The ceramic elements mainly used in the chip wide-area oxygen sensor include a porous protective layer, a solid electrolyte, a diffusion barrier, and a support layer. The solid electrolyte is a high-temperature conductive ceramic, and compactness is an important factor in reducing the impedance of the solid electrolyte, increasing the conductivity, and improving the mechanical strength. The porous protective layer and diffusion barrier require ceramics to have a certain open porosity to ensure smooth gas diffusion. The support layer is the heater matrix, which can withstand high temperature impact on the one hand, and requires good mechanical properties on the other hand. The functions of each layer are different, but they are tightly integrated. How to ensure the independence and unity of these components needs to be considered from the selection and preparation of materials.
Zirconia-based ceramics have excellent properties such as high ionic conductivity, high fracture toughness, low thermal conductivity, high thermal expansion coefficient and high hardness. In the research of automotive oxygen sensors, zirconia-based ceramics with different chemical ratios can be used to prepare solid electrolytes, support layers, diffusion barriers, and porous protective layers, respectively, to ensure the independent characteristics of each layer of materials and similar interface compatibility , In order to facilitate the integration of multilayer materials.
The ceramic preparation process mainly includes three steps: powder preparation, solid molding and high temperature sintering. The preparation of powder is the primary factor affecting the performance of ceramics.
2.1 Preparation of zirconia powder
There are three main methods for preparing zirconia-based ceramic powder: mechanical method, gas phase method and liquid phase method. The mechanical method is difficult to prepare ultrafine powder, and it is easy to introduce impurities. Gas phase method can get very good ultrafine powder, but the cost is higher. The liquid phase method is more popular, and the obtained ceramic powder has better uniformity and low cost. It is currently the most commonly used method for preparing ceramic powder in experiments and industry.
Liquid phase methods mainly include solvent evaporation method, hydrolysis method, molten salt method, microemulsion method, liquid phase precipitation method, hydrothermal method and sol-gel method.
l Liquid-phase precipitation method: dissolve soluble salt in a solvent, then add a precipitation agent to generate solvent-insoluble salts or hydroxides precipitate, and then filter, wash, dry or heat decomposition and other processes to obtain powder .
l Sol-gel method: first dissolve the inorganic salt or metal alkoxide in a solvent and mix it uniformly to form a solution; then proceed to hydrolysis and condensation chemical reactions to form a stable sol; the sol is aged and condensed to form a three-dimensional network structure, A gel is formed. The gel is dried, ground and calcined to obtain nano powder. The sol-gel method is one of the most common methods for preparing ceramic powder. The ceramic powder prepared by this method has a small particle size, high purity, easy control of the reaction process and chemical composition, low reaction temperature, and simple operation.
l Hydrothermal method: The hydrothermal method is a soft chemical preparation method, which refers to the synthesis by the chemical reaction of the solute in the aqueous solution under a certain temperature and pressure in a closed system. The precursor is fully dissolved in a closed system, and reaches a certain degree of supersaturation, thereby forming atomic or molecular growth elements, and then nucleating and crystallization to generate powder or nanocrystals.
2.2 Zirconia ceramic molding
The molding process is the key to ensuring the reliability of ceramic material performance and mass production. The molding process is divided into dry molding and wet molding. Dry molding is divided into dry pressing molding and isostatic pressing molding; wet molding is divided into grouting molding, hot die casting molding, casting molding, injection molding, injection molding and screen printing.
We mainly take the tape casting process of the zirconia sensor substrate as an example to describe. The casting method is to configure the ground powder and organic solvent into a slurry with a certain viscosity according to a certain ratio, and then use a doctor blade to coat the slurry on a special base tape according to a certain thickness, and then dry, Curing, peeling off the film of the green tape from the base tape, punching, laminating, and other processing treatments on the green tape to form a finished product of a certain size and shape. The products produced by the casting process have the advantages of high production efficiency, simple equipment, stable process, uniform structure, and good product quality. With the development of process technology, the application results of tape casting have continued to expand. In 1996, the Japanese used a tape casting method to prepare a capacitor film with a thickness of 5 nm, and in 1997, a film with a thickness of 3 nm was obtained. Now tape casting has occupied a dominant position in the field of multilayer co-fired ceramics (HTCC, LTCC).
The measuring principle and structural characteristics of the zirconia sensor:
The core component of the zirconia sensor is zirconia solid electrolyte, which is composed of multiple oxides. Commonly used such electrolytes are ZrO2·Y2O3, which is composed of binary oxides, among which ZrO2 is called the matrix and Y2O3 is called the stabilizer. ZrO2 is a monoclinic crystal at room temperature. It becomes a cubic crystal (fluorite type) at high temperature, but it becomes a monoclinic crystal when it is cooled, so the crystal form of pure zirconia is unstable. Therefore, when a certain amount of stabilizer Y2O3 is added to ZrO2, since Y3+ replaces the position of Zr4+, on the one hand, oxygen ion holes are left in the crystal, and on the other hand, due to the change in internal stress of the crystal, the crystal cools down. Cubic crystals remain afterwards, so it is also called stabilized zirconia. According to the above analysis, stabilized zirconia is a good conductor of oxygen ions at high temperatures (above 650°C).
Systech Instruments Ltd
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