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Name: Tom
Status: educator
Grade: N/A
Location: CT
Country: N/A
Date: 6/21/2005


Question:
The big debate in our department is regarding insulators and density. Is there a definite relationship? On one hand a higher density material can have particles that have greater inertias and are harder to move, meaning the can have some insulative properties. However, higher density materials usually have closer packed particles which make them more conducive to being conductors. What is the answer?


Replies:
It is not clear from the way you framed the question whether you referring to the relationship between density and electrical conductivity or thermal conductivity. If you mean electrical conductivity the relation is convoluted and indirect. There are several different mechanisms for electrical conductivity depending upon whether the material is a metal, a semi-conductor, an ionic solution electrolyte, a gas (plasma), a superconductor, a nominal insulator, or even a vacuum. Each type of material has a different mechanism.

The electrical conductivity depends upon the mobility of electrons in the material. This mobility is only indirectly related to the density of the material, in the most simple approximation, the electron mobility is independent of the density of the material. You can see that the density varies from essentially zero (for a vacuum of low pressure plasma) up to about 20 gm/cm^3 for a heavy metal. The web sites below goes into more detail on the subject.

http://en.wikipedia.org/wiki/Electrical_conduction

http://en.wikipedia.org/wiki/Effective_mass

Vince Calder


Tom,

Structure of the material is more important than density. The only truly perfect insulator is a vacuum: a vacuum has no molecules to transmit heat.

Liquids tend to be very poor insulators due to convection. Although they might or might not transmit the heat from molecule to molecule very well, the hot molecules can travel through the liquid quite easily. For a gas, higher density tends to transmit heat better. Higher density results in more heat carriers (molecules) to travel through the gas.

For solids, metals are the best conductors. The loose electrons can carry the heat through the material quite easily. For other materials, what matters most is how well adjacent molecules respond to each other. Many crystals are poor conductors. The molecules have specific frequencies at which they vibrate. The random patterns of heat do not inspire a resonance. However, light of the proper frequency will enter and transmit through the material quite easily.

As for plastics, I am not familiar enough with the structure and behavior to comment reliably.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Tom-

Unfortunately the ancient Greek philosophers could not settle your debate without somebody first figuring out electrons and quantum-mechanics. Or at least electrons-and-atoms. Two populations, together making up one solid. Only seen apart on special occasions which the Greeks never saw much, namely: vacuum tubes. Batteries, generators, and galvanometers helped too.

Density is a composite parameter, the final result of a few more-basic factors:

- the atomic weight of the elements,

- the crystal form in which the elements like to pack themselves together, and

- the void volume: percentage of exceptions from that favored crystal structure.

Density is history-dependent: diamond, graphite, glassy carbon, and soot are all pure carbon 12. Diamond, glassy carbon, and soot are insulating, and the electrically conducting one, graphite, has density in between diamond and glassy carbon.

So there must be some other picture dominating electrical conductivity.

That picture is made of about two things:

- wave-interference of free electrons trying to coast over the wash-board ripples of the atom lattice in the substance (it can force electrons to require higher-than-thermal energies)

- the "sea-level" of electrons in that lattice, compared with said ripples.

These in turn depend on element valence, and structure details of the particular crystal form. So electrical conductivity has no simple relationship with density or atomic number.

Void volume: Any electrically conductive substance becomes gradually less conductive if it has many voids or other flaws which the electrons must swim around to proceed. "Ideal" or "theoretical" density is slightly better for conductivity. But it is also better for insulating strength, too. Voids mean gas or vacuum space, and internal surfaces, all of which help plasma discharges (sparks) to skip on through.

When I first read your question, I asked, "what kind of 'conductivity'?" Electrical and thermal are possibilities. Electrical conductivity varies over more than 15 orders of magnitude. Things that pass 1 amp/cm2 given only 10^-5 volt/cm are metals, definitely conductors. Things that admit less than 10^-12 amp/cm2 with 1000 volt/cm, are definitely insulators.

This degree of change must have a picture that includes "trapping" of free carriers. The particles all have dimples to be stuck in, or they do not, and they drift around in any breeze. Some insulators such as diamond, if they have very low density of traps, begin to be slightly conductive even though the free electrons are rare compared with the atoms.

Thermal conductivity has no such sharp distinction, no real trapping effects, and it is more compatible with the kind of thinking you are doing. The best and worst thermal conductors vary by only a factor of 1000 or so. Everything is a thermal conductor, the question is one of degree. "Particle inertia" can matter here. Heavy elements conduct less heat per gram, but similar amounts of heat per atom, or per unit volume. Voids can locally reflect, globally diffuse, the random mechanical waves that make up heat in a solid.

Jim Swenson



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