Electrical Discharges Process Materials, Destroy Solids

A. A. Vorob'ev

When the electric-field strength at an electrode reaches a certain definite value, electric discharges occur. The dielectric surrounding the electrode loses its electrical strength and breaks down.

At normal pressure and temperature, discharges in air—which is a good dielectric—occur at a field strength equal to 30 kV/cm. This value is equal to the electrical strength of air and of many gases. The electrical strength of technically purified liquid dielectrics exceeds the electrical strength of air by ten or several tens of times and is approximately 200–400 kV/cm. Semiconductors have the same order of strength. Dielectrics have an electrical strength on the order of 1000–2000 kV/cm.

Figure 1 shows a breakdown channel in a rock-salt crystal.

Fig. 1. Photograph of a breakdown channel in a rock-salt crystal.

Figure 2 shows the breakdown channel in a piece of paraffin 30 mm thick under an impulse voltage of 150 kV with a 2-microsecond front.

If the electric field in the dielectric is nonuniform, then breakdown begins when the value of the electric strength is reached at the point where the electric-field intensity is greatest. After the onset of breakdown of the dielectric, its destruction spreads further through the volume. Destruction of a dielectric in a nonuniform field occurs at

Fig. 2. Breakdown in a piece of paraffin 30 mm thick by an impulse voltage of 150 kV

an average intensity that is several times less than its electric strength as determined in a uniform field. The sharpest concentration of the field lines occurs at the tip of a point for point-to-plane electrodes, as shown in Fig. 3.

The greatest reduction in the breakdown voltage of dielectrics and in the average field intensity is obtained with positive polarity of the point. In this case the space charges formed in the dielectric intensify the local nonuniformity of the field and, consequently, the overloading of the dielectric, promoting the development of a discharge in it and its destruction.

Fig. 3. Concentration of field lines in a nonuniform field.

Electrical destruction of dielectrics proceeds at high speed and is completed in a time measured in hundredths of a microsecond. Breakdown develops fastest in gas, where a discharge velocity of \(10^{7}—10^{8}\ \text{cm/sec}\) has been observed. A discharge in solid dielectrics develops at a velocity of \(10^{6}\ \text{cm/sec}\). The velocity of the discharge depends on the composition and properties of the dielectric and on the form of the electric field in which breakdown occurs. The greatest discharge velocity in a dielectric is observed in a uniform electric field, when destruction of the dielectric begins—

…occurs simultaneously along its entire length between the electrodes, after the field strength reaches a value equal to the electric strength of the dielectric. The lowest rate of development of a discharge in a dielectric is observed in a nonuniform field. In this case the discharge, having begun at the point where the field strength has reached a value equal to the electric strength, propagates through the volume of the dielectric at the rate at which mechanical destruction of the body develops.

It has been established that, for large dielectric thicknesses, the lowest value of the electric strength of solid dielectrics is observed with a positive point and with a voltage action time on the order of several microseconds.

The degree of destruction of a solid dielectric during electrical breakdown depends on the amount of energy released in the discharge paths and on the duration of the discharge. The passage, in the discharge, of a large amount of energy over a short interval of time is accompanied by the destruction of the entire volume of the dielectric into separate pieces. The size of the pieces into which a block of dielectric is broken decreases as the power of the electrical discharge increases.

With an insufficient amount of energy introduced into the discharge channel, or an insufficient duration of the voltage action, a breakdown that has begun may cease. This is shown in Fig. 4, where, at a magnification of 5 times, a photograph is given of a discharge (incomplete breakdown) in glass under the action of a voltage pulse of duration 0.1 μsec.

Fig. 4. Photograph of incomplete breakdown of glass. Pulse duration 0.1 μsec.

Figure 5 shows a piece of hard coal, and Fig. 6 shows how this piece was destroyed after the application of a single pulse of electric voltage with an amplitude of 600 kV.

The phenomenon of electrical breakdown can be effectively applied to low-operation processes for the treatment of solid bodies, the extraction of minerals, secondary crushing, drilling of wells, and other production processes whose technology is associated with the destruction of solid bodies.

In modern pulse-current generators it is possible to accumulate energy of hundreds of kilojoules at voltages on the order of one hundred kilovolts. Since the discharge lasts one hundred-thousandth of a second, the power in the discharge considerably exceeds the power of the Dnepropetrovsk Hydroelectric Station.

When cutting materials or crushing them, a large part of the supplied energy is lost in the transmission links of the mechanisms; considerable energy is expended on plastic deformation of the material being processed and on heating it. Strictly speaking, only a smaller part of the energy supplied to the processing unit is spent on separating one piece of material from another. Many proposals have been made to use pulsed spark discharges for processing materials and breaking them down. Such methods of processing materials will be very economical energetically, since the greater part of the supplied energy, with the exception of small losses, will be expended on processing the material.

