Meng Naying ¹, Gou Roujia ², Cao Jianwu ¹

(1. College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China; 2. National Engineering Research Center for Domestic & Building Ceramics, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:

[Significance] With the rapid increase in the number of automobiles, automotive exhaust emissions have become a severe environmental issue. Over the past two decades, the industry has made numerous efforts to reduce automotive exhaust emissions, among which the efficiency of the aftertreatment system has played a crucial role in reducing exhaust emissions. Within this system, the oxygen sensor plays a vital role, while the choice of solid electrolyte material significantly impacts the sensor's performance. Among various candidate materials, YSZ has been as the leading one, due to its excellent ionic conductivity, mechanical stability and high chemical stability. However, in the actual working environment of oxygen sensors, YSZ electrolyte materials are subjected to intense thermal shocks caused by rapid temperature fluctuations over prolonged periods. During this process, significant thermal stress continuously accumulates within the material, necessitating that YSZ possess sufficiently high thermal shock resistance. Conducting in-depth research on the thermal shock resistance of YSZ electrolyte materials is of great significance for ensuring their long-term stable operation, improving the efficiency of oxygen sensors and extending their service life. This paper was aimed to systematically review the classification and principles of oxygen sensors, the characteristics of zirconia as an electrolyte material, the influencing factors of YSZ thermal shock resistance, advancements in thermal shock resistance optimization and preparation processes, in order to provide a forward-looking perspective on the future development directions of YSZ electrolyte thermal shock resistance.

[Progress] Oxygen sensor electrolytes mainly include zirconia-based, perovskite-type and ceria-based materials. Among them, yttria-stabilized zirconia (YSZ) is the most technically mature and widely commercialized electrolyte. Pure zirconia undergoes crystal phase transformation, so doping with Y₂O₃ fundamentally stabilizes its crystal structure and avoids phase-change damage. Meanwhile, phase transformation toughening can be achieved under applied pressure, laying a foundation for the thermal shock resistance of YSZ. The thermal shock resistance of YSZ is synergistically affected by multiple factors such as stabilizer doping and microstructure, while its performance can be improved through compositional modification and optimization of preparation processes. In terms of stabilizers, Y₂O₃ is the most suitable dopant, with an optimal doping content of approximately 7 mol%, which balances phase-transformation toughening and mechanical properties. Both insufficient and excessive doping degrade thermal shock resistance. In addition, the doping concentration also influences the ionic conductivity of YSZ, which reaches a maximum at about 8 mol%. Regarding microstructure, grain refinement enhances grain boundary strength and fracture toughness, while suppressing crack propagation. Moderate porosity provides stress-relief paths and alleviates thermal stress concentration. A suitable combination of grain size and porosity can simultaneously improve thermal shock resistance and oxygen ion transport. However, the thermal shock resistance of YSZ electrolytes used in oxygen sensors degrades significantly with prolonged service. Therefore, further research on enhancing the thermal shock resistance of YSZ electrolytes is highly necessary. Current studies on the thermal shock behavior of YSZ mainly focus on two aspects. The first is compositional modification. Introducing cations, such as Mg²⁺, Al³⁺ and Fe³⁺, refines grains via lattice distortion, induces crack deflection, increases thermal conductivity and generates oxygen vacancies, thereby improving both mechanical and electrical properties. Multielement co-doping enables synergistic regulation of the microstructure, stabilizes the crystal phase and reduces thermal stress accumulation. The second is innovative preparation processes. Traditional dry and wet forming methods have their respective advantages and limitations. Emerging techniques, such as photocuring 3D printing and atmospheric plasma spraying, allow precise control over fabrication and sintering parameters to achieve high densification and grain refinement of YSZ. Moreover, controllable microcracks can be introduced to realize crack-deflection toughening, significantly enhancing thermal shock resistance.

[Conclusions and prospects] Due to the harsh working environment of oxygen sensors, considerable attention has been paid to the thermal shock resistance of YSZ, which is the most mature and widely used electrolyte material for oxygen sensors. This paper was aimed to review the characteristics of solid electrolyte materials for oxygen sensors, the factors affecting the thermal shock resistance of YSZ electrolytes and the current research status of improving the thermal shock resistance of YSZ through material modification and innovative preparation processes. Although remarkable breakthroughs have been made in improving the thermal shock resistance of YSZ electrolytes in the above two aspects, few studies have been carried out under actual working conditions. In the future, research on YSZ-based electrolytes for oxygen sensors should be combined with practical service conditions to investigate the effects of different thermal shock parameters on microstructure, mechanical properties and core electrical properties. With microstructure as a bridge, the coupling relationship among mechanical properties, microstructure and electrical properties should be established. Meanwhile, in terms of material modification, multi-cation co-doping should be actively explored. For preparation process optimization, it is necessary to regulate processing parameters and explore the corresponding relationship among preparation process, microstructure and performance.

Key words: oxygen sensor; yttria-stabilized zirconia; thermal shock resistance