Sensor Operating Conditions Research Articles

Transport and gas sensing properties of In 2O 3 nanocrystalline thick films: A Hall effect based approach

Undoped nanosized In 2O 3 with n-type conduction was produced in both polymorphic forms (cubic and rhombohedral) and deposited by screen-printing as thick films. These films show high sensitivity to low O 3 concentration levels. They have been investigated by four point conductance and Hall effect measurements under sensor operating conditions (elevated temperature and ozone exposure). The effective values of the charge carrier concentration and mobility have been calculated from the experimental records using the recipe for the single crystals. The response to O 3 is discussed in the frame of the standard models for gas sensors. The observed deviations from the model are explained in connection with the film crystalline structure and microscopic parameters spread.

Impact of high- g and high vibration environments on piezoresistive pressure sensor performance

The impact of acceleration and vibration loads on a piezoresistive pressure sensor and its packaging components is investigated in some detail. For the sake of definiteness, sensor operation conditions in a helicopter rotor blade are considered. There, the sensor chip is loaded with static high- g accelerations up to 1000 g and vibrations up to 10 g. The sensor diaphragm, the chip adhesive and the bond interconnections all behave like elastic springs and therefore determine the functional behaviour as well as the reliability of the sensor under acceleration loads. Although an encapsulated pressure sensor chip design has been investigated, the results obtained are sufficiently generic to be useful also in other fields of pressure sensor operation in acceleration-loaded environments as for instance in aircraft or in automobiles.

On-the-Go Mapping of Soil Mechanical Resistance Using a Linear Depth Effect Model

An instrumented blade sensor was developed to map soil mechanical resistance as well as its change with depth. The sensor has become a part of the Integrated Soil Physical Properties Mapping System (ISPPMS), which also includes an optical reflectance and a capacitor-based sensor implemented to determine spatial variability in soil organic mater and water content, respectively. The instrumented blade of the ISPPMS was validated in laboratory conditions by applying known loads. It was also tested in the field by comparing sensor-based estimates with measurements produced using a standard vertical cone penetrometer and another on-the-go sensor, the Soil Strength Profile Sensor (SSPS), consisting of five prismatic-tip horizontal penetrometers located at fixed depths. The comparison resulted in reasonable linear relationships between corresponding parameters determined using the three different methods. The coefficient of determination (r2) for average soil mechanical resistance was 0.32 and 0.57 when ISPPMS-based estimates were compared with the standard cone penetrometer and the alternative on-the-go sensor (SSPS), respectively. Depth gradients of soil mechanical resistance obtained using cone penetrometer and ISPPMS methods were correlated with r2 = 0.33. Observed differences in estimated parameters were due in part to the difficulties with obtaining data representing the same depths and in part to differences in sensor geometry and operating conditions, particularly when comparing the on-the-go sensors to the cone penetrometer. Based on its operation during Missouri field mapping, the instrumented blade proved to be a rugged and inexpensive sensor suitable for studying the spatial variability of the physical state of soils in the upper 30 cm of the profile.

Gold Film Amperometric Sensors for NO and NO 2

A porous gold membrane working electrode was evaluated for amperometric sensing of NO or in air. The Au electrode was evaporated by E-beam onto a porous Teflon substrate and tested in two different geometries: a lab-built universal (U) and a housing (C) used in building sensors for commercial sale. The optimum bias for reduction was determined to be −150 mV vs. Pt/air quasi-reference electrode (Pt/air-QRE) and was +400 mV for NO oxidation. Sensor signals were independent of gas flow rate in both housings at flow rates over 30 cm3/min. NO sensitivity in the U and C housing was 180 and 43 nA/ppm, respectively, and the sensitivity was −100 and −38 nA/ppm, respectively. The U sensor had a limit of detection (LOD) for of 1.3 ppb (signal-to-noise ratio 3σ). The sensor housing design and operating conditions can be directly related to observed analytical performance characteristics. © 2003 The Electrochemical Society. All rights reserved.

Backscatter Fe-57 Mössbauer studies of iron(II) phthalocyanine

Phthalocyanines are established gas sensitive materials. We have successfully recorded backscatter conversion electron Mössbauer spectra (CEMS) of iron(II) phthalocyanine. Further, we have shown that the introduction of nitrogen dioxide, a typical analyte for these materials, into the detector system can be achieved without deleterious effects. The method allows surface sensitive spectra of these materials to be recorded under normal sensor operating conditions of temperature and pressure. The high vacuum conditions of other surface sensitive techniques used to study materials of this type are not required by this method.

Gas mixture analysis using silicon micro-reactor systems

A novel way of operating metal-oxide gas sensors is demonstrated that extends the gas-analyzing facilities of conventional thin-film gas sensors. In our approach, thin-film metal-oxide gas sensors on micro-machined heater substrates are embedded into tiny silicon micro-chambers to form micro-reactor devices. Analyzing samples of polluted air, such micro-reactors can be operated either in constant-flow or no-flow modes. Whereas in the first mode, essentially normal sensor behavior is observed, gas depletion reactions are observed in the latter. Such depletion reactions are shown to provide, in a straightforward way, analytical information about gas mixtures which is difficult to obtain under normal sensor operating conditions. As an application example, we demonstrate how a micro-reactor device can be used to analyze samples of polluted air for their O/sub 3/ and NO/sub 2/ contents.

