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Q & A: metallic conduction

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Most recent answer: 08/26/2012
Q:
What is the definition of metallic conduction?
- Anonymous
A:
Usually we think of metallic conduction as coming from some moving charged particles which don't get stuck as the material is cooled down. That's different from, for example, a semiconductor in which there are also moving charges but they do get stuck or rejoin a non-conducting collection as the material is cooled.
A standard, not too rigorous, practical guide is that materials whose conductivity goes up as they are cooled are metallic and ones whose conductivity goes down are not. The reason that metals typically have higher conductivity at low temperature is that one of the main obstacles to free movement of their charge carriers is the thermal jiggling of the atoms, and this is reduced at low temperature.

Mike W.

(published on 10/22/2007)

Follow-Up #1: How does temperature affect metal thermal conductivity?

Q:
Q1 : In one of your answers, you wrote "In most metals, the conductivity goes down as they get hotter, because the electron waves bounce off of the thermal juggle waves." Does this mean that "lattice vibrational waves" disrupt the movement of "free moving electrons", thus lowering thermal conductivity? Q2: Why is the thermal conductivity of water maximum at around 400K and decreases at higher temperature? Thank you again in advance.
- Tony Hue (age 23)
Northridge, CA, USA
A:
Hi Tony- I've marked your question as a follow-up to a related one.

It's fair to say that lattice vibrations scatter electrons lowering the electrical conductivity and lowering their contribution to the thermal conductivity. However, the Wiedemann-Franz law has an extra factor of T (absolute temperature) in the thermal conductivity to electrical conductivity ratio. Therefore whether the thermal conductivity goes up or down as T is increased depends on how fast the electrical conductivity goes down.  If impurity scattering is more important than phonon scattering, the thermal conductivity goes up as T goes up.

Mike W.

(published on 08/24/2012)

Follow-Up #2: thermal conductivity of water

Q:
Thank you very much for your detailed answer!! I apologize for another question, but why is the thermal conductivity of water maximum at around 400K and decreases at higher temperature? Thank you again in advance.
- Tony (age 23)
Northridge, CA, USA
A:
Here is a plot of that, for other readers;
.

The thermal conductivity can be expressed as the product of the thermal diffusion rate (how fast heat randomly moves around) and the heat capacity per unit volume. The thermal diffusion rate is the product of the typical distance heat flows before randomly changing direction and the typical speed it flows. I guess that typical distance is pretty much a single molecular spacing. The speed should be about the thermal speed of a molecule, which grows as the square root of the absolute temperature. The heat capacity of liquid water doesn't change much with temperature. So I'd expect the thermal conductivity to grow a bit as temperature rose. It does, up to 100C, the boiling point of water, although not following the particular form one might guess. The differences, as I guessed, aren't especially big.

What happens above 100C, where it starts down again? Beats me. I'm not sure how those measurements are made, since water has to be pressurized to form a stable liquid above 100C. Pressure could also affect the thermal conductivity.

Maybe one of our readers knows more about this.

Mike W.

(published on 08/26/2012)

Follow-up on this answer.