High temperature potentiometric carbon dioxide sensor with minimal interference to humidity

We report on the development of a potentiometric sensor with Li₃PO₄ electrolyte and BaCO₃-coated Li₂CO₃ sensing electrode for measuring CO₂ over a wide range of concentrations (500 ppm to 20%) at 500 °C with minimal interference to humidity. The active element in the sensing electrode for CO₂ detection is Li₂CO₃. However, sensors with just Li₂CO₃ electrodes showed interference from humidity, which was eliminated by use of the BaCO₃ layer. Infrared spectroscopy as well as electrode preparations involving heating above the eutectic temperature of BaCO₃–Li₂CO₃ suggests that the BaCO₃ layer wets the Li₂CO₃ electrode surface, making it more hydrophobic and thereby reducing the interference from humidity.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

Tetrakis(alkylthio)-substituted lutetium bisphthalocyanines for sensing NO2 and O3

In this study, the nitrogen dioxide (NO₂) and ozone (O₃) sensing properties of a series bis[tetrakis(alkylthio) phthalocyaninato] lutetium(III) complexes [(C_nH_2n+1S)₄Pc]₂Lu(III) (n = 6, 10, 16) are investigated as a function of concentration in the temperature range between 25 °C and 150 °C. The concentration ranges were 1–10 ppm for NO₂, and 50 ppb–1 ppm for O₃. The response time and the sensor response to NO₂ are measured for approximately 1 min and 100% ppm⁻¹, respectively, for compound 1 at room temperature. At room temperature, all compounds are in the solid phase. The response time decreases to a few seconds with increasing operation temperature to 150 °C. At this temperature, all compounds are in the liquid crystal phase. The fastest response to oxidizing gases is observed at the liquid crystal phase of the Pcs. It has also been observed that the response time and the sensor response depend on the alkyl chain lengths of the Pcs. The doping effect of oxygen has been determined under high purity nitrogen N₂ flow, after exposure to dry air, at a different period of time and after annealing. It has been found that the conductivities of [(C_nH_2n+1S)₄Pc]₂Lu(III) thin films increased after exposure to dry air and the conduction mechanism also changed from ohmic behavior to space-charge-limited conduction.     

Gas sensitivity of Zn doped α-Fe2O3(SO4, Sn, Zn) to carbon monoxide

It is shown that the doping of Zn and Sn can improve the gas sensitivity of α-Fe₂O₃-based sensing material to CO. X-ray photo-electron spectroscopy analysis suggests that this is mainly due to the fact that the simultaneous doping of Zn and Sn can increase the S and hence SO₄²⁻ contents in the α-Fe₂O₃(SO₄²⁻, Sn, Zn) sensing material. The results also suggest that under a given condition, the gas sensitivity of α-Fe₂O₃(SO₄²⁻, Sn, Zn) to CO can be optimised by properly adjusting the doped Zn content.     

Preparation and characterization of nanocrystalline α-Fe2O3 by a sol-gel process

α-Fe₂O₃ ultra-fine powder with an average particle size of 6–26nm has been prepared by a sol-gel process. Thermal analysis, X-ray diffraction and transmission electron microscope were used to study its formation process and micro-structure. The temperature dependence of the electric conductance of the elements made of nanocrystalline α-Fe₂O₃ shows that the gas-sensing properties are strongly related to its surface. The elements exhibited good sensitivity and selectivity to ethyl alcohol, indicating it is a promising alcohol-sensing material.     

A technique for monitoring SO2 in combustion exhausts: Use of a non-Nernstian sensing element in combination with an upstream catalytic filter

