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CHEMICAL SENSORS Figure 7 Finished sensor with zeolite membrane over an active electrode. 3.4. Sensor testing The initial sensor testing was performed using a spe- cific sensor specimen, which did not have the added zeolite gas-selective membrane. This same sensor then had the zeolite film grown over its working electrode to run the final tests with the membrane in tact. This sensor with its zeolite membrane is seen in Fig. 7. A typical run of four hours is seen for the uncoated sen- sor in Fig. 8. In this run, oxygen and carbon dioxide were added individually and together into an inert gas stream. The sensor responded positively when either gas was present, and more strongly when both were added simultaneously. For oxygen containing gases, the sensor response was linear at lower oxygen content and showed a di- minishing response at higher oxygen content. This is illustrated in Fig. 9. Our experience with these sensors that utilize solid composite electrodes is that coarsen- ing the microstructure of the composite and provid- ing a less active and more current limiting electrode can expand the linear region. In this case, the elec- trode was intentionally produced to be highly active Figure 8 Sensor response to oxygen and CO2 in a four-hour test run. 4314 Figure 9 Response of the basic amperometric sensor to a range of oxygen and carbon dioxide concentrations. since the zeolite layer was anticipated to limit gas diffusion. A similar test using oxygen and carbon dioxide was performed on the sensor after zeolite coating. Fig. 10 shows the response of the zeolite-coated sensor to vari- ous atmospheres. The sensor is highly sensitive to even small concentrations of oxygen, showing a 0.32 mA re- sponsewhenchangingfromnitrogen(PO2 =1E−4),to 3 percent oxygen, seen at the 1100 s mark in Fig. 10. In contrast to the previous tests, the sensor showed virtu- ally no response to changes in carbon dioxide content of 30 percent (at 3000 s and again at 5000 s), while giving a marked response to as little as 3 percent of oxygen. So the initial appearance is that the zeolite film effectively blocks carbon dioxide from the sensor surface. 3.4.1. Zeolite selectivity The theoretical selectivity of zeolites is very specific. Add to this, the fact that our estimated 1 micron layerPDF Image | Development of a selective gas sensor utilizing zeolite membrane
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