Nickel titanium

From Wikipedia, the free encyclopedia

Nickel titanium (Ni Ti) is a shape memory alloy also commonly referred to by the name, Nitinol, derived from its place of discovery (Nickel Titanium Naval Ordinance Laboratory). William Buehler along with Frederick Wang, discovered its properties during research at the Naval Ordnance Laboratory in 1962.[1]

The discovery of the shape-memory effect dates to 1932 when a Swedish researcher named Arne Olader first observed the property in gold-cadmium alloys. The material, if deformed while cool, returned to its undeformed shape when warmed. Other shape-memory alloys (SMA) have since been discovered. Some of these alloys include CuSn, InTi, TiNi, and MnCu. The TiNi alloys, better known as Nitinol, and the copper-based alloys, are the most commonly used. It has been found that they can recover substantial amounts of strain, and/or generate significant force, when changing shape.

A reversible, solid phase transformation known as martensitic transformation is the force behind shape memory alloys. The alloy material forms a crystal structure, which is capable of undergoing a change from one form of crystal structure to another. Temperature change or/and loading initiate this transformation.^[1]

Above its transformation temperature, Nitinol is superelastic, able to withstand a small amount of deformation when a load is applied and return to its original shape when the load is removed. Below its transformation temperature, it displays the shape memory effect. When it is deformed it will remain in that shape until heated above its transformation temperature, at which time it will return to its original shape.

Nitinol is typically composed of approximately 50 to 55.6% nickel by weight. Making small changes in the composition can change the transition temperature of the alloy significantly. For this reason, Nitinol may or may not be superelastic at room temperature. These unique properties and tailorability of Nitinol to be used in a wide range of temperatures makes it suitable for many applications, particularly in medicine.

The wire can be heated with electricity making it an example of an electrical actuator.

[edit] Applications

There are hundreds of alloys that exhibit the shape memory effect (SMA), but only two alloy systems, TiNi and CuZnAl, have found many practical applications. Other alloys are considered too expensive or they exert insufficient force.

In 1989 a survey was conducted in the United States and Canada that involved seven organizations. The survey focused on predicting the future technology, market, and applications of SMA's. The companies predicted the following uses of Nitinol in a decreasing order of importance: (1) Couplings, (2) Biomedical and medical, (3) Toys, demonstration, novelty items, (4) Actuators, (5) Heat Engines, (6) Sensors, (7) Cryogenically activated die and bubble memory sockets, and finally (8) lifting devices. ^[2].

In dentistry, the material is used in orthodontics for brackets and wires and in endodontics, where Nitinol files are used to clean and shape the root canals during the root canal procedure.

Due to the fact it can change shapes it is also used as a golf club insert.

Another significant application of Nitinol in medicine is in stents: A collapsed stent can be inserted into a vein and heated (returning to its original expanded shape) helping to improve blood flow. Also, as a replacement for sutures where Nitinol wire can be weaved through two structures then allowed to transform into its preformed shape which should hold the structures in place.

Nitinol is highly biocompatible and has properties suitable for use in orthopaedic implants.

Nitinol is also popular in extremely resilient glasses frames. It is also used in some mechanical watch springs.

It can be used as a temperature control system; as it changes shape, it can activate a switch or a variable resistor to control the temperature.

It is used in cell-phone technology as a retractible antenna due to its highly flexible & mechanical memory nature.

It is used in some novelty products, such as self-bending spoons which can be used by amateur and stage magicians to demonstrate "psychic" powers or as a practical joke, as the spoon will bend itself when used to stir tea, coffee, or any other other warm liquid.