The Development of New Type of Carbon Dioxide Gas Sensor Based on Lithium Conductor

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The object of this research is to develop a portable carbon dioxide gas sensor by using a solid electrolyte based on lithium ionic conductor. The electromotive force (EMF) measurements were carried out with several CO_2 gas concentrations. The sensor was set up by using LiTi_2 (PO_4)_3+0.2Li_3PO_4 sintered material as the solid electrolyte, Li_2CO_3 as the detecting electrode, and CaCO_3 as the reference electrode. The measured EMF was in good agreement with the calculated EMF for the gas content from 80 ppm to 1 %. The response time was within several minutes.In a practical application, this type of sensor is expected to be installed in exhaust gas atmosphere. The influence of several gases coexistence with carbon dioxide was investigated. No influence was observed with NO_2 gas copresence from 100 ppm to 450 ppm. This is beca. Mse that lithium nitrate decomposes at the operating temperature of 650ﾟC. The EMF value becomes lower than the calculated EMF even at the SO_2 gas content of 20 ppm. This is ascribed to the formation of lithium sulfate on the electrolyte surface.One another gas should be investigated is water vapor. The sensor can detect the C0_2 concentration from 1000 ppm to 1 % at the water vapor concentration of 0-220g/m^3. The EMF output decreses in the CO_2 gas content lower than 1000 ppm. This phenomenon occurs by the chemical reaction between water vapor and lithium carbonate. In order to lower the operating temperature from 650ﾟC, several electrodes were selected as candidates. The sensor can detect the CO_2 gas concentration from 100 ppm. to 1 % at 350ﾟC in the case that Li_2CO_3-K_2CO_3-Na_2CO_3 eutectic mature was applied as the electrode.