PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 176 1970

THERMAL DECOMPOSITION OF LEAD AZIDE OBTAINED UNDER VARIOUS CONDITIONS

R. A. VASYUTKOVA, G. G. SAVELEV, Yu. A. ZAKHAROV

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Experimental Part

As is known, the properties and, in particular, the thermal stability of crystalline solids depend to a significant extent on the conditions of their preparation: supersaturation, the rate of obtaining the preparation, the medium, the precipitation temperature, the degree of dispersion, aging, and the introduction of impurities into the lattice [1, 2].

We carried out studies on the influence of some of the listed conditions on the rate of thermal decomposition of lead azide in solid solutions based on lead azide with additions of divalent copper. Thermal decomposition was studied under different experimental conditions:

a) using a McBain balance in the apparatus described in [3]; the pressure in the reaction vessel was maintained constant (\(\sim 2 \cdot 10^{-2}\) mm Hg) as a result of the continuous removal, during the course of the process, of gaseous decomposition products;

b) in an automatic differential apparatus; in this case the preparations were conditioned at an initial pressure in the system of \(\sim 10^{-5}\) mm Hg. The decomposition curves of \(\mathrm{PbN_6}\) obtained in the balance apparatus are presented in Fig. 1. As can be seen from the figure, a change in the method of preparation noticeably changes the thermal stability of \(\mathrm{PbN_6}\). Thus the finely crystalline azide obtained by rapid mixing of 0.2 N solutions of \(\mathrm{Pb(NO_3)_2}\) and \(\mathrm{NaNO_3}\) (curve 1) decomposes considerably more slowly than the coarsely crystalline \(\mathrm{PbN_6}\) obtained from a solution of ammonium acetate by slow cooling according to the Miles method [4].

The experiments performed showed that the character of the kinetic decomposition curve for pure lead azide (preparation 1) depends on the experimental conditions. When the experiments were carried out in the balance apparatus, the rate of the process proved to be maximal at the initial stage of the reaction and decreased continuously over the course of the process. Observation of decomposition in high vacuum showed, on the contrary, that in this case S-shaped decomposition curves are realized and, in addition, the latter technique makes it possible to record the decomposition of basic lead carbonate, which gives an initial maximum in the coordinates rate—time of the process. As indicated in [6], well-aged crystals of \(\mathrm{PbN_6}\) always have on their surface a film of basic lead carbonate, as a result of which the decomposition process is accelerated in the initial stage.

The medium used in the synthesis of pure $\mathrm{PbN_6}$ also has a noticeable effect on the course of the kinetic curves during the thermal decomposition of $\mathrm{PbN_6}$ (Fig. 3). As is seen from the figure, when $\mathrm{PbN_6}$ is precipitated from a medium containing 0.5 g of $\mathrm{CH_3COOH}$, the magnitude of the carbonate peak decreases, and with 2 g of $\mathrm{CH_3COOH}$ it is not observed at all.

Previous studies [7] have shown that divalent copper ions can be incorporated into the $\mathrm{PbN_6}$ lattice, forming solid interstitial solutions. We introduced $\mathrm{Cu^{++}}$ ions into the $\mathrm{PbN_6}$ lattice during the synthesis of lead azide by two methods:

1) by slow mixing (stream into stream) of 0.2 N solutions of $\mathrm{Pb(NO_3)_2}$ and $\mathrm{NaN_3}$ with a calculated amount of 0.2 N $\mathrm{Cu(NO_3)_2}$ solution.

2) by rapid mixing of the initial solutions.

By the first method, solid solutions based on $\mathrm{PbN_6}$ were obtained with a calculated $\mathrm{Cu^{++}}$ content of 5, 10, and 15 mol %.

By the second method—with a $\mathrm{Cu^{++}}$ content of 2, 4, 6, 8, and 10 mol %.

Fig. 1. Effect of the method of preparation on the decomposition of lead azide. 1 — thermal decomposition of lead azide at 240°C, obtained by rapid mixing from 0.2 N solutions of $\mathrm{Pb(NO_3)_2}$ and $\mathrm{NaN_3}$; 2 — thermal decomposition at 235°C of coarse-crystalline lead azide obtained by slow cooling from a solution of ammonium acetate.

The influence of the method of precipitation of the solid solutions on the effect of the impurity can be seen from a comparison of Figs. 4 and 5. Figure 4 shows the change in the rate of thermolysis of $\mathrm{PbN_6}$ with additions of $\mathrm{Cu^{++}}$ in preparations obtained by the first method; Fig. 5, by the second method. In the latter case, unlike the first, the slowing of the thermolysis of $\mathrm{PbN_6}$ by additions of $\mathrm{Cu^{++}}$ is not directly dependent on the amount of additive introduced, but has a maximum value when 4–6 mol % $\mathrm{Cu^{++}}$ is introduced; with an impurity content of 10 mol %, the preparation explodes (throughout, calculated amounts of additive are given; no analysis of its content in the crystals was carried out).

Discussion of the Results

There are three types of decomposition curves for exothermic reactions in the solid phase that proceed with the formation of a solid reaction product and a gas [8]:

$\mathrm{A_{solid} \to B_{solid} + C_{gas}}$.

