Tin Oxide Thick Film Research Articles

Discrimination of individual gas/odor using responses of integrated thick film tin oxide sensor array and fuzzy-neuro concept

A new neural network classifier for an Intelligent Gas Sensor (IGS) application is presented. The classifier is trained on fuzzyfied training set. Its superior classification and learning performance is demonstrated for discrimination of alcohols and alcoholic beverages using published data of thick film tin oxide sensor array fabricated and characterized at our laboratory. The new model proposed in this article not only gives a steep and monotone learning curve, but also exhibits lower sensitivity to learning parameter choices.

Response of oxygen plasma-treated thick film tin oxide sensor array for LPG, CCl 4, CO and C 3H 7OH

In the present paper, studies have been made to analyze the effect of oxygen plasma and dopants on the room temperature sensitivity of tin oxide, thick film gas sensors for various gases. To achieve this, an array of sensors was fabricated by thick film technology using tin oxide paste doped with Pd, Pt, CuO, ZnO, Cd and SnO 2. Subsequently, the fabricated array was annealed in oxygen plasma for 15 min. The response of the array has been studied for different concentrations of CO, LPG, CCl 4 and C 3H 7OH. The response–recovery time analysis of the fabricated array has also been carried out. These studies reveal that the sensitivity of oxygen plasma-treated array is much higher at room temperature compared with the untreated array. Moreover, addition of different dopants enhance the sensitivity and selectivity of the sensor to various gases/odours. The Pt-doped sensor has been found to be more sensitive to CO and LPG, whereas the Pd-doped sensor shows a good response towards CCl 4 and C 3H 7OH, and the Cd-doped sensor was found to show a fairly good response to all four gases.

Development of high sensitivity tin oxide based sensors for gas/odour detection at room temperature

An effort has been made to develop thick film tin oxide gas sensors which could detect various gases/odours at room temperature. To achieve this, the fabricated sensors were annealed in oxygen plasma for various durations. It was then found that, the room temperature sensitivity of such sensors was increased to about ten times as compared to the sensitivity of the non-annealed sensors. Further, plasma annealed sensors are found to be practically independent of temperature and the room temperature sensitivity of these sensors are found to be about 1.5 times the sensitivity of the conventional sensors at its operating temperature of 300°C. Studies on the variation of d.c. resistance, sensitivity, temporal response, current–temperature characteristics and impedance spectroscopy with the annealing time have also been made. These studies reveal that, with the increase in annealing time, there is a permanent gradual reduction in the d.c. resistance of annealed sensors. Further, it is also observed that with the increase in annealing time, the response time improves, barrier height reduces, barrier capacitance increases and the dependence of the sensitivity with temperature reduces while the sensitivity itself improves many-fold.

Transient response of thick-film tin oxide gas-sensors to multicomponent gas mixtures

Abstract In this study we analyse the conductance transient of thick-film tin oxide sensors under a step change in a multicomponent gas mixture concentration. We discuss a new nonlinear diffusion model which accurately depicts experimental sensor behaviour in the low-medium gas concentration range. This model allows a set of n concentration-independent parameters ( τ ′ 1 , τ ′ 2 , ..., τ ′ n ) to be estimated. Each parameter is characteristic of a species present in the n -component mixture and varies from one sensor to another. By adjusting conductance transient measurements of binary mixtures (ethanol, toluene and ethylbenzene) to theoretical transients, we obtained τ ′ i values. This new set of constants may give useful information for identifying the components of gas mixtures.

Thick film tin oxide sensors for detecting carbon monoxide at room temperature

Further studies are reported on a novel room temperature tin oxide–platinum sensor for carbon monoxide (CO). The expected porous nature has been confirmed by scanning electron microscope pictures of these sensors. Pore sizes were 0.1–10 μm. Recovery studies confirm that high resistance sensors recover more quickly. During the recovery a second fire can be detected and there are no signs of self-poisoning by CO. Preliminary results are presented for temperature and humidity cycling.

