Search

We discuss the synthesis of cuboid-monoclinic tungsten oxide (C-m-WO3) particles and hexagonal-plate-like-hexagonal tungsten oxide (H-h-WO3) particles by acidification and hydrothermal treatment with (NH4)10W12O41·5H2O as a starting material. From the WO3 particles, thin- and thick-film gas sensors were fabricated for NO2 sensing applications and the effect of the morphology and crystal structure of the WO3 particles on the NO2 sensing properties was investigated. The extremely high sensor responses of 160 to 20 ppb NO2 and 10 120 to 500 ppb NO2 were obtained for a C-m-WO3 thin-film sensor at an operating temperature of 200 °C. The crystal structure features of as-prepared particles were studied by XRD, XPS, SEM, TEM, and solid state 1H and 13C NMR measurements. The presence of residual impurities, cations, and high density of surface OH− in the H-h-WO3 particles was demonstrated, which could be one of the key reasons for the crystal structure dependence of the sensing properties in C-m-WO3 and H-h-WO3 gas sensors.

Macroporous (mp-) In2O3-based microspheres as a NO2 sensing material were prepared by the pyrolysis of atomized In(NO3)3 aqueous solutions containing polymethylmethacrylate (PMMA) microspheres (150 nm in diameter) as a template. Well-developed spherical macropores (less than 100 nm in diameter) reflecting the morphology of the PMMA microsphere templates could be formed in the In2O3-based microspheres. The introduction of macropores into In2O3-based microspheres was very effective in improving the NO2 response of their thick films fabricated on an alumina substrate equipped with interdigitated Pt electrodes (gap size: ca. 200 μm) by screen-printing. In addition, the addition of a little amount of SnO2 to the mp-In2O3 microspheres not only lowered the resistance in air but also improved the NO2 response. NO2 sensing properties of non-stacked microspheres of the mp-In2O3 mixed with SnO2 were also investigated by utilizing nano-gap Au electrodes (gap size: ca. 200 nm). The non-stacked microspheres showed fast response and recovery speeds to NO2, because of better diffusion capability of NO2., Sensors and Actuators B: Chemical, 151(1), pp.265-273; 2010

Carbon nanofibers (CNFs) mat was synthesized from Au–Pd catalyst at 700 °C for 60 min in 10 kPa of the gas mixture of C2H2, H2 and Ar by thermal chemical vapor deposition (CVD). Fine Au–Pd catalyst as admixture on the growth substrate of SiO2 was adopted for the growth of CNFs mat. Fine Pd particles without Au particles were detected at the top/inside of as-grown CNFs by TEM-EDS analysis. The Au component within Au–Pd particles with ca. 1.5 nm did not contribute to the CNF growth. On the other hand, the growth scale of individual CNF was ranged from 5 to 150 nm in diameter and less than 100 μm in length, by controlling H2 or Ar concentration in carbon source gas of C2H2/Ar or C2H2/H2/Ar. It seemed that Ar gas accelerated the growth of the CNFs mat and the length of each CNF was increased with increasing the supplying amount of H2. In contrast, H2 induced the lower growth density of CNFs mat and the decrease in the diameter of those. The CNFs mainly grow through adsorption and decomposition of acetylene on the surface of Au–Pd catalyst and precipitation of carbon on the surface of those.

Ultrahigh-sensitive WO3 nanosensors have been developed by using an interdigitated Au nano-electrode for dilute NO2 detection. The interdigitated Au nano-electrodes with a line spacing of 200 nm and various teeth numbers were fabricated on a SiO2/Si substrate by means of electron beam lithography. A WO3 thin film was deposited on the interdigitated Au nano-electrode to be a WO3 nanosensor. The nanosensor exhibited an extremely high sensor response (Rg/Ra = 28) to 0.01 ppm NO2 with excellent response-recovery characteristics. Interestingly, the sensor response increased with increasing the number of teeth in the interdigitated nano-electrode, i.e., the length of interface between oxide grain and Au electrode. This suggests that the nano-design of electrode structure is important for development of a highly sensitive gas sensor.

