| Abstract |
This study focuses on the investigation of ozone (O3) gas sensing properties of metal oxides. The sensing material applied was zinc oxide (ZnO) and was selected for various reasons, such as natural availability, non-toxicity, high stability and piezoelectric properties. The deposition of the material was carried out via DC magnetron sputtering at different thicknesses. The structural and optical properties of ZnO were studied via X-Ray diffraction and spectrophotometry techniques respectively. These techniques revealed that the metal oxide fabricated had a preferred growth orientation along the c-axis, a high transparency exceeding 80% and an energy gap (Eg) of ~3.31eV. The sensing properties of ZnO were tested via two different techniques, of which the conductometric was used as a bench mark for the study of the material sensing responses through surface acoustic wave (SAW) utilizing commercial transducers at very high frequency (~1GHz). A set of different O3 concentrations levels ranging from 6ppb to 2ppm for the conductometric and from 110ppb to 2ppm for the SAW tests were applied. The motivation behind the use of acoustic sensors was their small size, inexpensiveness, sensitivity and their response to a large range of oxidizing and reducing gases. The investigation has shown that there is an operating limit for the said commercial transducers which restricted the sensing operation thicknesses on the SAW to below 100nm. The testing gas selected was ozone due to its critical importance in our environment and its impact on human health. Ozone becomes harmful for human health for concentrations above 50ppb according to EPA and WΗΟ. The conductometric tests lead to the result that most of the samples exhibit an up to 5 orders of magnitude response and for thicknesses above 100nm the sensor (on glass substrates) can distinguish among different ozone concentrations with relatively adequate accuracy. Similarly, results obtained through the SAW technique did show a good response also, with a maximum frequency shift value of 8-9MHz, during their first photoreduction- oxidation cycle. Detailed study of the SAW sensing responses demonstrated, for the first time, the role of sensing substrates on their response rate and form. This study revealed the complexity of acoustoelectric and mechanical properties for the SAW transducers in the process of accurate and reproducible sensing results. Nevertheless both techniques were found to have a good reversibility during their photoreduction–oxidation processes which merits them as durable sensing devices for room temperature applications.
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Study of n-type metal oxide gas sensors utilizing Surface Acoustic Waves (SAW)
