Effect of Impurities on the Thermal Stability of Silver Azide

E. S. Kurochkin, Yu. A. Zakharov, G. G. Savel’ev, L. G. Cherepova

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

In works [1—3] we studied the effect of a number of semiconducting oxides and sulfides of certain metals, as well as dyes, on the thermal stability of silver azide. A significant catalytic effect of cobalt and nickel oxides, Bi₂S₃, and a number of dyes on the thermal decomposition of silver azide was found. A mechanism of catalysis was also proposed there. However, at present this question cannot be considered sufficiently studied, and the accumulation of experimental data is expedient for understanding the mechanism of catalysis.

The present work gives the results of an investigation of the effect of heterogeneous impurities—oxides, sulfides, and copper selenide—on the thermal stability of silver azide.

Experimental procedure

Unlike in works [1—3], the experiments were carried out on silver azide obtained by the interaction of an AgNO₃ solution with hydrazoic acid. A number of experiments were performed on silver azide obtained by the usual method—rapid mixing of 0.2 N solutions of AgNO₃ and NaN₃. The experiments were carried out on a weighing vacuum apparatus with automatic recording, described in [4]. A weighed portion of silver azide (10 mg) with a specified amount of additive was placed in a cup made of aluminum foil, and the substances were thoroughly mixed. The experiments were carried out at a temperature of 235°C in a vacuum of \(3 \cdot 10^{-3}\) mm Hg.

Experimental results and discussion

The results on the thermal decomposition of silver azide obtained by rapid mixing, with additions of CuO (4.5÷10 wt %), Cu₂Se, Bi₂S₃—10 wt % each, Cu₂O (5÷10 wt %), Cu₂S and CuS—10 wt % each, showed that at a decomposition temperature of 235°C a flash of the azide occurs. Moreover, for silver azide with CuO, Cu₂Se, and Cu₂S introduced in an amount of 10 wt %, a flash was observed at 200, 190, and 170°C, respectively.

At lower temperatures (below the flash temperature) we were unable to carry out thermal decomposition and obtain kinetic curves for silver azide with the additives indicated above.

Further investigations were carried out on silver azide obtained...

...by interaction of an \(AgNO_3\) solution with hydrazoic acid; this azide proved to be more thermally stable. The results are presented in Fig. 1 in \(\alpha-\tau\) coordinates (degree of decomposition—time), from which it is seen that all the investigated additives accelerate the process of thermal decomposition of the azide; moreover, the catalytic effect upon introduction of such additives as \(CuO\) and \(Cu_2O\) increases regularly with increasing amount of additive: with \(CuO\)—up to 9.9 wt. %, and with \(Cu_2O\)—up to 8%. At higher concentrations of the above amounts, ejection of the azide occurs. In the case of \(CuO\), it should be noted that a considerable catalytic effect is observed at 3.4% of the introduced impurity, and at 4.7% the weight loss in the first 2 min exceeds the theoretical value by a factor of two. This effect is evidently the result of the substance being thrown out of the cup.

For \(AgN_3\) with additives \(Cu_2S\) (4 wt. %) and \(Cu_2Se\) (10 wt. %), as in the case of azide obtained by rapid mixing, a flash occurs at \(235^\circ C\).

Fig. 1. Thermal decomposition of silver azide at \(235^\circ C\) in vacuum: pure (1) and with additives: \(CuO\)—3.3; 4.76 and 9.9 wt. %—curves 2, 3, 4, respectively; \(Cu_2O\)—3.4 wt. %—curve 5; and \(AgN_3\) with \(Cu_2O\) (8%), \(Cu_2S\) (4–10%), \(Cu_2Se\) (10%), \(CuO\) (20%)—curves 6–9—flash.

The data obtained show that the accelerating effect of copper catalysts strongly depends on their concentration. This is apparently associated with an increase in the contact area. Of interest is the previously undescribed fact of ejection of \(AgN_3\) from the reaction cup during thermal decomposition in the presence of catalysts. In order to verify that the substance is ejected, and to obtain additional data on this phenomenon, we carried out investigations by microscopic observation of the thermal decomposition of \(AgN_3\) in vacuum. It was found that ejection of the substance occurs both during decomposition of pure azide and in silver azide with catalytic additives, beginning at a temperature of about \(200^\circ C\). However, in the specimen with an additive the ejection proceeds more intensely than in the pure one. Under the microscope it is seen that individual microcrystals instantaneously disappear from the field of view and are then found on the quartz window located at a distance of 4 mm from the cup.

The causes of the ejection may be: a) cracking of the crystals or b) rapid decomposition of individual microcrystals.

The second cause is in better agreement with the influence of heterogeneous impurities on this process, since it is difficult to imagine an influence of these impurities on cracking.

REFERENCES

1. Yu. A. Zakharov, E. S. Kurochkin, G. G. Savel'ev, Yu. N. Rufov. Kinetika i kataliz, 7, 425, 5, 1966.
2. Yu. A. Zakharov, E. S. Kurochkin. Zh. neorgan. khim., 13, issue 7, 1762, 1968.
3. E. S. Kurochkin, G. G. Savel'ev, Yu. A. Zakharov, O. I. Chizhikova. Izv. TPI, vol. 199, 95, 1968.
4. G. G. Savel'ev, V. V. Bordachev. Izv. TPI, vol. 158, 80, 1967.