Mecanismo De Ruptura En Solidos

Proceedings of the 3rd International Conference on
Properties and Applications of Dielectric Materials
J uly 8-12, 1991 Tokyo, J apan

BREAKDOWN MECHANISM OF THE CHARGED SOLID DIELECTRIC DURING SHORT CIRCUIT DISCHARGE
Yang B aitun L i u f u D e T u Demin L i u Yaonan
Dept. of E l e c t r i c a l Engineering
Xian Jiaotong U niversity
X ian, P. R. China

Abstract
In this paper, thephenomenon of PET breakdown
during the short circuit discharge is studied. A
theoretical model of detrapping by collision ionization
is proposed for interpretation of the breakdown
mechanism.
The coefficient for detrapping by
collision ionization and the relation between it and
the velocity of the discharging is obtained from the
theory to the experiment practice. The critical
electrical fieldof the short circuit breakdown by the
space charges due to collision ionization by detrapping
is also suggested for the solid dielectrics by the
further investigation, and it is born out in the
tests. The results in the paper is important for the
developing of dielectrics and the study of space
charges.

When the carriers (electrons or holes) are
injected into the dielectric and capturedby traps till
the stable state reaches, during the processes of short
circuit, the rate of detrapping by the collision
ionization of free carriers with trapped carriers is
(a%/at)-=

-n f3

+

(1)

where n is the density of practically movable free
carriers; p is the detrapping coefficient of collision
ionization which is dependent on the velocity of t he
discharge; nt is thedensity of the carriers in the
traps.
Consider the retrapping of carriers may o ccur
in the process, its rate is

Introduction
Many applications are found for polymers in
almost all technological fields. With these application
one can also encounter problems that arise when
utilizing these materials. In electrical devices,such
as cables and capacitors, it is the large accumulation
of chargethat leads to electrical breakdown and
therewith to damage or destruction of the equipment.
The charges in polymers formed by field emission or
electron beam generally consist of trapped and free
charges, which is formed due to the capture of carriers
by the traps and the free carrier charge. The
breakdown can take place during the short circuit
discharge for a charged polymer[ll. The treecan also
occur in a PMMA sample in the short circuit discharge
after it is radiation by a electron beam[2]. The above
phenomenon shows that the dielectric breakdown is
obviously affected by the space charge. To answer the
question clearly is a active research project in the
developing of dielectrics and insulating techniques.
There are at least two problems needed to understand.
First, whydielectric breakdown is taken place during
the short circuit discharge but not in the process of
the formation of space charge in the sample. Second,
how is the space charge released for a charged
dielectric?

where N is the density of trap in the dielectric; 6 is
the capture cross section of the trap for carriers; v
is the thermal velocity of carriers.
Combining Eqn. ( 1 ) with Eqn. (2) and
considering the possibility of recombination of
positive and negative carrier, the variation rate of
trapped carrier per unit volume is
%/dt

-n[g+

(3)

For a discharging capacitor, the variation
rate of mobile charge with time is

Q

Qoexp(-t/z)

(4)

where ‘C=RC, is time constant of the discharging
circuit; C is the capacitance of the sample; R is the
resistance ofthe discharging circuit; Qo is the
initial total charge on the capacitor.

Theory

1048 --

6v(N-%)l-R

where R is the rate of recombination.

In this paper, the phenomenon mentioned above
is studied. The mechanism of short circuit breakdown by
the detrapping of space charge in a charged dielectric
is explored by theoretical analysis and verified by the
experiments.

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