Chemically deposited zinc oxide thin film gas sensor

Zinc oxide (ZnO) thin films were prepared by a low cost chemical deposition technique using sodium zincate bath. Structural characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) indicate the formation of ZnO films, containing 0.05–0.50 m size crystallites, with preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–450 K temperature range after repeated thermal cyclings in air. Palladium sensitised ZnO films were exposed to toxic and combustible gases e.g., hydrogen (H₂), liquid petroleum gas (LPG), methane (CH₄) and hydrogen sulphide (H₂S) at a minimum operating temperature of 150 °C; which was well below the normal operating temperature range of 200–400 °C, typically reported in literature for ceramic gas sensors. The response of the ZnO thin film sensors at 150 °C, was found to be significant, even for parts per million level concentrations of CH₄ (50 ppm) and H₂S (15 ppm).     

Sensing behaviour of nanosized zinc-tin composite oxide towards liquefied petroleum gas and ethanol

A chemical route has been used to synthesize composite oxides of zinc and tin. An ammonia solution was added to equal amounts of zinc and tin chloride solutions of same molarities to obtain precipitates. Three portions of these precipitates were annealed at 400, 600 and 800 °C, respectively. Results of X-ray diffraction and transmission electron microscopy clearly depicted coexistence of phases of nano-sized SnO₂, ZnO, Zn₂SnO₄ and ZnSnO₃. The effect of annealing on structure, morphology and sensing has been observed as well. It has been observed that annealing promoted growth of Zn₂SnO₄ and ZnSnO₃ at the expense of zinc. The sensing response of fabricated sensors from these materials to 250 ppm LPG and ethanol has been investigated. The sensor fabricated from powder annealed at 400 °C responded better to LPG than ethanol.     

Structure and CO gas sensing properties of electrospun TiO2 nanofibers

Titanium dioxide (TiO₂) nanofibers were fabricated by electrospinning a hybrid solution, which is a mixture of the TiO₂ sol precursor, polymer, and solvent. The structure and gas sensing properties of TiO₂ nanofibers were investigated. By calcining at 600 °C, the polymeric components were decomposed and a multi-layered random network structure of TiO₂ nanofibers was obtained. Polycrystalline TiO₂ nanofibers consist of tetragonal anatase and rutile TiO₂ phases. The diameter ranged from 400 nm to 500 nm and the grain size was about 15 nm. The TiO₂ nanofibers-based sensor exhibited response to CO concentration as low as 1 ppm at 200 °C.     

Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3, and H2S. Part II: Application as gas sensors

The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350–800 °C for 1–3 h in O₂) to NO₂, O₃ and H₂S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (W _n O_3n–2 or W _n O_3n–1) and WO₃ as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO₂ (0.05–0.5 ppm in N₂ and synthetic air), ozone (25–90 ppb) and H₂S (3–15 ppm in N₂ and synthetic air) at very low working temperatures (80–160 °C). The films annealed at 400 °C for 1–2 h are very selective to ozone at 120–160 °C; the films annealed at 400 °C for 1–3 h and at 800 °C for 1 h are very sensitive to NO₂ (in N₂).     

Preparation of cobalt oxide films by plasma-enhanced metalorganic chemical vapour deposition

Co oxide films were prepared on glass substrates at 150–400°C by plasma-enhanced metalorganic chemical vapour deposition using cobalt (II) acetylacetonate as a source material. NaCl-type CoO films were formed at low O₂ flow rate of 7cm³ min^–1 and at a substrate temperature of 150–400°C. The CoO films possessed (100) orientation, independent of substrate temperature. Deposition rates of the CoO films were 40–47 nm min^–1. The CoO film deposited at 400 °C was composed of closely packed columnar grains and average diameter size at film surface was 60 nm. At high O₂ flow rate of 20–50 cm³ min^–1, high crystalline spinel-type Co₃O₄ films were formed at a substrate temperature of 150–400°C. The Co₃O₄ film deposited at 400°C possessed (100) preferred orientation and the film deposited at 150°C possessed (111) preferred orientation. Deposition rates of the Co₃O₄ films were 20–41 nm min^–1. Both Co₃O₄ films with (100) and (111) orientation had columnar structure. The shape and average size of the columnar grains at the film surface were different; a square shape and 35 nm for (100)-oriented Co₃O₄ film and a hexagonal shape and 60 nm for (111)-oriented film, respectively.     

