Oxygen Concentration Dependence Research Articles

Excited state properties of 5-fluoro-4-thiouridine derivative†.

The excited state properties of thionated 5-fluorouridine (2',3',5'-tri-O-acetyl-5-fluoro-4-thiouridine; ta5F4TUrd), synthesized with Lawesson's reagent, have been intensively investigated with nanosecond transient absorption spectroscopy, time-resolved thermal lensing, near-infrared emission, and quantum chemical calculation. The intrinsic triplet lifetime of ta5F4TUrd was determined to be μs in acetonitrile, and the formation quantum yield of the excited triplet state was as large as . The quenching rate constants of the triplet ta5F4TUrd by the dissolved oxygen molecule and by the self-quenching process were found to be nearly equal to the diffusion-controlled rate of acetonitrile. The quantum yield of the singlet molecular oxygen produced through energy transfer between the triplet ta5F4TUrd and the dissolved oxygen, , was successfully determined to be under the oxygen-saturated condition. From the oxygen concentration dependence of the value, the fraction of triplet ta5F4TUrd quenched by dissolved oxygen which gives rise to the 1 O2 * formation, , was successfully obtained to be , which was the largest among the thionucleobases and the thionucleosides reported so far. This could be due to the lower energy and/or the ππ* character of the triplet state.

Pressure effects on reactivity and extinction of n-dodecane diffusion cool flame

Low-temperature combustion at high pressure has gained increasing interest owing to the growing demand for clean and efficient propulsion and energy systems. In particular, cool flame structure, extinction limits, and fuel reactivities at high pressure are critical in affecting the engine performance at near-limit conditions. This paper investigates the effects of pressure on the cool flame extinction limit, structure, radical index, reactivity, and oxygen concentration dependence by experiments, analysis, and modeling. A large n-alkane, namely n-dodecane (n-C12H26), is selected to study its diffusion cool flame dynamics and reactivity in a high-pressure counterflow burner up to 10 atm. The experimental results show that higher pressure increases the cool flame extinction strain rates, and that the pressure-weighted extinction strain rate (aP) is proportional to the square of pressure, P2. A scaling analysis explains the relationship between the dependence of flame structure, heat release rate, and pressure-weighted cool flame strain rate on pressure. Furthermore, radical indexes at different pressures are measured by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The radical index clearly shows that the low-temperature reactivity increases with pressure. In addition, due to the critical role of multiple oxygen addition reactions in low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is explored. It is found that the cool flame extinction limits are proportional to the nth power of the oxygen concentration, [O2]n, and increasing pressure leads to stronger extinction limit dependence (larger n) on the oxygen concentration. The present experiment and detailed kinetic analysis show clearly that increasing pressure promotes the low-temperature chemistry including the oxygen addition reactions, while the scaling analysis explains well the experimental results.

Study on cool flame radical index and oxygen concentration dependence of oxygenated fuels

The growing utilization of alternative fuels has sparked an interest in understanding the combustion characteristics of oxygenated fuels at low temperature. Cool flame extinction limits provide direct measures of the low-temperature reactivity of fuels and critical information for advanced low-temperature combustion engine design. This paper investigates the effects of fuel and oxygen concentrations on the extinction limits of diffusion cool flames for oxygenated fuels, including dimethyl ether, methyl decanoate, and 1-dodecanol in an atmospheric counterflow burner. The cool flame radical indexes of these oxygenated fuels are developed by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The results show that the ranking of low-temperature reactivities of long carbon chain oxygenated fuels, compared with n-alkane, is ether > n-alkane > alcohol > ester for a similar carbon number. Furthermore, due to the critical role of multiple oxygen addition reactions in the low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is also explored. The results show that the cool flame extinction limits of the oxygenated fuels are proportional to a nth power of the oxygen concentration, [O2]n, due to the combined effects of multiple oxygen addition reactions and the negative temperature coefficient in low-temperature chemistry. Additionally, the measured n number of 1-dodecanol is found to be larger than 2, which suggests the existence of the third oxygen addition reactions in low-temperature chemistry for large alcohols.

