Carbon-film-based humidity sensor containing sodium or potassium. Recovery effect

The paper describes the recovery effect of some modified carbon films applied as a humidity-sensitive resistor. The carbon films were prepared using mixtures of furfuryl alcohol and NaOH (or KOH) solution. The films have been tested as humidity-rensitive resistors. FT-IR investigations revealed that due to oxygen and carbon dioxide chemisorption the carbon surface was covered by polar surface functional groups. The electric resistance of such films with oxidised surface is extremely sensitive to contact with water vapour. On the contrary, the sensitivity of newly obtained Na (or K)-modified carbon films towards water vapour was found to be relatively minor before ageing. The process of humidity sensitivity development (caused by oxidation of carbon surface) is reversible, i.e., the sensitivity disappears upon HCl and/or heat treatment but recovers after a sufficiently long storage in air.