Instructor Information
JCE Classroom Activity: #74
Memory Metal In this Activity, students compare the properties of nitinol metal wire (known as “memory” metal) and ordinary wire. Using the observed properties, they then design (and possibly make) a toy that would use memory metal to operate. Nitinol metal is an alloy of nickel and titanium that exhibits interesting “shape memory” properties. When heated above 500–550 ⬚C the metal undergoes a solid-to-solid phase change. In the high-temperature, or austenite phase, the metal can be bent into a shape that can be fixed into the metal’s memory. The mechanical stress of bending produces gaps in the metal’s orderly crystalline atomic array. At the high temperature, atoms surrounding these defects gain enough thermal energy to shift their position in the crystalline array to fill these openings, thus fixing the austenite into its new atomic and macroscopic structure. When cooled, the metal undergoes another solid-tosolid phase change. The low temperature phase is called the martensite phase. When the new configuration that has been fixed into the metal’s memory is pulled out of shape, and then reheated (usually to at least 50 ⬚C), the remembered shape of the metal is restored. Upon heating, the thermal motion of the metal atoms is increased. To accommodate the increased motion, the atoms slip back into the austenite phase, which restores the remembered shape. The gentle heating used to return to the austenite phase from the martensite phase is well below the temperature usually used to set the metal’s memory, and does not provide enough energy to allow the defects to readjust and reprogram the memory metal. Nitinol metal rods also have interesting acoustic properties; a rod in the austenite phase “rings” when dropped, while a rod in the martensite phase “thuds” (1).
Integrating the Activity into Your Curriculum This Activity complements the National Chemistry Week 2005 theme, “The Joy of Toys”. The Activity connects toys with science, and also allows students to become inventors as they design a toy of their own. A JCE article describes a scientific toy powered by memory metal (2). The Activity ties in well with a discussion of the properties and structure of metals and alloys, and is an example of a solid-to-solid phase change.
About the Activity In a laboratory situation, instructors may wish to use Bunsen burners instead of candles to investigate the metal wires. A hair drier could be used in place of hot water to heat the memory metal to return it to its remembered shape. Nitinol metal wire and nitinol metal demonstration items can be purchased from various vendors (3). Different nitinol wire may vary in how difficult it is to bend into a particular shape, such as a coil. A QuickTime movie shows nitinol wire and is accompanied by background information and other experiments (4). 1. The memory metal wire moves in the area that it is being heated to try to return to its original shape, almost to the point that it becomes difficult to hold it in a new shape, even with tongs. 2. Copper and galvanized steel wire are easier to coil and bend, and do not show an ability to “remember” a set shape as memory metal wire does. 3. Two examples are braces and anti-scalding devices. Braces can be shaped so that as they are warmed in the mouth, they remain in a position that exerts the proper pressure on the teeth. Anti-scalding devices can be placed on hot water faucets. When the water becomes too hot, the memory metal device changes shape to block the water flow.
References, Additional Related Activities, and Demonstrations 1. Gisser, K. R. C.; Geselbracht, M. J.; Cappellari, A.; Hunsberger, L.; Ellis, A. B.; Perepezko, J.; Lisensky, G. C. Nickel–Titanium Memory Metal: A “Smart” Material Exhibiting a Solid-State Phase Change and Superleasticity. J. Chem. Educ. 1994, 71, 334–340. 2. Kauffman, G. B.; Mayo, I. The Thermobile: A Nitinol-Based Scientific Toy. J. Chem. Educ. 1998, 75, 313–314. 3. Examples of sources: Educational Innovations, http://www.teachersource.com, 888/912–7474; a 10 ft package for $24.95 or $5 per foot. Institute for Chemical Education, http://ice.chem.wisc.edu; memory metal kit contains piece of memory metal in shape of the letters ICE. Images SI Inc., http://www.imagesco.com/catalog/nitinol/nitinol.html; $5 per foot and other nitinol products such as springs and demonstration items. 4. Memory metal. http://mrsec.wisc.edu/Edetc/memmetal/, http://mrsec.wisc.edu/Edetc/ActivityGuides/ MemoryMetal_3_11_03.pdf (The URLs in references 3 and 4 were all accessed Jul 2005.) A piece of memor y metal spelling the word “ICE” is first pulled apart. When dipped in warm water, it “remembers” its original shape.
