Laboratory Study of Rock Deformation and Fracture - Ultra and Extremely Low Frequency Electromagnetic Fields - page 333

drop at the load resistance, R. The current and voltage suddenly rise at the moment
when the SW enters the sample and it abruptly decreases at the moment when the
SW escapes the sample, as indicated by the vertical arrows.
The onset time of the SW front was 0.02-0.10 s. Basically, this value is defined
by non-simultaneity of the SW entrance into different parts of the sample. Tests
with NaCl and KBr showed that the initial jump of the current is proportional to
SV s =l. The effective density of surface charges at the front of the SW amounted to
10 4 -10 3 C/m 2 , and this value depends on the crystallographic direction of SW
propagation. The relaxation time of polarization is 0.1-0.2 s. These parameters
are typical of all kinds of different ionic crystals that were tested (NaCl, LiF, KBr,
RbCl, MgO, LiD, CsCl, and others). It was found that the features of the electric
signals depend on the magnitude of the SW, characteristics of the atomic lattice of
the tested crystal, concentration and type of alloy admixture, density of dislocations
in the sample, and other parameters.
This shock polarization has been observed for many polar dielectrics: poly-
methylmetacrylate, polyethylene, trinitrotoluene, polyamides pitch, dibutylftalate
and water, as well as for a number of semi-conductors of p- and n-types (silicon,
germanium and others). For these materials, the effective density of the surface
charges at the SW varies between 10 7 -10 2 C/m 2 and the relaxation time is about
0.1-1 s (Eichelberger and Hauver 1962 ).
The orientation of polar molecules along the direction of SW propagation is
considered to be a plausible cause of shock polarization in polar dielectrics. Rotation
of the molecules under the influence of mechanical stress could happen when the
mass of one part of the molecule is greater than another. As a result, there occurs an
induced dipole moment in the element of volume, i.e., the medium is polarized in
the zone of SW compression. Thermal motion of the molecules eventually disorients
the molecules and leads to relaxation of the shock polarization (Eichelberger and
Hauver 1962 ). Some dielectrics acquire features of a conductor at the shock front
that causes a decrease in the shock polarization. In this case, the polarization charges
are shielded by the carriers of electric current.
A reverse effect, referred to as shock depolarization, has been observed in polar-
ized seignette-electrics/ferroelectrics and piezoelectrics (Neilson 1957 ; Neilson and
Benedick 1960 ). It was found that this phenomenon arises due to partial or total loss
of the seignette-electrics/ferroelectric properties of a solid under shock compression.

9.1.3 Theory of Shock Polarization of Ionic Monocrystals

One possible cause of the shock polarization could be connected with the presence
of charged dislocations (aged) and compensating clouds of point defects in the
crystal (Linde et al. 1966 ; Wong et al. 1969 ). Mineev et al. ( 1967 ) assumed that
the shock polarization in the ionic crystal could result from the diffusion of point
defects through the SW front. Under the low pressure (up to 40-50 GPa) the key role
had to be played by positively charged vacancies, because they are the carriers of
electric current in the ionic crystal under normal conditions. It was also established