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.     

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.     

Modelling of chemical surface acoustic wave sensors and comparative analysis of new sensing materials

Comparative analysis of different, new gas sensing materials in surface acoustic wave chemical sensors is presented. Different gas sensing materials as polyaniline (PANI), Teflon AF 2400, polyisobutylene (PIB), polyepichlorohydrin (PECH) are considered. They are chosen according to the type of gas to be detected and the desired accuracy: Teflon AF 2400 thin film for the detection of CO₂, PANI nanocomposites film that belongs to the group of conductive polymers for the detection of CO, NO₂ and phosgene (COCl₂), and PECH and PIB for the detection of dichloromethane (CH₂Cl₂, DCM). In the analysis, the simple and useful method of the complete analyses of gas chemical sensors is used. The method is based on the electrical equivalent circuit of the surface acoustic wave sensor. The method is very efficient and can be used for the optimal design of CO₂ sensors. The results are compared with those presented in public literature and good agreement is obtained, demonstrating the validity of modelling. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

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.     

Gas sensors: New materials and processing approaches

To satisfy demands for detecting chemicals in the environment, versatile sensors are required to detect a rapidly growing range of chemical species. In this paper, focus is directed on progress being made to develop temperature-independent oxygen sensors based on the perovskite solid solution system SrTi_1−x Fe _x O_3−δ,and on improving the sensitivity of thin film gas sensors integrated on Si-based self heated substrates. A novel strategy to produce macroporous films with high surface area for enhanced chemical activity is described, and how this processing strategy results in markedly improved sensitivity of gas sensors based on a novel material, CaCu₃Ti₄O₁₂, is demonstrated.     

Gas discrimination method for detecting transformer faults by neural network

This paper describes a method available for early detection of abnormality in an oil-filled transformer. In this method, four gas sensors having different characteristics and neural network are used to identify gas species (H₂, CH₄, C₂H₄, C₂H₂ and mixture of two species). To improve the selectivity of gas sensors, the time response patterns induced by changing sensor temperature and the stationary sensor output are identified by neural network. Furthermore, the mixture ratio of gases is derived by using the stationary sensor output in response to the changing sensor temperature. Gas species are well discriminated, and the mixture ratio derived from the sensor output agrees well with the measurement by gas chromatography. Therefore, it is confirmed that our method is applicable to the transformer diagnostic technology.     

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.     

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.     

Influence of Water Vapor on the Response of Mg-Doped SrTio3 Ceramic Oxygen Sensors

Mg-doped SrTio₃ thick film sensors fabricated by screen-printing proved to be very promising for the use as oxygen sensors. A study of the influence of water on the response of these sensors gives an important basis for understanding their behavior in practical applications. The influence of water on the sensor response was measured in the oxygen partial pressure region from air (0.21 bar) to pure N₂ (2.5 × 10^–5)and the temperature range from 600 to 800°C. The relative humidity was varied from 1 to 95% RH. The resistance variation as a function of temperature and the activation energy were evaluated under different dry and wet conditions. The results obtained show that the resistance of these sensors generally decreases with increasing water content in the carrier gas and that the effect of water was strongest at lower temperatures as well as at lower oxygen pressures. To explain this behavior, it is proposed that a partial proton conduction is introduced in the water-containing atmospheres and that this contributes to the total conductivity leading to a reduction of the total resistance. Finally, the measurements also show that the response of these sensors still depends on the oxygen partial pressure according to the standard expression even in the presence of water vapor. Therefore, these sensors can still be used as oxygen sensors in humid atmospheres.     

Effects of Ag nanoparticles decorated on ZnO nanorods under visible light illumination on flexible acetylene gas sensing properties

In this work, piezoelectric and plasmonic effects on a flexible acetylene (C₂H₂) gas sensor based on silver (Ag) nanparticles (NPs)-coated ZnO nanorods (Ag-ZnO) were realized. Using visible light illumination, the sensing properties can be modulated and the power consumption can be reduced significantly. Upon exposure to 1000 ppm C₂H₂ under 8.36 mW cm^?2 light illumination, the power consumption of the sensor noticeably reduced from 3.48 W (in dark) to 1.64 W. A large number of light-induced chemisorbed oxygen ions were generated in the Ag-ZnO forest due to the strong coupling effect between the plasmonic Ag NPs and the ZnO NRs. This resulted in increased surface charge densities, which facilitated the sensor to react with the C₂H₂ molecules at lower operating temperature, hence reduced the power requirement. Moreover, the sensor exhibited reliable detection of C₂H₂ gas within the concentration of 3–1000 ppm including a maximum sensor response of 26.2, response-recovery time of 66/68 s, the excellent mechanical stability at a bending angle up to 90^o, and 10⁴ cycles of repeated deformation processes. These results might facilitate research in developing a low power C₂H₂ gas sensor and will open up new approaches for future light modulated gas sensors.     

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.     

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.     

Characterization of Ba0.55Sr0.45TiO3 films as light and temperature sensors and its implementation on automatic drying system model

The growth of Ba_0.55Sr_0.45TiO₃ films on p-type silicon substrate with depletion and enhancement treatments have been conducted in this research. The aims were to examine film sensitivity as light sensor and value range, resolution, acuracy level, and their hysteresis as temperature sensor. The films were annealed at 800, 850, and 900 °C for 15 hours. In this work, enhancement BST of 850 °C was the best quality film and utilized as light and temperature sensors. Its implementation has been successfully conducted on ATMega8535 microcontroller-based automatic drying system model by exploiting the working principle of the BST films as automatic switch.     

