Shape-memory metals

A photoelectric conversion device 10 is formed in a p-type HgCdTe substrate 11 and a CCD 20 is formed in a p-type silicon substrate 13. Indium electrodes 4a and 4b, which are connected to the photoelectric conversion device and the CCD, respectively, are covered on their sides with shape-memory metal layers 5a and 5b. Pressure bonding is used to couple the photoelectric conversion device to the CCD. Thereafter, the temperature is raised to return the shape-memory metal to its original shape, at the same time releasing mechanical stress in the indium electrodes. 

Explain how shape-memory metals are able to manipulate their shapes in response to temperature fluctuations. Are there other alloy candidates for this type of behavior ... 

Damping Structural Vibrations with Shape-Memory Metals, NASA Publication, University Press of the Pacific, 2004. 

Metallic solids Recall from Chapter 8 that metallic solids consist of positive metal ions surrounded by a sea of mobile electrons. The strength of the metallic bonds between cations and electrons varies among metals and accounts for their wide range of physical properties. For example, tin melts at 232°C, but nickel melts at 1455°C. The mobile electrons make metals malleable—easily hammered into shapes—and ductile—easily drawn into wires. When force is applied to a metal, the electrons shift and thereby keep the metal ions bonded in their new positions. Read Everyday Chemistry at the end of the chapter to learn about shape-memory metals. Mobile electrons make metals good conductors of heat and electricity. Power lines carry electricity from power plants to homes and businesses and to the electric train shown in Figure 13-21a. 

The first materials known to exhibit shape memory were shape memory metal alloys. Shape memory pol)nners are being developed to replace the use of Shape memory alloys, in part because the polymers are light, high in shape recovery ability, easy to manipulate, and economical in comparison to shape memory alloys... 

A NiTiNOL shape memory metal alloy can exist in two different temperature-dependent crystal structures or phases called martensite (i.e., lower-temperature phase) and austenite (i.e., higher-temperature or parent phase). Several properties of the austenite and martensite phases are notably different. When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called the austenite start temperature A). The temperature at which the phenomenon is complete is called the austenite finish temperature (A). When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called the martensite start temperature (M ). The temperature at which martensite is again completely reverted is called the martensite finish temperature (Mj). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTiNOL can have three different forms ... 

To see how a shape memory metal behaves, suppose we bend a bar of NiTi alloy into a semicircle [Figure 23.20(a) ] and then heat it to about 500°C. We then cool the metal to below the transition temperature for the phase change to the low-temperature, flexible form. Although the cold metal remains in the semicircular form, as in Figure 2320(b), it is now quite flexible and can readily be straightened or bent into another shape. When the metal is subsequently warmed and passes through the phase... 

There are many uses for such shape memoiy alloys. The curved shape in a dental brace, for example, can be formed at high temperature into the curve that the teeth are desired to follow. Then at low temperature, where the metal is flexible, it can be shaped to fit the mouth of the wearer of the braces. When the brace is inserted in the mouth and warms to body temperature, the metal passes into the stiff phase and exerts a force against the teeth as it attempts to return to its original diape. Other uses for shape memory metals include heat-actuated shutoff valves in industrial process lines, which need no outside power source. Inserted into the face of a golf club, diape memory metals are said to impart more spin to the ball and greater control of the dub. 

Ryhanen, J., Niemi, E., Serlo, W., et al., Biocompatibility of Nickel-Titanium Shape Memory Metal and Its Corrosion Behavior in Human Cell Cultures, Journal of Biomedical Materials Research, Vol. 35, 1997, pp. 451 57. 

Hombogen, E. (2006), Comparison of shape memory metals and polymers. Advanced Engineering Materials, 8(1-2) pp. 101-106. 

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