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Answers to Questions
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Background
JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 82 No. 10 October 2005 •
Journal of Chemical Education
1488A
JCE Classroom Activity: #74
Student Activity
Memory Metal
Try This
photos: J. J. Jacobsen, J. Harris, E. K. Jacobsen
Nitinol metal (sometimes called “memory” metal) is a well-known example of a “smart” material that is able to sense changes in itself and its surroundings, and respond to those changes. Its name is short for Nickel Titanium Naval Ordnance Laboratory—providing information both on the metals it contains and its place of discovery. It was discovered during a search for an alloy to use in missile nose cones. Nitinol’s smart properties are a result of its ability to undergo a solid-to-solid phase change. The relative positions of nickel and titanium atoms shift slightly as the metal changes from a low-temperature martensite phase to a high-temperature austenite phase and vice versa. In this Activity, you will investigate the properties of wire made from nitinol metal and compare its properties to wire made from other metals. Using the observed properties of nitinol metal, you will then design (and maybe make!) a toy that uses this memory metal to operate.
You will need: three ~6-in. (~15-cm) pieces of wire: one memory metal (nitinol), one 24 gauge copper, and one 24 gauge galvanized steel; pencil or glass rod; candle; candle holder; matches; nonflammable surface; tongs or needle-nose pliers; shallow glass container; water; and hot plate or microwave. __1. Coil a ~6-in. (~15-cm) piece of memory metal wire tightly around a pencil or glass rod. How difficult is it to coil? Carefully try to slide the coiled wire off. Record your observations, including the shape of the wire. __2. Place a candle in a candle holder over a nonflammable surface. Light the candle. __3. Using tongs or needle-nose pliers, hold one end of the wire you coiled in step 1. Move the entire length of the wire through the candle flame for a few seconds. Remove the wire from the flame. Record your observations. (Be sure to allow the wire to cool thoroughly before touching it.) What happens to the wire? __4. Create a small loop or bend in the middle of the wire. Using tongs or a needlenose pliers, hold the two ends of the loop or bend tightly so the ends remain together. Hold the loop or bend in the candle flame for 10–15 seconds. How does the loop or bend behave? Remove the wire from the flame. Record your observations. Be Safe! Cut wires may have sharp __5. After allowing the wire to cool, pull the wire into a straight line. Using tongs or ends. Memory metal wire can move in needle-nose pliers, hold one end of the wire. Move the entire length of the wire unexpected ways. Goggles should be through the candle flame for a few seconds. Remove the wire from the flame. worn at all times. Be careful when workRecord your observations. What happens to the wire and its shape? ing with an open flame. Only work __6. Fill a shallow glass container two-thirds full of water. Using a hot plate or microover a nonflammable surface. wave, heat the water so that it is hot to the touch, but not boiling. After allowing the wire to cool, pull the wire into a straight line. Place the straight wire in the hot water. Record your observations. What happens to the wire and its shape? __7. Pull the memory metal wire into a straight line and place it in the hot water again. Repeat this step 5–10 times. Does the effect remain the same or change? __8. Repeat steps 1–6 with pieces of 24 gauge copper wire and 24 gauge galvanized steel wire. __9. Using the properties of memory metal you observed in the procedure above, design a toy or toy part that uses memory metal. Include a drawing of the toy and describe how it works. You may wish to make the toy or toy part, using a candle flame to set the shape. For example, how would you design a toy that would spring up from the table? How might the toy be activated?
Questions 1. How does the portion of the nitinol wire that is directly in the candle flame act as it is heated? 2. How do the properties of nitinol wire compare to an ordinary metal wire such as copper or galvanized steel? 3. What are some possible application for nitinol metal? How would it be useful for those particular applications?
Information from the World Wide Web (accessed Jul 2005) Toy story: materials engineering at play. http://www.tms.org/pubs/journals/JOM/0212/Roncone-0212.html Shape memory alloys and their applications. http://www.stanford.edu/~richlin1/sma/chem.html Technology transfer program: success stories. http://techtran.msfc.nasa.gov/new/memmetal.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
1488B
Journal of Chemical Education •
Vol. 82 No. 10 October 2005 •
www.JCE.DivCHED.org