In an electric field of high intensity, a large concentration of energy can be achieved in a small volume. During a short interval of time, in a condensed electrical discharge, a large power is released in a small volume. As a result, high efficiency in the processing of materials can be obtained. Pri-

Fig. 5. Photograph of a piece of hard coal before destruction.

Fig. 6. Photograph of a piece of hard coal after the application of one pulse of electric voltage with an amplitude of 600 [[unclear: unit not visible]].

The use, for the destruction of solids, of an electric discharge during which a large amount of power is released may be economically more advantageous than destruction by the action of explosives.

Proposals to use electric discharges for processing solids have been made repeatedly. Some types of electric discharges and the electrical destruction of solids under the action of high voltage have found successful and ever-growing application.

In 1938, at the All-Union Electrotechnical Institute, B. R. Lazarenko and N. P. Lazarenko proposed the electric-spark (electroerosion) method of metal processing and successfully introduced it into practice.

When a high pulsed voltage is applied between electrodes and a certain field strength is reached, breakdown occurs and a channel of through conductivity is formed in the interelectrode gap. During breakdown of the gap, electrons coming from the cathode reach the anode, which is the workpiece being processed, and bombard its surface, pulverizing the metal. The authors of the method indicate that, on the bombarded areas of the anode, the observed concentration of energy in a pulsed discharge is 10 times greater than the concentration of energy in the most powerful explosions. The electric-spark method has become widespread in the processing of metals, hard alloys, and other materials. Electric-spark processing is successfully used in grinding, cutting, the manufacture of shaped dies, etc. Specialized electric-spark installations for various kinds of material processing have been developed and are widely used in industry. The simpler, more precise, cheaper, and faster electric-spark processing of materials in some cases displaces methods of processing materials by cutting.

In 1955 L. A. Yutkin proposed an original method of using a high-voltage spark electric discharge (up to 70 kV) in a liquid for drilling holes in rocks. If an electric discharge occurs inside a liquid, and the electric pulse has a very steep front, then near the discharge there arises a zone of destruction with a diameter of up to 100 mm, within which all hard brittle bodies are destroyed, plastic bodies are strongly deformed, i.e., change their shape, and metals undergo work hardening, i.e., strong strengthening of the surface layer. The cause of these phenomena is the strong pressure in the channel of the electric discharge, which, without being weakened, is transmitted by the liquid. The hydrostatic shock leads to destruction or deformation of the surrounding solids.

Fig. 7 gives a photograph of the electrohydraulic drill described by L. A. Yutkin and a diagram of the device (Fig. 8).

In Fig. 7 are shown: 1) the bent end of the central rotating electrode; 2) projections of the drill column; 3) windows for the exit of liquid from the drill; 4) the drill bit.

In Fig. 8 are designated:

1. Current supply to the central rotating electrode. 2. Supply of flushing and working liquid. 3. Channel for supplying liquid. 4. Current supply to the drill bit. 5. Windows in the drill bit for the exit of gas and liquid. 6. Drill bit. 7. Projections of the drill bit. 8. Bent end of the central rotating electrode. 9. Channel for supplying liquid. 10. Insulator. 11. Central rotating electrode.

The impulse currents of lightning exert a destructive action when striking objects on the ground. A large lightning current, flowing through a conductor, can fuse, melt, and even vaporize it. Strong magnetic fields caused by large currents deform conductors. Strong

the electric field formed around a conductor can cause changes in the surrounding medium, for example, a corona in air. The destruction of dielectrics under the action of the impulse voltage and current of lightning depends on various conditions.

Fig. 7. Photograph of an electro-hydraulic drill

Fig. 3. Diagram of an electro-hydraulic drill device

Lightning, striking objects made of materials that do not conduct electric current, destroys them. When lightning strikes a tree, it splits it. This happens because the strong electric current of the lightning, flowing along the fibers of the tree, causes evaporation of water and large-scale release of vapor and gas, producing an increase of pressure inside the woody mass and rupture of the tree. Once, lightning struck an old poplar 30 m high and 3 m in circumference and smashed it into small pieces. The splitting of pieces of wood under the action of electric-current impulses (Fig. 9) is also readily observed in the laboratory. Often lightning, striking concrete, reinforced-concrete, and brick structures, destroys them. Striking a brick chimney, lightning completely destroyed its upper end over a length of 30 m. The next 15 m of the chimney were partially destroyed, and a crack formed at the base of the chimney. Sometimes lightning, like a bullet, pierces glass, leaving a small hole in it. Porcelain insulators of power-transmission lines are often shattered into small pieces by a lightning strike.

Lightning that struck the Château de Clermont in Beauvais (France) knocked a hole in a stone wall 65 cm wide and 60 cm deep. Stone fragments from the wall flew more than 16 m.

