Page 395 - Organic Electronics in Sensors and Biotechnology

372 Chapter Ten

charge falls to zero is large compared to the particle size. In this con-
dition, the particles will move relative to the liquid phase when the
electric field is applied. Colloidal particles that are 1 μm or less in
diameter tend to remain in suspension for long periods due to brownian
motion. Particles larger than 1 μm require continuous hydrodynamic
agitation to remain in suspension. The suspension stability is charac-
terized by the settling rate and the tendency to undergo or avoid floc-
culation. Stable suspensions show no tendency to flocculate, settle
slowly, and form dense and strongly adhering deposits at the bottom
of the container. Flocculating suspensions settle rapidly and form low-
density, weakly adhering deposits. If the suspension is too stable, the
repulsive forces between the particles cannot be overcome by the elec-
tric field, and deposition does not occur. According to a model for EPD,
the suspension should be unstable in the vicinity of the electrodes. 32
This local instability can be caused by the formation of ions from elec-
trolysis or discharge of the particles; these ions then cause flocculation
close to the electrode surface. It is desirable to find suitable physical/
chemical parameters that characterize a suspension sufficiently so that
its ability to deposit can be predicted.

Parameters Related to the EPD Process

Effect of Deposition Time At constant voltage, initially there is a linear
relationship between the amount of material deposited and the depo-
sition time, but later the deposition rate decreases with increased or
prolonged deposition time. This is expected because while the poten-
tial difference between the electrodes is maintained constant, the
electric field influencing electrophoresis decreases with deposition
time due to formation of an insulating layer of ceramic particles on
the electrode surface. 33

Applied Voltage Normally the amount of deposit increases with
34
increase in applied potential. Basu et al. have found that there is more
uniform deposition of films at moderate applied fields (25 to 100 V/cm)
and the quality of the films deteriorates if relatively higher applied
fields (>100 V/cm) are used. Since the formation of a particulate film
on the electrode is a kinetic phenomenon, the accumulation rate of the
particles influences their packing behavior in the coating. A higher
applied field may cause turbulence in the suspension; the coating may
be disturbed by flow in the surrounding medium, even during its
deposition. In addition, particles can move so fast that they cannot find
enough time to settle in their best positions to form a close-packed
35
structure. Negishi et al. have observed that the current density of
n-propanol solvent in the absence of any powder is proportional to the
applied voltage, and it tends to become unstable with increasing applied
voltages (Fig. 10.3). Such stability data serve as a good guideline for
deciding the deposition parameters and consequently the quality of