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.     

Microstructure, electrical and ethanol-sensing properties of perovskite-type SmFe0.7Co0.3O3

Ultrafine SmFe_0.7Co_0.3O₃ powder, prepared by a sol–gel method, shows a single-phase orthogonal perovskite structure. The influence of annealing temperature upon its crystal cell volume, microstructure, electrical and ethanol-sensing properties was investigated in detail. When the annealing temperature increases from 600 to 950 °C, the unit cell volume of the SmFe_0.7Co_0.3O₃ sample reduces, and its average grain size increases. When the annealing temperature increases from 600 to 850 °C, the optimal working temperature and response to ethanol of the SmFe_0.7Co_0.3O₃ sensor increase, and the response–recovery time shortens. But when the annealing temperature further increases from 850 to 950 °C, there are decreases of the optimal working temperature and sensor response, and the response–recovery time is prolonged. The results indicate that, as for sensor response, its optimal annealing temperature is about 850 °C, and the sensor based on SmFe_0.7Co_0.3O₃ annealed at 850 °C shows the highest response S = 80.8 to 300 ppm ethanol gas, and it has the best response–recovery and selectivity characteristics. When the ethanol concentration is as low as 500 ppm, the curve of its optimal response versus concentration is nearly linear. Meanwhile, the influence mechanisms of annealing temperature upon the conductance, the optimal working temperature and sensor response for SmFe_0.7Co_0.3O₃ were studied.     

High aspect ratio In2O3 nanowires: Synthesis, mechanism and NO2 gas-sensing properties

Flexural In₂O₃ nanowires with high aspect ratios were synthesized via a hydrothermal–annealing route. The as-synthesized In₂O₃ nanowires had diameters of 30–50 nm and length up to several microns. Various reaction parameters, such as the kind of reagents, the time of hydrothermal treatment, annealing time and annealing temperature, were investigated by a series of control experiments. The as-synthesized In₂O₃ nanowires showed excellent gas-sensing properties to NO₂ in terms of sensor response and selectivity.     

NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition

In₂O₃ nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In₂O₃ particle containing layers to NO_x and O₂ gases is investigated. The lowest detectable NO_x concentration is 200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NO_x is found to be above 10⁴. Furthermore, the cross-sensitivity against O₂ is very low, demonstrating that the In₂O₃ nanoparticles are very suitable for the selective NO_x detection.     

Sol–gel derived polycrystalline Cr1.8Ti0.2O3 thick films for alcohols sensing application

The powder sample of Cr_1.8Ti_0.2O₃ (CTO) was obtained by a sol–gel method. The thick films were developed on identical ceramic tubes of 4 mm length comprising of two Au-electrodes and printing an eight-layer film prepared by mixing CTO with glass powder and -terpinol as an organic vehicle. X-ray powder diffraction (XRD) patterns showed the formation of a single phase. The scanning electron microscope (SEM) images of the ceramic sensor treated at 850 °C revealed that the grain size was larger than 400 nm for the individual isolated grains on the surface, and the agglomerated dense spheroidal platelets had the size of 1–4 μm in diameter. The AC impedance measurement in ambient air showed that the resistance decreased nearly by two orders of magnitude with an increase in temperature in the range of 400–600 °C for both the powder sample and the thick film, and the activation energy E_a derived from the measurement was found to be 0.35 and 0.36 eV for the powder and the film, respectively. The films were exposed to various concentrations of alcohols (0.4–1.2 ppm of methanol and 1.0–5.0 ppm of ethanol), followed by determination of sensor response, sensitivity and reversibility and reproducibility. The origin of the gas response was attributed to the surface reaction of R-OH (R = methyl and ethyl group) with O⁻_(ads) to form adsorbed R-CHO, which was desorbed as a gas at 400 °C after the sensor departing from the gas.     

