Electrical Conduction and Humidity Sensing Properties of NiCr

Sensors & TransducersVolume 101February 2009www.sensorsportal.com ISSN 1726-5479Editor-in-Chief: professor Sergey Y. 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Sensors & Transducers JournalContentsVolume 101Issue 2February 2009www.sensorsportal.com ISSN 1726-5479Research ArticlesPreliminary Characterization of a Commercial Chiral Stationary Phase as a Selector forChemical Sensor Applications Using a Quartz Crystal MicrobalanceW. J. Buttner, C-L. Lu, V. Perez-Luna, J. R. Stetter and G. K. Webster............................................ 1New Copolymers Containing Charge Carriers for Organic Devices with ITO Films Treatedby UV-Ozone Using High Intensity Discharge LampEmerson Roberto Santos, Fábio Conte Correia, Elvo Calixto Burini Junior, Shu Hui Wang, MarciaAkemi Yamasoe, Pilar Hidalgo, Fernando Josepetti Fonseca, Adnei Melges de Andrade ............... 12Aquaregia and Oxygen Plasma Treatments on Fluorinated Tin Oxide for Assembly ofPLEDs Devices Using OC1C10-PPV as Emissive PolymerEmerson Roberto Santos, Elvo Calixto Burini Junior, Fernando Josepetti Fonseca.......................... 22Electrical Conduction and Humidity Sensing Properties of NiCr 2 O 4 -ZnO-CeO 2 CompositesL. Regina Mary, K. S. Nagaraja.......................................................................................................... 31Humidity and Electrical Sensing Properties of NiCr 2 O 4 –ZnO–MnO 2 CompositesRegina Mary L., Jeyaraj B. and Nagaraja K. S................................................................................... 42Poly (3, 4-Ethylenedioxythiophene) - Poly (4-Styrenesulfonate) for Humidity Sensing UsingInk-jet Printing Technique on Flexible Polyimide SubstrateHee C. Lim, Yew Fong Hor, Yew L. Hor, James L. Zunino III and John F. Federici.......................... 52Cobalt Doped SnO 2 Thick Film Gas Sensors: Conductance and Gas ResponseCharacteristics for LPG and CNG GasV. Kumar, S. K. Srivastava, Kiran Jain............................................................................................... 60Study on Gas Sensing Performance of TiO 2 Screen Printed Thick FilmsC. G. Dighavkar, A. V. Patil and R. Y. Borse...................................................................................... 73Metal Oxides Doped PPY-PVA Blend Thin Films Based Gas SensorD. B. Dupare, M. D. Shirsat and A. S. Aswar..................................................................................... 82Surface Morphology Dependent Copper Sulphide Ammonia Gas Sensor Working at RoomTemperature: Effect of SHI IrradiationRamphal Sharma, Abhay A. Sagade, J. C. Vyas , P. K. Nema, Anil Ghule and Sung-Hwan Han .... 90NO 2 Gas Sensing Properties of Screen Printed ZnO Thick FilmsA. V. Patil, C. G. Dighavkar and R. Y. Borse...................................................................................... 96Loss of Capacitance Ideality in Label-Free Immuno-ChipSandro Carrara, Vijayender Bhalla, Luca Benini, Bruno Samorì ....................................................... 104Development of an Optical Urea Biosensor Using Polypyrrole-polyvinyl Sulphonate FilmH. J. Kharat, K. Datta, P. Ghosh, Mahendra D. Shirsat ..................................................................... 112