The micro gap effect has been investigated on dilute H2S sensing using SnO2 thin film microsensors equipped with Au or Pt micro gap electrode. The micro gap electrode was fabricated by means of MEMS techniques (photolithography and FIB) and the gap size was varied in the range of 0.1–1 μm. For the SnO2 sensors equipped with Au micro gap electrode, the sensor response to dilute H2S was slightly increased with decreasing gap size, suggesting the small micro gap effect. The sensor response was divided into that at oxide–electrode interface (Si) and at oxide grain boundary (Sgb). It was found that the small Sgb/Si ratio was responsible for the small micro gap effect and that the high sensor response at grain boundary was characteristic in SnO2–H2S system. On the other hand, the SnO2 sensors with Pt micro gap electrode, the clear and large micro gap effect was obtained on dilute H2S sensing. In the Pt electrode sensor, the large Sgb/Si value induced the large micro gap effect. It was found that the Pt/SnO2 interface showed much higher sensor response than the Au/SnO2 interface. The electrode material strongly affected the sensing properties of semiconductor gas sensor with micro gap electrode.

The CNTs with 20–50 nm in diameter were directly grown on Au microgap electrode by means of thermal CVD at700∘C for 60 minutes under EtOH-Ar-H2atmosphere (6 kPa). The CNTs with entangled shape formed the network structure with contacting each other. In the CNTs-WO3composite,WO3grains with disk shape (50–200 nm) were independently trapped. The CNTs-WO3composite sensor showed the fairly good sensor response (Ra/Rg = 3.8 at200∘C). The sensor response was greatly improved with CNTs-WO3composite, comparing with that of CNT sensor (Ra/Rg = 1.05). This phenomenon can be explained by formation of p-n junction, between CNT(p) andWO3(n), and thus improvement ofNO2adsorption. The sensor response was decreased with increasing theWO3amount in CNTs-WO3composite, suggesting the electronic conduction due toWO3connection.

Noble metal (Au, Pd, Pt) loaded SnO2 sol solutions have been synthesized under hydrothermal condition. The mixed solution of noble metal chloride (HAuCl4, H2PtCl6, or PdCl2) and SnCl4 was neutralized with NH4HCO3 solution. The precipitate gel obtained was thoroughly washed and finally treated under hydrothermal condition (200 oC for 3 h at pH10.5) to obtain transparent sol solution. From SEM and TEM images of powder or thin film derived from sol, any noble metal particles could not be observed. However, the grain growth was suppressed when calcined at 900 oC for noble metal loaded SnO2. The peaks of Sn3d and O1s levels in XPS were shifted to the lower binding energy side. The electrical resistances of thin films prepared from noble metal loaded SnO2 sol were larger than that of pure SnO2 thin film. These results suggested that the noble metals were certainly loaded on SnO2 surface or included in the film to modify the surface state of SnO2.

This paper deals with a pyrolyzed polymer as a kind of carbon materials obtained by pyrolysis of polymers. Through a combination of micromachining of polymers and following pyrolysis process, we can obtain pyrolyzed polymer MEMS structures for various applications. This work focuses on the effect of NO2 gas adsorption on electrical properties of pyrolyzed polymers. The resistance of the sensor is dramatically decreased when the sensor is exposed to the NO2 gas. It is found that low pyrolysis temperature and high operating temperature contribute to higher sensitivity against gas mixture of NO2.

Effects of gap size on dilute Cl 2 gas sensing properties have been investigated for the In 2 O 3 thin film microsensors equipped with micro-gap electrode. The micro-gap electrodes with various gap sizes (0.1–2 μm) were fabricated on SiO 2 /Si substrate by means of MEMS techniques (photolithography and FIB). Then the In 2 O 3 thin film was deposited on the micro-gap electrode to be micro gas sensor. The resistance of In 2 O 3 microsensor was increased upon exposure to dilute Cl 2 gas of ppm-level, while the sensor responded to Cl 2 gas less than 0.5 ppm with resistance decrease. Interestingly, in both cases the sensitivities were increased with decreasing gap size, although the starting gap size of sensitivity increase were different from each other. The high sensitivity was obtained for In 2 O 3 microsensor with 0.1 μm gap. The number of grains was decreased with decreasing gap size, inducing the increasing contribution of interface between electrode and oxide. The sensitivity was increased due to the high sensitivity at interface. The similar effect of number of grains in the micro-gap was examined for the grains with different size packed in the 0.1 μm gap electrode. The different grain size was obtained after calcinations at 600–850 °C. With decreasing number of grains, the sensitivity to 1 ppm Cl 2 (resistance increase) was increased, while the sensitivity to 0.5 ppm Cl 2 (resistance decrease) was decreased. It is assumed that the condition of oxide–electrode interface or In 2 O 3 surface is changed with calcination.

