Temporal distribution T(t) of rapid heating within a stack of isotopically purified silicon multilayer (e. g. isotope 28 (light gray) and 30 (dark gray)) covered by a thin gold transducer layer. The speed of heat flow into the substrate allows deducing the cross-plane thermal conductivity and thermal resistance at the gold-silicon interface [3].

X-rays shed light on isotopically engineered multilayers for tailoring thermoelectric efficiency

The role of interfaces in the conduction of heat in multilayered samples has been elucidated by a cooperation between scientists from U. Münster, U. Giessen, U Essen, the Institute for Photonics and Synchrotron Radiation (IPS), and the synchrotron facilities ANKA and ESRF. Heat conductivity in surface-near regions is probed by the cooling time of a rapidly heated transducer film of gold on top of the investigated material. The cooling kinetics can only be understood by taking into account the role of thermal barriers across the interfaces [1].
A structural interplay between the Au layer and the underlying Si layers was found to determine the thermal resistance at the Au/Si interface [2]. Meanwhile the interfaces between the Si isotope layers are crystallographically perfect, thus only acoustic impedances contribute to heat conductivity. Stacking a number of these interfaces leads to a surprisingly strong suppression of the thermal conductivity [3]. This may open ways to achieve a better control on thermal properties for thermoelectric applications.



Temporal distribution T(t) of rapid heating within a stack of isotopically purified silicon multilayer (e. g. isotope 28 (light gray) and 30 (dark gray)) covered by a thin gold transducer layer. The speed of heat flow into the substrate allows deducing the cross-plane thermal conductivity and thermal resistance at the gold-silicon interface [3].

References:
1. H. Bracht, N. Wehmeier, S. Eon, A. Plech , D. Issenmann , J. Lundsgaard Hansen, A. Nylandsted Larsen, J.W. Ager III, E.E. Haller, Reduced thermal conductivity of isotopically modulated silicon multilayer structures, Appl. Phys. Lett. 101 (2012) 064103.
2. D. Issenmann , S. Eon, N. Wehmeier, H. Bracht, G. Buth, S. Ibrahimkutty, A. Plech , Determination of nanoscale heat conductivity by time-resolved x-ray scattering, Thin Solid Films 541 (2013) 28.
3. H. Bracht, S. Eon, R. Frieling, A. Plech , D. Issenmann , D. Wolf, J. Lundsgaard Hansen, A. Nylandsted Larsen, J.W. Ager III, and E.E. Haller: Thermal conductivity of isotopically controlled silicon nanostructures, New J. Physics 16 (2014) 015021.