Objective: · To demonstrate how atoms in a solid arrange themselves.
|Science Process Skills:
MATERIALS AND TOOLS
Eventually, the atoms begin to separate and move chaotically. This simulates melting. Reducing the amount of vibration brings theatoms back together where they "bond" with each other. In this demonstration, gravity pulls the BBs together to simulate chemical bonds. By observing the movement of BBs, a number of crystal defects can be studied as they form and transform. Because of movements in the pan, defects can combine (annihilation) in such a way that the ideal hexagonal structure is achieved and new defects form.
The model is viewed best with small groups of students standing around the device. After the solid "melts," diminish the motor speed gradually to see the ways the atoms organize themselves. It is important that the platform be adjusted so it is slightly out of level. That way, as the motor speed diminishes the BBs will move to the low side of the pan and begin organizing themselves. If this does not happen, apply light finger pressure to one side of the pan to lower it slightly. This will not affect the vibration movements significantly. While doing the demonstration, also stop the vibration suddenly. This will simulate what happens when molten material is quenched (cooled rapidly) .
The motor collar required in the materials list is available from a hardware store. The purpose of the collar is to provide an offcenter weight to the shaft of the motor. The set screw in the collar may have to be replaced with a longer one so that it reaches the motor shaft for proper tightening.
Constructing The Vibrating Platform
Note: Specific sizes and part descriptions have not been provided in the materials list because they will depend upon the dimensions of the surplus electric motor obtained. The motor should be capable of several hundred revolutions per minute.
Conducting The Experiment
Crystalline solids are substances whose atoms or molecules are arranged into a fixed pattern that repeats in three dimensions. Crystalline materials generally begin as a fluid of atoms or molecules in either the liquid or gaseous state. As they change to the solid state, the atoms or molecules join together in repeating patterns. Materials that do not form these patterns are called amorphous. Glass is a good example of an amorphous material.
The usefulness of a crystal depends on its structure. All crystalline materials have varying degrees of defects. Defects can take many forms. Gem-quality diamonds sometimes have small inclusions of carbon (carbon spots) that diminish their light refraction and thereby reduce their value. In other crystalline materials, defects may actually enhance value. Crystals used for solid state electronics have impurities deliberately introduced into their structure that are used to control their electrical properties. Impurity atoms may substitute for the normal atoms in a crystal's structure or may fit in the spaces within the structure. Other defects include vacancies, where atoms are simply missing from the structure, and dislocations, in which a half plane of atoms is missing. The important thing about crystal defects is to be able to control their number and distribution. Uncontrolled defects can result in unreliable electronic properties or weaknesses in structural metals.
Sample Crystal Defects
Many forces can affect the structure of a crystal. One of the most important forces that can influence the structure of a growing crystal is gravity. Growing crystals in microgravity can reduce gravity effects to produce crystals with better defined properties. The information gained by microgravity experiments can lead to improved crystal processing on Earth.
The connection between the force of gravity and the formation of defects varies from very simple and straightforward to complicated and nonintuitive. For example, mercury iodide crystals can form from the vapor phase. However, at the growth temperature (approximately 125 C) the crystal structure is so weak that defects can form just due to the weight of the crystal. On the other hand, the relationship between residual fluid flows caused by gravity and any resulting crystalline defects is not well understood and may be very complex.