Applications of Poly (3-Hexylthiophene) Thin Film as a Hydrazine-Sensitive Chemiresistor

Abstract

Hydrazine is a hypergolic compound that when combined with mixed oxides of nitrogen self ignites. This compound is used for the steering (guidance and course correction) of satellites and spacecraft. Hydrazine is a highly toxic and carcinogenic species exhibiting toxic effects in humans at very low levels of exposure. Both the American Conference of Government Industrial Hygienists (ACGIH) and the Air Force Office of Safety and Health (AFOSH) have set the hydrazine exposure threshold limit value (TLV) to 10 ppb in air for an 8-hour period. Hence a sensor capable of hydrazine detection at the ppb level is required to protect individuals working around device containing hydrazine. In this thesis, a type of passive, low cost, Poly (3-Hexylthiophene) (P3HT) thin-film chemiresistor micro-sensor was fabricated and investigated for the detection of hydrazine and associated compounds. The sensor works on the principle that interaction with hydrazine reduces the number of charged couple pairs in the P-type doped P3HT thin film. This causes the thin film to undergo a permanent and large increase in resistance. Standard microelectronic manufacturing techniques were used to form a micro-sensor composed of silicon substrate, interdigitated gold electrodes, and P3HT sensing film. Responses of the micro-sensor to hydrazine at different temperatures and concentration levels are reported. The effect of different doping levels of P3HT thin film was investigated to improve the sensor stability. Thermally-induced effects on performance and thermal stability of the P3HT thin film micro-sensor are also explored. The experiments show that the micro-sensor resistance increases from 1 to 2 ohms to over 106 ohms upon exposure to 25 ppm concentration of hydrazine at a flow rate of 4 liters/min. Experiments also showed that the sensor’s sensitivity to hydrazine vapor at low concentration levels can be enhanced by thermal treatment. In addition, thermally annealed and heavily doped P3HT micro-sensors had better thermal stability at high temperature. The sensors exhibited good specificity to hydrazine with no response to NO2 and N2O.