Conductance-transient analysis of thick-film tin oxide gas sensors under successive gas-injection steps

Dynamically characterizing the electrical response of chemical sensors is a strategy to improve their selectivity. In this paper, the conductance transient of thick-film tin oxide gas sensors under successive step rises in the gas concentration is analysed. The transient response of three thick-film tin oxide gas sensors in the presence of organic solvent vapours (toluene, o-xylene and ethanol) has been measured. As a main result, it has been found that, although the first step in the conductance-transient rise time is concentration independent, for the second and successive steps, decreases and becomes concentration dependent. This behaviour has been modelled successfully with a nonlinear diffusion - reaction model. The measured set of rise times is characteristic for each sensor - gas pair and may give useful information for gas recognition. Furthermore, the successive injection of gas samples during the sensor-calibration procedure may be a time-saving technique.

Qualitative and quantitative analysis of volatile organic compounds using transient and steady-state responses of a thick-film tin oxide gas sensor array

Quantitative analysis of gases, by means of semiconductor sensor arrays and pattern-recognition techniques such as artificial neural networks, has been the goal of a great deal of work over the last few years. However, the lack of selectivity, repeatability and drifts of the sensors, have limited the applications of these systems to qualitative or semi-quantitative gas analysis. While the steady-state response of the sensors is usually the signal to be processed in such analysis systems, our method consists of processing both, transient and steady-state information. The sensor transient behaviour is characterised through the measure of its conductance rise time ( Tr), when there is a step change in the gas concentration. Tr is characteristic of each gas/sensor pair, concentration-independent and shows higher repeatability than the steady state measurements. An array of four thick-film tin oxide gas sensors and pattern-recognition techniques are used to discriminate and quantify among ethanol, toluene and o-xylene [concentration range: 25, 50 and 100 ppm]. A principal component analysis is carried out to show qualitatively that selectivity improves when the sensor behaviour is dynamically characterised. The steady-state and transient conductance of the array components are processed with artificial neural networks. In a first stage, a feed-forward back-propagation-trained ANN discriminates among the studied compounds. Afterwards, three separate ANN (one for each vapour) are used to quantify the previously identified compound. Processing data from the dynamic characterisation of the sensor array, considerably improves its identification performance, rising the discrimination success rate from a 66% when only steady-state signals are used up to 100%.

Development and Characterisation of Palladium Doped Tin Oxide Thick Film Paste for Gas Sensors

This paper describes the development of palladium doped tin oxide thick film paste using indigenously available chemicals. Tin oxide powder is prepared by white precipitated method. Subsequently antimony oxide (10%) by weight is mixed with tin oxide powder in a ball mill for one hour, followed by the doping of palladium (1%) by weight to the mixture. After that the mixture is calcined at 800°C for an hour. To get the suitable paste, the proper composition of glass frit, tin oxide powder and fluidising agent are optimized.The developed paste has been used for the fabrication of gas sensor. The response of the sensor for propanol, methanol and acetone has been studied. The electrical properties of the film is found to be dependent on the relative proportion of tin oxide and palladium chloride. Also the sensing mechanism of tin oxide sensor has been studied.

Analysis of the conductance transient in thick-film tin oxide gas sensors

In this paper, we analyse the conductance transient of a Taguchi TGS-822 sensor under a step change in the vapour concentration. A diffusion-limited range in the conductance transient is observed. Adjustments between the theoretical calculations based on a non-linear diffusion-reaction model and the experimental results alloy a constant, τ′, to be estimated, which is independent of the final conductance value and depends, among other parameters, on the effective diffusion coefficients of vapours in the porous tin oxide sensor. From transient measurements of organic solvents (benzene and o-xylene) we have obtained τ′ values that are independent of concentration and characteristic for each vapour. This new parameter can give useful information for gas/vapour recognition.

Factors Determining the Carbon Monoxide Sensing Properties of Tin Oxide Thick Films Calcined at Different Temperatures

Effects of calcinations temperature on the gas‐sensing properties of SnO2 thick films were explored by measuring the variations in resistance with CO concentration. As the calcining temperature increased from 500° to 1100°C, the absolute values of the slopes in the log (concentration)vs log (resistance) curves gradually decreased from −0.9 to −0.2. The data were analyzed with the equations proposed by Madou and Morrison. It was suggested that the reduction in the formation rate of oxygen adsorbates was responsible for the decrease in the absolute slope, and hence a relatively high sensitivity was attained in a low gas concentration with a largely crystallizes sample.