Cases are quite frequent in which stones broken by thunderstorm discharges are found in the mountains. There is a legend that the large granite block—the Thunder Stone, on which Falconet erected his magnificent monument to Peter the Great on the bank of the Neva in Leningrad—was split off by a lightning strike. When a lightning discharge occurs in sand having high electrical resistance, the sand is melted by the lightning current. Fulgurites, “devil’s fingers,” are formed, whose length reaches tens of centimeters.

It should be noted that, in the destruction of an object under such a strike, only part of the energy released in the lightning discharge is expended. Part of the total store of lightning energy is spent on the formation of pro-

conducting channel in the air, and part of the energy is carried away by the current as it spreads through the ground.

Practically important operations are the crushing and destruction of solids, which can be carried out by their electrical breakdown.

The effectiveness of applying electrical destruction of solids by electrical pulses will be especially great when the path by which energy is transmitted from the source to the workpiece being processed is long. In deep drilling, energy is transmitted from the drive motor to the cutting tool by means of a pipe of appropriate length. The properties of such a system do not permit drilling at high speed and with great force on the cutting tool, not to mention that replacement of the cutting tool requires periodic lifting of the entire column. These operations take up a large part of the staff’s total time.

Fig. 9. At left, lightning destroyed a tall chimney; at upper right, lightning pierced a windowpane; at lower right, lightning split a stump.

A method may be considered for driving deep exploratory boreholes by means of electrical discharges, using electrical insulation of the current-carrying wire and removing the products of destruction from the borehole by means of transformer oil, which is delivered to the face under pressure. The circulation of the oil streams is arranged, like the circulation of a clay solution, to remove the products of destruction from the borehole. For insulation purposes, clean oil is supplied between the walls of the inner and outer pipes and is pumped out through the central pipe.

Experience in designing and manufacturing cables under oil pressure indicates the possibility of making a concentric cable for an operating voltage of hundreds of thousands of kilovolts.

Figure 10 presents a possible layout of an installation for driving deep boreholes. A is a copper current-carrying pipe ending in a flat or conical tip, on the surface of which there are projections--spark electrodes. Discharges from these electrodes destroy the rock beneath them. After a depression has formed under some point--a projection, for example, 1, on the conical surface, a pointed cor...

the recess is filled with oil. When the next voltage pulse is applied, the discharge will occur, for example, from point 2, then 3—4, etc. The volume of the cavity will be determined by the properties of the soil, the shape of the electrode, and the voltage and energy of the pulse. The oil entering from the pipe into the openings C, arranged along the conical surface, will carry the products of destruction through pipe A and leave them in the settling tank. If necessary, before entering pipe B the oil may be purified. The proposed drilling method may be especially effective when passing through hard rocks, where the ordinary driving tool has low productivity because of its instability.

After a certain volume has been excavated near the electrode, the entire connected system—namely, the casing pipe and the electrode—moves downward under the action of gravity. The remaining irregularities of the rock excavation will stop the movement of the system until subsequent discharges knock off these projections.

Fig. 10. Possible installation diagram for driving deep boreholes

The proposed method of driving is free from the defects and malfunctions of the rotary drilling used, and of turbodrills.

For a given value of the electrical strength of the rock, by selecting the appropriate value of the discharge voltage of the oil and the velocity of discharge development, it is possible to ensure that breakdown will occur predominantly or exclusively in the rock being worked, and not in the casing pipe.

Mechanical crushing of hard rocks presents well-known difficulties. Electrical destruction of solid dielectrics occurs with the expenditure of a small amount of energy and in a short time, on the order of microseconds. It may be proposed to carry out the crushing of large pieces of rock electrically, and the smaller pieces subsequently formed—mechanically. The size of the piece is determined by the mesh size of the sieve. A diagram of an electrical crusher for pieces of a definite size is shown in Fig. 11.

The material is fed onto the sieve and passes between flexible pointed electrodes. The closing of the gap between the electrodes by a piece of rock and the increase in the electrical conductivity of this section, or the pressure of the piece on a sensor of the circuit, may be used for synchronous application of a voltage pulse to the electrodes at the moment when the piece of rock arrives. By selecting the magnitude of the pulse energy, it is possible to vary the degree of crushing of the piece.

Fig. 11. Diagram of an electrical crusher.

In the crushing of pieces of rock by means of an electrical discharge, dielectric materials are destroyed and materials conducting electric current, enclosed in the dielectric, are freed. Thus, during the electrical crushing of pieces of rock, petrographic concentration and separation of rock from ore will take place. In the subsequent

During transportation, materials are separated by a known method; for example, magnetic materials are separated with the aid of a magnetic field.

The extraction of minerals, their beneficiation, and the processing of materials by means of electric spark discharges will make it possible to greatly facilitate many labor-intensive operations and to increase labor productivity. The new processing technology makes it possible to manufacture many parts from stronger materials, which, in turn, will make it possible to reduce the dimensions and weight of machines and many products and to change their appearance. Taken together, all this will improve and expand the possibilities for satisfying the material needs of people and society.

ERRATA

Izv. TPI, vol. 95.