We investigate the formaldehyde gas sensing properties of poly(5,6,7,8-tetrahydro-1-naphthylamine)-intercalated MoO₃ thin films ((PTHNA)_xMoO₃). The resistance responses of (PTHNA)_xMoO₃ to formaldehyde increase with increasing intercalation temperature. X-ray photoelectron spectroscopy reveals that the molar ratio of Mo⁵⁺ decreases with increasing intercalation temperature.

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In this study, we investigated the Pt catalyst effects on two structures through loading a Pt catalyst onto a resistive oxygen sensor using Ce_0.9Zr_0.1O₂ thick film. We used (1) Ce_0.9Zr_0.1O₂ thick film with a Pt/alumina layer, and (2) Pt/Ce_0.9Zr_0.1O₂ thick film, in which Pt catalysts were prepared by heating platinum chloride. In the case of sensor structure (1), it became clear that the Pt/alumina layer with a Pt concentration of 40 wt% was the most effective in the Pt/alumina layers fabricated in this study, where R(λ = 0.6)/R(λ = 1.4) was 0.0099, and R(λ = 0.6)/R(λ = 1.4) of the 40 wt% Pt/alumina layer decreased with decreasing temperature. In the case of sensor structure (2), the minimum values of R(λ = 0.6)/R(λ = 1.4) at 600 and 700°C were the same at 0.035. The ultrasonic treatment in the Pt loading process improved R(λ = 0.6)/R(λ = 1.4) to 0.02. The Pt particle size in this study was in a range from 20 to 100 nm because of the high temperature annealing in the Pt loading process.

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At present, lean-burn type engines or diesel engines for passenger cars have been developed and installed into automobiles to improve fuel efficiency. In these cases, NOx storage catalysts or selective catalytic reduction (SCR) systems are also installed for reducing NOx emission. In order to control these systems more efficiently, NOx sensor which responds selectively, quickly, and quantitatively are necessary. Here we focused on the NO direct decomposition function of La-based perovskite oxides. We applied them to a sensing electrode for the amperometric NOx sensor using yttria-stabilized zirconia (YSZ). La_0.8Sr_0.2MO₃ (M = Co, Mn, Fe, Ni) powders were synthesized by means of spray pyrolysis method. Among four kinds of oxides studied, the sensor using La_0.8Sr_0.2MnO₃-SE showed the highest sensitivity to NO₂ as well as low base current even in the presence of excess O₂ (21 vol%). The sensor also exhibited excellent NO₂ selectivity at 500°C. The sensitivity was increased with an increase of NO₂ concentration in an examined range between 50 to 800 ppm, and did not change in the examined O₂ concentration range (5-21 vol%).

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In order to reduce the electric resistance of the WO₃ sensing film fabricated from the ion-exchange method, lamellar-structured and Re-doped WO₃ was prepared through the acidification method. The mixed solution of Na₂WO₄ and Re₂O₇ was dropped into the mixed solution of H₂SO₄ and HCHO. Then the obtained gel was deposited on alumina substrate with Au electrode for thick film devices and calcined at 300°C. The size of the Re-doped WO₃ particles was smaller in one order in magnitude than that of the WO₃ by the ion-exchange method. The electric resistance of Re-doped WO₃ device in air showed minimum value at the 4 at% Re doping due to the increase in the donor density in the WO₃ crystal. As a result, the Re-doped WO₃ device exhibited low electric resistance less than 10⁸ Ω even in the 800 ppb NO₂. Such a low electric resistance device is suitable for the detection of the wide range of NO₂.

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Micro-machined thermoelectric hydrogen sensors (micro-THS) with catalyst combustors were fabricated using Si and SiGe thin films prepared using the sputtering or chemical vapor deposition methods, and the relationship between the thermoelectric property of the films and the performance of the sensors were investigated. The Seebeck coefficients of the Si and SiGe films were evaluated as 107-246 and 82-126 μV/K at temperatures of 50-480°C. The combustion heat estimated by increasing the temperature of the catalyst on the micro-THS with the Si thin film was 7-20% smaller than that of SiGe thin film. Moreover, the voltage signal of the micro-THS with the Si thin film was 8-13% larger than that of SiGe thin film. Both the Seebeck coefficient of the Si thin films and the voltage signal of the micro-THSs with the Si TE films were better than those of the SiGe thin film, although the combustion heat of the catalyst on the micro-THSs with the Si thin films was smaller compared to that with the SiGe thin film. For the micro sensor application using the thermoelectric thin film, the Seebeck coefficient is the most important factor to improve the sensor performance.

