Re: Cross sectional and internal characteristics
Posted by EARL WAYNE HARNER on 5/1/2008, 5:36 pm, in reply to "Re: Cross sectional and internal characteristics"
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: I'd sure like to know about the internal structure of these features. Is
: material erroded along the paths or changed (melted) or fractured (making
: the paths permiable)?
: You also mention that the charge leaks from the exteroir surfaces, have
: you ever used a large round sphere to rapidly discharge a blocks surface
: with a lower potential difference between to two surfaces. Does surface
: etching result? Thanks in advance. dz
: Eventually I hope to add some pictures of the microscopic structure of the
: discharge paths to the web site. The short answer to your first question
: is yes. The main discharge paths are roughly tubular where some of the
: Plexiglas has been evaporated and blasted out of the block. The channel
: walls are often discolored to a caramel color, or even blackened from
: carbonization if the specimen discharges while it is being irradiated by
: the electron beam. Because Plexiglas is fairly brittle, there is also
: considerable fracturing around the discharge paths. This causes the larger
: discharge paths to look almost crystalline when externally illuminated.
: The sudden expansion (actually a shock wave) of the spark channel creates
: localized stresses and cracking within surrounding acrylic. This can be
: easily observed as color gradients around the discharge zone if the
: specimen is illuminated by polarized light and viewed through another
: polarizing sheet (cross polarizers). The combination of microscopic
: tubules and fractures does allow gas to enter the interior of the
: Plexiglas. The yellowish discoloration (solarization) from the
: irradiation process fades with time as oxygen diffuses into the Plexiglas.
: In older specimens, the solarized region fades both from the outside
: surface inward, and from the discharge layer outward. This causes the
: solarized region to become a thin sliver in the middle, eventually
: disappearing entirely.
: When we charge the specimen, the interior acquires a huge negative charge.
: This attracts a correspondingly large number of positive ions from the air
: to collect on the outer surfaces of the specimen. The result is a very
: large voltage stress between the outside and interior of the Plexiglas. In
: addition, the radiation itself causes temporary increase in electrical
: conductivity (an effect called Radiation Induced Conductivity or RIC). The
: electrical conductivity also increases in regions that are subjected to
: extremely high electrical stress. The combination of these effects results
: in the discharge-free regions seen along the periphery of all specimens.
: The outside of a charged specimen is actually relatively near ground
: potential since there is an approximate balance of charge between the
: interior space charge and exterior surface charges. Poking the specimen
: allows positive surface charges to penetrate more deeply into the specimen
: which hopefully induces electrical breakdown by increasing the local
: E-field beyond the material's dielectric strength. The actual shape of the
: rest of the discharge terminal or E-field configuration is not critical.
: When the specimen discharges, there are also a set of external surface
: discharges that neutralize most of the positive surface charges residing
: on the exterior of the specimen. These surface discharges can sometimes be
: seen in photographs discharging specimens. These discharges alter the
: surface energy characteristics of the acrylic. Although these surface
: energy alterations are normally invisible, they can occasionally be made
: visible when water vapor lightly condenses onto the surface of a cold
: Wow Bert, That is a great set of answers and the icing on the cake was the
: surface characteristics. I'd bet that if loose material was on the surface
: and lingering in the air afterwards there would be some interesting
: features developing. I hope you've described these details to Dave, if not
: may I. The rocks thread might like to hear about the fracturing detail for
: the sake of permiability reasons. Thanks so much. dz