol % CuN\(_6\); 3 — PbN\(_6\) + 1.1 mol % CuN\(_6\); 4 — PbN\(_6\) + 0.17 mol % CuN\(_6\).
Fig. 2. Thermolysis of silver azide containing Cu\(^{++}\), at \(T = 232^\circ\)C.
1 — pure AgN\(_3\); 2 — AgN\(_3\) + 0.2 mol % CuN\(_6\); 3 — AgN\(_3\) + 0.4 mol % CuN\(_6\); 4 — AgN\(_3\) + 0.6 mol % CuN\(_6\).
Electron paramagnetic resonance showed the presence of signals in silver and lead azides containing Cu\(^{++}\). The EPR signal disappeared both in lead azide containing Cu\(^{++}\) ions and in silver azide containing Cu\(^{++}\) ions after the initial period of decomposition (degree of decomposition \(\alpha < 0.05\)). In pure copper azide no EPR signal was detected, probably because of strong line broadening. Mass-spectrometric analysis carried out by us also showed the effect of the copper impurity on the composition of the decomposition products in lead azide at 230°C. For
of pure lead azide after the initial decomposition, the following gaseous products were found: N₂, CO₂, HN₃, NH₃, and N₂O. For the composition PbN₆ + 0.05 mol % Cu⁺⁺, in addition to the products listed above, water was found. Apparently, during thermolysis of pure lead azide, water is present in such small amounts that it is completely consumed in the hydrolysis of the azide, yielding HN₃, NH₃, and other products.
Fig. 3. EPR spectra for undecomposed PbN₆ + 0.1 mol % CuN₆—curve 1, and AgN₃ + 0.4 mol % CuN₆—curve 2
Freezing the decomposition products of lead azide at the temperature of liquid nitrogen led to a sharp decrease in the first maximum of the thermolysis rate, which may be due to freezing out the vapors of CO₂, H₂O, NH₃, and HN₃. In this case the nature of the influence of Cu⁺⁺ on the thermal decomposition of PbN₆ changes: Cu⁺⁺ becomes a catalyst.
Discussion of the results
An analysis of the physicochemical properties carried out in works [1, 2] showed that solid solutions are formed in the PbN₆–CuN₆ systems. The character of the dark-conductivity isotherm of the AgN₃–CuN₆ system, specially measured by us, is qualitatively similar to the character of the conductivity isotherm of the PbN₆–CuN₆ system. The fact that azides with a copper impurity give an EPR signal, while the concentration of unpaired electrons proved, with an accuracy of ±50%, to be equal to the copper concentration found polarographically [5], and that pure copper azide gives no EPR signal, indicates that CuN₆ is highly dispersed in the matrices of silver and lead azides. The EPR results thus confirm the formation of solid solutions in the systems studied. However, definitive proof of the formation of solid solutions and elucidation of the influence of microimpurities on thermolysis require additional EPR studies in single crystals and determination of the miscibility limits in the systems PbN₆–CuN₆ and AgN₃–CuN₆.
The increase in the first rate maximum in the PbN₆–CuN₆ system is explained if it is assumed that part of the copper impurity was sorbed on the surface of the lead azide microcrystals and, owing to its lower thermal stability, decomposes. On the other hand, it may be assumed that CuN₆ entered the PbN₆ lattice in the form of two-dimensional formations (i.e., a set of monomolecular microplanes), so that “burnout” of copper may occur, analogous to the “burnout” of lead azide and other explosives placed in liquids whose boiling temperatures considerably exceeded the temperature of the experiment, as observed by Ubbelohde and Woodward [6]. In this case the first rate maximum should also increase. The absence of an EPR signal in partially decomposed lead and silver azides containing Cu⁺⁺, as well as the sharp increase in the intensity of the signal with mass number 28 (N₂) in the mass spectra of PbN₆ + 0.05 mol % Cu⁺⁺, compared with pure lead azide, confirm the decomposition of copper azide assumed by us.
However, it cannot be ruled out that true solid solutions are formed in these systems and that, upon heating, individual $Cu^{++}$ ions “burn out.” The presence of a residual rate maximum after freezing-out of the decomposition products of pure lead azide is probably associated with the occurrence of the surface decomposition reaction of $PbN_6$ [7]. The suggestion made by some investigators [8] that the first maximum is due to decomposition of $CO_2$ released during decomposition of the surface film of lead carbonates is unlikely, since thermolysis of lead azide containing 2 mole % $CO_3^{--}$, specially prepared by us by the coprecipitation method, produced practically no changes in the magnitude of the first maximum.
The delay in the time of appearance of the second rate maximum in the thermolysis of lead azide containing $Cu^{++}$ may be associated with $Cu^{++}$ remaining undecomposed in the azide lattice or with the inhibiting effect of metallic copper arising during the burnout of $CuN_6$. At the same time, the possibility cannot be excluded that, for the composition $PbN_6 + 0.05$ mole % $Cu^{++}$, thermolysis is retarded by water sorbed on the surface or incorporated into the lattice and detected by us in this preparation mass-spectrometrically. Retardation of the thermolysis of lead azide by water is described in [9], and the possibility of entry of the OH group into the $PbN_6$ lattice was assumed in [10]. However, water may also enter the lead azide lattice in molecular form. In the case of the $AgN_3$-$CuN_6$ system, the copper impurity also rapidly “burns out,” with two rate maxima appearing. The first rate maximum is apparently due to the joint decomposition of copper oxide azide and some amount of silver azide located around the impurity. The second maximum is the decomposition of silver azide. It is evident that the decomposition rate of $AgN_3$-$CuN_6$ after the “burnout” of the impurity increases strongly in comparison with pure silver azide. This may be associated with the influence of the products formed in the first period of decomposition.
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