Research On Preparation And Properties Of The Planer Oxygen Sensor

Planar automotive oxygen sensors, which prepared with multilayer by superimposing and co-sintering process, are the dominating products in the market for their fast light-off, small size, low cost and dependable performance. The performance of each layer determines oxygen sensorsYttria and scandia co-stabilized zirconia (2Y-11ScSZ) electrolyte has many advantages, such as good chemical stability, high oxygen conductivity and good mechanical strength, but the relevant research of its application in the sensor has not been reported yet.2Y-11ScSZ powders were synthesized with a glycine-nitrate process. The powers were characterized via SEM、XRD、BET methods. The results showed that2mol%yttria additions could improve the phase stability of scandia stabilised zirconia (11ScSZ) and stabilise the cubic structure at room temperature.2Y-llScSZ powders had good dispersity by importing PEG400as dispersant. Thin films were fabricated by tape-casting, the optimal proportion of the slurry(based on the powder)was15%of butanone,11%of isopropyl alcohol,2.8%of terpineol,4.5%of polyvinyl butyral,15%of diethyl o-phthalate and7.5%of polyethylene400.. The tapes were characterized by TG-DTA in sintering process. Dense ceramic thin film could be obtained at sintering temperature of1350℃and the density was at98%of theoretical density.A porous layer can avoid the failure of Pt electrode. MgAl2O4has been used as main material until now, but it is easy to fall off from the surface of electrolyte for different thermal expansion coefficient compared with ZrO2. So it is very important to improve the porous protective materials and enhance the thermal shock resistance of oxygen sensors.According to liquid phase sintering theory, three recipes of porous protective layers were studied. The porous protective layers of recipe1which had good thermal shock resistance and strong adhesive property were prepared by sintering at1350℃, where Al2O3and ZrO2were used as main raw materials, aluminium fluoride and barium fluoride as fluxing agent. The porous protective layer had porosity of66.77%and thermal expansion coefficient of10.29×10-6/℃. The porous protective layers of recipe2were mainly composed of Al2O3and MgAl2O4. The porous protective layer with good thermal shock resistance and strong adhesive property was obtained when the ratio of mMgAl2O4and mAl2O3(0.3-5μm) was about3%～5%, which had thermal expansion coefficient of8.25×10-6/℃. The more the content of MgAl2O4, the denser the structure of the sample had. According to the difference of sintering activity, recipe3was mainly composed of two kinds of Al2O3with different grain sizes. The porous protective layer with good thermal shock resistance and strong adhesive property was obtained when the ratio of mAl2O3(10nm) and mAl2O3(0.3-5μm) was about0.5%～1.5%, its thermal expansion coefficient was8.42×10-6/℃. The more the content of Al2O3(10nm), the easier the sample sintered. Each porous protective layer of the fore-mentioned recipes had good thermal shock resistance and strong adhesive property. Although recipe1was best on the basis of thermal expansion coefficient, the others met the demand of oxygen sensors too.Besides, when engines are warmed up or closed, the temperature of oxygen sensors changes severely, porous ZrO2transition layers could improve the thermal shock resistance of oxygen sensors. We had been prepared the porous ZrO2transition layers with the addition of different pore forming materials by tape casting and studied the effects of shapes and sizes of the pore forming materials on their morphology. And co-sintering process was researched of solid electrolyte with porous transition layers. The results indicated that the shapes of pore forming materials decided the morphology of the porous layers. In order to get better porous materials, the pore forming materials must not dissolve into solvent. Besides, because materials of electrolyte were the same as the porous layers, the remaining stress was small and the thermal shock resistance of oxygen sensors was improved.The electrical properties of the electrolyte sintered were analyzed by the electrochemical analysis system. The thermal shock resistance and response time of sensors prepared were surveyed. The results showed that, the conductivity of8YSZ at800℃was0.040S·cm-1, while2Y-11ScSZ film was0.091S·cm-1, showed two times as high electrical conductivity as8YSZ. The thermal shock resistance of planar oxygen sensors was better than the thimble shape oxygen sensor and2Y-11ScSZ better than8YSZ. The response time of2Y-11ScSZ planar oxygen sensors was shortest, the time of300mV to600mV was84ms and173ms from600mV to300mV.Thus,2Y-11ScSZ solid electrolyte in automobile oxygen sensors had more shortter response time, higher sensitivity for high oxygen conductivity and good chemical stability.