Morphological and Relative Humidity Sensing ...

Sensors & Transducers Volume 122, Issue 11, November 2010

www.sensorsportal.com

ISSN 1726-5479

Editors-in-Chief: professor Sergey Y. Yurish, tel.: +34 696067716, fax: +34 93 4011989, e-mail: [email protected] Editors for Western Europe Meijer, Gerard C.M., Delft University of Technology, The Netherlands Ferrari, Vittorio, Universitá di Brescia, Italy Editor South America Costa-Felix, Rodrigo, Inmetro, Brazil

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Kumar, Subodh, National Physical Laboratory, India Kung, Chih-Hsien, Chang-Jung Christian University, Taiwan Lacnjevac, Caslav, University of Belgrade, Serbia Lay-Ekuakille, Aime, University of Lecce, Italy Lee, Jang Myung, Pusan National University, Korea South Lee, Jun Su, Amkor Technology, Inc. South Korea Lei, Hua, National Starch and Chemical Company, USA Li, Genxi, Nanjing University, China Li, Hui, Shanghai Jiaotong University, China Li, Xian-Fang, Central South University, China Liang, Yuanchang, University of Washington, USA Liawruangrath, Saisunee, Chiang Mai University, Thailand Liew, Kim Meow, City University of Hong Kong, Hong Kong Lin, Hermann, National Kaohsiung University, Taiwan Lin, Paul, Cleveland State University, USA Linderholm, Pontus, EPFL - Microsystems Laboratory, Switzerland Liu, Aihua, University of Oklahoma, USA Liu Changgeng, Louisiana State University, USA Liu, Cheng-Hsien, National Tsing Hua University, Taiwan Liu, Songqin, Southeast University, China Lodeiro, Carlos, University of Vigo, Spain Lorenzo, Maria Encarnacio, Universidad Autonoma de Madrid, Spain Lukaszewicz, Jerzy Pawel, Nicholas Copernicus University, Poland Ma, Zhanfang, Northeast Normal University, China Majstorovic, Vidosav, University of Belgrade, Serbia Marquez, Alfredo, Centro de Investigacion en Materiales Avanzados, Mexico Matay, Ladislav, Slovak Academy of Sciences, Slovakia Mathur, Prafull, National Physical Laboratory, India Maurya, D.K., Institute of Materials Research and Engineering, Singapore Mekid, Samir, University of Manchester, UK Melnyk, Ivan, Photon Control Inc., Canada Mendes, Paulo, University of Minho, Portugal Mennell, Julie, Northumbria University, UK Mi, Bin, Boston Scientific Corporation, USA Minas, Graca, University of Minho, Portugal Moghavvemi, Mahmoud, University of Malaya, Malaysia Mohammadi, Mohammad-Reza, University of Cambridge, UK Molina Flores, Esteban, Benemérita Universidad Autónoma de Puebla, Mexico Moradi, Majid, University of Kerman, Iran Morello, Rosario, University "Mediterranea" of Reggio Calabria, Italy Mounir, Ben Ali, University of Sousse, Tunisia Mulla, Imtiaz Sirajuddin, National Chemical Laboratory, Pune, India Nabok, Aleksey, Sheffield Hallam University, UK Neelamegam, Periasamy, Sastra Deemed University, India Neshkova, Milka, Bulgarian Academy of Sciences, Bulgaria Oberhammer, Joachim, Royal Institute of Technology, Sweden Ould Lahoucine, Cherif, University of Guelma, Algeria Pamidighanta, Sayanu, Bharat Electronics Limited (BEL), India Pan, Jisheng, Institute of Materials Research & Engineering, Singapore Park, Joon-Shik, Korea Electronics Technology Institute, Korea South Penza, Michele, ENEA C.R., Italy Pereira, Jose Miguel, Instituto Politecnico de Setebal, Portugal Petsev, Dimiter, University of New Mexico, USA Pogacnik, Lea, University of Ljubljana, Slovenia Post, Michael, National Research Council, Canada Prance, Robert, University of Sussex, UK Prasad, Ambika, Gulbarga University, India Prateepasen, Asa, Kingmoungut's University of Technology, Thailand Pullini, Daniele, Centro Ricerche FIAT, Italy Pumera, Martin, National Institute for Materials Science, Japan Radhakrishnan, S. National Chemical Laboratory, Pune, India Rajanna, K., Indian Institute of Science, India Ramadan, Qasem, Institute of Microelectronics, Singapore Rao, Basuthkar, Tata Inst. of Fundamental Research, India Raoof, Kosai, Joseph Fourier University of Grenoble, France Reig, Candid, University of Valencia, Spain Restivo, Maria Teresa, University of Porto, Portugal Robert, Michel, University Henri Poincare, France Rezazadeh, Ghader, Urmia University, Iran Royo, Santiago, Universitat Politecnica de Catalunya, Spain Rodriguez, Angel, Universidad Politecnica de Cataluna, Spain Rothberg, Steve, Loughborough University, UK Sadana, Ajit, University of Mississippi, USA Sadeghian Marnani, Hamed, TU Delft, The Netherlands Sandacci, Serghei, Sensor Technology Ltd., UK Schneider, John K., Ultra-Scan Corporation, USA Sengupta, Deepak, Advance Bio-Photonics, India Shah, Kriyang, La Trobe University, Australia Sapozhnikova, Ksenia, D.I.Mendeleyev Institute for Metrology, Russia Saxena, Vibha, Bhbha Atomic Research Centre, Mumbai, India