Performance Comparison of SPR Sensors Based on Chalcogenide and Silica Glass PrismsRajan Jha and Anuj K. Sharma .......................................................................................................... 123Non-invasive Blood Glucose Quantification Using a Hybrid SensorSundararajan Jayapal, Dr. V. Palanisamy, Sandeep Mandyam ........................................................ 132Authors are encouraged to submit article in MS Word (doc) and Acrobat (pdf) formats by e-mail: editor@sensorsportal.comPlease visit journal’s webpage with preparation instructions: http://www.sensorsportal.com/HTML/DIGEST/Submition.htmInternational Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41Sensors & TransducersISSN 1726-5479© 2009 by IFSAhttp://www.sensorsportal.comElectrical Conduction and Humidity Sensing Propertiesof NiCr 2 O 4 -ZnO-CeO 2 CompositesL. Regina Mary, 1 K. S. NagarajaDepartment of Chemistry, Loyola Institute of Frontier Energy, Loyola CollegeChennai-600 035, Tamil Nadu, India1 Tel.: +91-44-28178200, fax: +91-44-28175566E-mail: dr.ksnagaraja@gmail.comReceived: 31 December 2008 /Accepted: 20 February 2009 /Published: 28 February 2009Abstract: Nickel chromite spinels NiCr 2 O 4 was prepared by mixing nitrates of Cr(III) and Ni(II)solutions with urea as fuel at 350 o C by combustion method. NiCr 2 O 4 -ZnO-CeO 2 (NiCZCe)composites with mole ratios 1:1:1, 2:1:1, 1:2:1, 1:1:2, 1:0:1, 0:1:1 and 1:1:0 were prepared and theirhumidity sensitivity measurements were studied. The samples were sintered at 950 o C for 6h and weresubjected to DC resistance measurements as a function of relative humidity in the range of 5-98 %.The NiCr 2 O 4 -ZnO-CeO 2 112 composite showed a resistance of 9.6 x10 9 Ω at RH 5 % and 7.5x10 6 Ω atRH 98 % with a humidity sensitivity factor of 1273. The 2 mol% Li + doped NiCr 2 O 4 -ZnO-CeO 2 112composite showed an enhanced humidity sensitivity factor of 8471 with a resistance of 3.6x10 9 Ω atRH 5 % and 4.2x10 5 Ω at RH 98 %. The response and recovery time of the highest humidity sensingcomposites were studied. X-ray diffraction, FT-IR spectroscopy, scanning electron microscopicstudies, nitrogen adsorption/desorption isotherm at 77 K and thermoelectric power measurements wereemployed to the composites to study the structural phases, vibrational frequencies, surfacemorphology, SBET surface area of the composites and to find the semiconducting nature of the highesthumidity sensing composite. Copyright © 2009 IFSA.Keywords: Ceramics, Composites, X-ray diffraction, Microporous materials, Electrical, Properties1. IntroductionHumidity sensors have gained increasing applications in industrial processing and environmentalcontrol [1]. For manufacturing highly sophisticated integrated circuits in semiconductor industry,31

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41humidity or moisture levels are constantly monitored in wafer processing. There are many domesticapplications, such as intelligent control of the living environment in buildings, cooking control formicrowave ovens, and intelligent control of laundry etc. In automobile industry, humidity sensors areused in rearwindow defoggers and motor assembly lines. In medical field, humidity sensors are used inrespiratory equipment, sterilizers, incubators, pharmaceutical processing, and biological products. Inagriculture, humidity sensors are used for green-house air-conditioning, plantation protection (dewprevention), soil moisture monitoring, and cereal storage. In general industry, humidity sensors areused for humidity control in chemical gas purification, dryers, ovens, film desiccation, paper andtextile production, and food processing. Oxides with the spinel structure are some of the most studiedcompounds in solid-state sciences due to their wide range of applications. The structure of spinel oxide isresponsible for a variety of interesting properties [2-5].In this paper, an investigation of electrical and humidity sensing properties of a triphasic thick filmcomposites of NiCr 2 O 4 -ZnO-CeO 2 with different mole ratios (1:1:1, 2:1:1, 1:2:1, 1:1:2, 1:0:1, 0:1:1,1:1:0) are presented. From the literature, diphasic oxides in mole ratios or doping oxides in percentagemole ratio has been reported [2, 5]. Triphasic oxides in mole ratios have not been reported so far ashumidity sensors. The variation of the DC resistance as a function of relative humidity was studied.Thick film in the form of pellet is made of compressed powder and therefore can be considered as acollection of tightly packed micro-crystalline grains. When heat-treated or sintered, the grains growtogether into a three-dimensional network of semiconductor surfaces, grain boundaries, and air-filledpores. In the presence of humid air, the conductance of the pellet can be altered in a number of ways.Electrical conductance can occur through internal surface layers of adsorbed water [6], the tiny porescan fill with water as determined by the Kelvin equation [7] leading to electrolytic conduction throughbulk water, or the conductance of the semiconductor itself can change due to an interaction of itssurface energy states with hydroxyl radicals, especially at the grain boundaries [8, 9].2. Experimental Procedure2.1. Sample PreparationAqueous solutions of Cr(NO 3 ) 3 .9H 2 O and Ni(NO 3 ) 2 .6H 2 O of appropriate mole ratio with requiredamount of urea as fuel by single-step combustion reaction at 350 o C yielded voluminous nickelchromite (NiCr 2 O 4 ) powder [10]. The composites of NiCr 2 O 4 -ZnO-CeO 2 (NiCZCe) with the moleratios 1:1:1, 2:1:1, 1:2:1, 1:1:2, 1:0:1, 0:1:1, 1:1:0 (Table 1) were prepared by mixing MnCr 2 O 4 , ZnOand CeO 2 and vibromilling and grinding using acetone and cetyl alcohol [5,11].Table 1. Sample code, humidity sensitivity factor, energy ofactivation of NiCr 2 O 4 -ZnO-CeO 2 composites (* 2 mol% Li + doped)CompositescodeNiCZCeR (Ω)RH 5 %R (Ω)RH 98 % S f E a110 3.5 x10 7 1.4 x10 6 24 0.504101 6.6 x10 8 2.7 x10 7 25 0.401011 7.6 x10 9 8.7 x10 8 9 0.483111 4.9 x10 9 5.8 x10 7 84 0.260211 8.4 x10 7 6.7 x10 5 124 0.197121 3.5 x10 10 5.6 x10 8 63 0.280112 9.6 x10 9 7.5 x10 6 1273 0.167*doped 112 3.6 x10 9 4.2 x10 5 8471 0.15032

