Enhancing the Responsiveness of Thermoelectric Gas Sensors with Boron-Doped and Thermally Annealed SiGe Thin Films via Low-Pressure Chemical Vapor Deposition

Thermoelectric gas sensor (THGS) devices with catalysts and Si0.8Ge0.2 thin films of different boron doping levels of 1018, 1019, and 1020 cm-3 were fabricated, and their transport properties are investigated. SiGe films were deposited on Si3N4/SiO2 multilayers on Si substrates using low-pressure chemical vapor deposition (LPCVD) and thermally annealed at 1050 °C. The Seebeck coefficients of the SiGe films were increased after thermal annealing, ranging from 191 to 275 μV/K at temperatures of 74 to 468 °C in air, and reaching the highest power factor of 6.78 × 10-4 W/mK2 at 468 °C. The thermal conductivity of the SiGe films varied from 2.4 to 3.0 W/mK at 25 °C. The THGS detection performance was tested for the H2 gas in air from 0.01 to 1.0%, and compared to the thermoelectric properties of the SiGe films. The high-temperature annealing treatment process was successful in enhancing the thermoelectric performance of both the SiGe films and sensor devices, achieving the best THGS performance with the sensor device fabricated from the annealed SiGe film with 1018 cm-3 boron-doped Si0.8Ge0.2.

Erratum: Effect of evaporation and condensation on a thermoacoustic engine: A Lagrangian simulation approach [J. Acoust. Soc. Am. 141 (6), 4398-4407 (2017)]

In the original paper [Yasui and Izu, J. Acoust. Soc. Am. 141(6), 4398-4407 (2017)], the temperature gradient in Rott equations was assumed as zero by mistake as an author error, although temperature gradient was adequately taken into account in the numerical simulations of thermal conduction between a fluid parcel and the wall of a stack. In the present erratum, the results of the corrected numerical simulations are shown. The results show that the pV work done by a fluid parcel is larger in a wet stack compared to that in a dry stack not only in a traveling-wave thermoacoustic engine but also in a standing-wave thermoacoustic engine. The pV work is determined not only by the volume oscillation amplitude of a fluid parcel but also by the change in the mean volume of a fluid parcel.

Is surface tension reduced by nanobubbles (ultrafine bubbles) generated by cavitation?

Nanobubbles (ultrafine bubbles) are produced by hydrodynamic or acoustic cavitation. They work as cavitation nuclei. Is the experimentally reported considerable reduction of surface tension of liquid water by nanobubbles real? It is theoretically suggested that nanobubbles partly covered with hydrophobic materials are concentrated at a surface of liquid water. A hydrophobic cap is directed toward a gas phase above a liquid surface. Uncovered surface of a nanobubble is directed into liquid water underneath the liquid surface. It is suggested that a liquid film is more easily ruptured by the presence of nanobubbles at the liquid surface, which reduces the value of surface tension in du Noüy ring method. A role of ionic surfactants on accumulation of nanobubbles at a liquid surface is also discussed.

Self-assembly patterning of ultrafine zirconia nanocrystal films fabricated on chemically patterned templates

Ultra-thin zirconia (ZrO₂) nanocrystal films were fabricated by using a controlled dip-coating process. ZrO₂ nanocrystals possess a cubic crystalline phase and large surface-to-volume area. The film composite with only several layers of nanocrystals were obtained by controlling the withdrawal speed and mass concentration of the colloidal solution. The optical properties of ZrO₂ nanocrystal films were accessed by UV-vis spectroscopy, which indicated the dense and uniform structure of the nanocrystal films. The high reflection index suggested that the films could be used in the reflection coating industry. Furthermore, a micro-pattern of self-assembled monolayers of silane molecular was used as a chemical mold for selective deposition of ZrO₂ nanocrystals. As a result, a self-assembly patterning of ZrO₂ nanocrystals with a neat edge was fabricated on silicon substrate. The low-cost fabricating method is compatible with conventional very-large-scale integration processes and can be extended to other kinds of nanocrystals.

