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Sensors and Actuators B 222 (2016) 1043–1049

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

anoporous �-alumina based novel sensor to detect trace moisture inigh temperature and high pressure environment

ebdulal Saha, Dilip Kr. Ghara, Mrinal Pal ∗

ensor and Actuator Division, CSIR-Central Glass & Ceramic Research Institute, 196, Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India

r t i c l e i n f o

rticle history:eceived 14 May 2015eceived in revised form0 September 2015ccepted 11 September 2015vailable online 12 September 2015

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 delineatesthe synthesis and characterization of nanoporous �-alumina thin film through sol-gel followed by dipcoating technique and fabrication of trace moisture sensor capable to work in high temperature and highpressure 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

eywords:ol–gelhin filmanoporous-Al2O3 humidity sensor

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 (�-Al2O3) films, respectively, could playan important role for achieving high and ultrafast sensitivity.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

Humidity sensors play a vital role to maintain a specific environ-ent conditions in numerous fields like food processing, medicine,

ea industries, household appliances, automotive control systems,onditioning systems, meteorology and research labs [1–4]. The rel-tive humidity (RH) [5], which is the ratio of actual water vapourressure to the saturated vapour pressure at a given temperature,

s 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 goalf optimizing performance. Various types of humidity sensors arevailable in market for different applications [22,23]. They rangerom low-cost, single point systems including portable battery-perated models to multi point, microprocessor-based systems.owever, available humidity sensors have many limitations such as

ow sensitivity, reproducibility, aging, relatively bulky in size and

ifficult to digitalize its signal [24,25]. Also, no single sensor canatisfy the requirements of every application [26]. Therefore, fur-her investigation is requiring in order enhancing the performancef humidity sensors toward sensitivity, reversibility, long-term

∗ Corresponding author.E-mail address: palm@cgcri.res.in (M. Pal).

ttp://dx.doi.org/10.1016/j.snb.2015.09.068925-4005/© 2015 Elsevier B.V. All rights reserved.

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. Nohumidity sensors are available in the market which is capable towork under high temperature and/or high pressure environmentfor some precise applications like in nuclear power plant for leakbefore break (LBB) detection, in tea industry during drying [27–32].The leak before break (LBB) concept is well known in nuclear powerindustry [27,30]. This is based on the premise that a detectable leakwill develop in the hot water pipe before catastrophic break occurs.Tea industry produces various category of tea by drying the rawmaterials in different temperature where humidity is a matter ofconcern [28,29,32].

In this work successful preparation of a �-Al2O3 based ceramicnanoporous thin film sensor using sol-gel technique and humid-ity sensing performance at high temperature and high pressureis reported. The sol–gel process is a convenient and inexpensivetechnique for the production of films of a variety of metal oxides[33–35]. �-Al2O3 has been explored since it is very stable both phys-ically and chemically. We have developed a novel procedure forpreparing 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).

1044 D. Saha et al. / Sensors and Actuators B 222 (2016) 1043–1049

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which is again feed to an analog to digital converter (ADC). This dig-ital signal is taken up by the microcontroller, the main processingunit of the electronic circuit. Microcontroller process the DC sig-nal to determine the moisture level detected by the sensor using a

Fig. 1. �-Alumina thin film se

. Experimental procedure

.1. Preparation of alumina sol for thin film coating

The sol–gel method similar to that proposed by Yoldas [37]as used for preparation of a boehmite sol. Hydrolysis waserformed at 90 ◦C by introducing aluminum tri-sec.-butoxide97%, Sigma–Aldrich) into excess amount of distilled waterwater:alkoxide molar ratio = 100:0.8) under vigorous stirring.fter 45 min of hydrolysis, the colloidal solution was peptized bydding a given amount of 1.6 (N) HNO3 acids of Sigma Aldrich makeacid: alkoxide molar ratio = 0.071:1) and then maintained at 90 ◦Cor 1 h with constant stirring. A transparent sol was finally obtainedy continuing refluxing the solution at 90–95 ◦C for 16 h. The elec-roless gold coated �-alumina support was dip in to the bindernd dispersant mixed boehmite sol by using a dip coater (modeleltadip-auto, Kolkata). The support was fixed on the bottom of

rotating shaft and immersed into the solution for 1 min of slowotating and then followed by fast spinning to remove the resid-al sol. After drying in air, the support with xerogel was calcinedo 500 ◦C with a heating rate of 0.5 ◦C/min. and soaking for 2 h tobtain a thin top layer on it. The process was repeated five timesith dipping and calcinations in order to get a crack-free supported-alumina thin film.

