Copper-Substituted Calcium Orthophosphate (CaxCu1-x)HPO4.nH2O for Humidity Detection

Calcium orthophosphate material (Ca1-xCux)HPO4.nH2O (0.4 ≤ x ≤ 1) with the gradual replacement of Ca2+ with Cu2+ ions were synthesized by a chemical precipitation technique. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Then, the prepared powders were deposited onto an alumina substrate with interdigitated Pt electrodes by the spin coating method and polyvinyl alcohol (PVA) as a binder. Successively, the sensors were investigated from 0% to 90% at room temperature under various conditions, including humidity, nitrogenous oxide, methane, carbon dioxide and ammonia. The results evidenced that at 90% RH, the sensitivity of sensors significantly increased with the increase in the Cu content. Moreover, the sensors exhibited good repeatability and, after 1 year of aging, the sensor response was equal to 34% that of the freshly prepared sensor. Finally, there was no interference in the presence of other gases (nitrogenous oxide 2.5 ppm, methane 10 ppm, carbon dioxide 500 ppm and ammonia 4 ppm).