Kinetics and extinction of non-premixed cool and warm flames of dimethyl ether at elevated pressure

The growing demand of clean and efficient propulsion and energy systems has sparked an interest in understanding low-temperature combustion at high pressure. Cool flame transition and extinction limits as well as oxygen concentration dependence at elevated pressures provide insights of the low-temperature and high-pressure fuel reactivity. A new experimental high-pressure counterflow burner platform was designed and developed to achieve the studies of high-pressure cool flames. Dimethyl ether (DME) was chosen to study its non-premixed cool flame in high-pressure counterflow burner at pressure up to 5 atm, perhaps for the first time. This paper investigates the effects of pressure on cool flame structure, extinction and transition limits, and oxygen concentration dependence as well as ozone assisted warm flames of DME in experiments and numerical simulations. The results show that the reignition transition from cool flame to hot flame occurs either with the decrease of the strain rate at a given fuel concentration and pressure or with the increase of fuel mole fraction or pressure at a given strain rate. Furthermore, it is shown that the higher pressure shifts the cool flame to higher strain rates and results in higher cool flame extinction strain rates. However, the existing kinetic model of DME fails in predicting the cool flame extinction limit at elevated pressures. Besides, the cool flame extinction limits are proportional to nth power of the oxygen concentration, [O2]n, and the increase of pressure leads to stronger extinction limit dependence (larger n) on oxygen concentration. The present experiment and detailed kinetic analysis show clearly that increasing pressure promotes the low-temperature chemistry including the oxygen addition reactions. In addition, stable warm flame was first experimentally observed by using DME at elevated pressure with ozone sensitization.

Memristive Characteristics of the Single-Layer P-Type CuAlO2 and N-Type ZnO Memristors.

Memristive behaviors are demonstrated in the single-layer oxide-based devices. The conduction states can be continually modulated with different pulses or voltage sweeps. Here, the p-CuAlO2- and n-ZnO-based memristors show the opposite bias polarity dependence with the help of tip electrode. It is well known that the conductivity of p-type and n-type semiconductor materials has the opposite oxygen concentration dependence. Thus, the memristive behaviors may attribute to the oxygen ion migration in the dielectric layers for the single-layer oxide based memristors. Further, based on the redox, the model of compressing dielectric layer thickness has been proposed to explain the memristive behavior.

Oxygen Concentration Dependence in Microwave Plasma‐Enhanced Chemical Vapor Deposition Diamond Growth in the (H, C, O, N) System

The dependence of nitrogen incorporation and crystalline quality of the nitrogen‐doped single‐crystal diamond on oxygen concentration is investigated. Nitrogen‐doped diamond is grown by microwave plasma‐enhanced chemical vapor deposition in the (H, C, O, N) system. Nitrogen concentration (N/H) and methane concentration (CH4/H2) are fixed at 100 ppm and 4%, respectively. Oxygen concentration (O/C) is controlled from 12.5 to 50% (O/H = 0.25–1%). The growth rate of diamond decreases from 38 to 7 μm h−1. The nitrogen concentration in the epitaxial layer increases with increasing oxygen concentration and starts to decrease when O/C > 25% (O/H > 0.5%). The full width at half maximum (FWHM) of the Raman peak at 1332 is 2.0–2.2 cm−1. Introduction of oxygen affects incorporation of nitrogen into diamond. There is an optimum value of oxygen concentration to give the highest nitrogen concentration. This is attributed to the enhancement of the effective concentration of nitrogen‐related species in the gas phase and reduction of the growth rate both due to oxygen introduction. The results suggest that the (H, C, O, N) system could give higher concentration of nitrogen than that of the (H, C, N) system without degradation of the crystal quality.