Fabrication and H2-Sensing Properties of SnO2 Nanosheet Gas Sensors

Vertically formed and well-defined SnO₂ nanosheets are easy to fabricate, involving only a single process that is performed under moderate conditions. In this study, two different sizes of a SnO₂ nanosheet were concurrently formed on a Pt interdigitated electrode chip, with interconnections between the two. As the SnO₂ nanosheets were grown over time, the interconnections became stronger. The ability of the fabricated SnO₂ nanosheets to sense H₂ gas was evaluated in terms of the variation in their resistance. The resistance of a SnO₂ nanosheet decreased with the introduction of H₂ gas and returned to its initial level after the H₂ gas was replaced with air. Also, the response-recovery behaviors were improved as a result of the growth of the SnO₂ nanosheets owing to the presence of many reaction sites and strong interconnections, which may provide multipassages for the electron transfer channel, leading to the acceleration of the reaction between the H₂ gas and SnO₂ nanosheets.

Effect of Core-shell Ceria/Poly(Vinylpyrrolidone) (PVP) Nanoparticles Incorporated in Polymer Films and Their Optical Properties (2): Increasing the Refractive Index

We investigated the preparation of well-dispersed core-shell ceria-poly(vinylpyrrolidone) (PVP) nanoparticles with an average particle size of around 20 nm which were used to produce a hybrid film with a polymer coating of dipentaerythritol hexaacrylate (DPHA). We obtained good dispersion of the nanoparticles in a mixed solvent of 48% 1-methoxy-2-propanol (MP), 32% 3-methoxy-3-methyl-1-butanol (MMB), and 20% methyl i-butyl ketone (MIBK). An ink of the polymer coating consisting of 68.7 wt% nanoparticles and 31.3 wt% DPHA with a polymerization initiator was prepared using this solvent mixture. The surface of the hybrid film showed low roughness and the nanoparticles formed a densely packed structure in the DPHA matrix. The resulting coating possessed excellent transparency and a high refractive index of 1.69.

Effect of evaporation and condensation on a thermoacoustic engine: A Lagrangian simulation approach

Acoustic oscillations of a fluid (a mixture of gas and vapor) parcel in a wet stack of a thermoacoustic engine are numerically simulated with a Lagrangian approach taking into account Rott equations and the effect of non-equilibrium evaporation and condensation of water vapor at the stack surface. In a traveling-wave engine, the volume oscillation amplitude of a fluid parcel always increases by evaporation and condensation. As a result, pV work done by a fluid parcel is enhanced, which means enhancement of acoustic energy in a thermoacoustic engine. On the other hand, in a standing-wave engine, the volume oscillation amplitude sometimes decreases by evaporation and condensation, and pV work is suppressed. Presence of a tiny traveling-wave component, however, results in the enhancement of pV work by evaporation and condensation.

Rapid Synthesis and Formation Mechanism of Core-Shell-Structured La-Doped SrTiO₃ with a Nb-Doped Shell

To provide a convenient and practical synthesis process for metal ion doping on the surface of nanoparticles in an assembled nanostructure, core-shell-structured La-doped SrTiO₃ nanocubes with a Nb-doped surface layer were synthesized via a rapid synthesis combining a rapid sol-precipitation and hydrothermal process. The La-doped SrTiO₃ nanocubes were formed at room temperature by a rapid dissolution of NaOH pellets during the rapid sol-precipitation process, and the Nb-doped surface (shell) along with Nb-rich edges formed on the core nanocubes via the hydrothermal process. The formation mechanism of the core-shell-structured nanocubes and their shape evolution as a function of the Nb doping level were investigated. The synthesized core-shell-structured nanocubes could be arranged face-to-face on a SiO₂/Si substrate by a slow evaporation process, and this nanostructured 10 μm thick thin film showed a smooth surface.

