Benedykt W. Licznerski

Simultaneous quantification of carbon monoxide and methane in humid air using a sensor array and an artificial neural network

Abstract The simultaneous quantification of carbon monoxide and methane in humid air is presented. The response of a three-sensor array, including an undoped and a platinum doped tin dioxide sensor showing non-ideal selectivity and a humidity sensor is fed into an optimised feed forward back propagation artificial neural network in order to obtain the carbon monoxide and methane concentration as network output. The gaseous environments under study were ternary mixtures in the concentration ranges of 0–0.5% methane, 0–1000 ppm carbon monoxide and 0–60% relative humidity at 20°C. The network structure, network output with respect to a priori known test concentrations and the influence of the size of the training data set is discussed.

New design of an SnO2 gas sensor on low temperature cofiring ceramics

Abstract A new design for thick film gas sensors was investigated. The sensors were made by the low temperature cofiring ceramics (LTCC) technique with a platinum heater buried inside the multilayer structure. SnO2 or SnO2 with Pd as catalyst thick films were used as gas sensitive materials. The properties of the gas sensors were measured with methane and carbon monoxide. The results of our study show that LTCC can be successfully applied in sensor technology as well as confirming the correctness of the sensor design.

Thick-film humidity sensors

The results of an investigation of thick-film humidity sensors based on ceramic materials are discussed in the paper. Thick-film technology is very promising for the production of low cost sensors based on ceramic materials. The technology gives the possibility for reproducible production of sensors with a defined microstructure, determined porosity and proper structure of grains and grain boundaries. Ceramic sensors have advantages over polymer sensors due to their better thermal stability and resistance to chemicals. Properties of thick-film planar humidity sensors based on - (ZCT) ceramics are presented. The influence of and Si additives, as well as firing temperature, on the sensor characteristics are discussed. Impedance spectroscopy measurements determined the correlation between the technological parameters and the electrical properties of humidity sensors. The role of each part of the sensitive material in the electrical conduction process is determined on the basis of measurements and calculated equivalent circuits. The proposed model describes the frequency characteristics at various relative humidities with very good fit to the experimental data. A new approach to the modelling of the impedance frequency dependence by means of an equivalent circuit yields very promising results for sensors.

APPLICATION OF SENSOR ARRAY AND NEURAL NETWORKS FOR QUANTIFICATION OF ORGANIC SOLVENT VAPOURS IN AIR

Abstract Organic solvents represent significant class of air pollutants. Mixtures of butanol and aromatic compounds are classified among the most commonly used there. The concept of two sensor systems capable of measurements and analysis of butanol/xylene and butanol/toluene mixtures is presented. Sensor array consists of four commercial, tin oxide based, semiconductor type gas sensors (TGS 800 series). For the sensors characterisation manually operated gas installation is constructed. Feedforward neural networks are developed for appropriate analysis of sensors responses. Both systems provide measurements of several single compounds concentrations with inaccuracy not exceeding 15% of range. Humidity influence on system responses is rejected. Possibility of portable realisation make the systems potential replacement of traditional gas analysing devices, especially in off-laboratory applications.

Thick-film resistive temperature sensors

This paper discusses thick-film resistive temperature sensors investigated at the Technical University of Wroclaw. The technology and electrical properties of resistance temperature detectors, thermistors, low-temperature thermometers and heating elements are presented. The R(T) curve of chosen air-fireable Ni - P based films agrees with Ni wire. The initially aged thermistors from the system can operate in the range from room temperature up to 673 K. The commercial thick-film resistors modified by the negative TCR drivers (e.g. powder) are fully suitable for low-temperature measurements in the range from 20 to 100 K. The integrated gas sensors need heaters because temperature influences their sensitivity, selectivity and response time. The thick-film compatible system based on commercially available resistive and conductive inks allows continuous long-term electrical heating of the sensor up to 673 K. The admissible operating temperature is much higher for the heaters made from conductive inks; for example fritless platinum heaters are satisfactory up to 1073 K.

The influence of Rh surface doping on anomalous properties of thick-film SnO2 gas sensors☆

Abstract Here there are presented new results from the investigations of SnO2 thick-film sensor. The effects of rhodium surface doping on dc and ac properties of SnO2 gas sensors were studied. The measurements reveal anomalous behaviour of the sensors. Below characteristic temperature, the resistance of the sensors increases in presence of reducting gases. The mechanism associated with the presence of rhodium can be connected with the increase of acceptor surface states concentration and causes the inversion of near-surface layers.

Anomalous behaviour of new thick film gas sensitive composition

Abstract Anomalous properties of gas sensitive film based on SnO2 are presented. The gas sensitive film was prepared by adding CeO2 and rhodium. The gas sensor was made on alumina substrate by the screen printing technique. The resistance characteristics of these gas sensors were investigated. It was found that resistance of the sensors increased in the presence of reducing gases. This behaviour was in contrast to typical properties of SnO2 sensors. Our sensors, based on the new composition, exhibited very short response time in CO and CH4.

Application of sensor dynamic response analysis to improve the accuracy of odour-measuring systems

A system consisting of a matrix of three semiconductor gas sensors was applied to the classification of different orange juices. The sensor matrix responses were sampled in short time intervals. Such responses were processed by discrete wavelet transform (DWT) together with the k-nearest neighbour (kNN) classification algorithm or by the probabilistic neural network (PNN). The obtained results show that both types of signal processing (DWT + kNN and PNN) applied provide very good class separation for time response analysis, while in the case of the static response analysis the correct classification coefficients are much lower. It is shown that the analysis of the sensors time response can be an efficient way of increasing both the accuracy level and the immunity to external noise in e-nose systems. The possibility of reducing the number of sensors without decreasing the system performance is also demonstrated. Additional experiments have shown that for both processing methods, the results obtained with the dynamic response of a single sensor were better than those reached with the three-sensor array measured in static conditions.

Humidity insensitive thick film methane sensor based on SnO2/Pt

Abstract The influence of environment humidity on electrical parameters is one of the basic disadvantages of the described SnO2 sensors. Due to complex research, the authors have managed to work out cermet composition (SnO2–Pt-black) which eliminates humidity influence on methane detection. In this paper, the basic characteristics of this latest methane sensor are presented.

Heaters for gas sensors from thick conductive or resistive films

Abstract The temperature of gas sensors influences their sensitivity, selectivity and response time. Most of them are operated at significantly elevated tempertures (e.g. electrochemical sensors based on semiconducting metal oxided require up to 900 °C). The possibility of using thick-film heaters in smart planar sensors if often reported. This paper, however, presents results of long-term high-temperature stability examinations of different resistor/conductor systems (in the temperature range 400–600 °C) and several conductive films with different metallurgy (for temperatures between 600 and 1000 °C). It is shown that the RuO2/Au thick-film system based on commercially available resistive and conductive inks allows continuous, long-term operation up to 400 °C. Moreover, the allowable operating temperature is much higher for heaters made form conductive films; for example PtAu heaters can operate successfully up to 600 °C whereas fritless platinum heaters are satisfactory in the whole range demanded by electrochemical sensors.