In potentially hazardous atmospheres, the speedy detection of combustible gases is a priority, and for the natural gas industry early detection of methane is essential. One common method of detecting combustible gases is with pellistor sensor technology. Pellistors detect a rise of temperature in a gas on combustion. Typical pellistor construction has a coil of fine platinum wire embedded in a refractory bead that is loaded with a catalyst, usually palladium.
What the Sensor’s Platinum Wire Does
The platinum wire heats the catalyst to its operating temperature. The platinum wire also detects any extra heat produced if gas burns on the catalyst, by a change in its resistance as the catalyst temperature increases. However, as the platinum wire is very fine (10-50 mm in diameter), pellistors are fragile. The power consumption of the device is high (120-500 mW) and they also have to be individually produced.
Fabricating Micromachined Planar Sensor Structures - Problems with the Catalyst Layer
Recently, micromachined ‘hotplate’ planar sensor structures, where a supported thin etched SiO2 or SiN membrane carries a platinum track on one side and a catalyst layer on the other, have been fabricated. The technology has resulted in smaller structures, less power use and should allow parallel production on the wafer level. However, due to the poor performance of the catalyst layer, reliable devices have not been achieved.
Micromachined Pellistor Structures with Low Power Consumption and a Controllable Catalyst
Now, scientists at the University of Southampton have produced a micromachined pellistor structure that has low power consumption and a controllable catalyst structure (P. N. Bartlett and S. Guerin, Anal. Chem., 2003, 75, (1), 126-132). Nanostructured palladium films were electrochemically deposited (e) from the hexagonal (H1) lyotropic liquid crystalline phase of a nonionic surfactant, octaethyleneglycol monohexadecyl ether, on to micromachined Si hotplate structures. (NH4)2PdCl4 served as the source of palladium. The electrodeposited nanostructured palladium catalyst layer can be formed into metal or alloy powders and films with regular nanoarchitecture. The H1-e palladium films have high surface areas (~ 28 m2.g-1) and are effective and stable catalysts for the detection of methane in air on heating to 500ºC.
Response of the Palladium-Coated Planar Pellistors, Palladium Adhesion and Detection Limits
The response of the H1-e palladium-coated planar pellistors was linearly proportional to a concentration of 0 to 2.5% methane in air with sensitivity of ~ 35 mV/% methane and good stability. Palladium adhesion to the structure is excellent. The detection limit for devices is < 0.125% methane in air. There is optimism that practical commercial devices can be achieved from this technology.
Source: Platinum Metals Review, April 2003, Volume 47, Number 2, page 72.
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