Aluminum phosphate is useful catalyst for many reactions. There are a lot of reports about catalytic activities of aluminum phosphate, however little reports about the relationship among the preparation, composition, catalytic activity of this phosphate. Phosphates transform to various other phosphates with hydrolysis and dehydration reactions by heating. In this work, some aluminum orthophosphates and condensed phosphates were prepared, and their compositions were estimated. Furthermore, specific surface area, acid strength, the amount of acidic sites, and the catalytic activities for the decomposition of trifluoromethane of these salts were investigated. Aluminum ortho‐ and polyphosphates were prepared in the systems of ortho‐ and poly‐phosphates. The catalytic activity for decomposition of CHF3 was related with a certain degree of specific surface area, acid strength, and the amount of acidic sites. Aluminum orthophosphate with a small amount of sodium cation had higher catalytic activity at lower temperature.

Neodymium oxide was added in the formation systems of various nickel phosphates (orthophosphate, diphosphate, and cyclo-tetraphosphate) from phosphoric acid and basic nickel carbonate. The thermal products were estimated by differential thermal analyses, X-ray diffraction, and Fourier transform infrared spectroscopy. Specific surface areas of phosphates were calculated from the amount of nitrogen gas adsorbed at the temperature of liquid nitrogen by BET method and catalytic activities for decomposition reaction of NF3 were examined. From the obtained results, the influence of the addition of neodymium oxide on formation and catalytic properties of various nickel phosphates was clarified.

The anodic porous alumina with a cell size of 25 to 40 nm was prepared by applying a voltage of 20 V for 90 s in 20 wt % sulfuric acid at 15°C. The nickel catalyst for the growth of carbon nanotubes (CNTs) was cathodically deposited at the bottom of the porous alumina at a constant current density of 10 mA/cm2. Highly ordered CNT arrays with a diameter of 15 to 20 nm were linearly grown at 625°C for 60 min along the inner wall of the porous alumina by ethanol CVD. On the other hand, the CNT arrays obtained using acetylene had twisted shapes and disordered carbon layers irrespective of the configuration of the porous alumina. This suggests that the alignment of CNTs depends on the crystallinity.

The WO3 thin film nanosensors have been fabricated by dropping and calcination of H2WO4 suspension onto Au nano-gap electrode (100 nm of gap-size and 10 µm in width), which was fabricated by means of MEMS (Micro Electro-Mechanical Systems) techniques, photolithography and FIB. The amount of WO3-electrode interface was controlled by changing amount of WO3 grains packed in the nano-gap electrode. The sensitivity to dilute NO2 was increased with increasing amount of interface, suggesting the importance of oxide-electrode interface in NO2 sensing. Further, the response behavior was affected by the amount of WO3 grains packed in nano-gap electrode.

The micro-gap electrodes with various gap sizes (0.1–1.5 μm) were fabricated on Si substrate by using MEMS techniques (photolithography and FIB) and the WO3 thin film was deposited on the micro-gap by suspension dropping to be micro-gas sensor. The effect of gap size on sensing properties to dilute NO2 (0.05–0.5 ppm) was investigated at 200 °C. The sensitivity to dilute NO2 almost unchanged irrespective of gap size larger than 0.8 μm. On the other hand, the sensitivity tended to increase with decreasing gap size less than 0.8 μm. The sensitivity (S = Rg/Ra) to 0.5 ppm NO2 was as high as 57 at gap size of 0.11 μm and was expected to increase further when the gap size was decreased less than 0.1 μm. The behavior was explained with the number of WO3 grains in the micro-gap and the resistance changes at two kinds of boundaries (grain boundary and electrode–grain interface).

