EFFECT OF IMPURITIES ON THE INITIATION AND DEVELOPMENT OF DETONATION IN SOME INORGANIC PRIMARY AZIDES

Yu. N. Sukhushin, Yu. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry, Faculty of Chemical Technology)

The introduction of impurities into inorganic primary azides is a powerful means both for changing a number of the most important characteristics of slow thermal decomposition, photolysis, and radiolysis [1, 2], and for studying the mechanism of these processes. In studying the influence of impurities on radiolysis, photolysis, and thermolysis, the role of electronic factors in the development of slow decomposition processes was revealed, and a number of methods were developed for purposeful modification of certain properties of primary explosives. In view of this, it is of interest to determine the influence of impurities on the processes of initiation and development of detonation, with the aim of clarifying the mechanism of rapid decomposition and determining the explosive properties of substances modified by additives.

We determined the impact sensitivity and detonation velocity in primary compositions of the systems: lead azide—silver azide \((\mathrm{Pb}(\mathrm{N}_3)_2—\mathrm{AgN}_3)\), at the angles rich in lead azide in its system with copper azide /II/ \((\mathrm{Pb}(\mathrm{N}_3)_2—\mathrm{Cu}(\mathrm{N}_3)_2)\), and at the angles rich in silver azide in its system likewise with copper azide /II/ \((\mathrm{AgN}_3—\mathrm{Cu}(\mathrm{N}_3)_2)\). The compositions were synthesized by mixing 0.2 N solutions of the corresponding salts: \(\mathrm{NaN}_3\), \(\mathrm{Pb}(\mathrm{NO}_3)_2\), \(\mathrm{Cu}(\mathrm{NO}_3)_2\), \(\mathrm{AgNO}_3\), of chemically pure grade. The compositions were dried in vacuum and stored over \(\mathrm{P}_2\mathrm{O}_5\). Measurements were carried out on compositions pressed to a density of \(1.8\ \mathrm{g/cm^3}\), the same in all experiments.

Impact sensitivity was determined on a K-4-44-1 drop hammer in the geometry “needle against plane,” or “Russian piercing,” with 10 specimens for each point. The detonation velocity was determined by photographic recording on an SFR-L high-speed photorecorder at a sweep speed of 45,000 rpm. Samples for measuring detonation velocity were prepared by pressing the primary composition into a cellophane casing \(0.1\ \mathrm{mm}\) thick and \(5\ \mathrm{mm}\) in diameter to a base of \(2\ \mathrm{cm}\), at constant charge weight. The streak records had the form of straight, clear lines without a noticeable section of detonation run-up.

The accuracy of determining the detonation velocity was \(50\ \mathrm{m/sec}\), with five determinations made for each composition.

Figure 1 shows the dependence of the detonation velocity in the system \(\mathrm{Pb}(\mathrm{N}_3)_2—\mathrm{AgN}_3\). The composition is given according to the composition of the nitrate solutions before precipitation. It should be noted that there is a significant increase in detonation velocity in the initial sections of the diagram, that is, in the region of formation—

Fig. 1. Dependence of the detonation velocity on composition in the system
$\mathrm{Pb(N_3)_2—AgN_3}$

of solid solutions. As the solid solution decomposes, a decrease in the detonation velocity is observed. In the region of existence of the mechanical mixture $\mathrm{Pb(N_3)_2(Ag^+)}$ and $\mathrm{AgN_3(Pb^{2+})}$, a monotonic increase in the detonation velocity is observed in the middle part of the diagram.

The dependence of impact sensitivity in this same system is presented in Fig. 2. The corridor of incomplete initiation of the compositions is hatched.

Fig. 2. Dependence of impact sensitivity on composition in the system
$\mathrm{Pb(N_3)_2—AgN_3}$

Here, in the region of formation of solid solutions, a drop in the curve is observed, that is, an increase in sensitivity, and then, as the solid solutions decompose, a decrease in sensitivity; in the region of mechani-

Fig. 3. Comparison of the dependence of detonation velocity and impact sensitivity on composition in the system \(\mathrm{Pb(N_3)_2—Cu(N_3)_2}\).

