SHAPE CHANGE MATERIALS

Shape change materials:


- shape memory materials
- piezo-electric materials
- magneto-strictive materials
- electro-rheological fluids
- electro-strictive materials


Shape memory alloys are a class of materials that undergo a significant shape change (the “shape memory effect”) as a result of one of two external stimuli; temperature or magnetic field. SMAs also display a property known as pseudo-elasticity, a remarkable property for a metal alloy, which allows elastomer-like flexibility providing the material is above a critical temperature. Strains of up to 10% can be accommodated as a result of pseudo-elasticity and the SMA will return to the original shape.

It is important to realise that the Shape Memory Effect is not simply a version of thermal expansion, which most materials display to some extent or other. The first material to be observed to have shape memory was a Gold-Cadmium alloy back in the 1930s. This material was difficult and expensive to make, however, and it wasn’t until the 1960s that the Naval Ordnance Laboratories in the U.S. developed a Nickel : Titanium alloy, known as “Nitinol”. Nitinol and derivatives are amongst the most commonly used shape memory alloys, together with CuZnAl and CuAlNi alloys.

The shape memory effect is a solid state phase change. That is a rearrangement of the molecular structure of the material, resulting in a different sized unit cells and an overall change in length (for example). The two phases in SMAs are martensite and austenite with austenite being the higher temperature phase.


Piezoelectric Materials

Piezoelectric materials are crystalline materials that generate an electric charge across their surface when subjected to a mechanical stress, with the charge produced being proportional to the stress applied. This behaviour is known as the piezoelectric effect. The materials also display the opposite effect, changing their shape when an electric field is applied across them. This behaviour means these materials can be used as electromechanical transducers, converting between electrical and mechanical energy.

Many materials show piezoelectric properties, including both ceramics and polymers, and these materials can be found to occur naturally (e.g. quartz) or be produced commercially (e.g. lead zirconate titanate, polyvinylidene fluoride). The essential feature which enables a material to be piezoelectric is that the crystalline structure is not symmetrical about its centre. This provides a net electric dipole moment within the crystal structure, which enables the structure to respond to an applied field or pressure.

The most widely used piezoelectric materials are those in which the electrical and mechanical axes (i.e. the directions within the crystal structure along which the electric field and mechanical pressure are applied) can be precisely oriented in relation to the shape of the sample. This allows the user to effect expansion and contraction of the material through application of an appropriate field, or to produce a change in polarity of the induced voltage through compressive or tensile loading. For these materials the non-centrosymmetric structure only exists up to a certain temperature, which is specific to the material; above this temperature the structure becomes centrosymmetric and the piezoelectric effect cannot be displayed.

Piezoelectric materials can be ideal candidates for applications where a lightweight, solid state system is required; the main advantages are an instantaneous and reliable response up to MHz range, and a small (~ 0.1% strain) but precise deformation. An example of one of the most popular piezoelectric ceramics is lead zirconate titanate (PZT). Through careful modifications to the basic PZT composition the physical and piezoelectric properties have been optimised to meet a vast range of actuator and sensor applications.

Magneto Strictive Materials

Magnetostriction is displayed by ferromagnetic materials, such as iron, nickel, cobalt and various alloys, which will be introduced later). It is important to have a basic understanding of the mechanisms of ferromagnetism, such that magnetostriction can be appreciated.



A ferromagnetic material contains regions known as domains. These domains will have associated with them a magnetisation, which is a vector quantity – That is it has both magnitude and direction. In an ideal, demagnetised iron bar the domains will be orientated randomly and will also be approximately the same size.



When a magnetic field is applied, some domains tend to grow with the applied field. Some domains will not be able to grow, owing to microstructural features or their alignment with respect to the applied field, and those domains will shrink, in preference to others that can grow around them. Once domain growth has completed then domain rotation occurs at progressively higher fields, leading to magnetic saturation.





Because magnetisation is a function of the electron orbits in the atoms that make up the basic unit cells of ferromagnetic material, a change in the magnetisation direction is associated with a change in the crystal lattice lengths, and a change in length of the bulk material is observed, which we know as magnetostriction.



It is useful to define a parameter that is used in describing magnetostriction, that of the saturation magnetostriction. This is the change in length observed when a material is fully saturated magnetically. All values given here are saturation magnetostriction values which will vary from zero to the maximum, depending on the magnetisation level in the material, which depends, in a non-linear fashion, on the applied field.




Electro Rheological Fluids Case Study

Electro-Rheological (ER) fluids were invented in the 1940s, and can change their physical properties in the presence of an electric field. The fluids can change from a free-flowing liquid to one with a finite static yield stress, giving properties consistent with a solid or gel when the field is turned on. The speed of the transition between two states is typically less than 1 millisecond (more than 1 kHz). This gives rise to new possibilities of fast mechanical control techniques. ER valves are fast enough, for example, to play music through a hydraulically operated ER loudspeaker. (Smart Technology Limited)

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