Seif, Selemani, Alabama A & M University, USA Seifter, Achim, Los Alamos National Laboratory, USA Silva Girao, Pedro, Technical University of Lisbon, Portugal Singh, V. R., National Physical Laboratory, India Slomovitz, Daniel, UTE, Uruguay Smith, Martin, Open University, UK Soleymanpour, Ahmad, Damghan Basic Science University, Iran Somani, Prakash R., Centre for Materials for Electronics Technol., India Srinivas, Talabattula, Indian Institute of Science, Bangalore, India Srivastava, Arvind K., Northwestern University, USA Stefan-van Staden, Raluca-Ioana, University of Pretoria, South Africa Sumriddetchka, Sarun, National Electronics and Computer Technology Center, Thailand Sun, Chengliang, Polytechnic University, Hong-Kong Sun, Dongming, Jilin University, China Sun, Junhua, Beijing University of Aeronautics and Astronautics, China Sun, Zhiqiang, Central South University, China Suri, C. Raman, Institute of Microbial Technology, India Sysoev, Victor, Saratov State Technical University, Russia Szewczyk, Roman, Industrial Research Inst. for Automation and Measurement, Poland Tan, Ooi Kiang, Nanyang Technological University, Singapore, Tang, Dianping, Southwest University, China Tang, Jaw-Luen, National Chung Cheng University, Taiwan Teker, Kasif, Frostburg State University, USA Thirunavukkarasu, I., Manipal University Karnataka, India Thumbavanam Pad, Kartik, Carnegie Mellon University, USA Tian, Gui Yun, University of Newcastle, UK Tsiantos, Vassilios, Technological Educational Institute of Kaval, Greece Tsigara, Anna, National Hellenic Research Foundation, Greece Twomey, Karen, University College Cork, Ireland Valente, Antonio, University, Vila Real, - U.T.A.D., Portugal Vanga, Raghav Rao, Summit Technology Services, Inc., USA Vaseashta, Ashok, Marshall University, USA Vazquez, Carmen, Carlos III University in Madrid, Spain Vieira, Manuela, Instituto Superior de Engenharia de Lisboa, Portugal Vigna, Benedetto, STMicroelectronics, Italy Vrba, Radimir, Brno University of Technology, Czech Republic Wandelt, Barbara, Technical University of Lodz, Poland Wang, Jiangping, Xi'an Shiyou University, China Wang, Kedong, Beihang University, China Wang, Liang, Pacific Northwest National Laboratory, USA Wang, Mi, University of Leeds, UK Wang, Shinn-Fwu, Ching Yun University, Taiwan Wang, Wei-Chih, University of Washington, USA Wang, Wensheng, University of Pennsylvania, USA Watson, Steven, Center for NanoSpace Technologies Inc., USA Weiping, Yan, Dalian University of Technology, China Wells, Stephen, Southern Company Services, USA Wolkenberg, Andrzej, Institute of Electron Technology, Poland Woods, R. Clive, Louisiana State University, USA Wu, DerHo, National Pingtung Univ. of Science and Technology, Taiwan Wu, Zhaoyang, Hunan University, China Xiu Tao, Ge, Chuzhou University, China Xu, Lisheng, The Chinese University of Hong Kong, Hong Kong Xu, Tao, University of California, Irvine, USA Yang, Dongfang, National Research Council, Canada Yang, Shuang-Hua, Loughborough University, UK Yang, Wuqiang, The University of Manchester, UK Yang, Xiaoling, University of Georgia, Athens, GA, USA Yaping Dan, Harvard University, USA Ymeti, Aurel, University of Twente, Netherland Yong Zhao, Northeastern University, China Yu, Haihu, Wuhan University of Technology, China Yuan, Yong, Massey University, New Zealand Yufera Garcia, Alberto, Seville University, Spain Zakaria, Zulkarnay, University Malaysia Perlis, Malaysia Zagnoni, Michele, University of Southampton, UK Zamani, Cyrus, Universitat de Barcelona, Spain Zeni, Luigi, Second University of Naples, Italy Zhang, Minglong, Shanghai University, China Zhang, Qintao, University of California at Berkeley, USA Zhang, Weiping, Shanghai Jiao Tong University, China Zhang, Wenming, Shanghai Jiao Tong University, China Zhang, Xueji, World Precision Instruments, Inc., USA Zhong, Haoxiang, Henan Normal University, China Zhu, Qing, Fujifilm Dimatix, Inc., USA Zorzano, Luis, Universidad de La Rioja, Spain Zourob, Mohammed, University of Cambridge, UK

