Smart Materials

Abstract

Materials play a vital role in human life. Materials are the base for any process. They are backbone of industry.Materials are growing day by day .But all the materials are not applicable for all purposes.so we have to use materials which suit for all purposes. One such type of materails is the smart materials.the name itself implies they are smart.They have a wide variety of applications in surgical,aerospace,civiletc.

So in this paper we are going to discuss about smart materials its types,properties applications.

Key words: Materials, smart materials, surgical,aerospace,civil

Introduction

Smart materials are materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields.

There are a number of types of smart material, some of which are already common. Some examples are as following:

· Piezoelectric
· Shape memory alloys and shape memory polymers

· Magnetic shape memory

· pH-sensitive polymers .

· Temperature-responsive polymers

· Halochromic materials.

· Ferrofluid

Piezoelectric Materials 

Piezoelectricity is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric potential in response to applied mechanical stress.. The word is derived from the Greek piezo or piezein, which means to squeeze or press.

Piezoelectric materials have two unique properties which are interrelated. When a piezoelectric material is deformed, it gives off a small but measurable electrical discharge. Alternately, when an electrical current is passed through a piezoelectric material it experiences a significant increase in size (up to a 4% change in volume)
Piezoelectric materials are most widely used as sensors in different environments. They are often used to measure fluid compositions, fluid density, fluid viscosity, or the force of an impact. An example of a piezoelectric material in everyday life is the airbag sensor in your car. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of an electric potential when stress is applied) also exhibit the reverse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.

Applications

· High voltage and power sources

· Sensors

· Actuators

· Frequency standard

· Piezoelectric motors

· Reduction of vibrations

Shape memory alloy

shape memory alloy (SMA, also known as a smart metal, memory alloy, or muscle wire) is an alloy that "remembers" its shape, and can be returned to that shape after being deformed, by applying heat to the alloy. The three main types of SMA are the copper-zinc-aluminum-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys. SMA's can also be used by alloying zinc, copper, gold, and iron.

One-way vs. two-way Shape Memory

Shape memory alloys may have different kinds of shape memory effect. The two most common memory effects are the one-way and two-way shape memory.

One Way Memory Effect

When a shape memory alloy is in its cold state (below A_s), the metal can be bent or stretched into a variety of new shapes and will hold that shape until it is heated above the transition temperature. Upon heating, the shape changes back to its original shape, regardless of the shape it was when cold. When the metal cools again it will remain in the hot shape, until deformed again.

Two Way Memory Effect

The two-way shape memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature shape. A material that shows a shape memory effect during both heating and cooling is called two-way shape memory.

Pseudo-Elasticity

Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; The loading is absorbed by the softer Martensite, but as soon as the loading is decreased the Martensite begins to transform back to Austenite since the temperature of the wire is still above Af, and the wire springs back to its original shape.

Applications

• Eyeglass Frames
• Bra Underwires
• Medical Tools
• Cellular Phone Antennae

Working of Shape Memory Alloys

In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite. Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration Upon deformation this phase takes on the second form Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic. The temperatures at which each of these phases begin and finish forming are represented by the following variables: Ms, Mf, As, Af.

Shape Memory Effect

The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the wire above the temperature Af. The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed Martensite is now transformed to the cubic Austenite phase, which is configured in the original shape of the wire.

The Shape memory effect is currently being implemented in:

• Coffepots
• The space shuttle
• Thermostats
• Vascular Stents
• Hydraulic Fittings (for Airplanes)

Properties

The copper-based and Ni Ti (nickel and titanium)-based shape memory alloys are considered to be engineering materials. These compositions can be manufactured to almost any shape and size.The yield strength of shape memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti can reach 500 MPa. The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the super elastic properties or the shape memory effect can be exploited. The most common application is in actuation.One of the advantages to using shape memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.

Advantages of Shape Memory Alloys

· Bio-compatibility

· Diverse Fields of Application

· Good Mechanical Properties (strong, corrosion resistant)

Industrial Applications

· Piping

· Robotics

· Medicine

· Optometry

· Aircraft

· Orthopaedic surgery

· Dentistry

Aeronautical applications

The flaps on a wing generally have the same layout shown on the left, with a large hydraulic system like the one shown. "Smart" wings, which incorporate shape memory alloys, this system is much more compact and efficient, in that the shape memory wires only require an electric current for movement.

