Potentiometric Gas Sensors for Oxidic Gases

Solid electrolyte-based electrochemical devices combined with an auxiliary phase of oxyacid salt have, in this decade, emerged as new attractive sensors to detect oxidic gases of CO₂, NO, NO₂ and SO₂. Various combinations of solid electrolytes and auxiliary phases as well as various new single or multi-component auxiliary phases have been exploited to improve the gas sensing properties and stability of these devices. Some of the potentiometric sensors developed e.g., CO₂ sensors using NASICON and Li₂CO₃-CaCO₃, NO₂ sensors using NASICON and NaNO₂-Li₂CO₃ and SO₂ sensors using MgO-stabilized zirconia and Li₂SO₄-CaSO₄-SiO₂, exhibit excellent gas sensing performances in laboratory tests and appear to be promising for monitoring the respective gases in ambient environments and/or combustion exhausts. This paper aims at describing our exploratory works on and the state of the art of these potentiometric gas-sensing devices.     

Capacitive Type Gas Sensors

Current status of capacitive type gas sensor were reviewed in this paper. Although the number of publications on capacitive type sensors has been limited so far, capacitive type sensors have good prospects given that the capacitor structure is so simple enabling miniaturization and achieving high reliability and low cost. Among the reported capacitive type sensors, detection of gas based on a change in dielectric layer thickness is most promising. On this point of view, capacitive type CO₂ and NO sensors using depletion layer formed at p-n junction of oxide semiconductor were introduced in detail. In addition, commercial capacitive type sensors for monitoring CO₂ based on this principle were mentioned. CO₂ concentration in office can be successfully monitored by the developed capacitive type CO₂ sensor.     

Surface acoustic wave sensors to detect volatile gases by measuring output phase shift

This paper presents properties of saw acoustic wave (SAW) gas sensors to detect volatile gases such as acetone, methanol, and ethanol by measuring phase shift. A dual-delay-line saw sensors with a center frequency of 100 MHz were fabricated on 128^∘ Y-Z LiNbO₃ piezoelectric substrate. In order to improve sensitivity of SAW sensors, a thin titanium (Ti) film as mass sensitive layer was deposited using e-beam evaporation on the surface of the SAW sensors. In our investigation the response time and sensitivity of SAW sensors were measured. The response time and sensitivity of SAW sensor with thin Ti film were strongly improved because of changing electrical and mechanical properties in the mass sensitive layer. As a result, high sensitivity and fast response time could be achieved by deposition of thin Ti film as mass sensitive layer on the surface of SAW sensor. It can be applied for high performance electronic nose system by assembling an array of different sensors.     

Effects of different morphologies of ZnO films on hydrogen sensing properties

This paper describes the characteristics of chemiresistor hydrogen (H₂) sensors with different ZnO film structures in which ZnO dense films, nanoparticles (NPs), and nanorods (NRs) were prepared by RF magnetron sputtering, the sol–gel method, and the hydrothermal method, respectively. These were decorated with a Pt NP catalyst to investigate the performance of devices comprised of these structures. The effects of the ZnO morphology and operating temperature on the gas sensing behavior of the sensor are reported in detail. The various ZnO film morphologies, which contributed significantly to differences between sensors, play a very important role in enhancement of the supported Pt catalyst area and initial oxygen absorption on the ZnO surface. ZnO dense films prepared by sputtering showed the fastest response with a 13.5 % resistance variation at 1,000 ppm H₂ because gas adsorption occurred only on the film surface. The sensor with ZnO NRs showed a slower response, but the highest change in resistance of 65.5 % occurred at 1,000 ppm H₂ at room temperature. H₂ sensing performance of the chemiresistor sensors was improved due to the Pt catalyst, which was more efficient in dissociating H₂ gas molecules even at low temperature. The best chemiresistor sensor was fabricated using ZnO NRs and had a response time of approximately 10 s, a 27 s recovery time, and an 81.5 % change in resistance at 200 °C.     