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41The composite code NiCZCe 111 represents (Table 1) one mole of NiCr 2 O 4 , one mole of ZnO and onemole of CeO 2 . The resulting mixtures were dried and compacted into cylindrical pellets of about10mm diameter and 5 mm thickness at a pressure of 400 MPa. These pellets were sintered at 850 o Cfor 6h in ambient air atmosphere. These pellets were sintered at 850 o C for 6 h in ambient airatmosphere. The heating rate was 10 o C min -1 up to 600 o C. The pellets were kept at 600 o C for 30 minto remove binder. The heating rate from 600 o C to 950 o C was 10 o C min -1 . The samples were cooleddown at room temperature at the natural cooling rate of the furnace. Lithium was doped in the basecomposite material by adding 2 mol% LiOH.H 2 O while mixing different mole ratio of the oxides. Thesamples were cooled down to room temperature at the natural cooling rate of the furnace. As lithiumwas shown to enhance the sensitivity of humidity sensors [12] the base matrices were doped with2 mol % of LiOH.H 2 O. Since our interest is to prepare a material for humidity sensor, the materialswere used as synthesized.2.2. Characterization and Humidity Sensing StudiesThe DC electrical resistance at different relative humidity levels of the samples in the form of pelletswas determined by a two-probe method [5, 11] as the present work is to measure the changes insurface conductivity as a function of applied field and current. The electrical contacts were made onthe surface of the pellet by means of two thin copper wires affixed with silver paint. Given the highresistivity of the materials under investigation, the potential inaccuracy due to contact resistance isassumed negligible. The pellet was inserted in the middle of the pyrex tube of 5 cm diameter on whichkanthal wire was uniformly wounded externally. The kanthal wire ends were connected to a varian tovary the temperature and a copper- constantan thermocouple kept at the pellet was used to measure thetemperature of the sample. The electrodes were connected to DC power supply and the Keithley6485 picoammeter in series. The temperature dependent conductance experiments in the temperaturerange of 120–300 ºC under ambient conditions were carried out to determine the activation energiesfor electrical conduction of the samples using the linearized form of the expression I = Io exp −Ea/kT ,where I is the current, Ea the activation energy, k the Boltzmann constant and T is the temperature.Controlled humidity environments of 5, 31, 51, 79 and 98 % relative humidity were achieved by usinganhydrous P 2 O 5 , saturated aqueous solution of CaCl 2 .6H 2 O, Ca(NO 3 ) 2 .4H 2 O, NH 4 Cl and CuSO 4 .5H 2 Oin a closed vessel at an ambient temperature of 298 K. Prior to the saturation of the pellets in the abovebuffers, the pellets were heat cleaned at 393 K for 12 h to remove adsorbed water. A degassed glasschamber of about 200 cm 3 was used for evaluating the response and recovery characteristics. Thischamber has a provision for a two-way inlet, one for transpiring the dry air and the other for moist airfrom a wet candle. Air drying was accomplished by transpiring the air stream through drying columnspacked with anhydrous CaCl 2 and dry P 2 O 5 connected in series. The resistance measurements in thedry air as well as in moist air alternatively helped to establish the response and recovery time of thecomposites [5].The structural studies were carried out using a Philips X`pert diffractometer for 2values ranging from10 to 80 o using CuK radiation at =1.54 A o . A Perkin-Elmer Infrared spectrometer was used for thedetermination of the functional groups. The samples were dispersed in spectroscopic grade KBr pelletsand were scanned in the range of 4000–400 cm -1 .The surface morphology of the samples was observed on a JSM-6360 SEM analyzer operating at anaccelerating voltage of 15 kV using platinum coated samples. The nitrogen adsorptiondesorptionisotherms of the composites were measured using an automatic adsorption instrument. Prior to themeasurements, the composites were degassed at 150 o C overnight. The surface area of the compositeswas calculated using BET equation which is the most widely used model for determining the specificsurface area (m 2 g¯1).33

