Electrical Characteristics of Explosives

M. A. Melnikov, N. P. Volkov

(Presented by the scientific seminar of the Research Institute of High Voltages)

In a number of technological processes associated with the production and use of explosives, charges of static electricity may appear. At a certain voltage, sufficient for breakdown of the explosive, and at a certain critical discharge power, a danger of ignition of the explosive arises. In this connection, it is of interest to evaluate the electric strength of some secondary explosives under the action of an impulse voltage.

In the present work, the electric strength of the most widely used explosives—TNT and tetryl—was investigated. For this purpose, pressed samples of explosive of thickness \(h = 1—3\) mm and density \(\rho = 1.3—1.6\) g/cm\(^3\) were used.

To record the volt–second characteristics, the electrical circuit shown in Fig. 1 was used. Breakdown of the samples was carried out on the front of a single voltage impulse. The investigations were conducted in a nonuniform electric field, using a point–plane electrode system with impulses of negative polarity. The curves were constructed for a breakdown probability \(\Psi = 50\%\). Each point of the curve was obtained on the basis of statistical processing of 10–15 oscillograms.

Figure 2 presents the volt–second characteristics for TNT (curves 1, 2, 3) and tetryl (curves 4, 5) at different sample densities.

The breakdown voltages of explosives depend to a considerable degree on the density of the samples, as is evident from Fig. 3. With increasing density \(\rho\), caused by an increased specific pressing pressure, the breakdown voltages increase; moreover, as \(\rho\) approaches limiting values, the increase in \(U_{\mathrm{br}}\) occurs more rapidly. This can be explained as follows. With an increase in the specific pressing pressure in the explosive samples, the sizes of air inclusions decrease, reaching, at limiting density values characteristic of each explosive, linear dimensions on the order of several microns. It is quite probable that the air in these inclusions is under elevated pressure and possesses higher electric strength. This leads to the discharge occurring directly through the explosive crystals. At low values of sample density, the number and sizes of air inclusions increase, and their electric strength decreases. In this case, there appears a greater probability of passage-

Fig. 1. Electrical schematic of the setup

Labels in the diagram: \(Tr_1\), \(Tr_2\), \(R_1\)–\(R_5\), \(C_1\)–\(C_7\), \(P_1\), \(P_2\), kV, “\(\sim 220\) V,” “triggering the oscilloscope,” “to osc.,” “O.I.,” “A.N.”

Fig. 2. Volt-second characteristics of trotyl and tetryl: curves 1, 2, 3—trotyl, specimen thickness \(h = 1\) mm, density \(\rho\) respectively equal to 1.4; 1.5 and 1.60 g/cm\(^3\). Curves 4, 5—tetryl, \(h = 2\) mm, \(\rho\) respectively 1.5 and 1.6 g/cm\(^3\).

Fig. 3. Dependence of the breakdown voltage of tetryl and trotyl on density: curves 1, 2, 3, 4—tetryl, \(h\) respectively 1.0; 1.5; 2.0 and 3.0 mm; curves 5, 6, 7—trotyl, \(h\) respectively 1.0; 1.5 and 2.0 mm.

of discharge over the surface of individual crystals of the explosive, a so-called surface discharge occurs, which leads to a significant reduction in breakdown voltages, analogous to what is observed also for homogeneous solid dielectrics.

As is known, breakdown of a solid dielectric is accompanied by the formation, in the arc stage of the discharge, of a region with a high temperature (several thousand degrees). In this case, melting and evaporation of the material occur in the channel of the solid dielectric, while the arc discharge takes place in a gaseous medium and volume bounded by the unmelted part of the solid dielectric [1].

Fig. 4. Typical oscillogram of breakdown of an explosive: TNT, \(h = 1.5\) mm, \(\rho = 1.4\) g/cm\(^3\). GIS 100 MHz, \(U_{\mathrm{br}} = 19.5\) kV

A characteristic feature of the electrical breakdown of explosives is that, when explosives are subjected to high temperatures in the region of the discharge channel, ignition of the substance occurs, accompanied by considerable gas formation and additional heat release. Thus, from 1 l of ordinary explosives up to 1000 l of gaseous products are formed. Consequently, during breakdown of an explosive, high pressures develop in the discharge channel, which leads to higher values of the active resistance of the discharge channel and causes a slower drop of the voltage across the specimen during breakdown as compared with solid dielectrics; this is confirmed by oscillograms of explosive breakdown (Fig. 4).

Intense local heating inside explosives, caused by the passage of the discharge current, can lead to a stable self-propagating chemical reaction, i.e., detonation of the explosive.

The sensitivity of explosives to a thermal impulse is usually measured by their flash temperature. For flash of an explosive to occur, the necessary condition is that the heat input, which is provided by the course of the chemical reaction, exceed the heat losses. The rate of heating has a strong influence on the flash temperature. As the heating rate increases, the flash temperature of the explosive increases and the ignition-delay period decreases, i.e., the interval of time from the beginning of heating to the instant of flash of the explosive.

Between the delay period and the flash temperature there is a relation, obtained by N. S. Semenov [2],

\[ \tau = C \cdot e^{\frac{E}{RT}}, \]

where

\(\tau\) — delay period, sec.;

\(E\) — activation energy of the explosive corresponding to the explosion reaction, in kcal/mole;

\(R\) — gas constant, equal to 1.986 kcal/mole·degree;

\(C\) — constant depending on the composition of the explosive;

\(T\) — flash temperature of the explosive in °K.

If the experimental data on the dependence between $T$ and $\tau$ are extrapolated to very short time intervals using the above formula, one can obtain an estimate of the order of the flash temperature of explosives under our experimental conditions. Thus, for tetryl ($E = 23100$ cal/mole, $C = 2.785 \times 10^{-10}$ sec), taking the duration of the arc discharge as $\tau = 10^{-6}$ sec, we obtain $T = 1150^\circ$K. The temperature in the discharge channel is an order of magnitude higher than the value of $T$ obtained above; however, no initiation of an explosion occurred under the experimental conditions. This apparently occurred because, during breakdown of the thin tablets of explosive used in our studies, significant heat losses took place owing to convection and thermal conduction from the discharge channel. With some sealing of the explosive specimens (placing them in glass or brass tubes) and recessing the electrodes into the explosive, the electrical discharge initiates stable detonation of the explosive substance.

Conclusions

1. From the obtained volt-second characteristics of TNT and tetryl, these substances may be classified as dielectrics.

2. The possibility of the occurrence of an explosion under an electrical discharge in the mass of an explosive has been demonstrated.

REFERENCES

1. A. A. Vorob’ev, G. A. Vorob’ev. Electrical Breakdown and Destruction of Solid Dielectrics. “Vysshaya Shkola” Publishing House, Moscow, 1966.

2. F. A. Baum, K. P. Stanyukovich, B. I. Shekhter. Physics of Explosion. State Publishing House of Physico-Mathematical Literature, Moscow, 1959.