Semiconducting-oxide chemical sensors

The Taguchi and thick-film forms of semiconducting metal-oxide sensors and their reproducibility and stability problems are discussed. An attempt to circumvent these problems by producing a modified thin-film sensor based on oxygen absorption rather than adsorption is described. The sensor is made with evaporated bismuth molybdate, which has a very high diffusion constant for oxygen vacancies. The combustion gas to be sensed extracts lattice oxygen from the bismuth molybdate; atmospheric oxygen restores the lattice oxygen. Because the oxygen vacancies move rapidly in the material, the bulk stoichiometry (i.e. metal/oxygen ratio) follows the surface stoichiometry and controls the resistance. Bismuth molybdate films are stable and, as prepared, are highly sensitive to alcohols. Techniques for improving selectivity are considered. The use of electronic circuitry to circumvent inherent sensor problems is examined.<>