1. Decomposition with the greatest rate, observed at the beginning of decomposition. In this case the rate of formation of nuclei in the initial period is high, and the surface is almost instantaneously covered with a film of the solid product.

1. The presence of an induction period, when the rate of formation of nuclei is small.

2. Rapid, but insignificant, gas evolution in the initial period of heating and a subsequent acceleration of the reaction, as in the second case.

For lead azide, a curve of type a [8] is characteristic, but other types of decomposition curves are also possible [6]. We obtained all three types of curves (Figs. 1, 2, 3), and it turned out that the form of the kinetic curve for $\mathrm{PbN_6}$ is largely determined by the experimental conditions. The difference in the form of the curves can be explained by the presence of two mechanisms [9] of decomposition of lead azide. There are several works confirming this point of view. Thus, by X-ray and chemical analysis, and also on the basis of analysis of the form of the kinetic curves, it was shown [10] that the thermolysis of lead azide in air at temperatures of 150, 175, and 200° proceeds through several intermediate phases, probably in the diffusion region. We calculated the apparent activation energy for the data presented in [10] and found that it is relatively low ($\sim 11$ kcal/mol). This value differs from the data of other works [6, 11, 12], where values from 23 to 52 kcal/mol were found. The low activation energy confirms the probable diffusion character of the process.

In [9] the authors conclude that two mechanisms of decomposition of lead azide are possible: either by the usual reaction

$$ \mathrm{PbN_6 \to Pb + 3N_2} $$

at high temperature and in the absence of moisture, or through basic lead azides in the presence of traces of moisture. The latter mechanism is diffusion-controlled.

Curve 1 in Fig. 1 represents the thermal decomposition of pure $\mathrm{PbN_6}$; the course of the curve is characteristic of a diffusion mechanism of decomposition. The initial acceleration on the kinetic curve can be explained by the presence of basic lead carbonates [6] in $\mathrm{PbN_6}$. The course of curve (2) indicates the possibility of decomposition of $\mathrm{PbN_6}$ by another mechanism, with a barely noticeable induction period and a significant subsequent acceleration. In both cases the decomposition was carried out on a gravimetric apparatus at $P \approx 10^{-2}$ mm Hg. In the first case the $\mathrm{PbN_6}$ preparation was microcrystalline, obtained from aqueous solutions; having a larger surface area than the coarse-crystalline preparation obtained from ammonium acetate (as in the latter case), it is more strongly subject to the action of moisture and $\mathrm{CO_2}$ in the air, with the formation of basic carbonates that give a larger number of nuclei. Consequently, a difference in the decomposition mechanism can be observed with different methods of preparing one and the same preparation.

Experiments carried out on a volumetric apparatus for the thermal decomposition of $\mathrm{PbN_6}$ (Figs. 2, 3) indicate that the type of the decomposition curve of $\mathrm{PbN_6}$ is also affected by the experimental conditions. A type-b curve is realized for lead azide only under conditions of a higher vacuum: decomposition begins at a pressure of $\sim 1 \cdot 10^{-5}$ mm Hg and proceeds at a pressure of $10^{-3}$—$10^{-2}$ mm Hg, with the decomposition temperature increasing significantly. In all probability, this is due not only to the influence of water vapor, which, as shown in a number of works [6, 9], can change the course of the thermolysis curve of $\mathrm{PbN_6}$, but also to changes in the conditions of heat transfer between the substance and the surrounding medium; since the decomposition of lead azide proceeds with self-heating, this should affect the reaction rate. In our experiment the conditions of heat transfer change significantly not only as a result of the difference in air pressure during thermal decomposition, but also owing to different

designs of the gravimetric and volumetric apparatuses: in the volumetric apparatus heat removal is effected mainly by contact with the metallic heated part of the apparatus, whereas in the gravimetric apparatus it occurs only through diffusion of gases and radiation. As a result of these differences, thermal decomposition in the volumetric apparatus proceeds calmly at such temperatures (about \(300^\circ\)C) at which an explosion occurs in the gravimetric apparatus. Thus, if, following the authors of [9], one assumes that decomposition

Fig. 2. Effect of the experimental conditions on the course of the kinetic curve. Thermal decomposition of PbN\(_6\) in the volumetric apparatus at \(280^\circ\)C

of PbN\(_6\) proceeds by two mechanisms, then it should also be accepted that at low temperatures lead azide decomposes by a diffusion mechanism through basic carbonates, with a small activation energy, and at high temperatures by the ordinary reaction with a large activation energy. (This is valid under the given experimental conditions.)

In the gravimetric apparatus it is not possible to observe decomposition by the second mechanism on finely crystalline preparations, since, as a result of self-heating due to poor heat removal, the preparation explodes as soon as the rate of the process

\[ \mathrm{PbN_6 \to Pb + 3N_2} \]

becomes sufficiently high.

The appearance of the initial peak (Fig. 3) during the thermal decomposition of PbN\(_6\) in the volumetric apparatus can be explained by the presence of an impurity having a lower decomposition temperature than PbN\(_6\).

