|
Shape Memory Materials
|
|
|
|
|
- Shape-memory materials attempt to recover their original fabricated
shape when they surpass a transition temperature (a narrow temperature
band, not a single point) between a low –temperature phase and
a high-temperature phase.
- Some shape-memory materials can exert tremendous force as they attempt
to return to their original shape. Some significant changes in the properties
of the material can accompany the transition.
Metal shape-memory alloys (SMA) have found significant commercial use
and currently are seeing rapid growth in medical applications.
- SMA actuators offer the highest pressure and greatest energy density
of any smart materials but are not well suited to rapid cycling.
- Two other shape-memory categories, shape-memory polymers (SMPs) and
shape-memory ceramics (SMC), have not found much use to date. SMPs offer
very little recovery force and have slow reaction times.SMCs can tolerate
much higher operating temperatures than can other shape-memory materials,
but their recoverable strain is quite small.
|
|
|
|
|
Shape-Memory Alloys
- The useful shape-recovery and superelastic properties of shape-memory
alloys stem from a transition between two crystal forms: a malleable
martensitic phase below the transformation temperature band and a stiff
austenitic phase above Ttr .The transition is rapid and readily reversible.
- SMAs require relatively little energy and - unlike most alloys - do
not require atomic diffusion to make the transition between phases.Most
commercial SMAs are nickel-titanium (also called Nitinol), copper-zinc-aluminum,or
copper-aluminum-nickel alloys.
- The copper-based SMAs have lower processing and raw material costs
than do NiTi SMAs, but NiTi offers superior performance. The materials
are available in many forms including bars, strips, wires, tubing, foils,
thin films and even a porous NiTi.
- Shape recovery occurs when an SMA piece undergoes deformation while
in the malleable low-temperature phase and then encounters heat greater
than Ttr. Recovery pressures can exceed 400 megapascals (60 000 psi).
Recoverable strain is as much as 8% (4%to 5%for the copper alloys) for
a single recovery cycle and drops as the number of cycles increases.
- One can use any heat source; magnetic induction and direct resistance
heating (passing current through the SMA) offer electronic control of
heating.
- SMAs with two-way shape recovery - the ability to recover both a high-temperature
form and a low-temperature form - exist, but they have low-recoverable
strain and the low-temperature recovery force is very small.
|
|
|
|
|
|
Shape-Memory Polymers
- Shape memory polymers (SMP) are special blends of two or more polymers.
SMP varieties include polynorborene -, polyisoprene -, styrene butadiene
-, and polyurethane-based materials and vinyl acetate - and polyester-based
compounds.
- When an SMP encounters a temperature above the lowest melting point
of the individual polymers, the blend makes a transition to a rubbery
state. The elastic modulus can change more than two orders of magnitude
across the transition.One can easily form an SMP into a desired shape
by heating it above the Ttr , fixing the SMP into the new shape,and
cooling the material below Ttr.
- The most suitable applications appear to be custom-fit devices, toys,and
novelty items. For example, one could form a spoon handle for the disabled,
the handle of a surgical tool, a cast or splint for orthopedic applications,
helmet linings, shoe linings or a variety of sporting equipment to improve
the fit for a particular user.
- SMP developers hope to encourage development of medical applications
and have put SMP samples through standard biocompatibility tests.
|
|
|
|
|
|
|
|
|