SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 118

shock absorbers. But a shock absorber must operate under varying road
conditions, from new and glassy asphalt to an old road full of potholes.
Conventional shock absorbers work hydraulically—using fluid pressure
and valves—but in 2002, car and truck manufacturer General Motors
began introducing shock absorbers based on magnetorheological fluids
in a few of their models. These shock absorbers are expensive, but the
fast-acting smart fluids adjust more efficiently and effectively to varying
road conditions and vehicle speeds. According to a 2003 General Mo-
tors product description, “[T]his design offers superior handling, con-
trol and ride quality on the roughest road surfaces because it automati-
cally minimizes damping forces as needed for improved road isolation
and ride smoothness. It can respond to inputs in one millisecond, or 10
times faster than systems on the market today.”
SHAPEMEMoryAlloyS
Imagine a material smart enough to “remember” its original shape. If it
gets deformed, it can regain its former structure with the application of
a little bit of heat. The figure opposite illustrates this process.
A type of material known as shape memory alloy (SMA) can per-
form this trick. SMAs are more complicated than electrorheological flu-
ids and the other smart materials previously described in this chapter.
An SMA does not only react or respond to environmental conditions,
it also has a memory that enables it to return to a specific structure, or
sometimes switch between two different structures. After the material
has been set, it can recover from a deformation that would be perma-
nent in other materials. When the temperature is raised by an amount
that depends on the specific material, it snaps back into shape auto-
matically. The memory is based on phase transitions, as described in the
sidebar on page 120.
The most common SMAs are nickel-titanium alloys and copper
alloys of various kinds. Nitinol, a specific alloy of nickel (Ni) and tita-
nium (Ti), is probably the most widely used. (The word nitinol comes
from the chemical symbols of its two metal components, along with
an abbreviation for the Naval Ordnance Laboratory, where this al-
loy was discovered and studied in the early 1960s.) Although nickel
and titanium alloys tend to be more expensive than copper materi-