Electrical treeing first occurs and propagates when a dry dielectric material is subjected to high and divergent electrical field stress over a long period of time. Electrical treeing is observed to originate at points where impurities, gas voids, mechanical defects, or conducting projections cause excessive electrical field stress within small regions of the dielectric. This can ionize gases within voids inside the bulk dielectric, creating small electrical discharges between the walls of the void. An impurity or defect may even result in the partial breakdown of the solid dielectric itself. Ultraviolet light and ozone from these partial discharges (PD) then react with the nearby dielectric, decomposing and further degrading its insulating capability. Gases are often liberated as the dielectric degrades, creating new voids and cracks. These defects further weaken the dielectric strength of the material, enhance the electrical stress, and accelerate the PD process.
Let’s take another look at that GIF…
What is happening in the GIF above? We observe the nail, in contact with some material, tapped on the head by hammer. Almost instantaneously, a branched structure and a bright flash of light appear, followed by small electrical arms flickering throughout the material.
What role does the nail play in initiating this process? The crack-like structures diverges from the point where the nail makes contact with the material–a first indication that mechanical stress due to the nail may be a trigger. The nail appears to introduce a crack, which propagates through the dielectric in a branching, fractal pattern.
But how might the flash of light be produced from the process of cracking?
Perhaps we can answer this by first posing another question:
Is it possible to cause the emission of light from a material with mechanical force?
Sure thing. Light is emitted when excited electrons return their energy to their surrounding by spitting out energy in the form of photons. Thermal and photon energy may excite electrons and induce an emission of visible light in a material, but a rubbing motion may also loosen charges and send them streaking off the material in neat little arcs creating sparks. However, there is yet another way to produce excitation followed by emission.
Remember that E&M physics lab where you charged pieces of Scotch tape by layering them atop one another and tearing off the upper tape? The upper piece would be left with a negative charge and the bottom tape with a positive charge. In this way, electrons can be mechanically separated from their burrows in the ground state. How much energy could be emitted if these electrons were to be returned to their holes?
“Sticky tape generates xrays”, declares an article published in 2008. The words nature: International weekly journal of science, hover above this announcement impressively like a guardian angel or a Harvard diploma, protecting it from eye-rolls and other quirks of skepticism. The casual reader may now advance…
Researchers at the University of California, Los Angeles, have shown that simply peeling ordinary sticky tape in a vacuum can generate enough X-rays to take an image — of one of the scientists’ own fingers
This kind of energy release — known as triboluminescence and seen in the form of light — occurs whenever a solid (often a crystal) is crushed, rubbed or scratched. It is a long-known, if somewhat mysterious, phenomenon, seen by Francis Bacon in 1605. He noticed that scratching a lump of sugar caused it to give off light.
The leading explanation posits that when a crystal is crushed or split, the process separates opposite charges. When these charges are neutralized, they release a burst of energy in the form of light.
Astute reader, you have already guessed it. The nail’s initial tap causes the material to crack. When the split forms and the crack’s wall are pulled apart, electrons are jerked from their holes. The resultant electric field yanks electrons back into the holes, and energy is emitted as a flash of light. New cracks race through the material in a branching pattern and the process is repeated. Within the cracks, collisions of charges with gases result in small electrical discharges observed as fragile, miniature lightning bolts.
electrical treeing excerpt from http://en.wikipedia.org/wiki/Electrical_treeing
picture retrieved from http://www.studentbeans.com/picture/electrical-treeing.html
sticky tape article from http://www.nature.com/news/2008/081022/full/news.2008.1185.html
special, special thanks to M.V. for all the rest