Noriya Izu

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.

Resistive Oxygen Gas Sensors for Harsh Environments

Resistive oxygen sensors are an inexpensive alternative to the classical potentiometric zirconia oxygen sensor, especially for use in harsh environments and at temperatures of several hundred °C or even higher. This device-oriented paper gives a historical overview on the development of these sensor materials. It focuses especially on approaches to obtain a temperature independent behavior. It is shown that although in the past 40 years there have always been several research groups working concurrently with resistive oxygen sensors, novel ideas continue to emerge today with respect to improvements of the sensor response time, the temperature dependence, the long-term stability or the manufacture of the devices themselves using novel techniques for the sensitive films. Materials that are the focus of this review are metal oxides; especially titania, titanates, and ceria-based formulations.

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.

NO and NO2 Sensing Properties of WO3 and Co3O4 Based Gas Sensors

Semiconductor-based gas sensors that use n-type WO3 or p-type Co3O4 powder were fabricated and their gas sensing properties toward NO2 or NO (0.5–5 ppm in air) were investigated at 100 °C or 200 °C. The resistance of the WO3-based sensor increased on exposure to NO2 and NO. On the other hand, the resistance of the Co3O4-based sensor varied depending on the operating temperature and the gas species. The chemical states of the surface of WO3 or those of the Co3O4 powder on exposure to 1 ppm NO2 and NO were investigated by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. No clear differences between the chemical states of the metal oxide surface exposed to NO2 or NO could be detected from the DRIFT spectra.

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.

Application of V2O5/WO3/TiO2 for Resistive-Type SO2 Sensors

A study on the application of V2O5/WO3/TiO2 (VWT) as the sensitive material for resistive-type SO2 sensor was conducted, based on the fact that VWT is a well-known catalyst material for good selective catalytic nitrogen oxide reduction with a proven excellent durability in exhaust gases. The sensors fabricated in this study are planar ones with interdigitated electrodes of Au or Pt. The vanadium content of the utilized VWT is 1.5 or 3.0 wt%. The resistance of VWT decreases with an increasing SO2 concentration in the range from 20 ppm to 5,000 ppm. The best sensor response to SO2 occurs at 400 °C using Au electrodes. The sensor response value is independent on the amount of added vanadium but dependent on the electrode materials at 400 °C. These results are discussed and a sensing mechanism is discussed.

Thermodynamic Behavior of Various Phases Appearing in the CeZrO4 ‐ CeZrO3.5 System and the Formation of Metastable Solid Solutions

In order to investigate the thermodynamic behavior of t{prime}, t{sup *}, and {kappa} phases with the composition, CeZrO{sub 4}, the equilibrium oxygen partial pressure, p{sub O{sub 2}}, over their mixtures with pyrochlore (Ce{sub 2}Zr{sub 2}O{sub 7+x}) was measured utilizing an electrochemical cell: Pt, {l_brace}CeZrO{sub 4}(t{prime}, t{sup *}, or {kappa} phase) + Ce{sub 2}Zr{sub 2}O{sub 7+x}{r_brace}/ZrO{sub 2}(+Y{sub 2}O{sub 3})/air, Pt. The conclusions described below were derived: the thermodynamic stability of the {kappa} phase is the lowest in the CeO{sub 2}0ZrO{sub 2} system. The {kappa} phase forms metastable solid solutions with the pyrochlore phase; it was virtually stable around 1,123 K. Two kinds of tetragonal phases exist, tet. (high temp.) and tet. (low temp.), which may correspond to t{prime} and t{sup *}, respectively. A change in p{sub o{sub 2}} corresponding to the phase transitions: {kappa} {yields} t{prime} and t{prime} {r_reversible} t{sup *} was observed. The standard Gibbs energies of formation, {Delta}G{degree}, of {kappa}(CeZrO{sub 4}) and t{sup *}(CeZeO{sub 4}) for the reaction: 1/2Ce{sub 2}Zr{sub 2}O{sub 7} + 1/4 O{sub 2} {yields} CeZrO{sub 4} were evaluated from the emf data.

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.

Hydrogen sensor based on RF-sputtered thermoelectric SiGe film

Si0.8Ge0.2 thin film was sputtered on an alumina substrate by the RF-sputtering method. After annealing in flowing Ar atmosphere, platinum film, which acts as a catalyst of the combustible sample gas, was further sputtered on half the surface area of SiGe film. The hydrogen-sensing properties were investigated for the development of potential applications of the device structure as a hydrogen sensor that makes use of the thermoelectric (TE) effect. The measurement results indicate that a reliable output voltage signal was successfully realized when the element was exposed to an environment with a certain hydrogen concentration. The operating temperature for the device was around 100°C, and the response and recovery time corresponding to 90% voltage change were both shorter than 50 s on switching the atmosphere from synthetic air to 3% H2. The detectable concentration of the device ranged from 0.01% to 3%. Furthermore, a good selectivity to hydrogen was also exhibited.