Ceria-doped SnO2 sensor highly selective to ethanol in humid air

In our earlier study, we reported that at 300 °C, a 2.0 wt.% CeO₂-doped SnO₂ sensor is highly selective to ethanol in the presence of CO and CH₄ gases [F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, CeO₂ doped SnO₂ sensor selective to ethanol in presence of CO, LPG and CH₄, Sens. Actuators B 108 (2005) 172–176]. In the present investigation, we report the influence of ambient air humidity on the ethanol selective SnO₂ sensor doped with 2.0 wt.% CeO₂. Maximum response to ethanol occurs at 300 °C which decreases with the relative humidity. The relative humidity was changed from 0 to 80% for different ambient air temperatures of 30, 40 and 50 °C and the response of the sensor was monitored in a 250–450 °C temperature range. As the relative humidity in 50 °C air increased from 0 to 30%, a 15% reduction in the maximum response to ethanol was observed. A further increase in the relative humidity no longer reduced the response significantly. The presence of humidity improved the sensor response to both CO and CH₄ up to 350 °C after which the extent of improvement became smaller and at 450 °C was almost diminished. The sensor is shown to be quite selective to ethanol in the presence of humid air containing CO and CH₄. The selectivity passes a maximum at 300 °C; however it declines at higher operating temperatures.     

Simulation and fabrication of piezoresistive membrane type MEMS strain sensors

Two piezoresistive (n-polysilicon) strain sensors on a thin Si₃N₄/SiO₂ membrane with improved sensitivity were successfully fabricated by using MEMS technology. The primary difference between the two designs was the number of strips of the polysilicon patterns. For each design, a doped n-polysilicon sensing element was patterned over a thin 3 μm Si₃N₄/SiO₂ membrane. A 1000×1000 μm² window in the silicon wafer was etched to free the thin membrane from the silicon wafer. The intent of this design was to fabricate a flexible MEMS strain sensor similar in function to a commercial metal foil strain gage. A finite element model of this geometry indicates that strains in the membrane will be higher than strains in the surrounding silicon. The values of nominal resistance of the single strip sensor and the multi-strip sensor were 4.6 and 8.6 kΩ, respectively. To evaluate thermal stability and sensing characteristics, the temperature coefficient of resistance [TCR=(ΔR/R₀)/ΔT] and the gage factor [GF=(ΔR/R₀)/] for each design were evaluated. The sensors were heated on a hot plate to measure the TCR. The sensors were embedded in a vinyl ester epoxy plate to determine the sensor sensitivity. The TCR was 7.5×10⁻⁴ and 9.5×10⁻⁴/°C for the single strip and the multi-strip pattern sensors. The gage factor was as high as 15 (bending) and 13 (tension) for the single strip sensor, and 4 (bending) and 21 (tension) for the multi-strip sensor. The sensitivity of these MEMS sensors is much higher than the sensitivity of commercial metal foil strain gages and strain gage alloys.     

Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO₂ sensing properties of SnO₂-based varistor-type sensors have been investigated in the temperature range of 400-650°C and in the NO₂ concentration range of 15–30 ppm. Pure SnO₂ exhibited a weak nonlinear I–V characteristic in air, but clear nonlinearity in NO₂ at 450°C. The breakdown voltage of SnO₂ shifted to a high electric field upon exposure to NO₂ and the magnitude of the shift was well correlated with NO₂ concentration. Thus, SnO₂ exhibited some sensitivity to NO₂ as a varistor-type sensor. When SnO₂ particles coated with a SiO₂ thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO₂ and the sensitivity to 15 ppm NO₂ was enhanced slightly. However, the sensitivity to 30 ppm NO₂ decreased. The Cr₂O₃-loading on SnO₂ also led to an increase in the breakdown voltage in air, but the Cr₂O₃ addition was not effective for promoting the NO₂ sensitivity under the present experimental conditions.     

High-performance solid-electrolyte SOx sensor using MgO-stabilized zirconia tube and Li2SO4---CaSO4---SiO2 auxiliary phase

Solid-electrolyte-based electrochemical SO_x sensors fabricated with MgO-stabilized zirconia and Li₂SO₄---CaSO₄---SiO₂ (4:4:2 in molar ratio) exhibit fairly good sensing characteristics for 2–200 ppm SO₂ in air at 600–750 °C, with the e.m.f. responses following the Nernst equation for the two-electron reduction of SO₂. The 90% response and 90% recovery times to 20 ppm SO₂ are 10 s and 7 min at 650 °C, and 10 s and 3 min at 700 °C, respectively. It is further found that the sensor exhibits excellent selectivity to SO_x in the coexistence of CO₂ and NO_x, and good long-term stability. The sensor is simple in structure, easy to prepare, and quite tough chemically and mechanically. These features should ensure practical use for this SO_x sensor.     

