Millisecond-order response measurement for fast oxygen gas sensors

A novel method for the evaluation of fast response of oxygen gas sensors in terms of milliseconds is developed. Using the method of modulating oxygen partial pressure by changing the total pressure inside the test chamber, the millisecond-order measurement was succeeded. For the 100 Hz sign-wave operation, the measurement system generated the pressure changes from 180 to 220 kPa, corresponding to the change of oxygen partial pressure from 36 to 44 kPa. Using both jump-method and pressure modulation method, the response of the resistive oxygen sensors of cerium oxide thick films were evaluated at 1173 K and the kinetic mechanism of gas sensing was discussed. Their response times of t₉₀ obtained by square-wave change were measured to be 37 and 22 ms for high-to-low oxygen partial pressure and vice versa transition, respectively. The log–log plot of resistance of sensor and the frequency, pressure modulation spectra, was also evaluated at the same time and the kinetics of oxygen sensing was suggested to be diffusion-limited.     

Ni诱导SiGe薄膜晶化烧结制备与热电势氢传感器件

在玻璃衬底上采用RF溅射法制备非晶SiGe薄膜的基础上，采用金属Ni诱导法对所制备的薄膜进行晶化烧结，并以Pt催化氢氧化放热反应和SiGe薄膜的热电势效应为原理制备出了小型热电势氢传感器。测试表明：以500℃烧结所得SiGe薄膜材料为敏感基体，在100℃工作温度下所制备的传感器对3％H2灵敏度为0．7mV，检测的浓度范围大约为0．02％-5％，且输出电压随着烧结温度的升高而升高。     

Formation mechanism of monodispersed spherical core-shell ceria/polymer hybrid nanoparticles

Very unique core-shell ceria (cerium oxide)/polymer hybrid nanoparticles that have monodispersed spherical structures and are easily dispersed in water or alcohol without the need for a dispersant were reported recently. The formation mechanism of the unique nanoparticles, however, was not clear. In order to clarify the formation mechanism, these nanoparticles were prepared using a polyol method (reflux heating) under varied conditions of temperature, time, and concentration and molecular weight of added polymer (poly(vinylpyrrolidone)). The size of the resultant nanoparticles was strongly and complicatedly dependent on the set temperature used during reflux heating and the poly(vinylpyrrolidone) molecular weight. Furthermore, the size of the nanoparticles increased by a 2-step process as the reflux heating time increased. The IR spectral changes with increasing reflux time indicated the increase in the number of cross-linked polymers in the shell. From these results, the formation mechanism was discussed and proposed.     

Development of Resistive Oxygen Sensors Based on Cerium Oxide Thick Film

It is important to shorten the response time of resistive oxygen sensor in order to reduce harmful emission of automobiles. The diffusion and surface reaction theory tells us that reducing particle size leads to shortening the response time. The fine ceria powder was prepared a by new precipitation method and the oxygen sensors having ceria thick film with the particle size of 120 nm were fabricated using fine ceria (cerium oxide) powder. The thick film exhibited good adhesion to alumina substrate. The value of n in R P(O₂)^1/n at 1073 and 1173 K were 6.2 and 6.4 in the oxygen partial pressures range from 10^{– 13} to 10⁵ Pa, respectively. The response time for the sensor was 22 and 12 ms at 1073 and 1173 K, respectively. The sensor fabricated in this study showed fast response.     

Kinetic behavior of resistive oxygen sensor using cerium oxide

In order to decrease exhaust gas emissions, oxygen gas sensors with fast response are required. We evaluated two kinds of fast response time (<1 s) for two oxygen sensors with different cerium oxide particle sizes and crystallite sizes, using two methods: the commonly used jump method and the so-called dynamic method. The dynamic method consists of comparing the amplitude of oxygen partial pressure with that of the sensor output, following the changes in oxygen partial pressure produced by periodic modulation of the hydrostatic pressure with the composition of the atmosphere kept constant. The response times obtained with the jump method and dynamic method are defined as t₉₀ and t_b, respectively. Further, we evaluated the relationship between the amplitude magnitude of the oxygen sensor output (A_n) and the frequency of the oxygen partial pressure (f), using the dynamic method. The results obtained were as follows. The value of t_b for the oxygen sensor with a crystallite size and grain size of about 100 nm was 134 ms or less at 1173 K. The value of t₉₀ was 20 and 1 ms when the oxygen partial pressure changed from high to low and from low to high, respectively. From a plot of log A_n versus log f, it was concluded that the kinetics of a sensor using cerium oxide with crystallite and grain sizes from 100 to 300 nm were controlled by diffusion when the oxygen partial pressure was periodically changed in the shape of a sine wave. It was found that the newly developed equipment was able to evaluate two kinds of response times less than 50 ms.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Adaptive particles for incompressible fluid simulation

We propose a particle-based technique for simulating incompressible fluid that includes adaptive refinement of particle sampling. Each particle represents a mass of fluid in its local region. Particles are split into several particles for finer sampling in regions of complex flow. In regions of smooth flow, neighboring particles can be merged. Depth below the surface and Reynolds number are exploited as our criteria for determining whether splitting or merging should take place. For the fluid dynamics calculations, we use the hybrid FLIP method, which is computationally simple and efficient. Since the fluid is incompressible, each particle has a volume proportional to its mass. A kernel function, whose effective range is based on this volume, is used for transferring and updating the particle’s physical properties such as mass and velocity. Our adaptive particle-based simulation is demonstrated in several scenarios that show its effectiveness in capturing fine detail of the flow, where needed, while efficiently sampling regions where less detail is required.     

Preparation of SnO2 nanoparticles less than 10 nm in size by precipitation using hydrophilic carbon black powder

Tin oxide powder has recently been prepared by a modified precipitation method in which the precipitate is mixed with carbon black powder, a method which offers low cost and mass production. In this study, we prepared tin oxide powder by the modified precipitation method using hydrophilic and non-hydrophilic carbon black powders. When using the hydrophilic carbon black powder, the crystallite size of the tin oxide after firing at 600 °C was 7.5 nm. In contrast, when using the non-hydrophilic carbon black powder, the crystallite size was 18 nm. Thus the crystallite size was shown to depend on the hydrophilicity of the carbon black powder. The smaller crystallite size obtained may be a result of the gel precipitate (stannic acid) particles being dispersed more uniformly on the surface of the hydrophilic carbon black powder than those on the surface of the non-hydrophilic carbon black powder.