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41The thermoelectric effect (TEP) is a direct conversion of thermal differentials into electric voltage andvice versa. The TEP measurements were carried between 100 and 1000 o C. The thick pellets weresandwiched between two circular platinum discs where the sample on either face served as electrodes.The temperature difference between the two ends of the sample was measured through two K-typethermocouples and EMF generated across the two ends of the sample was measured. TEPmeasurements were carried out to understand the nature of the charge carriers in the sample using theformula: ,where is the Seebeck coefficient and is the thermo EMF produced across the sample due to thetemperature difference. The sign of enables to distinguish n- and p- type conduction.3. Results and Discussion3.1. X-Ray Diffraction StudiesThe powder XRD patterns of NiCZCe 112 (Fig. 1) showed the characteristic peaks corresponding toNiCr 2 O 4 -ZnO-CeO 2 confirming the absence of new phases and impurities. The XRD pattern of theNiCZCe 112 composite NiCr 2 O 4 (JCPDS-896615), ZnO (JCPDS-800075) and CeO 2 (JCPDS81-0792). For the 2 mol% Li + NiCZCe 112 composite, the threshold for the detection of Li +(~0.04 mass %) lies well below the detection limit of XRD (5 mass %) and hence the correspondingpeaks are not observed [13].Fig. 1. X-ray diffraction patterns of NiCZCe 111, undoped NiCZCe 112 and NiCZCe 211 composites.34

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-413.2. FT-IR SpectroscopyThe FT-IR spectra of NiCZCe 112, NiCZCe 211 composites (Fig. 2) showed spectral bands from400 to 1000 cm –1 belonging to metal oxide stretching vibrations of Cr-O, Ni-O, Ce-O and Zn-O bonds.The common broadband near 3468 and 1654 cm –1 observed were assigned to υ(OH¯)and δ(H 2 O)[14,15] for the composites. The broadness of the peaks was reduced for chromites [14, 15, 16] and thesharp bands around 625 and 512 cm -1 are characteristic of metal chromite spinels. The FT-IR spectraof NiCZCe 112 composite has broader band around 3400 cm -1 and 1600 cm -1 indicating moreadsorption capacity for water than that of NiCZCe 211.Fig. 2. FT-IR spectra of NiCZCe 211 and undoped NiCZCe 112 composites.3.3. Scanning Electron Microscopy (SEM)Fig. 3 (a–c) depicts the intergranular porous structure of the composite materials qualitatively. In themicrograph of NiCZCe 211 composite the size of the grains are larger than that of the NiCZCe 112.The 2 mol% Li + doped NiCZCe 112 composite showed regular well defined formation of grains. Thissuggests that the addition of 2 mol% Li + can reduce the grain size with the intergranular pores, leadingto microporosity in addition to the presence of mesopores. Furthermore, it is observed for 2 mol% Li +doped NiCZCe 112 composite that the grains connect each other and/or aggregate to some extent withwell-developed porosity that is very important to humidity sensing.35