Sensors & Transducers Journal (ISSN 1726-5479) is a peer review international journal published monthly online by International Frequency Sensor Association (IFSA). Available in electronic and on CD. Copyright © 2010 by International Frequency Sensor Association. All rights reserved.

Sensors & Transducers Journal

Contents Volume 122 Issue 11 November 2010

www.sensorsportal.com

ISSN 1726-5479

Research Articles New Theoretical Results for High Diffusive Nanosensors Based on ZnO Oxides Paolo Di Sia ........................................................................................................................................

1

Morphological and Relative Humidity Sensing Properties of Pure ZnO Nanomaterial N. K. Pandey, Karunesh Tiwari ..........................................................................................................

9

Humidity Sensing Behaviour of Nanocrystalline α-PbO Synthesized by Alcohol Thermal Process Sk. Khadeer Pasha, L. John Kennedy, J. Judith Vijaya, K. Chidambaram........................................

20

Electrochemical Detection of Mn(II) and Cd(II) Mediated by Carbon Nanotubes and Carbon Nanotubes/Li+ Modified Glassy Carbon Electrode Muhammed M. Radhi, Wee T. Tan, Mohamad Z. AbRahman and Anuar Kassim ............................

28

Engineering of Highly Susceptible Paramagnetic Nanostructures of Gd2S3:Eu3+: Potentially an Efficient Material for Room Temperature Gas Sensing Applications Ranu K. Dutta, Prashant K. Sharma and Avinash C. Pandey.............................................................

36

Synthesis and Physical Properties of Nanocomposites (SnO2)x(In2O3)1-x (x = 0 – 1) for Gas Sensors and Optoelectronics Stanislav Rembeza, Pavel Voronov, Ekaterina Rembeza .................................................................

46

Structural, Electrical Properties of Nanocrystalline Co Doped- LaxCe1-XO2 for Gas Sensing Applications A. B. Bodade, Minaz Alvi, N. N. Gedam, H. G. Wankhade and G. N. Chaudhari..............................

55

Studies of CNT and Polymer Based Gas Sensor Monika Joshi and R. P Singh .............................................................................................................

66

Excess Noises in (Bio-)Chemical Nanoscale Sensors Ferdinand Gasparyan.........................................................................................................................

72

Catechol Biosensor Based on Gold Nanoparticle Modified Tetrabutylammoniumtetrafluoroborate Doped Polythiophene Films Suman Singh, S. Praveen Kumar, D. V. S. Jain, M. L. Singla ...........................................................

85

Electromechanical TiO2 Nanogenerators Valerio Dallacasa, Filippo Dallacasa ..................................................................................................

102

Lead Oxide- PbO Humidity Sensor Sk. KhadeerPasha, K. Chidambaram, L. John Kennedy, J. JudithVijaya..........................................

113

Ammonia Gas Sensing Properties of Nanocrystalline Zn1-xCuxFe2O4 Doped with Noble Metal S. V. Jagtap, A. V. Kadu, N. N. Gedam and G. N. Chaudhari ...........................................................

120

LPG and NH3 Sensing Properties of SnO2 Thick Film Resistors Prepared by Screen Printing Technique A. S. Garde .........................................................................................................................................

128

Effect of Ni Doping on Gas Sensing Performance of ZnO Thick Film Resistor M. K. Deore, V. B. Gaikwad, R. L. Patil, N. K. Pawar, S. D. Shinde, G. H. Jain ................................

143

Fabrication and Analysis of Tapered Tip Silicon Microneedles for MEMS based Drug Delivery System Muhammad Waseem Ashraf, Shahzadi Tayyaba, Nitin Afzulpurkar, Asim Nisar..............................

