| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 99 === Gas ionization sensors are physical devices that work by fingerprinting the ionization characteristics of distinct gases. Conventional ionization sensors were limited by the huge and bulky architecture (ex: FID, PID), risky high-voltage operation and high power consumption. In this thesis, carbon nanotubes (CNTs) with relatively low work function, extremely sharp nanotips, and structural and chemical stability under high electrical field were therefore used to improve these issues of gas ionization sensors.
In the beginning of this thesis, the effects on gas breakdown characteristics of different surface morphology of CNTs film are presented. For the Random oriented CNTs film, the variations of the breakdown voltages are especially large at high voltage region and their error bars in the high voltage region are as wide as 100 volts. These variations are associated with the nonuniformity of the CNTs’ length. On the other hand, the gas breakdown characteristics of the Uniform CNTs film were relatively stable from the measurement results. However, for both of the two samples, the shift-up of their breakdown voltages (Vbr) were fairly severe after the high-voltage process in stability tests. One could find that the Vbr of the Random oriented CNTs film lifts up from 365V to 605V after 1000 cycles, i.e., 68% increase. And for the Uniform CNTs film, it lifts up from 395V to 575V after 1000 cycles, i.e., 46% increase. Observed from the SEM images, the pull-off and evaporation of CNTs resulted from the high local electric field difference were considered as the main reason for the shift-up of breakdown voltages.
In order to acquire a better stability in the CNTs gas ionization sensor, the improvement of the adhesion and the contact resistance between CNTs and substrate under high electric field was obtained using Co-Ti co-deposited catalyst structure. The Vbr of the CNTs film synthesized from Co-Ti co-deposited catalyst lifts up from 375V to 435V after 1000 cycles, i.e., only 16% increase, which is much more reduced than that of the first two conventional CNTs film.
In addition, to improve the issue of high power consumption, pillar arrays of vertical aligned CNTs bundles with different spacer height ratios (R/H) were utilized to investigate the optimal local electrical field on the nanotubes that has the most efficient field emission, namely, the earliest gas breakdown and lowest breakdown voltage. In this thesis, the lowest breakdown voltages were approached by changing H while maintaining R and the optimal R/H ratio was around 2.91. This optimal R/H ratio would lessen the high operating-voltage and thus improve high power consumption issues of the ionization sensors.
Next, the optimized samples were exploited to explore their gas ionization characteristics under different gases environment. From the experiment, dissimilar trends of Paschen’s curve for distinct gases was obtained due to that different gas molecules have different mean free path, ionization energy and recombination rate. With a proper selection of the p×d product value, CNT gas ionization sensor can not only operate under low voltages but also provide enough space to distinguish between different gases.
Finally, the breakdown voltages of Ar and CO2 gases in mixture with air as a function of concentration were investigated. Take the R/H = 2.91 optimized patterned sample for example. It was found that the Vbr increases 50V as the concentration of CO2 in the mixture with air reaches 15 %, and decreases 100V as the concentration of Ar in the mixture with air reaches 11 %.
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