|MadSci Network: Physics|
There have been several questions concerning shape memory alloys and the crystal transformations that cause them to return to a previous shape, so I won't attempt to go over that information again. Instead, let's examine atomic and molecular bonding to try and get a better understanding of the forces involved. As you suggest, the properties of water are dominated by the hydrogen bonding that takes place between adjacent molecules. Ithica City School District has provided a number of animated pages that illustrate different types of chemical bonding. The following link: http://ithacasciencezone.com/chemzone/lessons/03bonding/mleebonding/defaul leads to 8 antimated pages that illustrate the different types of chemical bonds. Other pages that provide further discussion concerning chemical bonding can be found at: http:// www.newi.ac.uk/buckleyc/bonding.htm and http://www.tcm.phy.cam.ac.uk/~cjp20/lectures/topic1.pdf Let's examine the characteristics of hydrogen bonding in water as a way of understanding metallic bonding. Oxygen is highly electronegative, in other words, it attracts hydrogen's one electron quite strongly. As a result, the two hydrogens in a water molecule have a strong positive charge, since their electrons seem to spend most of their time hanging around the oxygen atom. If water was a linear molecule, i.e. if the hydrogen atoms were on opposite sides of the oxygen atom, these electrostatic forces would cancel out. But water, as most people know, looks like a Mickey Mouse head (oxygen as the round head, and the hydrogen atoms as two large ears). So each water molecule has a strong electrostatic polarity because the water molecule is not linear. The hydrogen atoms of adjacent water molecules are attracted to the negative oxygen atoms, and the negative oxygen atoms are attracted to the positive hydrogen atoms. In metals, the electrons are not tightly bound to a particular atom. Instead the electrons move about pretty freely between atoms. The metal nucleus has a positive charge because of the protons contained within the nucleus which attract any and all electrons near the nucleus. So, in metallic bonding, the nucleus is similar to the hydrogens in a water molecule (because they tend to have a positive charge), and the electrons are similar to the oxygen (because it tends to have a negative charge). Both hydrogen bonding in water and metallic bonding are from similar electrostatic forces, with the only difference being that you have molecule to molecule forces in water, and atom to atom forces in metals. Pure metals have unique properties because all of the atoms look alike. If you change the position of one row of atoms with respect to another, you can make this transformation without breaking any chemical bonds. This movement of one row of metal atoms with respect to another is referred to as "dislocation movement." As a result, metals exhibit ductility, or the ability to bend prior to breaking. One of the things that is done to strengthen metals is to combine different metals into alloys. At this link: http://web.mit.edu/ 3.091/www/pt/pert1.html you can see the change in atomic radius for each element. When you combine two different metals into an alloy, different atomic radius makes it more difficult for the atoms to move with respect to each other; it makes it more difficult for a dislocation to move through the metal. Let's think of an alloy as having a box full of baseballs with a few softballs thrown in. The softballs pin the rows of baseballs together, preventing them from moving as easily as they would without the softballs being present. If you have a certain ratio of baseballs to softballs, they will try and arrange themselves into a regular pattern. A typical shape memory alloy may have an atomic composition of 50% titanium and 50% nickel. The atomic radius of nickel is 1.24 angstroms and the atomic radius of 1.45 angstroms. In this case, we can expect that in a shape memory alloy crystal, every other atom will be nickel, and every other atom will be titanium. In the case of shape memory alloys, you can only move them a small amount before they no longer can remember their initial shape. At http://pubs.acs.org/cen/topstory/7906/7906notw1.html and http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTA-0103- we learn that shape memory alloys can only be deformed between 3 and 8% before they undergo inelastic deformation. So in the case of shape memory alloys, the amount of deformation that they undergo appears to be too small for the atoms to actually jump from one position to another. Instead, it appears that we can deform the alloy a bit, but the softballs are still close to the whole that they were budged out of and want to settle back into that hole if they are given a chance. If you deform the shape memory alloy too much, the softballs lose track of the places they were located, and therefore, they cannot return to their initial shape. So essentially the force generated by a shape memory alloy is caused by the attraction for metal atoms to settle back into their old, regular position, rather than being slightly displaced from that position. Although I haven't checked it out, Phil, I bet that a shape memory alloy decreases in density a bit when it is strained, and increases in density when it returns to it's original position. You might wonder how much force is generated by materials changing from one crystal form to another. You mentioned that a force is generated when water freezes into ice. The early settlers in Utah (and possibly other locations) mined granite blocks out of the mountains by drilling holes in the granite, filling those holes with water, and letting them freeze overnight. The force of the water freezing and increasing in volume was sufficient to split the granite. I will leave you with one last example. Graphite has a density of 2.2 grams per cubic centimeter; diamond has a density of 3.5 grams per cubic centimeter. To convert graphite to diamond, all one has to do is to put the graphite under a pressure of a bit over one million pounds per square inch and heat it up to increase the speed of the reaction. So, I suspect that if you confined a diamond, if you heat it up to maybe 1500C, it will exert a force on its surroundings of more than a million pounds per square inch. Thanks for a most interesting question.
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