Detection of sulfur dioxide (SO₂) at high temperature (600–750 °C) in the presence of some interferents found in combustion exhausts (NO₂, NO, CO₂, CO, and hydrocarbon (C₃H₆)) is described. The detection scheme involves use of a catalytic filter in front of a non-Nernstian (mixed-potential) sensing element. The catalytic filter was a Ni:Cr powder bed operating at 850 °C, and the sensing elements were pairs of platinum (Pt) and oxide (Ba-promoted copper chromite ((Ba,Cu)_xCr_yO_z) or Sr-modified lanthanum ferrite (LSF)) electrodes on yttria-stabilized zirconia. The Ni:Cr powder bed was capable of reducing the sensing element response to NO₂, NO, CO, and C₃H₆, but the presence of NO₂ or NO (“NO_x”, at 100 ppm by volume) still interfered with the SO₂ response of the Pt–(Ba,Cu)_xCr_yO_z sensing element at 600 °C, causing approximately a 7 mV (20%) reduction in the response to 120 ppm SO₂ and a response equivalent to about 20 ppm SO₂ in the absence of SO₂. The Pt–LSF sensing element, operated at 750 °C, did not suffer from this NO_x interference but at the cost of a reduced SO₂ response magnitude (120 ppm SO₂ yielded 10 mV, in contrast to 30 mV for the Pt-(Ba,Cu)_xCr_yO_z sensing element). The powder bed and Pt–LSF sensing element were operated continuously over approximately 350 h, and the response to SO₂ drifted downward by about 7%, with most of this change occurring during the initial 100 h of operation.     

Selectivity of flame-spray-made Nb/ZnO thick films towards NO2 gas

Unloaded ZnO and Nb/ZnO nanoparticles containing 0.25, 0.5 and 1 mol.% Nb were produced in a single step by flame-spray pyrolysis (FSP) technique. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. FSP yielded small Nb particles attached to the surface of the supporting ZnO nanoparticles, indicating a high SSA_BET. The morphology and accurate size of the primary particles were further investigated by TEM. Nb/ZnO nanoparticles paste composed of ethyl cellulose and terpineol as binder and solvent respectively was coated on Al₂O₃ substrate interdigitated with gold electrodes to form thick films by spin coating technique. After the sensing tests, the morphology and the cross-section of sensing film were analyzed by SEM and EDS analyses. The influence on a low dynamic range of Nb concentration on NO₂ response (0.1-4 ppm) of thick film sensor elements was studied at the operating temperatures ranging from 250 to 350 °C in the presence of dry air. The optimum Nb concentration was found be 0.5 mol.% and 0.5 mol.% Nb exhibited an optimum NO₂ response of ∼1640 and a short response time (27 s) for NO₂ concentration of 4 ppm at 300 °C.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

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.     

Sensing of CO2 at room temperature using work function readout of (hetero-)polysiloxanes sensing layers

Organically modified silicates based on primary amino groups are known to be CO₂ sensitive, as they can undergo a reversible acid base reaction. In order to generate detectable CO₂ signals and to limit the cross-sensitivity to humidity, some sources suggest that these materials should be operated at higher temperatures (50-70 °C). In this paper, a new variant of CO₂ sensing is to be presented, namely a combination of work function readout and organically modified silicates, which yields CO₂ detection even at room temperature. Kelvin probe measurements are used for work function readout. The layers are intended to be used in a “Floating Gate Field Effect Transistors” (FGFETs) sensing platform (mySens) by Micronas. The reversible interaction of CO₂ with spin-coated heteropolysiloxane sensitive layers results in changes of the work function with typical signal heights of 15-20 mV (change from 400 to 4000 ppm CO₂) and response times of only a few minutes. Also, results will be presented regarding variations in the chemical nature of the films. The findings summarized in this paper point towards the possibility of a new room temperature CO₂ sensor, which comprises fast response times and sufficient sensitivity for ambient CO₂ variations.     

Zeolite cover layer for selectivity enhancement of p-type semiconducting hydrocarbon sensors

Screen-printed thick films of the p-type semiconducting materials family SrTi_1−xFe_xO_3−δ have been investigated for hydrocarbon sensing. Among the different compositions tested, the formulations containing 10 and 20% of iron are found to perform best for this purpose. A pronounced cross-interference of NO persisted at operating temperatures of about 400 °C. In order to eliminate this problem, the application of a zeolite cover layer was studied. The properties of this cover layer were optimized with respect to layer thickness and Pt content. Using initial results of a catalytic study on the zeolite powder in addition to a simple diffusion–reaction model, the effect of the zeolite layer with respect to NO cross-interference could be explained satisfactorily.     