158

Authors are encouraged to submit article in MS Word (doc) and Acrobat (pdf) formats by e-mail: [email protected] Please visit journal’s webpage with preparation instructions: http://www.sensorsportal.com/HTML/DIGEST/Submition.htm International Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol. 122, Issue 11, November 2010, pp. 9-19

Sensors & Transducers ISSN 1726-5479 © 2010 by IFSA http://www.sensorsportal.com

Morphological and Relative Humidity Sensing Properties of Pure ZnO Nanomaterial 1

N. K. Pandey, 2Karunesh Tiwari

Sensors and Materials Research Laboratory, Department of Physics University of Lucknow, Lucknow, 226007, U. P. India Tel.: 1 +91 9415003998, 2 +091 9451037737 E-mail: [email protected], [email protected]

Received: 30 March 2010 /Accepted: 18 November 2010 /Published: 30 November 2010 Abstract: In this paper we report the resistive type humidity sensing properties of pure ZnO nanomaterial prepared by solid-state reaction method. Pellets of pure ZnO nanocrystalline powder have been made with 10 weight % of glass powder at pressure of 260 MPa by hydraulic press machine for 3 hours. These pellets have been sintered at temperatures 200 °C - 500 °C in an electric muffle furnace for 3 hours at heating rate of 5°C/min. After sintering, these pellets have been exposed to humidity in a specially designed humidity chamber at room temperature. It has been observed that as relative humidity increases, resistance of the pellets decreases for entire range of humidity i.e. 10 % to 90 %. The sensing element of ZnO shows best results with sensitivity of 11.13 MΩ/%RH for the annealing temperature of 400 °C. This sensing element manifests lower hysteresis, less effect of aging and high reproducibility for annealing temperature 400 °C. SEM micrographs show that the sensing elements manifest porous structure with a network of pores that are expected to provide sites for humidity adsorption. The average grain size calculated from SEM micrograph is 236 nm. XRD pattern shows peaks of hexagonal zincite. As calculated from Scherer’s formula, the average crystalline size for this sensing element is 59.4 nm. For this sensing element, the values of activation energy from the Arrhenius plot is 0.041 eV for temperature range 200 °C - 400 °C and 0.393 eV for temperature range 400 °C - 500 °C. The adsorption of water molecules on the surface takes place via a dissociative chemisorption process leading to release of electrons. ZnO has electron vacancy. Hence, because of this reaction, the electrons are accumulated at the ZnO surface and consequently, the resistance of the sensing element decreases with increase in relative humidity. Copyright © 2010 IFSA. Keywords: Zinc oxide, Electrical properties, Humidity sensor, Annealing.

9


1. Introduction The use of humidity control systems has greatly increased in quality control of production processes. Metal oxides have shown advantages in terms of their resistance to chemical attack and their thermal and physical stability. These materials possess a unique structure consisting of grains, grain boundaries surfaces and pores, which make them suitable for this kind of sensors. The working mechanism of metal oxide sensors lies in change of the resistance of the sensing element from chemisorption, physisoption and catalytic reaction on exposure to a stimuli like humidity [1-2]. Humidity sensors not only can be used to test humidity in some atmosphere but also to automatically control humidifiers, dehumidifiers and air-conditioner so as to adjust to a required humidity value. Humidity sensing materials include ceramics, composites and polymers, each of which has its own merits and specific conditions of application [1-3]. Even though the use of sensors is well established in process industries, agriculture, medicine, and many other areas, the development of new sensing materials with high sensing capabilities is proceeding at an unprecedented rate. Numerous materials have been utilized for humidity sensing, of which the metal oxides that are physically and chemically stable have been extensively investigated at both room and elevated temperatures [4-6]. Sensors based on changes in resistance and capacitance is preferred owing to their compact size, which could facilitate miniaturization required for electronic circuitry. Of these, a disc type sensor would offer higher sensitivity towards humidity than a thin film type because of its larger capacity towards water chemisorptions. Materials based on ZnO are well studied for electrical and sensing applications [7-9]. Metal chromates have also been reported to be potential materials from a technological point of view [10]. In recent years, ceramic humidity sensors based on porous metal oxides compared with other materials have received much attention due to their chemical and physical stability [11]. In the present research paper, the investigation on pure ZnO material, operating in the dc mode as humidity sensor, is reported.

2. Sample Preparation ZnO (Qualigens, 99.9 % pure) with 10 weight % of glass binder have been mixed uniformly for half an hour. Glass powder has been added as binder to increase the strength of the sample. The powders have been pressed in pellet shape by uniaxially applying pressure of 260 MPa in hydraulic press machine at room temperature. The samples are having diameter of 8 mm and thickness 4mm. The samples have been sintered in air at temperature of 200 ˚C, 300 ˚C, 400 ˚C and 500 ˚C for 3 hours in an electric muffle furnace (Ambassador, India).

