Spontaneous ignition limits of silane and phosphine

Spontaneous ignition limits of silane and phosphine have been investigated at relatively low concentrations. For silane, the spontaneous ignition occurs if the mixture concentrations is such that the silane/oxygen ratio is higher than a certain threshold limit value. In other words, the mixture is not stable if the ratio is higher than a certain value. On the other hand, in the case of phosphine the threshold limit line has been found to be a little curved, though the reason for the fact is not clear. At any rate, it is concluded that the spontaneous ignition of both silane and phosphine occurs as a result of a competition of chain branching and chain breaking reactions, in a way that is qualitatively similar to that in hydrogen oxidation.