Effect Of Mechanical Stress On Resistivity

CorticalChaos

15th November 2009 - 01:51 AM

i looked around for some info and couldn't find any (google scholar, web etc~) anyway, im wondering what would be the effect of mechanical stress on the thermal or electrical properties of a material, a metal in particular.

in piezoelectric ceramics a current can actually be generated through mechanical stress (and the opposite) but for metals for example, if you were to measure the thermal/electrical properties at the bottom of the ocean(material under extreme compression) and then measure the properties at SATP wouldn't there be a difference?

thermal properties depend on the molecular structure & atomic weights of the material & how well they can transfer vibration between each other (im not sure about the electrical properties)

under extreme pressures the atoms would be forced closer together so i expect that the thermal & electrical properties would improve, does anyone have information on this?

Enthalpy

17th November 2009 - 09:32 PM

It's called the piezoresistive effect.
http://en.wikipedia.org/wiki/Piezoresistive_effect

This effect is small for a metal: the electric resistance of a piece of metal varies about as much as its dimensions (length divided by area). Example for steel: tensile (for elongation) modulus =210GPa, and bulk modulus (for volume variation) over 300GPa. So at -9,000m or 90MPa you get a variation of about 300ppm, easy to measure.

Just for the fun: human technology knows to produce pressures exceeding not only the ocean's bottom but the Earth's centre as well. Search for "diamond anvil".

For bigger effects, take a semiconductor. Their resistivity varies for instance 300 or 2000 times more than their dimensions. Used for sensors, not only strain cells, but also accelerometers and pressure sensors, and many more.

Heat is conducted mainly by electrons in a metal. This explains why metals are good heat conductors (but some ceramics are good as well!) and why their conductivities for heat and electricity have a constant ratio.

By the way, this constant ratio is broken in a superconducting metal, and I've never read why. I don't see neither how BCS explains this, even for an old-fashioned superconductor.

Plumb Bob

30th November 2009 - 10:21 PM

QUOTE (Enthalpy+Nov 17 2009, 09:32 PM)

Heat is conducted mainly by electrons in a metal. This explains why metals are good heat conductors (but some ceramics are good as well!) and why their conductivities for heat and electricity have a constant ratio.

By the way, this constant ratio is broken in a superconducting metal, and I've never read why. I don't see neither how BCS explains this, even for an old-fashioned superconductor.

This might help (from Wikipedia).

In a normal conductor, an electrical current may be visualized as a fluid of electrons moving across a heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat, which is essentially the vibrational kinetic energy of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance.

The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force between electrons from the exchange of phonons. Due to quantum mechanics, the energy spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum amount of energy ΔE that must be supplied in order to excite the fluid. Therefore, if ΔE is larger than the thermal energy of the lattice, given by kT, where k is Boltzmann's constant and T is the temperature, the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid, meaning it can flow without energy dissipation.

Bob

Enthalpy

3rd December 2009 - 12:40 AM

In Wiki's citation, I strongly disagree at least with
"In a conductor, the electronic fluid can be resolved into individual electrons"
(transformed the sentence from "in a superconductor, it cannot).

Electrostatic interaction between a metal's mobile electrons is huge since a metal has in the order of one mobile electron per atom. This repulsion, about 10eV, preclude external field nor heat to move each electron relative to its neighbours. The "electron fluid" must consist of huge bunches of electrons moving together in a rather rigid manner.

Now, what is possible is that these bunches can vibrate at room temperature, allowing to transport heat and to dissipate power when moving, but can't in superconducting state (equivalent to an energy gap). What I don't get is how Cooper pairs introduce the gap.

Anyway, I don't understand properly how pairing fermions produces bosons, except through an addition of half-integers. For instance, why 1 mole of 4He has a volume, as opposed to 1 mole of photons. Maybe I should begin with this.

Are there some simple experimental results (I explicitly exclude superfluidity from "simple") that show this bosonic behaviour? Like, for instance, elastic scattering between two 4He giving different figures from 3He?

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