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Interfacial thermal resistance, also known as thermal boundary resistance, or Kapitza resistance, is a measure of resistance to thermal flow at the interface between two materials. While these terms may be used interchangeably, Kapitza resistance technically refers to an atomically perfect, flat interface whereas thermal boundary resistance is a more broad term. This thermal resistance differs from contact resistance because it exists even at atomically perfect interfaces. Owing to differences in electronic and vibrational properties in different materials, when an energy carrier attempts to traverse the interface, it will scatter at the interface. The probability of transmission after scattering will depend on the available energy states on side 1 and side 2 of the interface.
Assuming a constant thermal flux is applied across an interface, this interfacial thermal resistance will lead to a finite temperature discontinuity at the interface. From an extension of Fourier's law, we can write
Q = Δ T R = G Δ T {\displaystyle Q={\frac {\Delta T}{R}}=G\Delta T}
where Q {\displaystyle Q} is the applied flux, Δ T {\displaystyle \Delta T} is the observed temperature drop, R {\displaystyle R} is the thermal boundary resistance, and G {\displaystyle G} is its inverse, or thermal boundary conductance.