An integrated chemical sensor with multiple ion and gas sensors

An integrated chemical sensor with multiple ion and gas sensors, composed of four ISFETs (pH, Na⁺, K⁺ and Cl⁻) and two gas sensors (P_O2 and P_CO2) on a 4 mm × 4 mm chip, is realized using semiconductor processing. The ISFETs are based on an Si₃N₄-gate ISFET, and use polymeric membrane except for the pH ISFET. The P_O2 sensor is a miniaturized Clark-type sensor, consisting of a Pt cathode and Ag/AgCl anode patterned by the lift-off process. The P_CO2 sensor is a miniaturized Severinghaus-type sensor using a pH ISFET. All of the ISFETs show sensitivities over 50 mV/decade, and a linear range between 1 × 10⁻⁴ and 5 × 10⁻¹ mol/l. The sensitivities of the P_O2 and P_CO2 sensors are 0.35 nA/mmHg and 42 mV/decade, respectively, and their response times are 30 s and 1 min, respectively. The integrated chemical sensor with multiple ion and gas sensors could be used for clinical analysis.     

Rank Jumps in Codimension 2A-hypergeometric Systems

The holonomic rank of the A -hypergeometric systemH_A (β) is shown to depend on the parameter vector β when the underlying toric ideal I_Ais a non-Cohen–Macaulay codimension 2 toric ideal. The set of exceptional parameters is usually infinite.     

SnO2-based R134a gas sensor: Sensing materials preparation, gas response and sensing mechanism

Undoped SnO₂ and porous Al₂O₃ powders were obtained through a simple chemical precipitation process. SnO₂-based gas sensing materials and Al₂O₃ catalytic coating loaded with a noble metal were prepared by impregnation. The SnO₂ and Al₂O₃ powders were characterized by TEM, SEM, nitrogen adsorption-desorption experiment, FT-IR and in situ XRD. Gas responses of the SnO₂-based gas sensors were measured in a static state. The experimental results indicated that the response towards R134a of the SnO₂-based gas sensor can be significantly enhanced by loading noble metal and using catalytic coating. The sensor based on a double layer film SnO₂ (Au)/Al₂O₃ (Au) showed satisfactory results including large response, good selectivity, high long-term stability, fast response and recovery, revealing its potential application in the detection of refrigerants and the maintenance of air condition systems. Finally, a gas sensing mechanism for R134a is suggested and proved by bond energy data, FT-IR spectrum and in situ XRD.     

Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO₃ piezoelectric substrate for the simultaneous detection of CO₂, NO₂, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO₂ sensitive film (Teflon AF 2400), and a NO₂ sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO₂ sensitive film because it provides a high CO₂ solubility, with good permeability and selectivity. For the NO₂ sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S₁₁ in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO₂ and 51.5°/ppm for NO₂, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.     

pH sensor based on the crown heteropolyanion K28Li5H7P8W48O184·92H2O immobilized using a layer by layer assembly process

A new sensitive pH sensor based on immobilization of the crown heteropolyanion K₂₈Li₅H₇P₈W₄₈O₁₈₄·92H₂O (P₈W₄₈) on a electrode surface through a layer by layer assembly process is described. The immobilization is based on the electrostatic adsorption of the complex in layers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH). The deposited P₈W₄₈/LBL film was investigated by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. Compared to the electrochemical behavior of dissolved P₈W₄₈, a slight shift in the redox peak towards negative potentials is observed, which have been attributed to a slight decrease in the acidity of the interior of the P₈W₄₈/LBL film compared to the testing buffer solution. The relationship between the peak currents of the deposited P₈W₄₈/LBL film and the number of layers is shown to be linear, which demonstrates that equal amounts of P₈W₄₈ are adsorbed in each deposition layer. The P₈W₄₈/LBL modified electrode showed high sensitivities toward pH. Therefore, such electrodes were tested as pH sensors using the titration method. The resulting pH sensor has a detection range of pH 1–13, a sensitivity of 69 ± 2 mV/pH, high repeatability (<3 mV), fast response time (<7 s), low sensitivity toward change in ionic strength and nature of the supporting electrolyte, low internal resistance and a working life time of at least 3 months. Moreover, the sensor is easy to manufacture and can be easily miniaturized for measurements in micro- and nano-systems.     

