Improvement of the Sensitivity for Silica Humidity Sensors with a Multiple-layered Silica Bead Coating

Abstract

Many investigations on humidity sensors have attracted great attention to mesoporous thin films because of their high surface area and high porosity. The sensing response strongly depends on the pore properties of the sensing film because a larger interfacial area provides better accessibility to external changes from foreign chemical species in external environment. Multiple-layered silica beads (MLSBs) with high surface area and high porosity were first employed as sensing films in order to solve a problem arising from the density of silica films. Improved sensitivity of the MLSB-coated silica humidity sensor was successfully obtained: 1.85, 3.06 and 81.14 in the relative humidity (RH) ranges from 15% to 60%, 15% to 75%, and 15% to 90%, respectively. The capacitance sensitivity was as large as 2.3 pF/% RH at 50% to 90% RH. These results are due to better accessibility through an increase in the pore dimension of the MLSB-coated silica humidity sensor over that of previously reported porous thin-film silica humidity sensors fabricated by using the aerogel method.