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Name: Jack
Status: other
Grade: 9-12
Location: CA 
Country: N/A
Date: 7/25/2005


Question:
Why do thermistors release more electrons when the temperature is increased? I know it is something to do with covalent bonds.


Replies:
I am not sure that "releasing more electrons" is the right way to say it, but it is essentially correct. As the temperature is increased, more electrons are available to conduct electricity in a thermistor.

Most materials conduct electricity with electrons (or holes, which is sort of an absence of an electron). So we need to look at electron mobility in solids.

First, metals easily conduct electricity, and the resistance usually increases with temperature. One can view the metal as having many free electrons that can move in an electric field. Sort of a "sea" of electrons. As the temperature goes up, the atoms in the solid vibrate more and more and interfere with the movement of the conduction of the electrons. Resistance goes up. Strictly from a quantum mechanical point of view, the electrons scatter off lattice phonons (basically vibrations) and the higher the temperature, the more vibrations to interfere.

On the other hand, some materials have no or few electrons that can move easily in an electric field. These are insulators and pure semiconductors like silicon. At cryogenic temperature, the electrons basically cannot move. (But here I put a warning: electrons in solids move terrifically fast some of them near the speed of light. They never stop moving. But an electric current is a net or overall motion of electrons, and in insulators there is no overall movement.)

Insulators and semiconductors are full of electrons, so why do they not move? You pretty much need to think in quantum mechanical terms like band structure, which you can read about on the Internet. The bands are not physical things, but a way of thinking about how the electrons are distributed in energy.

A semiconductor is full of electrons, but quantum mechanics prevents them from going anywhere when cold. As heat is applied, electrons are induced to move from one band to another, and they can move and start to conduct electricity. A thermistor is a semiconductor, and its resistance goes down with increasing temperature, as expected.

This thermistor property may or may not have anything to do with covalent bonds. Roughly speaking, bonds are the way a chemist looks at how atoms are stuck together in a solid. Band structure is how a physicist looks at electrons in a solid. Same solid; different ways of looking at it.

If you are going to melt something, or look at how hard it is, then you may need to look at the strength of the bonds. If you are interested in electrical resistance, or heat conduction, then you may need to look at the band structure.

Bob Erck



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