Koichiro Suekuni (Assistant Professor) and Mikio Koyano (Associate Professor) of the School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Michihiro Ohta (Researcher) and Atsushi Yamamoto (Leader) of Thermoelectric Conversion Group, Energy Technology Research Institute, the National Institute of Advanced Industrial Science and Technology (AIST), and Eiji Nishibori (Director) of RIKEN RSC-Rigaku Collaboration Center, RIKEN SPring-8 Center, RIKEN, have confirmed that a naturally occurring mineral, tetrahedrite^*2, which mainly consists of non-toxic and earth-abundant elements, copper (Cu) and sulfur (S), exhibits high thermoelectric performance at approximately 400 ℃. They also clarified that the high performance is attributed to its extremely low lattice thermal conductivity caused by the complex crystal structure of tetrahedrite and the vibration of Cu atoms with anomalously large amplitude. These efforts will significantly contribute to realizing environmentally friendly thermoelectric power generation^*1 using materials composed of non-toxic and earth abundant elements.

Thermoelectric power generation is a technology that can convert thermal energy (in the form of a temperature difference) into electrical energy using solid devices. With the recent growing interest in energy problems, thermoelectric power generation has been attracting attention as a possible means of effectively utilizing the huge amount of unused waste heat. In particular, it is desired to recover and use intermediate-temperature (300-500 ℃) waste heat from automobiles and plants. Promising thermoelectric materials for the temperature range consists of toxic elements such as lead (Pb) and this fact is an obstacle to practical use.

In 2011, the research group of JAIST started searching for thermoelectric materials containing Cu and S, which are non-toxic and earth-abundant elements. About a year ago, the group artificially synthesized a material with a composition almost identical to that of tetrahedrite, a naturally occurring sulfide mineral, and reported its relatively high thermoelectric performance at room temperature in an international journal published by the Japan Society of Applied Physics. The scientists of JAIST, in cooperation with those from AIST with remarkable achievements in the research on thermoelectric sulfides, continued the research and found that nickel(Ni)-substituted Cu₁₂Sb₄S₁₃, Cu_12-xNi_xSb₄S₁₃, exhibits a high dimensionless thermoelectric figure of merit^*3 of ZT=0.7 (equivalent to conversion efficiency of 7 %) at approximately 400 ℃. This value is the highest among p-type^*4 Pb-free sulfides so far. Tetrahedrite is now attracting worldwide attention as a promising thermoelectric material. At the end of 2012, a research group from Michigan State University (USA) also reported high thermoelectric performance in tetrahedrite Cu_12-xM_xSb₄S₁₃ (M=Zn:zinc or Fe:iron).

The high performance of tetrahedrite is caused by its extremely low lattice thermal conductivity^*5, which is approximately half that of silica glass. A powder X-ray diffraction experiment was carried out using synchrotron radiation at SPring-8, which has been used to analyze the structural properties of various thermoelectric materials, to investigate the crystal structure of the Ni-substituted tetrahedrite and the vibrational properties of its atoms in detail. The results indicate that the Cu atom on the CuS₃ triangular plane vibrates with low frequency and large amplitude in the direction perpendicular to the triangle plane. From this finding, the vibration of Cu atoms with anomalously large amplitude is considered to inhibit heat transfer through the hard Cu-Ni-Sb-S network, resulting in the low thermal conductivity.

In this study, the sulfide mineral tetrahedrite, a well-known mineral resource, was found to be a promising thermoelectric material. Clarification of the characteristics of the crystal structure for the mineral resulting in its low thermal conductivity will lead to the development of high-performance and environmentally friendly thermoelectric sulfide minerals.

The research results were published online in Journal of Applied Physics on January 28, 2013.

This study was supported by a Grant-in-Aid for Research Activity Start-up and a Grant-in-Aid for Scientific Research by Japan Society for the Promotion of Science and by a research grant from the Thermal and Electric Energy Technology Foundation.

Figure 1 : Natural tetrahedrite (Cu, Fe, Ag, Zn)12Sb4S13 (sample owned by Geological Museum, AIST)(left) and dimensionless thermoelectric figure of merit, ZT, for the tetrahedrite Cu12-xNixSb4S13 and various p-type Pb-free sulfides(right)

Figure 2 : Part of crystal structure of tetrahedrite Cu atoms vibrate with large amplitude in a direction perpendicular to the CuS3 triangle plane and toward the Sb atom.

*1 Thermoelectric power generation

Metal or semiconductor materials create voltage when there is a temperature difference between both sides of the specimen. The generated voltage is proportional to the temperature difference. This phenomenon is known as the Seebeck effect and power generation using the effect is called thermoelectric power generation. When one end of a material touch with a heat source and the other end is kept at a certain temperature (e.g., a room-temperature or cooled bath), the material generates power.  [ back ]

*2 Tetrahedrite

Tetrahedrite is a naturally occurring mineral that generally has a composition of (Cu, Fe, Ag, Zn)₁₂Sb₄S₁₃. It has a cubic crystal structure with 58 atoms in a unit cell. The crystal structure is complex because the atoms form CuS₄ tetrahedra, CuS₃ triangles, and SbS₃ trigonal pyramids.  [ back ]

*3 Dimensionless thermoelectric figure of merit (ZT)

The dimensionless thermoelectric figure of merit ZT of a material is defined as ZT=S²Tρ^-1(κ_el+κ_L)^-1, where T (K) is the absolute temperature, ρ (Ω m) the electrical resistivity, S (V K^-1) the Seebeck coefficient, κ_el (W K^-1 m^-1) the electronic thermal conductivity, and κ_L (W K^-1 m^-1) the lattice thermal conductivity. High-performance thermoelectric energy conversion requires that materials have a low ρ, a high S, and a low κ_el and κ_L. A low ρ and high S of a thermoelectric material correspond to a low internal resistance and high output voltage of battery, respectively. A low κ is required to maintain the temperature difference, which generates a voltage.  [ back ]

*4 p-type (n-type)

p-type and n-type are terms generally used to indicate the type of carriers for charge transfer in semiconductors. In p- and n-type semiconductors, holes and electrons are charge carriers, respectively. In this report, materials with a positive Seebeck coefficient are called p-type materials.  [ back ]

*5 Lattice thermal conductivity<

Thermal conductivity is a physical quantity indicates the ease of heat transfer through a material. In general, heat is transferred by charge carriers and vibrations of crystal lattices. The component of the thermal conductivity attributed to the vibrations is called the lattice thermal conductivity.  [ back ]