Nickel pyrophosphates containing metal cations were prepared from aqueous solutions. The obtained phosphates were characterized with inductively coupled plasma (ICP), differential thermal analyses (DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), specific surface area, acid strength, amount of acidic sites, and adsorbed carbon measurements. The phosphates were used as a catalyst for decomposition of trifluoromethane. The kinds and ratios of metal cation added were discussed. The nickel pyrophosphate catalysts containing magnesium had higher selectivity to CO 2 and longer lifetimes than aluminum orthophosphate for decomposition of trifluoromethane.

WO 3 thin film sensors have been fabricated by suspension dropping on SiO 2 /Si substrate equipped with Au microelectrodes, and subjected to the detection of dilute NO 2 less than ppm level. The WO 3 thin film with the thickness of ca. 0.6 μm showed the extremely high sensitivity to dilute NO 2 at 200 °C (for example, S =9.4 to 0.2 ppm NO 2 ). The Au electrodes were visible for the thin film of 0.6 μm, inducing the easy access of NO 2 molecules to a part between electrodes. Moreover, the high sensitivity suggests the possibility of environmental monitoring using WO 3 thin film sensors.

Indium oxide based thin films have been prepared by an electron beam evaporation and subjected to the detection of dilute Cl2 gas less than ppm-level. Among various In2O3 based sensors tested, the In2O3 thin film modified with Fe2O3 (1 wt%) showed extremely high sensitivity to dilute Cl2 gas of 0.2 – 5 ppm. The Fe203-In2O3 sensor showed the sensitivity as high as 4.5 to 0.2 ppm Cl2 in air. Effects of Fe2O3 addition were assumed to be the stabilization of lattice oxygen and the increase in number of adsorption site of Cl2.

Electrodeposition was used to grow epitaxially BaTiO3 thin films on SrTiO3 single-crystal substrates with La0.7Sr0.3MnO3 (LSMO) conducting buffer layers. The epitaxial films appeared to consist of very small (ø10 nm) particles. The film completely covered the substrate when the reaction was performed at temperatures between 60 and 90 °C with LSMO potentials of –0.5 to –1.0 V against a Pt counter-electrode. It appeared that an electrophoretic force, acting on BaTiO3 nuclei within the solution, facilitated the deposition of the film.

The ZnSb 2 O 6 thick-film was prepared by dip-coating into sol solution (30.4 and 15.2 wt.%), in which fine particles of ZnSb 2 O 6 were dispersed, followed by calcination at 500–1000°C for 1 h in air. The sensing properties to dilute H 2 S were examined for the films prepared under various conditions. The 0.7-μm-thick film prepared from the twice-diluted sol solution (15.2 wt.%) and calcined at 900°C exhibited the excellent sensing properties to dilute H 2 S. The film could detect H 2 S as dilute as 0.01 ppm, which was lower than the legal regulation value for H 2 S. The film showed good selectivity to H 2 S and the response was stable for repeated exposure to H 2 S. The film consisted of large grains (60–100 nm), which had contact with each other, and open pores (100–150 nm), which formed the well-developed channel in the whole film. This is attributable for the excellent sensing properties.

A semiconductor gas sensor using SnO 2 was loaded with acidic or basic oxides (5 wt.%) to investigate ethanol-gas sensing properties and related catalytic properties. The sensitivity to ethanol gas at 300°C increased tremendously with an addition of a basic oxide (e.g., La 2 O 3 ), while it hardly changed with that of an acidic oxide (WO 3 ). It turned out that the addition of the basic metal oxide to SnO 2 brought about enhancement of catalytic activity not only for the dehydrogenation of ethanol gas to CH 3 CHO but also for the consecutive oxidation of CH 3 CHO to CO 2 . On the other hand, the acidic metal oxide enhanced only the dehydration reaction, showing even an adverse effect on the consecutive oxidation. Based on these results, it was concluded that the enhancement of the catalytic oxidation activity to an appropriate level could be a reason for the high sensitivity to ethanol gas for the sensors loaded with basic oxides, particularly one loaded with La 2 O 3 .

Trivalent cation-doped sodium zirconium phosphates Na1−xMxZr2−xP3O12 (M: In, Yb, Er, Y, Dy, Tb, and Gd; 0≦x≦2) were prepared, and their solubility limit and electrical conductivity were examined. There exist two types of NASICON-related phases; previously reported (M-NZP) and newly observed ones (NMP; x=1.0–1.9 for M=Yb, Er, Y, Dy, and Tb). NMP is a metastable phase and easily absorbs water. Arrhenius plots show linearity within the M-NZP phase region while some kinks within the NMP phase region, indicating the difference of the conduction mechanism between both phases.

