Smart+Materials+-+MOhamed+Adel+El-Said+Mahmoud


 * __S __****__mart __****__M __****__aterials __**

==== The technological development points towards a synergic performance of a combination of different materials. It means that the merging of distinct materials will lead to novel materials that can preserve the properties of the individual components, but furthermore will exhibit characteristics that would not be possible individually. Such composite materials impact upon may areas, including transport, bioengineering and medical instrumentation, civil engineering, mechanical tools, fashion, packaging, fire-retardant electrical enclosures, and sport. ====

//**__ We will identify some of this materials: __**//

 * __ Shape Memory Alloyes Materials __
 * __ Piezoelectric Materials __
 * __ MR Fluid Materials (Magnetorheological Fluid) __
 * __ Auxetic Materials __

** S **** hape **** M **** emory **** A **** lloyes **** M **** aterials **

Shape Memory Alloys (SMA) are novel and special materials. They can "remember" their shape when heated above a certain transition temperature, Also called __muscle wire__. = · //__Shape Memory Alloys History __// =

//** First discovered by Arne Olande in1938 **//** : **

He observed the shape and recovery ability of a gold-cadmium alloy (Aucadmium alloy (Au-Cd)

//**W.J. Buehler and Wang at the US Naval Ordinance Laboratory 1963: **//

observed the shape memory effect in a nickel and titanium alloy, today known as nitinol (“Night in All” Nickel Titanium Naval aval Ordinance Lab). = · //__How do they work __// = - SMAs change shape based on a solid state phase transformation - Atomic level changes (Rearrangement of atoms) - Change in shape occurs at a specific temperature (Shape Memory Effect) - We all know the most common phase changes



__ BUT __

NiTiNOL Says No ....

===**Nitinol is the name of one member of a class of metals known as SMA. The general phenomenon was discovered in the 1930's. Shape Memory Alloys can take two different crystal structures. They have a "hot" phase, in which the material is generally stiffer and has a higher yield point, and a "cool" phase, which is less stiff and has a lower yield strength. In the lower crystal phase they are generally superelastic. This means they can be deformed far more than other metals of the same general family-approximately 10 times more. They can be formed into a shape at higher temperature, cooled, then formed to a different shape around room temperature. When heated, they return to the shape they had at the high temperature. There are several known metal combinations that have these properties.**=== ===**Nitinol was developed by the Naval Ordnance Laboratory. The name comes from its composition and the discovery team who first recognized the potential of this powerful alloy (NIckel/TItanium/Naval Ordinance Laboratory).**=== ===**Nitinol alloys are typically made of 55%-56% Nickel and 44%-45% Titanium. Small changes in composition can significantly impact the properties of the material. There are two primary categories of Nitinol. The first, known as "SuperElastic", is characterized by extraordinary kink resistance and flexibility. The second category, "Shape Memory" alloys, is valued for the Nitinol's capacity to recover a pre-set shape when heated above its transformation temperature. The first category is often used for orthodontics (braces, wires, etc) and eyeglasses. Dynalloy® makes shape memory alloys, which are primarily useful for actuators, used in many diffrent mechanical devices.**=== media type="youtube" key="Mh31B4Ryn9U" height="315" width="420" align="center"

** P **** izoelectric ** ** Materials **

=== 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. ===

Some examples of piezoelectric materials are given in table 1.
**Table 1.** Piezoelectric constants of materials. || ** Materials ** ** x10-12m/V ** ||
 * ** Piezoelectric Constant **
 * Quartz || 2.3 ||
 * Barium titanate || 100-149 ||
 * Lead niobate || 80-85 ||
 * Lead zirconate titanate || 250-365 ||

Key Properties

 * 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. ====

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** MR F **** luid ****Magnetorheological Fluid ** **Above, MR fluid prior to magnetization. Below, the fluid turned into a solid after it was magnetized. Notice the shiny surface of the liquid in the top photo and the dull surface in the bottom photo.** =What is MR Fluid= Looking at it in a beaker, [|MR fluid] doesn't seem like such a revolutionary substance. It's a gray, oily liquid that's about three times denser than water. It's not too exciting at first glance, but MR fluid is actually quite amazing to watch in action. A simple demonstration by David Carlson, a physicist at the North Carolina lab, shows the liquid's ability to transform to solid in milliseconds. He pours the liquid into the cup and stirs it around with a pencil to show it's liquid. He then places a magnet to the bottom of the cup, and the liquid instantly turns to a near-solid. To further demonstrate that it's turned to a solid, he holds the cup upside down, and none of the MR fluid drops out. Typical MR fluid consists of these three parts: So, what is it that gives MR fluid its unique ability to transform from liquid to solid and from solid to liquid quicker than you can blink an eye? The carbonyl iron particles. When a magnet is applied to the liquid, these tiny particles line up to make the fluid stiffen into a solid. This is caused by the dc magnetic field, making the particles lock into a uniform polarity. How hard the substance becomes depends on the strength of the magnetic field. Take away the magnet, and the particles unlock immediately. While scientists have just recently discovered many new applications for MR fluid, it has actually been around for more than 50 years. [|Jacob Rabinow] is credited with discovering MR fluid in the 1940s while working at the U.S. National Bureau of Standards (now the [|National Institute of Standards and Technology]). Until about 1990, there were few applications for MR fluid because there was no way to properly control it. Today, there are **digital signal processors** and fast, cheap [|computers] that can control the magnetic field applied to the fluid. Applications for this technology include Nautilus exercise equipment, [|clothes washing machine] dampers, shock absorbers for cars and [|advanced leg prosthetics]. In the next section, we will look at the seismic applications of this MR technology, which may have the biggest impact on saving lives and preventing the collapse of buildings.
 * **Carbonyl Iron Particles** -- 20 to 40 percent of the fluid is made of these soft iron particles that are just 3 to 5 micrometers in diameter. A package of dry carbonyl iron particles looks like black flour because the particles are so fine.
 * **A Carrier Liquid** -- The iron particles are suspended in a liquid, usually hydrocarbon oil. Water is often used in demonstrating the fluid.
 * **Proprietary Additives** -- The third component of MR fluid is a secret, but Lord says these additives are put in to inhibit gravitational settling of the iron particles, promote particle suspension, enhance lubricity, modify viscosity and inhibit wear.

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** A **** uxetic ** ** M **** aterials **

===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.===
 * ===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.===

//**__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.=== = = = = =__//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.=== || media type="youtube" key="hpIKjC59yj0" height="315" width="420" align="center" **//__ References: __//**
 * ==__What Types Of Materials Can Exhibit Auxetic Behaviour?__==
 * [[image:http://www.azom.com/work/VYA679dKq926O9JL800g_files/image004.gif width="394" height="481"]] ||
 * **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.===
 * [[image:http://www.azom.com/work/VYA679dKq926O9JL800g_files/image005.gif width="399" height="324"]] ||
 * **Figure 3.** Width versus length variations for polypropylene fibre ||


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