Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.     

Room-temperature H2S gas sensing at ppb level by single crystal In2O3 whiskers

In₂O₃ whiskers and bipyramidal nano-crystals were prepared by a carbothermal method. These were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), photoluminescence and Raman spectroscopy. These were studied for application to sensing of H₂S gas. The single crystal whiskers were found to be sensitive to as low as 200 ppb of H₂S gas at room temperature and showed saturation in response at 10 ppm. On the other hand, the films made of bipyramids were less sensitive to H₂S gas and the response was found to be a nearly linear function of concentration in a concentration range of 10–80 ppm.     

Influence of Cs doping in spray deposited SnO2 thin films for LPG sensors

Detection of low concentrations of petroleum gas was achieved using transparent conducting SnO₂ thin films doped with 0–4 wt.% caesium (Cs), deposited by spray pyrolysis technique. The electrical resistance change of the films was evaluated in the presence of LPG upon doping with different concentrations of Cs at different working temperatures in the range 250–400 °C. The investigations showed that the tin oxide thin film doped with 2% Cs with a mean grain size of 18 nm at a deposition temperature of 325 °C showed the maximum sensor response (93.4%). At a deposition temperature of 285 °C, the film doped with 3% Cs with a mean grain size of 20 nm showed a high response of 90.0% consistently. The structural properties of Cs-doped SnO₂ were studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films was found to be along the (3 0 1) directions. The crystallite sizes of the films determined from XRD are found to vary between 15 and 60 nm. The electrical investigations revealed that Cs-doped SnO₂ thin film conductivity in a petroleum gas ambience and subsequently the sensor response depended on the dopant concentration and the deposition temperature of the film. The sensors showed a rapid response at an operating temperature of 345 °C. The long-term stability of the sensors is also reported.     

Improved field emission characteristics of screen-printed CNT-FED cathode by interfusing Fe/Ni nano-grains

Carbon nanotube (CNT) cathode with and without interfusing nano-metal particles was prepared using screen-printing technology. For the good electric conductivity of metal, the turn-on electric field of the Fe/Ni and CNT composite film (Fe/CNT film) decreases to 1.42 V/μm comparing with the usual CNT film of 2.45 V/μm, and the emission current increases from 60 μA to 440 μA at an applied electric field of 2.3 V/μm. Furthermore, the field enhancement factor β increases from 1721 to 3242. By characterizing the prepared samples via X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), it is found that carbide Fe₃C phase is formed in Fe/CNT film, and the metal particles are filled in the interspaces of CNTs. It is evaluated that benefiting from good electrical conductivity and chemical inertness of metal carbide, Fe/CNT film achieves high emission characteristics and emission uniformity.     

Investigation on formaldehyde gas sensor with ZnO thick film prepared through microwave heating method

ZnO nanopowders were prepared through microwave heating method. ZnO thick film sensors were fabricated by using ZnO nanopowders as sensing materials. The phase composition and morphology of the material particles were characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The gas-sensing properties of the sensors based on ZnO nano-materials were investigated. It was found that the sensor based on ZnO nano-materials (low power, 10× 10 min) exhibited very high responses to benzene and toluene when operating at 440 and 370 °C, respectively; but the sensor based on ZnO (low power, 10× 10 min) showed very low responses to benzene and toluene when operating at 205–215 °C. The sensor based on ZnO (low power, 10× 10 min) showed high response and good selectivity to dilute formaldehyde when operating at 210 °C; especially, the response to 0.001 ppm HCHO attained 7.4 when operating at 210 °C.     

Au-doped WO3-based sensor for NO2 detection at low operating temperature

Pure and Au-doped WO₃ powders for NO₂ gas detection were prepared by a colloidal chemical method, and characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The NO₂ sensing properties of the sensors based on pure and Au-doped WO₃ powders were investigated by HW-30A gas sensing measurement. The results showed that the gas sensing properties of the doped WO₃ sensors were superior to those of the undoped one. Especially, the 1.0 wt% Au-doped WO₃ sensor possessed larger response, better selectivity, faster response/recovery and better longer term stability to NO₂ than the others at relatively low operating temperature (150 °C).     

