SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 114

are enough to allow the Keck telescopes to generate images that are 10
times sharper and perform nearly as well as the Hubble Space Telescope
under certain conditions, despite the effects of the atmosphere.
rESPondIngToElECTrICITyAnd
MAgnETISM
Light is electromagnetic radiation, and electricity and magnetism, along
with optics, are extremely important in science, engineering, and indus-
try. Smart materials and adaptive systems that respond to some aspect
of electricity and magnetism are in high demand. A circuit based on
electrical resistance was mentioned earlier in the chapter.
Materials that respond to electricity or magnetism were some of the
earliest smart materials to be discovered. The British researcher James
Joule (1818-89) found in 1842 that iron changes length in response to
a magnetic field, a process called magnetostriction. Since electric and
magnetic fields are easy to produce and control with precision, these
smart materials can be extremely useful.
Piezoelectric materials generate a voltage when pressed or squeezed.
(The name for this group of materials comes from a Greek word, pieze-
in, which means to press.) Voltage is an electric potential—an ability
to cause a current to flow—and in piezoelectric materials, the voltage
arises when the pressure causes electric charges to line up or separate.
The opposite effect occurs as well; when a voltage is applied to a piezo-
electric material, it compresses. These voltages and movements are
small (the compression is generally a fraction of 1 percent of the total
length), but reliable. Common piezoelectric materials include quartz
(silicon dioxide) and Rochelle salt (sodium potassium tartrate—the
name Rochelle salt comes from the French town La Rochelle, where it
was first separated).
But responses of small magnitude are not necessarily useful. Is there
a way of increasing the magnitude? An analogy with alloys may be help-
ful—combining atoms of different sizes, as in mixing elements to make
an alloy, often results in a harder material because the atoms do not
slide past one another so easily.
Chemical engineers can sometimes increase the size or alter the na-
ture of the response of a smart material by a process known as doping.