Nanocrystalline chemically modified CdIn2O4 thick films for H2S gas sensor

CdIn₂O₄ sensor with high sensitivity and excellent selectivity for H₂S gas was synthesized by using sol-gel technique. Structural, electrical and gas sensing properties of doped and undoped CdIn₂O₄ thick films were studied. XRD revealed the single-phase polycrystalline nature of the synthesized CdIn₂O₄ nanomaterials. Since the resistance change of a sensing material is the measure of its response, selectivity and sensitivity was found to be enhanced by doping different concentrations of cobalt in CdIn₂O₄ thick films. The sensor exhibits high response and selectivity toward H₂S for 10 wt.% Co doped CdIn₂O₄ thick films. The current-voltage characteristics of 10 wt.% Co doped CdIn₂O₄ calcined at 650 °C shows one order increase in current with change in the bias voltage at an operating temperature of 200 °C for 1000 ppm H₂S gas.     

Fine-tuning of temperature coefficients of capacitance (TCC) and dielectric constant (TCK) of Mg2SnO4 via second phase addition

Fine-tuning of the temperature coefficients of capacitance and dielectric constant of magnesium orthostannate (Mg₂SnO₄) has been attempted by means of Sn (IV) and Zr (IV) oxide incorporation as a second phase. The additives were also employed to enhance the density and minimize or eliminate porosity at lower sintering temperatures. Phase-pure magnesium stannate powder was synthesized via conventional solid-state reaction. It was mixed with ZrO₂ and/or SnO₂ and sintered in the temperature range 1500°–1600°C for up to 6 h. Electrical measurements using an AC immittance spectroscopic technique over the temperature range 25°–300°C, on Mg₂SnO₄ compacts containing 5 wt.% of additives and sintered at 1500 °C/ 6 h, were carried out. Data analyses revealed that the capacitance and the derived dielectric constant remained invariant over more than 3 decades of frequency in the kilo to megahertz regime. It was also found that addition of ZrO₂ and SnO₂ has a benign effect on both temperature coefficient of capacitance (TCC) and temperature coefficient of dielectric constant (TCK) as it resulted in smaller dependence of capacitance and dielectric constant compared to pure Mg₂SnO₄. Typically, the TCC values were 5 and 30 ppm/°C and TCK values were 20 and 30 ppm/°C for 5 wt.% ZrO₂- and 5 wt.% SnO₂- added Mg₂SnO₄, respectively, in the temperature range 25°–300°C.     

Structure, properties and gas sensing behavior of Cr2 − xTixO3 films fabricated by electrostatic spray assisted vapour deposition

Single-phase eskolaite crystalline Cr_2 − xTi_xO₃ films (CTO) with a uniform porous microstructure were fabricated via an electrostatic spray assisted vapour deposition (ESAVD) method. The sensing behavior upon exposure to ammonia and ethanol was characterized in a CTO film-based sensor device in terms of response, reproducibility, humidity constraints and sensor stability. The ESAVD process has been shown to be capable of producing CTO films at low temperature (650 °C) and more importantly, it results in a more uniform titanium distribution and better microstructural control than processes based on solid-state chemical reactions. The material with a nominal composition of Cr_1.7Ti_0.3O₃ exhibited the highest sensitivity among the different Cr_2 − xTi_xO₃ compositions examined towards ammonia over the temperature range of 200-500 °C with a peak sensitivity of 2.90 at 200 °C. The CTO materials, when used as sensors, also exhibit excellent responses to ethanol concentration in air. The sensitivity was 0.64 for 10 ppm ethanol, 0.85 for 25 ppm, and 0.92 for 50 ppm, respectively.     

Proton conduction at the surface of Y-doped BaCeO3 and its application to an air/fuel sensor

25 mol% Y³⁺-doped BaCeO₃ (BCY25) showed an extremely low activation energy of 0.3 eV for proton conduction at the surface. The resulting overall conductivity at the surface reached 8.24 × 10^–3 S cm^–1 at 400°C, which was 3, 8, and 28 times higher than those in the bulk of BCY25, 20 mol% Sm³⁺-doped ceria, and 8 mol% yttria-stabilized zirconia, respectively. Such fast proton conduction enabled an air/fuel (A/F ) sensor using BCY25 as the solid electrolyte to work above 150°C for H₂ and above 250°C for C₂H₄.     

Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing

Ti thin films were anodized in aqueous HF (0.5 wt.%) and in polar organic (0.5 wt.% NH₄F + ethylene glycol) electrolytes to form TiO₂ nanotube arrays. Ti thin films were deposited on microscope glass substrates and then anodized. Anodization was performed at potentials ranging from 5 V to 20 V for the aqueous HF and from 20 V to 60 V for the polar organic electrolytes over the temperatures range from 0 to 20 °C. The TiO₂ nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It has been observed that anodization of the deposited Ti thin films with aqueous HF solution at 0 °C resulted in nanotube-type structures with diameters in the range of 30-80 nm for an applied voltage of 10 V. In addition, the nanotube-type structure is observed for polar organic electrolyte at room temperature at the anodization voltage higher than 40 V. The volatile organic compound (VOC) sensing properties of TiO₂ nanotubes fabricated using different electrolytes were investigated at 200 °C. The maximum sensor response is obtained for carbon tetrachloride. The sensor response is dependent on porosity of TiO₂. The highest sensor response is observed for TiO₂ nanotubes which are synthesized using aqueous HF electrolyte and have very high porosity.     

