smart+materials+-+Ahmed+Sabry

=__**//Shape Memory Alloys ://**__=

shape memory alloys (SMA's) are alloys that remember their original shapes. SMA's are useful for materials that change shape,stiffness,position,natural frequency and other mechanical characteristics in response to temperature or elctromagnetic fields.

Nickel-titanium "NiTinol" alloys are the most useful of all SMA's alloys. Other shape memory alloys include copper, aluminium, nickel, zink and iron. In 1961, Nitinol, which stands for "Nickel Titanium Naval Ordnance Laboratory", was discovered to possess the unique property of having shape memory. Wiliam J.Buheler, a reasearsher at the naval ordnary laboratory is the first one who discovered these alloys. The real discovery of SMAs alloys came 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.

Exactly what made these metals remember their original shapes was a 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. 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". Under the transition temperature, Nitinol is in the martensite phase. The transition temperature varies for different compositions from about -50 ° C to 166 ° C. 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. 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.



There are various ways to manufacture nitinol. i am going to speak about two ways which are : 1- techniques of producing include vacuum melting techniques such as electron-beam melting at temperetures between 700 - 900 C'. 2- process of cold working. 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.Cold working of Nitinol causes marked changes in the mechanical and physical properties of the alloy.media type="youtube" key="fsBHF_j2FJ4" width="425" height="350"

The propertties of SMAs are :

1- melting point around 1240 - 1310 C'. 2- density is around 6.5 g/cm3.

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). Mechanical properties tested include (hardness, impact toughness, fatigue strength, and machinability ).

Nitinol is used in a variety of application. Some of them are :

1- at millitary, nitinol couplers are used in F14 planes. These couplers join hydraulic lines tightly and easily. 2- at medicine, in tweezers to remove foreign objects through small incisions. 3- orthodontic wires made of nitinol reduces the need to retighten and adjust the wire. These wires also accelerate tooth motion as they revert to their original shapes. and more of applications

the reference is www.stanford.edu. www.youtube.com

=**//__Auxestic Materials :__//**=

This material being more fatter when you stretch it and being more thinner when you stress it. That is very strange and we should ask ourselves "How?".

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 ( n ), is defined so that most `normal' materials have a positive value.
 * ==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 strange materials can actually become fatter when stretched, a phenomenon which is now attracting the practical interest of many materials scientists.

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. || But it is not just structures - some materials have an intrinsically negative n. Auxetic behaviour is found in materials from the molecular level, up through the microscopic, and right up to genuinely macroscopic structures. Figure 2 shows that all the major classes of materials (polymers, composites, metals and ceramics) now exist in auxetic form and that natural and synthetic auxetic materials are known over several orders of magnitude of stiffness, or Young's modulus, E. || Enhanced indentation resistance has been demonstrated in auxetic foams. Auxetic carbon-fibre reinforced composites and microporous polymers have also shown similar benefits. The indentation resistance of auxetic ultra-high molecular-weight polyethylene (UHMWPE) is enhanced by up to a factor of three when compared with conventional UHMWPE. Other important properties known to be positively affected include shear resistance, fracture toughness, sound and vibration damping, and ultrasonic energy absorption.media type="youtube" key="a1Q-ReSdipY" width="425" height="350"
 * **Figure 1.** Non-auxetic (a) and auxetic (b) deformation due to fibril hinging in a nodule-fibril microstructure. ||
 * ==What Types Of Materials Can Exhibit Auxetic Behaviour?==
 * ==What Types Of Materials Can Exhibit Auxetic Behaviour?==
 * [[image:http://www.azom.com/work/VYA679dKq926O9JL800g_files/image004.gif width="394" height="481" align="center"]] ||
 * **Figure 2.** Auxetic materials form the molecular to the macroscopic level. ||
 * There is good reason to get excited about auxetic materials. As well as their novel behaviour under deformation, many other material properties can be enhanced as a result of having a negative Poisson's ratio. Typically such properties are inversely proportional to (1 - n 2) or (1 + n ), eg indentation resistance and shear modulus respectively. The negative limit of n for isotropic materials is -1, and approaching this, the (1 - n 2) and (1 + n ) factors tend to zero, leading to enhancements in the related material property for auxetic over non-auxetic materials.

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 fibre-reinforced 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.

