A Novel Polymer - Derived - nanoCeramic for Ultrahigh Temperature MEMS Micro

Polymer–derived ceramics (PDCs) constituted from silicon, carbon, nitrogen and oxygen embody not only remarkable resistance to creep, thermal shock and oxidation at very high temperatures, but also functional properties such as semiconductivity which extends up to 1350C. In this report we describe the extraordinary sensitivity of the electrical resistance of the PDCs to hydrogen, at temperatures up to 1000C. Two types of structures: carbon–nanotube paper coated with a monolayer of PDCs (Type A), and free standing MEMS–scale PDC sensors (Type B) were studied. The first type of specimens showed a huge change in resistance when cycled between hydrogen and nitrogen; the sensitivity increased with temperature. The Type B specimens showed the opposite behavior, being more sensitive at the lower temperatures; nevertheless, these sensors showed reproducible and reversible change of resistance in the presence of hydrogen at temperatures up to 1000C. It is inferred that the response of Type A sensors was limited by kinetics, while that of Type B sensors was limited by the thermodynamic solubility of hydrogen in PDC materials. However, a more robust scientific understanding of the underlying chemical and nanostructural basis for PDC conductometric–sensors, is essential. The potential of Type B sensors may well constitute a new paradigm for the measurement of hydrogen gas activity at very high temperatures.