A smart fluid developed in labs at the Michigan Institute of Technology
Science and technology have made amazing developments in the design of electronics and
machinery using standard materials, which do not have particularly special properties (i.e.
steel, aluminum, gold). Imagine the range of possibilities, which exist for special materials
that have properties scientists can manipulate. Some such materials have the ability to
change shape or size simply by adding a little bit of heat, or to change from a liquid to a
solid almost instantly when near a magnet; these materials are called smart materials.
Smart materials have one or more properties that can be dramatically altered. Most
everyday materials have physical properties, which cannot be significantly altered; for
example if oil is heated it will become a little thinner, whereas a smart material with
variable viscosity may turn from a fluid which flows easily to a solid. A variety of smart
materials already exist, and are being researched extensively. These include piezoelectric
materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory
alloys. Some everyday items are already incorporating smart materials (coffeepots, cars,
the International Space Station, eyeglasses) and the number of applications for them is
growing steadily.
Each individual type of smart material has a different property which can be
significantly altered, such as viscosity, volume, and conductivity. The property that
can be altered influences what types of applications the smart material can be used
shape memory alloys
Shape memory alloys (SMAs) are metals that "remember" their original shapes.
SMAs are useful for such things as actuators which are materials that "change shape,
stiffness, position, natural frequency, and other mechanical characteristics in response
to temperature or electromagnetic fields" (Rogers, 155). The potential uses for SMAs
especially as actuators have broadened the spectrum of many scientific fields. The
study of the history and development of SMAs can provide an insight into a material
involved in cutting-edge technology. The diverse applications for these metals have
made them increasingly important and visible to the world.
History
Nickel-titanium alloys have been found to be the most useful of all SMAs. Other
shape memory alloys include copper-aluminum-nickel, copper-zinc-aluminum, and
iron- manganese-silicon alloys.(Borden, 67) The generic name for the family of
nickel-titanium alloys is Nitinol. In 1961, Nitinol, which stands for Nickel Titanium
Naval Ordnance Laboratory, was discovered to possess the unique property of having
shape memory. William J. Buehler, a researcher at the Naval Ordnance Laboratory in
White Oak, Maryland, was the one to discover this shape memory alloy. The actual
discovery of the shape memory property of Nitinol came about by accident. At a
laboratory management meeting, a strip of Nitinol was presented that was bent out of
shape many times. One of the people present, Dr. David S. Muzzey, heated it with his
pipe lighter, and surprisingly, the strip stretched back to its original form. (Kauffman
and Mayo, 4)
Crystal Structures
Exactly what made these metals "remember" their original shapes was in question
after the discovery of the shape-memory effect. Dr. Frederick E. Wang, an expert in
crystal physics, pinpointed the structural changes at the atomic level which
contributed to the unique properties these metals have. (Kauffman and Mayo, 4)
He found that Nitinol had phase changes while still a solid. These phase changes,
known as martensite and austenite, "involve the rearrangement of the position of
particles within the crystal structure of the solid" (Kauffman and Mayo, 4). Under the
transition temperature, Nitinol is in the martensite phase. The transition temperature
varies for different compositions from about -50 ° C to 166 ° C (Jackson, Wagner, and
Wasilewski, 1). In the martensite phase, Nitinol can be bent into various shapes. To
fix the "parent shape" (as it is called), the metal must be held in position and heated to
about 500 ° C. The high temperature "causes the atoms to arrange themselves into the
most compact and regular pattern possible" resulting in a rigid cubic arrangement
known as the austenite phase (Kauffman and Mayo, 5-6). Above the transition
temperature, Nitinol reverts from the martensite to the austenite phase which changes
it back into its parent shape. This cycle can be repeated millions of times (Jackson,
Wagner, and Wasilewski, 1).
Manufacture
There are various ways to manufacture Nitinol. Current techniques of producing
nickel-titanium alloys include vacuum melting techniques such as electron-beam
melting, vacuum arc melting or vacuum induction melting. "The cast ingot is pressforged and/or rotary forged prior to rod and wire rolling. Hot working to this point is
done at temperatures between 700 ° C and 900 ° C" (Stoeckel and Yu, 3).