Production of Auxetic Fibres
Until recently only auxetic cylinders could be produced, which are difficult to use in real applications. However, at the ECCM9 conference in June 2000, the development of a continuous process for producing auxetic microporous polymer fibres was reported. Again, this has the distinct advantage of being based on a conventional processing route melt spinning, but with novel processing conditions required to achieve the nodule-fibril microstructure. Figure 3 shows the variation in width plotted against length variation for two polypropylene (PP) fibres stretched axially. Fibre 1 is a conventional PP fibre and shows a contraction in width as it is extended, corresponding to a positive n. Fibre 2 is processed using extruder temperatures which lead to the nodule-fibril microstructure. Its width now increases upon stretching - it displays auxetic behaviour. ||
 * [[image:http://www.azom.com/work/VYA679dKq926O9JL800g_files/image005.gif width="399" height="324" align="center"]] ||
 * **Figure 3.** Width versus length variations for polypropylene fibres. ||
 * The ability to tailor the response of the fibre to give auxetic properties through processing is again evident. Polyproplylene filaments with diameters from 180pm to 1 mm, and Young's moduli in the range 0.2 to 2GNm-2 have been produced. ||

the reference are: www.azom.com www.youtube.com

=__//Magnetorheological fluid://__=

A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming vicoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force can be controlled with an electomagnet, which gives rise to its many possible control-based applications. MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are primarily on the micrometre-scale and are too dense for Brownian Motion to keep them suspended (in the lower density carrier fluid). ferrofluid particles are primarily nanoparticles that are suspended by Brownian Motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.

A Magneto-rheological fluid is similar to a ferrofluid in the way that there are magnetic particles suspended in a fluid medium. This type of fluid does not use nano sized particles, but they must be small enough to remain suspended in the liquid. They are typically 2 or 3 times larger than the particles in Ferrofluids and are on the micrometer scale. The particles in a magnetorheological fluid are magnetically polarisable. This means that when an external field is applied the micron sized particles will line up and form chain like structures. the alignment of the particles will increase the viscosity of the fluid. A simple magnetorheological fluid can be made at home. Micron sized ferrous particles can be collect from sand or lake beds. By placing a magnet in a plastic bag and dragging it through sandy sediment many particles will be separated out. Turning the bag inside out and removing it from the magnet prevents the particle from becoming permanently stuck to its surface. These particles can be mixed with a small amount of oil such as vegetable oil. By holding a magnet to the outside of t he container and poring off excess oil you will be left with a basic magnetorheological fluid. This fluid will not remaining stable for long periods due to the lack of a suffricant, but it serves well to demonstrate the scientific principles involved. Magnetorheological fluids are being used mostly for controlled damping of oscillations. They are ideal for use in the suspension in large vehicles. In its liquid state it will provide limited damping, but when a magnetic field is brought near to the fluid it will greatly dampen any oscillations. This means that a large mechanical force can be controlled with a much smaller mechanical force.

How it works
The magnetic particles, which are typically micrometre or nanometrescale spheres or ellipsoids, are suspended within the carrier oil are distributed randomly and in suspension under normal circumstances, as below. When a magnetic field is applied, however, the microscopic particles (usually in the 0.1–10 µm range) align themselves along the lines of magnetic flux, see below. When the fluid is contained between two poles (typically of separation 0.5–2 mm in the majority of devices), the resulting chains of particles restrict the movement of the fluid, perpendicular to the direction of flux, effectively increasing its viscosity. Importantly, mechanical properties of the fluid in its “on” state are anisotopic. Thus in designing a magnetorheological (or MR) device, it is crucial to ensure that the lines of flux are perpendicular to the direction of the motion to be restricted.

Material behavior
To understand and predict the behavior of the MR fluid it is necessary to model the fluid mathematically, a task slightly complicated by the varying material properties (such as yield stress). As mentioned above, smart fluids are such that they have a low viscosity in the absence of an applied magnetic field, but become quasi-solid with the application of such a field. In the case of MR fluids (and ER), the fluid actually assumes properties comparable to a solid when in the activated ("on") state, up until a point of yield (the shear stress above which shearing occurs). This yield stress (commonly referred to as apparent yield stress) is dependent on the magnetic field applied to the fluid, but will reach a maximum point after which increases in magnetic flux density have no further effect, as the fluid is then magnetically saturated. The behavior of a MR fluid can thus be considered similar to a Bingham plastic, a material model which has been well-investigated. However, a MR fluid does not exactly follow the characteristics of a Bingham plastic. For example, below the yield stress (in the activated or "on" state), the fluid behaves as a viscoelastic material, with a complex modulus that is also known to be dependent on the magnetic field intensity. MR fluids are also known to be subject to shear thinning, whereby the viscosity above yield decreases with increased shear rate. Furthermore, the behavior of MR fluids when in the "off" state is also non-Newtonian and temperature dependent, however it deviates little enough for the fluid to be ultimately considered as a Bingham plastic for a simple analysis. Thus our model of MR fluid behavior becomes: Where τ =shear stress; τ//y// = yield stress; //H// =Magnetic field intensity η = Newtonian viscosity; is the velocity gradient in the z-direction.

