INVESTIGATION OF PULSED CURRENTS IN PRESSED LEAD AZIDE IN PRE-BREAKDOWN FIELDS

A. I. Gavrilin, N. I. Dimova

(Presented by the scientific seminar of the Research Institute of Nuclear Physics)

Very few experimental works are known that are devoted to the study of phenomena preceding the electrical breakdown of solid dielectrics, and there are no works at all in which pre-breakdown phenomena in primary explosives have been investigated.

For primary explosives, up to the present time the experimental study of electrical breakdown has been reduced mainly to investigation of the dependence of electric strength on various factors (temperature, impurities, the form of the applied voltage, etc.) [1, 2]. The magnitude of the electric strength depends in many respects on the specific conditions of the experiment (the presence of defects and microcracks in the samples, the quality of contact with the electrodes, field nonuniformity in the sample, etc.). It is difficult to control the fulfillment of these conditions, which often makes it impossible to compare the data of different experiments.

From this point of view, pre-breakdown currents are characteristics of the real substance, taking into account the interaction of current carriers with the structural elements of the primary explosive, and are of interest for studying the phenomenon of electrical breakdown.

In the present work, pre-breakdown currents were investigated in pressed samples of lead azide, \(\mathrm{PbN_6}\), in the form of tablets.

Experimental Method and Discussion of Results

The investigations were carried out using the circuit described in [3]. Single rectangular pulses with a rise time of \(10^{-5}\) sec and an amplitude of up to 2.5 kV were used. Capacitive currents were compensated, and only the active current flowing through the sample was measured. A wide-band amplifier USh-10 and a two-beam oscilloscope OK-17M were used to amplify and record the active current.

The object of the investigation was pressed tablets of lead azide with a thickness of 50–350 microns. The pressing pressure was varied during the experiments from 25 atm to 1600 atm.

The samples were prepared according to the method described in another article of ours in this collection. As many works have shown (a review is given in [4]), the electric strength of porous ceramic dielectrics has a reduced value. Moreover, the larger the pores in the samples, the lower their electric strength.

Pressed lead azide likewise constitutes a multiphase system consisting of individual crystallites surrounded by an oxide film, and of internal air inclusions—pores. Thus, in addition to defects in single crystals, such as, for example, Frenkel and Schottky defects, the following defects are present:

1) micropores,
2) oxide films at the boundary between individual crystallites.

In the gaseous inclusions of porous substances placed in a strong electric field, discharges arise. It is precisely these discharges that are often the main process leading to breakdown of the porous substance.

Figure 1 gives oscillograms of the voltage on the specimen and of the current flowing through the specimen; the specimen thickness is \(d = 140\ \mu\text{m}\), and the pressing pressure is \(P = 25\ \text{atm}\).

Fig. 1. Oscillograms of current and voltage during discharges in the pores of pressed lead azide:
1 — current, 2 — voltage

As is seen from these oscillograms, at a certain electric-field strength discharges begin in the pores. The electric-field strength at which breakdown of the air layer occurs does not depend on whether voltage is applied to the layer directly by metallic electrodes or whether a dielectric is situated between the air layer and the electrodes [5].

It should be noted that the decrease in the electric strength of inhomogeneous dielectrics has been experimentally confirmed by many authors only for materials with relatively large pores (diameter not less than \(10\ \mu\text{m}\)). As for the influence of microporosity (diameter \(1\text{–}10\ \mu\text{m}\)) on \(E_{\mathrm{br}}\), this question requires special investigation. It is quite possible that very small pores will not cause a noticeable decrease in \(E_{\mathrm{br}}\), if only because breakdown of air in such pores requires a very high field strength.

By increasing the pressing pressure to \(1600\ \text{atm}\), we were able to eliminate discharges in the pores and to measure the active current flowing through the specimen in the pre-breakdown stage.

The current and voltage oscillograms obtained are shown in Fig. 2. A study was made of the dependence of the current amplitude on the pellet thickness at the same field strength for all thicknesses, lying in the region of pre-breakdown values. The dependence \(J = f(d)\) for lead azide pellets is shown in Fig. 3. As follows from this figure, the current amplitude increases with increasing pellet thickness.

Fig. 2. Oscillograms of current and voltage:
1 — current, 2 — voltage

Fig. 3. Dependence of the current amplitude on the sample thickness in the region of pre-breakdown values \((E = 1 \cdot 10^5\ \mathrm{V/cm})\)

Consequently, the current is due to processes occurring inside the sample in a strong electric field.

From the oscillogram (Fig. 2) it is seen that the current varies nonlinearly with voltage. On the basis of the obtained oscillogram, the current–voltage characteristic for a sample of thickness $d = 150$ microns, shown in Fig. 4, was constructed.

The current–voltage characteristic in Fig. 4 can be divided into three regions. In the region at field strengths from $2.63 \cdot 10^4$ to $4 \cdot 10^4$ V/cm, the current increases slowly with increasing field. At a certain field strength critical for the given sample ($E \sim 4 \cdot 10^4$ V/cm), the current begins to increase sharply. The reason for this may be the appearance of a new mechanism leading to a rapid increase in the number of current carriers. At a field strength $E = 5 \cdot 10^4$ V/cm, the current reaches its amplitude value and then begins to decrease.

Fig. 4. Current–voltage characteristic for pressed lead azide. ($d = 150$ microns, $P = 1600$ atm)

Conclusions

1. The current amplitude increases with increasing sample thickness. This fact is evidence that the current is due to processes inside the sample in strong electric fields.

2. At a certain electric-field strength critical for the given sample, a sharp increase in current begins, indicating the appearance of a new mechanism leading to a rapid increase in the number of current carriers.

In conclusion, the authors express their gratitude to Prof., Dr. G. A. Vorob’ev and to Candidate of Physico-Mathematical Sciences A. V. Baranov for valuable comments during the discussion of the results obtained.

References

1. Yu. A. Zakharov. Dissertation, Tomsk, 1963.

2. Yu. A. Zakharov, G. G. Savel’ev, V. V. Boldyrev. Kinetics and Catalysis, vol. 5, no. 5, 1964.

3. A. V. Baranov. Collection of Proceedings of the Interuniversity Conference on Breakdown of Dielectrics and Semiconductors, GEI, 1964.

4. G. I. Skanavi. Physics of Dielectrics (Region of Strong Fields). GIFML, 1958.