Elimination of flammable gas effects in cerium oxide semiconductor-type resistive oxygen sensors for monitoring low oxygen concentrations

We have investigated the catalytic layer in zirconium-doped cerium oxide, Ce_0.9Zr_0.1O₂ (CeZr10) resistive oxygen sensors for reducing the effects of flammable gases, namely hydrogen and carbon monoxide. When the concentration of flammable gases is comparable to that of oxygen, the resistance of CeZr10 is affected by the presence of these gases. We have developed layered thick films, which consist of an oxygen sensor layer (CeZr10), an insulation layer (Al₂O₃), and a catalytic layer consisting of CeZr10 with 3 wt% added platinum, which was prepared via the screen printing method. The Pt-CeZr10 catalytic layer was found to prevent the detrimental effects of the flammable gases on the resistance of the sensor layer. This effect is due to the catalytic layer promoting the oxidation of hydrogen and carbon monoxide through the consumption of ambient O₂ and/or the lattice oxygen atoms of the Pt-CeZr10 catalytic layer.

Sensing properties of Pd-loaded Co3O4 film for a ppb-level NO gas sensor

We prepared 0.1 wt%–30 wt% Pd-loaded Co₃O₄ by a colloidal mixing method and investigated the sensing properties of a Pd-loaded Co₃O₄ sensor element, such as the sensor response, 90% response time, 90% recovery time, and signal-to-noise (S/N) ratio, toward low nitric oxide (NO) gas levels in the range from 50 to 200 parts per billion. The structural properties of the Pd-loaded Co₃O₄ powder were investigated using X-ray diffraction analysis and transmission electron microscopy. Pd in the powder existed as PdO. The sensor elements with 0.1 wt%–10 wt% Pd content have higher sensor properties than those without any Pd content. The response of the sensor element with a 30 wt% Pd content decreased markedly because of the aggregation and poor dispersibility of the PdO particles. High sensor response and S/N ratio toward the NO gas were achieved when a sensor element with 10 wt% Pd content was used.

Thermal balance analysis of a micro-thermoelectric gas sensor using catalytic combustion of hydrogen

A thermoelectric gas sensor (TGS) with a combustion catalyst is a calorimetric sensor that changes the small heat of catalytic combustion into a signal voltage. We analyzed the thermal balance of a TGS to quantitatively estimate the sensor parameters. The voltage signal of a TGS was simulated, and the heat balance was calculated at two sections across the thermoelectric film of a TGS. The thermal resistances in the two sections were estimated from the thermal time constants of the experimental signal curves of the TGS. The catalytic combustion heat Q_catalyst required for 1 mV of ΔV_gas was calculated to be 46.1 μW. Using these parameters, we find from simulations for the device performance that the expected Q_catalyst for 200 and 1,000 ppm H₂ was 3.69 μW and 11.7 μW, respectively.

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.

CO responses of sensors based on cerium oxide thick films prepared from clustered spherical nanoparticles

Various types of CO sensors based on cerium oxide (ceria) have been reported recently. It has also been reported that the response speed of CO sensors fabricated from porous ceria thick films comprising nanoparticles is extremely high. However, the response value of such sensors is not suitably high. In this study, we investigated methods of improving the response values of CO sensors based on ceria and prepared gas sensors from core-shell ceria polymer hybrid nanoparticles. These hybrid nanoparticles have been reported to have a unique structure: The core consists of a cluster of ceria crystallites several nanometers in size. We compared the characteristics of the sensors based on thick films prepared from core-shell nanoparticles with those of sensors based on thick films prepared from conventionally used precipitated nanoparticles. The sensors prepared from the core-shell nanoparticles exhibited a resistance that was ten times greater than that of the sensors prepared from the precipitated nanoparticles. The response values of the gas sensors based on the core-shell nanoparticles also was higher than that of the sensors based on the precipitated nanoparticles. Finally, improvements in sensor response were also noticed after the addition of Au nanoparticles to the thick films used to fabricate the two types of sensors.

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.