Impedancemetric gas sensor based on Pt and WO3 co-loaded TiO2 and ZrO2 as total NOx sensing materials

Pt-loaded metal oxides [WO₃/ZrO₂, MO_x/TiO₂ (MO_x = WO₃, MoO₃, V₂O₅), WO₃ and TiO₂] equipped with interdigital Au electrodes have been tested as a NO_x (NO and NO₂) gas sensor at 500 °C. The impedance value at 4 Hz was used as a sensing signal. Among the samples tested, Pt-WO₃/TiO₂ showed the highest sensor response magnitude to NO. The sensor was found to respond consistently and rapidly to change in concentration of NO and NO₂ in the oxygen rich and moist gas mixture at 500 °C. The 90% response and 90% recovery times were as short as less than 5–10 s. The impedance at 4 Hz of the present device was found to vary almost linearly with the logarithm of NO_x (NO or NO₂) concentration from 10 to 570 ppm. Pt-WO₃/TiO₂ showed responses to NO and NO₂ of the same algebraic sign and nearly the same magnitude, while Pt/WO₃ and WO₃/TiO₂ showed higher response to NO than NO₂. The impedance at 4 Hz in the presence of NO for Pt-WO₃/TiO₂ was almost equal at any O₂ concentration examined (1–99%), while in the case of Pt/WO₃ and WO₃/TiO₂ the impedance increased with the oxygen concentration. The features of Pt-WO₃/TiO₂ are favorable as a NO_x sensor that can monitor and control the NO_x concentration in automotive exhaust. The effect of WO₃ loading of Pt-WO₃/ZrO₂-based sensor is studied to discuss the role of surface W-OH sites on the NO_x sensing.     

Calorimetric hydrocarbon sensor for automotive exhaust applications

We have developed a calorimetric sensor utilizing a thermoelectric device supported on a planar alumina substrate. By using a highly selective carbon monoxide (CO) oxidation catalyst and a non-selective platinum (Pt) catalyst, the device can be built to detect either CO or hydrocarbons with high selectivity. The CO oxidation catalyst comprises lead-modified platinum and exhibits excellent selectivity over the 200–400 °C temperature range. The thermoelectric device consists of two thick film junctions made of niobium pentoxide (Nb₂O₅)-doped titanium dioxide (TiO₂) and a lithiated nickel (Ni), which are supported on a planar alumina substrate. The thermocouple detects the difference in temperature due to different catalytic reactions over the two junctions and shows a high output signal because of the high Seebeck coefficient of Nb₂O₅-doped TiO₂ (−400 μV/°C). In gas bench tests, the sensor has a linear output of 0–2.75 mV over 0–1000 ppm of propylene and a response time of 2.5 s (at 90% of amplitude) at a gas temperature of 350 °C. An engine dynamometer evaluation shows that the response of the sensor parallels the change in CO and hydrocarbon constituent concentrations when the engine air-to-fuel ratio is varied.     

Room temperature hydrogen response kinetics of nano–micro-integrated doped tin oxide sensor

The nano–micro-integrated sensor has been fabricated by sol–gel depositing the nanocrystalline indium oxide (In₂O₃)-doped tin oxide (SnO₂) thin film on microelectromechanical systems (MEMS) device having interdigitated electrode configurations with two different electrode spacing (10 μm and 20 μm) and two different number of fingers (8 and 20). The present nano–micro-integrated sensor exhibits high H₂ sensitivity range (S = 3–10⁵) for the H₂ concentration within the range of 100–15,000 ppm at room temperature. It has been demonstrated that, the room temperature response kinetics of the present nano–micro-integrated sensor is a function of finger spacing, H₂ concentration and air-pressure, but independent of number of fingers. Such dependence has been explained on the basis of Le Chatelier's principle applied to the associated H₂ sensing mechanism and the role of above parameters in shifting the dynamic equilibrium of the involved surface reactions under the described test conditions. A new definition of the response time has been proposed, which is not only suitable for the theoretical analysis but also for the practical applications, where a gas-leak detection alarm is required to be triggered.     

