the metal to return to the stiff, preformed shape, exerting pressure on the teeth. This same idea of changing to an original shape at higher temperatures finds applications in valves that will shut off if temperatures of liquids passing through the valve go higher than desired. Another ChemMatters article on how memory metals work at the molecular level (with diagrams) is found in the October, 1993 issue, pp.4-7 ( “Memory Metal”) . An application of futuristic proportions is the construction of a robot bat that can fly, having flexible wings that, if “distorted”, will return to the original shape, using the characteristics of memory metal for muscle function. The distortion in shape of the memory metal, which is highly elastic, is done using heat from an electric current sent to the wires that are the size of a human hair. Heating causes contraction (going to the higher temperature’s original shape). Contraction of the wires increases electrical resistance and a change in current flow that can be used as a sensory input to control electronics. Cooling allows the metal to return to another shape or length because of the wire’s elasticity. ( ) and ( ) More on transistors A description of the hafnium-based transistor (referred to in the Metals article) is found at as well as . The latter reference describes the method, at the nanoscale, of depositing individual atoms for the N- and P-layers of a transistor that allows for much thinner transistors. Also, there is a different use for transistors—to control the flow of molecules rather than of electrons, thereby moving molecules themselves without using moving parts such as valves and pumps. The concept is based on the early work of moving ions between electrically charged plates. This technique can be used to analyze molecules such as DNA, proteins, etc. at a nanoscale. ( ) More on alloys Alloys come in a variety of mixes (both homogeneous and heterogeneous) that provide a variety of useful characteristics. Something like alloy steels have various small amounts of metals added to iron. These include: Chromium for increased hardness and resistance to corrosion Tungsten and molybdenum for increased heat resistance Nickel adds toughness (as in armor plating) 59
Vanadium adds springiness Manganese improves resistance to wear This website, provides information on the various alloys of aluminum, copper, iron, lead, titanium and so-called smart alloys (another term for memory metals such as Nitinol). More on ores From where do metals come in nature? Students may not be aware of the idea that metals usually occur in nature as compounds (minerals) in rock (the ore), rather than as a pure substance such as gold and silver. The mining industry and subsequent processes of refining a
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