Sensors and Actuators B 191 (2014) 572–578 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/ locate/snb Alteration of pore size distribution by sol–gel impregnation for dynamic range and sensitivity adjustment in Kelvin condensation-based humidity sensors F. Hossein-Babaei a,∗ , S. Rahbarpour b a Electronic Materials Laboratory, Electrical Engineering Department, K.N. Toosi University of Technology, Tehran, Iran b Electronic Engineering Department, Shahed University, Tehran 159-18155, Iran a r t i c l e i n f o Article history: Received 21 May 2013 Received in revised form 29 August 2013 Accepted 7 October 2013 Available online 14 October 2013 Keywords: Relative humidity Humidity sensor Dynamic range Pore size distribution Titanium dioxide Sol–gel impregnation a b s t r a c t It has been theoretically established that the dynamic range of sensitivity in porous dielectric ceramic- based resistive or capacitive humidity sensors is mainly determined by the open pore size distribution in their sensing pallets. Here, we directly apply the concept on ceramic resistive humidity sensors and utilize the results for the experimental verification of the theoretical model. The open pore size distribution in a number of identical titanium dioxide pallets, fabricated by the sintering of the slip-cast powder at 1073 K, is modified in two different directions: (a) impregnation with titanium tetraisopropoxide, followed by a heat treatment at 773 K increases the proportion of the finer pores and (b) re-sintering the original pallets at 1223 K closes the finer pores and substantially shifts the distribution towards larger pore dimensions. Sensitivity measurement results are consistent with the theoretically established concept: increasing the population of larger pores heightens the sensitivity to high relative humidity (RH) levels and shifts the dynamic range of the pallets from the original 20–85% to 50–95% RH. On the contrary, increasing the proportion of the finer pores in the microstructure of the ceramic pallet enhances the sensitivity at the lower RH range, shifting the dynamic range of the sensor to the 2–25% RH range. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The laboratory, industrial, and domestic applications of porous solid-based resistive and capacitive humidity sensors are wide spread. In the most common type of these sensors, the elec- tric conductivity and/or the dielectric constant of a porous ceramic or polymer pallet changes due to the adsorption of the water molecules to its effective surface. These changes are, then, translated to relative humidity fluctuations in the surrounding atmosphere [1]. As a result, the operation mechanism and the qual- ity factors of these devices are strongly dependent on the process of water molecules diffusion into the open pores of the porous pallet and their absorption to the pore walls. The relationship between the pore radius and water adsorption mechanism in a porous solid has been theoretically established more than a century ago by Lord Kelvin: r k = 2M RT ln(P s /P) (1) ∗ Corresponding author. Tel.: +98 21 8873 4172; fax: +98 21 8876 8289. E-mail addresses: fhbabaei@kntu.ac.ir, fhbabaei@yahoo.com (F. Hossein-Babaei), s.rahbarpour@shahed.ac.ir (S. Rahbarpour). wherein, M,  and  are the molecular weight, density and surface tension of water, respectively. P is the partial pressure of water vapor in the surrounding atmosphere and P s is the saturation pres- sure of water in the same ambient conditions. Kelvin equation has been verified experimentally by two different optical observa- tion techniques which facilitated direct observation of the surface adsorbed water layers [2,3]. Based on the nature of the porous solid used for pallet fab- rication, humidity sensor elements are divided into three main categories of ceramics, polymer and electrolyte [4,5]. Ceramic pallets are of higher physical and chemical stabilities and are advan- tages from the view point of many quality factors: they are of longer life times [6,7], provide more reproducible RH measurement results [8], can be applied for water content measurements at elevated temperatures [9], and can be recovered after poisoning in a con- taminated environment by thermal cleaning [6]. Bulk [10,11] and thin film [12,13] ceramic elements are both utilized for pallet fab- rication, but in all configurations the sensing mechanism is based on the measurement of the humidity-sensitive conductivity and/or dielectric constant of the ceramic element. Humidity sensitive ceramic pallets are mostly made of oxide- based ceramic dielectrics and wide band gap semiconductors such as TiO 2 [14–16], SnO 2 [17,18], ZnCr 2 O 4 [19,20], Al 2 O 3 [21,22] and MgCr 2 O 4 [23,24]. The physical property monitored in the majority 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.10.032