There is also a process of cold working of Ni-Ti alloys. The procedure is similar to
titanium wire fabrication. Carbide and diamond dies are used in the process to
produce wires ranging from 0.075mm to 1.25mm in diameter. (Stoeckel and Yu, 4)
Cold working of Nitinol causes "marked changes in the mechanical and physical
properties of the alloy" (Jackson, Wagner, and Wasilewski, 21). These processes of
the production of Nitinol are described in greater detail in Jackson, Wagner, and
Wasilewski's report (15-22).
Properties
The properties of Nitinol are particular to the exact composition of the metal and the
way it was processed. The physical properties of Nitinol include a melting point
around 1240 ° C to 1310 ° C, and a density of around 6.5 g/cm³ (Jackson, Wagner,
and Wasilewski, 23). Various other physical properties tested at different
temperatures with various compositions of elements include electrical resitivity,
thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity,
magnetic susceptibility, and thermal conductivity (Jackson, Wagner, and Wasilewski,
23-55). Mechanical properties tested include hardness, impact toughness, fatigue
strength, and machinability (Jackson, Wagner, and Wasilewski, 57-73).
The large force generated upon returning to its original shape is a very useful
property. Other useful properties of Nitinol are its "excellent damping characteristics
at temperatures below the transition temperature range, its corrosion resistance, its
nonmagnetic nature, its low density and its high fatigue strength" (Jackson, Wagner,
and Wasilewski, 77). Nitinol is also to an extent impact- and heat-resistant (Kauffman
and Mayo, 4). These properties translate into many uses for Nitinol.
Applications
Nitinol is being used in a variety of applications. They have been used for military,
medical, safety, and robotics applications. The military has been using Nitinol
couplers in F-14 fighter planes since the late 1960s. These couplers join hydraulic
lines tightly and easily. (Kauffman and Mayo, 6)
Many of the current applications of Nitinol have been in the field of medicine.
Tweezers to remove foreign objects through small incisions were invented by NASA.
Anchors with Nitinol hooks to attach tendons to bone were used for Orel Hershiser's
shoulder surgery. Orthodontic wires made out of Nitinol reduces the need to retighten
and adjust the wire. These wires also accelerate tooth motion as they revert to their
original shapes. Nitinol eyeglass frames can be bent totally out of shape and return to
their parent shape upon warming. (Kauffman and Mayo, 6) Nitinol needle wire
localizers "used to locate and mark breast tumors so that subsequent surgery can be
more exact and less invasive" utilize the metal's shape memory property. Another
successful medical application is Nitinol's use as a guide for catheters through blood
vessels (Stoeckel and Yu, 9-10).
There are examples of SMAs used in safety devices which will save lives in the
future. Anti-scalding devices and fire-sprinklers utilizing SMAs are already on the
market. The anti-scalding valves can be used in water faucets and shower heads. After
a certain temperature, the device automatically shuts off the water flow. The main
advantage of Nitinol-based fire sprinklers is the decrease in response time. (Kauffman
and Mayo, 7)
Nitinol is being used in robotics actuators and micromanipulators to simulate human
muscle motion. The main advantage of Nitinol is the smooth, controlled force it exerts
upon activation. (Rogers, 156)
Other miscellaneous applications of shape memory alloys include use in household
appliances, in clothing, and in structures. A deep fryer utilizes the thermal sensitivity
by lowering the basket into the oil at the correct temperature. (Falcioni, 114)
According to Stoeckel and Yu, "one of the most unique and successful applications is
the Ni-Ti underwire brassiere" (11). These bras, which were engineered to be both
comfortable and durable, are already extremely successful in Japan (Stoeckel and Yu,
11). Nitinol actuators as engine mounts and suspensions can also control vibration.
These actuators can helpful prevent the destruction of such structures as buildings and
bridges. (Rogers, 156)
Other applications:
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European Space Agency
SMA INC.