The following salts may be present as such an impurity:

1) lead carbonate, formed as a result of the exchange reaction:

\[ \mathrm{Na_2CO_3 + Pb(NO_3)_2 = 2NaNO_3 + PbCO_3,} \]

\[ \mathrm{2NaHCO_3 + Pb(NO_3)_2 = 2NaNO_3 + PbCO_3 + CO_2 \uparrow + H_2O.} \]

The salts Na₂CO₃ and NaHCO₃ are present as impurities in sodium azide, which is the starting reagent in the preparation of PbN₆. It is quite probable that lead carbonate is formed in the surface

Fig. 3. Thermal decomposition of PbN₆ obtained from solutions containing different amounts of CH₃COOH. 1 — 0.5 g CH₃COOH, \(t_{\mathrm{decomp.}} = 270^\circ\mathrm{C}\). 2 — PbN₆ without acid, \(t_{\mathrm{decomp.}} = 275^\circ\mathrm{C}\). 3 — 2 g CH₃COOH, \(t_{\mathrm{decomp.}} = 270^\circ\mathrm{C}\).

layer of PbN₆ under the action of moisture and carbon dioxide from the air. Addition of acetic acid to the sodium azide solution leads to decomposition of sodium carbonate and bicarbonate and thereby reduces the amount of lead carbonate in PbN₆, as is evidenced by the decrease of the initial peak in Fig. 3;

2) the second possible cause of the appearance of the initial peak during the thermal decomposition of PbN₆ is the presence of an impurity of lead hydroxide Pb(OH)₂ or Pb₂O(OH)₂, which has a decomposition temperature in air of 145°C and is readily soluble in acetic acid.

Fig. 4. Dependence of the rate constant of the thermal decomposition of PbN₆ on the amount of added Cu++. The samples were obtained by slow mixing of the initial 0.2 N solutions.

Formation of the hydroxide may proceed according to the reaction

\[ 2\mathrm{NaOH} + \mathrm{Pb(NO_3)_2} = \mathrm{Pb(OH)_2} + 2\mathrm{NaNO_3}, \]

\[ 2\mathrm{NaOH} + 2\mathrm{Pb(NO_3)_2} = \mathrm{Pb_2O(OH)_2} + 2\mathrm{NaNO_3} + \mathrm{H_2O}. \]

Alkali may be present as an impurity in sodium azide;

3) it is also possible that basic lead salts, in particular PbO·PbN₆, serve as the impurity giving rise to the initial peak, cor—

together with lead carbonate. These salts have been little studied; it is known only that their formation in \( \mathrm{PbN_6} \) is associated with the action of moisture and carbon dioxide from the air [9, 10].

The mechanism of the influence of the homophase impurity \( \mathrm{Cu^{++}} \) on the thermal decomposition of lead azide is described in [13], and therefore we shall not dwell on it. The cause of the change in the effectiveness of the action of the impurity (Figs. 4, 5), depending on the method of obtaining the preparation, may be the accompanying change in the amount of impurity entering the crystal lattice.

Fig. 5. Thermal decomposition of \( \mathrm{PbN_6} \) with additions of \( \mathrm{Cu^{++}} \) at 240°C. 1—pure \( \mathrm{PbN_6} \); 2—\( \mathrm{PbN_6} \) with 2 mol % \( \mathrm{Cu^{++}} \); 3—\( \mathrm{PbN_6} \) with 4 mol % Cu and \( \mathrm{PbN_6} \) with 6 mol % Cu; 4—\( \mathrm{PbN_6} \) with 8 mol % Cu; 5—\( \mathrm{PbN_6} \) with 10 mol % Cu. The preparations were obtained by rapid mixing of 0.2 N solutions.

Further investigations are necessary in order to determine definitively the reasons for this behavior of solid solutions.

REFERENCES

1. V. V. Boldyrev. Methods for Studying the Kinetics of Thermal Decomposition of Solids. Tomsk, 1958.

2. V. V. Boldyrev. Influence of Defects in Crystals on the Rate of Thermal Decomposition of Solids. Tomsk, 1963.

3. Yu. A. Zakharov. Candidate’s dissertation. Tomsk, 1963.

4. Miles. In: Collection of Initiating Explosives, 62. Moscow, 1935.

5. G. G. Savel’ev, V. V. Bordachev. Present collection.

6. P. Griffiths and Groocock. J. Chem. Soc., No. 8, 3380, 1957.

7. G. G. Savel’ev, Yu. A. Zakharov, G. T. Shchechkov. Izvestiya TPI, 151, p. 39, 1965.

8. Solid-State Chemistry, edited by V. Garner. IL, Moscow, 1961.

9. M. Stammier, J. E. Abel, R. C. Ling. Nature, 192, No. 4803, 626, 1962.

10. M. Stammier, J. Abel, J. Kaufmann. Nature, 185, 4711, 1960.

11. Hawkes, Winkler. Canad. J. of Res., (B)25, No. 6, 1957.

12. J. Jach. Trans. Farad. Soc., 59, 4, 1963.

13. Yu. A. Zakharov, G. G. Savel’ev, E. B. Chernov. Izv. TPI, 151, 32, 1965.