Synthesis and ethanol sensing properties of Fe-doped SnO2 nanofibers

Fe-doped SnO₂ nanofibers are synthesized through an electrospinning method and characterized by scanning electron microscopy and transmission electron microscopy. The sensor fabricated from these nanofibers exhibits high sensitivity and rapid response/recovery to ethanol at 300 °C. The sensitivity is up to 15.3 when the sensor is exposed to 100 ppm ethanol, and the response and recovery time is about 1 and 3 s, respectively. The linear dependence of the sensitivity on the ethanol concentration is observed in the range of 10-300 ppm. These results demonstrate that Fe-doped SnO₂ nanofibers can be used as the sensing material for fabricating high performance ethanol sensors.     

Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate Part I Thermo-oxidative stability of Cs2MoOS3

Cesium oxythiomolybdate (Cs₂MoOS₃) is a potential high temperature solid lubricant. It undergoes complex oxidation reactions at elevated temperatures, but continues to provide lubrication above the oxidation temperature. Therefore, in order to determine the nature of the lubricant at elevated temperature, it is necessary to understand the thermal chemistry of Cs₂MoOS₃in an air environment. The thermo-oxidative stability of Cs₂MoOS₃was evaluated between room temperature and 800°C in air. Melting and phase transition temperatures were determined. X-ray photoelectron spectroscopy, micro-Raman scattering and x-ray diffraction were used to identify the chemical species evolved at increasing temperatures. As-received Cs₂MoOS₃was not pure. It also contained cesium molybdates, molybdenum oxides, and Cs₂SO₄. Between 300–400°C, the material began to decompose forming Cs₂SO₄and MoS₂. Between 400–600°C, Cs₂MoOS₃also formed cesium molybdates and molybdenum oxides. In addition, the Cs₂SO₄began to oxidize to cesium oxides (which melted) and SO _x gas. Also, MoS₂oxidized to MoO₃. At approximately 700°C, MoO₃began to sublime. Upon cooling from 800°C, the material was primarily cesium oxides and Cs₂MoO₄, with small amounts of complex cesium molybdates and molybdenum oxides.     

Improved ethanol sensing properties of Cu-doped SnO2 nanofibers

Pure and Cu-doped SnO₂ nanofibers are synthesized via a simple electrospinning method, and characterized by transmission electron microscopy and X-ray diffraction. The sensor fabricated from Cu-doped SnO₂ nanofibers exhibits improved sensing properties to ethanol at 300 °C. The sensitivity is up to 3 when this sensor is exposed to 5 ppm ethanol. The response and recovery times are about 1 and 10 s, respectively. The linear dependence of the sensitivity on the ethanol concentration is observed in the range of 5-500 ppm. Good selectivity is also observed in our studies. The results make Cu-doped SnO₂ nanofibers good candidates for fabricating high performance ethanol sensors.     

Specific Features of Anodic Dissolution of Copper in H2SO4 + H2O and H2SO4 + CuSO4 + H2O Systems

We investigate anodic dissolution of copper in H₂SO₄ + H₂O and H₂SO₄ + H₂O + CuSO₄ systems, which model solutions for the electrochemical production of copper (+2) sulfate. Ultimate densities of anodic current in the temperature range 20–80°C for a voltage up to 8 V were found. We show that a concentration of copper ions ( Cu²⁺) of 1.5–2.0 moles/liter in the anolyte is the limiting one in the electrochemical production of solutions of copper (+2) sulfate.     

Study of YSZ-based electrochemical sensors with oxide electrodes for high temperature applications

Potentiometric sensors based on yttria stabilized zirconia (YSZ) with WO₃ as sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 550–700°C in the presence of different concentrations (300-1000 ppm) of NO₂ and CO in air. The response to NO₂ was very stable with fast response time (20-40 s). The best sensitivity (18.8 mV/decade) using Pt electrodes was observed at 600°C. At the same temperature a cross-sensitivity (-15 mV/decade) to CO gas was also noticed. The response to CO was decreased (-4 mV/decade) using Au electrode. The role played by WO₃ on the sensing electrode was discussed.     