… mixtures, the course of the curve of impact sensitivity, as well as the course of the detonation-velocity curve, is monotonic. The fact that an increase in detonation velocity corresponds to an increase in impact sensitivity is noteworthy. A comparison of the impact sensitivity of the corner of the diagram rich in \(\mathrm{Pb(N_3)_2}\) of the system \(\mathrm{Pb(N_3)_2—Cu(N_3)_2}\) with the detonation velocity in this system is given in Fig. 3.

The increase in detonation velocity with increasing impurity content occurs more smoothly than in the corners of the \(\mathrm{Pb(N_3)_2—AgN_3}\) system. The increase in detonation velocity also corresponds to an increase in the impact sensitivity of the compositions. The character of the dependence of detonation velocity and impact sensitivity of the corner rich in \(\mathrm{AgN_3}\) in the system \(\mathrm{AgN_3—Cu(N_3)_2}\) (Fig. 4) is different from that in the systems described above: in the region of low impurity contents, a sharp drop in detonation velocity is observed. The increase in impact sensitivity coincides with an increase in detonation velocity, and the decrease in detonation velocity—with a decrease in impact sensitivity, as also in the \(\mathrm{AgN_3}\), \(\mathrm{Pb(N_3)_2—Cu(N_3)_2}\) systems.

Compositions rich in copper(II) azide were not studied because of the extreme danger of handling them: spontaneous explosions occurred at all stages of processing the compositions and samples.

Fig. 4. Comparison of the dependence of detonation velocity and impact sensitivity on composition in the system $\mathrm{AgN_3—Cu(N_3)_2}$

Similar patterns of change in the detonation velocity have already been encountered by us twice; in these cases the compositions were obtained by other methods and from other starting materials.

In discussing the results obtained, it should be noted that, in view of our attempt to maintain a constant macroscopic density of the specimens, it may be assumed that the observed changes in detonation upon introduction of microimpurities are not determined by the factor of macroscopic density. The following may act as other causes:

1) the microscopic or X-ray density of individual microcrystals may have a strong influence if detonation develops without the participation of air gaps [5];

2) introduction of impurities may change the sizes of individual microcrystals; however, direct observation showed that a noticeable change in crystal habit occurs only in the system $\mathrm{Pb(N_3)_2—Cu(N_3)_2}$, where at small amounts of impurity there is a transition of the $\alpha$-form of $\mathrm{Pb(N_3)_2}$ into the $\beta$-form of $\mathrm{Pb(N_3)_2}$;

3) the influence of impurities (especially small amounts) cannot appreciably change the energetics of the development of the process; therefore, as one of the possible causes of such an influence one may, with a certain degree of caution, name the influence of the changing thermal stability of the compositions.

and, consequently, since the change in thermal stability is explained by us from the standpoint of the processes of electronic excitation [2], by the influence of impurities on the electronic structure of the substance.

However, according to the viewpoint of [3], a change in the electronic structure of an explosive substance may directly affect the conditions for the excitation and development of detonation. For example, in the case of initiating inorganic azides, the influence of electron donors and acceptors on the electronic excitation of the azide radical \(N_3^0\),

\[ N_3^- \rightleftarrows N_3^0 + e \]

may have a significant effect on the kinetics of decomposition, which proceeds according to the equation

\[ N_3^0 + N_3^0 \to 3N_2 \uparrow + h\nu . \]

However, the present lack of data at our disposal on the band structure of the systems described above does not permit a detailed discussion of this question. It should be noted once again that there is a parallelism between the detonation velocity and impact sensitivity, which apparently confirms the view current in the physics of explosion that both of these processes have an essentially thermal nature [5].

REFERENCES

1. Yu. A. Zakharov. Dissertation, Tomsk, 1963.
2. Yu. A. Zakharov, G. G. Savel’ev. Kinetics and Catalysis. 7, 1 (1965).
3. F. Bowden, A. Yoffe. Fast Reactions in Solids, IL, Moscow, 1962.
4. J. W. Mitchell, N. F. Mott, Phil. Mag. 2, 1149 (1957).
5. K. K. Andreev, A. F. Belyaev. Theory of Explosives, Oborongiz, Moscow,