Trends in the development of solid state amperometric and potentiometric high temperature sensors

High temperature sensors based on solid electrolytes are well established in many applications. In this context, stabilized zirconia as an oxygen ion conductor plays the key role to monitor oxygen. Recent trends in the use of stabilized zirconia aim at the detection of other exhaust or environmental gases like NO x , CO or hydrocarbons. This is possible by, e.g. monitoring currents of specific electrode reactions at a given voltage (‘amperometric devices’) or monitoring voltages between different electrodes which result from specific electrode reactions including the kinetically determined formation of surface oxygen during different catalytic reactions [‘mixed-potential (non-Nernstian) devices’]. In all applications, the catalytic and electrochemical properties of the electrode materials play a key role for achieving reproducible sensor signals. This requires controlled microstructures down to the atomic scale. Therefore, extensive work now focuses on the structural as well as functional optimization of known, and on the development of new electrode materials. In this promising trend particular emphasis is put on the in-situ spectroscopic and microscopic characterization of the various interfaces under sensor operation conditions.

Properties of sputter and Sol-Gel deposited PZT thin films for sensor and actuator applications: Preparation, stress and space charge distribution, self poling

In this paper, properties of sputter– and sol–gel deposited PZT thin films are reviewed and compared. It is shown that film properties are influenced by type and parameter of fabrication technology. By means of determining both stress and space charge distribution in the PZT films, it can be seen that an oxygen vacancy leads to a strong self–polarization which is stable under sensor operation conditions and makes these films very advantageous for pyroelectric IR sensors.

Dependence of Image Transient Behavior on Operating Parameters of Amorphous Silicon Image Sensors

AbstractThe growing interests for real-time applications of our amorphous silicon X-ray image sensors requires a better understanding of transient behavior in imaging characteristics. In our pixel structure, which consists of a p-i-n photodiode and an addressing thin film transistor (TFT), the image lag can stem from a number of mechanisms, including charge retention on the photodiode, TFT charge transfer efficiency, and data-line charge sharing. The exact prevailing mechanism depends on the array design and the operating conditions of the sensors. This paper describes the results of our systematic studies on image lag, and its dependence on various operating conditions of the image sensors, such as applied bias, light intensity, frame time, and applied gate voltages. We find that the charge retention in the intrinsic region of the photodiode is an important source of the image lag, and it is correlated to the forward transient characteristics of the diode. This correlation can be explained by the common underlying mechanisms leading to depletion region formation and charge transport through the diode. Furthermore, we notice that the time dependent decay of the image lag, originated mainly from charge retention in the diode, presents power-law time dependence, which is consistent with the release of trapped charges from a continuous distribution of defect states. We also find only a small variation in the image lag, <%3, for a wide range of gate on-periods, 10μs-80μs, and gate on-voltages, 15V- 30V, which indicates an efficient charge transfer by the switching TFT. These results demonstrate that in our devices, the image lag is controlled by the intrinsic defect levels in the a-Si:H photodiode.

Fe-57 Mössbauer study of iron phthalocyanine†

Abstract Phthalocyanines have been noted for their use as chemical sensors. Iron phthalocyanine (FePc) has been used as a model to elucidate the gas sensing mechanism using transmission Mossbauer spectroscopy to characterize bulk properties and conversion electron Mossbauer spectroscopy to study surface interactions. This technique allows an FePc sample to be monitored before, during and after exposure to various gases at ambient temperatures and pressures, hence mimicking gas sensor operating conditions.

Effect of the sensor structure on the stability of Ga 2O 3 sensors for reducing gases

Gas sensors based on semiconducting polycrystalline Ga 2O 3 thin films can be used either for sensing oxygen ( T⩾900 °C) or reducing gases ( T<900 °C), depending on the operating temperature. Whereas the stability of these sensors under purely oxidizing conditions has been proven, up to now there have been no systematic investigations on their material stability in the presence of reducing gases. The following study deals with sensor films on Al 2O 3 and BeO substrates; the sensors are equipped with interdigital electrodes made of sputtered platinum or gold thin films. Tests were carried out at the operating conditions of the Ga 2O 3 hydrogen sensor (600 °C, 1% H 2 in synthetic air), with the gas atmosphere remaining the same but at elevated temperatures, and also under purely reducing conditions (5% H 2 in N 2). Ga 2O 3 sensor films on a BeO substrate are stable under the operating conditions of hydrogen sensors, namely a temperature of 600 °C (H 2 in air), even under sustained exposure to hydrogen. No degradations have been found due to changes in the Ga 2O 3 film even at operating temperatures up to 900 °C. Generally speaking, films on Al 2O 3 substrate have poorer stability characteristics. In using gold electrodes, there is a limit with respect to thermal stressability of gold contacts at about 800 °C. When used in a purely reducing atmosphere, these Ga 2O 3 thin film sensors are stable so long as the oxygen-defect equilibrium of the Ga 2O 3 crystal lattice is frozen ( T<700 °C). Upon starting bulk-defect diffusion, oxygen depletion occurs in the lattice, which in the case of sensors on a BeO substrate leads to an increase in conductivity that is reversible by subsequent oxidation, while in the case of sensors on Al 2O 3 it leads to the destruction of the film. At the usual operating temperatures therefore, H 2 sensors can be operated in both oxidizing and reducing atmospheres.

