Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation. It offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat. The conversion efficiency in thermoelectric technology is governed by the dimensionless thermoelectric figure of merit zT=S2T/r(kL+kc),  where S is the thermopower (Seebeck coefficient), r is the electrical resistivity, T is the absolute temperature, kL is the lattice thermal conductivity, and kc is the carrier thermal conductivity. The traditional high efficiency thermoelectric materials are solid crystalline semiconductor compounds.Good electronic properties are maintained by crystalline semiconductor structure while low lattice thermal conductivity is achieved by varies of methods and approaches to scatter heat transfer phonons. Laboratory results suggest that high zT values can be realized in several families of materials, such as caged compounds and nano-structured materials.　

Recently, a novel high efficiency thermoelectric material is identified and a new concept of phonon-liquid electron-crystal is proposedby scientists from Shanghai Institute of Ceramics, Chinese Academy of Science in collaboration with researchers from Brookhaven National Laboratory, the University of Michigan, and California Institute of Technology. They are published in the journal Nature Materials titled "Copper ion liquid-like thermoelectrics" with the authors of Liu Huili, Shi Xun, Chen Lidong, Xu Fangfang, Zhang Linlin, and Zhang Wenqing from Shanghai Institute of Ceramics, Chinese Academy of Sciences; Li Qiang from Brookhaven National Laboratory; Uher Citrad from the University of Michigan; Day Tristanand Snyder G. Jeffrey from California Institute of Technology.

Solid state materials like traditional glasses and crystals propagate heat through longitudinal and transverse waves. The reduction of lattice thermal conductivity in these traditional solid materials is limited to the value of a glass. However, a liquid does not propagate sheer vibrations. As a result, a liquid material should have less heat conductive. Semiconductor Cu2-xSe has a rigid face-centered cubic Se sublattice to provide a crystalline pathway for semiconducting electrons (or more precisely holes). The copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary ‘liquid-like’ behavior of copper ions around a crystalline sublattice of Se could eliminate some of the vibrational modes in Cu2-xSe as well as strongly scatter heat transferred phonons. As a result, intrinsically very low lattice thermal conductivity and high zT value of 1.5 in this otherwise simple semiconductor could be achieved. This is among the highest values for any bulk thermoelectric materials.　This unusual combination of properties leads to an ideal thermoelectric material within the new concept of ‘Phonon-liquid electron-crystal’. The results point out a new strategy and direction for high efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.

This research was partially funded by ‘Bairen program’ of Chinese Academy of Sciences, National Natural Science Foundation of China, Shanghai Science and Technology Commission, and CAS/SAFEA International Partnership Program for Creative Research Team.

Figure 1 Crystal structure of Cu2Se at high temperatures with cubic anti-fluorite structure. (a) Unit cell with only the 8c and 32f interstitial positions are shown with Cu atoms. (b) Projected plane representation of the crystal structure along the cubic direction.  (Image by Shi Xun et al.)