Study on CuO–BaTiO3 semiconductor CO2 sensor

The prototype of a CO₂ sensor made of CuO–BaTiO₃, which has capacitance sensitive effect, is designed based on the pn heterojunctions of CuO and BaTiO₃ semiconductors. The preparation of BaTiO₃ semiconductor powders is pointed out, using the coprecipitation and semiconducting techniques. The characteristic quantities relating to the capacitance sensitive effect of the sensor are studied systematically with the aid of a gas tester. A reasonable mechanism of the sensor is proposed.     

Humidity sensing and electrical properties of a composite material of nano-sized SiO2 and poly(2-acrylamido-2-methylpropane sulfonate)

A composite material of nano-sized SiO₂ and poly(2-acrylamido-2-methylpropane sulfonate) (poly(AMPS)) was used to make a humidity sensor. The infrared (IR) spectra and microstructure of the material were analyzed, and the humidity sensing and electrical properties of the sensor were measured. The sensor well responded to humidity with a relative good linearity, though it depended on the applied frequency. The temperature influence between 15 and 35 °C was −0.71 and −0.15% RH/°C at 30 and 90% RH, respectively. The sensor showed a negligible hysteresis and fast response time upon humidification and desiccation. The stability of the sensor in a highly humid and alcoholic environment increased with increasing the SiO₂ content. The activation energy for conduction reduced with water adsorption. The different impedance plots observed at low and high relative humidity suggested different sensing mechanisms of the SiO₂/poly(AMPS) composite material.     

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.     

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.     

Doping behavior of CdO in BaTiO3-based PTCR ceramics

Impurities have great influence on the PTCR effect in BaTiO₃ ceramics, especially a donor and an acceptor. When Cd replaces Ba as equivalent impurity, neither donor nor acceptor does it act as. However, it was found that doping with either solid CdO or vapor one resulted in the enhancement of the PTCR effect of BaTiO₃-based semiconducting ceramics. The mechanism of the enhancement of the PTCR effect is also discussed.     

The effect of humidity on the electrical conductivity of mesoporous polythiophene

Commercial samples of bromine terminated polythiophene powder have been compacted to a mesoporous state in order to measure their electrical properties at different relative humidities by impedance spectroscopy, in this case the complex admittance versus frequency. The sample powders were compacted between the electrodes of a co-axial tube capacitor arrangement and connected to a standard impedance bridge with a frequency range of 20–1 MHz. The low humidity conductivity shows a dc limit at about 4 × 10⁻⁶ S/m. When the conductivity measurements were recalculated by subtracting the low humidity response, assumed dry, from the medium or high humidity response a peak was seen at values ranging from 20 to 60 kHz with a sharp fall off at about 100 kHz. Plots of the change in susceptibility did not show peaks, but the excess response vanished at a lower frequency than seen for the conductivity. The log–log plots of the data showed good low frequency fits to a straight line, implying power law dependencies similar to those seen in ionic dielectric materials. I conclude that in humid air there is no significant conduction through the adsorbed water layer itself, but that the presence of surface water molecules affects the polythiophene conduction that occurs through the overlap of adjacent π-bonds. In this model the presence of surface water enhances the π-bond overlap, but has a strong frequency dependence.     

Selective NH3 gas sensor based on Langmuir-Blodgett polypyrrole film

Polypyrrole thin films have been deposited onto a glass substrate by the Langmuir-Blodgett technique to fabricate a selective ammonia (NH₃) gas sensor. The d.c. electrical resistance of the sensing elements is found to exhibit a specific increase upon exposure to different gases such as NH₃, CO, CH₄, H₂ in N₂ and pure O₂. The polypyrrole thin-film detector shows a considerable increase of resistance when exposed to NH₃ in N₂, and negligible response when exposed to comparable concentrations of interfering gases such as CO, CH₄, H₂ in N₂ and pure O₂. The calibration curve for NH₃ in N₂ at room temperature is measured in the concentration range from 0.01 to 1%. The relative change of the electrical resistance is about 10% for the lower detectable limit of 100 ppm of NH₃ in N₂. The sensitivity of the Langmuir-Blodgett polypyrrole towards ammonia is considerably higher than that of the electrochemical polypyrrole. The fast rise time and the high sensitivity of the detector are reported as a function of number of the polypyrrole layers. Long-term aging tests of the selective NH₃ gas sensor are performed.     

