SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 115

Doping is an unattractive term for the introduction of a trace amount
of other elements or substances into a material. For example, one of
the most commonly used piezoelectric materials is lead zirconate tita-
nate, abbreviated PZT (the P stands for Pb, the chemical symbol of lead,
which derives from the Latin word for lead, plumbum ). PZT contains
lead, oxygen, titanium, and zirconium. Adding a tiny amount of an ele-
ment such as niobium introduces some slight instability in the materi-
al's structure—the niobium atoms infiltrate the structure and alter its
geometry in a few places. These instabilities are like “tipping points”
or places where the structure is not quite “balanced,” and an applied
electric or magnetic field can initiate more pronounced changes than it
could for a stable, steady structure.
Piezoelectric materials make good sensors, and movements induced
by applying voltages to piezoelectric materials form the basis for a lot of
actuators. These movements can be quickly and accurately controlled
with electrical devices such as computers. Adaptive systems require such
components. Actuators in deformable mirrors used in many adaptive
optics systems are piezoelectric. (The actuators in the deformable mir-
ror at Keck Observatory, however, operate by a similar though slightly
different principle known as electrostriction: Nonconducting materi-
als—materials that can carry little or no electric current—commonly
change shape under the application of an electric field. But unlike the
piezoelectric effect, electrostriction is not proportional to the applied
voltage, nor does it reverse the response when the sign—plus or mi-
nus—of the voltage is reversed. Electrostrictive actuators generally have
a faster response time, though the responses are more complicated.)
Piezoelectric materials are also used in alarm systems and medi-
cal imaging such as ultrasound. In some ultrasound machines such as
those used to image the internal organs of a patient or the fetus of a
pregnant woman, the high-frequency waves come from applying al-
ternating high-frequency electric fields to a piezoelectric crystal. This
causes the crystal to vibrate, pushing against air and producing high-
frequency sound waves (which are beyond the range of human hearing,
and are therefore called ultrasound).
Magnetostriction, mentioned above, was an early discovery. As
illustrated in Part A of the figure, application of a magnetic field, such
as that created by a powerful magnet, induces the tiny magnetic par-
ticles in a magnetostrictive material to align, changing the material's