Heat energy spontaneously goes from higher temperature to lower temperature. However, recently, a team from China and the USA (Kezhao Xiong and Zonghua Liu from East China Normal University, Chunhua Zeng from Kunming University of Science and Technology, and Baowen Li from University of Colorado Boulder) revealed that in some complex network structures, heat energy can transfer from a node with lower temperature to another node with higher temperature, which we call thermal siphon effect. The team even discovered that this effect becomes more evident with the decrease of network assortativity.
To understand this abnormal phenomenon (Figure a), the team studied power spectra of the nodes (Fig c and d), and the transport of the energy within the spectra range. They found that, within the power spectrum range, heat energy still transfers from (effective) high temperature node to (effective) low temperature node.
Moreover, an optimal network structure is discovered, that displays a small thermal conductance and a large electrical conductance simultaneously.
It is well known that realistic systems for heat management and control are not regular lattices but complex networks such as the thermal devices of nanotube and nanowire networks, whose topologies are fundamentally different from the cases of 1D and 2D lattices. In particular, the ideal materials for thermoelectric applications are phonon glass and electric crystal, namely good electric conduction and poor thermal conduction.
Therefore, the study may shed a new light on the search of good thermoelectric materials.
See the article:
Thermal siphon phenomenon and thermal/electric conduction in complex networks
Kezhao Xiong, Zonghua Liu, Chunhua Zeng, Baowen Li
Natl Sci Rev, 2019, doi: 10.1093/nsr/nwz128