Influences of ball-milling time on gas-sensing properties of Co3O4-SnO2 composites

The relevance of sensor response to CO and H2 with the microstructure of the sensing body was investigated for thick-film devices using Co3O4(0.5 mass%)-SnO2 composites. When the composite was prepared from constituent oxides by milling in a satellite ball mill for various spans of time (1-12 h), the sensor response to 100 ppm CO, defined as the ratio of the electrical resistance in air to that in the sample gas, was found to reach a maximum as large as about 800 at 6 h of ball-milling (BM). The response to H2, in contrast, showed no such marked promotion. SEM observation of the granular state and pore size distribution analyses indicated that increasing BM time gave rise especially to an increase in the volume of pores in the pore size range of 20-35 nm. It is suggested that such a change in microstructure is responsible for the marked promotion of the response to CO, while the same change is not so much effective on the response to the gas (H2) with a much larger diffusion coefficient. For comparison, the composite prepared by an impregnation method was also subjected to the BM treatments. In this case, the response to CO could not be promoted so much even after BM for 18 h, because the heavy coagulations of grains formed in the impregnation process were prevented from being pulverized by BM.