Modification of the Radiation-Chemical Stability of \(PbN_6\) by Heterophase Additives

Yu. A. Zakharov, S. M. Rabin, V. D. Minaev

(Presented at the scientific seminar of the Department of Radiation Chemistry).

Some observations of recent years \([1–8]\) suggest that a promising possibility for the directed modification of the radiation-chemical stability of solid salts consists in introducing heterophase additives of semiconductors into them. In the present work the influence of such additives—semiconductor oxides—on the radiolysis of \(PbN_6\) is considered. The choice of \(PbN_6\) for investigation is determined by its practical importance as an explosive substance, and by the fact that \(PbN_6\) is a convenient model object for studying solid-phase decomposition. To obtain information on the direction of electron transfer during radiolysis, the electrical conductivity and \(\gamma\)-transmission of \(PbN_6\)–oxide layers were investigated.

MATERIALS AND EXPERIMENTAL CONDITIONS

Polycrystalline \(PbN_6\) was obtained by an exchange reaction between aqueous solutions of \(NaN_3\) and \(Pb(NO_3)_2\). The content of \(N_3^-\) in \(PbN_6\), determined by chemical analysis, was \(98 \pm 2\%\). The heterophase additives used were the oxides \(ZnO\), \(CaO\), \(Co_2O_3\), and \(NiO\). To obtain the required composition, \(PbN_6\) and the oxide were thoroughly mixed mechanically. In the experiments a powder fraction of 30–60 mesh was used. The technical \(NaN_3\) employed was recrystallized twice; the other reagents were of “chemically pure” grade.

The irradiation source was a BSV-200 X-ray tube operating at 200 kV, 20 mA; the dose rate was determined with a ferrosulfate dosimeter. Radiolysis was carried out in vacuum at a residual pressure of \(10^{-2}\)–\(10^{-4}\) mm Hg in the temperature range \(0\)–\(100^\circ\mathrm{C}\), with the amount of gas released during irradiation being recorded. The sensitivity of the volumetric setup was \(0.7\cdot10^{-6}\) mol.

In studying the electrophysical properties of the \(PbN_6\)–oxide systems, pressed tablets of \(PbN_6\) and of the corresponding oxide were brought into tight mechanical contact. Measuring leads into the irradiation field, under conditions analogous to those of radiolysis, were used to measure the \(T\)-conductivity and \(T\)-emf. The current sensitivity of the measuring circuit was \(10^{-14}\) A.

RESULTS

1. When the oxides \(ZnO\), \(CdO\), \(Co_2O_3\), and \(NiO\) are introduced into \(PbN_6\), the radiation-chemical stability of \(PbN_6\) changes. Additives of \(ZnO\) and \(CdO\) markedly accelerate the evolution of nitrogen from \(PbN_6\) under irradiation; additives of \(NiO\) and \(Co_2O_3\) slow it down (see Fig. 1). Both the catalytic and inhibitory effects are rapidly saturated as the amount of added additive is increased. The rate of gas evolution for all the mixtures studied in the dose range \(1\cdot10^{17}\)–\(1\cdot10^{19}\) eV/g remains constant, as is the case for the radiolysis of pure \(PbN_6\) [3,5]. Study of the temperature dependence of the rate of gas evolution showed that the introduction of the indicated heterophase additives has no effect on the activation energy of \(PbN_6\) radiolysis: in all cases the activation energy of the gas-evolution process during radiolysis was \(5\pm1\) kcal/mol, as for pure \(PbN_6\). After irradiation is stopped, gas-

excitation from the \(PbN_6\)–oxide compositions continued, the regularities being analogous to those found for pure \(PbN_6\) [9].

1. In the absence of irradiation, the layers \(PbN_6\)–oxide are not a source of any noticeable e.m.f. However, when layers of contacting tablets are irradiated, appreciable \(\gamma\)-e.m.f.’s are recorded. In this case \(PbN_6\), irradiated in contact with \(ZnO\) and \(CdO\), is charged to a positive potential, and in contact with \(NiO\)—to a negative one. The current–voltage characteristics for the \(\gamma\)-currents in the layers studied are given in Fig. 2. For the layers \(PbN_6\)–\(ZnO\) and \(PbN_6\)–\(CdO\), the magnitude of the \(\gamma\)-current depends little on the direction of the current, whereas in the layers \(PbN_6\)–\(Co_2O_3\) and \(PbN_6\)–\(NiO\) a rather considerable rectification effect is observed, the reverse direction being the current direction from the oxide to \(PbN_6\).