Oxygen permeation properties of Sr{sub 1{minus}x}A{prime}{sub x}CoO{sub 2.5+{delta}} (A{prime} = Ca, Sr, Ba, and Mg) were investigated. The oxides with x {le} 0.2 had hexagonal 2H-BaNiO{sub 3}-type structure at room temperature. The disk membranes (1.0 mm thick) made of these oxides, when cooled from high temperature, showed significant rates of oxygen permeation down to critical temperatures characteristic of the kind of oxides, below which the rates sharply diminished. Among the oxides tested, Sr{sub 0.9}Ca{sub 0.1}CoO{sub 2.5+{delta}} showed the largest rate (2.0 cm{sup 3} at standard temperature and pressure min{sup {minus}1} cm{sup {minus}2}) at 900 C. As indicated by the X-ray diffraction and thermogravimetry-differential thermal analysis, this oxide underwent phase transformation between the above structure (low-temperature phase) and a cubic perovskite structure (high-temperature phase), which were oxygen nonpermeable and oxygen permeable, respectively. On heating, the oxygen content (2.5 + {delta}) of the oxide decreased in two steps, i.e., first from 2.75 to 2.6 over the temperature range 400 to 600 C without changing the structure, and then from 2.6 to 2.5 at the temperature of transformation to the high-temperature phase (about 900 C). The rate of oxygen permeation of the oxide at 900 C was found to depend on the membranemore » thickness. Based on these results, oxygen permeation and its relevance to phase transformation are discussed.« less

A low pressure evaporation method was adopted for preparing SnO2 and CuO–SnO2 thin films. Metallic tin and copper were evaporated on the alumina substrate under a low pressure (1 torr) of air atmosphere. As observed with AFM, these evaporated films had unique microstructure in which discrete clusters of SnO2 grains (30 nm) contacted to each other two-dimensionally with large mesopores penetrating between. The film added with a small amount of CuO exhibited very high sensitivity to H2S in air, being able to detect dilute H2S close to 0.02 ppm at 300 °C. The high sensitivity seems to result from the unique promoting effect of CuO coupled with the unique microstructure of the film.

Aiming at developing a semiconductor CO sensor applicable to the safety control of gas appliances, we carried out an extensive material search for promoters to In2O3-based elements. Among the elements added (0.5 wt%) with each of 12 transition metal oxides, the Co3O4-added one was outstanding in sensitivity and selectivity to CO at 200°C. Remarkably the sensing properties of this element could be improved further by the addition of Au (0.04 wt%). The doubly promoted element, Au (0.04 wt%)-Co(0.5 wt%)-In2O3, gave excellent characteristics in sensitivity to CO, selectivity to CO over H2 and other selected gases, and response rates at 250°C. The promoting effects of the additives were shown to originate from the proper enhancement of catalytic activity for CO oxidation.

Metal oxide-based semiconductor gas sensors have been exploited for detecting various odorous components generated in the process of aging and/or putrefaction of beef, such as ethyl acetate, isoamyl alcohol, acetoin, and so on. Among various sensor elements tested, WO3(5wt%)-ZnO showed the highest sensitivity to ethyl acetate (20ppm in wet air) at 500°C. On the other hand, acetoin could be detected with excellent sensitivity and selectivity by WO3(0.5wt%)-ZnO element at 300°C. The effect of WO3 additive was evalutated in the catalytic oxidation of acetoin over ZnO-based materials. As a result, the increase in acetoin sensitivity was found to be correlated with the catalytic activity in oxidative dehydrogenation of acetoin to diacetyl. These two elements obtained appeared to be applicable to evaluate beef freshness. Actually, it was confirmed that the sensitivities of these sensors to the headspace volatiles of the beef kept at 5°C increased gradually during the preservation term of 5 days.