3. Sample Characterization 3.1. XRD Study X-Ray diffraction has been studied using XPERT PRO-Analytical XRD system (Netherlands). The wavelength of the source used is 1.54060 Å. Fig. 1 shows X-ray pattern for the sensing element of pure ZnO annealed at 500 ˚C. The pattern shows extent of crystallization of the sensing element in the form of powder. The average crystallite size for the sensing elements of pure ZnO calculated from Scherer’s formula is 59.4 nm. XRD pattern manifests presence of large number of peaks of ZnO (zincite).

10


Fig. 1. X-Ray Diffraction pattern of sensing element of pure ZnO for annealing temperature 500 ˚C.

3.2. Scanning Electron Micrograph (SEM) Study The study of surface morphology of sensing elements has been carried out using Scanning Electron Microscope (LEO-430, Cambridge, England). Fig. 2 shows SEM micrograph of nanomaterial of sensing elements of pure ZnO, annealed at temperature 500 ˚C. The micrograph has been taken for 30.00 Kx magnification. SEM micrograph shows that the sensing element manifests porous structure having granulation and tendency to agglomerate. Scanning micrograph shows flakes of ZnO scattered throughout forming a network of pores that are expected to provide sites for humidity adsorption. The grain size for the sensing elements of pure ZnO calculated from SEM micrographs is 236 nm. Values of crystallite size and grain size have been shown in Table 1.

Fig. 2. SEM Micrograph of sensing element of pure ZnO for annealing temperature 500 ºC.

11


Table 1. Values of crystallite and grain size for the sensing element of ZnO. Sensing element of Pure ZnO

Crystallite size by XRD 59.4 nm

Grain size by SEM 236 nm

3.3. Kelvin Radius The Kelvin radius at 20 ˚C room temperature has been calculated [8]. The value has been found to be in the range of 4.6 Ả to 100 Ả for relative humidity from 10 to 90 % RH. The Kelvin radius is given by the formula: rk = 2γM / ρRT ln(ρs/ρ)

(1)

Here γ is the surface tension of water, M is the molecular weight of water, ρ is the density of water and ρs/ρ is the relative humidity. Take values, γ = 0.0756 Nm-1 (at 20 ˚C), M = 18.02 Kg., ρ =999.97 Kgm-3. Values of Kelvin radii have been calculated in accordance with equation (1) at ambient temperature of 20 ˚C. Variation of Kelvin radius with % RH has been shown in Fig. 3.

Fig. 3. Variation of Kelvin radius with change in % RH.

3.4. Activation Energy For an n-type semiconductor, the variation of resistance with temperature is given by the relation [12]: σ = σo e–E/kt

(2)

Here, σo = 2μe[2πm* (kT/ħ2)]3/2, E is the activation energy of electronic transport, m* is the effective mass, μe is the mobility of electron, e = electronic charge, k = Boltzmann constant, ħ=h/2π, where h is the Planck’s constant. Equation (2) in terms of resistance may be written as R = Ro e–E/kt

(3)

ln (R) = ln(Ro) – E⁄kT

(4)

12


From the slope of the plot log(R) versus 1⁄T values of activation energy for electronic transport may be calculated. Fig. 4 shows graph between log(R) and 1000⁄T. The graph shows two slopes corresponding to two different regions (region I for the range of temperature 200-400 ˚C and region II for the range of temperature 400-500 ˚C), indicating that there are two different inter-band transitions for electrons. The values of activation energy calculated are shown in Table 2. Sample of pure ZnO shows lower values of activation energies in the two regions indicating that this sensing element has lower operating temperatures and may be used at room temperature as well (kT≈0.023 eV at room temperature).

