Examining the Gas Sensing Properties of the Nanocrystalline Sm2O3-WO3

第4卷增刊过程工程学报Vol 4 Suppl2004年8月The Chinese Journal of Process engineeringAug.2004Examining the Gas Sensing Properties of the NanocrystallineSm2O3-WO3Tsai- Ta Teng(邓粲达,Kuan- Nan Lin(林冠男,Hong- Ming Lin(林鸿明,Tsung-Yeh Yang(杨宗烨)(Department of Materials Engineering, Tatung University, Taipei 104, China)Abstract: In this study, nanocrystalline(NC)wO, and Sm2O3-wO3 gas sensors have been synthesizedand deposited on AlO3 substrate by gas condensation method, then sintering to 600C. SEM is usedobserve the morphology of the surface. TEM is used to measure the particle size. the resistance of sensoris measured in different temperatures and gas concentration to evaluate the sensing properties of NCwO, and NC Sm20]wO, sensor. The results indicate that wO, is a usability gas sensor material. AfterSm20, is doped in WO,, it does not obviously influence CO detection; it owns better sensitivity andstability. To UV irradiation on WO,, the sensitivity of wO, is enhanced and it maintains wO, recoveryproperties in NO: detecting. UV irradiation on Sm2O, doped wO, does not show enhanced wellproperties as UV illuminate on wOKey words: nanocrystalline: WO; Sm,; gas condensation method1 INTRODUCTIONThe charge carrier concentration on the surface of a semiconductor is sensitive to the compositionof the surrounding atmosphere, which was found half a century ago, considerable research has beencarried out on the development of novel solid state gas sensors based on smiconducting metal oxideIn general, semiconducting gas sensors are single phase ceramics that can monitor surface adsorbedgases through changes in surface conductivity from electrons transfer between the gas molecules and thesensor surface. However, these kinds of sensors generally have disadvantages of poor selectivity.insensitivity to the very low gas concentrations required for certain applications and require periodicconditioning cycles to ensure consistency of performarSeveral systems have been proposed for hybridized system sensors, mostly based on rectifyingjunctions formed between n-type and p-type semiconducting ceramics, that including Cuo (p-type)uno(n-type) La2 CuO4(p-type)Zno (n-type)2. A hybridized system gas sensor consists of twosemiconducting oxides in contact, with the reactions giving rise to the enhanced sensing behavioroccurring at the interface between the two materials. gases that are adsorbed onto the sensor surface itscharge transfer characteristics by changing the structure of the interfacial barrier. This process gives to agas detection mechanism distinct from that of single phase ceramic sensors中国煤化工2 EXPERIMENTALCNMHGAlumina substrates (0.63 mm thick)with gold electrodes and an Ag-Pt heater printed on both sidesare used in this study. the chamber is evacuated by means of rotary and diffusion pump then purged and168过程工程学报第4backfield with helium gas. After repeat previous procedure three times, the chamber was pumped tobelow 5x10- torr and then 5 mbar helium and 5mbar oxygen is introduced. After preheating, tungstenboat is evaporated under this atmosphere by a resistance-heated. During evaporation, metal tungsten wasoxidized to form NC wO, is deposited on the liquid-N2-cooled cold trap. After the temperature of coldtrap raises to room temperature, open the chamber, exchange tungsten boat to tantalum boat and putsamarium powder on tantalum boat. After the chamber was pumped below 5x10 torr, keep it in highvacuum, heat tantalum boat to evaporate samarium, and evaporate samarium, then it well deposit on NCwO,. After temperature of cold trap raise to room temperature, the powers on cold trap are collected andcoated on the clean Al2O3 substrate by spin coating. Then, step-heating to 600C to enhance the bondingbetween NC Sm2Oj-wO3 and substrate, and the samarium will oxidize to samarium oxide(Nc Sm2O3)A setup of measurement equipment is shown in Fig 1. The sensor is settled in a chamber. The detectedgases flowing through the chamber are passed continuously and in dynamic equilibrium with atmosphereA thermocouple is used for monitoring the operation temperature. Agilent 34970 A is used to record thechange of resistance and Agilent E3646 A is used to heat sample by supporting step voltage. The crystalstructure and ceramic microstructure of the particles are determined by Scanning Electron Microscope(SEM). The phase identify was obtained from X-ray diffraction(XRD)patterns. The size of granules andsurface morphology were analyzed by Transmission Electron Microscopy (TEM)emperatureFig. I The electronic measurement system2 Theta ()Fig 2 X-ray diffraction of NC WO,, NC Sm2ONC Sm-W-O, and NC Sm2 Oy-wO33 RESULTS AND DISCUSSIONX-Ray diffractions patterns as shown in Fig. 2, show that there is Sm20, phase in NC Sm-W-Opowders. They are two possible reasons for this condition. Samarium is evaporated and condensed on thecold trap It is oxidize to samarium oxide when open to air. The Sm is a strong oxidizer. When NC Smattach on NC wO ,. it will react with WO, and draw away oxygen from wO, to