Nanocrystalline Materials as Potential Gas Sensing Elements
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ABSTRACT Sensors and Transducers, and in the specific context of this paper gas sensors, are currently amongst the largest growth areas in the modem electronics industry and this seems likely to continue for the foreseeable future. Nanocrystalline materials posses many properties that could make them ideal as potential gas sensing elements with many advantages over their microcrystalline counterparts. Most importantly these include increased surface area coupled with reduced sintering temperatures and times. However, it should also be noted that there are several disadvantages including the comparatively high cost of materials and increased electrical resistance. This paper reviews the operating mechanisms of semiconductor gas sensors and the possible advantages of using nano sized powders to produce gas sensitive devices. Results are presented which have been obtained from several materials produced by laser evaporation including alumina (A120 3), zirconia (ZrO 2), and tin dioxide (Sn0 2) in contaminated atmospheres incorporating carbon monoxide, hydrogen and methane.
INTRODUCTION Sensors and Transducers are amongst the largest growth areas in modem electronics. This is prompted by an ever increasing need to measure and monitor a wide spectrum of measurands including flow, pressure, speed, temperature, position and various chemical species. It has been estimated that in the decade from 1990 the world market for sensors will grow from $16 billion to $39 billion. This represents an annual growth approaching ten per cent. Demand is greatest in the USA, which accounts for approximately 39% of the world market, and is followed by Western Europe (38%) and Japan (23%). The specific area of gas sensors shows a similar growth [1]. In this case the demand is driven by increasing public awareness and associated legislation covering the dangers arising from exposure to toxic and potentially explosive gases in both domestic and industrial environments. Although there are a very large number of potentially toxic, explosive and environmentally sensitive atmospheric contaminants, the majority of scenarios in these three categories are covered by a surprisingly small number of gases as follows:-
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Toxic -Carbon Monoxide Explosive -Lower Hydrocarbons (Methane, Ethane, Propane, Butane) Environmentally Sensitive -Carbon Dioxide, Oxides of Nitrogen and Sulphur
33 Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
Several technologies have been exploited commercially to detect these gases and these are presented in the table with a brief description of their mode of operation [2,3]. Table. Common types of gas sensor with a brief description of their mode of operation. Gas sorption
Photophysical
Catalytic or Pellistor
Electrochemical Ion Selective Electrodes Micro Balances
Field-Effect Transistors
Semiconductor
Species are irreversibly adsorbed onto a material, often by way of a chemical reaction, which produces a change in a physical property such as colour. A specific molecular transition or vibra
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