Selective acetone sensor based on dumbbell-like ZnO with rapid response and recovery

Dumbbell-like ZnO microcrystals have been obtained through a facile solution method. The structure, morphology and optical properties of the as-prepared ZnO microcrystals have been characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and photoluminescence. The as-prepared ZnO microcrystals exhibit excellent sensing properties against acetone at an operating temperature of 300 °C. The response and recovery times are found to be 1.5 and 3 s, respectively. Moreover, the sensor holds the successful discrimination between acetone and ethanol, which makes our product a good candidate in fabricating highly selective sensors in practice.     

Novel tunable laser sources with 1.5-μm and 1.57-μm cascaded DFB reflectors for in situ gas monitoring applications

Novel tunable lasers based on 1.5-μm and 1.57-μm cascaded distributed-feedback reflectors are realized for real-time monitoring of H₂O and CO gas mixtures immediately in multi-gas sensor systems. With simple fabrication procedures, the new design allows the realization of a widely tunable laser source that can cover the H₂O and CO absorption wavelength bands. With the temperature tuning of 0.1 nm/°C and current tuning of 0.014 nm/mA, the laser can be tuned to cover over 3 nm wavelength range in each wavelength band. Experiments verify that the lasers can have more than 38 dB SMSR over the tuning range. The characteristics of high power, excellent spectral purity, and simple wavelength switching control can simplify the analysis procedures of gas sensing and thus reduce the cost. By direct absorption method, the tunable laser has been successfully adopted in a diode laser sensor system for monitoring of water vapor concentration near 1.5 μm and carbon monoxide near 1.57 μm. Less than 15% error in the line strength is observed between the measured data and HITRAN database.     

Structural, optical and electrical properties of CdO/Cd(OH)2 thin films grown by the SILAR method

Successive Ionic Layer Adsorption and Reaction (SILAR) was used to form Cd(OH)₂ thin films from aqueous cadmium–ammonia complex on glass substrates at room temperature and the thermal annealing effect on thin films was studied. The as-deposited films were annealed at 200, 300 and 400 °C for 1 h in an oxygen atmosphere for conversion from Cd(OH)₂ to CdO and change in the structural, optical and electrical properties of the films and the effect of the light on the electrical properties of the films were investigated. The structural and surface morphological properties of the films were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that Cd(OH)₂ phase is converted into the cubic CdO films by annealing. The band gap energy values of films decreased from 3.59 to 2.13 eV through increasing annealing temperature. It was found that the current increased with increasing light intensity and CdO films were more conductive than the as-deposited films.     

Electrical response of Sm2O3-doped SnO2 to C2H2 and effect of humidity interference

Pure and Sm₂O₃-doped SnO₂ are prepared through a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sensor based on 6 wt% Sm₂O₃-doped SnO₂ displays superior response at an operating temperature of 180 °C, and the response magnitude to 1000 ppm C₂H₂ can reach 63.8, which is 16.8 times larger than that of pure SnO₂. This sensor also shows high sensitivity under various humidity conditions. These results make our product be a good candidate in fabricating C₂H₂ sensors.     

Phase change and cooling characteristics of microjets measured using microcantilever heaters

This paper reports novel microcantilever metrology tools to investigate free microjets emanating from a micromachined nozzle of diameter 10 μm. Microcantilevers with an integrated resistive heater-thermometer were used to study microjet cooling and phase change characteristics. While the microcantilever heater was aligned to and impinged upon by hydrocarbon microjets, the heater temperature was modulated by a constant voltage source. Successive heating and cooling cycles captured the microjet phase change hysteresis properties. When the microcantilever heater temperature was controlled, it was possible to construct a full boiling curve. Measurements were made on microjets of butane, hexane, and octane. The convective heat fluxes accompanied by microjet impingement boiling were 2.9–7.6 kW/cm² for heater temperatures of 25–350 °C. Overall, the techniques reported herein show promise for characterizing microscale jet flows.     

Pulsed laser deposited Y-doped BaZrO3 thin films for high temperature humidity sensors