Electrochemical Insight into NaxCoO2 for the Oxygen Evolution Reaction and the Oxygen Reduction Reaction

Layered NaxCoO2 provides multiple degrees of freedom for manipulating its structure and physical properties by tuning the Na concentration, leading to specific functionalities including thermoelectricity, superconductivity, and potentiality in Li-/Na-ion batteries. However, the contribution of varied Na to charge transfer, electrocatalytic kinetics, and energetics in terms of the electrochemical interface reaction for the oxygen evolution reaction (OER) in water splitting and the oxygen reduction reaction (ORR) in fuel cells is not yet fully understood. This work reveals that varied Na concentrations indirectly affect the electrochemical OER or ORR activity by changing the Co–O bond in the constituent CoO6 octahedron of NaxCoO2. Tuning the Na concentration gives rise to the unique evolution of the electronic configuration and subsequently further enhances the Co–O bond’s covalency, which results in promoting the catalytic kinetics of OER and ORR. As the Fermi level descends deeper into the O 2p orbitals with increasing Na extraction, the lattice oxygen becomes active in the proton–electron transfer process, which is reflected in the pH and oxygen-concentration dependence of the OER activity. Based on the characterization of its electrochemical properties, the high electrocatalytic activity of Na0.75CoO2, which exhibits competent OER activity superior to that of IrO2, is rationalized. Meanwhile, intrinsic Na0.75CoO2 reveals a half-wave potential of 0.74VRHE for ORR. The evolution of the structure and the electronic configuration of NaxCoO2 related to its electrochemical properties enables further improved NaxCoO2-based catalysts for efficient electrochemical OER and ORR.

Oxygen concentration dependence of as-grown defect formation in nitrogen-doped Czochralski silicon single crystals

The interstitial oxygen concentration ([OI]) dependence of the as-grown defect (voids and dislocation clusters) behavior in nitrogen-doped silicon crystals fabricated by Czochralski technique (Cz-Si) have been investigated. Nitrogen-doped Cz-Si crystals with [OI] values of 7–8 and 1–3 × 1017 atoms cm−3 are evaluated, and it was found that the range of v/G in which as-grown defect-free (AGDF) region can be obtained become wide with decreasing [O I], where v is the crystal growth rate and G is the axial temperature gradient at the solid–liquid interface. This result indicates that as-grown defect suppression effect by nitrogen doping is enhanced at low [OI].By the analysis based on a thermodynamic model, it is shown that nitrogen–vacancy (NV) and nitrogen–oxygen (NO) complexes compete in the nitrogen-doped Cz-Si crystals and their concentration fraction changes with [OI] value. At low [OI], the concentration of the NO complex becomes low; thus, the concentration of nitrogen that can form NV complexes increases, thereby enhancing the as-grown defect suppression effect. Silicon wafers for insulated gate bipolar transistors (IGBTs) are required AGDF and oxide precipitate-free, therefore nitrogen-doped Cz-Si crystals with low [OI] values are suitable for the fabrication of silicon wafers for IGBTs.

Origin of carrier lifetime degradation in floating-zone silicon during a high-temperature process for insulated gate bipolar transistor

We report on the relationship between carrier lifetime degradation of floating-zone silicon (FZ Si) and the guard ring formation process for silicon integrated gate bipolar transistor (Si-IGBT). A clear carrier lifetime degradation was observed through the guard ring oxidation and annealing processes for Si-IGBT and showed interstitial oxygen (Oi) concentration dependence, which was obtained by Fourier transform infrared absorption spectroscopy. Based on the carrier lifetime measurements through the step etching of the FZ Si substrate, it has been suggested that the carrier lifetime degradation is not only caused by the Oi itself near the FZ Si surface region but also the other defects induced by the Oi injection. Diffused interstitial Si atoms kicked-out by the Oi into the FZ Si substrate, which has a longer diffusion length than Oi, can be considered to be the origin of the carrier lifetime degradation.

Rate-Limiting Steps Elucidated By Electrochemical Impedance Spectroscopy of a Yttria-Stabilized Zirconia 2-in-1 Temperature and Oxygen Gas Sensor Measured at Extreme Temperatures