Fig. 4. Variation of log R with 1000/T. Table 2. Activation energy for sensing element of pure ZnO from Arrhenius graph. Activation Energy Range of Temperature

Pure ZnO Sample

Region I (200 ⁰C-400 ⁰C)

0.041 eV

Region II (400 ⁰C-500 ⁰C)

0.393 eV

4. Principle of Operation for Humidity Sensing As dry oxides of pure ZnO nanomaterial are brought in contact with humid air, water molecules chemisorb on the available sites of the oxide surface. The adsorption of water molecules on the surface takes place via a dissociative chemisorption process which may be described in a two-step process as given below: (i) Water molecules adsorbed on grain surface reacts with the lattice A (A→ Zn or Ti) as H2O + Oo + A ↔ 2OH–A + Vo + 2e− , 13


where Oo represents the lattice oxygen and Vo is the vacancy created at the oxygen site according to the reaction. Oo↔ O2− + Vo (ii) Doubly ionized oxygen, displaced from the lattice, reacts with the H+ coming from the dissociation of water molecules to form a hydroxyl group as given below: H+ + O2− ↔ OH− Because of this reaction, the electrons are accumulated at the ZnO surface and consequently, the resistance of the sensing element decreases with increase in relative humidity. This chemisorbed layer, once formed is not further affected by exposure to humidity, but it can be thermally desorbed. Once the first layer is formed, subsequent layers of water molecules are physically adsorbed. The first layer of physisorbed water molecules is characterized by double hydrogen bonding of a single water molecule. The physisorption changes from monolayer to multilayer as the water vapor pressure increases. Water molecules in the succeeding physisorbed layers are only singly bonded and form a liquid like network. The physisorbed water dissociates due to high electrostatic field in the chemisorbed layer by the following mechanism: 2H2O ↔ H3O+ + OH− The hydronium ion (H3O+) releases a proton to a neighboring water molecule according to the following dissociation: H3O+→H2O + H+ Charge transport occurs when neighboring water molecule accepts this proton while releasing another proton, and so on. This proton moves freely in water and is responsible for conduction in the sensing element which in turn determines the sensitivity of the sensing element. This is known as Grotthuss chain reaction [13-14]. The sensing material is highly porous, hence it provides large surface area for adsorption. Since ZnO have electron vacancies, they attract H+ ions released due to dissociation of water molecule.

5. Experimental After sintering, variation in resistance is recorded with change in % RH in a humidity control cabinet at room temperature. The resistance of the pellet has been measured normal to the cross-section of the pellet using copper electrode. Standard solution of potassium sulphate has been used as humidifier and potassium hydroxide as de-humidifier. Inside the humidity chamber, a thermometer (±1 °C) and standard hygrometer (Huger, Germany, ±1 % RH) are placed for the purpose of calibration. Relative humidity has been measured using the standard hygrometer. Variation in resistance of the pellets has been recorded using a resistance meter (Sino meter, ±0.001 MΩ, model: VC-9808). Copper electrode has been used to measure the resistance of the pellet. To see the effect of ageing, the samples have again been exposed to humidity after six months and variation in the value of resistance recorded with change in the value of relative humidity. The stability of the sensing element has been checked by keeping the sensing element at fixed values of % RH in the chamber and the values of resistance is recorded as a function of time [6]. The values have been found to be stable within ±3 % of the measured values. 14


6. Results and Discussion Fig. 5 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in relative humidity after the sample has been annealed at 200 °C. This figure shows the graphs both for the increasing cycle of relative humidity and also for decreasing cycle of relative humidity. In increasing cycle the resistance is found to decrease by 23.6 % for increase in relative humidity from 10 to 50 % RH. In decreasing cycle the resistance is found to decrease by 37.7% in this range of 10 to 50 % RH. In the range of 50 to 90 % RH resistance decreases by 34.2 % and 18.6 % for increasing and decreasing cycles respectively. The sensitivity in the range of 10 to 50 % RH is 8.20 MΩ/%RH and 9.00 MΩ/%RH in the range of 50 to 90 % RH. Here the sensitivity of humidity sensor has been defined as the change in resistance (ΔR) of sensing element per unit change in relative humidity (RH %). For calculation of sensitivity, the humidity from 10 to 90 % RH has been divided in equal intervals of 5 % RH each. Difference in the value of the resistance for each of this interval has been calculated and then divided by 5. The average has been taken for all these calculated values. Formula for calculation of sensitivity of the sensing elements may be written as given below: Sensitivity = (∆R)/(∆%RH)

(5)

Fig. 6 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in relative humidity after the sample has been annealed at 300 °C. In the increasing cycle of relative humidity, the resistance is found to decrease by 24.3 % for increase in relative humidity from 10 to 50 % RH. In decreasing cycle of relative humidity, the resistance decreases by 33.4 % in this range of 10 to 50 % RH. In the range of 50 to 90 % RH resistance decreases by 51.5 % and 43 % respectively for increasing and decreasing cycle of relative humidity. The sensitivity in the range of 10 to 50 % RH is 7.8 MΩ/%RH and it rises to 12.6 MΩ/%RH in the range of 50 to 90 % RH.

Fig. 5. Variation of resistance with % RH for sensing element of pure ZnO annealed at 200 ˚C.


