Thin films of (K+, Na+)-β-ferrites were prepared by the partial replacement of K+ ion in K+-β-ferrite with Na+ ion to obtain a humidity sensing element having almost linear characteristic in terms of log-impedance vs. relative humidity, with humidity-sensitive characteristic controlled by changing the K+/Na+ proportion. Replacing 4mol% of K+ ion with Na+ ion yielded the thin films showing almost linear characteristic. The impedance of the film changed by more than two orders of magnitude with an increase in relative humidity. For a change in relative humidity from 43% to 80%, 95% of the total change was completed within a response time of only 10s. For the reverse change, the response time was less than 15s. These values were faster than expected. Further, the element had excellent durability in terms of humidity-sensitive characteristic. This suggests that thin films with an K:Na alkali ratio 0.96:0.04 is applicable to humidity sensors in terms of linearity in log-impedance change, magnitude of impedance change, response time and durability.
