A. Introduction
Crystals have been valued by man throughout history as building materials or ornamental objects. Today they are further valued as technological materials. Solar cells and semiconductors are two recent developments based on the unique properties of certain crystals. Liquid crystals are another source of exciting possibilities for the future.
B. Solar Cells
Solar cells are perhaps the most familiar to the student, as many of them own solar-powered calculators, or have seen the solar panels on various space vehicles. Inexpensive solar toys are available for student assembly which emphasize the rather simple nature of their operation. The energy produced by the solar cell is also called
photovoltaic energy
. The solar cell is made of silicon which has been infused with an impurity such as boron. Silicon has a strongly bonded tetragonal crystal with pairs of shared electrons uniting it to its four neighbors. When the boron is added as a substitutional impurity (see section II D) it will share its three valence electrons with three of its silicon neighbors but is lacking a fourth electron to share with the fourth silicon neighbor. This absence of an electron is called a hole. So the addition of boron results in a “
hole
” which is considered to be a positive charge. Next we diffuse a very thin layer of phosphorus into the top of the boron silicon. Phosphorus is different from boron in that it has five valence electrons, one more than it needs to join into the stable bonding with silicon. So this extra electron is free to move among the electron shells, in some cases filling in boron holes, but usually resulting in an excess of electrons moving around. These are called
conduction electrons
. The silicon region with holes is called the “p-type layer” and the region with the excess of conduction electrons is called the “n-type layer.”
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(figure available in print form)
As the conduction electrons move in to fill in the holes, a peculiar thing happens. The boron atoms which take up the electrons in their holes now become negative ions! And the phosphorous which has given the conduction electron up now becomes a positive ion! This results in a boundary called the
p-n junction
where holes with their positive charge and electrons with their negative charge are repelled by the charged ions that set them loose. So the excess conduction electrons are unable to move across the boundary to fill in the holes. This results in electrons building up in the n-type layer and holes will collect in the p-type layer.
(figure available in print form)
Electrons move toward the p-type side and holes (positive) move toward the n-type side until a dynamic equilibrium is reached. Ionized boron repels further movement of electrons and ionized phosphorus repels further movement of holes (positives).
When photons strike the solar cell, bonded electrons are bounced right out of their positions creating many more conduction n-electrons and holes on both sides of the junction. Since there are already so many electrons on the n-type side and so many holes on the p-type side, the additional new ones are only a tiny proportion; but by making new holes on the n-type side and new conduction electrons on the p-type side, the solar cell is unbalanced. The electrons from the p-type side move across the junction creating a flow of electrons, or electricity! This flow moves electrons out through the n-type layer onto a conductive wire grid which is connected to a circuit that is completed by an attachment to the p-type layer.
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Read more about this fascinating but complex topic in Chalmers’ article or Swan’s book.
(figure available in print form)
Solar cell circuit in which conduction electrons move toward the n-type crystal where they travel to the current collector on the surface of the cell and move through the external circuit to the p-type crystal area.
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C. Transistors
Transistors are constructed of the same types of materials as described in the section on solar cells. These materials are called semi-conductors and when bonded together form a p-n junction which is influenced by the addition of a voltage differential placed across it rather than the effects of photons of sunlight. When these thin layers of semi-conductors are sandwiched together they make up a transistor which can be used to regulate the flow of electrons in circuits, detect and amplify radio signals, produce oscillations in transmitters, and act as digital switches. These tiny solids were the first electrical components in which materials with different electrical characteristics were physically joined by structural contact rather than by wires.
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D. Liquid Crystals
Another interesting crystal technology involves a crystal which is not a solid. The melting point of a pure crystalline substance is very precise. And in some cases the crystalline properties of a solid crystal carry over into the liquid state. there are many compounds that will do this. Their atomic structure remains quite orderly even while taking on the other characteristics of typical liquids such as pouring or taking on the shape of its container. Selective reflection of white light into its various spectral components by these liquid crystals can be directed and controlled by thermal, acoustical, electrical, magnetic, and even mechanical means. Wrist watches and clocks whose numbers melt from one display into another are liquid crystals. Window glass can be made dim or clear depending on the intensity of the sunlight passing through. An exciting futuristic application of the liquid crystal would be flat plates or screens hung on walls which display television images.
The simplest or first generation display capability with a single electrical lead connected to a single picture element forms the seven-segment number displays as seen on clocks and watches. Second generation displays attaches four picture elements to one electrical lead and displays the seven segment numbers and also star-burst shapes which can form letters or numbers, useful for pocket calculators.
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More complex wiring and liquid crystals with helical (spiral) axis positions can display 5-32 elements per electrical lead and are used for personal computers. The newest experimental versions are capable of producing TV picture displays on a flat substrate.
Liquid crystals are true liquids but also have some solid properties. Their internal order is very delicate and can be changes by a weak electrical field, magnetism, or even temperature variations. Noticeable optical effects are the result of re-arrangement of the molecules and the resulting changes in refraction (light-bending), reflection, absorption, scattering, or coloring of the visible light from their surface. Liquid crystals modify the ambient light rather than emit their own light and therefore require minimal amounts of power. A typical LCD (liquid crystal device) uses one microwatt per square centimeter of display area.
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A very simplified diagram below shows the effect of an electric current on liquid crystal molecules. This change is visible due to its effect on light waves.
(figure available in print form)
Some liquid crystals are sensitive to temperature, and are used as a component in thermometers. They can be used in diagnostic tools to detect cancers, pulmonary disease, and vascular diseases. Their dramatic color variations are caused by an actual helical swing of 360 degrees by the molecules! The diagram below shows this 360 degree re-orientation process.
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(figure available in print form)
A uniform starting position for the crystals is vital to their usefulness. Liquid crystals can be aligned by “rubbing” of the substrate. Until recently this was poorly understood but used as a standard practice anyway. It is now known that the rubbing results in microgrooves which serve to orient the molecules.
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Although liquid crystal technology has only recently been exploited by man for diagnostic tools, displays, new materials such as kevlar, and oil-recovery technology; nature has used these peculiar molecules in living systems right along. The structure of cell membranes and some tissues are liquid crystals. Hardening of the arteries is a result of the deposition of liquid crystals of cholesterol, cells involved in sickle cell anemia have liquid crystal structure, and on a brighter note, it may soon be possible to change the solid form of a gall stone into a liquid crystal form that can be flushed from the body.
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For further information see the Brown or Kahn articles.
The technological development of crystals has taken off in this generation from semiconductors to transistors to integrated circuits to microchips. Always getting smaller but with vastly increased information handling abilities. They are the mainstay of the space and military industries, making possible the impossible in distant space travel, satellite technology, and weapons’ accuracy.
Solar cells are becoming more efficient and more common, and as our energy problems increase, student interest in solar technology has also increased. And the liquid crystals in our students’ watches and pocket calculators are just one more bit of technology waiting to be explained to our students.
There are many very helpful books and articles which can aid the reader in his or her understanding of these interesting, complex topics. Many of these have been listed in the bibliography, but I would like to specifically recommend the Holden book,
The Nature of Solids
, and the Chalmers article, “Photovoltaic Generation of Electricity,” as two excellent readings with which to begin. They will provide you with the information you will want to feel confident about teaching about crystal technology.
All of the books and articles that are specifically referred to in the text of this unit are available in book or reprint form at the Yale-New Haven Teachers Institute office on Wall Street, New Haven.