H2 selective gas sensor based on SnO2

A highly selective H2 gas sensor was developed based on a ‘hot wire type’ gas sensor commercially available. Tin oxide semiconductor was sintered in a bead (0.5 mm in diameter) covering over a platinum wire (0.02 mm in diameter) coil. The gas sensor was operated by a bridge electric circuit. A dense layer of ≈0.1 mm in depth near the surface of the porous bead, was formed by chemical vapor deposition of hexamethyldisiloxane (HMDS). The dense layer functioned as a ‘molecular sieve’, thereby the diffusion of gases with large molecular diameters, except for H2, was effectively controlled, resulting in a prominent selectivity for H2. On the other hand, diffusion control of O2 caused a strong H2 reduction of the tin oxide in the inner layer covered with the dense layer, resulting in decay in sensor output and irreversible reduction of the hydrogen sensitivity (deactivation of tin oxide). As a countermeasure against the damage, addition effect of cerium oxide from 0.5 to 5.0 at% was investigated. The H2 selective gas sensor thus obtained, had a significantly minor humidity dependence and a prominent long term stability.