Introduction Yttria-stabilized zirconia (YSZ) is known for its good ionic conductivity, essentially being synonymous with the term “lambda sensor”. Automotive lambda sensors have been fabricated with YSZ as the electrolyte for 30+ years and have operated in a temperature region of 600oC to 800oC. There is an increasing demand for higher operating temperature sensors, specifically a 2-in-1 temperature and oxygen sensor which is stable, selective, and sensitive in extreme environments. However, there remains a disagreement among scholars about the effects on oxygen vacancy activation energy at temperatures higher than 800oC. Also, the rate-limiting step of oxygen reduction on the platinum electrode is not inherently clear. A 2-in-1 temperature and oxygen sensor was fabricated and characterized with the analysis of electrochemical impedance spectroscopy (EIS), DC resistance, and open circuit potential measurements at temperatures between 600oC and 1200oC. The temperature dependence on individual activation energies within the sensor was elucidated directly through these techniques. Methods Yttria-stabilized zirconia (MTI Corp) was employed as the solid-state electrolyte in sensor fabrication, with the primary electrode material as porous platinum (ESL 5570). Platinum paste was screen printed onto planar YSZ substrates in an interdigitated electrode configuration, then fired at 1500oC for 2 hours in nitrogen atmosphere (1 atm). The sensor was placed in a ThermoScientific tube furnace capable of reaching 1500oC, with mass flow controllers varying oxygen gas concentration throughout electrical measurements. Platinum foil acted as a Faraday cage around the sensor to shield from furnace inductive fluctuations and minimize noise during low frequency EIS measurements. A thermocouple placed next to the sensor validated furnace operating temperature and cooling endcaps on the furnace tube created a natural temperature gradient between sensor placement and the electrical connections outside of the furnace. Figure 1 displays the environmental testing setup. Results and Conclusions Figure 2a displays an impedance Nyquist spectrum that contains real and imaginary components of impedance measured at 0.21 atm PO2, where electrolyte resistance decreases with an increase in temperature. Figure 2b displays the Arrhenius plot of isolated YSZ electrolytic resistance extracted from figure 2a, where there are distinct activation regions at moderate and high temperatures. In agreement with Badwal [1], the shift to lower EA at higher temperatures was due to a gradual transition between an association region where oxygen vacancies are bound to dopant cations, and a dissociation region where vacancies are free and mobile.At a known temperature, EIS also elucidates oxygen concentration dependence on sensor electrode resistance. Figure 3a shows sensor electrode resistance, fitted from figure 2a, as a function of oxygen concentration. Investigating the oxygen reduction reaction (ORR) variation on platinum at various temperature regions, it is suggested that ORR on platinum is co-limited by oxygen adsorption and by oxygen transport along the surface of the electrode, in agreement with Adler [2]. Figure 3b shows that electrode 2 is dominant at temperatures below 1050oC, indicative of oxygen diffusion limitation on the surface and through platinum. Electrode 1 is dominant at temperatures above 1050oC, where dissociative adsorption of oxygen on the electrode surface is predominant.

Oxygen concentration dependence of microstructure formed on Ni by backward pulsed laser deposition

In the automotive industry, direct joining between resin and metal without additional material is expected due to a growing need for hybrid structures composed of resin and metal. Roughening the metal surface before joining can improve the adhesion with the resin, and forming a microstructure on the metal surface by backward pulsed laser deposition could be a useful method. In the present study, we investigated the dependence of microstructure on the oxygen concentration in the ambient gas during surface processing for controlling the morphology of the microstructure. In the experiments, the oxygen concentration in the ambient gas composed of nitrogen and oxygen was controlled under atmospheric pressure, and microstructure characteristics, such as the shape and hardness, were analyzed. As a result, it was demonstrated that the formation range of the microstructure was constant regardless of the oxygen concentration, whereas a rougher microstructure was formed at higher oxygen concentrations, and a dense, flat microstructure was formed at lower oxygen concentrations. These results implied that the oxidation between the nanoparticles in the laser-induced plume and the ambient gas affects the mobility of the nanoparticles on the metal surface, leading to a transmutation in the morphology of the microstructure. Finally, it was shown that it is important to reduce the surface mobility of nanoparticles, such as processing under high oxygen concentration, to form a microstructure that improves adhesion.

Oxygen Concentration Dependence of the Optical and Electrical Properties of Mn-Doped Tin Oxide Thin Films Deposited on a Polyethylene Terephthalate Substrate.