Pulsed laser deposited (PLD) Y-doped BaZrO₃ thin films (BaZr_1-xY_xO_3-y/2, x = 0.2, y > 0), were investigated as to their viability for reliable humidity microsensors with long-term stability at high operating temperatures (T > 500 °C) as required for in situ point of source emissions control as used in power plant combustion processes. Defect chemistry based models and initial experimental results in recent humidity sensor literature [1] and [2]. indicate that bulk Y-doped BaZrO₃ could be suitable for use in highly selective, high temperature compatible humidity sensors. In order to accomplish faster response and leverage low cost batch microfabrication technologies we have developed thin film deposition processes, characterized layer properties, fabricated and tested high temperature humidity micro sensors using these thin films. Previously published results on sputtering Y-doped BaZrO₃ thin films have confirmed the principle validity of our approach [3]. However, the difficulty in controlling the stoichiometry of the films and their electrical properties as well as mud flat cracking of the films occurring either at films thicker than 400 nm or at annealing temperature above 800 °C have rendered sputtering a difficult process for the fabrication of reproducible and reliable thin film high temperature humidity microsensors, leading to the evaluation of PLD as alternative deposition method for these films.X-ray Photoelectron Spectroscopy (XPS) data was collected from as deposited samples at the sample surface as well as after 4 min of Ar⁺ etching. PLD samples were close to the desired stoichiometry. X-ray diffraction (XRD) spectra from all as deposited BaZrO₃:Y films show that the material is polycrystalline when deposited at substrate temperatures of 800 °C. AFM results revealed that PLD samples have a particle size between 32 nm and 72 nm and root mean square (RMS) roughness between 0.2 nm and 1.2 nm. The film conductivity increases as a function of temperature (from 200 °C to 650 °C) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. Humidity measurements are presented for 200–500 nm thick films from 500 °C to 650 °C at vapor pressures of between 0.05 and 0.5 atm, with 0.03–2% error in repeatability and 1.2–15.7% error in hysteresis during cycling for over 2 h. Sensitivities of up to 7.5 atm⁻¹ for 200 nm thick PLD samples at 0.058 atm partial pressure of water were measured.     

Tensile and high cycle fatigue test of Al–3% Ti thin films

This paper presents the results of tensile and high cycle fatigue tests with stress ratio R = 0.1 for an Al–3% Ti thin film of 1 μm thickness in atmospheric air at room temperature. Specimens with three different widths (50, 100, and 150 μm) were fabricated to study the width effects of each sample. Test results show that tensile and fatigue properties for the Al–3% Ti thin film with different widths are very close, and, thus, width effects were found to be minimal. The elastic moduli ranged from 80 to 82 GPa, and the tensile strengths ranged from 369 to 379 MPa. Fatigue strength coefficients of the specimens with 50, 100, and 150 μm width were 193, 181, and 164 MPa, respectively. In addition, fatigue strength exponents of the specimens with 50, 100, and 150 μm width were −0.023, −0.020, and −0.013, respectively. When present test results are compared with typical properties of bulk aluminium, the Al–3% Ti thin film is found to have longer life at the same stress, but it is more sensitive to the stress level.     

A novel biosensor using entangled carbon nanotubes layer grown on an alumina substrate by CCVD of methane on FeOx–MgO

Glucose oxidase (GOx) was immobilized on entangled and high surface area carbon nanotubes (CNTs) grown on an alumina substrate, and direct electron transfer reaction between GOx and the electrode was revealed. Fe/MgO catalyst layer was spin-coated on the insulating alumina substrate and the CNT layer was grown on the catalyst by chemical vapor deposition of methane at 950 °C for 15 min. About 20–30 nm bundles of about 1 nm single-wall as well as 10 nm multiwall CNTs are formed. The redox process was surface-controlled and electron transfer coefficient and the rate constant were estimated to be 0.35 and 0.64 s⁻¹, respectively. In addition GOx immobilized on CNT layer showed a linear response range between 12 and 62 μM of glucose concentration. A detection limit and sensitivity of 0.1 μM and 635 μA mM⁻¹ cm⁻², respectively, were obtained for the biosensor.     

Controlled synthesis and gas-sensing properties of hollow sea urchin-like α-Fe2O3 nanostructures and α-Fe2O3 nanocubes

Hollow sea urchin-like α-Fe₂O₃ nanostructures were successfully synthesized by a hydrothermal approach using FeCl₃ and Na₂SO₄ as raw materials, and subsequent annealing in air at 600 °C for 2 h. The hollow sea urchin-like α-Fe₂O₃ nanostructures with the diameters of 2–4.5 μm consist of well-aligned α-Fe₂O₃ nanorods with an average length of about 1 μm growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 μm. α-Fe₂O₃ nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl₃ at 140 °C for 12 h. The response S_r (S_r = R_a/R_g) of the hollow sea urchin-like α-Fe₂O₃ nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, respectively, which was excess twice that of the α-Fe₂O₃ nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like α-Fe₂O₃ nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics.     

