Sensors and Actuators B 222 (2016) 1043–1049 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb Nanoporous -alumina based novel sensor to detect trace moisture in high temperature and high pressure environment Debdulal Saha, Dilip Kr. Ghara, Mrinal Pal ∗ Sensor and Actuator Division, CSIR-Central Glass & Ceramic Research Institute, 196, Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India a r t i c l e i n f o Article history: Received 14 May 2015 Received in revised form 10 September 2015 Accepted 11 September 2015 Available online 12 September 2015 Keywords: Sol–gel Thin film Nanoporous -Al2O3 humidity sensor a b s t r a c t Development of trace moisture sensor which can perform in high temperature and high pressure environ- ment is always a challenging task though it is needed for some specific applications. This article delineates the synthesis and characterization of nanoporous -alumina thin film through sol-gel followed by dip coating technique and fabrication of trace moisture sensor capable to work in high temperature and high pressure environment. The structure and morphology of the films were properly characterized using XRD, FESEM and AFM. High sensitivity and good quasi-linearity were obtained at the low frequency region and best linearity, which may be suitable for device application, was exhibited at 1 kHz. The linearity of sen- sor response at different moisture level is comparable with commercially available sensors. In addition, sensors are ultrasensitive having response and recovery time around 1 s and 1.5 s, respectively. A combi- nation of micro and nano pores in the electrode and -alumina (-Al 2 O 3 ) films, respectively, could play an important role for achieving high and ultrafast sensitivity. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Humidity sensors play a vital role to maintain a specific environ- ment conditions in numerous fields like food processing, medicine, tea industries, household appliances, automotive control systems, conditioning systems, meteorology and research labs [1–4]. The rel- ative humidity (RH) [5], which is the ratio of actual water vapour pressure to the saturated vapour pressure at a given temperature, is the most important parameter in specifying humidity. Many humidity sensors, based on polymers [6–9], ceramics [10–15] and composites [16–21] have been developed with the goal of optimizing performance. Various types of humidity sensors are available in market for different applications [22,23]. They range from low-cost, single point systems including portable battery- operated models to multi point, microprocessor-based systems. However, available humidity sensors have many limitations such as low sensitivity, reproducibility, aging, relatively bulky in size and difficult to digitalize its signal [24,25]. Also, no single sensor can satisfy the requirements of every application [26]. Therefore, fur- ther investigation is requiring in order enhancing the performance of humidity sensors toward sensitivity, reversibility, long-term ∗ Corresponding author. E-mail address: palm@cgcri.res.in (M. Pal). stability as well as fast response and recovery times. Moreover, majority of humidity sensors are basically polymer based compos- ites which are not suitable at high temperature environment. No humidity sensors are available in the market which is capable to work under high temperature and/or high pressure environment for some precise applications like in nuclear power plant for leak before break (LBB) detection, in tea industry during drying [27–32]. The leak before break (LBB) concept is well known in nuclear power industry [27,30]. This is based on the premise that a detectable leak will develop in the hot water pipe before catastrophic break occurs. Tea industry produces various category of tea by drying the raw materials in different temperature where humidity is a matter of concern [28,29,32]. In this work successful preparation of a -Al 2 O 3 based ceramic nanoporous thin film sensor using sol-gel technique and humid- ity sensing performance at high temperature and high pressure is reported. The sol–gel process is a convenient and inexpensive technique for the production of films of a variety of metal oxides [33–35]. -Al 2 O 3 has been explored since it is very stable both phys- ically and chemically. We have developed a novel procedure for preparing nanoporous -alumina films which is based on dip coat- ing of diluted sol solutions on electroless gold plated substrates [36]. The porosity, quality of deposited films and thickness were studied using field effect scanning electron microscope (FESEM), atomic force microscope (AFM) and Brunauer–Emmett–Teller (BET). http://dx.doi.org/10.1016/j.snb.2015.09.068 0925-4005/© 2015 Elsevier B.V. All rights reserved.