Mikio Koyano

Photoluminescent and Structural Properties of Precipitated ZnO Fine Particles

The correlation between photoluminescence (PL) and structural properties of ZnO crystals has been investigated using fine particles prepared by a precipitation method and subsequent heat treatment. The ZnO particles heat-treated at temperatures less than 600°C exhibited red emission centered at 1.9 eV for a weak light excitation similar to that observed in ZnO thin films sputter-grown under thermally non-equilibrium conditions. Heat treatment at high temperatures reduced the red PL while it enhanced the green PL around 2.45 eV. Raman scattering and X-ray diffraction measurements showed that the ZnO particles obtained at lower temperatures have a greater lattice disorder accompanied by a specific lattice expansion along the c-axis of the ZnO crystal which would introduce defect states in the band gap. In addition, electrical conductivity measurements suggested that the lattice disorder is associated with interstitial Zn. These results indicate that the lattice disorder along the c-axis of the ZnO crystal, i.e., the interstitial Zn is most likely responsible for the red PL observed for both ZnO particles and thin films. On the other hand, recombination centers such as singly charged oxygen vacancies contributing to the green emission may be too low in concentration to affect the structural quality.

High-performance thermoelectric mineral Cu12−xNixSb4S13 tetrahedrite

X-ray structural analysis and high-temperature thermoelectric properties measurements are performed on polycrystalline samples of artificial mineral Cu12−xNixSb4S13 tetrahedrite. Analysis of the atomic displacement parameter manifests low-energy vibration of Cu(2) out of CuS3 triangle plane. The vibration results in low lattice thermal conductivity of less than 0.5 W K−1 m−1. By tuning of the Ni composition x and decrease of electronic thermal conductivity, dimensionless thermoelectric figure of merit for x = 1.5 achieves 0.7 at 665 K, which is a considerably high value among p-type Pb-free sulfides. Because the tetrahedrite is an environmentally friendly material, it constitutes a good thermoelectric material for use in support of a sustainable society.

Thermoelectric Properties of Mineral Tetrahedrites Cu10Tr2Sb4S13 with Low Thermal Conductivity

We have investigated thermoelectric properties of synthesized mineral Cu10Tr2Sb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn) tetrahedrites, which have a cubic and complex crystal structure. The mother phase Tr = Cu shows metal–semiconductor transition and anomalous hysteresis. Through various Tr substitutions, the thermopower was increased and thermal conductivity was decreased. Results show that Tr = Ni had the largest dimensionless figure of merit Z T of 0.15 at 340 K. The main advantage for the large Z T is the quite low lattice thermal conductivity. Because of the large Z T and the environmentally friendly components, tetrahedrites are anticipated as a good thermoelectric material.

Photoluminescence and Raman spectra of ZnO thin films by charged liquid cluster beam technique

Zinc oxide (ZnO) thin films have been deposited on glass substrates from isopropyl solutions of zinc acetate by using the Charged Liquid Cluster Beam (CLCB) technique. The structure of the films is determined by the temperature of the substrate. Raman scattering spectra of the films obtained by non-resonant excitation are similar to those of bulk polycrystalline ZnO. The photoluminescence (PL) spectra of the films exhibit at 10 K four broad emission bands in the visible and near infrared spectral range. The effect of post-deposition annealing on the red emission band is also reported.

Electrical resistivity and photoluminescence spectrum of layered oxysulfide (LaO)CuS

Abstract On growth conditions the dependence of the electrical resistivity and the photoluminescence (PL) spectrum of a layered oxysulfide (LaO)CuS have been investigated. The electrical resistivity shows semiconducting behavior and its magnitude decreases with the increase in the sintering temperature and time, which is considered to introduce structural defects such as Cu or La vacancies. The PL spectrum consists of six emission bands which are assigned to a direct interband transition and the transitions originating in two kinds of donor and acceptor levels corresponding to defect centers in the band gap. The PL spectra depend on the growth conditions. The introduction of lattice imperfection increases the intensity of the wide emission bands, and the (LaO)CuS sample visually appears to be ‘white’ under UV excitation. The white luminescence is an important property for the application to display back-light.

Systematic study of electronic and magnetic properties for Cu12–xTMxSb4S13 (TM = Mn, Fe, Co, Ni, and Zn) tetrahedrite

Substitution effects of 3d transition metal (TM) impurities on electronic and magnetic properties for Cu12Sb4S13 tetrahedrite are investigated by the combination of low-temperature experiments and first-principles electronic-structure calculations. The electrical resistivity for the cubic phase of Cu12Sb4S13 exhibits metallic behavior due to an electron-deficient character of the compound. Whereas that for 0.5 ≤ x ≤ 2.0 of Cu12−xNixSb4S13 exhibits semiconducting behavior. The substituted Ni for Cu is in the divalent ionic state with a spin magnetic moment and creates impurity bands just above the Fermi level at the top of the valence band. Therefore, the semiconducting behavior of the electrical resistivity is attributed to the thermal excitation of electrons from the valence band to the impurity band. The substitution effect of TM on the electronic structure and the valency of TM for Cu11.0TM1.0Sb4S13 are systematically studied by the calculation. The substituted Mn, Fe, and Co for Cu are found to be in ...

Evaluation of the Figure of Merit on Thermoelectric Materials by Harman Method

The figure of merit ZT of thermoelectric materials has been evaluated by the Harman method in the temperature region below room temperature. In this method only resistance measurements by both dc and ac methods are required to obtain the ZT values. ZT is given by ZT=(Rdc/Rac-1)/x, where Rdc, Rac and x are the resistance value by the ac and dc methods and the rate of the heat to the heat bath, respectively. The heat effect is experimentally confirmed to be negligibly small and we can use x=1 which corresponds to a sufficient adiabatic condition. Because an ambiguity due to experimental errors such as the length between the measurement terminals in the resistivity and the thermal conductivity measurements is removed, ZT can be determined very simply and precisely by the Harman method.

Low-field magnetization and AC magnetic susceptibility of spin- and cluster-glasses of itinerant magnet FexTiS2

Magnetizations and ac magnetic susceptibilities at low magnetic fields using a SQUID magnetometer have been measured for spin-glass (SG; x =0.1 and 0.2) and cluster-glass (CG; x =1/4 and 1/3) phases of Fe x TiS 2 over the frequency range 0.5–100 Hz and temperature range 4.2–80 K. The frequency and temperature variations of the ac magnetic susceptibilities have different behaviors for SG and CG; in particular, the estimated distribution function of relaxation time g (ω, T ) for x =1/4 has a double-peaked shape. The dimensionality and criticality of the CG phases are also discussed. Monte Calro simulations using a site-percolation model with a two-dimensional triangular lattice are made by taking into account the 1st, 2nd, and 3rd nearest neighbors to obtain the percolation probability P .

Evaluation of the Figure of Merit of Thermoelectric Modules by Harman Method

The equipment for the evaluation of the figure of merit ZT has been fabricated and ZT values have been estimated for thermoelectric devices and modules which were simultaneously prepared using the same materials. It is pointed out that the thermoelectric loss from an electrode and a solder as the binder between the p- and n-type devices cannot be neglected in the module and is easily and quantitatively estimated by the Harman method. The loss of ZT is approximately 7% in the present module and a possible improvement of the figure of merit in the module is proposed.

Electronic properties of intercalation compound FexTiS2

Mesure des coefficients de Hall et des pouvoirs thermoelectrique entre 1,5 et 300 K, pour X≤1/3