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41(a)(b)(c)Fig. 3. SEM micrographs of (a) 2mol% Li + doped NiCZCe 112 (b) undoped NiCZCe 112and (c) NiCZCe 211.3.4. Nitrogen Adsorption/Desorption StudiesNitrogen adsorption/desorption isotherms (at 77 K) of the 2 mol% Li + doped and undopedNiCZCe 112 composites which possessed maximum sensitivity were studied. The BET surface areafor similar type of chromites and for composites has already been reported by Manoharan et al [10]and Saha et al [17]. The isotherms of 2 mol% Li + doped and undoped NiCZCe 112 composites showeda hysteresis effect with the slope of the plateau increasing with a significant increase in the nitrogenuptake through the entire pressure range. The increase in uptake of nitrogen in the samples is a resultof the major increase in porosity created within the 2 mol% Li + doped and undoped NiCZCe 112composites. The 2 mol% Li + doped NiCZCe 112 composite exhibited the most prominent hysteresiseffect, which can be characterized by the formation of intergranular pores as a result of Li + addition.The amount of nitrogen adsorbed increased corresponding to the BET surface area of 56.34 m 2 /g for 2mol% Li + doped NiCZCe 112 composite while it was only 32.47 m 2 /g for undoped NiCZCe 112composite. The addition of 2 mol% Li + retards the growth of the bulk phase, leading to an increase inthe surface area, an enhancement in the formation of a considerable amount of micro and mesopores.36

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41Fig. 4. Nitrogen adsorption/desorption isotherm of (a) 2mol% Li+ doped NiCZCe 112and (b) undoped NiCZCe112 composite.3.5. Humidity MeasurementsAll the NiCZCe composites showed a decrease in resistance with an increase in %RH, showing thatthe conduction occurred mainly at the grain surface, which was governed by the total water moleculecontent [18]. The resistance changes in porous spinel type oxides with increasing the humidity leveloccur because of adsorption and capillary condensation of water. At low humidity levels,chemisorption takes place, leading to formation of two surface hydroxyls with the charge transportoccurring by the hopping mechanism [18], while at high humidity levels, water is physisorbed on thetop of the chemisorbed layer.The humidity sensitivity factor for undoped NiCZCe 112 composite is 1273, whose resistance atRH 5 % is 9.6 x10 9 Ω and at RH 98 % is 7.5 x10 6 Ω. The undoped NiCZCe 112 composite has a BETsurface area of 32.47 m 2 /g. The sintered porous semiconductor has a large internal surface area for theadsorption of water vapor. The humidity sensitivity of the composites with various molar ratios ofNiCr 2 O 4 -ZnO-CeO 2 is shown in Table 1. Thus, the 2 mol% Li + NiCZCe 112 composite possessedcomparatively a high humidity sensitivity factor of 8471 has a BET surface area of 56.34 m 2 /g with theresistance of 3.6 x10 9 Ω at RH 5% and 4.2 x10 5 Ω at RH 98%, while that of undoped NiCZCe 112 andNiCZCe 211 was only 1273 and 124, respectively. The higher surface area of 2mol% Li +NiCZCe 112 composite is due to the formation of micropores along with mesopores in the ceramicbody on sintering at high temperature which is also evident from scanning electron micrographs (seeFig. 2). Decrease in resistance due to Li + doping to metal oxide thick film ceramics have already beenreported [19, 20]. In doped semiconducting oxides the conduction mechanism is due to surfaceconduction at low temperature [21].Usually, the main charge carriers are protons obtained by the dissociation of physisorbed watermolecules. The addition of alkali cations like Li + that also act as charge carriers along with protonsincreases the conduction. Further the coordination of water molecule to Li + increases the acidity of thecomposite thereby fast release of H + . Good linearity in the log R versus RH% plot is an importantcriterion for good humidity sensitivity material (Fig. 5). The results suggest that the more linear theplot, the better the response, recovery and sensitivity of the material.37