Partial phase diagram of the ternary reciprocal system KF-AlF3-Al2O3-K2O

A K₂ZrF₆ treatment improves the wettability of SiC (or C) fibers by aluminum alloys. A possible mechanism involves the formation of K₃AlF₆ followed by the dissolution of the alumina film. DTA analyses of K₃AlF₆--Al₂O₃ mixtures show that a temperature invariant transition occurs at 1132K, the solid containing trace amounts of KAlF₄ and β-alumina at room temperature. Part of the ternary reciprocal system KF-AlF₃-Al₂O₃-K₂O is calculated, the Gibbs free energy of the liquid being described according to the CIS theory. The K₃AlF₆-Al₂O₃ isopteth section is given. It is characterized by a liquidus with a deep minimum at 1132K and 11.6 wt % alumina.     

Potentiometric CO2 gas sensor with lithium phosphorous oxynitride electrolyte

Potentiometric cell, Au/LiCoO₂ 5 m/o Co₃O₄/Li_2.88PO_3.73N_0.14/Li₂CO₃/Au, has been fabricated and investigated for monitoring CO₂ gas. A LiCoO₂–Co₃O₄ mixture was used as the solid-state reference electrode instead of a reference gas. The idea is to keep the lithium activity constant on the reference side using thermodynamic equilibrium at a given temperature. The thermodynamic stability of the reference electrode was studied from the phase stability diagram of Li–Co–C–O system. The Gibb’s free energy of formation of LiCoO₂ was estimated at 500°C from the measured value of the cell emf. The sensors showed good reversibility and fast response toward changing CO₂ concentrations from 200 to 3000 ppm. The emf values were found to follow a logarithmic Nernstian behavior in the 400–500°C temperature range. CH₄ gas did not show any interference effect. Humidity and CO gas decreased the emf values of the sensor slightly. NO and NO₂ gases affect this sensor significantly at low temperatures. However, increased operating temperature seems to reduce the interference.     

Comprehensive study of hydrogen sensing characteristics of Pd metal–oxide–semiconductor (MOS) transistors with Al0.24Ga0.76As and In0.49Ga0.51P Schottky contact layers

The interesting hydrogen sensing characteristics of two transistors with an Al_0.24Ga_0.76As (device A) and In_0.49Ga_0.51P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H₂/air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H₂/air at room temperature. On the other hand, device B exhibits more stable hydrogen-sensing characteristics at high temperatures. Even at a low concentration of 14 ppm H₂/air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al_0.24Ga_0.76As and In_0.49Ga_0.51P materials are lattice-matched to the GaAs substrate, the studied devices can be integrated as sensor arrays to obtain superior hydrogen sensing characteristics including higher sensing signals, lower detection limit, shorter response time, and widespread detection and temperature regimes.     

Highly sensitive and fast responding CO sensor using SnO2 nanosheets

A highly sensitive and fast responding CO sensor was fabricated from a sheet-like SnO₂. The SnO sheets were prepared by a room temperature reaction between SnCl₂, hydrazine and NaOH, and they were subsequently oxidized into SnO₂ sheets at high temperature (600 °C). The morphology and size of the SnO₂ sheets could be controlled during the formation of SnO, which influence the sensor response (R_a/R_g) and response time to a great extent. The sensor response of SnO nanosheets to 10 ppm CO was enhanced up to 2.34, and the 90% sensor response time could be reduced to 6 s, which are significantly higher and shorter than those of SnO₂ powders (1.57 and 88 s), respectively. The realization of both a high sensitivity and rapid response were explained in terms of rapid gas diffusion onto the entire sensing surface due to the less-agglomerated and very thin structure of SnO₂ nanosheets and the catalytic effect of Pt.     

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.     

