Solid Electrolyte Gas Sensors Research Articles

小特集・化学センサー材料と検知機構 固体電解質ガスセンサー

小特集・化学センサー材料と検知機構 固体電解質ガスセンサー

Solid Electrolyte Gas Sensors for So<SUB>x</SUB> and CO<SUB>2</SUB>

Alkali metal sulfates are basically applicable to SOx sensors as solid electrolytes. The electro-motive force (emf) of the solid electrolyte cells is expressed by the Nernst equation. The lowering of the emf is noticed at a large SOx pressures difference between the anode and the cathode. This is caused by the physical permeation of gases in the electrolyte through cracks and pores which arise from volume change during the III-I phase transition and from low sinterability, The use of NASICON (Na3Zr2Si2PO12) which is easily sintered to high densities provides an SOx sensor with no permeability. The generation of emf is essentially due to the formation of Na2SO4 both on the anode and on the cathode. The solid reference electrodes for SOx sensors are reviewed briefly. A small insert-type CO2 sensor has been constructed using Na2CO3 with a tip of NASICON or β-alumina, and is proved to function satisfactorily. The activity of Na2O in NASICON or β-alumina is taken as a reference state in this sensor.

The Speed of Response of Solid Electrolyte Galvanic Cells for Gas Sensing

The factors affecting the speed of response of solid electrolyte gas sensors are considered. The uptake or release of sample material (gas) by such cells is the main reason for sluggishness in response and for variation with time in the emf developed. Special attention is given to the effect on the emf caused by the presence of a second phase with electrical properties different from those of the main material. For a model cell, consisting of a solid electrolyte containing an intergrain phase with mixed conductor properties, the diffusion equation is solved numerically. The ambipolar diffusion coefficient is used in the calculations. The numerical parameters required for the calculation of concentration profiles and emf as a function of time are those of a typical calcia‐stabilized zirconia. The results have a more general validity.