Effect of an Electric Field on the Rate of Thermal Decomposition of Certain Solids

G. G. Savel’ev, Yu. A. Zakharov, V. B. Spitsa

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

The totality of experimental data on the effect of additives on the thermal decomposition of silver oxalate, lead azide, and silver azide shows that the limiting stage in the decomposition of these salts is either the stage of formation of holes (anion radicals) or the stage of their decay. Accordingly, electron donors decrease, and acceptors increase, the rate of decomposition [1–3]. On the other hand, in [4] it was found that irradiation with ultraviolet light or X-rays during decomposition does not affect the decomposition rate of AgN₃. From this fact one may conclude that the above-mentioned stages are not rate-limiting.

However, since irradiation increases the concentration of both electrons and holes, while the magnitude and even the sign of the effect must depend on the irradiation conditions and on the position of the effective Fermi levels for electrons and holes in the crystal [5], it would be of interest to change the electron concentration by some other method. This can be done by applying an electric field to the decomposing substance. It is known, for example, that a transverse field changes the surface electrical conductivity of solids (“field effect”) [6]. Application of a field leads to quenching or enhancement (depending on the direction of the field) of cathodoluminescence [7] and photoluminescence [8], and to a change in the rate of photolysis of AgBr [9]. In [10, 11] the question of the influence of an electric field on gas adsorption on the surface of a crystal was considered theoretically. It has been found that an electric field applied to a catalyst changes the rate of the catalytic reaction of oxidation of methane and alcohols [12–13].

Experimental Method and Results

If an electric field is applied to the decomposing substance by means of two identical electrodes, as in [14], then in the substance the Fermi level of electrons will rise at one electrode and fall at the other. When the rate of thermal decomposition is measured by the total gas evolution or by weight loss, the acceleration and deceleration will completely or partially compensate one another; in addition, the effect will not depend on the direction of the field because of the symmetry of the system.

To reveal the dependence of the thermolysis rate on the direction of the field and to increase the effect, it is necessary to break the symmetry of the system, in particular—

placing between one of the electrodes and the substance being decomposed a layer of another substance with electrical properties similar to those of the decomposed substance, or by using electrodes of different shapes.

The experimental apparatus (Fig. 1) was a vacuum system in which a weighed portion of the substance (4 mg) was placed in a Teflon cup. A massive steel block served as the lower electrode. The cup was lowered onto it; above the weighed portion of the substance in the cup was an upper electrode made of foil. Thus a system was obtained: metal—decomposed substance—Teflon—metal.

The decomposition kinetics were studied by a volumetric method from the change in pressure in the vessel. Typical curves for the thermal decomposition of silver oxalate in pressure–time coordinates are presented in Fig. 2. It is seen from this figure that for one direction of the field (minus on the upper electrode) the preparations decompose faster, while for the opposite direction they decompose more slowly than without a field. In the case of silver oxalate, the field affects mainly the induction period. Silver azide decomposes without an induction period; the influence of the field in this case was smaller (Fig. 3). In the case of PbN₆, the same results were obtained as in the case of silver azide.

It was observed in this connection that if Teflon is placed under the upper electrode, the field effect is somewhat reduced. If a tablet of pressed ZnO is used as the upper electrode, the effect increases considerably. If a concave electrode is used, the effect decreases to zero.

Discussion

The data obtained can serve as confirmation of the ideas we have developed concerning the nature of the limiting stage of the thermal decomposition of solids. Indeed, when an electric field is applied, as a result of polarization or nonuniformity of the electric field, or as a result of both of these causes, the substance will become charged positively or negatively.

In our case the sign of the charge of the substance is opposite to the sign of the charge of the upper electrode.

The charging of the substance evidently occurs because of the nonuniformity of the electric field and because of charge transfer between the cup and the substance, as shown by experiments in which the substance was blocked with Teflon on two sides.

Thus, when the substance is positively charged, the rate of thermal decomposition increases, and when it is negatively charged it decreases.

Fig. 1. Schematic of the apparatus for studying the effect of an electric field on the rate of thermal decomposition.
1 — vacuum vessel; 2 — molybdenum lead-ins for applying voltage; 3 — furnace; 4 — LT-2 lamp of the thermal vacuum gauge; 5 — Teflon cup; 6 — copper wire; 7 — vacuum stopcock; 8 — ground joint for lowering the cup; 9 — massive metallic electrode.

Fig. 2. Effect of an electric field on the rate of thermal decomposition.
1 — field directed from bottom to top; 2 — without field; 3 — field directed from top to bottom.

Fig. 3. Effect of an electric field on the rate of thermal decomposition of silver azide. The relative rate is plotted on the ordinate, and temperature on the abscissa. Notation as in Fig. 2.

decreases. This result can be explained if the rate of decomposition is limited by the stage of radical decay, while the stage of electron excitation into the conduction band is reversible. It is interesting to note that, if, following Vol'kenshtein, Gorban', and Sokolov [7], one assumes that the impurity level does not shift under the influence of the electric field relative to the ceiling of the valence band, then the electric field can change the reaction rate also at the interface between the metal and the substance being decomposed, since under the influence of the electric field the energy gap between them will change.

Conclusions

1. An influence of the electric field on the rate of thermal decomposition of $\mathrm{Ag_2C_2O_4}$, $\mathrm{PbN_6}$, and $\mathrm{AgN_3}$ has been found. Cathodic polarization accelerates, while anodic polarization slows, the thermal decomposition of these substances.

2. The change in rate under the influence of an electric field may serve as one of the methods for determining the rate-limiting stage of decomposition.

References

1. Yu. A. Zakharov. Candidate’s dissertation, Tomsk, 1963.

2. A. S. McLaren, G. T. Rogers, Proc. Roy. Soc., A 240, 484, 1957.

3. G. G. Savel'ev, Yu. A. Zakharov. Izv. vysshikh uchebnykh zavedenii, khimiya i khim. tekhnologiya, 7, No. 5, 1964.

4. V. V. Boldyrev, A. I. Skorik, DAN SSSR, 156, No. 5, 1964.

5. F. F. Vol'kenshtein. Kinetics and Catalysis, 2, issue 4, 1961.

6. G. Henisch, W. Reingold, P. Tipl. In the collection of articles Electrophysical Properties of Germanium and Silicon, Sov. Radio, Moscow, p. 371, 1956.

7. F. F. Vol'kenshtein, A. N. Gorban', V. A. Sokolov. Kinetics and Catalysis, 4, No. 1, 1963.

8. G. Henisch. Electroluminescence, Moscow, 1964.

9. E. Klein, R. Matejec, Z. Electrochem., 63, 883, 1959.

10. F. F. Vol'kenshtein, V. V. Sandomirskii. Dokl. AN SSSR, 118, 960, 1958.

11. F. F. Vol'kenshtein. Electronic Theory of Catalysis on Semiconductors, Moscow, 1960.

12. P. M. Stadnik, V. P. Fentsik. Kinetics and Catalysis, 2, 4, 1961.

13. P. M. Stadnik, E. Yu. Sekeresh. Kinetics and Catalysis, 5, 3, 1964.

14. A. A. Kabanov, Yu. A. Zakharov, A. F. Trubitsyn. Izv. vysshikh uchebnykh zavedenii, khimiya i khimich. tekhnologiya 8, 529, 1965.