Typical fuse and slag

Electromechanical Actuator

Hinge less shape memory alloy Flap

The shape memory wire is used to manipulate a flexible wing surface. The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current, which is easily supplied through electrical wiring, eliminating the need for large hydraulic lines. By removing the hydraulic system, aircraft weight, maintenance costs, and repair time are all reduced. The smart wing system is currently being developed cooperatively through the Defense Advanced Researched Project Agency (DARPA, a branch of the United States Department of Defense), and Boeing.

Aircraft maneuverability depends heavily on the movement of flaps found at the rear or trailing edge of the wings. The efficiency and reliability of operating these flaps is of critical importance. Most aircraft in the air today operate these flaps using extensive hydraulic systems. These hydraulic systems utilize large centralized pumps to maintain pressure, and hydraulic lines to distribute the pressure to the flap actuators. In order to maintain reliability of operation, multiple hydraulic lines must be run to each set of flaps. This complex system of pumps and lines is often relatively difficult and costly to maintain. Many alternatives to the hydraulic systems are being explored by the aerospace industry. Among the most promising alternatives are piezoelectric fibers, electrostrictive ceramics, and shape memory alloys

Surgical tools

Bone plates are surgical tools, which are used to assist in the healing of broken and fractured bones. The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets. Repairs like this fallnto an area of medicine known as osteosynthesis.

Currently osteotemy equipment is made primarily of titanium and stainless steel. The broken bones are first surgically reset into their proper position. Then a plate is screwed onto the broken bones to hold them in place, while the bone heals back together. This method has been proven both successful and useful in treating all manner of breaks, however there are still some drawbacks. After initially placing the plate on the break or fracture the bones are compressed together and held under some slight pressure, which helps to speed up the healing process of the bone. Unfortunately, after only a couple of days the tension provided by the steel plate is lost and the break or fracture is no longer under compression, slowing the healing process.

Musical wires

There have been many attempts made to re-create human anatomy through mechanical means. The human body however, is so complex that it is very difficult to duplicate even simple functions. Robotics and electronics are making great strides in this field, of particular interest are limbs such hands, arms, and legs. Many different solutions have been proposed for this problem, some include using "muscles" controlled by air pressure, piezoelectric materials, or shape memory alloys.
In order to reproduce human extremities there are a number of aspects that must be considered:

• The gripping force required to manipulate different objects (eggs, pens, tools)
• The motion capabilities of each joint of the hand
• The ability to feel or touch objects (tactile senses)
• The method of controlling movement within the limb
• Emulating real human movement (smoothness, and speed of response).

Shape memory alloys mimic human muscles and tendons very well. SMA's are strong and compact so that large groups of them can be used for robotic applications, and the motion with which they contract and expand are very smooth creating a life-like movement unavailable in other systems.
Shape Memory Polymers

Shape Memory Polymers (SMP) are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape induced by an external stimulus (trigger), such as e.g. temperature change.

Properties of shape memory polymers .In addition to temperature change, the shape memory effect of SMPs can also be triggered by an electric or magnetic field , light or a change in pH. SMPs include thermoplastic and thermo set (covalently cross-linked) polymeric materials. SMPs are known to be able to store up to three different shapes in memory .Two important quantities that are used to describe shape memory effects are the strain recovery rate (Rr) and strain fixity rate (Rf). The strain recovery rate describes the ability of the material to memorize its permanent shape, while the strain fixity rate describes the ability of switching segments to fix the mechanical deformation.

Types of Smps:

· Physically cross linked SMPs

· Linear Block Copolymers

· Thermoplastic polymers

· Chemically crosslinked SMPs

· PEO based crosslinked SMPs

· Light induced SMPs

· Electro-active SMPs

Applications

Industrial applications

· Robotics

· Polyurethane SMPs are also applied as an auto choke element for engines.

Potential medical applications

· intravenous cannula,

· self-adjusting orthodontic wires and

· selectively pliable tools for small scale surgical procedures where currently metal-based shape memory alloys such as Nitinol are widely used.