Structural and capacitive humidity sensing properties of nanocrystal magnetite/silicon nanoporous pillar array

A composite thin film was fabricated by coating nanocrystal magnetite (nc-Fe₃O₄) on silicon nanoporous pillar array (Si-NPA), which shows a regular hierarchical structure composed by the pillar array in micron dimension and the nanoporous structure in the film of Fe₃O₄. Capacitive humidity sensors were made based on nc-Fe₃O₄/Si-NPA and the corresponding sensing properties were investigated. The experimental results disclosed that nc-Fe₃O₄/Si-NPA sensor exhibits high sensitivity, strong output signal intensity, and short response times. These high performances of nc-Fe₃O₄/Si-NPA sensor are explained on the basis of the structural and compositional factors of nc-Fe₃O₄/Si-NPA.     

Gasochromic effect of palladium doped peroxopolytungstic acid films prepared by the sol-gel route

Gasochromic palladium doped peroxopolytungstic acid (Pd:P-PTA) films have been prepared using dip-coating deposition from peroxopolytungstic acid (P-PTA) sols into which PdCl₂ was added in molar ratios Pd:W=1:125, 1:100, 1:53, and 1:40. These films exhibit reversible colouring/bleaching changes when exposed to hydrogen or hydrogen/argon mixture (4%) and air, alternatively. Gasochromically coloured and bleached films were characterised using in-situ Fourier transform infrared (FT-IR) spectroscopy. The vibrational modes of as-deposited, coloured and bleached films were assigned and the polaron absorption, which characterises the IR spectra of coloured films, was detected. Colouring/bleaching kinetics of films exposed to H₂ and H₂/Ar mixture as a function of the concentration of the catalyst and temperature of heat-treatment is reported. Proton (σ_pr) and electronic (σ_el) conductivities determined from impedance spectra revealed an increase in σ_el from 10⁻⁵ S cm⁻¹ in bleached state, to 10⁻³ S cm⁻¹ in coloured state, while σ_pr remained constant (10⁻² S cm⁻¹).     

Fast response of undoped and Li-doped titania thick-films at low temperature

The fast response of undoped and Li-doped TiO₂ operating at low temperature to hydrogen and oxygen is investigated. The TiO₂ sensors are fabricated using thick-film technique. The prepared materials exhibit the presence of only rutile phase of TiO₂ but enlarged crystal lattice parameters were confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows that the grain size of the material has not obviously changed with different Li-doping (2–4 mol%), but the undoped is much smaller. Kroger–Vink model indicates that Li mainly substitutes for the lattice point of Ti. Because the material resistance decreases as the oxygen pressure increases, Li-doped samples can be regarded as a p-type semiconductor compared with pure TiO₂. The operating temperature of the Li-doped TiO₂ samples is found to be lower than that of pure TiO₂ in H₂ and O₂ environment. At less than 3 mol% Li content, the response time of the Li-doped TiO₂ gas sensors is much shorter than that of pure TiO_2, at the same temperature under both H₂ and O₂ environment. Moreover, the sample of 3 mol% Li-doping exhibits the best response characteristics. The response mechanism is suggested to arise from the conduction holes ionized by Li and the surface potential barrier change in different gas environments.     

Gas-sensing property and mechanism of CaxLa1−xFeO3 ceramics

LaFEO₃ and Ca_xLa_1−xFeO₃ ceramic powders have been prepared by the coprecipitation method from La(NO₃)₃, Fe(NO₃)₃ and Ca(NO₃)₂ aqueous solutions. The orthorhombic perovskite phases of LaFeO₃ and Ca_xLa_1−xFeO₃ are characterized by X-ray diffraction patterns. The sensors fabricated with those powders have high sensitivity to alcohol. Partial substitution of La³⁺ in LaFeO₃ with Ca²⁺ can enhance the sensitivity of the materials to reducing gases. The resistance of an LaFeO₃ sensor in air, vacuum and alcohol-containing air has been measured. Complex impedance spectroscopy has been used to try and analyse the gas-sensing mechanism. According to the experimental results, it can be deduced that the surface adsorptive and lattice oxygen govern the sensing properties of LaFeO₃ and Ca_xLa_1−xFeO₃ ceramics.     

