In order to observe the dependence of sensitivity of a $SnO_2$ gas sensor on microstructure thick films were prepared by a new film fabrication method of centrifugal separation. As a first step a well-dispersed slurry of $SnO_2$ powder was prepared, which was accumulated on an alumina substrate by a centrifugal separation method. Experiments showed that the sensor prepared by centrifugal separation has a higher and more stable sensitivity than the screen printed sensor. It is assumed that the difference between the centrifugal preparation and screen printing processes is the cause of the difference in sensitivity of the sensors. The centrifugal method and CIP, allowed us to prepare sensors with various microstructures from powders of different sizes. The microstructure of the film fabricated by the centrifugal method showed that the powder was accumulated according to its size because of the centrifugal force; it was possible to well characterize the microstructure. And it is also easy to analyze sensors according to their microstructures, because the microstructure of the film fabricated by the centrifugal separator can be changed by varying CIP conditions and the particle size distribution. The sensitivity of the sensor showed a tendency toward saturation with increasing thickness, which was attributed to the diffusion of oxygen into a porous thick film. The sensitivity of sensor with their atomic packing factor increased by CIP showed a similar tendency to that without CIP, but the effective for their sensitivity saturation decreased. This result can be explained by the decrease in pore size with increasing APF causing the effective thickness to decrease. So, it was inferred that the size of the pores could affect the sensitivity of sensors. The sensitivity of sensors with APF change by the particle size distribution of powder increased with an increased APF. This is a result of the increase of both coordination number of particles and the volume of the small-sized powder. The measured decrease of air resistance confirmed this result. But in case of sensors made of small powder, the resistance increased with by the increase of reaction area than by the increase of the coordination number among particles.
The present investigation confirmed that the pore size, coordination number among particles and reaction area affect the sensitivity of a gas sensor. The optimum conditions for the sensitivity of sensor are increased pore size, coordination number among particles and reaction area, which are all dependent of each other.
