With the aim of characterizing the thermal conductivity for nanometer-scale thermoelectric materials, we have constructed a new measurement system based on ac calorimetry. Analysis of the obtained data requires time-evolution of temperature distribution in nanometer-scale material under periodic heating. In this study, we made a simulation using a C#-program for time-dependent temperature distribution, based on 2-dimensional heat-diffusion equation including the influence of heat emission from material edges. The simulation was applied to AlN with millimeter-scale dimensions for confirming the validity and accuracy. The simulated thermal diffusivity for 10×75-mm2-area AlN was 1.3×10-4 m2/s, which was larger than the value set in the heat-diffusion equation. This overestimation was also observed in the experiment. Therefore, our simulation can reproduce the unsteady heat conduction and be used for analyzing the ac calorimetry experiment.

In order to optimize the performance of thermoelectric devices, we have fabricated and characterized the micrometer-scaled Si thermopile preserving the phonon-drag effect, where the Si thermopile consists of p- and n-type Si wire pairs. The measured Seebeck coefficient of the p-type Si wire was found to be higher than the theoretical value calculated only from the carrier transport, which indicates the contribution of phonon-drag part. Moreover, the measured Seebeck coefficient increased with increasing the width of Si wire. This fact is considered due to dependency of phonon-drag part on the wire width originating from the reduction of phonon-boundary scattering. These contributions were observed also in measured output voltage of Si-wire thermopile. Hence, the output voltage of Si-wire thermopile is expected can be enhanced by utilizing the phonon-drag effect in Si wire by optimizing its size and carrier concentration.

The Seebeck coefficient of Si wire co-doped with P and Ga atoms is investigated for applying thermoelectric devices. The observed Seebeck coefficient is closed to the theoretical values of electronic part of Seebeck coefficient due to the electronic transport. From the estimation of phonon scattering processes, it is found that the phonon-drag contribution to the Seebeck coefficient in co-doped Si wire is mainly governed by the phonon-boundary scattering.

The phonon-drag contribution to the Seebeck coefficient (Sph) for p-type Si, Ge and Si1-xGex is investigated for thermoelectric applications. The Sph in Si and Ge is found to mainly determined by the phonon velocity, phonon mean free path and carrier mobility associated with acoustic deformation potential scattering. Moreover, the Sph in Si1-xGex is predictable by the above-mentioned material parameters interpolated with those in Si and Ge.

We herein investigate the operation stability of the single-electron-pump (SEP) refrigerator with respect to thermal and dimensional fluctuations. The SEP refrigerator was found to successfully demonstrate single-electron extraction and injection at temperatures up to 2 K. Although the dimensional fluctuation in junction capacitance will seriously affect operation, the effect of the gate capacitance fluctuation is unlikely to be severe.