DISCUSSION OF RESULTS

Studies of the catalysis and inhibition of the radiolysis of nitrates and \(NH_4ClO_4\) by heterophase additives [1–3], as well as our results in this direction [8], make it possible to suppose that the change, by semiconductor additives, in the radiolysis rate of solid radiation-unstable salts is the result of donor–acceptor interactions between the contacting partners. The meaning of the influence of the contacts consists in a shift of the equilibrium of the electronic stages of radiolysis. This is consistent with the circumstance that the oxides do not change the activation energy of salt decomposition.

In works [4] a scheme of the radiolysis of \(PbN_6\) was substantiated, including the following principal elementary stages:

\[ N_3^- \;\rightsquigarrow\; N_3^0 + e \tag{1} \]

\[ N_3^0 + e \;\longrightarrow\; N_3^- \tag{2} \]

\[ e + T \;\longrightarrow\; Te \tag{3} \]

\[ 2N_3^0 \;\longrightarrow\; 3N_2 + 2\Box^- + h\nu \tag{4} \]

Stationarity was also shown under irradiation with respect to the intermediate products—the radicals \(N_3^0\) and conduction electrons. On this basis one can obtain the following expressions for the rates of change of the concentrations of \(N_3^0\) and electrons:

\[ \frac{d[N_3^0]}{dt} = \kappa_1 J [N_3^-] - \kappa_2 [N_3^0][e] - \kappa_4 [N_3^0]^2 =0 , \tag{I} \]

\[ \frac{d[e]}{dt} = \kappa_1 J [N_3^-] - \kappa_2 [N_3^0][e] - \kappa_3 [T][e] =0 . \tag{II} \]

Taking into account that the rate of radiolysis is equal to \(v=\kappa_4[N_3^0]^2\), from equation (II) it is easy to find the stationary concentration of electrons:

\[ [e] = \frac{\kappa_1\sqrt{\kappa_4}\,J[N_3^-]} {\kappa_2\sqrt{\kappa_4}[T]+\kappa_3\sqrt{v}}, \tag{III} \]

where \(J\) is the irradiation intensity,

\([T]\) is the concentration of electron traps.

If \(PbN_6\) during radiolysis is in contact with a semiconductor that is an electron donor or acceptor, then the stationary electron concentration changes by a constant amount \(A\) (it may be positive or negative):

\[ [e] = \frac{\kappa_1\sqrt{\kappa_4}\,J[N_3^-]} {\kappa_3\sqrt{\kappa_4}[T]+\kappa_2\sqrt{v}} + A . \tag{IV} \]

Substituting this value of \([e]\) into equation (I), one can express the dependence of the radiolysis rate on the quantity \(A\) in the form:

\[ A = \frac{\kappa_1\kappa_3\kappa_4\,J[N_3^-][T]} {\kappa_2\sqrt{v}\left(\kappa_3\kappa_4[T]+\kappa_2\sqrt{v}\right)} - \frac{\sqrt{\kappa_4}}{\kappa_2}\sqrt{v}. \tag{V} \]

Graphically, the function \(v=f(A)\) has the form shown in Fig. 3. From this graph it follows that, when an additive that is an electron donor is introduced into \(PbN_6\), the radiolysis rate of \(PbN_6\) should decrease, while upon introduction of an acceptor it should increase.

Fig. 1. Temperature dependence of the radiolysis rate of \(PbN_6\)–oxide compositions.
1 — \(PbN_6 + 10\%\,NiO\);
2 — \(PbN_6 + 10\%\,Co_2O_3\);
3 — \(PbN_6\);
4 — \(PbN_6 + 10\%\,ZnO\);
5 — \(PbN_6 + 10\%\,CdO\).

