Interfacial thermal resistance
This is even more critical for nanoscale systems where interfaces could significantly affect the properties relative to bulk materials.
Low thermal resistance at interfaces is technologically important for applications where very high heat dissipation is necessary.
As stated above, thermal boundary resistance is due to carrier scattering at an interface.
Molecular dynamics (MD) simulations are a powerful tool to investigate interfacial thermal resistance.
Recent MD studies have demonstrated that the solid-liquid interfacial thermal resistance is reduced on nanostructured solid surfaces by enhancing the solid-liquid interaction energy per unit area, and reducing the difference in vibrational density of states between solid and liquid.
[7] The AMM and DMM models should apply for interfaces where at least one of the materials is electrically insulating.
[8] According to the AMM and DMM models, a crucial factor in determining the thermal resistance at an interface is the overlap of phonon states.
Specifically, the models completely disregard the effects of inelastic scattering and multiple phonon interactions.
In reality, however, this is not the case and the interaction probability of two phonons can be calculated using perturbation theory (quantum mechanics).
Thus as the overlap between phonon dispersions is small, there are fewer modes to allow for heat transfer in the material, giving at a high thermal interfacial resistance relative to materials with a high degree of overlap.
[9] Neither model is very effective for predicting the thermal interface resistance (with the exception of very low temperature), but rather for most materials they act as upper and lower limits for real behavior.
is the group velocity which is approximated to be the speed of sound in the material for the AMM and DMM models,
is the number of phonons at a given wavevector, E is the energy, and α is the probability of transmission across the interface.
n is determined based on the dispersion relation for the materials (for example, the Debye model) and Bose–Einstein statistics.
The assumption of elastic scattering makes it more difficult for phonons to transmit across the interface, resulting in lower probabilities.
While this idea was first proposed in 1936,[9] it wasn't until 1941 when Pyotr Kapitsa (Peter Kapitza) carried out the first systematic study of thermal interface behavior in liquid helium.
Studies around 1960 surprisingly showed that the interfacial resistance was nearly independent of pressure, suggesting that other mechanisms were dominant.
[12] Fortunately such a large thermal resistance was not observed due to many mechanisms which promoted phonon transport.
In liquid helium, Van der Waals forces actually work to solidify the first few monolayers against a solid.
The final dominant mechanism to anomalously low thermal resistance of liquid helium interfaces is the effect of surface roughness, which is not accounted for in the acoustic mismatch model.
When these waves eventually scatter, they provide another mechanism for heat to transfer across the interface.
Similarly, phonons are also capable of producing evanescent waves in a total internal reflection geometry.
As a result, when these waves are scattered in the solid, additional heat is transferred from the helium beyond the prediction of the acoustic mismatch theory.
The free electron gas found in metals is a very good conductor of heat and dominates thermal conductivity.
Interfacial thermal conductance is a measure of how efficiently heat carriers flow from one material to another.
Diamond on the other hand is a very good electrical insulator (although it has a very high thermal conductivity) and so electron transport between the materials is nil.
Further, these materials have very different lattice parameters so phonons do not efficiently couple across the interface.
Diamond on the other hand has a very high Debye temperature and most of its heat-carrying phonons are at frequencies much higher than are present in bismuth.
The largest phonon mediated thermal conductance measured to date is between TiN (Titanium Nitride) and MgO.
While there are no free electrons to enhance the thermal conductance of the interface, the similar physical properties of the two crystals facilitate a very efficient phonon transmission between the two materials.