Characteristics of thick-film CO2 sensors based on NASICON with Na2CO3-CaCO3 auxiliary phases

Potentiometric CO₂ sensors were fabricated using a NASICON (Na_1+x Zr₂Si_xP_3−x O₁₂) thick film and auxiliary layers. The powder of a precursor of NASICON with high purity was synthesized using the sol-gel method. Using the NASICON paste, an electrolyte was prepared on the alumina substrate through screen printing and then sintered at 1000^∘C for 4 h. In the present study, as new auxiliary phases, a series of Na₂CO₃-CaCO₃ system was deposited on the Pt sensing electrode. The electromotive force (EMF) values were found to be linearly dependent on the logarithm of the CO₂ concentration in the range of 1000–10000 ppm. The device to which Na₂CO₃-CaCO₃ (1:2) was attached showed good sensing properties at low temperatures.     

MEMS-based microheaters integrated gas sensors

Abstract

In the present work efforts have been made to develop microheater integrated gas sensors with low power consumption. The design and simulation of a single-cell microheater is carried out using ANSYS. Low power consumption (<35?mW) platinum micro-heater has been fabricated using bulk micromachining technique on silicon dioxide membrane (1.5?μm thin), which provided improved thermal isolation of the active area of 250?×?250?μm². The micro-heater has achieved a maximum temperature of ～950?°C at an applied dc voltage of 2.5 V. Fabricated mircro-heater has been integrated with SnO₂ based gas sensors for the efficient detection of H₂ and NO₂ gases. The developed sensors were found to yield the maximum sensing response of ～184 and ～2.1 with low power consumption of 29.18 and 34.53?mW towards the detection of 1?ppm of NO₂ gas and 500?ppm of H₂ gas, respectively.     

Thin Film Praseodymium-Cerium Oxide Langasite-Based Microbalance Gas Sensor

Langasite (La₃Ga₅SiO₁₄) has proven to be a viable piezoelectric material for use in high temperature bulk acoustic wave gas sensors. To detect changes in pO₂ under oxidizing conditions, we utilized a PLD deposited Pr_0.15Ce_0.85O_{2 –} film as the gas sensitive layer given its ready reduction under these conditions. The sensor was operated at 600C and showed strong sensitivity to changes in oxygen partial pressure, saturating at a frequency shift of –360 Hz below 1%O₂/Ar (pO₂ = 10³ Pa). The frequency shift was calculated to be too large to be solely accounted for by the corresponding change of mass in the PCO film. Stress induced by dilation of the PCO lattice upon reduction is viewed as being a likely source of sensor sensitivity.     

Fabrication and characterization of co-planar type MEMS structures on SiO2 /Si 3 N 4 membrane for gas sensors with dispensing method guided by micromachined wells

MEMS structures for micro gas sensors had advantage for lower power consumption, reducing size, and easily making cavity structures. Also, co-planar type MEMS structures (CPMS) for gas sensors with low power consumption heater and dispensed sensing materials were newly proposed and investigated. CPMS, which were formed with micro heater and sensing electrodes at the same layer, to reduce process steps, diffusions between upper layer and lower layer, and thermal differences between the center and the periphery of the sensing layer compared with stacked structure. Dispensing method guided by back-side etched well was good for forming sensing material on sensing electrode and had advantage that various sensing materials could be applied for array type sensors. CPMS were fabricated on four-inch diameter and double side polished (100) silicon wafers and using anisotropic bulk silicon micromachining for membrane formation and etched well. A size of chips with 1.15 mm × 1.15 mm membrane was 4.8 mm × 4.8 mm. And co-planar type sensing electrodes were located in the middle of low stress SiO₂/Si₃N₄ (400 nm /1 μm) membranes. Membranes are thermally isolated from the chip frame because they have low thermal conductivity, generally. Temperatures were measured using IR thermometer with linearly increasing applied power. Power consumption at 400^∘C was 150 mW. Membranes of CPMS were withstood up to 730^∘C at the power of 350 mW. Characteristics of micro heaters for various heater widths of 50 μm, 75 μm, 100 μm and ratios of membrane dimension to heater dimension were measured. Sensing materials guided by micromachined well were dispensed on sensing electrodes. CPMS were mounted on a TO-8 package. From these results, fabricated and characterized CPMS could be used for applications in portable gas sensors for detection of CO, NO_x, CH_x, H₂S, and so on.     