Simulating adsorption of n-heptane in the Pt/Al2O3 model: influence of platinum

The molecular modeling method refered to in the literature as Grand Canonical Monte Carlo was used to analyze the phenomenon of n-heptane adsorption on the Pt/γ-Al₂O₃ catalyst. In order to describe relevant interactions, use was made of the forcefield methods (UFF and CVFF). With the conditions adopted for the purpose of the study, Pt was found to exert an advantageous effect on the adsorption of n-heptane. The number of adsorbed molecules was related to the content of the noble metal, and the relation was directly proportional, when temperature and pressure were constant. The contribution of Pt was most distinct at 573 K and 100 kPa.     

Characterization of transition metal oxide ceramic material for continuous thermocouple and its use as NTC fire wire sensor

The ceramic powder prepared from the mixture of Mn₃O₄ and La₂O₃ have been characterized for NTC behavior and the same have been used as CT²C (continuous thermocouple) sensor in the form of a thin metal cable to detect over-heating. These materials have mega ohm resistance at room temperature and showed exponential drop in resistance with the rise in temperature over a temperature range of 100–400 °C. It has been observed that as the concentration of La₂O₃ increases from 0 to 10% the NTC behavior drops from (400–260 °C) ±10%. The material was pressed into pellets and sintered at 700 °C for 3 h resulting in good bonding strength. Electrical characterization of the material was done by measuring the resistance over a temperature range of 100–400 °C. The material showed reproducible NTC characteristics over the temperature range 400, 370, 340, 280, and 260 °C with decreasing thermistor constant values (B = 9588, 9210, 8500, 5170, 3330 K⁻¹) and activation energy (ΔE = 826, 794, 733, 445, 287 meV), respectively. The decrease in activation energy of the ceramic powder with increase in La₂O₃ concentration makes it possible to fabricate thermal sensors which can be used in different temperature ranges. The microstructure was studied using SEM and evidence of a sintered body with grain size around 1 μm was observed in the material. XRD analysis indicated the single-phase tetragonal structure of the ceramic material. The process of using this ceramic material for fabrication of 10 ft continuous fire wire sensor has been explained.     

Ozone sensors on the base of SnO2 films deposited by spray pyrolysis

The gas-sensing properties of SnO₂-based thin films designed for ozone detection are discussed in this paper. The influence of film characteristics on sensor performance is analyzed. SnO₂ films with thickness 30–200 nm were deposited by spray pyrolysis. The SnO₂ films have a response to ozone that is quantitative and rapid and sufficient for use in ozone control and monitoring applications. Sensor performance is compared with similarly prepared sensors fabricated from In₂O₃- and WO₃-based films. The mechanism of the processes controlling the sensor response characteristics is proposed. The data support our conclusion that the reaction with ozone using the SnO₂-film sensors is limited by the adsorption/desorption processes.     

Manufacture and characterization of sol–gel V1−x−yWxSiyO2 films for uncooled thermal detectors

V_1−x−yW_xSi_yO₂ films for uncooled thermal detectors were coated on sodium-free glass slides with sol–gel process, followed by the calcination under a reducing atmosphere (Ar/H₂ 5%). The V_1−x−yW_xSi_yO₂ films as prepared inherit various phase transition temperatures ranging from 20 to 70 °C depending on the dopant concentrations and the fabrication conditions. Compared to the hysteresis loop of plain VO₂ films, a rather steep loop was obtained with the addition of tungsten components, while a relaxed hysteresis loop with the tight bandwidth was contributed by Si dopants. Furthermore, the films with switching temperature close to room temperature were fabricated to one-element bolometers to characterize their figures of merit. Results showed that the V_0.905W_0.02Si_0.075O₂ film presented a satisfactory responsivity of 2600 V/W and detectivity of 9 × 10⁶ cm  Hz^1/2/W with chopper frequencies ranging from 30 to 60 Hz at room temperature. It was proposed that with appropriate amount of silicon and tungsten dopants mixed in the VO₂, the film would characterize both a relaxed hysteresis loop and a fair TCR value, which effectively reduced the magnitude of noise equivalent power without compromising its performance in detectivity and responsivity.     

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.     

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.     

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.