An experimental study of Love-wave acoustic sensors operating in liquids

It has been shown that properly designed Love-wave acoustic sensors are very promising for sensing in gaseous and liquid environments because of their high sensitivity. Since Love-wave devices do not incur a radiative loss when used in a liquid, they have many potential applications in biosensing. We have successfully manufactured a range of 40 μm wavelength Love-wave devices based on SiO₂/ST-quartz, with the SiO₂ thickness ranging from 0 to 7.3 μm. The relationship of the mass-loading sensitivity to the thickness of the SiO₂ layer has been obtained experimentally. High sensitivity (≥ 300 cm⁻² g⁻¹) is achieved at an SiO₂ thickness between 3.5 and 6.5 μm. The Love-wave oscillators have operated efficiently in various liquids with excellent stability and low noise. In this paper, we report the experimental results for the devices operating in various liquids. The mass sensitivity, insertion loss, oscillation frequency stability, noise level, liquid viscous loading and acoustoelectric coupling have been studied. The influence of the thickness of the SiO₂ layer on some of these properties has also been investigated.     

CuO/SnO2 thin film heterostructures as chemical sensors to H2S

CuO/SnO₂ heterostructures as well as SnO₂(CuO) polycrystalline films have been studied for H₂S sensing. Gas sensing properties of these materials have been compared in conditions: 25–300 ppm H₂S in N₂ at 100–250°C. A shorter response time of the heterostructures as compared to that of the SnO₂(CuO) films has been found. It is suggested that the improvement of dynamic sensor properties of SnO₂/CuO heterostructures is caused by the localization of electrical barrier between CuO and SnO₂ layers.     

H2 sensors using Fe2O3-based thin film

This paper describes the fabrication procedure as well as the sensing properties of new hydrogen sensors using Fe₂O₃-based thin film. The film is deposited by the r.f. sputtering technique; its composition is Fe₂O₃, TiO₂(5 mol%) and MgO(0–12 mol%). The conductance change of the film is examined in various test gases. The sensitivity to hydrogen gas is enhanced by treating the film in vacuum at 550 °C for 4 h and then in air at 700 °C for 2 h. The sputtered film is identified to be polycrystalline -Fe₂O₃ based on X-ray diffraction patterns. However, the surface layer is considered to be changed to Fe₃O₄ after heating in vacuum and then to γ-Fe₂O₃ after heating in air. The film is thus a multilayer one with a thin γ-Fe₂O₃ layer on a -Fe₂O₃ layer. The sensing mechanism is discussed based on measurements of the physical properties of the film, such as the temperature dependence of the sensor conductance, X-ray diffraction pattern, surface morphology, RBS (Rutherford back-scattering) spectrum and optical absorption spectrum.     

Humidity sensing properties of Nation and sol-gel derived SiO2/Nafion composite thin films

Nafion, sol-gel derived SiO₂ and SiO₂/Nafion composite films were prepared through casting and dip-coating methods. Impedance response of the films to relative humidity and the stability of the response against different aging environments were examined. The response to relative humidity of each film was analyzed with their complex impedance spectra at different humidities.     

Tin oxide microsensor for LPG monitoring

A silicon-based SnO₂ gas sensor has been fabricated for monitoring liquified petroleum gas (LPG), commonly used as town gas. The gas sensor is made by silicon IC technology together with SnO_f2 thin-film processing. The whole chip with a size of 9 mm x 9 mm consists of nine sensors (three by three array). each sensor is supported by a thin membrane of SiO₂/Si₃N₄/SiO₂ layers that provides a low thermal mass and prevents heat conduction through the surrounding substrate material. Tin oxide thin film is prepared by thermal evaporation of metallic tin granules and subsequent thermal oxidation of the metallic film at 600 °C. To form the SnO₂(Pt) thin film, a layer of Pt with a thickness of several tens of angstroms is sputtered onto the tin oxide film and heat treated at 500 °C in air for several hours in order to stabilize its electrical response. The fabricated SnO₂(Pt) microsensors exhibit about 85 and 92% sensitivities to 5000 ppm C₃H₈ and 5000 ppm C₄H₁₀ (the main components of LPG) at 250 °C, respectively, and show a rapid response time of less than 5 s.     

Highly sensitive gas sensors based on hollow SnO2 spheres prepared by carbon sphere template method

Hollow SnO₂ spheres were prepared in dimethylfomamide (DMF) by controlled hydrolysis of SnCl₂ using newly made carbon microspheres as templates. The phase composition and morphology of the material particles were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The gas sensing properties of sensors based on the hollow SnO₂ spheres were investigated. It was found that the sensor exhibited good performances, characterized by high response, good selectivity and very short response time to dilute (C₂H₅)₃N operating at 150 °C, especially, the response to 1 ppb (C₂H₅)₃N attained 7.1 at 150 °C. It was noteworthy that the response to 0.1 ppm C₂H₅OH of the sensor was 2.7 at 250 °C.