Presented at the University of Wisconsin - Madison Engineering Expo on April 18-20, 1997, this exhibit contains demontrations and experiments illustrating many aspects of materials science.
Materials:
CRYSTALS
The arrangement and identity of atoms in a solid control its physical and chemical properties. Many solids are crystalline, repeating arrays of atoms. The repeating building block of the crystal is called the "unit cell." Many models are on display at this table. The spheres in these models represent atoms in the crystal. The big model represents a face-centered cubic crystal structure. Try to duplicate this structure on the smaller model set in less than 20 seconds.
The other small model sets show other crystal structures:
Techniques:
DIFFRACTION
Scientists use X-ray diffraction to find the crystal structure of a material. Atoms in crystals scatter X-rays into diffraction patterns. In an analogous fashion, a square array of dots scatters laser light into a square diffraction pattern on the screen. Pull gently on the lever. As the square array of dots on the plastic (a polymer called polydimethylsiloxane) is distorted into a retangular array, the diffraction pattern also becomes rectangular but with its long direction perpendicular to the long direction of the structured array. The diffraction spots in the same direction become closer together.
SCANNING TUNNELING MICROSCOPY (STM)
Scientists use STM to examine the surface of a material. The STM works by measuring electrical current flowing between a tip terminating in a single atom and a surface. Some STMs can even image individual atoms on a surface. This STM is probing the surface atoms of a piece of graphite.
MAGNETIC FORCE MICROSCOPY (MFM) WITH LEGOS
Scientists use MFM to measure the location of magnetic fields on a surface. We have built a larger version of a MFM with LEGO®s. In MFM, a magnet at the tip of a lever moves up and down as it interacts with magnetic fields. A laser beam reflects from the lever and measures the lever movement. Push the button on the remote control. The tip of the MFM lever moves up and down as it moves over alternating north-south magnetic regions, called domains, of the refrigerator magnet. The laser spot on the screen correspondingly moves up and down.
MAGNETIC FORCE MICROSCOPY (MFM) WITH REFRIGERATOR MAGNETS
A magnetic strip moves up and down as it interacts with different magnetic fields on the magnetic sheet. Dragging the strips across the sheet reveals different responses in different directions, corresponding to a striped pattern of magnetic domains. Kits demonstrating these properties are available from MRSEC in limited supply. Special thanks to Dan Dahlberg from the Magnetic Microscopy Center for introducing this experiment to us.
Properties:
ELECTRICAL RESISTANCE
Resistance (measured in ohms) impedes the flow of electricity in materials. Materials that control the flow of electricity are called resistors. Measuring the resistance of graphite pencil lines reveals that resistance increases as the line length increases and decreases as the line width increases.
THERMAL CONDUCTIVITY
Different materials transfer or conduct heat at different rates. Most people realize that plastic is a poor conductor of heat and metal is a good conductor of heat, but it may not be obvious that diamond is an extremely good thermal conductor due to its rigid bonds that transmit thermal energy easily.
Applications:
MAGNETORHEOLOGICAL FLUID
These fluids are now being researched for use in shock absorbers. This mix of mineral oil and iron filings flows normally but becomes more rigid in a magnetic field.
Developed by NASA as a fluid that can be controlled in zero gravity, ferrofluids are now used in computer hard drives and stereo speakers. Ferrofluid is a colloid - a suspension of tiny magnetic particles. In this case the ferrofluid is composed of magnetite (Fe3O4) nanoparticles of ~10 nm diameter surrounded by soap-like molecules, which enable them to stay suspended in mineral oil. High quality ferrofluids exhibit "spikes" in magnetic fields, as the ferrofluid marks the location of magnetic field lines. Pennies, which normally sink in ferrofluid, can be buoyantly displaced by the attraction of ferrofluid to a magnet.
LIGHT-EMITTING DIODES (LEDs)
LEDs are semiconductor devices that emit light by combining negative charges (electrons) and positive charges (holes). Improved fabrication of gallium nitride (GaN) and related solids has been a recent technological advance toward the development of blue LEDs, which are important for solid-state lasers and flat panel displays. The "fish" has two eyes: a red phosphide-based LED and a blue nitride-based LED. When the "fish" is dunked into liquid nitrogen, the cold temperature causes the semiconductor crystal lattices to shrink, which causes the red LED to give an orange glow and the blue LED to give a violet glow.