Masayoshi Uno

Ag9TlTe5: A high-performance thermoelectric bulk material with extremely low thermal conductivity

We studied a high-performance thermoelectric material whose chemical formula is Ag9TlTe5. Ag9TlTe5 is simple and easy to prepare. Its highest dimensionless figure of merit (ZT) value is 1.23, obtained at 700K. The values of individual thermoelectric properties at 700K are 2.63×10−4Ωm for electrical resistivity, 319μVK−1 for Seebeck coefficient, and 0.22Wm−1K−1 for thermal conductivity. Ag9TlTe5 is a unique material combining extremely low thermal conductivity and relatively low electrical resistivity.

Thermoelectric properties of CoSb3

Abstract The typical skutterudite structure compound CoSb 3 was prepared by arc melting followed by sintering. The samples were characterized by powder X-ray diffraction method, electron probe microanalysis (EPMA), and the thermoelectric properties such as the thermal diffusivity, electrical resistivity, and Seebeck coefficient were measured in the temperature range from room temperature to about 750 K. The thermal conductivity of CoSb 3 was estimated from the heat capacity, the experimental density, and the thermal diffusivity measured by the laser flash method. The calculated dimensionless figure of merit, ZT of CoSb 3 was lower than that of state of the art thermoelectric materials. In order to enhance the ZT , it was attempted to reduce the lattice thermal conductivity of CoSb 3 . The electronic contribution to the thermal conductivity was estimated by Wiedemann–Franz law, and the lattice thermal conductivity was determined. It was found that the lattice thermal conductivity of CoSb 3 can be decreased, and ZT of CoSb 3 can potentially be enhanced.

Molecular dynamics study of mixed oxide fuel

Abstract In order to develop new techniques to calculate the physicochemical properties of MOX fuel, molecular dynamics methods were applied to UO2, PUO2, and (U,Pu)O2. These methods enabled us to obtain the heat capacity and thermal conductivity from basic properties, viz., the lattice parameter, linear thermal expansion coefficient, and compressibility. Results for UO2 showed both the existence of a Bredig transition and a peak in the heat capacity at high temperature. The lattice parameter, heat capacity, and thermal conductivity of MOX fuel were calculated from basic properties of UO2 and PuO2. These results showed that molecular dynamics techniques can be usefully applied to determine physicochemical properties of MOX fuel.

Thermal properties of zirconium hydride

Abstract Zirconium hydride specimens with hydrogen contents of 1.45–1.70 H/Zr were fabricated directly from zirconium metal in a modified UHV Sieverts apparatus. All the zirconium hydride fabricated in the present study were found from X-ray diffraction analysis to be CaF2-type δ-ZrH2−x. At temperatures of 350–700 K, the heat capacity of the zirconium hydride was measured using an enthalpy method by means of a differential scanning calorimeter (DSC), and the experimental results were consistent with the values estimated from the sound velocities and the thermal expansion coefficient. In the temperature range of 300–700 K, the thermal diffusivity of the zirconium hydride was examined by a laser flash method. The thermal diffusivity decreased with increasing temperature and was not markedly influenced by the hydrogen content. The thermal conductivity of the zirconium hydride was calculated from thermal diffusivity and heat capacity and found to have slightly lower thermal conductivity than pure zirconium metal.

Characteristics of zirconium hydride and deuteride

The electrical and thermal properties of zirconium hydride and deuteride have been measured. The lattice parameter of δZrD2−x was smaller than that of δZrH2−x, and the deuteride had higher elastic moduli than the hydride. The electrical and thermal conductivities of δZrH2−x were slightly different from pure Zr metal. The electronic structure of the zirconium hydride was found from XPS measurements to differ from those in the pure Zr metal, and there was a peak due to the Zr–H bond at 6.4 eV below Fermi energy in the XPS spectra. The density of states was estimated by a molecular orbital calculation, and the agreement between the XPS valence band measurements and the calculations was quite satisfactory. Some of the mechanical and thermal properties of zirconium hydride and deuteride were interpreted in terms of the results of the molecular orbital calculation.