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In order to perform a high throughput exploration of sensor materials using surface plasmon resonance (SPR), the gas sensing property of a ZnO/Au/SiO₂ chip with SPR and the enhancing effect of UV irradiation on the desorption rate of NO₂ from the ZnO surface were investigated. When the ZnO/Au/SiO₂ chip was exposed to a high concentration of NO₂ (1000 ppm), a large peak shift was observed in the SPR curve. However, this sensing signal for NO₂ gas did not recover to the baseline. In the case of low-concentration NO₂ (10 ppm), the peak shift of the SPR curve was lower than that in the case of the high-concentration gas, but recovery to the baseline was observed. From the X-ray photoelectron spectra for N 1s of the ZnO thin films exposed to 1000- and 10-ppm NO₂, two chemisorption states—NO₂^- (403.7 eV) and NO₃^- (407 eV)—were confirmed. After the ZnO film was irradiated by UV rays, exposed to 10-ppm NO₂, all peaks related to N 1s disappeared. However, in the case of the ZnO film exposed to 1000-ppm NO₂, adsorbed NO₃^- remained on the surface of ZnO. From these results, it was found that UV irradiation effectively assisted NO₂ desorption from the surface of the ZnO thin film exposed to 10-ppm NO₂.

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A stabilized zirconia-based sensor attached with an oxide sensing-electrode (SE) was developed for the selective detection of propylene (C₃H₆), representative hydrocarbon (HC), at high temperature. Among six kinds of commercial oxides examined, NiO-SE gave the highest sensitivity to C₃H₆ at 600°C under the wet condition. However, other sensing characteristics, such as the C₃H₆ selectivity, the response and recovery times, of this sensor were not satisfactory. Thus, to improve the C₃H₆ sensing characteristics, NiO-SE was successively mixed with different oxides. For better reproducibility of response, all the single- or mixed-oxide SEs were prepared by ball-milling. As a result, the sensor attached with NiTiO₃-SE (the initial mixture of 50 mol% NiO and 50 mol% TiO₂) exhibited highly selective and sensitive response to C₃H₆ at 650°C, accompanying with relatively quick response and recovery. In addition, by the comparison of sensing performances of NiTiO₃-SEs fabricated by the different methods, the ball-milling method was found to be the most appropriate from the viewpoint of obtaining high selectivity and sensitivity to C₃H₆.

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This paper reports a micro gas sensor assembled with tin oxide, SnO₂, nano-powders. The micro gas sensors are fabricated by a modified micro molding in capillary (MIMIC) technique using an ethanol suspension with 2.0 vol% SnO₂ nano-powders. A micro pattern is formed as a set of parallel lines 23 μm in width and 1.2 μm in depth. The micro pattern is heat treated at 300°C for 5 h. The micro-sensor was able to detect ethanol gas by measuring the electrical resistance of the tin oxide a glass tube furnace. When ethanol vapor flows through the furnace the resistance decreases and the response time is 1.9 s. The sensor resistance ratio was determined to be 19.5 at 450°C. Our modified MIMIC technique enables easy and simple fabrication of ethanol vapor micro-sensors in a cost-effective way.

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The current gas sensors have a problem in selectivity of gas and using the dynamic response is one solution to identify the gas type. In the dynamic response, the information from the non-linear phenomena under the transient temperature conditions was used to recognize various gas types with a single sensor element. The mechanism was slightly revealed so far and this work aimed at determining the key factors of the dynamic response. Non-doped SnO₂ films were prepared by a liquid phase deposition (LPD) method to control the morphology of the film and the components of the sensor. Nano-scale control in the thickness was demonstrated by the simple LPD process. The gas type was successfully identified even with the non-doped SnO₂ films. The film thickness did not largely affect the profile of the signal and the dynamic response was considered to be due to basic phenomena between the gas and the sensor material itself such as adsorption and desorption.

