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.     

LTCC compatible PLZT thick-films for piezoelectric devices

The production of PLZT thick-films for piezoelectric devices prepared from perovskite-type (Pb, La)(Zr, Ti)O₃ powders is described. The powder manufacture, paste preparation and thick-film production compatible with the low temperature co-fired ceramics (LTCC) process are detailed. The maximum firing temperature of applied technology is 850 °C. Measurements of the dielectric and piezoelectric properties of the produced films are carried out. The low dielectric loss (0.022) and high d₃₃ and d₃₁ piezoelectric coefficients (85.7×10⁻¹² and −34.6×10⁻¹² m/V, respectively) of the material, together with a relatively low sintering temperature, make it suitable for various applications, e.g. ultrasonics.     

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.     

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.     

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.     

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.     

Solidification structure of CaO–SiO2–MgO–Al2O3 (–CaF2) systems and computational phase equilibria: Crystallization of MgAl2O4 spinel

The solidification behaviour of the CaO–SiO₂–Al₂O₃–CaF₂–10% MgO system, which is similar to the inclusion compositions in the stainless steel and the crystallization of spinel have been investigated using XRD, SEM-EDS, and an image analyser. The solidification mode and the phase equilibria were computed by employing thermochemical software. The liquidus temperature of the oxides containing 5% CaF₂ increases with increasing alumina content from 10% to 30%, while the solidus temperature has little dependence on alumina content. The size of spinel crystals in the final microstructure increases on increasing the content of alumina, resulting from that the oxides spending more time at higher temperatures below the liquidus temperature, where crystal growth is generally faster than nucleation, during slow cooling. The liquidus temperature of the oxides containing 30% Al₂O₃ is scarcely varied, while the solidus temperature decreases by increasing the content of CaF₂ to 10%. The size of spinel crystals decreases as the content of CaF₂ increases, resulting from the fact that the oxides could spend more time at relatively lower temperatures, where nucleation is faster than growth, during the cooling process.     

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.     

PZT-polymer composites fabricated with YAG laser cutter

Piezoelectric composites have been constructed by YAG laser cutting of PZT ceramics and back-filling with silicone rubber. Composites containing 46–76 vol.% PZT have been prepared with several different rod sizes. The measured dielectric constant ranges from 310 to 598, d₃₃ from 297 to 305 pC/N, and g₃₃ from 87 × 10⁻³ to 62 × 10⁻³ V m/N. The influence of the new fabrication method on the piezoelectric properties of the composite is evaluated, and it is confirmed that an elastomer with low elastic modulus increases g₃₃ and suppresses the radial mode vibration in resonance. For the PZT-silicone rubber composites made by YAG laser cutting, g₃₃ can be enhanced in 1–3 composites to about three times the value for solid PZT. The piezoelectric properties are compared with those of 1–3 composites made by ultrasonic cutting. The g₃₃ values of composites fabricated by ultrasonic cutting are larger than those fabricated by laser cutting. These results indicate that to get higher values of g₃₃ it is necessary to perforate PZT with ditches of the same width.     

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.     

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.     

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.     

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.     

A hydrogen peroxide biosensor based on direct electrochemistry of hemoglobin in Hb–Ag sol films

Hemoglobin (Hb) was used as a template to fabricate hemoglobin–silver (Hb–Ag) sol in which the hemoglobin showed direct electrochemistry on a glass carbon (GC) electrode. Ultraviolet–visible (UV–vis) spectra and reflectance absorption infrared (RAIR) spectra suggested that hemoglobin in Hb–Ag sol retained its native secondary structure. Scanning electron microscopy (SEM) demonstrated that the morphology of the Hb film was much different from the Hb–Ag sol film. The Hb–Ag film proved to exhibit a good electrocatalytic activity for the reduction of hydrogen peroxide. Based on this, a novel amperometric hydrogen peroxide biosensor was developed, which showed a sensitive response to the reduction of H₂O₂ without any electron mediator. Under optimum conditions, the biosensor responded linearly to H₂O₂ in the concentration range of 1 × 10⁻⁶ to 2.5 × 10⁻² M with detection limit of 1 × 10⁻⁷ M at 3σ. Moreover, the studied biosensor exhibited high sensibility, good reproducibility, and long-term stability.     

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.     

Computation of hydrostatic piezoelectric coefficients for 1–3 composites by the finite-element method

The hydrostatic piezoelectric charge coefficients (d_h) and hydrostatic piezoelectric voltage coefficients (g_h) of 1–3 PZT/polymer composites have been calculated by two equations containing the stress tensors of each element. A composite model is divided into 162 elements, and the stress distributions are computed under 0.7 MPa hydrostatic pressure using the finite-element method. The higher d_h value is found for the composite with 30.9% PZT and the higher g_h value for the composite with 19.8% PZT.     

Direct electrochemistry and electrocatalysis of myoglobin immobilized in zirconium phosphate nanosheets film

Myoglobin (Mb) is incorporated on a novel matrix—zirconium phosphate nanosheets (ZrPNS) and immobilized at a glassy carbon electrode surface. UV–vis spectra and electrochemical measurements show that the matrix is well biocompatible and can retain the bioactivity of immobilized Mb. The direct electron transfer between Mb and electrode exhibits a couple of well-defined redox peaks. The cathodic and anodic peaks are located at −0.340 and −0.280 V vs. Ag/AgCl, respectively. The ZrPNS can improve the electron transfer between Mb and electrode with an electron transfer constant of 5.6 s⁻¹. Meanwhile, the catalytic ability of the protein toward the reduction of H₂O₂, O₂, NaNO₂, trichloroacetic acid (TCA) is also studied and a third-generation biosensor is subsequently fabricated. The linear range of biosensor to H₂O₂ is from 8 × 10⁻⁷ to 1.28 × 10⁻⁵ M with the limit detection of 1.4 × 10⁻⁷ M. The small apparent Michaelis–Menten constant (34 μM) suggests that Mb/ZrPNS film performs good affinity with H₂O₂. The biosensor also exhibits acceptable stability and reproducibility. This work paves a way to develop other biologic active materials in this kind of nanosheets for constructing novel biosensors.