Sm2O3 and reduce wO3to wO3-x中国煤化工The morphology of the samples after coating is a poreCNMHbetween NC particles and substrate is weakly. If the heateds, wav pva ous structure willcrash down and form a dense film. To increase the gas sensitivity, it is necessary to maintain the porousAH Tsai-Ta Teng, et al. Examining the Gas Sensing Properties of the Nanocrystalline Sm:Oj-wO3structure to retain the large specific surface area. The sintering process with step-heating up to 350oC isused to oxidize material to stable state, and then heat up to 600oC to retain the porous structure of Ncmaterial. SEM images indicate the porous structure is maintained after sintering which show in Fig. 3(a)and (b).Fig 3 SEM images of NcTEM images of NC wO, and NC Sm2O, hybridized NC wO] are shown in Fig 4(a) to( d). Fig 4(a)and(b)are the images of as prepared NC WO3 and NC wO, after sintering, respectively. Fig. 4(c)and(d)are the images of as prepared NC Sm hybridized NC wO(or NC Sm-w-O)and NC Sm hybridized NcwO, after sintering(or NC Sm2O3-WO3), respectively. The mean Nc wO, or NC Sm2O, particle sizesas prepared and after sintering are 43 nm and 60 nm or 22 nm and 30 nm respectively. The NC wO, andNC Sm2O, particle size is controlled by the different vacuum pressures and surface tension. It is betterhat small size of NC Sm2O3 can attach firmly on nc wO3 surface to enhance sensitivity.100nmroOm(a)as preparation woyb)wO, after sintering (c)as preparation Sm-W-0(d)Sm2Or-wOFig 4 TEM images of NCThe change of sensitivity varies time with 10. 15, 20, 30 and 40 ppm NO/air is measured at 200oCThe results are presented in Fig. 5(a). It shows the resistance versus time curve. We can see the resistanceincreases with the concentration of NO2 increases, and the response time and recovery time is fast. Fig5(b)shows the sensitivity of NC wo, varies with concentration NOz at 200oC. The sensitivity is goodfor the various concentrations of NO 2 gas, and is clear to reveal a linear tendency of sensitivity with theincreasing of NO2 concentrations. The sensitivity of NC Sm203-wO, gas sensor is obviously higher thathat of pure NC wO, gas sensor with similar sensing properties as shown in Fig. 6(a). But it is observedthat it can' t well recover when concentrations of NO2 great than 40 ppm. It may imply the adsorption anddesorption of NO2 for NC Sm2Oj-WO3 gas sensor car中国煤化 rum quickly at highconcentration NO2 gas. Fig. 6(b)shows the saturated serCNMH G&as sensor at 200Cin different NO2 concentrations. The sensitivity is increasegIs Noz concentrations7a)shows the sensitivity of UV illumination NC wO3 varies with time at 200C in 10, 15, 20,170过程工程学报第4卷30 and 40 ppm NOz/air, respectively, and Fig. 7(b)shows the sensitivity of UV illumination NC WO,varies with NO2 concentrations at 200 C. They are compare with Fig. 5(a)and( b), the sensitivity of NCWO, becomes obviously higher when UV illumination on wO, surface. It is useful to enhance thesensitivity so that we can measure lower concentration NO2. The sensitivity of UV illumination NCSm2Or-wO, varies with 10, 15, and 20 ppm NO2 at 200C as shown in Fig 8(a). Compare with Fig. 6(a)and(b), the sensitivity of NC Sm2O3-wO, is not obvious difference at the 200 C. the results indicatethat NC Sm2O-wO, sensor can't be obvious enhanced the sensitivity by UV illumination. Because theabsorption wave of Sm2O, is not 368 nm that no UV absorption occurs at UV illuminate on Sm2O200c)-50000500100001500020000250003000035000Fig 5(a) The sensitivity of NC WO, varies with time in different NO2 concentrations at 200oC:(b)The sensitivity of NC WO, varies with NOz concentrations at 200C050001000015000200002500300035Time (sec)NO. Concentraton (opm)Fig6(a)The sensitivity of NC Sm2O]WO, varies with time in different NO2 concentrations at 200C:(b) The sensitivity of NC Sm2Oj.wO, varies with NO2 concentrations at 200C中国煤化工CNMHGFig. 7(a)The sensitivity of UV illumination NC wO, varies with time at 200oC in different NO: concentrations万方數据 e sensitivity of UV illumination NC wO: varies with No: concentrations at200°C增刊Tsai-Ta Teng, et al. Examining the Gas Sensing Properties of the Nanocrystalline Sm2Oj-woNO. Concentration (ppm)Fig 8(a)Sensitivity of UV illumination NC Sm2Oj-WO, varies with time at 200oC in different NO2 concentrations(b)Sensitivity of UV illumination NC Sm20]wO, varies with NO2 concentrations at 200C4 CONCLUSIONSNC WOjbased gas sensor undergoing sintering by step-heating up to 600oC can retain a porousnetwork structure and enhance the bonding strength between particles and al2O3 substrate, that canincrease the gases sensitivity and response time. NC Sm20, on NC wO, gas sensor can increase thesensitivity, response time and decrease the recovery time in different concentrations of NOz. In the casef UV illuminated on NC wO3 gas sensor, it can increase the sensitivity and maintain short recoverytimeAcknowledmemtsFinancial support of this research by Tatung University, Taipei, under the grant B91-T15-073 isgratefully acknowledged[I] Jung, S J and Yanagida, H. 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