Mn (2.59 wt.%)-doped tin oxide (MTO) was deposited on a polyethylene terephthalate (PET) substrate using RF magnetron sputtering at room temperature. Oxygen was introduced during the sputtering process to prevent the reduction of transmittance and the formation of nonstoichiometric compositions. The dependence of the optical and electrical properties of MTO thin films on the O₂/(Ar+O₂) ratio was systematically investigated. The transmittance of the MTO single layer (24 nm) at 550 nm gradually increased from 85.9% to 88.2% with an increase of the O₂/(Ar+O₂) ratio from 0% to 7.9%. The optical band gap was also affected by the O₂/(Ar+O₂) ratio, varying from 3.30 to 3.64 eV. The MTO film deposited at an O₂/(Ar+O₂) ratio of 2.7% showed the minimum resistivity (0.03 Ω·cm). The XPS spectra indicated that the SnO₂ thin films become almost stoichiometric as the O₂/(Ar+O₂) ratio increased. Thus, this sputtering process can be used to prepare flexible MTO films with a wider optical band gap, improved transmittance, and decreased resistivity.

Transition metal oxide (Ni, Co, Fe)-tin oxide nanocomposite sensing electrodes for a mixed-potential based NO2 sensor

A mixed-potential based sensor utilizing transition metal oxide (Ni, Co, Fe)-tin oxide nanocomposite sensing electrodes are fabricated for the first time and investigated for the gas sensing performance towards the highly toxic nitrogen dioxide. The nanocomposites are synthesized by solvo-combustion route and characterized for the physical, gas sensing and electrochemical properties in a temperature range of 600–700 ℃. The sensor equipped with Fe2O3-SnO2 (Fe:Sn = 2:1) nanocomposite sensing electrodes sintered at 1000 ℃ shows the maximum response of 60 mV towards 100 ppm NO2 with a relatively fast response and recovery dynamics at an operating temperature of 650 ℃. The sensor also shows a linear dependence of response over the logarithm of NO2 concentration with a sensitivity of ∼44 mV/decade. Additionally, the oxygen concentration dependence, cyclability and cross-sensitivity towards interfering gases are also investigated. Finally, the sensing mechanism and electrochemical activity of the sensing electrodes are studied using polarization curve measurement and electrochemical impedance spectroscopy.

Effects of oxygen concentration on the electron energy distribution functions in atmospheric pressure helium/oxygen and argon/oxygen needle-electrode plasmas

In this work, a numerical investigation on the oxygen concentration effects of the electron energy distribution function (EEDF) in the atmospheric pressure helium/oxygen and argon/oxygen plasmas has been performed based on a typical needle-electrode discharge system and a Particle-in-cell Monte-Carlo collision model. Specially, the corresponding mechanisms have been analyzed in detail with the use of the energy conservation of electrons in the development of discharge. The present work gives the significant results below. In helium/oxygen mixture, the oxygen concentration dependence of EEDF presents the different behaviors in the three phases of the discharge. Increasing oxygen concentration, in the first phase, the peak of EEDF decreases and its distribution becomes wide, and then, EEDF presents a transition to the third phase, where the peak of EEDF increases, but its distribution gets narrow. The mechanisms governing these dependences can be attributed to the different effects of oxygen concentration on the electron energy obtained from the electric field, the energy loss of electrons due to the inelastic collisions, and the contribution of the electron density to reducing the average energy of electrons, at the different phases of the discharge. In argon/oxygen mixture, for different time ts in the development of discharge, the oxygen concentration dependences of EEDF are similar, i.e. the rising peak and the distribution narrowing down with the increase in oxygen concentration, but the dependence is weak. In comparison with helium/oxygen mixture, this weak dependence is mainly due to the especial distributions of the excitation and ionization thresholds of O2, O, and Ar, namely, the ionization thresholds of O2 and O are located between the excitation and ionization thresholds of argon, which leads to the unobvious increase of the inelastic collision frequencies of electrons with O2 and O, when increasing oxygen concentration, and thus leads to weak effect on the EEDF in argon/oxygen mixture.

