In this decade, several notable developments have been brought in the field of thermoelectric energy conversion. Firstly, several new materials having thermoelectric figure of merit exceeding that of state-of-the-art thermoelectric materials have been found. Secondly, several general theoretical approaches have clarified electronic structures, which good thermoelectric materials should have. Namely, they suggest that materials having narrower differential conductivity offer larger thermoelectric figure of merit. It is truly interesting that recent interesting topics, such as high-Tc superconductivity and giant magnetoresistance achieved in Manganite, are found among materials having narrow band width, where electron-electron correlation is essential. Low dimensional carrier systems achieved in semiconductor heterostructures are expected to be another possible good thermoelectric materials, since they are expected to present narrower differential conductivity. However, careful theoretical examination of such systems have not been made. In this paper, recent progress on thermoelectric theory has been reviewed. Furthermore, thermoelectric figure of merit of low dimensional systems will be discussed based on numerical simulation of thermoelectric properties of these systems.
Thin films of Bi2Te3 were grown on mica (001) substrates by hot wall epitaxy (HWE). Reflection high energy electron diffraction (RHEED) patterns of deposited films indicated that the film condition depends on the substrate temperature in the film growth process, and the epitaxial Bi2Te3 single crystal films were grown with two-steps substrate temperature control. The X-ray diffraction (XRD) pattern indicated that the Bi2Te3(0001) single crystal film was grown on the mica substrate with c-axis oriented perpendicular to the substrate plane. The microstructure of the Bi2Te3 film was examined by scanning electron microscope (SEM), and the layered structure was observed. The deviation from stoichiometric composition causes the change in the conduction character, the film with an excess of tellurium is n-type and with an excess of bismuth is p-type. Hall concentrations for n- and p-type films were 2.4×1025/m3 and 3.1×1025/m3 respectively: The Seebeck coefficient α and the power factor α2/ρ for n-type film were -237μV/K and 2.6×10-3W/(K2m) at 293K, and those for p-type, 141μV/K and 0.55×10-3W/(K2m).
Silicon carbide (SiC) is a wide gap semiconductor which has enough mechanical strength and chemical stability at a high temperature region. These properties are favorable to use as high temperature thermoelectric conversion materials. Several SiC based materials were fabricated in our previous works. In this study, Fe was selected as an additive element to reduce the electrical resistivity. Electrical resistivity and Seebeck coefficient were measured from room temperature to around 750°C. Thermal conductivity was measured from room temperature to around 300°C. By addition of Fe, electrical resistivity decreased drastically and they showed n-type semiconductor properties. The electrical resistivity decreased to 1.8×10-4Ωm for the Fe 10.0wt.% doped sample at 750°C. The maximum value of figure of merit Z was estimated about 1.0×10-5K-1 around at 750°C.
The thermal conductivity κ and the electric properties of Si1-xGex (x≤0.03) samples doped with Group 3B and 5B elements which were prepared by arc melting were measured as functions of carrier (or dopant) concentration n and of temperature in the range from 298K to 1273K. Consequently it was found that the combination of alloying with 3at% Ge and doping with 0.5at% P or 0.3at% B decreases the values of κ to as low as about 12W/m·K at 298K. The electrical resistivities ρ of doped Si and Si0.97Ge0.03 samples decrease linearly with increase of n. However the Seebeck coefficients S of doped Si have a local maximum or a hump at a concentration of about 3×1019cm-3, while no such maxima appeared in doped Si0.97Ge0.03 although its S values are considerably higher than those obtained by Dismukes et al, for Si0.7Ge0.3 alloys over the whole concentration range from 1018cm-3 up to 1021cm-3. At 1073K, the maximum values of the figure of merit ZT were 0.62 for n-type Si0.97Ge0.03 doped to 1.8×1020cm-3 electrons and 0.47 for p-type Si0.97Ge0.03 doped to 1.7×1020cm-3 holes, so that they attained to about 70% of those obtained by Dismukes et al. for Si0.7Ge0.3 alloys.
Effects of doping on the thermal conductivity of CoSb3 have been investigated using Ni, Pd and Pt as donor impurities. The lattice thermal conductivity decreases with increasing carrier concentration, almost independently of these impurities. The lattice thermal conductivity as low as 0.04 Wcm-1K-1 can be achieved by heavy doping (the carrier concentration range is more than 1020cm-3). The comparison of results to theoretical calculations indicates that the coupling of point-defect (alloy) scattering with electron-phonon scattering plays an important role in reducing the lattice thermal conductivity. However, the degrees of these two scattering effects for Ni doping are essentially different from those for Pd or Pt doping due to the impurity dependence of material properties (atomic mass, atomic size, deformation potential, and carrier effective mass).
Zn2-xMnxSiO4 green phosphors have been prepared by the solid state reaction, and photoluminescence, color purity and decay time were investigated as a function of both the firing conditions and the activator concentrations (X=0.005-0.12) using MnCO3, MnO, MnSO4 and MnC2O4 as activators. For the phosphors doped with various activator compounds, there were no distinct differences in the color purity, FWHM values of emission peaks and the color coordinate in the CIE chromaticity diagram. However, Manganese concentration affected greatly the photoluminescence and the decay time. The decay time decreased from 30ms to 6ms as Mn concentration increased from X=0.005 to X=0.12. The decay time was measured at about 9ms for the phosphors doped with X=0.08 showing the highest luminance.