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Potentiometric CO₂ sensors based on NASICON (Na_1+xZr₂Si₂PO₁₂) were fabricated by combining with a Li₂CO₃-BaCO₃ (1:2 in molar ratio) auxiliary phase. The CO₂ sensing properties of NASICON (x = 0, 1, 2, 2.75) sensors were investigated under dry condition at 450°C. The electromotive force (EMF) values of NASICON sensors were proportional to the logarithm of CO₂ concentrations in the range of 250 to 2500 ppm. When exposed to dry air containing 250 ppm CO₂, the EMF values of each sensor continuously changed toward a negative potential. From a X-Ray Diffraction (XRD) measurement, it was confirmed that new crystal phases such as Li₂ZrO₃, ZrO₂, Na₂Zr(PO₄)₂ and Na₅Zr(PO₄)₃ were formed at the interface between sensing electrode and solid-state electrolyte, because of reaction between Li₂CO₃ and NASICON. The result of XRD measurement suggested that the base-EMF change closely related with the decrease of Li⁺ activity in auxiliary phase and Na₂O activity in NASICON.

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Silver implantation (75 keV, 2 × 10¹⁴ ions/cm²) in ZnO ceramics was carried out at room temperature to characterize the diffusion phenomenon. Annealing caused Ag evaporation from the sample. Results suggest that it is difficult to incorporate Ag into Zn site by annealing. The Ag profile with the monotonous decrease in concentration below 3 × 10¹⁷/cm³ was observed in ZnO annealed at 900°C. Above results become the basic results for producing the ZnO sensor used the reaction at surface and the grain boundary.

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Li₂O-Al₂O₃-SiO₂ (LAS) system-based ceramics have been prepared with petalite and kaolin and their thermal shock resistances investigated. Feed slurries in five combinations of petalite and kaolin composition between 40% and 60% were first prepared with 5, 15 and 35 μm fractions of petalite and slip casted into moulds. Then the casts were subjected to 2-stage sintering: in the first stage, at 850°C for 2 h and in the second stage, at 1200, 1250 and 1300°C for 2 h. Heating rate was 3°C per minute. The characteristics of sintering, microstructure, X-ray diffraction pattern and water absorption rate of the LAS ceramics produced were studied. Their thermal shock resistances were also measured by a directly flame contact method. Petalite was transformed into β-eucryptite and β-spodumene at temperatures higher than 1200°C. The LAS ceramics were found to have the highest thermal shock resistance at 1250°C. Even at the reduced addition of 45% petalite, the more the particle size of petalite increase, the better is thermal shock resistance of the LAS system ceramics.

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Eu doped Li-Nb-Ti oxide (LNT) solid solutions were prepared by heating mixed powders at temperatures of 1120°C for up to 24 h. Despite the fact that these materials often possess a crystallographic superstructure, no such structure was observed for a sintering time of 10 h. For the longer sintering period of 24 h, no superstructure was observed for samples with less additions of TiO₂, but it was confirmed by X-ray diffraction and a High-Resolution Transmission Electron Microscopy that samples with compositions containing 20 and 25 mol%-TiO₂ did possess superstructures. Heating for 24 h resulted in an improvement of the photoluminescence (PL) intensity of the LNT solid solutions, and this increase was more marked in the samples where the superstructure was developed. For the Eu-doped LNT solid solution containing 25 mol%-TiO₂, the PL intensity of the sample with the superstructure was almost 3 times as high as that of a sample with the same doping but without the superstructure.

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The aim of this study is to obtain highly densified cordierite ceramics using a coal fly ash as one of the starting raw materials. In the previous studies, however, only porous cordierite ceramics were obtained since the utilized coal fly ashes were not finely pulverized. It was because fly ash was generally so hard to be finely pulverized. In addition, no detailed information for the properties of the obtained cordierite ceramics was available. We succeeded in pulverizing the fly ash by improving pulverizing procedure. Namely, its original grain size of 12 μm could be decreased to 1.2 μm. The pulverized fly ash was mixed with magnesia and alumina sources in a wet ball mill. X-ray diffraction revealed the presence of cordierite phase only for the sample heat-treated at 1150°C. The density of the sintered powder reached 97% theoretical, which was beyond the value required for HIP treatment = 95%. The sintered samples using the pulverized powder revealed sufficiently high thermo-mechanical and electrical properties compared with those of the sintered samples using the unpulverized powder. For instance, flexural strength of 132 MPa, fracture toughness of 3.0 MPa·m^1/2 , thermal expansion coefficient of 2.1 × 10^-6/K and dielectric constant of 6.5 were obtained for the sample using the pulverized powder. HIP treatment enhanced flexural strength of the treated samples and suppressed bubble formation in them.