.2. Fabrication of sensor

The porous �-alumina thin film was fabricated on non-porouslumina substrate sandwiched between two parallel gold platelectrodes, detail shown in Fig. 1 (1a is the schematic model and 1bs the exact fabricated sensor). Gold coating on non-porous aluminaubstrate was fabricated by depositing thick gold layer (typicalhickness 2-3 �m) through electroless gold plating process. A dipoating technique was utilized to grow porous �-alumina film onold coated �-alumina substrate. This was followed by drying at0 ◦C and 50–60% relative humidity condition. In a small bowl goldaste was mixed with organic binder and then printed the mix-ure on �-alumina film by using screen printing technique (model

T-500M, Sky hill, Thiwan) to make top electrode. Binder burnt outhen electrode was fired at 950 ◦C and the gold electrode on the top

ecomes micro porous as clearly visible under field effect scanninglectron microscope (FESEM, model 35 Supra VP, Carl Zeiss), pre-

ented in Fig. 2. The micro porous top gold electrode helps for easybsorption/adsorption and desorption of water molecules insidehe dielectric nanoporous �-alumina thin film. X-ray diffractionattern (XRD) from 20 to 90 (2�/deg) was recorded at a step of

a) schematic, (b) photograph.

0.05 using PANalytical (X’pert pro) instrument fitted with Cu-K�

target (wavelength 1.5418 A) and nickel filter. Capacitance as wellas dissipation factors of the sensor were measured using standardtwo-probe technique. Copper wires were connected with gold coat-ing on both surfaces for connecting purpose and measurementwere carried out using a LCR meter (HP 34970A). Frequency wasvaried from 5 Hz to 13 MHz using a computer controlled HewlettPackard impedance analyzer (HP4192A) fitted with a test fixture(Agilent 16034E). Different salt solution bath has been used for gen-erating various % RH below 100 ◦C. Water vapor has been used forthe measurement at 100 ◦C.

2.3. Development of electronic circuit and high temperaturemeasurement setup

We have developed complete sensor device by attachingrequired electronic circuit in the sensor module (refer. Fig. 6(a)).The capacitative sensor is connected to a one shot multivibratorwhich is trigger by a constant frequency source results a pulsewidth modulation (PWM) waveform. The duty cycle of the PWMis varied in tune with change of capacitance due to variation ofRH. This PWM is feed to a RC filter for the generation of DC signal

Fig. 2. FESEM image of top surface of porous gold electrode.

D. Saha et al. / Sensors and Actuators B 222 (2016) 1043–1049 1045

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�-Al2O3 was also cross checked using X-ray diffraction technique.All the peaks of XRD pattern as presented in Fig. 5 matched well

Fig. 3. FESEM image of nanoporous �-alumina thin film.

ook-up table, stored inside the microcontroller. Result is displayedn the LCD panel and stored in an EEPROM.

As developed humidity sensors (refer. Fig. 11) in six numbersere mounted vertically in a stainless steel chamber (refer. Fig. 12)

acketed by two inch thick insulated glass wool to protect heat lossy radiation. This chamber was placed above another steel platehamber having same length and breadth. A heater was placedn this lower chamber to keep the temperature of the chamber at65 ◦C. Sensors were mounted in such a way that their head toucheshe top surface of this chamber. A slit at the center of the top face ofhis chamber was made for steam outlet to flow to upper chamberontaining the sensors. At the lower part of the bottom chamber aalve was installed for entering the super heated steam from gener-tor. Super heated steam generator is an electrical, table top boileraving one boiler shell and one super heater. A 3 phase, 1 kW elec-rical heater was used each for the boiler shell and super heater toroduce superheated steam having a pressure and temperature of0 bar and 175 ◦C, respectively. The unit requires around 4 h and0 min to build up a steam pressure of 10 bar from ambient condi-ions. A thermocouple was placed at the top surface of the bottomhamber near the slit for measuring the temperature of the outlet

team as well as the chamber. Super heated steam will come out ofhe generator while opening the valve.