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41Fig. 5. Plots of log R vs. RH% for NiCZCe (a) 011 (b) 111 (c) 101 (d) 110 (e)121(f) undoped 112 (g) 2mol% Li + doped NiCZCe 112 (h) 211 composites.3.6. Electrical Conductance StudiesThe room temperature electrical conductance measurements of the composites prior to relativehumidity measurements signified that the current increased linearly with the applied voltage,indicating the ohmic contact of the electrodes. The temperature dependence of electrical conductancecarried out in the temperature range 120–350 ºC suggested that the current (I) increased with anincrease in temperature (T). The activation energies calculated from the temperature dependence ofconductance data are also shown in Table 1. The activation energy for electrical conduction inpolycrystalline materials generally involves the combination of the energy required to raise the carriersfrom the dominant levels to their corresponding transport bands and the energy required to create thecarriers in the dominant levels [22]. The low activation energy predicts that the small polaronconduction dominates in the studied temperature range.3.7. Response and Recovery CharacteristicsThe response and recovery time obtained from the plots of log R vs. time (Fig. 6) for undopedNiCZCe 112 composite were found to be 250 s and 300 s, respectively, and for 2 mol% Li + dopedNiCZCe 112 composite they were found to be 320 s and 400 s, respectively. The longer time taken forthe restoration of the resistance to that in dry air could be understood in the light of the fact that theseexperiments are conducted at 25 °C at which temperature the desorption kinetics is expected to be slowthus evidencing a surface controlled phenomena. The increase in the response and recovery time for2 mol% Li + doped NiCZCe 112 compared to that of their undoped composites are due to the increasedmicropore formation in the doped composite which results in slow adsorption and desorption of watermolecules. The 2 mol% Li + doped NiCZCe 112 composite showed increase in response and recovery38

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41time showing that the Li + doping enhances porosity in the ceramic material. It was found that the “wet”and “dry” run data curves did not exactly match. It was important to be certain that this difference wasa sample hysteresis effect due to differing rates of water adsorption and desorption from the pelletsurface. Adsorption is an exothermic process, whereas desorption needs external energy for watermolecules to depart from the sample surface [23, 24]. This explains why the recovery time is longerthan the response time.Fig. 6. Response and recovery time plots of undoped and 2 mol% Li + dopedNiCZCe 112 composite.3.8. TEP MeasurementsFrom the TEP measurements, a plot of Seebeck coefficient vs. temperature (Fig. 7) gave a negativevalue of which implied that the highest humidity sensing undoped NiCZCe 112 composite werewith majority negative charge carriers that are responsible for the conduction. Usually n-type ceramicsare observed to increase humidity sensitivity [25]. This is due to the donation of electrons from thechemisorbed water molecules on the ceramic surface.Fig. 7. TEP studies - Plot of Seeback coefficient (S) vs. Temperatureof undoped NiCZCe 112.ConclusionNiCr 2 O 4 -ZnO-CeO 2 composites were prepared and humidity measurements, FT-IR, BET, SEM, XRD,TEP studies were done. XRD studies confirmed the composite nature and the absence of the formationof new phases. FT-IR study showed the characteristic metal oxide vibrational frequencies. TheNiCZC-112 composite showed the highest humidity sensitivity factor of 1.2 x 10 3 . The micropore39

