A Comparative Performance Analysis of Nanoporous Alumina Moisture Sensors Fabricated by Anodizing Aluminum in Aqueous Oxalic and Sulphuric Acid Solutions

摘要

Moisture sensors fabricated with H_2C_2O_4-grown films exhibited a relatively inadequate sensitivity at low humidities (0-45% RH) but superior sensitivity at high humidities (45-100% RH) than H_2SO_4-film sensors. In contrast, the sensors with H_2SO_4-grown films showed a moderate sensitivity but >20 times larger than H_2C_2O_4-film sensors in the low-humidity range; and sufficient sensitivity but approximately 1.6 times smaller than H_2C_2O_4-film sensors in the high-humidity range. The opposite behaviours of moisture sensors fabricated by growing anodic alumina films in different electrolytic solutions have been elucidated in terms of the film porosities, a geometrical shape parameter characterizing the pores, the humidity-induced physicochemical phenomena taking place in the honeycomb-like nanoporous structure, and the resultant surface conductivity changes in it. Deterioration in moisture sensitivity of both types of sensors after long-term storage in normal atmospheric humidity conditions has been interpreted in terms of hydration of alumina surface in humid environments, as the primary mechanism. The theoretical analysis presented herein will provide useful insights for tailoring sensor properties according to practical applications, and help in deployment of sensors by appreciating their shortcomings.