Norimitsu Murayama

Nano-structured thin-film Pt catalyst for thermoelectric hydrogen gas sensor

A hydrogen gas sensor using thermoelectric thin-film of nickel oxide with platinum catalyst thin-film on half of its surface Pt/NiO/alumina was fabricated and showed prominent selectivity for hydrogen gas. The platinum catalyst thin-film plays the most important role for its detection mechanism and its catalytic activity which depends both on surface morphology and thickness was investigated using Pt/Si. The surface morphology of platinum film deposited on Si by sputtering with low r.f. power showed that the small grains with the diameter around 20 nm agglomerated. The catalytic activities of the Pt films were increased with the film thickness and saturated above a thickness of 60 nm. By increasing the r.f. power, Pt films of the larger grains with the diameter around 40 nm were obtained and their catalytic activities were less than those of smaller grains. However, the catalytic performance of the Pt film with the thickness over 150 nm was unaffected by the grain size. In order to investigate the temperature dependence of the catalytic activity and the properties of the Pt/NiO/alumina sensor, a non-contacting in situ monitoring experimental-set equipped with an infra-red camera was employed in this study.

Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics

Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity.

Thermoelectric Performance of Magnetically c-Axis Aligned Ca-based Cobaltites

We report the preparation and thermoelectric properties of highly grain-oriented and densified [Ca2CoO3-δ]0.62CoO2 and [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 bulks. The grain-alignment and densification were performed by magnetic alignment and spark-plasma-sintering (SPS) methods, respectively. Both obtained samples with strong c-axis orientation (F>0.94) and high relative density (>98%) which showed highly conductive behaviors up to 1000 K, and their resistivities were almost half of those for samples processed only by SPS. Consequently, the power factors were enhanced up to 5.5×10-4 and 4.5×10-4 Wm-1K-2 for [Ca2CoO3-δ]0.62CoO2 and [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 at 1000K, respectively.

Thermoelectric Thick-Film Hydrogen Gas Sensor Operating at Room Temperature

A sensor of thick film NiO doped with alkali ions was fabricated and coated with Pt as the catalyst on half of its surface. When this sensor was exposed to air mixed with hydrogen gas, the catalytic reaction heated up the Pt-coated surface, and then thermoelectric voltage appeared across the hot and cold region of the oxide film. At 22°C, the sensor showed the built-up temperature difference and voltage signal of 0.12°C and 0.087 mV, respectively, for the 100 ccm flow of 3% hydrogen/air mixed gas, with the full response time, T90, of 55 s.

Li-Doped Nickel Oxide as a Thermoelectric Material

Thermoelectric properties of ceramic samples of NiO doped with Li are studied in the temperature range of 450–1280 K, with different Li contents. The substitution of Li for Ni atoms in a cubic NiO lattice results in a high thermoelectric power factor over a wide temperature range. Furthermore, Li substitution decreased the thermal conductivity resulting in an increase in the thermoelectric figure of merit, Z = 0.75 ×10-4 K-1 over 1000 K, suggesting that NiO is a promising oxide-base material for high-temperature thermoelectric devices.

Effect of grain size on electric resistivity and thermopower of (Ca2.6Bi0.4)Co4O9 thin films

(Ca2.6Bi0.4)Co4O9 thin films were deposited on MgO single crystal substrates by the spin coating method. Films with different grain sizes (50–500 nm) were obtained by preparing at different temperatures. Films were polycrystalline in nature and oriented preferentially in the (001) direction. Electrical resistivity and thermopower were measured to study the effect of the grain size on the thermoelectric properties. Hall carrier concentrations of the films are dependent on the grain size. The film with a smaller grain size has a lower carrier concentration, a larger resistivity, and a higher thermopower. The thermoelectric power factor is related to the grain size and shows a maximum for the film with grain size of about 100 nm.

Densification and Grain-Orientation of Bi-Pb-Sr-Ca-Cu-O Superconductors by Hot-Pressing

The bulk density of a normal sintered sample of Bi1.6Pb0.4Sr1.6Ca2Cu2.8Oy with a Tc of 103 K was 4.81 g/cm3 and shrinkage of the sample was barely observable. By hot-pressing at a pressure of 39 MPa at 800°C in a vacuum of 5×10-5 Torr for 2 h, the bulk density of the hot-pressed sample reached 6.21 g/cm3, which was over 95% of the theoretical density, and the Tc was 72 K. The grains were oriented with the c-axis along the pressing axis. After annealing at 835°C in air for 40 h, the Tc was improved to 108 K and the critical current density at 77 K was found to be 731 A/cm2.

Cation Ordering in LaBa2Cu2TaO8+y

LaBa2Cu2TaO8+y crystallizes in a tetragonal unit cell with the dimensions a=3.9674(2) and c=12.052(1) A, showing semiconductivity. In this compound, the Ta ions were found to preferentially occupy the Cu1 site between the two Ba layers in the LaBa2Cu3O7-y-type structure.

Fabrication of Oxide Thermoelectric Generator Element

A prototype oxide thermoelectric element has been fabricated for the first time, for the application of power generation at high temperatures in air. Sintered bodies of Li0.025Ni0.975O(p-type) and Ba0.4Sr0.6PbO3 (n-type) were directly joined under the uniaxial pressure of 4 MPa at 1123 K for 10 h in air. The thermoelectric properties of this coupled element were measured in the temperature range of 440 to 1060 K. This element was stable and showed no appreciable contact resistance at the junction after a thermal cycle of over 1000 K. The output power and efficiency of the coupled element were calculated to be 14 mW and 0.64%, respectively, on the assumption that the temperatures of the hot and cold sides of the element were 1000 K and 500 K, respectively.

Thermoelectric Properties of RF-Sputtered SiGe Thin Film for Hydrogen Gas Sensor

Phosphorus-doped Si0.8Ge0.2 thin films were deposited on the Si3N4/SiO2/Si substrate by the RF sputtering. Thermal annealing was carried out to crystallize as-deposited, amorphous-like SiGe thin films. With increasing annealing temperature and time, the crystallization of the SiGe thin films progressed, resulting in a high carrier mobility and a large absolute value of Seebeck coefficient. The SiGe thin film deposited on the Si3N4/SiO2/Si substrate and then annealed at 850°C for 5 h at an argon flow rate of 150 cc/min showed a Seebeck coefficient of -198 µV/K, a Hall mobility of 10.54 cm2/Vs, a carrier concentration of 1.1×1018 cm-3 at 100°C. The thermoelectric hydrogen sensor with the SiGe thin film annealed at 850°C for 5 h showed a voltage signal of 5.81 mV, a catalyst activity of 16.17°C and a response time, corresponding to 90% voltage signal of 50 s for 3% H2 in air. The sensor operating at 100°C detected hydrogen in air at concentrations from 0.01 to 3%, and showed a good linearity between voltage signal and gas concentration.