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41formation in the highest humidity sensing composites of NiCZCe is evident from nitrogen adsorptionand desorption and SEM studies. Temperature-dependent studies showed the low activation energy ofall the composites confirming the involvement of small polaron hopping mechanism in the conduction.From TEP measurements it is confirmed that the NiCZCe 112 composite has n-type semi-conductingnature. Good response and recovery time of NiCZCe 112 composite and its highest sensitivity conveythat it could be considered as a good commercial humidity sensor.AcknowledgementsThe authors thank the Directorate of Collegiate Education (DCE), Tamil Nadu for giving permissionand the University Grant Commission for the award of Faculty Improvement Programme (FIP)carrying out research for two years.References[1]. E. Traversa, Ceramic sensors for humidity detection: the state-of-the-art and future developments,Sens. Actuators B, 23, 1995, pp. 135-156.[2]. Y. Yokomizo, S. Uno, M. Hirata, H. Hiraki, Microstructure and humidity- sensitive properties of ZnCr 2 O 4 -LiZnVO 4 ceramics sensors, Sens. Actuators B, 4, 1983, pp. 599-606.[3]. Z. V. Marinkovic et al., Preparation of nanostructured Zn-Cr-O spinel powders by ultrasonic spraypyrolysis, J. Euro. Ceram. Soc., 21, 2001, pp. 2051-2055.[4]. T. A. S. Ferreira et al., Structural and morphological characterization of FeCo 2 O 4 and CoFe 2 O 4 spinelsprepared by a coprecipitation method, Solid State Sciences, 5, 2003, pp. 383-392.[5]. S. Pokhrel, B. Jeyaraj, K. S. Nagaraja, Humidity-sensing properties of ZnCr 2 O 4 -ZnO composites, Mater.Lett., 57, 22-23, 2003, pp. 3543-3548.[6]. J. H. Anderson, G. A. Parks, The electrical conductivity of silica gel in the presence of adsorbed water,J. Phys. Chem., 72, 1968, pp. 3662-3668.[7]. T. Seiyama, N. Yamazoe, H. Arai, Ceramic humidity sensors, Sens. Actuators B, 4, 1983, pp. 85-96.[8]. S. L. Yang, J. M. Wu, ZrO 2 -TiO 2 ceramic humidity sensors, J. Mater. Sci., 26, 1991, pp. 631-636.[9]. J. L. Zhang, Electrical conduction of Ba 0.5 Sr 0.5 TiO 3 ceramics under d. c. voltage, J. Mater. Sci. Lett., 11,1992, pp. 294-295.[10]. S. S. Manoharan, C. Patil Kashinath, Combustion synthesis of metal Chromite powders, J. Am. Ceram.Soc., 75, 4, 1992, pp. 1012-1015.[11]. S. Pokhrel, K. S. Nagaraja, Solid state electrical conductivity and humidity sensing properties ofCr 2 O 3 -MoO 3 composites, Phy. Stat. Sol. (A), 194, 2002, pp. 140-146.[12]. M. K. Jain, M. C. Bhatnagar, G. L. Sharma, Effect of Li + doping on ZrO 2 -TiO 2 humidity sensor, Sens.Actuators B, 55, 1999, pp. 180-185.[13]. A. M. E. Suresh Raj et al, Electrical and humidity sensing properties of tin(IV)oxide- tin (II) molybdatecomposites, Mater. Res. Bull., 36, 2001, pp. 837-845.[14]. D. Dvoranova, V. Nrezova, M. Mazur, M. A. Malati, Investigations of metal doped titanium dioxide photocatalysts, Appl. Catal. B: Environ., 37, 2002, pp. 91-105.[15]. M. F. Zawrah, Investigation of lattice constant, sintering and properties of nano Mg-Al spinels, Mater. Sci.Eng. A, 382, 2004, pp. 362-370.[16]. J. Preudhomme, P. Tarte, Infrared study of spinels-III, Spectrochim. Acta, 27A, 1971, pp. 1817-1835.[17]. D. Saha, R. Giri, K. K. Mistry, K. Sengupta, Magnesium chromate-TiO 2 spinel tape cast thick film ashumidity sensor, Sens. Actuators B, 107, 2005, pp. 323-331.[18]. S. Pokhrel, K. S. Nagaraja, Electrical and humidity sensing properties of molybdenum(VI) oxide andtungsten (VI) oxide composites, Phy. Stat. Sol., A, 198, 2, 2003, pp. 343-349.[19]. J. Judith Vijaya, L. John Kennedy, G. Sekaran, B. Jeyaraj, K. S. Nagaraja, Effect of Sr addition on thehumidity sensing properties of CoAl 2 O 4 composites, Sens. Actuators B, 123, 2007, pp. 211-217.[20]. E. Joanni, J. L. Baptista, ZnO-Li 2 O humidity sensors, Sens. Actuators B, 17, 1993, pp. 69-75.[21]. M. L. Zang et al., Fast response of undoped and Li-doped titania thick films at low temperatures, Sens.Actuators B, 131, 2008, pp. 680-686.40

Sensors & Transducers Journal, Vol. 101, Issue 2, February 2009, pp. 31-41[22]. Z. A. Ansari, T. G. Ko, J. H. Oh, Humidity sensing behavior of thick films of strontium-doped leadzirconium-titanate,Surf. Coat. Technol., 179, 2004, pp. 182-187.[23]. K. Arshaka, K. Twomey, D. Egan, A ceramic thick film humidity Sensor Based on MnZn Ferrite, Sensors,2, 2002, pp. 50-61.[24]. W. Qu, J. U. Meyer, A novel thick-film ceramic humidity sensor, Sens. Actuators B, 40, 1997, pp. 175-182.[25]. K. S. Chou, T. Lee, F. Liu, Sensing mechanism of a porous ceramic as humidity sensor, Sens. Actuators B,56, 1999, pp. 106-111.___________________2009 Copyright ©, International Frequency Sensor Association (IFSA). All rights reserved.(http://www.sensorsportal.com)41

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