Fig. 4. AFM images of (a) gold surface and (b) nanoporous

Fig. 5. XRD plot of �-alumina thin film.

3. Results and discussion

3.1. Microscopy studies

Surface morphology of porous �-Al2O3 dielectric layer wasinvestigated using different surface scanning techniques beforethe application of top gold layer. Fig. 3 shows the FESEM imageof surface coating layer of �-alumina film grown on gold sur-face. Figure clearly delineates that lot of nano pores distributedthroughout the surface. Nano pores were generated when binderand dispersant were burnt out at 500 ◦C. We have generated poresof average diameter between 12 and 15 nm. Fig. 4 is the atomicforce microscopy (Model Nanonics, Multivew 3000, Israel) imageof the bottom gold electrode (Fig. 4 (a)) and thin film �-aluminacoating (Fig. 4(b)). It was observed that gold coated surface isalmost smooth. However, the growth of �-Al2O3 crystals alongwith pores is very prominent as displayed in Fig. 4(b). The depthof the pores is around 181 nm as estimated from the Fig. 4(b),which favors the low level moisture sensing process. Growth of

with the standard pattern which indicates the growth of pure �-Al2O3.

�-alumina thin film coating grown on gold surface.

1046 D. Saha et al. / Sensors and Actuators B 222 (2016) 1043–1049

Fig. 6. (a) Photograph of sensor module, (b) hysteresis plot measured at 1 kHz.

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ig. 7. Dynamic response curve for moisture of 100 ◦C measured at normal pressure.

.2. Sensitivity study

Sensor modules were fabricated by mounting the preparedorous film as shown in Fig. 6(a). Sensitivity of the sensors (capac-

tative) in terms of change of capacitance was measured using arecision LCR meter. Variation of capacitance with relative humid-

ty for 1 kHz frequency measured at 30 ◦C is presented in Fig. 6(b). Itas observed that the capacitance value increases with the increase

f humidity percentages due to more and more water moleculeondensed in the nano pore. No hysteresis loss was observed as its clear from Fig. 6(b). The response and recovery time were mea-ured from the dynamic response curve of the sensor. Fig. 7 is aypical dynamic response curve for moisture of 100 ◦C measured atormal pressure. Figure delineates that our sensor is really ultra-ensitive having a response and recovery time around 1 s and 1.5 s,espectively.

The stability of sensor has been checked by measuring the dis-ipation factor. Fig. 8 is a typical plot of dissipation factor withumidity measured at 1 kHz. From the graph it is observe that dissi-ation factor of the sensor increases with rise in humidity. A vapor

o liquid state change happens inside the pores due to condensa-ion. Energy loss due to this change of state is responsible for thencrease of capacitance as well as dissipation factor. It is also toe noted that the high sensitivity exists at 1 kHz with respect to

Fig. 8. Dissipation factor vs. temperature plot of �-alumina thin film sensor at nor-mal pressure.

whole range of humidity [38]. To compare the performance, thedeveloped sensor was calibrated by a using a secondary standardhumidity meter (Model HI8564 Hanna Instrument, USA). Measuredrelative humidity value in our sensor matched well with standardmeter. This typical characteristic is the key factor for selecting �-Al2O3 film as a suitable candidate to be use in high temperatureand high pressure region.

Porous layer of sensors absorbed moisture molecules from theenvironment and condenses inside the pores. Interaction betweenporous structure and moisture depends on the diameter of thepores and molecular dimensions. The dimensions of the vapormolecules must be less than the Kelvin radius [39]. Kelvin equa-tion is valid when average pore diameter ≥ 8-10 nm. Non-uniformpores lead to difference in absorption and desorption process whichproduces hysteresis. The pores of prepared �-Al2O3 are almost uni-form and average diameter is more than 12 nm hence we did notobserve any hysteresis. The capacitance of the layer changes dueto the difference in permittivity as a function of vapour uptake and

depends on ambient vapour concentrations [25,40]. The sensitivity,linearity, response and recovery time depend on pore morphol-ogy, thickness of the film and geometry of the electrode contacts.