Thermoelectric properties of NaCo2O4

Abstract The thermoelectric properties such as the thermal conductivity, electrical resistivity and Seebeck coefficient of NaCo 2 O 4 and NaCo 1.9 M 0.1 O 4 (M=Ti, Rh, Pd) were evaluated in the temperature range from room temperature to 723 K. Polycrystalline samples were prepared by sintering in air followed by hot pressing. The thermal conductivity was calculated from the heat capacity, the experimental density, and the thermal diffusivity measured by the laser flash method. The electrical resistivity and Seebeck coefficient were measured simultaneously using ULVAC ZEM-1 in He atmosphere. The dimensionless figure of merit, ZT , of NaCo 1.9 Pd 0.1 O 4 was higher than that of NaCo 2 O 4 over a wide range of temperatures, and reached ZT =0.045 at 723 K.

Study of the thermodynamic properties of (U, Ce)O2

Abstract The X-ray diffraction analysis of (U, CO)2 with the CeO2 contents ranging from 0 to 20 mol% CeO2 was performed at room temperature to obtain the variation in the lattice parameter with the CeO2 content. Ultrasonic pulse echo measurements were also carried out to estimate the change in the mechanical properties of (U, Ce)O2 with the CeO2 content. The lattice parameter of (U, Ce)O2 was found to decrease with increasing CeO2 content. The variation in the lattice parameter with the CeO2 content closely followed the Vegard law. The shear and longitudinal velocities in (U, Ce)O2 were found to decrease with increasing CeO2 content. The Youngs and shear moduli, and Poissons ratio estimated from the wave velocities decreased with the CeO2 content. No mechanical property showed anomaly in low CeO2 content region.

Thermoelectric properties of BaUO3

The thermoelectric properties of BaUO3 were evaluated in the temperature range from room temperature to about 1000 K. In order to clarify the phonon glass property of BaUO3, the heat conduction mechanism were also studied. The electrical resistivity was higher by approximately four orders of magnitude than that of the state-of-the-art thermoelectric materials. The Seebeck coefficient was negative in the whole temperature range, and the maximum absolute value bore comparison with the currently used thermoelectric materials. The thermal conductivity was extremely low and like glasses in spite of its simple crystal structure, indicating that BaUO3 has a phonon glass property. The heat conduction of BaUO3 was mainly composed by the phonon contribution at low temperatures and the excitonic contribution at high temperatures. The dimensionless figure of merit ZT was extremely lower than that of the state-of-the-art thermoelectric materials. In order to utilize BaUO3 as an actual thermoelectric module, optimization of the electrical properties are required.

On the zirconium-oxygen-hydrogen ternary system

Abstract Thermodynamic studies on the Zr–O–H ternary system have been carried out, based on the hydrogen solubility for αZr(O), βZr(O) and ZrO2. The dissolved oxygen in zirconium strongly affected both the hydrogen solubility and the phase diagram. The effect of the interstitial oxygen on the hydrogen solubility was discussed on the basis of the partial molar quantities of hydrogen in the Zr–O–H ternary solid solutions. The hydrogen dissolution into the ZrO2 was studied in O2/H2O atmosphere using a thermal desorption method. The hydrogen solubility in the ZrO2 was evaluated from the thermal desorption spectra to range from 10−5 to 10−4 mol H/mol oxide and decreased with increasing temperature.

Some properties of a lead vanado-iodoapatite Pb10(VO4)6I2

A lead vanado-iodoapatite Pb 10 (VO 4 ) 6 I 2 was synthesized as a potential waste form to immobilize radioactive iodine and some thermal, mechanical and chemical properties were measured. Thermogravimetry-differential thermal analysis (TG-DTA) showed that the apatite was stable up to about 800 K. The thermal conductivity of the hot-pressed sample, with the theoretical density of 82%, increased gradually with increasing temperature from 0.65 W/m.K at room temperature to 0.78 W/m.K at 523 K. The micro-Vickers hardness and Youngs modulus were lower than those of a typical glass waste form. The leaching rate of iodine for apatite was two orders of magnitude higher than that of an AgI glass waste form. Despite the high leaching rate (compared to AgI embedded in glass), the high chemical stability up to 800 K and acceptable mechanical properties of this apatite suggest that it will be a good waste form when embedded in a suitable matrix material.