Fig. 7 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in the value of the relative humidity after the sample has been annealed at 400 °C. Fig. 7 shows the graphs both for the increasing and decreasing cycles of relative humidity. In the increasing cycle of relative humidity, the resistance is found to decrease by 38.6 % for increase in relative humidity from 10 to 50 % RH. In decreasing cycle, the resistance decreases by 44.4 % in this range of 10 to 50 % RH. In the range of 50 to 90 % RH resistance decreases by 44.0 % and 34.7 % respectively for increasing 15


and decreasing cycle of relative humidity. The sensitivity is the range of 10 to 50 % RH is 13.0 MΩ /%RH and 9.15 MΩ /%RH in the range of 50 to 90 % RH. Fig. 8 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in relative humidity after the sample has been annealed at 500 °C. This Fig. 8 shows the graphs both for the increasing and decreasing cycles of relative humidity. In the increasing cycle of relative humidity, the resistance is found to decrease by 62.2 % for increase in relative humidity from 10 to 50 % RH. In the decreasing cycle, the resistance decreases by 32.6 % in this range of 10 to 50 % RH. In the range of 50 to 90 % RH resistance decreases by 77.6% and 87.7 % respectively for increasing and decreasing cycle of relative humidity. The sensitivity is the range of 10 to 50 % RH is 17.5 MΩ /%RH and 8.25 MΩ /%RH in the range of 50 to 90 % RH. Table 3 shows values of sensitivity in the range of relative humidity from 10 to 90% RH. Column a in this table gives the values of sensitivity for the increasing cycle of % RH from 10 to 90% RH and column b gives corresponding values for the decreasing cycle of % RH.



Table 3. Values of sensitivity for ZnO sample in the range of relative humidity from 10 to 90 % RH. Sensitivity (MΩ/%RH)

a

Annealing Temp.

(a)

(b)

(c)

200 ⁰C 300 ⁰C 400 ⁰C 500 ⁰C

8.63 10.26 11.13 12.89

8.49 9.78 10.25 13.25

9.08 10.23 10.25 13.25

increasing cycle of % RH; bdecreasing cycle of % RH; cincreasing cycle of % RH after six months.

Fig. 9 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in the value of the relative humidity for the experiment performed after six months for the annealing temperature 200 ˚C. The graph marked ‘original’ in the Fig. 9 is for an initial cycle of increasing relative humidity and the graph marked ‘repeated after 6 months’ is for the increasing cycle of the relative humidity corresponding to the same experiment repeated after six months. For the experiment performed after 6 months, the sensitivity in the range of 10 to 50 % RH is 7.3 MΩ /%RH 16


and the value of sensitivity rises to 10.8 MΩ/%RH in the range 50 to 90 % RH. It is found that variations in both the range (i.e. 10 to 50 % and 50 to 90 % RH) are nearly same after six months. It is seen that both the graphs are showing almost similar behavior in the entire range of relative humidity from 10-90 % RH. Fig. 10 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in relative humidity for the annealing temperature 300 ˚C. For the experiment repeated after six months, the sensitivity in the range of 10 to 50 % RH is 8.30 MΩ/%RH and the value of sensitivity rises to 12.10 MΩ/%RH in the range 50 to 90 % RH. Thus when observations have been repeated after six months, it is found that variations in both the ranges (i.e. 10 to 50 % and 50 to 90 % RH) are nearly same.

Fig. 9. Variation of resistance with RH % for sensing element of pure ZnO after six months for annealing temperature 200 ˚C.


Fig. 11 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in relative humidity for the annealing temperature 400 ˚C. For the experiment repeated after six months, the sensitivity in the range of 10-50 % RH is 14.7 MΩ/%RH and the value of sensitivity becomes 8.40 MΩ/%RH in the range 50 to 90 % RH. Fig. 12 is the graph between the variation of the resistance of the sensing element of pure ZnO with change in the value of the relative humidity for the annealing temperature 500 ˚C. For the experiment repeated after six months, the sensitivity in this range of 10-50 % RH is 15.4 MΩ/%RH and the value of sensitivity becomes 9.35 MΩ/%RH in the range 50 to 90 % RH. It is found that variations in both the ranges (i.e. 10 to 50 % and 50 to 90 % RH) are nearly same after six months. Column c in Table 3 shows the values of sensitivity of the sensing element of pure ZnO for different annealing temperatures for the experiment repeated after six months. The values of sensitivity in this table has been shown for the relative humidity range 10 to 90 % RH. Along side in the same table, as mentioned earlier, the initial values of sensitivity have also been shown for the increasing and decreasing cycles of relative humidity from 10 to 90 % RH in columns a and b respectively.