D. Saha et al. / Sensors and Actuators B 222 (2016) 1043–1049 1047

Fig. 9. Relative humidity (%) vs. capacitance (pF) with different frequencies.

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Fig. 11. Photograph of six ready to use sensors device.

Fig. 10. Block diagram of electronic circuit used in sensor device.

ypical combination of micro and nano pores in the electrode and �-lumina films, respectively, could play important role for achievingigh and ultrafast sensitivity.

As porous nature plays the important role, we have also per-ormed nitrogen sorption analysis of the �-Al2O3 film. We havestimated the average pore radius of the sample using N2 adsorp-ion/desorption isotherms which is found to be in nanometer range.his is in good agreement with FESEM study which indicates theresence of nano pores. The surface areas of the porous film werestimated using the Brunauer–Emmett–Teller (BET) method andound to be 184 m2 g−1 and the pore volume is around 0.45 cc/g.

We have measure capacitance (pF) with respect to relativeumidity (%) for different frequencies using a typical sensor (no.) and presented in Fig. 9. It clearly delineates that in high fre-uency range, 50 kHz and 100 kHz sensitivity is very poor which isot suitable for device application. For 500 Hz and 1 kHz sensitivity

s pretty high which is acceptable. However, for 500 Hz responses very nonlinear, which is not acceptable for circuit developmentecause noise will be very high. It is observed that only for 1 kHzensitivity is almost linear which can be utilized for device purpose.

.3. Performance of device

We have developed complete sensor device by attachingequired electronic circuit in the sensor module (Fig. 6(a)). Achematic of the electronic circuit is presented in Fig. 10 in theorm of block diagram for better understanding. Measured capaci-ance has been converted to electrical dc signal using a pulse width

odulation (PWM) converter and a RC filter. Further this dc signaleed into a microcontroller (AT89S52) through analog to digital con-erter ((ADC), MCP3202) for RH calculation (as shown in Fig. 10).

he photographs of six such complete sensor devices used to detectrace moisture in typical environment of high temperature and highressure is presented in Fig. 11. Each device is consists of a sensoread (metal part) containing the sensor and a teflon pipe (around

Fig. 12. Experimental setup for sensitivity measurement at high temperature andhigh pressure.

eight inch in length) with electronic circuit attached at the top. Theteflon pipe was used to keep the electronic circuit at a distance toprotect from high temperature.

6 5 4 1 2 3

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Arrangement of six sensors with respect to the crackAlso a special experimental setup was developed indigenously

as delineated in Fig. 12 to check the trace moisture detection capa-bility of our developed sensors in typical environment of hightemperature and high pressure. Super heated steam having 175 ◦Ctemperature and 10 bar pressure has been generated in a tank. Thissuper heated steam was leaked slit by making a slit arrangementin the tank (shown in inset of Fig. 12). Sensitivity response of thesensors toward this super heated steam is presented in Fig. 13. Itcan be observed that saturation was achieved very fast. Here satu-ration (which can be the maximum value of RH at 175 ◦C and 1 barpressure) corresponds to around 10% RH (about 1500 ppm) whichis the actual value of relative humidity at around 175 ◦C. Among the

six sensors five has responded during the high temperature study.The six sensors were placed vertically in the test set-up (as shownin the above sketch). Due to proximity to the slit, sensors 1 and 4

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capability of detecting low humidity, Sens. Actuators B 165 (2012) 7–12.

ig. 13. Sensitivity plot of �-alumina thin film sensor at 165 ◦C and 1 atmosphereressure.

ense quickly but sensor no. 2 and 5 less sensitive and no. 6 doesot sense at all.

