Fabrication and characterization of low temperature sintering PMN–PZN–PZT step-down multilayer piezoelectric transformer

For the fabrication as step-down multilayer piezoelectric transformer, piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃–Pb(Zn_1/3Nb_2/3)O₃–Pb(Zr_0.52Ti_0.48)O₃ (PMN–PZN–PZT) ceramics were optimized by ZnO–Li₂CO₃ (ZL) and Pb₃O₄ content. Effects of the additions on the structure, bulk density and electrical properties of ceramics were investigated. The results revealed that the proper additions of ZL with Pb₃O₄ content could modify the electrical properties of the PMN–PZN–PZT ceramics. The composition sintered at 995 °C with 0. 01 wt.% ZL and 0.10 wt.% Pb₃O₄ content showed higher values, which were listed as follows: d₃₃ = 256 pC/N, K_p = 0.60, Q_m = 1910, _r = 1032, tan δ = 0.0070 and r = 2.09 Ω. In addition, the step-down piezoelectric transformers with optimized PMN–PZN–PZT composites were fabricated and the characteristics as the output power and resistance loads were measured. Meanwhile, the step-down piezoelectric transformers sintered at 995 °C showed the favorable characteristics with a higher gain G of 0.204 and a lower temperature rise of 6 °C when the output power was 5 W, and the driving frequency were approximately constant (≈126 kHz) when the output power was from 5 to 13 W. Moreover, the maximum efficiency (90.2%) was obtained at load resistance of 10 Ω.     

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.     

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.     

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.     

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.     

Optimization of NASICON composition for Na recognition

Na_1+xZr₂Si_xP_3−xO₁₂ are ceramic materials which are fast ionic conductors by Na⁺. The present work concerns the characterizations of such materials in the range x = 1.4–3.0 for their use as ion sensitive membranes. Samples were prepared by sol-gel route to obtain pellets with a high density. Characteristics, such as lattice parameters and bulk conductivity are given. Results on detection limit and selectivity of such membranes used in ISE devices are presented. The effect of the stoichiometry on electrochemical characteristics is discussed. The best performances are obtained for x = 2.0–2.2, with samples sintered at 1200°C. No influence of sintering temperature is noticeable for the selectivity, excepted for proton which is less interfering after sintering at 1200°C.     

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.     

Room temperature synthesis of γ-Fe2O3 by sonochemical route and its response towards butane

Nanocrystalline gamma iron oxide (γ-Fe₂O₃) has been synthesized at room temperature through sonication-assisted precipitation technique. The key in obtaining γ-Fe₂O₃ at room temperature lies in exploiting high-power ultrasound (600 W). The gas-sensing properties to n-butane of pure γ-Fe₂O₃ were investigated by studying the electrical properties of the sensor elements fabricated from the synthesized powder. The maximum response (90%) of the sensor to 1000 ppm n-butane at 300 °C can be explained on the basis of catalytic activity of the nanocrystallites. The response and recovery time of the sensor to 1000 ppm n-butane were less than 12 s and 120 s, respectively.     

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.     

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.     

Characterization of protective coatings for planar automotive gas sensors

Alumina support material suitable for use as a planar automotive gas sensor support was coated in thin films of yttria-stabilised zirconia (YSZ) and titania. The morphology, composition, thickness and homogeneity of the coating was measured. The coating was applied to the ‘green’ form of a tape cast alumina substrate which was subsequently fired at 1500 °C to produce the final form of the coated alumina. The YSZ coating gave a continuous 5 μm thick coating with no evidence of mixed oxide formation between the YSZ and the alumina substrate. XRD indicated a face centred cubic Y doped ZrO₂ or primitive tetragonal Zr_0.9Y_0.1O_1.95 phase. The titania coatings were much thinner (<1 μm) with signs of trace amounts of aluminium titanium oxide (Al₂TiO₅) as well as rutile titania in XRD. Spot analysis using X-ray photoelectron spectroscopy showed a fairly regular titania coverage. Atomic force microscopy analysis showed a particle size of 1–3 μm for the YSZ coating and 0.5 μm for titania.     

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.     

