Development of Oxygen Sensors Utilizing SPE Composite Electrodes

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A new type of oxygens sensor was developed by utilizing SPE composite electrodes. As an SPE material, perfluorinated ion-exchange membrane, Nafion, was selected because of its chemical and thermal stability. On the Nafion, thin, porous platinum was deposited for the use of a detector electrode by an electroless plating method using hydrazine or sodium tetrahydroborate as reductant. Polyehtylen film of 10 mum was hot-pressed on the platinum electrode as a permeation limiting membrane for oxygen. The thickness of the layer was controlled by changing the number of polyethylene films. This sensor showed the proportional response in its reduction current to the oxygen content up to 50%. The sensor was stable at least until 2,000 hours.Nafion particles of about 1mm in diameter was also used as an SPE material in order to prepare miniaturized oxygen sensors. Platinum was deposited in the same manner as above. Since any conventional method was not available to cover on such a small platinum layer uniformly, plasma polymerization technique was selected. Hexafluoropropylene was selected as a starting material for plasma polyner layer. Under the selected conditions, very stable thin layers of perfluorinated pofymer was coated uniformly even in the thickness less than 1/mum. Using this technology, miniaturized sensor of 1.2mm was fabricated. This sensor showed linear response to both gaseous oxygen and dissolved oxygen. The response time of the sensor was longer than the value expected from the thickness of the oxygen perm-limiting membrane.In order to fabricated the smaller and more stable sensors, new ultra-thin ion-exchange membranes was prepared by a Plasma Polymerization technology. Nafion was treated to enhance the perm-selectivity for the sake of fabrication of more stable sensors.