IR detectors using Pb(Mg1/3Nb2/3)O3 and Pb(Sc1/2Ta1/2)O3 ceramics under DC bias

Recently, one type of thermal-type IR detector, the pyroelectric sensor, has become of great interest in commercial applications, because of its ability to operate without cooling, its constant detectivity independent of wavelength, and its low cost. The conventional pyroelectric materials are usually normal ferroelectric materials with a first and second phase transition. The working temperatures are sufficiently below the Curie temperature T_c for stable responsivity to temperature. Electric field induced-type pyroelectric sensors have also been proposed. Relaxor ferroelectric materials such as Pb(Mg_1/3Nb_2/3)O₃ (PMN) and Pb(Sc_1/2Ta_1/2)O₃ (PST), which have a glassy Curie temperature near room temperature, are used in this type of sensor. This paper describes the sensor properties of electric field induced-type pyroelectric sensors prepared by using PMN and PST ceramics as compared with the conventional type sensors. Material evaluations of PMN and PST ceramics were made to determine their dielectric and pyroelectric properties. PMN shows excellent induced pyroelectric properties for the sensors over a wide range of temperatures. On the other hand, PST seems to be inadequate for an IR detector because of a very narrow high-response temperature range. The sensors with PMN and PST ceramics show enhanced pyroelectric activities under dc bias field. The measured sensor voltage responsivities agree with the calculated values for the PMN case. The electric field induced-type infrared sensor with thick or thin film materials seems to be satisfactory as linear array IR detectors for thermal imaging, with application of a higher electric field.     

Optical and sensor properties of ZnO nanostructure grown by thermal oxidation in dry or wet nitrogen

Zn films on glass were oxidized at 390°C in dry or wet N₂. As-prepared ZnO oxides were characterized by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), a transmission electron microscope (TEN) and photoluminescence (PL) measurements. Gold electrodes were deposited on ZnO films to obtain ZnO gas sensors, which were characterized by exposure to ethanol at room temperature. The results show that the specimen oxidized in dry N₂ exhibits a sharp PL peak at 380 nm whereas that oxidized in wet N₂ exhibits a broad peak at 490 nm. The sensors demonstrate significant sensitivity to ethanol vapor at room temperature with a detection limit of 200 ppm for the sensor with ZnO oxidized in wet N₂. Recovery for the sensor with ZnO oxidized in wet N₂ can be achieved by illumination with natural light. However, for the sensor with ZnO oxidized in dry N₂, ultraviolet (UV) radiation is needed for recovery.     

Selective CO Gas Detection of Zn2SnO4 Gas Sensor

The ability of semiconductor gas sensors to differentiate between gases is essential but difficult to obtain. In this study, Zn₂SnO₄ was made to be CO selective and the possible mechanism for the selectivity was studied.The electrical and the gas-sensing properties of uncoated and CuO-coated Zn₂SnO₄ were investigated. In order to obtain an ohmic contact to Zn₂SnO₄, a ZnO layer was stacked on top of Zn₂SnO₄ and co-fired. CuO was coated by immersing the sintered sample in Cu-containing solution. Both uncoated and CuO-coated samples showed the higher sensitivity to 200 ppm CO gas than to 200 ppm H₂ gas. However, the CuO-coated Zn₂SnO₄ showed much enhanced sensitivity and thus good selectivity for CO gas (S _CO/S _{H 2} 6) compared to the uncoated sample. The excellent selectivity of Zn₂SnO₄-based materials for CO gas was explained by the difference in the mechanisms of CO and H₂ oxidation.     

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.     

On the Relationship Between the Grain Size and Gas-Sensitivity of Chemo-Resistive Metal-Oxide Gas Sensors with Nanosized Grains

In this work we elaborate the effect of grain size on the sensitivity of chemo-resistive metal-oxide gas sensors with nanosized grains. The effective carrier concentration in nanocrystalline SnO₂ sensors with various grain sizes is calculated as a function of the surface state density. This involves numerical computation of the charge balance equation (i.e., the electroneutrality condition) using approximated analytical solutions of Poissons equation for small spherical crystallites. The calculations demonstrate a sharp decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with the gas phase, these calculations enable to simulate the response curves of nanocrystalline SnO₂ gas sensors. The simulations show that the conductivity increases linearly with decreasing trapped charge densities, and that the sensitivity to the gas-induced variations in the trapped charge density is proportional to 1/D, where D is the average grain size.     

Improvement of Dielectric and Interface Properties of CeO2 Buffer Layer by Using the Metal Seed Layer and N2 Plasma Treatment

In this paper, we investigated the feasibility of cerium oxide (CeO₂) films as buffers layer of MFIS (metal ferroelectric insulator semiconductor) type capacitors. CeO₂ layer were prepared by a two-step process of a low temperature film growth and subsequent RTA (rapid thermal annealing) treatment. By applying a cerium (Ce) metal seed layer of 4 nm, unwanted SiO₂ layer generation was successfully suppressed at the interface between the buffer layer and the Si substrate. After N₂ plasma treatment, the leakage current was reduced by about 2-orders. By employing a N₂ plasma treatment, we were able to successfully obtain good properties at the interface between the buffer layer and the Si substrate.