Highly sensitive SnO2 porous film gas sensors fabricated by plasma spray physical vapor deposition

Synergic effect of film structure and sensor geometry on the gas sensor properties was investigated for porous thick film gas sensors fabricated by plasma spray physical vapor deposition (PS-PVD). Sensor model was also developed taking into account of both gas permeation/reaction and spatial current density distributions. It is found that higher responses are fundamentally attained with thinner sensor films, regardless of other parameters. At a fixed thickness, the response is less dependent on the applied voltage or the electrode gap at wider gaps, but it changes significantly when the gap becomes smaller than the critical gap size. This tendency becomes more pronounced for the thicker and denser films. This is explained by the fact that the gas/film reaction is localized near the film surface and the current change upon exposure to the target gas is reduced resulting in the decrease in the response. Furthermore, especially for the sensors with narrow electrode gap and at high applied voltage, a local Joule heating is no longer ignored and increases the temperature at least near the electrode. This affects the response to either increase or decrease, depending on the local temperature that is determined by the sensor design, i.e., gas diffusion speed and reaction, with respect to the optimal sensor operating temperature. As a result of the geometry optimization, the PS-PVD sensors have exhibited the noticeable response of formaldehyde at a concentration as low as 40 ppb.