Heat transfer in solid/superfluid and in micro-nano junctions

J. Amrit, A. Ramière, L. Yu

Our research activities focus on micro/nanoscale thermal transfer and transport at interfaces and in MEMS structures at temperatures below 2K.

Interface between Silicon and solid 4He

The analysis of our measurements of the thermal boundary (Kapitza) resistance between a silicon crystal and pressurized superfluid Helium revealed that thermal transfer at the interface is dominated by resonant scattering of phonons. This mechanism results when the phonon thermal wavelengths (2-10nm) match surface roughnesses of the same order of magnitude. This investigation was extended above the solidification pressure of He and a preliminary measurement of the Kapitza resistance was conducted at the minimum point of the melting curve of He (0.778K and ~25 bars). The results display a sharp drop in the Kapitza resistance. This first order transition is due to a change in the He density (order parameter), which in the solid phase has phonons of transverse polarizations, thereby  facilitating coupling between transverse modes across both media. Our analysis shows that in order to fully explain the results based on the acoustic mismatch model, the extent of the critical cone within which phonons are transmitted across the interface must be spread out.

Thermal transport in Silicene: role of flexural modes

We extended our numerical studies using Monte Carlo method, of thermal transport in ribbons (Ramière et al., Nanoenergy Ltrs, 2013) to investigate phonon transport in junctions formed between suspended Silicene membranes, a structural configuration often met in thermoelectricity and applications involving graphene or silicene. Silicene is a novel 2D material (discovered in 2010) composed of Si atoms. In addition to longitudinal and in-plane transverse polarizations of phonons, the ability of Silicene to be deformed mechanically induces an out-of-plane flexural mode polarization (ZA) of phonons. Our measurements at 1K demonstrate the role of the ZA polarization. In addition, existence of a constriction thermal resistance is evidenced, with a dependence in (d/D) 1/2 where d is the junction width and D the membrane length (Ramière et al., accepted J. Phys.: Conf. Series, probably 2015). The influence of surface roughness is also taken into account. The average and frequency dependent transmission coefficients also are determined as a function of (d/D).

Junction of width d between two suspended Silicene membranes of sides D (left), Transmission of L modes for different widths (center), Transition in the Kapitza resistance at Si crystal/4He upon solidification (right).
Temperature evolution in the left membrane as a function of time. The temperature field depends on the magnitude of phonon scattering at the boundary and on the presence of the junction on the right side.

Thermal transport across 3D constrictions in Silicon

After Silicon ribbons, we will investigate thermal transport across 3D micro/nano-constrictions (master’s and doctoral research of L. Yu).