Ab Initio Prediction of Potassium Partitioning Into Earth's Core

AbstractPotassium partitioning between molten silicates and liquid iron alloys is the fundamental process determining its incorporation into the Earth's core. In this study, it is investigated using the method of the ab initio molecular dynamics simulation combined with the thermodynamic integration technique. Results suggest that the potassium incorporation into iron alloys positively depends on temperature, while the effect of pressure is insignificant. Moreover, the existence of oxygen in liquid iron alloys significantly enhances the potassium solubility therein, whereas sulfur and silicon only have negligible effects. Electronic structure analyses reveal that potassium remains alkali‐metallic in liquid iron alloy systems under all conditions in this study, which is distinct from the characteristics reported for solid potassium. Atomic structure analyses indicate that the oxygen coordination number around potassium atom increases with oxygen concentration in liquid iron alloys, supporting the oxygen concentration dependence of the potassium partitioning between molten silicates and liquid iron alloys. Using the obtained partitioning coefficients combined with geochemical property, the maximum potassium concentration in the Earth's core is estimated to be ~30 ppm, which would be unlikely to affect the thermal evolution of the Earth's core significantly.

Oxygen concentration dependence of silicon oxide dynamical properties

To understand oxidation in three-dimensional silicon, dynamic characteristics of a SiOx system with various stoichiometries were investigated. The calculated results show that the self-diffusion coefficient increases as oxygen density decreases, and the increase is large when the temperature is low. It also shows that the self-diffusion coefficient saturates, when the number of removed oxygen atoms is sufficiently large. Then, approximate analytical equations are derived from the calculated results, and the previously reported expression is confirmed in the extremely low-SiO-density range.

The Lithium and Oxygen Concentration Dependence in Li-O2 Batteries Studied with Electrochemical Quartz Crystal Microbalance and Cylindrical Microelectrodes

The Li-O2 battery is a promising chemistry suggesting very high energy densities. In the literature it is agreed upon that during the discharge process Li2O2 is the final product formed via the intermediate LiO2. However, the nature of the intermediate is not fully understood, e.g. how long is the lifetime and how does it proceed to form the final product (chemical or electrochemical reaction). This study sets out to shed light on the reaction mechanism of the Li-O2 battery by studying the effects of different concentrations of lithium and oxygen. Experimentally this is done using an electrochemical quartz crystal microbalance (EQCM) and a cylindrical microelectrode. The EQCM results show that the system is unable to return to its initial mass, i.e. there is a buildup of reaction product during cycling. Further, it is not possible to recover all the discharge electrons during recharge. The intermediate is believed to diffuse away from the electrode rendering it unavailable for recharge. This result is further strengthened by the measurements on microelectrodes where intermediate is believed to diffuse away from the electrode more easily. There is also a discrepancy when looking at the change in mass and the expected mass change from the corresponding current suggesting that both electrochemical and chemical processes occurs. In conclusion it can be said that this study provides additional knowledge to the surface confined processes in the Li-O2 battery. The EQCM provides information that otherwise would go unnoticed, i.e. solely measuring the current response will not provide detailed quantitative information of the mass response.