Shear strength
Low shear strength has been the primary reason for limited range of applications. In the absence of external pressure the maximum shear strength is about 100 kPa. If the fluid is compressed in the magnetic field direction and the compressive stress is 2 MPa, the shear strength is raised to 1100 kPa. If the standard magnetic particles are replaced with elongated magnetic particles, the shear strength is also improved.

Particle sedimentation
Ferroparticles settle out of the suspension over time due to the inherent density difference between the particles and their carrier fluid. The rate and degree to which this occurs is one of the primary attributes considered in industry when implementing or designing an MR device. Surfactants are typically used to offset this effect, but at a cost of the fluid's magnetic saturation, and thus the maximum yield stress exhibited in its activated state.

Common MR fluid surfactants
MR fluids often contain surfactants including, but not limited to: These surfactants serve to decrease the rate of ferroparticle settling, of which a high rate is an unfavorable characteristic of MR fluids. The ideal MR fluid would never settle, but developing this ideal fluid is as highly improbable as developing a perpetual motion machine according to our current understanding of the laws of physics. Surfactant-aided prolonged settling is typically achieved in one of two ways: by addition of surfactants, and by addition of spherical ferromagnetic nanoparticles. Addition of the nanoparticles results in the larger particles staying suspended longer since to the non-settling nanoparticles interfere with the settling of the larger micrometre-scale particles due to Brownian motion. Addition of a surfactant allows micelles to form around the ferroparticles. A surfactant has a polar head and non-polar tail (or vice versa), one of which adsorbs to a nanoparticle, while the non-polar tail (or polar head) sticks out into the carrier medium, forming an inverse or regular micelle,respectively, around the particle. This increases the effective particle diameter. Steric repulsion then prevents heavy agglomeration of the particles in their settled state, which makes fluid remixing (particle redispersion) occur far faster and with less effort. For example, magnetorheological dampers will remix within one cycle with a surfactant additive, but are nearly impossible to remix without them. While surfactants are useful in prolonging the settling rate in MR fluids, they also prove detrimental to the fluid's magnetic properties (specifically, the magnetic saturation), which is commonly a parameter which users wish to maximize in order to increase the maximum apparent yield stress. Whether the anti-settling additive is nanosphere-based or surfactant-based, their addition decreases the packing density of the ferroparticles while in its activated state, thus decreasing the fluids on-state/activated viscosity, resulting in a "softer" activated fluid with a lower maximum apparent yield stress. While the on-state viscosity (the "hardness" of the activated fluid) is also a primary concern for many MR fluid applications, it is a primary fluid property for the majority of their commercial and industrial applications and therefore a compromise must be met when considering on-state viscosity, maximum apparent yields stress, and settling rate of an MR fluid.
 * oleic acid
 * tetramethylammonium hydroxide
 * citric acid
 * soy lecithin

Modes of operation and applications
An MR fluid is used in one of three main modes of operation, these being flow mode, shear mode and squeeze-flow mode. These modes involve, respectively, fluid flowing as a result of pressure gradient between two stationary plates; fluid between two plates moving relative to one another; and fluid between two plates moving in the direction perpendicular to their planes. In all cases the magnetic field is perpendicular to the planes of the plates, so as to restrict fluid in the direction parallel to the plates.

Squeeze-Flow Mode
The applications of these various modes are numerous. Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied. Shear mode is particularly useful in clutches and brakes - in places where rotational motion must be controlled. Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces. This particular flow mode has seen the least investigation so far. Overall, between these three modes of operation, MR fluids can be applied successfully to a wide range of applications. However, some limitations exist which are necessary to mention here.

Limitations
Although smart fluids are rightly seen as having many potential applications, they are limited in commercial feasibility for the following reasons: Commercial applications do exist, as mentioned, but will continue to be few until these problems (particularly cost) are overcommedia type="youtube" key="7LArS6tlVNg" width="425" height="350"media type="youtube" key="Or-ugO0rkro" width="425" height="350"
 * High density, due to presence of iron, makes them heavy. However, operating volumes are small, so while this is a problem, it is not insurmountable.
 * High-quality fluids are expensive.
 * Fluids are subject to thickening after prolonged use and need replacing.
 * Settling of ferro-particles can be a problem for some applications.

Applications
The application set for MR fluids is vast, and it expands with each advance in the dynamics of the fluid.

Mechanical engineering
Magnetorheological dampers of various applications have been and continue to be developed. These dampers are mainly used in heavy industry with applications such as heavy motor damping, operator seat/cab damping in construction vehicles, and more. As of 2006, materials scientists and mechanical engineers are collaborating to develop stand-alone seismic dampers which, when positioned anywhere within a building, will operate within the building's resonance frequency, absorbing detrimental shock waves and oscillations within the structure, giving these dampers the ability to make any building earthquake-proof, or at least earthquake-resistant.

