Modification by Additives of the Radiation Stability of Lead Azide

Yu. A. ZAKHAROV, S. M. RYABYKH

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

In contrast to the reaction of thermal decomposition of lead azide, which has been studied in detail in a whole series of works [1—3], questions of the radiolysis of $\mathrm{PbN}_6$ should be regarded as completely unstudied. In this area there are only a few reports, mainly in the nature of qualitative observations of the process of radiation decomposition of $\mathrm{PbN}_6$ [4—5]. In connection with the above, we undertook a study of the radiolysis of $\mathrm{PbN}_6$ under the action of X-radiation, and also of the influence both of heterophase semiconductor additives and of homophase impurities forming solid solutions with $\mathrm{PbN}_6$ on the radiation stability of $\mathrm{PbN}_6$. The first results obtained by us in this respect are reported in the present work.

Materials and Experimental Conditions

The lead azide used in the work was prepared by precipitation upon mixing 0.2 $N$ solutions of chemically pure $\mathrm{Pb(NO_3)_2}$ and recrystallized technical $\mathrm{NaN_3}$. Heterophase semiconductor additives—$\mathrm{Ni_2O_3}$, $\mathrm{CoO}$, $\mathrm{CdO}$, and $\mathrm{ZnO}$—were introduced into the initial $\mathrm{Pb(NO_3)_2}$ solution in suspended form immediately before synthesis. To introduce homophase impurities—$\mathrm{Ag}^{+}$ and $\mathrm{Cu}^{++}$—the corresponding nitrates were dissolved together with $\mathrm{Pb(NO_3)_2}$ before precipitation.

Radiolysis was carried out in a thermostated cell evacuated to a pressure of $10^{-4}$ mm Hg and then sealed off from the vacuum system. The weight of the irradiated sample was 500 mg. The degree of decomposition was judged from the increase in the pressure of nitrogen in the reaction cell. The pressure was measured with an LT-2 thermocouple vacuum gauge, whose readings were recorded by an EPP-09 potentiometer. The choice of this method was due to the very small degree of decomposition of the azides during irradiation. The radiation source was a RUP-200—20—5 X-ray apparatus; the dose rate was $2 \cdot 10^{16}$ eV/g·sec.

Results and Discussion

To determine the influence of heterophase impurities on the rate of radiolysis of $\mathrm{PbN}_6$, a series of experiments was carried out at 20°C. The decomposition curves of pure $\mathrm{PbN}_6$ and of $\mathrm{PbN}_6$ containing semiconductor additives are shown in Fig. 1.

To estimate the donor–acceptor properties of the additives used, measurements were made of the electron work function \((\varphi)\) from \(\mathrm{PbN_6}\) and from the semiconductors used as additives [6–7]. The results of measurements of \(\varphi\) by the dynamic-capacitor method are given in Table 1.

By special experiments it was also established that irradiation of the oxides with doses close to those used in the present work most likely does not lead to changes in \(\varphi\) that could cause changes in the donor–acceptor properties of the semiconductors.

From the character of the arrangement of the curves in Fig. 1 it is evident that additives of semiconductors with a low electron work function—CdO and ZnO—accelerate radiolysis, whereas additives of \(\mathrm{Ni_2O_3}\) and CoO, which have a considerable electron work function, noticeably reduce the rate of radiolysis of \(\mathrm{PbN_6}\).

Fig. 1. Radiolysis of \(\mathrm{PbN_6}\) \((t^\circ = 20^\circ\mathrm{C})\).
1 — pure \(\mathrm{PbN_6}\); 2 — \(\mathrm{PbN_6} + 10\%\ \mathrm{Ni_2O_3}\); 3 — \(\mathrm{PbN_6} + 10\%\ \mathrm{CoO}\); 4 — \(\mathrm{PbN_6} + 10\%\ \mathrm{ZnO}\); 5 — \(\mathrm{PbN_6} + 10\%\ \mathrm{CdO}\).

Figure 2 shows the influence of homophase impurities—\(\mathrm{Ag^+}\) and \(\mathrm{Cu^{++}}\)—on the radiation stability of \(\mathrm{PbN_6}\). The presence of \(\mathrm{Ag^+}\) ions in the crystal lattice of \(\mathrm{PbN_6}\) very noticeably accelerates radiation decomposition; moreover, the effect tends toward saturation at an impurity content of about 10 mole %. When \(\mathrm{Cu^{++}}\) ions are introduced into the \(\mathrm{PbN_6}\) lattice, the opposite effect is observed—a noticeable decrease in the radiolysis rate, proportional to the content of \(\mathrm{Cu^{++}}\) in the system.