Improved methodology for testing and characterization of piezodelectric gas sensors

An apparatus used for generating test vapours, exposing coated piezoelectric crystals and recording the responses in real time is described. In designing the apparatus, special care was taken to prevent any changes in crystal resonant frequency that could be caused by factors other than the presence of chemical stimulus. Two three-way solenoid valves were used to enable the sensor to be exposed alternatively to clean carrier gas and carrier gas containing sample vapour, while keeping the sensor operating conditions as constant as possible. The experimental data were collected automatically and, simultaneously, stored in a computer memory and sent to a recorder. The described methodology provides sensor response curves that are convenient for display and analysing sensor behaviour. Organophosphorus compounds were used for testing.

A microprocessor-controlled nitrogen dioxide sensing system

The electrical conductivity of semiconducting phthalocyanine films shows excellent sensitivity to electron-acceptor gases and in principle provides the basis for a low-cost, low power-requirement, compact sensing system for nitrogen dioxide. However, such films suffer a number of disadvantages, including slow response and reversal, reduced response in humid conditions and, in some cases, chemical reactivity following exposure to high NO 2 concentrations or for long periods. We now report the development of a microprocessor-controlled sensing system which has been shown to minimize the influence of these disadvantages. The instrument operates on the principle of controlling the sampling and sensor operating conditions so as to maintain reproducible kinetics, optimum conditions for minimizing humidity effects and minimal risk of reaction with NO 2. The results of experiments designed to determine these optimum operating conditions will be summarized and the construction and evaluation of a prototype instrument incorporating these features will be described.

Atomistic understanding of sensor mechanisms: Spectroscopic studies at prototype electronic and ionic conductors

A brief introduction is given for an unequivocal characterization of geometric, electronic and chemical structures of the different interfaces to be optimized for chemical sensors. These involve the chemically sensitive layers, substrates, contracts, surfaces and passivating overlayers. Systematic concepts are described to characterize these interfaces phenomenologically and spectroscopically with particular emphasis on in situ measurements of atomistic structures under sensor operation conditions. An atomistic understanding about elementary steps of gas/solid interactions is derived which involve one, two and more molecules interacting simultaneously with the sensor. “Prototype” sensor materials discussed here are inorganic electron or mixed conductors (such as ZnO, TiO 2, and SnO 2), inorganic ion conductors (such as ZrO 2 and LaF 3) and organic electron or mixed conductors (such as Pb phthalocyanine). The results make possible to build reliable microstructured solid state devices.

Analogy between electrical and structural properties of electron-beam-deposited SnO 2

The electrical properties of electron-beam-evaporated SnO 2 films were studied under various evaporation and annealing conditions. These electrical properties were explained by correlating them with the structural properties of films prepared under the same conditions. The long-term stability of these films was then studied under sensor operating conditions to determine the deposition conditions for gas sensor applications. From correlation of electrical and structural properties and long-term stability experiments, it was established that, because of their superior electrical and structural properties, the films deposited at 25 and 350 °C were more suitable for gas sensor applications than the films deposited at intermediate substrate temperatures.

Investigation of mercury adsorption on gold films by STM

SUMMARYScanning tunnelling microscopy, combined with complementary electrical and analytical measurements, provides a powerful method for examining the behaviour of mercury on gold‐film sensors under actual sensor operating conditions. The films exhibit a linear increase in resistance upon adsorption of mercury, and this resistive change is accompanied by a decrease in measured barrier height at submonolayer coverages of mercury. STM studies of changes in the effective barrier height upon mercury adsorption provide evidence that mercury migrates to grain boundaries for films exposed at submonolayer coverages. Coverages in excess of a monolayer result in a nearly constant, reduced effective barrier height across the entire surface, which is indicative of a more uniform distribution of mercury over the surface. This behaviour is interpreted in terms of the adsorption of mercury onto defective, contaminated gold surfaces.

Transient engine performance with water ingestion

The immediate effects on the transient performance of a generic, high bypass ratio jet engine on account of water ingestion are discussed. The air compression subsystem has been analyzed with respect to four aerothermodynamic and mechanical processes associated with two-phase fluid flow and the engine simulation has been carried out under three limiting cases of interest in practice, one pertaining to draining of water at the end of compression and the other two, to partial evaporation at two different locations in the burner. General observations are made on engine operability as a function of engine and control design under various engine and (control input) sensor operating conditions, with various mass fractions of water in the air-water mixture entering the engine, during various pilot-initiated power demand changes.