Potential industrial applications

· Self-repairing structural components, such as automobile fenders in which dents are repaired by application of temperature.

· Clothing.

Magnetic shape-memory alloys

Magnetic shape-memory (MSM) alloys are ferromagnetic materials exhibiting large changes in shape and size in an applied magnetic field

PH-sensitive polymers

pH sensitive polymers are materials which will respond to the changes in the pH of the surrounding medium by varying their dimensions. Such materials swell or collapse depending on the pH of their environment. This behaviour is exhibited due to the presence of certain functional groups in the polymer chain.

There are two kinds of ph sensitive materials: one which have acidic group (-COOH, -SO₃H) and swell in basic pH, and others which have basic groups (-NH₂) and swell in acidic pH. Polyacrylic acid is an example of the former and Chitosan is an example of the latter.

Halochromic material

A halochromic material is a material which changes colour when pH changes occur. The term ‘chromic’ is defined as materials that can change colour reversibly with the presence of a factor.Halochromic substances are suited for use in environments where pH changes occur frequently, or places where changes in pH are extreme. Halochromic substances detect alterations in the acidity of substances, like detection of corrosion in metals.Halochromic substances may be used as indicators to determine the pH of solutions of unknown pH. The colour obtained is compared with the colour obtained when the indicator is mixed with solutions of known pH. The pH of the unknown solution can then be estimated. The colour change of halochromic substances occur when the chemical binds to existing hydrogen and hydroxide ions in solution. Such bonds result in changes in the conjugate systems of the molecule, or the range of electron flow. This alters the amount of light absorbed, which in turn results in a visible change of colour. Halochromic substances do not display a full range of colour for a full range of pH because, after certain acidities, the conjugate system will not change. The various shades result from different concentrations of halochromic molecules with different conjugate systems.

Ferrofluid

A ferrofluid, is a liquid which becomes strongly polarised in the presence of a magnetic field.Ferrofluids are colloidal mixtures composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals forces and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale)paramagnetism, and are often described as "superparamagnetic" due to their large magnetic susceptibility. Permanently magnetized fluids are difficult to create at present.

Smart Materials in Aerospace
Some materials and structures can be termed ‘sensual’ devices. These are structures that can sense their environment and generate data for use in health and usage monitoring systems (HUMS). To date the most well established application of HUMS are in the field of aerospace, in areas such as aircraft checking.An aircraft constructed from a ‘sensual structure’ could self-monitor its performance to a level beyond that of current data recording, and provide ground crews with enhanced health and usage monitoring. This would minimise the overheads associated with HUMS and allow such aircraft to fly for more hours before human intervention is required.

Smart Materials in Civil Engineering Applications
‘sensual structures’ could be used in the monitoring of civil engineering structures to assess durability. Monitoring of the current and long term behaviour of a bridge would lead to enhanced safety during its life since it would provide early warning of structural problems at a stage where minor repairs would enhance durability, and when used in conjunction with structural rehabilitation could be used to safety monitor the structure beyond its original design life. This would influence the life costs of such structures by reducing upfront construction costs (since smart structures would allow reduced safety factors in initial design), and by extending the safe life of the structure. ‘Sensual’ materials and structures also have a wide range of potential domestic applications, as in food packaging for monitoring safe storage and cooking.The above examples address only ‘sensual’ structures. However, smart materials and structures offer the possibility of structures which not only sense but also adapt to their environment. Such adaptive materials and structures benefit from the sensual aspects highlighted earlier, but in addition have the capability to move, vibrate, and exhibit a multitude of other real time responses.Potential applications of such adaptive materials and structures range from the ability to control the aeroelastic form of an aircraft wing, thus minimising drag and improving operational efficiency, to vibration control of lightweight structures such as satellites, and power pick-up pantographs on trains. The domestic environment is also a potential market for such materials and structures, with the possibility of touch sensitive materials for seating, domestic appliances, and other products.

Conclusion:

Materials are an component for any type of process.material selection is an important aspect of any industry.So the materials should be chosen in such a way that supports in all aspects.Smart materails helps in all these aspects. So smart materials are gaining popularity day by day.