Optics
Magnetorheological Finishing, a magnetorheological fluid-based optical polishing method, has proven to be highly precise. It was used in the construction of the Hubble Space Telescope's corrective lens.

Automotive and aerospace
If the shock absorbers of a vehicle's suspension are filled with magnetorheological fluid instead of plain oil, and the whole device surrounded with an electromagnet, the viscosity of the fluid, and hence the amount of damping provided by the shock absorber, can be varied depending on driver preference or the weight being carried by the vehicle - or it may be dynamically varied in order to provide stability control. This is in effect a magnetorheological damper. For example, the MagneRide active suspension system permits the damping factor to be adjusted once every millisecond in response to conditions. General Motors (in a partnership with Delphi Corporation) has developed this technology for automotive applications. It made its debut in both Cadillac (Seville STS build date on or after 1/15/2002 with RPO F55) as "Magneride" (or "MR") and Chevrolet passenger vehicles (All Corvettes made since 2003 with the F55 option code) as part of the driver selectable "Magnetic Selective Ride Control (MSRC)" system) in model year 2003. Other manufacturers have paid for the use of it in their own vehicles. As of 2007, BMW manufactures cars using their own proprietary version of this device, while Audi and Ferrari offer the MagneRide on various models. General Motors and other automotive companies are seeking to develop a magnetorheological fluid based clutch system for push-button four wheel drive systems. This clutch system would use electromagnets to solidify the fluid which would lock the driveshaft into the drive train. Porsche has introduced magnetorheological engine mounts in the 2010 Porsche GT3 and GT2. At high engine revolutions, the magnetorheological engine mounts get stiffer to provide a more precise gearbox shifter feel by reducing the relative motion between the power train and chassis/body. As of September 2010, Acura (Honda) has begun an advertising campaign highlighting its use of MR technology in passenger vehicles manufactured for the 2011 model year. Magnetorheological dampers are under development for use in military and commercial helicopter cockpit seats, as safety devices in the event of a crash. They would be used to decrease the shock delivered to a passenger's spinal column, thereby decreasing the rate of permanent injury during a crash.

Human prosthesis
Magnetorheological dampers are utilized in semi-active human prosthetic legs. Much like those used in military and commercial helicopters, a damper in the prosthetic leg decreases the shock delivered to the patients leg when jumping, for example. This results in an increased mobility and agility for the patient.

the references are www.wikipedia.com www.rmcybernetics.com www.youtube.com

=__**//piezoelectic Materials://**__=

Background
Simply stated, piezoelectric materials produce a voltage in response to an applied force, usually a uniaxial compressive force. Similarly, a change in dimensions can be induced by the application of a voltage to a piezoelectric material. In this way they are very similar to electro-strictive materials. These materials are usually ceramics with a perovskite structure (see figure 1). The perovskite structure exists in two crystallographic forms. Below the Curie temperature they have a tetragonal structure and above the Curie temperature they transform into a cubic structure. In the tetragonal state, each unit cell has an electric dipole, i.e. there is a small charge differential between each end of the unit cell.

**Figure 1.** Shows the (a) tetragonal perovskite structure below the Curie temperature and the (b) cubic structure above the Curie temperature. A mechanical deformation (such as a compressive force) can decrease the separation between the cations and anions which produces an internal field or voltage. Some examples of piezoelectric materials are given in table 1. **Table 1.** Piezoelectric constants of materials.
 * ** Materials ** || **Piezoelectric Constant**

**x10-12m/V** ||
 * Quartz || 2.3 ||
 * Barium titanate || 100-149 ||
 * Lead niobate || 80-85 ||
 * Lead zirconate titanate || 250-365 ||

Key Properties
> ==media type="youtube" key="laSQ6yd7jaE" width="425" height="350"== > ==media type="youtube" key="YFITXmzB9EI" width="425" height="350"==
 * The ability to produce a voltage output in response to an applied stress
 * The ability to produce a strain output (or deformation) in response to an applied voltage.

Transducers
Piezoelectric materials are used in electromechanical devices. In the case of a microphone transducer, sound of a particular frequency results in a strain in the material, which in turn induces an electric field. Similarly in speakers, a voltage input into the piezoelectric material can be converted into a mechanical strain, such as in a speaker transducer.

Other Applications
In radios, piezoelectric devices can be used in tuners, where the correct strain in the crystal will amplify only the desired frequency. They are also employed in fine watch circuits, ones with “quartz movements”.

the reference is www.azom.com www.youtube.com