Low operating temperature CO sensor prepared using SnO<Subscript>2</Subscript> nanoparticles

A low operating temperature CO (carbon monoxide) sensor was fabricated from a nanometer-scale SnO₂ (tin oxide) powder. The SnO₂ nanoparticles in a size range 10–20 nm were synthesized as a function of surfactant (tri-n-octylamine, TOA) addition (0–1.5 mol%) via a simple thermal decomposition method. The resulting SnO₂ nanoparticles were first screen-printed onto an electrode patterned substrate to be a thick film. Subsequently, the composite film was heat-treated to be a device for sensing CO gas. The thermal decomposed powders were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD), and surface area measurements (BET). The CO-sensing performance of all the sensors was investigated. The experimental results showed that the TOA addition significantly decreased the particle size of the resulting SnO₂ nanoparticle. However, the structure of the powder coating was crucial to their sensing performance. After heat-treatment, the smaller particle tended to cause the formation of agglomeration, resulting in the decline of surface area and reducing the reaction site during sensing. However, the paths for the sensed gas entering between the agglomerated structure may influence the sensing performance. As a CO sensing material, the SnO₂ nanoparticle (~12 nm in diameter) prepared with 1.25 mol% TOA addition exhibited most stable electrical performance. The SnO₂ coating with TOA addition >0.75 mol% exhibited sensor response at a relatively low temperature of <50°C.     

CO2 Sensing Property of CuO-BaTiO3 Mixed Oxide Film Prepared by Self-Assembled Multibilayer Film as a Precursor

Preparation of CuO-BaTiO₃ mixed oxide thin film by the decomposition of a self-assembled multibilayer film as a molecular template was investigated in this study. Furthermore, CO₂ sensing property of the resultant thin film was investigated as a capacitive type sensor. The self-assembled bilayer film of few 1000 layers thickness can be obtained easily by casting an aqueous suspension consisting of dimethyldihexadecylammoiun bromide (DC1-16), Cu(ClO₄)₂, Ba(TiO(C₂H₄)₂), 2,6-dimetyle-3,5heptadione (DHP), and polyvinyl alcohol. Divalent copper ion (Cu²⁺)) which is associated with 2 DHP molecules was incorporated into the molecular bilayer film and BaTiO₃ precursor exists at the interspace of molecular bilayer film by coordinating with polyvinyl alcohol. Upquenching the organic-inorganic film at 1173 K leads to the uniform film of CuO-BaTiO₃ oxide mixture. Although operating temperature shifted to higher temperature, the resultant film exhibits the capacitance change upon exposure to CO₂. Consequently, it is concluded that the mixed oxide film of CuO-BaTiO₃ prepared by the decomposition of multibilayer film was also an appropriate capacitive type CO₂ sensor.     

CO2 Sensing Property of CuO-BaTiO3 Mixed Oxide Film Prepared by Self-Assembled Multibilayer Film as a Precursor

Preparation of CuO-BaTiO₃ mixed oxide thin film by the decomposition of a self-assembled multibilayer film as a molecular template was investigated in this study. Furthermore, CO₂ sensing property of the resultant thin film was investigated as a capacitive type sensor. The self-assembled bilayer film of few 1000 layers thickness can be obtained easily by casting an aqueous suspension consisting of dimethyldihexadecylammoiun bromide (DC1-16), Cu(ClO₄)₂, Ba(TiO(C₂H₄)₂), 2,6-dimetyle-3,5heptadione (DHP), and polyvinyl alcohol. Divalent copper ion (Cu²⁺)) which is associated with 2 DHP molecules was incorporated into the molecular bilayer film and BaTiO₃ precursor exists at the interspace of molecular bilayer film by coordinating with polyvinyl alcohol. Upquenching the organic-inorganic film at 1173 K leads to the uniform film of CuO-BaTiO₃ oxide mixture. Although operating temperature shifted to higher temperature, the resultant film exhibits the capacitance change upon exposure to CO₂. Consequently, it is concluded that the mixed oxide film of CuO-BaTiO₃ prepared by the decomposition of multibilayer film was also an appropriate capacitive type CO₂ sensor.     