Among various single-metal oxides, In 2 O 3 , SnO 2 and ZnO were found to exhibit rather high sensitivities to CO at 300°C and above. Especially In 2 O 3 appeared to be most attractive in view of the selectivity to CO over H 2 . It was further found that modifications of In 2 O 3 with the addition of alkali metal carbonates, especially Rb 2 CO 3 , were very effective for enhancing the sensitivity and the selectivity to CO. The sensor element using Rb-In 2 O 3 could detect 500-4000 ppm CO in wet air at 300°C satisfactorily, while its cross-sensitivities to other gases such as H 2 , CH 4, C 3 H 6 , and NO were comparatively small. Catalytic activity test and TPD measurements showed that, as a reason for the promoting effects, the addition of Rb 2 CO 3 to In 2 O 3 enhances the catalytic activity of CO oxidation. In addition, XPS data indicated that, as another reason, Rb 2 CO 3 forms the strongest p-n junctions with In 2 O 3 among alkali metal carbonates.

A new design concept of semiconductor gas sensors, which introduces a foreign antenna material to a gas in problem, is described for the detection of dilute odorous gases such as HN 3 and H 2 S. The antenna material, dispersed on the surface of a semiconducting oxide, interacts with the gas sensitively and selectively, and the resulting change in its chemical state induces a change in semiconducting properties of the underlying oxide through hetero-junctions. Typical examples of such sensors include NH 3 sensors using Au and Pt WO 3 and an H 2 S sensor using CuO--SnO 2 .

Tin dioxide thin films of various thicknesses up to 150 nm were prepared on quartz glass substrates from a sol solution of SnO2 (particle size 3 nm) by a spin-coating method and subjected to calcination at different temperatures up to 800°C for 30 min. The grain size of SnO2 was found to be far smaller than those obtained from the SnO2 sol-derived powder under the same calcination conditions. The suppression of grain growth of SnO2 was more conspicuous as the film thickness decreased so that in the thinnest film (20 nm thick) the SnO2 grain size remained as small as 6 nm after calcination at 800°C. It is suggested that the SnO2 grains in the ultrathin film deposited on the substrate are restrained from moving and coalescing with each other, resulting in the suppression of grain growth.

The sensitivity of a semiconductor sensor using SnO2 to H2S in air could be enormously promoted by loading SnO2 with a small amount of CuO. However, response kinetics began to deteriorate rather sharply as the H2S concentration decreased below a certain limit, which depended on CuO loadings. XPS measurements on a series of CuO-SnO2 samples calcined in air revealed that the binding energies (BEs) for O1s and Sn3d5/2 levels shifted downward from those of pure SnO2, while Cu2p3/2 level showed an upward shift from that of pure CuO. With increasing CuO loading, the shifts in BE increased or decreased linearly for O1s and Sn3d5/2 levels or Cu2p3/2 level, respectively, indicating continuous shifts in Fermi levels of SnO2 and CuO. For a fixed CuO loading, the magnitudes of the BE shifts were dependent on the methods of CuO loading, reflecting differences in the dispersion of CuO particles on SnO2 grains. These phenomena were well consistent with the formation of p-n contacts between the finely dispersed CuO (p) and the underlying SnO2 (n) grains. The sensitivity to H2S was shown to be well correlated with the magnitudes of the BE shifts, indicating that the formation of the p-n contacts in air and the rupture of them upon exposure to H2S are the origin of the high H2S sensitivity.

A sensor array system was composed by using four gas sensor elements based on semiconducting oxides, i.e., Rh(0.4wt%)-WO3, MoO3(5wt%)-ZnO, Er2O3(5wt%)-ZnO and Pr6O11(5wt%)-Fe2O3. These elements were sensitive and selective to the important flavor components of beef consomme soup of methylpyrazine, acetone, capronaldehyde and dimethyldisulfide, respectively. Upon exposure to the vapor of consomme soup, the sensor array was found to give a sensitivity pattern which changed sensitively depending on the quality of consomme soup. This suggests the possibility of applying such a multi-sensor array system for the soup-quality control.

A dense thick film of mixed-conductive perovskite-type oxide (La0.6Sr0.4Co0.8Fe0.2O3) was prepared by means of a slip-casting technique. Film thickness could be easily controlled by changing the casting time. Oxygen semipermeability through the film increased with decreasing film thickness. An acid treatment of the film was found greatly to improve the oxygen semipermeability, because the treatment could remove surface impurities like SrO.

