Thermal conductivity: dependence on temperature

[javispain]

I performed simulations with the non-equilibrium Green’s function method to calculate the lattice thermal conductivity. According to my results (and the ones shown in the example), the lattice thermal conductivity increases as the temperature increases, and I dont understand why this trend happens. For most of crystals, it is usual to have a lower thermal conductivity as the temperature increases, because of the anharmonic effects. What I do is to calculate the spectrum and the obtain the thermal conductance for different temperatures.

With the reverse non-equilibrium method, the results are obtained as expected.

Could you please help me on this?

[berna]

Which material did you use?

[javispain]

It is PbTe (300-800K)

[berna]

These two articles have different results for lattice conductivity of PbTe
http://pubs.rsc.org/en/Content/ArticleLanding/2012/EE/C2EE22495J#!divAbstract (fig. 7c)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.205203 (fig. 5)
Is there any setting about volume in your calculation?

[javispain]

No, it is the same code as the examples, but for different supercells -3x3x30, 4x4x48, 8x8x64. But I always obtain an opossite trend. This is, the conductivity increases as temperature increases, instead of decreasing

[Julian Schneider]

Anharmonic effects are not included in phonon transmission calculated via non-equilibrium Green's functions. Everything is purely harmonic. Therefore NEGF is primarily suited for interfacial thermal conductance (where scattering at the interface is the main contribution to thermal resistance) and not so much for bulk conductivity (where inelastic phonon-phonon-scattering is the main contribution to resistance).

Non-equilibrium MD, on the other hand fully includes anharmonic effects and can therefore be used to calculate both interface conductance and bulk conductivity.
So what you observe makes perfect sense.

Have a look at our recent webinar on thermal transport simulations http://docs.quantumwise.com/webinars/webinars.html