WO3/BTO heterostructures based NO2 sensor with enhanced response characteristics

Abstract

In the present work, an efficient NO₂ gas sensor has been realised using single phase Barium titanate, BaTiO₃, (BTO) thin film, grown by chemical solution deposition technique (CSD). The gas sensing characteristics of BTO thin film were enhanced by integrating WO₃ modifier in the form of uniformly distributed circular nano-clusters and continuous overlayer. The WO₃ nanoclusters/BTO sensing element exhibited enhanced sensor response (～156) with fast response speed (16?s) at a relatively low operating temperature (140?°C) towards 50?ppm NO₂ gas. An attempt has been made to explain the sensing mechanism involving the twin effect of “Fermi-level exchange mechanism” and “spill over mechanism” upon interaction with target NO₂ gas. The obtained results in the present work are encouraging for the realization of hand-held NO₂ gas sensor.     

Application of TiO<Subscript>2</Subscript> thin film with nanorods to surface acoustic wave type ultraviolet photo detection

TiO₂ thin films with nanorods grown on 128° Y?cut LiNbO₃ and 90° rotated 42°45′ ST?cut quartz were used to fabricate surface acoustic wave ultraviolet photodetectors. TiO₂ thin film was deposited by radio?frequency magnetron sputtering and TiO₂ nanorods were then synthesized on the thin film via the hydrothermal method. 128° Y?cut LiNbO₃ is a Rayleigh wave substrate with a high electromechanical coupling coefficient, whereas 90° rotated 42°45′ ST?cut quartz is a surface skimming bulk wave substrate with a high wave velocity. The effects of substrate characteristics and TiO₂ nanorod morphology on the ultraviolet sensitivity of the surface acoustic wave photodetectors were investigated. The variations of insertion loss, phase, resistance, and capacitance under ultraviolet illumination were examined. The performance of the TiO₂ thin film with nanorods deposited on 128° Y?cut LiNbO₃ is much greater than that of the film deposited on 90° rotated 42°45′ ST?cut quartz, which can be attributed to the former’s high electromechanical coupling coefficient.     

Thick Film Planar CO<Subscript>2</Subscript> Sensors Based on Na β-Alumina Solid Electrolyte

Practical small-sized thick film CO₂ sensor with self-heater was fabricated with Na β -Alumina (NBA), Na₂Ti₆O₁₃-TiO₂, and Na₂CO₃ as a solid electrolyte, reference electrode, and a sensing electrode, respectively. The measured EMF from the sensor followed the Nernstian behavior with CO₂ concentration change in the range of 400 to 600^∘C (350–580 mW power consumption). However, in the aspect of stability, densification of the NBA thick film and prevention of Na₂CO₃ evaporation were needed. In this study, an Al₂O₃ porous layer deposited on Na₂CO₃ was effective in improving the durability during operation of the sensor. It is thought that Al₂O₃ suppresses evaporation of Na₂CO₃.     

Gas Sensors Based on Piezoelectric Micro-Diaphragm Transducer

ABSTRACT

We have fabricated high sensitive gas sensor based on piezoelectrically driven micro-diaphragm transducers. The micro-diaphragm transducer was fabricated using micro-electro-mechanical-system (MEMS) technique. The diol based sol-gel derived Pb(Zr_0.52,Ti_0.48)O₃(PZT) film was used as a piezoelectric actuating layer. We have used the resonant frequency change of micro-diaphragm transducer upon mass increase as a sensing signal. The resonant frequency values were measured by analysis of electrical signals from the micro-diaphragm transducer. The fundamental resonant frequency of the micro-diaphragm was in the range of 250 to 360 kHz, depending on their physical boundary conditions. The mass sensitivity of bare micro-diaphragm transducer was 66.5 Hz/ng. Two polymer sensing layers such as the polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) films were used to estimate the gas sensing behavior of microtransducers for various vapors of organic compounds. PMMA was used to detect primary alcohols while PDMS was used for toluene and benzene. The resonant frequency of micro-diaphragm transducer was shifted toward lower frequency range as the vapor concentration increased. With PMMA gas sensing layer, the micro-diaphragm showed a gas sensitivity of 0.456 Hz/ppm for ethanol vapor. When the PDMS gas sensing layer was used, the micro-diaphragm showed a gas sensitivity of 0.143 Hz/ppm for toluene vapor. When the test vapors were removed from the reaction chamber, the resonant frequencies of micro-diaphragm sensors were completely recovered to their initial state.     