ZnO-based gas sensors have been exploited for the selective detection of acetone and capronaldehyde in the vapour of consomme´soup. The sensitivity to acetone is much improved by the addition (5 wt.%) of two acidic oxides, MoO3 and WO3, the highest sensitivity being attained at 500 and 550 °C, respectively. These sensors show fairly good selectivity to acetone over other important flavour components, methylpyrazine, dimethyldisulfide and capronaldehyde. The promoting effects are suggested to result from changes in rate and path of the catalytic oxidation of acetone. For the detection of capronaldehyde, Gd2O3- and Er2O3-ZnO are found to show excellent sensitivities and selectivities at 300 °C.

Metal tungstates and their composites with carbonates have been investigated for application to semiconductor-type sensors capable of detecting nitrogen oxides at elevated temperature. Among the metal tungstates tested at 500°C, MgWO4 and ZnWO4 are highly sensitive to NO2 only, while BaWO4 is remarkably sensitive to both NO and NO2. The sensitivity to NO and NO2 is greatly promoted when BaWO4 is added to BaCO3. The resulting composite, BaWO4-BaCO3 (2:1 in molar ratio) exhibits the most excellent sensing characteristics: its signal output is the same to both NO and NO2 and increases almost linearly with increasing gas concentration in the range 0–400 ppm NO and 0-200 ppm NO2 at 600°C.

Iron oxide-based semiconductor gas sensors were exploited for detecting dimethyldisulfide (DMDS) gas, an important flavor component of consomme soup. Among the various metals and metal oxides tested, Pr6O11 was the most effective additive to Fe2O3 for promoting the sensitivity to DMDS vapor (20 ppm) in air at 300°C, giving the highest sensitivity at a Pr6o11 content of 5wt%. The sensor using 5wt%Pr6O11-added Fe2O3 was also fairly good in selectivity to DMDS among various flavor components of consomme soup. As characterized by XRD, 5wt%Pr6O11-added Fe2O3 system contained no crystalline phases other than Fe2O3, while PrFeO3 was observed at Pr6O11 contents of 1Owt% and above. The Pr4+/Pr3+ ratio as revealed from XPS analysis remained far higher for Pr6O11-added systems than that for pure Pr6O11, though it tended to decrease with increasing Pr6O11 content. These results suggest that, at low Pr6O11 contents, a major part of Pr components added exist as finely dispersed Pr4+ species on the surface of Fe2O3 particles and that such surface Pr4+ species are responsible for the observed enhancement of gas sensitivity to DMDS.

We tried to fabricate WO 3 thin-film sensors using four kinds of WO 3 particles with very different crystalline morphologies (spherical, disk, cuboid, and hexagonal). Using highly dispersive suspensions prepared, WO 3 thin films of 1.5–3.0 μm in mean thickness were obtained by micro-injecting film formation method. NO 2 sensing properties of these thin-film sensors suggested that the sensitivity to NO 2 as well as the rate of response-recovery depended markedly on morphology and microstructure of the WO 3 crystals contained.

The effects of the grain size of WO[sub 3] on WO[sub 3]-based sensor for NO[sub x] were investigated. By hydrolyzing ammonium paratungstate with hot nitric acid in solution and calcining the resulting precipitate in air at prescribed temperatures in the range of 300 to 600 C, the mean crystallite sizes (D) of WO[sub 3] were varied in the range of 16 to 57 nm. The sensitivities of the WO[sub 3] sensor elements to NO[sub 2] (10 ppm) as well as to NO (200 ppm) remained independent of D down to D = 33 nm, below which the sensitivities increased steeply with a decrease in D, reaching at D = 25 nm the sensitivity values three to four times as large as those for D > 33 nm.

The influence of the loading and dispersion of CuO on the H2S-sensing properties of CuO–SnO2 sensors has been investigated. The response rates to H2S depend on various factors such as CuO loading, specific surface area (or size) of SnO2 grains, method of loading the SnO2 grains with CuO and the operating temperature. For the CuO–SnO2 sensor prepared by a chemical fixation method, the response rates to dilute H2S (1–10 ppm) increased with decreasing CuO loading, while the sensitivity to H2S decreased monotonically with decreasing amount of CuO dispersed per unit surface area of SnO2. By optimizing these factors, it was possible to obtain a CuO–SnO2 sensor which responded to H2S above 1 ppm at 160 °C with sufficient sensitivity and fairly good response kinetics.

