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CHEMICAL SENSORS Figure 10 Response of the zeolite coated sensor to oxygen and CO2 containing gases. Figure 11 Anticipated selectivity of potassium and sodium zeolite-A membranes. would repeatedly filter any gas passing through more than 300 times before a molecule could pass completely through. The selectivity expected for two of the A-type zeolites is as shown in Fig. 11. It can clearly be seen that no absolute selectivity is expected within the gases tested if the layer consists of zeolite Na+-A, which is indeed the case here. But selectivity can also be achieved by near size ef- fects. When the porosity is on the order of the length of the mean free path of a gas molecule, selectivity is ac- cording to Knudson diffusion, wherein the permeation is proportional to the square root of the reciprocal of the molecular weight. This would indicate that CO2 would have about one-third lower permeation than oxygen or NO, which would be essentially the same. In the case where the pore size is smaller than the mean free path, but large enough to admit the molecule, the situation is difficult to predict. In this case, the affinity of the gas molecule for specific sites within the zeolite may aid in surface diffusion through the pores. Limited data was taken before the sensor failed, and there is no clear an- swer on the mechanism for diminished sensor response to CO2, but it is clear that some limiting of the CO2 gas interaction with the sensor has occurred. Quantifying the selectivity attained is based on lim- ited experimentation and some difficulties. Ideally more, and better planned experiments were to have taken place, but the data shows the uncoated sensor with an average response of 0.43 mA/percent of oxygen and 0.22 mA/percent of CO2. This yields a selectivity of about 2 for oxygen over CO2. This is anticipated since the reduction potential for CO2 is −1.6 V and the reduction potential for O2 is −0.67 V. At the surface of the amperometric sensor, oxygen is more likely to dissociate and traces of oxygen were always present in the gas mixtures. For the zeolite coated sensor, the response to all gases was diminished. The response to oxygen by the coated sensor was one tenth that of the uncoated sensor (0.041 mA/percent) and even more di- minished for CO2 at 0.001 mA/percent. The selectivity of the coated sensor for oxygen was 40 times greater than for CO2. 3.5. Film analysis As stated earlier, analysis of the zeolite film by SEM is generally not practical for samples to be used as sen- sors. After failure of the sensor, a careful analysis of the zeolite film was performed. Images of the surface, shown in Fig. 12, indicate that the zeolite had devel- oped cracks at some unknown time. The nature of the cracks, permeating across the individual zeolite grains, suggests that the crack evolved from the sensor, thereby splitting a previously intact zeolite film. This also in- dicates that the film was likely to be well attached to the sensor given that it did not spall away from the sen- sor under stress and that the film itself was coherently bonded together. As estimated during analysis of glass slides pro- cessed along side the sensor, the film formed was in- deed on the order of 1 micron in thickness as shown in the cross-sectional view shown in Fig. 13. The film showed significant surface roughness, but no apparent open pores through the surface other than the cracks. The point during testing at which the cracks appeared is unknown, so their influence on the data is left to experimental replication. The dimensions of the crack were sufficiently large to produce no selectivity based on their gross scale. 4315PDF Image | Development of a selective gas sensor utilizing zeolite membrane
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