Investigation of Humidity Sensing Performance of Mg-doped CCTO Films

Calcium copper titanate (CaCu3Ti4O12: CCTO) is known for its colossal dielectric constant, which remarkably depends on the structural form: ~105 for single crystals, ~104-105 for bulk ceramic and ~103 for thin films. Besides the structural forms, other important factors including growth conditions e.g. annealing temperatures, growth rates, types of substrates used and doping could affect the dielectric properties of CCTO. These growth parameters influence properties like density, phase, texture, stress, and the presence of impurities and defects. More recently, humidity sensing applications have been addressed. These studies are still much fewer than those that investigated bulk CCTO ceramics. In this work, Mg-doped CCTO thin films on alumina substrates were synthesized via the sol-gel spinning technique to investigate their humidity sensing performance. Capacitive humidity sensors with interdigitated electrodes with gaps of 50 µm and 100 µm were fabricated. Structural analysis by X-ray diffraction (XRD) confirmed their crystal structures, while energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) verified the successful substitution of Mg2+ into Cu2+ sites in the CCTO lattices. Humidity sensing tests performed in the range of 20 – 90% relative humidity (RH) at 20, 50, and 100 kHz demonstrated that humidity sensors performed the best at 20 kHz. Furthermore, humidity sensors with a 100 µm electrode gap showed more sensitivity than a 50 µm electrode-gap sensor. Additionally, these Mg-doped CCTO films exhibited less hysteresis errors compared to those reported for Mg-doped CCTO ceramics. The optimal doping levels of Mg-doping are in the range of 5 – 15 mol% for humidity sensing applications.