Empirical Bayes estimation of the scale parameter in a Pareto distribution

Let f(xθ) = αθ^αx^−(α+1)I(x>θ) be the pdf of a Pareto distribution with known shape parameter α>0, and unknown scale parameter θ. Let {(X_i, θ_i)} be a sequence of independent random pairs, where X_i's are independent with pdf f(xα_i), and θ_i are iid according to an unknown distribution G in a class of distributions whose supports are included in an interval (0, m), where m is a positive finite number. Under some assumption on the class and squared error loss, at (n + 1)th stage we construct a sequence of empirical Bayes estimators of θ_n+1 based on the past n independent observations X₁,…, X_n and the present observation X_n+1. This empirical Bayes estimator is shown to be asymptotically optimal with rate of convergence O(n^−1/2). It is also exhibited that this convergence rate cannot be improved beyond n^−1/2 for the priors in class .     

Ultrahigh ethanol response of SnO2 nanorods at low working temperature arising from La2O3 loading

The influences of La₂O₃ loading on the ethanol sensing properties of SnO₂ nanorods were investigated. An obvious enhancement of response was obtained. The response of 5 wt% La₂O₃ loaded SnO₂ nanorods was up to 213 for 100 ppm ethanol at low working temperature of 200 °C, while that of pure SnO₂ nanorods is 45.1. The improvement in response might be attributed to the presence of basic sites, which facilitated the dehydrogenation process. While the working temperature was increased to 300 °C, the sensor response decreased to 16 for 100 ppm ethanol. Additionally, the La₂O₃ loaded SnO₂ nanorods sensors showed good selectivity to ethanol over methane and hydrogen. Our results demonstrated that the La₂O₃ loaded SnO₂ nanorods were promising in fabricating high performance ethanol sensors which could work at low temperature.     

All-optical hydrogen sensor based on a high alloy content palladium thin film

Optical reflectance measurements were performed to determine the hydrogen response characteristics of 20 nm thick Pd_0.6Au_0.4 films. The response time and signal change characteristics were determined as a function of hydrogen concentrations ranging from 0.05% to 4% in a balance of dry CO₂ free air. The detection limit was determined to be 0.05%, with a corresponding response time of 130 s, while at 4% hydrogen concentrations the response time was 5 s at ambient temperatures. A linear decrease of both the signal change and response time was measured within an operating temperature range between 25 °C and 100 °C for a 1% hydrogen in air gas mixture. The sensor response dependence of the Pd_0.6Au_0.4 film with a change in humidity was determined between ambient levels and 95% relative humidity (RH). While the signal change was independent of humidity the response time increased due to water adsorption on the Pd alloy sensing layer. A similar increase in response time was shown for 100 ppm of background CO mixed with 1% hydrogen in nitrogen at room temperature. At an elevated operating temperature of 80 °C, 100 ppm of CO did not affect the sensor response towards 1% hydrogen in a balance of nitrogen. Reliability tests have been performed over a 1-year time period and the sensing specifications have not drifted beyond 2% and 13% of the calibrated signal change and response time, respectively. A response time on the order of seconds and the proven stability of the high alloy content Pd thin film demonstrate the promising attributes of this material for use in an all-optical hydrogen sensor.     

Detection of three-axis angles by an optical sensor

This article describes an optical method for measurement of three-axis angles (roll angle Δθ_X, pitch angle Δθ_Y, yaw angle Δθ_Z). The laser autocollimation method is improved from two-axis angle measurement to three-axis angle measurement by employing a diffraction grating instead of a plane mirror as the target reflector. The three-axis angle components can be calculated by three methods of using different diffraction light spots reflected from the diffraction grating. Method 1 uses three beams, which are the 0th-order and the ±1st-order diffraction light spots. Both Method 2 and Method 3 use two light spots. The former uses the ±1st-order diffraction light spots and the latter combines the 0th-order with the +1st-order or the −1st-order diffraction light spots. A prototype three-axis angle sensor is also designed and fabricated to compare the characteristics of the three methods from the viewpoints of sensitivity, linearity and resolution of the sensor output.