Here are some of the pictures available from this section:
Future Applications
There are many possible applications for SMAs. Future applications are envisioned to
include engines in cars and airplanes and electrical generators utilizing the mechanical
energy resulting from the shape transformations. Nitinol with its shape memory
property is also envisioned for use as car frames. (Kauffman and Mayo, 7) Other
possible automotive applications using SMA springs include engine cooling,
carburetor and engine lubrication controls, and the control of a radiator blind ("to
reduce the flow of air through the radiator at start-up when the engine is cold and
hence to reduce fuel usage and exhaust emissions") (Turner, 299).
SMAs are "ideally suited for use as fasteners, seals, connectors, and clamps" in a
variety of applications (Borden, 67). Tighter connections and easier and more
efficient installations result from the use of shape memory alloys (Borden, 72).
Conclusion
The many uses and applications of shape memory alloys ensure a bright future for
these metals. Research is currently carried out at many robotics departments and
materials science departments. With the innovative ideas for applications of SMAs
and the number of products on the market using SMAs continually growing, advances
in the field of shape memory alloys for use in many different fields of study seem
very promising.
Piezoelectric Materials:
Introduction: the piezoelectric effect
The piezoelectric effect describes the relation between a mechanical stress
and an electrical voltage in solids.
It is reversbile: an applied mechanical stress will generate a voltage and an
applied voltage will change the shape of the solid by a small amount (up to a
4% change in volume).
In physics, the piezoelectric effect can be described as the the link between
electrostatics and mechanics.
History
The piezoelectric effect was discovered in 1880 by the Jacques and Pierre
Curie brothers. They found out that when a mechanical stress was applied on
crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane
sugar, electrical charges appeared, and this voltage was proportional to the
stress.
First applications were piezoelectric ultrasonic transducers and soon swinging
quartz for standards of frequency (quartz clocks).
An everyday life application example is your car's airbag sensor. The material
detects the intensity of the shock and sends an electricla signal which triggers
the airbag.
Piezoelectric materials
The piezoelectric effect occurs only in non conductive materials. Piezoelectric
materials can be divided in 2 main groups: crystals and cermaics. The most
well-known piezoelectric material is quartz (SiO2).
Magnetorheological fluid (MR)
When exposed to a magnetic field, the particles in magnetorheological fluid align
along the field lines.
Magnetorheological Fluid
The other fluid that can reinforce Kevlar armor is magnetorheological (MR) fluid.
MR fluids are oils that are filled with iron particles. Often, surfactants surround the
particles to protect them and help keep them suspended within the fluid. Typically,
the iron particles comprise between 20 and 40 percent of the fluid's volume.
The particles are tiny, measuring between 3 and 10 microns. However, they have a
powerful effect on the fluid's consistency. When exposed to a magnetic field, the
particles line up, thickening the fluid dramatically. The term "magnetorheological"
comes from this effect. Rheology is a branch of mechanics that focuses on the
relationship between force and the way a material changes shape. The force of
magnetism can change both the shape and the viscosity of MR fluids.
The hardening process takes around twenty thousandths of a second. The effect can
vary dramatically depending on the composition of the fluid and the size, shape and
strength of the magnetic field. For example, MIT researchers started with spherical
iron particles, which can slip past one another, even in the presence of the magnetic
field. This limits how hard the armor can become, so researchers are studying other
particle shapes that may be more effective.
As with STF, you can see what MR fluids look like using ordinary items. Iron filings
mixed with oil create a good representation. When no magnetic field is present, the
fluid moves easily. But the influence of a magnet can cause the fluid to become
thicker or to take a shape other than that of its container. Sometimes, the difference is
very visually dramatic, with the fluid forming distinctive peaks, troughs and other
shapes. Artists have even used magnets and MR fluids or similar ferrofluids to create
works of art.
With the right combination of density, particle shape and field strength, MR fluid can
change from a liquid to a very thick solid. As with shear-thickening fluid, this change
could dramatically increase the strength of a piece of armor. The trick is activating the
fluid's change of state. Since magnets large enough to affect an entire suit would be
heavy and impractical to carry around, researchers propose creating tiny circuits
running throughout the armor.
Magnetorheological fluid before and after exposure to a magnetic
field
Without current flowing through the wires, the armor would remain soft and flexible.
But at the flip of the switch, electrons would begin to move through the circuits,
creating a magnetic field in the process. This field would cause the armor to stiffen
and harden instantly. Flipping the switch back to the off position would stop the
current, and the armor would become flexible again.
