Manufacture of Tin oxide Pellets for Gas Sensing Applications

Department of Electrical Engineering – SEES, CINVESTAV - IPN, Zacatenco, D.F.,C.P 07360, Mexico * - krishnakarthik.tv@gmail.com

Prof. (Dr.) M. de la L. Olvera, T.V.K.Karthik*

ABSTRACTTin oxide powders are prepared by using Homogeneous Precipitation Method (Solution with a mixture of SnO2.5H2O with a solvent, addingammonia solution produces precipitate and centrifuge, drying and heating results powders). Powders were prepared from two different solvents,namely, water and Alcohol, and three different concentrations, 0.1M, 0.5M and 1M. For all the above concentrations, calcinations of powdersfor 2 hours at 6000C results the phase transformation of the sample which is observed by the X-ray Diffraction studies and XRD patterns alsoshows that all synthesized powders are polycrystalline, fitting well with the tin oxide Tetragonal type structure. Variation in the intensity of thepeaks, associated with the characteristic planes, depending on the concentration and also the type of solvent, was determined. The synthesizedtin powders are ball milled with alumina (Ceramic material) powders with fixed milling parameters but by varying the tin/alumina ratios. Then,obtained milled powders are pressed with a pressing machine to make pellets, and finally probed their sensing properties in CO.

INTRODUCTION

Two representative TEM photographs of Pd particles loaded on the surface of SnO2

GAS SENSING MECHANISM:Materials that change their properties depending on the ambient gas can be utilized as gas sensing materials.Pure Metal Oxide Gas Sensors: Essentially trapping of electrons at adsorbed molecules and band bending induced by these charged moleculesare responsible for a change in conductivity. When a molecule adsorbs at the surfaceØElectrons can be transferred to this molecule, if the acceptor levels of the molecule lie below the Fermi level of the solid.ØElectrons are donated to the solid, if the donar levels of the molecule are above the Fermi-level of the solid.Effect of Additives: Additives are used for sensitizing and increasing the gas response to particular gases. Generally it is assumed that the metalor metal-oxide additives reside on the surface of the semiconducting gas sensing oxide in form of dispersed clusters.

EXPERIMENTAL PROCEDURE

SampleSnCl2.5H2O

Concentration (M)

Solvent type

Solvent volume

(ml)

Volume of Ammonia (ml)

Powders Yield (gr)

A 1 Water 35 25 3

B 0.5 Alcohol 500 170 30

C 0.5 Water 500 110 30.6

D 0.1 Water 500 110 5.8

E 0.1 Alcohol 500 175 6.8

Molarity (M) Solvent SnO2:Al SnO2:Al ratio

(gr)Number of balls used for milling

1 Water 5:11 2.5:5.5 50.5 Alcohol 3:1 6:2 50.5 Alcohol 5:1 10:2 80.5 Water 1:1 4:4 50.5 Water 5:1 10:2 80.1 Water 1:1 4:4 50.1 Alcohol 1:1 4:4 5

Pellet preparation by pressing the milled powders:vThe ball milled powders are then pressed at very high pressure of 8 tons for 1 hour. The pressing machine consists of a base, a die of 12mm diameter with two die pellets and also a piston. One die pellet is placed in the die and then powder for pressing is added on top ofthat the other die pellet is placed which is closed by the base. Then by placing the piston on the top, required pressure is applied.vThe pressure should be increased in a timely manner, initially 2 tons is applied and for every increase of next 2 tons, 10mins time gapshould be maintained. Once the required pressure is reached then leave the die at that pressure for required pressing time.

Synthesis of Tin Oxide powders by Homogeneous Precipitation:v Precursors : 0.1-1M of SnCl2.5H2O and ammonia. SnCl2.5H2O+ammonia until the PH of the

solution reaches 11 or 12.v Centrifuge the precipitate for 20mins at 4500rpm and Remove the supernatant, add water to

the precipitate (washing the precipitate in order to remove chlorine ions) and centrifugeagain. Finally dry precipitates at 1000C for 24 hours and calcinate in furnace at 6000C for2hours.

v Ball milling tin oxide powders with alumina (PM 400 type manufactured by RETSCH) :v Milling speed: 4500rpmv Milling time: 45minsv Ball used for milling: 8gms andv Balls to powder ratio: 5:1

RESULTS AND DISCUSSION

CONCLUSION

ACKNOWLEDGEMENT

CONTACT INFORMATION

Characterization:The sensitivity comparison of pellets were obtained calculating pellets electrical conductance from the measured resistance in vacuum (0.23torr) and in the presence of different partial pressures of CO, from 0 to 300ppm. The resistance was measured by using a Keithley 2001multimeter. The partial pressures were calculated from the value of the chamber pressure, which was measured with a TM20 Leybold detector.The sensitivity of the films is defined as the ratio of its conductance in the presence of the gas to be detected to that in vacuum, i.e.:S = Ggas / Gvac , This value changes from gas to gas.

vPills prepared with alcohol as solvent (dried totally without any agglomeration) shows more sensitivity then the pills prepared with water assolvent (little agglomeration of particles ) and at 0.1M the sensing is more than 0.5M if Alcohol is the solvent and it is reversed in case ofwater as a solvent.vIncrease in the tin oxide ratio in the pellets doesn’t bring much change in sensitivity but change in alumina ratio showed very high increaseof sensitivity from 11 to 1100.

vCrystalline tin oxide powders are formed from hydrous tin oxide after calcinations (6000C, 2 hours, both 1M and 0.5M concentrations).Calcination will remove the solvents from the powders and helps for phase transformation of powders.vXRD pattern of tin oxide powders of different concentrations with same solvent (water/ Alcohol) are more narrow and intense at 0.5M than0.1M (particles growth broadens the peak ) and 1M (Particles agglomeration broadens the peak) concentrations. 0.5M are more finely groundthan 0.1M and 1M.vXRD of powders prepared with 0.5M and 0.1M for different solvents (water and alcohol) have almost same phases and intensity too. So thesolvent used to prepare the powders does not affect the phases of the powders but it may affect the sensitivity of the pellets.

vHomogeneous precipitation has been successfully utilized to synthesize tin oxide powders. The powders with Calcination at 6000C for 2 hhas exact phase match with JCPDS values of tin oxide, hence it is proved that calcinations is a compulsory process.vIt has been shown that initial concentration has a significant effect on crystallite size. Optimal concentration (0.5M) is more finely groundthan the lower (0.1M) and higher (1M) concentrations.vIt is also proved that the solvent used to prepare the powders does not affect their phases but it affects the sensitivity, as the concentrationincreases the yield increases but sensitivity decreases due to agglomeration of the particles.vIncrease in the tin oxide ratio in the pellets doesn’t bring much change in sensitivity but change in alumina ratio showed very high increaseof sensitivity of a factor of 100.vSo we conclude from this result that the homogeneous precipitation method is effective in synthesizing tin oxide powders with a highersurface area which is expected to increase the sensitivity of chemical gas sensors.

T.V.K.Karthik, M.Tech,(Phd)Student in Electrical department,SEES Department, CINVESTAV-IPN,Mexico City, Mexicokrishnakarthik.tv@gmail.com

We acknowledge the financial support given by CONACYT. We appreciate the below members from CINVESTAV-IPN, MIGUEL ANGELLUNA ARIAS from SEES department for lab assistance and ADOLFO TAVIRA FUENTES from SEES department for XRD measurements andBLANCA ESTELA ZENDEJAS LEAL from Physics Department for pressing the powders