The Influence of Electrode Surface Conditions on the Bouncing Behaviour of Low-Velocity Charged Microspheres in a High Field Gap

  • Sunita Mohindra

Student thesis: Doctoral ThesisDoctor of Philosophy


This study presents an experimental and theoretical investigation into
the influence of electrode surface conditions on the bouncing impact
behaviour of charged microparticles in a high voltage gap, It uses a
simulation technique based on a microparticle gun that can inject
positively charged, low velocity microparticles into a planar UHV gap.
Experiments have been performed on different combinations of particle/
target materials (e.g. carbonyl - iron, gold, titanium, copper and nickel )
and a variety of surface states (e.g. mechanically polished, argon ion
etched, electron bombarded and electropolished) which are monitored

It is found firstly, that the mean mechanical behaviour as characterized
by the coefficient of restitution or e-values exhibits general trends which
are similar to those found with corresponding macro systems, The observed
differences in the critical velocity for elastic impact for the micro and
macro systems and the large scatter in the measured e-values are interpreted
in terms of the microtopography of a surface, Secondly, the electrical
behaviour, as characterized by the ratio reversed/initial charge or Q-values,
is found i) to exhibit overall trends that are similar for all particle/
electrode combinations and ii) to vary in magnitude for individual surfaces,
This is interpreted in terms of the electrical properties of the
contaminating films, in particular the tunnelling resistivity and relative
work functions of the junction oxide layers which are shown to be
influential in determining the magnitude of charge transferred (a) and
hence the Q-values.

Theoretically, the evaluation and assessment of an existing model based on
quantum mechanical tunnelling across the oxide junction has resulted in a
modified expression for which predicts theoretical values that are in
close agreement with experimental values.

Finally, in considering the technological implications of these findings
it has been shown that although both the mechanical and electrical
properties determine the breakdown criterion for a material it is the
electrical properties which are the more dominant.
Date of Award1981
Original languageEnglish


  • coefficient of restitution
  • surface conditions
  • electrical breakdown
  • high field gap
  • microparticle-low-velocity bouncing
  • work functions

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