SELF

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and S.B. Karavashkin

5. Physical processes promoting metallic bridge formation

To reveal the physical phenomena able to promote bridge formation, we have to take into account few factors analysed above. First of all, we have to account that the gap is small and can be easily strapped, if we reveal the forces promoting the surface areas of contacts to close in. Second, as we revealed above, contact surfaces in the region of showering arcs growth are melted by quite current-powerful discharge which is provided by the inductive load. Third, production of bridges is preceded by a high-frequency high-voltage oscillation process able to develop in the showering arc region considerable electrodynamic and magnetodynamic force. And forth, as we saw above, bridges are able to be produced with any voltage polarity at the contacts.

Proceeding from the listed conditions, we can say, such processes as condensation of material of cathode and anode, as well as an occasional strapping of contact gap by a foreign body, are unable to provide the whole amount of mentioned properties. Because, for example, with occasional strapping of contact gap we would not have so stable and regular sequence of showering arc growth. Foreign conducting bodies occasionally appeared in the gap region would always add to this process irregular pattern, which is not observed in experiments. And condensation of material of cathode and anode would be possible only in case if before the monotonous section of showering arc a short arc would take place, whose decay could initiate condensation. But as we saw in oscillograms, the bridge formation is preceded by a high-frequency discharge which is simply unable to produce quite much amount of plasma whose material could condense. And this discharge would last until conditions supporting it would exist. But as we could make sure, the bridge is produced out of any relevance to the fall of energy maintaining the discharge. The more, the very fact of attenuating oscillations in the beginning of monotonous section evidences that a part of oscillation energy scatters after the bridge formation.

Basically, after most popular factors appear useless to describe the bridge formation, there remain two most real possibilities - electrostatic constriction of liquid drops of material of electrode and plasma jets strapping the gap. Most probably, both phenomena take place, dependently on conditions at contacts, and can be realised even in one showering arc at different stages. So we will briefly substantiate the phenomenology of both physical processes.

In order to substantiate the possibility of liquid bridge formation by way of electrostatic constriction of the sections of contact surface, we have to draw our attention to the within-gap voltage strength arising at the stage of high-frequency high-voltage oscillations preceding the bridge production. Before we have found the size of gap where the bridge is produced, it was about (40- 80) mcm. Potentials arising at the contacts can differ in 1- 2 kV. Thus, even in supposition of even field, the average strength can be estimated as 2,510⁸ V/m. This is quite large strength and not occasionally the change of capacitance of capacitors in high-frequency fields (capacitor flickering) is well known in the theory of radio elements reliability. "In many constructions of fixed capacitors we watch a special kind of instability characterised by short-term changes (splashes) of capacitance. These changes usually do not exceed the limits of tenth and hundredth parts of percent, but they are inadmissible if the capacitors worked in circuits setting the frequency for generators. This phenomenon was first revealed in ceramic capacitors made of materials with large and with electrodes of silver faced by ignition. Later the same phenomenon was revealed also in mica, enamel glass, film capacitors. A common property of all these constructions were electrodes of leaky metallic films produced by ignition, sputtering either chemical deposition" [34, p. 183]. "It was also noticed that flickering does not arise at quite small voltage at the capacitor and is most intensive at large voltage, when we can see even sparking on the electrodes surface" [34, p. 183].