Nitinol is a type of memory shape alloy or 'smart' material. It consists of an alloy of Nickel Titanium (NiTi), hence its name, formed into a small spring shape. These have recently been made available for schools to cheaply purchase and are required in some of the newer schemes of work in GCSE/14-16 science and further science.
The 'smart' property of a Nitinol spring is that when extended and then heated directly it will contract with some force to its original length. Heating can involve directly heating with a flame or connecting to a circuit, whereupon the current flow can cause the wire to become hot enough to produce the same effect.
Investigations may be conducted to see how much force the spring can pull when it contracts, as different voltages are applied to the spring circuit. Also the relationship between the extended and contracted length can be investigated.
As a demonstration of the properties of this smart material, a simple thermal actuator can be constructed in one of many ways. Some modern fireproof bins may contain smart springs which contract, closing the lid, when heated by fire. This is a good example of how smart materials are being used in modern applications and a simple 'desktop' demonstration fireproof bin can easily be constructed to show this. A bit of imagination can be used to construct similar demonstration pieces.
If you anchor one end of the spring and attach an 'arm' or dial hand to the free end, this will move when the extended spring is heated either directly or by applying electricity. This simple approach forms the basis of some robotic and mechanical uses for Nitinol.
The level of nickel in the alloy is usually around the 50-55% mark although this may be adjusted to produce a wire which contracts at a different temperature range. Most available to schools through the main suppliers will contract at a reachable temperature however commercial applications use various types which reflect their intended applications and as such if purchasing Nitinol springs from commercial suppliers, check the temperature range is within what you are able to produce easily in school.
Be aware that although these springs are intended to be extended to a certain point, over-extension may result in the spring remaining slightly extended after being heated to the desired temperature. Even if you do not exceed the maximum extension, after a lot of extension and contraction the springs may show wear and tear and so do not expect them to always return to their 'as new' state.
Commercial applications include:
- Dentistry - bracing wires and brackets
- Medicine - vein stents
- Optics - frames for eye glasses
- Heat switches
- Fire safety devices