Modelling and simulation of abnormal behaviour of thick-film tin oxide gas sensors in CO

A new model is proposed that takes into account oxygen vacancies in tin oxide resistive-type, assuming that lattice oxygen modifies the rate of oxygen adsorption. Applying this hypothesis in a Monte Carlo simulation, effects observed in thick-film samples are explained. Morevoer, computational techniques have been used in order to simulate different thick-film structures, and the role of both surface coverage and reduction in the sensing mechanism is investigated. The simulation results are in good qualitative agreement with the experimental results obtained from our samples. In particular, phenomena like undershoot and overshoot of the sample's resistance, very long recovery times and poisoning of the sensor surface are discussed.

A non-linear diffusion-reaction model of electrical conduction in semiconductor gas sensors

A non-linear diffusion-reaction model of electrical conduction in homogeneous semiconductor gas sensors has been developed. It is shown that the transient response time of a device may be classified according to the relative importance of diffusion and reaction rates. Experimental results on tin oxide thick films show that the response time is predominantly diffusion limited, suggesting that an improvement in performance may be achieved through a modification in physical properties, such as film porosity, instead of the underlying chemical kinetics.

Tin oxide gas sensor and countermeasure system against accidental gas leaks

Gas sensors are used in both domestic and commercial environments. In Japanese apartment buildings, installation of gas sensors is required by law, even though the sensor elements still need improvement. One problem was that sensors could not preferentially detect a specific gas. We have, however, developed three types of tin oxide gas sensors, namely, sintered, tin film and thick film types. The combined use of these three sensors and a microcomputer makes up a gas sensing and countermeasure system. The system preferentially detects leaking gases in ambient atmosphere and stops the gas leak by means of an electromagnetic valve. The kind of gas that is leaking is displayed on a CRT.

Sensing characteristics of tin oxide thick film gas sensor

Thick film gas sensor of tin oxide based materials has been developed by screen printing method, and its sensing characteristics have been examined for some gases. To perform the method, tin oxide fine power with the size about 0.5 μm in diameter was prepared through the fine crystallization in liquid phase, and was mixed with glass frits as a binder and palladium or platinum as a catalyst. This mixed material is used as the paste. With the screen printing method using the paste, the mass production and the sufficient mechanical strength of the sensor can be achieved. The gas sensor of tin oxide incorporated with palladium black as the catalyst shows an excellent selective detection for ethanol and hydrogen gases at the specimen temperature of 250 °C. And the platinum black-doped tin oxide sensor has also a high sensitivity to both gases at the temperature over 350 °C.

Studies on Photoelectrochemical Properties of SnO2 Films Prepared from Organic Resinate Solution

Tin oxide thick films have been prepared on glass surfaces from organometallic tin resinate solutions, and their photoelectrochemical properties have been studied. The antimony‐doped tin oxide film showed similar photoelectrochemical properties to those of the single crystal electrode. No anodic dark current was observed for the electrolysis of water up to the bias potential of 2.0V vs. SCE and saturation currents were observed upon illumination. These and other results indicate that films prepared from the resinate show purer semiconducting properties with less degenerate energy levels than those prepared from aqueous . Possible reasons are discussed from experimental observations.

Physical properties of antimony-doped tin oxide thick films

The physical properties of Sb-doped SnO2 thick films, prepared by a repeating chemical spray deposition method, have been investigated. The films 1000–14 000-Å thick were deposited on fused quartz, borosilicate glass, and soda lime glass substrates at 600 °C using an aqueous solution of a mixture of SnCl4 and SbCl3. The films prepared by the method are homogeneous, and the electrical resistivity of the films on fused quartz and borosilicate glass substrates were found to be independent of the film thickness, and are 9.5×10−4 Ω cm, and 8.6×10−4 Ω cm, respectively. The resistivity of the films thicker than 4000 Å on soda lime glass substrates is almost constant, and is 1.8×10−3 Ω cm, although a large increase in the resistivity of the thinner films was observed. The optical band gap of the films on fused quartz and borosilicate glass substrates is also independent of the film thickness, and is almost the same: 3.75 eV. But the band gap of the films on soda lime glass substrates depends on the film thickness, and increases from 2.85 to 3.08 eV with increasing thickness from 2250 to 13 000 Å. The Hall mobility and carrier concentration of the films were also measured. The results of x-ray diffraction analysis and observations by SEM are described.