It is to be noted here that below 100 ◦C it is possible to achieve00% relative humidity. However; above 100 ◦C (in the super heatedrea) the maximum possible relative humidity plunges rapidly andome down to ppm level as the temperature increases [41]. Pureteam (100 ◦C) will register only around 20% on the relative humid-ty scale at 150 ◦C and around 8–9% (around 1335 ppm) at 175 ◦C at

bar pressure [41]. Interestingly, our developed sensors can detecthis low ppm moisture very efficiently and saturates less than 5 shich is quite remarkable. We have utilized same sensor materialreviously to measure trace moisture (ppm level) in transformeril [42]. These sensors are also re-useable if washes with isopropyllcohol (IPA) which is not possible for polymer based compositeensors.

At higher temperature range, though mechanism is same butondensation happens due to different reason. The temperaturend pressure of the super heated steam are 175 ◦C and 10 barespectively (in our experiment), which is higher than the tem-erature (165 ◦C) and pressure (1 atmosphere) of the place whereensors were kept. Because of this difference of temperature andressure water molecules condensed in the pores of sensor elementhen super heated steam is leaked through the slit.

Conventionally radioactive radiation is used in nuclear plant toetect LBB in reactor pipes where superheated steam comes outrom 10 bar pressure to 1 atmosphere if any crack develops in theine [27]. As demonstrated experimentally, our developed sensor

ill be capable enough to detect this type of crack within few sec-nd and can save any nuclear plant in case of any leak. This deviceas tested in nuclear plant at Bhaba Atomic Research Centre in

ndia and gives satisfactory result. Our developed sensors demon-trate the capability to be a good replacement of use of radiationor LBB detection in future.

. Conclusion

�-Al2O3 based nanoporous thin film sensors were successfullyrepared using sol-gel followed by dip coating technique. Sensorsemonstrate a quasi-linear response with humidity and maximum

inearity was obtained at 1 kHz. Also, the response was very sharpnd enhanced at high temperature without any drift over a longemperature range. The reproducibility of the sensors was cross

hecked with SHAW dew point meter (UK) in trace moisture rangend % relative humidity below 100 ◦C was checked using a humidityeter (Hanna Instrument, Model no. HI 8564). Our fabricated sen-

ors based on �-Al2O3 nanoporous thin film were highly sensitive

[

rs B 222 (2016) 1043–1049

and capable to detect presence of trace moisture at high temper-ature (165 ◦C) and high pressure. These sensors are very suitablefor application to detect leak before break in nuclear plant and intea processing industries to detect trace moisture during drying athigh temperature.

Acknowledgements

The authors are thankful to Board of Research in NuclearSciences (BRNS), Department of Atomic Energy, Government ofIndia for financial support. Technical help from Dr. Sandip Bysakhof CSIR-CGCRI in doing the AFM highly acknowledged. We alsoacknowledge Prof. S. Das of Heat Power Laboratory of JadavpurUniversity for help in conducting the test.

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Biographies

Debdulal Saha received the Ph.D. degree from JadavpurUniversity, Kolkata, India, in 2010. He is currently a Scien-tist at CSIR-Central Glass and Ceramic Research Institute,Kolkata, India. His research interest is ceramic materialsfor moisture and gas sensor application. He has more thantwenty published paper in peer reviewed journals.

Dilip Kr. Ghara received the M.Sc. degree from JadavpurUniversity, Kolkata, India, in 2004. His research interestis sensor characterization & device fabrication. He hasalready fabricated moisture and LPG leak detection sen-sors. He has several publications paper in peer reviewedjournals.

Mrinal Pal Principal Scientist of Sensor and Actuator divi-sion at CSIR-Central Glass and Ceramic Research Institute(CSIR-CGCRI), India. He is the recipient of prestigious STAfellowship (regular) of Govt. of Japan. He has researchexperience of more than two decades and thrust areais design and advanced applications of nanomaterials inthe field of gas sensors, magnetic materials, multiferroics,etc. He has more than 80 peer-reviewed research publica-tions in the field of materials science and nanotechnology.He has contributed several book chapters on advancedmaterials and technology for esteemed publishers in USAand India. He has guided several students for their Ph.D.

tific agencies and member of panels of experts to various Ph.D. committees, P.G.Board of studies. He has been invited for keynote and invited talks in many nationaland international conferences. Dr. Pal has been actively involved in wide nationaland international collaboration in different area of his interest.