Lacunar pyrochlore-type tungsten oxides as humidity-sensing materials

The humidity-sensing characteristics of lacunar pyrochlore-type tungsten oxides containing small cations were investigated. Pyrochlore-type tungsten oxides having the general formula (M(2)O)(x)WO(3)zH(2)O, where M = H+, Li+, Na+, Ag+ and NH4+, were prepared by hydrothermal reaction followed by ion exchange. The sensing elements were fabricated by screen printing the oxides on top of a pair of comb-shape electrodes or between 2 electrode layers. It was found that the hydrogen form of the pyrochlore-type tungsten oxide (H-pyroWO(3)) has the lowest resistivity, whereas the lithium form (Li-pyroWO(3)) produced the best linearity between the relative humidity and the logarithm of its resistance. The sensors showed good durability and reproducibility for detecting changes in relative humidity in the range 5-95%. The response time of the sensor for 90% of the total impedance change (T-90) is about 2 min for both adsorption and desorption processes.