Fig. 2. Radiolysis of \(\mathrm{PbN_6}\) \((t^\circ = 20^\circ\mathrm{C})\).
1 — pure \(\mathrm{PbN_6}\); 2 — \(\mathrm{PbN_6} + 1\%\ \mathrm{Cu^{+2}}\); 3 — \(\mathrm{PbN_6} + 1\%\ \mathrm{Ag^+}\); 4 — \(\mathrm{PbN_6} + 10\%\ \mathrm{Ag^+}\); 5 — \(\mathrm{PbN_6} + 15\%\ \mathrm{Ag^+}\).

Table 1

Values of the contact potential difference (K.R.P.) relative to the gold reference electrode \(\varphi^{Au}=4.8\ \mathrm{eV}\) at \(20^\circ\mathrm{C}\) and \(p = 10^{-4}\ \mathrm{mm\ Hg}\).

Measurements of the dark and photoconductivity of PbN₆ with additions of Ag and Cu, carried out in [8], lead one to believe that these additives form solid interstitial solutions with PbN₆. In this case Ag⁺ serves as a donor, while the Cu⁺⁺ ion—because of its small size and high value of the second ionization potential—serves as an electron acceptor in the corresponding solid solutions.

From a comparison of Figs. 1 and 2 it follows that the dependence of the character of the effect of an impurity on the rate of radiolysis of PbN₆ on the donor–acceptor properties of the additives is qualitatively similar for heterophase semiconducting impurities and homophase impurities.

In both cases radiolysis is accelerated by impurities that increase the concentration of electrons in the substance, and is slowed by impurities that reduce the number of free electrons. In making this assumption we suppose that placing the PbN₆–semiconductor system in a radiation field does not fundamentally change the donor–acceptor properties of the impurities or the direction of transfer of free charges in their mixture with PbN₆. In experiments carried out by us earlier, this point of view was confirmed by the regular change in the catalytic activity of semiconductors when their φ was changed by doping with impurity ions [10–11].

Fig. 3. Radiolysis of pure PbN₆. 1 — 8°C; 2 — 15°C; 3 — 60°C; 4 — 80°C

Kinetic curves showing an increase in the rate of radiolysis with increasing process temperature for pure PbN₆ and with the CdO additive are given in Figs. 3 and 4.

Within the dose ranges used by us, it may be considered that nitrogen evolution increases linearly with time. This approximation makes it possible to determine easily the reaction rate and the rate constant \(k\). The dependence of \(\ln k\) on \(1/T^\circ K\), shown in Fig. 5, made it possible to determine the activation energy in both cases; it proved to be 1600 cal/mole for pure PbN₆ and 1790 cal/mole for PbN₆ with a CdO impurity. It may be concluded that the addition of CdO has no noticeable effect on the activation energy of the process.

Fig. 4. Radiolysis of PbN₆ + 10% CdO. 1 — 4°C; 2 — 20°C; 3 — 60°C; 4 — 80°C

It should be noted that, in studying the radiolysis of PbN₆ with a CdO additive after cessation of

after irradiation a continued evolution of \(N_2\) was observed at a noticeable rate, especially significant at high temperatures (60–80°C).

It is unlikely that this “dark reaction” is due to improved conditions for diffusion of nitrogen—the product of radiolysis—to the surface of the \(PbN_6\) crystals.

Fig. 5. Graphical determination of activation energy. 1 — pure \(PbN_6\); 2 — \(PbN_6 + 10\%\ CdO\)

It is more plausible to relate the dark evolution of \(N_2\) to the decomposition of unstable radicals and intermediate products that accumulate in the crystals during irradiation.

A similar phenomenon, caused by the decomposition of \(N_4^{-}\) radicals, was observed in [12] in a study of the photolysis of alkali-metal azides.

REFERENCES

1. D. A. Goung. J. Chem. Soc., 1341, 1964.
2. B. Reitzner, I. V. R. Kanfman, E. F. Bartell. J. Amer. Chem. Soc., 66, 421, 1962.
3. M. Stammler, J. E. Abel. Adv. in X-ray Analys., 4, 421, 1962.
4. V. V. Sviridov. Photochemistry and Radiation Chemistry of Solid Inorganic Substances, 254, Minsk, 1964.
5. Collection: Effect of Ionizing Radiations on Organic Materials, pp. 458–461, Moscow, 1965.
6. Yu. A. Zakharov, G. G. Savel’ev. Kinetics and Catalysis, 8, 1966.
7. G. G. Savel’ev, Yu. A. Zakharov. Scientific Reports of Higher Education Institutions, series Chemistry and Chemical Technology, 7, No. 5, 1964.
8. Yu. A. Zakharov, G. G. Shchekov. ZhFKh, 41, 1691, 1967.
9. R. A. Vasyutkova, Yu. A. Zakharov, G. G. Savel’ev. This collection, p. 131.
10. Yu. A. Zakharov, V. A. Nevostruev. Kinetics and Catalysis, 8, 211, 1967.
11. V. A. Nevostruev, Yu. A. Zakharov. ZhFKh, 42, No. 2, 1968.
12. P. W. M. Jacobs, T. Kureichi. J. Chem. Soc., 4723, 1964.