Abstract
Long term reliability up to 10, 000 hours has been tested for the capacitive-type humidity sensors using cellulose acetate butyrate (CAB) film as the humidity sensitive material. When these sensors using as-received commercial unpurified CAB were tested outdoor in the thermometer screen box, their capacitance values descended abruptly within 3, 000 hours from about hundreds pF to nearly zero pF. The cause of this deterioration was found to be originated from the hydrogen-ion releasable impurity included in CAB.
Then, on the substrate type-humidity sensors, four kinds of tests, i.e., humidity cycle test, high temperature and high humidity test, and so on, were carried out in accelerated conditions.
As a result, the sensors made of the purified CAB with a water-vapor permeable Au thin film electrode on the glass substrate showed long term stability, i.e., fluctuation of their capacitance values corresponded to that of humidity less than 1% RH/1000h. It was suggested that the Au electrode acted as a filter and it protected the elements of sensor from injurious materials present in the environment.