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Single crystals of an organically templated holmium hydrogen phosphate sulfate, [enH₂]_0.5[Ho(HPO₄)(SO₄)(H₂O)] (en; ethylenediamine) were prepared by hydrothermal reaction and its crystal structure was determined by single crystal X-ray diffraction data. This compound crystallized in the monoclinic system, space group P2₁/a (No. 14) and the unit cell parameters are a = 12.938(5), b = 6.834(3), c = 9.100(2) Å, and β = 88.12(2)°. The crystal structure is similar to that of [enH₂]_0.5[Ce(HPO₄)(SO₄)(H₂O)] which had a layered structure built up by Ce centered polyhedra and HPO₄ and SO₄ tetrahedra, and the diprotonated ethylenediamine cations were located in the interlayer. For other rare earth elements two types of compounds were obtained under hydrothermal conditions. The compounds of R = La, Pr, Nd, Sm and Eu are isostructural with [enH₂]_0.5[Ce(HPO₄)(SO₄)(H₂O)] and those of Gd, Tb, Dy, Er and Yb with [enH₂]_0.5[Ho(HPO₄)(SO₄)(H₂O)]. The unit cell volumes of these compounds decrease by lanthanide contraction. The compounds of R except La, Sm and Eu exhibited the Curie-Weiss paramagnetic behavior.

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The cohesive energy was found to be nearly proportional to the melting point that has correlated empirically with the linear thermal expansion coefficient of solids. So, the relationship between the linear thermal expansion coefficient and the cohesive energy was examined at 295, 500, 1000 and 1200 K by using the experimental data of various metals, oxides, borides, carbides and nitrides. As a result, it was revealed that the linear thermal coefficient is inversely proportional to the cohesive energy. Therefore, there is a great possibility of evaluating the linear thermal expansion coefficient of these materials from the ab-initio calculation of the cohesive energy. This proposed method will be very practical and effective in designing new materials.

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First, we succeeded in synthesizing and sintering cordierite ceramics via the solid state reaction route without the melting process by using single phase oxide powders. They are highly pure and fine amorphous silica, α-alumina and magnesia powders. Usually, cordierite ceramics was synthesized by a sol-gel route. Alternatively, it was prepared by a solid state reaction route using impure minerals such as talc, or the other which required melt-quenching and crystallization processes, though using single phase raw material powders. This is probably because single phase raw material powders especially alumina are deficient in reactivity. We have succeeded in synthesizing cordierite ceramics via the solid state reaction route without melt-quenching and crystallization processes, by adopting a reactive α-alumina as one of raw material powders. As a result, cordierite formation of the starting raw material mixture went to completion at 1270°C. The mixture was sintered at 1430°C for 2 h. Its bulk density reached 98% theoretical, its flexural strength reached 243 MPa, which agreed with the best value already reported, its fracture toughness was 3.5 MPa·m^1/2 far larger than the reported value of 2.3 MPa·m^1/2, its thermal expansion coefficient (from RT. to 800°C) is 1.8 × 10^-6/K, which might be probably the ultimately smallest value experimentally expected and its dielectric constant was sufficiently small as 4.8 to 5.2 between extremely a wide frequency range, namely, from 1 MHz to 1 GHz with low loss tangent of < 6 × 10^-3. These properties were found more excellent or at least comparable compared with the best properties reported already and suitable for substrate materials on micro-electronic applications.

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This paper describes the Vickers hardness (H_v) of dense β-Si_6-zAl_zO_zN_8-z (z = 1, 2, and 3) prepared by the combination of combustion synthesis (CS) and spark plasma sintering (SPS). The Vickers hardness was measured by a Vickers microhardness tester in the applied load ranging from 0.981 N (0.1 kg) to 19.614 N (2.0 kg) at room temperature. A significant indentation size effect (ISE) is observed for each sample at low indentation test load. The ISE shows more clearly on the β-SiAlON with z = 3, while it reaches a saturation value when the applied indentation load F ≥ 4.903 N for z = 1 and 2. In conclusion, the Vickers hardness is 17.6 GPa for z = 1, 16.2 GPa for z = 2, and 14.7 GPa for z = 3 under the load of 19.614 N.

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Hollow cadmium molybdate microtubes were successfully prepared via a sacrificial template route at a low temperature assisted by a ionic liquid, 1-butyl-3- methylimidazolium chloride. The morphology and properties of the CdMoO₄ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescent spectra techniques (PL), respectively. The forming of microtube precursors plays a key role in the shaping process of hollow structure. The special morphology CdMoO₄ showed a distinct optical properties in PL spectra analysis, which can be potentially applied on fabrication of solar batteries and optical devices, and as material encapsulators or carriers.

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