In addition to making stronger, lighter, more flexible armor, fabrics treated with
shear-thickening and magnetorheological fluids could have other uses as well. For
example, such materials could create bomb blankets that are easy to fold and carry
and can still protect bystanders from explosion and shrapnel. Treated jump boots
could harden on impact or when activated, protecting paratroopers' boots. Prison
guards' uniforms could make extensive use of liquid armor technology, especially
since the weapons guards are most likely to encounter are blunt objects and
homemade blades.
However, the technologies do have a few pros and cons. Here's a rundown:
Neither type of armor is quite ready for battlefield use. STF-treated Kevlar armor
could be available by the end of 2007 [Source: Business Week]. MR fluid may
require another five to 10 years of development before it can consistently stop bullets.
[Source: Science Central]. Check out the links on the next page to learn more about
military technology, body armor and related topics.
Other Uses for MR Fluids
MR fluids have numerous uses besides strengthening body armor. Their ability to
change from liquids to semisolids almost instantly makes them useful for dampening
impacts and vibrations in items like:
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Car shock absorbers
Washing machines
Prosthetic limbs
Bridges
Since it can instantly and reversibly change shape, it could also be used to create
scrolling Braille displays or reconfigurable molds.
Auxetic Materials
Background
Pick up an elastic band and stretch it lengthways as if you were going to `ping' it at somebody.
Before letting fly, look at the width of your elongated missile - it's thinner than an unstretched
band, as you'd expect. Try this with an auxetic elastic band and you'd be in for a surprise. These
bizarre materials can actually become fatter when stretched, a phenomenon which is now
attracting the practical interest of many materials scientists.
Many people are sceptical about whether auxetic behaviour really exists, because we expect
materials to behave like our elastic band and get thinner when stretched. Why is this? Do we
base our expectation on a knowledge of the deformation mechanisms within the material? Or on
classical elasticity theory? No, the only reason we think like this is down to everyday experience.
Even the property relating directly to this behaviour, Poisson's ratio (), is defined so that most
`normal' materials have a positive value.
How do Auxetic Materials Work?
Poisson's ratio is the ratio of the contractile lateral strain to the tensile axial strain for a material
stretched axially, and is typically around +0.2 to +0.4. However, when we look into classical
elasticity theory we find that the Poisson's ratios of isotropic materials can not only take
negative values, but can have a range of negative values twice that of positive ones.
A study of the structure of materials, and how it deforms, demonstrates that auxetic properties
are entirely feasible. Figure 1 shows a 2D structure consisting of a regular array of rectangular
nodules connected by fibrils. Deformation of the structure is by `hinging' of the fibrils. For the
`open' geometry shown in figure 1a, the cells elongate along the direction of stretch and
contract transversely in response to stretching the network, giving a positive v. However,
modify the structure to adopt a `re-entrant' geometry, figure 1b, and the network now
undergoes elongation both along and transverse to the direction of applied load. In other words,
this is an auxetic structure.
Production of Auxetic Materials
It is clear that auxetic materials have beneficial properties, so the next step is to produce them,
and there are as many different routes as there are materials. Some routes rely on transforming
non-auxetic materials into an auxetic form (foams), whereas others employ standard techniques
but with novel material architecture to achieve the auxetic effect (honeycombs and fibrereinforced composites). Still others require novel development of existing processing routes for
conventional materials to produce auxetic functionality.
As an example of this latter scenario, take the processing of auxetic microporous polymers. The
key is to produce a three-dimensional version in the polymer of the two-dimensional schematic
in figure 1, i.e. to achieve nodules interconnected with fibrils. The route used to achieve this
complex microstructure, for UHMWPE, is a batch process based on conventional polymer powder
processing techniques of sintering and extrusion, but adapted so that there is only partial
melting of the starting powder, which gives rise to fibrillation. An additional initial compaction
stage can be used solely to impart some degree of structural integrity to the extrudates. This
means the properties of the polymers can be tailored to fit the applications required and, by
varying certain processing parameters, to produce everything from structural auxetic polymers
with a Young's modulus of 0.2GNm-2, down to very auxetic, low modulus polymers
applications
Biomedical Industry
Key areas of application are seen in the biomedical field. Prosthetic materials, surgical implants,
suture/muscle/ligament anchors and a dilator to open up blood vessels during heart surgery are
all possible. Another area relates to the use of auxetic materials in piezoelectric sensors and
actuators. Auxetic metals could be used as electrodes sandwiching a piezoelectric polymer, or
piezoelectric ceramic rods could be embedded within an auxetic polymer matrix. These are
expected to increase piezoelectric device sensitivity by at least a factor of two, and possibly by
ten or a hundred times. The development of auxetic materials for micro- and nano-mechanical
and electromechanical devices is also being investigated.
