PART 2 SHAPE MEMORY ALLOYS e.g. Nitinol & Magnetic Shape Memory Alloys

Doc Brown's Chemistry Revision Notes - Smart Materials and Materials Science

PART 2 SHAPE MEMORY ALLOYS (metals)

Shape memory alloys remember their shape! Return to their desired shape - smart material!

• SHAPE MEMORY ALLOYS (SMAs) are metallic alloy materials in which deformation (change in shape) can be induced and recovered through temperature changes or stress changes.

• An SMA subjected to a mechanical stress or shear force will undergo a reversible deformation, usually over a narrow temperature range.

  • Shape-Memory Alloys are metals that, after being strained, at a certain temperature revert back to their original shape.

    • The crystal structure changes above their transformation temperature causes them to return to their original shape.

      • The transition temperature is characteristic of a particular alloy composition.

    • SMAs can involve relatively large forces produced by any resistance encountered in their transformation and, reversibly, a large mechanical movement effect as the strain released-relaxed.

  • This enable the shape to be recovered if the temperature is altered back.

  • The overall phenomena is that a material is mechanically deformed, then heated, and then rapidly reverts to its 'remembered' shape.

  • Basically, bend a piece of shape memory alloy so it is out of shape, warm it up, and it returns to its original shape!

• Shape memory alloys cab used in various devices e.g.

  • thermostats for temperature control

  • on-off switches for clamping devices such as pipe couplings where a loose sleeve can be caused to contract and clamp tightly by heating or cooling to induce the shape recall dimensions.

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NITINOL

• Titanium and nickel metals are the two main components of Nitinol 'smart alloys'.

• Nitinol belongs to a group of shape memory alloys (SMA) which can 'remember their original shape'.

• For example they can regain there original shape on heating e.g. used in thermostats in cookers , coffer makers etc.

  • When nitinol is cool you can bend it, i.e. it can be quite flexible, BUT if you bend it (stress it) to far, it might stay bent and not regain its correct original shape.

  • However, if you heat the 'stressed' nitinol object above a certain threshold temperature, it relaxes and regains its original remembered shape!

• They can be designed to regain their original shape after release of a physical stress e.g. used in 'bendable' eyeglass frames, very handy if you tread on them. Place them into a bowl of warm water and your glasses frame regains its shape!

• Nitinol is used in dental braces.

  • The dental brace is designed to return to the correct dental shape on warming.

  • The cold dental brace is stressed on placing in position on the teeth.

  • As the nitinol brace warms up, it 'remembers' and returns to its 'designed shape' and pulls the teeth into their correct position in your mouth!

• Nitinol shape memory alloys are used to make stents.

  • Stents are fine tubes used to treat damaged blood vessels which restrict the flow of blood.

  • Fine cylindrical mesh tubes are inserted into blood vessels to maintain the inner diameter of a blood vessel.

  • Once inserted, the fine tubes warm up and expand to allow the flow of blood.

• Nitinol is an acronym for 'Nickel Titanium Naval Ordinance Laboratory' betraying, like so many technological developments, its military origins, but now acquiring many 'peaceful' uses.

MAGNETIC SHAPE MEMORY ALLOYS - MAGNETOSTRICTIVE MATERIALS

• Magnetostrictive materials change in shape under the influence of externally applied magnetic field.

• The reverse can also occur so that the magnetization of the material can change under the influence of a mechanical stress stimulus.

• For example, magnetic shape memory alloys are metallic materials that change their shape in response to a significant change in the external magnetic field.

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INDEX of SMART MATERIALS PAGES

PART 1 CHROMOGENIC MATERIALS - Thermochromic, Photochromic & Electrochromic Materials

PART 2 SHAPE MEMORY ALLOYS e.g. Nitinol & Magnetic Shape Memory Alloys

PART 3 SHAPE MEMORY POLYMERS, pH and temperature sensitive-responsive polymers, self-healing materials

Part 4 LYCRA-SPANDEX

PART 5 High performance polymers like KEVLAR

PART 6 GORETEX, THINSULATE and TEFLON-PTFE

PART 7 PIEZOELECTRIC EFFECT (PIEZOELECTRIC MATERIALS) and PHOTOMECHANICAL MATERIALS

PART 8 CARBON FIBRES