Hydrogen and Methane Gas Sensors Synthesis of Multi-Walled Carbon Nanotubes

The electrical resistance of multi-walled carbon nanotubes increases or decreases in methane (CH4) or hydrogen (H2) gas, respectively. The gas sensitivity was measured in a mixture of methane and H2 gases with different partial concentrations for the very first time. The marginal value of the ratio between the methane and H2 gases concentrations in a gas mixture at which the resistance variation changes sign was studied at several measuring temperatures. This marginal value depends on the measuring temperature. The best sensitivity, fastest response, and fastest recovery could be observed at 230 or 300 � C in methane or H2 gas, respectively. These were the highest possible measuring temperatures for each gas, and samples did not indicate stable properties at operating temperatures above 230 or 300 � C for methane or H2 gas, respectively. Furthermore, the change of resistance in the H2 partial concentration of 1.3% in air can be inverted by introducing 2.7% of methane into this mixture of H2/air at 230 � C.