Filters
Auxetic foam and honeycomb filters offer enhanced potential for cleaning fouled filters, for
tuning the filter effective pore size and shape, and for compensating for the effects of pressure
build-up due to fouling. These benefits rely on the pores opening up both along and transverse
to the direction of a tensile load applied to an auxetic filter. The pores of a non-auxetic filter
open up in the stretching direction but close up in the lateral direction, leading to poorer filter
performance, figure 1. However, stretching an auxetic filter improves performance by opening
pores in both directions. The effect of stretching on the de-fouling of an auxetic polymeric
honeycomb fouled with glass beads has been investigated. For the particular honeycomb studied
the value of  is dependent on the stretching direction. The studies clearly demonstrate that
defouling is enhanced when the filter is loaded in the direction with the largest negative .
Figure 1. Schematic of particulate de-fouling capabilities of non-auxetic and auxetic materials
Auxetic Fibres
The breakthrough development of a continuous process to produce auxetic materials in fibrous
form has created the opportunity to apply their unique characteristics in a wide range of
applications previously not possible. Fibres can be used in single or multiple filament structures
and can be used to produce a woven structure. Typical performance characteristics expected of
auxetic fibres and structures are detailed in the table of applications, (table 1), together with a
list of the applications in which these characteristics could offer significant benefits. For
example, by analogy with the filter de-fouling scenario of figure 4, biomedical fibrous drugrelease materials could be made from auxetic fibres. Extending the fibres opens the micropores
and a specific dose of drug is released.
Advanced auxetic fibres will include multi-filament yarns in which an auxetic filament is wrapped
with one or more other yarns, perhaps high stiffness/strength, dyeable or conductive filaments,
so that the benefits of the auxetic material are combined with other beneficial properties for
smart technical textiles applications. This will lead to the possibility of hierarchical composites
displaying auxetic behaviour at more than one lengthscale. Current research on auxetic
composites is concentrated on the use of non-auxetic constituents and so benefits due to the
auxetic effect occur at a macrostructural level. Employing auxetic fibres as the reinforcement
will enable benefits, such as impact energy and acoustic energy absorption, to be achieved at
the microstructural level.
Auxetic Fibre Reinforced Composites
Auxetic fibre reinforcements should also enhance the failure properties of composites. Fibre pullout is a major failure mechanism in composites. A unidirectional composite loaded in tension will
undergo lateral contraction of both the matrix and fibre materials, leading to failure at the
fibre/matrix interface. Auxetic fibres, on the other hand, allow the possibility of maintaining the
interface by careful matching of the Poisson's ratios of the matrix and fibre, figure 2.
Figure 2. Fibre pull-out in composites
The Future
So what does the future hold for auxetics? Despite the very significant developments to date we
have only scratched the surface of this exciting and multi-disciplinary field. The successful
synthesis and development of molecular and multi-functional auxetics represent key
opportunities for the future. In addition to leading to materials with extreme properties such as
high modulus and strength, these advanced materials will have potential in sensor, drug-release
and separations applications. By accepting a negative Poisson's ratio as a positive property we
are truly expanding the applications of these fascinating materials
General references:
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Stanford university Website: http://www.stanford.edu/~richlin1/sma/sma.html
the institute of material website: http://www.iom3.org
http://www.azom.com/article.aspx?ArticleID=168
Shape Memoy Alloy information from Mide Technology Corporation
European Space Agency on SMAs
NASA Langley Research Center on SMAs
http://www.piezomaterials.com