THERMAL DECOMPOSITION OF BARIUM AND COPPER AZIDES IN THE PRESENCE OF SEMICONDUCTORS

Yu. A. ZAKHAROV, E. B. CHERNOV

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

In a number of previous works we studied the influence of heterophase semiconductor additives on the thermal [1—4], photo- [5], and radiation [6—7] stability of solid salts, including azides [1, 4, 5], and showed the possibility of changing the rate of these processes over relatively wide limits by means of solid additives.

In the present article, which belongs to this series of works, data are presented on changes in the thermal stability of $\mathrm{BaN}_6$ and $\mathrm{CuN}_3$ caused by certain semiconductor oxides. The azide compositions investigated in the work were synthesized by the methods described in [8]. In preparing mixtures of azides with additives, the finely ground oxides were introduced beforehand into a solution of $\mathrm{NaN}_3$, and precipitation of the azides was carried out in a suspension of the semiconductors.

The initial stage of decomposition of the azide compositions was studied, where the influence of additives should appear most distinctly [2]; decomposition was carried out in vacuum ($10^{-5}$—$10^{-1}$ mm Hg), with the pressure of the evolved nitrogen being recorded.

Figure 1 presents decomposition curves for pure barium azide and for barium azide containing semiconductor additives. Among the contacts tested were both active inhibitors of thermolysis—$\mathrm{NiO}$, $\mathrm{Co}_2\mathrm{O}_3$—and impurities experimentally found to accelerate the decomposition process quite distinctly—$\mathrm{ZnO}$, $\mathrm{Ag}_2\mathrm{S}$.

On the rate of thermolysis of $\mathrm{CuN}_3$ (Fig. 2), the listed semiconductors exert an influence qualitatively opposite to that observed in the case of $\mathrm{BaN}_6$. In this case, the retardation of the decomposition process by the additives $\mathrm{ZnO}$ and $\mathrm{Ag}_2\mathrm{S}$ is especially clearly expressed in the initial stage of decomposition.

Earlier [1] we studied certain regularities in the catalysis by semiconductors of the thermolysis of $\mathrm{AgN}_3$ and introduced the assumption that the direction and degree of the change in the decomposition rate caused by them are determined by the character of the limiting stage of azide decomposition and by the donor-acceptor properties of the additive, the latter being capable of qualitative characterization when measuring the work function of electrons from the decomposing compound and the impurity.

The measurements performed [1, 4] showed that two groups of semiconductors—$\mathrm{ZnO}$, $\mathrm{Ag}_2\mathrm{S}$ and $\mathrm{Ni}$, $\mathrm{Co}_2\mathrm{O}_3$—which change the rate of azide thermolysis in opposite directions, possess substantially different values of the contact potential difference (c.p.d.) with respect to-

relative to the gold electrode—\(\Delta\varphi_{\mathrm{Al-ZnO}}=-0.4\) V; \(\Delta\varphi_{\mathrm{Au-Ag_2S}}=-0.6\) V; \(\Delta\varphi_{\mathrm{Au-Co_2O_3}}=+0.93\) V (measured by the vibrating-capacitor method at \(T=200^\circ\mathrm{C}\) and \(p=10^{-2}\) mm Hg).

Fig. 1. Change by semiconductors (10 wt.%) in the rate of thermal decomposition of \(\mathrm{BaN_6}\) at \(t=125^\circ\mathrm{C}\). 1, 3—respectively pure \(\mathrm{BaN_6}\), and with an admixture of 10% \(\mathrm{Ag_2S}\) and \(\mathrm{NiO}\), irradiated with a dose of \(10^5\) roentgens.

The data presented show that the electron work function from \(\mathrm{Ag_2S}\) and \(\mathrm{ZnO}\) is 1.3–1.5 V lower than the analogous value for \(\mathrm{Co_2O_3}\). The c.p.d. value for \(\mathrm{NiO}\) was not measured by us; however, judging from the literature data, the electron work function from \(\mathrm{NiO}\) is very high (\(\varphi_{\mathrm{NiO}}=5.55\), according to [9]).

In accordance with the theory of contact phenomena [10], semiconductors with a small \(\varphi\) should be regarded as probable electron donors, and impurities with a large \(\varphi\) as possible electron acceptors upon contact; moreover, the very fact of a two-sided effect of impurities on the decomposition rate of azides, correlating with the \(\varphi\) values of the additives, may be regarded as evidence of real electron exchange during thermolysis between azides and semiconductors.

Fig. 2. Thermal decomposition at \(100^\circ\mathrm{C}\) of pure \(\mathrm{CuN_3}\) and containing 10 wt.% \(\mathrm{Co_2O_3}\) (1); \(\mathrm{ZnO}\) (3) and \(\mathrm{Ag_2S}\) (4)

A comparison of the data reported in the present work and those given in [1] shows that, for silver and copper azides, a qualitative similarity is observed in the influence of semiconductors on the rate of thermolysis—acceleration of the process by additives with a large \(\varphi\) (electron acceptors) and retardation of decomposition by impurities that are electron donors,

possessing a small \(\varphi\) (characteristic of \(\mathrm{CuN_3}\)), or has no effect on the rate of the process (in the case of \(\mathrm{AgN_3}\)). The latter gives grounds for believing that the thermal decomposition of these azides is limited by a qualitatively similar stage; such a stage may probably be the reaction of two radicals

\[ \mathrm{N_3^0 + N_3^0 \to 3N_2,} \]

which is tentatively accepted as the limiting one in the thermal decomposition of \(\mathrm{AgN_3}\) [1, 4].

It is known [8, 11] that many physicochemical (in particular, electrophysical) properties, as well as the postulated mechanism of thermolysis of barium azide, differ substantially from the properties and mechanism of thermolysis of \(\mathrm{AgN_3}\) and \(\mathrm{CuN_3}\). These differences are once again emphasized by the fact that the effect of impurities on the decomposition of \(\mathrm{CuN_3}\) and \(\mathrm{BaN_6}\) is opposite in character.

A very interesting experimental fact is the absence of a catalytic effect of semiconductors on the thermolysis of \(\mathrm{BaN_6}\) after preliminary irradiation of mixtures of \(\mathrm{BaN_6}\) plus impurity with X-radiation (200 keV), with exposure doses of not less than \(10^5\) roentgens (Fig. 1).

At present it does not appear possible to speak with sufficient certainty about the causes of this effect.

They may be associated, in particular, with shielding of the impurities from the azide by radiolytic metal or with the formation in \(\mathrm{BaN_6}\), upon irradiation, of a large number of electron traps. Preliminary experiments established that “poisoning” of catalysts by irradiation is also characteristic for smaller doses (on the order of \(10^4\) roentgens).

Conclusions

1. Mechanical mixing of inorganic semiconductors with barium and copper azides causes noticeable changes in the rate of thermal decomposition of the latter. Additions of \(\mathrm{NiO}\) and \(\mathrm{Co_2O_3}\) retard, while \(\mathrm{ZnO}\) and \(\mathrm{Ag_2S}\) accelerate the thermolysis of \(\mathrm{BaN_6}\); the impurities change the rate of decomposition of \(\mathrm{CuN_3}\) in the opposite direction.

2. The question of the reasons for the change, by semiconductors, in the rate of thermolysis of azides is discussed. In doing so, the donor–acceptor properties of the impurities and the probable character of the stage limiting the process of azide decomposition are taken into account.

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