Page 272 - Electrical Properties of Materials

254 Dielectric materials

) a ( White ) b ( White
light light

Polarizer

Glass plates

Liquid crystal
molecules
V V

Liquid crystal
molecules

Glass plates

Fig. 10.27 Polarizer
A schematic representation of the
operational principles of a twisted
nematic display device.

very successful in supplanting cathode ray tubes. Their main competitors are
∗
∗ The way this book is organized, plasma plasma displays at present, and organic light-emitting diodes (to be discussed
effects are often discussed but there is in Appendix I) in the future.
no single chapter, not even a single sec-
tion, devoted entirely to their properties.
Hence this point is probably the best
for briefly describing the operation of 10.18 Dielectrophoresis
plasma displays. The essential elements
are as follows. There are two sets of A phenomenon related to polarization is dielectrophoresis (or DEP), in which
electrodes perpendicular to each other, an electric field is used to generate a force on a dielectric particle, often when
to which voltages can be applied by the it is suspended in a liquid. Unlike the dielectric effects we have encountered so
control circuits. Between each pair of
crossed electrodes there is a cell filled far, DEP requires a nonuniform field. Also unlike most ‘classical’ effects on
with gas (xenon and neon), in which the matter, it was only investigated in detail relatively recently (by Herbert Pohl, in
gas is ionized by the applied voltage. the 1950s).
The ionized gas gives rise to ultraviolet Consider a single dielectric particle, which we assume contains separable
photons, which, on impact, cause red,
green, and blue phosphors (just as in a charges ±q. In the absence of any field, the centres of positive and negative
colour cathode ray tube) to emit visible charge coincide. If a uniform electric field E is applied in the x-direction, there
light at the right amplitude, in the right will be a force ±qE on each charge. These forces cause the charge centres to
proportion of colours.
separate, moving apart by distance dx. This polarization is clearly responsible
for any dielectric effects. However, in total the forces cancel, so there is no net
force on the particle.
Consider now the case when the electric field is nonuniform. How could
we generate such a field? One simple method is to apply a potential between
a pair of shaped electrodes. For example, we could apply a voltage V between
a cylindrical wire located at the origin and a co-axial cylindrical conducting
shell, as shown in Fig. 10.28. At any radius r around the centre, the electric
flux and field must both act in the radial direction. Symmetry and continuity