Monofunctionalized β-cyclodextrins as sensor elements for the detection of small molecules

β-Cyclodextrins functionalized by different moieties that were tethered to a single 6-deoxyaminoglucose unit were investigated with respect to their suitability for sensor applications. Derivatizing the cyclodextrins with hydrophobic moieties like dipalmitoylglycerol and cholesterol allowed us to study packing density and orientation of the cyclodextrin tori at the air–water interface. From the pressure-area isotherms, it was concluded that the cyclodextrins are positioned towards the water subphase, with their molecular axis predominately parallel to the interface. By introducing a disulfide group, we managed to immobilize cyclodextrins on gold surfaces via self-assembly. MALDI mass spectrometry (MALDI MS) and XPS confirmed that the molecules are chemisorbed on the gold substrate displaying high surface coverage as determined by means of impedance spectroscopy. The inclusion of various charged guest molecules was monitored by changes in the charge transfer resistance of the redox couple [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻. The charge transfer resistance is sensitive to the surface potential, which leads to either repulsion or attraction of the redox active species.     

A novel thick-film ceramic humidity sensor

The paper describes the results of studies on the fabrication and characterisation of a thick-film humidity sensor based on the semiconducting metal oxide MnWO₄. The sensor element possesses a novel ‘sandwich’-configuration with a 40 μm porous MnWO₄ ceramic layer sandwiched by two 10 μm polarity-reversed, interdigitated metal films. Instead of traditional glass frits, LiCl powders are used as adhesion promoters for sintering the sensor paste. With this method, MnWO₄ powders with an average particle size of 3.0 μm are sintered at the standard thick-film firing temperature of 850°C. The sintered ceramic layer exhibits a porous structure. The novel electrode arrangement combines the advantages of humidity sensors in the form of a parallel capacitor with those in the form of an interdigital capacitor, permitting a high sensitivity and a fast response. The influence of temperature on the sensor characteristics has been compensated for by integrating a thick-film NTC resistor. The humidity sensor shows no cross-sensitivity to organic vapour. The organic contamination on the sensor surface can be burned out by heating the sensor element at about 400°C with the refresh heater printed on the back side of the substrate.     

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.     

High-resolution high-voltage sensor based on SAW

A surface acoustic wave (SAW)-based high-voltage sensor is described. The sensor consists of a SAW oscillator fabricated on a 10 mm × 10 mm 128° rotated Y-cut, X-propagating LiNbO₃ substrate. The voltage is applied to electrodes on the substrate, and the resulting electric field changes the propagation time of the SAW. The propagation time is directly related to the output frequency of the SAW oscillator. The high-voltage sensor offers a small-sized high-voltage measurement device with several attractive features: a high resolution (better than 0.2 V up to 2.4 kV, better than 0.4 V for higher voltages), a large range (−10 to +10 kV), a high input impedance (> 10¹³ ω) and a low input capacitance (< 10 pF). The sensitivity amounts to 16 Hz V⁻¹.     

A precise and robust quartz sensor based on tuning fork technology for (SF6)-gas density control

A novel sensor technology for measuring and monitoring gas density is described. The gas density sensor is self-calibrating and specifically designed to operate as a sulfur hexafluoride (SF₆) monitor and control device in gas insulated high-voltage switchgears (GIS), but can also be applied for density measurement of any kind of gas. It comprises a pair of tuning forks oscillating at their resonance frequency. One oscillator is exposed to the gas to be monitored, the other one is used for comparison and temperature compensation. Exposure to gas leads to a shift in the resonance frequency proportional to the gas density. A density standard based on a combined weight and mass measurement has been performed with a precision better than 0.02%, giving proof of exact sensor calibration. The gas density sensor works with a precision better than 0.5% over a range of 50 kg/m³ SF₆ gas density. Sensor response time is less than 50 ms. The real gas equation of Beattie and Bridgman fits the gauged values with a mean error of 0.125%.     

