Thickness effects of aerosol deposited hygroscopic films on ultra-sensitive humidity sensors

• Published in: Sensors and actuators. B, Chemical Chemical, 2018-07, Vol.265, p.632-643
• Main Authors: Liang, Jun-Ge, Kim, Eun-Seong, Wang, Cong, Cho, Myung-Yeon, Oh, Jong-Min, Kim, Nam-Young
• Format: Article
• Language: English
• Subjects: Aerosol deposition; Aerosols; Barium titanates; Chemical vapor deposition; Density; Detection; Feasibility studies; Gas detectors; Heat treatment; Humidity; Humidity sensor; Hygroscopicity; Microstructure; Morphology; Recovery time; Response/recovery time; Sensitivity; Solidification; Thickness; Thickness effect; Water vapor
• ISSN: 0925-4005; 1873-3077
• DOI: 10.1016/j.snb.2018.03.093

•Aerosol deposition can be used to prepare ultra-sensitive ceramic humidity sensors.•Film-based sensors were prepared using a shock-loading solidification mechanism of aerosol deposition.•Their sensitivity depends on the hydrophilicity, pore volume, and open-pore ratio.•The thickness of the film regularly affects the humidity-sensing properties. Hygroscopic film that has a super mesoporous structure exhibits high sensitivity and fast response/recovery in humidity sensing applications. Aerosol deposition (AD) via a shock-loading-solidification preparation mechanism, which is an unexplored potential hygroscopic film preparation technique, can produce a porous microstructure, and the AD hammering effect creates various interior and surface microstructures in different-thickness films. The objective of our study was to verify the feasibility of using AD as a humidity-sensing film preparation technique, and to investigate the thickness effects of the film on its microstructure and hygroscopic properties. Hygroscopic films with thicknesses ranging from 0.1–10.0 μm were aerosol deposited with BaTiO3 powders before undergoing a 400 °C thermal treatment. The resulting surface morphology exhibited roughness increases for thicknesses from 0.1–6.0 μm, and a decrease at 10.0 μm. The cross-sectional structure was characterized by a transitional-density grain distribution in which higher density small-sized grains could be seen in the bottom layer. Films with thicknesses of 1.5 μm and 3.0 μm achieved excellent sensitivities of 178.6 ± 7.3 pF/%RH and 299.9 ± 5.4 pF/%RH, respectively. Further, the response and recovery time for the 1.5 μm film were both less than 5 s. The current study determined the feasibility of AD-based humidity-sensing film preparation and provided a reference for optimal thickness control. The surface hydrophilicity, pore volume, and open-pore ratio were analyzed as critical factors of the thickness related humidity sensing effects, and physical modeling indicated that AD-prepared films exhibited an expanded humidity detection range, enhanced water vapor adsorption and desorption, and improved sensitivity to humidity.