KOYANO Laboratory
<Major Research Areas>Solid state physics, thermoelectric conversion, nano-composites, low-dimensional conductor physics

Energy Conversion in Nano-space
- Thermoelectric Conversion and
Low-dimensional Properties -

•The Seebeck effect, which is the basis of thermoelectric conversion technology: velocity distribution in conduction electron changes by thermal difference on thermoelectric material

•Thermoelectric conversion materials consisting of layered sulfide: single crystals and an element for measurement

•Magnetic nanoparticles dispersed in conducting oxide: Tunnel conduction of electrons is observed through oxide layers on the surface of the ferromagnetic nanoparticles.

Research activity

　　Thermoelectric conversion technology has been attracting a great deal of attention as a means to solve current energy problems. This technology enables direct alternative conversion between poor-quality thermal energy and high-quality electric energy. We are identifying various properties of thermoelectric materials and their relevant compounds and developing new nanocomposites using advanced equipment and innovative methods.

1Development of new energy conversion materials and thermoelectric conversion physics

　　In specific types of semiconductors (thermoelectric materials), the conduction electron velocity distribution changes due to thermal difference, and voltage occurs at both ends of the specimen. This is a phenomenon called the Seebeck effect, which is used for basic thermoelectric conversion technology. If the electrical resistivity of thermoelectric material can be kept low, a great quantity of electric power can be obtained owing to this effect. This is the principle of thermoelectric power generation, which is expected to become next generation technology for reuse of waste heat. On the other hand, when direct current is applied to thermoelectric materials, a thermal difference occurs at both ends of the material due to the Pertier effect. This phenomenon, called thermoelectric cooling (electronic cooling), is used in a wide range of areas, such as chlorofluorocarbon-free refrigerators and the temperature stabilization of laser diodes for optical communication.
　　Our laboratory has developed new thermoelectric materials with layered structures, and investigated energy conversion mechanisms in thermoelectric phenomena. For example, we investigated in detail the thermoelectric properties of a layered compound of Tantalum disulfide (TaS2), and through experiments proved for the first time that thermoelectric efficiency is enhanced by Coulomb repulsion or electron correlation of conduction electrons in the low-dimensional conductor.
　　We have recently been conducting further research on these materials, and developing Bi-based low-temperature thermoelectric materials and point-contact equipment for experiments on the thermoelectric energy conversion process in nano-space, using various innovative measuring methods that we developed at JAIST.

2Development of new nano-composites and investigation of their properties

　　A number of low-dimensional compounds that have layered (two-dimensional) or needle (one-dimensional) structures and show interesting properties are present in nature. We are examining the electric properties of such materials with a “nano structure produced in nature” to develop new functions.
　　Although these compounds do not have magnetism themselves, the fact that they acquire magnetic and electric properties by being compounded with magnetic materials is interesting. We are producing new magnetic materials by applying various technologies to study the ferromagnetic or antiferromagnetic interaction in the magnetic moments and galvanomagnetic effects in the nano field.

Equipment

Helicon sputtering equipment, electric furnace, X-ray diffractmeter, transmission electron microscope (TEM), SQUID magnetometer, low-energy Raman spectrometer, physical property measurement system (PPMS), thermal transport option (TTO), galvanomagnetic effect measuring system, liquid helium

The main research achievements in the past five years

1. M. Koyano, D. Kito, K. Sakai and T. Ariga, Synthesis and Electronic Properties of Thermoelectric and Magnetic Nanoparticle Composite Materials, J. Electronic Materials, DOI: 10.1007/s11664-011-1545-9 (2011).
2. K. Suekuni, K. Tsuruta, T. Ariga and M. Koyano, Variable-Range-Hopping Conduction and Low Thermal Conductivity in Chalcogenide Spinel C_yFe₄Sn₁₂X₃₂ (X = S, Se) (in press).
3. T. Ariga, M. Koyano and A. Ishida, Thermomagnetic Effects in High-mobility PbTe Films, J. Electronic Materials, DOI: 10.1007/s11664-011-1512-5(2011).
4. M. Koyano and N. Akashi, Measurement of local Peltier constant at a micro contact, J. Electron, Mater., 37, 1037-1040 (2009).
5. K. Suga, A. Ohnishi, M. Koyano, M. Sasaki, and K. Kindo, Magnetic-field-induced quantum oscillation in η-Mo₄ O₁₁ , J. Phys. Soc. Jpn., 77, 074605 (2008).