Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 480

3.1.3 Single crystal piezoelectrics
A fi nal possible alternative is called Langasite [33]. This is a piezoelectric crystal
that is grown under very high pressure. The crystal has a lower coupling coeffi -
cient than PZT, but a very high Curie temperature and depolarization fi eld. Thus,
when high voltages are applied, similar performances can be attained. It is also
much less brittle and lighter than PZT. However, the crystal is very limitedly avail-
able and more expensive than PZT. The structure of the d -matrix is as follows:

dd 0 d

dd

0 0

⎡ ⎤

11 12 14

⎢ ⎥

(8)

0 0 0 0

⎢ ⎥

25 26

⎢ ⎥

0 0 0 0 0 0

⎣ ⎦

Bohm et al . [71] reports that due to the symmetry of the crystal, only two param-
eters are independent. Usually d 11 and d 14 are mentioned.

3.1.4 Piezoelectrics as actuators
Because the coupling between a fi eld in the poling direction is the strongest, it is
most obvious to employ this effect. But since the mutual distance between elec-
trodes must remain small to attain a high electric fi eld for a given voltage, special
confi gurations have been developed for employing the d 33 -effect. First of all, one
can stack a series of disks or small patches on top of each other, each patch with
its own set of electrodes. This is called a 'piezo stack' (see Fig. 7). By applying
the same voltage to each patch, a high fi eld in the poling direction is attained
and because the dimension of the piezoelectric material in its poling direction is
increased, so is its displacement. However, the displacements are still very small;
a typical stack of several centimeters can only attain displacements in the order of
magnitude of 10 2

m. But large forces can be exerted this way.
Another concept in which the d 33 -effect is implemented, is the embedding of
fi bers that are poled in their longitudinal direction in a polymer with the applica-
tion of interdigitated electrodes on the surface. These materials are called active
fi ber composite [73] (see Fig. 8). Because these electrodes are placed at small
instances, a high electrical fi eld in the poling direction per unit applied voltage is
attained. These plies can in turn be embedded into fi ber reinforced polymers.

μ

Figure 7: Piezo stack confi guration (fi gure adapted from [72]).