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All the sensing elements show large decrease in the values of the resistance for initial values of the relative humidity from 10 to 50 % RH. It is understood that the large decrease in the value of resistance or increase in the conductivity of the sample with relative humidity is due to the adsorption of water molecules on the pellet surface with capillary nanopores. Higher porosity increases surface to volume ratio of the materials and enhances diffusion rate of water into or out off the porous structures and thus helps in getting good sensitivity. At high relative humidity (>50 %), liquid water condenses in the capillary like pores, forming a liquid like layer. A further rise in electrical conductivity occurs due to electrolytic conduction in addition to the protonic transport in the adsorbed layer.

7. Conclusions As relative humidity increases, there is decrease in the resistance of pellets in the range 10-90 % RH. Sensing element of pure ZnO shows best results with sensitivity value at 11.13 MΩ/%RH in 10-90 % relative humidity range for annealing temperature 400 ˚C. This sensing element shows good reproducibility, low hysteresis and less effect of aging. XRD pattern of this sensing element shows large number of peaks of ZnO. SEM micrographs show that the sensing elements manifest porous structure. The Kelvin radius at 20 ˚C room temperature is in the range of 4.6 to 100 Ả.

Acknowledgements Authors would like to thank the University Grant Commission for providing the Financial Assistance for carrying out the Research work.

References [1]. E. Traversa, Ceramic sensors for humidity detection: the state-of-the-art and future developments, Sens. and Act. B, 23, 1995, pp. 135-156. [2]. B. M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc., 74, 1991, pp. 697-708. [3]. N. Parvatikar, S. Jain, S. V. Bhoraskar, M. V. N. Ambika Prasad, Electrical and humidity sensing properties of polyaniline/WO3 composites, Sens. Actuator B: Chem., 114, 2006, pp. 599-603. 18


[4]. T. Zhang, R. Wang, W. C. Geng, Analysis of dc and ac properties of humidity sensor based on polypyrrole materials, Sens. Actuator B: Chem., 128, 2008, pp. 482-487. [5]. M. T. Wu, H. T. Sun, L. Ping, CuO-doped ZnCr2O4–LiZnVO4, thick film humidity sensors, Sens. Actuators B, 17, 1994, pp. 109—112. [6]. M. K. Jain, M. C. Bhatnagar, G. L. Sharma, Effect of Li+ Doping on ZrO2-TiO2 Humidity, Sensor Sens. Actuators, B. Chem., 55, 1999, pp. 180. [7]. C. N. Xu, K. Miyasaki, T. Watanabe, Humidity sensors using manganese oxides, Sens. Actuators, B. Chem., 46, 1998, pp. 87. [8]. Y. Shimizu, H. Arai, T. Seiyama, Theoretical studies on the impedance-humidity characteristics of ceramic humidity sensors, Sen. & Act. B, 7, 1985, pp. 11-22. [9]. A. M. Edwin Suresh Raj, C. Mallica, O. M. Sridharan, K. S. Nagaraja, Manganese oxide- manganese tungstate composite humidity sensors, Mater. Lett., 53, 2002, pp. 316. [10].Y. Yokomizo, S. UnO, M. Harata, H. Hiraki, K. Yuki, Microstructure and Humidity-Sensitive Properties of ZnCr2O4–LiZnVO4 Ceramic Sensors, Sens. Actuators, 4, 1983, pp. 599. [11].E. Traversa, A. Bearzotti, M. Miyayama, H. Yanagida, Study of the conduction mechanism of La2CuO4— ZnO heterocontacts at different relative humidities, Sens. Actuators, B. Chem., 24–25, 1995, pp. 714. [12].N. K. Pandey, A. Tripathi, K. Tiwari, A. Roy, Relative Humidity Sensing Studies of WO3-ZnO Nanocomposite, Advanced Materials Research Vols., 79-82, 2009, pp. 365-368. [13].N. K. Pandey, A. Tripathi, K. Tiwari, A. Roy, A. Rai, P. Awasthi, A. Mishra, A. Kumar, Morphological and Humidity Sensing Studies of WO3 Mixed With ZnO And TiO2 Powders, Sen. and Trans., 96, 9, 2008, pp. 42-46. [14].B. C. Yadav, N. K. Pandey, A. Srivastava, P. Sharma, water: a unique matter, J. Meas. Sci. Tech., 18, 2007, pp. 260-264. ___________________ 2010 Copyright ©, International Frequency Sensor Association (IFSA). All rights reserved. (http://www.sensorsportal.com)

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