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Crystal Growth

Activity 14

OBJECTIVE:

To observe crystal growth phenomena in a 1-g environment.

BACKGROUND:

A number of crystals having practical applications, such as L-arginine phosphate (LAP) and triglycine sulfate (TGS), may be grown from solutions. In a one-gravity environment, buoyancy-driven convection may be responsible for the formation of liquid inclusions and other defects which can degrade the performance of devices made from these materials. The virtual absence of convection in a microgravity environment may result in far fewer inclusions than in crystals grown on Earth. For this reason, solution crystal growth is an active area of microgravity research.

Crystal growing experiments consist of a controlled growth environment on Earth and an experimental growth environment in microgravity on a spacecraft. In this activity, students will become familiar with crystal growing in 1-g.

illustration of procedure
One or more crystals of alum (aluminum potassium sulfate or AlK(SO4)212H20 will be grown from seed crystals suspended in a crystal growth solution. With the use of collimated light, shadowgraph views of the growing crystals will reveal buoyancy-driven convective plumes in the growth solution. (Refer to activity 6 for additional background information.)

PROCEDURE:

Step 1. Create a seed crystal of alum by dissolving some alum in a small quantity of water in a beaker. Permit the water to evaporate over several days. Small crystals will form along the sides and bottom of the beaker.
Step 2. Remove one of the small crystals of alum and attach it to a short length of monofilament fishing line with a dab of silicone cement.
Step 3. Prepare the crystal growth solution by dissolving powdered or crystalline alum in a beaker of warm water. The amount of alum that can be dissolved in the water depends upon the amount of the water used and its temperature. Refer to the table (Alum Solubility in Water) for the quantity required.
Step 4. When no more alum can be dissolved in the water, transfer the solution to the growth chamber acrylic box.
Step 5. Punch a small hole through the center of the lid of the box. Thread the seed crystal line through the hole and secure it in place with a small amount of tape. Place the seed crystal in the box and place the lid on the box at a 45 degree angle. This will expose the surface of the solution to the outside air to promote evaporation. It may be necessary to adjust the length of the line so that the seed crystal is several centimeters above the bottom of the bottle.
Step 6. Set the box aside in a place where it can be observed for several days without being disturbed. If the crystal should disappear, dissolve more alum into the solution and suspend a new seed crystal.
Step 7. Record the growth rate of the crystal by comparing it to a metric ruler. The crystal may also be removed and its mass measured on a balance.
Step 8. Periodically observe the fluid flow associated with the crystal's growth by directing the light beam of a slide projector through the box to a projection screen. Observe plumes around the shadow of the crystal. Convection currents in the growth solution distort the light passing through the growth solution.

MATERIALS NEEDED


Aluminium Potassium Sulfate
  ALK ((SO4 )2 12H2O*
Square acrylic box **
Distilled water
Stirring rod
Monofilliment fishing line
Silicone cement
Beaker
Slide projector
Projection screen
Eye protection
Hot plate
Thermometer
Balance

    * Refer to the chart on the next page for the amount of alum needed for the capacity of the growth chamber (bottle) you use
    ** Clear acrylic boxes, about 10x10x13 cm are available from craft stores. Select a box that has no optical distortions.
QUESTIONS:

1. What is the geometric shape of the alum crystal?
2. What can cause more than one crystal to form around a seed?
3. What do shadowgraph plumes around the growing crystal indicate? Do you think that plumes would form around crystals growing in microgravity?
4. Does the growth rate of the crystal remain constant? Why or why not?
5. What would cause a seed crystal to disappear? Could a crystal decrease in size? Why?
6. What are some of the possible applications for space-grown crystals?

FOR FURTHER RESEARCH:

1. Grow additional alum crystals without the cap placed over the box. In one experiment, permit the growth solution to evaporate at room temperature. In another, place the growth chamber in a warm area or even on a hot plate set at the lowest possible setting. Are there any differences in the crystals produced compared to the first one grown? How does the growth rate compare in each of the experiments?
2. Experiment with growing crystals of other chemicals such as table salt, copper sulfate, chrome alum, Rochelle salt, etc. Caution: Become familiar with potential hazards of any of the chemicals you choose and take appropriate safety precautions.
3. Review scientific literature for results from microgravity crystal growing experiments.

graph of alum solubility in water Shadowgraph image of a growth plume rising from a growing crystal

 
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