Zeolite-coated interdigital capacitors for humidity sensing

Inter-digital capacitors (IDCs) with electrode gaps of 10 or 50 microns have been coated with zeolite films consisting of different zeolites with Si/Al ratios ranging from 1.5 (zeolite A) to infinite (silicalite). The performance of the sensor in the measurement of humidity has been related to the electrical properties of the zeolites (relative permittivity, ɛr), which in turn is a function of their Si/Al ratio. With zeolites of a high Al content the limit of detection was under 0.5 ppmV.