The titania nanotube arrays (TNAs) with smooth surface was synthesized by anodization of titanium foil with 3 cm2in square area using the electrolyte composed of 0.2 wt% NH4F and 0.5 vol% H2SO4in ethylene glycol in order to evaluate the methylene blue photodegradation under ultra-violet irradiation. The tube length and inner diameter as a size parameter were controlled by the anodization time from 5 to 10 h and applied voltage from 10 to 50 V. The titania nanotube arrays (TNAs) annealed at 300 to 500°C were assigned to anatase phase, and TNAs at 600°C had both phase of anatase and rutile. The crystallite size and the apparent rate constant were increased with the increase in the annealing temperature of TNAs from 300 to 500°C. The bigger crystallite size of TNAs is suggested to be related to the increase in the amount of hole at the valence band, leading to the decrease in the apparent rate constant of MB degradation. Interestingly, the four kinds of linear relationship with the apparent rate constant were seen in both the inner diameter of TNAs and the length. Consequently, the apparent rate constant strongly depended on inner diameter of TNAs.

In order to improve the sensitivity to NO in air, WO3 based sensor elements were loaded with a small amount (0.004 wt%) of noble metals. Among the various noble metals added, Au and Ru were found to be effective promoters. The addition of Au enhanced the sensitivity to 200 ppm NO in air by about one order of magnitude, making the Au-loaded element attached with Pt electrodes the most sensitive to NO in the temperature range (200∼500°C). Ru-WO3 element also exhibited high NO sensitivity at 250 and 300°C, but the promoting effect of Ru quickly decreased as temperature increased. Such promoting effects were hardly consistent with the catalytic activity of the noble metals for the oxidation of NO to NO2, which were in the order Pt>Ru>Au=0. TPD experiments provided evidence supporting that the enhancement of the oxidative adsorption of NO as NO2 would be a major reason for the promoting effects of Au and Ru.

A miniaturized semiconducting metal oxide NO 2 gas sensor with integrated micro heater is reported in this paper. The sensor has the size of 1×1×0.4mm3 and detects NO 2 based on the electrical conductance change of porous nanoparticles WO 3 thin film, which is deposited on the interdigitated Pt electrodes. Integrated micro heater designed to provide the operating temperature above 200°C for the sensing material WO 3 . In this work, a 2 µm-thick Si diaphragm is suspended on four surrounding beams and separated from the substrate, therefore, the thermal isolation is very efficient. Furthermore, the thermal stress is also reduced in the diaphragm, because it could expand or contract easily in all directions when the temperature changed. The fabrication is simple since the heater and electrode are fabricated in the same process. The sensor can detect dilute NO 2 gas as low as 0.05 ppm with high sensitivity up to 13. The design and electro-thermo-structural coupled field simulation as well as the fabrication and testing of the sensor are presented.

From a screening test for the sensing properties of 26 single oxides, an In 2 O 3 sensor element was found to exhibit rather high sensitivity to both CO and H 2 in wet air at 300°C. In order to improve the selectivity to CO vs. H 2 , each of 33 metal oxides was added to In 2 O 3 . As a result, the addition of alkaline metal oxides was found to enhance markedly both sensitivity and selectivity to CO. Particularly, Rb 2 O (5 weight percent)-In 2 O 3 element showed the highest CO selectivity. It could detect 500 to 4000 ppm CO in wet air at 300°C satisfactorily.

Among the In{sub 2}O{sub 3}-based elements loaded with each of 12 metal oxides [0.5 weight percent (w/o) each], the Co{sub 3}O{sub 4}-added element was outstanding in sensitivity and selectiveness to CO at 250 C. Remarkably the sensing properties of this element could be increased further by the addition of Au (0.04 W/o). The element thus obtained, Au (0.04 w/o)-Co (0.5 w/o)-In{sub 2}O{sub 3}, gave excellent characteristics in sensitivity to CO, selectivity to CO against H{sub 2} and other selected gases, and responses rate at 250 C, giving promise of being applied as a CO sensor for safety control of gas appliances.