Fig. 2. Volt–ampere characteristics of \(PbN_6\)–oxide layers in the radiation field \((t = 50^\circ C)\).
1 — \(PbN_6 - Co_2O_3\);
2 — \(PbN_6 - NiO\);
3 — \(PbN_6 - CdO\);
4 — \(PbN_6 - ZnO\).

Fig. 3. Dependence of the radiolysis rate of \(PbN_6\) on the value \(A\) (according to equation V).

Such conclusions were to be expected, since a change in the steady-state concentration of electrons should lead to a shift of the equilibrium in the recombination reaction (2) to the right when electron acceptors are introduced and to the left when electron donors are introduced; this, in turn, changes the steady-state concentration \(N_s^e\) and ultimately the rate of radiolysis, as described above.

The character (direction and magnitude) of charge exchange at an irradiated contact is determined by the quality of the contact and by the ratio of the electron work functions of the contacting partners. However, when a contact is irradiated with hard radiation, the properties of the crystals that determine their donor–acceptor interactions may change. A direct experiment to observe the direction of electronic transitions in the contacting layers consists in measuring a group of electrophysical properties of \(PbN_6\)–oxide layers, and above all in measuring \(\gamma\)-emf and rectification effects in the layers directly in the radiation field.

The electrophysical measurements described are, in essence, a study of charge separation in a photocell with a \(p\)–\(n\) junction (or with a contact inhomogeneity of another type), operating in two regimes: photovoltaic (measurements of \(\gamma\)-emf) and one close to photodiode operation (measurements of current–voltage characteristics).

In \(PbN_6\)–\(NiO\) and \(PbN_6\)–\(Co_2O_3\) layers, under irradiation, the blocking direction is the direction of current from the azide to the oxide. Since \(PbN_6\) is a \(p\)-type photoconductor [5], such a pattern can be observed only in the case of electron transfer from the oxide into \(PbN_6\) (hole transfer is impeded by the contact barrier). In \(PbN_6\)–\(ZnO\) and \(PbN_6\)–\(CdO\) layers, where the contact region is anti-blocking, the reverse transfer of electrons—from \(PbN_6\) into the oxide—should occur.

Irradiation of the layers in the absence of an external field should lead to separation of electron–hole pairs at the contacts and, as a consequence, to the appearance of a \(\gamma\)-emf. Minority carriers (electrons in \(PbN_6\)), formed at a distance from the con-

also, not exceeding the free path length, are picked up by the contact field and transferred from one substance to another until an established equilibrium is reached, at which the current through the contacts ceases. The experimental data showed that \(PbN_6\) in contact with \(ZnO\) and \(CdO\) acquires a positive potential, while in contact with \(NiO\) it acquires a negative one. These data correlate well with the pattern of current measurements and also support the opinion expressed above that, in the radiation field, in the layers \(PbN_6 - ZnO\) and \(PbN_6 - CdO\) the oxide acts as an electron acceptor, while in the layers \(PbN_6 - NiO\) and \(PbN_6 - Co_2O_3\) it acts as a donor.

In accordance with the assumptions stated earlier, \(ZnO\) and \(CdO\) should accelerate, while \(NiO\) and \(Co_2O_3\) should slow, the radiolysis of \(PbN_6\). The experimental facts presented in this work are in full agreement with this.

LITERATURE

1. D. A. Zakharov, V. A. Novostruev. In the collection “Radiation Physics of Nonmetallic Crystals,” Kiev, 1965, Nauka Publishers, 1967, p. 397.

2. D. A. Zakharov, V. A. Novostruev, ZhFKh, 41, No. 4, 1968; Kinetics and Catalysis, 8, 210, 1967.

3. Yu. A. Zakharov, V. A. Novostruev, S. M. Ryabykh, in the collection “Proceedings of Young Scientists—Chemists, Tomsk, 1967” (in press).

4. S. M. Ryabykh, D. A. Zakharov, ZhFKh (in press).

5. G. P. Savel’eva, Yu. V. Gandrichenko, D. A. Zakharov, Izvestiya VUZov, “Physics,” No. 7 (1968).