A tetracene-based single-electron transistor as a chlorine sensor

An organic molecular single-electron transistor (SET) based on a tetracene quantum dot has been modeled and employed for sensing of chlorine gas, within the framework of density functional theory. The sensing behavior of the SET is estimated through a charge-stability diagram and total energy as a function of gate potential (TE vs. V_g) for varying distances of chlorine from the SET quantum dot, which could be used as an electronic fingerprint for detection. The better sensing ability, high power efficiency and large operational temperature range of tetracene SET, in comparison to conventional sensors, makes it a very powerful candidate for a chlorine gas sensor.     

NO2 adsorption behaviour on LaFeO3 electrodes of YSZ-based non-nernstian electrochemical sensors

X-ray photoelectron spectroscopy (XPS) was used to examine the NO₂ adsorption behaviour on the LaFeO₃ and Pt electrodes of planar yttria stabilized zirconia non-Nernstian gas sensors. The electrochemical sensors were exposed to the same gas atmosphere containing 1000 ppm NO₂ at 650°C. XPS of the as-prepared sensors and sensors after exposure to NO₂ revealed bonded nitrogen peaks on the surface of the semiconducting oxide but no nitrogen peaks on the Pt electrode. Therefore, NO₂ adsorption on a LaFeO₃ electrode plays an important role in the NO₂ detection mechanism.     

Developing novel gas sensors for NO2 detection based on Ce(1-x)MXO2, {M?=?Ru, In} solid solutions

CeO₂, Ce_(1-x)M_XO₂, {M = Ru, In} compounds with sensing properties were fabricated using the sol–gel route. The main purpose was to compare the efficiency of CeO₂ vs. Ce_(1-x)M_XO₂ doped compounds as gas sensors for NO₂ detection. Characterization was performed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area determination (BET). Measurements of electrical resistance under different conditions of time, concentration and temperature in the presence of NO₂ were carried out. Ruthenium inclusion increased the CeO₂ sensor response in a great extent, gas response (S) = 1.8 for CeO₂ vs. gas response (S) = 350 for Ce_0.95Ru_0.05O₂ and gas response (S) = 35 for Ce_0.95In_0.05O₂. This behavior is reported by the first time. Our results demonstrate that ruthenium or indium inclusion has been beneficial for CeO₂. Conclusively the materials herein described could be applied as NO₂ gas sensors.     

Electrochemical NOx sensors based on stabilized zirconia: comparison of sensing performances of mixed-potential-type and impedancemetric NOx sensors

A tubular sensor was fabricated by using yttira stabilized zirconia (YSZ) and ZnCr₂O₄ sensing-electrode (SE) aiming for detection of NOx at high temperature. The sensing characteristics of this YSZ-based sensor were evaluated at 700^∘C by means of potentiometric (mixed-potential) and impedancemetric methods with the variation of thickness of SE. A correlation between the thickness and the sensing performances was obtained for both types of NOx sensors. The mixed-potential-type sensor using ZnCr₂O₄-SE exhibited high NOx sensitivity when the SE thickness was small (4 μm). On the other hand, the impedancemetric sensor, employing the same oxide-SE, provided almost equal sensitivity to NO and NO₂ when the SE thickness was large (39 μm). In this case, the total concentration of NOx can be measured. The comparison of sensing mechanisms for the both sensors was briefly discussed.     

CO gas sensing properties in Pd-added ZnO sensors

Unadded and 0.5 mol% Pd-added ZnO bulk and thin films were prepared by sintering and sputtering, respectively, and their CO gas sensing properties were investigated. The effects of Pd addition, sensing temperature (100–500 °C), and humidity on the CO gas response were discussed. In the bulk sensors, Pd-addition lowered the temperature for the maximum CO gas response (sensitivity) from 400 to 300 °C, whereas the thin film sensors (unadded and Pd-added) exhibited maximum gas response at 200 °C. The Pd-addition enhanced the CO gas response in thin film sensors, and it was also effective for reducing the interference from humidity in both bulk and thin film sensors.