In2O3 doped WO3 powders were prepared by a polymer solution route and their NO2 gas sensing properties were analyzed. The synthesized powders showed nano-sized particles with specific surface areas of 6.01~21.5 m2/g and the particle size and shape changed according to the content of In2O3. The gas sensors fabricated with the synthesized powders were tested at operating temperatures of 400~500 oC and 100~500 ppm concentrations of NO2 atmosphere. The particle size and In2O3 content affected on the initial sensor resistance in an air atmosphere. The highest sensitivity (8.57 at 500 oC), which was 1.77 higher than the sensor consisting of the pure WO3 sample, was measured in the 0.5 mol% In2O3 doping sample. In addition, the response time and recovery time were improved by the addition of In2O3.

Nano-sized SnO2 thick films were prepared by a screen-printing method onto Al2O3 substrates. The sensing characteristics were investigated by measuring the electrical resistance of each sensor in a test box as a function of the detection gas. The nano-sized SnO2 thick film sensors were treated in a N2 atmosphere. The structural properties of the nano SnO2with a rutile structure according to XRD showed a (110) dominant SnO2 peak. The particle size of SnO2:Ni nano powders at Ni 8 wt% was about 45 nm, and the SnO2 particles were found to contain many pores according to the SEM analysis. The sensitivity of the nano SnO2-based sensors was measured for 5 ppm CH4 gas and CH3CH2CH3 gas at room temperature by comparing the resistance in air with that in the target gases. The results showed that the best sensitivity of SnO2:Ni and SnO2:Co sensors for CH4 gas and CH3CH2CH3 gas at room temperature was observed in SnO2:Ni sensors doped with 8 wt% Ni. The response time of the SnO2:Ni gas sensors was 10 seconds and recovery time was 15 seconds for the CH4 and CH3CH2CH3 gases.