(Invited) Design of Highly Sensitive and Selective Diode-Type H2 Sensors

Numerous efforts have been directed to developing highly sensitive and selective H2 sensors, because H2, which is the most promising alternative to fossil fuels, is highly flammable and explosive in a wide concentration range (4–75% in air). However, general gas sensors (e.g., semiconductor-type and catalytic combustion-type sensors) cannot operate under O2-free atmosphere, because the effective reaction of H2 with negatively-charged oxygen adsorbates on the oxide surface is essential for the large H2 response. We have thus far demonstrated that diode-type gas sensors using noble-metal (N) sensing electrodes and an anodized titania film (N/TiO2 sensors) show quite large H2 response under oxygen-free atmosphere as well as relatively excellent H2 selectivity to other inflammable gases, in comparison with other gas sensors. Use of Pd as the sensing-electrode material especially resulted in quite large H2 response1 , 2 ), and alloying of Pd with Pt (Pd-Pt) effectively improved the H2 response and the long-term stability3). However, the H2 response of these sensors abruptly decreased with an increase in oxygen concentration in target gas. On the other hand, a Pt/TiO2 sensor showed lower H2 response than the Pd/TiO2 and Pd-Pt/TiO2 sensors, but the coating of the Pt-sensing electrode with a slight amount of Au drastically enhanced the magnitude of H2 response in air, probably due to a decrease in the amount of oxygen adsorbates on the Pt electrode4,5). Therefore, effects of the Au coating of the Pt-sensing electrode on the H2-sensing properties have been investigated in this study. A polished Ti plate (10 mm × 5 mm × 0.5 mm) was annealed at 600°C for 1 h in air, and the half part was anodized at 20°C for 30 min at a current density of 50 mA cm-2 in 0.5 M H2SO4 aqueous solution. A pair of Pt electrodes (3 mm × 3 mm) was fabricated on the surface of both the TiO2 thin film and the Ti plate by magnetron sputtering for 6 min at 300 W in Ar. Subsequently, Au was coated on the Pt electrodes by magnetron sputtering at 40 W in Ar (sputtering time (n): 10–120 s) and Au wires were attached on both the electrodes by using Au paste. The sensing properties of the obtained sensors (Au(n)/Pt/TiO2 sensors, after annealing at 600°C for 1 h in air) to 5–8000 ppm H2 balanced with air or N2 under dry or wet atmosphere were measured at 250°C, while applying a dc voltage of 100 mV to the sensors under forward bias condition (Au(n)/Pt(+)–TiO2–Ti(−)). The response of a Pt/TiO2 sensor to 8000 ppm H2 in dry air was three orders of magnitude smaller than that in dry N2, but Au coating on the Pt electrode drastically enhanced the forward current of the Pt/TiO2 sensor in air. The Au(20)/Pt/TiO2 sensor showed the largest H2 response in dry air among all Au(n)/Pt/TiO2 sensors, but the magnitude of H2 response in dry air was still smaller than that in dry N2. The addition of moisture to the atmosphere further enhanced the H2 response of the Au(20)/Pt/TiO2 sensor in air, while it decreased the H2 response in N2. Consequently, the response of the Au(20)/Pt/TiO2 sensor to 8000 ppm H2 in air was comparable to that in N2 under the most humidified atmosphere (AH: 12.8 g m-3). The Au(20)/Pt/TiO2 sensor showed linear relationship between the H2 response and the logarithmic H2 concentration, and the sensor could respond to 50 ppm H2 in dry and wet air, while the sensor easily responded to even 5 ppm H2 in dry and wet N2. In addition, the Au(20)/Pt/TiO2 sensor showed quite excellent H2 selectivity against propane and propene. These results indicate that the optimal Au coating on the Pt electrode was quite effective in improving both the H2 response and H2 selectivity in air as well as reducing oxygen-concentration dependence of the H2response, especially under wet atmosphere. 1) Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, Sens. Actuators B 83(2002) 195. 2) Y. Shimizu, K. Sakamoto, M. Nakaoka, T. Hyodo, M. Egashira, Adv. Mater. Res. 47-50(2008) 1510. 3) T. Hyodo, M. Nakaoka, Y. Shimizu, M. Egashira, Sens. Lett. 9(2011) 641. 4) T. Hyodo, T. Yamashita, Y. Shimizu, ECS Transactions 50(12) (2012) 171. 5) T. Hyodo, T. Yamashita, Y. Shimizu, Sens. Actuators B 207 (2015) 105.

(Invited) Design of Highly Sensitive and Selective Diode-Type H2 Sensors

Diode-type gas sensors employing an anodized TiO2 film and Pt electrodes modified with and without a small amount of Au (Au/Pt/TiO2 and Pt/TiO2, respectively) have been fabricated, and their H2-sensing properties have been investigated in detail. The Pt/TiO2 sensor showed much larger H2 response in N2 than that in air, but the surface modification of the Pt electrode with Au was quite effective in reducing the oxygen concentration dependence of the H2 response and enhancing the H2 selectivity against hydrocarbons such as C3H8 and C3H6 in both wet air and wet N2, at a forward bias of +100 mV at 250°C. In addition, the Au/Pt/TiO2 sensor showed relatively good H2-sensing properties at a low forward bias (+ 1 mV) at 250°C as well as at a forward bias of +100 mV at a low temperature (50°C) in both wet air and wet N2.

Oxygen Concentration Dependence of Dust Explosion Hazard and Example of Application of Safety Measures

Oxygen Concentration Dependence of Dust Explosion Hazard and Example of Application of Safety Measures