Talanta 66 (2005) 1247–1253 A resistive-type humidity sensor using composite films prepared from poly(2-acrylamido-2-methylpropane sulfonate) and dispersed organic silicon sol Pi-Guey Su ∗ , Chyi-Long Uen General Education Center, Chungchou Institute of Technology, No. 6, Lane 2, Sec. 3, Shanjiao Road, Yuanlin Township, Changhua County 510, Taiwan Received 3 September 2004; received in revised form 7 January 2005; accepted 20 January 2005 Available online 24 February 2005 Abstract A composite material of dispersed organic silicon sol and poly(2-acrylamido-2-methylpropane sulfonate) (poly-AMPS) was used to make humidity sensor without protective film or complicated chemical procedures. The organic silicon sol was dispersed well in the poly-AMPS without using dispersion agent. Parameters that may affect the water-resistive but humidity-sensitive characteristic of composite material, the adding amount of organic silicon sol solution and the film of thermal treatment time, were investigated. The microstructure of the material was analyzed, and the humidity sensing and electrical properties of the sensor were measured. The sensor well responded to humidity with a relatively good linearity, though it depended on the applied frequency. The temperature influence between 15 and 35 ◦ C was within -0.17 % relative humidity (RH)/ ◦ C in the range of 30–90% RH. The activation energy was maximum around 40% RH. The sensor showed the hysteresis within 5.9%, fast response time, long-term stability (75 days at least) and satisfactory resistance to high humidity atmosphere (97% RH) and chemical environment (20% C 2 H 5 OH vapor). Analyzing the structure and complex impedance plots of organic silicon sol/poly-AMPS was used to explain improvement in humidity sensing properties in comparison with nano-sized SiO 2 powder/poly-AMPS films. © 2005 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; Composite material; Dispersion; Organic silicon sol; Poly-AMPS; Impedance spectra 1. Introduction There are many kinds of ceramics and organic polymers have been studied for the development of humidity sen- sors for measurement and control of humidity in human comfort and a myriad of industrial processes. The polymer electrolytes are one of interesting materials because they have a better humidity-sensitive characteristic, such as long-term stability, reliability, ease of processing and the low fabrication cost, than the ceramic materials or hygroscopic resin containing an electroconductive powder distributed therein. The resistive-type humidity sensors are fabricated with polymer electrolytes. The conductivity of polymer ∗ Corresponding author. Tel.: +886 4 8311498x1736; fax: +886 4 8314515. E-mail address: pigueysu@dragon.ccut.edu.tw (P.-G. Su). electrolyte usually increase with humidity due to that the polymer electrolyte ionizes and produces conductive ions migration inside the material [1]. For example, poly(2- acrylamido-2-methylpropane sulfonate) (poly-AMPS) polymer electrolyte with acidic groups or their salts, such as –SO 3 - H + or –SO 3 - M + , where M is an alkali metal has been used for humidity sensors material [2–6]. However one of serious shortcomings of the hydrophilic electrolytic-type polymer is that when exposed to a highly humid atmosphere for a long time, then the humidity sensing layer is liable to swell, shrink or peel off from the substrate [7,8]. Many kinds of methods have been studied to solve the problem of resistant to high humidity atmosphere using polymer electrolyte as sensing material. Sakai et al. [3,9] used the graft polymerization method to prepare the polymer electrolyte on the surface of the pores of porous polyethylene 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.01.039