THICK-FILM HUMIDITY SENSORS

The results of an investigation of thick-film humidity sensors based o n ceramic materials are discussed in the paper. Thick-film technology is very promising far the production of low cost sensors based on cera mic materials. The technology gives the possibility for reproducible p roduction of sensors with a defined microstructure, determined porosit y and proper structure of grains and grain boundaries. Ceramic sensors have advantages over polymer sensors due to their better thermal stab ility and resistance to chemicals. Properties of thick-film planar hum idity sensors based on ZnCr2O4-TiO2 (ZCT) ceramics are presented. The influence of Li2O, B2O3 and Si additives, as well as firing temperatur e, on the sensor characteristics are discussed. Impedance spectroscopy measurements determined the correlation between the technological par ameters and the electrical properties of humidity sensors. The role of each part of the sensitive material in the electrical conduction proc ess is determined on the basis of measurements and calculated equivale nt circuits. The proposed model describes the frequency characteristic s at various relative humidities with very good fit to the experimenta l data. A new approach to the modelling of the impedance frequency dep endence by means of an equivalent circuit yields very promising result s for sensors.