MoO3 and WO3 based thin film conductimetric sensors for automotive applications

Un-doped semiconducting oxides suitable for automotive gas sensor applications have been studied in this work. Thin films of MoO₃ and WO₃ were fabricated by ion beam deposition on alumina substrates with gold interdigitated electrodes. The process pressure inside the deposition chamber was 1.6 × 10^–4 Torr. The oxygen to argon ratio in the secondary plasma was maintained at 5:5 sccm. A stabilization heat treatment of 500°C for 8 h was performed for each set of films that produced nanocrystalline structures. Gas sensing tests were carried out at 450°C with nitrogen dioxide/ammonia with synthetic air background similar to those realized in diesel automotive exhausts. XRD and electron microscopy studies were performed to understand the microstructure of the thin films following the sensing tests. The MoO₃ films were selective to ammonia whereas the WO₃ films showed high senstitivity towards NO₂ with respect to NH₃. An attempt is made to correlate the structural characteristics to the sensing behavior of the materials.     

Trimethylamine sensing properties of nano-LaFeO3 prepared using solid-state reaction in the presence of PEG400

LaFeO₃ precursors are prepared using solid-state reaction in the presence of PEG400, and then LaFeO₃ nano-powders are obtained through heating these precursors under different conditions. Eventually, LaFeO₃ thick film sensors are fabricated by using LaFeO₃ nano-materials as sensing materials. The phase composition and morphology of particles in these materials are characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The XRD analysis results reveal that LaFeO₃ can be obtained by heating at 400–900 °C. TEM images manifest that the average particle sizes increase with heating temperature increasing, the particle sizes are in the range of 50–80 nm when heating temperature is increased to 800 °C. Furthermore, the influence of the heating duration and the heating temperature on the gas-sensing properties of the sensors based on LaFeO₃ nano-materials is also investigated in this work. The sensitivities to several organic gases, such as (CH₃)₃N and (CH₃)₂CO are studied. It is found that the sensor based on LaFeO₃ nano-material (800 °C, 2 h) exhibits best performance in all sensors investigated in this work. In detail, the sensitivities of the sensor based on LaFeO₃ nano-material (800 °C, 2 h) to 1000 and 0.001 ppm (CH₃)₃N at 208 °C are as high as 2553 and 1.6, respectively; and the response time and recovery time for 10 ppm trimethylamine are 8 and 50 s, respectively.     

ZnO thin films for VOC sensing applications

ZnO thin films were prepared by reactive RF sputtering on thermally oxidized Si for gas sensing applications. Three VOC vapors were chosen to investigate the response behavior of the prepared ZnO. Acetone, isopropanol and ethanol were tested, and the sensitivity of the sensor toward acetone was the highest (S ∼ 100) for 500 ppm acetone at 400 °C. The largest sensitivity was achieved at 400 °C for all the above vapors. The sensor shows a stable, reversible and repeatable behavior in the acetone concentration ranging from 15 up to 1000 ppm. The mechanism of the sensing was explained according to the ionosorption model.     

Titanium oxy-nitride films deposited on stainless steel by an ion beam assisted deposition technique

Surfaces of stainless steel SUS304 were coated with titanium oxy-nitride (TiON) films at temperatures of 400–770°C using an ion-beam assisted deposition technique constructed from an electron beam evaporator for Ti evaporation and a microwave ion source for ionizing nitrogen gas. The N ions were accelerated at energies of 0.5–2.0 keV. Most of the deposited TiON films consisted of (60–80)% TiN and (40–20)% TiO₂, and the fraction of TiO₂ increased with increasing substrate temperature. Hardness of the TiNO films varied in the range from 160 GPa to 260 GPa with increasing substrate temperature. The titanium oxy-nitride film could be deposited on stainless steel without a significant deterioration surface layer at 600°C. However, when TiNO films were deposited at temperatures higher than 700°C, the thickness of the TiNO films were significantly thinner and a thick layer containing nitride such as Cr₂N, CrFe, Fe₂N and Fe₄N was formed in a near surface region of stainless steel because more nitrogen diffused into stainless steel.     

Potentiometric SO2 gas sensor based on Ca2+ conducting solid electrolyte

Abstract

A new SO₂ sensor based on Ca²⁺ conductor CaO.0.6MgO.6Al₂O₃ (CMA) with a Na₂SO₄ auxiliary electrode and Pt/O₂ reference electrode has been fabricated and tested. In this design, both the electrodes are exposed to the same test gas thus eliminating the need to separate electrode chambers. Experimental results showed that the sensor response to SO₂ is rapid and stable and the 90% response time is about 1 min. The measured emf of the sensor is a linear function of the logarithm of partial pressure of SO₂ over a concentration range 1-1000 ppm and a temperature range 873-1073 K. In this cell, the CMA/Pt interface was sensitive to oxygen, while the Na₂SO₄/Pt interface was sensitive to SO₃.