Screen printed thick films of NiO and LaFeO3 as gamma radiation sensors

Thick films of nickel oxide (NiO) and perovskite type LaFeO₃ were investigated for γ-radiation dosimetry purposes. Samples were fabricated using the thick film screen printing technique. These films were exposed to a ⁶⁰Co γ-radiation source with a dose rate of 6 Gy/min. The effects of γ-radiation on both the optical and the electrical properties of NiO and LaFeO₃ films were studied. Absorption spectra for the specimens were recorded and the values of the optical band gap for as-printed and γ-irradiated films were calculated. It was found that the optical band gap values decreased with the increase in radiation dose. To study changes in the electrical properties of the materials, two types of specimens having sandwich metal–semiconductor–metal structure were made with layers of NiO and LaFeO₃, respectively. Commercial DuPont 4929 silver paste was used to manufacture electrical contacts. It was observed that radiation induced changes in the electrical properties for both types of thick films, e.g. values of current decreased with an increase in radiation dose. It was found that annealing restored both the electrical and the optical properties of the samples.     

Comparative study of spin coated and sputtered PMMA as an etch mask material for silicon micromachining

SiO₂ and Si₃N₄, are usually used to mask the selected portions during etching of silicon in anisotropic etchants like KOH but polymers are expected to be very good alternative to SiO₂ and Si₃N₄ as masking materials for MEMS applications. An adherent spin coated PMMA layer is reported to work as a mask material. It is a low temperature process, cheaper and films can be easily deposited and removed. One of the problems in its use is its adhesion to the substrate. Our previous experience in the field made us feel that sputtered PMMA will act as better mask because of its better adhesion to silicon. In the present article, a comparative study of spin coated PMMA with sputtered PMMA as an etch mask for silicon micromachining is reported. Structural and adhesive characteristics of the films are determined and compared with those available in the literature. These films deposited on silicon wafer were exposed to anisotropic etchant, KOH, to estimate the masking behavior. The maximum masking time of 32 min in 20 wt.% KOH at 80 °C was obtained for spin coated PMMA samples, which were prebaked at 90 °C. Masking time of sputter deposited PMMA films was found to be 300 min under similar conditions such as 20 wt.% KOH at 80 °C. This masking time is sufficient for fabrication of various MEMS structures, thus indicating candidature of sputtered PMMA as masking material. Various properties of the films are discussed and compared with the ones obtained through literature.     

Trace detection of nitroaromatic compounds with layer-by-layer assembled {SBA/PSS}n/PDDA modified electrode

The {SBA/PSS}_n/PDDA films modified electrode was prepared by layer-by-layer (LBL) assembly with mesoporous SiO₂ (SBA), poly(sodium 4-styrene-sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) in this paper. SBA is a large pore-size mesoporous material with highly ordered hexagonally arranged mesochannels and high thermal stability etc. The electrochemical characteristics of the {SBA/PSS}_n/PDDA films have been studied by electrochemical impedance spectroscopy in 0.1 M KCl solution containing 5.0 mM Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ at the formal potential of 0.230 V. The ultratrace nitroaromatic compounds (NACs) such as TNT, TNB, DNT and DNB were determined by differential pulse voltammetry (DPV) measurement. The sensitivities for NACs determination with {SBA/PSS}_n/PDDA modified electrode were dependent on the number of layers, pH and ionic strength of electrolyte, based on which a set of optimized conditions for film fabrication was inferred. The current responses were linear with NACs ranging from 10⁻⁹ to 10⁻⁷ mol/l. The results showed that the {SBA/PSS}_n/PDDA modified electrode established a new way for fast, simple and sensitive analysis of NACs.     

A comparative study of the physical properties of CdS, Bi2S3 and composite CdS–Bi2S3 thin films for photosensor application

Thin films of CdS, Bi₂S₃ and composite CdS–Bi₂S₃ have been deposited using modified chemical bath deposition (M-CBD) technique. The various preparative parameters were optimized to obtain good quality thin films. The as-deposited films of CdS, Bi₂S₃ and composite were annealed in Ar gas at 573 K for 1 h. A comparative study was made for as-deposited and annealed CdS, Bi₂S₃ and composite thin films. Annealing showed no change in crystal structure of these as-deposited films. However, an enhancement in grain size was observed by AFM studies. In addition change in band gap with annealing was seen. A study of spectral response, photosensitivity showed that the films can be used as a photosensor.