Electronic Noses, a Dierent Approach to the Sensitivity and Selectivity Issues P. M. Faia,* M. A. Pereira, A. M. Nunes and C. S. Furtado Department of Electrical Engineering, University of Coimbra, Pinhal de Marrocos, 3030 Coimbra, Portugal Abstract Many approaches have been made during the past years to the gas sensing task. It may be addressed using one sensor or a set of arranged sensors. Solid state chemical sensing has been systematically used for the development of gas sensor devices, based on semi conducting metal oxides. There are three major issues related to chemical sensing: sensitivity, selec- tivity and stability. Selectivity may be improved by the use of sensor arrays (also referred as electronic noses), while the use of additives may improve sensi- tivity (pure doped materials or porosity control). In this paper we describe our current work about the sensitivity and selectivity issues. A tentative explana- tion of the observed resistance and impedance changes by porosity control is given. # 1999 Elsevier Science Limited. All rights reserved Keywords: porosity, electrical conductivity, sensors. 1 Introduction The sense of smell is perhaps the least appreciated of the ®ve senses. While many animals rely heavily on their acute sense of smell, it is becoming decreas- ingly important to man. As Doty notes: 1 `The ®eld of olfaction science only recently began to enjoy the bouquet of respectability it now has. An instrument that could perform simple odour discrimination and provide measurement of odour intensity, would be most useful'. Such an instrument, an Electronic nose, has received much attention in recent years both from the academic and industrial world. The ®rst use of the term Electronic nose has been ascribed to Gardner who uses the following de®ni- tion: 2 `An Electronic nose is an instrument, which comprises an array of electronic chemical sensors and an appropriate pattern-recognition system, capable of recognising simple or complex odours'. 2 Chemical Sensing The use of ceramic oxides in gas detection devices involves dierent issues. The most important char- acteristics in gas detection sensors are sensitivity, selectivity and stability of the sensing devices. Sen- sitivity is the capacity that a device has of detecting the presence of a certain type of gas, in dierent concentrations. Selectivity is the capacity that a sensing device has to detect dierent types of gases. Stability refers to the capacity that a device has to give the same information under certain condi- tions, independently of the number of times this device has already been used. In this paper we deal with the sensitivity and selectivity issues. 3 Towards an Integrated Sensing System 3.1 Selectivity issue A sensor array is a system involving a suitable data analysis procedure, constituted by a number of sensors, which are generally based on dierent chemical and physical working principles. It is well known that the use of sensor arrays is advanta- geous for the identi®cation of gases because of the poor selectivity of many gas sensors, when only one sensor alone is applied. 3±5 The ®rst use of a sensor array was reported with semiconductor gas sensors. 6 As to the shape of our electronic nose, we are trying a dierent type of design, as seen in Fig. 1. We chose this shape due to a unique reason. Some of our research partners are working on an injection moulding project with ceramics. Based on their knowledge on that subject we designed a ceramic substrate where we will implant the sensors with this conical shape (in opposition with the traditional planar sensor disposition), similar to a real human nostril. It will have 16 sensor elements Journal of the European Ceramic Society 19 (1999) 883±886 # 1999 Elsevier Science Limited Printed in Great Britain. All rights reserved PII:S0955-2219(98)00337-9 0955-2219/99/$ - see front matter 883 *To whom correspondence should be addressed. Fax: +351- 39-796-247; e-mail: faia@dee.uc.pt