is about tens of electronvolts and is comparable with the
in Figure 35a. A smaller ice sample was then cut off
defects' activation energy. That should result in signifi-
the bent specimen for tensile tests (Fig. 35b and c). The
cant buildup of defects near a dislocation and in total
electrodes were formed from an Hg-In amalgam,
electrical charge if αi differs for positive and negative
which adheres well to the surface of ice but remains
liquid down to 30C, so that it does not interfere with
defects. Such a mechanism of dislocation charge cre-
ation was first suggested and estimated in Petrenko and
deformation. Dislocation currents were measured with
an electrometer at T = 20C during tensile creep of
Ryzhkin (1986a,b), but has never been accurately calcu-
lated.
the ice samples. A typical recording of both the current
I and the tensile elongation ∆l are shown in Figure
Electrically charged dislocations can manifest them-
35d. The abrupt changes in the current ∆I were caused
selves in three different electromechanical phenomena.
First, their motion during plastic deformation can result
by the starts and stops of charged dislocations. It was
shown that q can be calculated as
in an electrical current, a so-called dislocation current.
Second, application of an external electrical field may
∆Ib
cause the dislocation motion and, hence, small plastic
q= ˙ z
(54)
deformation. Third, the introduction of dislocations
lpd
may change concentrations of charge carriers and, as
a result, cause a change in electrical conductivity and
where bz = component of the Burgers' vector along
dielectric permittivity of ice. We will consider below all
the length of the specimen
three groups of phenomena.
d = interelectrode distance
∆I = dislocation current
Dislocation currents in ice
˙ = rate of the tensile plastic deformation.
lp
Dislocation currents in ice were measured and used
for determination of the linear density of dislocation
The dislocation charge may be expressed in terms of
charge q (Petrenko and Whitworth 1983). We used
qa/e, i.e., in the number of elementary charges per in-
termolecular distance a. We found that the dislocation
specimens of very pure single crystals of ice in which
the majority of edge dislocations of one mechanical
charge in ice was positive, with an absolute magnitude
of qa/e = 2 103. That corresponded to two proton
sign was introduced by preliminary bending, as shown
[0001]
a)
a. Bent crystal containing an excess of disloca-
E1
tions of one sign. When deformed by tension the
majority of dislocations move toward the upper
electrode E1.
E2
b)
b. How the tensile specimen is cut from the bent crystal.
c)
d)
50
30
0
2
0
electrometer
2
4
0
100
time (s)
d. Recordings of change of length ∆l and
c. Ice specimen mounted for ten-
current I during tensile deformation. Load is
sile deformation experiment.
applied and removed at times indicated by the
arrows. ∆l was measured in micrometers.
Figure 35. Dislocation currents in ice (after Petrenko and Whitworth 1983).
24
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