Articles

    A Shape Memory Metal

    Kids, did you ever imagine that there might be chemistry involved in braces? How about eyeglass frames? There is a special metal alloy called Nitinol that is often involved in both of these applications. It is a nickel-titanium (Ni-Ti) alloy developed by chemists at the Naval Ordinance Lab (NOL) ­hence the name of NiTiNOL. This unique metal displays the very rare phenomenon of the shape memory effect. It seems to remember its original shape after being bent.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Let¹s see how it works in braces. The bracket part of braces is a tiny metallic or porcelain anchor glued to a tooth. As many as 12-14 of these minihandles are fastened in each jaw. Next a thin flexible Nitinol wire is fastened to each bracket and bent back and forth as necessary to connect the misplaced teeth. This special wire is elastic (actually, "superelastic") and wants to, very slowly, return back to its original straight shape. Moving at about one millimeter per month, the wire brings the teeth with it. Eventually, a stronger stiffer wire is needed to move teeth into their permanent final alignment, and orthodontists often use stainless steel for this. Both Nitinol and stainless steel (an alloy of iron and carbon with some chromium and nickel) are resistant to acids and corrosion. You never see rust on braces even though they are in a wet environment and exposed to weak acids (citric acid, decomposition of sugars, etc.).

    Another example of Nitinol¹s properties is in the springs of eyeglass frames whose arms can flex slightly out, away from the body, without damage. One of the most amazing demonstrations of shape memory is when it is temperature dependent. Nitinol can do this as well. A straight wire can be wound into a tight coil, put into hot water, and instantly it springs back to its straight shape. Eyeglasses with this Nitinol in the temple piece can be squashed or severely bent, but restored by immersing in hot water. This effect is caused by a change in the crystal form of the metal. It transforms between the high temperature phase called austenite and the low temperature form called martensite. Because the phase change occurs with just a simple shearing motion of the atoms, and no diffusion or large atomic movement is required, the transformation occurs virtually instantly and can cycle many times.

    Most applications of Nitinol are in aerospace engineering and hydraulics. For $35, Shape Memory Applications, Inc. will supply a kit of demo wires and springs (2380 Owen St., Santa Clara, CA 95054, (408) 727-2221; www.sma-inc.com). The demo wire is especially fun ­ bend it, dip into hot water, and bam! it snaps back to straight, over and over again. Their website also has a very helpful review document. Also published for demo purposes is the "Thermobile" (G. Kauffman & I. Mayo, J. Chem. Educ. 1998, 75(3), 313), an engine with no visible power source that converts thermal energy to mechanical energy using a Nitinol loop wrapped around two pulleys. I encourage you to research this amazing metal. Some other alloys showing this effect are gold-cadmium (Au-Cd) and brass (copper-zinc).

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    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    April 2000

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    References: M. W. McClure, ChemMatters, American Chemical Society, 2/00, p. 7.