L.M. Cavanagh

Front‐end signal conditioning used for resistance‐based sensors in electronic nose systems: a review

The overall performance of an electronic nose system will depend on the individual performance of its constituent elements. Although often overlooked, it is clear that careful design/selection of the front‐end signal conditioning circuit is of critical importance if optimal performance of the odour sensing system is to be achieved. In this paper circuits are reviewed, which have been employed as front‐end signal conditioners for resistance‐based sensors in electronic nose systems, with many of the conclusions drawn being equally applicable to other resistor sensors. The relevant equations governing the behaviour of each circuit methodology are derived and advantages and disadvantages are discussed. The performance of the circuit is then quantitatively assessed in a specific test case, in which the maximum sensitivity of the circuit is calculated in relation to the task of interfacing to a theoretical thin‐film conducting‐polymer sensor.

A novel approach to electronic nose-head design, using a copper thin film electrode patterning technique

Presented in this paper is a process for manufacturing copper electrode patterns on alumina substrates using thin film deposition, spin coating and printed circuit board (PCB) etching techniques. The process was used for the design and manufacture of an array of gas sensors for use in an electronic nose system. This approach was executed in three phases. Firstly a 500nm layer of copper was deposited onto the alumina substrate. Secondly photoresist was applied by spin coating onto the copper layer and finally the PCB etching process was used to achieve the final electrode pattern. Conducting polymer composite materials were deposited onto the resulting electrode patterns producing an array of sensors for vapour detection. The sensor array showed good responses to Propanol at concentrations ranging from 5000ppm to 30000ppm with fast recovery times. The sensor array was slotted into an electronic nose system and an illustrative analysis of the sensor arrays ability to discriminate between different solvents was carried out with promising results

Influence of the Poly(Vinyl Butyral) Binder in Screen-Printed Thick-Film Gas Sensors Containing

This study investigates gas sensitive resistors fabricated by screen-printing inks created from a mixture of NiO-TiO2 and containing a poly(vinyl butyral) binder for the detection of alcohol vapors at room temperature. It was found that these films exhibited a significant resistance change in response to the vapors, while also displaying an inherent selectivity, proving most sensitive to toluene and propanol vapor; with comparatively low responses exhibited towards ethanol and methanol vapors. It is proposed that swelling of the polymer binder used in the fabrication of the NiO-TiO2 thick-films, rather than oxygen ion induced majority carrier modulation in the oxides, is the significant factor in causing the electrical resistance of the films to increase upon exposure to the organic solvent vapors. Very fast response and recovery times of 9 s and 16 s, respectively were recorded for the devices upon exposure to 4000 ppm step changes in propanol concentration. For comparative analysis, poly(vinyl butyral)/carbon black composites were fabricated from a by a drop-coating technique. The superior sensitivities and response times of the screen-printed sensors were attributed to the (SEM confirmed) highly porous structure attained by the thick-film devices, caused by the excessive free-volume induced in the polymer-binder matrix by the oxide materials. The results suggest that the effects of the polymer binder, even when present in comparatively small amounts, cannot be ignored in certain sensing applications

{\rm NiO}{\hbox{-}}{\rm TiO}_{2}

The response of screen-printed thick-films of NiO/TiO/sub 2/ to organic solvent vapours was studied. It was found that these films displayed a significant resistance change in response to the vapours even at room temperature. The sensors displayed an inherent selectivity, proving most sensitive to toluene and propanol vapour; with comparatively low responses exhibited towards ethanol and methanol vapours. Very fast response and recovery times of 9 s and 16 s respectively were recorded for the devices upon exposure to 4000 ppm step changes in propanol concentration.

Oxides

The effect of ethanol vapour and temperature was investigated on gas sensors fabricated from poly(vinyl acetate)/carbon black composites based around a predetermined percolation threshold. Samples with 8% carbon black loading displayed the best response to the ethanol vapour. Typical response and recovery times of 140s and 45s respectively were recorded. In addition, bridge structures were fabricated, where all four resistive elements were prepared from the same composite material and in which a novel passivation process was employed. It was observed that these bridge structures were significantly less affected by variations in temperature in comparison to the single sensor structures.

NiO-TiO/sub 2/ thick-films for detection of alcohol vapours at room temperature

This study presents an analysis of electrode patterns suitable for use with drop coated conducting polymer gas sensors. A thin-film technique was used to efficiently fabricate the copper electrode patterns (Arshack, 2005). Conducting polymer composite (CPC) materials were deposited using a 500 nano-litre syringe onto the electrode patterns to produce an array of sensors for organic solvent vapour detection. The sensors were exposed to propanol vapour in steps of 3000 ppm from a minimum concentration of 5000 ppm up to a maximum concentration of 20,000 ppm. Empirical results showed that a non-parallel electrode configuration produces a marginally larger response and is also less noisy than the interdigitated or parallel electrode configurations. Results show that increasing the baseline resistance of the sensing material gives a larger response

Response of poly(vinyl acetate)/carbon black composites to ethanol vapour and temperature

Statistical design of experiment (DOE) is a critically important tool in the engineering, physical and chemical science worlds for improving existing products and also for the development of new products. This paper investigates the use of DOE for the design and characterisation of conducting polymer composite (CPC) vapour sensors. DOE has the advantage of providing a large amount of information on a process using a minimum amount of experimentation, equipment and time. This analysis tool is therefore ideal for the development of composite sensors, where more than one material determines the properties of the sensor. The effects of % plasticiser, polymer type, alcohol type and concentration on the response and response times of these materials were investigated. The results showed that plasticiser significantly improved the response but also increased the response time. PVB was the best polymer to use to achieve a high response but again it had a very long response time. The DOE analysis technique provided a large amount of information on the response of the chosen materials with three alcohols that using standard testing methods would have required a lot more experimental work