High performance CVD-diamond-based thermocouple for gas sensing

A new thermochemical sensor was developed exploiting the unique properties of CVD-diamond. The thermal transduction step was based on the electric signal produced by a semiconductor-metal thermocouple. A thin film of boron doped polycrystalline diamond was grown by plasma enhanced CVD technique with CH3OH + B2O3 vapour addition. The high melting point, the chemical inertness and the extreme hardness make diamond an interesting candidate for sensing applications in harsh environments. The active thermocouple was completed evaporating a 120 nm palladium film on the diamond film. The sensor was then tested for different temperature drops and showed good stability and repeatability in addition to high values of the thermoelectric power (S > 350 μV/°C). The catalytic properties of thin palladium films allow the thermocouple to be used for gas detection. In particular, hydrogen could be detected due to the palladium Fermi level modification, induced by the absorption process. To this purpose, the diamond-palladium sensor was exposed to different hydrogen concentrations, varying from 0.5 to 2%. The signal due to hydrogen absorption increases linearly with the gas concentration and is superimposed to the one produced by temperature changes.