EPR and DRS evidence for NO2 sensing in Al-doped ZnO

Zinc oxide (ZnO) is a well-known semiconducting multifunctional material wherein properties right from the morphology to gas sensitivity can be tailor-made by doping or surface modification. Aluminum (Al)-incorporated porous zinc oxide (Al:ZnO) exhibits good response towards NO₂ at low-operating temperature. The NO₂ gas concentration as low as 20 ppm exhibits S = 17% for 5 wt.% Al-incorporated ZnO. The NO₂ response increases with operating temperature and concentration and reaches to its maximum at 300 °C without any interference from other gases such as SO₃, HCl, LPG and alcohol. Physico-chemical characterization likes differential thermogravimetric analysis (TG-DTA) electron paramagnetic resonance (EPR) and diffused reflectance spectroscopy (DRS) have been used to understand the sensing behavior for pure and Al-incorporated ZnO. The TG-DTA depicts formation of ZnO phase at 287 °C. The EPR study reveals distinct variation for O⁻ (g = 2.003) and Zn interstitial (g = 1.98) defect sites in pure and Al:ZnO. The DRS studies elucidate signature of adsorbed NO_x species in aluminium-incorporated zinc oxide indicating its tendency to adsorb these species even at low temperatures. This paper is an attempt to correlate the gas sensing behavior with the physico-chemical studies such as EPR and DRS.     

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.     

Pyroelectric and dielectric properties of sol–gel derived barium–strontium–titanate (Ba0.64Sr0.36TiO3) thin films

The barium–strontium–titanate (BST, Ba_0.64Sr_0.36TiO₃) thin films have been prepared by the sol–gel method on a platinum-coated silicon substrate. The resulting thin films show very good dielectric and pyroelectric properties. The dielectric constant and dissipation factor for Ba_0.64Sr_0.36TiO₃ thin film at a frequency of 200 Hz were 592 and 0.028, respectively. The dependence of the capacitance as a function of the voltage shows a strongly non-linear character, and two peaks characterizing spontaneous polarization switching can be clearly seen in this curve, indicating that the films have a ferroelectric nature. The capacitance changed from 495 to 1108 pF with the applied voltage in the −5 to +5 V range at a frequency of 100 kHz. The peak pyroelectric coefficient at 30 °C is 1080 μC/m² K. The pyroelectric coefficient at room temperature (25 °C) is 1860 μC/m² K, and the figure-of-merit of this film is 37.4 μC/m³ K. The high pyroelectric coefficients and the greater figures-of-merit of Ba_0.64Sr_0.36TiO₃ thin films make it possible to be used for thermal infrared detection and imaging.     

Pulsed laser deposition of Y2O3 : Eu thin film phosphors

Thin films of Y₂O₃ : Eu cathodoluminescent (CL) phosphors were deposited using pulsed laser deposition using deposition temperature between 250°C and 800°C, O₂ pressures between residual vacuum (2×10⁻⁵ Torr) and 6 Torr, and post annealing up to 1200° for 1 h in air. The CL efficiency of the best thin film was about one third that of the starting powder. The brightness and efficiency of the thin films improved as the deposition temperature, O₂ pressure and post annealing temperature were increased, except that O₂ pressures above 600 mTorr did not significantly improve the CL properties. At deposition temperatures >600°C, the surface morphology changed from a smooth film to a nodular deposit for O₂ pressures >200 mTorr, with nodule dimensions ≈100 nm. Simultaneously, the CL properties improved dramatically because of enhanced optical scattering out of the thin film. Optical scattering was discussed in terms of anomalous diffraction. The CL properties also improved dramatically with high temperature post annealing. This effect was interpreted in terms of improved crystallinity and activation of the Eu. The low brightness and efficiency of thin films versus powder was affected by depletion of the Eu in the thin films owing to the deposition process.     

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.     

Hydrolyzed colloid reaction (HCR) technique for phosphor powder preparation

Undoped and Eu³⁺, Ce³⁺ and Tb³⁺-doped YVO₄ YPO₄, LaPO₄ and YV_xP_1−xO₄ were prepared in H₂O by the recently introduced hydrolyzed colloid reaction (HCR) technique working at low temperature (<100°C) and atmospheric pressure. Two intermediate — partially hydrophobic — complex colloidal mixtures with metastable characteristics can transform into the stable orthovanadate–orthophosphate phase due to intensive hydrolysis. In contrast with the other low temperature reacting processes — like the sol-gel technique, which makes an amorphous structure — the HCR method can produce crystalline structures in nanometer size ranges. The reaction, morphology, incorporation of activators and different luminescent characteristics are surveyed in this letter-type paper selected from our previous results.     

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.