Batron P–Si microsensor for methane and its derivatives

Baytron P–Si chip was made by depositing Baytron P onto a gap of two parallel gold plates on a silicon wafer separated by 150 μm such that the gold plates contact the polymer only in the gap region. The resistance of the polymer was examined at ambient room temperature before and after the exposure of the chip to different concentrations of methane or its derivatives such as chloroform, carbon tetrachloride and methylene chloride. The chip response to methane gas opens up the prospects of its usage at ambient temperature for the industrial and environmental detections in the range of 200–1300 ppm. Among the derivatives of methane, methylene chloride showed the highest response for detection. The mechanism of sensing is discussed.     

A large-displacement 65Pb(Mg1/3Nb2/3)O3–35PbTiO3/Pt bimorph actuator prepared by screen printing

In this paper we present a novel approach to preparing large-displacement 65Pb(Mg_1/3Nb_2/3)O₃–35PbTiO₃/Pt (65/35 PMN–PT/Pt) bimorph actuators. These “substrate-free”, bending-type actuators were prepared by screen-printing the 65/35 PMN–PT and Pt thick-film pastes as the electrodes on alumina substrates. After this screen printing and the subsequent firing the 65/35 PMN–PT/Pt composites were peeled off from the substrates. Displacements of nearly 100 μm at 18 V were achieved for actuators with dimensions of 1.8 cm × 2.5 mm × 50 μm for the 65/35 PMN–PT layer. The normalized displacement (the displacement per unit length) was 40 μm/cm at 18 V. The experimental results together with a computation procedure were used to obtain the material parameters for a finite-element analysis of the 65/35 PMN–PT/Pt bimorph actuators.     

Thin film polypyrrole/SWCNTs nanocomposites-based NH3 sensor operated at room temperature

A PPY/SWCNTs nanocomposite-based sensor with relatively high sensitivity and fast response–recovery was developed for detection of NH₃ gas at room temperature. The gas-sensitive composite thin film was prepared using chemical polymerization and spin-coating techniques, and characterized by Fourier transformed infrared spectra and field-emission scanning electron microscopy. The results reveal that the conjugated structure of the PPY layer was formed and the functionalized SWCNTs were well-embedded. The effects of film thickness, annealing temperature, and SWCNTs content on gas-sensing properties of the composite thin film were investigated to optimize the gas-sensing performance. The as-prepared thin film PPY/SWCNTs composite sensor with optimized process parameters had a response of 26–276% upon exposure to NH₃ gas concentration from 10 to 800 ppm, and their response and recovery times were around 22 and 38 s, respectively.     

An efficient cell count method using a lattice molded on indents of a culture dish

Cell count is an important task for obtaining biological and medical information. In this paper, a novel cell count method is presented for improving the efficiency of the procedure as well as reducing microbial contamination compared to the conventional cell count method using a hemocytometer. The proposed method involves a lattice array consisting of a 50 μm × 50 μm square with lines of 2-μm width and 1.4-μm depth on a surface indented from a culture dish bottom. This configuration enables observation of cells at the same focus as the lattice. Therefore, an instant cell count during incubation is possible without the tedious, error-prone preparation steps such as harvesting and loading required in conventional methods. In addition, cells can be preserved with minimal contact with the external environment. These advantages become magnified with a periodic long-term cell count. A polystyrene culture dish, 35 mm in diameter, was fabricated by injection molding using a nickel mold, wherein indents of 3 mm × 3 mm in area and 1 mm in height were electroplated based on microfabrication technology. For easy separation from the nickel mold, the four sides of the indents in the mold are inclined at 54.74° via anisotropic silicon etching. The usefulness of the suggested method was verified using adhering HeLa (cervical carcinoma cell) cells and floating Jurkat cells. Both were placed in culture dishes and cultivated for 3 days in a carbon dioxide incubator (5% CO₂, 95% air) at 37 °C, and then successfully observed through the divided lattice using an inverted microscope. The dish was also assessed with a hemocytometer by counting HEK 293T (human embryonic kidney) cells and yeast cells.     

Detection of amines with chromium-doped WO3 mesoporous material

Chromium-doped mesoporous tungsten trioxide – with KIT-6 structure – was prepared through a chemical route. The resulting material was deposited on a micromechanically fabricated hot-plate and tested as a sensor for ammonia and trimethylamine in the temperature range of 200–500 °C. Maximum response was reached at 350 and 450 °C for ammonia and TMA, respectively. It was also found that the sensor shows a non-linear cross-sensitivity to the gases.